Schematic drawing of the active site suggested by the calculations and Abberation-corrected HRTEM images of Cu particles in the conventionally prepared, most active Cu/ZnO/Al2O3 catalyst.
Catalysts employed in heterogeneous catalysis are complex materials, often consisting of a transition metal, a support and a promoter. One of the main stumbling blocks in the development of rational catalyst design strategies, whether guided by experimental observations or theory and computation, is that the complexity of the catalysts makes it difficult to characterize the active sites where the catalysis takes place. And without detailed knowledge of the active site on an atomic scale it is very difficult to provide guidance towards improvements. In a recently published article in Science, SLAC theorists at the SUNCAT Center for Interface Science and Catalysis teamed up with experimental researchers at the Fritz-Haber-Institute in Berlin/Germany in an effort to shed light on the structure of the active site of the industrial catalyst used to produce methanol.
Using a combination of experimental evidence from bulk- and surface-sensitive imaging methods together with density functional theory calculations the researchers showed that there is a crucial atomic structure motif of the Cu/ZnO/Al2O3 catalyst that is responsible for the high catalytic performance of the system. The active site is found to consist of Cu steps decorated with Zn atoms, all stabilized by a series of well-defined bulk defects. Creation of the bulk defects and their vicinity to zinc depends crucially on the catalyst preparation method. Thus, catalysts with the same Cu/Zn/Al ratio can vary significantly in their performance depending on their preparation history. Identification of the active site of the industrially employed methanol syntheses catalyst has been a challenge for more than a decade and will allow researchers to search for improved catalysts by modeling methanol synthesis on the atomic scale. Design strategies for improved methanol catalyst, based on density functional theory calculations, are currently under development at SLAC.