Applied Catalysis B, Environmental (v.52, #4)
NO reduction with ammonia employing Co/Pt supported on a mesoporous silica containing zirconium as a low temperature selective reduction catalyst by Ramón Moreno-Tost; José Santamarı́a-González; Enrique Rodrı́guez-Castellón; Antonio Jiménez-López (241-249).
Selective catalytic reduction (SCR) of NO with ammonia in an excess of oxygen was carried out over cobalt–platinum impregnated on a zirconium containing mesoporous silica (Si/Zr molar ratio of 5) with a 12 wt.% of Cobalt and Co/Pt molar ratios of 10, 20 and 30. These catalysts were compared with two catalysts containing 12 wt.% of Co and 1.5 wt.% of Pt, respectively. The presence of platinum improves the reducibility of Co(III) to Co(II) and at the same time the presence of cobalt ions impedes the reduction of Pt(II) to Pt(0). This gives rise to a synergetic effect, especially in the case of the catalyst with a Co/Pt molar ratio of 30, exhibiting a good performance in the SCR of NO by ammonia, even when water or SO2 are added to the feed on stream, and maintaining high conversion and selectivity at moderately low temperatures (250 °C). In fact, this catalyst maintains a conversion of 84% at 300 °C in the presence of water during 24 h.
Keywords: NO selective catalytic reduction; Platinum; Cobalt; SO2; Mesoporous silica; Zirconium;
Kinetics of the selective catalytic reduction of NO by NH3 on a Cu-faujasite catalyst by Gérard Delahay; Stéphane Kieger; Nathalie Tanchoux; Philippe Trens; Bernard Coq (251-257).
The kinetics of the selective catalytic reduction (SCR) of NO by NH3 in the presence of O2 has been studied on a 5.5% Cu-faujasite (Cu-FAU) catalyst. Cu-FAU was composed of cationic and oxocationic Cu species. The SCR was studied in a gas phase-flowing reactor operating at atmospheric pressure. The reaction conditions explored were: 458<T R<513 K, 250<NO (ppm) < 3000, 1000<NH3 (ppm) < 4000, 1<O2 (%) < 4. The kinetic orders were 0.8–1 with respect to NO, 0.5–1 with respect to O2, and essentially 0 with respect to NH3. Based on these kinetic partial orders of reactions and elementary chemistry, a wide variety of mechanisms were explored, and different rate laws were derived. The best fit between the measured and calculated rates for the SCR of NO by NH3 was obtained with a rate law derived from a redox Mars and van Krevelen mechanism. The catalytic cycle is described by a sequence of three reactions: (i) CuI is oxidized by O2 to “CuII-oxo”, (ii) “CuII-oxo” reacts with NO to yield “CuII-N x O y ”, and (iii) finally “CuII-N x O y ” is reduced by NH3 to give N2, H2O, and the regeneration of CuI (closing of the catalytic cycle). The rate constants of the three steps have been determined at 458, 483, and 513 K. It is shown that CuI or “CuII-oxo” species constitute the rate-determining active center.
Keywords: Nitric oxide; Ammonia; Selective catalytic reduction; Copper; Zeolite; Kinetics;
Interface species and effect of hydrogen on their amount in the CO oxidation on Au/ZnO by Maela Manzoli; Anna Chiorino; Flora Boccuzzi (259-266).
A FTIR study of the CO oxidation on Au/ZnO from 90 to 300 K in the absence and in the presence of hydrogen has been performed. An insight on the origin of deactivation during CO oxidation at room temperature and on the regeneration effect of hydrogen is given. FTIR spectra show that at 90 K only carbonylic species on the metallic particles and on the support cations are produced. Carbonates and/or carbonyls at the interface between the metal and the oxide are produced by increasing the temperature from 90 to 300 K. The presence of hydrogen in the mixture inhibits in some extent the formation of transient intermediates and of stable carbonates adsorbed on the support. The amount of stable species at the interface is reduced as a consequence of the lowered basicity of the reactive oxygen species at the borderline between the metal and the oxide.
Keywords: Gold; Zinc oxide; FTIR; CO adsorption; CO oxidation; CO oxidation in the presence of H2; Interface species;
Photocatalytic transformations of aminopyrimidines on TiO2 in aqueous solution by Paola Calza; Claudio Medana; Claudio Baiocchi; Ezio Pelizzetti (267-274).
The photocatalytic degradation in aqueous solution of aminopyrimidines (2-aminopyrimidine, 4-aminopyrimidine, 4-methyl-2-aminopyrimidine, 2,6-dimethoxy-4-aminopyrimidine) on TiO2 has been investigated. Their transformation pathways seem to be closely related to the position of the amino group in the heterocyclic ring; especially the fate of the organic nitrogen is deeply influenced.Both the ratio [NH4 +]/[NO3 −] and the extent of their formation are connected to the nature of the substituents and the amino group position in the heteroaromatic ring. When the amino group is located in C4, the transformation pathways proceed mainly through the cleavage of the CN bonds, with the formation of oxygenated structures as intermediates and the release of nitrogen mainly as ammonium ions. In this case, the stoichiometric conversion of nitrogen into inorganic ions and the complete mineralization is achieved within few hours of irradiation.On the contrary, when the amino group is held in C2, a lack in both nitrogen and carbon mineralization is observed. It is attributable to the stoichiometric conversion of the NC(NH2)N moiety into guanidine, a very stable compound for which the mineralization is obtained only at long irradiation times (>70 h). Interestingly, in this latter case the nitrogen is mainly released as nitrate ions.
Keywords: Photocatalysis; Titanium dioxide; Aminopyrimidine;
Enhanced photocatalytic decomposition of 4-nonylphenol by surface-organografted TiO2: a combination of molecular selective adsorption and photocatalysis by Kei Inumaru; Mika Murashima; Takashi Kasahara; Shoji Yamanaka (275-280).
n-Octyl-grafted TiO2 (C8-TiO2) was prepared as a model of a photocatalyst with high molecular adsorption selectivity, and its photocatalytic activity for the decomposition of dilute 4-nonylphenol (an endocrine disrupter, ca. 2 ppm) in water was investigated. The catalyst was highly active in the presence of concentrated phenol (1000 ppm), and the 4-nonylphenol was decomposed in 180 min, while pristine TiO2 (P-25) under the same conditions showed much lower activity. The high C8-TiO2 activity was ascribed to the molecular selective adsorption of the organic molecules on the alkyl-grafted hydrophobic surface. Infrared spectra showed that the grafted alkyl groups were gradually decomposed under photoirradiation. The model catalyst demonstrated that molecular selective adsorption is important for removal of low-concentration contaminants in the presence of other, more concentrated compounds.
Keywords: Photocatalysis; Molecular selectivity; Endocrine disrupter; Titanium oxide;
Effect of Na-addition on catalytic activity of Pt-ZSM-5 for low-temperature NO–H2–O2 reactions by M Machida; T Watanabe (281-286).
The effect of additives on Pt-ZSM-5 catalysts was studied for the selective NO reduction by H2 in the presence of excess O2 (NO–H2–O2 reaction) at 100 °C. The reaction of NO in a stream of 0.08% NO, 0.28% H2, 10% O2, and He balance yielded N2 with less than 10% selectivity, which could not be increased by changing Pt loading or H2 concentration in the gas feed. Co-impregnation of NaHCO3 and Pt onto ZSM-5 decreased the BET surface area and the Pt dispersion. Nevertheless, the Na-loaded catalyst (Na-Pt-ZSM-5) exhibited the higher NO x conversion (>90%) and the N2 selectivity (ca. 50%). Such a high catalytic activity even at high Na loadings (≥10 wt.%) is completely contrast to other Na-added Pt catalyst systems reported so far. Further improvement of N2 selectivity was attained by the post-impregnation of NaHCO3 onto Pt-ZSM-5. In situ DRIFT measurements suggested that the addition of Na promotes the adsorption of NO as NO2 −-type species, which would play a role of an intermediate to yield N2. The introduction of Lewis base to the acidic supports including ZSM-5 would be applied to the catalyst design for selective NO–H2–O2 reaction at low temperatures.
Keywords: Additive; Na; NO–H2–O2 reaction; Platinum; ZSM-5;
Investigation on modification of Ru/CNTs catalyst for the generation of CO x -free hydrogen from ammonia by S.J. Wang; S.F. Yin; L. Li; B.Q. Xu; C.F. Ng; C.T. Au (287-299).
The modification of Ru/CNTs (CNTs denotes carbon nanotubes) with rare earth, alkali, and alkaline earth compounds were systematically studied. The loading of modifying agents leads to decrease in pore volume and surface area; but improvement in thermal stability of Ru/CNTs. The size and morphology of Ru particles are not affected by the modification. For the catalysts modified by metal nitrates, activities were found in the order of K–Ru>Na–Ru>Li–Ru>Ce–Ru>Ba–Ru>La–Ru>Ca–Ru>Ru, signifying that within the same groups (K, Na and Li; Ba and Ca), the higher the electronegativity of the promoter, the lower is the NH3 conversion. Of all the potassium salts adopted, KNO3, KOH, and KCO3 show similar promotional effect, suggesting that KOH is the active promoter for catalytic activity. The maximum promotional effect was observed at an atomic ratio of K/Ru=2. The electron-withdrawing groups, F−1, Cl−1, Br−1, SO4 2−, and PO4 3− are inhibitors of Ru catalysts. The results of N2-TPD investigation revealed that the promotional effects of a modifier is a combined result of: (i) enhancing combinative desorption of nitrogen atoms, and (ii) decreasing of the apparent activation energy of the decomposition reaction.
Keywords: Ammonia decomposition; Hydrogen generation; Ruthenium catalyst; Carbon nanotubes; Catalyst promoter;
Liquid phase hydrodechlorination of chlorophenols over Pd/C and Pd/Al2O3: a consideration of HCl/catalyst interactions and solution pH effects by Guang Yuan; Mark A. Keane (301-314).
The liquid phase hydrodechlorination (HDC) of 2-chlorophenol (2-CP) and 2,4-dichlorophenol (2,4-DCP) has been studied over 1% (w/w) Pd/C and Pd/Al2O3 under conditions of minimal mass transport constraints. The HDC of 2,4-DCP generated HCl and 2-CP as the only intermediate partially dechlorinated product which reacts further to yield phenol; cyclohexanone was formed over Pd/Al2O3, but not over Pd/C, prior to complete dechlorination. Pd/Al2O3 is characterized (on the basis of TEM analysis) by a narrow distribution of smaller Pd particles to give a surface area weighted mean particle diameter = 2.4 nm that is appreciably lower than the value of 13.2 nm established for Pd/C, where the latter is characterized by a broader distribution of larger (spherical) particles. The addition of NaOH served to increase fractional dechlorination by suppressing HDC inhibition due to the HCl that is generated. Reuse of the catalysts revealed an appreciable deactivation of Pd/C and a limited loss of activity in the case of Pd/Al2O3. Deactivation of Pd/C can be linked to a decrease (up to ca. 60%) in the initial BET surface area allied to appreciable leaching (up to ca. 40%) of the starting Pd content through the corrosive action of HCl and, while the average Pd diameter is essentially unaffected, there is evidence of a preferred leaching of larger Pd particles. The stronger metal/support interactions prevalent in Pd/Al2O3 results in limited Pd leaching and comparable initial HDC activities during catalyst reuse with/without NaOH addition. Inclusion of HCl in the reaction mixture (pH 5–1.5) resulted in a marked decline in the initial HDC rate associated with Pd/Al2O3 and a lesser drop in HDC activity for Pd/C. The difference in response to bulk solution pH variations are discussed in terms of the nature of the reactive species in solution and the amphoteric behavior of the Pd supports.
Keywords: Liquid phase hydrodechlorination; 2,4-Dichlorophenol; 2-Chlorophenol; Pd/C; Pd/Al2O3; HCl/catalyst interaction(s);