John P.A. Neeft

Catalysts for the oxidation of soot from diesel exhaust gases. I. An exploratory study

Abstract A large number of candidate soot oxidation catalysts were screened on their catalytic activity with a model soot. It was found that the intensity of contact between soot and catalyst is one of the major parameters that determine the soot oxidation rate. Two types of contact were studied; many catalysts increase the rate of soot oxidation considerably when the contact is intimate (‘tight’), whereas under conditions of poor (‘loose’) contact only some catalysts accelerate this oxidation reaction. It is tentatively suggested that (i) mobility of the catalyst is a major parameter determining the loose contact activity of catalysts and (ii) this mobility correlates with the melting point or the partial pressure of the catalyst.

Diesel particulate emission control

This paper reviews the emission control of particulates from diesel exhaust gases. The efficiency and exhaust emissions of diesel engines will be compared with those of otto engines (petrol engines). The formation of particulates (or “soot”), one of the main nuisances of diesel exhaust gases, will be briefly outlined. The effects of various emission components on human health and the environment will be described, and subsequently the emission standards for particulates and for NOx, which have been introduced worldwide, will be summarized. Possible measures for reducing exhaust emissions of particulates and NOx will be discussed, such as the use of alternative fuels, modifications to the engine and the use of aftertreatment devices. It will be made clear that aftertreatment devices may become necessary as diesel emission standards become more stringent, in spite of important progress in the other fields of reducing exhaust emissions. Selective catalytic reduction via hydrocarbons, ammonia or urea, a possible aftertreatment method for NOx emission control, will be discussed briefly. Filters for collecting particulates from diesel exhaust gases will be examined in more detail and aftertreatment control systems for particulate removal will be reviewed. These can be divided into (i) non-catalytic filter based systems which use burners and electric heaters to burn the soot once it has been collected on the filter; (ii) catalytic filter-based systems which consist of filters with a catalyst coating, or filters used in combination with catalytically active precursor compounds added to the diesel fuel; and (iii) catalytic non-filter-based systems in which gaseous hydrocarbons, carbon monoxide and part of the hydrocarbon fraction of the particulates are oxidized in the exhaust gases. Finally, recent trends in diesel particulate emission control will be discussed, indicating the growing importance of the catalytic solutions: the fast introduction of non-filter-based catalysts for diesel engines and the possible application of filters in combination with catalytically active precursor compounds added to diesel fuel.

Kinetics of the oxidation of diesel soot

The kinetics of the uncatalysed oxidation of a flame soot and diesel soot were studied. Oxidation of the flame soot could be described by an nth-order model, with an order in carbon of 0.7. The order in molecular oxygen concentration was found to be 1 for the flame soot and slightly lower than 1 for the diesel soot. This order in oxygen concentration was also found to be a function of conversion. Water caused a significant increase in oxidation rate of the flame soot, which was accompanied by an increase in reaction order in carbon and a much higher CO2/CO ratio, whereas the activation energy did not change. The oxidation rate of the diesel soot was not significantly influenced by the addition of water. Experimental results were more reproducible for the oxidation of the flame soot than for the oxidation of diesel soot, and the flame soot appeared to be a good model for diesel soot in oxidation studies. Therefore it is recommended to use a model soot in (kinetic) studies of soot oxidation.

Catalysts for the oxidation of soot from diesel exhaust gases II. Contact between soot and catalyst under practical conditions

Abstract In part I of this study [Appl. Catal. B 8 (1996) 57–78] it was shown that many catalysts are active for the oxidation of soot, and that the contact between soot and catalyst is an important parameter for the activity of the catalyst. In this study, the contact between soot and catalyst is studied under practical conditions. Soot from a one-cylinder diesel engine is deposited either on catalyst material which is supported by a sheet of filter paper, or on catalytic coatings on segments of wall-flow monolith. It is shown that under these practical conditions, the activity of the catalysts for oxidation of soot is low. Therefore, it is concluded that under practical conditions the contact between soot and catalyst is poor. It is shown that this contact resembles the contact denoted as ‘ loose contact ’ that had earlier been used in part I of this study. This finding has major consequences for the use of catalytic coatings for the removal of soot from diesel exhaust gases, because only when the contact between catalyst and soot can be increased, the use of catalytic coatings seems to be feasible.

Soot oxidation catalyzed by a Cu/K/Mo/Cl catalyst: evaluation of the chemistry and performance of the catalyst

Several non-supported oxidic compounds potentially present in a Cu/K/Mo/Cl catalyst (copper molybdates, potassium molybdates, and a mixed copper-potassium molybdate (K2Cu2(MoO4)3)) have been tested individually on their activity in the oxidation of a model soot (Printex-U, which non-catalytically oxidizes at 875 K). These oxidic compounds are active between 665 and 720 K, but only after establishment of ‘tight contact’ between the catalyst and soot in a ball mill. Without the ball mill procedure (‘loose contact’) these oxides are less active (the soot oxidation temperature is shifted to about 790 K), while a ZrO2 supported Cu/K/Mo/Cl catalyst still shows a high activity around 670 K. Hence, the ‘loose contact’ activity of the supported Cu/K/Mo/Cl catalyst is not explained by the presence of an active oxidic compound. DRIFT and XRD analyses have shown that addition of KCl to CuMoO4 (two compounds present within the Cu/K/Mo/Cl catalysts) followed by calcination at 950 K in air, eventually results in the formation of a mixed potassium-copper molybdate. Simultaneously several volatile copper, potassium and chlorine containing compounds (e.g. K2CuCl4) are formed. These copper and chlorine containing compounds possess a high ‘loose contact’ soot oxidation activity between 600 and 690 K. A catalytic cycle, involving Cu2OCl2, is proposed to explain the high ‘loose contact’ activity of copper chlorides and supported Cu/K/Mo/Cl catalysts. The activity of the latter catalyst will be maintained as long as Cu2OCl2 can be reformed by reaction of copper molybdates with KCl, which serves as a chlorine supplier.

The formation of carbon surface oxygen complexes by oxygen and ozone. The effect of transition metal oxides

Abstract Various surface oxygen complexes (SOCs) have been identified by DRIFT spectroscopy on the surface of carbon black (Printex-U) after partial non-catalytic conversion in 10% O2 in Ar and ozone. An in situ DRIFT analysis of the oxidation of fullerene C60 showed the formation of similar functionalities and validated the use of C60 as a carbon black model compound for DRIFT spectroscopic studies, although C60 is more reactive towards oxygen than carbon black. Ex situ DRIFT analyses of partially converted catalyst/carbon black mixtures and in situ analyses of catalytic fullerene C60 oxidation, revealed that several transition metal oxides (Cr2O3, MoO3, V2O5 and CuO) promote the formation of SOCs. Fe2O3 and Co3O4 do not enhance the formation of SOCs in 10% O2 in Ar and prevent the formation of SOCs on carbon black samples in ozone. Reaction of carbon black with oxygen associated with metal oxides (carbothermic reduction) does not yield SOCs. Apparently, lattice oxygen is not directly involved in the catalytic formation of these complexes. Indications for chemical interactions between metal oxides and either carbon black or C60, such as M–O–C bonds, have not been found. Spill-over of activated oxygen from the Cr2O3, MoO3, V2O5 and CuO surfaces onto the carbon black surface is likely to explain the catalytic formation of SOCs.

Catalytic oxidation of carbon black — I. Activity of catalysts and classification of oxidation profiles

The oxidation of carbon black in the presence of catalytically active compounds has been studied under isothermal conditions in a flow reactor. In the presence of metal oxides, oxidation rates of the carbon black are found to be up to a few hundred times higher compared with the non-catalysed oxidation. Alkali metal carbonates increase this oxidation rate even up to a factor of over 100 000. The combustion curves (reaction rate as a function of carbon conversion) of isothermal, catalytic oxidation of carbon black can be classified in four distinct profile types. From these combustion curves, it can be concluded that a relative ranking of catalysts depends on the carbon conversion level and on the catalyst-to-carbon black ratio. The contact between catalyst and carbon black is an essential parameter in the catalysed oxidation of carbon black. When this contact is poor, the catalytic effect of the metal oxides and alkali metal carbonates is low or even absent. For alkali carbonate catalysts, the very high reaction rates were attributed to redistribution of the catalyst during reaction. Also for some of the metal oxides (i.e., Ag2O, CuO, MoO3, PbO, Sb2O3) it is suggested that mobility of the catalyst can increase the contact between the catalyst and the carbon black, resulting in higher oxidation rates.

The effects of heat and mass transfer in thermogravimetrical analysis. A case study towards the catalytic oxidation of soot

Abstract A convenient model has been developed which qualitatively describes the heat and oxygen mass transport within a TGA sample crucible. The major conclusion from this model and from experimental observations was that for exothermal reactions, heat- and mass-transfer limitations can significantly influence the results of thermogravimetrical analysis. Therefore, it is crucial to use small, diluted samples, in particular for quantitative thermo-analytical measurements. It is recommended that the occurrence of heat- and mass-transport limitations be assessed by variation of the sample mass by at least a factor of ten. The model provides an explanation for the double-peak-shaped curves which are often observed when studying the catalysed oxidation of a model soot. The first peak is attributed to a thermal runaway reaction, which occurs when the sample is relatively large and when it is not diluted with silicon carbide. It is shown that the occurrence of the two peaks is caused by a combination of heat- and mass-transport limitations within the TGA sample.

Feasibility study towards a Cu/K/Mo/(Cl) soot oxidation catalyst for application in diesel exhaust gases

Abstract The activity of a supported catalyst containing copper, potassium and molybdenum was studied. Model experiments revealed that chlorine plays an essential role in the activity of this catalyst. It was shown that the active species of the catalyst probably consist of copper chloride compounds. Deactivation of the catalyst was studied, and was found to be more pronounced for a poor contact between soot and catalyst compared with a tight contact. Deactivation rates were found to be low, which was tentatively suggested to be caused by loss of active species that are formed by solid-solid reactions (e.g., KCl + CuMoO4 → K-molybdates + Cu-(oxy)chlorides) which are slow as a result of low solid-state diffusion rates. A Cu/K/Mo-catalyst as a coating on small segments of a wall flow monolith downstream of a diesel engine showed a relatively low activity. Besides, the catalyst was found to deactivate rather fast, which corroborated the outcome of the above mentioned model study. As a result, the feasibility of this Cu/K/Mo-catalyst for use in practical applications is low due to a progressive loss of catalytic material by high vapour pressures of active components formed by solid-solid reactions of less volatile compounds present in the catalyst.

Metal oxides as catalysts for the oxidation of soot

Abstract A large number of candidate soot oxidation catalysts were screened on their catalytic activity with a model soot. It was found that the intensity of contact between soot and catalyst is one of the major parameters that determine the soot oxidation rate. Two types of contact were studied: many catalysts increase the rate of soot oxidation considerably when, under model conditions, the contact is intimate (“tight”); whereas under conditions of poor (‘loose”) contact, which can be resemblant of the contact in practice, only some catalysts are able to accelerate this oxidation reaction. It is tenatively suggested that (i) mobility of the catalyst is a major parameter determining the loose contact activity of catalysts, and (ii) this mobility correlates with the melting point or the partial pressure of the catalyst.