Raj Rao Rajaram

Mechanism of the lean NOx reaction over Cu/ZSM-5

Abstract Transient techniques (including TAP, temporal analysis of products) have been used to probe the mechanism of the lean NO x reaction over Cu/ZSM-5. The activation of propene and nitric oxide by Cu/ZSM-5, in the presence and absence of oxygen, have been investigated by TAP to elucidate the nature of the reducing species involved in the formation of nitrogen. Propene is converted to long-lived (carbon-rich)_ species on the zeolite whether oxygen is present or not, and it is these deposited coke species which act as the reductant in this catalyst system. The ability of the coke to reduce nitric oxide is significantly enhanced by the presence of oxygen. The crucial role of the oxygen appears to be to induce the generation of an adsorbed state of NO x on the exchanged-Cu sites; these NO 2 -type species exist at temperatures characteristic of high lean NO x activity. During temperature-programmed reaction under a fuel-lean (oxidising) exhaust-gas, both nitric oxide and propene are retained at lower temperatures; as the temperature rises, so propene retention (as coke) and total oxidation begin to compete. However, there are sufficient reducing species still present on the surface to allow substantial reduction of the -NO 2 species, as the latter approach their limit of thermal stability.

Regeneration of NOx trap catalysts

We have investigated the regeneration of a nitrated or sulphated model Pt/Ba-based NO x trap catalyst using different reductants. H 2 was found to be more effective at regenerating the NO x storage activity especially at lower temperature, but more importantly over the entire temperature window after catalyst ageing. When the model NO x storage catalyst is sulphated in SO 2 under lean conditions at 650 °C almost complete deactivation can be seen. Complete regeneration was not achieved, even under rich conditions at 800 °C in 10% H 2 /He. Barium sulphate formed after the high temperature ageing was partly converted to barium sulphide on reduction. However, if the H 2 reduced sample was exposed to a rich condition in a gas mixture containing CO 2 at 650 °C, the storage activity can be recovered. Under these rich conditions the S 2- species becomes less stable than the CO 3 2-, which is active for storing NO x . Samples which were lean aged in air containing 60 ppm SO 2 at <600°C, after regeneration at λ = 0.95 at 650°C, have a similar activity window to a fresh catalyst. It is, therefore, important that CO 2 is present during the rich regenerations of the sulphated model samples (as of course it would be under real conditions), as suppression of carbonate formation can lead to sulphide formation which is inactive for NO x storage.

The Role of Ceria in Three-Way Catalysts

Abstract The role of ceria in three-way catalysts (TWC) for the control of gaseous exhaust emissions has variously been described as: (a) oxygen storage under transient conditions; (b) catalytic promoter of precious metals for certain reactions such as water gas shift; (c) structural promoter for the stabilisation of precious metals and alumina against particle growth. However, it would appear that the interaction of ceria with the precious metal can significantly affect activity depending on pretreatment conditions and the precious metal involved. In this paper, we shall consider the role of the individual precious metals in TWCs and how their interaction with ceria affects performance. Catalysts containing Pt or Rh, with or without ceria, dispersed on alumina, have been prepared and pretreated in different atmospheres before characterisation and activity measurement in a simulated exhaust environment. Under oxidising conditions, a Pt-ceria complex is formed which maintains Pt stability against sintering. The complex fixes Pt in a high oxidation state and produces sites of lower activity than on alumina under lean conditions. However, mild reduction of a ceria-containing Pt catalyst produces sites which are highly active for the removal of CO and NO. Such sites are sufficiently stable to allow characterisation by XPS, TEM etc. but, following mild oxidation treatment, the high activity is destroyed. It is believed that the active sites are formed by surface Pt crystallites at the metal-support interface, inducing changes in the surface properties of ceria through the formation and oxidation of anionic vacancies. The presence of ceria also modifies the activity of Rh-containing catalysts by promoting the reduction of Rh.

Origins of low-temperature three-way activity in Pt/CeO2

Abstract The ability of Pt/CeO 2 to exhibit three-way catalytic activity at low temperatures results from a strong metal-support interaction, which is induced by activating the catalyst under a reducing atmosphere. Carbon monoxide chemisorption measurements show that a loss of exposed metal area occurs during the activation procedure, even though the conditions do not favour platinum sintering. Following activation (and subsequent exposure to air at ambient temperature), the surface ceria is left in a highly reducible state, which is characterised by a subambient peak during temperature-programmed reduction. We envisage that the strong interaction arises according to a traditional SMSI model, by the migration of partially reduced ceria over the surface of the platinum particles (at ⩾ 600°C). The resultant high degree of contact between the metal (with high work-function) and the metal oxide (with high band gap) promotes the formation of oxygen vacancies on the ceria surface. Although the presence of this highly reducible ceria-covering blocks conventional three-way sites on the platinum, it provides new sites that are active even at low temperatures. Therefore, unlike most previous explanations of promotion caused by a strong interaction, we propose that the ‘support’ becomes the active phase. Re-oxidation at elevated temperatures causes the ceria covering to coalesce, leading initially to its partial thinning, and subsequently to the re-exposure of the platinum particles.

Low-temperature redox activity in co-precipitated catalysts: a comparison between gold and platinum-group metals

Abstract We have identified two main mechanisms for inducing low-temperature activity in precious metal catalysts: normal-support activation (NSA) and active-phase enhancement (APE). The usual roles of a precious metal and its metal oxide support are reversed in NSA, resulting in highly active sites being created on the metal oxide. This applies when metallic gold is well dispersed within a defect-forming metal oxide (such as CeO2 or even ZrO2), and leads to formation of new sites for CO, NO and alkene conversion (to CO2, N2 and H2O). In APE, established metal oxide catalysts can be deliberately made to operate at lower temperatures, by providing precious metal sites for oxygen activation. An example is iron(III) oxide, where incorporation of palladium leads to substantial lowering of the temperatures for CO oxidation, water-gas shift and oxidative dehydrogenation. Gold does not have the same effect, except for CO oxidation at high metal loading. However, the presence of gold or palladium in a hydroxylated metal oxide can lead to another route to low-temperature oxidation, in which highly reactive peroxide-like intermediates are believed to be formed on exposed metal surfaces.

The development of a model capable of predicting diesel lean NOx catalyst performance under transient conditions

Abstract Steady state kinetics data from a commercial Pt-based lean NOx catalyst have been used to formulate a kinetic model to describe the performance of the catalyst. It is clear from this analysis that steady state kinetics in isolation are not sufficient to provide a full picture of the operational performance of such a catalyst. However, when this kinetic analysis is combined with mechanistic information obtained over the catalyst, the resulting model is extremely powerful. Within this paper, the development of the kinetic model is described, and the requirement for both accurate mechanistic information and detailed kinetic measurements is clearly demonstrated. The use of the model to predict the performance of a light-duty diesel vehicle under light-off conditions is described, and the power and flexibility of the model within the lean NOx area are emphasised.

Effects of SO2 on the alkane activity of three-way catalysts

Over current Pt-Rh/CeO2-Al2O3 catalysts, the conversion of alkanes occurs by two principal mechanisms: direct oxidation (HC + O2) and steam reforming (HC+H2O). Sulphur dioxide can influence both these mechanisms. Direct oxidation, which predominates when the exhaust-gas is fuel-lean, ispromoted by the adsorption of SOx species by the support. Under fuel-rich atmospheres, the presence of SO2 severelyinhibits steam reforming. The poisoning is associated with the formation of S2− on the platinum and of SO42− on the support, but there is no indication of S-species being retained by the rhodium. It is proposed that each of the two mechanisms is sensitive to a different type of interaction at the metal-support interface. Direct oxidation is enhanced by the transfer of electrons from the precious metal to the support; steam reforming occurs at interfacial sites, which can be blocked by adsorbed SOx species.

Catalysis at Lower Temperatures

Operating heterogeneous catalytic reactions at low temperatures offers the prospects of improved selectivity, better catalyst durability and lower running costs. In practice, though, low-temperature catalysis can be unpredictable and transitory. It often arises from fragile nano-scale interactions, which can be difficult to induce,and even more difficult to measure. In trying to understand these interactions some of the long accepted tenets of catalysis are brought into question. We are seeing, however, the emergence of some generic patterns of behavior, which may ultimately allow us to custom-design durable low-temperature catalysts.

The Development of a Model Capable of Predicting Diesel Lean NOx Catalyst Performance Under Transient Conditions

Steady state kinetics data from a commercial Pt-based lean NOx catalyst have been used to formulate a kinetic model to describe the performance of the catalyst. It is clear from this analysis that steady state kinetics in isolation are not sufficient to provide a full picture of the operational performance of such a catalyst. However, when this kinetic analysis is combined with mechanistic information obtained over the catalyst, the resulting model is extremely powerful. Within this paper, the development of the kinetic model is described, and the requirement for both accurate mechanistic information and detailed kinetic measurements is clearly demonstrated. The use of the model to predict the performance of a light-duty diesel vehicle under light-off conditions is described, and the power and flexibility of the model within the lean NOx area are emphasised.