V. Pitchon

The current state of research on automotive lean NOx catalysis

A predominant challenge to both industrial and academic research institutions is the selective reduction of NOx during lean exhaust conditions. Despite a proliferation of publications, papers and reviews, a large number of catalysts fall short of a credible and consistent diesel application. Herein are reviewed systems designed specifically with the goal of assisting researchers in their assessment of catalytic performance. By way of conclusion, three principal types of catalysts active for HC-SCR are discussed.

Absorption/desorption of NOx process on perovskites: performances to remove NOx from a lean exhaust gas

Abstract NOx adsorption/desorption capacities of perovskites (ABO3) were measured under representative exhaust gas mixture conditions at temperatures below 550°C, with A=Ca, Sr, Ba and B=Sn, Zr, Ti. The solids exhibited good NO2 sorption capacities with a reversible adsorption to desorption process according to the sequence Ba>Sr>Ca for A, while for element B the sequence Sn>Zr>Ti is observed. In the case of alkaline earth metals, the absorption behaviour proved to be directly related to their electropositivity (Ba>Sr>Ca). The key factors which control the absorption of NO2 are the bonding energy between the element B and the oxygen atom on one hand and the electropositivity of the element A on the other. The best result was obtained with the perovskite BaSnO3. The absorption of NO2 is favoured at low temperature and in the presence of water. The addition of platinum has no significant influence upon any NO2 absorption.

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

Abstract 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 NO 2 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. SO 2 poisoning was also studied.

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

Abstract2% 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.

Alkali addition to silica-supported palladium: Infrared investigation of the carbon monoxide chemisorption

Abstract Alkali nitrates (lithium, sodium, potassium, caesium) are added to a Pd n + /SiO 2 precursor. After reduction under hydrogen the addition leads to a moderate increase in the metallic particle size but to drastic changes in the infrared spectrum of carbon monoxide adsorbed on palladium. A direct interaction between the alkaline ion and carbon monoxide bonded to palladium is invoked. The extent of the interaction increases with the reduction temperature. The particular behaviour of lithium-doped solids is discussed.

Support Effects in the Gold-Catalyzed Preferential Oxidation of CO

The study of support effects on the gold‐catalyzed preferential oxidation of carbon monoxide in the presence of hydrogen (PROX reaction) is possible only with careful control of the gold particle size, which is facilitated by the application of the direct anionic exchange method. Catalytic evaluation of thermally stable gold nanoparticles, with an average size of around 3 nm on a variety of supports (alumina, titania, zirconia, or ceria), clearly shows that the influence of the support on the CO oxidation rate is of primary importance under CO+O2 conditions and that this influence becomes secondary in the presence of hydrogen. The impact of the support surface structure on the oxidation rates, catalyst selectivity, and catalyst activation/deactivation is investigated in terms of oxygen vacancies, oxygen mobility, OH groups, and surface area on the oxidation rates, catalyst selectivity and catalyst activation/deactivation. It allows the identification of key morphological and structural features of the support to ensure high activity and selectivity in the gold‐catalyzed PROX reaction.

Absorption/desorption of NOx process on perovskites: Nature and stability of the species formed on BaSnO3

Abstract The formation of various species during the adsorption of NOx, issuing from a synthetic, Lean–Burn exhaust gas upon BaSnO3 was studied using FTIR and TGA. An exposure to CO2 does not lead to the formation of carbonates yet contact with NO2 does produce nitrates, which accounts for the high NOx capacity of such solids. N-bounded nitrate and bulk nitrate species were identified. These nitrates decompose upon heating with no loss of the perovskite structure. The process of absorption/desorption is reversible, repeatable and can be explained by the following equations: Absorption phase: BaSnO 3 +3 NO 2 ↔ Ba ( NO 3 ) 2 + SnO 2 + NO , Desorption phase: Ba ( NO 3 ) 2 + SnO 2 ↔ BaSnO 3 +2 NO 2 + 1 2 O 2 .

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

Abstract 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.

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

NOx absorption/desorption capacities of 12-tungstophosphoric acid hexa-hydrate were measured under representative exhaust gas mixture conditions. The amounts of NOx 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.

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 (H 3 PW 12 O 40 6H 2 O, 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 N 2 by a reaction between the sorbed NO 2 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 NO 2 consumption and CO 2 production shows that the principal pathway is the reaction CO + NO 2 → CO 2 + NO. In a mixture of reducing gas (CO, HC, H 2 ), the competition is strongly in favour of CO though in its absence the reaction observed was the hydrogenation of propene to propane.