Search

The steady-state kinetics of the selective catalytic reduction (SCR) of nitrogen oxides (NO and NO 2 ) with ammonia over a commercial iron zeolite catalyst were studied in the temperature range of 250 to 450 °C using an integral tubular reactor. Special attention was paid to the stoichiometric ratio of the conversion of ammonia and nitrogen oxides. For this purpose, both systematic SCR measurements at different feed gas compositions and independent studies of the catalytic oxidation of ammonia in the absence of NO and NO 2 were carried out. Under all reaction conditions, a considerable deviation from the expected 1:1 stoichiometry was observed. The steady-state kinetics of the reacting system could be described by global Langmuir–Hinshelwood-type rate equations for standard SCR, fast SCR, NO/NO 2 equilibrium and NH 3 oxidation. For the correct calculation of the ammonia oxidation it was necessary to include two terms. A first one describing the reaction of NH 3 with O 2 , which becomes important at temperatures above 400 °C, and a second rate equation which is not only proportional to NH 3 and O 2 but also to the NO concentration and which is of particular relevance at low reaction temperatures. Additional measurements with different catalyst particle sizes including industrial extrudates could be successfully described with the aid of a reactor model which took film and pore diffusion phenomena into account. [ABSTRACT FROM AUTHOR]

Abstract: To broaden and lower the narrow temperature window of the selective non-catalytic reduction (SNCR) of nitric oxide (NO), the use of activated ammonia as the reduction agent was examined. A 172nm VUV (vacuum ultraviolet) excimer lamp was employed as the excitation source for molecular ammonia. Activated ammonia generated by VUV radiation in a photochemical reactor was injected into a model flue gas (NO/O2/N2) at room temperature. The effects of reaction temperatures, oxygen concentrations, and NH3/NO molar ratios (MRs) on NO removal were investigated in a lab-scale plug flow reactor. Temperatures ranged from 500°C to 850°C. A temperature window enlargement of 150°C was achieved at the lower boundary of the temperature window. The results are the first clear observations of low temperature SNCR by activated ammonia. Above 600°C, NO removal was affected by injection of activated ammonia, while around 750°C, conventional SNCR by injection of molecular ammonia was effective. An approximate 80% NO removal was attained at 700°C with an MR =2.0 and 8.3% O2. Although the chemical compositions of activated ammonia remained unknown during the present study, the results suggested that effective chemical species for NO removal such as NH2 and H2 were produced from activated ammonia above 600°C. The number of photons emitted per mole of molecular ammonia, Np , was proposed to evaluate the effects of NH3 concentrations and residence times of NH3 gas in the photochemical reactor. The optimum Np range was found to be the point at which maximum NO removal was attained. [Copyright &y& Elsevier]

Abstract: NO x storage-reduction experiments were performed using a coupled LNT-SCR system consisting of a low-precious metal loaded Pt/Rh LNT catalyst and a commercial Cu–zeolite SCR catalyst. Cycling experiments revealed that when a CO+H2 +C3H6 mixture or C3H6 by itself was used as the reductant, the NO x conversion over the SCR catalyst exceeded the conversion of NH3 over the same catalyst. This is explained by the presence of propene, which slipped through the LNT catalyst and reacted with the LNT NO x slip. Separate experiments, conducted under continuous flow and lean-rich cycling conditions, confirmed the ability of propene, as well as ethene, to function as a NO x reductant over the SCR catalyst. Cycling experiments also revealed that the SCR catalyst was able to store propene, such that NO x reduction by stored propene continued into the lean phase (after the switch from rich conditions). According to adsorption experiments, significant co-adsorption of NH3 and propene occured in the SCR catalyst, while under lean-rich cycling conditions the contributions of NH3 and C3H6 to NO x conversion were found to be essentially additive. These findings suggest that under actual driving conditions, NO x reduction by non-NH3 reductants (olefins and possibly other hydrocarbons) in the SCR catalyst can contribute to the mitigation of lean and rich phase NO x . [Copyright &y& Elsevier]

A series of cerium-tungsten oxide catalysts was prepared by the co-precipitation method and was evaluated for the selective catalytic reduction of NO by ammonia (NH-SCR) over a wide temperature range. These catalysts were characterized by BET, XRD, XPS and H-TPR analyses. The experimental studies demonstrated that, among cerium-tungsten oxides, CeO-WO with a Ce/W molar ratio of 3/2 exhibited the best activity toward NH-SCR reactions, N selectivity and SO durability over a broad temperature range of 175-500 °C at a space velocity of 47,000 h. The strong interaction between Ce and W could be the main factor leading to the high activity of the CeO-WO mixed oxide catalyst. Graphical Abstract: A series of cerium-tungsten oxide catalysts was prepared by the co-precipitation method and was evaluated for the selective catalytic reduction of NO by ammonia (NH-SCR) over a wide temperature range. These catalysts were characterized by BET, XRD, XPS and H-TPR analyses. The experimental studies demonstrated that, among cerium-tungsten oxides, CeO-WO with a Ce/W molar ratio of 3/2 exhibited the best activity toward NH-SCR reactions, N selectivity and SO durability over a broad temperature range of 175-500 °C at a space velocity of 47,000 h. The strong interaction between Ce and W could be the main factor leading to the high activity of the CeO-WO mixed oxide catalyst.[Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]