Tuning the structural dynamics of Pt/CeO2 catalysts for accelerating the water gas shift reaction
Résumé
Context and purpose
Pt/CeO2 is a promising catalyst for the low temperature water gas shift reaction (WGS). Moreover, it is “dynamic” since Pt can be dispersed during an oxidative treatment at moderate temperature leading to single atoms catalysts (SACs) while a reducing treatment triggers 3D particles formation. Starting from SACs, suitable redox treatments were previously shown to tailor nanoclusters more active for the low-temperature CO oxidation in the presence of water.
In this work, the influence of different redox treatments on the WGS activity of Pt/CeO2 catalysts has been investigated depending on the Pt surface density as well as the doping by alkali cations. Relationships between physicochemical and catalytic properties have been established based on several complementary characterization methods including in situ/operando techniques.
Material and methods
Pt/CeO2 catalysts with different loadings were prepared by wet impregnation using Pt(NH3)4(NO3)2 as precursor and high surface area CeO2 as support, followed by calcination in air at 500 °C. A wet impregnation by alkali nitrates was further achieved on selected calcined samples for doping. The catalysts were characterized before, during and after the reaction by various techniques, including ICP-OES, XRF, BET, XRD, STEM-HAADF, H2 and CO-TPR, CO-DRIFT, and operando Raman and DRIFTS. The catalytic performances were measured using a fixed-bed reactor flowing a H2O/CO/Ar:32/8/60 mixture at atmospheric pressure and 235 °C.
Main results
The Pt/CeO2 catalysts were shown to be predominantly SACs after calcination. Catalytic data revealed a quick ceria reduction under the reaction feed, while STEM images showed the formation of 3D Pt nanoparticles (NPs) of ca 1.5 nm after only a few minutes on stream. Interestingly, an optimal H2 productivity was observed after a few minutes likely due to intermediate active sites transiently formed. An optimal H2 productivity was also observed for a Pt coverage of 0.12 at/nm2 (figure 1A), revealing two different behaviors for high and low loadings. They are attributed to a balance between the Pt dispersion and the reducibility of ceria (figure 1B).
Furthermore, different pretreatments were applied to tune the Pt NPs size. A reductive treatment at 500 °C improves the catalytic performances at low Pt loadings by strongly improving the surface ceria reducibility (figures 1B and C), while it decreases the catalytic activity at high loadings by Pt sintering. Various oxidative treatments were also performed to redisperse Pt particles after 1 h on stream. For low Pt loadings, the initial activity is recovered after an oxidative treatment at 230 °C, while this catalyst is strongly activated after an oxidative treatment at 500 °C (figure 1A).
Alkali dopants increase the activity of the Pt/CeO2 catalysts with high Pt loadings, presumably by stabilizing ultradispersed Pt species and modifying the ceria reducibility. However, they strongly poison the catalysts at low Pt loadings.
Major conclusions
Both structural dynamics and ceria redox properties of Pt/CeO2 control its catalytic activity in WGS. These two parameters strongly depend on the Pt surface density and the presence of alkali dopants, allowing the use of redox treatments as a key strategy to (re-)activate the catalyst.