Catalysts for selective propane oxidation in the presence of carbon monoxide: Mechanistic aspects

Abstract

The competitive combustion of propane and carbon monoxide was investigated with regard to its selectivity for propane oxidation. Based on variations in the sol–gel synthesis and the composition, the TiCrOx catalysts were optimized. Specifically, the addition of small amounts of cerium led to a catalyst, which converted 89% propane at 375°C but no carbon monoxide. The origin of this excellent and unusual selectivity has been investigated. The method of choice was diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS), which allowed recording adsorbed species on the catalysts surface. Hopcalite was taken as reference catalyst due to its selectivity (although opposite to TiCrOx) for the combustion of carbon monoxide in the presence of hydrocarbons. The spectroscopic investigations showed that carbon monoxide is linearly adsorbed on Cu+ at lower temperatures and formed carbonates at higher temperatures, whereas propane formed no detectable adsorbed species on hopcalite. The TiCrOx catalyst formed formate species with carbon monoxide above 250°C and carboxylates with propane above 150°C. The mixture of propane and carbon monoxide led to the formation of both species. By the addition of oxygen to the gas feed, only propane formed adsorbates on the catalysts surface, whereas CO seemed not to interact with the surface. Pre-adsorbed oxygen probably inhibits the adsorption of carbon monoxide but is not necessary for propane activation. The formation of carboxylates from propane also takes place in the absence of molecular oxygen, pointing to a Mars-van-Krevelen-like mechanism, in which lattice oxygen is involved in the oxidation process. A physical mixture of TiO2 and Cr2O3 showed no catalytic activity, and spectroscopic investigations showed no adsorbates on the sole oxides. Since only anatase and eskolaite could be identified in the powder diffraction patterns of the catalysts, a third phase has to be responsible for the propane oxidation, most likely surface decorations or solid solutions of the metal ions.