Time-Resolved APXPS with Chemical Potential Perturbations: Recent Developments at the MAX IV Laboratory

Abstract

Vol. 35, No. 3, 2022, Synchrotron radiation newS Technical RepoRT Time-Resolved APXPS with Chemical Potential Perturbations: Recent Developments at the MAX IV Laboratory Andrey ShAvorSkiy,1 eSko kokkonen,1 evgeniy redekop,2 giulio d’Acunto,3,4 JoAchim SchnAdt,1,3,4 And JAn knudSen1,3,4 1MAX IV Laboratory, Lund University, Lund, Sweden 2Department of Chemistry, Centre for Materials Science and Nanotechnology (SMN), University of Oslo, Oslo, Norway 3Division of Synchrotron Radiation Research, Department of Physics, Lund University, Lund, Sweden 4NanoLund, Lund University, Lund, Sweden Introduction Heterogeneous systems made up of gas-solid interfaces are common in nature and in industrial processes. They play a critical role in heterogeneous catalysis, formation of weather patterns, atmospheric phenomena, and corrosion. For many of these systems, the activity and structure of the interface are intimately related and rapidly responding to changes in the gas phase composition. To understand such activity-structure relationships, correlated measurements of activity and interfacial structure and composition are required. Often, studies of heterogeneous systems focus on experiments under steady-state conditions. Under such conditions, however, the acquisition of correlated data is often complicated by the dynamic nature of the heterogeneous processes and their complexity [1]. For example, under steady-state conditions only the slowest reaction steps and most abundant intermediates are characterized. It is only when the system is driven away from its steady state and allowed to relax that the true time evolution of its key features becomes observable. The literature often discusses that such transient methods result in a better mechanistic understanding of surface reactions than steady-state experiments [2]. The general scheme for a time-resolved experiment thus should follow the classical pump-probe scheme: the transient conditions are created on demand by driving the system away from the steady state via an external perturbation (the pump). Among the multitude of methods to excite/perturb the system, chemical potential perturbations are desirable for investigating activity-structure relationship and can be implemented via changes in gas composition above the solid surface. The structural/compositional response is then measured (the probe) during subsequent relaxation of the system into the previous (for reversible processes) or new (for nonreversible processes) resting state (Figure 1) [3]. The simplest way to perform a time-resolved measurement is to decrease the acquisition time for a measurement. However, the signal-tonoise ratio required for extraction of meaningful information sets the lower limit achievable in this approach. The analysis of a consequent series of such measurements allows extraction of the time evolution of the signal. During such an experiment, the measurements of the sample response occur in real time, which gives the advantage of studying nonreversible processes as well as spontaneously oscillating reactions. It thus becomes possible to precisely match composition data with the activity characteristics. Both the perturbation and measurement must occur on a much shorter time scale than the subsequent relaxation to provide meaningful data. During the past two decades, Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS) has developed to become one of the most popular and powerful experimental techniques for studying gas-solid interfaces under realistic conditions. Currently, the time resolution of XPS experiments with chemical perturbations is limited to a few hundred milliseconds when performed at modern synchrotron facilities; reaching a time resolution far better than this is currently a challenge, due to the relatively low efficiency of the collection of photoelectrons and the presence of the gas phase at few mbars. To overcome this issue and achieve a breakthrough in the time resolution of the APXPS experiment, it must be combined with an averaging approach in which the same process is repeated multiple times and the data are summed/ averaged to improve the signal-to-noise ratio. In the following examples, we demonstrate the current state-of-the art time-resolved in situ APXPS measurements performed at the SPE-