Bulk bismuth molybdate mixed oxide catalysts have revolutionized the industrial production of acrolein and acrylonitrile from propylene. Cerium-rich bismuth molybdate (Bi0.5Ce1.5(MoO4)3) has been shown to be superior in catalytic activity, yield, productivity and service life to pure bismuth molybdates. Yet, in situ spectroscopic surface information critical to decipher the fundamental surface structure-performance relations of these complex mixed oxide catalysts is still missing from the literature. In the present work, a combination of bulk and surface sensitive in situ characterization techniques were combined to elucidate the surface-bulk dynamics under reaction conditions. Ex situ Aberration-Corrected Transmission Electron Microscopy (ACTEM) indicated the presence of a thin amorphous layer (1-2 nm) on the surface of the crystalline Bi0.5Ce1.5(MoO4)3. In situ high sensitivity low-energy ion scattering (HS-LEIS) spectroscopy revealed the elemental composition of the outermost surface layer (~0.3 nm) and enabled depth profiling the Bi0.5Ce1.5(MoO4)3 catalyst (~3 nm). The HS-LEIS analysis of the topmost layer demonstrated that the surface is enriched in Mo and Bi that are part of the amorphous overlayer. In situ near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) was used to monitor the elemental composition and cation oxidation states in the near-surface region (~1-3 nm) of the catalyst under reaction conditions. The surface region was dynamic since Mo underwent the Mo6+ ↔ Mo5+ redox cycle while Bi3+ and Ce3+ did not undergo redox during the selective oxidation of propylene. The pronounced surface enrichment of Mo and redox behavior of MoOx confirms that the surface Mo6+Ox sites are the catalytic active sites while the Bi3+ and Ce3+ sites are not active sites and only indirectly participate as ligands by moderating the MoOx redox cycle in the selective propylene oxidation reaction. The corresponding bulk-phase of the Bi0.5Ce1.5(MoO4)3 catalyst was monitored under reaction conditions with in situ Raman and XRD and was found to be stable during propylene oxidation even after 60 minutes under propylene in the absence of molecular O2. These new molecular level insights about the surface characteristics of the bulk Bi0.5Ce1.5(MoO4)3 mixed oxide catalyst during propylene oxidation reaction conditions shed new light on the origins of the superior performance of this system compared to traditional bismuth molybdate catalysts.

Active surface of cerium-rich bismuth molybdate mixed oxide catalyst during the selective oxidation of propylene

ACS Spring 2022

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