Platinum/Apatite Water-Gas Shift 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.

Pt-Co Alloy Nanoparticles on a γ-Al2 O3 Support: Synergistic Effect between Isolated Electron-Rich Pt and Co for Automotive Exhaust Purification

There is interest in minimizing or eliminating the use of Pt in catalysts by replacing it with more widely abundant and cost-effective elements. The alloying of Pt with non-noble metals is a potential strategy for reducing Pt use because interactions between Pt and non-noble metals can modify the catalyst structure and electronic properties. Here, a γ-Al₂ O₃ -supported bimetallic catalyst [Pt(0.1)Co(1)/Al₂ O₃ ] was prepared which contained 0.1 wt % Pt and 1 wt % Co and thus featured an extremely low Pt : Co ratio (<1 : 30 mol/mol). The Pt and Co in this catalyst formed alloy nanoparticles in which isolated electron-rich Pt atoms were present on the nanoparticle surface. The activity of this Pt(0.1)Co(1)/Al₂ O₃ catalyst for the purification of automotive exhaust was comparable to the activities of 0.3 and 0.5 wt % Pt/γ-Al₂ O₃ catalysts. Electron-rich Pt and metallic Co promoted activation of NO_x and oxidization of CO and hydrocarbons, respectively. This strategy of tuning the surrounding structure and electronic state of a noble metal by alloying it with an excess of a non-noble metal will enable reduced noble metal use in catalysts for exhaust purification and other environmentally important reactions.

Construction of stabilized bulk-nano interfaces for highly promoted inverse CeO2/Cu catalyst

As the water-gas shift (WGS) reaction serves as a crucial industrial process, strategies for developing robust WGS catalysts are highly desiderated. Here we report the construction of stabilized bulk-nano interfaces to fabricate highly efficient copper-ceria catalyst for the WGS reaction. With an in-situ structural transformation, small CeO₂ nanoparticles (2–3 nm) are stabilized on bulk Cu to form abundant CeO₂-Cu interfaces, which maintain well-dispersed under reaction conditions. This inverse CeO₂/Cu catalyst shows excellent WGS performances, of which the activity is 5 times higher than other reported Cu catalysts. Long-term stability is also very solid under harsh conditions. Mechanistic study illustrates that for the inverse CeO₂/Cu catalyst, superb capability of H₂O dissociation and CO oxidation facilitates WGS process via the combination of associative and redox mechanisms. This work paves a way to fabricate robust catalysts by combining the advantages of bulk and nano-sized catalysts. Catalysts with such inverse configurations show great potential in practical WGS applications.

Cu-CeO₂ has been considered as promising alternative to Cu-Zn-Al catalyst for water-gas shift (WGS) reaction, but it still suffers from low activity caused by Cu sintering. Here, the authors develop inverse CeO₂/Cu catalyst with remarkable activity and stability in WGS via construction of stabilized bulk-nano interfaces.

Efficient catalytic hydrogen generation by intermetallic platinum-lead nanostructures with highly tunable porous feature

The water-gas shift (WGS) reaction is an essential industrial reaction for upgrading hydrogen (H₂) by removing carbon monoxide (CO), while highly efficient platinum (Pt)-based catalysts for WGS with simultaneously high activity and stability are still yet to be developed due to the poisoning issue during the reaction. Herein, we report on the porous PtPb peanut nanocrystals (porous PtPb PNCs) and porous PtPb octahedron nanocrystals (porous PtPb ONCs) with controllable ratios of Pt/Pb as extremely active and stable catalysts towards WGS reaction. It exhibits the composition-dependent activity with porous PtPb PNCs-40/ZnO being the most active for WGS to H₂, 16.9 times higher than that of the commercial Pt/C. The porous PtPb PNCs-40/ZnO also display outstanding durability with barely activity decay and negligible structure and composition changes after ten successive reaction cycles. X-ray photoelectron spectroscopy (XPS) results reveal that the suitable binding energy of Pt 4f_7/2 and the high ratio of Pt(0) to Pt(II) in porous PtPb PNCs/ZnO and porous PtPb ONCs/ZnO are crucial for the enhanced WGS activity. The CO stripping results indicate the optimized CO adsorption strength on the Pt surface ensure the excellent WGS activity and the outstanding durability. The present work demonstrates an important advance in tuning the porous metal nanomaterials as highly efficient and durable catalysts for catalysis, energy conversion and beyond.

Molecular-level understanding of reaction path optimization as a function of shape concerning the metal–support interaction effect of Co/CeO2 on water-gas shift catalysis

In this work, the active sites generated in hydrogen reduction and the reaction pathways for the water gas shift (WGS) reaction over Co/CeO₂ catalysts were studied by in situ XAS and XPS coupled with DFT+U calculations.

Improving the activity of gold nanoparticles for the water-gas shift reaction using TiO2-Y2O3: an example of catalyst design

In the last ten years, there has been an acceleration in the pace at which new catalysts for the water-gas shift reaction are designed and synthesized. Pt-based catalysts remain the best solution when only activity is considered. However, cost, operation temperature, and deactivation phenomena are important variables when these catalysts are scaled in industry. Here, a new catalyst, Au/TiO2-Y2O3, is presented as an alternative to the less selective Pt/oxide systems. Experimental and theoretical techniques are combined to design, synthesize, characterize and analyze the performance of this system. The mixed oxide demonstrates a synergistic effect, improving the activity of the catalyst not only at large-to-medium temperatures but also at low temperatures. This effect is related to the homogeneous dispersion of the vacancies that act both as nucleation centers for smaller and more active gold nanoparticles and as dissociation sites for water molecules. The calculated reaction path points to carboxyl formation as the rate-limiting step with an activation energy of 6.9 kcal mol-1, which is in quantitative agreement with experimental measurements and, to the best of our knowledge, it is the lowest activation energy reported for the water-gas shift reaction. This discovery demonstrates the importance of combining experimental and theoretical techniques to model and understand catalytic processes and opens the door to new improvements to reduce the operating temperature and the deactivation of the catalyst.

Catalytic Performance of Ce0.6Y0.4O2-Supported Platinum Catalyst for Low-Temperature Water-Gas Shift Reaction

In this study, Pt/Ce_0.6Y_0.4O₂ catalyst was prepared using a citric sol-gel method and was used as a catalyst for a water-gas shift (WGS) reaction. Compared to 1 wt % Pt/CeO₂ and Pt/Y₂O₃ catalysts, the Pt/Ce_0.6Y_0.4O₂ catalyst showed a much higher WGS catalytic activity. At 250 °C, the conversion of carbon monoxide was 86.35% at a weight hourly space velocity of 30 000 cm³ g_cat ^-1 h^-1. The physicochemical properties of the catalysts were investigated via X-ray diffraction, transmission electron microscopy, chemisorption, H₂ and CO temperature-programmed reduction, and in situ diffuse reflection infrared Fourier transform spectroscopy. These results confirmed that the catalytic activity did not depend on the dispersion and particle size of platinum. The high reducibility of the Ce_0.6Y_0.4O₂ support plays a crucial role in improving the activity of the Pt/Ce_0.6Y_0.4O₂ catalyst, and this improvement can also be explained by the reduction in CO adsorption strength.