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A novel but fairly simple approach has been used to elucidate the effect 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, isobutylene hydrogenation, and ethane hydrogenolysis) in addition to H2 TPD, CO chemisorption, XRD, and TEM. The results were used to indicate the degree of metal surface blockage by the promoter and to better understand how Li+ goes about affecting Pd catalysis, especially CO hydrogenation which is known to be greatly modified. Li+ promotion decreased the strength of H2 adsorption and enhanced CO adsorption on Pd/SiO2. Additionally, it significantly decreased the hydrogenation (in the presence of CO) and ethane hydrogenolysis activities of Pd/SiO2 relative to those of the unpromoted catalyst, with the activities decreasing monotonically with increasing Li+ loading. Isobutylene hydrogenation results suggest Li+ blockage of some of the active Pd sites. Steady-state isotopic transient kinetic analysis (SSITKA) of isobutylene hydrogenation in the presence of CO indicates that surface coverage by CO was significantly enhanced by Li+ promotion. The simultaneous enhancement and suppression of CO and H2 adsorption, respectively, coupled with active site blockage by Li+ resulted in olefin hydrogenation appearing to behave like a structure sensitive reaction. Ethane hydrogenolysis results 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 hydrogenation 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 be attributed to an increase in the number of active sites or in the coverage of the active sites by the surface intermediates since TEM indicated no change in Pd particle size distribution. Higher loadings of Li+ (Li/Pd ≥ 2) resulted in a decrease in methanol formation. |
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