Ken-ichi Tanaka

Adsorption kinetics and patterning of a Si(111)-7×7 surface by dissociation of methanol

CH(3)OH undergoes dissociation on a Si(111)-7 x 7 surface via a two dimensionally free precursor. The sticking probability attained by the STM (scanning tunneling microscopy) was entirely coverage independent, where the observed image represented the final state of the adsorption. CH(3)OH dissociates equally on the faulted and unfaulted halves at room temperature. However, the dissociation at the center adatom-rest atom site is four times preferential to that at the corner adatom-rest atom site in each half unit cell. Such site selectivity, center/corner, changes with the occupation of adatoms in corresponding half unit cell, that is, center/corner=4 for the half unit cell with one reacted adatom, but 2.6 and 1.8 for the half unit cells with two and three reacted adatoms, respectively. Such site selectivity is well rationalized by the dissociation depending on the local conformation of the site instead of the local density of states (LDOS). The site selectivity of center/corner is well reproduced by considering the occurrence probability of the whole dissociation pattern. As the STM image represents the final state of the adsorption, if the final step of adsorption involves dissociation of molecule or precursor, the STM image reflects the dissociation probability depending on the local structure. On the other hand, if no dissociation of molecule or precursor is involved at the final step, the adsorption probability might depend on the LDOS. The adsorption of H(2)S, H(2)O, and NH(3) is also discussed from this general viewpoint of adsorption. The concept of a two dimensionally free precursor will be important to understand the kinetics of heterogeneous catalysis.

Dissociation mechanism of 2-propanol on a Si(111)-(7×7) surface studied by scanning tunneling microscopy

Adsorption of 2-propanol, (CH3)2CHOH, on a Si(111)-7x7 surface was studied by scanning tunneling microscopy. (CH3)2CHOH adsorbs equally on the faulted and unfaulted half unit cells by forming Si-OCH(CH3)2 and Si-H on an adatom and rest atom pair. Si-OCH(CH3)2 is consecutively increased in each half unit cell, and the adsorption is saturated when every half unit cell has three Si-OCH(CH3)2, which corresponds to 0.5 of the adatom coverage. The sticking probability for the dissociation of (CH3)2CHOH is independent of the adatom coverage from 0 to 0.4, but it depends on coverage at higher than 0.4. By counting the darkened adatoms, Si-OCH(CH3)2 on the center adatom (m) and that on the corner adatom (n), it was found the m/n ratio is ca. 4 for the first dissociation of (CH3)2CHOH in virgin half unit cell, but it becomes ca. 1.9 and 1.8 when two and three Si-OCH(CH3)2 are contained in a half unit cell. This result reveals that the dissociation probability of (CH3)2CHOH at the adatom-rest atom pair site is influenced by the nearest Si-OCH(CH3)2 in the half unit cell.

Adsorbed atoms and molecules destined for a reaction

The idea of an activation complex is popular for explaining reaction rates, but the characteristics of reactions and catalysis may not be explained in this way. A predestined state for each reaction composed of surface atoms and adsorbed species is responsible for these features. Two single Sn atoms trapped in adjacent half-unit cells of an Si(111) 7 × 7 surface is an example of a predestined state. An isolated Sn atom in a half-unit cell does not migrate to other half-unit cells at room temperature, but when two single Sn atoms are in adjacent half-unit cells they undergo rapid combination to form an Sn2 dimer. In addition, these two single Sn atoms replace the center Si adatoms and an Si4 cluster is formed. The spatial distribution of molecules desorbing from surfaces may reflect the predestined states for the desorption processes. The spatial distribution in the temperature-programmed desorption (TPD) of NO on Pd(110) and Pd(211) surfaces and that in the temperature-programmed reaction (TPR) of NO + H2 were studied. N2 desorbing from Pd(110) by the recombination of N atoms obeys cos6θ − cos7θ but the N2 produced by a catalytic reaction of NO with H2 obeys cosθ. In contrast, the N2 desorbing with NO at 490 K in the TPD of Pd(110) shows a sharp off-normal distribution expressed by cos46(θ − 38). The adsorption of NO on Pd(211) predominantly occurs on the (111) terrace but the spatial distribution suggests that the predestined states for the reaction and desorption are formed on both the (111) terrace and (100) step surfaces.

DRIFTS investigation and DFT calculation of the adsorption of CO on Pt/TiO2, Pt/CeO2 and FeOx/Pt/CeO2.

Molecular structures and vibrational spectra of the CO species adsorbed on the Pt/TiO2, Pt/CeO2 and FeOx/Pt/CeO2 have been investigated by means of density functional theory (DFT) calculation and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The geometrical structures and vibrational frequencies were obtained at the MPW1PW91/SDD level. Theoretical calculation shows that the calculated IR spectra were in good agreement with the experimental results. The calculated results clarify the assignment of the adsorbed CO species on the surface of Pt/TiO2, Pt/CeO2 and FeOx/Pt/CeO2.

Surface Nano-Structuring by Adsorption and Chemical Reactions

Nano-structuring of the surface caused by adsorption of molecules or atoms and by the reaction of surface atoms with adsorbed species is reviewed from a chemistry viewpoint. Self-assembly of adsorbed species is markedly influenced by weak mutual interactions and the local strain of the surface induced by the adsorption. Nano-structuring taking place on the surface is well explained by the notion of a quasi-molecule provided by the reaction of surface atoms with adsorbed species. Self-assembly of quasi-molecules by weak internal bonding provides quasi-compounds on a specific surface. Various nano-structuring phenomena are discussed: (i) self-assembly of adsorbed molecules and atoms; (ii) self-assembly of quasi-compounds; (iii) formation of nano-composite surfaces; (iv) controlled growth of nano-materials on composite surfaces. Nano-structuring processes are not always controlled by energetic feasibility, that is, the formation of nano-composite surface and the growth of nano-particles on surfaces are often controlled by the kinetics. The idea of the “kinetic controlled molding” might be valuable to design nano-materials on surfaces.

Controlled growth of Zn nano-dots on a Si(111)-7×7 surface saturated with C2H5OH

Metal atoms bonded with Si adatoms on the Si(111)-(7x7) surface undergo migration by hopping adjacent Si-rest atoms with dangling bond. By saturated adsorption of Si(111)-(7x7) surface with C(2)H(5)OH, the whole Si-rest atoms and a half of Si adatoms are occupied with Si-H and Si-OC(2)H(5), so that the Zn atoms adsorbed on this surface cannot migrate by hopping. When Zn atoms were deposited on this surface, ca. 5 nm Zn dots were grown in the hexagonal spacing of ca. 5.4 nm width around the corner holes, which work as a mold. This is quite different from the growth of honeycomb layers composed of Zn(3) clusters on the clean Si(111)-(7x7) surface. The dots grow up to nine (1.97 nm) to 13 layers (2.64 nm) by keeping their size, which implies a layer-by-layer growth of dots in the mold, where the growth is controlled by the kinetics instead of energetic feasibility.

Catalytic oxidation of CO on metals involving an ionic process in the presence of H2O: the role of promoting materials

A new catalytic oxidation of CO involving an ionic process in the presence of H2O is proposed on a Pt-catalyst with specific promoting materials (co-catalysts). Oxidation of CO is very slow at room temperature on ordinary Pt-catalysts such as Pt/SiO2, Pt/Al2O3, Pt/TiO2, Pt/Graphite, and Pt/carbon nano-tube (CNT), and H2 or H2O have no effect on the reaction. However, in the presence of specific co-catalysts, the oxidation of CO is markedly enhanced by H2 or H2O, so that highly selective preferential oxidation (PROX) of CO in H2 is attained. The role that co-catalysts play in the oxidation of CO enhanced by H2 or H2O was clarified by the experiments with Pt supported on CNT and carbon nano-fiber (CNF) that had Ni–MgO and FeOx at their one terminal end, respectively. Oxidation of CO was markedly enhanced by H2 on the Pt/CNT and Pt/CNF, but no enhancement was observed on the Pt/CNT-p and Pt/CNF-p, where the CNT-p and CNF-p were purified by removing Ni–MgO and FeOx. Similar enhancement of the oxidation of CO by H2 or H2O was observed on FeOx/Pt/TiO2 and FeOx/Au/TiO2, although no enhancement was observed on the Pt/TiO2 and Au/TiO2 catalysts. The in situ DRIFT spectra of the FeOx/Pt/TiO2 catalyst (Fe:TiO2:Pt = ca. 100:100:1) during reaction in a flow of (CO + O2 + H2) suggested the rate-determining slow step was HCOO(a) + OH(a) → CO2 + H2O. The oxidation of CO enhanced by H2O/D2O and H2/D2 showed a common hydrogen isotope effect of rH/rD = 1.4–1.5. The highly selective oxidation of CO in H2 on the Pt/CNT and Pt/CNF catalysts strongly suggests efficient transport of ionic intermediates from Ni–MgO or FeOx to Pt over the hydrophobic CNT and CNF surface according to the local potential gradient. According to this mechanism, selectivity in the preferential oxidation of CO in H2 is defined by the turnover number of a H2O molecule forming CO2 during its residence time on the catalyst, which is essentially different from the selectivity based on competitive adsorption and/or reaction. The role of the H2O molecule is as a kind of messenger molecule or a molecular catalyst to promote the reaction on the surface expressed by the equation n(CO + 1/2O2) + H2O → nCO2 + H2O. In this mechanism, the selectivity is given by n/(n + 1). Curious phenomena previously observed in the PROX reaction of CO in H2 on various catalysts are well explained by the mechanism including an ionic process proposed in this paper.

Study on Formation Process and Models of Linear Fe Cluster Structure on a Si(111)-7 × 7-CH3OH Surface

STM results showed that Fe atoms were deposited on a Si(111)-7 × 7 reconstructed surface, which was saturated with CH3OH molecules. Fe atomic linear structure was composed of stable clusters and in-situ observed by the scanning tunneling microscopy (STM). The aim to improve its application of magnetic memory material, both formation process and models, has been explored in this paper. By combining surface images and mass spectrometer data, an intermediate layer model was established. In terms of thermal stability, the most favorable adsorption sites of CH3OH were further explored. After that, Fe atoms were deposited on the Si(111)-7 × 7-CH3OH surface, forming a linear cluster structure. On the one hand, a new Fe cluster model was put forward in this paper, which was established with height measurement and 3D surface display technology. This model is also affected by the evaporation temperature, which can be consistent with the atomic stacking pattern of face centered cubic structures. On the other hand, the slight height change suggested the stability of linear structures. Even in the condition of thin air introduction, Fe cluster showed a good performance, which suggested the possibility of magnetic memory application in the future. These investigations are believed to have, to a certain extent, increased the probability of forming Fe linear clusters on the surface of silicon substrate, especially according to the models and surface technology we adjusted.

Development of a Highly Active Pt-Catalyst for Selective Oxidation of CO in Excess H2 at Room Temperature

Conference Name:7th International Forum on Advanced Material Science and Technology, IFAMST-7. Conference Address: Dalian, China. Time:June 26, 2010 - June 28, 2010.

Hydrogen Storage in Nitrides by the Use of Ammonia as a Hydrogen Carrier

Hydrogen storage by calcium nitride or magnesium nitride has been undertaken by the use of ammonia, in which the possibility of ammonia as a vector for hydrogen carriers has been studied. When the calcium imide ornitride obtained by thermal decomposition of calcium amide dispersed on active carbon (AC) was brought into contact with ammonia gas (300 Torr) at room temperature, NH3 uptake readily occurred. When the sample after NH3 uptake was heated, the absorbed ammonia was released in the form of hydrogen and nitrogen. The ammonia is possibly absorbed in the form of the decomposed state in the imide ornitride. This type of hydrogen storage has been extensively studied and characterized.For magnesium nitride, ammonia was absorbed and desorbed without the decomposition.