Probing the morphological effects of ReOx/CeO2 catalysts on the CO2 hydrogenation reaction

Abstract

The performance optimization of ceria (CeO₂)-supported catalysts in heterogeneous reactions can be enabled by the control of crystal morphology. However, insights into how the CeO₂ morphology affects the CO₂ hydrogenation performance are still lacking. Here, in situ diffuse reflectance infrared Fourier transform spectroscopy and temperature-programmed surface reaction techniques are utilized to identify and characterize the morphological effect of CeO₂ supported rhenium (Re) catalysts, including CeO₂ nanocube supported Re (ReO_x/CeO₂-C), CeO₂ nanorod supported Re (ReO_x/CeO₂-R), and CeO₂ nanopolyhedron supported Re (ReO_x/CeO₂-P). ReO_x/CeO₂-R exhibits enhanced CO₂ conversion (14.7%) compared to ReO_x/CeO₂-C (1.5%) and ReO_x/CeO₂-P (1.3%) at 2 MPa and 613 K. The CH₄ selectivity on ReO_x/CeO₂-R (40.9%) also far exceeds those on ReO_x/CeO₂-C and ReO_x/CeO₂-P (∼5%). The electronic metal–support interaction between Re and CeO₂ is found to be largely dependent on the CeO₂ morphology and closely linked with H₂ activation and CO₂ adsorption. The strongly adsorbed carbonate (CO₃²⁻) is identified to cover the surface of ReO_x/CeO₂-P and ReO_x/CeO₂-C during CO₂ hydrogenation but is absent over ReO_x/CeO₂-R. This stable and irreversible intermediate locally modifies the catalytic performance. The metallic Re⁰ in ReO_x/CeO₂-R favors H₂ dissociation to generate oxygen vacancies and thus promotes chemisorption of CO₂ as CO₂^δ− rather than CO₃²⁻. Moreover, the facile elimination of CO₃²⁻ in ReO_x/CeO₂-R is a key step for highly catalytic performance. This work uncovers the CeO₂ morphological effects which play a significant role in CO₂ hydrogenation. These findings offer a new understanding of the morphology-dependent catalytic mechanism and open a new avenue to control the reactivity of CeO₂-supported metal catalysts for CO₂ hydrogenation.