Soot-catalysts mixtures

The catalytic combustion of Diesel soot was studied performing reactivity tests of soot-catalyst mixtures in a tubular flow reactor. The dependence of the reaction rate on the temperature was found. With respect to the uncatalysed combustion the reactivity of the soot in the presence of catalyst increased of some orders of magnitude while the apparent activation energy was found to be less than half. 

Hirano et al. [13] studied strontium-substituted LaFeOs perovskites for soot combustion, and the activity of these perovskites was reported to be only slightly lower than that of a Pt/Al20s reference catalyst. Taniguchi et al. [14] also reported that some Lai tK Fe03 perovskite catalysts outperform the soot combustion capacity of Pt catalysts in experiments performed with air and with soot-catalyst mixtures prepared in loose contact. 

Ura et al. [35] also reported that the partial substitution of strontium by potassium in Sri J< ,Ti03 perovskites improves the catalytic combustion of soot in N0 /02 mixtures with soot-catalysts mixtures prepared in loose contact. It was observed that potassium favors the creation of oxygen vacancies on the perov-sldte surface where molecular oxygen or NO are adsorbed forming basic surface oxygen species active for soot oxidation. In this line, it was confirmed that potassium substitution into the perovskite framework is more effective than a potassium salt impregnation [36]. 

The study of soot oxidation on a laboratory scale faces the issue of soot-catalyst mixture preparation. This is a relevant problem since the reaction rate is determined by the level of interaction between the two solids and the gas mixture. Contact between carbon and... 

Saab et investigated the contact points between ceria and soot by electron paramagnetic resonance (EPR) and found that under tight contact conditions, a new paramagnetic species is formed, which acts as a fingerprint of the soot and catalyst interface. This species is responsible for the increase of activity of soot-catalyst mixtures. 

Figure 2. DTA profiles obtained for soot-lCulOOCe mixture samples with different catalyst calcination temperatures.

Figure 6 TPO profiles of soot impregnated with catalysts (catalyst/soot) and mechanical mixtures of catalysts with soot (catalyst + soot)...

The results of soot oxidation tests performed with temperature-programmed methods (either TPO or TG) are strongly affected by the choice of the reaction conditions. In a nice study, Peralta et al investigated the effect of the main parameters on the soot oxidation reaction under laboratory conditions. The most important variables that might influence final results are the composition of soot and soot/catalyst ratio, the type of soot/catalyst contact and the amount of oxygen in the feed mixture. 

It has been reported that SWCNTs form in the soot generated in the arc-discharge chamber when metal catalysts are present. This preparation technique was published almost concurrently by two groups [2,3]. Thus lijima and Ichihasi [2], employing an Fe catalyst and a mixture of CH4 (10 Torr) and Ar (40 Torr), obtained ca. 1 nm diameter SWCNTs, whereas Bethune et al. [3] produced ca. 

The carbon removal reaction supposedly takes place at two-phase boundary of a solid catalyst, a solid reactantfcarbon particulate) and gaseous reactants(02, NO). Because of the experimental difficulty to supply a solid carbon continuously to reaction system, the reaction have been exclusively investigated by the temperature programmed reaction(TPR) technique in which the mixture of a catalyst and a soot is heated in gaseous reactants. 

Arc discharge [25] is initially used for producing C60 fullerenes. Nanotubes are produced by arc vaporization of two carbon rods placed in a chamber that is filled with low pressure inert gas (helium, argon). The composition of the graphite anode determines the type of CNTs produced. A pure graphite anode produce preferably MWNT while catalyst (Fe, Co, Ni, Y or Mo) doped graphite anode produces mainly SWNT. This technique normally produces a complex mixture of components, and requires further purification to separate the CNTs from the soot and the residual catalytic metals present in the crude product. 

Adsorption of nitric and sulfuric acids on ice particles provides the sol of the nitrating mixture. An important catalyst of aromatic nitration, nitrous acid, is typical for polluted atmospheres. Combustion sources contribute to air pollution via soot and NO emissions. The observed formation of HNO2 results from the reduction of nitrogen oxides in the presence of water by C—O and C—H groups in soot (Ammann et al. 1998). As seen, gas-phase nitration is important ecologically. 

The hydrocarbon feedstock is reacted with a mixture of oxygen or air and steam in a sub-stoichiometric flame. In the fixed catalyst bed the synthesis gas is further equilibrated. The composition of the product gas will be determined by the thermodynamic equilibrium at the exit pressure and temperature, which is determined through the adiabatic heat balance based on the composition and flows of the feed, steam and oxygen added to the reactor. The synthesis gas produced is completely soot-free [28]. 

Mixtures of the Iron-Group Metals. It was first reported by Seraphin et al. (50) that a binary mixture of Fi and Ni yielded more abundant SWNTs than Fe or Ni alone catalyst. Saito et al. (51) showed that a mixture of Fe and Ni with the ratio of 1 1 (by weight) gave the highest yield of SWNTs, and deviation from the 1 to 1 composition reduced the yield. Approximately 10% of all the carbon in the raw soot (both the chamber and cathode soot) was incorporated into SWNTs at the highest yield. Diameters of SWNTs produced from Fe/Ni range from 0.9 to 1.4 nm, and the mode diameter is located in 1.1-1.2 nm. 

Supported electrodes. The mixture of catalyst and charcoal is poured into the space between two mechanically rigid walls, with asbestos paper as support and a graphite felt or metal sheet as current collector. No binder is necessary. With such electrodes, both liquid and gaseous working materials can be studied. For the experiments with dissolved fuels described in Section 4.2, we used modified electrodes of this type 6 mg chelate was mixed with 6 mg soot and poured between two graphite felt discs. 

The influence of the inlet concentration of NO was studied with four soot samples mixed with a supported platinum catalyst (I wt% Pi on ASA) at 650 K. The oxidation rate at 50% soot conversion is plotted as a function of the NO inlet concentration in Figure 12.2.a. From this figure it is clear that the influence of the NO concentration on the oxidation rate of the synthetic Printex-U and the diesel soots activated with copper or iron is comparable. There is a first order relation between the NO inlet concentration and the oxidation rate. For cerium activated soot, there is also a first order relation between the NO inlet concentration and the oxidation rate. In this case, however, the effect of NO is approximately twice as large as is the case with Printex-U, Phntex-U with a physical mixture of a cerium catalyst (not shown), and copper- or iron-activated soot. 

Molten eutectic salt mixtures have been reported to be active catalysts in graphite and coal char gasification [4, 5]. A major reason probably is the contact between soot and catalyst which is increased by wetting of the soot with the catalyst. 

In this paper results of soot reactivity tests and of catalysed and uncatalysed ceramic filter regeneration experiments are presented. The aim is to investigate the influence of the catalyst features and of the carbon-catalyst contact on the catalyst performances. This is accomplished by comparing the results of combustion tests of mixtures of carbon and catalyst particles, specifically prepared by thorough pounding of the two components in a mortar, with those attainable in a more realistic filter regeneration system. 

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