3. Results and discussion
3.1. Effect of combining Pd and Cu on CO2 hydrogenation
3.1.1. Effects of Pd and Cu loadings
To identify the effect of Cu and Pd loadings, two series of the amorphous BMY 7378 supported Pd–Cu bimetallic catalysts with different metal loadings, Pd(X)–Cu/SiO2 and Pd–Cu(Y)/SiO2, were tested for CO2 hydrogenation at 523 K and 4.1 MPa. This is the same condition as in our early work  for comparison. These bimetallic catalysts yielded only CO and CH3OH as carbon-containing products. Both CO2 conversion and products formation rates were practically constant from 5 h to 100 h on stream. Thus, the activity and selectivity of the bimetallic catalysts reported here were collected at 10–12 h on stream.
Fig. 1A illustrates the changes in the CO2 conversion and product formation rate over Pd–Cu(Y)/SiO2 catalysts as a function of the Cu/(Pd + Cu) atomic ratio. The ratio was varied by changing Cu loading from 0 to 50 wt%. The Pd loading was initially fixed at 5.7 wt% based on our previous work . CO2 conversion on monometallic Pd catalyst is 3%. Only a small amount of CH3OH was obtained in the product stream and Eubacteria catalyst yields CO selectively. The CO2 conversion increased gradually with increasing Cu loading and maximized at Cu/(Pd + Cu) ratio of 0.9. The formation rate of CO changed in a similar way; however, CH3OH formation rate maximized at slightly lower Cu/(Pd + Cu) atomic ratio, which is 0.75 (Cu loading 10 wt%). Therefore, 10 wt% Cu loading was employed hereafter in this work.