Molecules adsorbed on rough metallic surfaces give very strong infrared absorption. The infrared spectroscopy using this effect, surface-enhanced infrared absorption spectroscopy (SEIRAS), has successfully been applied to in situ, time-resolved monitoring of reactions at the electrochemical interface and provided several new insights into electrochemical surface science. Mechanistic studies on the fundamental electrocatalytic reactions relevant to fuel cells are focused in this review. The reactions treated in this review are hydrogen evolution reactions on Pt and Ag, and electrochemical oxidation of methanol, formaldehyde, and formic acid on Pt, through which it is highlighted that time-resolved analysis of reactions is quite essential for understanding reaction mechanisms at the molecular scale.
Although the dynamic behavior of reactant molecules adsorbed on catalyst surfaces can be monitored by several spectroscopic methods, dynamic structural changes in solid catalysts themselves have not been studied. Recent developments in in-situ time-resolved XAFS techniques enabledthe study on structural kinetics of catalytically active structures, which shows the roles of catalysts in catalysis. In this paper, recent in-situ time-resolved XAFS studies on catalysis at solid surfaces are introduced.
The interaction between metals and oxides is an important key factor governing the electronic state and structure of the deposited metals and the physical and chemical properties of metal-oxide systems. This review describes our successful elucidation of two principles in the interaction between metals (single metal atom and also metal clusters) and metal oxide surfaces. Precise structures which have never before been obtained can be revealed using a combination of modern surface science techniques such as Polarization-dependent Total Reflection EXAFS, Scanning Tunneling Microscopy and Kelvin Force Probe Microscopy. Our results demonstrate that surface oxygen atoms and the metal oxide geometric arrangement play a key role in both metal immobilization and self-regulated growth on metal oxide surfaces.
In order to get deeper insights into Co-MoS2 hydrodesulfurization (HDS) catalysts, we tried to prepare surface designed Co-Mo HDS catalysts, in which the active sites, CoMoS, are selectively and fully formed, by means of a CVD technique using Co(CO)3NO as a precursor to Co sulfide. The CVD catalysts were characterized by XPS, XAFS, NO adsorption, and magnetic measurements. Thus CVD-Co/MoS2 catalysts allow us to study the structure of CoMoS, the reaction mechanism of HDS, and the effects of additives and sulfidation conditions on HDS activity in terms of TOF. The structure of CoMoS is described as dinuclear Co sulfide clusters located on the edge of MoS2 particles. The electronic state of CoMoS and its TOF are determined by the interaction between MoS2 particles and the support.
Although IR and UV-Vis are conventionally used spectroscopies, in-situ analysis gives new and effective insights of the reaction mechanism of catalysts. In this manuscript, application of in-situ IR and UV-Vis to the reaction mechanism of automobile catalysts is described. The first topic is kinetic analysis of stored nitrates on NSR (NOx Storage-Reduction) catalyst by in-situ IR. The application of in-situ IR to the study on reaction mechanism of HC-SCR (Selective Catalytic Reduction of NO by hydrocarbons) over Ag/alumina catalyst is also reported. The effect of carbon number in hydrocarbon reductants is clarified. Finally, observation of morphology change in Ag species of Ag/alumina catalyst by in-situ UV-Vis is described. From the comparison of dynamic change in morphology of Ag species and catalytic activity, the mechanism of hydrogen effect on HC-SCR is clarified.
Our recent progress in the study on zeolite acidity is reviewed. An IRMS-TPD (infrared spectroscopy/mass spectroscopy-temperature programmed desorption) of ammonia experiment provides us not only number and strength of the acid site, but the structure of Brønsted or Lewis character. When more than two kinds of the Brønsted acid sites exist, these acid sites can be measured individually and directly. This is an important advantage of the method, because any other methods cannot provide such precise characterization data. Furthermore, these findings are supported by the DFT (density functional theory) calculation. Three techniques, i.e., TPD, IR, and DFT, therefore, are combined into a powerful technique to study the zeolite acidity. Studies on four kinds of Brønsted acid sites in HY and cation exchanged HY zeolites, known as catalysts for the cracking of hydrocarbons, are explained as examples.
Activated carbons (ACs) oxidized and out-gassed were prepared by introducing acidic functional groups in the warming nitric acid and eliminating them in He flow at 1000oC, respectively. Adsorption time course of 4-chlorophenol (4CP) on the carbons was examined in aqueous solutions. Though any significant pH variation could not be observed for the oxidized AC, the pH for the out-gassed AC obviously decreased as a result of concurrently releasing chloride ions from the carbon surface into the aqueous solution. Measuring surface functional groups on the 4CP adsorbed carbon rinsed with methanol to thoroughly remove physically adsorbed 4CP indicated that the hydroxyl groups increased by 2.5 times as many as the released chloride ions, and the basic sites decreased in contrast. 4CP is estimated to degrade on the out-gassed carbon, releasing chloride ions in the aqueous solution and forming chemical bonding between decomposed phenolic compound and some basic sites on the surface.