Robert M. Palomino

In Situ Investigation of Methane Dry Reforming on Metal/Ceria(111) Surfaces: Metal-Support Interactions and C−H Bond Activation at Low Temperature

Studies with a series of metal/ceria(111) (metal = Co, Ni, Cu; ceria = CeO2) surfaces indicate that metal–oxide interactions can play a very important role for the activation of methane and its reforming with CO2 at relatively low temperatures (600–700 K). Among the systems examined, Co/CeO2(111) exhibits the best performance and Cu/CeO2(111) has negligible activity. Experiments using ambient pressure X-ray photoelectron spectroscopy indicate that methane dissociates on Co/CeO2(111) at temperatures as low as 300 K—generating CHx and COx species on the catalyst surface. The results of density functional calculations show a reduction in the methane activation barrier from 1.07 eV on Co(0001) to 0.87 eV on Co/CeO2(111), and to only 0.05 eV on Co/CeO2@x(111). At 700 K, under methane dry reforming conditions, CO2 dissociates on the oxide surface and a catalytic cycle is established without coke deposition. A significant part of the CHx formed on the Co /CeO2@x(111) catalyst recombines to yield ethane or ethylene. Natural gas can transform the energy landscape of the world since it is a cheap and abundant fuel stock and a good source of carbon for the chemical industry. Methane (CH4) is the primary component of natural gas but is difficult to convert to upgraded fuels or chemicals because of the strength of the C@H bonds in the molecule (104 kcalmol@1) and its non-polar nature. Enabling low-temperature activation of CH4 is a major technological objective. It is known that enzymes, such as the CH4 monooxygenase, and some copperand zinc-based inorganic compounds can activate C@H bonds near room temperature. In recent studies, we found that a Ni/CeO2(111) system activates CH4 at room temperature as a consequence of metal–support interactions. The dry reforming of CH4 with CO2 (DRM; [Eq. (1)]): CH4 þ CO2 ! 2 COþ 2 H2 ð1Þ then takes place at a moderate temperature of about 700 K. Over this surface, Ni and O sites of ceria (CeO2) work in a cooperative manner during the dissociation of the first C@H bond in CH4. We pondered whether this useful phenomenon might be seen with other admetal/CeO2 combinations. Herein, we compare the behavior of Co, Ni, and Cu on CeO2(111) using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), kinetic testing, and theoretical calculations based on density functional theory. The deposition of small amounts of Co (< 0.3 ML) on a CeO2(111) film at 300 K produced a partial reduction of the oxide surface and adsorbed Co/CoOx species (Supporting Information, Figure S1). Upon annealing from 300 to 700 K, most of the Co transformed into Co (Supporting Information, Figure S2). This particular type of metal/oxide surface was exposed to CH4 at 300, 500, and 700 K. Figure 1 shows C 1s XPS spectra collected before and after exposing a Co/CeO2(111) surface to 1 Torr of CH4 at 300 K for 5 minutes. The strong peak near 285 eV is attributed to CHx groups formed by the partial dissociation of CH4 on the metal/oxide interface. 6] This peak was not seen when a pure CeO2(111) substrate was exposed to CH4 at 300 K. In Figure 1 there is a second strong peak near 289.5 eV. This likely corresponds to a COx species. [5, 6] Some of the CH4 molecules fully dissociated, producing C atoms that eventually reacted with oxygen atoms of the CeO2 to yield COx species. The intensity of the C 1s peak for the CHx species increased with Co coverage up to 0.15–0.2 ML, and then decreased at higher admetal coverages. Thus, small clusters of Co on CeO2 are the best for C@H bond activation. The dissociative adsorption of CH4 on the Co/CeO2(111) surface at room temperature did not induce a change in the oxidation state of Co or Ce. Such changes [*] Dr. Z. Liu, Dr. R. M. Palomino, Dr. D. C. Grinter, Dr. S. D. Senanayake,

In situ Investigation of Methane Dry Reforming on M-CeO2(111) {M= Co, Ni, Cu} Surfaces: Metal-Support Interactions and the activation of C-H bonds at Low Temperature

Studies with a series of metal/ceria(111) (metal=Co, Ni, Cu; ceria=CeO2 ) surfaces indicate that metal-oxide interactions can play a very important role for the activation of methane and its reforming with CO2 at relatively low temperatures (600-700 K). Among the systems examined, Co/CeO2 (111) exhibits the best performance and Cu/CeO2 (111) has negligible activity. Experiments using ambient pressure X-ray photoelectron spectroscopy indicate that methane dissociates on Co/CeO2 (111) at temperatures as low as 300 K-generating CHx and COx species on the catalyst surface. The results of density functional calculations show a reduction in the methane activation barrier from 1.07 eV on Co(0001) to 0.87 eV on Co2+ /CeO2 (111), and to only 0.05 eV on Co0 /CeO2-x (111). At 700 K, under methane dry reforming conditions, CO2 dissociates on the oxide surface and a catalytic cycle is established without coke deposition. A significant part of the CHx formed on the Co0 /CeO2-x (111) catalyst recombines to yield ethane or ethylene.

New In-Situ and Operando Facilities for Catalysis Science at NSLS-II: The Deployment of Real-Time, Chemical, and Structure-Sensitive X-ray Probes

The start of operations at the National Synchrotron Light Source II (NSLS-II) at Brookhaven National Laboratory heralded a new beginning for photon-science-based research capabilities in catalysis. This new facility builds on many years of pioneering work that was conducted at the NSLS synergistically by many scientists from academia, government labs, and industry. Over several decades, numerous discoveries in catalysis were driven through the emergence of an arsenal of tools at the NSLS that exploited the power of emerging X-ray methods encompassing scattering, spectroscopy, and imaging. In-situ and operando methodologies that coupled reactor environments directly with advanced analytical techniques paved a rapid path towards realizing an improved fundamental understanding at the frontiers of chemical science challenges of the day.

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.

Unexpected visible light driven photocatalytic activity without cocatalysts and sacrificial reagents from a (GaN)1–x(ZnO)x solid solution synthesized at high pressure over the entire composition range

Optical and photocatalytic properties were determined for the solid solution series (GaN)1–x(ZnO)x synthesized at high pressure over the entire compositional range (x = 0.07 to 0.9). We report for the first time photocatalytic H2 evolution activity from water for (GaN)1–x(ZnO)x without cocatalysts, pH modifiers and sacrificial reagents. Syntheses were carried out by reacting GaN and ZnO in appropriate amounts at temperatures ranging from 1150 to 1200 °C, and at a pressure of 1 GPa. ZnGa2O4 was observed as a second phase, with the amount decreasing from 12.8 wt% at x = 0.07 to ∼0.5 wt% at x = 0.9. The smallest band gap of 2.65 eV and the largest average photocatalytic H2 evolution rate of 2.31 μmol h−1 were observed at x = 0.51. Samples with x = 0.07, 0.24 and 0.76 have band gaps of 2.89 eV, 2.78 eV and 2.83 eV, and average hydrogen evolution rates of 1.8 μmol h−1, 0.55 μmol h−1 and 0.48 μmol h−1, respectively. The sample with x = 0.9 has a band gap of 2.82 eV, but did not evolve hydrogen. An extended photocatalytic test showed considerable reduction of activity over 20 hours.

Direct Conversion of Methane to Methanol on Ni-Ceria Surfaces: Metal–Support Interactions and Water-Enabled Catalytic Conversion by Site Blocking

The transformation of methane into methanol or higher alcohols at moderate temperature and pressure conditions is of great environmental interest and remains a challenge despite many efforts. Extended surfaces of metallic nickel are inactive for a direct CH4 → CH3OH conversion. This experimental and computational study provides clear evidence that low Ni loadings on a CeO2(111) support can perform a direct catalytic cycle for the generation of methanol at low temperature using oxygen and water as reactants, with a higher selectivity than ever reported for ceria-based catalysts. On the basis of ambient pressure X-ray photoemission spectroscopy and density functional theory calculations, we demonstrate that water plays a crucial role in blocking catalyst sites where methyl species could fully decompose, an essential factor for diminishing the production of CO and CO2, and in generating sites on which methoxy species and ultimately methanol can form. In addition to water-site blocking, one needs the effects of metal-support interactions to bind and activate methane and water. These findings should be considered when designing metal/oxide catalysts for converting methane to value-added chemicals and fuels.