Correlation of Rh Particle Size with CO Chemisorption: Effect on the Catalytic Oxidation of MTBE

Abstract

This study was conducted to identify the correlation between the CO chemisorption (linear interaction and gem dicarbonyl) and the specific size of rhodium particles, and further to determine the influence of this relationship on the catalytic oxidation reaction of methyl tert-butyl ether (MTBE). During the synthesis, first, TiO2 was developed by the sol-gel method under acidic conditions. Second, Rh was deposited (1 wt %) by the incipient wetness impregnation method. Later, with the aim of controlling the particle size, the Rh/TiO2 materials were crystallized at different reduction conditions during 3 h heat treatment. The results obtained by TEM micrographs indicated that the average particle size varies between 1.0 and 8.1 nm, depending on the conditions of heat treatment. From the histogram analysis of each TEM micrograph, two correlations were made: (i) the gem-dicarbonyl interaction was typical of Rh particles ≤ 1.5 nm, and (ii) a linear interaction, bridged interaction and dentate interaction were observed in particles ≥ 1.6 nm. The gem-dicarbonyl interaction (particle size ≤ 1.5 nm) was the most active in the oxidation reaction of MTBE.

Catalytic Reduction of Esters over Zirconia-Supported Metal Catalysts

“…For instance, CO might chemisorb on Rh atoms primarily in three configurations: geminal dicarbonyl, linear monocarbonyl, and bridged carbonyl species, which depends on the oxidation state of the active site and the degree of dispersion of metal nanoparticles. 54 The results presented in this section are consistent with the work of Miura et al, 61 who studied the dehydrogenative coupling (reverse reaction of hydrogenolysis) of octanol over 1% mol metal/ZrO 2 catalysts at 170 °C under an Ar atmosphere. They demonstrate the self-esterification of octanol over Cu/ZrO 2 with a 68% yield, which differed with the low yields obtained for Co/ZrO 2 (1%), Ni/ZrO 2 (11%), Ru/ZrO 2 (9%), Pt/ZrO 2 (3%), and ZrO 2 (1%).…”

“…Such an assumption might not be valid for metals where CO has multiple adsorption configurations over the active site, which yields undercounting of metal active sites and overestimation of the intrinsic activity of the metals. For instance, CO might chemisorb on Rh atoms primarily in three configurations: geminal dicarbonyl, linear monocarbonyl, and bridged carbonyl species, which depends on the oxidation state of the active site and the degree of dispersion of metal nanoparticles . The results presented in this section are consistent with the work of Miura et al, who studied the dehydrogenative coupling (reverse reaction of hydrogenolysis) of octanol over 1% mol metal/ZrO 2 catalysts at 170 °C under an Ar atmosphere.…”