Applied Catalysis B, Environmental (v.56, #3)
Kinetics of malonic acid degradation in aqueous phase over Pt/graphite catalyst by Z.P.G. Masende; B.F.M. Kuster; K.J. Ptasinski; F.J.J.G. Janssen; J.H.Y. Katima; J.C. Schouten (189-199).
This work aims at describing quantitatively the catalytic decarboxylation of malonic acid over a 5.0 wt.% Pt/graphite catalyst. The study was carried out using a slurry phase continuous flow stirred slurry reactor (CSTR) at a temperature range of 120–160 °C and at a reactor pressure of 1.8 MPa. The conversion of malonic acid during catalytic oxidation was found to proceed via decarboxylation to CO2 and acetic acid, and also oxidation to CO2 and H2O. No indication of deactivation of the platinum catalyst was observed at a maximum residual oxygen pressure in the reactor up to 150 kPa. A reaction mechanism involving elementary steps has been suggested to explain the decarboxylation and oxidation of malonic acid. A kinetic model that accounts for both non-catalysed and catalysed decarboxylation of malonic acid has been developed and validated. The non-catalysed reaction is first order in malonic acid. The activation energies and adsorption enthalpies have been determined. The model is able to describe the experimental data adequately.
Keywords: Wastewater treatment; Malonic acid; Acetic acid; Decarboxylation; Kinetics; Platinum catalyst; Multiphase reactors;
On the mechanism of sulphur poisoning and regeneration of a commercial gasoline NO x -storage catalyst by F. Rohr; S.D. Peter; E. Lox; M. Kögel; A. Sassi; L. Juste; C. Rigaudeau; G. Belot; P. Gélin; M. Primet (201-212).
A commercial NO x -storage catalyst for gasoline applications containing platinum, palladium and rhodium together with Ba/CeO2/Al2O3 has been sulfated on the engine bench at 390 and 510 °C with nominal exposures of 0.3 and 1.3 g sulphur/l catalyst. Exposures of 0.3 g/l are too low to have a notable impact on the catalytic performance. At 390 °C, the sulphur is quantitatively adsorbed as opposed to 510 °C. Sulphur is mainly adsorbed at the inlet side of the catalyst thereby shielding the downstream region. Desulfation on synthetic gas bench at 700 °C leads to a removal of 50% of the sulphur. The residual sulphur is more evenly distributed along the length of the catalyst compared to the sulphur profile in the sulfated catalyst. This leads to an improvement of the NO x -activity at the inlet side and a drop in NO x -performance at the outlet side after desulfation. Results from SEM–EDX, TPD, TPR and IR suggest that the barium component is selectively poisoned by the sulphur while alumina and ceria are not affected significantly. The insufficient desulfation of the catalyst is almost entirely due to the incomplete sulphur removal from the barium sites. In the desulfated state the IR and TPD-data suggest the presence of barium carbonate species of low thermal stability together with bulk carbonate. After sulfation the IR data indicate the presence of different sulphate species, both bulk and surface phases. Thermal ageing and sulphur poisoning affect different parts of the operational temperature window of the NO x -storage catalyst. While thermal stress alone mainly affects low temperature NO x -performance, sulphur poisoning mainly reduces high temperature activity. The results of this work help to identify strategies to improve both the sulfur resistance of the catalyst and the operational conditions for a better management of the NO x -trap system.
Keywords: NO x storage; Sulphur; Sulphur poisoning;
Decomposition of gas-phase benzene using plasma-driven catalyst (PDC) reactor packed with Ag/TiO2 catalyst by Hyun-Ha Kim; Seung-Min Oh; Atsushi Ogata; Shigeru Futamura (213-220).
This paper describes the decomposition of gas-phase benzene using a plasma-driven catalyst (PDC) reactor packed with 1.0 wt.% Ag/TiO2 catalysts. The decomposition of benzene preferentially produced CO2 and CO, and formic acid as minor one. Carbon balance based on these products was satisfactory at around 100%. For the concentration lower than 110 ppm, the PDC reactor successfully decomposed benzene with specific input energy of around 130 J/l, where the formation of nitrogen oxides was small. The plasma-induced catalytic activity appeared only during plasma application and disappeared as the plasma was turned off. Thermal catalysis showed that temperature was not an important parameter in the decomposition of benzene using the PDC reactor. Comparison of the effects of dilution gases (Ar, N2) on the benzene decomposition revealed that the contribution of UV light from the plasma to the activation of Ag/TiO2 catalyst was negligible. The contribution of catalytic reaction became dominant as increasing specific input energy. The removed amount of benzene showed zero-order to the initial concentration of benzene and determined mostly by specific input energy to the PDC reactor.
Keywords: Nonthermal plasma; Corona discharge; TiO2; Benzene; Plasma-driven catalysis;
La2O3 as primer for supporting La0.9Ce0.1CoO3 ± δ on cordieritic honeycombs by L. Fabbrini; I. Rossetti; L. Forni (221-227).
To understand better the effect of La2O3 as primer for the preparation of honeycomb supported La0.9Ce0.1CoO3 ± δ catalyst, some samples were prepared starting from different La-precursors. Physical and morphological properties of both the primer-coated honeycomb and its precursors (as-prepared and after thermal treatment at different temperature) have been investigated by TGA, TPD–MS, XRD and SEM. Calcination at high temperature brought about a rearrangement of both primer and active phase morphology. When La acetate was used as precursor, such a transformation seemed not to alter substantially the morphology of the primer layer, leading to a poorer anchoring to the support, with deep cracking and exfoliation of the La oxide platelets so formed. By contrast, when the primer precursor was La nitrate, the higher reactivity of the hydroxynitrates, forming during calcination, with respect to both the active phase and the honeycomb, led to a more uniform and compact La2O3 layer, anchoring much better the perovskite to the monolith support.
Keywords: Methane; Catalytic combustion; Lanthanum oxide; Perovskite supporting on honeycomb monoliths;
A comparative study of the C3H6 + NO + O2, C3H6 + O2 and NO + O2 reactions in excess oxygen over Na-modified Pt/γ-Al2O3 catalysts by I.V. Yentekakis; V. Tellou; G. Botzolaki; I.A. Rapakousios (229-239).
The performance of 0.5 wt.% Pt (varying wt.% Na)/γ-Al2O3 catalysts in catalysing the C3H6 + NO + O2, C3H6 + O2 and NO + O2 reactions under simulated lean-burn engine exhaust conditions, was investigated over a wide range of temperature (∼100–500 °C) and sodium loadings (0–4.2 wt.%). For the first two reactions, depending on the Na loading, both promoting and poisoning effects were obtained: optimal promotion was achieved at a sodium loading of 2.6 wt.%. On the other hand, NO + O2 → NO2 reaction was inhibited over the whole range of Na loadings used. In the promoting regime, Na widened the temperature window of the C3H6 + NO + O2 reaction towards lower temperatures by ∼50 °C, accompanied by an enhancement in N2-selectivity by ∼40 additional percentage points. For the C3H6 + O2 reaction the propene light-off temperature and the temperature for 100% propene conversion decreased by 64 °C. Na loadings higher than the optimal loading caused a dramatic decrease in NO conversion over the whole temperature range in the case of C3H6 + NO + O2 reaction, and a substantial decrease in hydrocarbon conversion for both reactions. Comparison of the C3H6 conversion profiles for these two reactions indicates significant inhibition of hydrocarbon oxidation by NO for all catalysts: nitrogen oxide increases the propene light-off temperature and the temperature of 100% C3H6 conversion by ∼80 °C. The promoting and poisoning effects of Na for all three reactions are understandable in terms of the influence of alkali modifier on the relative adsorption strengths of reactant species. The inhibition of hydrocarbon oxidation caused by NO and the propene-induced inhibition of NO2 formation at low temperatures are understandable in terms of the competition of reaction intermediates for active surface sites.
Keywords: Platinum; Sodium; Propene; NO x ; Propene oxidation; Selective catalytic reduction; SCR; Lean-burn conditions; NO2 formation; N2-selectivity;
Evaluation of a spinel based pigment system as a CO oxidation catalyst by S. PalDey; S. Gedevanishvili; W. Zhang; F. Rasouli (241-250).
Commercially available black pigment consisting of mixed manganese, copper and iron oxides was tested as a catalyst as well as an oxidant for CO oxidation. The crystalline structures of the catalyst were determined by XRD as Cu1.5Mn1.5O4 mixed with Fe2O3 and Mn3O4 oxides. The fresh catalyst with 30–300 nm size and a BET surface area of 18.5 m2 g−1 was able to completely convert CO at 525 °C even at a significantly high CO―O2 ―He gas flow rate of 1000 ml min−1 (corresponding space velocity being ∼310,000 h−1). While the reaction rate was independent of oxygen concentration in the range tested (0.8–9.9 vol.% of O2 at a constant CO concentration of 0.85%), it depended on CO concentration. The reaction order over fresh catalyst was measured to be 0.85 with respect to CO with an activation energy value of 47.9 kJ mol−1. Application of a reduction followed by oxidation type of heat treatment on fresh catalyst induced the formation of fine clusters or domains of ∼5 nm on the surface of the catalyst particles. This refined morphology with high density of defects led to a great improvement in catalytic activity. Complete CO conversion was achieved at 180 °C over a heat treated catalyst. This change in morphology also led to higher reducibility of mixed oxide system after heat treatment as indicated by TPR results. The mixed oxides of transition metals can be a viable alternative to precious metal and noble metal containing catalysts for oxidation of CO.
Keywords: Oxidation catalyst; CO oxidation; Heat treatment; Cu1.5Mn1.5O4; Copper oxide; Iron oxide; Manganese oxide; Transition metal oxides;
Proton tunneling-induced bistability, oscillations and enhanced performance of PEM fuel cells by A. Katsaounis; S. Balomenou; D. Tsiplakides; S. Brosda; S. Neophytides; C.G. Vayenas (251-258).
Proton migration through hydrated Nafion membranes in polymer electrolyte membrane (PEM) fuel cells occurs both in the aqueous phase of the membrane and on the sulfonate groups on the surface of the membrane pores. Here we show using D2 and H2 fuel and basic quantum mechanical equations that this surface proton migration is largely due to proton tunneling between adjacent sulfonate groups, leading to an exponential variation of Nafion conductivity with cell potential. This amphibious mode of proton migration, particle-like in the aqueous phase and wave-like in the narrow pores, is shown to be the major cause of cell overpotential, bistability and oscillations of state-of-the-art PEM fuel cells operating on H2, reformate or methanol fuel. We also show that this phenomenon can be exploited via introduction of a third auxiliary electrode to independently control the anode–cathode potential difference and dramatically enhance fuel cell power output even in absence of noble metals at the anode.
Keywords: PEM fuel cell; Pt–Ru anode; Nafion conductivity; Proton tunneling; Fuel cell bistability; Fuel cell oscillations; Pt cathode;