Screening of Cu-Based Catalysts for Selective Methane to Methanol Conversion

Developing selective catalysts for methane conversion to value-added chemicals has been considered a promising approach for efficient utilization of abundant natural gas resources. Here, we present a descriptor-based screening for high methane conversion and methanol selectivity at the interfaces between metal oxide clusters and Cu2O/Cu­(111) support [MOx/Cu2O/Cu­(111), M = Mg, Fe, Co, Ni, Cu, Zn, Ti, Zr, Sn, and Ce] on exposure to the mixture of CH4/O2/H2O. The activation energy of direct methane to methanol conversion, *OH + *CH4 → *CH3OH + *H, and the ratio for adsorption energy of oxygen and water, Eads(O2)/Eads(H2O), are identified as the effective descriptors for methane conversion and methanol selectivity, respectively. By providing exceptional oxygen and hydroxyl sites with high-lying O 2p states, the MgO/Cu2O/Cu­(111) catalyst is found to be the most promising system among the systems studied, being able to provide the highest methane conversion with high methanol selectivity

Activation and Conversion of Methane to Syngas over ZrO₂/Cu(111) Catalysts near Room Temperature

Enzymatic systems achieve the catalytic conversion of methane at room temperature under mild conditions. In this study, varying thermodynamic and kinetic parameters, we show that the reforming of methane by water (MWR, CH4 + H2O → CO + 3H2) and the water–gas shift reaction (WGS, CO + H2O → H2 + CO2), two essential processes to integrate fossil fuels toward a H2 energy loop, can be achieved on ZrO2/Cu(111) catalysts near room temperature. Measurements of ambient-pressure X-ray photoelectron spectroscopy and mass spectrometry, combined with density functional calculations and kinetic Monte Carlo simulations, were used to study the behavior of the inverse oxide/metal catalysts. The superior performance is associated with a unique zirconia–copper interface, where multifunctional sites involving zirconium, oxygen, and copper work coordinatively to dissociate methane and water at 300 K and move forward the MWR and WGS processes

Catalytic Tandem CO₂–Ethane Reactions and Hydroformylation for C3 Oxygenate Production

The strategy of the tandem hydroformylation reaction for C3 oxygenate production from CO2 and ethane represents an opportunity to simultaneously upgrade greenhouse gas CO2 and the large-reserved shale gas into value-added liquid products. One of the challenges is how to tune and achieve the appropriate ethylene/CO/H2 ratios for the downstream hydroformylation. Herein, we analyze and identify the desired ethylene/CO/H2 ratios by considering different combinations of main and side reactions of CO2 and ethane, based on which the PtSn3/γ-Al2O3 catalyst was identified as promising to enable the catalytic tandem hydroformylation reaction. The combined studies of reactor evaluation, in situ and ex situ characterizations, and theoretical calculations revealed that the Pt cluster/SnOx interfacial structures dominated the simultaneous dehydrogenation and dry reforming of ethane, thereby allowing the coformation of ethylene, CO, and H2 that were subsequently converted into C3 oxygenates in the tandem hydroformylation reactor. The current work not only demonstrates the design principles of suitable catalysts for the tandem-reactor strategy but also highlights the utilization of CO2 and shale gas to produce value-added oxygenate products

Microscopic Investigation of H₂ Reduced CuO_<i>x</i>/Cu(111) and ZnO/CuO_<i>x</i>/Cu(111) Inverse Catalysts: STM, AP-XPS, and DFT Studies

Understanding the reduction mechanism of ZnO/CuOx interfaces by hydrogen is of great importance in advancing the performance of industrial catalysts used for CO and CO2 hydrogenation to oxygenates, the water-gas shift, and the reforming of methanol. Here, the reduction of pristine and ZnO-modified CuOx/Cu(111) by H2 was investigated using ambient-pressure scanning tunneling microscopy (AP-STM), ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), and density functional theory (DFT). The morphological changes and reaction rates seen for the reduction of CuOx/Cu(111) and ZnO/CuOx/Cu(111) are very different. On CuOx/Cu(111), perfect “44” and “29” structures displayed a very low reactivity toward H2 at room temperature. A long induction period associated with an autocatalytic process was observed to enable the reduction by the removal of chemisorbed nonlattice oxygen initially and lattice oxygen sequentially at the CuOx–Cu interface, which led to the formation of oxygen-deficient “5–7” hex and honeycomb structures. In the final stages of the reduction process, regions of residual oxygen species and metallic Cu were seen. The addition of ZnO particles to CuOx/Cu(111) opened additional reaction channels. On the ZnO sites, the dissociation of H2 was fast and H adatoms easily migrated to adjacent regions of copper oxide. This hydrogen spillover substantially enhanced the rate of oxygen removal, resulting in the rapid reduction of the copper oxide located in the periphery of the zinc oxide islands with no signs of the reduction of ZnO. The deposited ZnO completely modified the dynamics for H2 dissociation and hydrogen migration, providing an excellent source for CO2 hydrogenation processes on the inverse oxide/metal system

Highly Selective Methane to Methanol Conversion on Inverse SnO₂/Cu₂O/Cu(111) Catalysts: Unique Properties of SnO₂ Nanostructures and the Inhibition of the Direct Oxidative Combustion of Methane

Direct methane to methanol (CH4 → CH3OH) conversion in heterogeneous catalysis has been a long-standing challenge due to the difficulties in equalizing the activation of methane and protection of the methanol product at the same reaction conditions. Here, we report an inverse catalyst, consisting of small structures of SnO2 (0.5–1 nm in size) dispersed on Cu2O/Cu(111), for highly selective CH3OH production from CH4. This system was investigated by combining theoretical [density functional theory calculations (DFT) and kinetic Monte Carlo simulations (KMC)] and experimental methods [scanning tunneling microscopy (STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS)]. The DFT and AP-XPS studies showed that on SnO2/Cu2O/Cu(111), the conversion of CH4 by oxygen (O2) preferred complete combustion to carbon dioxide (CO2). The addition of water (H2O) enhanced the production of CH3OH to nearly 100% selectivity in KMC simulations. This trend was consistent with the results of AP-XPS. The presence of water in the reaction environment rendered an extremely high amount of methoxy species (*CH3O), a precursor for CH3OH production. The high CH3OH selectivity of SnO2/Cu2O/Cu(111) reflected the unique atomic and electronic structure of the supported SnO2 nanoparticles. As a result, the O2 adsorption and dissociation, and thus the full combustion of CH4 to CO2, were completely suppressed, while the H2O dissociative adsorption was still feasible, providing active hydroxyl species for a truly selective CH4 to CH3OH conversion

Selective Methane Oxidation to Methanol on ZnO/Cu₂O/Cu(111) Catalysts: Multiple Site-Dependent Behaviors

Because of the abundance of natural gas in our planet, a major goal is to achieve a direct methane-to-methanol conversion at medium to low temperatures using mixtures of methane and oxygen. Here, we report an efficient catalyst, ZnO/Cu2O/Cu­(111), for this process investigated using a combination of reactor testing, scanning tunneling microscopy, ambient-pressure X-ray photoemission spectroscopy, density functional calculations, and kinetic Monte Carlo simulations. The catalyst is capable of methane activation at room temperature and transforms mixtures of methane and oxygen to methanol at 450 K with a selectivity of ∼30%. This performance is not seen for other heterogeneous catalysts which usually require the addition of water to enable a significant conversion of methane to methanol. The unique coarse structure of the ZnO islands supported on a Cu2O/Cu­(111) substrate provides a collection of multiple centers that display different catalytic activity during the reaction. ZnO–Cu2O step sites are active centers for methanol synthesis when exposed to CH4 and O2 due to an effective O–O bond dissociation, which enables a methane-to-methanol conversion with a reasonable selectivity. Upon addition of water, the defected O-rich ZnO sites, introduced by Zn vacancies, show superior behavior toward methane conversion and enhance the overall methanol selectivity to over 80%. Thus, in this case, the surface sites involved in a direct CH4 → CH3OH conversion are different from those engaged in methanol formation without water. The identification of the site-dependent behavior of ZnO/Cu2O/Cu­(111) opens a design strategy for guiding efficient methane reformation with high methanol selectivity