Water formation on Pd(111) by reaction of oxygen with atomic and molecular hydrogen

In this work we have studied the steady-state reaction of molecular and atomic hydrogen with oxygen on a Pd(111) surface at a low total pressure (<10(-7) mbar) and at sample temperatures ranging from 100 to 1100 K. Characteristic features of the water formation rate Phi(pH2; pO2; TPd) are presented and discussed, including effects that are due to the use of gas-phase atomic hydrogen for exposure. Optimum impingement ratios (OIR) for hydrogen and oxygen for water formation and their dependence on the sample temperature have been determined. The occurring shift in the OIR could be ascribed to the temperature dependence of the sticking coefficients for hydrogen (SH2) and oxygen (SO2) on Pd(111). Using gas-phase atomic hydrogen for water formation leads to an increase of the OIR, suggesting that hydrogen abstraction via hot-atom reactions competes with H2O formation. The velocity distributions of the desorbing water molecules formed on the Pd(111) surface have been measured by time-of-flight spectroscopy under various conditions, using either gas-phase H atoms or molecular H2 as reactants. In all cases, the desorbing water flux could be represented by a Maxwellian distribution corresponding to the surface temperature, thus giving direct evidence for a Langmuir-Hinshelwood mechanism for water formation on Pd(111).     

Photochemical reaction of magnesium tetraphenyl porphyrin with sulfur dioxide

<正>The photochemical reaction of magnesium tetraphenyl porphyrin(MgTPP) with sulfur dioxide(SO_2) was investigated in dichloromethane(CH_2Cl_2) by steady-state fluorescence,UV-vis absorption,MS,and XRD spectroscopic techniques.These spectra showed that under irradiation MgTPP reacted with SO_2 to form 1:1 molecular adduct at first step.During the process of keeping irradiation and maintaining the flow of SO_2,SO_2 was reduced into S~(2-) by MgTPP and the results were detected using MS and XRD techniques.Understanding the photochemical reaction of MgTPP with SO_2 is of key interest in elucidating fundamental photochemical reaction mechanisms associated with this class of chlorophyll in the presence of SO_2.     

High-pressure STM of the interaction of oxygen with Ag(111)

To identify surface phases that could play a role for the epoxidation of ethylene on Ag catalysts we have studied the interaction of Ag(111) with O(2) at elevated pressures. Experiments were performed using high-pressure scanning tunneling microscopy (STM) at temperatures between 450 and 480 K and O(2) pressures in the mbar range. Below p(O(2)) approximately 1 mbar the surface largely showed the structure of bare Ag(111). At p(O(2)) above approximately 1 mbar the (4 x 4)O structure and the closely related (4 x 5 radical 3)rect structure were observed. The findings confirm theoretical predictions that the (4 x 4)O structure is thermodynamically stable at the oxygen partial pressure of the industrial ethylene oxide synthesis. However, in other experiments only a rough, disordered structure was observed. The difference is caused by the chemical state of the STM cell that depends on the pretreatment and on previous experiments. The surface was further analyzed by X-ray photoelectron spectroscopy (XPS). Although these measurements were performed after sample transfer to ultra-high vacuum (UHV), so that the surface composition was modified, the two surface states could still be identified by the presence of carbonate or a carbonaceous species, and by the absence or presence of a high-binding energy oxygen species, respectively. It turns out that the (4 x 4)O structure only forms under extremely clean conditions, indicating that the (4 x 4)O phase and similar oxygen-induced reconstructions of the Ag(111) surface are chemically unstable. Chemical reactions at the inner surfaces of the STM cell also complicate the detection of the catalytic formation of ethylene oxide.     

Surface Pourbaix diagrams and oxygen reduction activity of Pt, Ag and Ni(111) surfaces studied by DFT

Based on density functional theory calculations we investigate the electrochemically most stable surface structures as a function of pH and electrostatic potential for Pt(111), Ag(111) and Ni(111), and we construct surface Pourbaix diagrams. We study the oxygen reduction reaction (ORR) on the different surface structures and calculate the free energy of the intermediates. We estimate their catalytic activity for ORR by determining the highest potential at which all ORR reaction steps reduce the free energy. We obtain self-consistency in the sense that the surface is stable under the potential at which that particular surface can perform ORR. Using the self consistent surfaces, the activity of the very reactive Ni surface changes dramatically, whereas the activity of the more noble catalysts Pt and Ag remains unchanged. The reason for this difference is the oxidation of the reactive surface. Oxygen absorbed on the surface shifts the reactivity towards the weak binding region, which in turn increases the activity. The oxidation state of the surface and the ORR potential are constant versus the reversible hydrogen electrode (RHE). The dissolution potential in acidic solution, on the other hand, is constant vs. the standard hydrogen electrode (SHE). For Ag, this means that where the potential for dissolution and ORR are about the same at pH = 0, Ag becomes more stable relative to RHE as pH is increased. Hence the pH dependent stability offers an explanation for the possible use of Ag in alkaline fuel cell cathodes.     

Determination of phenol in the presence of sulfite (sulfur dioxide) by the 4-aminoantipyrine spectro-photometric method

The interference of sulfite (sulfur dioxide) with the 4-aminoantipyrine (4-AAP) spectrophotometric method for phenol is discussed for procedures without distillation, and with distillation in the absence and presence of copper(II) sulfate. Sulfite represses the color development in all these procedures. Without chloroform extraction, the maximum tolerable amounts of sulfite in the procedures without distillation, distillation without copper(II) sulfate, and distillation with copper(II) sulfate are 15, 10, and 20 mg/100 ml, respectively. For the extraction method, the limits are 4.0, 4.0, and 10 mg/ 100 ml, respectively. Copprer(II) sulfate catalyzes the air-oxidation of sulfite. Phenol can be determined in the presence of large amounts of sulfite by treating with sulfide to form polythionates and thiosulfate; excess of sulfide is removed with copper(II) sulfate, sulfuric acid is added, the phenol is distilled, and the 4-AAP method is applied. Improvements are made in the overall accuracy of the 4-AAP method.     

On the characteristics and mechanism of the radiation-induced chain reaction between sulfur dioxide and molecular oxygen in acid solutions

Characteristics of the -induced chain reaction between sulfur dioxide and molecular oxygen in perchloric and sulfuric acid media in the presence of Ce(III) ions have been studied. The concentration effects of dissolved oxygen (0.2·10^–3–9.4·10^–3 mol/dm³, sulfur dioxide (0.3·10^–1–2.0·10^–1 mol/dm³ and Ce(III) (0.2·10^–3–4.8·10^–3 mol/dm³) and dose rate (0.26·10¹⁹–1.0·10¹⁹ eV/dm³·s) on the radiation — chemical yield of oxygen consumption G(–O₂) and accumulation of sulfate G(HSO ₄ ^– ), have been investigated. The reaction proceeds with G(–O₂) reaching 10²–10³ molecule/100eV in a catalytic regime. The reaction rate in perchloric acid medium is 3–4 times lower than that in the sulfuric acid medium and depends on the SO₂, O₂ and Ce(III) concentrations, the reaction order varying from 1.0 to 0 and/or in the reverse direction. The mechanism of the process involves chain propagation with 3 stages and 3 intermediates: SO₃H, HSO₅ and Ce(IV). The catalytic effect is caused by the interaction of HSO₄ with Ce(IV) ions followed by their reduction when interacting with SO₂, yielding SO₃H radicals. Chain termination may be due to one or two of the three intermediates or due to all three particles, the kinetics depending on this. Kinetic equations describing the experimental data have been obtained.     

Diffusional study of the reaction of sulfur dioxide with reactive porous matrices

A single-pellet high-temperature diffusion cell reactor is used to study the sulfation of calcined Greer limestone pellets. The effective diffusivities of gases through the reactive pellets, during the calcination and sulfation are determined. The experimentally determined effective diffusivity of sulfur dioxide through the pores of the product shell is used in the modified expanding grain model to obtain the diffusivity of sulfur dioxide through the product shell of the grains as a function of the conversion and the reaction temperature. The activation energy for the initial diffusivity of sulfur dioxide through the product shell of grains, is found to be 34.13 kcal mol^?1; the diffusivity values decreased with increasing conversion. Additionally it is found that the ratio of the tortuosity of reacting shell of pellet to the initial pellet tortuosity before any sulfation was increased with increasing conversion.     

Interactions of oxygen and ethylene with submonolayer Ag films supported on Ni(111)

We investigate the oxidation of, and the reaction of ethylene with, Ni(111) with and without sub-monolayer Ag adlayers as a function of temperature. The addition of Ag to Ni(111) is shown to enhance the activity towards the ethylene epoxidation reaction, and increase the temperature at which ethylene oxide is stable on the surface. We present a systematic study of the formation of chemisorbed oxygen on the Ag-Ni(111) surfaces and correlate the presence and absence of O(1-) and O(2-) surface species with the reactivity towards ethylene. By characterizing the samples with low-energy electron microscopy (LEEM) in combination with X-ray photoelectron spectroscopy (XPS), we have identified specific growth of silver on step-edge sites and successfully increased the temperature at which the produced ethylene oxide remains stable, a trait which is desirable for catalysis.     

Aromatic Azide Transformation on the Ag(111) Surface Studied by Scanning Probe Microscopy

Chemical transformation of 9‐azidophenanthrene on the Ag(111) surface was studied by nc‐AFM in UHV. High‐resolution imaging supported by first‐principle calculations revealed the structure of the final products that originated from a common and elusive 9‐phenanthryl nitrenoid intermediate chemisorbed on the Ag(111) surface. A formal nitrene insertion into the C?H bond along with its dimerisation and hydrogenation were identified as main reaction channels. Thus, the ability of aryl azides to form covalent σ‐ and π‐bonds between their transformation products on a solid surface was demonstrated at a single‐molecule level.     

The photochemically induced reaction of sulfur dioxide with alkanes and carbon monoxide

Abstract— The gas phase photochemical reactions of SO₂ induced by 3130 Å radiation have been studied in the presence of added alkanes or added CO. The quantum yields obtained in the reactions with the low molecular weight alkanes employed are lower than those obtained by previous workers. The quantum yields were found to be pressure dependent increasing slowly with increasing pressure. A stoichiometric ratio of one SO₂ removed per molecule of hydrocarbon consumed was observed only under experimental conditions of [SO₂] < [RH]. For reaction mixtures where [SO₂] < [RH] the ratio of [SO₂]/[RH] reacted always exceeded unity. The quantum yields decreased slightly with increasing temperature. In all the alkane reaction systems studied, the deposition of viscous, nonvolatile reaction products was observed. In the experiments with added CO, the quantum yields were computed with respect to the rate of CO₂ formation. At 25°C and equal pressures of SO₂ and CO, φco₂ was observed to be 0.005 and it decreased slightly with increasing temperature. The results obtained are interpreted in terms of the sulfoxidation of the alkanes and the oxidation of CO proceeding by way of a ³SO₂ reaction intermediate.     

Effect of subsurface oxygen on the reactivity of the Ag(111) surface

Periodic, self-consistent, density functional theory calculations have been performed to demonstrate that subsurface oxygen (O(sb)) dramatically increases the reactivity of the Ag(111) surface. O(sb) greatly facilitates the dissociation of H2, O2, and NO and enhances the binding of H, C, N, O, O2, CO, NO, C2H2, and C2H4 on the Ag(111) surface. This effect originates from an O(sb)-induced upshift of the d-band center of the Ag surface and becomes more pronounced at higher O(sb) coverage. Our findings point to the important role that near-surface impurities, such as O(sb), can play in determining the thermochemistry and kinetics of elementary steps catalyzed by transition metal surfaces.     

Stable sulfinic acids from the ene reaction of methyl-substituted allenes with sulfur dioxide

Allenes 2,4-dimethyl-2,3-pentadiene and 3-methyl-1,2-butadiene in liquid sulfur dioxide undergo ene addition with the solvent giving the stable 3-(2,4-dimethyl-1,2-pentadienyl) and 2-(3-methyl-1,2-butadienyl) sulfinic acids.     

Vinyl acetate formation by the reaction of ethylene with acetate species on oxygen-covered Pd(111)

The reaction pathway of vinyl acetate synthesis is scrutinized by reacting gas-phase ethylene (at an effective pressure of 1 x 10-4 Torr) with eta2-acetate species (with a coverage of 0.31 +/- 0.02 monolayer) on a Pd(111)-O(2x2) model catalyst surface in ultrahigh vacuum. It is found that the 1414 cm-1 infrared feature due to the symmetric OCO stretching mode of the acetate species decreases in intensity due to reaction with gas-phase ethylene, while temperature-programmed desorption experiments demonstrate that vinyl acetate is formed. The formation of ethylidyne species is detected when almost all of the acetate species have been removed. The experimental removal kinetics are reproduced by a model in which adsorbed acetates react with an ethylene-derived (possibly ethylene or vinyl) species, where ethylene adsorption is blocked by the acetate present on the surface.     

Theoretical Investigation on the Adsorption of Ag^＋ and Hydrated Ag^＋ Cations on Clean Si（111） Surface

In this paper, the adsorption of Ag^＋ and hydrated Ag^＋ cations on clean Si（111） surface were investigated by using cluster （Gaussian 03） and periodic （DMol^3） ab initio calculations. Si（111） surface was described with cluster models （Si14H17 and Si22H21） and a four-silicon layer slab with periodic boundary conditions. The effect of basis set superposition error （BSSE） was taken into account by applying the counterpoise correction. The calculated results indicated that the binding energies between hydrated Ag^＋ cations and clean Si（111） surface are large, suggesting a strong interaction between hydrated Ag^＋ cations and the semiconductor surface. With the increase of number, water molecules form hydrogen bond network with one another and only one water molecule binds directly to the Ag^＋ cation. The Ag^＋ cation in aqueous solution will safely attach to the clean Si（111） surface.     

Surface chemistry of polymers. The adsorption of carbon dioxide and sulfur dioxide on polyvinylidene chloride

Isotherms for the adsorption of nitrogen (77 K), carbon dioxide (195–247 K) and sulfur dioxide (254–293 K) on polyvinylidene chloride have been measured volumetrically. The B.E.T. cross-sectional areas of 18 Ų (CO₂) and 24 Ų (SO₂) are comparable to liquid density values. The isosteric heat of adsorption of CO₂ is constant for 0.2 < θ < 0.4 and is lower than the latent heat of condensation. For SO₂, the two are practically identical up to the monolayer. Entropy calculations show ‘supermobility’ in the case of CO₂.     

Microbial removal of sulfur dioxide from a gas stream with net oxidation to sulfate

A study has been conducted of the feasibility of utilizing the sulfate reducing bacteriumDesulfovibrio desulfuricans and the chemoautotrophThiobacillus denitrificans as a basis of a microbial process for the removal of sulfur dioxide from a gas with net oxidation to sulfate. In reactors-in-series, SO₂ was reduced to H₂S in the first stage by D.desulfuricans. The H₂S was then stripped with nitrogen and sent to a second stage where it was oxidized to sulfate by T.denitrificans. A sulfur balance demonstrated complete reduction of SO₂ to H₂S in the first stage and complete oxidation of H₂S to sulfate in the second stage.     

Mechanistic study on the atmospheric formation of acid rain base on the sulfur dioxide

The reaction pathways of acid rain formation from reaction of sulfur dioxide vapor and water vapor on the singlet potential energy surface have been investigated theoretically. The calculated results show that the reactants are initially associated with the adduct SO₂–H₂O through a barrier less process. Subsequently, via a variety of transformations of isomer SO₂–H₂O, three kinds of products H₂SO₃, SO₃ + H₂, and H₂O₂ + ³SO are obtained. The cleavage and formation of the chemical bonds in the reaction pathways have been discussed using the structural parameters. Also, by means of the transition states and their connected intermediates or products at the CCSD(T)//B3LYP level, mechanism of H₂O + SO₂ reaction on the singlet potential energy surface are plotted. The calculation results show that the most suitable reaction pathways are the formation of H₂SO₃. Finally, the rate constants have been calculated only for these suitable pathways by the RRKM and TST theories at temperature range of 250–2500 K.