Unraveling structure-functionality relationships of shape-defined Cu2O nanocrystal model catalysts for methanol decomposition

Abstract

Methanol is one of the centerpieces of the chemical industry as a C1 building block and an intermediate producing high-value chemicals such as formaldehyde, methyl methacrylate, methyl tertiary-butyl ether/ tertiary-amylmethylether, and acetic acid. The global demand for methanol is expected to grow exponentially due to its applications in hydrogen production, direct methanol fuel cells, and olefin production via the Methanol to Olefins (MTO) processes. Cu-based catalysts have been widely studied both in academia and in the industry to transform methanol into value-added chemicals at the industrial scale. Some of these academic fundamental research studies have been performed either under ultra-high vacuum (UHV), cryogenic temperature conditions utilizing single-crystal nanocatalysts or under industrially relevant high temperature-pressure conditions utilizing complex mesoporous catalysts resulting in complex data which is challenging to analyze in a conclusive manner to obtain reliable mechanistic information due to the presence of the well-known limitations in heterogenous catalysis called “the materials gap” and “ the pressure gap”. Thus, uniquely defined model catalysts are required to bridge these gaps by offering well-ordered surfaces that can be studied under ambient conditions. This thesis focuses on the structure-functionality relationships of shape-defined Cu2O model catalysts for methanol decomposition. Cubic and octahedral Cu2O nanocrystal catalysts were synthesized and characterized by various ex-situ methods such as Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), X-Ray Absorption Near Edge (XANES), Extended X-Ray Absorption Fine Structure (EXAFS), Attenuated Total Reflectance Infrared Spectroscopy (ATR-IR), X-Ray Photoelectron Spectroscopy (XPS) and H2-Temperature Programmed Reduction (H2-TPR). The nature of the surface-active sites were characterized by CO adsorption via in-situ Fourier Transform Infrared Spectroscopy (in-situ FTIR) and the morphology-dependent methanol and formaldehyde decomposition properties were studied via in-situ FTIR and Temperature Programmed Desorption (TPD). The results showed that c-Cu2O and o-Cu2O have distinct structure-functionality relationships for methanol decomposition. It is proposed that the labile surface oxygens that can be readily donated from the c-Cu2O surface can facilitate low-temperature (T ≤ 250 °C) methanol/methoxy oxidation to formates which in turn yield predominantly CO2 and H2O as the total oxidation products. In contrast, limited reducibility of the c-Cu2O surface only allows methanol/methoxy oxidation to first formaldehyde and then to dioxymethylene, eventually yielding predominantly CO and H2 as the thermal decomposition products, indicating the predominance of dehydrogenation catalytic pathways rather than total oxidation thus, unraveling the structure-functionality relationships of shape-defined Cu2O nanocrystal model catalysts for methanol decomposition.