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Applied Catalysis B, Environmental (v.65, #3-4)

EDITORIAL BOARD (pp. co2).

FULL TITLE (pp. i).

Contents Volume 65 (pp. i-ii).

Calendar of Forthcoming Events (pp. iii).

Influence of NO₂ on the hydrolysis of isocyanic acid over TiO₂ by Gaia Piazzesi; Martin Elsener; Oliver Kröcher; Alexander Wokaun (pp. 169-174).

Investigation of the catalytic hydrolysis of isocyanic acid (HNCO) over TiO₂ revealed that the rate of hydrolysis was considerably reduced in the presence of NO₂ at temperatures below 200°C. Transient low temperature investigations of the reaction, Temperature Programmed Desorption (TPD) experiments and Diffuse Reflectance Infrared Fourier Transform (DRIFT) investigations showed that part of the NO₂ adsorbed strongly on the surface while part reacted with NH₃ to form ammonium nitrate on the catalyst surface. Since at high temperatures NO₂ was desorbed and NH₄NO₃ was decomposed, the catalytic activity was completely restored by thermal treatment at high temperature.

Keywords: Isocyanic acid; HNCO; Hydrolysis; NO; ₂; NH; ₄; NO; ₃; Ammonium nitrate; Urea; SCR

Preparation of Cu-ion-exchanged Fe-PILCs for the SCR of NO by propene by Fernando Dorado; Antonio de Lucas; Prado B. García; José L. Valverde; Amaya Romero (pp. 175-184).

Cu-ion-exchanged iron-pillared interlayer clays (Fe-PILCs) were prepared under different pH conditions to analyze the influence on the distribution of the copper species over their structure, and on the catalytic performance for the selective catalytic reduction (SCR) of NO_x by propene. It was observed that for those samples prepared without pH control, the copper was as isolated Cu²⁺ ions. When the samples were prepared under acid pH, the catalytic activity decreased and an appreciable CO production was observed, likely due to the low amount of Cu²⁺ cations in those catalysts. Finally, for the samples prepared under alkaline pH, the copper was as Cu²⁺ ions and CuO clusters. Their catalytic tests showed the best results for the SCR of NO_x. The presence of CuO species led to an improvement in NO_x yield to N₂. With the catalytic tests and a study by in situ FTIR of SCR of NO, a reaction mechanism has been proposed, where the reaction intermediates are mainly acetates, organic nitro compounds and nitrous oxide species.

Keywords: Cu; NO-SCR; Iron-pillared clays; Infrared spectroscopy; TPD

MFI zeolite as a support for automotive catalysts with reduced Pt sintering by Takaaki Kanazawa (pp. 185-190).

The noble metals used as catalysts in automotive exhaust systems are subject to sintering at extreme temperatures, leading to deterioration of catalytic activity. Zeolite with the MFI (ZSM5) structure is examined as a support for Pt particulate catalysts. The MFI structure is composed of agglomerates of single-crystal zeolite with interstitial mesoporosity. Pt fixed within these mesopores is shown through high-temperature aging tests in air to be highly resistant to sintering due to the mechanical constraints on particle size imparted by the mesoporous structure. The deterioration of catalytic activity after aging is significantly lower than that for comparable γ-alumina supported catalyst.

Keywords: Catalyst; Durability; MFI zeolite; Grain boundary; Pt; Sintering

Catalytic purification of waste gases containing VOC mixtures with Ce/Zr solid solutions by José I. Gutiérrez-Ortiz; Beatriz de Rivas; Rubén López-Fonseca; Juan R. González-Velasco (pp. 191-200).

This study has been undertaken to investigate the efficiency of ceria, zirconia, and Ce_xZr1−_xO₂ mixed oxides as catalysts for the vapour-phase destruction in air of single model VOCs ( n-hexane, 1,2-dichloroethane and trichloroethylene) and non-chlorinated VOC/chlorinated VOC binary mixtures. Considering all catalyst compositions examined for the individual destruction of these compounds, activity for complete oxidation decreased in the following order: n-hexane<1,2-dichloroethane0.5Zr_0.5O₂ and Ce_0.15Zr_0.85O₂) were different than that with the best performance for n-hexane oxidation (CeO₂). Concerning chlorinated VOCs conversion, it was observed that notable improvements in catalyst activity of CeO₂ could be achieved through structural doping with Zr ions. Mixed oxides exhibited promoted redox and acid properties, which resulted catalytically relevant for the oxidation of 1,2-dichloroethane and trichloroethylene. In contrast, the combustion of n-hexane was essentially controlled by surface oxygen species, which were more abundant on CeO₂. Attainment of high n-hexane conversions with CeO₂ was also attributed in part to the hydrophobicity of the support and the reduced interaction with carbon dioxide.Significant ‘mixture effects’ on both activity and selectivity were noticed when a given chlorinated feed was decomposed in the presence of n-hexane. On one hand, each VOC decreased the reactivity of the other relative to that of the pure compound resulting in higher operating temperatures to achieve adequate destruction. Competitive adsorption played an important role in the reciprocal inhibition effects detected with all catalysts. On the other hand, the selectivity to HCl was noticeably enhanced when n-hexane was co-fed, probably due to the increased presence of water generated as an oxidation product.

Keywords: VOC; Catalytic combustion; Ceria–zirconia mixed oxides; Binary mixtures; 1,2-Dichloroethane; Trichloroethylene; n; -Hexane

Exhaust-gas reforming using precious metal catalysts by Sylvain Peucheret; Mark Feaviour; Stan Golunski (pp. 201-206).

Rh-only and Rh bimetallic catalysts have been screened for exhaust-gas reforming, under conditions that mimic the output of an autoignition gasoline engine. Propane has been used as a model fuel, with simulated exhaust-gas providing the co-reactants (O₂ and H₂O) needed to generate hydrogen. Based on oxygen-conversion as a measure of light-off, Pt–Rh on ceria–zirconia shows the highest activity. In the presence of SO₂, adsorbed sulphur species do not inhibit the oxidation reactions that induce light-off, but suppress the major pathway to hydrogen (steam reforming). By excluding platinum and using silica-enriched alumina as the underlying support, light-off is delayed, but the steam reforming reaction becomes much more insensitive to the presence of sulphur. The Pt–Rh catalyst is most suited to exhaust-gas reforming systems in which the engine runs on a sulphur-free fuel, whereas the Rh-only catalyst is the better choice when the fuel is conventional gasoline.

Keywords: Exhaust-gas reforming; Platinum; Rhodium; Sulphur poisoning

Preferential oxidation of CO in rich H₂ over CuO/CeO₂: Details of selectivity and deactivation under the reactant stream by A. Martínez-Arias; A.B. Hungría; G. Munuera; D. Gamarra (pp. 207-216).

A CuO/CeO₂ catalyst is examined with respect to its performance for preferential oxidation of CO in H₂-rich streams. Catalytic activity results are explained on the basis of characterization by operando-DRIFTS and complemented with the analysis of redox properties by electron paramagnetic resonance (EPR) and X-ray photoelectron spectra (XPS). General catalytic activity features are accounted for by comparative analysis of the activities for individual CO and H₂ oxidation, for which similar CuO and CeO₂ interfacial active sites appear to be involved. An interesting particularity is related to observation of a low temperature hydrogen oxidation process in which CO apparently acts as gaseous promoter. A deactivation process taking place rapidly under the reactant stream is evidenced and attributed to accumulation of hydroxyls on the interfacial active sites and/or to copper sintering in the course of the run.

Keywords: CuO catalyst; CeO; ₂; CO-PROX; Hydrogen; Operando; -DRIFTS; XPS; EPR

Photocatalytic degradation of organic pollutants catalyzed by layered iron(II) bipyridine complex–clay hybrid under visible irradiation by Mingming Cheng; Wanhong Ma; Chuncheng Chen; Jiannian Yao; Jincai Zhao (pp. 217-226).

An organic–inorganic layered hybrid was prepared by intercalation of Fe(bpy)₃²⁺ into laponite clay. UV–vis diffuse reflectance, X-Ray diffraction, and SEM confirmed the intercalation and the strong host–guest interaction of Fe(bpy)₃²⁺ molecules with the clay matrix. Compared with laponite, the hybrid formed a solid layered structure due to the linking of laponite platelets by Fe(bpy)₃²⁺ molecules. Upon visible light irradiation ( λ>420nm), the hybrid was found to be highly effective for the degradation of nonbiodegradable cationic organic pollutants such as Rhodamine B (RhB) and N, N-dimethylaniline by activating H₂O₂ at neutral pH values, but inactive toward anionic organic compounds such as Orange II and Sulforhodamine-B. The adsorption and degradation of organics on the hybrid could be controlled by changing the pH value of the suspension. The total organic carbon (TOC) removal yield of RhB was 41%. pH effect trials and the final degraded products further indicate that unless the target is adsorbed onto the clay layers the reaction could not occur. NeitherOH norOOH/O₂⁻ EPR signals were detected during the reaction. The solid support of laponite not only alters the photochemical properties of Fe(bpy)₃²⁺ but also provides a rigid microenvironment for the enrichment of local substrate molecules and thus enhances the interaction of the active center with the substrate.

Keywords: Iron complex; Laponite; Photocatalytic; Pollutant

Use of unsupported and silica supported molybdenum carbide to treat chloroarene gas streams by Antonio de Lucas Consuegra; Patricia M. Patterson; Mark A. Keane (pp. 227-239).

The gas phase catalytic hydrodechlorination (HDC) of mono- and di-chlorobenzenes (423K≤ T≤593K) over unsupported and silica supported Mo carbide (Mo₂C) is presented as a viable means of detoxifying Cl-containing gas streams for the recovery/reuse of valuable chemical feedstock. The action of Mo₂C/SiO₂ is compared with MoO₃/SiO₂ and Ni/SiO₂ (an established HDC catalyst). The pre- and post-HDC catalyst samples have been characterized in terms of BET area, TG-MS, TPR, TEM, SEM, H₂ chemisorption/TPD and XRD analysis. Molybdenum carbide was prepared via a two step temperature programmed synthesis where MoO₃ was first subjected to a nitridation in NH₃ followed by carbidization in a CH₄/H₂ mixture to yield a face-centred cubic (α-Mo₂C) structure characterized by a platelet morphology. Pseudo-first order kinetic analysis was used to obtain chlorobenzene HDC rate constants and the associated temperature dependences yielded apparent activation energies that decreased in the order MoO₃/SiO₂ (80±5kJmol⁻¹)≈MoO₃ (78±8kJmol⁻¹)>Ni/SiO₂ (62±3kJmol⁻¹)≈α-Mo₂C (56±6kJmol⁻¹)≈α-Mo₂C/SiO₂ (53±3kJmol⁻¹). HDC activity was lower for the dechlorination of the dichlorobenzene reactants where steric hindrance influenced chloro-isomer reactivity. Supporting α-Mo₂C on silica served to elevate HDC performance, but under identical reaction conditions, Ni/SiO₂ consistently delivered a higher initial HDC activity. Nevertheless, the decline in HDC performance with time-on-stream for Ni/SiO₂ was such that activity converged with that of α-Mo₂C/SiO₂ after three reaction cycles. A temporal loss of HDC activity (less extreme for the carbides) was observed for each catalyst that was studied and is linked to a disruption to supply of surface active hydrogen as a result of prolonged Cl/catalyst interaction.

Keywords: Hydrodechlorination; Chlorobenzene(s); Molybdenum carbide; Nickel/silica

Characterization by Mössbauer spectroscopy of trimetallic Pd–Sn–Au/Al₂O₃ and Pd–Sn–Au/SiO₂ catalysts for denitration of drinking water by Anthony Garron; Károly Lázár; Florence Epron (pp. 240-248).

The reduction of nitrate using a catalytic process is one of the most interesting ways to solve the problem of drinking water pollution by this compound. The key parameter of this technique is the selectivity toward nitrogen formation. Palladium/tin-based bimetallic catalysts are well suited for this purpose, but the selectivity of these catalysts is not high enough for a direct application of this process. In the present study, alumina- and silica-supported catalysts were prepared by successive deposition of tin and gold onto palladium particles by using controlled surface reaction. The characterization of trimetallic Pd–Sn–Au catalyst evidenced that trimetallic catalysts supported on silica present a palladium/tin/gold phase. The catalytic test showed that this type of catalyst is very active and selective in nitrate and nitrite reduction. Moreover, the addition of gold improves the stability and the selectivity toward nitrogen formation of the catalyst compared to the parent Pd–Sn catalyst.

Keywords: Pd–Sn–Au catalysts; Catalytic reduction in water; Trimetallic catalyst; Gold; Support effect; Nitrate reduction

Sulphur trap materials based on mesoporous Al₂O₃ by E. Schreier; R. Eckelt; M. Richter; R. Fricke (pp. 249-260).

Different sulphur trap materials based on mesoporous Al₂O₃ supports modified with the storage component Ca or Ba and the oxidation components Pt or Cu or Mn were prepared and the SO₂ uptake behaviour examined in the temperature range 50–600°C. A comparison between the different oxidation components shows that the presence of Pt is not a necessary precondition for attaining a good storage behaviour under these test conditions. Mn is the most suitable oxidation component.Tests performed over four temperature cycles show that the removal efficiency for SO₂ gradually decreases over the course of temperature cycles. Moreover, a progressive diminution of the SO₂ uptake is observed especially in the lower temperature range during the temperature cycles. A further modification with Na prevents this drawback. Especially a distinct improvement is observed with the Ca-containing material. It stored 22wt.% sulphate during the four temperature cycles. This material is regarded as being most suitable for application as sulphur trap material.

Keywords: SO; _x; trap; SO; _x; adsorption; Alumina; Ba; Ca; Mn; Cu; Pt

Catalytic wet peroxide oxidation of phenol with a Fe/active carbon catalyst by J.A. Zazo; J.A. Casas; A.F. Mohedano; J.J. Rodríguez (pp. 261-268).

A Fe on activated carbon catalyst has been prepared and tested for phenol oxidation with H₂O₂ in aqueous solution at low concentration (100mg/L). Working at 50°C, initial pH 3 and a dose of H₂O₂ corresponding to the stoichiometric amount (500mg/L) complete removal of phenol and a high TOC reduction (around 85%) has been reached. Oxidation of phenol gives rise to highly toxic aromatic intermediates which finally disappear completely evolving to short-chain organic acids. Some of these last showed to be fairly resistant to oxidation being responsible for the residual TOC. In long-term continuous experiments the catalyst undergoes a significant loss of activity in a relatively short term (20–25h) due to Fe leaching, this being related with the amount of oxalic acid produced. Deactivation may also be caused by active sites blockage due to polymeric deposits on whose formation some evidences were found. Washing with 1N NaOH solution allows to recover the activity although complete restoration was not achieved.

Keywords: CWPO; Hydrogen peroxide; Phenol oxidation; Active carbon iron supported catalyst; Fe/AC

Wet oxidation of phenol, cresols and nitrophenols catalyzed by activated carbon in acid and basic media by Aurora Santos; Pedro Yustos; Sergio Rodriguez; Felix Garcia-Ochoa (pp. 269-281).

A commercial activated carbon, Industrial React FE01606A, was used as catalyst in the wet oxidation, in both acid and basic media, of phenolic pollutants, such as phenol, cresols and nitrophenols, currently found in industrial wastewaters. Reaction runs were carried out in a fixed-bed reactor (FBR) with concurrent upflow by feeding a 1000mgL⁻¹ aqueous solution on each pollutant concurrently with a gas oxygen flow. Temperature and oxygen pressure of the reactor were set to 160°C and 16bar, respectively. The basic medium was maintained by using 500ppm sodium bicarbonate as buffer reagent to keep the pH in the range 7–8. The initial pH 3.5 was set by adding sulphuric acid. Oxidation intermediates were identified and their distribution with respect to the pollutant oxidation progress was measured. Utilizing these results, oxidation routes for each phenolic compound were deduced. The intermediates produced were diverse in acid and basic media and their composition explains the evolution of the corresponding toxicity measured at the reactor effluent. While under acidic conditions hydroxybenzoic acids, dihydroxyl benzenes and quinones were obtained as primary products, these last two compounds (more toxic than the original pollutant) were not detected in basic conditions, and with such media lower toxicities at the reactor exit were obtained. Moreover, the catalyst was found to be stable in the time range studied (300h).

Keywords: Catalytic wet oxidation; Activated carbon; Substituted phenols; pH; Toxicity

Catalytic oxidation of HCN over a 0.5% Pt/Al₂O₃ catalyst by Haibo Zhao; Russell G. Tonkyn; Stephan E. Barlow; Bruce E. Koel; Charles H.F. Peden (pp. 282-290).

The adsorption of HCN on, its catalytic oxidation with 6% O₂ over 0.5% Pt/Al₂O₃, and the subsequent oxidation of strongly bound chemisorbed species upon heating were investigated. The observed N-containing products were N₂O, NO and NO₂, and some residual adsorbed N-containing species were oxidized to NO and NO₂ during subsequent temperature programmed oxidation. Because N-atom balance could not be obtained after accounting for the quantities of each of these product species, we propose that N₂ and was formed. Both the HCN conversion and the selectivity towards different N-containing products depend strongly on the reaction temperature and the composition of the reactant gas mixture. In particular, total HCN conversion reaches 95% above 250°C. Furthermore, the temperature of maximum HCN conversion to N₂O is located between 200 and 250°C, while raising the reaction temperature increases the proportion of NO_x in the products. The co-feeding of H₂O and C₃H₆ had little, if any effect on the total HCN conversion, but C₃H₆ addition did increase the conversion to NO and decrease the conversion to NO₂, perhaps due to the competing presence of adsorbed fragments of reductive C₃H₆. Evidence is also presented that introduction of NO and NO₂ into the reactant gas mixture resulted in additional reaction pathways between these NO_x species and HCN that provide for lean-NO_x reduction coincident with HCN oxidation.

Keywords: HCN; Oxidation; Desorption; Supported Pt catalyst; C; ₃; H; ₆; NO; N; ₂; O; NO; ₂; NO; _x

Generating hydrogen-rich fuel-cell feeds from dimethyl ether (DME) using physical mixtures of a commercial Cu/Zn/Al₂O₃ catalyst and several solid–acid catalysts by Troy A. Semelsberger; Kevin C. Ott; Rodney L. Borup; Howard L. Greene (pp. 291-300).

Homogeneous physical mixtures containing a commercial Cu/ZnO/Al₂O₃ catalyst and a solid–acid catalyst were used to examine the acidity effects on dimethyl ether hydrolysis and their subsequent effects on dimethyl ether steam reforming (DME-SR). The acid catalysts used were zeolites Y [Si/Al=2.5 and 15: denoted Y(Si/Al)], ZSM-5 [Si/Al=15, 25, 40, and 140: denoted Z(Si/Al)] and other conventional catalyst supports (ZrO₂, and γ-Al₂O₃). The homogeneous physical mixtures contained equal amounts, by volume, of the solid–acid catalyst and the commercial Cu/ZnO/Al₂O₃ catalyst (BASF K3-110, denoted as K3). The steam reforming of dimethyl ether was carried out in an isothermal packed-bed reactor at ambient pressure.The most promising physical mixtures for the low-temperature production of hydrogen from DME contained ZSM-5 as the solid–acid catalyst, with hydrogen yields exceeding 90% ( T=275°C, S/ C=1.5, τ=1.0s and P=0.78atm) and hydrogen selectivities exceeding 94%, comparable to those observed for methanol steam reforming (MeOH-SR) over BASF K3-110, with values equaling 95% and 99%, respectively ( T=225°C, S/ C=1.0, τ=1.0s and P=0.78atm). Large production rates of hydrogen were directly related to the type of acid catalyst used. The hydrogen production activity trend as a function of physical mixture wasK3+Z(25)K3+Z(15)K3+Z(40)>K3+Z(140)K3+Y(15)>K3+Y(2.5)≫K3+γ-Al2O3>K3+ZrO2

Keywords: Dimethyl ether; Hydrolysis; Zeolites; Methanol; Alumina; Zirconia; Acidity; ZSM-5; Y; Steam reforming; Hydrogen; Fuel cells

Nitrogen-containing TiO₂ photocatalysts by C. Belver; R. Bellod; A. Fuerte; M. Fernández-García (pp. 301-308).

A series of N-substituted Ti isopropoxide precursors were synthesized by using three different amine-type ligands. The resulting Ti-complexes were characterized by nuclear magnetic resonance (NMR) and used to obtain solid precipitates by a reverse microemulsion method. These N-rich solid precipitates were subjected to three calcination treatments differing in the gas atmosphere allowed to contact the solid, yielding nanosized materials. A thermogravimetric analysis of the solid precipitates, combined with a mass spectrometry/infrared study of the evolving gaseous products, were able to show the influence of preparation parameters, e.g. Ti-precursor nature and treatment conditions, in the decomposition process of the solid precursors and the formation of the final nanoparticulated solid catalysts. Both parameters affect the interaction between solid oxygen species (O²⁻; OH⁻) and N,C-containing fragments present in the solid precursors. The chemical (e.g. nitrogen content), structural (phase, cell parameters and volume, and defect structure) and morphological (BET area and primary particle size) properties of the catalytic final solids were studied as a function of the preparation conditions.

Keywords: Photocatalysis; Microemulsion method; NMR; TG-MS; DRIFTS; TiO; ₂; anatase materials; N-doping and impurity; Visible light absorption; Pollutant mineralization and degradation

Nitrogen-containing TiO₂ photocatalysts by C. Belver; R. Bellod; S.J. Stewart; F.G. Requejo; M. Fernández-García (pp. 309-314).

A series of nanosized N-containing TiO₂-based materials with Anatase-type structure and synthesized by a microemulsion method were tested in the photocatalytic degradation of methylcyclohexene, a representative example of volatile organic compound (VOC) present in urban atmospheres. A combined diffuse reflectance infrared (DRIFTS) and X-ray absorption (XAS) spectroscopic study allows to analyze the nature and number of N-containing species and other defects (particularly oxygen vacancies) present in the solid catalysts. The structural characterization was used to interpret the UV–vis spectra of the solids and the resulting joint information allows to rationalize the photocatalytic activity differences observed through the samples under sunlight-type excitation. We founded that our samples contain substitutional and interstitial N-containing impurities and a significant number of oxygen vacancies. Photocatalytic activity is correlated with an optimum of oxygen vacancies, above and below which a decrease of the steady state reaction rate is observed. The physico-chemical bases of this behavior are discussed on the light of the above mentioned experimental results.

Keywords: Photocatalysis; Microemulsion method; XAS; DRIFTS; TiO; ₂; Anatase materials; N-doping and impurity; Visible light absorption; Pollutant mineralization and degradation

Kinetics of dye decolorization in an air–solid system by Alison J. Julson; David F. Ollis (pp. 315-325).

The photocatalytic decolorization of adsorbed organic dyes (Acid Blue 9, Acid Orange 7, Reactive Black 5 and Reactive Blue 19) in air was examined, applicable to self-cleaning surfaces and catalyst characterization. Dye-coated Degussa P25 titanium dioxide (TiO₂) and dye-coated photo-inert aluminum oxide (Al₂O₃) particles, both of sub-monolayer initial dye coverage, were illuminated with 1.3mWcm⁻² of near-UV light. Visual evidence of color removal is reported with photographic images. Two methods, Indirect and Direct Analysis, were employed to quantitatively examine the decolorization kinetics of dyes using UV–visible transmission and diffuse reflectance spectroscopy, respectively. A decrease in dye concentration with time was observed with near-UV illumination of dye-coated TiO₂ powders for all dyes. Dyes did not photodegrade significantly on photo-inert Al₂O₃.UV–visible spectroscopy data was used to model the kinetics of the photocatalytic degradation. Two first-order reactions in series provided the most convincing rate form for the photodegradation of dyes adsorbed to TiO₂, with a first step the conversion of colored dye to colored intermediate, and the second the conversion to colorless product(s). The first rate constant was of similar magnitude for all dyes, averaging k₁=0.13min⁻¹. Similarly, for the second, k₂=0.0014min⁻¹.

Keywords: Photocatalysis; Dye; Air–solid; Decolorization

Erratum to “Proton and electron wave-particles in chemical and physical environments�? [Appl. Catal. B: Environ. 64 (2006) 111–120] by Costas G. Vayenas; Alexandros D. Katsaounis (pp. 326-327).

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Applied Catalysis B, Environmental (v.65, #3-4)