Soot deactivation

Under the conditions used in this study, the catalytic activities were stable for NO reduction for all catalysts. However, in NOj reduction, deactivation was observed. For catalyst 1-7, there was a rapid, reversible deactivation that was more noticeable at lower temperatures. The activity could be restored by removing propene from the feed. Therefore, it was likely due to carbonaceous deposits on the catalyst. In addition, there was slow deactivation. For example, afto the experiment in Table 2 and cleaning in a flow ofN0/O2/H20 (0. l%/4.7%/1.5%, balance He) at SOOT, the catalyst showed an NO conversion of 33% and propene conversion of 42% at 450°C, versus 53 and 99%, respectively, before deactivation. For catalyst 1-5, only slow deactivation was observed. 

Carbon Deposition. The processing of hydrocarbons always has the potential to form coke (soot). If the fuel processor is not properly designed or operated, coking is likely to occur. Carbon deposition not only represents a loss of carbon for the reaction but more importantly also results in deactivation of catalysts in the processor and the fuel cell, due to deposition at the active sites. 

Fuels— The antioxidant and detergent funetions of Mannieh bases are the ones mainly exploited. They are also claimed to assist in improving storage stability,as metal deactivators," and in reducing soot ignition temperatures." ... 

The technologies for production of syngas from hydrocarbons are based on either steam-reforming or partial oxidation. In the former case, the hydrocarbons react with steam with considerable addition of heat to produce a syngas with a H2/CO ratio of 3 or more. Partial oxidation may be carried out either thermally or catalytically (or by a combination) to produce a syngas with an H2/CO ratio less than 2. Regardless of technology, CO2 may be added to the feed to adjust the gas composition to a low H2/CO ratio. In all cases, limits for the formation of carbon on catalysts or soot in the condensate must be considered to avoid rapid deactivation and low on-stream factors. 

Cerium oxide based catalysts have been widely studied during the last two decades. Main catalytic application was the elimination of automotive exhaust emissions (1,2). The catalytic properties of this oxide has often been related with the mobility of oxygen vacancies in the solid (3,4) and hence with its capacity to release stored oxygen under reducing conditions tests (5,6).Moreover, A.F. Ahlstrom and C.U.I. Odenbrand (7) reported the deactivation by sulphur dioxide of supported copper oxide during the oxidation of soot. 

The presence of sulphur in diesel exhaust gases or particles has to be considered as a poisoning agent for the catalysts used in soot combustion reactions. Copper oxide has been reported to be sensitive towards sulphur dioxide (7) which implies a deactivation of the solid and then eventual modifications of its sinface properties. In this way, lCulCel073 sample was treated in a microflow reactor under SO2 flow (2L.h ) at room temperature for 30 minutes. 

In addition, a useful by-product - clean solid carbon in the form of soot is produced. Obviously, this can be captured and stored more easily than gaseous carbon dioxide. Whereas thermocatalytic cracking offers the benefit of operating at a much lower temperature than direct thermolysis, it does suffer from progressive catalyst deactivation through carbon build-up. Moreover, reactivation would result in unwanted emissions of carbon dioxide. 

However, currently no industrial technology for the CO2 reforming of methane is established. The reasons for this are the rapid deactivation of conventional catalysts if used without the presence of steam, and the relatively high soot formation [68]. 

If the steam is completely or partly replaced by carbon dioxide, the H2 to CO ratio is shifted towards more CO. This involves an environmental benefit, because two greenhouse gases (CH4 and CO2) are combined, resulting in a product gas which might be more favorable for certain applications, e.g., methanol. However, currently no industrial technology for the CO2 reforming of methane is established. The reasons for this are the rapid deactivation of conventional catalysts if used without the presence of steam, and the relatively high soot formation. 

The composition/purity of the product gas from gasification is critical for the bio-SNG synthesis for example, soot formation has been found to be the catalyst deactivation factor at ECN [7], while deactivation was due to sulfur deposition on the catalyst surface for the lOkW, , pilot-scale tests COSYMA in Gussing [25]. 

The concentration of NO required to form the NO2 must be high enough to oxidize the trapped soot. In addition, CO and HC were also oxidized in this same catalyst however, any SO2 was oxidized by the precious metals to SO3. Because of the complex effects of fuel sulfur in this technology (deactivation of NO oxidation catalyst and formation of SO3), this technology is limited to low... 

One anticipates that the major source of deactivation of the DPT is likely to be high temperature occurring during soot regeneration (149—151). In extreme cases, the cordierite DPF may melt (>1300°C). For this reason, alternative structures based on SiC and aluminum titantate have been developed however, there are structures that more expensive than cordierite (8). 

Colombo et al. [48] performed a simulation study of the de-NOj activity effect on soot regeneration. Simulations were carried out at a fixed temperature of 500 °C and with a feed mixture containing equal amounts of ammonia and NOj. The initial soot load was set to 8 g/1 and simulations were repeated with two different NO2/NO , ratios, namely 0 and 1. For both reacting systems, simulations were first performed considering the SCR reactivity and then repeated by deactivating the de-NO reactions. The computed evolution of the total soot mass and of wall soot was followed as a function of time, as shown in Fig. 13.16. 

Similar light-induced production of HONO upon exposure to NO2 has been observed on soot [129]. The source strengths estimated for atmospheric conditions are comparable to those for humic acids. The process therefore represents only a small source of HONO in the gas phase. However, Monge et al. demonstrated that HONO production on soot does not cease quickly due to deactivation of reactive species under irradiation as it does under dark conditions, and so soot may act as a photoactive substrate over its entire life cycle in the atmosphere. 

Deactivation of Soot Combustion Catalysts by Perovskite Structure Formation... 

The formation of perovskite structures does not always have a positive effect on the soot combustion activity of mixed oxides, and a few examples of catalyst deactivation due to perovskite structure formation have been reported. The stability of Ba, Isoot combustion, was studied. This catalyst was thermally stable after 30 h at 800 °C, but above 830 °C catalyst deactivation due to BaCe03 perovskite structure formation was observed [46]. Ba/Mn-Ce catalysts were also tested for soot oxidation in the presence of NO,c> concluding that the formation of perovskite-type oxides after the high-temperature calcination caused the loss of NO storage capacity and a small increase in soot oxidation temperature, but did not seem to affect the NO oxidation activity [47]. 

Krishna, K. and Makkee, M. (2006). Soot Oxidation over NOx Storage Catalysts Activity and Deactivation, Catal Today, 114, pp. 48-56. 

The graphitic carbtMi (coke) is formed in most chemical processes and causes deactivation of catalysts or clogging in the reactors. Moreover, huge amounts of soot particles are released from vehicle engines due to the incomplete combustion... 

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