Selective catalytic reduction

About: Selective catalytic reduction is a research topic. Over the lifetime, 10502 publications have been published within this topic receiving 226291 citations.

Urea thermolysis and NOx reduction with and without SCR catalysts

Abstract: Urea–selective catalytic reduction (SCR) has been a leading contender for removal of nitrogen oxides (deNOx) from diesel engine emissions Despite its advantages, the SCR technology faces some critical detriments to its catalytic performance such as catalyst surface passivation (caused by deposit formation) and consequent stoichiometric imbalance of the urea consumption Deposit formation deactivates catalytic performance by not only consuming part of the ammonia produced during urea decomposition but also degrading the structural and thermal properties of the catalyst surface We have characterized the urea thermolysis with and without the urea-SCR catalyst using both spectroscopic (DRIFTS and Raman) and thermal techniques (thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC)) to identify the deposit components and their corresponding thermal properties Urea thermolysis exhibits two decomposition stages, involving ammonia generation and consumption, respectively The decomposition after the second stage leads to the product of melamine complexes, (HNCNH)x(HNCO)y, that hinder catalytic performance The presence of catalyst accompanied with a good spray of the urea solution helps to eliminate the second stage In this work, kinetics of the direct reduction of NOx by urea is determined and the possibility of using additives to the urea solution in order to rejuvenate the catalyst surface and improve its performance will be discussed

Evaluation of a catalytic reduction technique for the measurement of total reactive odd-nitrogen NOy in the atmosphere

Abstract: A catalytic reduction technique for the measurement of total reactive odd-nitrogen NO y in the atmosphere was evaluated in laboratory and field tests. NO y component species include NO, NO2, NO3, HNO3, N2O5, CH3COO2NO2(PAN), and particulate nitrate. The technique utilizes the reduction of the higher oxides to NO in reaction with CO on a metal catalyst and the subsequent detection of NO by chemiluminescence produced in reaction with O3. The efficiency and linearity of the conversion of the principal NO y species were examined for mixing ratios in the range of 0.1 to 100 parts per billion by volume (ppbv). Results of tests with Au, Ni, and stainless steel as the catalyst in the temperature range of 25–500°C showed Au to be the preferred catalyst. NH3, HCN, N2O, CH4, and various chlorine and sulfur compounds were checked as possible sources of NO y interference with the Au catalyst. The effects of pressure, O3, and H2O on NO y conversion were also examined. The results of the checks and tests in the laboratory showed the technique to be suitable for initial NO y measurements in the atmosphere. The technique was subsequently tested in ambient air at a remote ground-based field site located near Niwot Ridge, Colorado. The results of conversion and inlet tests made in the field and a summary of the NO y data are included in the discussion.

Influence of sulfation on iron titanate catalyst for the selective catalytic reduction of NOx with NH3

Abstract: Iron titanate catalyst (FeTiOx) is a potential candidate for the substitution of conventional V2O5–WO3 (MoO3)/TiO2 and Fe/Cu-zeolite catalysts for the selective catalytic reduction (SCR) of NOx with NH3 because of its high SCR activity and N2 selectivity in the medium temperature range. Due to the presence of small amount of SO2 in typical diesel exhaust derived from combustion of sulfur-containing fuels, it is very important to investigate the influence of sulfation on SCR activity, catalyst structure and reaction mechanism. After sulfation under the SCR condition, the surface area and pore volume of FeTiOx catalyst decreased to a certain extent due to the formation of sulfate species. According to the characterizations of FeTiOx catalyst using X-ray diffraction, X-ray absorption fine structure spectroscopy, and in situ diffuse reflectance infrared Fourier transform spectroscopy of SO2 + O2 treatment, the sulfate species mainly formed on iron sites in a chelating bidentate conformation, resulting in the enhancement of Bronsted acidity and Lewis acid strength simultaneously. NH3 adsorption was greatly enhanced in the high temperature range, while NOx adsorption was severely inhibited due to the stronger acidity of sulfate species. The operation temperature window of the sulfated catalyst shifted ca. 50 °C towards high temperature range accordingly. The reaction mechanism study shows that the Langmuir–Hinshelwood reaction pathway was cut off by the sulfation process, resulting in the activity loss at low temperatures; only Eley–Rideal reaction pathway between adsorbed NH3 species and gaseous or weakly adsorbed NO dominated in the SCR reaction, which made this catalyst resistant to SO2 poisoning at relatively high temperatures.

Selective catalytic reduction of nitrogen oxides by ammonia on iron oxide catalysts

Abstract: In this study, new Fe 2 O 3 based materials are developed for the selective catalytic reduction (SCR) of NO x by NH 3 in diesel exhaust. As a result of the catalyst screening, performed in a synthetic model exhaust, ZrO 2 is considered to be the most effective carrier for Fe 2 O 3 . The modification of the Fe 2 O 3 /ZrO 2 system with tungsten leads to drastic increase of SCR performance as well as pronounced thermal stability. These results show that tungsten acts as bifunctional component. The highest catalytic activity is observed for ZrO 2 that is coated with 1.4 mol% Fe 2 O 3 and 7.0 mol% WO 3 (1.4Fe/7.0W/Zr). By the use of this catalyst quantitative conversion of NO x is obtained between 285 and 430 °C with selective formation of N 2 . Here, the turnover frequency of NO x per Fe atom is found to be 35 × 10 −5 s −1 that indicates a high catalytic performance. The SCR activity of the 1.4Fe/7.0W/Zr material is decreased in the presence of H 2 O and CO 2 , whereas it is increased by NO 2 . Temperature programmed reduction by H 2 (HTPR) analyses show that the Fe sites of the 1.4Fe/7.0W/Zr catalyst are mainly in the form of crystalline Fe 2 O 3 , whereby relatively small oxide entities are also present. The strongly aggregated Fe 2 O 3 species are associated with the presence of the promoter tungsten. Based upon stationary catalytic examinations as well as diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) studies we postulate an Eley Rideal type mechanism for SCR on 1.4Fe/7.0W/Zr catalyst. The mechanistic model includes a redox cycle of the active Fe sites. As first reaction step, we assume dissociative adsorption of NH 3 that leads to partial reduction of the iron as well as to production of very reactive amide surface species. These amide intermediates are supposed to react with gaseous NO to form N 2 and H 2 O. In the final step, the reduced Fe sites be regenerated by oxidation with O 2 . As a side reaction of SCR, imide species, originated from decomposition of amide, are oxidized by NO 2 or O 2 into NO.