In situXRD and HRTEM studies on the evolution of the Cu/ZnO methanol synthesis catalyst during its reduction and re-oxidation

Abstract

During the formation of the CuZnO solid solutions, the foreign anions in anion-modified (a.m.-) oxides give rise to: (i) extended stacking faults of (002) ZnO lattice plane, which are occupied by copper ions in the form of small clusters; and (ii) vacant inner holes of the a.m.-ZnO crystal. The main part of copper ions in the clusters is reduced to Cu⁰ with hydrogen at 473 K. According to HRTEM studies, the reduction of Cu_0.08Zn_0.92O results in the formation of copper metal species of two types: (i) particles of 3–10 nm in size on the a.m.-ZnO surface; and (ii) small (no more than 3 nm in size) atomic copper metal clusters in defect voids of the a.m.-ZnO structure. The copper metal clusters are coherent inclusions in the bulk of the ZnO, and the large copper metal particles are epitaxially bonded to the surface of the ZnO matrix. Copper metal particles on the surface of a.m.-ZnO are reoxidized to Cu⁺² at 523 K in the helium flow containing 0.05 vol% oxygen, and they come back to the extended stacking faults. The copper metal clusters in the holes of a.m.-ZnO are inaccessible to the oxygen and are not reoxidized.