(673h) A Paradigm Shift in the Nature of the Active Surface of the Bismuth Molybdate Catalyst during the Selective Oxidation of Propylene

Bulk bismuth molybdate mixed oxide catalysts have revolutionized the industrial production of acrolein and acrylonitrile from propylene. Despite their impact, in situ spectroscopic surface information vital to decipher the fundamental surface structure-performance relations of these catalysts is still missing from the literature. In the present work, in situ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is used to monitor the elemental composition and oxidation states in the near-surface region (~1-3 nm) of the α-Bi₂(MoO₄)₃ catalyst under propylene oxidation reaction conditions. The in situ NAP-XPS measurements are complemented with in situ high sensitivity low-energy ion scattering spectroscopy (HS-LEIS) to determine the elemental composition of the outermost surface layer (~0.3 nm) and depth profiling down to ~1.2 nm of the α-Bi₂(MoO₄)₃ catalyst. The corresponding bulk-phase of the α-Bi₂(MoO₄)₃ catalyst under reaction conditions is monitored with in situ Raman and XRD. The bulk phase of the α-Bi₂(MoO₄)₃ catalyst is stable during propylene oxidation and even after 60 minutes of propylene in the absence of molecular O₂. In contrast, the surface region is dynamic since Mo undergoes the Mo⁶⁺ ↔ Mo⁵⁺ redox cycle while Bi³⁺ is always present and does not undergo redox during the selective oxidation of propylene. Furthermore, the topmost layer of α-Bi₂(MoO₄)₃ is significantly surface enriched in Mo. The pronounced surface enrichment of Mo and redox behavior of MoO_x suggests that the surface Mo⁶⁺O_x sites are the catalytic active sites while the Bi³⁺ sites are not active sites and only indirectly participate as ligands that moderate the MoO_x redox cycle in the selective oxidation of propylene. These new molecular level insights about the surface characteristics of α-Bi₂(MoO₄)₃ catalysts during propylene oxidation reaction conditions are creating a paradigm shift of the previously proposed models for propylene oxidation by α-Bi₂(MoO₄)₃ catalysts that are based on the bulk phase of bismuth molybdate.