Bridging the pressure gap: adsorbate-induced surface restructuring of the model catalysts at the atomic scale

Abstract

Advanced technologies have led to more improvements in our lives in the past several decades than occurred over the entire last century. Science has also benefited from those advances. Human beings have gained the opportunity to explore in detail a microscopic world that is invisible to our naked eyes. Among scientific fields, chemistry, which covers a broad range of phenomena, including many related to industrial mass production, has crucially contributed to our basic understanding, and especially the basic unit known as a molecule. Various chemical reactions used in the mass production of commodities, and to the production and conversion of energy, are now indispensable. Catalysts, for example, are used in a wide range of applications, where they have a decisive influence on reaction rates, and by decreasing activation energy barriers, also reduce energy consumption. In the study of catalytic reactions, the catalyst’s surface has been found to play a large role in efficiency and performance. Prof. Dr. Gerald Ertl, who is a Nobel prize winner in Chemistry (2008), explained the detailed process of ammonia gas production on the catalyst surface during catalysis, and Prof. Gabor A. Somorjai, who is called a father of surface science, discovered the mechanisms behind diverse industrial chemical reactions over model and real catalysts, and proposed many strategies for enhancing industrial catalysts. Many studies have been carried out to elucidate the principles of chemical reactions on a material surface. Many of the discovered principles can be applied in ways that benefit our lives. But while countless studies have been performed in the field of surface science, until now there has been a clear gap between the active surface area and the dimension of the material surface. Moreover, many catalytic reactions are influenced by pressure gap between in an ultra-high vacuum (UHV) condition and ambient pressure environments, which is faced challenging problem to be solved in the surface science field at present. In the well-known Langmuir-Hinshelwood surface reaction mechanism, the chemical reactants undergo a series of processes, such as adsorption on the surface of the material, diffusion, dispersion, intermediate formation reaction, and desorption. As a result, the size of the surface area and the distribution of effective active sites can have a great influence on the chemical reactivity of the catalysts. One of the fundamental goals of surface science is to investigate the effects of catalyst and gas pressure in relation to the dimensions of the materials. For example, a flat surface with a two-dimensional shape has a limited number of exposed atoms, ~ 10$^{15}$/cm$^2$, regardless of the type of material. In contrast, powder catalysts have an unlimited number of exposed atoms, and the effect is not comparable to a material having a two-dimensional surface area. In addition, various surface shapes as well as the size of the exposed surface can affect the configuration of the reaction sites that can be reacted with molecules, leading to very complicated reaction mechanisms. If such factors are not taken into consideration, it can be extremely difficult to predict performance. It is a well-known proposition that the chemical reaction results obtained from model catalysts in the laboratory do not necessarily coincide with those used in actual mass production chemical processes. For example, to investigate most single crystal materials with a two-dimensional surface morphology, most modern surface science techniques require an ultra-high vacuum environment. However, the actual processes are performed in atmospheric environments. Processes that involve reactions that must cross a high activation energy barrier are performed at high pressures in the range of tens to hundreds of atmospheres. As a result, there are large differences in chemical potentials compared with those observed under experimental conditions in the laboratory. Importantly, this difference in chemical potential is directly related to the rendering of the different free energies on the surface, which has resulted in the publication of surprising results that tend to ignore surface restructuring or reconstructions of the material surface. Structural changes lead to modifications of active sites, and so results on these altered surfaces will inevitably differ from the previously predicted reaction mechanisms. To accurately discuss the chemical reactions occurring in the actual process, and explain the exact mechanistic processes in heterogeneous catalysis, measurement results need to be obtained in real world conditions. To accomplish this, recently developed ambient pressure scanning tunneling microscope (AP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), can provide more advanced analysis at elevated pressure conditions than previous surface science techniques. In this thesis, the surface reactions and morphologic changes under near-atmospheric pressure environments for Pt(557), Pt(111), Ni(111) and Pt$_3$Ni(111) single crystal model catalysts are discussed. It was determined that the surface structure of the Pt(557) model catalyst, which theoretically has a finite width difference of 1.3 nm, showed dramatic surface restructuring at elevated pressure and at elevated temperature, because the catalyst has a very dense step-terrace structure. This unstable surface is affected by the adsorption process of reactants, which affects the adsorption of CO prior to CO oxidation, one of the well-known industrial reactions. As a result, a disordered-ordered reversible phenomenon was observed, depending on changes in the thermodynamic parameters. The results for the Pt(111) and Ni(111) model catalysts were compared with reported model adsorption structures on the surface during exposure to carbon monoxide (CO) or oxygen (O$_2$) gas at atmospheric pressure, and showed how the surface changed with elapsed-time during the CO oxidation reaction. For the Pt(111) model catalyst at near-atmospheric pressure conditions, CO and O$_2$ molecules were competitively adsorbed, and theoretically, so were substances which can be regarded as intermediates of the predicted super-oxo formations. It was confirmed in our AP-STM experiments that the intermediate formations and desorption are performed repeatedly every time. For the Ni(111) model catalyst, we discovered that the well-known Ni oxidation model was no longer appropriate in a near-atmospheric pressure environment. The thickness and cluster size of the nickel oxide changed depending on the level of O2 pressure. However, during CO oxidation, selective surface depressions were observed for the O2-assisted Ni(CO)4, which is consistent with theoretical predictions in the atmospheric O2 condition. The surface of the Pt$_3$Ni(111) model catalyst, which is a type of alloyed material, is unlike those of the above Pt(111) and Ni(111) model catalysts, and so were the observed chemical reactions under CO, O$_2$, and CO oxidation. In particular, the well-known layer of platinum film was not stable under O$_2$ atmospheric pressure, and subsurface nickel atoms were segregated on the surface, forming oxidized nickel clusters, and creating an interfacial Pt-NiO$_{1-x}$ nanostructure, which is considered to be a critical reason for the enhanced catalytic performance observed during CO oxidation. These various types of model catalyst surfaces are also expected having the results depending on the pressure and temperature, their studies for comprehensive understanding helps to identify chemical mechanisms in actual industrial reaction process.