γ-Fe2O3 is considered to be a promising catalyst for the selective catalytic reduction (SCR) of nitrogen oxide (NOx). In this study, first-principle calculations based on the density function theory (DFT) were utilized to explore the adsorption mechanism of NH3, NO, and other molecules on γ-Fe2O3, which is identified as a crucial step in the SCR process to eliminate NOx from coal-fired flue gas. The adsorption characteristics of reactants (NH3 and NOx) and products (N2 and H2O) at different active sites of the γ-Fe2O3 (111) surface were investigated. The results show that the NH3 was preferably adsorbed on the octahedral Fe site, with the N atom bonding to the octahedral Fe site. Both octahedral and tetrahedral Fe atoms were likely involved in bonding with the N and O atoms during the NO adsorption. The NO tended to be adsorbed on the tetrahedral Fe site though the combination of the N atom and the Fe site. Meanwhile, the simultaneous bonding of N and O atoms with surface sites made the adsorption more stable than that of single atom bonding. The γ-Fe2O3 (111) surface exhibited a low adsorption energy for N2 and H2O, suggesting that they could be adsorbed onto the surface but were readily desorbed, thus facilitating the SCR reaction. This work is conducive to reveal the reaction mechanism of SCR on γ-Fe2O3 and contributes to the development of low-temperature iron-based SCR catalysts.

Selective catalytic reduction technology is one of the most effective methods to control NOx emissions from diesel engines. However, the generation of solid urea and by-products, which will lead to the incomplete decomposition of urea and reduce NOx reduction efficiency. The accumulation of deposits will also affect the back pressure and even block the catalyst, causing a vicious cycle. In this paper, a numerical model coupled with a detailed decomposition mechanism of urea was established. Based on this model, four heat transfer methods and chemical reactions in the film were calculated. The results show that the main heat transfer methods between the film and the plate are conduction and evaporation, both of which have maximum values higher than 107 W/m2. The mass of the film varies with heat flux can be divided into four stages. In the first stage, the film forms rapidly, and intense heat exchange occurs. In the second stage, the film increases slowly and the decomposition mass of urea increases. The main component of the film is solid urea. In the third stage, the solid urea mass begins to decrease. In the fourth stage, film mass decreases, and a large amount of urea is decomposed. The main byproduct of urea decomposition is biuret in the first three stages and cyanuric acid in the fourth stage. The solid urea and byproducts are mainly distributed in regions with low heat flux. With an increase in the initial plate temperature, the heat flux increases, the film and solid urea masses decrease, the byproducts appear earlier and their mass increases faster. When the temperature is 523 K, 528 K, and 533 K, the maximums of biuret mass occur at about 3.1s, 1.4s and 0.5s, respectively. The present work provides guidance understanding the relationship between heat transfer characteristics and byproducts formation of urea decomposition.

Nitrogen oxides, mercury and chlorobenzene are important air pollutants emitted by waste incineration and other industries. Coordinated control of multiple pollutants has become an important technology for air pollution control. Through solid-phase structure control, the catalytic performance of the WCeMnOx/TiO2-ZrO2 catalyst for simultaneous catalytic removal of NO, mercury and simultaneous removal of NO and chlorobenzene were improved. MnWO4 improved the solid acidity of the catalyst and improved the catalytic activity at high temperature. The formation of Ce0·75Zr0·25O2, Ce2WO6, Ce2Zr2O7 and Ce2Ti2O7 improved the catalytic activity at low temperature. The presence of TiOSO4 would affect the valence of metal ions and the reduction of chemisorbed oxygen, thereby reducing the catalytic activity at low temperature. Within the same size range of nanoparticles, cyclic nanoparticles exposed more active sites due to their hollow structure, and their catalytic performance was better than spherical nanoparticles. The thickness of the circular nanoparticles of WCM/TZ-14 catalyst was about 14 nm, and the diameter was about 40 nm Ce0.75Zr0.25O2 and MnWO4 were also present in the phase composition. Therefore, it exhibited the best performance for simultaneous catalytic removal of NO, mercury and simultaneous removal of NO and chlorobenzene. The coincidence temperature window was 347-516 °C. Finally, WCM/TZ-14 catalyst followed both E-R and L-H mechanisms in the NH3-SCR reaction.

The MOx (M = Cu, Mn, Co)/CePO4 support was firstly prepared via the hydrothermal and impregnated method. Selective catalytic reduction of NO with NH3 (NH3-SCR) results showed that the MnOx modifications greatly improved the SCR activities at low temperatures. The NOx conversion of the MnOx/CePO4 catalyst was above 80% even at 180 °C. In-situ DRIFTS results suggest that the SCR reaction is majorly conducted between the absorbed monodentate nitrate and NH3 species (i.e., the Langmuir-Hinshelwood mechanism). MOx (M = Cu, Mn, Co) exists in the formation of nano-size particles obtained by SEM and TEM directly. These nano-size particles can provide active surface adsorbed oxygen and thus improve the NO oxidation ability as indicated by the O2-TPD and NO oxidation tests. The process of NO oxidation to NO2 plays a key role to produce the absorbed monodentate nitrate as indicated by the In-situ DRIFTS. The support CePO4 acts as the acid sites to form highly active NH4+ species. The synergic effect between the MnOx and CePO4 contributed to the high SCR activity over the MnOx/CePO4 catalyst. Additionally, the MOx/CePO4 catalyst exhibits an excellent water tolerance and N2 selectivity. Consequently, the MnOx/CePO4 catalyst becomes the potential catalyst for the practical process.

A particulate oxidation catalyst (POC) was employed to perform experiments on the engine test bench to evaluate the effects on the nitrogen dioxide (NO2) and particulate matter (PM) emissions from diesel engine. The engine exhaust was sampled from both upstream and downstream of the POC. The results showed that the POC increased the ratios of NO2/NOx significantly in the middle and high loads, the ratio of NO2/nitrogen oxides (NOx) increased 4.5 times on average under all experiment modes with the POC. An engine exhaust particle sizer (EEPS) was used to study the particle number-weighted size distributions and the abnormal particle emissions with the POC. The results indicated that the average reduction rate of particle number (PN) was 61% in the operating range of the diesel engine. At the engine speed of 1,400 r/min, the reduction rates of PN tended to decrease with the larger particle size. In the long time run under the steady mode (520 Nm, 1,200 r/min), abnormal particle emissions after the POC happened seven times in the first hour, and the average PN concentration of these abnormal emission peaks was much higher than that in normal state. The particle emissions of peaks 1-5 equaled the particles emitted downstream of the POC in normal state for 1.9h in number concentration, and for 3.6h in mass concentration. The PN concentrations tended to increase over time in 5h under the steady engine mode and the increase of the PN in the size range of 6.04-14.3 nm was more evident.