In Situ Optical Reflectance Difference Observations of CO Oxidation over Pd(100)

Abstract

Using a home-built reflectometer, we have investigated the changes in the optical reflectivity of a Pd(100) model catalyst during CO oxidation under high-pressure, high-temperature conditions. We observe changes in optical contrast when exposing the surface to CO oxidation conditions at 200 mbar from room temperature up to 400 °C. These changes in reflectivity are a result both of the formation of a surface oxide layer and of a change in surface roughness because of gas exposure. However, the reflectivity is more sensitive to the presence of a thin, flat oxide layer than to surface roughness. CO oxidation plays an important role in the decrease of the reflectivity. Since adding a reducing agent to the gas mixture renders it unlikely that the oxide thickness increases, we conclude that the observed decrease in reflectivity is dominated by increased surface roughness because of the catalytic reaction. We contribute this observed surface roughening to a Mars–van Krevelen-type reaction mechanism.

Operando Reflectance Microscopy on Polycrystalline Surfaces in Thermal Catalysis, Electrocatalysis, and Corrosion

“…They further stipulate that even very thin oxides with a thickness of only a few nanometers can be detected using this method. 15 Since it works with visible light, 2D-SOR can be used under high-pressure conditions and when the sample is submerged in an electrolyte. We have since developed this method further and have also used it in combination with high-energy surface X-ray diffraction (HESXRD) where we correlate changes in the surface reflectance to changes in the surface oxide thickness and roughness on a single-crystal Pd(100) sample.…”

Real-time imaging of surface chemical reactions by electrochemical photothermal reflectance microscopy

“…This phenomenon can be explained by the CO adsorption/oxidation on the Pt electrode and the oxidation of the Pt surface. 8 At the beginning of the potential sweep, a part of formic acid is dissociated to form adsorbed CO (CO ad ) on the Pt surface. In this case, the CO to Pt forward donation of electron greatly outweighs the backdonation, the net charge of the Pt is negative, which leads to a high free electron density of metal, large reflectance of the probe light and low photothermal intensity (Figure 2b, 0.5 V).…”

“…2,3 However, the imaging speed of traditional scanning electrochemical microscopy ranges from a few minutes to a few hours per frame, due to the move-stop-measure scan mode and a trade-off between probe response time and current detection sensitivity. 4 In situ spectroscopic methods such as electro-reectance, [5][6][7][8][9] infrared spectroscopy, 10,11 Raman spectroscopy, 12 sum-frequency generation spectroscopy, 13,14 second harmonic generation, 15 and their surface-enhanced derivatives need relative long signal integration time for each measured spot. To real-time monitor the complete electrochemical processes, including the intermediate species, on the whole surface, an electrochemical imaging approach with a temporal resolution compatible with the process on the electrode surface is required.…”

Photoreduction of a Pd-Doped Mesoporous TiO2 Photocatalyst for Hydrogen Production under Visible Light

“…In this case, layers of PdO could still be present on the TiO 2 , shielding the TiO 2 from light absorption. The formation of such sites could increase the photocatalyst reflectivity leading to visible light scattering [61]. As well, this phenomenon could also be attributed to the partial blocking of semiconductor pores, which may decrease the TiO 2 specific surface area, as reported in Table 1.…”