Nanocrystalline Non-stoichiometric CeO_x on Pt Heterogeneous Catalysts for Selective Oxidation of Methanol to Methyl Formate

Due to the increasing energy and environmental crisis, the utilization of C1 chemistry has gained much attention as a reliable pathway for “green chemistry” over several decades. Among various intermediates in C1 chemistry, methyl formate is one of the key components to produce more than 50 chemical products in the field of industry. In general, most methyl formate is produced by the carbonylation and dehydrogenation reaction of methanol. Since these reactions proceed at elevated temperatures, the selectivity for methyl formate generation is inevitably low, producing unintended by-products such as carbon dioxide. In this regard, the catalytic methanol oxidation reaction (MOR) capable of forming methyl formate even at room temperature can be considered an efficient alternative route. However, because MOR can generate both carbon dioxide and methyl formate via full oxidation and partial oxidation respectively, it is essential to maximize the partial oxidation selectivity of MOR for achieving the desired component.<br/>Recently, the formation of heterogeneous catalysts with platinum group metals (PGMs) and reducible metal oxides has been attempted because they provide enhanced reaction rate and partial oxidation selectivity in MOR, which is derived from the strong metal-support interactions (SMSI). The charge transfer between PGM and metal oxide occurs by forming the interfaces and this electronic interaction at the interface is the origin of the SMSI. Especially, several studies already revealed that the metal-oxide interface plays an important role to accelerate MOR performance and partial oxidation selectivity is correlated with the concentration of the interface. However, it is still unclear how the oxidation state or chemical composition of constituent materials in heterogeneous catalysts correlates with partial oxidation selectivity and methyl formate generation.<br/>Here, to explore the effect of the oxidation states of support oxides on partial oxidation selectivity in MOR, we fabricate heterogeneous catalysts composed of nanocrystalline non-stoichiometric CeO_x nanowires and Pt film (CeO_x/Pt). The oxidation state of CeO_x nanowires was controlled with varying annealing atmospheres (vacuum, Ar, Air, O₂). Although there was no noticeable difference in the crystallinity of CeO_x nanowires depending on the annealing conditions, it was confirmed that the ratio of O/Ce and the amount of Ce (III) changed after the annealing process depending on the processing conditions. To evaluate the turnover frequency (TOF) and the selectivity to methyl formate generation toward MOR, a batch reactor experiment was carried out. We verified that all the CeO_x/Pt regardless of annealing conditions exhibit higher TOF and methyl formate selectivity compared to a Pt film used as a reference. Interestingly, although there is no TOF difference among the samples, CeO_x/Pt with vacuum treatment achieved almost 70% of significantly enhanced selectivity to methyl formate compared to the others. Further analysis revealed that a high fraction of the Ce (III) in CeO_x/Pt obtained with vacuum treatment boosted the catalytic selectivity. Our findings provide a new pathway to design heterogeneous catalysts with exceptional selectivity for achieving desired product by engineering the chemical composition in the metal oxide.