Rh/C catalysts for methanol hydrocarbonylation: I. Catalyst characterisation Original Research Article

Commercially available activated carbon supports, and carbon supported rhodium catalysts were characterised by BET, XPS, TPD, SEM, TEM and chemisorption measurements to elucidate the effect of the pore structure and chemical nature of the carbons on the Rh/C catalysts. During impregnation, both of the parameters had a significant effect on the Rh/C catalysts. First, the meso- and macropores were important for the mass transfer of the metal precursor within the support particle; the larger the pores the better the distribution of rhodium within the support particle. Second, the chemical composition of the carbon surface determined the amount of interaction of the rhodium species and the carbon surface; the pH influenced the attraction of the species, and the oxygen containing surface groups acted as adsorption sites for rhodium. During reduction, the thermal decomposition of the oxygen containing surface groups was essential for dispersion. The thermally stable (CO evolving, weakly acidic, neutral or basic) surface groups remained intact, whereas the thermally unstable (CO2 evolving, acidic) surface groups decomposed inducing agglomeration of rhodium. Thus, it is not only the amount of oxygen containing adsorption sites that affected dispersion but also their nature and stability. Evidently, the degree of agglomeration depends strongly both on the type of carbon and on the reduction conditions. Accordingly, TEM provides a good measure for the particle size since it also accounts for the hydrogen induced agglomeration, whereas hydrogen chemisorption only affords a less informative average value.