第574回のスポットライトリサーチは、北海道大学 触媒科学研究所 触媒構造研究部門 高草木研究室のCan Liu（劉燦）博士にお願いしました。
Can Liu, Bang Lu, Hiroko Ariga-Miwa, Shohei Ogura, Takahiro Ozawa, Katsuyuki Fukutani, Min Gao, Jun-ya Hasegawa, Ken-ichi Shimizu, Kiyotaka Asakura, and Satoru Takakusagi
J. Am. Chem. Soc. 2023, 145, 36, 19953-19960
The mechanisms of surface reactions are at the core of understanding heterogeneous catalysis. The dynamic behavior of intermediate adsorbates, such as their diffusion, spillover, and reverse spillover, determines the activity and product selectivity of oxide-supported metal catalysts. Understanding these surface processes at the atomic level is critical, but challenging due to the limitations of conventional characterization techniques.
We have previously observed the dissociation of methanol molecules on Pt nanoparticles into methoxy intermediates, which then spilled over to the five-fold coordinated Ti4+ sites (Ti5c) of the TiO2(110) surface (see Fig. 1a). In this work, through temperature programmed desorption (TPD) experiments, we found that most of the methoxy intermediates were thermally decomposed to CO and CH4 above 350 K (see Fig. 2). The activity and product selectivity for methoxy decomposition depended on the Pt nanoparticle density. It suggests that the Pt nanoparticles acted as active sites and the decomposition property was controlled by the diffusion and reverse-spillover of the methoxy intermediates (see Fig. 1b).
Then, how do the methoxy intermediates reach the Pt active sites? We performed sequential scanning tunneling microscopy (STM) imaging to monitor the movement of individual adsorbates. As is shown in Fig. 3, the methoxy intermediates diffuse along the  or direction and can thus move throughout the entire surface. Based on the visual evidence, we further investigated the atomistic details of the diffusion paths using density functional theory (DFT) calculations. Figure 4 shows the atomic model of diffusion in the  direction, as well as the energy diagram showing a reasonable energy barrier. The temporal formation of molecularly adsorbed methanol is the key to the easy diffusion along the  direction. As for the diffusion along the direction, apart from the formation of molecularly adsorbed methanol, the presence of an additional proton on the adjacent bridging oxygen is also necessary to reduce the energy barrier to a reasonable value. Therefore, we have resolved the diffusion pathways and proposed the model in which the methoxy intermediates diffuse and reach the Pt active sites for their decomposition (see the scheme in Fig. 1b).
This study successfully elucidates the complex elementary steps in heterogeneous catalysis at the atomic level. It shows how the intermediate adsorbates diffuse on the catalyst surface and reach the active sites to yield the corresponding products. It also provides new inspiration for catalyst design that activity and selectivity can be modulated by considering the surface diffusion.
The direct STM visualization of the movement of the intermediate adsorbates on the catalyst surface is certainly one of the most attractive features of this study. Our group has a long history of research on methanol adsorption on the Pt/TiO2(110) surface, which can be traced back to our first paper published in Langmuir in 2010 (S. Takakusagi et al., Langmuir 26 (2010) 16392). Since then, we have devoted much effort to the systematic study of the preparation and observation of the methoxy intermediates. This study focused on the investigation of their dynamic behavior and decomposition property, which was realized by precisely controlling the experimental conditions and by combining various surface science techniques, such as TPD and XPS.
STM imaging of the behavior of the molecules at the atomic level naturally bridges the gap between experiment and theory by pinpointing the rational diffusion pathways for DFT calculation. In addition, the comparison between high and low densities of Pt nanoparticles effectively proves that the decomposition is controlled by diffusion and reverse-spillover, and the main active sites are the Pt nanoparticles. Imaging the diffusion of methanol on Pt/TiO2 catalyst helps to reveal more accurate decomposition mechanism. In this study, fundamental surface science approach showed its power and beauty by resolving a surface catalytic reaction at atomic level.
Although STM is a well-established technique, obtaining high-resolution images on a routine basis is still a difficult and time-consuming task. First, high-resolution STM images require sharp and robust tungsten tips. However, the shapes of the tips prepared by electrochemical etching are very sensitive to the preparation conditions, resulting in a low yield. To make matters worse, the only effective way to check the quality of the tips is to use them in a condition similar to a practical experiment. Therefore, ensuring a perfect tip is time-consuming. Second, the scanning condition is also critical. Any surface contamination could alter the dissociative adsorption of methanol and the movement of the adsorbates. In addition, scanning parameters such as area, speed, and bias voltage were optimized to follow the movement of the methoxy intermediates. A large number of sequential images had to be taken to capture the moment of hopping. The TPD measurements of the methoxy intermediates on the Pt/TiO2(110) were also not easy because their coverage was rather small and high sensitivity detection of the desorption products was required. I performed them in collaboration with the group of Prof. Katsuyuki Fukutani (Univ. of Tokyo), an expert in TPD measurements, and then succeeded in obtaining the reliable results.
Chemistry is closely related to the chemical industry and has great value, but is ultimately complex. The theory of physical chemistry in homogeneous phase has been well developed over years. However, it is difficult to establish mechanism and theory for the complex surface reactions such as heterogeneous catalysis and electrocatalysis due to limited experimental evidence. Surface science techniques have the potential to solve this problem. The surface chemical processes can be studied at the atomic/molecular level and accurately compared with theoretical analysis. I will try my best to connect experiment and theory of surface chemistry in my future research career.
Repeated experimentation is tedious. Scientific discovery, on the other hand, is exciting. Hard work deserves success. In particular, I would like to thank my supervisor Professor Takakusagi for his dedicated support and guidance of the project, my associate supervisor Professor Asakura for his helpful discussions and suggestions, and all the collaborators for their efforts and contributions with professional skills and knowledge.
Can Liu 劉燦
Institute for Catalysis, Hokkaido University
2009.6, BEng in Materials Physics at University of Science and Technology Beijing
2014.7, PhD in Materials Science and Engineering at Tsinghua University; Supervisor Prof. Zhengjun Zhang; Research theme: Preparation and electrochemical property adjustment of molybdenum oxide film
2014.9 -2017.3 , Postdoctoral researcher at Institute for Catalysis, Hokkaido University; Supervisor Prof. Shen Ye; Research theme: In-situ studies on the electrode reactions of lithium-oxygen battery
2018.4 -present, Postdoctoral researcher at Institute for Catalysis, Hokkaido University; Supervisor Prof. Satoru Takakusagi; Research theme: Molecular adsorption and reaction on the model catalyst surfaces