Abstract
Molecular structures of the unpromoted and K2O-promoted supported WO3/Al2O3 catalysts were studied with in situ Raman and UV–vis spectroscopy. In situ Raman spectra revealed that supported 20% WO3/Al2O3 corresponds to near monolayer coverage of isolated and oligomeric surface WOx species on Al2O3. Above monolayer surface WOx coverage (21% WO3/Al2O3), crystalline WO3 nanoparticles are also present. The addition of K2O to the supported WO3/Al2O3 catalyst increased the concentration of isolated surface WOx species and did not form K2WO4 nanoparticles. The reducibility of the tungsten oxide structures depends on their structures (2D or 3D) and the K2O promoter. Their interaction with acidic CO2 and SO2 gases was also investigated. Adsorption of CO2 creates several surface carbonate species of varying acidity that were detected using a combination of in situ IR and mass spectroscopy. Adsorbed bicarbonate form on weakly basic surface sites on tungsten oxide monolayer WO3/Al2O3 catalyst as well as in the presence of low 2.5% K2O loading. At high 5% K2O loading, the presence of the strong surface basic sites yields adsorbed carbonates. After SO2 pretreatment, however, new strongly adsorbed sulfate appears on the surface that inhibits CO2 adsorption.
