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Deep oxidation of methane on particles derived from YSZ-supported Pd–Pt-(O) coatings synthesized by Pulsed Filtered Cathodic Arc
Abstract
Methane total oxidation was studied between 250 and 800 °C in a specific quartz reactor for Pd, PdOy, Pd0.6Pt0.4Oy and Pd0.4Pt0.6Oy thin films deposited on yttria stabilized zirconia (YSZ) substrates. Pt containing films exhibited poor activity and a rapid thermal decomposition. Pd and PdOy films showed good activity and transformed to particles dispersed on the YSZ substrates. The higher reaction rate of PdOy films was explained by a better dispersion of the catalyst. A drop of the reaction rate was observed when the temperature exceeded 735 °C and 725 °C for initially Pd and PdOy, respectively, which can be associated with the decomposition of PdO into Pd and O2. The good cycling stability and catalytic efficiency of the latter films render it promising for the pollution control in methane combustion.
... A fortiori in the case of thin films, size effects may also lead to a more complex interpretation of the results and require more specific attention in their advanced microstructural design as well as the detailed understanding of dependencies on the material response. The thermal decomposition of metal oxide and nitride thin films can be exploited to generate original microstructures such as high specific surface area metal islands on an oxide matrix [1] or dispersed particles with different core and surface compositions [2]. Thereby, physico-chemical properties of the as-grown films are strongly impacted by thermal treatments. ...
... Palladium oxide (PdO) and platinum oxide (PtO 2 ) are stable in normal conditions but decompose in air around 850 • C [9] and 450 • C [10], respectively. Thus, the ther-mal decomposition of complex Pd 1−y Pt y O x thin films produced by physical vapour deposition methods [2,3] is rather complex but is characterized for y ≥ 0.4 by the crystallisation of metallic nanoparticles below 750 • C in a mixture of methane and oxygen [2,3]. ...
... Palladium oxide (PdO) and platinum oxide (PtO 2 ) are stable in normal conditions but decompose in air around 850 • C [9] and 450 • C [10], respectively. Thus, the ther-mal decomposition of complex Pd 1−y Pt y O x thin films produced by physical vapour deposition methods [2,3] is rather complex but is characterized for y ≥ 0.4 by the crystallisation of metallic nanoparticles below 750 • C in a mixture of methane and oxygen [2,3]. ...
Laser interference patterning (also known as “laser interference metallurgy”) is used to locally tailor the microstructure of oxide (Pd0.25Pt0.75Ox) and nitride (Cu3N) thin films to induce a chemical decomposition, which is responsible for a decrease of electrical resistivity. This technique allows hereby a laser-induced chemical decomposition of as-deposited oxide and nitride films, resulting locally in a porous microstructure due to the simultaneous emission of gaseous nitrogen and oxygen. The process locally generates at the nanometer scale metal precipitatation of Pt or Cu in the oxide or nitride matrix. Thus, isolated metallic clusters with low resistivity coexist with a high resistivity phase, establishing a preferential electrical conduction path and giving the system a lower effective resistivity. The decomposition process is investigated by four-point probe method, X-ray diffraction, spectrophotometry, white light interference, scanning and transmission electron microscopies.Research highlights▶ One step laser interference patterning of Cu3N and Pd0.25Pt0.75Ox. ▶ Thermal decomposition of metal nitride and oxide thin films. ▶ Laser-induced precipitation of metallic nanoparticles. ▶ Significant lowering of electrical resistivity.
... Noble metals are fascinating for various aspects, among which is their poor reactivity towards oxygen. The association of noble metals with other metallic elements can produce original microstructures and morphologies, more particularly upon thermal annealing and in the presence of oxygen [1,2]. We showed recently that the electrical conductivity of noble metal-based oxides and nitrides could be tailored by laser interference patterning [3]. ...
... The present study is dedicated to the effect of laser interference patterning on the microstructure of sputter-deposited Pd 0.25 Pt 0.75 O x and goldincorporated yttria-stabilized zirconia (Au-YSZ) thin films. These two chemical systems are of interest for their potential applications in the field of catalysis [2,8,9]. Moreover Au-YSZ nanocomposites exhibit improvements in tribological properties due to microstructure adaptive changes influencing the friction coefficient [10]. ...
Laser interference patterning-induced microstructural modifications have been investigated in two noble metal-incorporated oxide thin film systems: Pd0.25Pt0.75Ox and gold-incorporated yttria-stabilized zirconia - Au-YSZ. Transmission electron microscopy was used to investigate the influence of the laser treatment on the microstructure of the samples. In the case of Pd0.25Pt0.75Ox, the formation of a nanocomposite arrangement resulted from the precipitation of metal nanograins in the oxide matrix triggered by laser irradiation. In Au-YSZ, the starting microstructure consisted of gold nanograins embedded in a YSZ matrix. A noticeable growth and coalescence of gold nanograins occurred near the surface in the region of maximum interference. Simultaneously, a foamy morphology, mostly consisting of gold crystals, was formed at the film surface. In contrast to thermal annealing, the laser treatment proposed here is a fast procedure to partially relocate gold at the film surface and provide a local solid lubrication.
The preparation of WTe2 by isothermal tellurium annealing of sputtered W or WO3 precursors is reported. X‐ray diffraction, energy dispersive, and Raman spectroscopies confirm the presence of 1T́ WTe2 phase in the annealed films which are preferentially oriented with the [001] crystallographic direction normal to the surface. Atomic force microscopy images indicate noticeable morphology differences between thin samples tellurized from different precursors. Rounded small grains or small platelets are obtained when W and WO3 precursor are used, respectively. WTe2 is an important member of the transition metals dichalcogenides family. In this paper, WTe2 films are prepared by isothermal tellurium annealing of sputtered W or WO3 films in a close space configuration. Despite the used precursor, films show preferential orientation with the [001] direction oriented normal to the surface. Rounded grains/platelets are obtained when using W/WO3 precursors.
Laser annealing (LA) of AlN/Ag multilayers was proven to be an effective process to control the structure and dispersion of Ag into the AlN resulting in intense coloration via the localized surface plasmon resonance, which is of particular importance for decorative applications. In this work we present a study of the structural changes occurring in various AlN/Ag multilayers after LA, in an effort to establish firm knowledge of the diffusion and re-nucleation mechanisms that occur during the laser process. We investigate the effect of the basic LA parameters, such as the laser wavelength (193 and 248 nm), fluence (400–700 mJ/cm2), pressure (1 and 10 Bar) and number of pulses (1 and 2) and we show that the main processes is the Ag particle enhancement close to the film surface as a result of additive outidiffusion Ag and the formation of nanoparticles of varying size.
Laser interference patterning (LIP) and the hereby induced microstructure modifications have been
investigated in gold/yttria-stabilized zirconia nanocomposite films. Transmission electron microscopy
was used to study the influence of the laser treatment on the structure and microstructure of the
samples. The impact of LIP on the friction coefficient has been evidenced. The initial microstructure
consisted of gold nanograins homogeneously distributed in the yttria-stabilized zirconia matrix. A
noticeable growth and coalescence of gold nanograins occurred near the surface in specific regions.
Simultaneously, a foamy morphology, mostly consisting of gold crystals, was formed at the surface
and is responsible for a drastic diminution of the friction coefficient after patterning. Furthermore,
the influence of the film topography on the friction behavior is analyzed using Abbott–Firestone
curves. In contrast to thermal annealing, the laser treatment proposed here is a fast procedure to
partially relocate gold at the film surface and provide a local solid lubrication.
The chemistry, microstructure, and catalytic activity of thin films incorporating palladium were studied using scanning and transmission electron microscopies, X-ray diffraction, spectrophotometry, 4-point probe and catalytic tests. The films were synthesized using pulsed filtered cathodic arc and magnetron sputter deposition, i.e. techniques far from thermodynamic equilibrium. Catalytic particles were formed by thermally cycling thin films of the Pd–Pt–O system. The evolution and phase formation in such films as a function of temperature were discussed in terms of the stability of PdO and PtO2 in air. The catalytic efficiency was found to be strongly affected by the chemical composition, with oxidized palladium definitely playing a major role in the combustion of methane. Reactive sputter deposition of thin films in the Pd–Zr–Y–O system allowed us forming microstructures ranging from nanocrystalline zirconia to palladium nanoparticles embedded in a (Zr,Y)4Pd2O matrix. The sequence of phase formation is put in relation to simple thermodynamic considerations.
Noble metal oxide surfaces with metals of the platinum
group are of prime importance for catalytic reactions
such as the reduction of nitrogen oxides and the catalytic
combustion of methane. Pt3O4 is theoretically highly efficient
for the conversion of methane but is stable only in a narrow
range of temperatures around 900 K. To overcome that
problem, solid solutions of Pt3O4 and sodium can be stabilized
in NaxPt3O4 compounds with 0 < x < 1. The ternary compound
is one of the most intriguing oxides with a unique combination
of peculiar physical and chemical properties such as a
bifunctional catalytic behavior, electronic conductivity, and
mixed valency. Its study remains difficult due to very small
amounts of the material produced by extremely severe
conditions of synthesis. We present a simple and easily
upscalable way to synthesize large quantities of NaxPt3O4.
Plasma deposition of catalytic thin films is reviewed in view of highlighting some interesting features. Plasma sputtering, plasma enhanced chemical vapor deposition and plasma enhanced metalorganic chemical vapor deposition and their preferential use in various kind of catalytic films are described. Fuel cell electrodes, gas sensors and photocatalytic films are emphasized as significant applications. As example, magnetron sputtering deposition is successfuly used for growing fuel cell electrodes with high performances. Doping doped TiO2 photocatalysts are deposited using various kind of plasma depending on the expected film morphology. Gas sensors are well designed when using plasma deposition. Plasma treatment of catalysts offers a suitable altertive to thermal treatments. Finally, associated simulations, especially recent progress in molecular dynamics simulations of catalytic films growth are surveyed. This is a suitable way to understand basic mechanisms of catalytic film growth.
In this work, we study the influence of the incorporation of different metals (Me = Au, Ag, Cu, Mo) on the electronic structure of amorphous carbon (a-C:Me) films. The films were produced at room temperature using a novel pulsed dual-cathode arc deposition technique. Compositional analysis was performed with secondary neutral mass spectroscopy whereas X-ray diffraction was used to identify the formation of metal nanoclusters in the carbon matrix. The metal content incorporated in the nanocomposite films induces a drastic increase in the conductivity, in parallel with a decrease in the band gap corrected from Urbach energy. The electronic structure as a function of the Me content has been monitored by x-ray absorption near edge structure (XANES) at the C K-edge. XANES showed that the C host matrix has a dominant graphitic character and that it is not affected significantly by the incorporation of metal impurities, except for the case of Mo, where the modifications in the lineshape spectra indicated the formation of a carbide phase. Subtle modifications of the spectral lineshape are discussed in terms of nanocomposite formation.
A dual-cathode arc plasma source was combined with a computer-controlled bias amplifier to synchronize substrate bias with the pulsed production of plasma. In this way, bias can be applied in a material-selective way. The principle has been applied to the synthesis of metal-doped diamondlike carbon films, where the bias was applied and adjusted when the carbon plasma was condensing and the substrate was at ground when the metal was incorporated. In doing so, excessive sputtering by energetic metal ions can be avoided while the sp(3)sp(2) ratio can be adjusted. It is shown that the resistivity of the film can be tuned by this species-selective bias; Raman spectroscopy was used to confirm expected changes of the amorphous ta-C:Mo films. The species-selective bias principle could be extended to multiple material plasma sources and complex materials.
Cathodic arc deposition is an established and industrially applied technique for the formation of nitrides (e.g. TiN); it can also be used for metal oxide thin film formation. A cathodic arc plasma source with the desired cathode material is operated in an oxygen atmosphere of appropriate pressure, and metal oxides of various stoichiometric composition can be formed on different substrates. We report here on a series of experiments on metal oxide formation by cathodic arc deposition for different applications. Black copper oxide has been deposited on accelerator components to increase the radiative heat transfer between the parts. Various metal oxides such as tungsten oxide, niobium oxide, nickel oxide and vanadium oxide have been deposited on ITO glass to form electrochromic films for window applications. Optical waveguide structures can be formed by refractive index variation using oxide multilayers. We have synthesized multilayers of Al2O3-Y2O3-Al2O3-Si as possible basic structures for passive optoelectronic integrated circuits, and Al2-xErxO3 thin films with a variable Er concentration which is a potential component layer for the production of active optoelectronic integrated devices such as amplifiers or lasers at a wavelength of 1.53 μm. Aluminum and chromium oxide films have been deposited on a number of substrates to impart improved corrosion resistance at high temperature. Titanium sub-oxides which are electrically conductive and corrosion resistant and stable in a number of aggressive environments have been deposited on various substrates. These sub-oxides are of great interest for use in electrochemical cells. Common features of all these depositions are the high deposition rate typical for cathodic arc deposition, the good adhesion of the films due to the high metal ion energy, and the advantage of an environmentally clean method in comparison to wet-chemical oxide formation techniques.
The catalytic combustion of methane has been investigated over eight different bimetallic palladium catalysts, comprising the co-metals Co, Rh, Ir, Ni, Pt, Cu, Ag, or Au. The catalysts were characterized by TEM, EDS, PXRD, and temperature-programmed oxidation (TPO). It was found that a catalyst containing both Pd and Pt was the most promising, as it had a high activity that did not decline with time. The catalyst containing Pd and Ag was also a promising candidate, but its activity was slightly lower. For PdCo and PdNi, the co-metals formed spinel structures with the alumina support, with the result that the co-metals did not affect the combustion performance of palladium. For PdRh, PdIr, PdCu, and PdAg, the co-metals formed separate particles consisting of the corresponding metal oxide. These catalysts, except PdRh, showed a stable activity. For PdPt and PdAu, the co-metals formed alloys with palladium, and both catalysts showed a stable activity.
The catalytic combustion of CH4has been investigated over Pd/ZrO2with the aim of identifying how the state of oxidation of the Pd affects the catalytic activity, particularly at low temperatures. It is found that fully reduced Pd is inactive and that the activity increases at 533 K until a plateau in activity is achieved when 6–7 ML of PdO have formed over the surface of the Pd crystallites. The maximum activity is identical to that for fully oxidized Pd. Examination of the PdO formed under reaction conditions and during oxidation in O2at 533 K reveals some differences. Temperature-programmed reduction in CH4and Raman spectroscopy demonstrate that the oxide formed under reaction conditions is largely crystalline PdO, whereas the oxide formed in O2is a mixture of amorphous and crystalline PdO. The crystalline form of PdO is found to reduce more readily than the amorphous form. It is observed that the low-temperature CH4combustion activity of PdO can be enhanced by producing a small amount of metallic Pd on the surface of PdO. The metallic Pd is more effective than PdO for the dissociative adsorption of CH4. It is hypothesized that the products of CH4adsorption on Pd, Hsand CHx,s(x=3–1), rapidly diffuse to the Pd/PdO interface and then reduce the PdO to Pd. The effects of metallic Pd cannot be sustained, though, under steady-state conditions, because the metallic Pd is oxidized back to PdO.
The catalytic combustion of methane at low temperature under lean conditions was investigated over bimetallic palladium-platinum catalysts supported on alumina. Pd-Pt catalysts with constant 2wt.% metal loading and varying compositions in Pt and Pd were prepared by successive impregnations of the metal salts. The catalysts were characterised by powder X-ray diffraction, transmission electron microscopy/electron dispersion X-ray spectroscopy (TEM/EDX), volumetry of H2 chemisorption, FTIR study of CO adsorption and temperature-programmed oxidation (TPO). In the absence of water added to the feed, the methane conversion over Pd-rich bimetallic catalysts (Pt/Pt+Pd molar ratios less than 0.3) was found to be the same as that of the reference Pd/Al2O3 catalyst. Interestingly, under wet conditions, these bimetallic catalysts exhibited an improved performance with respect to Pd/Al2O3. This effect was found to be maintained upon mild steam ageing. An interaction between both metals was suggested to explain the enhanced activity of bimetallic catalysts. This was confirmed by TPO experiments indicating that formation and decomposition of PdO is affected upon Pt addition even for very low amounts of Pt. The adsorption of CO on reduced catalysts studied by FTIR revealed new types of adsorbed CO species, suggesting again an interaction between two metals.
A catalytic combustor is described for a 1.5MW gas turbine engine. The catalyst temperature is limited and the high combustor outlet temperatures required by the turbine are generated downstream of the catalyst. The combustor design places a low NOx preburner upstream of the catalyst and uses this preburner to achieve optimum catalyst operation by providing the desired catalyst inlet temperature. The combustor system employs the catalyst during engine acceleration and loading. The catalyst design has been tested on a sub-scale rig under full pressure and flow conditions simulating turbine operation over the entire operating range including acceleration and loading. The design should achieve emissions at full load operation of
In this work, different procedures, namely carbonate coprecipitation and modified solid–solid diffusion, were used to prepare hexaaluminate samples, unsupported or supported onto θ-Al2O3. These samples were used as catalyst for the methane total oxidation as synthesized or after impregnation of 1wt% Pd. It was observed that the modified solid–solid diffusion procedure is an efficient method to obtain the hexaaluminate structure. At a theoretical ratio x of hexaaluminate onto Al2O3 less than 0.6 (xLa0.2Sr0.3Ba0.5MnAl11O19+(1−x)·Al2O3, with x=0.25, 0.60), samples with high specific surface area and θ-Al2O3 structure are then obtained. Large differences in catalytic activity can be observed among the series of sample synthesized. All the pure oxide samples (i.e. without palladium) present low catalytic activity for methane total oxidation compared to a reference Pd/Al2O3 catalyst. The highest activity was obtained for the samples presenting a θ-Al2O3 structure (with x=0.60) and a high surface area. Impregnation of 1wt% palladium resulted in an increase in catalytic activity, for all the solids synthesized in this work. Even if the lowest light-off temperature was obtained on the reference sample, similar methane conversions at high temperature (700°C) were obtained on the stabilized θ-Al2O3 solids (x=0.25, 0.60). Moreover, the reference sample is found to strongly deactivate with reaction time at the temperature of test (700°C), due to a progressive reduction of the PdOx active phase into the less active Pd° phase, whereas excellent stabilities in reaction were obtained on the pure and palladium-doped hexaaluminate and supported θ-Al2O3 samples. This clearly showed the beneficial effect of the support for the stabilization of the PdOx active phase at high reaction temperature. These properties are discussed in term of oxygen transfer from the support to the palladium particle. Oxygen transfer is directly related to the Mn3+/Mn2+ redox properties (in the case of the hexaaluminate and stabilized θ-Al2O3 samples), that allows a fast reoxidation of the metal palladium sites since palladium sites reoxidation cannot occur directly by gaseous dioxygen adsorption and dissociation on the surface.
The kinetics of methane combustion at low temperatures are consistent with a Mars–van Krevelen redox mechanism involving the activation of methane on site pairs consisting of oxygen atoms and oxygen vacancies on the surface of PdOxcrystallites. H2O strongly inhibits methane oxidation rates by titrating surface vacancies in a quasi-equilibrated adsorption–desorption step. Initial activation periods during methane oxidation are related to the presence of oxygen-deficient PdOxcrystallites, which contain stronger Pd–O oxygen bonds and increase their oxygen content during steady-state combustion reactions. Strong Pd–O bonds in small crystallites and in oxygen-deficient PdOxalso lead to the observed decrease in methane oxidation turnover rates as crystallite size decreases or as samples are treated at temperatures above those required for PdO-to-Pd decomposition.
The contribution of a redox mechanism involving lattice oxygen in the catalytic combustion of methane over PdO/ZrOâ catalysts, prepared from amorphous Pd-Zr alloys, has been studied by means of gas pulse methods, including a novel technique `pulse thermal analysis`, and using labeled catalysts containing Pd¹â¸O. Special emphasis was devoted to the influence of the isotope exchange (scrambling) of reactants and products, especially Oâ and COâ, with the catalyst on the quantity of ¹â¸O-containing reaction products. Substantial amounts of Hâ¹â¸O and C¹â¸O¹â¶O were detected during pulses of a reactant mixture consisting of methane and ¹â¶Oâ in a ratio 1:4 at 300 and 500°C. The effect of the oxygen exchange of molecular oxygen with the solid phase proved to be negligible due to its low extent. At 300°C, at least 20% of the COâ formed originated from the redox mechanism involving lattice oxygen. At 500°C, oxygen exchange of COâ with the catalyst became predominant and precluded determining reliably the proportion of COâ formed by the redox process. The results indicate that a substantial part of methane is oxidized via a redox process. Consequently, this reaction has to be taken into account when interpreting the catalytic behavior of palladium-based catalysts and explaining the structure-activity relations previously observed. 66 refs., 9 figs., 2 tabs.
Possible relationships between the structure and activity of supported palladium catalysts for the oxidation of methane have been explored. Samples have been exposed to a wide range of oxidation and reduction pretreatments and the activity for methane oxidation has been determined as a function of time-on-stream, both for pre-oxidised and for pre-reduced samples. Parallel experiments on the rate and extent of oxidation at 300°C have been performed. The results show that the rate of oxidation of pre-reduced palladium samples is very sensitive to the previous thermal history and samples retain a ‘memory’ of their treatment. In all cases a monolayer of adsorbed oxygen is taken up very rapidly but further oxidation is slower, although the samples approach the bulk oxide state within a few minutes. There are no obvious correlations between the catalytic activity for methane oxidation and the previous treatment of the samples, suggesting a sensitive dependence of activity on the morphology of the samples. It is clear, however, that metallic palladium is not active for methane oxidation. Similarly, chemisorbed oxygen is not as active as oxidised palladium. Moreover, it is found that there is no loss of activity when the palladium is fully oxidised, suggesting that previously reported changes in activity may reflect long-term morphological effects rather than simple over-oxidation of palladium particles.
Palladium is one of the most active catalysts for the catalytic combustion of methane. Since Pd is oxidized during methane combustion to PdO and PdO is required for high activity, it is of interest to understand the dynamics of Pd oxidation and the structure of the oxide formed as well as the dynamics of PdO reduction by H2and CH4. In the present study isothermal and temperature-programmed oxidation and reduction were used to probe the dynamics of the oxidation and reduction of zirconia-supported Pd. During uptake in oxygen, a monolayer of oxide is generated immediately, and upon further oxidation, the oxide forms a shell around a core of metal with thicknesses that increase with the oxidation temperature. The initial oxide is amorphous and subsequently transforms to crystalline PdO. The dynamics of Pd oxidation suggest that oxidation follows the Cabrera–Mott theory. Reduction of PdO by H2occurs in a shellwise manner, consistent with a shrinking core mechanism, while reduction in CH4occurs via an autocatalytic, nucleation mechanism. In the latter case, small particles of Pd must first be formed on which CH4can dissociate. The fragments (H and CHx(x=3–1)) diffuse to the metal–oxide boundary where reduction of the oxide occurs. Consistent with this picture, the kinetics of PdO reduction are first order in Pd initially, but then they become zero order in Pd.
Pt(O) films having compositions ranging from pure Pt to amorphous platinum oxide a-PtOx (x ∼ 1.4) were prepared by reactive sputtering and examined during and after heating to temperatures used for deposition and processing of high-epsilon (HE) and ferroelectric (FE) materials (400–650 °C). A two stage decomposition process was observed for a-PtOx (x ∼ 1.4) films heated in N2, with the first stage of decomposition beginning at temperatures well below 400 °C. In an O2 ambient, decomposition was accompanied by formation of a crystalline Pt3O4 phase prior to complete decomposition to metallic Pt. However, the relatively slow rate of oxygen loss from a-PtOx suggests that significant amounts of oxygen should remain in Pt(O) electrodes after HE/FE layer deposition. © 1999 American Institute of Physics.
The nonfaradaic electrochemical promotion modification of catalytic activity (NEMCA) was investigated in the total oxidation of propane and propene on Pt films deposited on 8 mol% Y2O3-stabilized ZrO2 solid electrolyte. It was found that propane and propene oxidation exhibit opposite behaviors with respect to the applied overpotentials. Propane oxidation exhibits an electrophobic enhancement (reaction increases upon positive polarization of the catalyst-electrode) while propene oxidation is electrophilic. Upon applied negative overpotential and under an oxygen-rich atmosphere, the rate of propane oxidation strongly decreases, with high values of faradaic efficiency, up to 10,000. Propane and propene oxidation rates were found to be controlled over a wide range of voltage in a reversible way. Catalytic activity measurements under open-circuit voltage conditions, using solid electrolyte potentiometry and temperature-programmed desorption, have been used in order to understand and investigate the origins of this electrochemical promotion. They seem related to different behaviors of propane and propene with respect to their competitive adsorption with oxygen on platinum. Propene adsorbs strongly in competition with oxygen, whereas propane adsorbs weakly. This indicates that the NEMCA behavior of a catalytic reaction is dependent on the oxygen coverage on the catalyst-electrode under open-circuit conditions.
Catalytic oxidation of hydrocarbons in lean-burn natural-gas engine exhaust has been studied for Pt and Pd supported on alumina. A Pt–Pd/alumina catalyst exhibited higher and longer-lasting hydrocarbon oxidation activity than Pt–Rh/alumina, Pt/alumina, and Pd/alumina catalysts. Increasing the palladium content in Pt–Pd/alumina catalyst increased the oxidation activity and had more durability. While increasing the platinum content a little bit also improved the activity, adding much more did not. Supporting the platinum on alumina retarded the sintering of Pd and PdO, thus lengthening the oxidation activity of the Pt–Pd/alumina catalyst.
The combustion of light hydrocarbons finds an important application in volatile organic compounds (VOCs) abatement. Catalytic combustion is interesting in this domain, because it may be carried out at relatively low temperatures and under large air excess. For this purpose, precious metal-based catalysts are very performing, but the quest for a lower-cost alternative solution is necessary. In this field, conducting mixed oxides such as perovskites are good candidates, especially when they are electrochemically promoted. The present work shows that electrochemical promotion of catalytic activity (NEMCA effect) in propene total combustion can be carried out with La0.8Sr0.2Co0.8Fe0.2O3, an electron-conducting perovskite-type oxide, deposited on YSZ.
Electrochemical catalysts based on Pd deposited by Physical Vapour Deposition on YSZ were used for methane deep oxidation.
Different thicknesses of Pd films varying from 11 to 75nm were catalytically characterized between 150 and 750°C. The Pd
loadings were extremely low. Catalytic and EPOC experiments were carried out on those electrochemical catalysts. Their catalytic
activities were compared with the performances of a reference catalyst. It was found that the catalytic activity can be in
situ tuned by applying an anodic polarization thus supplying oxygen ions at the surface of the catalyst. Faradaic efficiency
values up to 258 were observed and the induced modifications of the catalytic rate were typically 100 times higher than the
corresponding ionic current. The influence of the polarization on the temperature of decomposition of the palladium oxide
was also examined. The polarization was found to enhance the thermal stability of the oxide and turn palladium oxide into
metallic palladium at higher temperatures.
The origin of the electrochemical promotion of catalysis (EPOC) was investigated via oxygen temperature-programmed desorption
(O2-TPD) from a polycrystalline Pt film interfaced with YSZ. TPD experiments were carried out under operating conditions similar
to those used for catalytic activity measurements. This study has clearly shown that an anodic current generates the migration
of “backspillover” ionic oxygen species from YSZ toward the Pt surface. These ionic species act as promoters and enable the
formation of weakly adsorbed oxygen species coming from the gas phase which are more reactive and thus responsible for the
activity enhancement. The effect of polarization is to carry or to remove the promoting ionic species on the Pt surface. Therefore,
electrochemical promotion of catalysis can be considered as an electrically controlled metal support interaction, where the
support is an O2− conducting solid electrolyte.
The performance of Al2O3, ZrO2 and ZrO2 stabilized with SiO2 (ZrO2-s) supported palladium catalysts for the methane combustion was studied between 473 and 873 K. The nature of the surface species of palladium catalysts under reaction conditions were detected by FT-IR and microcalorimetry of CO adsorbed. The different behavior of palladium catalysts under reaction conditions is attributed to support effects associated to differences in thermal conductivity and oxygen mobility of supports. Prereduction of the catalysts enhances their activity. Under reaction conditions, the prereduced sample becomes partially oxidized by preferential adsorption/reaction of oxygen both on Pd ( 1 1 1) planes and on the sites that can multibondedly adsorb CO. The reconstruction of the metallic particles and the formation of PdOx (0<x 1) phase were directly observed by FT-IR and microcalorimetry of adsorbed CO. Combination of different characterization techniques with reaction results suggests that a mixed phase, Pd-0/PdOx, is the most active phase for methane combustion, and that a redox mechanism may occur on this phase. (C) 2000 Elsevier Science B.V. All rights reserved
The operating requirements for practical catalytic combustion systems are presented. Catalytic materials for methane combustion are then reviewed in light of these operating requirements. Measured catalytic rates for methane oxidization for a number of active metal and oxide catalyst systems are reported and compared. The precious metals, particularly Pd, are most active. The oxides can exhibit high surface areas but in all cases have a much lower areal activity resulting in a substantially lower weight specific activity. Data on the thermal stability and volatility of both support and active components are presented and discussed in terms of the required operating temperatures. It is concluded that at the required operating temperature for modern gas turbines, most catalyst systems would not have sufficient stability and life. An alternative approach is to limit the catalyst temperature and to react a portion of the fuel after the catalyst. This process has substantial advantages. This latter system will be described and the important catalyst performance characteristics discussed. Test results demonstrate NOx levels below 2 ppm even at combustor outlet temperatures as high as 1500°C.
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.
The high-temperature catalytic chemistry of supported palladium for methane oxidation has been studied. Palladium oxide supported on alumina decomposes in two distinct steps in air at one atmosphere. The first step occurs between 750 and 800 ° C and is believed to be a decomposition of palladium-oxygen species dispersed on bulk palladium metal designated (PdOx/Pd). The second decomposition is between 800 and 850 ° C and behaves like crystalline palladium oxide designated (PdO). To reform the oxide, the temperature must be decreased well below 650 ° C. Thus, there is a significant hysteresis between decomposition to palladium and re-formation of the oxide. Above 500 ° C, methane oxidation occurs readily when the catalyst contains PdO. However, when only palladium metal is present no oxygen adsorption occurs and no methane activity exists. One may conclude that the high temperature (> 500 ° C) activity of a supported palladium containing catalyst is due to the ability of palladium oxide to chemisorb oxygen. Palladium, as a metal, does not chemisorb oxygen above 650 ° C and thus, is completely inactive toward methane oxidation.
Black CuO films are used for improved radiation cooling of, e.g. an r.f. kicker at the Advanced Light Source of Lawrence Berkeley Laboratory. The films are required to be transparent to r.f. fields particularly in the range 1–1.25 GHz, UHV compatible, and very adhesive even after a number of baking cycles. Usual techniques such as wet-chemical coating do not fulfil the requirements for UHV compatibility. A cathodic arc plasma source with a copper cathode was operated in an oxygen atmosphere to form CuO films on the substrate, which was either the anode or was mounted on a substrate holder using a separate anode. The films are single-phase polycrystalline CuO. They are slightly oxygen-rich, adhere very well (pull strength exceeding 85 MPa) and fulfil all UHV requirements. The deposition method is very efficient, and large workpieces up to a surface area of 5000 cm2 have been coated.
This review examines recent developments in the complete oxidation of methane at low temperature over noble metal based catalysts in patents and open literature. The abatement of natural gas vehicle (NGV) methane emissions is taken as one example among possible applications. The review develops current ideas about the properties of palladium and platinum catalysts supported on silica and alumina supports in the complete oxidation of methane under oxidising conditions, focusing on low-temperature reaction conditions. The influence of residual chloride ions on the catalytic activity, the kinetic aspects of the oxidation of methane over these catalysts, the nature of the active sites, the influence of metal particle size and reaction products on the activity, the observed changes in catalytic activity with reaction time and the effect of sulphur containing compounds are examined. The latest studies concerned with improved palladium and platinum supported catalysts which would exhibit enhanced and stable catalytic activity at low temperature in the presence of water and sulphur containing compounds are reported. Possible routes for preparing catalysts able to meet future regulations concerning methane emissions from lean-burn NGV vehicles are discussed.
The complete oxidation of methane by supported Pd was studied in a reaction mixture of 2% CH4 in air at 550 K, and atmospheric pressure. The catalysts were Pd supported on Al2O3 and ZrO2 deposited from PdCl2 or Pd(NH3)2(NO2)2 precursors. For the determination of how the turnover rate varies with the structure of the catalyst, the average Pd particle size on the series of catalysts was varied in the critical size range (1-10 nm). The number of active sites were measured before and after reaction by the method of H2 titration of adsorbed oxygen at 373 K. The number of active sites remained the same, although the rate per gram of catalyst increased with time on stream for all catalysts. At steady-state, the turnover rate for the series of catalysts tested varied in the range 2 × 10−2 to 8 × 10−2 s−1 under the conditions of these experiments. It is concluded that the reaction is structure insensitive. In an investigation of the kinetics of methane oxidation on palladium, the reaction products were found to strongly inhibit the oxidation of methane.
Palladium is the active component in several catalytic formulations for environmental technologies, due to its superior performances in the conversion of some hydrocarbons (for example, methane) and halocarbons, and the thermal stability and low volatility of Pd species. The properties and reactivity of Pd-based catalysts in the conversion of methane catalytic combustion for gas turbine applications, reduction of greenhouse gas (methane, N2O) emissions, hydrodehalogenation and oxidative destruction of halocarbons and their applications in the elimination of other pollutants from gaseous emissions are reviewed, with emphasis on the structure-activity relationships, reaction mechanism and sensitivity to poisoning.
In this study, we have grown silver-containing hydrogenated (a-C:H) and non-hydrogenated (a-C) amorphous carbon coatings by two plasma immersion ion implantation methods: I) chemical vapor deposition of methane combined with pulsed filtered cathodic arc deposition of silver, and II) by alternating arc pulses from graphite and silver in a dual cathodic arc plasma source. This unique “bias selective” feature of the deposition system allowed the deposition of silver with the substrates at ground and avoided the sputtering of the grown a-C film. Chemical composition of the samples was analyzed by acquiring their compositional depth-profiles using radio-frequency Glow Discharge Optical Emission Spectroscopy (rf-GDOES), while the microstructural properties were analyzed by X-ray absorption near edge spectroscopy (XANES) and Raman spectroscopy. In this contribution, we compare mechanical and biomedical properties by means of nanoindentation and cell viability tests, respectively, of a-C(H) films obtained by two different plasma immersion ion implantation techniques.
The results presented in this work clearly show that zirconia synthesized by precipitation of zirconyl chloride with NaOH leads to a zirconia with high surface area (224 m(2)/g), which is relatively stable after high temperature exposure. Pd supported on this high surface area zirconia exhibit higher turnover frequency for methane combustion than when supported on zirconia obtained from zirconyl chloride precipitated with ammonium hydroxide. We also found that when the commercial zirconium hydroxide is subjected to a NaOH reflux treatment, we obtained a zirconia of high and stable surface area (157 m(2)/g) even after heating it at 700 degrees C. Pd supported on the T a treated commercially derived support shows the highest dispersion and the highest intrinsic activity than on any of the other preparations studied. The results presented in this paper open up a new venue for obtaining high surface area zirconia using the commercially available hydroxide. This work shows that it is possible to stabilize zirconia using a Na treatment and also obtain high catalytic activity for methane oxidation on Pd stabilized on such support. Kinetic results and the corresponding mechanistic interpretation and the IR results support that the effect is consistent with increasing water support interactions which in term affect the reaction kinetics.
The catalytic oxidation activity of platinum particles in automobile catalysts is thought to originate from the presence of highly reactive superficial oxide phases which form under oxygen-rich reaction conditions. Here we study the thermodynamic stability of platinum oxide surfaces and thin films and their reactivities toward oxidation of carbon compounds by means of first-principles atomistic thermodynamics calculations and molecular dynamics simulations based on density functional theory. On the Pt(111) surface the most stable superficial oxide phase is found to be a thin layer of alpha-PtO2, which appears not to be reactive toward either methane dissociation or carbon monoxide oxidation. A PtO-like structure is most stable on the Pt(100) surface at oxygen coverages of one monolayer, while the formation of a coherent and stress-free Pt3O4 film is favored at higher coverages. Bulk Pt3O4 is found to be thermodynamically stable in a region around 900 K at atmospheric pressure. The computed net driving force for the dissociation of methane on the Pt3O4(100) surface is much larger than that on all other metallic and oxide surfaces investigated. Moreover, the enthalpy barrier for the adsorption of CO molecules on oxygen atoms of this surface is as low as 0.34 eV, and desorption of CO2 is observed to occur without any appreciable energy barrier in molecular dynamics simulations. These results, combined, indicate a high catalytic oxidation activity of Pt3O4 phases that can be relevant in the contexts of Pt-based automobile catalysts and gas sensors.
Several recent reports implicate an important role played by c-Jun N-terminal kinases (JNKs) in long-term potentiation (LTP). However, little is known about how the isoforms of JNKs participate in synaptic plasticity. Here we showed that short-term synaptic plasticity was impaired in the hippocampal area CA1 of JNK1-deficient (JNK1-/-) mice; these mice showed normal LTP in response to a strong tetanus and no alteration of N-methyl-D-aspartate receptor-dependent long-term depression (LTD) in the hippocampus. However, LTD induced either by group I metabotropic glutamate receptors (mGluRs) agonist dihydroxyphenylglycine or by paired-pulse low-frequency stimulation was absent in both the JNK1-/- slices and in JNK inhibitor anthrax [1, 9-cd] pyrazol-6(2H)-1 (SP600125)-pretreated slices. Induction of mGluR-dependent LTD resulted in an increase in phosphorylation of JNK1 substrates, including p-c-Jun and p-ATF2 in wild-type (WT) mice, and these increases failed to occur in the JNK1-/- or SP600125-pretreated mice. These results demonstrated that JNK1 played a crucial role in the short-term synaptic plasticity and mGluR-dependent LTD, whereas hippocampus LTP was not affected by JNK1 deficiency.