| AVS 63rd International Symposium & Exhibition | |
| In-Situ and Operando Spectroscopy and Microscopy for Catalysts, Surfaces, & Materials Focus Topic | Thursday Sessions |
| Session IS-ThA |
| Session: | Ambient Pressure Photoelectron Spectroscopy and Scanning Probe Techniques |
| Presenter: | Robert Weatherup, Lawrence Berkeley National Laboratory (LBNL) |
| Authors: | R.S. Weatherup, Lawrence Berkeley National Laboratory (LBNL) B. Eren, Lawrence Berkeley National Laboratory (LBNL) Y. Hao, Lawrence Berkeley National Laboratory (LBNL) H. Bluhm, Lawrence Berkeley National Laboratory (LBNL) M.B. Salmeron, Lawrence Berkeley National Laboratory (LBNL) |
| Correspondent: | Click to Email |
Determining the chemical state of a catalyst under realistic reaction conditions is of crucial importance in designing catalytic systems with improved activity and selectivity towards sought after products, and a key step in developing or improving existing industrial processes. Ambient pressure X-ray photoelectron spectroscopy APXPS has has proved a powerful technique for providing quantitative and surface sensitive (within a few nm) information on the chemical composition of surfaces/interfaces, with commercial analyzers available that allow measurements at pressure in the tens of mbar regime.[1] However numerous reactions of interest occur at atmospheric pressures and above, and thus the behavior observed in existing APXPS systems may not be truly representative of such reactions.
Here we demonstrate atmospheric pressure XPS using single-layer graphene membranes as photoelectron-transparent barriers that sustain pressure differences in excess of 6 orders of magnitude.[2] The graphene-based membranes are produced by transferring graphene grown by chemical vapor deposition,[3] onto metal (Au or Al) coated silicon nitride grids using a polymer-free transfer technique. The graphene serves as a support for catalyst nanoparticles under atmospheric pressure reaction conditions (up to 1.5 bar), where XPS allows the oxidation state of Cu nanoparticles and gas phase species to be simultaneously probed. We thereby observe that the Cu2+ oxidation state is stable in O2 (1 bar) but is spontaneously reduced under vacuum. We further demonstrate the detection of various gas-phase species (Ar, CO, CO2, N2, O2) in the pressure range 10–1500 mbar including species with low photoionization cross sections (He, H2). Pressure-dependent changes in the apparent binding energies of gas-phase species are observed, attributable to changes in work function of the metal-coated grids supporting the graphene. We expect atmospheric pressure XPS based on this graphene membrane approach to be a valuable tool for studying nanoparticle catalysis.
[1] Starr, D. E.; Liu, Z.; Hävecker, M.; Knop-Gericke, A.; Bluhm, H. Investigation of Solid/vapor Interfaces Using Ambient Pressure X-Ray Photoelectron Spectroscopy. Chem. Soc. Rev.2013, 42, 5833–5857.
[2] Weatherup, R. S.; Eren, B.; Hao, Y.; Bluhm, H.; Salmeron, M. B. Graphene Membranes for Atmospheric Pressure Photoelectron Spectroscopy. J. Phys. Chem. Lett.2016, 7, 1622–1627.
[3] Hofmann, S.; Braeuninger-Weimer, P.; Weatherup, R. S. CVD-Enabled Graphene Manufacture and Technology. J. Phys. Chem. Lett.2015, 6, 2714–2721.