Compression of a Stearic Acid Surfactant Layer on Water Investigated by Ambient Pressure X-ray Photoelectron Spectroscopy.

We present a combined Langmuir-Pockels trough and ambient pressure X-ray photoelectron spectroscopy (APXPS) study of the compression of stearic acid surfactant layers on neat water. Changes in the packing density of the molecules are directly determined from C 1s and O 1s APXPS data. The experimental data are fit with a 2D model for the stearic acid coverage.

Enhanced Methanol Synthesis from CO Hydrogenation Achieved by Tuning the Cu-ZnO Interaction in ZnO/CuO Nanocube Catalysts Supported on ZrO and SiO.

The nature of the Cu-Zn interaction and especially the role of Zn in Cu/ZnO catalysts used for methanol synthesis from CO hydrogenation are still debated. Migration of Zn onto the Cu surface during reaction results in a Cu-ZnO interface, which is crucial for the catalytic activity. However, whether a Cu-Zn alloy or a Cu-ZnO structure is formed and the transformation of this interface under working conditions demand further investigation.

Crystallographic Orientation Dependence of Surface Segregation and Alloying on PdCu Catalysts for CO Hydrogenation.

The influence of the crystallographic orientation on surface segregation and alloy formation in model PdCu methanol synthesis catalysts was investigated using near-ambient pressure X-ray photoelectron spectroscopy under CO hydrogenation conditions. Combined with scanning tunneling microscopy and density functional theory calculations, the study showed that submonolayers of Pd undergo spontaneous alloy formation on Cu(110) and Cu(100) surfaces in vacuum, whereas they do not form an alloy on Cu(111). Upon heating in H, inward diffusion of Pd into the Cu lattice is favored, facilitating alloying on all Cu surfaces.

Operando high-pressure investigation of size-controlled CuZn catalysts for the methanol synthesis reaction.

Although Cu/ZnO-based catalysts have been long used for the hydrogenation of CO to methanol, open questions still remain regarding the role and the dynamic nature of the active sites formed at the metal-oxide interface. Here, we apply high-pressure operando spectroscopy methods to well-defined Cu and CuZn nanoparticles supported on ZnO/AlO, γ-AlO and SiO to correlate their structure, composition and catalytic performance. We obtain similar activity and methanol selectivity for Cu/ZnO/AlO and CuZn/SiO, but the methanol yield decreases with time on stream for the latter sample.

Water-polyamide chemical interplay in desalination membranes explored by ambient pressure X-ray photoelectron spectroscopy.

Reverse osmosis using aromatic polyamide membranes is currently the most important technology for seawater desalination. The performance of reverse osmosis membranes is highly dependent on the interplay of their surface chemical groups with water and water contaminants. In order to better understand the underlying mechanisms of these membranes, we study ultrathin polyamide films that chemically resemble reverse osmosis membranes, using ambient pressure X-ray photoelectron spectroscopy.

Quantitative Characterization of a Desalination Membrane Model System by X-ray Photoelectron Spectroscopy.

Aromatic polyamide films form the active layer in reverse osmosis desalination membranes. Despite widespread use of this technology, it suffers from low rejection rates for certain water contaminants and from membrane fouling. Through a better understanding of the fundamental surface chemical processes during reverse osmosis desalination, advances in membrane and material design are expected.

Water (Non-)Interaction with MoO.

Molybdenum(VI) oxide (MoO) is used in a number of technical processes such as gas filtration and heterogeneous catalysis. In these applications, the adsorption and dissociation of water on the surface can influence the chemistry of MoO and thus the course of heterogeneous reactions. We use ambient pressure X-ray photoelectron spectroscopy to study the interaction of water with a stoichiometric MoO surface and a MoO surface that features oxygen defects and hydroxyl groups.

Structure of Copper-Cobalt Surface Alloys in Equilibrium with Carbon Monoxide Gas.

We studied the structure of the copper-cobalt (CuCo) surface alloy, formed by Co deposition on Cu(110), in dynamic equilibrium with CO. Using scanning tunneling microscopy (STM), we found that, in vacuum at room temperature and at low Co coverage, clusters of a few Co atoms substituting Cu atoms form at the surface. At CO pressures in the Torr range, we found that up to 2.

Dimethyl methylphosphonate adsorption and decomposition on MoO as studied by ambient pressure x-ray photoelectron spectroscopy and DFT calculations.

Organophosphonates range in their toxicity and are used as pesticides, herbicides, and chemical warfare agents (CWAs). Few laboratories are equipped to handle the most toxic molecules, thus simulants such as dimethyl methylphosphonate (DMMP), are used as a first step in studying adsorption and reactivity on materials. Benchmarked by combined experimental and theoretical studies of simulants, calculations offer an opportunity to understand how molecular interactions with a surface changes upon using a CWA.

Water Adsorption and Dissociation on Polycrystalline Copper Oxides: Effects of Environmental Contamination and Experimental Protocol.

We use ambient-pressure X-ray photoelectron spectroscopy (APXPS) to study chemical changes, including hydroxylation and water adsorption, at copper oxide surfaces from ultrahigh vacuum to ambient relative humidities of ∼5%. Polycrystalline CuO and CuO surfaces were prepared by selective oxidation of metallic copper foils. For both oxides, hydroxylation occurs readily, even at high-vacuum conditions.

Direct Mapping of Band Positions in Doped and Undoped Hematite during Photoelectrochemical Water Splitting.

Photoelectrochemical water splitting is a promising pathway for the direct conversion of renewable solar energy to easy to store and use chemical energy. The performance of a photoelectrochemical device is determined in large part by the heterogeneous interface between the photoanode and the electrolyte, which we here characterize directly under operating conditions using interface-specific probes. Utilizing X-ray photoelectron spectroscopy as a noncontact probe of local electrical potentials, we demonstrate direct measurements of the band alignment at the semiconductor/electrolyte interface of an operating hematite/KOH photoelectrochemical cell as a function of solar illumination, applied potential, and doping.

Unravelling the Chemical Influence of Water on the PMMA/Aluminum Oxide Hybrid Interface In Situ.

Understanding the stability of chemical interactions at the polymer/metal oxide interface under humid conditions is vital to understand the long-term durability of hybrid systems. Therefore, the interface of ultrathin PMMA films on native aluminum oxide, deposited by reactive adsorption, was studied. The characterization of the interface of the coated substrates was performed using ambient pressure X-ray photoelectron spectroscopy (APXPS), Fourier transform infrared spectroscopy in the Kretschmann geometry (ATR-FTIR Kretschmann) and time-of-flight secondary ion mass spectrometry (ToF-SIMS).

In Situ Characterization of the Initial Effect of Water on Molecular Interactions at the Interface of Organic/Inorganic Hybrid Systems.

Probing initial interactions at the interface of hybrid systems under humid conditions has the potential to reveal the local chemical environment at solid/solid interfaces under real-world, technologically relevant conditions. Here, we show that ambient pressure X-ray photoelectron spectroscopy (APXPS) with a conventional X-ray source can be used to study the effects of water exposure on the interaction of a nanometer-thin polyacrylic acid (PAA) layer with a native aluminum oxide surface. The formation of a carboxylate ionic bond at the interface is characterized both with APXPS and in situ attenuated total reflectance Fourier transform infrared spectroscopy in the Kretschmann geometry (ATR-FTIR Kretschmann).

Ambient pressure photoelectron spectroscopy: Practical considerations and experimental frontiers.

Over the past several decades, ambient pressure x-ray photoelectron spectroscopy (APXPS) has emerged as a powerful technique for in situ and operando investigations of chemical reactions under relevant ambient atmospheres far from ultra-high vacuum conditions. This review focuses on exemplary cases of APXPS experiments, giving special consideration to experimental techniques, challenges, and limitations specific to distinct condensed matter interfaces. We discuss APXPS experiments on solid/vapor interfaces, including the special case of 2D films of graphene and hexagonal boron nitride on metal substrates with intercalated gas molecules, liquid/vapor interfaces, and liquid/solid interfaces, which are a relatively new class of interfaces being probed by APXPS.

Concentration and chemical-state profiles at heterogeneous interfaces with sub-nm accuracy from standing-wave ambient-pressure photoemission.

Heterogeneous processes at solid/gas, liquid/gas and solid/liquid interfaces are ubiquitous in modern devices and technologies but often difficult to study quantitatively. Full characterization requires measuring the depth profiles of chemical composition and state with enhanced sensitivity to narrow interfacial regions of a few to several nm in extent over those originating from the bulk phases on either side of the interface. We show for a model system of NaOH and CsOH in an ~1-nm thick hydrated layer on α-Fe2O3 (haematite) that combining ambient-pressure X-ray photoelectron spectroscopy and standing-wave photoemission spectroscopy provides the spatial arrangement of the bulk and interface chemical species, as well as local potential energy variations, along the direction perpendicular to the interface with sub-nm accuracy.