5 years ago

Redox Dynamics of Rh Supported on ZrP2O7 and ZrO2 Analyzed by Time-Resolved In Situ Optical Spectroscopy

Redox Dynamics of Rh Supported on ZrP2O7 and ZrO2 Analyzed by Time-Resolved In Situ Optical Spectroscopy
Hiroshi Yoshida, Satoshi Hinokuma, Masato Machida, Riichiro Kakei, Haris Puspito Buwono, Masahiro Yamamoto
In situ time-resolved diffuse reflectance spectroscopy was first applied to supported Rh catalysts (0.4 wt % Rh/ZrO2 and Rh/ZrP2O7) under dynamic three-way catalysis conditions fluctuating between fuel-lean and fuel-rich gas atmospheres. The optical absorption at 650 nm was found to decrease upon lean-to-rich switching of the gas feed, which led to the reduction of Rh oxide (Rh3+) to metallic Rh (Rh0), followed by a reversible increase upon back switching rich-to-lean. The kinetic analysis suggested that the reduction of Rh3+ to Rh0 was faster than the reoxidation over Rh/ZrP2O7, whereas the reduction was comparable with or slower than the reoxidation over Rh/ZrO2. The activation energy of Rh/ZrP2O7 for the reduction, 13.6 kJ mol–1, was smaller than that for the oxidation, 48.7 kJ mol–1, which contrasted with those of Rh/ZrO2 (21.4 and 34.1 kJ mol–1, respectively). These results were closely associated with the higher NO reduction activity of Rh/ZrP2O7 than Rh/ZrO2 under a lean-gas atmosphere because Rh was more active in the metallic state than in the oxide state. Applying fast lean–rich perturbation of the gas feed with 1 s intervals led to an immediate and significant drop of the optical absorption intensity, suggesting that the reduction of Rh substantially penetrated to deeper layers under the surface. This study provided the first in situ evidence for the formation of active metallic Rh species under high-frequency lean–rich oscillations.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b05260

DOI: 10.1021/acs.jpcc.7b05260

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