Mechanism for the water–gas shift reaction on monofunctional platinum and cause of catalyst deactivation

Towards the rational design of Pt-based alloy catalysts for the low-temperature water-gas shift reaction: from extended surfaces to single atom alloys

The rational design of Pt-based catalysts for the low-temperature water-gas-shift (LT-WGS) reaction is an active research field because of its important role played in the fuel cell-based hydrogen economy, especially in mobile applications. Previous theoretical analyses have suggested that Pt alloys, leading to a weaker CO binding affinity than the Pt metal, could help alleviate CO poisoning and thus should be promising catalysts of the LT-WGS reaction. However, experimental research along this line was rather ineffective in the past decade. In the present work, we employed the state-of-the-art kinetic Monte Carlo (KMC) simulations to examine the influences of the electronic effect by introducing sub-surface alloys and/or core–shell structures, and the synergetic effect by introducing single atom alloys on the catalytic performance of Pt-alloy catalysts. Our KMC simulations have highlighted the importance of the OH binding affinity on the catalyst surfaces to reduce the barrier of water dissociation as the rate determining step, instead of the CO binding affinity as has been emphasized before in conventional mean-field kinetic models. Along this new direction of catalyst design, we found that Pt–Ru synergetic effects can significantly increase the activity of the Pt metal, leading to Ru_1–3@Pt alloys with a tetrahedron site of one surface-three subsurface Ru atoms on the Pt host, showing a turnover frequency of about five orders of magnitude higher than the Pt metal.

KMC simulations show that decreasing the barrier of H₂O decomposition is more beneficial than decreasing the CO binding affinity in LT-WGS, while the latter was overemphasized by MF-MKM. Here Ru_1–3@Pt alloy is proposed as a promising catalyst.

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.

Impact of small promoter amounts on coke structure in dry reforming of methane over Ni/ZrO2

Choosing the correct alkali metal as a promoter not only reduces coke formation in dry reforming of methane but also removes coke via gasification.

An electronic perturbation in TiC supported platinum monolayer catalyst for enhancing water-gas shift performance: DFT study

The water-gas shift (WGS) reaction behaviors over the TiC(0 0 1) supported Pt monolayer catalyst (Pt_ML/TiC(0 0 1)) are investigated by using the spin-unrestricted density functional theory calculations. Importantly, we find that the Pt_ML/TiC(0 0 1) system exhibits a much lower density of Pt-5d states nearby the Fermi level compared with that for Pt(1 1 1), and the monolayer Pt atoms undergo an electronic perturbation when in contact with TiC(0 0 1) support that would strongly improve the WGS activity of supported Pt atoms. Our calculations clearly indicate that the dominant reaction path follows a carboxyl mechanism involving a key COOH intermediate, rather than the common redox mechanism. Furthermore, through the detailed comparisons, the results demonstrate that the strong interactions between the monolayer Pt atoms and TiC(0 0 1) support make Pt_ML/TiC(0 0 1) a highly active catalyst for the low-temperature WGS reaction. Following the route presented by Bruix et al (2012 J. Am. Chem. Soc. 134 8968-74), the positive effect derived from strong metal-support interaction in the metal/carbide system is revealed.

Ceria-based model catalysts: fundamental studies on the importance of the metal–ceria interface in CO oxidation, the water–gas shift, CO2 hydrogenation, and methane and alcohol reforming

Model metal/ceria and ceria/metal catalysts have shown to be excellent systems for studying fundamental phenomena linked to the operation of technical catalysts.

High-temperature water gas shift reaction on Ni–Cu/CeO2 catalysts: effect of ceria nanocrystal size on carboxylate formation

The WGS mechanism strongly depends on the Ni–Cu surface composition.

Highly Effective Pt-Based Water-Gas Shift Catalysts by Surface Modification with Alkali Hydroxide Salts

Herein, we describe an economical and convenient method to improve the performance of Pt/alumina catalysts for the water-gas shift reaction through surface modification of the catalysts with alkali hydroxides according to the solid catalyst with ionic liquid layer approach. The results are in agreement with our findings reported earlier for methanol steam reforming. This report indicates that alkali doping of the catalyst plays an important role in the observed catalyst activation. In addition, the basic and hygroscopic nature of the salt coating contributes to a significant improvement in the performance of the catalyst. During the reaction, a partly liquid film of alkali hydroxide forms on the alumina surface, which increases the availability of H₂O at the catalytically active sites. Kinetic studies reveal a negligible effect of the KOH coating on the rate dependence of CO and H₂O partial pressures. TEM studies indicate an agglomeration of the active Pt clusters during catalyst preparation; restructuring of Pt nanoparticles occurs under reaction conditions, which leads to a highly active and stable system over 240 h time on stream. Excessive pore fillings with KOH introduce a mass transfer barrier as indicated in a volcano-shaped curve of activity versus salt loading. The optimum KOH loading was found to be 7.5 wt %.

In situ and theoretical studies for the dissociation of water on an active Ni/CeO2 catalyst: importance of strong metal-support interactions for the cleavage of O-H bonds

Water dissociation is crucial in many catalytic reactions on oxide-supported transition-metal catalysts. Supported by experimental and density-functional theory results, the effect of the support on OH bond cleavage activity is elucidated for nickel/ceria systems. Ambient-pressure O 1s photoemission spectra at low Ni loadings on CeO2 (111) reveal a substantially larger amount of OH groups as compared to the bare support. Computed activation energy barriers for water dissociation show an enhanced reactivity of Ni adatoms on CeO2 (111) compared with pyramidal Ni4 particles with one Ni atom not in contact with the support, and extended Ni(111) surfaces. At the origin of this support effect is the ability of ceria to stabilize oxidized Ni(2+) species by accommodating electrons in localized f-states. The fast dissociation of water on Ni/CeO2 has a dramatic effect on the activity and stability of this system as a catalyst for the water-gas shift and ethanol steam reforming reactions.

Effect of annealing in oxygen on alloy structures of Pd–Au bimetallic model catalysts

Annealing in an oxygen ambient stabilizes Pd atoms on Pd–Au surfaces, resulting in higher oxygen uptake and CO oxidation activity.