Chromium Oxides as Structural Modulators of Rhodium Dispersion on Ceria to Generate Active Sites for NO Reduction
Satoru Ikemoto, Satoshi Muratsugu, Takanori Koitaya, Mizuki Tada
Abstract
The role of Cr species on Cr- and Rh-incorporated ceria catalysts (Cr0.19Rh0.06CeOz), which exhibit reversible redox performances at a temperature less than 373 K, was investigated for NO reduction catalysis. The incorporation of the Cr species to Cr0.19Rh0.06CeOz produced dispersed Rh species, whereas only the Rh-incorporated ceria catalyst without Cr (Rh0.04CeOz) produced small Rh0 aggregation, as characterized by high-angle annular dark-field scanning transmission electron microscopy coupled with electron energy loss spectroscopy or energy-dispersive X-ray spectroscopy, X-ray absorption fine structure (XAFS), and X-ray photoelectron spectroscopy. Cr0.19Rh0.06CeOz with the dispersed Rh species mediated the efficient conversion of NO to N2 with CO at 473 K for up to 216 h and exhibited better catalytic performance than Rh0.04CeOz and Cr0.17CeOz. Kinetic analysis, in situ Fourier-transform infrared spectroscopy analysis, and in situ XAFS analysis revealed that the co-existence of Rh and Cr was essential for the NO reduction performance with CO on Cr0.19Rh0.06CeOz. Chromium oxides played a role not only to produce the highly dispersed Rh species, responsible for the co-activation of CO and NO, but also to contribute to the oxygen transfer from the cleaved NO to CO on the CeO2 surface, promoting the efficient NO reduction with CO at low temperature.