Resolving the Oxygen Species on Ozone Activated AgAu Alloy Catalysts for Oxidative Methanol Coupling

Bimetallic alloy catalysts frequently demonstrate distinct performances that are superior to their monometallic counterparts, yet their surface chemistry needs to be carefully studied to understand their structure-activity relationships. The nanoporous Ag0.03Au0.97 alloy catalyst becomes highly active and selective for oxidative methanol coupling to methyl formate after O3 activation. HS-LEIS reveals the O3 treatment results in enrichment of Ag (>30%) on the outermost surface layer, while oxygen treatment additionally leads to segregation of a larger portion of Cu impurity on the surface. A series of characteristic Raman bands at 395, 577, 867, and 904 cm-1 only form under oxidative methanol coupling reaction on O3-activated AgAu catalyst. These bands correspond to Ag3-O* (395 cm-1), M-O* on O-Au(111) and AgAu alloy (577 cm-1), CH3OH* (867 cm-1), and HOOH* (904 cm-1), as revealed by DFT calculations. The cyclic in situ Raman and reactivity studies indicate the detected oxygen species could be related to a "memory effect" of the catalyst upon pretreatment. The current study highlights the importance of applying surface-specific techniques for investigation of compositions of outermost surface layers of alloy catalysts, as well as integration of in situ spectroscopies and computational investigations for understanding surface structures at the molecular level under reaction conditions.