Successive Strong Electrostatic Adsorptions of [RhCl6](3-) on Tungstated-Ceria as an Original Approach to Preserve Rh Clusters From Sintering Under High-Temperature Reduction

Supported transition metal materials have been identified as ideal candidates for decades to catalyze various reactions of interest in the fields of hydrocarbon refining, energy, and environmental catalysis. As the resources in transition metals on Earth are known to be limited, it appears to be of utmost interest to improve the efficiency of the catalytic materials in achieving the best use of these transition metals, in other words, the highest metal accessibility when supported on oxide carriers. The present study demonstrates that the deposition of oxotungstates (with a W surface density of 1.3 W/nm(2)) coupled with the selective deposition of Rh on the CeO2 surface (via successive strong electrostatic adsorptions of an anionic [RhCl6](3-) precursor) significantly reduces Rh sintering after reduction at 600 degrees C under H-2 compared to a W-free CeO2 support. Oxotungstates are found to decrease the number of nucleation sites of CeO2 and, therefore, to act as spacers leading to the isolation of the nucleation sites. Oxotungstates may also act as a physical barrier preventing the sintering of the Rh metallic phase in the course of the reduction step at high temperature. Various morphologies of sub-nanometric Rh clusters exhibiting a limited atomicity (4-13 Rh atoms) were considered, and most were found to be consistent with the very low coordination numbers of 4.0 +/- 0.2 determined experimentally by EXAFS. The coordination numbers and the H-2 chemisorption data led the conclusion that the Rh sub-nanometric clusters must be at the maximum two to three Rh layers thick and not larger than five Rh atoms. The benzene hydrogenation turnover frequencies at 50 degrees C were found to be similar on both the sub-nanometric Rh clusters and the 2.5 nm Rh nanoparticles (0.12 +/- 0.3 s(-1)).