Morphology Dependent Reactivity of CsOx Nanostructures on Au(111): Binding and Hydrogenation of CO2 to HCOOH.

Cesium oxide (CsOx) nanostructures grown on Au(111) behave as active centers for the CO2 binding and hydrogenation reactions. The morphology and reactivity of these CsOx systems were investigated as a function of alkali coverage using scanning tunneling microscopy (STM), ambient pressure X-ray photoelectron spectroscopy (AP-XPS), and density functional theory (DFT) calculations. STM results show that initially (0.05-0.10 ML) cesium oxide clusters (Cs2O2) grow at the elbow sites of the herringbone of Au(111), subsequently transforming into two-dimensional islands with increasing cesium coverage (>0.15 ML). XPS measurements reveal the presence of suboxidic (CsyO; y ≥ 2) species for the island structures. The higher coverages of cesium oxide nanostructures contain a lower O/Cs ratio, resulting in a stronger binding of CO2. Moreover, the O atoms in the CsyO structure undergo a rearrangement upon the adsorption of CO2 which is a reversible phenomenon. Under CO2 hydrogenation conditions, the small Cs2O2 clusters are hydroxylated, thereby preventing the adsorption of CO2. However, the hydroxylation of the higher coverages of CsyO did not prevent CO2 adsorption, and adsorbed CO2 transformed to HCOO species that eventually yield HCOOH. DFT calculations further confirm that the dissociated H2 attacks the C in the adsorbate to produce formate, which is both thermodynamically and kinetically favored during the CO2 reaction with hydroxylated CsyO. These results demonstrate that cesium oxide by itself is an excellent catalyst for CO2 hydrogenation that could produce formate, an important intermediate for the generation of value-added species. The role of the alkali oxide nanostructures as active centers, not merely as promoters, may have broad implications, wherein the alkali oxides can be considered in the design of materials tuned for specific applications in heterogeneous catalysis.

The Interaction of K and O2 on Au(111): Multiple Growth Modes of Potassium Oxide and Their Catalytic Activity for CO Oxidation.

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 (K2 O2 , K2 O and KOy with y<0.5) were observed on top of the gold substrate at 300-525 K. Initially, small aggregates of K2 O2 /K2 O (1-2 nm in size) were seen at the elbows of the herringbone structure. After increasing the K coverage (>0.15 ML), large islands of the oxide (20-40 nm in size) appeared. These islands contained a mixture of K2 O and KOy (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.

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.

Surface characterization and methane activation on SnOx/Cu2O/Cu(111) inverse oxide/metal catalysts.

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. Herein, we report the preparation and characterizations of novel SnOx/Cu2O/Cu(111) interfaces that enable low-temperature methane activation. Scanning tunneling microscopy identified small, well-dispersed SnOx 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 (CHx groups), the product of methane activation, were formed on SnOx/Cu2O/Cu(111) at a temperature as low as 300 K. An essential role of the SnOx-Cu2O interface was evinced by the SnOx coverage dependence. Systems with a small amount of tin oxide, 0.1-0.2 ML coverage, produced the highest concentration of adsorbed CHx 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 SnOx/Cu2O/Cu(111). On the supported SnOx, 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.

Surface structure of mass-selected niobium oxide nanoclusters on Au(111).

The structures formed by the deposition of mass-selected niobium oxide clusters, Nb3Oy(y = 5, 6, 7), onto Au(111) were studied by scanning tunneling microscopy. The as-deposited Nb3O7clusters assemble into large dendritic structures that grow on the terraces as well as extend from the top and bottom of step edges. The Nb3O6cluster 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. The dendritic assemblies for both the Nb3O7and Nb3O6clusters have fractal dimensions of about 1.7 which is very close to that expected for simple diffusion limited aggregation. Annealing the Nb3O7,6/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.

Water-promoted interfacial pathways in methane oxidation to methanol on a CeO2-Cu2O catalyst.

Highly selective oxidation of methane to methanol has long been challenging in catalysis. Here, we reveal key steps for the pro-motion of this reaction by water when tuning the selectivity of a well-defined CeO2/Cu2O/Cu(111) catalyst from carbon monoxide and carbon dioxide to methanol under a reaction environment with methane, oxygen, and water. Ambient-pressure x-ray photoelectron spectroscopy showed that water added to methane and oxygen led to surface methoxy groups and accelerated methanol production. These results were consistent with density functional theory calculations and kinetic Monte Carlo simulations, which showed that water preferentially dissociates over the active cerium ions at the CeO2-Cu2O/Cu(111) interface. The adsorbed hydroxyl species blocked O-O bond cleavage that would dehydrogenate methoxy groups to carbon monoxide and carbon dioxide, and it directly converted this species to methanol, while oxygen reoxidized the reduced surface. Water adsorption also displaced the produced methanol into the gas phase.

Morphology and chemical behavior of model CsOx/Cu2O/Cu(111) nanocatalysts for methanol synthesis: Reaction with CO2 and H2.

Cs is a promoter of Cu-based catalysts for the synthesis of alcohols from CO2 hydrogenation. Scanning tunneling microscopy and ambient-pressure x-ray photoelectron spectroscopy were used to study the morphology and chemical properties of surfaces generated by the deposition of cesium on Cu2O/Cu(111) and Cu(111) substrates. CsOx nanostructures were formed after Cs metal was deposited on Cu2O/Cu(111) at 300 K. The formed CsOx protrude over the surface of copper oxide by 2-4 Å, with the dimension at the base of the nanostructures being in the range of 1-3 nm. Heating to elevated temperature induced significant changes in the size and dispersion of the CsOx nanostructures, and there was a clear reconstruction of the copper oxide substrate, which then exhibited long range order with a hexagonally packed structure. The as-deposited and annealed surfaces of CsOx/Cu2O/Cu(111) were more reactive toward CO2 than plain Cu2O/Cu(111) or clean Cu(111). However, none of them were stable in the presence of H2, which fully reduced the copper oxide at 400-450 K. In CsOx/Cu(111), the CsOx nanoclusters were dispersed all over the metallic copper in no particular order. The CsOx species had an average width of 2 nm and ∼1 Å height. The CsOx/Cu(111) systems exhibited the highest activity for the binding and dissociation of CO2, suggesting that the CsOx-copper interface plays a key role in alcohol synthesis.

Growth and structural studies of In/Au(111) alloys and InOx/Au(111) inverse oxide/metal model catalysts.

Indium oxide has received attention as an exciting candidate for catalyzing the CO2 hydrogenation to methanol due to its high selectivity (>80%). Compared to the extent of research on the activity of indium oxide-based powder catalysts, very little is known about the phenomena associated with the formation of surface alloys involving indium or the growth mechanism for indium oxide nanoparticles. In this report, scanning tunneling microscopy and X-ray photoelectron spectroscopy (XPS) were employed to elucidate the growth mode, structure, and chemical state of In/Au(111) alloys and InOx/Au(111) inverse model catalysts. Our study reveals distinct morphological differences between In/Au(111) and InOx/Au(111), and the InOx structure also depends strongly on the preparation conditions. In/Au surface alloy systems with extremely low coverage (0.02 ML) form islands preferentially on the elbow sites of reconstructed Au(111) herringbone, regardless of hexagonally closed packed and face centered cubic stacking. At higher coverage (0.1 ML), the In islands expand over the herringbone in the ⟨110⟩ direction and create two dimensional domain structures over the entire surfaces. Moreover, this 2D domain structure is disturbed by temperature with high dispersion of indium atoms observed during the annealing process. Oxidation of the In/Au(111) surface alloys with O2 at 550 K produces InOx/Au(111) systems which contain various sizes of InOx aggregates (from 0.7 nm to 10 nm). On the other hand, InOx/Au(111) surfaces prepared by vapor deposition of In at 550 K in an O2 background exhibit highly dispersed and uniformly small InOx particles (∼1 nm). Both InOx systems were confirmed to be partially oxidized by XPS.

Imaging the ordering of a weakly adsorbed two-dimensional condensate: ambient-pressure microscopy and spectroscopy of CO2 molecules on rutile TiO2(110).

Disorder-Order transitions in a weakly adsorbed two-dimensional film have been identified for the first time using ambient-pressure scanning tunneling microscopy (AP-STM) and X-ray photoelectron spectroscopy (AP-XPS). As of late, great effort has been devoted to the capture, activation and conversion of carbon dioxide (CO2), a ubiquitous greenhouse gas and by-product of many chemical processes. The high stability and non-polar nature of CO2 leads to weak bonding with well-defined surfaces of metals and oxides. CO2 adsorbs molecularly on the rutile TiO2(110) surface with a low adsorption energy of ∼10 kcal mol-1. In spite of this weak binding, images of AP-STM show that a substantial amount of CO2 can reside on a TiO2(110) surface at room temperature forming two-dimensionally ordered films. We have employed microscopic imaging under in situ conditions, soft X-ray spectroscopy and theory to decipher the unique ordering behavior seen for CO2 on TiO2(110).

Hydrogenation of CO2 on ZnO/Cu(100) and ZnO/Cu(111) Catalysts: Role of Copper Structure and Metal-Oxide Interface in Methanol Synthesis.

The results of kinetic tests and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) show the important role played by a ZnO-copper interface in the generation of CO and the synthesis of methanol from CO2 hydrogenation. The deposition of nanoparticles of ZnO on Cu(100) and Cu(111), θoxi < 0.3 monolayer, produces highly active catalysts. The catalytic activity of these systems increases in the sequence: Cu(111) < Cu(100) < ZnO/Cu(111) < ZnO/Cu(100). The structure of the copper substrate influences the catalytic performance of a ZnO-copper interface. Furthermore, size and metal-oxide interactions affect the chemical and catalytic properties of the oxide making the supported nanoparticles different from bulk ZnO. The formation of a ZnO-copper interface favors the binding and conversion of CO2 into a formate intermediate that is stable on the catalyst surface up to temperatures above 500 K. Alloys of Zn with Cu(111) and Cu(100) were not stable at the elevated temperatures (500-600 K) used for the CO2 hydrogenation reaction. Reaction with CO2 oxidized the zinc, enhancing its stability over the copper substrates.

Enhanced hydrogenation activity and diastereomeric interactions of methyl pyruvate co-adsorbed with R-1-(1-naphthyl)ethylamine on Pd(111).

Unmodified racemic sites on heterogeneous chiral catalysts reduce their overall enantioselectivity, but this effect is mitigated in the Orito reaction (methyl pyruvate (MP) hydrogenation to methyl lactate) by an increased hydrogenation reactivity. Here, this effect is explored on a R-1-(1-naphthyl)ethylamine (NEA)-modified Pd(111) model catalyst where temperature-programmed desorption experiments reveal that NEA accelerates the rates of both MP hydrogenation and H/D exchange. NEA+MP docking complexes are imaged using scanning tunnelling microscopy supplemented by density functional theory calculations to allow the most stable docking complexes to be identified. The results show that diastereomeric interactions between NEA and MP occur predominantly by binding of the C=C of the enol tautomer of MP to the surface, while simultaneously optimizing C=O····H2N hydrogen-bonding interactions. The combination of chiral-NEA driven diastereomeric docking with a tautomeric preference enhances the hydrogenation activity since C=C bonds hydrogenate more easily than C=O bonds thus providing a rationale for the catalytic observations.

Chemisorptive enantioselectivity of chiral epoxides on tartaric-acid modified Pd(111): three-point bonding.

The chemisorption of two chiral molecules, propylene oxide and glycidol, is studied on tartaric-acid modified Pd(111) surfaces by using temperature-programmed desorption to measure adsorbate coverage. It is found that R-glycidol shows preferential enantioselective chemisorption on (S,S)-tartaric acid modified Pd(111) surfaces, while propylene oxide does not adsorb enantioselectively. The enantioselectivity of glycidol depends on the tartaric acid coverage, and is exhibited for low tartaric acid coverages indicating that the bitartrate phase is responsible for the chiral recognition. The lack of enantioselectivity when using propylene oxide as a chiral probe implies that the enantiospecific interaction between glycidol and bitartate species is due to hydrogen-bonding interactions of the -OH group of glycidol. Scanning tunneling microscopy images were collected for tartaric acid adsorbed on Pd(111) under the same experimental conditions as used for enantioselective experiments. When tartaric acid is dosed at room temperature and immediately cooled to 100 K for imaging, individual bitartrate molecules were found. Density functional theory (DFT) calculations show that bitartrate binds to Pd(111) through its carboxylate groups and the -OH groups are oriented along the long axis of the bitartrate molecule. An enantiospecific interaction is found between glycidol and bitartate species where R-glycidol binds more strongly than S-glycidol to (S,S)-bitartate species by simultaneously forming hydrogen bonds with both the hydroxyl and carboxylate groups, thereby providing three-point bonding.