Mechanism of CO formation in reverse water–gas shift reaction over Cu/Al2O3 catalyst

The mechanism of the reverse water–gas shift reaction over a Cu catalyst was studied by CO₂ hydrogenation, temperature-programmed reduction of the Cu catalyst and pulse reaction with QMS monitoring. In comparison with the reaction of CO₂ alone, hydrogen can significantly promote the CO formation in the RWGS reaction. The formate derived from association of H₂ and CO₂ is proposed to be the key intermediate for CO production. Formate dissociation mechanism is the major reaction route for CO production. Cu(I) species were formed from the oxidation of Cu⁰ associated with CO₂ dissociation. This revised version was published online in July 2006 with corrections to the Cover Date.     

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     

On the mechanism of the selective catalytic reduction of NO to N2 by H2 over Ru/MgO and Ru/Al2O3 catalysts

Steady-state and transient kinetic experiments were performed in a versatile microreactor flow set-up with magnesia- and alumina-supported ruthenium catalysts in order to elucidate the mechanism of the selective catalytic reduction (SCR) of nitric oxide with hydrogen. Both Ru/MgO and Ru/γ-Al₂O₃ were found to be highly active catalysts converting NO and H₂ into N₂ and H₂O with selectivities close to 100% at full conversion, although Ru-based catalysts are known to be active in the synthesis of NH₃ from N₂ and H₂. Frontal chromatography experiments with NO at room temperature revealed that NO and its dissociation products displace adsorbed atomic hydrogen (H−*) almost completely from hydrogen-precovered Ru surfaces. Obviously, NO and H₂ compete for the same adsorption sites, H−* being the weaker bound adsorbate. Temperature-programmed surface reaction (TPSR) experiments in H₂ subsequent to NO exposure demonstrated that higher heating rates and lower partial pressures of H₂ shift the selectivity from NH₃ to N₂. Therefore, the coverage of H−* is concluded to govern the branching ratio between the rate of associative desorption of N₂ (2N−*→N₂ + 2*) and the rate of hydrogenation of N−* (N−* + 3H–* →NH₃ + 4*). Finally, the steady-state coverages of N- and O-containing adsorbates were derived by interrupting the SCR reaction and hydrogenating the adsorbates off as NH₃ and H₂O. By solving the site balance, the Ru surfaces were found to be essentially N₂ is attributed to the very low coverage of H−* due to site blocking by a N + O coadsorbate layer, favouring the recombination of N−* instead of its hydrogenation to NH₃. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of ZnO Contained in Supported Cu-Based Catalysts on Their Activities for Several Reactions

The specific activity of Cu-based catalyst supported on Al₂O₃, ZrO₂ or SiO₂ for methanol synthesis and reverse water–gas shift reactions was improved by the addition of ZnO to the catalyst. On the other hand, the specific activity of the supported Cu-based catalyst for methanol steam reforming and water–gas shift reactions was not improved by the addition of ZnO to the catalyst.     

Comparison of the activity of Au/CeO2 and Au/Fe2O3 catalysts for the CO oxidation and the water-gas shift reactions

We compare the activity and relevant gold species of nanostructured gold–cerium oxide and gold–iron oxide catalysts for the CO oxidation by dioxygen and water. Well dispersed gold nanoparticles in reduced form provide the active sites for the CO oxidation reaction on both oxide supports. On the other hand, oxidized gold species, strongly bound on the support catalyze the water-gas shift reaction. Gold species weakly bound to ceria (doped with lanthana) or iron oxide can be removed by sodium cyanide at pH ≥12. Both parent and leached catalysts were investigated. The activity of the leached gold–iron oxide catalyst in CO oxidation is approximately two orders of magnitude lower than that of the parent material. However, after exposure to H₂ up to 400 °C gold diffuses out and is in reduced form on the surface, a process accompanied by a dramatic enhancement of the CO oxidation activity. Similar results were found with the gold–ceria catalysts. On the other hand, pre-reduction of the calcined leached catalyst samples did not promote their water-gas shift activity. UV–Vis, XANES and XPS were used to probe the oxidation state of the catalysts after various treatments.     

Ockham's razor for paring microkinetic mechanisms: Electrical analogy vs. Campbell's degree of rate control

Elucidation of the key molecular steps and pathways in an overall reaction is of central importance in developing a better understanding of catalysis. Campbell's degree of rate control (DRC) is the leading methodology currently available for identifying the germane steps and key intermediates in a catalytic mechanism. We contrast Campbell's DRC to our alternate new approach involving an analysis and comparison of the “resistance” and de Donder “affinity,” that is, the driving force, of the various steps and pathways in a mechanism, in a direct analogy to electrical networks. We show that our approach is as just rigorous and more insightful than Campbell's DRC. It clearly illuminates the bottleneck steps within a pathway and allows one to readily discriminate among competing pathways. The example used for a comparison of these two methodologies is a DFT study of the water–gas shift reaction on Pt–Re catalyst published recently. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4332–4346, 2015     

Improved Performance of Au/Fe2O3 Catalysts Promoted with ZrO2 and Nb2O5 in the WGS Reaction under Hydrogen-rich Conditions

Effect of addition of the ZrO₂ and Nb₂O₅ promoters on the activity and stability of the Au/Fe₂O₃ catalysts in the WGS reaction under hydrogen-rich conditions was studied. Results showed that this new catalyst possesses enhanced activity and stability under conditions common in fuel processors. Its CO conversion almost reached the maximum value 99% at 200 °C and maintained better stability compared with unmodified samples within 50 h on-stream. Detailed characterization including BET, XRD, HRTEM, XPS, XRF and H₂-TPR revealed that ZrO₂ and Nb₂O₅ acted as structural promoters and a strong interaction between ZrO₂ and Nb₂O₅ existed. The enrichment of Zr and Nb on the surface kept the gold and magnetite particles apart delaying sintering. More active gold sites, larger surface area and smaller magnetite particles were the main reasons for the enhanced performance.     

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.