Bulk bismuth molybdate mixed oxide catalysts have found application in the industrial production of acrolein and acrylonitrile from oxidation/ammoxidation of propylene since the early 1960s, thereby, revolutionizing the textile manufacturing industry in a short period of time. Despite the impact of bismuth molybdate catalysts, operando 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, operando 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 operando 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 t 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.

Jih-Mirn Jehng

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