Nature and Dynamic Evolution of Rh Single Atoms Trapped by CeO2 in CO Hydrogenation

Abstract

Metal single-atom catalysts (SACs) have attracted extensive interest because of their maximum atom efficiency and potentially good performances. However, the understanding of the catalytic behavior and the stability of SACs in high-temperature catalytic reactions remains a major challenge, especially under reducing conditions. In this work, we investigated the nature and stability of Rh/CeO2 SACs with different Rh loadings (0.2–4 wt % Rh) during syngas conversion (260 °C, 2 MPa). Using CO-DRIFTS, we found that atomically dispersed Rh was dispersed on Rh/CeO2 catalysts with an Rh loading up to 3 wt %. The Rh/CeO2 catalysts were stable in H2 up to 350 °C, while Rh nanoclusters were formed in CO at 200 °C. During syngas conversion at 260 °C, the Rh single atoms (Rhiso) on a 0.5 wt % Rh/CeO2 SAC slowly agglomerated to form Rh nanoclusters (RhNC) and a Rhiso/RhNC ratio of ∼1:2.5 was obtained after ∼10 h time-on-stream run. Furthermore, the Rhiso/RhNC ratio did not change with further reaction, indicating that the reaction reached steady state after a 10 h run. After the catalysts reached steady state (∼10 h), the Rhiso/RhNC ratio on the spent Rh/CeO2 catalysts decreased with the increase in Rh loadings, while the reactivity increased first and then decreased. We observed an optimal value of Rhiso/RhNC ≈ 1:3 on the 1 wt % Rh/CeO2 catalyst, showing the maximum reaction rates of CO (rCO) and ethanol selectivity (SEtOH) of 85.8 μmol/s/gRh and 26.2%, respectively. This indicated that the combination of Rh single atoms and Rh nanoclusters is beneficial to the ethanol selectivity and reaction rate, which was confirmed by DFT simulations. Moreover, the ratios of Rhiso/RhNC on the spent catalysts under the steady state have good relationships with the CO conversion rate and product selectivity in the reaction.