Ambient Pressure X-ray Photoelectron Spectroscopy Studies Research Articles

Surfactants Used in Colloidal Synthesis Modulate Ni Nanoparticle Surface Evolution for Selective CO2 Hydrogenation.

Colloidal chemistry holds promise to prepare uniform and size-controllable pre-catalysts; however, it remains a challenge to unveil the atomic-level transition from pre-catalysts to active catalytic surfaces under the reaction conditions to enable the mechanistic design of catalysts. Here, we report an ambient-pressure X-ray photoelectron spectroscopy study, coupled with in situ environmental transmission electron microscopy, infrared spectroscopy, and theoretical calculations, to elucidate the surface catalytic sites of colloidal Ni nanoparticles for CO2 hydrogenation. We show that Ni nanoparticles with phosphine ligands exhibit a distinct surface evolution compared with amine-capped ones, owing to the diffusion of P under oxidative (air) or reductive (CO2 + H2) gaseous environments at elevated temperatures. The resulting NiPx surface leads to a substantially improved selectivity for CO production, in contrast to the metallic Ni, which favors CH4. The further elimination of surface metallic Ni sites by designing multi-step P incorporation achieves unit selectivity of CO in high-rate CO2 hydrogenation.

Near ambient-pressure X-ray photoelectron spectroscopy study of CO2 activation and hydrogenation on indium/copper surface

Indium-based catalysts exhibit excellent performance for CO2 hydrogenation to methanol, yet their nature and chemical evolution under reaction conditions are still elusive, thus hindering an understanding of their reaction mechanism. In this work, near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) is employed to investigate the chemical properties and the catalytic role of indium/copper model catalysts under CO2 hydrogenation conditions. We found that the deposition of In on the surface of a Cu foil led to the formation of Cu-In alloy, whereas upon CO2 exposure, In was partially oxidized to In2O3-x and Cu remains metallic. Due to the presence of In2O3-x, CO2 was activated on the surface of In/Cu samples mainly in the form of carbonate. In addition, compared with the pure In foil reference, both the fraction of oxygen vacancies and the coverage density of carbonate were higher on the In/Cu samples, indicating the promotion effect of Cu-In alloy in the activation of CO2. These results reveal the evolution of the active sites of indium/copper catalysts and inspire the design of advanced In-based bimetallic catalysts for CO2 hydrogenation.

Controlled growth of uniform two-dimensional ZnO overlayers on Au(111) and surface hydroxylation

Ultrathin ZnO nanostructures present interesting two-dimensional (2D) graphene-like structure in contrast to wurtzite structure in bulk ZnO. Growth on Au(111) has been regarded as a well-established route to the 2D ZnO layers while controlled growth of uniform ZnO nanostructures remains as a challenge. Here, reactive deposition of Zn in O3 and NO2 was employed, which is investigated by scanning tunneling microscopy and X-ray photoelectron spectroscopy (XPS). We demonstrate that uniform ZnO monolayer nanoislands and films can be obtained on Au(111) using O3 and uniform ZnO bilayer nanoislands and films form on Au(111) using NO2, respectively. Formation of atomic oxygen overlayers on Au(111) via exposure to O3 is critical to the formation of uniform ZnO monolayer nanostructures atop. Near ambient pressure XPS studies revealed that nearly full hydroxylation occurs on monolayer ZnO structures upon exposure to near ambient pressure water vapor or atomic hydrogen species, while partial surface hydroxylation happens on bilayer ZnO under the same gaseous exposure conditions.

Dimethyl methylphosphonate adsorption and decomposition on MoO2 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. However, most calculations of DMMP and CWAs on surfaces are limited to adsorption studies on clusters of atoms, which may differ markedly from the behavior on bulk solid-state materials with extended surfaces. We have benchmarked our solid-state periodic calculations of DMMP adsorption and reactivity on MoO2 with ambient pressure x-ray photoelectron spectroscopy studies (APXPS). DMMP is found to interact strongly with a MoO2 film, a model system for the MoOx component in the ASZM-TEDA© gas filtration material. Density functional theory modeling of several adsorption and decomposition mechanisms assist the assignment of APXPS peaks. Our results show that some of the adsorbed DMMP decomposes, with all the products remaining on the surface. The rigorous calculations benchmarked with experiments pave a path to reliable and predictive theoretical studies of CWA interactions with surfaces.

Ambient-Pressure X-ray Photoelectron Spectroscopy Study of Cobalt Foil Model Catalyst under CO, H2, and Their Mixtures

Ambient-pressure X-ray photoelectron spectroscopy (XPS) was used to investigate the reactions of CO, H2, and their mixtures on Co foils. We found that CO adsorbs molecularly on the clean Co surface and desorbs intact in vacuum with increasing rate until ∼90 °C where all CO desorbs in seconds. In equilibrium with 100 mTorr gas, CO dissociates above 120 °C, leaving carbide species on the surface but no oxides, because CO efficiently reduces the oxides at temperatures ∼100 °C lower than H2. Water as impurities or produced by reaction of CO and H2 efficiently oxidizes Co even at room temperature. Under 97:3 CO/H2 mixture and with increasing temperatures, the Co surface becomes more oxidized and covered by hydroxyl groups until ∼150 °C where surface starts to get reduced, accompanied by carbide accumulation indicative of CO dissociation. A similar trend was observed for 9:1 and 1:1 mixtures, but surface reduction begins at higher temperatures.

Sub-nanosecond time-resolved ambient-pressure X-ray photoelectron spectroscopy setup for pulsed and constant wave X-ray light sources.

An apparatus for sub-nanosecond time-resolved ambient-pressure X-ray photoelectron spectroscopy studies with pulsed and constant wave X-ray light sources is presented. A differentially pumped hemispherical electron analyzer is equipped with a delay-line detector that simultaneously records the position and arrival time of every single electron at the exit aperture of the hemisphere with ~0.1 mm spatial resolution and ~150 ps temporal accuracy. The kinetic energies of the photoelectrons are encoded in the hit positions along the dispersive axis of the two-dimensional detector. Pump-probe time-delays are provided by the electron arrival times relative to the pump pulse timing. An average time-resolution of (780 ± 20) ps (FWHM) is demonstrated for a hemisphere pass energy E(p) = 150 eV and an electron kinetic energy range KE = 503-508 eV. The time-resolution of the setup is limited by the electron time-of-flight (TOF) spread related to the electron trajectory distribution within the analyzer hemisphere and within the electrostatic lens system that images the interaction volume onto the hemisphere entrance slit. The TOF spread for electrons with KE = 430 eV varies between ~9 ns at a pass energy of 50 eV and ~1 ns at pass energies between 200 eV and 400 eV. The correlation between the retarding ratio and the TOF spread is evaluated by means of both analytical descriptions of the electron trajectories within the analyzer hemisphere and computer simulations of the entire trajectories including the electrostatic lens system. In agreement with previous studies, we find that the by far dominant contribution to the TOF spread is acquired within the hemisphere. However, both experiment and computer simulations show that the lens system indirectly affects the time resolution of the setup to a significant extent by inducing a strong dependence of the angular spread of electron trajectories entering the hemisphere on the retarding ratio. The scaling of the angular spread with the retarding ratio can be well approximated by applying Liouville's theorem of constant emittance to the electron trajectories inside the lens system. The performance of the setup is demonstrated by characterizing the laser fluence-dependent transient surface photovoltage response of a laser-excited Si(100) sample.

Organometallic Ruthenium Nanoparticles as Model Catalysts for CO Hydrogenation: A Nuclear Magnetic Resonance and Ambient-Pressure X-ray Photoelectron Spectroscopy Study

We present a study of the structure and reactivity of Ru nanoparticles of different sizes (1.3, 1.9, and 3.1 nm) for CO hydrogenation using gas-phase nuclear magnetic resonance and mass spectroscopy. In addition, the nanoparticles were characterized under reaction mixtures in situ by ambient-pressure X-ray photoelectron spectroscopy. We found that during reaction the Ru is in the metallic state and that the diphosphine ligands [bis(diphenylphosphino)butane (dppb)] on the surface of 1.9 and 3.1 nm nanoparticles not only act as capping and protecting agents but also stay on the surface during reaction and improve their activity and selectivity toward C2–C4 hydrocarbons.