星 永宏

Structural effects on the rate of the dechlorination of chlorobenzene have been studied using the low index planes of platinum and silver in acetonitrile solution. Chlorobenzene is electrolyzed on platinum electrodes using naphthalene as a mediator at - 2.94 V vs. Ag/Ag+. The main product is benzene. A reaction via a mediator does not depend on electrodes in general, because it is a homogeneous catalytic reaction. Surface structure, however, affects the partial current density of benzene (j(C6H6)) on platinum electrodes: Pt(111)< Pt(110)similar to Pt(100). On silver electrodes, j(C6H6) shows no structural effect in the electrolysis using naphthalene. Electrolysis without naphthalene gives the same structural effect as that with naphthalene on platinum electrodes, whereas no silver electrode reduces chlorobenzene at - 2.94 V vs. Ag/Ag+ without naphthalene. Therefore, the anomalous structural effect on the electrolysis with naphthalene is attributed to the existence of a direct reduction of chlorobenzene on platinum electrodes. Silver electrodes reduce chlorobenzene to benzene at more negative potential - 3.17 V vs. Ag/Ag+ without naphthalene. The value of j(C6H6) depends on the surface structure: Ag(110)similar to Ag(111)< Ag(100). The flat (100) terrace has the highest activity for the dechlorination of chlorobenzene.

Electrochemical reduction of chlorobenzene has been studied on a silver polycrystalline electrode in acetonitrile solution containing various concentrations of water. The main product is benzene. The partial current density of benzene j(C6H6) is enhanced with the increase of the concentration of chlorobenzene linearly. The value of j(C6H6) is independent of the concentration of water. This result differs from the electrolysis of chloroform in which the partial current density of the product (methane) is enlarged with the increase of the water concentration remarkably. The electrolysis in acetonitrile containing D2O shows that the proton source of benzene is acetonitrile as well as water.

Dechlorination of chlorobenzene has been studied in acetonitrile on metal electrodes (Pt, Cd, Sn, In, Pb, Zn, Ag) using naphthalene as a mediator. Voltammograms of the metal electrodes, except Pt, give redox peaks at -2.94 V (Ag\Ag+) in the solution containing 10 mM of naphthalene. The cathodic peak at -2.94 V is enhanced with an increase of the concentration of chlorobenzene, evidence that chlorobenzene is also reduced at this potential. The current density of the enhanced cathodic peak depends on the metals in the following order: Cd=Sn < In=Pb < Zn < Ag. Chlorobenzene has been electrolyzed on Zn and Ag and Pt electrodes at -2.94 V (Ag\Ag+) in 0.1 M tetraethyl ammonium perchlorate (TEAP) + acetonitrile containing 10 mM of chlorobenzene and naphthalene. Main product of the electrolysis is benzene. The partial current density of benzene also depends on the metals as follows: Pt=Zn < Ag. (C) 2004 Elsevier B.V. All rights reserved.

Adsorption of sulfuric acid anions (HSO<Sub>4</Sub><Sup>&minus;</Sup> or SO<Sub>4</Sub><Sup>2&minus;</Sup>) on single crystal electrodes of platinum and palladium in 0.05 M H<Sub>2</Sub>SO<Sub>4</Sub> has been studied using infrared reflection absorption spectroscopy (IRAS). Both Pt(111) and Pd(111) electrodes provide a single IRAS band around 1200 cm<Sup>&minus;1</Sup> in the region above 1000 cm<Sup>&minus;1</Sup>. Two bands appear around 1200 and 1100 cm<Sup>&minus;1</Sup> with Pt(100), Pt(110) and Pt(S)-[n(111)&times;(111)] electrodes. The intensity of the lower frequency band is enhanced with the increase of the step atom density. The IRAS spectra of Pd(100), Pd(110) and stepped surface of Pd (Pd(311)) differ completely from those of Pt electrodes: a single band is observed around 1200 cm<Sup>&minus;1</Sup>. Adsorbed geometry and adsorption site of the anion are discussed. The reduction of CO<Sub>2</Sub> is deactivated remarkably by the adsorption of sulfuric acid anion on the Pt(S)-[n(111)&times;(111)] electrodes. Active site for the CO<Sub>2</Sub> reduction is also discussed.

Electrochemical reduction of carbon dioxide was studied with various series of copper single crystal electrodes in 0.1 M KHCO3 aqueous solution at constant current density 5 mA cm(-2); the electrodes employed are Cu(S)-[n(1 0 0) x (1 1 1)], Cu(S)-[n(1 0 0) x (1 1 0)], Cu(S)-[n(1 1 1) x (1 0 0)], Cu(S)-[n(1 1 1) x (1 1 1)] and Cu(S)-[n(1 1 0) x (1 0 0)]. The electrodes based on (1 0 0) terrace surface give ethylene as the major product. The ethylene formation is further promoted by the introduction of (1 1 1) or (1 1 0) steps to the (1 0 0) basal plane. The highest C2H4 to CH4 formation ratio amounts to 10 in terms of the current efficiency for the (7 1 1) (=4(1 0 0) - (1 1 1)) surface as compared with the value 0.2 for the (I 11) electrode. The n (1 1 1) - (1 1 1) surfaces favor the formations of acetic acid, acetaldehyde and ethyl alcohol with the increase of the (1 1 1) step atom density. CH4 formation at the n (1 1 1) - (1 1 1) electrodes decreases with the increase of the (111) step atom density. The n (1 1 1) - (1 0 0) surfaces give higher gaseous products; the major product is CH4 with lower fraction of C-2+ compounds. (C) 2003 Elsevier Science B.V. All rights reserved.

Structural effects on the rate Of CO2 reduction are studied on kinked stepped surfaces of Pt inside the stereographic triangle. The surfaces examined are Pt(431), Pt(531) and Pt(532), which consist of two or three atomic rows of (I 11) terraces and (I 11) or (100) steps incorporated with kink atoms. The rate of CO2 reduction is enhanced with the increase of the kink atom density. The kinked stepped surfaces have a higher activity for CO2 reduction than the analogous stepped surfaces that have similar terrace and step structures without kink sites. Introduction of kink atoms to Pt(S)-[3(111) x (100)] surface changes the potential dependence of the reduction rates significantly. (C) 2002 Elsevier Science B.V. All rights reserved.

The sulfuric acid anion (HSO4- or SO42-) adsorbed on Pd single-crystal electrodes was studied using infrared reflection absorption spectroscopy (IRAS) in 0.05 M H2SO4 solution. Surfaces examined are flat ones (Pd-(111) and Pd(100)) and stepped ones (Pd(110) (= 2(111 )-(111)) and Pd(311) (= 2( 111)-(100))). All of the surfaces give a single IRAS band around 1200 cm-1 that is assigned to S-O stretching vibration. The integrated band intensity depends on the crystal orientation significantly, i.e., Pd(311) ∼ Pd(110) ≪ Pd(100) &lt Pd-(111), showing that the flat surfaces adsorb more sulfuric acid anion than the stepped ones. Magnitude of the band shift (dv/dE) on the flat surfaces is nearly twice as high as that on the stepped ones.

Adsorption of sulfuric acid anion (HSO4- or SO42-) was studied using IRAS (infrared reflection absorption spectroscopy) on Pt(S)-[n(111) × (111)] and Pt(S)-[n(100) × (111)] electrodes in 0.05 M H2SO4. Pt(S)[n(111) × (111)] electrodes give two IRAS bands. One is from a sulfuric acid anion adsorbed on the terrace with 3-fold symmetry (∼1200 cm-1), and another is from that on the step with 2-fold symmetry (∼1200 cm-1 and ∼1100 cm-1). Relative band intensity from the 2-fold sulfuric acid anion (∼1100 cm-1) gets higher with the increase of step atom density. Pt(S)-[n(100) × (111)] electrodes also show two IRAS bands around 1200 and 1100 cm-1 which originate from the sulfuric acid anion adsorbed on the terrace and the step with 2-fold symmetry. The relative intensity of the lower frequency band increased with the increase of the step atom density.

Electrochemical reduction of CO2 was studied using single-crystal electrodes, Cu(111), Cu(100), Cu(S)-[n(100) × (111)], and Cu(S)-[n(100) × (110)] at a constant current density 5 mA cm-2 in0.1 M KHCO3 aqueous solution. Copper single crystals were prepared from 99.999% copper metal in graphite crucibles by the Bridgeman method. The crystal orientation was determined by the X-ray back reflection method. The Cu(111) electrode yields mainly CH4 from CO2, and the Cu(100) favorably gives C2H4. Introduction of (111) steps to Cu(100) basal plane, leading to Cu(S)-[n(100) × (111)] orientations, significantly promoted C2H4 formation and suppressed CH4 formation. The selectivity ratio C2H4/CH4 on Cu(711) (n = 4) amounted to 14, 2 orders of magnitude higher than that on Cu(111).

The effects of underpotential-deposited lead on the adsorption of CO on Pt(111) surfaces have been investigated in 0.1 M HClO4 by in situ infrared reflection absorption spectroscopy (IRAS). Lead coverages of about 0.4 on Pt(111) electrodes polarized at 0.1 V vs RHE prevent CO from adsorbing on multiply bonded sites, reducing the overall coverage to about a third of that observed in the absence of coadsorbed Pb under otherwise identical conditions, yielding a single, sharply-defined feature in the infrared reflection absorption spectra characteristic of atop-bonded CO. However, the integrated absorption band of atop CO at 0.1 V in the presence of coadsorbed Pb was found to be about 3 times higher than that predicted from the corresponding spectral features observed for Pb-free surfaces at that specific coverage corrected for dipole-dipole coupling interactions. Although much of this effect may be due to a Pb-induced decrease in the dielectric screening within the CO adlayer, the gains in intensity are too large to be explained solely on this basis, pointing to chemical factors, such as Pt-Pb charge polarization, as an important factor.

Electrocatalytic activity in the reduction of CO2 to adsorbed CO was studied systematically on a series of Pt single crystal electrodes using voltammetry. The single cryst;al electrodes examined were stepped surfaces (Pt(S)-[n(111) x (111)]), Pt(S)-[n(lll)x (100)], Pt(S)-[n(100)x (111)]), and kinked step surfaces (Pt(S)-[n(110)x (100)] and Pt(S)[n(100) x (110)]). Atomically flat surfaces, Pt(lll) and Pt(100), show poor activity for CO2 reduction. Introduction of step sites to (111) or (100) surface significantly enhances the electrocatalytic activity in CO2 reduction. The rate increases proportionally with the step atom density. The order of the activity series is obtained for the stepped surfaces: Pt(111) &lt; Pt(100) &lt; Pt(S)-[n(lll) x (100)] &lt; Pt(S)-[n(lll) x (100)] &lt; Pt(S)-[n(lll) x (111)] &lt; Pt(110). The most active site in the stepped surface is derived from the psudo-4-fold bridged site in Pt(S)-[n(111) x (111)]. Kinked step surfaces show higher activity than stepped surfaces: the reduction rate per kink atom is more than twice as high as the value per step atom. Densely packed kink atoms along the step line greatly promote the reduction of CO2. (C) 2000 Elsevier Science Ltd. All rights reserved.

Structural effects on the rate of CO2 reduction were studied on the low index planes of Pt (Pt(111), Pt(100) and Pt(110)) using macro-electrolysis in acetonitrile containing 0.1 M (M=mol dm(-3)) tetraethyl anmmonium perchrolate. CO2 was reduced at -3.1 V (vs. Ag/Ag+) with the concentration of water controlled at 25+/-2 mM. Oxalic acid was the main reduction product on each electrode. Formic acid and trace of CO were also produced. The order of the partial current density of oxalic acid formation is Pt(110) &lt; Pt(111) &lt; Pt(100): the partial current on Pt(100) is twice as high as that on Pt(110).

CO is adsorbed on Cu electrode with a simultaneous transfer of charge. The apparent charge transfer is derived from displacement of anions specifically adsorbed on the electrode during the adsorption of CO. This paper presents the results of voltammetric measurements of the adsorption of CO on single crystal electrodes of Cu[110] zone in phosphate buffer solution (pH 6.8)at 0 degrees C. The profile of the charge transfer wave depends significantly on the crystal orientation of Cu single crystals. The potential of the charge transfer is well correlated with the work functions and the potential of zero charge of the Cu single crystals. We propose a model that H2PO4 is specifically adsorbed on (100) steps of Cu[110] zone crystal electrodes occupying 3 to 4 atomic sites. (C) 1998 Elsevier Science Ltd. All rights reserved.

Structural effects on the rates of CO2 reduction were studied on Ag(111), Ag(100) and Ag(110) electrodes in 0.1 M KHCO3 using macroelectrolysis. The partial current density of each reduction product (CO, HCOO-, H-2) was measured at various potentials. All the Ag single crystal electrodes mainly gave CO at any potential. The partial current density of CO on the atomically stepped Ag(110) was remarkably higher than those on the flat Ag(111) and Ag(100), as is the case with Pt group metals. The partial current density of HCOO- gave a smaller orientation dependence. (C) 1997 Elsevier Science S.A.

The structural effect on the rate of CO2 reduction was studied with voltammetry on single crystals of Pd (Pd(111), Pd(100), Pd(110)) in 0.1 M HClO4 saturated with CO2. CO2 was reduced to an adsorbed product at potentials more negative than -0.1 V vs. RHE. The rate of CO2 reduction depends remarkably on the crystal orientation: Pd(100) &lt; Pd(lll) &lt; Pd(110). The Pd(111) surface shows uniquely high activity, whereas (111) is the surface of lowest activity in CO2 reduction for other Pt group metals (Pt, Ph, Ir). The rate of CO2 reduction at -0.5 V vs. RHE on Pd(110) is two orders of magnitude higher than that of Pt(110).

Electrochemical measurements were conducted for investigation of the dynamical process of adsorbed CO formation from CO2 and adsorbed hydrogen on Pt single crystal surfaces in 0.5 M H2SO4. The adopted monocrystal surfaces were Pt(111) and Pt(110) [= 2(111)-(111)], whose atomic orientations are composed of (111) type arrangement on the surface. The rate of the CO formation on Pt(110) was more than 10 times as high as those on Pt(111). Oxidation charge of CO at the final stage of the reaction was almost constant between 0.05 and 0.20 V (vs. rhe) on Pt(110), whereas the amount of adsorbed hydrogen, source material of the reaction, greatly varied in this potential range. This fact suggests that hydrogen atoms are regenerated on Pt(110). Such regeneration was not observed on Pt(111). The difference of the reactivity between Pt(111) and Pt(110) may be attributed to the four-fold hollow site on Pt(110).

Final products of the photochemical reactions of quinazoline, and naphthalene in 1,2,4,5-tetramethylbenzene (durene) mixed crystals are investigated on the basis of the phosphorescence and fluorescence spectra of the products. Quinazoline in durene is considered to transform into a benzene derivative, whereas naphthalene and quinoline tranform into anthracene derivatives. These products are different from a pyrazine derivative found in the case of quinoxaline, which is specific to the reaction in a durene mixed crystal. The cause of this difference is discussed in terms of the configurations of guests in the mixed crystals and the difference of the molecular structures.

A pressure effect on the phosphorescence decay rate constants (k(T)) of quinoxaline and quinoline in mixed single crystals of durene-h14 and durene-d14 is observed up to 20 kbar. k(T)'s increase drastically under high pressure in the temperature region where measurable photoinduced hydrogen abstraction reactions are known to occur at 1 atm. This increase can be attributed to the enhancement of the reaction rate constants (k(TR)) of photoinduced hydrogen abstraction. The absolute values of k(TR) increase and the slopes of the Arrhenius plots decrease as the applied pressure increases. The Arrhenius plots of k(TR) at high pressures deviate from straight lines as they do at 1 atm, suggesting that tunneling remains important at high pressures. The pressure dependence of k(TR)'s can be explained reasonably well on the basis of the Siebrand model by taking account of the decrease of the tunneling distance and the increase of the vibrational energy of the low-frequency vibration with increasing pressure.