The dimethyl maleate hydrogenation activity of Cu, ZnO-X and physically mixed Cu+ZnO-X samples was systematically investigated to probe the essential role of ZnO in ester hydrogenation processes. Cu samples exhibited high C=C bond hydrogenation ability with dimethyl succinate as the main product. Comparatively, ZnO was inactive in hydrogenation due to its weak ability to dissociate hydrogen while the C=O group could be activated and adsorbed on the ZnO surface. Interestingly, physical mixing with ZnO significantly improved the C=O hydrogenation activity of Cu samples. The H2-TPD results reveal the origin of "Cu–ZnO synergy": hydrogen atoms formed on the copper surface can spill over to the ZnO surface and react with the adsorbed C=O group. [ABSTRACT FROM AUTHOR]