Comparison of the catalytic activity of Au3, Au4+, Au5, and Au5- in the gas-phase reaction of H2 and O2 to form hydrogen peroxide: a density functional theory investigation

We report a detailed density functional theory (B3LYP) analysis of the gas-phase H2O2 formation from H2 and O2 on Au3, Au4+, Au5, and Au5-. We find that H2, which interacts only weakly with the Au clusters, is dissociatively added across the Au-O bond, upon interaction with AunO2. One H atom is captured by the adsorbed O2 to form the hydroperoxy intermediate (OOH), while the other H atom is captured by the Au atom. Once formed, the hydroperoxy intermediate acts as a precursor for the closed-loop catalytic cycle. An important common feature of all the pathways is that the rate-determining step of the catalytic cycle is the second H2 addition to form H2O2. The H2O2 desorption is followed by O2 addition to AunH2 to form the hydroperoxy intermediate, thus leading to the closure of the cycle. On the basis of the Gibbs free energy of activation, out of these four clusters, Au4+ is most active for the formation of the H2O2. The 0 K electronic energy of activation and the DeltaGact at the standard conditions are 12.6 and 16.6 kcal/mol respectively. The natural bond orbital charge analysis suggests that the Au clusters remain positively charged (oxidic) in almost all the stages of the cycle. This is interesting in the context of the recent experimental evidence for the activity of cationic Au in CO oxidation and water-gas shift catalysts. We have also found preliminary evidence for a correlation between the activation barrier for the first H2 addition and the O2 binding energy on the Au cluster. It suggests that the minimum activation barrier for the first H2 addition is expected for the Au clusters with 7.0-9.0 kcal/mol O2 binding energy, i.e., in the midrange of the expected interaction energy. This represents a balance between more favorable H2 dissociation when the Aun-O2 interaction is weaker and high O2 coverage when the interaction is stronger. On the basis of this work, we predict that the hydroperoxy intermediate formation can be both thermodynamically and kinetically viable only in a narrow range of the O2 binding energy (10.0-12.0 kcal/mol)-a useful estimate for computationally screening Au-cluster-based catalysts. We also show that a competitive channel for the OOH desorption exists. Thus, in propylene epoxidation both OOH radicals and H2O2 can attack the active Ti in/on the Au/TS-1 and generate the Ti-OOH sites, which can convert propylene to propylene oxide.     

Investigation of the reaction products of 5-amino-1,3-disubstituted-pyrazoles with aromatic aldehydes. synthesis of new fluorinated 1,3,4-trisubstituted-4H-pyrazolo[3,4-e][1,4]thiazepin-7-ones

The condensation products of 5-amino-1,3-disubstituted-pyrazoles with aromatic aldehydes were identified as 2,4-dihydro-2,5-diphenyl-4-(phenylmethylene)-3H-pyrazol-5-imine derivatives Treatment of these products with mercaptoacetic acid gave new fluorine containing 1H-pyrazolo[3,4-e][1,4]thiazepin-7-ones.     

Simple preparation of alpha-diazo esters

[reaction: see text] The TiCl4-mediated reaction of an ester with benzoyl chloride results in high yields of the alpha-benzoylated ester. Diazo transfer of the benzoylated ester utilizing p-acetoamidobenzenesulfonyl azide affords the alpha-diazo ester in good yield. Using this simplified procedure, it is easy to prepare gram quantities of alpha-diazo esters.     

Spectrophotometric determination of indium using gallion

Summary The formation of violet coloured chelate of indium with Gallion has been studied. Indium forms only one chelate with Gallion at p_H 4.5 having _max 610 nm. The composition of the complex formed has been determined by various methods and is found to be 11 (MR). The values of stability constant have been calculated by three methods and the average value of logK obtained is 5.6 ± 0.1. A procedure for the spectrophotometric determination of indium has been suggested.

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Zusammenfassung Die Bildung eines violetten Chelates von Indium mit dem Reagens Gallion wurde studiert. In bildet damit ein einziges Chelat bei p_H=4,5 und _max=610 nm. Die Zusammensetzung des entstandenen Komplexes wurde nach verschiedenen Methoden ermittelt und gleich 11 (MetallReagens) gefunden. Die Stabilitätskonstante wurde nach drei Verfahren berechnet, wobei sich als Mittelwert logK=5,6±0,1 ergab. Eine Arbeitsvorschrift für die spektrophotometrische Bestimmung des In wird vorgeschlagen.

     

Studies in spiroheterocycles. Part V. Synthesis of new fluorinated spiro[3H-indole-3,2′-oxiran]-2(1H)-ones

Reinvestigation of the reaction of 3-aroylmethyleneindol-2-ones with hydrogen peroxide has revealed that a 1:1 isomeric mixture of spiro[3H-indole-3,2′-oxiran]-2(1H)-ones is formed. The structures of the two isomers have been confirmed by their ir, pmr and mass spectral studies. Mass fragmentation patterns of these compounds are discussed.     

2,4,6-Trichloro-1,3,5-triazine Catalyzed Chemoselective Transthioacetalization of Aldehyde Acetals and Oxathioacetals

Summary. A mild and efficient transthioacetalization of aldehyde acetals and oxathioacetals was carried out using 2,4,6-trichloro-1,3,5-triazine as a mild and inexpensive catalyst. Chemoselective transacetalization is impressive as aldehyde O,O- and O,S-acetals are converted into the corresponding S,S-acetals in the presence of ketones or their acetals and oxathiocetals in nearly quantitative yields.     

Potentiometric estimation of sulfide ion using the Fe(III)-EDTA complex

The Fe(III)-EDTA complex reacts with sulfide ion in a fast electron transfer reaction, oxidising the latter to elemental sulfur and getting itself reduced to Fe(II). The reaction has been developed for the quantitative estimation of sulfide ion by titration against the Fe(III)-EDTA complex, measuring the redox potential of the system. Repeated use of a given quantity of the complex solution by the process of regeneration has been demonstrated. The possibility of its practical application in liquid phase oxidation processes of recovering sulfur from H₂S is emphasized.     

Nonlinear dynamics of confined polymer melts with attractive walls

A scaling model is presented to analyze the reversible strain-hardening phenomenon in end-tethered polymer clay nanocomposites (Krishnamoorti, R.; Giannelis, E. P. Langmuir 2001, 17, 1448). It is assumed that for attractive clay-polymer interactions a fraction of the polymer chains that span the space between opposite clay plates get adsorbed on them, thereby bridging the plates. Under large amplitude oscillatory shear, such bridges are stretched, and this results in the strain-hardening behavior. The onset of strain hardening is predicted to be dependent only on the average distance separating the two plates and is independent of the frequency of the oscillations and the polymer molecular weight.