Support Effect on the Water Gas Shift Activity of Chemical Vapor Deposition-Tailored-Pt/TiO2 Catalysts

Platinum Based Catalysts in the Water Gas Shift Reaction: Recent Advances

The water gas shift (WGS) is an equilibrium exothermic reaction, whose corresponding industrial process is normally carried out in two adiabatic stages, to overcome the thermodynamic and kinetic limitations. The high temperature stage makes use of iron/chromium-based catalysts, while the low temperature stage employs copper/zinc-based catalysts. Nevertheless, both these systems have several problems, mainly dealing with safety issues and process efficiency. Accordingly, in the last decade abundant researches have been focused on the study of alternative catalytic systems. The best performances have been obtained with noble metal-based catalysts, among which, platinum-based formulations showed a good compromise between performance and ease of preparation. These catalytic systems are extremely attractive, as they have numerous advantages, including the feasibility of intermediate temperature (250–400 °C) applications, the absence of pyrophoricity, and the high activity even at low loadings. The particle size plays a crucial role in determining their catalytic activity, enhancing the performance of the nanometric catalytic systems: the best activity and stability was reported for particle sizes < 1.7 nm. Moreover the optimal Pt loading seems to be located near 1 wt%, as well as the optimal Pt coverage was identified in 0.25 ML. Kinetics and mechanisms studies highlighted the low energy activation of Pt/Mo2C-based catalytic systems (Ea of 38 kJ·mol−1), the associative mechanism is the most encountered on the investigated studies. This review focuses on a selection of recent published articles, related to the preparation and use of unstructured platinum-based catalysts in water gas shift reaction, and is organized in five main sections: comparative studies, kinetics, reaction mechanisms, sour WGS and electrochemical promotion. Each section is divided in paragraphs, at the end of the section a summary and a summary table are provided.

Recent Advances in Structured Catalysts Preparation and Use in Water-Gas Shift Reaction

The water-gas shift reaction plays a key role in hydrogen production processes from fossil sources and renewable biomass feedstock and can be considered as the first purification process of syngas. The water gas shift process is normally carried out in two adiabatic stages, of high and low temperature with an intersystem cooling. The two stages use two different catalytic systems, which present some critical issues, thus making extremely attractive the designing and implementing of new configurations. Innovative and highly active catalytic formulations along with more efficient reactor systems could provide the basis for the design of a single-stage process, resulting in a noticeable process intensification. In the last decades, much attention has been paid to the use of structured catalysts, which have numerous advantages, related to both fluid dynamics and heat transfer phenomena. Numerous papers have been published in which the competitive performances of structured catalysts have been shown with respect to conventional catalytic systems. In this brief review, we provide an overview of the most recent developments in the preparation of structured catalysts and use in the water gas shift reaction.

Engineering Stable Surface Oxygen Vacancies on ZrO2 by Hydrogen-Etching Technology: An Efficient Support of Gold Catalysts for Water-Gas Shift Reaction

The surface structure of supports is crucial to fabricate efficient supported catalysts for water-gas shift (WGS). Here, hardly reducible ZrO₂ was etched with hydrogen (H), aiming to modify surface structures with sufficient stable oxygen vacancies. After deposition of gold species, the obtained khaki ZrO₂-H notably improved WGS catalytic activities and stabilities in comparison to the traditional white ZrO₂. The characterization results and quantitative analysis indicate that sufficient surface oxygen vacancies of ZrO₂-H support give rise to more metallic Au⁰ species and higher microstrain, which all boost WGS catalytic activities. Furthermore, optoelectronic properties were successfully used to correlate with their WGS thermocatalytic activities, and then a modified electron flow process was proposed to understand the WGS pathway. For one thing, the introduction of surface oxygen vacancies narrowed the band gap of ZrO₂ and decreased the Ohmic barrier, which facilitated the flow of "hot-electron". For another thing, the conduction band electrons can be easily trapped by oxygen vacancies of ZrO₂ supports, and then these trapped electrons immediately take part in reduction of H₂O to H₂. Thus, the electron recombination was suppressed and the WGS catalytic activity was improved. It is worth extending H₂-etching technology to improve other thermocatalytic reactions.

CO oxidation on Ru-Pt bimetallic nanoclusters supported on TiO2(101): The effect of charge polarization

Pt-based catalyst is widely used in CO oxidation, while its catalytic activity is often undermined because of the CO poisoning effect. Here, using density functional theory, we propose the use of a Ru-Pt bimetallic cluster supported on TiO₂ for CO oxidation, to achieve both high activity and low CO poisoning effect. Excellent catalytic activity is obtained in a Ru₁Pt₇/TiO₂(101) system, which is ascribed to strong electric fields induced by charge polarization between one Ru atom and its neighboring Pt atoms. Because of its lower electronegativity, the Ru atom donates electrons to neighboring Pt. This induces strong electric fields around the top-layered Ru, substantially promoting the adsorption of O₂/CO + O₂ and eliminating the CO poisoning effect. In addition, the charge polarization also drives the d-band center of the Ru₁Pt₇ cluster to up-shift to the Fermi level. For surface O₂ activation/CO oxidation, the strong electric field and d-band center close to the Fermi level can promote the adsorption of O₂ and CO as well as reduce the reaction barrier of the rate-determining step. Meanwhile, since O₂ easily dissociates on Ru₁Pt₇/TiO₂(101) resulting in unwanted oxidation of Ru and Pt, a CO-rich condition is necessary to protect the catalyst at high temperature.

Black TiO2−x with stable surface oxygen vacancies as the support of efficient gold catalysts for water-gas shift reaction

H₂-etching engineered oxygen vacancies on black TiO_2−x to enhance the hot-electron flow and water-gas shift catalytic performance of Au catalysts.