Mechanistic Investigation of Enhanced Catalytic Selectivity toward Alcohol Oxidation with Ce Oxysulfate Clusters

Abstract

Ceria-based materials have been highly desired in photocatalytic reactions due to their redox properties and strong oxygen storage and transfer ability. Herein, we report the structures of one CeCe 70 oxysulfate cluster and four MCe 70 clusters (M = Cu, Ni, Co, and Fe) with the same Ce 70 core. As noted, single-crystal X-ray diffraction confirmed the structures of CeCe 70 and the MCe 70 series, while Raman spectroscopy indicated an increase in oxygen defects upon the introduction of Cu and Fe ions. The clusters catalyzed the oxidation of 4-methoxybenzyl alcohol under ultraviolet light. CuCe 70 and FeCe 70 exhibited enhanced reactivity compared to CeCe 70 and improved aldehyde selectivity compared to control experiments. In comparison with their homogeneous congeners, the CeCe 70 /MCe 70 clusters altered the location of radical generation from the bulk solution to the clusters' surfaces. Mechanistic studies highlight the role of oxygen defects and specific transition metal introduction for efficient photocatalysis. The mechanistic pathway in this study provides insight into how to select or design a highly selective catalyst for photocatalysis.

Interfacial Unit-Dependent Catalytic Activity for CO Oxidation over Cerium Oxysulfate Cluster Assemblies

“…The family of MCe 70 torus clusters (M = Ce 3+/4+ , Cd 2+ , Co 2+ , Cu 2+ , Fe 2+/3+ , Ni 2+ , and Zn 2+ ) was synthesized according to published procedures. , Notably, the MCe 70 clusters feature the shared formula of repeating Ce 70 toroids as previously reported through single-crystal X-ray diffraction (SC-XRD): [Ce 70 (OH) 36 (O) 64 (SO 4 ) 60 (H 2 O) 10 ] 4– (see Table S1 for detailed formula listings) . The toroid clusters are linked through sulfate and monomeric cation-based interfacial units (Figure ).…”

“…28−30 Nitrogen physisorption measurements were performed at 77 K to measure the apparent surface areas of all clusters (Figure S3), which are in agreement with previously reported surface areas of MCe 70 clusters. 30 Following the characterization of the MCe 70 cluster family, we explored the clusters' performance in CO oxidation in a packed bed reactor across a selected temperature regime (Figures S5−S11). Additionally, we investigated CeO 2 (commercially obtained) to contextualize the conversions of the MCe 70 materials (Figure S12).…”

“…CeCe 70 , CoCe 70 , CuCe 70 , FeCe 70 , and NiCe 70 were synthesized according to published procedures for microcrystalline samples. 30 CdCe 70 was synthesized according to published procedure for CdCe 70 �standard, yet was left to sit undisturbed for 5 days. 28 ZnCe 70 was synthesized according to modifications from published procedure and is detailed below.…”

“…Recent work from the Farha and Nyman teams further expanded the series to include a Ce 70 torus structure interconnected by Ce monomers and examined the photocatalytic and radical scavenging character of these clusters. 30 To date, most discrete Ce-oxo species have been studied in photocatalytic settings, given that the monodentate alkyl or aromatic capping agents present in other reported Ce-oxo clusters limit their thermal stability. 24,31 However, the limited reports of utilizing discrete Ce-oxo clusters for thermal-based catalysis starkly contrasts the substantial volume of studies investigating bulk ceria as a catalyst and/or catalyst support in applications such as CO oxidation or CO 2 hydrogenation.…”

“…Additionally, work from the Nyman group expanded the metal oxysulfate torous motif, first reported in U and Zr, , to Ce while providing insights to their assembly. , In these crystallographically defined systems, every Ce atom is exposed to the surface in a donut-like structure (Figure ) with a range of monomeric metal linkage moieties interconnecting the clusters to form one dimensional (1D) channels. Recent work from the Farha and Nyman teams further expanded the series to include a Ce 70 torus structure interconnected by Ce monomers and examined the photocatalytic and radical scavenging character of these clusters . To date, most discrete Ce-oxo species have been studied in photocatalytic settings, given that the monodentate alkyl or aromatic capping agents present in other reported Ce-oxo clusters limit their thermal stability. , However, the limited reports of utilizing discrete Ce-oxo clusters for thermal-based catalysis starkly contrasts the substantial volume of studies investigating bulk ceria as a catalyst and/or catalyst support in applications such as CO oxidation or CO 2 hydrogenation. , Thus, the Ce 70 torus family, which instead features capping sulfates, is more amenable to interrogate as catalysts for more thermally demanding conditions.…”