Abstract
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.
Highlights
Water-gas shift (WGS) is a mildly exothermic equilibrium reaction (∆H0298K = −41.2 kJ·mol−1), which can be conducted both heterogeneously and homogeneously, the homogeneous process is not of commercial interest [1]
Summarizing, the supports play a crucial role in regulating the catalytic activity; the particle size and the porosimetric characteristics of the support can modulate the dispersion of the active phases, the interactions between active phases and support can modify the electronic characteristics of the catalytic systems, regulating both the activity and the selectivity
Summarizing, the catalytic activity is strongly related to the active phase loading, the best performance was reported for Pt loading ≈ 1 wt%, an enhanced stability can be reached by incorporating the active phase, the best stability was reported for particle sizes < 1 nm
Summary
Several papers have been published on the use and the role of platinum as catalyst in the WGS reaction, focusing on the structural characteristics and morphology of the catalysts, on the chemical nature of the species and intermediates in the reaction mechanisms and on the effect of promoters in reaction conditions [18] These works have involved both theoretical and experimental studies and Metals 2020, 10, 866 and innovative reactor configurations. Together with an eventual poisoning effect due to the presence of sour gasses, such as H2S, in the process stream, most of the deactivation problems are mainly related to the cycling operation which the process requires For this reason, an interesting topic in the research of new catalytic formulations is the catalyst stability under different reaction conditions and operating cycles. The review article has been divided in five main sections, while the articles have been distributed, considering the closest correspondence to the topic: comparative studies (monometallic, polymetallic and promoted catalysts), kinetics studies, reaction mechanism studies, deactivation studies and electrochemical promotion
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