HC-SCR

Hyeon Kim, M. and Nam, l.-S. (2004) New opportunity for HC-SCR technology to control NO emission from advanced internal combustion engines, in Catalysis, 18, (ed. J.J. Spivey), Royal Society of Chemistry, pp. 116-185. 

An alternative use of NTP is to generate H2 (by reforming a part of the fuel) to be added to the auto-exhaust to promote low-temperature activity in HC-SCR. Quite interesting results have been recently shown from Ag-A12Q3 catalysts whose performances... 

Various other classes of catalysts have been investigated for NH3-SCR, in particular, metal-containing clays and layered materials [43 15] supported on active carbon [46] and micro- and meso-porous materials [31b,47,48], the latter also especially investigated for HC-SCR [25,3lb,48-53], However, while for NH3-SCR, either for stationary or mobile applications, the performances under practical conditions of alternative catalysts to V-W-oxides supported on titania do not justify their commercial use if not for special cases, the identification of a suitable catalyst, or combination of catalysts, for HC-SCR is still a matter of question. In general terms, supported noble metals are preferable for their low-temperature activity, centred typically 200°C. As commented before, low-temperature activity is a critical issue. However, supported noble metals have a quite limited temperature window of operation. 

At higher reaction temperatures (>300°C), micro- or meso-porous materials and/or oxides containing transition metals are preferable. The performances are considerably dependent on the type of reductant, besides the characteristics of the catalyst and the type of transition metal. Although all possible combinations have been explored, including the usage of high-throughput methods, identification of a suitable catalyst formulation active in HC-SCR under practical conditions, especially to decrease by more than... 

NTP and both NH3- and HC-SCR was observed to be present under real diesel engine exhaust conditions [90],... 

The impact of oxygen storage in DeNO catalysis in 02 excess is more complex and strongly depends on the process used for NO reduction. The impact of OSC materials will be examined on two processes the selective reduction by HC (HC-SCR) and the NO -trap process. 

At present the most effective available after-treatment techniques for NO, removal under lean conditions are ammonia selective catalytic reduction (SCR) [1-3] and NO, storage reduction (NSR) [4—6]. Indeed, three-way catalysts (TWCs) are not able to reduce NO, in the presence of excess oxygen, because they must be operated at air/ fuel ratios close to the stoichiometric value. Also, non-thermal plasma (NTP) and hydrocarbon-selective catalytic reduction (HC-SCR) are considered, although they are still far from practical applications. 

The NOx reduction for the exhaust from lean-burn engine is one of the greatest challenges in environmental protection, and a lot of researchers have strived to develop more effective catalysts by many ways. Their efforts could be possibly categorized into four approaches (1) NOx direct decomposition, (2) selective catalytic reduction on NOx with hydrocarbons (HC SCR),... 

NOx reduction conversions met in the DOC are quite low. Excess of air in burned lean fuel mixture results in excess of oxygen in the exhaust. Under such conditions, the reducing components naturally present in diesel exhaust (CO, H2 and HC) are readily oxidized by the excessive oxygen and NOx remains unreduced. However, the unburned hydrocarbons still exhibit a certain activity for NO reduction on NM/y-Al203 and NM/zeolites catalysts under lean conditions (HC-SCR). Many efforts have been put into the investigation of different NM-based or alternative catalysts tailored for the HC-SCR reaction and the development of reliable reaction mechanisms—cf., e.g., Joubert et al. 

HC-SCR under specific situations, but none of them exhibits sufficient activity and durability in the entire range of operating conditions met in automobile exhaust treatment (cf. the reviews by Burch, 2004 Burch et al., 2002). 

Figure 1. Dependence of durability of Cu ZSM5 during HC-SCR on Cu ion exchange rate. Reaction condition NO 150 ppm, CO 500 ppm, H2 250 ppm, 02...

Copper ions exchanged microporous molecular sieves, in particular Cu-ZSM-5, are active catalysts for the selective catalytic reduction of NO and N2O with hydrocarbons in the presence of O2 (HC-SCR). It has been reported that the catalytic activity may be controlled by intra-crystalline diffiisivity and by geometry-limited diffusion depending on the hydrocarbon molecular size and the zeolite pore size [1]. Therefore, it is of interest to prepare Cu-Al-MCM-41 mesoporous molecular sieves and to compare their activity with that of Cu-ZSM-5. 

The plots in Figs. 5 and 6 show the HC-SCR activity and selectivity of Cu-containing mesoporous samples together with data obtained for Cu-containing microporous Cu-S-1 and Cu-ZSM-5 samples of similar Cu content and Si/Al ratio. 

Fig.5. HC-SCR Arrhenius plots (rwo expressed as NO molecules s Cu mole ) of microporous and mesoporous Cu-containing catalysts.

At our knowledge, this is the first time that the catalytic behavior of Cu-Al-MCM-41 for the HC-SCR of NO is reported. HC-SCR of NO was previously reported on the Pt-MCM-41, Rh-MCM-41 and Co-MCM-41 catalysts [8]. Pt-MCM-41 resulted the most active catalyst, but no comparison was made with the activity of Pt-ZSM-5 catalysts measured under the same experimental conditions. 

New Opportunity for HC-SCR Technology to Control NOx Emission from Advanced Internal Combustion Engines... 

A few classicaV studies on the reactivity of HCs to reduce NOx with catalysts indicated that the use of such reductants for controlling mobile NOx emissions was quite attractive to the automotive industry, thereby the advent of a new type of HC-SCR technology in the mid-1980s. An example may be the treatment process of the tail gas from nitric acid production plant via ammonia oxida-tion. The process includes the usual injection of excessive amounts of HCs over supported noble metals such as Pt, Pd and Rh to eliminate the yellowish stack plume due to 0.1 - 0.5% NOx, mainly NO2, from the nitric acid plant. 

Challenges in HC-SCR Technology up to Early 1980s. - In this period, an attempt to use HCs as a possible reductant for the removal of NOx in exhaust streams from stationary and mobile sources was established mainly with supported and unsupported metal oxides, in addition to supported noble metals. Several catalysts could promote this type of NOx reduction under appropriate conditions, among them being those consisting of these metal constituents. 

Because of such strong demand for advanced HC-SCR technology applicable to automobile NO emission controls, Volkswagen AG and Bayer AG in Germany examined a variety of zeolitic catalysts on which NO from ICEs reacts... 

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