Simultaneous in situ monitoring of surface and gas species and surface properties by modulation excitation polarization-modulation infrared reflection-absorption spectroscopy: CO oxidation over Pt film

Abstract

A method for in situ monitoring of surface and gas species utilizing separately the difference and sum reflectivity of two polarizations, normal and parallel to the surface, measured by polarization-modulation infrared reflection-absorption spectroscopy is presented. Surface and gas-phase spectra were separately but simultaneously obtained from the reflectivities. The technique is combined with modulation excitation spectroscopy to further enhance the sensitivity, and a small-volume cell was designed for this purpose. CO oxidation over a 40 nm Pt film on aluminum was investigated under moderate pressure ͑atmospheric pressure, 5% CO, and 5%-40% O 2 ͒ at 373-433 K. The surface species involved in the oxidation process and the gas-phase species, both reactant ͑CO͒ and product ͑CO 2 ͒, could be simultaneously monitored and analyzed quantitatively. In addition, the reflectivity change of the sample during the reaction was assigned to a near-surface bulk property change, that is, surface reconstruction to the oxide phase. Under an O 2 -rich atmosphere, two reactive phases, denoted as low-and high-activity phases, were identified. A large amount of atop CO was observed during the low-activity phase, while the adsorbed CO completely disappeared during the high-activity phase. The presence of an infrared-inactive CO 2 precursor formed by the reaction between surface oxide and gaseous CO during the high-activity phase was inferred. The desorption of the CO 2 precursor is facilitated under a CO-rich atmosphere, most likely, by surface reconstruction to metallic Pt and a competitive adsorption of CO on the surface.

Modulation Excitation X-Ray Absorption Spectroscopy to Probe Surface Species on Heterogeneous Catalysts

“…Therefore, catalytic activity appears generally larger when the state of Pt changes from partially oxidized to reduced. This is in good agreement with reports showing the requirement for partially oxidized Pt to best run the reaction [30,[34][35][36]. Additional pulse experiments with different pulse size and pulse compositions should be carried out to provide substantial confirmation of these preliminary results.…”

Time Resolved Operando X-ray Techniques in Catalysis, a Case Study: CO Oxidation by O2 over Pt Surfaces and Alumina Supported Pt Catalysts

“…Whilst the mathematical basis of this approach [184][185][186][187] is well beyond the scope of this review the major benefits of this method may be summarized as follows. In the first instance it permits a considerable enhancement in the sensitivity of any given technique to changes that are related to the applied perturbation and can therefore reveal aspects of the chemistry at hand that are otherwise invisible.…”

“…For the moment, however, MES is predominantly used as a qualitative enhancer of the sensitivity of the basic XAFS or XRD based experiment. To the author's knowledge, quantitative restoration of reaction kinetics using the full frequency response capacity of MES has yet to be demonstrated for X-ray based experiments, though examples do exist in the world of infrared spectroscopy (see for instance [186]). …”

“…MES, using phase sensitive detection (PSD), is founded upon the principle that the behavior of a system that may be reversibly perturbed (for instance redox cycling) can then be analyzed on the basis that an applied square wave perturbation may be expressed as Fourier series wherein the square wave is viewed as an infinite summation of odd-frequency sine functions [184][185][186][187]. Almost by definition, therefore, any decent (i.e., continually reformed) catalytic system should be amenable to this form of experimentation and subsequent analysis.…”

“…The infrared spectroscopic community were the first to adopt this type of approach to studying catalysis [183][184][185][186][187][188][189] and hence the soubriquet MES. However, this is an a posteriori method that is not limited to spectroscopic techniques; providing the requisite reversible modulation may be realized, any data, be it spectroscopic or scattering in nature, may be subsequently handled in this manner.…”