Host: The Japan Society of Vacuum and Surface Science
Name : Annual Meeting of the Japan Society of Vacuum and Surface Science 2023
Location : [in Japanese]
Date : October 31, 2023 - November 02, 2023
Introduction
The utilization of biomass-derived polyols includes the conversion to alkene molecules through catalytic removal of oxygen atoms known as deoxy-dehydration (DODH) reaction. For this reaction, ReOx-Au/CeO2 catalyst, consisting of rhenium (Re) oxide and gold (Au) nano-particles (NPs) supported on cerium dioxide (CeO2), was found highly active [1]. It is proposed that the ReOx is partially reduced via hydrogen spillover from the Au NPs under the presence of molecular hydrogen and that the partially reduced ReOx acts as an active site for the DODH reaction [2]. It has not been unveiled, however, how the Au NPs promotes the hydrogen-induced reduction and how the Au-NP-activated hydrogen reduces the ReOx and the CeO2 surfaces. In this study, the chemical states of Re and Ce of the ReOx-Au/CeO2 model catalyst under exposure to molecular hydrogen were studied using near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS).
Experiment
The ReOx-Au/CeO2 model catalysts were prepared by vacuum deposition of Re and Au on CeO2 thin films with a thickness of 10 nm formed on Si substrates. After co-deposition of Re and Au, the CeO2 thin films were thermally treated at 300 ºC in air. Characterization of the sample surfaces with XPS and AFM revealed that the ReOx is deposited with coverages of sub-monolayer and the Au atoms form NPs with a diameter of approximately 10-12 nm and an average spacing of 45 nm. The hydrogen-induced reduction process was traced with NAP-XPS at BL-13B of the Photon Factory under 0.1 Torr H2 and at three different temperatures (100 ºC, 140 ºC and 250 ºC). According to a recent report [3], hydrogen spillover takes place at 100 ºC and 140 ºC on CeO2, while it does not occur at 250 ºC and instead of that water desorption takes place.
Results and Discussion
In the beginning, the ReOx is pre-treated by heating in gaseous oxygen to be oxidized to a Re oxidation state of 7+. Then the oxidized surfaces were exposed to molecular hydrogen with a pressure of 0.1 Torr. Fig.1(a) shows time evolution of Re 4f7/2 XPS taken at 140 ºC. The oxidation state of Re was reduced from 7+ to 6+ and 4+, while components of the lower oxidation states such as 2+ and 0 were not observed. Comparison of evolution of the reduced component of Re between with and without the Au NPs revealed that the presence of the Au NPs obviously enhances the reduction rate of Re, which supports hydrogen activation and following hydrogen spillover by the presence of the Au NPs. It was also confirmed that the higher temperature induces the higher reduction rate as shown in Fig. 1(b). The higher reduction rate observed at 250 ºC suggest that direct reduction of ReOx by molecular hydrogen might contribute dominantly at such a high temperature. While at low temperatures below 150 ºC, where the hydrogen spillover takes place [3], the hydrogen-spillover-induced reduction should be taken into accounts, which will be discussed in the presentation.
References
[1] Ota, N. et al. ACS Catal. 2016, 6, 3213-3326. [2] Nakagawa, Y. et al. ACS Catal. 2018, 8, 584-595. [3] Beck, A. et al. ACS Nano 2023, 17, 1091-1099.
