Mixed Oxide Support Research Articles

ZrxMg1-xO supported cobalt catalysts for methane decomposition into COx-free hydrogen and carbon nanotubes

ABSTRACTZirconia-magnesia supported cobalt catalysts with various Zr/Mg atomic ratios were prepared and evaluated for non-oxidative catalytic decomposition of methane to produce COx-free hydrogen and carbon nanotube. The catalytic performance of the catalysts was performed in a continuous fixed bed flow reactor at 700°C under atmospheric pressure. The fresh and spent catalysts were characterized by XRD, TPR, BET, TEM, and Raman spectroscopy. The results showed that the change in Zr/Mg ratio of the mixed oxide support has a significant effect on the catalytic performance of the active Co metal. The catalyst 30%Co/Zr0.8Mg0.2 showed the highest activity and stability within the used series of catalysts with hydrogen yield reached up to 79%. Both Co/Mg1.0 and Co/Zr1.0 showed poor stability due to strong Co-Mg interaction and aggregation of Co species on Zr support, respectively. All catalysts produced mainly MWCNTs with different diameters depending on the Zr/Mg ratio. The outer diameter increased with increasing Zr content in the catalyst due to the enlargement of the particle size of cobalt as a result of aggregation.

Enhancement of Electrocatalytic Reduction of Carbon Dioxide at Electrodes Modified with Highly-Acidic Mixed-Metal-Oxide Films

Electroreduction of carbon dioxide to simple organic fuels and chemicals is a topic of growing scientific and technological interest. The reaction provides means for both reducing emissions of CO2 into atmosphere and storing renewable energy. The presentation will address low-temperature CO2-conversion processes based on electrocatalytic and photoelectrochemical approaches. Among important issues are choice of the catalytic or semiconducting materials, their morphology and operating conditions including temperature, solvent, electrolyte, pH etc. There is a need to improve the reaction dynamics and selectivity toward specific products. In practical electrolysis cells, the CO2-reduction (at cathode) is accompanied by water oxidation (at anode or photoanode). Given the fact that the CO2 molecule is very stable, its electroreduction processes are characterized by large overpotentials. It is often postulated that, during electroreduction, the rate limiting step is the protonation of the adsorbed CO product to form the CHO adsorbate. In this respect, the proton availability and its mobility at the photo(electro)chemical interface has to be addressed. On the other hand, competition between such parallel processes as hydrogen evolution and carbon dioxide reduction has also to be considered. Recently, mixed oxide systems stabilized through Zr-O-W bonds have been demonstrated to be very attractive acid catalysts exhibiting high catalytic activities and good stabilities in many demanding industrial reactions. The mixed WO3/ZrO2 systems are characterized by fast charge (electron, proton) propagation during the system’s redox transitions. By dispersing metallic Cu electrocatalytic nanoparticles over, under or within, such active WO3/ZrO2 matrices, the electrocatalytic activities of the respective systems toward the reduction of carbon dioxide have been significantly enhanced even at decreased loadings in acid media. The enhancement effects should be attributed to features of the mixed metal oxide support such as porosity and high population of hydroxyl groups (due to presence of ZrO2), high Broensted acidity of sites formed on mixed WO3/ZrO2, fast electron transfers coupled to unimpeded proton displacements (e.g. in HxWO3), as well as strong metal-support interactions between nanosized metals (Cu) and the metal (W,Zr) oxo species. Application of the hybrid system composed of both above-mentioned oxides resulted in high current densities originating from electroreduction of carbon dioxide mostly to methanol (CH3OH) as demonstrated upon identification of final products.

Co-Based Catalysts Derived from Layered-Double-Hydroxide Nanosheets for the Photothermal Production of Light Olefins.

Solar-driven Fischer-Tropsch synthesis represents an alternative and potentially low-cost route for the direct production of light olefins from syngas (CO and H2 ). Herein, a series of novel Co-based photothermal catalysts with different chemical compositions are successfully fabricated by H2 reduction of ZnCoAl-layered double-hydroxide nanosheets at 300-700 °C. Under UV-vis irradiation, the photothermal catalyst prepared at 450 °C demonstrates remarkable CO hydrogenation performance, affording an olefin (C2-4= ) selectivity of 36.0% and an olefin/paraffin ratio of 6.1 at a CO conversion of 15.4%. Characterization studies using X-ray absorption fine structure and high-resolution transmission electron microscopy reveal that the active catalyst comprises Co and Co3 O4 nanoparticles on a ZnO-Al2 O3 mixed metal oxide support. Density functional theory calculations further demonstrate that the oxide-decorated metallic Co nanoparticle heterostructure weakens the further hydrogenation ability of the corresponding Co, leading to the high selectivity to light olefins. This study demonstrates a novel solar-driven catalyst platform for the production of light olefins via CO hydrogenation.

In situ DRIFT studies of alkane adsorption on vanadia supported titania-doped catalysts

In-situ DRIFT studies of alkane adsorption (ethane and propane) are performed over well characterized 90Ti-M (M = Si, Al, and W) and 2V90Ti-M catalysts at different temperatures and partial pressures of alkanes. The titania-doped mixed oxide supports are synthesized by the sol-gel method and supported vanadia catalysts are prepared by the incipient wet impregnation method. The supported vanadia catalysts possess only surface vanadia species coordinated with titania and/or silica, alumina and tungsten in the corresponding catalysts. Analysis of the in situ DRIFT spectra of alkane adsorption confirms the formation of various surface oxygenated species, such as ethoxide/isopropoxide, acetaldehyde/ketone, acetate, formate and cyclic anhydride. The alkoxides (ethoxide and isopropoxide) are formed due to the breaking of the CH bond of the parent alkane and are the precursor for the formation of olefins (ethylene and propylene). The unstable alkoxide species are further oxidized to form aldehyde or ketone, respectively. The carboxylates species (acetate and formate) are formed due to further oxidation of acetaldehyde and acetone at high temperatures. Deposition of vanadia suppresses the formation of cyclic anhydride, a precursor for the formation of CO2. In-situ DRIFT studies of propane ODH reaction reveal that isopropoxide is not a readily detectable surface species in the presence of oxygen even at low temperature. However, the carbonate species and propylene are identified during propane ODH reaction. Furthermore, the intensity of overtone VO vibration decreases and shifts from 2030 cm−1 to 2025 cm−1 during alkane adsorption.

Highly-Acidic Mixed-Metal-Oxides (WO3-ZrO2) As Active Supports for Dispersed Metal Centers: Enhancement of Electrocatalytic Reduction of Oxygen and Carbon Dioxide

Application of metal oxides as active matrices in electrocatalysis is particular importance. The hydrous behavior, which favors proton mobility and affects overall reactivity, reflects not only the oxide’s chemical properties but its texture and morphology as well. What is of importance to electrochemical science and technology, certain nonstoichiometric mixed-valence oxides could exhibit pseudo-metallic conductivity and possess appreciable catalytic activity. Recently, mixed oxide systems stabilized through Zr-O-W bonds have been demonstrated to be very attractive acid catalysts exhibiting high catalytic activities and good stabilities in many demanding industrial reactions. For electrocatalytic applications, mixed-valent tungsten(VI,V) oxide and zirconium(IV) oxide have been sequentially deposited and integrated through voltammetric potential cycling to form sub-microstructured films on glassy carbon electrode. The mixed WO3/ZrO2 systems are characterized by fast charge (electron, proton) propagation during the system’s redox transitions. By dispersing metallic Pt or Cu electrocatalytic nanoparticles over such active WO3/ZrO2 supports, the electrocatalytic activities of the respective systems toward the reduction of oxygen or carbon dioxide have been enhanced even at decreased loadings in acid media. The enhancement effects should be attributed to features of the mixed metal oxide support such as porosity and high population of hydroxyl groups (due to presence of ZrO2), high Broensted acidity of sites formed on mixed WO3/ZrO2, fast electron transfers coupled to unimpeded proton displacements (e.g. in HxWO3), as well as strong metal-support interactions between nanosized metals (Pt or Cu) and the metal (W, Zr) oxo species. The fact that WO3/ZrO2 nanostructures are in immediate contact with the metallic catalytic sites leads to the specific interactions (via the surface hydroxyl groups) with the reaction intermediates (H2O2 or CO adsorbates). Further mechanistic studies involving rotating ring disk voltammetry and gas-diffusion electrode measurements are planned in addition to spectroscopic and XPS measurements. The mixed-metal systems will also be considered together with nanostructured carbon carriers such as reduced graphene oxide, carbon nanotubes or carbon black.

Partial oxidation of methane to syngas using Co/Mg and Co/Mg-Al oxide supported catalysts

Partial oxidation of methane (POM) was studied over a series of Co/Mg-Al catalysts. Various commercial hydroxides with different Mg/Al ratios were used as precursors of the oxides employed as catalytic supports, as well as MgO prepared in the laboratory. The effects of Co loading, Mg content, and calcination temperature (400–750 °C) were studied on the POM reaction. The catalytic performance was evaluated at 800 °C in a fixed-bed tubular quartz reactor under a high space velocity of 300 L N CH4/(gcat h) and a O2/CH4 molar ratio of 0.5. A very high activity was obtained during 4 h on-stream with a 20 wt. % Co catalyst prepared with a Mg-Al mixed oxide support having a MgO content of 63 wt. %. This catalyst gave CH4 conversions (91.3%) very close to the maximum corresponding to the thermodynamic equilibrium. Such notable performance could be attained with the catalyst precursor subjected to calcination at 500 °C for 6 h and subsequent in situ reduction under H2 flow at 800 °C for 2 h. Two main types of deactivation mechanisms were generally observed in most of the samples that suffered either a very rapid cobalt re-oxidation upon their exposure to the POM reaction atmosphere or significant deposition of carbonaceous deposits of different kinds. The characterization of the spent samples by transmission electron microscopy (TEM) revealed the presence of carbon deposits of different nature in the Mg/Al samples having a high MgO content, including filamentous carbon (whiskers), onion-shell like sheets and/or amorphous encapsulating layers. Conversely, no carbon deposits were observed in the spent Co/MgO catalyst, which underwent oxidation of the relatively less dispersed surface Co sites to form clearly defined crystals of Co oxides with particle diameters typically ranging between 25 and 100 nm.

Effects of Zr doping on Fe2O3/CeO2 oxygen carrier in chemical looping hydrogen generation

The CeO2-supported Fe2O3 is a satisfactory alternative oxygen carrier for chemical looping hydrogen generation (CLHG), which demonstrates good reactivity and stability with no carbon deposition observed due to the oxygen mobility property of CeO2. However, sintering always takes place because of the low thermal stability of CeO2. In the present work, the Fe2O3/CeO2 oxygen carriers doped by Zr cation, resulting in the CexZr1−xO2 mixed oxide supports, were prepared by co-precipitation method. To optimize the Fe2O3/CexZr1−xO2 oxygen carriers, the Ce/Zr ratio was systematically changed. The reactivity, redox stability, carbon deposition, and sintering characteristics of the oxygen carriers were analyzed to investigate the effects of Zr doping as well as the fundamental mechanism, and the results showed that Fe2O3/Ce0.75Zr0.25O2 was the best oxygen carrier considering the comprehensive characteristics with no CO or CO2 (detection limit 0.01% in volume) observed in the obtained hydrogen. The generated solid solution Ce0.75Zr0.25O2 could improve the oxygen mobility and thermal stability of the oxygen carrier. In addition, the Zr cation in Ce0.75Zr0.25O2 can inhibit the migration of the Fe element from the bulk to surface of the particles, enhancing its resistance to sintering, which can further promote the reactivity and redox stability. The Ce0.75Zr0.25O2 is a potential support to improve the redox characteristics of iron oxygen carrier in CLHG, although the phase segregation was observed after redox cycles, which can be inimical to the reactivity of the Fe2O3/Ce0.75Zr0.25O2 oxygen carrier. Furthermore, the oxygen carriers were characterized by X-ray diffraction patterns, hydrogen temperature-programmed reduction measurement, scanning electron microscopy images, energy dispersive X-ray spectrometer analysis, and Raman spectra before and after the redox cycles.

Elucidating the Role of CO2 in the Soft Oxidative Dehydrogenation of Propane over Ceria-Based Catalysts

A mixed oxide support containing Ce, Zr, and Al was synthesized using a physical grinding method and applied in the oxidative dehydrogenation of propane using CO2 as the oxidant. The activity of the support was compared with that of fully formulated catalysts containing palladium. The Pd/CeZrAlOx material exhibited long-term stability and selectivity to propene (during continuous operation for 140 h), which is not normally associated with dehydrogenation catalysts. From temperature-programmed desorption of NH3 and CO2 it was found that the catalyst possessed both acidic and basic sites. In addition, temperature-programmed reduction showed that palladium promoted both the reduction and reoxidation of the support. When the role of CO2 was investigated in the absence of gas-phase oxidant, using a temporal analysis of products (TAP) reactor, it was found that CO2 dissociates over the reduced catalyst, leading to formation of CO and selective oxygen species. It is proposed that CO2 has the dual role of regener...

Characterization and reactivity of vanadium oxide supported on TiO2-SiO2 mixed oxide support

A TiO2-SiO2 support, 90Ti-Si, containing 90 wt.% TiO2 was synthesized by the sol-gel method. Several supported vanadium oxide (vanadia) catalysts, xV90Ti-Si (x = 2–12.5 wt.%) were synthesized by incipient wetness impregnation method using this support. The catalysts and support were characterized and probed with the propane oxidative dehydrogenation (ODH) reaction. The support possesses no rutile phase and retains 30–40% of its initial surface area even after calcination at 1073 K. The surface area loss is accelerated in the presence of vanadia and the rutile phase appears below 1073 K. Surface vanadia species are present for the supported vanadia catalysts calcined at 673 K and the propane ODH activity and propene yield increases with vanadia loading. The ODH activity of the 2V90Ti-Si sample at 673 K is retained even after the catalyst is pre-calcined at 1023 K. Interestingly, the activity of the 4V90Ti-Si sample at 673 K reaches a maximum at an intermediate pre-calcination temperature. The higher vanadia loading samples deactivate with an increase in calcination temperature. The ODH reactivity reveals that the propene yield at iso-conversion increases with loading. Analysis of kinetic parameters also reveals that the rate constant ratio increases with vanadia loading.

CO2 Reforming of Methane over Ni/MgO Catalysts Promoted with Zr and La Oxides

AbstractAn investigation on carbon dioxide reforming of methane (DRM) to synthesis gas (H2 and CO) was carried out with several synthesized Ni‐based catalysts. Mixed oxide supports (MgO‐ZrO2 and MgO‐La2O3), were prepared using co‐precipitation method with K2CO3 as the precipitant. The findings indicated a high level of catalytic activity and stability of synthesized Ni‐based catalysts for DRM. Different techniques, namely, XRD, XPS, H2‐TPR, BET, TEM, FESEM, and TGA, were employed to perform the characterization of synthesized catalysts. It was found that with the addition of promoter Zr, the catalyst Ni/Mg0.85Zr0.15O showed a high level of activity (CH4; 86%, and CO2; 98%) and better stability. The significant anti‐coking activity of the reduced catalyst with low Ni metal was evaluated. Furthermore, this research study also examined the performance of the catalyst activity and stability. The results obtained from the temperature program reduction (H2‐TPR) revealed that the presence of ZrO2 with MgO support can enhance Ni reducibility in comparison to La2O3, which is an important indicator of high activity and stability of the Ni catalysts for DRM. It was also found that the presence of La2O3 did not show any effects on the stability of the catalyst.

Facile synthesis of Mo/Al2O3–TiO2 catalysts using spray pyrolysis and their catalytic activity for hydrodeoxygenation

In this study, spherical mixed oxides of Al2O3 and TiO2 supported molybdenum (Mo) catalysts, Mo/Al2O3−TiO2, were successfully prepared using a novel approach combining sol-gel and spray pyrolysis (SP) methods. First, both boehmite sol and titania sol were prepared by a sol-gel process, and then molybdate salt was dispersed in the sol mixture with the assistance of citric acid, followed by spray pyrolysis of the mixed precursor solution. Structural analyses of prepared catalysts showed that the dispersion of active sites was affected by the TiO2 concentration in the Al2O3−TiO2 mixed oxide support. The catalytic activities of reduced catalysts were investigated for hydrodeoxygenation (HDO) of palmitic acid, which is the main component of microalgae–derived biocrude. The results show that the Mo/Al2O3–TiO2 catalysts exhibited excellent catalytic performance for the HDO of palmitic acid.

Production of renewable hydrogen through aqueous-phase reforming of glycerol over Ni/Al2O3[sbnd]MgO nano-catalyst

In this study, a series of Ni nano-catalysts supported on Al2O3 and MgO were prepared through the co-precipitation technique. Effects of the Al/Mg ratio on physicochemical characteristics of Ni/Al2O3MgO catalysts were examined. Moreover, catalytic performance was investigated in order to determine the optimum catalyst for H2 production in aqueous phase reforming (APR) of glycerol. It was revealed that, the APR activity of synthesized catalysts strongly depended on the aforementioned ratio. In addition, it was observed that, the catalytic activity of Ni/MgO and Ni/Al2O3 samples were both lower than that of the corresponding mixed oxide supports. Furthermore, it was shown that, amongst the compositionally different prepared mixed oxide materials, the respective catalytic activities increased through enhancing of the Al/Mg ratio. It was demonstrated that the Ni/Al2Mg1 catalyst possessed highest catalytic activity of 92% glycerol conversion and selectivity towards hydrogen production of 76%. Ultimately, it was concluded that, the APR activity lowered in the following order: Ni/Al2Mg1 > Ni/Al1Mg1 > Ni/Al1Mg2 > Ni/Al > Ni/Mg for the understudied synthesized materials.

CO2 methanation over Ni catalysts based on ternary and quaternary mixed oxide: A comparison and analysis of the structure-activity relationships

Methanation of CO2 was studied in a high throughput reactor (Amtech SPIDER 16) over Ni catalysts supported on ternary and quaternary alumina-zirconia-titania-ceria mixed oxides, in order to compare them under industrial relevant conditions and derive indications about the structure-activity relationships and specifically the role of ceria. The samples were characterized by BET, XRD, H2-TPR, CO-chemisorption, XPS and CO-TPD analyses. Catalytic activity was evaluated towards CO2 methanation at 5bar pressure, temperature range of 300–400°C and different Gas Hourly Space Velocities (GHSVs). The results showed that enhanced catalytic activity depends on both textural improvements (for the ternary oxide supported Ni) and reducibility and metal dispersion (for the quaternary oxide supported Ni). The comparison between both groups of catalysts revealed that addition of CeO2 to 20%Ni/Al2O3-ZrO2-TiO2 further improves the catalytic performance. The catalyst supported on a mixed oxide support with 15wt% of ceria, titania and zirconia (the remaining 55% is alumina) exhibits the highest CO2 conversion (82%) and methane selectivity (98%) at 350°C and GHSV of 4000h−1. The role of CeO2 as promoter is to increase Ni dispersion and stabilize the presence of β-type NiO species which are reduced at lower temperatures. Other promoters (TiO2, ZrO2) have mainly a textural effect. Moreover, CeO2 promotes also the stability decreasing the deactivation rate of about one order of magnitude with respect to the best ternary system (Ni/C5) investigated.

Preparation and characterization of Ce1−xPrxO2 supports and their catalytic activities

In this work, the addition of praseodymium (Pr) into ceria as a mixed oxide support in a form of Ce1−xPrxO2 (x = 0.01, 0.025, 0.050, 0.075 and 0.10) was prepared using a co-precipitation method. The structural and textural properties of the synthesized supports were characterized by X-ray diffraction (XRD), N2 adsorption–desorption, Raman spectroscopy, H2-temperature programmed reduction (H2-TPR) and H2-chemisorption. Upon addition of Pr, XRD patterns and Raman spectra indicated an enlargement of ceria unit cell and the characteristics Raman broad peak at 570 cm−1 which was attributed to the existence of oxygen vacancies in the ceria lattice. This indicated that some Ce4+ ions in ceria were replaced by larger Pr3+ cations. To evidence the incorporation of Pr3+ cations into ceria lattice, X-ray absorption near edge structure (XANES) was employed. The results showed that the oxidation states of Ce in mixed oxide supports were slightly lower than 4+ while those of Pr were still the same as a precursor salt. Therefore, the incorporation of Pr3+ into ceria lattice would lead to strain and unbalanced charge and result in oxygen vacancies. The reducibility of Ce1−xPrxO2 mixed oxide supports was investigated by H2-TPR and temperature-resolved X-ray absorption spectroscopy experiment under reduction conditions. XANES spectra of Ce L3 edges showed a lower surface reduction temperature (Ce4+ to Ce3+) of Ce0.925Pr0.075O2 than that of CeO2 which agreed with H2-TPR results. H2-chemisorption indicated that Pr promoted the dispersion of the metal catalyst on the mixed oxide support and increased the adsorption site for CO. For WGS reaction, 1% Pd/mixed oxide support had higher WGS activity than 1% Pd/ceria. The increase of WGS activity was due to the increase of Pd dispersion on the support and the existence of oxygen vacancies produced from incorporation of Pr into the ceria lattice.

Harnessing the Beneficial Attributes of Ceria and Titania in a Mixed-Oxide Support for Nickel-Catalyzed Photothermal CO 2 Methanation

Abstract Solar-powered carbon dioxide (CO 2 )-to-fuel conversion presents itself as an ideal solution for both CO 2 mitigation and the rapidly growing world energy demand. In this work, the heating effect of light irradiation onto a bed of supported nickel (Ni) catalyst was utilized to facilitate CO 2 conversion. Ceria (CeO 2 )-titania (TiO 2 ) oxide supports of different compositions were employed and their effects on photothermal CO 2 conversion examined. Two factors are shown to be crucial for instigating photothermal CO 2 methanation activity: ① Fine nickel deposits are required for both higher active catalyst area and greater light absorption capacity for the initial heating of the catalyst bed; and ② the presence of defect sites on the support are necessary to promote adsorption of CO 2 for its subsequent activation. Titania inclusion within the support plays a crucial role in maintaining the oxygen vacancy defect sites on the (titanium-doped) cerium oxide. The combination of elevated light absorption and stabilized reduced states for CO 2 adsorption subsequently invokes effective photothermal CO 2 methanation when the ceria and titania are blended in the ideal ratio(s).

Chemoselective Hydrazine-mediated Transfer Hydrogenation of Nitroarenes by Co3 O4 Nanoparticles Immobilized on an Al/Si-mixed Oxide Support.

Cobalt oxide nanoparticles (size 2 to 3.5 nm) were successfully impregnated on an alumina-silica (mixed oxide) support through an experimentally viable and easily reproducible protocol. The prepared material was well characterized by XRD, HR-TEM, BET surface area, EDX and XPS analyses. Porous alumina-silica having a high surface area served as a protective heterogeneous support on which the well-dispersed Co3 O4 nanoparticles served as an active catalytic species for the hydrazine-mediated transfer hydrogenation of nitroarenes. About 2 mol % of the active catalyst in ethanol at 60 °C was adequate for a successful conversion. Moreover, transfer hydrogenation of nitroarenes by the catalyst was found to take place chemoselectively in the presence of other labile functional groups such as halide, alkene, nitrile, carbonyl, and ester. This inexpensive catalyst was also able to catalyze the reaction on a gram scale reaction and found to be robust and recyclable up to eight runs.

Manipulating ceria-titania binary oxide features and their impact as nickel catalyst supports for low temperature steam reforming of methane

Mixed oxides of titanium with cerium were examined for the impact of their reducible oxide properties as a support for nickel-catalysed low temperature steam reforming of methane. Modifications to characteristics of the mixed oxide supports were obtained by varying the relative cerium/titanium composition and utilising different synthesis routes. Sol-gel and flame spray pyrolysis synthesis techniques were employed to achieve this purpose. The sol-gel-prepared supports exhibited improved methane conversion rates with an increased presence of cerium within the support while the catalysts with flame-made supports demonstrated poor stability upon incorporation of cerium at certain ratios. Despite the consistent high surface area and fine nickel dispersion across all flame-synthesised supports, the findings revealed that while these attributes are beneficial for steam reforming catalysis, they do not necessarily assure effective conversions. In addition to the above qualities, the sol-gel-synthesised materials demonstrated that crystalline features of the support also play a vital role in maintaining stable conversions at 500°C.

The promotion of CO dissociation by molybdenum oxide overlayers on rhodium

A considerable promotional effect of MoOx species observed at high pressures on the catalytic activity of rhodium initiated the present UHV model study. The MoOx overlayers formed on Rh films (0.15–20.0ML) supported by TiO2(110) substrate were characterized by AES, TPD, work function (WF) measurements and CO adsorption. On the mixed oxide support produced by depositing 1.2ML Mo onto TiO2(110), a new recombinative CO desorption state was observed with Tp=700K, assigned as β-CO and related to the promotional effect of MoOx species diffused onto Rh particles of 1.0ML coverage. The development of β-CO needs 0.5–0.7ML threshold Rh coverage, attributable to particle size effect and geometric factors governing the CO adsorption. The β-CO state with Tp=725–742K could also be detected on Rh films covered by MoOx moiety formed by the oxidation of Mo overlayers in O2. Remarkably, recombinative CO desorption with Tp=700K could be observed on the Rh nanoparticles covered by MoOzCy produced from pure Mo deposits by CO adsorption, too. In harmony with the promotional effect of MoOx overlayer found at high pressures, it is established that the dissociation of CO is maximal at 0.2–0.3ML Mo coverage, attributed to the presence of active sites at the oxide-metal interface. The low desorption peak temperature (700K) of associative CO desorption observed in the presence of MoOx and MoOzCy overlayers indicates a low activation energy for the reactions of Oa and Ca atoms, allowing high reaction rates for these intermediates. The MoOx species exerted both promotion and inhibition effects on CO adsorption at sub-monolayer coverages, but above 1ML it completely suppressed the reactivity of rhodium layers towards CO, suggesting that its surface concentration is a critical factor.

Novel Highly Acidic Mixed-Metal (W, Zr) Catalytic Matrices for Dispersed Noble Metal Nanoparticles: Electrooxidation of Hydrogen and Small Organic Molecules

Application of metal oxides as active matrices in electrocatalysis is particular importance. The hydrous behavior, which favors proton mobility and affects overall reactivity, reflects not only the oxide’s chemical properties but its texture and morphology as well. What is of importance to electrochemical science and technology, certain nonstoichiometric mixed-valence oxides could exhibit pseudo-metallic conductivity and possess appreciable catalytic activity. Recently, mixed oxide systems stabilized through Zr-O-W bonds have been demonstrated to be very attractive acid catalysts exhibiting high catalytic activities and good stabilities in many demanding industrial reactions. For electrocatalytic applications, mixed-valent tungsten(VI,V) oxide and zirconium(IV) oxide have been sequentially deposited and integrated through voltammetric potential cycling to form sub-microstructured films on glassy carbon electrode. The mixed WO3/ZrO2 systems are characterized by fast charge (electron, proton) propagation during the system’s redox transitions. By dispersing metallic Pt and bimetallic platinum-ruthenium (PtRu) electrocatalytic nanoparticles over such active WO3/ZrO2 supports, the electrocatalytic activities of the respective systems toward the oxidation of hydrogen and small organic molecules (formic acid, methanol, ethanol or dimethyl ether) have been enhanced even at decreased loadings of noble metal nanostructures (for hydrogen oxidation) as well as in terms of both increasing the electrocatalytic currents and lowering the onset potentials of organic fuels’ reactions. The enhancement effects should be attributed to features of the mixed metal oxide support such as porosity and high population of hydroxyl groups (due to presence of ZrO2), high Broensted acidity of sites formed on mixed WO3/ZrO2, fast electron transfers coupled to unimpeded proton displacements (e.g. in HxWO3), as well as strong metal-support interactions between nanosized noble metals (Pt, Pd or PtRu) and metal oxo species. The fact that WO3/ZrO2 nanostructures are in immediate contact with the metallic catalytic sites leads to the competitive interactions (via the surface hydroxyl groups) with undesirable reaction intermediates (including CO adsorbates). Thus their desorption (“third body effect”) or even oxidative removal (e.g. of CO to CO2) are feasible. Furthermore, during oxidation of ethanol, when rhodium nanoparticles have been dispersed in between WO3 and ZrO2 layers (toward formation of the nanoreactor system), significant electrocatalytic current enhancements are observed. The result can be rationalized in terms of the formation of sub-microstructured nano-reactors in which Rh induces splitting of C-C bonds in C2H5OH molecules toward CHx/CH4 and CO adsorbate species, or even methanol-type intermediates, that could be further electrooxidized at PtRu catalytic centers. Further mechanistic studies along this line are planned.

Growth of transition metals on cerium tungstate model catalyst layers

Two model catalytic metal/oxide systems were investigated by photoelectron spectroscopy and scanning tunneling microscopy. The mixed-oxide support was a cerium tungstate epitaxial thin layer grown in situ on the W(1 1 0) single crystal. Active particles consisted of palladium and platinum 3D islands deposited on the tungstate surface at 300 K. Both metals were found to interact weakly with the oxide support and the original chemical state of both support and metals was mostly preserved. Electronic and morphological changes are discussed during the metal growth and after post-annealing at temperatures up to 700 K. Partial transition-metal coalescence and self-cleaning from the CO and carbon impurities were observed.