The real active sites over Zn-Cr catalysts for direct synthesis of isobutanol from syngas: structure-activity relationship

A series of Zn-Cr oxides nanoparticles were prepared by a coprecipitation procedure. The structure of different catalysts was investigated by X-ray Absorption Fine Structure (XAFS), X-ray photoelectron spectroscopy (XPS), temperature programed reduction of hydrogen (H-2-TPR) and in situ infrared spectrum (in situ IR). Both EXAFS and XANES demonstrated the cation disorder distribution became more serious with decreasing annealing temperature and increasing Zn/Cr molar ratios. The cation distribution also affected the oxygen state on the surface over Zn-Cr spinel. The population of surface hydroxyl species increased with more serious cation disorder distribution and they facilitated the formate formation which was a significant intermediate C1 species for alcohol synthesis. This study was the first time to investigate the situation of cation distribution in Zn-Cr spinel by XAFS and related it to catalyst performance. The results revealed that the isobutanol productivity presented a linear relationship to the level of cation disorder distribution in Zn-Cr spinel, unambiguously revealing the real active sites and structure-activity relationship.