David C. Grinter

Structure of a model TiO2 photocatalytic interface

The interaction of water with TiO2 is crucial to many of its practical applications, including photocatalytic water splitting. Following the first demonstration of this phenomenon 40 years ago there have been numerous studies of the rutile single-crystal TiO2(110) interface with water. This has provided an atomic-level understanding of the water-TiO2 interaction. However, nearly all of the previous studies of water/TiO2 interfaces involve water in the vapour phase. Here, we explore the interfacial structure between liquid water and a rutile TiO2(110) surface pre-characterized at the atomic level. Scanning tunnelling microscopy and surface X-ray diffraction are used to determine the structure, which is comprised of an ordered array of hydroxyl molecules with molecular water in the second layer. Static and dynamic density functional theory calculations suggest that a possible mechanism for formation of the hydroxyl overlayer involves the mixed adsorption of O2 and H2O on a partially defected surface. The quantitative structural properties derived here provide a basis with which to explore the atomistic properties and hence mechanisms involved in TiO2 photocatalysis.

Dry Reforming of Methane on a Highly-Active Ni-CeO2 Catalyst: Effects of Metal-Support Interactions on C-H Bond Breaking.

Ni-CeO2 is a highly efficient, stable and non-expensive catalyst for methane dry reforming at relative low temperatures (700 K). The active phase of the catalyst consists of small nanoparticles of nickel dispersed on partially reduced ceria. Experiments of ambient pressure XPS indicate that methane dissociates on Ni/CeO2 at temperatures as low as 300 K, generating CHx and COx species on the surface of the catalyst. Strong metal-support interactions activate Ni for the dissociation of methane. The results of density-functional calculations show a drop in the effective barrier for methane activation from 0.9 eV on Ni(111) to only 0.15 eV on Ni/CeO2-x (111). At 700 K, under methane dry reforming conditions, no signals for adsorbed CHx or C species are detected in the C 1s XPS region. The reforming of methane proceeds in a clean and efficient way.

Inverse Oxide/Metal Catalysts in Fundamental Studies and Practical Applications: A Perspective of Recent Developments

Inverse oxide/metal catalysts have shown to be excellent systems for studying the role of the oxide and oxide-metal interface in catalytic reactions. These systems can have special structural and catalytic properties due to strong oxide-metal interactions difficult to attain when depositing a metal on a regular oxide support. Oxide phases that are not seen or are metastable in a bulk oxide can become stable in an oxide/metal system opening the possibility for new chemical properties. Using these systems, it has been possible to explore fundamental properties of the metal-oxide interface (composition, structure, electronic state), which determine catalytic performance in the oxidation of CO, the water-gas shift and the hydrogenation of CO2 to methanol. Recently, there has been a significant advance in the preparation of oxide/metal catalysts for technical or industrial applications. One goal is to identify methods able to control in a precise way the size of the deposited oxide particles and their structure on the metal substrate.

Ambient pressure XPS and IRRAS investigation of ethanol steam reforming on Ni–CeO2(111) catalysts: an in situ study of C–C and O–H bond scission

Ambient-Pressure X-ray Photoelectron Spectroscopy (AP-XPS) and Infrared Reflection Absorption Spectroscopy (AP-IRRAS) have been used to elucidate the active sites and mechanistic steps associated with the ethanol steam reforming reaction (ESR) over Ni-CeO2(111) model catalysts. Our results reveal that surface layers of the ceria substrate are both highly reduced and hydroxylated under reaction conditions while the small supported Ni nanoparticles are present as Ni(0)/NixC. A multifunctional, synergistic role is highlighted in which Ni, CeOx and the interface provide an ensemble effect in the active chemistry that leads to H2. Ni(0) is the active phase leading to both C-C and C-H bond cleavage in ethanol and it is also responsible for carbon accumulation. On the other hand, CeOx is important for the deprotonation of ethanol/water to ethoxy and OH intermediates. The active state of CeOx is a Ce(3+)(OH)x compound that results from extensive reduction by ethanol and the efficient dissociation of water. Additionally, we gain an important insight into the stability and selectivity of the catalyst by its effective water dissociation, where the accumulation of surface carbon can be mitigated by the increased presence of surface OH groups. The co-existence and cooperative interplay of Ni(0) and Ce(3+)(OH)x through a metal-support interaction facilitate oxygen transfer, activation of ethanol/water as well as the removal of coke.

Ordered Carboxylates on TiO2(110) Formed at Aqueous Interfaces

As models for probing the interactions between TiO2 surfaces and the dye molecules employed in dye-sensitized solar cells, carboxylic acids are an important class of molecules. In this work, we present a scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) study of three small carboxylic acids (formic, acetic, and benzoic) that were reacted with the TiO2(110) surface via a dipping procedure. The three molecules display quite different adsorption behavior, illustrating the different interadsorbate interactions that can occur. After exposure to a 10 mM solution, formic acid forms a rather disordered formate overlayer with two distinct binding geometries. Acetic acid forms a well-ordered (2 × 1) acetate overlayer similar to that observed following deposition from vapor. Benzoic acid forms a (2 × 2) overlayer, which is stabilized by intermolecular interactions between the phenyl groups.

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.

Lepidocrocite-like TiO2 and TiO2(110)–(1 × 2) supported on W(100)

Ultrathin films of TiO2 were grown on a W(100)–O(2 × 1) substrate and characterised with a combination of scanning tunneling microscopy (STM) and low energy electron diffraction. In addition to islands of rutile TiO2(110) with (1 × 1) termination that were reported previously, we also observed rutile TiO2(110) islands with a (1 × 2) film termination. A lepidocrocite-like TiO2 nanosheet was also observed on the W(100) surface. High resolution STM images show that the nanosheet grows in the principal orthogonal directions of the W(100) substrate and forms a commensurate (1 × 7) coincident cell.

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.

Visualization of Water-induced Surface Segregation of Polarons on Rutile TiO2(110)

Water-oxide surfaces are ubiquitous in nature and of widespread importance to phenomena like corrosion as well as contemporary industrial challenges such as energy production through water splitting. So far, a reasonably robust understanding of the structure of such interfaces under certain conditions has been obtained. Considerably less is known about how overlayer water modifies the inherent reactivity of oxide surfaces. Here we address this issue experimentally for rutile TiO2(110) using scanning tunneling microscopy and photoemission, with complementary density functional theory calculations. Through detailed studies of adsorbed water nanoclusters and continuous water overlayers, we determine that excess electrons in TiO2 are attracted to the top surface layer by water molecules. Measurements on methanol show similar behavior. Our results suggest that adsorbate-induced surface segregation of polarons could be a general phenomenon for technologically relevant oxide materials, with consequences for surface chemistry and the associated catalytic activity.