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.

Catalytic selective reduction of NO with ethylene over a series of copper catalysts on amorphous silicas

Abstract: Catalytic selective reduction of NO to N 2 was studied comparing a series of Cu-based catalysts (ca. 8 wt.%) supported over amorphous pure and modified silicas: SiO 2 , SiO 2 -Al 2 O 3 , SiO 2 -TiO 2 , SiO 2 -ZrO 2 . The catalysts were prepared by the chemisorption-hydrolysis method which ensured the formation of a unique copper phase well dispersed over all supports, as confirmed by scanning electron micrographs (SEMs). Temperature-programmed reduction (TPR) analyses confirmed the presence of dispersed copper species which underwent complete reduction at a temperature of about 220°C, independently of the support. It was found that the support affects the extent of NO reduction as well as the selectivity to N 2 formation. Maximum N 2 yield was found in the range 275–300°C. The catalyst prepared over SiO 2 -Al 2 O 3 was the most active and selective with respect to the other silicas. Competitiveness factors (c.f.’s) as high as 13–20% in the temperature range 200–250°C could be calculated. For all catalysts, the temperature of the N 2 peak maximum did not correspond to that of the maximum C 2 H 4 oxidation to CO 2 , suggesting the presence of two different sites for the oxidation and the reduction activity. On the catalyst prepared on SiO 2 -Al 2 O 3 , a kinetic interpretation of catalytic data collected at different contact times and temperatures permitted evaluating the ratio between kinetic coefficients as well as the difference between activation energies of NO reduction by C 2 H 4 and C 2 H 4 oxidation by O 2 .

Investigation of selective catalytic reduction impact on mercury speciation under simulated NOx emission control conditions

Abstract: Selective catalytic reduction (SCR) technology increasingly is being applied for controlling emissions of nitrogen oxides (NOx) from coal-fired boilers. Some recent field and pilot studies suggest that the operation of SCR could affect the chemical form of mercury (Hg) in coal combustion flue gases. The speciation of Hg is an important factor influencing the control and environmental fate of Hg emissions from coal combustion. The vanadium and titanium oxides, used commonly in the vanadia-titania SCR catalyst for catalytic NOx reduction, promote the formation of oxidized mercury (Hg2+). The work reported in this paper focuses on the impact of SCR on elemental mercury (Hg0) oxidation. Bench-scale experiments were conducted to investigate Hg0 oxidation in the presence of simulated coal combustion flue gases and under SCR reaction conditions. Flue gas mixtures with different concentrations of hydrogen chloride (HCl) and sulfur dioxide (SO2) for simulating the combustion of bituminous coals and subbit...

A highly effective catalyst of Sm-Mn mixed oxide for the selective catalytic reduction of NOx with ammonia: Effect of the calcination temperature

Abstract: For low temperature selective catalytic reduction (SCR) of NOx with ammonia, the highly effective Sm-Mn mixed oxide (Sm-Mn-O, the molar ratio Sm/Mn is 0.1) catalysts was developed. The effect of the calcination temperature on the structure, physicochemical and catalytic properties of the Sm-Mn-O catalysts was investigated in detail. The results showed that the Sm-Mn-O catalyst calcined at 450 °C exhibited the highest activity, and after been calcined at >550 °C its catalytic activity markedly decreased. With an increase in the calcination temperature, its surface area firstly increased and then decreased, and the poorly crystallized MnO 2 in Sm-Mn-O was transformed to highly crystalized Mn 2 O 3 phase. These changes influenced the adsorption ability of the Sm-Mn-O catalyst for NH 3 or NO + O 2 . After calcination at >550 °C, the contents of Mn 4+ and adsorbed oxygen (O S ) on the surface of Sm-Mn-O decreased obviously, resulting in the decrease in its catalytic properties for the NO oxidation and transformation of bridged nitrite to active intermediate of bidentate nitrate. Conversely, its catalytic activity for NH 3 oxidation was enhanced after being calcined at high temperature. These adverse factors cause the decrease in the SCR activities of Sm-Mn-O calcined at >550 °C.

Mechanistic investigation of hydrothermal aging of Cu-Beta for ammonia SCR

Abstract: The selective catalytic reduction of NO with NH3 over a Cu-BEA catalyst was studied after hydrothermal aging between 500 and 900 degrees C. The corresponding catalyst was characterized using XPS and XRD techniques in the aging interval of 500, 700 and 800 degrees C. No structural changes during the aging process were observed. However, the oxidation state of copper changed during aging and more Cu2+ was formed. We suggest that one of the deactivation mechanisms is the decrease of the Cu+ species. The NO oxidation and NH3 oxidation activity was decreased with increasing aging temperature. Further, we observed that the ammonia oxidation was decreased faster compared to the SCR reactions at low aging temperatures. The experiments from the calorimeter as well as from the ammonia TPD investigations indicate a trend towards more weakly bound ammonia with higher aging temperatures. From the results of the SCR experiments using different NO2/NOx ratios and ammonia oxidation experiments we suggest that most of the N2O is coming from side reactions of the SCR mechanism and not from reactions between NH3 and O-2 alone. Interestingly, we observe that after the 900 degrees C aging a quite large activity remained for the case with 75% NO2/NOx ratio. The N2O production shows a maximum at 200 degrees C, but increases again at higher temperatures. However, the N2O formed at low temperature is decreased after hydrothermal aging while the high temperature N2O is increased. We propose that the different reactions examined in this work do not all occur on the same type of sites, since we observe different aging trends for some of the reactions.

Phosphorus modification to improve the hydrothermal stability of a Cu-SSZ-13 catalyst for selective reduction of NOx with NH3

Abstract: Phosphorus is introduced to modify the Cu-SSZ-13 catalyst via incipient wetness impregnation, with P/Cu-SSZ-13 = 1 and 2 wt.%, and Si/Al = 4, for selective catalytic reduction of NOx with NH3. N2 physisorption and XRD results show the incorporation of phosphorus as phosphate acid enhanced the hydrothermal stability of Cu-SSZ-13 significantly, during hydrothermal aging in 10 vol.% H2O/air at 750 °C for 16 h. NMR and Raman results suggest that phosphate ions migrate and coordinate with the framework-bonded Al species, forming a framework silicoaluminophosphate interface, thus impeding further dealumination and structure collapse. Before the hydrothermal aging, the isolated Cu2+ ions partly interact with the phosphate ions, forming Cu-phosphate species and reducing the SCR performance. Nevertheless, the appropriate content of phosphate ions can prevent the structure collapse caused by the hydrothermal aging, remaining the isolated Cu2+ ions as well as excellent SCR performance.