Showing papers in "Journal of the Research Institute for Catalysis, Hokkaido University in 1977"

Application of the theory of stoichiometric number determination of the rate-determining step：the mechanism of water-gas shift reaction catalyzed by platinum

Abstract: The mechanism of water-gas shift reaction, CO+ H 20 = CO2+ H 2, catalyzed by evaporated platinum films was investigated by means of tracers of l3C and 180 over the temperature from 410 to 450°C on the basis of the theory of stoichiometric number determination of the rate-determining step. The stoichiometric number ~r of the rate-determining step was determined to be unity by tracing the transfer of carbon atoms between CO and COz by 13C, i. e. the rate-determining step consists in any constituent step of transfer of carbon atoms. The transfer of oxygen atoms among CO, H 20 and CO2 traced by 180, which was observed simultaneously with the former, showed that the ratio of the total forward rate of transfer of oxygen atoms from CO to CO2 and HzO to its reverse total rate is close to unity, which enables us to exclude several steps of the constituent steps of transfer of carbon atoms from being rate-determining. Assuming that the reaction 123 follows the sequence of steps, CO~CO(a), HzO~H(a)+OH(a), CO (a)+ OH (a).<-" X (a), 4 5 6 X(a).c--"COz(a)+H(a), COz(a).,--::>COz and 2H(a).,--::>Hz, it is concluded from the above results that step 4 or 5 is rate-determining, where (a) denotes the adsorbed state and X(a) is an intermediate; with reference to the rate equation of the reverse reaction previously observed, step 4 is inferred to be rate-determining. Other mechanisms are discussed, and inferred to be less plausible than the above mechanism.

Adsorption isotherm and the states of adsorption

Abstract: Existence of the two distinctly different states, the r- and s-states, of hydrogen adatom was previously deduced from the quantum-mechanical theory of adsorption developed by one of the present authorslO). The energy of the r-state adsorption of a single hydrogen adatom without interaction with other adatoms (an adatom free from interaction with other ones will be termed a single adatom in what follows) was shown lower than that of s-state one on any crystal plane of nickel and the energy of either state is lowest on the least dense (HO)-plane. Steps, kinks, and defects on a lattice plane furnish s-states of extraor­ dinarily low energy. The consequent distribution of sites over the energy of a single adatom should be uniquely deduced from isotherm observed at any temperature by the distribution function method, if the neglect of the interactions among adsorbates, implied in the latter method, were applicable at all. The distribution function exactly based on the observed isotherm was thus deduced by applying the SOMMERFELD's method of deducing the free energy of Fermi gas from the known distribution function of quantum states over energy inversely to the present problem. The distribution function of sites thus deduced for the dissociative adsorption of hydrogen shifted considerably with the observation temperature, from 0° to 300°C, of the isotherm. This contradicts the neglect of the interaction implied, since ex­ perimedtal facts exclude the change of the surface structure of the adsorbents over the temperature range in question from being alternatively responsible for the shift. Experimental informations indicated that the adsorbent's surface consists of crystal planes each big enough to secure the physical identity of metal atoms on it. The interac­ tion between r-adatoms was shown repulsive as caused by the exchange repulsion between hydrogen adatoms and their competition for metal electrons, besides by the dipole interac­ tion in case of charged adatom. There exists practically no interaction between s-adatoms nor between $- and r-adatoms. The model of exclusive adsorption of r-adatoms on (110)- *) In 1963 the Russian translation of the article was published in Vol. IV, No.1 of Kinetica i Katalis. The Editorial Board of the Journal of the Research Institute for Catalysis has the pleasure of presenting here the original English version of the article for the benefit of a larger circle of scientists. The permission of the Editorial Board of Kinetica i Katalis is gratefully acknowledged (Editor). **) Research Institute for Catalysis, Hokkaido University, Sapporo 060, Japan.

CHANGE OF MECHANISM OF THE HYDROGEN ELECTRODE REACTION WITH OVERPOTENTIAL：IV. Surface Coverage Effect

Abstract: Analysis of the distribution of the reaction affinity among constituent steps of the hydrogen electrode reaction and the current 1'S. overpotential relation, reported previously, was extended taking the impeding effect of the surface coverage into consideration. The discharge-combination and the discharge-electrochemical desorption routes were treated assuming the LANGMUIR and FRUMKIN-TEMKIN adsorption isotherms for the reaction intermediate. The surface coverage term does not much influence the affinity distribution but the current vs. overpotential relations are significantly modified. Comparison with experiments on Ni and Rh suggests that the kinetically effective surface coverage of the hydrogen intermediate would be much smaller than that observed electrochemically.

Electroreduction of acetone on platinum in sulfuric acid solution and catalytic action of platinum and mercury electrodes

Abstract: Electroreduction of acetone was conducted on smooth and platinized platinum electrodes in aqueous sulfuric acid solution. Acetone is reduced with a much slower rate, ca. a thousandth, than ethylene on a platinum electrode. Propane is produced with a selectivity of >90% on the platinized platinum electrode. Mechanistic study leads to the conclusion that (i) hydrogen atom and acetone adsorb on different facets (or different kind of sites), (ii) in the TAFEL line region, the combination of the adsorbed hydrogen and acetone at the boundary between the two facets is rate-controlling, and (iii) the limiting current is due to the surface diffusion of the adsorbed acetone to the boundary (or the change in the ad-sorption state). Difference of the reaction rate between the smooth and the platinized platinum electrodes is mainly attributed to that of the surface area. Hydrogen electrode reaction on a platinum electrode is discussed in term of the diffusion of the evolved hydrogen into the solution. Introduction ANTROPOVll concluded that metals of high hydrogen overvoltage are active in the reduction of polar bonds such as >C=O and metals of low hydrogen over voltage are active in the reduction of non-polar bonds such as >C=C<. One of the present authors2) compared catalytic actions of metal electrodes with respect to the hydrogen electrode reaction and the electroreduction of organic compounds and concluded that transition metals including gold, silver and copper (named d-metals) and metals after IIB in the periodic table (named sp-metals) show essentially different catalytic actions in harmony with ANTROPOV'S conclusion. In addition, he pointed out that steric selectivity is also different between the dand sp-metals; cis addition *) Dept. of Chern., Faculty of Science, Hokkaido University, Sapporo 060, Japan **) Res. lnst. for Catalysis, Hokkaido University, Sapporo 060, Japan

Water chemisorption and electrical conductivity of zinc oxide

Abstract: The effect of adsorption of water vapour on the electrical conductivity was investigated on zinc oxide, uoth pure as well as doped, containing 0.05, 0.1 and 1 mole percent of gallium and lithium. Water was found to undergo chemisorption at temperatures varying from 150-250°C. At 230°C and above chemisorption was accompanied by evolution of gas which was identified as hydrogen. The electrical conductivity decreased on water chemisorption and the volume of gas evolved subsequent to chemisorption was found to be related to the decrease in electrical conductivity. The experimental observations were interpreted to suggest that on n-type ZnO semiconductor, water undergoes dissociative chemisorption leading to the formation of 0 2and H+ ions and that the hydrogen ions suusequently get attached to quasi-free electrons and are liueratecl as gaseous hydrogen.

STUDIES ON COMOX CATALYST-PART 1：A study on fresh γ-Al2O3-CoO, γ-Al2O3-MoO3 and γ-Al2O3-CoO-MoO3 systems by ESR technique

Abstract: It is found that cobalt doped r-alumina tends to incorporate the cobalt ions in the bulk forming cobalt aluminate spinel. Molybdenum doped r-alumina contains molybdenum on the surface only and shows the presence of Mo (V) in small concentrations. Interaction between support and molybdenum starts at 300°C. In doubly doped (cobalt and molybdenum) samples two competitive processes occur. Previously present surface layer of molybdenum favours cobalt molybdate formation, otherwise cobalt diffuses in the bulk forming spinel. Magnetic susceptibility data are also reported.

Effect of surface potential on the rate of electron transfer step of hydrogen electrode reaction on metals

Abstract: The galvanostatic transient method has been applied to the hydrogen evolution reaction on gallium, gold, silver, nickel and platinum metals in aqueous sodium hydroxide and gold in sulfuric acid, in order to determine the overvoltage component 1)1 which is caused by charging up of the electric double layer at the metal-solution interface. It has been found that (i) the hydrogen overvoltage 1) is composed of two independent components 1J 1 and the other component 1)a which is caused by the change of the surface potential of the electrode due to specifically adsorbed Na atoms in alkaline solutions or H atoms in acid which is produced by the electron transfer step, and (ii) the polarization curve of the overall reaction log i vs. 1) is divided into two independent parts log i vs. 1)1 and log i V.I'. 1)a which correspond respectively to the polarization curve of the electron transfer step Na++e--Na(a) or H+ +e--H(a) and to that of the recombination of adsorbed H atoms. In alkaline solutions 1)a controls the activity of H (a) through the decomposition of water molecule by Na(a). On the basis of the FRUMKIN's electrode potential theory 1Ja has been given as the difference of the chemical potential of Na(a) or H(a) -LlPNa or -LIPH between the reversible and polarization states which means a compensation effect between the changes of the surface potential and the free energy of adsorption of Na (a) or H (a). As the result of the application of the compensation effect to the activation free energy of the electron transfer step which is given by the HORIUTI-POLANYI's rule, it has been shown by the rate theory developed by HORIUTI in the frame of the transition state method that the rate of the electron transfer step can be expressed by the Tafel equation in terms of '71 independent of the surface potential of the electrode in accordance with the experimental results. *) The experimental part of this paper was reported at the Second USSR-JAPAN Seminor on Electrochemistry held in Moscow, 15-20th, May, 1976. **) Research Institute for Catalysis, Hokkaido University, Sapporo, Japan.

EVALUATION OF THE EFFECTIVE CHARGES OF ATOMS FROM COMBINATION OF THE X-RAY Kα1 CHEMICAL SHIFTS AND THE ISOMER SHIFTS OF THE NΓR SPECTRA

Abstract: The chemical shifts of KaJ and K~l X-ray emission lines for a number of elements (I, Sn, Cd, Te, Ru .. ·) have been obtained with a 2 m bent-quartz-crystal spectrometer. An application of the "free ion" model for evaluation of the effective charges from the X-ray chemical shifts has been described and shown that for intransition elements with JlS and lip valence electrons the parameters of the theoretical equations describing dependence of the chemical shift on the number of removed electrons for KaJ and K~J lines are interdependent in HF approximation. The partial effective charges for 5s and 5p electrons of iodine and tin and for 55 and 4d electrons of ruthenium in their compounds were evaluated by combining X-ray KaJ chemical shifts with the isomer shifts of NI'R spectra. Additionally, the ligands of the hexacoordinated compounds of Ru were arranged into series according to their donor-acceptor abilities. The effective charges of atoms in about 90 compounds of considered elements are given in Tables.

Frequency response method for determination of the diffusion coefficient of hydrogen in metal

Abstract: A new electrochemical method, termed as "the frequency response method", for the determination of the diffusion coefficient D in metal is proposed. In the bielectrode system, there appears a phase lag in th.e hydrogen permeation current, when the hydrogen charging constant current is overlapped with a sinusoidal current. The relation between the phase lag and the D value is given by solving the diffusion equation. Measurements were carried out for hydrogen and deuterium in the a-phase systems of Pd/H and Pd/D in the temperature range of 0° to 40°C, and the following values were obtained:

Similarities between the intensity -energy relation in spectroscopic and electrode processes

Abstract: The similarities and differences between the spectroscopic dipole and electrochemical electron transitions are pointed out with the use of time-dependent perturbation theory. A proof of GURNEY condition of radiationless transition of electron at interfaces is given. An expression for the current density in quantum mechanical term is put forward. Introduction The object in this paper is to point out the correlations (and differences) between the quantum mechamcal aspects of the fundamental processes in spectroscopy and the lesser known similar aspects of the emission and ac­ceptance of electrons across the interfaces made by ionic solutions and metals or semi-conductors. Thus, phenomenologically, the method by which electrode-kinetic results are portrayed is only superficially different from that in which spectroscopic results are shown. The current density used in current-potential diagrams is the analogue of the intensity of the absorption or emission of photons plotted in a spectrum. For the transfer of an electron across an interface, the electrostatic work done is the potential difference multiplied by the elec­tronic charge and this is the analogue of the energy of the photon absorbed in spectroscopic processes. Thus, the intensity-frequency relation in absorp­tion spectroscopy (Fig. 1 a) is the analogue of the current-potential diagram in cathodic electrochemical processes (Fig. 1 b). Qualitatively, after a certain critical energy has been reached, further increase in the photon energy sud­denly increases the intensity of absorption of photons by particles in solution (or in the gas phase). Correspondingly, increase in the potential of an elec­trode beyond a critical value in the negative direction