performance was evaluated from the data collected between the first 3 and 20 h of reaction. Experiments with CO 2 /H 2 (HC) were performed at 523, 533 and 543 K; the temperature was increased consecutively after periods of 20 h of reaction at each temperature.After each temperature change the system was stabilised for 3 h, then the sample corresponding to the initial activity of the catalyst at a given temperature was taken. The CO 2 /H 2 (HC)feed contained 50 ppm of methane and 20 ppm of ethane. The reduction and reaction were carried out at a total flow rate of 9 l h-1 except for the catalytic tests at 5 MPa where theflow employed was 7.8 l h-1. The gaseous products were analysed using a suitable gas chromatograph (Varian Star) equipped with TCD and FID detectors connected on-line to the reactor. Results and discussion

Catalyst

characterization

Catalysts

were

characterized by TEM after reduction and after catalytic tests at increasing temperatures (523—543 K) with clean and hydrocarbon-containing CO2/H2 mixtures. For Cu-Zn-Ga/SiO2 catalyst before reaction, two different types of particles were visible on the amorphous hydrophobic silica, with mean particle sizes centered around either 30 or 6—8 nm. Fig .1 沈阳化工学院学士学位论文 附录 56 Fig. 1 shows high-resolution images of this sample.Electron di.raction analyses carried out on both types of particles determined that larger particles are metallic

copper whereas particles with size centered at 6—8 nm correspond to ZnO. It is interesting to note the simultaneous occurrence of both crystalline phases: no isolated copper particles were present, they were always accompanied by ZnO particles. On the other hand, EDX microanalysis performed over areas of the catalyst containing both Cu and ZnO particles always indicated the presence of gallium. However, no evidence for the presence of isolated gallium-bearing phases was found.Moreover, the exact localization of gallium was not possible by TEM techniques although several attempts were carried out using narrow electron beams in EDX transverse experiments.It is likely that gallium is extremely finely dispersed in the sample. After catalytic reaction under CO2/H2or CO2/H2 (HC),a slight sintering of copper-based particles (mean crystallite size of ca. 38 nm) was observed. In both cases, besides metallic copper, a new copper-based phase was identified: the tetragonal mixed oxide Cu 4 O 3. On the other hand, ZnO crystallites remained well-dispersed with an unchanged size centered at 6—8 nm. Table 2 Sample Ga 3d 5/2 a A’ Cu/Zn Ga/Zn Reduced, 573 Ka 19.4 1851.6 0.071 Post-reaction, 543 Ka CO2/H2 18.9 1849.6 Post-reaction, 543 K CO2/H2(HC) 19.0 1849.2 Sample Ga 3d5@2 aA{ Cu/Zn Ga/Zn Fig. 2 shows highresolution images of the catalyst after reaction under CO Both and ZnO crystallites are identified and selected-area