Enabling Quantitative Analysis of Surface Small Molecules for Exposomics and Behavioral Studies

Workplace chemical exposures are a major source of occupational injury. Although over half of these are skin exposures, exposomics research often focuses on chemical levels in the air or in worker biofluids such as blood and urine. Until now, one limitation has been the lack of methods to quantitatively measure surface chemical transfer. Outside the realm of harmful chemicals, the small molecules we leave behind on surfaces can also reveal important aspects of human behavior. In this study, we developed a swab-based quantitative approach to determine small molecule concentrations across common surfaces. We demonstrate its utility using one drug, cyclobenzaprine, on metal surfaces, and two human-derived metabolites, carnitine and phenylacetylglutamine, on four common surfaces: linoleum flooring, plastified laboratory workbench, metal, and Plexiglas. We observed peak areas proportional to surface analyte concentrations at 45 min and 1 week after deposition, enabling quantification of molecule abundance on workplace built environment surfaces. In contrast, this method was unsuitable for analysis of oleanolic acid, for which we did not observe a strong linear proportional relationship following swab-based recovery from surfaces. Overall, this method paves the way for future quantitative exposomics studies in analyte-specific and surface-specific frameworks

Combining CO₂ Reduction with Ethane Oxidative Dehydrogenation by Oxygen-Modification of Molybdenum Carbide

The surface properties that determine the selectivity of Mo2C catalysts in ethane oxidative dehydrogenation with CO2 as a soft oxidant were investigated using a combination of pulse experiments and in-situ spectroscopic methods. Oxygen modification was discovered to be crucial for inhibiting the cleavage of the C–C bond in ethane and enhancing the production of ethylene. The addition of the Fe promoter accelerated the formation of surface oxygen species and stabilized them from reduction by ethane, leading to a shorter induction period, higher ethylene yield, and improved stability

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 2.5 CO molecules can bind on a single Co atom, in carbonyl-like configurations. Based on high-resolution STM images, together with density functional theory calculations, we determined the most stable CuCo cluster structures formed with bound CO. Such carbonyl-like formation manifests in shifts in the binding energy of the Co core-level peaks in X-ray photoelectron spectra, as well as shifts in the vibrational modes of adsorbed CO in infrared reflection absorption spectra. The multiple CO adsorption on a Co site weakens the Co–CO bond and thus reduces the C–O bond scission probability. Our results may explain the different product distribution, including higher selectivity toward alcohol formation, when bimetallic CuCo catalysts are used compared to pure Co

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

Room-Temperature Activation of Methane and Dry Re-forming with CO₂ on Ni-CeO₂(111) Surfaces: Effect of Ce³⁺ Sites and Metal–Support Interactions on C–H Bond Cleavage

The results of core-level photoemission indicate that Ni-CeO₂(111) surfaces with small or medium coverages of nickel are able to activate methane at 300 K, producing adsorbed CH_<i>x</i> and CO_<i>x</i> (<i>x</i> = 2, 3) groups. Calculations based on density functional theory predict a relatively low activation energy of 0.6–0.7 eV for the cleavage of the first C–H bond in the adsorbed methane molecule. Ni and O centers of ceria work in a cooperative way in the dissociation of the C–H bond at room temperature, where a low Ni loading is crucial for the catalyst activity and stability. The strong electronic perturbations in the Ni nanoparticles produced by the ceria supports of varying natures, such as stoichiometric and reduced, result in a drastic change in their chemical properties toward methane adsorption and dissociation as well as the dry reforming of methane reaction. The coverage of Ni has a drastic effect on the ability of the system to dissociate methane and catalyze the dry re-forming process

Mechanistic Insights of Ethanol Steam Reforming over Ni–CeO_<i>x</i>(111): The Importance of Hydroxyl Groups for Suppressing Coke Formation

We have studied the reaction of ethanol and water over Ni–CeO_2‑<i>x</i>(111) model surfaces to elucidate the mechanistic steps associated with the ethanol steam reforming (ESR) reaction. Our results provide insights about the importance of hydroxyl groups to the ESR reaction over Ni-based catalysts. Systematically, we have investigated the reaction of ethanol on Ni–CeO_2‑<i>x</i>(111) at varying Ce³⁺ concentrations (CeO_1.8–2.0) with absence/presence of water using a combination of soft X-ray photoelectron spectroscopy (sXPS) and temperature-programmed desorption (TPD). Consistent with previous reports, upon annealing, metallic Ni formed on reduced ceria while NiO was the main component on fully oxidized ceria. Ni⁰ is the active phase leading to both the C–C and C–H cleavage of ethanol but is also responsible for carbon accumulation or coking. We have identified a Ni₃C phase that formed prior to the formation of coke. At temperatures above 600 K, the lattice oxygen from ceria and the hydroxyl groups from water interact cooperatively in the removal of coke, likely through a strong metal–support interaction between nickel and ceria that facilitates oxygen transfer

Highly Active Ceria-Supported Ru Catalyst for the Dry Reforming of Methane: In Situ Identification of Ru^δ+–Ce³⁺ Interactions for Enhanced Conversion

The metal–oxide interaction changes the surface electronic states of catalysts deployed for chemical conversion, yet details of its influence on the catalytic performance under reaction conditions remain obscure. In this work, we report the high activity/stability of a ceria-supported Ru–nanocluster (<1 nm) catalyst during the dry reforming of methane. To elucidate the structure–reactivity relationship underlying the remarkable catalytic performance, the active structure and chemical speciation of the catalyst was characterized using in situ X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS), while the surface chemistry and active intermediates were monitored by in situ ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Methane activates on the catalyst surface at temperatures as low as 150 °C. Under reaction conditions, the existence of metal–support interactions tunes the electronic properties of the Ru nanoclusters, giving rise to a partially oxidized state of ruthenium stabilized by reduced ceria (Ruδ+–CeO2–x) to sustain active chemistry, which is found to be very different from that of large Ru nanoparticles supported on ceria. The oxidation of surface carbon is also a crucial step for the completion of the catalytic cycle, and this is strongly correlated with the oxygen transfer governed by the Ruδ+–CeO2–x interactions at higher temperatures (>300 °C). The possible reaction pathways and stable surface intermediates were identified using DRIFTS including ruthenium carbonyls, carboxylate species, and surface −OH groups, while polydentate carbonates may be plain spectators at the measured reaction conditions

Striving Toward Noble-Metal-Free Photocatalytic Water Splitting: The Hydrogenated-Graphene–TiO₂ Prototype

Graphane, graphone, and hydrogenated graphene (HG) have been extensively studied in recent years due to their interesting properties and potential use in commercial and industrial applications. The present study reports investigation of hydrogenated graphene/TiO_2–<i>x</i> (HGT) nanocomposites as photocatalysts for H₂ and O₂ production from water without the assistance of a noble metal cocatalyst. By combination of several techniques, the morphologies, bulk/atomic structure, and electronic properties of all the powders were exhaustively interrogated. Hydrogenation treatment efficiently reduces TiO₂ nanoparticles, while the graphene oxide sheets undergo the topotactic transformation from a graphene-like structure to a mixture of graphitic and turbostratic carbon (amorphous/disordered) upon altering the calcination atmosphere from a mildly reducing to a H₂-abundant environment. Remarkably, the hydrogenated graphene–TiO_2–<i>x</i> composite that results upon H₂-rich reduction exhibits the highest photocatalytic H₂ evolution performance equivalent to low loading of Pt (∼0.12 wt %), whereas the addition of HG suppresses the O₂ production. We propose that such an enhancement can be attributed to a combination of factors including the introduction of oxygen vacancies and Ti³⁺ states, retarding the recombination of charge carriers, and thus, facilitating the charge transfer from TiO_2–<i>x</i> to the carbonaceous sheet