Abstract Cu/ZnO/Al 2 O 3 catalysts are the industrial standard for methanol synthesis. Their high activity stems from the synergy between Cu and Zn, but their precise structure under CO 2 hydrogenation conditions remains unknown. Here we show, using operando transmission electron microscopy, that the formation of ZnO x overlayers and CuZn surface alloys on Cu surfaces can be reversible and is mediated by the operating temperature and the chemical potential of the gas phase. Lower temperatures and more oxidative conditions lead to thicker ZnO x overlayers. At elevated temperatures, the overlayer coverage opens, exposing Cu nanoparticle surfaces to the feed and enabling CO 2 activation. Furthermore, we show that CuZn alloys are transient species and are re-oxidized by H 2 O formed during the reaction. This implies that, in CO 2 hydrogenation conditions, CuZn and Cu–ZnO surface states may coexist and continuously convert into one another as the local chemical potential oscillates throughout steady-state operation. Maintaining this fluctuation might be critical to the lifetime and performance of the catalyst.
Boniface et al. (2026) studied this question.