AVS2016 Session IS+HC-WeA: Ambient Pressure XPS Studies of Surface and Chemistry of Catalysts

In this presentation I will describe the application of near ambient pressure photoelectron spectroscopy (NAPXPS) to the investigation of electrochemically active gas-solid and liquid-solid interfaces during electrochemical processes. Understanding the oxygen evolution reaction (OER) on a molecular level has become increasingly important over the last few years, beacuse energy storage of renewables is becoming more relevant now that CO₂ emission has been identified as a source of climate change.

Nafion membrane based NAPXPS experiments performed on Pt electrodes during the OER demonstrate that Pt oxides are detrimental for the OER. An oxygen induced species characterized by an Pt4f binding energy 0.6 eV above the metallic Pt4f peak was observed. However, these experiments were done in water vapor and the relevance of the results for the OER in liquid water is questionable [1]. Therefore a new approach to study the liquid-solid interface during an electrochemical process in liquid water was developed.

In this new process the electrode material is deposited on a bilayer of graphene that is stabilized by a silicon nitride window with a pinhole structure. The electrode is exposed to the aqueous electrolyte and is irradiated at the same time by synchrotron light through the bilayer graphene membrane. The emitted photoelectrons have to pass the graphene membrane before being detected in the photoelectron analyzer [2]. Recent progress in the study of the electronic structure of noble metal electrodes used in OER reaction will be discussed [3,4].

References:

1. R. Arrigo et al., In Situ Study of the Gas-Phase Electrolysis of Water on Platinum by NAP-XPS, Angew. Chem. Int. Ed. 52, 11660-11664 (2013)

2. J. J. Velasco-Velez et al., Photoelectron spectroscopy at the graphene-liquid interface reveals the electronic structure of an electrodeposited cobalt/graphene electrocatalyst, Angew. Chem. Int. Ed. 54, 14554-14558 (2015)

3. V. Pfeifer et al., The electronic structure of iridium oxide electrodes active in water splitting, PCCP, 18, 2292-2296 (2016)

4. V. Pfeifer et al., The electronic structure of iridium and its oxides, Surf. Interface Anal., 48, 261-273 (2016)

Perovskite materials that are characterized by the composition ABO₃ can be formed by a wide variety of A and B cations. This enables the catalytic properties to be altered by selectively choosing the constituents while maintaining nominally the same structure. The adsorption and oxidation of simple alcohols such as methanol and ethanol have been identified as probe reactions to characterize and compare the catalytic properties of different oxide surfaces.

Methanol and ethanol oxidation on doped La_0.7Sr_0.3MnO₃(001) have been studied using ambient pressure x-ray photoelectron spectroscopy (APXPS). La_0.7Sr_0.3MnO₃(001) was grown on single crystal Nb-doped SrTiO₃(001) by pulse laser deposition. The growth of this film has been extensively characterized as a function of temperature, oxygen pressure and laser fluence in order to produce near ideal crystallinity and morphology in the film. The APXPS experiments were performed on the end station at the recently commissioned Beamline CSX-2 at NSLS II. In order to investigate the so-called “pressure gap” that may occur between reactions studied under vacuum conditions and at pressure approaching atmospheric pressure, experiments were conducted at nominally 10^-5 torr and at 0.1 torr between 250° C and 350° C.

Methanol forms methoxy when adsorbed on the perovskite surface at 250° C. The surface coverage was four times greater at 0.1 torr compared to 10^-5 torr. Methoxy was the only C-containing surface species observed at 10^-5 torr with or without O₂. Methoxy was also the dominant surface species at 0.1 torr in the absence of O₂. However, small amounts of formate and atomic C were also evident. At the higher pressure the Mn 2p spectra indicated that the alcohol partially reduced Mn³⁺ to Mn²⁺. There was also an indication in the O 1s spectra that O was removed from the surface.

When O was present at 0.1 torr formate became the dominant surface species with only trace amounts of methoxy and C also evident. Gas phase CO₂ and H₂O products were also detected in the C 1s and O 1s spectra.

Results using ethanol rather than methanol as the reactant were generally the same, i.e. only ethoxy on the surface at lower pressures, a mixture of ethoxy and acetate at higher pressure in the absence of O₂, and exclusively acetate on the surface when O₂ was present. The only significant difference between methanol and ethanol was a greater tendency for ethanol to form the carboxylate in the absence of O₂.

Operando experiments are planned to monitor the products with a mass spectrometer to determine whether the different pressures, and resulting surface species, lead to different products.

[1] H. Bluhm, M. Hävecker, A. Knop-Gericke, M. Kiskinova, R. Schlögl, M. Salmeron, MRS Bulletin 32, 2007, 1022.

[2] J. J Velasco-Velez, V. Pfeifer, M. Hävecker, R. S Weatherup, R. Arrigo, C.-H. Chuang, E. Stotz, G. Weinberg, M. Salmeron, R. Schlögl, A. Knop-Gericke, Angewandte Chemie International Edition 54, 2015, DOI: 10.1002/anie.201506044.

[3] Z. Liu, H. Bluhm, Hard X-ray Photoelectron Spectroscopy (HAXPES), 447-466. DOI: 10.1007/978-3-319-24043-5_17.

[4] R. Arrigo, M. Haevecker, M. E. Schuster, C. Ranjan, E. Stotz, A. Knop-Gericke, R. Schloegl, Angewandte Chemie International Edition, 52, 2013, 11660-11664.

[5] V. Pfeifer, T. Jones, J.-J. Velesco-Velez, M. Greiner, C. Massué, R. Arrigo, D. Teschner, F. Girgsdies, M. Scherzer, J. Allan, M. Hashagen, G. Weinberg, S. Piccinin, M. Haevecker, A. Knop-Gericke, R. Schlögl, Physical Chemistry Chemical Physics 18, 2015, DOI: 10.1039/C5CP06997A.

In order to comprehend the properties affecting the catalytic performance of metal nanoparticles (NPs), their dynamic nature and response to the environment must be taken into consideration. The working state of a NP catalyst might not be the state in which the catalyst was prepared, but a structural and/or chemical isomer that adapted to the particular reaction conditions. This talk provides examples of recent advances in the preparation and characterization of NP catalysts with well-defined sizes and shapes. It discusses how to resolve the shape of nm-sized Pt, Au, Pd, and Cu catalysts via a combination of in situ microscopy (AFM, STM, TEM), and in situ and operando spectroscopy (XAFS, GISAXS) and modeling, and how to follow its evolution under different gaseous or liquid chemical environments and in the course of a reaction. It will be highlighted that for structure-sensitive reactions, catalytic properties such as the reaction rates, onset reaction temperature, activity, selectivity and stability against sintering may be tuned through controlled synthesis.

Examples of catalytic processes which will be discussed include the gas-phase oxidation of alcohols (methanol, propanol, butanol), the oxidation and reduction of NO, the electrochemical oxidation of propanol and electrochemical reduction of CO₂. Emphasis will be given to elucidating the role of the NP size, shape and chemical state in the activity and selectivity of the former reactions.