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Capillary microreactors based on hierarchical SiO2 monoliths incorporating noble metal nanoparticles for the Preferential Oxidation of CO
... Silica supports are used for anchoring and dispersing metal nanoparticles on the functional silica surfaces [11], high surface area and tunable surface properties, good thermal stability, and varied morphologies [12]. Aerogel [7], mesoporous silica [13], rods, etc., have been widely explored also for CO oxidation [14,15]. ...
... Silica aerogels with high porosity consist of a silica network and have been used in many catalyzed reactions [9], wherein the aerogels were reported to show enhanced catalytic activity due to local heating and heat retention properties [10,14]. Fumed silica is used as a raw material for additives in various chemical industry products [16,17]. ...
The catalyst morphology, metal-support interaction, and reaction conditions greatly influence the catalytic performance and reaction dynamics. Similarly, the dispersion of the metal within the support plays a crucial role in the thermal stability and sintering of the catalyst. Furthermore, temperature-dependent conversion hysteresis is well-known to occur during ignition and extinction of exothermic CO oxidation over supported Pd catalysts due to the variation of CO adsorption on the surface or bulk oxidation of Pd and the ability of the catalyst to regenerate the active sites. Herein, the catalytic performance and the hysteresis behavior of mesoporous silica aerogel supported Pd (Pd/a-SiO2), and commercial fumed silica-supported Pd (Pd/f-SiO2) were investigated and compared using CO oxidation as a probe reaction under different reaction conditions and operating parameters (i.e., catalyst weight, ramp rate, and flow rate). Surface and morphologic examination using XPS, FTIR, and TEM of Pd/a-SiO2 and Pd/f-SiO2 reveal a strong correlation between the catalyst surface and structure and its catalytic performance and stability under different reaction parameters. Moreover, this study presents the effect of surface area, particle size, and size distribution on diffusion and mass transport of reactants (CO, O2) and products (CO2) and active sites accessibility.
This study showed that Pd/f-SiO2 had better efficiency under high (turbulence) flow. Moreover, intrinsic apparent activation energy (Ea) and the number of active sites were calculated from the Kinetics of CO oxidation fitted using Arrhenius plots indicate that the ramp rate has less effect on Pd/f-SiO2 catalytic behavior. Even though, Pd/f-SiO2 had higher relative active sites than Pd/a-SiO2, (Ea) was lower. Cyclic stability and long-term stabilities showed that both catalysts are stable and can regenerate the active sites. The current study contributes to understanding the catalysts' surface, structural and morphological properties on the catalysts' performance toward CO oxidation and other reactions under dynamic conditions.
... As it was previously reported, the stabilization efficiency of a polymer depends on its protective value, which is defined as the weight, in grams, of gold in a red gold sol which is protected by 1 g of protecting polymer against flocculation by 1% sodium chloride [41]. The protective value of PVP is much larger than those of other polymers used as capping agents (50.0, 5.0, 1.3, 0.07 and 0.04 for poly(N-vinyl-2-pyrrolidone) (PVP), poly(vinyl alcohol), poly(acrylamide), poly(acrylic acid) and poly(ethyleneimine), respectively) and, therefore, is one of the most vastly used capping agent found in the literature [27,28,[42][43][44][45][46][47][48]. ...
... Pd, well-known for its remarkable capacity in hydrogen absorption, is widely used for numerous important applications, such as low-temperature reduction of automobile pollutants, hydrogenation/dehydrogenation reactions, hydrogen purification, electrocatalysis, petroleum cracking, water treatment and a range or organic reaction, such as Suzuki, Heck, etc. [29,43,[153][154][155][156][157][158][159][160]. Due to the widespread use of Pd, innumerable investigations dealing with the control of the size and shape of Pd nanocrystals and their impact in the Pd-based catalysts have been addressed. ...
The pivotal role of size and morphology-controlled nanocrystals in catalysis is a recognized fact nowadays. Among the strategies developed to adjust such features, colloidal synthetic approaches have been proven to be a valuable alternative through which noble metal nanocrystals with tailored sizes and morphologies can be formed upon proper selection of the experimental conditions. This review summarizes some of the main aspects to be considered in the synthesis of colloidal noble metal and includes representative examples of their catalytic applications by spotlighting the experimental conditions used in the synthesis and how the size and/or shape of the nanocrystals influence in the final catalytic performance.
... At present, different types of microreactors are employed for heterogeneous catalysis, such as packed-bed, monolithic and wall-coated microreactors [22]. The former, composed by packed microparticles of a solid catalyst inside the microchannel, is very interesting for solid-liquid reactions, due to a highly improved micromixing performance compared with non-packed microchannels [23], and for catalytic reactions in gas phase since it assures a more efficient contact with the catalyst [24]. However, the packed-bed microreactor strategy presents as the main drawback the immobilization of the catalyst inside the capillary, since the synthesis in-situ of the catalyst filling is not straightforward and may require synthesis conditions which are not compatible with the microreactor materials. ...
... The incorporation of a benchmark TiO 2 sample such as P25 without treatment inside the fused silica capillaries was carried out by using a dispersion of the P25 in EtOH. This approach is a straightforward methodology to incorporate already synthesized catalysts compared to the methodology of synthesizing the catalyst inside the microreactor in packed-bed, monolithic or wall-coated configuration, which is the usual process reported in the literature [13,23,24]. ...
The elimination of volatile organic compounds (VOCs) at low concentration is a subject of great interest because these compounds are very harmful for the environment and human health. In this work, we have developed an easy methodology to immobilize a benchmark photocatalyst (P25) inside a capillary microreactor (Fused silica capillary with UV transparent coating) without any previous treatment. For this purpose, a dispersion of the sample (P25) in EtOH was used obtaining a packed bed configuration. We have improved the immobilization of the benchmark photocatalyst (P25) inside the capillary incorporating a surfactant (F-127) to generate porosity inside the microreactor to avoid severe pressure drops (∆P < 0.5 bar). The resulting capillaries were characterized by Scanning Electron Microscopy (SEM). These microreactors show a good performance in the abatement of propene (VOC) under flow conditions per mol of active phase (P25) due to an improved mass transfer when the photocatalyst is inside the capillary. Moreover, the prepared microreactors present a higher CO₂ production rate (mole CO₂/(mole P25·s)) with respect to the same TiO₂ operating in a conventional reactor. The microreactor with low pressure drop is very interesting for the abatement of the VOCs since it improves the photoactivity of P25 per mol of TiO₂ operating at near atmospheric pressure.
... As it was previously reported, the stabilization efficiency of a polymer depends on its protective value, which is defined as the weight, in grams, of gold in a red gold sol which is protected by 1 g of protecting polymer against flocculation by 1% sodium chloride [41]. The protective value of PVP is much larger than those of other polymers used as capping agents (50.0, 5.0, 1.3, 0.07 and 0.04 for poly(N-vinyl-2-pyrrolidone) (PVP), poly(vinyl alcohol), poly(acrylamide), poly(acrylic acid) and poly(ethyleneimine), respectively) and, therefore, is one of the most vastly used capping agent found in the literature [27,28,[42][43][44][45][46][47][48]. ...
... Pd, well-known for its remarkable capacity in hydrogen absorption, is widely used for numerous important applications, such as low-temperature reduction of automobile pollutants, hydrogenation/dehydrogenation reactions, hydrogen purification, electrocatalysis, petroleum cracking, water treatment and a range or organic reaction, such as Suzuki, Heck, etc. [29,43,[153][154][155][156][157][158][159][160]. Due to the widespread use of Pd, innumerable investigations dealing with the control of the size and shape of Pd nanocrystals and their impact in the Pd-based catalysts have been addressed. ...
Although black phosphorus (BP) is an interesting 2D nanosheet material with a high hole mobility, its application in the photocatalytic water oxidation for O2 evolution is not reported yet. Herein the use of BP coupled with a reductive hydroxide, Ni(OH)2, is reported for the first time. The developed photo‐assisted process confirms that the BP XPS measurements confirm that the oxidation state of BP is reduced through the photo‐assisted loading method. The obtained P‐Ni(OH)2 samples present the steady and efficient photocatalytic water splitting for O2 generation. Under the simulated sunlight irradiation in 0.1 M Na2S2O4 solution, the O2 generation rate can reach up to 15.7 μmol/(gcatalyst*h) or 224.3 μmol/(gBP*h). The density functional theory (DFT) calculation suggests that the electrons and holes move to surface of BP and Ni(OH)2, respectively. The merit of self‐polarization of P‐Ni(OH)2 ensures the stability of BP and achieves the photocatalytic O2 generation from water.
... Furthermore, monolithic devices (microreactors containing a porous network of typically silica or other oxide materials) can be functionalized extensively with enzymes [12], magnetic nanoparticles [11] and other functional groups [12][13][14] in order to tailor their catalytic properties such as selectivity and activity [10,15]. The incorporation of metal nanoparticles in monolithic structures in particular has received attention for its applications in, chromatography [15,16], metal adsorption for contaminant purification [17], C-C coupling reactions [18], reduction of nitrophenols [19] and CO oxidation reactions [20]. In the case of oxidation it has been reported that silica monoliths loaded with either Pt or Pd nanoparticles presented conversions 2.5 times higher in microreactors than when using powder catalysts. ...
... In the case of oxidation it has been reported that silica monoliths loaded with either Pt or Pd nanoparticles presented conversions 2.5 times higher in microreactors than when using powder catalysts. This shows that using microreactor technology for selective oxidation reactions enhances the catalytic ability of precious metal catalysts [20]. A major challenge in all cases however is to evenly functionalize the catalytically active species along the length of the monolithic microreactor, which is key to maximizing efficient reaction control. ...
... Microreactor technology has exhibited considerable potential in the synthesis of nanoparticles due to its high microscopic mixing rate [19][20][21][22][23][24][25]. The aggregation and morphology of nanoparticles can be controlled by enhancing the mass transfer process in microreactors, which strongly influences the specific surface areas, pore volumes and crystal sizes of the resultant nanoparticles. ...
In this work, several mesoporous γ-Al2O3 nanorods with high pore volume and narrow pore size distribution were controllable prepared via “gibbsite-AACH (ammonium aluminum carbonate hydroxide)” precursor route in a membrane dispersion microreactor. The effects of reaction temperature, (NH4)2CO3 concentration and calcination temperature on the samples were intensively studied. The highly uniformed morphology was assured due to the high mixing intensify in the membrane dispersion microreactor and the successful conversion of the precursor in the slurry. Under optimal conditions, these as-prepared γ-Al2O3 nanorods behaved high pore volume of 1.65 mL/g and narrow pore size distribution of 3–20 nm with a length and width of 150–200 nm and 30–60 nm, respectively. Furthermore, the adsorption experiment of the prepared samples were carried out under natural pH value, and the as-prepared sample of AACH-500 exhibited favorable adsorption performance for Congo red (CR) solution. The maximal adsorption capacity was up to 2006.7 mg/g, thus indicating that the as-prepared mesoporous γ-Al2O3 nanorods are in adsorption and other application potential of the field.
... Catalytic oxidation of carbon monoxide is a research area of dominant significance because of its application in critical spheres of CO preferential oxidation in hydrogen-rich gas, vehicle exhaust removal and pollution air purification [1]. CO is an odorless and colorless gas which is very hazardous and toxic at a concentration above 35 ppm for animals and humans [2] due to its high affinity toward hemoglobin. ...
As a result of the increase in industrial activities and the extensive use of automobiles, air pollutants, especially carbon monoxide, reach the alarm level. The removal of carbon monoxide through oxidation is still one of the best tactics so far. Therefore, looking for an active and durable catalyst is considered the key factor for improving the oxidation process. In this work, Pd nanoparticles (1.0, 3.0 and 5.0 wt.%) were supported onto reduced graphene oxide / copper metal organic framework nanocomposite ([email protected]) through one pot solvothermal synthesis method. The catalysts were thoroughly characterized by state-of-the-art techniques such as XPS, SEM, TEM, XRD, N2 physisorption, and FT-IR. The results revealed that the Pd nanoparticles were exceptionally dispersed on [email protected] nanocomposite surface and the Cu-BTC crystals showed excellent octahedral structure with smooth edges as indicated SEM images. The results revealed that 3.0 wt. % Pd/[email protected] acts as an excellent catalyst in the CO oxidation as indicated by T50 and T100 values of 71 and 82 °C, respectively. Additionally, the results showed that rGO has a vital role in the dispersion of Pd NPs on the catalyst surface as well as the presence of surface oxygen groups, which yields higher catalytic activity compared with the catalyst without rGO.
... Microreactor technology has exhibited considerable potential in the synthesis of nanoparticles due to its high microscopic mixing rate [19][20][21][22][23][24][25]. The aggregation and morphology of nanoparticles can be controlled by enhancing the mass transfer process in microreactors, which strongly influences the specific surface areas, pore volumes and crystal sizes of the resultant nanoparticles. ...
Mesoporous γ-Al2O3 nanofibers with high pore volume and uniform pore size distributions were successfully synthesized via a template-free method in a membrane dispersion microreactor followed by calcination. The effects of crystal temperature, pH values, continuous phase concentration and washing solvent on the γ-Al2O3 nanofibers were carefully studied. The results showed that the as-obtained γ-Al2O3 nanofibers showed a length of 40-60 nm and a width of 3.2-3.4 nm, which were attributed to the high microscopic mixing rate in the membrane dispersion microreactor. Moreover, the precursors of γ-Al2O3 nanofibers treated with deionized water and mixed deionized water/alcohol solution had high pore volumes, reaching to 1.60 mL/g and 2.00 mL/g, respectively. In addition, the adsorption performance of γ-Al2O3 nanofibers with high pore volumes was also investigated. These fibers showed an excellent adsorption capacity of 1323.68 mg/g for the removal of Congo red from aqueous solution, thereby indicating their potential for applications in adsorption and other related areas.
... Catalytic oxidation of carbon monoxide (CO) has drawn much research attention due to its significance in fundamental research and practical applications in pollution air purification, vehicle exhaust removal, and CO preferential oxidation in hydrogen-rich gas [1][2][3][4][5]. Noble metal catalysts, including supported Au, Pd, Pt, etc., exhibit excellent CO oxidation activity [6][7][8]. ...
... Sol-gel derived noble-metal-silica nanocomposites are among the most frequently investigated silica based composites, due to their wide range of applications. In some applications, for example the application as electrode materials for catalysis or cells, high loading of metal content is required [13][14][15][16][17][18][19]. For such applications, Ag is superior to other noble-metal-silica nanocomposites, due to its highest conductivity, much lower price (compared to gold (Au), platinum (Pt), palladium (Pd), et al), and higher chemical stability (compare to Cu). ...
Sol-gel derived noble-metal-silica nanocomposites are very useful in many applications. Due to relatively low price, higher conductivity, and higher chemical stability of silver (Ag) compared with copper (Cu), Ag-silica has gained much more research interest. However, it remains a significant challenge to realize high loading of Ag content in sol-gel Ag-silica composite with high structural controllability and nanoparticles’ dispersity. Different from previous works by using multifunctional silicon alkoxide to anchor metal ions, here we report the synthesis of Ag-silica nanocomposite with high loading of Ag nanoparticles by employing acetonitrile bi-functionally as solvent and metal ions stabilizer. The electrical conductivity of the Ag-silica nanocomposite reached higher than 6800 S/cm. In addition, the Ag-silica nanocomposite could simultaneously possess high electrical conductivity and positive conductivity-temperature coefficient by properly controlling the loading content of Ag. Such behavior is potentially advantageous for high-temperature devices (like phosphoric acid fuel cells) and inhibiting the thermal-induced increase of devices’ internal resistance. The strategy proposed here is also compatible with block-copolymer directed self-assembly of mesoporous material, spin-coating of film and electrospinning of nanofiber, making it more charming in various practical applications.
... Compared with conventional equilibrium apparatus, microapparatus is easily adapted to targeted applications and are a powerful tool for microscale processing due to their low reagent consumption rates, superior mass and heat transfer characteristics [19][20][21]. Raman spectroscopy is a useful optical analysis method, because it is nondestructive, sensitive, rapid and involves minimal sample preparation. In accordance with the band intensity of a Raman active species being linearly proportional to its concentration in a fluid [22,23], the Raman peak intensity ratio reflects the varying concentration of solute and solvent [19,24,25], which helps us to better grasp the change of components in system. ...
This work developed a new method for measuring the volume expansion of a CO2 + n-dodecane mixture, using a fused silica capillary cell (FSCC), combined with a heating-cooling stage and a confocal Raman spectrometer. The cell was constructed on the micron scale to reduce the temperature gradient and reagent consumption. The results confirmed the feasibility of this method that using Raman spectra to verify phase equilibrium, a digital camera to record images of the CO2 + n-dodecane mixture and a micrometer to measure the volume, a section of water (water seal) between n-dodecane and gas phase to prevent n-dodecane from evaporating. It was shown that the volume expansion factor of the mixture increased with pressure going from 1.0 to 10.0 MPa, but decreased as the temperature was raised from 30 to 80 °C. In addition, a positive quadratic relationship (R² > 0.99) was obtained between Raman peak intensity ratio and volume expansion factor, such that more volume expansion factors could be obtained according to the equations with measured Raman peak intensity ratio data. This experiment widens the ranges of temperature and pressure in the measurements of the volume expansion, and provides more volume expansion data for CO2-enhanced oil recovery.
... Pd NPs incorporated in support (typically porous silica) have been used widely as catalysts in chemical processes such as hydrogenation [24][25][26][27][28], CO oxidation [29][30][31][32][33] and C-C coupling reactions [34][35][36][37][38]. Generally, the incorporation of metal NPs within silica supports can be achieved by a post-synthetic deposition method and a direct synthesis method. However, the particle size and the dispersion of metal nanoparticles (NPs) of the supported metal catalysts prepared by the post-synthetic deposition method are difficult to control during the preparation processes, however, these two factors have great effects on the catalytic activity and selectivity [6,22,39]. ...
Pd incorporated in mesoporous silica (Pd-MS) catalysts were prepared by a direct synthesis method in the presence of 1, 3, 5-trimethylbenzene (TMB) and their formation mechanism was proposed. TMB not only acted as an effective expanding agent to enlarge pore size and pore volume of MS but also acted as a structure-directing agent to induce the formation of worm-like morphology. For solvent-free hydrogenation of p-chloronitrobenzene (p-CNB), a complete conversion of p-CNB with a selectivity towards p-chloroaniline of 99.9% over the Pd-MS catalyst with addition of TMB (Pd-MS-50-TMB, where 50 represented the addition of 5 mL of aqueous NH3) was obtained at 85 °C and 3.45 MPa of H2 in 2 h, while only 48.4% p-CNB conversion over Pd-MS-50 synthesized without TMB was obtained. The enhanced activity of the Pd-MS-50-TMB was not only attributed to the smaller size of Pd particles, but also to the worm-like morphology of the support with larger pore size and pore volume that could enhance mass transfer and allow the reactants more facile access to Pd for reaction.
... Monoliths can offer several benefits to conventionally prepared catalysts for the preferential oxidation of CO. For example, coating the active metal on monoliths for the CO PrOx reaction can result in an enhanced catalytic performance, compared to the catalyst in powder form [275]. In this recent study, the authors showed that supporting Pt and Pd nanoparticles on silica-monoliths led to an improved activity and selectivity for the reaction, possibly due to better mass and heat transfer properties. ...
Considerable research has been conducted on monolithic catalysts for various applications. Strategies toward coating monoliths are of equal interest and importance. In this paper, the preparation of monoliths and monolithic catalysts have been summarized. More specifically, a brief explanation for the manufacturing of ceramic and metallic monoliths has been provided. Also, different methods for coating γ-alumina, as a secondary support, are included. Techniques used to deposit metal-based species, zeolites and carbon onto monoliths are discussed. Furthermore, monoliths extruded with metal oxides, zeolites and carbon are described. The main foci are on the reasoning and understanding behind the preparation of monolithic catalysts. Ideas and concerns are also contributed to encourage better approaches when designing these catalysts. More importantly, the relevance of monolithic structures to reactions, such as the selective oxidation of alkanes, catalytic combustion for power generation and the preferential oxidation of carbon monoxide, has been described.
... Pd and CoPd nanoparticles were prepared by the reduction by solvent modifying the procedure described in other works of our research group [1,3]. For the synthesis of the nanoparticles, palladium and cobalt precursors were palladium acetate (Pd(OAc) 2 , 98%, Sigma-Aldrich) and cobalt(II) nitrate hexahidrate (Co(NO 3 ) 2 ·6H 2 O, Sigma-Aldrich). ...
Pd and CoxPd1-x nanoparticles (NPs), synthesized using the reduction by solvent method, were loaded on SiO2 and Ti-SiO2 supports. The resulting catalysts were tested in the ammonia-borane decomposition reaction under dark and UV-vis conditions. The synergistic promotion by Co (in the NPs) and Ti (in the support), combined with the UV-vis light irradiation, enhanced the catalytic activity showing very promising TOFs values for this kind of catalysis, from 1.53 to 49.5 mol H2 per mol Pd per min.
... However, considering that the vast majority of modern chemical processes utilise heterogeneous catalysts to replace stoichiometric reactions due to obvious economic and environmental advantages, the application of catalytic coatings in microreactors is disproportionately scarcely studied [17]. A few gas-phase heterogeneously catalysed reactions have been studied in various reactions such as Fisher-Tropsch synthesis, CO 2 hydrogenation, water gas shift [18], preferential CO oxidation [19], and complete oxidation of organic compounds [20] demonstrating a 2-5 fold increase in reaction rates compared to conventional reactors [21][22][23][24]. ...
A 2.5wt.% Pd/ZnO catalytic coating has been deposited onto the inner wall of capillary reactors with a diameter of 0.53 and 1.6mm. The coatings were characterised by XRD, SEM, TEM and elemental analysis. The performance of catalytic reactors was studied in solvent-free hydrogenation of 2-methyl-3-butyn-2-ol. No mass transfer limitations was observed in the reactor with a diameter of 0.53mm up to a catalyst loading of 1.0kg(Pd) m-3. The activity and selectivity of the catalysts has been studied in a batch reactor to develop a kinetic model. The kinetic model was combined with the reactor model to describe the obtained data in a wide range of reaction conditions. The model was applied to calculate the range of reaction conditions to reach a production rate of liquid product of 10-50kg a day in a single catalytic capillary reactor.
Catalytic performance is known to be influenced by several factors, with the catalysts’ surface oxidation state being the most prominent of all. Cobalt appears as one of the most promising materials for preferential oxidation of carbon monoxide in hydrogen rich mixtures (COPrOx). However, the oxidation state of the active sites on cobalt-based catalysts for COPrOx is a subject of intense debate. In this thesis, we use operando NAP-XPS and NEXAFS combined with DFT and other in situ and ex situ characterization methods to correlate COPrOx activity and cobalt oxidation state. Based on NAP-XPS and NEXAFs results, we identified CoO as the optimum cobalt oxidation state, while first principal calculations provided a rational explanation of this finding. We also noted that CoO is metastable and oxidizes fast under COPrOx conditions leading to catalyst deactivation.
The development of “selective” reaction catalysts has considerable research interest in synthesizing fine chemicals and bioactive compounds. We have developed four types of solid-state heterogeneous palladium catalysts immobilized on cellulose particles (CLP), monolithic cellulose (CLM), and monolithic silica particles (SM), 5% Pd/CLP, 5% Pd/CLM, 5% Pd/SM, and 0.25% Pd/SM(sc) [sc: supercritical], for chemoselective hydrogenation under batch and continuous-flow reaction conditions. These four catalysts indicate novel chemoselectivity toward hydrogenations based on the structure of supports and immobilization methods, even using the same techniques or materials. 5% Pd/CLM, 5% Pd/SM, and 0.25% Pd/SM(sc)-packed catalyst cartridges for continuous-flow hydrogenation were also developed as efficiently utilizing the monolithic structure of the solid-state catalysts. The continuous-flow system using such catalysts could achieve high productivities of the hydrogenated products. Furthermore, the tertiary amine-functionalized basic anion exchange resin WA30, as a solid-state organocatalyst, catalyzed synthesis of site-selectively deuterium-labeled β-nitroalcohols have been developed under both batch and continuous-flow conditions. The WA30-catalyzed deuteration of nitromethane in deuterium oxide and the subsequent nitroaldol reaction proceeded to provide the bis-deuterium-labeled β-nitroalcohols in high yields and high deuterium contents. The WA30-packed catalyst cartridge can be applied in continuous synthesis for at least 72 h without degradation of the catalyst activity.
Catalytic membrane plays an important role in environmental remedy. In this study, we reported an Ag coated membrane (PAN-Si-Cu-Ag) with a high catalytic activity to reduce 4-nitrophenol (4-NP) and methyl orange (MO) from water. The best performance is 99% reduction degree and 280 L.m⁻².h⁻¹.bar⁻¹ flux for (4-NP) reduction at 4-NP: NaBH4 =1:50 (mM) during a 12-hour filtration. The reduction degree for MO is above 90% and the flux is about 230 L·m⁻²·h⁻¹·bar⁻¹, which is almost the best report till now. The Ag coated membrane was prepared by metal displacement-epitaxial growth on silica covalent grafted PAN membrane (PAN-Si). Silica atoms were used as linker to ensure the good adhesion between polymer and metal NPs, the loss amount of Ag NPs from the coated catalytic membrane is loss about 2μg/cm² after one month storage. Cheap metal NPs were firstly reduced on the surface of PAN-Si membrane and then used to displace Ag ions. Thus the obtained catalytic membrane showed a very high loading (28%). Finally, the catalytic filtration mechanism of 4-NP was distinguished by Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and adsorption measurement.
Two different types of palladium catalysts supported on dual-pore monolithic silica beads [5% Pd/SM and 0.25% Pd/SM(sc)] for chemoselective hydrogenation were developed. Alkyne, alkene, azide, and nitro functionalities and the aromatic N-Cbz protecting group were chemoselectively hydrogenated using 5% Pd/SM. On the other hand, 0.25% Pd/SM(sc) showed unique and higher hydrogenation catalyst activity toward a wide variety of reducible functionalities. Furthermore, the catalyst activities of both 5% Pd/SM and 0.25% Pd/SM(sc) under continuous-flow hydrogenation conditions were also evaluated. A pre-packed 5% Pd/SM cartridge could be used continuously for at least 72 h without any loss of catalyst activity. The 0.2% Pd/SM(sc) catalyst prepacked in a cartridge showed high catalyst activity for the continuous-flow hydrogenation of trisubstituted alkenes under mild reaction conditions.
We describe a novel strategy to increase the unit-time-productivity of flow synthesis by using hierarchical bimodal porous silica gel (HBPSG) supported palladium column reactors. Because HBPSG has a significantly large surface area, the column reactors have low pressure drop, enabling high-volume production. We demonstrated flow synthesis of the precursor of adapalene, a pharmaceutical compound, at 5 g/h which is over 10-fold greater productivity than previous approaches.
Various carbon materials were used as support of polyvinylpyrrolidone (PVP)-capped Pd nanoparticles for the synthesis of catalysts for the production of hydrogen from formic acid dehydrogenation reaction. Among investigated, MWCNT-supported catalysts were the most promising, with a TOF of 1430 h⁻¹ at 80°C. The presence of PVP was shown to play a positive role by increasing the hydrophilicity of the materials and enhancing the interface contact between the reactant molecules and the catalytic active sites.
A facile synthetic protocol was used to prepare morphology controlled Pd nanocrystals with spherical and cubic shapes of different sizes. Carbon-supported catalysts were prepared from the as-synthesised nanocrystals and their catalytic ability in a tandem dehydrogenation/hydrogenation reaction composed by the dehydrogenation of ammonia borane, serving as a hydrogen source, and the subsequent hydrogenation of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) was assessed. The catalytic performance was strongly dependent on the nanocrystals morphology and the spherical nanoparticles with an average size of 5.5 nm displayed the best performance among investigated.
GRAPHICAL ABSTRACT
Synopsis: Colloidal synthesis was used to prepare morphology-controlled Pd nanocrystals with spherical and cubic shapes and different sizes. They were loaded on a carbon support and tested in a tandem dehydrogenation/hydrogenation reaction based on the hydrogen production from \(\hbox {NH}_{3}\hbox {BH}_{3}\) and hydrogenation of 4-nitrophenol. The catalytic activity was dependent on the nanocrystal morphology. Open image in new window
In this work, it is demonstrated that hierarchical mesoporous silica monoliths (HMSMs) with trimodal porosities can be facilely prepared using P123 as structure-directing agent and tetraethoxysilane as precursor. In such HMSMs, classic co-continuous macrostructures with interconnected macropores and smooth isotropic skeletons can be obtained in a tunable way, depending on the synthesis variables, including the amounts of P123 and 1,3,5-trimethylbenzene and acid concentrations. Skeletons feature randomly oriented SBA-15-type primary particles with 2D hexagonally ordered mesochannels largely accessible from bimodal macropores. Co-continuous macropore size of a typical HMSM (P1.2) is 19.6 μm, intra-skeleton macropore of 1.9 μm and mesopore size of 9.9 nm. The BET surface area by N2 sorption and total pore volume by Archimedes principle reach 590 m² g⁻¹ and 3.56 cm³ g⁻¹, respectively. Based on systematic study on the relationship between synthetic compositions and porous structures, the ternary diagram and formation mechanism of these HMSMs were analyzed.
Pd nanoparticles were successfully immobilized in the mesopores of hierarchical porous SiO2 and TiO2 using supercritical carbon dioxide (scCO2)-acetone solution. The hierarchical porous SiO2 with macropores and mesopores demonstrated remarkable properties as a support for noble metal nanoparticle catalysts. The nanoparticle precursor, palladium(II) acetate (Pd(OAc)2), was impregnated into the mesopores of hierarchical porous SiO2 and TiO2 microparticles using scCO2. These samples were subsequently reduced by treatment in a stream of H2 at 473 K. During reduction, the formed Pd nanoparticles were dispersed within the hierarchical porous SiO2, with a uniform three-dimensional distribution of nanoparticles in the mesopores revealed by TEM images. The resulting Pd@hierarchical porous SiO2 exhibited high activity for the heterogeneous catalytic hydrogenation reaction of styrene in methanol solution. The catalytic activity of the Pd@hierarchical porous SiO2 was higher than that of Pd@conventional porous SiO2 without hierarchical porosity or the Pd@hierarchical porous TiO2.
To investigate the volume expansion of a carbon dioxide (CO2) + n-hexane mixture at different temperatures and pressures, we developed a measurement set-up using an optically transparent fused silica capillary cell (FSCC) combined with Raman spectra. The results showed that the method is feasible, the water seal is effective in its ability to avoid vaporization of n-hexane, and Raman spectra can help ensure the system equilibrium. The volume expansion factor of the CO2 + n-hexane mixture increased with increasing pressure and decreased with increasing temperature from 30 °C to 80 °C and from 1 MPa to 10 MPa. It was found that the trend of the expansion factor of CO2 + n-hexane mixture was steeper at higher temperature. When temperature was 30°C, the volume expansion factor increased from 2.15 to 13.61 as pressure increased from 5.0 to 6.5 MPa. Meanwhile, the volume expansion factor increased from 2.36 to 19.40 as pressure increased from 8.0 MPa to 9.2 MPa at 60°C. In addition, we obtained a significant and positive relationship between the volume expansion factor and the Raman peak height ratio of CO2 and n-hexane using quadratic equation.
Simple, cost-effective, and reproducible devices have been prepared for the detection of H2 at room temperature. Pd, Cu and bimetallic Cu-Pd nanoparticles (prepared by the reduction-by-solvent method) have been loaded on a MWCNT deposit avoiding the use of surfactants or complicated techniques. The devices based on pure Pd nanoparticles have shown a high H2 sensitivity in the studied H2 concentration range with low response and recovery times. However, the addition of Cu in the metallic nanoparticles does not result in a considerable improvement. Furthermore, the Pd and Cu-Pd/MWCNTs-based sensors have kept part of their H2 sensitivity when they have been tested in an O2-depleted atmospheres, which is an altogether new finding compared to Pd-based systems. Through these experiments it has been possible to conclude how the main H2 detection mechanism is the electron transfer from the nanoparticles (when the PdHx is formed) to the carbon nanotubes.
Palladium nanoparticles (Pd NPs) were synthesised by the reduction-by-solvent method using polyvinylpirrolidone (PVP) as capping agent. The non-static interaction between PVP and the metallic surface may change the properties of the NPs due to the different possible interactions, either through the O or N atoms of the PVP. In order to analyse these effects and their repercussion in their catalytic performance, Pd NPs with various PVP/Pd molar ratios (1, 10 and 20) were prepared, deposited on silica and tested in the formic acid decomposition reaction. The catalytic tests were conducted using catalysts prepared by loading NPs with three different time lapses between their purification and their deposition on the silica support (1 day, 1 month, and 6 months). CO adsorption, FTIR spectroscopy, XPS and TEM characterisation were used to determine the accessibility of the Pd NPs surface sites, electronic state of Pd and the average NPs size, respectively. The H2 production from the formic acid decomposition reaction has a strong dependence with the Pd surface features, which in turn are related to the NPs aging time due to the progressive removal of the PVP.
Using atomic layer deposition (ALD), RuO2 was deposited on mesoporous Al2O3 with a mean pore size of ∼12 nm and particle diameter of ∼1 mm. The entire internal structure of mesoporous Al2O3 was coated by RuO2 nanostructures using ALD, forming a mesoporous bead of RuO2/Al2O3. Substantially high CO oxidation activity at reaction temperatures below 100 °C is also shown. There was a decrease in CO oxidation activity with time at 100 °C, and post-annealing at 700 °C fully recovered the catalytic activity of fresh RuO2/Al2O3. The RuO2/Al2O3 can be useful as an oxidation catalyst for eliminating harmful molecules.
Oxidized Ni, at very small loadings, was deposited on mesoporous Al2O3 using atomic layer deposition. The prepared structures were used as catalysts for CO oxidation between 30 and 250 °C. Ni particles with a mean size less than 1-2 nm were shown to be reactive, even at room temperature, for CO oxidation. The room temperature reactivity for CO oxidation was decreased with increasing reaction time. However, when post-annealing at 300 °C was applied to deactivated catalysts, a higher initial CO oxidation reactivity (compared with the value of the preceding catalytic operation) was observed. Furthermore, we observed that repeated cycles of room-temperature CO oxidation and post-annealing at 300 °C gradually increased the catalytic activity for room temperature CO oxidation. X-ray photoelectron spectroscopy was used to determine the origin of this behavior and the results are discussed in detail. © 2016 Korean Chemical Society, Seoul and Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim.
A small amount of NiO was deposited on mesoporous Al2O3 using atomic layer deposition and was subsequently annealed at different temperatures. We show that all of the samples prepared here show substantially high catalytic activity for CO oxidation near room temperature. Furthermore, as the pre-annealing temperature increased, an enhanced resistance toward poisoning during CO oxidation was observed. Particularly, the samples pre-annealed at 525 and 600°C showed almost no change in the CO oxidation reactivity for 12h at 30°C, and they are structurally stable at the temperature higher than 500°C.
Well-dispersed Pd nanoparticles have been synthesized inside the mesoporosity of a silica monolith featuring hierarchical porosity of homogeneous interconnected macropores (4 microns) and mesopores (11 nm). These monoliths have been implemented as microreactors for selective hydrogenation reactions. Conversion and selectivity can be tuned by adjusting the flow rates of hydrogen and substrates. In the selective hydrogenation of cyclooctadiene, a conversion of 95% and a selectivity of 90% in the monohydrogenated product, constant over a period of 70 h, have been reached. These figures correspond to a productivity of 4.2 mmol s(-1) g(-1)(MonoSil) (or 0.32 mol s(-1) g(-1)(Pd)). In the stereoselective hydrogenation of 3-hexyn-1-ol a constant conversion of 85% was observed, with however moderate selectivity into the cis isomer, over a test period of 7 h. These results open the route to the synthesis of important chemicals and intermediates via safe and green processes.
NH3-SCR honeycomb catalyst was prepared by coating powdery active metal components on the surface of blank monolith supports and tested at the temperatures of 250-350 degrees C, and superficial gas velocities of 5.3-6.3 m/s. The tests were performed with a simulated gas mixture containing NO, SO2, O-2, H2O and N-2 or a real flue gas produced on line. The results showed that the coated honeycomb catalyst has an extraordinary high activity for low-temperature deNO(x) in the presence of 5 vol.% steam and 1000 ppm SO2. Its activity increased with increasing the coating thickness and reached at 110 mu m thickness as comparable as that of the 100% active components-molded catalyst, suggesting the deNO(x) reaction proceeded mainly in the surface layers of the honeycomb cell wall and hence coated honeycomb catalyst should be more cost-efficient than 100% active components-molded one. In a real coal-burned flue gas stream the catalyst maintained essentially its low temperature activity and stability, further verifying its high industrial applicability. Then, the effects of NO concentration and NH3/NO ratio on NO conversion were investigated to seek a suitable kinetic model for prediction of the catalyst's deNO(x) rate and reactor design for pilot plant research. The data obtained showed that a simple power-law kinetic equation with first order in NO and zero order in ammonia can be applied to the coated honeycomb catalyst, and was used for determination of the kinetic parameters. Finally, simulation calculation was performed under arbitrary conditions to provide valuable information for design of pilot-plant scale reactors.
The catalytic properties of Pt supported on zeolite P (ZP)-based materials for the preferential CO oxidation in hydrogen atmosphere under mild conditions (from room temperature to 150 A degrees C), have been investigated. Pt catalysts (1-4 wt%) supported on a zeolitized pumice support (Z-PM) have been prepared. A series of bimetallic Pt-Fe on ZP, having 2 wt% Pt and different Fe loading (0.5-4 wt%), have been also prepared and used as model catalysts. A detailed characterization of the catalysts has been carried out by means of surface area and porosity measurements, X-ray diffraction, scanning electron microscopy and transmission electron microscopy in order to investigate the morphological and microstructural properties of both support and catalytic system. Pt/Z-PM exhibited complete CO conversion with 55 % selectivity at temperatures as low as 50 A degrees C, with no noticeable degradation of the catalytic performances, indicating that the Fe content present as an impurity in the zeolitized pumice support allows to obtain catalysts characterized by high activity and stability. On the basis of the characterization and kinetic tests, hypotheses on the role of Fe promoter have been formulated.
New catalyst and reactor designs are necessary to meet the expanding use of low temperature hydrogen based fuel cells. Residential and commercial power generation requires the reformer integrated to the fuel cell must be sufficiently small given that space is often a premium in many of these applications. Precious metal washcoated monolithic structures, similar to those successfully used since 1975 in automobile catalytic converters [1], provide high activity per unit reactor volume, low pressure drop and greater structural stability than traditional base metal catalysts in packed beds and thus are well suited for distributed hydrogen applications, New precious metal catalysts, with high activity densities, have been formulated for hydrogen generation since traditional base metal oxides have much lower activities per unit mass and thus are not sufficiently active when used in limited amounts as washcoats. The higher cost of precious metals is mitigated by savings due to reduced system size and existing metal recycling operations. Catalytic fuel processing, of infrastructure fuels (e.g. natural gas and LPG), are being reformed for the fuel cell to power homes, commercial and residential buildings, schools, hospitals. Such systems must operate safely and reliably in the user facilities while unattended. This brief review will illustrate the application of automotive washcoating technology to reformers for distributed hydrogen applications.
In the present work, novel isolated Ti-SBA-15-spherical and Ti-KIT-6 (Si/Ti = 200, 100 and 50) photocatalysts have been synthesized; optimized through N2-adsorption/desorption, SEM, EDX, UV–Vis, FT-IR, XPS and TEM analysis techniques; and explored for the photocatalytic reduction of greenhouse gas CO2 to renewable fuels. The Ti-KIT-6 (Si/Ti = 100) showed better CH4 production rate (4.15 μmol gcat.−1 h−1) than the corresponding Ti-KIT-6-dried (2.63 μmol gcat.−1 h−1) and the Ti-SBA-15-calcined/dried (1.85, 3.45 μmol gcat.−1 h−1, respectively) in the initial optimization reactions. CH3OH, CO, and H2 are the other main fuel products produced by the Ti-KIT-6-calcined (Si/Ti = 100). The increased surface concentration of OH groups found in the Ti-KIT-6-calcined (Si/Ti = 100) than the other two ratios (Si/Ti = 200, 50), the presence of more accessible surface reaction active sites due to the lower number of Ti–O–Ti or TiO2 agglomerates, and the more isolated Ti species which are uniformly dispersed on the 3-D KIT-6 mesoporous silica support without collapsing the mesoporous structure, have boosted the higher activity, which is even higher than the best commercial Aeroxide P25 TiO2. The reaction has been preceded by the competitive adsorption of CO2 and H2O vapors. The UV light source/intensity, H2O/CO2 ratios and catalyst shapes are the key factors that influence the performance of the catalyst, and therefore, these parameters have been optimized to boost the fuel products.
Different Pt-based catalyst layers have been prepared and tested in a stacked foil microreactor for CO oxidation and preferential oxidation of CO in presence of hydrogen. The reactions were performed on Pt without support by impregnation of a pre-oxidized microstructured metal plate, Pt/Al2O3 and Pt/CeO2 based on sol methods as well as Pt/nano-Al2O3, a combined method of sol-gel and nanoparticle slurry coating. The ceria based sol-gel catalyst was much more active for CO oxidation than alumina based sol-gel catalysts at low temperature. However, total oxidation was only obtained at higher temperature on the alumina based catalysts. The combined method seems to have advantages in terms of less internal mass transfer limitation when trying to increase the catalyst coating thickness based on sol-gel approaches due to no reduction of CO selectivity up to 300 °C reaction temperature. Experiments on CO oxidation with the Pt/CeO2 catalyst have been conducted in an oxygen supply microreactor to evaluate the catalyst performance under sequential oxygen supply to reaction zone (CO excess).
This work demonstrates that a small loading of Pt (0.2–0.5 wt.%) selectively located at the top of the Cu/Al2O3 solid is enough to obtain active catalysts for the COPrOx reaction. The characterizations of the catalysts prepared in this work using different Pt:Cu ratios show that the phases formed strongly depend on the Cu and Pt loadings. For the samples with 4 wt.% of Cu, the formation of defective CuAl2O4-like species is predominant, with Cu2+ occupying distorted sites in the alumina surface. For the samples with 8 wt.% of Cu, CuO and metallic Cu in the form of small crystals are observed besides Cu2+. For both 0.2 and 0.5 wt.% of Pt, some proportion of Cu–Pt alloy is found which increased with the Cu content. Metallic Pt particles are only observed by EXAFS for the samples with a higher loading of the noble metal.
The higher CO conversions at low temperatures were obtained for the Pt0.5Cu8/Al2O3 sample, probably due to the simultaneous formation of small Pt crystals in close contact with CuO. The XPS results show that a strong surface enrichment in Pt takes place in all the samples. Thus, a small loading of the noble metal is enough to obtain effective catalysts. The surface Pt enrichment observed is most probably due to the impregnation of the Pt salt onto the Cu/Al2O3 solid, with a relatively low calcination temperature (300 °C) that prevents the formation of bulky CuAl2O4 spinel crystals.
Bimetallic alumina and ceria supported Pt–Co catalysts were prepared via the decomposition of a [Co(H2O)6][Pt(NO2)4]·2H2O double complex salt. Catalysts were tested in CO preferential oxidation (PROX). PtCo/Al2O3 catalyst exhibited high activity and selectivity. It provided the CO conversion higher than 80% and selectivity above 60% in the temperature interval 90–165 °C at WHSV of 260,000 cm3 g−1 h−1 and O2/CO ratio of 0.6. XRD and TEM analysis indicated the formation of a platinum-rich Pt–Co alloy nanoparticles supported over alumina surface.
Amino-functionalized mesoporous silicas (AFMS) were synthesized using the anionic surfactant N-lauroylsarcosine sodium (Sar-Na) as template and 3-aminopropyltrimethoxysilane (APTMS) as co-structure directing agent (CSDA). Orthogonal experiments were applied to optimize the synthesis of AFMS with high amino loading, high surface area, and large pore size. The synthesized AFMS were characterized by FT-IR, TG-DTA, XRD, N2 adsorption–desorption, and TEM techniques. The removal of Cu2+ or Pb2+ from aqueous solutions by the synthesized AFMS was investigated in detail. The pH value of an aqueous solution containing Cu2+ or Pb2+, adsorption temperature, and the dosage of the used AFMS affect the removal efficiency of Cu2+ or Pb2+ greatly. The unary adsorption isotherms of Cu2+ and Pb2+ on the optimized AFMS adsorbent are well described by the Sips isotherm model, in which the adsorption capacities are 2.18 and 4.74 mmol/g for Cu2+ and Pb2+, respectively, under the optimized conditions, much higher than the literature data. Furthermore, the AFMS adsorbent shows a good stability, confirmed by adsorption–regeneration cycles. Based on these excellent properties, the application in the removal of Cu2+ and Pb2+ from wastewater is anticipated.
Mesoporous silica MCM-41 was successfully prepared by flow synthesis in a microreactor at shorter reaction times (i.e., minutes versus day) at high yield (i.e., 60% calcined sample) to give particles of more uniform size and shape compared to MCM-41 prepared by conventional batch synthesis. Magnetic iron oxide nanoparticles were incorporated and organic amines (i.e., propylamine and propyl diethylene amine) were grafted to obtain magnetic mesoporous catalysts for the Knoevenagel condensation reactions of benzaldehyde with ethyl cyanoacetate, ethyl acetoacetate and diethyl malonate. The incorporation of magnetic nanoparticles and large organic amines can hinder reactants access to the catalyst resulting in lower reactivity. NH2-magMCM-41 showed superb catalyst activity and selectivity for the all three Knoevenagel condensation reactions studied. The catalyst can be easily dispersed into solution and rapidly removed by a magnet for recovery and reuse.
In this work, the Buchwald–Hartwig aryl amination is reported in a home-made continuous plug flow tube microreactor, using a homogeneous well-defined palladium-N-heterocyclic carbene (NHC) complex. The microreactor enabled a 100% conversion of the reagents within 30 min, even at very low catalyst concentrations. A direct comparison between batch and continuous flow reactions is described and shows that the Buchwald–Hartwig reaction is faster in the microreactor than in the batch case. An investigation of the influence of the operating parameters of the microreactor on the reaction was carried out. Increasing temperature allowed a faster conversion of the reagents; moreover, no effect on microreactor performance was found by changing tube diameter. The dependence of reaction kinetics on reagents and [Pd(NHC)] pre-catalyst concentrations was investigated based on initial reaction rates. The resulting expression for the rate of reaction showed some similarities with the one reported for palladium–phosphine catalyst, but also some important differences.
In situ characterization of silicon micro channel reactor with supported platinum alumina catalyst prepared on the channel wall was conducted with FT-IR microscope under propane flow, propene flow and cyclohexane flow. Surface π-allylic species along with surface OH group were observed at 373K under propane or propene flow. These species were rather stable under N2 flow and H2 flow. The reactor was active for dehydrogenation of cyclohexane to produce benzene and cyclohexene. In situ FT-IR observation under flow of cyclohexane at 473K, decrease of reactant, increase of products, a maximum of π-allylic and CC species were observed from inlet toward outlet. The maximum absorbance of the intermediate shifted with flow rate. These suggested consecutive reaction path and existence of film resistance.
A fully-integrated micro PEM fuel cell system with a NaBH4 hydrogen generator was developed. The micro fuel cell system contained a micro PEM fuel cell and a NaBH4 hydrogen generator. The hydrogen generator comprised a NaBH4 reacting chamber and a hydrogen separating chamber. Photosensitive glass wafers were used to fabricate a lightweight and corrosion-resistant micro fuel cell and hydrogen generator. All of the BOP such as a NaBH4 cartridge, a micropump, and an auxiliary battery were fully integrated. In order to generate stable power output, a hybrid power management operating with a micro fuel cell and battery was designed. The integrated performance of the micro PEM fuel cell with NaBH4 hydrogen generator was evaluated under various operating conditions. The hybrid power output was stably provided by the micro PEM fuel cell and auxiliary battery. The maximum power output and specific energy density of the micro PEM fuel cell system were 250 mW and 111.2 W∙hr/kg, respectively.
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The adsorption of CO2 up to pressures of 4 MPa has been studied using two series of activated carbon fibers (ACFs) covering a wide range of burn-off. The relative fugacities covered in these experiments range from 3 × 10-4 to 0.76. Additionally, N2 adsorption at 77 K and CO2 adsorption at 273 and 298 K at subatmospheric pressures have been carried out. The experiments performed at high pressures allow us to compare both adsorptives at similar ranges of adsorption potential. The results obtained led to the following conclusions: (i) CO2 adsorption at 273 K at subatmospheric pressures is a suitable technique to characterize the narrow microporosity of the ACF. (ii) The use of N2 to characterize the narrow microporosity is not appropriate because its adsorption is limited by the existence of diffusional restrictions in this type of porosity. (iii) CO2 at 273 K (or 298 K) is an adsorptive that behaves quite similarly to N2 at 77 K at comparable relative pressure ranges; thus, CO2 adsorbs in the super-microporosity range (pore size: 0.7−2 nm) at 298 K if pressures of about 4 MPa are used.
Our previous work [T. Conant, et al., Wall coating behavior of catalyst slurries in non-porous ceramic microstructures. Chem. Eng. Sci. 61 (17) (2006) 5678] described how the inertial effects limit the catalyst wall-coating thickness in a single microchannel. In this work, we demonstrate the inherent benefit of multichanneled structures in avoiding inertial effects during the process of coating catalyst slurries. Our microreactor consists of 25 capillaries of 530μm in diameter housed within a 1/4in. o.d. stainless steel housing. The maximum fraction coated within the parallel-channel microstructure was found to be 40% which is a significant improvement over the 25% achieved with single 530μm capillaries. Transmitted-light optical microscopy shows that the channels are plug-free. The thick coatings may lead to some loss of cohesion within the coating layer, resulting in non-uniform coating thicknesses. The catalyst layer within the reactors was tested for the steam reforming of methanol, confirming the viability of coating catalysts slurries in pre-fabricated microreactors.
To enhance the inner surface of microchannel reactors, porous metal oxide coatings based on alumina, silica, and titanium oxide have been developed as support for catalytically active components. The oxides were made by the sol–gel process. Thin film coating was accomplished by dip coating using an Fe-based alloy as support. The influence of the sol composition, sol viscosity, and thermal treatment procedure on the surface enhancement factor and the porosity of the coating was investigated. The resulting surface enhancement factor reached 800m2/m2.
The selective oxidation (SELOX) of CO in the presence of H2, CO2 and H2O has been carried out on stainless steel microreactors with 500μm diameter microchannels. To this end, catalytic films consisting of Pt supported on zeolite Y (Pt-FAU) or ETS-10 (Pt-ETS10), were prepared by seeded hydrothermal synthesis on the surface of the reactor microchannels to give a uniform, mechanically stable catalytic coating on the channel surface. Activity tests were performed with feeds consisting of CO and H2, plus CO2 and/or H2O. The best performance was obtained on microreactors coated with Pt-FAU catalytic films. When feeds representative of real reformate streams (containing CO2 and H2O) were fed to Pt-FAU coated reactors, the microreactor clearly outperformed the fixed bed reactor giving light-off temperatures that were lower by almost 50°C.
Adsorption of CO2 at 273 K up to 4 MPa has been studied in activated carbons and carbon molecular sieves of different origins and pore size distribution. The materials selected for this study include carbon molecular sieves with a pore size (i.e., pore width) between 0.3 and 0.5 nm, activated carbons with supermicroporosity (pore size between 0.7 and 2 nm), and mesoporous and macroporous activated carbons. The relative fugacities covered in the experiments ranges from 10-4 to nearly 1. Additionally, N2 adsorption at 77 K at subatmospheric pressures has been also done. The experimental conditions used allow us to compare both measurements at similar adsorption potentials, in which both gases adsorb in the different ranges of porosity. The results obtained show that CO2 adsorbs at 273 K in the different ranges of porosity following a mechanism similar to that of N2 at 77 K. CO2 is sensitive to narrow micropores not accessible to N2 at 77 K, and hence, it is an adequate complement to N2 at 77 K. This is especially important for the characterization of the narrow micropores of carbonaceous solids and, especially, carbon molecular sieves. CO2 adsorbs in mesopores according to the capillary condensation mechanism.
Well-dispersed and stable colloidal dispersions of polymer-protected Ni/Pd bimetallic nanoclusters have been obtained over an entire composition range by an improved polyol reduction method, in which nickel(II) sulfate and palladium(II) acetate were reduced at high temperature by ethylene glycol in the presence of poly(N-vinyl-2-pyrrolidone). Transmission electron microscopy indicates that these bimetallic nanocluster particles have definitely monodispersed size-distributions, with each particle containing both nickel and palladium atoms. The alloy structure has also been shown by X-ray diffraction and extended X-ray absorption fine-structure analysis. X-ray absorption near-edge spectroscopic and X-ray photoelectron spectroscopic data have confirmed that the nickel in the bimetallic nanoclusters is in the zero-valence state, as stabilized by the presence of Pd. Dispersions of these bimetallic nanoclusters were used as homogeneous catalysts for hydrogenation of nitrobenzene at 30 °C under an atmospheric pressure of hydrogen. The catalytic activities are demonstrated to be dependent on the metal composition of the particles. The highest activity can be achieved for a bimetallic nanocluster with a molar ratio of Ni:Pd = 2:3, which exhibits 3.5 times greater activity than that of a typical colloidal palladium catalyst.
The different steps for manufacturing a microchannel reactor for the PROX reaction are discussed. Transient Liquid Phase bonding (TLP) using a Ni–B–Si amorphous melt spun is used for joining micromilled Al-alloyed ferritic stainless steel plates followed by recrystallization at 1200 °C for 5 h. A CuOx–CeO2 catalyst synthesized by the coprecipitation method was washcoated on the microchannel block resulting in a homogenous 20–30 μm thick layer. The catalytic activity for CO-PROX reaction is similar in both the powder catalyst and the microchannel coated reactor but the selectivity is higher in the microchannel reactor.
Gas permeable, photoactive and crack-free titania–silica aerogels of high titanium content (i.e., up to Ti/Si = 1) were prepared by two-steps acid–base catalyzed method involving an acid-catalyzed prehydrolysis of silicon alkoxide followed by a base-catalyzed hydrolysis/condensation reactions with a chelated titania precursor. The prepared titania–silica aerogels displayed good mechanical strength (>30 kN m−2), large surface area (>550 m2/g), mesoporous structure (8–11 nm) and good gas permeation. The porous aerogels trap and filter airborne particulates and the titania–silica aerogel have a fair performance for aerosol (65%) and bioaerosol (94%) filtrations. The photoactive anatase nano-TiO2 crystallized within the aerogel displays an order of magnitude higher reaction rate for UVA photooxidation of trichloroethylene compared to commercial Degussa P25 TiO2. The bactericidal activity of the titania–silica aerogel for Bacillus subtilis cells under UVA was also six orders of magnitude better.
Alumina supported Pt group metal monolithic catalysts were investigated for selective oxidation of CO in hydrogen-rich methanol reforming gas for proton exchange membrane fuel cell (PEMFC) applications. The results are described and discussed in the present paper and show that Pt/γAl2O3 was the most promising candidate to selectively oxidize CO from an amount of about 1 vol% to less than 100 ppm. We have investigated the effect of the O2 to CO feed ratio, the feed concentration of CO, the presence of H2O and/or CO2, and the space velocity on the activity, selectivity and stability of Pt/Al2O3 monolithic catalysts. Afterwards, the Pt/Al2O3 catalyst was scaled up and applied in 5 kW hydrogen producing systems based on methanol steam reforming and autothermal reforming. The hydrogen produced was then used as fuel for an integrated PEMFC.
To develop a portable power device using fuel cell technology, the miniaturization of the hydrogen production unit is indispensable. In this paper, preparation of a microchannel reactor concerning steam reforming of methanol (SRM) has been investigated by optimizing the parameters of diffusion bonding and the composition of catalyst coatings. The results show the microchannel reactor can be sealed successfully by diffusion bonding at the optimized parameters (900 °C, 20 MPa, 1 h). The most active and selective catalyst coating is Cu50/Zn50 [Ce5] (Cu/Zn/Ce = 50/50/5, molar). It has been found that CeO2 has an important influence on improving catalytic activity, decreasing the outlet CO concentration, and strengthening the stability. Based on the characterization of catalyst coatings, it can be attributed to the increase in copper area and to the change in oxidation state of copper that results in the synergetic effect of Cu0 and Cu+. In addition, the effects of reaction temperature, space velocity, molar ratio of methanol to water, and reaction time were also investigated in the developed microchannel reactor with the Cu50/Zn50 [Ce5]. The results show that the microchannel reactor can generate enough hydrogen for a power output of 11 W.
Silicon microreactors with Pt/Al2O3 thin-film wall catalyst were adopted for kinetic studies of CO preferential oxidation (PrOx). The activity of this catalyst was compared to that of other catalyst systems based on similar formulation. Internal and external mass-transport and heat-transport limitations of the microreactor were examined and comparisons were made to minimized packed-bed reactors (m-PBRs). We found that at lower temperatures (<220°C), the microreactor shows negligible mass- and heat-transport resistance, implying direct access to intrinsic kinetics. However, external mass transport begins to play a significant role in limiting overall reaction rates above 220°C for PrOx. A microkinetic reaction model for PrOx was used for the study of reaction pathways and the analysis of surface intermediate species, which are difficult to study experimentally. Reaction mechanisms are discussed with these modeling results as a guide. Afterward, the results of a separate, nonisothermal reactor model using the finite-difference method are discussed to understand differences in performance of the microreactor and m-PBRs with respect to the CO conversion vs. temperature characteristic. As a result, we discovered that the temperature gradients in m-PBRs favor the reverse water–gas shift (r-WGS) reaction, thus causing a much narrower range of permissible operating temperatures compared to that of the microreactor. Accordingly, the extremely efficient heat removal of the microchannel/thin-film catalyst system eliminates temperature gradients and efficiently prevents the onset of the r-WGS reaction. © 2005 American Institute of Chemical Engineers AIChE J, 2005
Stable mono and bimetallic nanoparticles have been prepared from colloids developed by a simple and reproducible method, based on a reduction-by-solvent process. This method allows for the preparation of metallic nanoparticles with different compositions (i.e., Ni-Pd, Fe-Pd, Mg-Pd, Pd, and Pt) and metallic ratios, and an average size of about 2 nm. Purified and nonpurified metallic species were used in the selective hydrogenation of phenylacetylene to styrene in liquid phase under very mild conditions (1 bar H2 pressure and ). The catalysts prepared show high selectivity and activity toward styrene at very high loadings of phenylacetylene (substrate-to-catalyst weight ratio near 7500). Therefore, these materials are highly interesting as selective hydrogenation catalysts for reactions of great industrial importance.
Carbon monoxide is known to be poisonous to the proton exchange membrane fuel cell catalyst. Selective oxidation of carbon monoxide in a hydrogen-rich reformate stream is considered to be a practical method with the most potential for reducing concentrations down to tolerant levels. In the present work, nine different noble metal catalysts were investigated using a microstructured reactor in the presence of excess hydrogen and carbon dioxide at a GHSV of 15 500 h−1 and at temperatures up to 160 °C. The most active were Pt–Ru/γ-Al2O3, Rh/γ-Al2O3 and Pt–Rh/γ-Al2O3 yet the most stable was Pt–Rh/γ-Al2O3. Its activity was also investigated using a wet feed and also at a GHSV of 31 000 h−1. Water was found to promote the catalyst activity while at higher GHSV higher temperatures were required to achieve full carbon monoxide conversion. The catalyst exhibited steady performance in the microstructured reactor for 50 h while reducing 1.12% carbon monoxide to 10 ppm with inlet oxygen to carbon monoxide ratio of 4.
Ethanol steam reforming (S/C = 3) was effectively performed at low temperature over catalytic wall reactors comprising a wide range of channel dimensions, ranging from conventional monoliths (channel width of 0.9 mm) to semicylindrical microchannels (0.35 mm radius) and to silicon micromonoliths (channel diameter of 3–4 μm). Co3O4 catalyst coatings were successfully prepared inside the channels from in situ thermal decomposition of two-dimensional layered cobalt hydroxide salts and showed a remarkable high homogeneity and mechanical stability. The Si-micromonolithic reactor proved extremely promising for hydrogen production for micro-fuel cell operation. Specific H2 production rates exceeding 50 LN of H2 per mL of liquid and cm3 of reactor were measured for ca. 42% ethanol conversion and residence times in the order of milliseconds.
This review on selective oxidations is split into two parts. The first part concerns catalytic gas-phase oxidation reactions in micro-reactors, typically being performed in wall-coated micro-channels [V. Hessel, G. Kolb, J.C. Schouten, V. Cominos, C. Hofmann, H. Löwe, G. Nikolaidis, R. Zapf, A. Ziogas, E.R. Delsman, M.H.J.M. de Croon, O. de la Iglesia, R. Mallada, J. Santamaria, in: S. Ernst, E. Gallei, J.A. Lercher, Rossini, E. Santacesaria (Eds.), Conference pre-prints of the DGMK/SCI-Conference “Oxidation and Functionalization: Classical and Alternative Routes and Sources”, Milan, Italy, October 12–14, 2005 [1] (see also acknowledgements at the end of the article)]. Liquid and gas–liquid oxidations are not included due to their different reactor design and way of processing. This comprises process development for fine chemical intermediates or bulk chemicals. By example of different reactions, the benefits of micro-chemical process engineering are shown. While there are numerous engineering reasons, one major driver is the increase of selectivity by diminishing side and follow-up reactions, most often the total oxidation to carbon dioxide, through preventing or at least decreasing hot spot formation. Another major advantage of micro-reactors is that mass-transfer resistances can be suppressed, thereby giving access to intrinsic kinetics. In the second part, we describe selective oxidation as one gas purification step in the framework of fuel processing for fuel cells, which is most often termed preferential oxidation in this context. Here, the selectivity towards carbon monoxide formation by diminishing the hydrogen oxidation is a major driver. The current developments are grouped so that all facets from kinetic modelling, heat transfer studies, catalyst testing, reactor and integrated-system engineering up to process engineering, exergy analysis, performance benchmarking and operation under real-case process flows are covered.
It is indispensable to remove CO at the level of less than 50 ppm in H2-rich feed gas for the proton exchange membrane (PEM) fuel cells. In this paper, catalyst with high activity and selectivity, and a microchannel reactor for CO preferential oxidation (PROX) have been developed. The results indicated that potassium on supported Rh metal catalysts had a promoting effect in the CO selective catalytic oxidation under H2-rich stream, and microchannel reactor has an excellent ability to use in on-board hydrogen generation system. CO conversion keeps at high levels even at a very high GHSV as 500 000 h−1, so, miniaturization of hydrogen generation system can be achieved by using the microchannel reactor.
Our recent studies of CO preferential oxidation (PrOx) identified systematic differences between the characteristic curves of CO conversion for a microchannel reactor with thin-film wall catalyst and conventional mini packed-bed lab reactors (m-PBR's). Strong evidence has suggested that the reverse water-gas-shift (r-WGS) side reaction activated by temperature gradients in m-PBR's is the source of these differences. In the present work, a quasi-3D tubular non-isothermal reactor model based on the finite difference method was constructed to quantitatively study the effect of heat transport resistance on PrOx reaction behavior. First, the kinetic expressions for the three principal reactions involved were formed based on the combination of experimental data and literature reports and their parameters were evaluated with a non-linear regression method. Based on the resulting kinetic model and an energy balance derived for PrOx, the finite difference method was then adopted for the quasi-3D model. This model was then used to simulate both the microreactor and m-PBR's and to gain insights into their different conversion behavior.Simulation showed that the temperature gradients in m-PBR's favor the reverse water-gas-shift (r-WGS) reaction, thus causing a much narrower range of permissible operating temperature compared to the microreactor. Accordingly, the extremely efficient heat removal of the microchannel/thin-film catalyst system eliminates temperature gradients and efficiently prevents the onset of the r-WGS reaction.
Indirect methanol fuel cells currently being investigated at General Motors for transportation applications require removal of carbon monoxide from the hydrogen-rich gas stream produced by the fuel processing section. A variety of catalytic materials, including noble metals (Pt, Pd, Rh, and Ru) and base metals (Co/Cu, Ni/Co/Fe, Ag, Cr, Fe, and Mn), were evaluated in a laboratory reactor feedstream containing CO, H2, and O2 in order to identify alternate catalysts which are more effective than currently used Pt/Al2O3 in selectively oxidizing CO in the presence of excess H2. Both Ru/Al2O3 and Rh/Al2O3 are among the most active catalysts for CO oxidation, achieving nearly complete CO conversion at temperatures as low as l00°C (compared to ∼200°C required for currently used Pt/Al2O3. Furthermore, the Ru/Al2O3 and Rh/Al2O3 catalysts were found to be exceptionally selective for CO oxidation, making it possible to purify the fuel cell feedstream with a minimum loss of the energy content associated with H2.
In preferential CO oxidation in H2-rich stream (PROX), the additive effect of potassium on Pt catalysts was more remarkable over Al2O3 than over SiO2, ZrO2, Nb2O5 and TiO2. The additive effect of potassium to Pt/Al2O3 was more effective than other alkali metals. Especially, the presence of H2 drastically promoted CO oxidation over Pt/Al2O3 modified with potassium. On the other hand, the suppressing effect of steam on the PROX was more significant over K-Pt/Al2O3 than over Pt/Al2O3, although the PROX activity of K-Pt/Al2O3 was much higher than that of Pt/Al2O3 even under the presence of steam. The different effect of steam addition to the PROX over K-Pt/Al2O3 suggests that the active site can be Pt surface modified with potassium ions.
Highly productive: Grafted monolith silica skeleton microreactors process bulky molecules more efficiently than a batch mode reactor. This efficiency is due to a higher contact area, shorter diffusion path, and lower inhibition by products in the thin monolith skeleton. These materials provide a new approach in the field of heterogeneous catalysis for the synthesis of fine chemicals.
Ordered mesoporous silicas such as micelle-templated silicas (MTS) feature unique textural properties in addition to their high surface area (approximately 1000 m2/g): narrow mesopore size distributions and controlled pore connectivity. These characteristics are highly relevant to chromatographic applications for resistance to mass transfer, which has never been studied in chromatography because of the absence of model materials such as MTS. Their synthesis is based on unique self-assembly processes between surfactants and silica. In order to take advantage of the perfectly adjustable texture of MTS in chromatographic applications, their particle morphology has to be tailored at the micrometer scale. We developed a synthesis strategy to control the particle morphology of MTS using the concept of pseudomorphic transformation. Pseudomorphism was recognized in the mineral world to gain a mineral that presents a morphology not related to its crystallographic symmetry group. Pseudomorphic transformations have been applied to amorphous spherical silica particles usually used in chromatography as stationary phases to produce MTS with the same morphology, using alkaline solution to dissolve progressively and locally silica and reprecipitate it around surfactant micelles into ordered MTS structures. Spherical beads of MTS with hexagonal and cubic symmetries have been synthesized and successfully used in HPLC in fast separation processes. MTS with a highly connected structure (cubic symmetry), uniform pores with a diameter larger than 6 nm in the form of particles of 5 microm could compete with monolithic silica columns. Monolithic columns are receiving strong interest and represent a milestone in the area of fast separation. Their synthesis is a sol-gel process based on phase separation between silica and water, which is assisted by the presence of polymers. The control of the synthesis of monolithic silica has been systematically explored. Because of unresolved yet cladding problems to evaluate the resulting macromonoliths in HPLC, micromonoliths were synthesized into fused-silica capillaries and evaluated by nano-LC and CEC. Only CEC allows to gain high column efficiencies in fast separation processes. Capillary silica monolithic columns represent attractive alternatives for miniaturization processes (lab-on-a chip) using CEC.
Silica monolithic capillaries (SMCs) were synthesized by a sol-gel process. First, a simplification of the synthesis was proposed by replacing the calcination and the drying steps which can have tremendous effects on chromatographic and physical properties, by a single water or methanol 2 h washing step. The efficiency of such a washing step was demonstrated and the comparison of the chromatographic and electrochromatographic properties between calcined and washed SMCs has shown that such a modification did not impair retention, efficiency, and stability of the monolith. This simplified procedure was carried out to synthesize SMCs with two different skeleton sizes. These capillaries were evaluated in electrochromatography and present high efficiencies (H = 5 microm) at least equal to the best ones reported in the literature. Furthermore, the influence of the skeleton size on the EOF of the second kind (EOF-2) was investigated with unmodified SMCs used under various experimental conditions including electrical field strength and buffer concentration. The ionic strength of the mobile phase and the applied electrical field that enable this EOF-2 were related to the size of the skeleton which was tuned by the synthesis conditions.
A new method of immobilizing Pd catalysts on the channel wall of a capillary by using polysilane with metal oxide has been developed, and applied to hydrogenation reactions.