Use of a batch rotating photocatalytic contactor for the degradation of organic pollutants in wastewater

Aqueous solutions of a chlorinated VOC, 3,4-dichlorobut-1-ene, as well as other pollutants, may be mineralised to carbon dioxide, water and hydrochloric acid using a sealed rotating photocatalytic reactor. The effect of pH, dissolved oxygen concentration, light intensity, pollutant concentration and rotation speed on the degradation rate have been investigated as well as competition kinetics with methanol. This reactor may be optimised to minimise competition effects in mixed solutions. (C) 2001 Elsevier Science B.V. All rights reserved.

A density functional theory study of the a-olefin selectivity in Fischer-Tropsch synthesis

We studied the alpha-olefin selectivity in Fischer-Tropsch (FT) synthesis using density functional theory (131717) calculations. We calculated the relevant elementary steps from C-2 to C-6 species. Our results showed that the barriers of hydrogenation and dehydrogenation reactions were constant with different chain lengths, and the chemisorption energies of alpha-olefins from DFT calculations also were very similar, except for C-2 species. A simple expression of the paraffin/olefin ratio was obtained based on a kinetic model. Combining the expression of the paraffin/olefin ratio and our calculation results, experimental findings are satisfactorily explained. We found that the physical origin of the chain length dependence of paraffin/olefin ratio is the chain length dependence of both the van der Waals interaction between adsorbed alpha-olefins and metal surfaces and the entropy difference between adsorbed and gaseous alpha-olefins, and that the greater chemisorption energy of ethylene is the main reason for the abnormal ethane/ethylene ratio. (c) 2008 Elsevier Inc. All rights reserved.

CO oxidation and NO reduction on metal surfaces: density functional theory investigations

This article reviews the accumulated theoretical results, in particular density functional theory calculations, on two catalytic processes, CO oxidation and NO reduction on metal surfaces. Owing to their importance in automotive emission control, these two reactions have generated a lot of interest in the last 20 years. Here the pathways and energetics of the involved elementary reactions under different catalytic conditions are described in detail and the understanding of the reactions is generalized. It is concluded that density functional theory calculations can be applied to catalysis to elucidate mechanisms of complex surface reactions and to understand the electronic structure of chemical processes in general. The achieved molecular knowledge of chemical reactions is certainly beneficial to new catalyst design.

On the need to use steady-state or operando techniques to investigate reaction mechanisms: An in situ DRIFTS and SSITKA-based study example

The nature of the surface species formed at the surface of 2 wt.% Pt/CeO2 catalyst during the forward water-gas-shift (WGS, CO + H2O -> CO2 + H-2) and the reverse reaction (RWGS) were essentially identical. More, the surface concentration of formate, carbonate and carbonyl species was similar in each case. The presence of well-resolved IR bands allowed an unequivocal relative quantitative analysis of each species, avoiding the use of the carboxylate stretching region (1600-1200 cm(-1)). However, the quantitative analysis in the case of an isotopic study was complicated due to the overlapping of the various isotope bands, yet this problem could be overcome by integrating the high-wavenumber part of the bands. The reactivity of the surface species formed under RWGS conditions was followed under two different gaseous streams. Firstly, the reactivity of these intermediates were followed under an inert gas (i.e., At), in which case carbonates were essentially stable and less reactive than formates. Secondly, the reactivity of the same surface species was followed when switching to the corresponding C-13-labelled feed (i.e., (CO2)-C-13 + H-2), in which case carbonates were exchanged significantly faster than formates. While carbonates species have been reported as reaction intermediate under reaction conditions, the increased stability or surface poisoning by these carbonates in the absence of reaction mixture was highlighted. Ultimately, this work re-emphasises the need to use steady-state conditions if the true operando reactivity of the adsorbates and structure of the solid are to be determined. (c) 2005 Elsevier B.V. All rights reserved.

Steam reforming of model compounds and fast pyrolysis bio-oil on supported noble metal catalysts

The production of hydrogen by steam reforming of bio-oils obtained from the fast pyrolysis of biomass requires the development of efficient catalysts able to cope with the complex chemical nature of the reactant. The present work focuses on the use of noble metal-based catalysts for the steam reforming of a few model compounds and that of an actual bio-oil. The steam reforming of the model compounds was investigated in the temperature range 650-950 degrees C over Pt, Pd and Rh supported on alumina and a ceria-zirconia sample. The model compounds used were acetic acid, phenol, acetone and ethanol. The nature of the support appeared to play a significant role in the activity of these catalysts. The use of ceria-zirconia, a redox mixed oxide, lead to higher H-2 yields as compared to the case of the alumina-supported catalysts. The supported Rh and Pt catalysts were the most active for the steam reforming of these compounds, while Pd-based catalysts poorly performed. The activity of the promising Pt and Rh catalysts was also investigated for the steam reforming of a bio-oil obtained from beech wood fast pyrolysis. Temperatures close to, or higher than, 800 degrees C were required to achieve significant conversions to COx and H-2 (e.g., H-2 yields around 70%). The ceria-zirconia materials showed a higher activity than the corresponding alumina samples. A Pt/ceria-zirconia sample used for over 9 h showed essentially constant activity, while extensive carbonaceous deposits were observed on the quartz reactor walls from early time on stream. In the present case, no benefit was observed by adding a small amount of O-2 to the steam/bio-oil feed (autothermal reforming, ATR), probably partly due to the already high concentration of oxygen in the bio-oil composition. (c) 2005 Elsevier B.V. All rights reserved.

Steric effects in the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol using an Ir/C catalyst

The liquid phase selective hydrogenation of cinnamaldehyde to cinnamyl alcohol has been carried out over a graphite-supported iridium catalyst. The effect of reaction parameters such as temperature, pressure, concentration of reactant, the effect of addition of product to the feed and pre-reduction of the catalyst were studied. In situ pre-reduction of the catalyst with hydrogen had a very significant enhancing effect on the conversion of cinnamaldehyde and selectivity of the catalyst to cinnamyl alcohol. Kinetic analysis of the pre-reduced catalyst showed that the reaction is zero order with respect to cinnamaldehyde and first order with respect to hydrogen. The reaction follows an Arrhenius behaviour with an activation energy of 37 kJ mol(-1). Detailed analysis of the reaction showed that hydrogenation of the C=C double bond to give hydrocinnamaldehyde predominantly occurred at low conversions of cinnamaldehyde (

The investigation of mechanisms in environmental catalysis using time-resolved methods

The formation of nitrogen oxides (NOx) during a combustion process is difficult to avoid because of the large exotherm and the consequent problem of avoiding local high-temperature spikes. Consequently, for many applications, such as for automotive power generation, there will be a continuing need to use catalytic after-treatment to reduce harmful emissions. The investigation of the mechanisms of the key catalytic reactions in environmental catalysis can provide an insight into the action of the catalyst, and time-resolved methods offer a powerful means to study these processes under realistic conditions. The use of Temporal Analysis of Products (TAP) and Steady State Isotopic Transient Kinetic Analysis (SSITKA) methods to investigate the reduction of NOx under various experimental conditions is described. From a detailed analysis of the SSITKA profiles, it is shown that at low temperatures the mechanism for the formation of N-2 and N2O from NO may differ from the conventional high-temperature mechanism. This is supported by density functional theory calculations, which show that the barrier to the formation of N2O from the reaction of N(ads) and NO(ads) may be too high to allow this process to occur at low temperatures. The alternative reaction of NO(ads) + NO(ads) = N2O(g) + O(ads) is shown to be much more favorable and is consistent with the SSITKA analysis. The remarkable effect of hydrogen as a reductant at low temperatures is described, and alternative interpretations of the role of hydrogen are discussed.

An investigation of alternative catalytic approaches for the direct synthesis of hydrogen peroxide from hydrogen and oxygen

Catalytic systems for the direct production of hydrogen peroxide from hydrogen and oxygen are investigated, and the factors which make a successful process identified. The use of low metal loadings, an organic co-solvent (such as ethanol) and reduced palladium as the catalytic metal all lead to good activity and selectivity. (C) 2002 Elsevier Science B.V. All rights reserved.

The effect of the addition of sodium compounds in the liquid-phase hydrodechlorination of chlorobenzene over palladium catalysts

The hydrodechlorination of chlorobenzene over supported palladium catalysts has been studied. The palladium catalysts: deactivate as the reaction proceeds due to the HCl formed as by-product. The effect of the addition of sodium compounds has been analysed for the neutralisation of HCl. When NaOH was added to the reaction mixture, no beneficial effect was observed due to the detrimental effect of the alkaline medium on the textural and metallic properties of the catalysts. Doping the support with NaOH prior to impregnation with the metal precursor leads (after calcination and reduction) to catalysts with better activity and tolerance to deactivation, especially those obtained when using PdCl2 as the metal precursor. Low metal dispersion and the capture of chloride by forming NaCl are the: main factors contributing to the: improved catalytic properties. Finally, doping the catalysts with NaOH or NaNO3, after reduction of the metal precursor leads to a moderate increase in initial activity and final conversion, although NaOH impregnation also gave rise to support corrosion and metal dispersion modification. (C) 2001 Elsevier Science B.V, All rights reserved.

Sheets of Large Superhydrophobic Metal Particles Self Assembled on Water by the Cheerios Effect

A copper-rich cereal: Superhydrophobic copper particles show a very large Cheerios effect and rapidly self-assemble into robust sheets on the surface of water. These sheets can support objects (including water drops, see photo) placed on them, even though the irregular geometry of the particles means that they contain macroscopic holes.

Enzymatic and chemoenzymatic synthesis of arene trans-dihydrodiols

Several potential approaches to the enzyme-catalysed synthesis of arene trans-diols have been examined including epoxidation/hydrolysis, bis-benzylic hydroxylation, cis-dihydroxylation/alcohol dehydrogenation/ketone reduction, cisdihydroxylation/cis-trans isomerisation. and multi-enzyme synthesis of trans-dihydrodiol secondary metabolites from primary metabolites. The lack of general applicability of these enzymatic methods has led to the development of several chemoenzymatic routes for the synthesis of a series of trans-dihydrodiols from the readily available cis-dihydrodiol precursors. Partial hydrogenation of cis-dihydrodiol metabolites to yield the corresponding cis-tetrahydrodiols followed by a regioselective Mitsunobu inversion process gave trans-tetrahydrodiols that were in turn converted to trans-dihydrodiols. The formation of anti-benzene dioxides or iron tricarbonyl complexes from the corresponding cis-dihydrodiol precursors provided shorter and more convenient chemoenzymatic routes to trans-dihydrodiols. The application of cis-dihydrodiol metabolites of polycyclic azaarenes in the synthesis of the corresponding arene oxides followed by chemical hydrolysis provides a convenient route to trans-dihydrodiols. (C) 2002 Elsevier Science B.V. All rights reserved.

The preparation and properties of coprecipitated Cu-Zr-Y and Cu-Zr-La catalysts used for the steam reforming of methanol

A series of Cu-zirconia catalysts containing various additives (Y2O3, La2O3, Al2O3 and CeO2) have been prepared by coprecipitation and their activities and stabilities under operating conditions have been obtained for the steam reforming of methanol. It has been found that an yttria-promoted catalyst containing 30 mol% Cu and 20 mol% of Y2O3 is not only very active but is also very stable under reaction conditions. The yttria appears to stabilise a high copper surface area and may also have a slight promotional effect on the copper. The results obtained with this material compare very favourably with data for the best catalysts reported in the literature. (C) 2007 Published by Elsevier B.V.

Sulfur-tolerant NOx storage traps: an infrared and thermodynamic study of the reactions of alkali and alkaline-earth metal sulfates

The sulfur tolerance of a barium-containing NOx storage/reduction trap was investigated using infrared analysis. It was confirmed that barium carbonate could be replaced by barium sulfate by reaction with low concentrations of sulfur dioxide (50 ppm) in the presence of large concentrations of carbon dioxide (10%) at temperatures up to 700 degreesC. These sulfates could at least be partially removed by switching to hydrogen-rich conditions at elevated temperatures. Thermodynamic calculations were used to evaluate the effects of gas composition and temperature on the various reactions of barium sulfate and carbonate under oxidizing and reducing conditions. These calculations clearly showed that if, under a hydrogen-rich atmosphere, carbon dioxide is included as a reactant and barium carbonate as a product then barium sulfate can be removed by reaction with carbon dioxide at a much lower temperature than is possible by decomposition to barium oxide. It was also found that if hydrogen sulfide was included as a product of decomposition of barium sulfate instead of sulfur dioxide then the temperature of reaction could be significantly lowered. Similar calculations were conducted using a selection of other alkaline-earth and alkali metals. In this case calculations were simulated in a gas mixture containing carbon monoxide, hydrogen and carbon dioxide with partial pressures similar to those encountered in real exhausts during switches to rich conditions. The results indicated that there are metals such as lithium and strontium with less stable sulfates than barium, which may also possess sufficient NOx storage capacity to give sulfur-tolerant NOx traps.

On the nature of the silver phases of Ag/Al2O3 catalysts for reactions involving nitric oxide

The nature of the silver phases of Ag/Al2O3 catalysts (prepared by silver nitrate impregnation followed by calcination) was investigated by X-ray diffractograms (XRD), transmission electron microscopy (TEM) and UV-VIS analyses and related to the activity of the corresponding materials for the oxidation of NO to NO2. The UV-VIS spectrum of the 1.2 wt.% Ag/Al2O3 exhibited essentially one band associated with Ag+ species and the NO2 yields measured over this material were negligible. A 10 wt.% Ag/Al2O3 material showed the presence of oxidic species of silver (as isolated Ag+ cations and silver aluminate), but the UV-VIS data also revealed the presence of some metallic silver. The activity for the NO oxidation to NO2 of this sample was moderate. The same 10% sample either reduced in H-2 or used for the C3H6-selective catalytic reduction (SCR) of NO showed a significantly larger proportion of silver metallic phases and these samples displayed a high activity for the formation of NO2. These data show that the structure and nature of the silver phases of Ag/Al2O3 catalysts can markedly change under reaction feed containing only a fraction of reducing agent (i.e. 500 ppm of propene) in net oxidizing conditions (2.5% O-2). The low activity for N-2 formation during the C3H6-SCR of NO (reported in an earlier study) over the high loading sample can. therefore, he related to the presence of metallic silver. which is yet a good catalyst for NO oxidation to NO2. The reverse observations apply for the oxide species observed over the low loading sample, which is a good SCR catalyst but do not oxidize NO to NO2. (C) 2002 Elsevier Science B.V. All rights reserved.

A modified commercial DRIFTS cell for kinetically relevant operando studies of heterogeneous catalytic reactions

This paper discusses a number of checks that should be carried out to ensure that the kinetic and spectroscopic measurements made using a DRIFTS cell are meaningful. The observations reported here demonstrate how an appropriately modified commercial DRIFTS cell can provide pertinent kinetic information about both gaseous products and the related surface intermediates. The oxidation of CO with 02 was used as a test to assess the catalyst bed bypass by the reaction mixture. Full CO conversion was obtained after the light-off temperature in the case of the modified cell, contrary to the case of the original cell, for which 80% of the reaction mixture bypassed the catalyst bed. The water-gas shift reaction over a Pt/CeO2 catalyst was used as a model reaction to further characterize the behavior of the cell under reaction conditions. The catalyst bed was shown not to be a dead-zone and was purged in essentially the same time as that needed to purge the cell. The reaction chamber globally operated in a quasi plug-flow mode and the gas composition in the thin catalyst bed appears to be homogeneous when operated under differential conditions. The production of the gas-phase reaction product CO2 could be simultaneously followed both by mass spectrometry and DRIFTS, both techniques leading to identical results. Various IR bands integration methods were discussed to allow a precise and accurate determination of the surface concentration of adsorbates during isotopic exchange. (c) 2008 Elsevier B.V. All rights reserved.