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

Abstract

Atomically precise cerium oxo clusters offer a platform to investigate structure−property relationships that are much more complex in the ill-defined bulk material cerium dioxide. We investigated the activity of the MCe 70 torus family (M = Cd, Ce, Co, Cu, Fe, Ni, and Zn), a family of discrete oxysulfate-based Ce 70 rings linked by monomeric cation units, for CO oxidation. CuCe 70 emerged as the best performing MCe 70 catalyst among those tested, prompting our exploration of the role of the interfacial unit on catalytic activity. Temperature-programmed reduction (TPR) studies of the catalysts indicated a lower temperature reduction in CuCe 70 as compared to CeCe 70 . In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicated that CuCe 70 exhibited a faster formation of Ce 3+ and contained CO bridging sites absent in CeCe 70 . Isothermal CO adsorption measurements demonstrated a greater uptake of CO by CuCe 70 as compared to CeCe 70 . The calculated energies for the formation of a single oxygen defect in the structure significantly decreased with the presence of Cu at the linkage site as opposed to Ce. This study revealed that atomic-level changes in the interfacial unit can change the reducibility, CO binding/uptake, and oxygen vacancy defect formation energetics in the MCe 70 family to thus tune their catalytic activity.

Solubility and Stability of Hexanuclear Ce(IV)–O Clusters

“…Due to the low-lying f-orbital, it is the only lanthanide with a stable +IV oxidation state. Ce­(IV) salts are used as oxidants in organic chemistry or in quantitative analysis (cerimetry), and the redox-couple Ce­(III/IV) is utilized in industrial applications, such as automotive exhaust emission control, where nanoparticles of ceria (CeO 2 ) act as an oxygen storage component in the three-way catalytic converter. , Cerium–oxo clusters or Ce–O clusters that resemble cerium dioxide present a fascinating platform for further research as these discrete crystallographically defined clusters enable more confident assignments of structure–activity relationships …”

“…Until today, various Ce–O clusters have been reported with variable numbers of metal cores, ranging from two ,,, to 13 cerium ions as well as some polynuclear Ce­(IV)–O clusters of higher order whose properties approach those of the parent oxide CeO 2 with an increasing number of metal cores. ,,− However, the majority of Ce­(IV)-O clusters exhibit a hexanuclear building unit in which the metal ions are coordinated by either eight or nine oxygen ions. Ce–O cluster materials reported so far have shown great potential in catalytic applications, including the oxidation of catechol, the photooxidation of 2-propanol or the conversion of CO to CO 2 . Apart from pure Ce–O clusters, some heterometallic clusters have also been described .…”

“…2,3 Cerium−oxo clusters or Ce−O clusters that resemble cerium dioxide present a fascinating platform for further research as these discrete crystallographically defined clusters enable more confident assignments of structure−activity relationships. 4 Ce−O clusters are usually crystallized from solutions of cerium nitrate and chelating molecules. Thus, the resulting Ce− O building units are known with different ligands, i.e., alkoxides, 5−9 carboxylates, 10−16 nitrates, 10−12 or carbonates.…”

“…Ce−O cluster materials reported so far have shown great potential in catalytic applications, including the oxidation of catechol, 10 the photooxidation of 2-propanol 21 or the conversion of CO to CO 2 . 4 Apart from pure Ce−O clusters, some heterometallic clusters have also been described. 14 Christou et al discovered various mixed-valence Ce X Mn X cluster compounds with interesting properties for use as single-molecule magnets.…”

Boosting the Ceramics with In Situ MOF-Derived Nanocarbons

“…Metal–organic frameworks are porous extended crystalline solids constructed by connecting metal ions or metal clusters with organic linkers through coordination bonds . The accessible surface area of MOFs can reach up to 8000 m 2 /g, surpassing the conventional materials such as zeolites and activated carbon. − Therefore, they are very promising materials for many applications including gas storage, − catalyst, − electrochemical applications, − and drug delivery, , to name a few. MOFs are also a suitable source for obtaining nanocarbon-based materials, particularly N-doped carbons .…”