HYDROGENATION OF CARBON-DIOXIDE OVER SILVER PROMOTED COPPER/ZIRCONIA CATALYSTS

Zirconia supported catalysts containing the IB metals copper and silver, either alone or in combination, were prepared by sequential precipitation and tested for methanol synthesis from CO2/H-2 in a continuous fixed bed microreactor at 1.7 MPa. The addition of 4.8 at.-% silver to binary copper/zirconia catalyst resulted in a distinct increase in the selectivity to methanol from 70 to over 80% for dried uncalcined samples, while the activity remained virtually unaffected. After calcination of the silver-promoted samples at 623 K in air, a marked loss in their activity was observed, while the selectivity remained above 80%. This behaviour could be due to the different decomposition behaviour of Ag2O in oxidizing and reducing atmospheres. In contrast, the activity and selectivity of the unpromoted Cu/ ZrO2 were only slightly influenced by calcination. Reduction and subsequent exposure of a silver promoted Cu/ZrO2 sample to nitrous oxide was investigated by means of X-ray photoelectron spectroscopy (XPS). Precipitated copper oxide and silver oxide were used as references. The XPS investigations indicate that N2O-titration does not provide reasonable metal surface areas for supported Cu/Ag catalysts as the stoichiometry of the oxygen adsorption on silver is ill defined.