Adsorption and oxidation of ethanol on colloid-based Pt/C, PtRu/C and Pt3Sn/C catalysts: in situ FTIR spectroscopy and on-line DEMS studies

The interaction of colloid-based, carbon supported Pt/C (40 wt%), PtRu/C (45 wt%) and Pt3Sn/C (24 wt%) catalysts with ethanol and their performance for ethanol electrooxidation were investigated in model studies by electrochemical, in situ infrared spectroscopy and on-line differential electrochemical mass spectrometry measurements. The combined application of in situ spectroscopic techniques on realistic catalysts and under realistic reaction (DEMS, IR) and transport conditions (DEMS) yields new insight on mechanistic details of the reaction on these catalysts under the above reaction and transport conditions. Based on these results, the addition of Sn or Ru, though beneficial for the overall activity for ethanol oxidation, does not enhance the activity for C-C bond breaking. Dissociative adsorption of ethanol to form CO2 is more facile on the Pt/C catalyst than on PtRu/C and Pt3Sn/C catalysts within the potential range of technical interests (<0.6 V), but Pt/C is rapidly blocked by an inhibiting CO adlayer. In all cases acetaldehyde and acetic acid are dominant products, CO2 formation contributes less than 2% to the total current. The higher ethanol oxidation current density on the Pt3Sn/C catalyst at these potentials results from higher yields of C2 products, not from an improved complete ethanol oxidation to CO2.     

Iron(II) sulfate oxidation with oxygen on a Pt/C catalyst: A kinetic study

The kinetics of iron(II) sulfate oxidation with molecular oxygen on the 2% Pt/Sibunit catalyst was studied by a volumetric method at atmospheric pressure, T = 303 K, pH 0.33–2.4, [FeSO₄] = 0.06?0.48 mol/l, and [Fe₂(SO₄)₃] = 0?0.36 mol/l in the absence of diffusion limitations. Relationships were established between the reaction rate and the concentrations of Fe²⁺, Fe³⁺, H⁺, and Cl^? ions in the reaction solution. The kinetic isotope effect caused by the replacement of H₂O with D₂O and of H⁺ with D⁺ was measured. The dependence of Fe²⁺ and Fe³⁺ adsorption on the catalyst pretreatment conditions was studied. A reaction scheme is suggested, which includes oxygen adsorption, the formation of a Fe(II) complex with surface oxygen, and the one-electron reduction of oxygen. The last step can proceed via two pathways, namely, electron transfer with H⁺ addition and hydrogen atom transfer from the coordination sphere of the iron(II) aqua complex. A kinetic equation providing a satisfactory fit to experimental data is set up. Numerical values are determined for the rate constants of the individual steps of the scheme suggested.     

Absence of Mn-centered oxidation in the S(2) --> S(3) transition: implications for the mechanism of photosynthetic water oxidation

A key question for the understanding of photosynthetic water oxidation is whether the four oxidizing equivalents necessary to oxidize water to dioxygen are accumulated on the four Mn ions of the oxygen-evolving complex (OEC), or whether some ligand-centered oxidations take place before the formation and release of dioxygen during the S(3) --> [S(4)] --> S(0) transition. Progress in instrumentation and flash sample preparation allowed us to apply Mn Kbeta X-ray emission spectroscopy (Kbeta XES) to this problem for the first time. The Kbeta XES results, in combination with Mn X-ray absorption near-edge structure (XANES) and electron paramagnetic resonance (EPR) data obtained from the same set of samples, show that the S(2) --> S(3) transition, in contrast to the S(0) --> S(1) and S(1) --> S(2) transitions, does not involve a Mn-centered oxidation. On the basis of new structural data from the S(3)-state, manganese mu-oxo bridge radical formation is proposed for the S(2) --> S(3) transition, and three possible mechanisms for the O-O bond formation are presented.     

Structure-relationships in electrocatalysis: oxygen reduction and hydrogen oxidation reactions on Pt(111) and Pt(100) in solutions containing chloride ions

Kinetics of the oxygen reduction reaction (orr) and the hydrogen evolution–oxidation reactions (her/hor) were studied on the Pt(111) and Pt(100) surfaces in 0.05 M H₂SO₄ containing Cl⁻. The orr is strongly inhibited on the (100) surface modified by adsorbed Cl⁻ (Cl_ad), and it occurs as a 3.5e⁻ reduction via solution phase peroxide formation. In the hydrogen adsorption (H_upd) potential region, the orr is even more inhibited, and corresponds only to a 2 e⁻ reduction at the negative potential limit where the electrode is covered by one monolayer of H_upd and some (unknown) amount of Cl_ad. On the Pt(111)---Cl_ad surface, the orr is inhibited relatively little (in addition to that caused by strong bisulfate anion adsorption on this surface), and the reaction pathway is the same as in Cl⁻ free solution. The kinetics of the hor on Pt(111) are the same in pure solution and in a solution containing Cl⁻, since Cl_ad does not affect platinum sites required for the breaking of the H---H bond. A relatively large inhibition of the hor is observed on the (100) surface, implying that strongly adsorbed Cl_ad is present on the surface even near 0 V.     

A note on ‘sub-Tg relaxation processes in poly(cyclohexylmethacrylate)’

We have critically examined the various relaxation processes occurring in poly(cyclohexylmethacrylate) using dielectric spectroscopy. In addition to the α- and γ-processes found earlier by other workers, we have detected a secondary (β-)process in the temperature range of 293-353 K with an activation energy of about 73 ± 5 kJ/mol.     

CO oxidation on inverse CeO(x)/Cu(111) catalysts: high catalytic activity and ceria-promoted dissociation of O2

A Cu(111) surface displays a low activity for the oxidation of carbon monoxide (2CO + O(2) → 2CO(2)). Depending on the temperature, background pressure of O(2), and the exposure time, one can get chemisorbed O on Cu(111) or a layer of Cu(2)O that may be deficient in oxygen. The addition of ceria nanoparticles (NPs) to Cu(111) substantially enhances interactions with the O(2) molecule and facilitates the oxidation of the copper substrate. In images of scanning tunneling microscopy, ceria NPs exhibit two overlapping honeycomb-type moire? structures, with the larger ones (H(1)) having a periodicity of 4.2 nm and the smaller ones (H(2)) having a periodicity of 1.20 nm. After annealing CeO(2)/Cu(111) in O(2) at elevated temperatures (600-700 K), a new phase of a Cu(2)O(1+x) surface oxide appears and propagates from the ceria NPs. The ceria is not only active for O(2) dissociation, but provides a much faster channel for oxidation than the step edges of Cu(111). Exposure to CO at 550-750 K led to a partial reduction of the ceria NPs and the removal of the copper oxide layer. The CeO(x)/Cu(111) systems have activities for the 2CO + O(2) → 2CO(2) reaction that are comparable or larger than those reported for surfaces of expensive noble metals such as Rh(111), Pd(110), and Pt(100). Density-functional calculations show that the supported ceria NPs are able to catalyze the oxidation of CO due to their special electronic and chemical properties. The configuration of the inverse oxide/metal catalyst opens new interesting routes for applications in catalysis.     

Electrochemistry of platinum single crystal surfaces: Part II. Structural effect on formic acid oxidation and poison formation on Pt (111), (100) and (110)

The geometrical arrangement of sites favourable for formic acid oxidation and the poison formation reaction is determined using low index platinum single crystal planes. For this determination, the least number of sites required for the reactions to occur, which was obtained in the study of electrocatalysis by adatoms, was used, that is three adjacent sites are required for formic acid oxidation and four adjacent sites are required for poison formation.The triplet of sites on a unit lattice of Pt (111) and that on a unit lattice of Pt (100) plane are equally very favourable for the main oxidation reaction, but that on a unit lattice of Pt (110) is not so favourable as those on the former two planes. The oxidation rate is more than one order of magnitude lower on the latter than on the former triplets.The poison formation reaction proceeds at a very high rate on the (100) and the (110) planes. The geometrical arrangement of four sites on a square unit lattice of the (100) plane and on a rectangular unit lattice of the (110) plane are favourable for the poison formation reaction, but that on a hexagonal unit lattice of the (111) plane is not so favourable as the former two.     

A new type of strong metal-support interaction and the production of H2 through the transformation of water on Pt/CeO2(111) and Pt/CeO(x)/TiO2(110) catalysts

The electronic properties of Pt nanoparticles deposited on CeO(2)(111) and CeO(x)/TiO(2)(110) model catalysts have been examined using valence photoemission experiments and density functional theory (DFT) calculations. The valence photoemission and DFT results point to a new type of "strong metal-support interaction" that produces large electronic perturbations for small Pt particles in contact with ceria and significantly enhances the ability of the admetal to dissociate the O-H bonds in water. When going from Pt(111) to Pt(8)/CeO(2)(111), the dissociation of water becomes a very exothermic process. The ceria-supported Pt(8) appears as a fluxional system that can change geometry and charge distribution to accommodate adsorbates better. In comparison with other water-gas shift (WGS) catalysts [Cu(111), Pt(111), Cu/CeO(2)(111), and Au/CeO(2)(111)], the Pt/CeO(2)(111) surface has the unique property that the admetal is able to dissociate water in an efficient way. Furthermore, for the codeposition of Pt and CeO(x) nanoparticles on TiO(2)(110), we have found a transfer of O from the ceria to Pt that opens new paths for the WGS process and makes the mixed-metal oxide an extremely active catalyst for the production of hydrogen.     

A new example of stereoisomerism in inorganic chemistry: PtRu chiral axis in [(η-p-cymene)RuCl(μ-N,N′-dialkyldithiox-amidato N,N′-Ru-S,S′Pt)Pt(2-diphenylphosphinopyridine)Cl]

The novel compounds [(η⁶-p-cymene)RuCl(μ-RR-DTO N,N′Ru S,S′-Pt)Pt(RR-DTO)(P-N)Cl] (P-N = 2-diphenylphosphino-pyridine, RR-DTO = N,N′-dialkyl-dithioxamidato, R = benzyl, 1; R = (R)-(−)-2-hydroxypropyl (coming from (R)-(−)-2-hydroxypropyl-amine), 2;) exhibit a Pt---Ru chiral axis which is a new example of stereoisomerism in inorganic chemistry. The crystal structure of 2 is also reported.     

Ba deposition and oxidation on theta-Al2O3/NiAl(100) ultrathin films. Part II: O2(g) assisted Ba oxidation

Ba deposition on a theta-Al(2)O(3)/NiAl(100) substrate and its oxidation with gas-phase O(2) at various surface temperatures are investigated using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and temperature programmed desorption (TPD) techniques. Oxidation of metallic Ba by gas-phase O(2) at 800 K results in the growth of 2D and 3D BaO surface domains. Saturation of a metallic Ba layer deposited on theta-Al(2)O(3)/NiAl(100) with O(2)(g) at 300 K reveals the formation of BaO(2)-like surface states. These metastable peroxide (O(2)(2-)) states are converted to regular oxide (O(2-)) states at higher temperatures (800 K). In terms of thermal stability, BaO surface layers (theta(Ba) < 5 ML) that are formed by O(2)(g) assisted oxidation on the theta-Al(2)O(3)/NiAl(100) substrate are significantly more stable (with a desorption/decomposition temperature of c.a. 1050 K) than the thick (2 < theta(Ba) < 10 ML) metallic/partially oxidized Ba layers prepared in the absence of gas-phase O(2), whose multilayer desorption features appear as low as 700 K.     

不同金属改性对Pt/β-分子筛催化柴油车尾气中HC、CO及SO_2氧化性能的影响

采用共浸渍法制备了Ce、Cr、Mo和Cu改性的Pt/β-分子筛催化剂,运用氮吸附、XRD、H_2-TPR、NH_3-TPD和XPS等手段对该催化剂进行了表征,研究了不同金属改性对催化剂的织构性质、骨架结构、表面酸性以及在模拟柴油车尾气中抑制SO_2氧化性能和催化HC、CO氧化活性的影响。结果表明,金属改性对催化剂织构性能和骨架结构影响较小。Cr、Mo和Cu的添加可以调变催化剂酸强度,进而抑制SO_2的氧化;Cu改性的催化剂具有最好的抑制SO_2氧化的能力,在350和450℃条件下,与未改性Pt/β-分子筛催化剂相比,Cu改性催化剂上SO_2转化率分别下降了70.4%和70.2%。然而,改性金属与Pt物种之间产生的相互作用,会使Pt物种更难还原,导致Pt对HC和CO氧化的催化活性降低。     

A carboxylate-bridged Mn(II) compound with 6-methylanthranilate/bipy: oxidation of alcohols/alkenes and catalase-like activity

Abstract

A novel manganese compound, [Mn₂(μ_1,3-6-CH₃-2-NH₂C₆H₄COO)₂(bipy)₄](ClO₄)₂ (bipy = 2,2′-bipyridine), was synthesized and used as a catalyst precursor in the oxidation of alkenes and primary alcohols to corresponding aldehydes, ketones, and acids. The six-coordinate compound has a binuclear structure in which two Mn(II) ions adopt a syn-anti μ_1,3-bridging mode with two carboxylate groups and two chelated bipy ligands. The compound exhibits good activity in the oxidation of cyclohexene to 2-cyclohexene-1-one as the major product (93% conv. in 3 h, 79.3% selectivity) and of cinnamyl alcohol to cinnamaldehyde as the major product with 46% selectivity (100% conv. in 1.5 h) with tert-butyl hydroperoxide (TBHP) in acetonitrile at 70 °C. Furthermore, the catalase-like activity of the compound was studied in different solvents (acetonitrile, methanol, Tris-HCl buffer; TOF = 29,910 h^?1 in Tris-HCl buffer).     

Accelerating effect of poly(methyl methacrylate) on rubber oxidation, Part 1: A chemiluminescence study

The oxidative stability of blends of poly(methyl methacrylate) (PMMA) with four types of rubber, poly(ethylene-stat-propylene-stat-5-methylene-2-norbornene) (EPDM), poly(ethylene-stat-vinyl acetate) (EVA), dicarboxy terminated poly(acrylonitrile-stat-butadiene) (NBR) and poly(1,3-butadiene-stat-styrene) (SBR), has been investigated as models for rubber-toughened PMMA. Chemiluminescence was used to monitor the oxidation of the rubber in the blends, revealing an unexpected accelerating effect of PMMA on the oxidation of all the rubbers investigated. The effect varies according to the type of rubber and the temperature. The thermo-oxidative stability of PMMA has also been found to decrease in the presence of the rubber as proved by thermogravimetry, confirming mutually antagonistic effects of PMMA and rubber in the blend oxidation. On the basis of results from all techniques, including identification of oxidation products by FTIR spectroscopy, a possible mechanism is suggested, involving the formation of mobile radicals in PMMA induced by rubber oxidation. These radicals are suggested to play a crucial role in enhancing rubber phase oxidation.     

Adsorption and reactions of C(6) hydrocarbons at high pressures on Pt(111) single-crystal surfaces studied by sum frequency generation vibrational spectroscopy: mechanisms of isomerization and dehydrocyclization of n-hexane

The adsorption geometries and surface reactions of various C(6) hydrocarbons (n-hexane, 2-methylpentane, 3-methylpentane, and 1-hexene) adsorbed on Pt(111) were investigated using sum frequency generation (SFG) surface vibrational spectroscopy. The adsorptions and reactions were carried out in 1.5 Torr of C(6) hydrocarbons in the absence and presence of excess hydrogen (15 Torr) and in the temperature range 296-453 K. At 296 K and in the presence of excess hydrogen, n-hexane and 3-methylpentane adsorbed molecularly on Pt(111) mostly in "flat-lying" geometries. Upon heating the sample up to 453 K, the molecules underwent dehydrogenation to form new surface species in "standing-up" geometries, such as hexylidyne and metallacyclic species. However, 2-methylpentane and 1-hexene were dehydrogenated to metallacyclobutane and hexylidyne, respectively, at 296 K in the presence of excess hydrogen. The dehydrogenated species remained unreacted on the surface upon heating the sample up to 453 K. The absence of excess hydrogen enhanced dehydrogenation of n-hexane and 3-methylpentane to form pi-allyl c-C(6)H(9) and metallacyclohexane, respectively, at 296 K. Upon heating to 453 K, the pi-allyl c-C(6)H(9) species underwent irreversible dehydrogenation, while hexylidyne and metallacyclic species remained unreacted. On the basis of these results, the mechanisms for catalytic isomerization and dehydrocyclization of n-hexane, which are the important "reforming" reactions to produce high-octane fuels over platinum, were discussed.     

Vibrational spectroscopy on platinum single-crystal electrodes: Part I. In-situ infrared spectroscopic studies of the adsorption and oxidation of CO on Pt (111) in sulphuric acid

Platinum single-crystal electrodes of 5 mm diameter were prepared for in situ infrared spectroscopic measurements by melting platinum wires. The linear potential sweep voltammograms of hydrogen adsorption/desorption on Pt (111), (110) and (100) in 0.5 M sulphuric acid are in excellent agreement with those observed on smaller platinum single-crystal surfaces.The adsorption and oxidation of CO on Pt (111) in 0.5 M sulphuric acid was studied by in situ polarization modulated infrared reflection absorption spectroscopy. The effects of the initial adsorption potential and surface reconstruction on the nature and oxidation mechanism of the adsorbed CO layer are reported.     

1H-NMR Studies on (6R)- and (6S)-deuterated (1–6)-linked disaccharides: assignment of the preferred rotamers about C5–C6 bond of (1–6)-disaccharides in solution

Conformational properties of the C5–C6 bond in the α- and β(1–6)-linked disaccharides in solution were clarified based on the ¹H-NMR spectra of the (6R)- and (6S)-deuterated derivatives.