Ambient-pressure photoelectron spectroscopy for heterogeneous catalysis and electrochemistry

Abstract

We describe the design and capabilities of a new ambient-pressure X-ray photoelectron spectroscopy system at the Stanford Synchrotron Radiation Lightsource. A unique feature of this system is that samples are illuminated at grazing incidence and with a tightly focused beam, which allows a 50 μm aperture to be placed in the first differential pumping stage of the lens system of the electron spectrometer. The low conductance of the aperture enables surface-sensitive electron spectroscopy of solid surfaces, liquids, and solid–liquid interfaces to be performed operando at pressures as high as 100 Torr. The instrument can also be used to obtain polarization-resolved X-ray absorption spectra using Auger-electron-yield detection. Results for Pt surfaces in ambient-pressure gas environments and for liquid water are presented.

Introduction

X-ray photoelectron spectroscopy (XPS) has many advantages compared with other techniques that probe surface chemistry and heterogeneous catalysis [1], [2], [3], [4]. It is, for example, element specific and provides information about the chemical state of systems through chemical shifts [5]. Importantly, XPS is also surface sensitive, because photoelectrons with kinetic energies between 100 and 1400 eV typically have inelastic mean free paths less than 20 Å in condensed media. A related disadvantage, however, is that conventional XPS measurements require ultra-high vacuum conditions (P < 1 × 10⁻⁸ Torr) to minimize scattering of emitted photoelectrons by gases and to prevent discharges on the electrostatic lens elements and the detectors, whereas many processes of interest in catalysis and electrochemistry take place at elevated pressure (>760 Torr) and temperature. This limitation was partially circumvented by the development of ambient-pressure XPS (APXPS), which began in the early 1980s [6] but later accelerated with the advent of third-generation synchrotron facilities [4]. Today, several synchrotron-based instruments allow XPS measurements at gas pressures of up to a few Torr. Concomitant improvements in conventional X-ray sources have also led to recent efforts to develop laboratory-based instruments for ambient-pressure spectroscopy [7], [8].

The basic design principle of ambient-pressure electron spectrometers is straightforward: a small aperture limits the conductance between a gas cell containing the sample and a differentially pumped electron-energy analyzer. Samples are placed very close to the aperture to reduce the path length of the emitted photoelectrons through the ambient-pressure gas environment; typically, the sample is separated from the aperture by ∼0.1–0.5 mm, which ensures that the local pressure at the sample is the same as the background pressure in the gas cell [9].

To date, three different electrostatic focusing schemes have been employed for the lens system of ambient-pressure spectrometers: (i) a conventional lens configuration optimized for fixed working distances (typically ∼40–50 mm) [7]; (ii) an additional focusing electrostatic lens in the differential pumping scheme [10], [11]; (iii) a pre-lens section incorporated into a conventional lens design [12]. In comparison with (i), the latter two methods provide more space between the sample and the spectrometer, so that additional differential pumping stages can be incorporated without losing detection efficiency. These design considerations and related studies performed in the early development stages of the technique have been reviewed in detail [9], [10], [13] and will not be recapitulated here.

In the past decade, APXPS has been primarily used to investigate solid surfaces in near-ambient gas environments [14], [15], [16], [17]. The unique capabilities of APXPS are exemplified by recent studies of surface-oxide growth on catalytically active metal surfaces [18], [19], [20], the interaction of water with metal and metal-oxide surfaces [21], [22], [23], the surface electronic-structure of operating solid-oxide fuel cells [24], and compositional changes of bimetallic nanoparticles in oxidizing and reducing atmospheres [25]. In addition, ambient-pressure spectrometers have been used to perform X-ray absorption spectroscopy (XAS) at the K edges of C, N and O, and at the L edges of 3d transition metals [26]; these studies provide information about the geometric structure of adlayers in equilibrium with near-ambient gas environments. It is noteworthy, however, that few of the in situ APXPS studies outlined above yielded significant additional information beyond what had been obtained from the corresponding ex situ measurements performed in UHV. These observations have encouraged researchers to improve the performance of APXPS systems, e.g., by extending the working pressure range.

In the present article, we demonstrate the capabilities of the new APXPS system now operating at beamline 13-2 at the Stanford Synchrotron Radiation Lightsource (SSRL). Although the working principle is comparable to other instruments in use around the world, the design of the gas cell allows us to perform in situ measurements at pressures of up 100 Torr. The design of the new spectrometer will be described in Section 2, and applications of the new spectrometer to adsorbate systems and fuel-cell reactions will be presented in Section 3.