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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

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 CO₂more » 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.« less

Due to 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 heterogenousmore » 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 towards methane conversion and enhance the overall methanol selectivity to over 80%. Thus, in this case, the surface sites involved in a direct CH₄ → to CH₃OH conversion are different from those engaged in methanol formation without water. Furthermore, the identification of the site-dependent behavior of ZnO/Cu₂O/Cu(111) opens a design strategy for guiding efficient methane reforming with high methanol selectivity.« 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

The milling of Palladium acetate and CeO₂ under dry conditions results in robust, environmentally friendly catalysts with excellent methane oxidation activity. These catalysts show superior performance compared to those prepared by milling metallic Pd and outperform Pd/CeO₂ catalysts prepared by traditional incipient wetness technology. Morphological investigation by HRTEM, Raman and DRIFT spectroscopic analysis, in-situ synchrotron X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) characterization techniques, coupled with ambient pressure XPS analysis, have been used to deeply characterize the samples, and allowed to identify the presence of Pd⁰/Pd²⁺ species with different degrees of interaction with ceria (Ce³⁺/Ce⁴⁺). We find thatmore » these Pd species are likely generated by the mechanical and electronic interplay taking place over the ceria surface during milling and are indicated as responsible for the enhanced catalytic activity.« 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