14 Search Results

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

Abstract In industrial catalysis, alkali cations are frequently used to promote activity or selectivity. Scanning tunneling microscopy, ambient‐pressure X‐ray photoelectron spectroscopy, and density‐functional calculations were used to study the structure and reactivity of potassium oxides in contact with the Au(111) surface. Three different types of oxides (K ₂ O ₂ , K ₂ O and KO _y with y <0.5) were observed on top of the gold substrate at 300–525 K. Initially, small aggregates of K ₂ O ₂ /K ₂ O (1–2 nm in size) were seen at the elbows of the herringbone structure. After increasing the K coveragemore » (>0.15 ML), large islands of the oxide (20–40 nm in size) appeared. These islands contained a mixture of K ₂ O and KO _y ( y <0.5). A key correlation was found involving the structure, oxidation state, and chemical activity of the alkali oxide. The small aggregates of potassium oxide had a very high catalytic activity for the oxidation of CO, being much more than plain promoters.« less

Abstract In industrial catalysis, alkali cations are frequently used to promote activity or selectivity. Scanning tunneling microscopy, ambient‐pressure X‐ray photoelectron spectroscopy, and density‐functional calculations were used to study the structure and reactivity of potassium oxides in contact with the Au(111) surface. Three different types of oxides (K ₂ O ₂ , K ₂ O and KO _y with y <0.5) were observed on top of the gold substrate at 300–525 K. Initially, small aggregates of K ₂ O ₂ /K ₂ O (1–2 nm in size) were seen at the elbows of the herringbone structure. After increasing the K coveragemore » (>0.15 ML), large islands of the oxide (20–40 nm in size) appeared. These islands contained a mixture of K ₂ O and KO _y ( y <0.5). A key correlation was found involving the structure, oxidation state, and chemical activity of the alkali oxide. The small aggregates of potassium oxide had a very high catalytic activity for the oxidation of CO, being much more than plain promoters.« 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

The structures formed by the deposition of mass-selected niobium oxide clusters, Nb₃O_y (y = 5, 6, 7), onto Au(111) were studied by scanning tunneling microscopy. The as-deposited Nb₃O₇ clusters assemble into large dendritic structures that grow on the terraces as well as extend from the top and bottom of step edges. The Nb₃O₆ cluster also forms dendritic assemblies but they are generally much smaller in size. The assemblies are composed of smaller discrete structures (<1 nm) which are likely to be single clusters. Furthermore, the dendritic assemblies for both the Nb₃O₇ and Nb₃O₆ clusters have fractal dimensions of about 1.7more » which is very close to that expected for simple diffusion limited aggregation. Annealing the Nb₃O_7,6/Au(111) surfaces up to 550 K results in changes in assembly sizes and increases in heights, while heating to 700 results in the disruption of the assemblies into smaller structures. By contrast, the as-deposited Nb₃O₅/Au(111) surface at RT exhibits compact cluster structures which become 3D nanoparticles when annealed above 550 K. Differences in the observed surface structures and thermal stability are attributed to differences in metal-oxygen stoichiometry which can influence cluster binding energies, mobility and inter-cluster interactions.« 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

Inverse ZnO/Cu catalysts are key systems in the conversion of CO₂, a common atmospheric pollutant, into methanol, a high-value chemical and fuel. The chemistry of methanol and methoxy groups over inverse ZnO/Cu₂O/Cu(111) catalysts was investigated employing Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS), Scanning Tunneling Microscopy (STM) and calculations based on Density Functional Theory (DFT). The results of AP-XPS show that the adsorption of methanol on the binary oxide substrate at 300 K leads to formation of *CH₃O and *HCOO species with a minor amount of *CH_x. Furthermore, most of the methoxy groups disappeared from the surface after heating to 450more » K, the onset temperature for the formation of methanol during the hydrogenation of CO₂. The results of AP-XPS, STM and DFT point to preferential adsorption of methoxy on the ZnO regions of the binary oxide. On the supported ZnO or on a ZnO-Cu₂O interface, the breaking of the O-H bond in methanol is an exothermic process with a negligible (1-2 kcal/mol) or non-existent energy barrier depending on the size and shape of the ZnO islands. STM« less

The dissociative adsorption of methanol was investigated on Cu(111) and ultrathin Cu₂O films. We employed synchrotron-based Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS) and Scanning Tunneling Microscopy (STM) to study the dynamics of gas–solid interactions, and calculations based on Density Functional Theory (DFT) were used to examine the reaction path. C 1s XPS spectra revealed that methanol underwent dissociative adsorption on plain Cu(111) to form methoxy (CH₃O), formaldehyde (H₂CO), and formate (HCOO) at a pressure range of 0.5–10 mTorr, with these species remaining on the surface after evacuation. This was accompanied by the appearance of a low coverage (~0.05 ML) ofmore » O_ads in the O 1s which can be considered a highly active site for methanol activation. The high activity is apparent by a coverage of 0.8 ML of methoxy at room temperature. STM was unable to image these species at room temperature as they were highly mobile on metallic copper. In contrast, for CH₃OH on Cu₂O/Cu(111), STM showed clear hot spots for reaction and a complex array of adsorption structures. On the oxide substrate, there was decomposition of methanol to H₂CO, CH₃O, HCOO, and hydrocarbon species (CH_x) due to the subsequent interactions of methanol with lattice oxygen. Cu(111) remained entirely saturated with decomposition products under 10 mTorr of methanol (θ ≈ 0.97 ML), whereas the Cu₂O overlayer was saturated at a much lower coverage (θ ≈ 0.30 ML). STM revealed rows and step edges of Cu₂O decorated with decomposition products and metallic Cu islands ~5 nm in size. The difference in activity between Cu(111) and Cu₂O/Cu(111) is attributed to the significant amount of O present on the oxide surface. Additionally, Density Functional theory (DFT) calculations described the XPS measurements well, showing a likely methanol dissociation to *CH₃O and therefore a surface reduction. More importantly, the DFT results revealed that it was the chemisorbed oxygen on Cu₂O/Cu(111) which oxidized the dissociated *CH₃O to *HCOO and eventually CO₂, while the reaction only involving upper oxygen on the Cu₂O hexagonal ring led to the formation of H₂CO.« less

CO₂ hydrogenation to methanol is regarded as a promising reaction to catalytically convert a major greenhouse gas (CO₂) into a value-added product (methanol). In the current study, scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) were applied to investigate the growth mode of low coverages (<0.2 ML) of ZrO₂ in an inverse ZrO₂/CuOx/Cu(111) system, which has the potential to achieve a high selectivity for a direct CO₂ to methanol transformation. It was found that the morphology of ZrO₂ was strongly affected by the preparation method. The ZrO₂/CuO_x/Cu(111) model catalyst prepared by the oxidation at 600 K of Zr pre-depositedmore » on Cu(111) exhibited substantial mixing of ZrO₂ and CuO_x. In contrast, the direct deposition of Zr under an O₂ ambient over CuO_x/Cu(111) at 600 K produced small ZrO₂ islands (10-12 nm in size) with a two-dimensional structure (i.e. only one layer of ZrO₂). XPS studies indicate that both preparation methods lead to ZrO₂/CuO_x/Cu(111) surfaces. The model catalyst prepared by the direct deposition of Zr in O₂ was annealed up to 700 K in ultra-high vacuum. Both STM and XPS results suggest no apparent change in ZrO₂, while CuO_x was reduced at such annealing conditions. The island size of 10-12 nm observed for ZrO₂ on Cu(111) is much smaller than island sizes seen for CeO₂ (30-50 nm) and ZnO (300-500 nm) on the same substrate, opening the possibility for unique chemical properties« less