Catalysis Today

Published by Elsevier
Print ISSN: 0920-5861
Single crystals of V2O5 and Li0.03V2O5 were imaged in ambient conditions by atomic force microscopy (AFM). Atomic-scale resolution images are compared with total electron-density plots of the surface calculated using the ab initio HartreeFock method. The calculated oxygen charge at the V2O5(0 1 0) surface suggests an increased local reactivity of the bridging oxygens with respect to electrophilic attacks by adsorbate molecules. The intercalation of lithium has no consequence on the reactivity of the surface. This is supported by results from electrostatic potentials calculated from the cluster charge distributions.
A number of Cu- and Fe-hydroxide containing catalysts, supported on oxide carriers, were prepared to provide the removal of 1,1-dimethylhydrazine from aqueous solutions via its oxidation by hydrogen peroxide and air oxygen. The Cu-containing samples as well as Fe/ZSM-5 are the most active catalysts in this reaction. The reaction products were analyzed by gas chromatography and UV–Vis spectroscopy. The effect of nature of the oxidizer and catalyst, pH and temperature on both the reaction rate and product composition was studied.
Optimisation of the performance of fuels has focused attention on the use of oxygen containing additives to improve octane or cetane numbers. A systematic study of the synthesis of one such additive (1,1-dimethoxyethane) from the catalysed reaction between acetylene and methanol has been carried out. An amorphous catalyst prepared from zinc oxide on silica has been found to be an efficient catalyst with excellent catalytic life. Methyl vinyl ether is the primary product, reacting further via an equilibrium reaction to form dimethoxyethane. The reaction kinetics of the first reaction has been measured at various temperatures as a basis for process optimisation.
The hydrodechlorination of CF3CCl2F over Pd and Pt supported on β-AlF3 and MgF2 with D2 gas has been investigated employing temperature programmed isotope exchange (TPIE) under static conditions. The isotope exchange observed between the H-loaded metal catalyst and the D2 gas phase demonstrates the significantly higher hydrogen uptake capability of Pd-based catalysts. Both Pd and Pt on β-AlF3, show significantly higher hydrogen/deuterium uptake and isotope exchange activity as compared with the MgF2 support, probably due to the presence of hexagonal channels in β-AlF3 and its higher Lewis acidity. The combination of these properties make Pd/β-AlF3 a superior catalyst for selective hydrodechlorination of CF3CCl2F. Based on the results of the hydrodechlorination of CF3CCl2F with D2, a competitive rather than a consecutive mechanism is proposed. The data from H/D-TPIE are best interpreted by the formation of surface carbene species as intermediates.
While 1,2-dichloroethane hydrodechlorination over pure palladium mainly produces ethane, increasing silver or copper content in bimetallic catalysts results in an increase in ethylene selectivity. The specific consumption rate of 1,2-dichloroethane decreases when silver or copper loading increases. The turnover frequency, that is, the number of catalytic cycle per active site (palladium atom and its surrounding silver or copper atoms) and per second, seems to be independent of surface composition of alloy particles and 1,2-dichloroethane hydrodechlorination is insensitive to the atom's nature (silver or copper).
The deep oxidation of 1,2-dichloroethane (DCE) over H-type zeolites (H-Y, H-ZSM-5 and H-MOR) was evaluated. Experiments were performed on conditions of lean chlorocarbon concentration (around 1000 ppmv) under dry and humid conditions, between 200 and 550°C in a conventional fixed-bed reactor. The high density of Brønsted acid sites, proved by temperature-programmed desorption (TPD) of ammonia and diffuse reflectance FT-IR of adsorbed pyridine measurements, make H-ZSM-5 zeolite an effective catalyst for DCE decomposition. Vinyl chloride was identified as an intermediate in 250–400°C range. When vinyl chloride was destroyed at higher temperatures, all the zeolites showed a great selectivity (>90%) to HCl. CO was promoted in quantity reflecting the difficulty of its oxidation over these zeolite catalysts. The activity of the zeolites was reduced in the presence of water vapour (15,000 ppmv). It was noticed that the addition of water to the feed stream did not alter the order of activity observed in the dry experiments. Moreover, the presence of water in the DCE decomposition changed significantly the reaction product distribution. Vinyl chloride formation was found to be significantly lowered over the three zeolites, and selectivity to CO2 formation was largely enhanced. The X-ray powder diffraction (XRD) analysis of the deactivated samples indicated partial destruction of the zeolite crystal structure during reaction.
A systematic investigation of the oxidation of 1,2-dichlorobenzene (o-DCB) was conducted over a series of vanadia/titania catalysts with different V2O5 loadings (i.e. 0.8, 3.6 and 5.8 wt%). The TiO2 support was also found to be active for this reaction, but its activity was significantly lower than that of the surface vanadia species. Reaction rates per vanadium atom (i.e. turnover frequencies) were calculated for these catalysts and found to be independent of the vanadia coverage. Kinetic measurements showed that under excess oxygen conditions, the reaction is approximately first order in o-DCB and zero order in oxygen. Carbon monoxide and carbon dioxide were the only products detected. The selectivity towards CO was approximately 40–45% and was found to be independent of vanadia loading and temperature. No deactivation of the catalyst was observed even after 100 h of operation at 723 K. Addition of WO3, MoO3 and ZnO to the V2O5/TiO2 system was found to have no significant effect on its activity for o-DCB oxidation.
The destruction mechanisms of C2-chlorinated hydrocarbons have been investigated. The adsorption of 1,2-dichloroethane (DCE) and trichloroethylene (TCE) on alumina and alumina supported catalysts was studied by FTIR-spectroscopy. Interpretation of the spectra suggests that the destruction of DCE occurs via HCl elimination to vinyl chloride, followed by an attack of a hydrogen and a surface oxygen on the double bond resulting in acetyl chloride. Subsequent elimination of a chloride ion gives acetaldehyde that can be further oxidized into acetate. With regard to the destruction of TCE, the spectra indicate that TCE is attacked by a basic oxygen and a hydrogen according to Markovnikov’s rule. This results in the formation of acyl chloride, which is further converted into acetate like species. In addition, the oxidation of TCE was examined over alumina supported catalysts. While alumina itself is not very active, palladium and chromium containing catalysts are. In the absence of water, the formation of tetrachloroethylene (PCE) was observed. Addition of water to the feed resulted in a decrease in the amount of PCE produced. Although water did not affect the TCE conversion over palladium it inhibited the oxidation reaction over chromium oxide. The activity of the chromia catalyst compared to alumina might be due to the supply of basic oxygen which can attack the double bond. The inhibitive effect of water on the conversion of TCE is probably due to blocking of active oxygen sites.
A systematic investigation of the catalytic oxidation of 1,2-dichlorobenzene (o-DCB) was conducted over a series of supported vanadia/alumina catalysts. The reaction was found to be first-order in o-DCB and zero-order in O2 under excess oxygen conditions. Deactivation studies conducted with a 5.6% V2O5/Al2O3 catalyst at 773 K indicated a 15% loss in activity after 75 h on-stream. In situ FTIR studies suggest that the benzene ring remains intact during the adsorption of o-DCB, while no surface species containing C–Cl bonds were detected. Thus, it is suggested that chlorine abstraction is the first step in the reaction. Several partial oxidation products, namely phenolates, maleates and carboxylates were observed on the catalyst surface under reaction conditions. All of these species with the exception of the maleates can react with gas phase oxygen.
Porous carbon xerogels synthesized in a previous study were investigated as catalysts supports. The support chosen was a micro-mesoporous carbon xerogel obtained from the pyrolysis of a resorcinol–formaldehyde resin whose synthesis variables were fixed at suitable values. Palladium and silver were deposited on this tailored texture carbon by co-impregnation using a solution of palladium and silver nitrates in nitric acid and water. Several catalysts were prepared with various Pd and Ag global contents, the latters being measured experimentally. Alloy particles, detected in all bimetallic samples, were studied by a combination of various techniques that enabled us to obtain their size as well as their bulk and surface composition. When present, the fraction of unalloyed silver was also calculated. The characterization data were related to the results of catalytic tests obtained for selective hydrodechlorination of 1,2-dichloroethane into ethylene. Results show that when the Ag content is too high, pure Ag particles are formed and the alloy composition remains constant. As a consequence, the surface composition of the alloy is constant as well and the catalytic tests lead to similar results.
Calcination eharacteristics and 13ET surface arcas for the different perovskites used
Several ABO3-type perovskite oxides (A = La, Y, Nd or Gd; B = Fe, Mn, Cr or Co) have been investigated as catalysts for the oxidation of 1,2-dichlorobenzene (o-DCB), a model compound for the highly toxic polychlorinated dibenzodioxins. Initial transient and steady state activity measurements were conducted with all catalysts in the absence and presence of water. Perovskites containing Cr in the B-site were more active than perovskites containing other transition metals, with YCrO3 being the most active catalyst among the different systems studied. Furthermore, YCrO3 did not show any loss of its initial activity after several hours on stream. Other perovskites lost 10–20% of their initial activity within the first 5–10 h on stream. This loss was associated with a corresponding loss in BET surface area. With the exception of LaCoO3, all perovskites retained their crystalline structure upon exposure to o-DCB under reaction conditions. LaCoO3 was converted to LaOCl and Co3O4. The presence of water appeared to enhance the catalytic activity of some perovskites. This effect can be attributed to a faster removal of Cl− ions from the catalyst surface via their reaction with water.
When Au is deposited as nano-particles on select metal oxides, it exhibits surprisingly high catalytic activity for many oxidation reactions. Therefore, there is also the possibility to improve the activities of Au catalysts for hydrogenation using the appropriate preparation methods like the gas-phase grafting method (GG) and the deposition precipitation method (DP). In this work, we investigated the hydrogenation of 1,3-butadiene and of crotonaldehyde over Au catalysts prepared by GG and DP and discussed the structure sensitivity of these reactions. From these experiments, it was found that the catalytic activities for the hydrogenation of 1,3-butadiene over Au catalysts was almost structure insensitive in terms of the size effect of Au particles and the influence of metal oxides supports and the crotonaldehyde hydrogenation over Au catalysts was slightly sensitive to the selection of the support in the view point of the product selectivity.
The hydrogenation of 1,4-pentadiene, E-1,3-pentadiene, and a Z/E-1,3-pentadiene mix have been studied over a Pd/alumina catalyst in the liquid phase at 313 K. 1,4-Pentadiene was found to react in a manner similar to 1-pentene. However, E-1,3-pentadiene caused rapid deactivation of the catalyst. When a Z/E-1,3-pentadiene mixture was hydrogenated no such deactivation was observed and the initial rate of E-1,3-pentadiene hydrogenation was reduced. It is proposed that the adsorbed Z-1,3-pentadiene disrupts π-stacking of the E-isomer. The pentene isomer selectivity indicated that 3,4-addition was favoured, which indicated that the internal double bond was more reactive than the terminal double bond. The reaction of 1,4-pentadiene in competition with 2-pentyne resulted in an increased rate of reaction for both reactants. Whereas the reaction of Z/E-1,3-pentadiene in competition with 2-pentyne or 1-pentyne resulted in a decreased rate of reaction for both reactants. In this case the alkene isomer selectivity could be modelled by simple averaging of the individual selectivities.
The present study was carried out to asses performance of a Pd-monolith downflow bubble column (DBC) reactor, and compare it with that of the slurry and the fixed bed DBC. The selective hydrogenation of butyne-1,4-diol to cis-2-butene-1,4-diol over palladium catalyst was chosen as a model reaction. In principle, the monolith DBC allowed the reaction to take place under kinetic control regime. Comparison with DBC employing 5% Pd/C powder and 1% Pd-on-Raschig ring catalysts revealed a better performance of the monolith DBC (1% Pd loading) with advantage of smaller reaction volume and intensified reaction rate. In the monolith DBC, improved hydrogen transport was possible, as the interface between bubbles and the channel wall was very thin, thus, the length of the diffusion path was very short. In addition, the interfacial surface area at both gas–liquid and liquid–solid interface in the monolith was also very high. The reaction kinetics was well represented by the Langmuir–Hinshelwood mechanism. As an alternative to conventional three-phase reactors, the monolith DBC was simple due to its inherent characteristic operation and no specially designed device.
Hydrogenation of 2-butyne-1,4-diol to 1,4-butanediol promoted by stainless steel reactor wall has been investigated in supercritical carbon dioxide (scCO2) as well as in conventional organic solvents in the absence of catalysts at 50 °C. scCO2 was found to be an effective medium at pressures higher than supercritical region (7.3 MPa). The influence of phase behavior, pressure and reaction time has been discussed. The first step hydrogenation of butynediol (CC) to butenediol (CC) is slower but the second-step hydrogenation of butenediol (CC) to butanediol (CC) is faster in scCO2 compared with the case of ethanol.
In early 2000, GE Energy launched a program to develop a catalytic combustion system for one of its small power generation gas turbines, the GE10-1 engine. The target was to release to the market a new combustor able to guarantee NOx emissions lower than 2.5 ppmvd (referred to 15 vol.% O2). Today, a full-scale engine test campaign has been completed, during which measured NOx emissions were as low as 1 ppmvd in the 90–100% load range.The article is aimed to illustrate the developed technology and the results obtained. The combustion system's configuration is briefly described, focusing on the XONON® catalyst module installed. Reported data show combustion system's performances, mainly in terms of pollutant emissions and operability. Perspectives for future development of such combustion system are outlined.
The surface chemistry of NOx on metal oxides is important to environmental catalysis. Here we employ plane-wave, supercell DFT calculations to characterize NOx chemistry at the RuO2(1 1 0) surface as a model of a catalytically active transition metal oxide surface. We identify a range of potential NOx intermediates, and use a thermodynamic analysis to characterize their stability as a function of gas exposure conditions. Adsorbed NO (nitrosyl) and to a lesser extent NO3 (nitrate) dominate the surface phase diagram. Computed vibrational spectra are in good agreement with observation and provide new assignments of observed surface species. NO2 is thermodynamically unstable at the surface and its desorption is never favored: in contrast to its activity towards CO oxidation, RuO2(1 1 0) is not an effective NO oxidation catalyst. Rather, it could be effective as a reversible NO adsorber. Finally, we characterize the kinetics of several NO surface reactions and identify a pathway that may contribute to the decomposition of NO to N2 and N2O over partially reduced surfaces.
Density functional theory calculations for NO adsorption and NO–CO reaction on a Co2+-dimer/γ-Al2O3(1 1 0) model catalyst were conducted to understand two new aspects of the increases in the amount and reduction rate of adsorbed NO by gas phase CO molecules which are undetectable at the Co2+-ensemble/γ-Al2O3 catalyst surface. Three kinds of dinitrosyl adsorbates were found and assigned to normal gem-dinitrosyl species (gem-dinitosyl I and II) and an unique cis-(NO)2 dimeric species. The gem-dinitrosyl II with a reconstructed structure involving Co–Osurf cleavage was considered as stable species experimentally observed, which is responsible for the spectator CO-promoted NO adsorption. We also found that the cis-(NO)2 dimeric species specific for the Co2+-ensemble structure possesses a much higher reactivity than the gem-dinitrosyl species, enabling the facile reaction with CO that is very weakly trapped at the surface. In the cis-(NO)2 dimeric species two adsorbed NO molecules on two adjacent Co2+ sites interact with each other due to the opposed orientation of the unoccupied d orbitals of the two Co2+, resulting in easy formation of a N–N bond through the 2π*–2π* hybridization. The NO adsorbates modify and activate the surface (Co2+ sites) to make pseudo-compounds (intermediates) with CO at the Co2+ sites in such a way that CO is not detected at the surface. The potential energy surface for the NO–CO reaction is presented and the transition states and intermediates are discussed.
This paper is a report of angle-resolved product desorption measurements in the course of catalyzed NO and N2O reduction on Pd(1 1 0). Surface-nitrogen removal processes show different angular distributions, i.e. normally directed N2 desorption takes place in process (i) 2N(a) → N2(g). Highly inclined N2 desorption towards the [0 0 1] direction is induced in process (ii) N2O(a) → N2(g) + O(a). N2O or NH3 desorption follows the cosine distribution characterizing the desorption after the thermalization in process (iii) N2O(a) → N2O(g) or (iv) N(a) + 3H(a) → NH3(a) → NH3(g). Thus, a combination of the angular and velocity distributions provides the analysis of most of surface-nitrogen removal processes in the course of catalyzed NO reduction.At temperatures below 600 K, processes (ii) and (iii) dominate and process (iv) is enhanced at H2 pressures higher than NO. Process (i) contributes significantly above 600 K. Only three processes except for NH3 formation are operative when CO is used. Only process (ii) was observed in a steady-state N2O + CO (or H2) reaction.
Two realistic mathematical models were developed which can reproduce almost quantitatively the region of existence and the properties of the experimentally observed oscillatory behaviour for the N2O + H2 and N2O + CO reactions over the Ir(1 1 0) single crystal surface. The peculiarity of the oscillatory behaviour in these systems is the phase shift between the oscillations of the partial pressures of the two reaction products. While the oscillation maximum for H2O is “delayed” compared to the maximum of N2 oscillation, nearly anti-phase oscillations of the N2 and CO2 production rates were observed. Moreover, not only the products N2 and CO2 oscillate in counter-phase, but also the reactants N2O and CO produce counter-phase oscillations. It was demonstrated that in both systems oscillatory behaviour could originate due to the lateral interactions in the adsorbed layer. The main feedback mechanism generating oscillations operates via the acceleration of N2O decomposition by oxygen. The result of mathematical modeling shows that the larger phase shift of oscillations of CO2 and N2 production rates in comparison with the H2O and N2 production rates originates due to the more complicated character of lateral interactions in the adsorbed layer.
Nanometer sized metal clusters dispersed on oxide supports often exhibit much higher activity than single-component metal catalysts. Their catalytic performance markedly depends on cluster size, shape and size distributions, along with support materials and support preparation methods. Supported metal nanoclusters can also easily rearrange and sinter during the course of thermally activated catalytic reactions even at moderate temperatures. An accurate assessment of the effects of cluster–support interactions on the growth, structure and reactivity of supported metal clusters, as well as the adsorbate-induced structural changes is therefore necessary to understand their catalytic performance under realistic operating conditions. The detailed understanding will also contribute to development of a new and reliable way to control their structural catalytic properties on the atomic scale. As a part of the effort to gain this atomic level understanding, we present our recent findings from density functional theory calculations, including: electronic structure of a reduced TiO2(1 1 0) surface and interactions between oxygen vacancies, with a brief introduction to the dynamics of oxygen molecules on the reduced surface, role of oxygen vacancies and oxygen adspecies in the nucleation of Au, Ag and Cu clusters.
Reviewed in this article is surface characterization of organic molecules adsorbed at well-defined Pt(111) electrode surfaces from aqueous solution. Among the metal electrodes and surface-structure sensitive properties studied in this work are those involving electrocatalytic activity, such as platinum metal. Platinum is one of the most interesting materials for study in view of its exceptional spectrum of catalytic properties and its good stability in various electrolytic media. The compounds investigated were: (1) 4-Phenylpyridine (4PPY), (II) 4,4′-Bipyridyl (44BPY), (III) 2-Phenylpyridine (2PPY), (IV) 2-2′ Bibyridyl (22BPY), (V) Pyridazine (PD), and (VI) 4-Pyridazinecarboxylic acid (4PDCA). For (I) and (II), the pyridine ring binds to the Pt(111) surface in a tilted, nearly vertical orientation having a pendant phenyl ring, and is virtually unreactive toward electrochemical oxidation. However, (III) and (IV) attach to Pt(111) with their pyridine ring vertically oriented and N-attached, while the other aromatic ring is oriented parallel to the electrode surface. Similarly, (V) and (VI) are attached to Pt(111) through their ring nitrogen atom, with a tilted-vertical orientation and average ring-to-surface angles ranging from 73° to 86°. Compounds (I-VI) were studied in order to understand the influence of molecular structure on surface bonding, molecular orientation, electronic structure, and electrochemical reactivity at well-defined surfaces such as Pt(111), as a function of pH, electrode potential and adsorbate concentration.
We present periodic density functional theory calculations on the adsorption modes of acetone over the (1 1 1) surface of Pt. The dominant species previously observed in low temperature HREELS and RAIRS experiments is an “end-on” η1(O)-ketone and the calculated adsorption energy and vibrational frequencies for this species agree well with the literature data. The secondary species reported previously has been assigned to a η2(C2,O)-ketone with the molecular plane parallel to the surface. We show that this is an unlikely assignment and suggest an alternative enolate structure which has calculated vibrational frequencies in agreement with the spectra. The implications of the adsorption modes proposed in this work for models of the enantioselective hydrogenation of pyruvate esters are discussed.
Efficient CO activation on Rh particles promoted by Mn cocatalysts is important to the activity for the conversion of the syngas (CO and H2) to hydrocarbon and oxygenates. To study the effect of the step edge and promotion of Mn cocatalysts on CO activation, we studied the CO dissociation on Mn-decorated Rh(1 1 1) and stepped Rh(5 5 3) surfaces using density functional theory calculations. We found that the presence of the step edge and Mn stabilizes the transition state and reaction products: compared to clean Rh(1 1 1), calculated barrier for CO dissociation on Mn-decorated Rh(5 5 3) is lowered by about 1.60 eV, and corresponding reaction energies with respect to CO in gas phase changes from endothermic (0.21 eV) to strong exothermic (−1.73 eV). The present work indicates that the addition of Mn cocatalysts and decrease of Rh particle sizes improves greatly the activity of CO dissociation.
Atomic-scale structures and dynamic behaviors of CeO2(1 1 1) surfaces were imaged by noncontact atomic force microscopy (NC-AFM) and scanning tunneling microscopy (STM). Hexagonally arranged oxygen atoms, oxygen point vacancies, multiple oxygen vacancies, and hydrogen adatoms at the surfaces were visualized by atom-resolved NC-AFM observations. Multiple defects were stabilized by displacement of the surrounding oxygen atoms around the multiple defects, which gave enhanced brightness in the NC-AFM image due to a geometric reason. Multiple defects without reconstruction of the surrounding oxygen atoms were reactive and were healed by exposure to O2 gas and methanol at RT. Successive NC-AFM and STM measurements of slightly reduced CeO2(1 1 1) surfaces revealed that hopping of surface oxygen atoms faced to the metastable multiple defects was thermally activated even at room temperature (RT) and more promoted at higher temperatures. Heterogeneous feature of the reactivity of surface oxygen atoms with methanol was imaged by successive NC-AFM observations. These observations gave a new insight for understanding the surface structures and behavior of CeO2−x with the facile oxygen reservoir and oxidation–reduction properties related to the unique catalysis.
Batch reactor results and analysis are reported for the reaction of NO with O2 to form NO2 over a Pt(1 1 1) single crystal at atmospheric pressure. The apparent activation energy and NO, O2, and NO2 reaction orders are found to be 80 kJ mol−1, 1.3, 1, and −2 and are comparable to previous studies on supported Pt catalysts which take inhibition by the product NO2 into account. The absolute rates on a per Pt atom basis are the highest yet reported 0.34 ± 0.02 s−1, at 300 °C, 73 ppm NO, 27 ppm NO2 and 5% O2. Auger electron spectroscopy and X-ray photoelectron spectroscopy are used to show that the surface chemisorbed oxygen coverage under reaction conditions is 0.76 ± 0.06 ML, consistent with a coverage controlled by NO2 dissociation. DFT calculations are used to compare the stability of possible surface intermediates on a clean Pt(1 1 1) surface with those on a p(√3 × √3)-2O (2/3 ML) ordering surface. In contrast to the clean surface, O2 adsorption and dissociation are endothermic at 2/3 ML oxygen, but a peroxynitrite intermediate OONO* is slightly stable and may provide an alternative, associative pathway to NO2 formation that is consistent with the observed first order reaction kinetics in O2.
The hybrid catalytic combustor concept proposed by the authors has an advantage concerned with catalyst durability, because the catalyst is maintained below 1000°C even for application to 1300°C class gas turbines. A full-scale hybrid catalytic combustor has been designed for a 200 MW (1300°C) class gas turbine. The catalyst bed was 450 mm in diameter and consisted of a Pd/ alumina washcoat on a cordierite monolith. In experiments, the combustor has demonstrated the capability of meeting the NOx emission level of SCR (selected catalytic reduction) during atmospheric pressure testing. To predict the catalyst performance at an elevated pressure, the characteristics of the catalyst were studied using a small scale reactor test, and a material property test using a DTA/TGA-Q.MASS system. The catalyst showed a higher activity in the oxidized state (PdO) than in the metallic state (Pd). This activity difference was governed by the equilibrium of the oxygen release from PdO in bulk. It was considered that oxidation rate of the metallic Pd in bulk was not so high and this caused self-oscillation for the Pd catalyst around the temperature of the oxygen release equilibrium. Even below the temperature of the oxygen release equilibrium, both surface and bulk (lattice) oxygen of the PdO was consumed by the methane oxidation reaction, and resulted in a lack of surface oxygen on the catalyst. This caused a reversible decrease in the catalyst activity during combustion testing, and indicated that the oxygen dissociation step was a rate limiting step in the catalytic combustion.
Mass (g) of fractions obtained during liquefied wood sample preparation scheme with time.
A blend of spruce (Picea spp.) and fir (Abies spp.) sawdust was liquefied by an optimized technique in the presence of multifunctional alcohols and p-toluene sulfonic acid monohydrate as a catalyst. The lignin behaviour during wood liquefaction was studied at different reaction times. Samples prepared by the new separation procedure were characterized by quantitative 31P, 13C NMR and by size-exclusion chromatography. The final products showed increased molar masses indicating the condensation reactions between the lignin and the multifunctional alcohols. Aliphatic chains were found to be incorporated into the lignin network after liquefaction. The resulting polymer showed an increased content of aliphatic hydroxyl groups with respect to the starting material and a high degree of solubility in multifunctional alcohols.
After reviewing some recent studies on the characterisation of coke deposits on fluid catalytic cracking (FCC) and hydroprocessing catalysts by solid state 13C-NMR, the quantitative structural information that has been obtained through the use of demineralisation of FCC catalysts to provide coke concentrates for analysis will be described. The deactivated catalysts investigated contain only approx. 1% (w/w) carbon and were obtained both from refinery units operating with heavy feeds and from laboratory fluidised-bed tests with n-hexadecane. As for other carbonaceous materials, the use of a low-field field strength in conjunction with the single pulse excitation (SPE or Bloch decay) technique has enabled most of the carbon to be detected and, therefore, NMR-invisible graphitic layers are not thought to be major structural features of the cokes. Although stripping the catalysts gives rise to highly aromatic cokes (aromaticity>0.95), even for n-hexadecane, differences in feedstock composition are still reflected in the structure of the resultant cokes with those derived from n-hexadecane containing less condensed aromatic nuclei than those from heavy feeds.
The adsorption and hydrogenation of ethyne over EUROPT-1 has been studied using 14C-ethyne and 14C-ethene as tracers at 295 K. Adsorption of both ethyne and ethene show typical primary and secondary regions. Only a fraction of the initially adsorbed radioactivity can be removed evacuation, treatment in hydrogen or under ethyne hydrogenation conditions at the adsorption temperature. Ethene cannot be adsorbed on an ethyne precovered surface, although ethyne will displace only 68% preadsorbed ethene. In the hydrogenation of ethyne, evidence has been obtained to show that the surface coverage by hydrocarbon progressively increases during the course of the reaction up to the acceleration point in the pressure-time curves. The initial selectivity for ethene formation is ca. 0.25. Hydrogenation of 12C-ethyne/14C-ethene mixtures shows that the further hydrogenation of ethene to ethane is of only minor importance in determining the selectivity. A reaction scheme is proposed in which three types of surface site are identified as being responsible for the hydrogenation of (a) ethyne to ethene; (b) ethyne to ethane and (c) ethene to ethane.
Changes effected in the isotopic composition of monolayer equivalent amounts of or equimolar probe gases by contact with pure or rhodia-promoted CeO2 and ZrO2 are reported and analysed. A prerequisite for efficient and selective promotion of the R0-type homophase exchange process, at 295 K over preoxidised samples was dispersal thereon of ≥2 wt% rhodia originated from chloride-free precursors. Temperatures for onset of heterophase isotope exchange over preoxidised CeO2 or ZrO2 under T-ramp were 790 or 873 K. However, dispersion of 0.5, 2.0 or 4.0 at.% rhodia thereon resulted in onset of heterophase exchange at progressively lower temperatures. Nature of the sites/mechanisms via which dispersed rhodia promotes those R0 or R2 changes in the dioxygen probe gases are considered. Also presented are results from analagous experiments in which replaced dioxygen as probe gas over preoxidised CeO2 or RhOx//CeO2. Observations under T-ramp demonstrated the onset at ca. 450 K of a decrease in mirrored by onset of an increase in . Continuation of both processes at similar rates brought about an equimolar mixture at 700 K. The 1:1 stoichiometry of that conversion would be consistent with either of the R1-type heterophase oxygen isotope exchange processes ; or over preoxidised CeO2 or 0.5% RhOx//CeO2. Modifications of their activity and selectivity in conversions following reductions in H2 at 423 or 823 K are described and interpreted.
The Pt supported on mesoporous silicate FSM-16 showed high and stable catalytic activity for the hydrodesulfurization of thiophene at 350°C and this activity was higher than that of commercial catalysts. The acid sites of FSM-16 and the spillover hydrogen formed on Pt particle in Pt/FSM-16 catalyst play an important role for the hydrodesulfurization of thiophene.
Aluminated mesoporous molecular sieves of FSM-16 type were functionalized by cation exchange with MnTMPyP metalloporphyrin species. Textural analysis showed that the method of alumination determines the distribution of metalloporphyrin cations between the internal mesopore system and the external support surface. Direct alumination leads to the preferential accumulation of MnTMPyP within the internal channels, while impregnation of the purely siliceous FSM-16 with aluminium results in the immobilization of a large part of metalloporphyrin at the outer surface of the support. This conclusion is supported by the selectivity patterns observed in the oxidation of cyclohexene with iodosylbenzene. The nanospace constraints experienced by MnTMPyP centres located in the channels of the directly aluminated supports suppress the epoxidation pathway leaving way for the allylic oxidation. The metalloporphyrin species abundant at the external surface of FSM-16 supports aluminated by impregnation of purely siliceous material are free of steric hindrances and yield predominantly epoxide. The results point to the crucial role of the alumination procedure in tailoring the catalytic properties of FSM-16 supported metalloporphyrins.
To characterize the oxygen mobility over metal supported catalysts on a dynamic and in situ base, isotopic exchange reaction combined with CO oxidation was designed and exemplified on three kinds of three way catalysts of Pt/CeO2-ZrO2 (CZ-O, CZ-D and CZ-R). The obtained oxygen diffusion coefficients, oxygen release rate, and oxygen storage capacity were discussed and correlated with XRD spectra and other physical parameters. It was found that the oxygen mobility and oxygen storage capacity were parallel to the structural homogeneity of Zr introduction into the CeO2 frame work, and decreased as: CZ-R > CZ-D > CZ-O. These results indicated that this combined isotopic exchange technique could be used to quantify the surface and bulk oxygen mobility, the oxygen storage capacity and oxygen release rate over the metal supported catalysts, and could be employed as a meaningful probe into the nature of CeO2-ZrO2 oxygen storage material. The oxygen mobility is also another important indicator for the development of oxygen storage materials.
The photocatalytic discoloration of Reactive Blue 19 (RB-19), a textile anthraquinone dye, were investigated in aqueous suspensions containing TiO2 or ZnO as catalysts. The reactions can be mathematically described as a function of parameters pH, amount of catalyst and dye concentration being modeled by the use of response surfaces methodology. Optimized values for the concentration of the catalyst and the pH for each reaction systems were determined. ZnO showed greater degradation activity than TiO2 when the reaction was carried out in a 4 l circulating reactor under optimized conditions. The figure-of-merit electric energy per order (EE/O) allows to determine that the electrical energy cost of reducing the color by one order of magnitude was three times higher for TiO2. In both reaction systems, the acute toxicity evolved from zero to around 50 toxicity units in the initial stages of irradiation, depleting to minimal values after 30 min of reaction.
The article describes how the application of experimental methods developed over the last 40 years (work function, photoemission, photoelectron spectroscopy and scanning tunnelling microscopy) enables our understanding of oxygen chemisorption to progress in a step-wise manner, with STM providing the ultimate atom-resolved evidence for the dynamics involved. Different oxygen states exhibit distinct reactivities and provide a stimulus for the design of catalysts with specific activities in oxidation catalysis. There is also evidence accruing that disordered oxygen states are in general more active in dehydrogenation reactions than the ordered states, with for example ammonia being ‘oxidised’ to nitrogen adatoms at Cu(1 1 0) at cryogenic temperatures.
Overview of the samples calcined at 673 K a
IS vs. QS plot showing the sets of values extracted from the measured spectra. The values for the bulk contributions are all coinciding with the point referred to as 'bulk Au 0 '. The grey background regions indicate the values that are most commonly measured for Au compounds, corresponding to the known valence states in gold: Au I , Au III and Au V. As can be seen in the plot two areas are present in which the values are distributed: an area close to the IS for bulk whose values can be referred to as metallic-like, and an area in the Au III-region close to IS = 2.6 mm/s, QS = 3.6 mm/s whose values can be referred to as ionic. 
Mössbauer parameters extracted from the measured spectra a
Gold catalysts supported on TiO2 and TiO2/SiO2 have been studied with 197Au Mössbauer spectroscopy. During the preparation of the Au/TiO2 catalyst the formation of metallic particles is observed. The presence of ionic gold in active gold catalysts is not observed. The spectra can be interpreted in terms of bulk-like contributions from the inner-core atoms plus contributions from the outermost atoms at the surface of the particles.
Mesoporous SBA-15 (Santa Barbara University at California) has been synthesized and functionalized with (4-chloromethyl)phenyltriethoxysilane, there after the surface silanols groups were passivated by dimethoxydimethylsilane (MeO2Me2Si) [Bz-SBA-15]. Further (1R,2S)-(+)-cis-1-amino-2-indanol [AIL] has been immobilized over functionalized SBA-15 (BzAIL-SBA-15). The heterogeneous catalysts were synthesized by complexation of BzAIL-SBA-15 by [RuCl2(benzene)]2 and [RuCl2(p-cymene)]2 in 2-propanol at reflux condition designated as Ru-Bn-BzAIL-SBA-15 (Ru-1) and Ru-Cy-BzAIL-SBA-15 (Ru-2). The synthesized materials have been characterized by small angle X-ray diffraction spectroscopy (SAXS), N2 adsorption–desorption isotherms, Fourier Transformed-Infra-red (FT-IR) spectroscopy, Transmission Electron Microscopy (TEM), intensively coupled plasma atomic emission spectroscopy (ICP-AES), Diffuse reflectance UV–vis and 13C CP MAS NMR analysis. Further the synthesized and characterized materials (Ru-1 and Ru-2) have been successfully applied in the asymmetric transfer hydrogenation (ATH) reaction of simple prochiral ketones, which gave a yield of 56% and 18% and enantiomeric excess (ee) values of 62% and 77% for Ru-1 and Ru-2, respectively, after 1 h for acetophenone. The reusability studies showed approximately the same enantiomeric excess value at lower conversions.
Au/TiO2–CeO2 photocatalysts were studied in the photodegradation of the 2,4-dichlorophenoxyacetic acid (2,4-D) in aqueous medium under UV light emission source. The TiO2–CeO2 supports (2.5–10 wt.% CeO2) were prepared by the sol–gel method using titanium alkoxide and cerium nitrate as starting precursors. Gold nanoparticles were prepared by the deposition–precipitation method with urea and deposed on the supports to obtain Au/TiO2–CeO2 photocatalysts. The BET area increases as function of the CeO2 content from 64 m2/g for the bare TiO2 solid to 71–137 m2/g for the TiO2–CeO2 mixed oxides. TEM and HRTEM images showed Au nanoparticles ranking from 8.1 to 3.4 nm. Additionally well defined gold plasmon and a red shift in the Eg band gap energy (3.14–2.47 eV) was observed by UV–vis spectroscopy on the mixed oxides. For the photodegradation of 2,4-D it has been observed an important effect of cerium oxide in the photoactivity, it varies from 78 to 99% of 2,4-D conversion and from 61 to 88% of total organic carbon conversion for the Au/TiO2 and Au/TiO2–CeO2 photocatalysts respectively. A synergetic effect where gold plays the role of electrons tramp and the cerium oxide the role of oxidizing agent is proposed.
Triphenylthiapyrylium cation (TPTP+) has been studied as solar photocatalyst and compared with other organic dyes (triphenylpyrylium, acridine yellow, methylene blue, rosolic acid and alcian blue) using ferulic acid as model pollutant. The best results were obtained with TPTP+ (ca. 85% elimination), although important photodegradation was also achieved with acridine yellow and triphenylpyrylium. Concentrations of TPTP+ in the range 10–30 mg/l had only a moderate effect on the reaction rate; on the other hand, the concentration of ferulic acid had a remarkable influence on the pseudo-first order rate constant, for which higher values were measured in diluted solutions of the substrate. The results from photophysical studies based on fluorescence quenching and laser flash photolysis were compatible with an electron transfer mechanism. Although the process is thermodynamically feasible from the first singlet excited state, triplet involvement cannot be ruled out at the present stage. The studies were extended to detoxification of Sevnol, a commercial pesticide, based on carbaryl as active ingredient. Total removal of carbaryl was accomplished, although mineralization was only 30%. However, activated sludge respirometry showed that elimination of carbaryl resulted in a complete detoxification of the solution and a significant enhancement of the biodegradability.
The biphasic palladium-catalysed cleavage of water-insoluble allylic substrates in the presence of methylated cyclodextrins has been investigated with the aim of performing substrate-selective catalytic reactions. While no substrate selectivity was observed in control experiments in which acetonitrile was used as mass transfer promoter, the use of DMCyD led in some cases to high substrate selectivities. For instance, a 97:3 product ratio was observed during the cleavage of a 50:50 mixture of N-dodecyl-O-allyl urethane and N,N-dihexyl-O-allyl urethane. The whole results demonstrate that the size-fit concept which postulates the highest reactivity for the best size-matched host–guest pair is limited to predict the substrate selectivity.
The scaling-up of the gas phase catalytic ammoxidation of 2,6-dichloro toluene (DCT) to 2,6-dichloro benzonitrile (DCBN) over a promoted vanadium phosphate (VPO) catalyst from first lab-scale experiments to pilot plant runs is reported. First experiments in a row of conversions of isomeric dichloro toluenes using simple, non-promoted VPO catalysts only show poor yield and selectivity. In particular, DCT ammoxidation is hindered due to bulky chlorine substituents probably preventing a sufficient interaction of the methyl group and lattice oxygen and/or N-containing surface species. Improved synthesis of VPO catalyst with the addition of promoters and γ-alumina or titania leads to significant increase in DCT conversion and DCBN yield. A Cr containing vanadyl pyrophosphate catalyst admixed with titania (anatase) showed conversion up to 97% with DCBN yields of ca. 80%. The same catalyst was also used for pilot plant runs, usually in the form of 5 mm × 3.5 mm shaped tablets that were prepared from a larger batch of solid synthesis. The scaling-up of the process using 100 ml of catalyst was investigated both by catalytic experiments and reactor simulations. The results showed that the temperature control will be crucial in scaling-up. Validation of simulation results with that of experimental results was also checked and a good agreement between measured and simulated results is observed.
A novel TiO2 immobilized hydrophobic montmorillonite photocatalysts were designed and prepared for the advanced oxidation of persistent organic pollutants in water, which combined the pre-adsorption and concentrated effects for aqueous micro-organic pollutants with the photocatalytic destruction of organic pollutants. The photocatalysts were synthesized by immobilizing TiO2 onto surfactant-pillared montmorillonite via ion exchange reaction between sodium montmorillonite with cation surfactant, cetyl trimethyl ammonium bromide (CTMAB). The Degussa P25 catalyst loading amounts varied between 20 and 80%. The composition and texture of the prepared composites were characterized with X-ray powder diffraction, scanning electron microscope, and energy-dispersive spectrometry. The adsorption performance and photocatalytic activities of prepared composite photocatalysts were evaluated using decabromodiphenyl ether (BDE 209) as a model pollutant in aqueous solution. The results were found that these composite photocatalysts can effectively degrade BDE 209, and the complete removal can be achieved within 180 min of irradiation. The removal efficiency of BDE 209 increased with the percentage of immobilized TiO2 on the hydrophobic clay. Highly hydrophobic substrates (BDE 209) can effectively adsorb (concentrate) in or near the hydrophobic surfactant regions of TiO2 immobilized CTMAB-pillared montmorillonite photocatalysts. The degradation pathways involving photochemical debromination and hydroxyl radical addition are proposed based on the identification of specific by-products.
Fischer–Tropsch catalysts, based on cobalt, are complex materials containing the catalyst support and reduction and structural promoters in addition to the cobalt metal itself. While this basic description covers both modern FT catalyst and those first commercialised, in Germany in 1930s, modern FT catalysts have vastly improved performance. While such improvements owe much to modern materials and manufacturing methods, the role of advanced characterisation techniques cannot be underestimated. Modern techniques discern details of catalyst structure that were not available during the early pioneering work, they guide current developments and offer the prospect of designing tailored catalysts.
This review paper first discusses the needs for fundamental changes in the energy system for major efficiency improvements in terms of global resource limitation and sustainable development. Major improvement in energy efficiency of electric power plants and transportation vehicles is needed to enable the world to meet the energy demands at lower rate of energy consumption with corresponding reduction in pollutant and CO2 emissions. A brief overview will then be given on principle and advantages of different types of low-temperature and high-temperature fuel cells. Fuel cells are intrinsically much more energy-efficient, and could achieve as high as 70–80% system efficiency (including heat utilization) in electric power plants using solid oxide fuel cells (SOFC, versus the current efficiency of 30–37% via combustion), and 40–50% efficiency for transportation using proton-exchange membrane fuel cells (PEMFC) or solid oxide fuel cells (versus the current efficiency of 20–35% with internal combustion (IC) engines). The technical discussions will focus on fuel processing for fuel cell applications in the 21st century. The strategies and options of fuel processors depend on the type of fuel cells and applications. Among the low-temperature fuel cells, proton-exchange membrane fuel cells require H2 as the fuel and thus nearly CO-free and sulfur-free gas feed must be produced from fuel processor. High-temperature fuel cells such as solid oxide fuel cells can use both CO and H2 as fuel, and thus fuel processing can be achieved in less steps. Hydrocarbon fuels and alcohol fuels can both be used as fuels for reforming on-site or on-board. Alcohol fuels have the advantages of being ultra-clean and sulfur-free and can be reformed at lower temperatures, but hydrocarbon fuels have the advantages of existing infrastructure of production and distribution and higher energy density. Further research and development on fuel processing are necessary for improved energy efficiency and reduced size of fuel processor. More effective ways for on-site or on-board deep removal of sulfur before and after fuel reforming, and more energy-efficient and stable catalysts and processes for reforming hydrocarbon fuels are necessary for both high-temperature and low-temperature fuel cells. In addition, more active and robust (non-pyrophoric) catalysts for water–gas-shift (WGS) reactions, more selective and active catalysts for preferential CO oxidation at lower temperature, more CO-tolerant anode catalysts would contribute significantly to development and implementation of low-temperature fuel cells, particularly proton-exchange membrane fuel cells. In addition, more work is required in the area of electrode catalysis and high-temperature membrane development related to fuel processing including tolerance to certain components in reformate, especially CO and sulfur species.
The very large reserves of methane, which often are found in remote regions, could serve as a feedstock for the production of chemicals and as a source of energy well into the 21st century. Although methane currently is being used in such important applications as the heating of homes and the generation of hydrogen for ammonia synthesis, its potential for the production of ethylene or liquid hydrocarbon fuels has not been fully realized. A number of strategies are being explored at levels that range from fundamental science to engineering technology. These include: (a) stream and carbon dioxide reforming or partial oxidation of methane to form carbon monoxide and hydrogen, followed by Fischer–Tropsch chemistry, (b) the direct oxidation of methane to methanol and formaldehyde, (c) oxidative coupling of methane to ethylene, and (d) direct conversion to aromatics and hydrogen in the absence of oxygen. Each alternative has its own set of limitations; however, economical separation is common to all with the most important issues being the separation of oxygen from air and the separation of hydrogen or hydrocarbons from dilute product streams. Extensive utilization of methane for the production of fuels and chemicals appears to be near, but current economic uncertainties limit the amount of research activity and the implementation of emerging technologies.
A comprehensive study on physical and chemical properties of Mo/MCM-22 bifunctional catalysts has been made by using combined analytic and spectroscopic techniques, such as adsorption, elemental analysis, and 129Xe and 31P NMR of adsorbed trialkylphosphine oxide probe molecules. Samples prepared by the impregnation method with Mo loadings ranging from 2–10 wt.% have been examined and the results are compared with that obtained from samples prepared by mechanical mixing using MoO3 or Mo2C as agents. Sample calcination treatment is essential in achieving a well-dispersed metal species in Mo/MCM-22. It was found that, upon initial incorporation, the Mo species tend to inactivate both Brønsted and Lewis sites locate predominantly in the supercages rather than the 10-membered ring channels of MCM-22. However, as the Mo loading exceeds 6 wt.%, the excessive Mo species tend to migrate toward extracrystalline surfaces of the catalyst. A consistent decrease in concentrations of acid sites with increasing Mo loading <6 wt.% was found, especially for those with higher acid strengths. Upon loading of Mo > 6 wt.%, further decreases in both Brønsted and Lewis acidities were observed. These results provide crucial supports for interpreting the peculiar behaviors previously observed during the conversion of methane to benzene over Mo/MCM-22 catalyst under non-oxidative conditions, in which an optimal performance was achieved with a Mo loading of 6 wt.%. The effects of Mo incorporation on porosity and acidity features of the catalyst are discussed.
Our recent research works on the selective catalytic reduction (SCR) of diesel engine NOx by hydrocarbons over alumina-supported silver (Ag/Al2O3) were reviewed. The reaction mechanism of the SCR of NOx by C2H5OH over Ag/Al2O3 was studied using in situ DRIFTS and DFT calculations. A novel enolic species originating from the partial oxidation of C2H5OH and C3H6, was found on the surface of Ag/Al2O3 during the SCR of NOx by in situ DRIFTS, which was also supported by DFT calculations. Based on this, a mechanism of the NOx reduction was proposed, which can successfully explain the high efficiency of the NOx reduction by C2H5OH over Ag/Al2O3. A palladium promoted Ag/Al2O3 catalyst (denoted Ag–Pd/Al2O3) was developed for the SCR of NOx by C3H6. The Ag–Pd/Al2O3 showed a higher NOx conversion than Ag/Al2O3, especially at temperatures ranging from 300 to 450 °C. The engine bench tests showed that the average NOx conversion was greater than 80% in the diesel engine exhaust temperature range of 300–400 °C using our catalytic converter with C2H5OH as reductant, which represents a leap from the Euro II standard to the Euro III standard for NOx emission control in diesel engines.
The reaction network of the partial oxidation of n-butane to maleic anhydride on (VO)2P2O7 has been investigated using steady-state and transient experiments in a Temporal-Analysis-of-Products (TAP) reactor under vacuum conditions to identify by mass spectrometry possible intermediate products and in a tubular fixed bed reactor at atmospheric pressure to derive information on the role of the detected and other potential intermediates in the reaction network. The oxidation of butane, butadiene, tetrahydrofuran, dihydrofuran, and furan has been studied in the TAP reactor and, additionally to these compounds, crotonaldehyde, crotonlactone, and malealdehydic acid were oxidized in the tubular flow reactor. From the results obtained it can be concluded that the main reaction pathway from butane to maleic anhydride proceeds via the intermediate products n-butenes, butadiene, crotonaldehyde, dihydrofuran, furan, and crotonlactone.
In order to investigate the effects of additives on the active sites of β-(VO)2P2C7 catalyst, a series of promoted VPO catalysts, prepared by the impregnation of VOHPO4.0.5H2O with solutions containing promoter elements, were characterized with BET, XRD, and NARP (NO-NH3 Rectangular Pulse) technique, and their catalytic properties were examined with the continuous flow reaction of the selective oxidation of n-butane. The structure of catalyst and the number of surface V=O species as the active sites are discussed in relation to the promoter and the preparation procedure, and the catalytic property is discussed in relation to the structural factors and the active sites.
Top-cited authors
Chunshan Song
  • The Chinese University of Hong Kong
Angelo Vaccari
  • University of Bologna
F. Trifirò
  • University of Bologna
S. Malato
  • Plataforma Solar de Almería
Gabriele Centi
  • Università degli Studi di Messina