Catalyst characterization and long-term stability

For our results the BET surface areas obtained for the CZC samples were considerably higher than those reported for ZnO supported Cu catalysts. As shown in Table 1, the values for the average pore diameter were very close for each sample. Although the Cu⁰ specific surface areas for the CZC materials proved to be lower than expected from the internal surface areas and the Cu contents, the values exhibited a similar trend as the BET results. However, an increasing Cu content resulted in a decreasing Cu^0 surface area roughly in agreement with an increasing Cu particle size. In addition, we found that the net decrease of the Cu⁰ specific surface area with increasing Cu content, observed for CZC15, CZC25 and CZC35, is in good agreement with earlier results reported in the literature for supported Cu catalysts.

According to the XRD pattern in Figure 1, the characteristic Cu⁰ peaks could be easily distinguished for samples with highest Cu contents as revealed. There were no Cu⁰ signals for CZC5 and CZC15 which was attributed in part as the result of lower Cu loadings. Furthermore, if the Cu crystallite size for the above samples is too small it will not be detected by XRD, as a result of pronounced line broadening. The broadening of the peaks also renders it difficult to establish whether the Zr, Ce and O atoms are arranged in one single mixed phase or in separate phases. The above reason has rendered it difficult to determine the shape of the corresponding crystal structures. The Cu crystallite sizes obtained from the diffraction peaks for CZC25 and CZC35 were 12.4 and 15.3 nm, respectively.