11 Search Results

Alkali oxides are typically used as promoters of heterogeneous catalysts for the water–gas shift (WGS; H₂O + CO → H₂ + CO₂) reaction. On Au(111), CsO_x exhibits diverse nanostructures at varying coverages, as revealed by scanning tunneling microscopy. Clusters of cesium oxide (Cs₂O₂) nucleate at elbow sites of the Au(111) herringbone when θ_Cs is less than 0.1 ML. Subsequently, these clusters transform into two-dimensional (2D) islands (Cs₂O, Cs₂O₂, CsO₂) as the cesium coverage increases (θ_Cs > 0.1 ML). Both types of CsO_x nanostructures enable the WGS process on Au(111). The highest activity was seen for the cesium oxide clusters whichmore » facilitated the partial dissociation of water and binding of CO. The CO_ads and OH_ads groups were not strongly bound and probably reacted to yield a short-lived HOCO intermediate that led to gaseous H₂ and CO₂. The 2D islands of CsO_x also enabled the WGS but their efficiency was reduced due to the formation of cesium hydroxide compounds (limiting mobility of OH groups) and the generation of CO₃ and C species (blocking of active centers). The fact that the performance of the CsO_x/Au(111) catalysts changed dramatically with variations in the chemical properties of the CsO_x nanostructures indicates that the alkali oxide was an integral part of the active phase, playing a central role in the activation and conversion of the reactants. To attach the label of “promoter” to CsO_x is a simplification that does not help in the design and optimization of catalysts for C1 chemistry. In conclusion, to achieve a rational design, one must consider the structural and chemical properties of the alkali oxide.« less

Understanding the reduction mechanism of ZnO/CuO_x interfaces by hydrogen is of great importance for advancing the performance of industrial catalysts for CO₂ hydrogenation to methanol. Here, the reduction of pristine and ZnO-modified CuO_x/Cu(111) by H₂ was investigated using ambient pressure scanning tunnelling 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 CuO_x/Cu(111) and ZnO/CuO_x/Cu(111) are very different. On CuO_x/Cu(111), perfect "44" and "29" structures displayed a very low reactivity towards H₂ at room temperature. A long induction period associated with an autocatalytic process was observedmore » to enable the reduction by the removal of chemisorbed non-lattice oxygen initially and lattice oxygen sequentially at the CuO_x-Cu interface, which led to 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 CuO_x/Cu(111) opened new 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 for 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.« less

Enzymatic systems achieve the catalytic conversion of methane at room temperature under mild conditions. Here, 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 ofmore » 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.« less

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. Here, to investigate the phenomena behindmore » this behavior, 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 In₂O_3–x aggregates, which encapsulate the Ru systems in a migration driven by thermodynamics. In this way, the Ru^O sites for CH₄ production are blocked while creating RuO_x–In₂O_3–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.« less

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), kinetic Monte Carlo simulations (KMC)] and experimental methods [scanning tunneling microscopy (STM), ambient-pressure X-ray photoelectron spectroscopy (AP-XPS)]. The DFT and AP-XPS studies showed that onmore » 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 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. Further, 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, was completely suppressed; while the H₂O dissociative adsorption was still feasible, providing active hydroxyl species for a truly selective CH₄ to CH₃OH conversion.« less

Understanding the surface chemistry of CH₃O species is essential for the production of methanol by CO₂ hydrogenation over Cu-based heterogeneous catalysts, as it facilitates the rational design of more efficient conversion processes. Recent research has identified inverse ZrO₂/Cu catalysts as highly active and selective systems for the transformation of CO₂ to methanol with a performance that can be better than that of commercial Cu/ZnO catalysts. Here, we employed synchrotron-based ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and calculations based on density functional theory (DFT) to understand the fate of CH₃O groups under dry and wet environments. AP-XPS spectra revealed that undermore » CO₂ hydrogenation conditions, formate and methoxy are two key intermediates to produce methanol. Furthermore, there are three different types of reactive sites on the surface: One is active for methoxy adsorption, which is stable and responsible for the methanol synthesis; Another one transforms CO₂ into CO; and a third one is active for CO₂ and methoxy dissociation, leading to C and methane formation. The theoretical calculations indicate that CH₃OH readily dissociates to CH₃O species following a highly exothermic (ΔE = -20.99 kcal/mol) and barrierless process. The water produced by the reverse water-gas shift reaction (CO₂ + H₂ → H₂O + CO) can prevent the decomposition of CH₃O species. We discovered that by introducing a tiny amount of water vapor (2 × 10^-6 Torr) into the reaction chamber, the energy barrier for the reaction CH₃O(ads) + H(ads) → CH₃OH(gas) is dramatically reduced. AP-XPS and computational modelling showed that water is quite capable of extracting adsorbed methoxy to form gaseous methanol. With this in mind, one could boost the methanol selectivity by adding appropriate amounts of water or steam, which is an inexpensive and feasible solution for industrial operations.« less

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. In this study, we report a well-defined inverse SnO_x/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 SnO_x/Au(111) surfaces. Nanostructures of SnO_x at the sub-monolayer level were preparedmore » by depositing Sn on Au(111) followed by O₂ oxidation. For the as-prepared SnO_x/Au(111), two-dimensionally formed SnO_x 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 SnO_x, respectively, in corroboration with XPS analysis. Furthermore, SnO_x/Au(111) was annealed under UHV to test its thermal stability. Upon annealing at 400–600 K, a disappearance of SnO_x NPs and reappearance of highly dispersed Sn clusters were clearly noticeable from the STM and XPS results, identifying the thermal decomposition of SnO_x 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 SnO_x/Au(111) surfaces toward CH₄, CO₂, and H₂. The SnO_x/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 SnO_x 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 SnO_x is a potential catalyst or additive to achieve CO₂ hydrogenation under mild conditions.« less

The conversion and utilization of carbon dioxide is a critical challenge for the control of greenhouse gas pollution and in the production of high value chemicals in C1 chemistry. ZrO₂/Cu(111) is an inverse oxide/metal catalyst that displays high activity and stability for the hydrogenation of CO₂ into methanol at 500-600 K. At elevated temperatures, ZrO₂ grows on a CuO_x/Cu(111) substrate forming islands of 10-12 nm in size and an average height of ~ 3 Å. Reaction with H₂ leads to the removal of the copper oxide producing ZrO₂/Cu(111) surfaces which are very active for the binding and dissociation of CO₂more » into CO and C. After exposing ZrO₂/Cu(111) to moderate or elevated pressures of a CO₂/H₂ mixture at 300 K, atomic C and minor amounts of CH_xO and CO_x are deposited on the catalyst surface. The adsorbed CH_xO and CO_x disappear upon heating above 400 K. The catalytic tests for CO₂ hydrogenation give CO as the main reaction product and CH₄ and CH₃OH as secondary products. The relative yields of methane and methanol change with time and track the amount of atomic C deposited on the active ZrO₂/Cu(111) surface. The formation of methane stops once the catalyst surface is saturated with C. Under steady-state conditions, ZrO₂/Cu(111) is a much better catalyst for methanol synthesis than ZnO/Cu(111). This trend reflects variations in the size of the oxide islands and in the strength of oxide-metal interactions. The use of an inverse oxide/metal configuration is an important synthetic tool when preparing active, selective, and stable catalysts for CO₂ hydrogenation.« less

To activate methane at low or medium temperatures is a difficult task and a pre-requisite for the conversion of this light alkane into high value chemicals. In this work, we report the preparation and characterizations of novel SnO_x/Cu₂O/Cu(111) interfaces that enable low-temperature methane activation. Scanning tunneling microscopy identified small, well-dispersed SnO_x nanoclusters on the Cu₂O/Cu(111) substrate with an average size of 8 Å, and such morphology was sustained up to 450 K in UHV annealing. Ambient pressure X-ray photoelectron spectroscopy showed that hydrocarbon species (CH_x groups), the product of methane activation, were formed on SnO_x/Cu₂O/Cu(111) at a temperature as lowmore » as 300 K. An essential role of the SnO_x–Cu₂O interface was evinced by the SnO_x coverage dependence. Systems with a small amount of tin oxide, 0.1–0.2 ML coverage, produced the highest concentration of adsorbed CH_x groups. Calculations based on density functional theory showed a drastic reduction in the activation barrier for C–H bond cleavage when going from Cu₂O/Cu(111) to SnO_x/Cu₂O/Cu(111). On the supported SnO_x, the dissociation of methane was highly exothermic (ΔE ~ –35 kcal mol^–1) and the calculated barrier for activation (~20 kcal mol^–1) could be overcome at 300–500 K, target temperatures for the conversion of methane to high value chemicals.« less

The discovery of new and versatile strategies for the immobilization of molecular water-oxidation catalysts (WOCs) is crucial for developing clean energy conversion devices [e.g., (photo)electrocatalytic cells for water splitting]. The traditional approach for surface attachment to transparent conductive oxides [e.g., fluorine doped tin oxide (FTO)] is via synthetic modification of the ligand architecture to incorporate functional groups such as carboxylic acids (-COOH) or phosphonates (-PO₃H₂) prior to immobilization. However, challenges arising from desorption and the cumbersome derivatizations steps have limited the scope and applications of surface-bound WOCs. Herein, we report the successful immobilization of underivatized Ru(II)-based WOCs (Ru–Cat1 = [Ru(tpy)more » (bpy) (H₂O)]²⁺ (tpy = 2,2':6'2"–terpyridine and bpy = 2,2;-bipyridine) and Ru–Cat2 = [Ru(Mebimpy) (bpy) (H₂O)]²⁺ (Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl) pyridine)) and the Ru(II) polypyridyl chromophore Ru–C3 = [Ru(bpy)₃]²⁺ onto a FTO plasma-grafted poly(acrylic acid) surface (PAA|FTO). Various characterization techniques such as attenuated total reflectance Fourier transform infrared spectroscopy, scanning electron microscopy, atomic force microscopy, and cyclic voltammetry measurements provide evidence for the plasma-induced grafted PAA|FTO film and immobilization. Surface stability and electrocatalytic properties of these new hybrid composite films upon cycling were investigated at different pH values. Immobilized Ru–Cat1 and Ru–Cat2 onto PAA|FTO displayed pH-dependent (RuIII/RuII) couples and onset potentials indicative of PCET (proton-coupled electron transfer) reactions. Based on cyclic voltammetry results and spectroscopic monitoring, the immobilized WOCs Ru–Cat1 and Ru–Cat2 exhibited a higher surface stability in neutral aqueous solutions relative to Ru–C3 upon electrochemical oxidation. Furthermore, we attribute the surface PCET and stability to the presence of a water ligand in the coordination sphere of immobilized Ru–Cat1 and Ru–Cat2 which can H-bond with negatively charged carboxylate groups of the cross-linked PAA brushes. Our findings demonstrate that the plasma-grafted polymeric network onto FTO offers a versatile platform to directly anchor unmodified homogeneous WOCs or chromophores for potential applications in solar-to-fuel energy conversion.« less