Rh/C catalysts for methanol hydrocarbonylation - I. Catalyst characterisation

Commercially available activated carbon supports, and carbon supported rhod ium catalysts were characterised by BET, XPS, TPD, SEM, TEM and chemisorpti on measurements to elucidate the effect of the pore structure acid 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 m eso- and macropores were important for the mass transfer of the metal precu rsor within the support particle; the larger the pores the better the distr ibution of rhodium within the support particle. Second, the chemical compos ition of the carbon surface determined the amount of interaction of the rho dium species and the carbon surface: the pH influenced the attraction of th e species, and the oxygen containing surface groups acted as adsorption sit es for rhodium. During reduction. the thermal decomposition of the oxygen c ontaining surface groups was essential for dispersion. The thermally stable (CO evolving, weakly acidic, neutral or basic) surface: groups remained in tact, whereas the thermally unstable (CO2 evolving, acidic) surface groups decomposed inducing agglomeration of rhodium. Thus, it is not only the amou nt 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. Accord ingly, TEM provides a good measure for the particle size since it also acco unts for the hydrogen induced agglomeration, whereas hydrogen chemisorption only affords a less informative average value. (C) 2001 Elsevier Science B .V. All rights reserved.