ACTIVITY AND STRUCTURE-SENSITIVITY OF THE WATER-GAS SHIFT REACTION OVER CU-ZN-AL MIXED-OXIDE CATALYSTS

The activity and structure-sensitivity of the water-gas shift (WGS) re action over Cu-Zn-Al mixed oxide catalysts were studied. Three sets of samples with different Cu/Zn and (Cu+Zn)/Al atomic ratios were prepar ed by coprecipitation. Depending on the cation ratio, the ternary hydr oxycarbonate precursors contained hydrotalcite, aurichalcite and/or ro sasite phases. The decomposed precursors contained CuO, ZnO, ZnAl2O4, and Al2O3. The relative proportion of these phases depended on both th e chemical composition of the sample and the calcination temperature e mployed for decomposing the precursor. After activation with hydrogen, samples were tested for the WGS reaction at 503 K. The turnover frequ ency of the eighteen samples tested was essentially the same (0.2-0.3 s(-1)) irrespective of changing the copper metal surface area between 3 and 35 m(2)/g Cu and the metallic copper dispersion between 0.5 and 5.0%. This indicated that the WGS reaction is a structure-insensitive reaction, as the specific reaction rate r(0) (mol CO/h/g Cu) is always proportional to the copper metal surface area. Preparation of mixed o xides with a high copper dispersion is therefore required for obtainin g more active catalysts. It was found that the value of the metallic c opper dispersion is related to the amount of hydrotalcite contained in the hydroxycarbonate precursor: the higher the hydrotalcite content i n the precursor, the higher the copper metal dispersion in the resulti ng catalyst and, as a consequence, the higher the catalyst activity. T ernary Cu/ZnO/Al2O3 catalysts exhibited a substantially faster WGS act ivity than binary Cu/ZnO catalysts. The addition of aluminium, althoug h inactive for the WGS reaction, is required for improving the catalys t performance.