Tuning Selectivity of CO₂ Hydrogenation via Support Composition Modification Adjusted the Activity Reduction of H Species over Ce_1–<i>x</i>Pr_<i>x</i>O_2−δ-Supported Metal (Ru, Rh) Nanoclusters

Abstract

Selectivity control of supported metal catalysts, which are most widely utilized in the field of heterogeneous catalysis, is of great scientific significance to obtaining the desired chemical product in a multipath reaction but has remained a grand challenging issue. In this work, we demonstrate that the selectivity of CO 2 hydrogenation from CH 4 to CO can be tuned by a robust and unique support doping strategy by changing the reduction activity of H species over M/Ce 1– x Pr x O 2−δ (M = Ru, Rh) in which metal (M) nanoclusters showed the same existence form on differently doped ceria nanorod supports. The CH 4 selectivity of the catalyst decreased with an increase in the Pr content in the support. The selectivity of CH 4 on Ru/CeO 2 was higher than 90%, while on Ru/Ce 0.2 Pr 0.8 O 2−δ, the selectivity of CO reached 80%. A variety of techniques, including steady-state isotope transient kinetic analysis (SSITKA) type in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)–mass spectrum (MS), temperature-programmed desorption (TPD) and temperature-programmed surface reaction (TPSR), had been applied in this work to analyze the structure–activity relationship between the doping of Pr and the selectivity of the CO 2 hydrogenation reaction. Ru sites were not directly involved in the hydrogenation of carbon-containing intermediate species (including bicarbonate and formate) during the CO 2 hydrogenation reaction. The active H species on the support sites, which are incorporated in RE 3+ –OH, directly contacted and reacted with the carbon-containing intermediate species. The introduction of Pr in the support weakened the reducing ability of the support, thus decreasing the reducing ability of H species on the surface of the catalyst, which further hindered the conversion of formate into CH 4, resulting in the declined CH 4 selectivity. Our study clearly revealed the important role of support in the CO 2 hydrogenation reaction and proposed a strategy to modulate the reaction selectivity via support doping. By changing the redox performance of the support, the activity of H species on the support can be adjusted. Thus, the conversion of important reaction intermediates (such as formate) can be affected, so as to achieve precise regulation of the reaction products. We have provided a broader perspective for the selective catalyst design of heterogeneous catalysis and the reaction mechanism study of supported metal catalysts.

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