The removal of NOx from a lean exhaust gas using storage and reduction on H3PW12O40·6H2O

NO_x sorption and reduction capacities of 12-tungstophosphoric acid hexahydrate (H₃PW₁₂O₄₀·6H₂O, HPW) were measured under representative alternating conditions of lean and rich exhaust-type gas mixture. Under lean conditions, the sorption of NO_x is large and is equivalent to 37 mg of NO_x/g_HPW. Although a part of these NO_x remains unreduced, HPW is able to reduce some of the NO_x to produce N₂ by a reaction between the sorbed NO₂ and hydrocarbon (HC), but this process is slow. The addition of 1% Pt affects strongly the chemical behaviour occurring during the course of a rich operation. The NO desorption observed at the beginning of the rich phase is strongly accelerated. The direct correlation between NO₂ consumption and CO₂ production shows that the principal pathway is the reaction CO+NO₂→CO₂+NO. In a mixture of reducing gas (CO, HC, H₂), the competition is strongly in favour of CO though in its absence the reaction observed was the hydrogenation of propene to propane.     

Removal of NOx: Part II. Species formed during the sorption/desorption processes on barium aluminates

The formation of various species formed during the adsorption of NO_x issued from a synthetic lean-burn exhaust gas upon barium aluminates was studied using FTIR and TGA. The results have been systematically compared to those obtained with bulk BaO. Two factors are responsible for the difference observed during adsorption/desorption tests on the two solids. On one hand, stable carbonates are formed on BaO whereas no carbonate is formed on barium aluminate. On the other hand, the structure of the nitrates formed on these two compounds is very different, an N-bounded nitrate is formed on barium aluminate and not on bulk BaO.     

Supported Heteropolyacids for NOx Storage and Reduction

TiO₂ and SnO₂ were studied as possible supports for HPW in order to produce a new type of sorbent for NO _x trap applications. SnO₂ was synthesised by various inorganic methods while TiO₂ was purchased commercially. The sorption capacities and efficiencies depend upon the BET surfaces. Best performances were obtained with support with large pores where the crystalline structure of HPW is maintained. The optimal loading of HPW is ca. 50% with BET surface in the range 20–80 m²g^–1.     

NOx sorption–desorption study: application to diesel and lean-burn exhaust gas (selective NOx recirculation technique)

NO_x adsorption/desorption capacities of barium aluminates and BaSnO₃ were measured under representative exhaust gas mixture at temperatures below 550°C and compared to those of bulk BaO. The capacities are high and the test of sorption–desorption is reproducible on barium aluminate and BaSnO₃, while this is not the case on BaO. The difference is due to the electronic environment of barium oxide. If BaO is not engaged in a chemical bond, progressive formation of high stability carbonates is observed. This is not the case with barium aluminate and BaSnO₃, where carbonation does not take place because the competition between nitrate and carbonate formation is in favour of the nitrate due to its chemical nature. An N-bounded nitrate, with IR frequencies at 1360 and 1415 cm⁻¹, is formed on barium aluminate and BaSnO₃ and not on bulk BaO.     

Role of support in the oxidation of acetylene over gold catalysts

The influence of the carrier (CeO₂, TiO₂ or ZrO₂) on the reaction of acetylene oxidation over a 2% gold catalyst has been investigated in the presence or in the absence of CO. XPS reveals that gold is metallic in every case and the percentage of gold on the surface is always higher than that in the bulk confirming the wide dispersion as evaluated by TEM. There is a significant effect of the support on the activity in the absence of CO, but the presence of CO affects the reactivity differently according to the support. In a mixture of CO and C₂H₂, the oxidation of CO is systematically inhibited by C₂H₂ whereas the behaviour is completely different for the oxidation of C₂H₂. CO has no effect upon Au/CeO₂, acts as an inhibitor with Au/TiO₂ and as a promoter with Au/ZrO₂. Kinetic study reveals that the order towards C₂H₂ remains unchanged whereas the order towards O₂ clearly depends upon the support. These effects on activity and kinetics may result from various factors discussed in this paper.     

Multifunctional catalysts for de-NOx processes: The case of H3PW12O40·6H2O-metal supported on mixed oxides

The role of a multifunctional catalyst for de-NO_x process has been investigated. The NO_x storage capacity of H₃PW₁₂O₄₀·6H₂O (HPW) was improved by the presence of a noble metal (Pt, Rh or Pd). Both HPW and noble metal were deposited on a specific support (based on Zr–Ce or Zr–Ti). The presence of noble metal in several oxidation states, as evidenced by TPR and IR, involves the possibility of forming different catalytic sites: (i) M⁰ (zero-valent metal) and perhaps (ii) (metal–H)^δ+ from specific interactions between noble metal and the HPW proton. Supports were also able to adsorb and activate NO_x and to generate cationic catalytic sites (M^x+). These cationic sites seem to be the clue for their important activity toward NO_x reduction. This catalyst presents an outstanding resistance to SO₂ poisoning which can be related to NO and NO₂ absorption mechanism in HPW. The use of alternating short cycles of lean/rich mixtures allows us optimising the performance of this catalytic system in terms of both NO_x reduction capacity and NO_x storage efficiency: up to 48 and 84%, respectively (with a 2% CO + 1% H₂ mixture for reducing). Experimental results sustain two hypotheses: first, HPW-metal-support catalyst includes several (independent) catalytic functions required for a de-NO_x process to occur and second, the formation of oxygenate active species must be indispensable for NO_x reduction into nitrogen.     

Removal of NOx: Part I. Sorption/desorption processes on barium aluminate

NO_x adsorption/desorption capacities of barium aluminates were measured under representative exhaust gas mixture at temperatures below 550°C. The solid doped with Pt or not, exhibits good NO₂ sorption capacities with a reversible adsorption to desorption process. With bulk BaO, desorption was observed at high temperature. The different behaviour between the two catalysts is explained by the fact that strongly bonded carbonates are formed on bulk BaO while they do not exist on barium aluminate, therefore allowing the formation of nitrates which can be decomposed by a thermal process. SO₂ poisoning was also studied.     

Application of heterogeneous gold catalysis with increased durability: Oxidation of CO and hydrocarbons at low temperature

2% Au/Al₂O₃ catalysts were prepared by a novel method involving Direct Anionic Exchange (DAE). The method produces strong bonding of the gold complex (HAuCl₄) to the alumina support with no loss of gold during the subsequent steps of preparation. The complete removal of chloride from the catalyst was achieved by washing with concentrated ammonia. This procedure ensures a better activity and prevents sintering during calcination as shown by TEM. The catalysts were tested for the oxidation of CO and of saturated and unsaturated hydrocarbons (C₁ to C₃). The catalysts showed high activities over a range of concentrations and temperatures relevant to applications in automotive exhaust cleaning. Furthermore, a remarkable resistance to thermal ageing at 600°C in the absence or presence of water was observed, due to the presence of the strongly anchored nanosized gold particles obtained during the preparation step.     

The Mechanism of the Selective NOx Sorption on H3PW12O40·6H2O (HPW)

NO _x absorption/desorption capacities of 12-tungstophosphoric acid hexa-hydrate were measured under representative exhaust gas mixture conditions. The amounts of NO _x absorbed and then desorbed are high and equal to 46 of NO₂ mgg^–1 of HPW. The mechanism of absorption proceeds by substitution of lattice water molecules with formation of a [H⁺(NO₂ ^–,NO⁺)] complex. During the cooling phase and in the presence of water, around 100°C, reverse substitution occurs. Two possibilities to wash-coat HPW on a monolith are presented. The first one consists in a partial substitution of H⁺ by a monovalent cation while the second one consists of supporting HPW on a high surface oxide. The anchorage quality is related to the Brønsted acidity, the best candidates for the role of support are SnO₂ and TiO₂.