Applied Catalysis B, Environmental (v.35, #2)
Total oxidation of CH4 on Sn-Cr composite oxide catalysts by Xiang Wang; You-Chang Xie (85-94).
A series of Sn-Cr composite oxide catalysts with various Sn/Cr mole ratios were prepared and used for CH4 deep oxidation. Compared with pure SnO2 and Cr2O3, the activity and thermal stability of Sn-Cr binary catalysts were significantly enhanced. The optimal activity was achieved on those samples with Sn/Cr mole ratios around 50:50. XRD results demonstrated that Cr3+ was doped into the crystal lattice of rutile structure SnO2 to form solid solution. As a result, the crystallization of the Sn-Cr samples was retarded, thus higher surface areas and lower crystallinity of them were maintained. TPR and XPS substantiated that more active oxygen species was formed in the Sn-Cr binary catalysts. Indeed, the high surface areas of the Sn-Cr composite oxides and the presence of more active oxygen species in them are considered as the two main reasons leading to their enhanced CH4 oxidation activity.
Keywords: CH4 deep oxidation; SnO2-based catalysts; Chromium oxides; BET; XRD; TPR; XPS;
Influence of adsorption on the photodegradation of various dyes using surface bond-conjugated TiO2/SiO2 photocatalyst by Hu Chun; Wang Yizhong; Tang Hongxiao (95-105).
The adsorption and photodegradation have been investigated in illuminated surface bond-conjugated TiO2/SiO2 suspension for five commercial dyes, including cationic dyes, cationic blue X-GRL (CBX) and cationic pink FG (FG), and anionic reactive brilliant red K-2G (K-2G), reactive yellow KD-3G (KD-3G), and acid red B (ARB). Two adsorption modes are proposed. In the TiO2/SiO2 system, CBX and FG (FG) adsorbed on the catalyst surface by a penta-heterocycle N group, whereas, the other three anionic dyes adsorbed on the catalyst surface by a sulfonate group. The mode of adsorption of substrates to the TiO2/SiO2 particle surface is relevant factor to the formation of NH4 +. The nitrogen of azo group was transformed to NH4 + ions and N2 gas, and the penta-heterocycle and benzene-ring-substitute N groups in CBX and FG were transformed predominantly to NH4 + ions. The triazine ring and its substitute groups are much stable in the period of 8 h irradiation.
Keywords: Anionic dye; Adsorption; Cationic dye; NH4 +; Photodegradation; TiO2/SiO2/water interface;
Low temperature catalytic NO x –H2 reactions over Pt/TiO2-ZrO2 in an excess oxygen by M Machida; S Ikeda; D Kurogi; T Kijima (107-116).
Platinum catalysts supported on noncrystalline TiO2-ZrO2 binary oxides were found to be highly active for the selective NO reduction in a stream of NO (0.08 vol.%)–H2 (0.08–0.56 vol.%)–O2 (10 vol.%) at low temperatures (<100°C). The NO conversion to N2/N2O occurred at >0.08 vol.% H2 and the selectivity to N2 increased with increasing H2 concentration. In situ DRIFTS measurement suggested that the high selectivity in this temperature range is closely related to a stoichiometric reaction between H2 and NO oxidatively adsorbed as nitrate (NO3 −). By comparison with the results from a parallel study of H2–O2 combustion, we propose that almost all of H2 reacted is consumed by the reduction of nitrate species, which covered the Pt surface to inhibit H2–O2 combustion. The NO reduction activity was sensitive to the catalyst pretreatment; the catalyst reduced in H2 allowed 89% NO conversion at 90°C, whereas the catalyst treated in O2 required 175°C to attain the lower conversion of 50%. The effect of the pretreatment is closely related to the reactivity of nitrate adsorbates produced via different routes.
Keywords: NO–H2–O2 reaction; H2–O2 combustion; Platinum; TiO2-ZrO2; Oxidative NO adsorption;
Influence of pH and chloride anion on the photocatalytic degradation of organic compounds by Antoine Piscopo; Didier Robert; Jean Victor Weber (117-124).
In the present work, we have studied the influence of pH (in the range 3–11) and chloride anions (in the range 0–0.8 mol/l) on the photocatalytic degradation (PCD), with titanium dioxide (TiO2), in aqueous solution of two aromatic compounds: benzamide (BA) and para-hydroxybenzoic acid (4-HBZ). No significant adsorption is detected for the BA; further the pH input has a very low effect on this photodegradation rate. However, the pH effect on the adsorption of 4-HBZ on TiO2 surface is significant and this is a factor to take into account in relation the photocatalytic decomposition of organic pollutants in water. For this compound, the degradation rate increases when the pH decreases. The effect of the pH on the degradation rate of 4-HBZ is not only dependent on the TiO2 charge surface but also on the formation of hydroxyl free radicals. At low chloride concentration, the behaviours of BA and 4-HBZ are sensitively different ([Cl−]<0.02 mol/l). 4-HBZ is only weakly dependent on the chloride concentration. By contrast, the degradation of BA is strongly affected by the chloride ions concentration.
Keywords: Photocatalysis; Titanium dioxide; Benzamide; para-Hydroxybenzoic acid;
FT-IR spectroscopic studies of hydrocarbon trapping in Ag+-ZSM-5 for gasoline engines under cold-start conditions by Xinsheng Liu; Jordan K. Lampert; Dmitrii A. Arendarskiia; Robert J. Farrauto (125-136).
A detailed study of adsorption of automobile exhaust hydrocarbons in Ag+-exchanged zeolites under cold-start conditions (room temperature and in the presence of water) was carried out with FT-IR spectroscopy, using toluene and propylene as probes. The results show that exchanged Ag+ in zeolites is unique for trapping olefin and aromatic hydrocarbons due to its resistance to water adsorption. In contrast, exchanged Cu2+ in zeolites, which has good hydrocarbon trapping properties under dry conditions, does not trap hydrocarbons under wet conditions. Here, solvation of Cu2+ screens the interactions of the cation with adsorbed hydrocarbons. The results also show that, in addition to the nature of the cation, the structure of the zeolite also plays a role in hydrocarbon trapping. Aging at high temperatures, with water vapor (∼10%) and SO2 (∼15 ppm), leads to a decrease of the trapping centers in the zeolites due to framework dealumination.
Keywords: FT-IR; Ag-ZSM-5; Hydrocarbon trap; Toluene; Propylene; Cold-start;
Preparation, characterisation and catalytic activity of pure and substituted La-hexaaluminate systems for high temperature catalytic combustion by Gianpiero Groppi; Cinzia Cristiani; Pio Forzatti (137-148).
La-Al-O, La-Mg-Al-O, La-Mn-Al-O and La-Mg-Mn-Al-O hexaaluminates have been prepared using the carbonates route previously developed for M-substituted and unsubstituted Ba-β-Al2O3. Starting from amorphous precursors, the formation of a final magnetoplumbite (MP) phase is observed upon calcination at T≥1100°C. In the case of LaMn1Al11O19, the MP phase already forms at 900°C evidencing a promotion effect of Mn ions. Upon calcination at 1300°C, monophasic samples can be obtained only for Mg-substituted samples (LaMg1Al11O19 and LaMg0.5Mn0.5Al11O19), whereas in the other samples the presence of LaAlO3 is always detected. This behaviour is associated with the stabilisation, via a charge compensation mechanism, of the MP phase due to the introduction of Mg2+ ions in the structure. The co-presence of Mg and Mn in the final catalyst has resulted in a higher specific catalytic activity per Mn mol. Such a behaviour is likely associated with the stabilisation of Mn ions at high oxidation state due to the co-presence of Mg2+.
Keywords: Catalytic combustion; Hexaaluminates; Mn-Mg substituted; CH4 combustion;
Catalytic properties of La0.8A0.2MnO3 (A = Sr, Ba, K, Cs) and LaMn0.8B0.2O3 (B = Ni, Zn, Cu) perovskites by Florina-Corina Buciuman; Emmanuel Joubert; Jean-Christophe Menezo; Jacques Barbier (149-156).
The selective catalytic reduction of nitrogen oxides in the presence of oxygen and water vapor was investigated over perovskite-type mixed oxides La0.8A0.2MnO3 (A=Sr, Ba, K, Cs) and LaMn0.8B0.2O3 (B=Ni, Zn, Cu) with propene as the reductant, in the temperature range of 150–550°C. The catalysts were contacted with a reactant mixture containing 1000 ppm NO, 500 ppm C3H6, 5% O2 and 5% H2O in helium at a space velocity of 14.4 cm3/g s. A maximum was observed in the conversion of nitrogen oxides to nitrogen with temperature in the range of 300–400°C. In the absence of propene, the activity of these perovskites for NO removal in the 300–550°C range increased from approximately 20% (with propene) to 40% (without propene). Water inhibited the NO x removal process as well, the conversion levels in the propene and water-free gas mixtures rising to about 50–55%. The removal of nitrogen oxides did not take place in the absence of oxygen. Two different reaction mechanisms were postulated: (i) the selective reduction with propene at 150–300°C; (ii) the direct decomposition, which is inhibited by the presence of propene and water, at 300–550°C. The nitrogen dioxide formed by the perovskite-catalyzed oxidation of nitrogen monoxide appears to play a key role in both mechanisms.
Keywords: Perovskite; Lanthanum manganite; Selective catalytic reduction; Nitrogen oxides; Decomposition; Auto-exhaust; Catalyst;