Characterization of supported and promoted silver epoxidation catalysts using molecular probes.

摘要

Silver supported on low surface area alpha-Al2O3 and promoted with Cs or Cs plus Cl was studied using CO, NO, 1,3-butadiene, and 3,4-epoxy-l-butene as probe molecules, and employing diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), chemisorption measurements, temperature programmed desorption (TPD). Scanning electron microscopy (SEM) provided information about the Ag crystallites. The objective was to gain insight into the nature of adsorption sites, the effect of promoters, and the adsorption behavior of butadiene and epoxybutene in regard to the butadiene epoxidation reaction.; IR absorption bands for CO chemisorbed on reduced Ag/alpha-Al 2O3 catalysts were observed for the first time; however, CO adsorption occurred on only 3--7% of the total Ag surface at 300 K, and it depended on both the pretreatment and CO pressure utilized. After a I-HT pretreatment (calcination at 773 K followed by reduction at 673 K), cesium suboxides appeared to be formed, and CO adsorption on these Cs suboxides gave bands at 2028, 1950, and 1869 cm-1.; NO was used for the first time to probe these Ag epoxidation catalysts. Volumetric uptake measurements and DRIFTS results showed that Ag-O sites were the principal surface species formed after NO adsorption at 300 K on reduced, unpromoted Ag catalysts, and this was indicated by the formation of gas-phase N2O as well as a value of unity for the NO/O uptake ratio. On unpromoted, O-covered Ag catalysts, a band between 1396--1420 cm-1 was observed which was assigned to a nitrate species.; Adsorption of butadiene on supported Ag catalysts at 300 and 473 K using in situ DRIFTS was studied for the first time. Weakly adsorbed molecular butadiene was the dominant species after a 30-min exposure to butadiene for all samples regardless of the adsorption temperature. After purging in He, no IR absorption features remained with alumina, whereas for an unpromoted Ag catalyst after a HT pretreatment and for unsupported Ag powder, butadiene adsorbed at 300 K gave dominant by bands in two spectral regions, i.e., CH 2/CH3 stretching modes at 2978, 2927, and 2882 and a C=C=C stretching mode between 1994 and 2017 cm-1.; Epoxybutene adsorption was studied at 300 and 473 K using both DRIFTS and TPD, and a significant adsorption on alpha-Al2O3 gave bands at 1680, 1724, and 1646 cm-1 due to the C=O stretching mode of crotonaldehyde, the C=O stretching mode of acrolein, and the C=C stretching mode of crotonaldehyde and acrolein, respectively, in addition to bands in the CHx stretching region due to 2,5-dihydrofuran. The band assignments were based on a TPD study of EpB on alumina and crotonaldehyde adsorption on alumina, unsupported Ag, and supported Ag catalysts. (Abstract shortened by UMI.)