ETHYLENE OXIDATION OVER SMALL SILVER CRYSTALLITES

Well-dispersed Ag/SiO2, Ag/η-Al2O3, and Ag/TiO2 catalysts were prepared and their activity and selectivity for ethylene oxide (EO) formation in the presence and absence of C2H4Cl2 (EDC) and CO2 were compared to those of a poorly dispersed Ag/α-Al2O3 catalyst. Under comparable reaction conditions, the smallest (3-4 nm) Ag crystallites had turnover frequencies (TOFs) that were over 100 times lower than those of the large crystallites on α-Al2O3. In the presence of 0.5 ppm EDC, selectivities over Ag/η-Al2O3 and Ag/TiO2 were near 10% compared to a value near 60% over the Ag/α-Al2O3 catalyst, and this was attributed to the presence of secondary oxidation reactions occurring on these reactive supports. In contrast, in the absence of EDC and CO2 at 523 K, selectivities of 17 and 55% were obtained over 4.4- and 7.6-nm Ag crystallites on SiO2, respectively, compared to 23% from the 1-μm crystallites on α-Al2O3. The addition of EDC at low levels of 150-200 ppb markedly increased the selectivity to over 60% for the Ag/α-Al2O3 catalyst whereas the selectivity remained essentially constant at 55% over the latter 1.06% Ag/SiO2 catalyst. The best SiO2-supported catalyst, 1.71% Ag/SiO2 with 13.3-nm crystallites, gave selectivities of 61-64% and a TOF for EO formation of 6.4 × 10-3 s-1 at 503 K in the presence of 500 ppb EDC compared to a selectivity of 61% and a TOF of 55 × 10-3 s-1 for the 18% Ag/α-Al2O3. Thus comparably high selectivities can be attained over both large and small Ag crystallites although the EDC levels for optimum values appear to differ, with larger crystallites requiring higher EDC concentrations. The lower TOF values on the Ag/SiO2 catalysts are compensated by their much higher dispersions, the these SiO2-supported catalysts also show much greater resistance to sintering. A comparison with literature results indicates that in the absence of moderators a 1000-fold decrease in TOF occurs as Ag crystallite size decreases to 3 nm from that in bulk samples (such as single crystals); however, structure sensitivity cannot be unequivocally claimed because of the unknown role of adsorbed oxygen on these small particles. © 1990.