LI+ PROMOTION OF PD SIO2 - THE EFFECT ON HYDROGENATION, HYDROGENOLYSIS, AND METHANOL SYNTHESIS/

A novel but fairly simple approach has been used to elucidate the effe ct of Li+ promotion of 5 wt% Pd/SiO2 [(Li/Pd)(atomic) = 0, 1, 2, 4] by using a set of three distinct reactions (CO hydrogenation, isobutylen e hydrogenation, and ethane hydrogenolysis) in addition to H-2 TPD, CO chemisorption, XRD, and TEM. The results were used to indicate the de gree of metal surface blockage by the promoter and to better understan d how Li+ goes about affecting Pd catalysis, especially CO hydrogenati on which is known to be greatly modified. Li+ promotion decreased the strength of H-2 adsorption and enhanced CO adsorption on Pd/SiO2. Addi tionally, it significantly decreased the hydrogenation (in the presenc e of CO) and ethane hydrogenolysis activities of Pd/SiO2 relative to t hose of the unpromoted catalyst, with the activities decreasing monoto nically with increasing Li+ loading. Isobutylene hydrogenation results suggest Li+ blockage of some of the active Pd sites. Steady-state iso topic transient kinetic analysis (SSITKA) of isobutylene hydrogenation in the presence of CO indicates that surface coverage by CO was signi ficantly enhanced by Li+ promotion. The simultaneous enhancement and s uppression of CO and H-2 adsorption, respectively, coupled with active site blockage by Li+ resulted in olefin hydrogenation appearing to be have like a structure sensitive reaction. Ethane hydrogenolysis result s indicate a nonuniform distribution of the promoter on Pd as well as its probable dispersion on the surface of the support as well. Despite the fact that site blockage resulted in a decrease in the hydrogenati on and ethane hydrogenolysis activities of Li-Pd/SiO2 (Li/Pd = 1), an increase in the rate of methanol formation was observed. This increase in the methanol formation rate with low levels of Li+ promotion can b e attributed to an increase in the number of active sites or in the co verage of the active sites by the surface intermediates since TEM indi cated no change in Pd particle size distribution. Higher loadings of L i+ (Li/Pd greater than or equal to 2) resulted in a decrease in methan ol formation. (C) 1995 Academic Press, Inc.