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

Understanding the Surface Structure and Catalytic Activity of SnO_<i>x</i>/Au(111) Inverse Catalysts for CO₂ and H₂ Activation

Carbon dioxide hydrogenation is a promising approach for the reduction of greenhouse gas pollution via the production of fuels and high-value chemicals utilizing C1 chemistry. In this process, the activation of nonpolar molecules, CO2 and H2, at mild conditions is challenging. Herein, we report a well-defined inverse SnOx/Au­(111) catalyst that shows the ability to activate both CO2 and H2 at room temperature. Scanning tunneling microscopy (STM) and ambient pressure X-ray photoemission spectroscopy (AP-XPS) are combined to understand the surface structure, growth mode, chemical state, and activity of SnOx/Au­(111) surfaces. Nanostructures of SnOx at the sub-monolayer level were prepared by depositing Sn on Au(111) followed by O2 oxidation. For the as-prepared SnOx/Au­(111), two-dimensionally formed SnOx thin films on a Au(111) substrate were observed with STM of two different moieties, discernible based on their height: clusters (∼0.4 Å) and nanoparticles (NPs, 1–2.5 Å), which are assigned to Sn–Au alloys and SnOx, respectively, in corroboration with XPS analysis. Furthermore, SnOx/Au­(111) was annealed under UHV to test its thermal stability. Upon annealing at 400–600 K, a disappearance of SnOx NPs and reappearance of highly dispersed Sn clusters were clearly noticeable from the STM and XPS results, identifying the thermal decomposition of SnOx and subsequent formation of Sn–Au alloys on the surface due to the recombination of Sn clusters with Au. We investigated the reactivity of the SnOx/Au­(111) surfaces toward CH4, CO2, and H2. The SnOx/Au­(111) surfaces have excellent CO2 and H2 activation abilities even at room temperature with negligible reactivity for methane activation. Our AP-XPS results show that H2 can be activated on the SnOx NPs by the reduction to Sn. For CO2, the activation and further dissociation are identified by a reoxidation of Sn with newly formed Sn–O bonds and the formation of surface carbon. Therefore, we propose that SnOx is a potential catalyst or additive to achieve CO2 hydrogenation under mild conditions

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

Tuning the Placement of Pt “Single Atoms” on a Mixed CeO₂–TiO₂ Support

Defect sites on the oxide supports can be used to anchor and activate “single-atom” catalysts (SACs). By engineering the anchoring sites for supporting SACs, one can alter their electronic and atomic structures which, in turn, define their activity, selectivity, and stability for catalytic reactions. To create and tune unique sites for Pt SACs on CeO2 support, in this work, we synthesized a system consisting of CeO2 decorated on TiO2 nano-oxides for supporting the Pt SACs and investigated the effect of Pt weight loading. A combination of multiple structural characterization methods including diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) was employed to characterize the distribution of charge states of single atoms and evaluate the heterogeneity of their binding sites. We have found that the placement of Pt atoms can be tuned on a mixed oxide surface by changing the weight loading of Pt

Aliovalent Doping of CeO₂ Improves the Stability of Atomically Dispersed Pt

Atomically dispersed supported catalysts hold considerable promise as catalytic materials. The ability to employ and stabilize them against aggregation in complex process environments remains a key challenge to the elusive goal of 100% atom utilization in catalysis. Herein, using a Gd-doped ceria support for atomically dispersed surface Pt atoms, we establish how the combined effects of aliovalent doping and oxygen vacancy generation provide dynamic mechanisms that serve to enhance the stability of supported single-atom configurations. Using correlated, in situ X-ray absorption, photoelectron, and vibrational spectroscopy methods for the analysis of samples on the two types of support (with and without Gd doping), we establish that the Pt atoms are located proximal to Gd dopants, forming a speciation that serves to enhance the thermal stability of Pt atoms against aggregation

Microwave-Assisted Synthesis of Cu@IrO₂ Core-Shell Nanowires for Low-Temperature Methane Conversion

A facile microwave-assisted synthesis was developed for the tunable fabrication of a Cu@IrO2 core@shell nanowire motif. Experimental parameters, such as (i) the reaction time, (ii) the method of addition of the Ir precursor, (iii) the capping agent, (iv) the reducing agent, and (v) the capping agent-to-reducing agent ratio, were subsequently optimized. The viability of other methods based on the previously reported literature, such as refluxing, stirring, and physical sonication, was studied and compared with our optimized microwave-assisted protocol in creating our as-prepared materials. It should be noted that the magnitude of the IrO2 shell could be tailored based on varying the Cu:Ir ratio coupled with judicious variations in the amounts of the capping agent and the reducing agent. Structural characterization techniques, such as XRD, XPS, and HRTEM (including HRTEM-EDS), were used to analyze our Cu@IrO2 motifs. Specifically, the shell could be reliably tailored from sizes of 10, 8, 6, and 3.5 nm with corresponding Cu:Ir ratios of 10:1, 15:1, 20:1, and 25:1, respectively. Moreover, the structural integrity of the motifs was probed and found to have been maintained after not only heat treatment but also the post-methane conversion process, indicative of an intrinsically high stability. Both components within the CuO-IrO2 interface were able to activate methane at temperatures between 400 and 500 K with a reduction of the associated metal cations (Cu2+ → Cu1+; Ir4+ → Ir3+) and the deposition of CHx fragments on the surface, as clearly observed in the ambient-pressure XPS results. Thus, on the basis of their stability and chemical activity, these core-shell materials could be very useful for the catalytic conversion of methane into “higher-value” chemicals

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

Investigating the Elusive Nature of Atomic O from CO₂ Dissociation on Pd(111): The Role of Surface Hydrogen

CO2 dissociation is a key step in CO2 conversion reactions to produce value-added chemicals typically through hydrogenation. In many cases, the atomic O produced from CO2 dissociation can potentially block adsorption sites or change the oxidation state of the catalyst. Here, we used ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory (DFT) calculations to investigate the presence of surface species from the dissociation of CO2 on Pd(111). AP-XPS results show that CO2 was dissociated to produce adsorbed CO, but dissociated atomic O was not observed at room temperature. We were only able to observe atomic O when CO2 was introduced at 500 K. Further investigations of O-covered Pd(111) revealed that chemisorbed O could be easily removed by low pressures of CO and H2. Notably, the effect of H2 is quite prominent since it could react with chemisorbed O at a pressure as low as 2 × 10–9 Torr, and the presence of H2 at ambient pressure prevented CO2 dissociation. DFT calculations showed that in the presence of background H2, facile CO2 dissociation took place via the reverse water–gas shift (rWGS) reaction, which resulted in the formation of adsorbed CO and removal of O by H2. DFT also identified the possible variation of surface species on simultaneous exposure of CO2 and H2 over Pd(111) depending on temperature and pressure, which opens alternative opportunities to tune the CO2 hydrogenation catalysis by controlling the reaction conditions

Atomic Structural Origin of the High Methanol Selectivity over In₂O₃–Metal Interfaces: Metal–Support Interactions and the Formation of a InO_<i>x</i> Overlayer in Ru/In₂O₃ Catalysts during CO₂ Hydrogenation

CO2 hydrogenation to methanol is of great environmental and economic interest due to its potential to reduce carbon emissions and produce valuable chemicals in one single reaction. Compared with the unmodified traditional Cu/ZnO/Al2O3 catalyst, an indium oxide (In2O3)-based catalyst can double the methanol selectivity from 30–50 to 60–100%. It is worth noting that over catalysts involving various active metals dispersed on indium oxide (M/In2O3, M = Pd, Ni, Au, etc.), although the methanol yield is boosted, the selectivity remains similar to that of plain In2O3 despite the distinct chemical properties of the added metals. To investigate the phenomena behind this behavior, here we used RuO2/In2O3 as a test catalyst. The results of ambient pressure photoelectron spectroscopy, in situ X-ray absorption fine structure, and time-resolved X-ray diffraction indicate that the structure of the RuO2/In2O3 catalyst is highly dynamic in the presence of a reactive environment. Specifically, under CO2 hydrogenation conditions, Ru clusters facilitate the reduction of In2O3 to generate In2O3–x aggregates, which encapsulate the Ru systems in a migration driven by thermodynamics. In this way, the Ru0 sites for CH4 production are blocked while creating RuOx–In2O3–x interfacial sites with tunable metal–oxide interactions for selective methanol production. In an inverse oxide/metal configuration, indium oxide has properties not seen in its bulk phase that are useful for the binding and conversion of CO2. This work reveals the dynamic nature of In2O3-based catalysts, providing insights for a rational design of materials for the selective synthesis of methanol