Gold catalysts supported on mesoporous titania for low-temperature water–gas shift reaction

Mesoporous titania with high surface area and uniform pore size distribution was synthesized using surfactant templating method through a neutral [C₁₃(EO)₆–Ti(OC₃H₇)₄] assembly pathway. The different gold content (1–5 wt.%) was supported on the mesoporous titania by deposition–precipitation (DP) method. The catalysts were characterized by X-ray diffraction, TEM, SEM, N₂ adsorption analysis and TPR. The catalytic activity of gold supported mesoporous titania was evaluated for the first time in water–gas shift reaction (WGSR). The influence of gold content and particle size on the catalytic performance was investigated. The catalytic activity was tested at a wide temperature range (140–300 °C) and at different space velocities and H₂O/CO ratios. It is clearly revealed that the mesoporous titania is of much interest as potential support for gold-based catalyst. The gold/mesoporous titania catalytic system is found to be effective catalyst for WGSR.     

Gold catalysts supported on mesoporous zirconia for low-temperature water–gas shift reaction

Mesoporous ZrO₂ with high surface area and uniform pore size distribution, synthesized by surfactant templating through a neutral [C₁₃(EO)₆–Zr(OC₃H₇)₄] assembly pathway, was used as a support of gold catalysts prepared by deposition–precipitation method. The supports and the catalysts were characterized by powder X-ray diffraction, scanning and transmission electron microscopy, N₂ adsorption analysis, temperature programmed reduction and desorption. The catalytic activity of gold supported on mesoporous zirconia was evaluated in water–gas shift (WGS) reaction at wide temperature range (140–300 °C) and at different space velocities and H₂O/CO ratios. The catalytic behaviour and the reasons for а reversible deactivation of Au/mesoporous zirconia catalysts were studied. The influence of gold content and particle size on the catalytic performance was investigated. The WGS activity of the new Au/mesoporous zirconia catalyst was compared to the reference Au/TiO₂ type A (World Gold Council), revealing significantly higher catalytic activity of Au/mesoporous zirconia catalyst. It is found that the mesoporous zirconia is a very efficient support of gold-based catalyst for the WGS reaction.     

Mechanistic aspects of the water–gas shift reaction on alumina-supported noble metal catalysts: In situ DRIFTS and SSITKA-mass spectrometry studies

Steady-state isotopic transient kinetic analysis (SSITKA) experiments coupled with mass spectrometry were performed for the first time to study essential mechanistic aspects of the water–gas shift (WGS) reaction over alumina-supported Pt, Pd, and Rh catalysts. In particular, the concentrations (μmol g⁻¹) of active intermediate species found in the carbon-path from CO to the CO₂ product gas (use of ¹³CO), and in the hydrogen-path from H₂O to the H₂ product gas (use of D₂O) of the reaction mechanism were determined. It was found that by increasing the reaction temperature from 350 to 500 °C the concentration of active species in both the carbon-path and hydrogen-path increased significantly. Based on the large concentration of active species present in the hydrogen-path (OH/H located on the alumina support), the latter being larger than six equivalent monolayers based on the exposed noble metal surface area (θ > 6.0), the small concentration of OH groups along the periphery of metal-support interface, and the significantly smaller concentration (μmol g⁻¹) of active species present in the carbon-path (adsorbed CO on the noble metal and COOH species on the alumina support and/or the metal-support interface), it might be suggested that diffusion of OH/H species on the alumina support towards catalytic sites present in the hydrogen-path of reaction mechanism might be considered as a slow reaction step. The formation of labile OH/H species is the result of dissociative chemisorption of water on the alumina support, where the role of noble metal is to activate the CO chemisorption and likely to promote formate decomposition into CO₂ and H₂ products. It was found that there is a good correlation between the surface concentration and binding energy of CO on the noble metal (Pt, Pd or Rh) with the activity of alumina-supported noble metal towards the WGS reaction.     

Study on the supported Cu-based catalysts for the low-temperature water–gas shift reaction

This paper presents a study on the influence of support (Al₂O₃, MgO, SiO₂-Al₂O₃, SiO₂-MgO, β-zeolite, and CeO₂) of Cu-ZnO catalysts for the low-temperature water–gas shift reaction. Supported Cu-ZnO catalysts were prepared by the conventional impregnation method, followed by the H₂ reduction. The activity of Cu-ZnO catalysts for the water–gas shift (WGS) reaction was largely influenced by the kind of support; Cu-ZnO catalysts supported on Al₂O₃, MgO, and CeO₂ showed high activity, while those on SiO₂-Al₂O₃, SiO₂-MgO and β-zeolite showed less activity in the temperature range 423–523 K. XRD analysis demonstrated that the copper species were highly dispersed on the supports used in the present study, except for a MgO support. TPR results of a series of supported CuO-ZnO catalysts suggest that the reducibility of CuO is one of the important factors controlling the activity of the WGS reaction over the supported catalysts.     

Kinetics of the water–gas shift reaction over nanostructured copper–ceria catalysts

Water–gas shift reaction was studied over two nanostructured Cu_xCe_1−xO_2−y catalysts: a Cu_0.1Ce_0.9O_2−y catalyst prepared by a sol–gel method and a Cu_0.2Ce_0.8O_2−y catalyst prepared by co-precipitation method. A commercial low temperature water–gas shift CuO–ZnO–Al₂O₃ catalyst was used as reference. The kinetics was studied in a plug flow micro reactor at an atmospheric pressure in the temperature interval between 298 and 673 K at two different space velocities: 5.000 and 30.000 h⁻¹, respectively. Experimentally estimated activation energy, E_af, of the forward water–gas shift reaction at CO/H₂O = 1/3 was 51 kJ/mol over the Cu_0.1Ce_0.9O_2−y, 34 kJ/mol over the Cu_0.2Ce_0.8O_2−y and 47 kJ/mol over the CuO–ZnO–Al₂O₃ catalyst. A simple rate expression approximating the water–gas shift process as a single reversible surface reaction was used to fit the experimental data in order to evaluate the rate constants of the forward and backward reactions and of the activation energy for the backward reaction.     

Effect of pretreatment on the activity of Ni catalyst for CO removal reaction by water–gas shift and methanation

An Ni metal catalyst manufactured by the tapecasting method for use as a structural catalyst did not exhibit catalytic activity for the carbon monoxide (CO) removal reaction. However, the catalyst pretreated by an oxidation and reduction process showed superior activity for CO removal via water–gas shift and methanation, resulting in a decrease of the CO concentration to below 1% in reformate gas. The catalytic activity was generated by the reorganization of the surface structure of Ni metal, and enhanced by surface oxygen intermediates such as Ni(OH)₂ and NiOOH promoted by NiO oxidized incompletely after the pretreatment. After the reorganization process induced by the pretreatment, the Ni metal on the surface was converted to active Ni and NiO which played the role of a promoter.     

Relating catalyst structure and composition to the water–gas shift activity of Cu–Zn-based mixed-oxide catalysts

In order to investigate the effect of cerium oxide on Cu–Zn-based mixed-oxide catalysts four catalyst samples were characterized by means of XRD, in situ XANES and thermogravimetric analysis. The activity of the catalyst samples was tested for the forward water–gas shift reaction. Cerium oxide was found to increase the crystallinity of the ZnO phase indicating a segregation of the Cu and ZnO phases. The TOF of the water–gas shift reaction based on chemisorption data was found to be independent of composition and preparation conditions of the four catalyst samples. In contrast, the catalyst stability depends on composition and preparation conditions. Cerium oxide impregnated before calcination of the hydrotalcite-based Cu–Zn precursors leads to a more stable water–gas shift catalyst.     

Kinetics of the water–gas shift reaction on Pt catalysts supported on alumina and ceria

We report the kinetic parameters for the water–gas shift (WGS) reaction on Pt catalysts supported on ceria and alumina under fuel reformer conditions for fuel cell applications (6.8% CO, 8.5% CO₂, 22% H₂O, 37.3% H₂, and 25.4% Ar) at a total pressure of 1 atm and in the temperature range of 180–345 °C. When ceria was used as a support, the turnover rate (TOR) for WGS was 30 times that on alumina supported Pt catalysts. The overall WGS reaction rate (r) on Pt/alumina catalysts as a function of the forward rate (r_f) was found to be: r = r_f(1 − β), where r_f = k_f[CO]^0.1[H₂O]^1.0[CO₂]^−0.1[H₂]^−0.5, k_f is the forward rate constant, β = ([CO₂][H₂])/(K_eq[CO][H₂O]) is the approach to equilibrium, and K_eq is the equilibrium constant for the WGS reaction. The negative apparent reaction orders indicate inhibition of the forward rate by CO₂ and H₂. The surface is saturated with CO on Pt under reaction conditions as confirmed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The small positive apparent reaction order for CO, in concert with the negative order for H₂ and the high CO coverage is explained by a decrease in the heat of adsorption as the CO coverage increases. Kinetic models based on redox-type mechanisms can explain the observed reaction kinetics and can qualitatively predict the changes in CO coverage observed in the DRIFTS study.     

High temperature water–gas shift reaction over hollow Ni–Fe–Al oxide nano-composite catalysts prepared by the solution-spray plasma technique

The effect of Fe content in Ni–Fe–Al oxide nano-composites prepared by the solution-spray plasma technique on their catalytic activity for the high temperature water–gas shift reaction was investigated. The composites showed a hollow sphere structure, with highly dispersed Fe–Ni particles supported on the outer surface of the spheres. When the water–gas shift reaction was performed over an Ni–Al oxide composite catalyst without Fe, undesired CO methanation took place predominantly compared to the water–gas shift reaction, and significant amounts of hydrogen were consumed. When appropriate amounts of Fe were added to the Ni–Al oxide composite catalyst during the plasma process, methanation was suppressed remarkably, without serious loss of activity for the water–gas shift reaction. The catalyst was characterized by STEM, XRD and H₂ chemisorption measurements.     

Ceria in catalysis: From automotive applications to the water–gas shift reaction

Ceria is a crucial component of automotive catalysts, where its ability to be reduced and re‐oxidized provides oxygen storage capacity. Because of these redox properties, ceria can greatly enhance catalytic activities for a number of important reactions when it is used as a support for transition metals. For reactions that use steam as an oxidant (e.g., the water–gas‐shift reaction and steam reforming of hydrocarbons), rates for ceria‐supported metals can be several orders of magnitude higher than that for ceria or the transition metal alone. Because the redox properties of ceria are strongly dependent on treatment history and the presence of additives, there are significant opportunities for modifying catalysts based on ceria to further improve their performance. This article will review some of the contributions from my laboratory on understanding and using ceria in these applications. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

A mechanistic model for the water‐gas shift reaction over noble metal substituted ceria

The water‐gas shift (WGS) reaction was carried out in the presence of Pd and Pt substituted nanocrystalline ceria catalysts synthesized by solution combustion technique. The catalysts were characterized by powder XRD and XPS. The noble metals were found to be present in ionic form substituted for the cerium atoms. The catalysts showed high activity for the WGS reaction with high conversions below 250°C. The products of reaction were only carbon dioxide and hydrogen, and no hydrocarbons were observed even in trace quantities. The reactions were carried out with different amounts of noble metal ion substitution and 2% Pt substituted ceria was found to be the best catalyst. The various possible mechanisms for the reaction were proposed and tested for their consistency with experimental data. The dual site mechanism best described the kinetics of the reaction and the corresponding rate parameters were obtained. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Effect of structural and acidity/basicity changes of CuO–CeO2 catalysts on their activity for water–gas shift reaction

The objective of this work was to investigate the influence of CuO loading and catalyst pretreatment procedure to derive an optimal CuO–CeO₂ catalyst for the water–gas shift reaction (WGS), and to study in detail structure– and surface acidity–activity relationships. Catalyst samples prepared by coprecipitation and a 10, 15 and 20 mol% CuO content were examined by XRD, BET and TPR/TPD analyses and subjected to pulse WGS activity tests in the temperature range of 180–400 °C. Strong structure–activity dependence in the WGS reaction was observed for all catalyst samples. It was established that increasing CuO content has a positive effect on H₂ production during the WGS reaction, due to favored CeO₂ reduction. Increasing calcination temperature on the other hand reduces the BET surface area, induced by CuO sintering and agglomeration of CeO₂ particles, resulting in a negative effect on H₂ production. Distinctive WGS activity dependence on surface acidity was observed and investigated.     

Hydrodeoxygenation of guaiacol on noble metal catalysts

Hydrodeoxygenation (HDO) performed at high temperatures and pressures is one alternative for upgrading of pyrolysis oils from biomass. Studies on zirconia-supported mono- and bimetallic noble metal (Rh, Pd, Pt) catalysts showed these catalysts to be active and selective in the hydrogenation of guaiacol (GUA) at 100 °C and in the HDO of GUA at 300 °C. GUA was used as model compound for wood-based pyrolysis oil. At the temperatures tested, the performance of the noble metal catalysts, especially the Rh-containing catalysts was similar or better than that of the conventional sulfided CoMo/Al₂O₃ catalyst. The carbon deposition on the noble metal catalysts was lower than that on the sulfided CoMo/Al₂O₃ catalyst. The performance of the Rh-containing catalysts in the reactions of GUA at the tested conditions demonstrates their potential in the upgrading of wood-based pyrolysis oils.     

W-promoted Co–Mo–K/γ–Al2O3 catalysts for water–gas shift reaction

The effects of incorporating tungsten into the traditional Co–Mo–K/γ–Al₂O₃ catalysts on the catalytic performances for water–gas shift reaction were investigated. Activity tests showed that W-promoted Co–Mo–K/γ–Al₂O₃ catalysts exhibited higher activity than W-free Co–Mo–K/γ–Al₂O₃ catalyst. Raman and H₂-TPR studies indicated that part of the octahedrally coordinated Mo–O species on Co–Mo–K catalysts transformed into tetrahedrally coordinated Mo–O species in the presence of W promoter.     

Support‐dependent activity of noble metal substituted oxide catalysts for the water gas shift reaction

The water gas shift reaction was carried out over noble metal ion substituted nanocrystalline oxide catalysts with different supports. Spectroscopic studies of the catalysts before and after the reaction showed different surface phenomena occurring over the catalysts. Reaction mechanisms were proposed based upon the surface processes and intermediates formed. The dual site mechanism utilizing the oxide ion vacancies for water dissociation and metal ions for CO adsorption was proposed to describe the kinetics of the reaction over the reducible oxides like CeO₂. A mechanism based on the interaction of adsorbed CO and the hydroxyl group was proposed for the reaction over ZrO₂. A hybrid mechanism based on oxide ion vacancies and surface hydroxyl groups was proposed for the reaction over TiO₂. The deactivation of the catalysts was also found to be support dependent. Kinetic models for both activation and deactivation were proposed. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Nano-structured CeO2 supported Cu-Pd bimetallic catalysts for the oxygen-assisted water–gas-shift reaction

The present work focuses on the development of novel Cu-Pd bimetallic catalysts supported on nano-sized high-surface-area CeO₂ for the oxygen-assisted water–gas-shift (OWGS) reaction. High-surface-area CeO₂ was synthesized by urea gelation (UG) and template-assisted (TA) methods. The UG method offered CeO₂ with a BET surface area of about 215 m²/g, significantly higher than that of commercially available CeO₂. Cu and Pd were supported on CeO₂ synthesized by the UG and TA methods and their catalytic performance in the OWGS reaction was investigated systematically. Catalysts with about 30 wt% Cu and 1 wt% Pd were found to exhibit a maximum CO conversion close to 100%. The effect of metal loading method and the influence of CeO₂ support on the catalytic performance were also investigated. The results indicated that Cu and Pd loaded by incipient wetness impregnation (IWI) exhibited better performance than that prepared by deposition–precipitation (DP) method. The difference in the catalytic activity was related to the lower Cu surface concentration, better Cu–Ce and Pd–Ce interactions and improved reducibility of Cu and Pd in the IWI catalyst as determined by the X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR) studies. A direct relation between BET surface area of the CeO₂ support and CO conversion was also observed. The Cu-Pd bimetallic catalysts supported on high-surface-area CeO₂ synthesized by UG method exhibited at least two-fold higher CO conversion than the commercial CeO₂ or that obtained by TA method. The catalyst retains about 100% CO conversion even under extremely high H₂ concentration.     

The role of cationic Au and nonionic Au species in the low-temperature water–gas shift reaction on Au/CeO2 catalysts

The roles of cationic and nonionic Au species in the water–gas shift (WGS) reaction on Au/CeO₂ catalysts were studied by comparing the reaction behavior of a cyanide leached catalyst, after removal of the Au nanoparticles by cyanide leaching, with that of non-leached catalysts, following the technique introduced by Q. Fu et al. [Science 301 (2003) 935]. Using rate measurements as well as in situ spectroscopic and structure-sensitive techniques, we found that based on the Au mass balance, cyanide leaching removed all Au except for ionic Au³⁺ species, and that leaching resulted in a pronounced decay of the catalyst mass normalized activity to 1–25% of that of a non-leached catalyst. The extent of the activity loss strongly depended on the post-treatment of the leached catalyst. Both the catalyst treatment after leaching and, in particular, the WGS reaction resulted in considerable reformation of Au⁰ species by thermal decomposition of Au oxides (Au³⁺) and subsequent nucleation and growth of very small Au⁰ aggregates and metallic Au⁰ nanoparticles, as indicated by Au(4f) signals at 85.9 eV (Au³⁺), 84.0–84.6 eV (up-shifted signal of small Au⁰ aggregates), and 84.0 eV (metallic Au⁰). In this work, correlations between ionic and nonionic Au species and between total WGS activity and activity for the formation/decomposition of bidentate formate species are evaluated, and the role of the respective Au species in the WGS reaction on Au/CeO₂ catalysts is discussed.     

Au–TiO2 catalysts stabilised by carbon nanofibres

The catalytic activity and stability in the water–gas shift reaction have been tested for Au-based catalysts prepared by deposition of Au from colloid solutions. The supports that have been used are TiO₂, TiO₂ supported on carbon nanofibres (CNF) and CNF. Thermal treatments of the samples show that the Au particle size depends on the support material and hence the interaction between the Au particles and the support. In situ X-ray absorption spectroscopic (XAS) measurements during the water–gas shift reaction show no changes in the first Au–Au coordination number for the catalysts containing CNF. Furthermore, improved short-time stability is obtained compared to the AuTiO₂ catalysts. The improved stability is achieved by the CNF stabilising small TiO₂ particles and hence prevent subsequent sintering of the Au particles.