Selective Catalytic Reduction Reaction Research Articles

Changes of Nitrate Activity and Byproduct Distribution Characteristics for Synergistic NOx and Dioxin Abatement over V2O5/AC Catalyst.

The simultaneous removal of NOx and dioxins has been considered an economical and effective technology of controlling multipollutant flue gas in the context of "carbon peaking and carbon neutrality". However, this technology has not yet been implemented in practical situations, because the interactive relationship between the selective catalytic reduction (SCR) reaction and dioxin catalytic oxidation lacks a deep understanding, especially on a carbon-based catalyst. In this research, the influence of NO and NH3 on the oxidation characteristics and byproducts distribution of dibenzofuran (DBF) was studied on V2O5/AC catalyst. Results indicated that NH3 has a stronger inhibition effect for DBF catalytic oxidation than NO due to obvious competitive adsorption between NH3 and DBF on the V2O5/AC catalyst. In addition, although both NO and NH3 inhibit the complete degradation of DBF, their effects on the byproduct distribution are not consistent. NO primarily affects the level of oxygen-containing byproducts, while NH3 primarily affects the level of alkane byproducts. Furthermore, the SCR reaction activity demonstrated a reduction when DBF was present. The occupation of V2O5 sites by DBF and its oxidizing intermediates has hindered the production of monodentate nitrate and the reactivity of bridged nitrate, resulting in a decrease in SCR activity via the L-H mechanism. This work aims to provide theoretical guidance for simultaneous removal of NOx and dioxins in industrial fumes.

In-situ interzeolite transformation synthesis Cu-SSZ-13 from coal fly ash-derived zeolite for NH3-SCR reaction

Cu-SSZ-13 catalyst has excellent performance for removing NOx. In this study, the in-situ inter-zeolite transformation (IZT) method was proposed to prepare Cu-SSZ-13 catalyst. The coal fly ash-derived zeolite acted as a raw material rather than the chemical silicon and aluminum source. A series of catalysts were prepared using the in-situ IZT method with different ratios of the two template agents. The NO conversion of Cu-SSZ-13 catalysts was above 90 % within the temperature range of 200–700 °C, whereas the N2 selectivity was maintained at about 100 %. The as-synthesized catalysts had higher conversion rates for Si and Al, higher copper loading and more isolated Cu2+. The selective catalytic reduction (SCR) reaction mechanisms of Cu-SSZ-13 catalyst prepared using the in-situ IZT method followed both the Eley-Rideal (E-R) mechanism and the Langmuir-Hinshelwood (L-H) mechanism, though it was mainly based on the E-R reaction. In this study, the Cu-SSZ-13 catalysts were synthesized using coal fly ash based zeolite in one step. This study further improved the synthesis process route from coal based solid waste to SCR catalyst, shortened the preparation cycle, and improved catalyst performance.

Effect of vanadium and tungsten loading order on the denitration performance of F-doped V2O5-WO3/TiO2 catalysts.

F-doped V2O5-WO3/TiO2 catalyst has been confirmed to have excellent denitration activity at low temperatures. Since the V2O5-WO3/TiO2 catalyst is a structure-sensitive catalyst, the loading order of V2O5 and WO3 may affect its denitration performance. In this paper, a series of F-doped V2O5-WO3/TiO2 catalysts with different V2O5 and WO3 loading orders were synthesized to investigate the effect of denitration performance at low temperatures. It was found that the loading orders led to significant gaps in denitration performance in the range of 120-240 °C. The results indicated loading WO3 first better utilized the oxygen vacancies on the TiF carrier promoting the generation of reduced vanadium species. In addition, loading WO3 first facilitated the dispersion of V2O5 thus enhanced the NH3 adsorption capacity of VWTiF. In situ DRIFT verified the rapid reaction between NO2, nitrate, and nitrite species and adsorbed NH3 over the VWTiF, confirming that the NH3 selective catalytic reduction (NH3-SCR) reaction over VWTiF at 240 °C proceeded by the Langmuir-Hinshelwood (L-H) mechanism. This research established the constitutive relationship between the loading order of V2O5 and WO3 and the denitration performance of the F-doped VWTi catalyst providing insights into the catalyst design process.

Promoting effect of polyoxometallic acid modification on the NH3-SCR performance of Mn-based catalysts

The strong oxidation capabilities of Mn sites render Mn-based catalysts showing high activities of selective catalytic reduction (SCR) at temperatures < 250℃, but simultaneously lead to the generation of by-product N2O with strong greenhouse effect. The activity-N2-selectivity trade-off was a prevalent observation in the development of Mn-based SCR catalysts. Here, a polyoxometallic acid enhancement strategy was devised to simultaneously promote the N2 selectivity and low-temperature SCR (LT-SCR) activity of Mn-based metal oxide catalysts, where the introduction of uniformly dispersed phosphomolybdic acid (HPMo) tuned the oxidation capabilities and enhanced NH3 adsorption-storage capability of catalysts. The combination of transient reaction, in-situ DRIFTS analysis and DFT calculations revealed that the main origin of N2O on the Mn-based catalysts was the deep oxidation of NH3, which was inhibited by the interaction between HPMo and Mn sites. Additional NH3 molecules adsorbed by the abundant acid sites supplied by HPMo further facilitated the SCR reaction. This strategy could guide the design of high-activity and high-selectivity LT-SCR catalysts for industrial denitration.

Effects of Cu species in Cu-SSZ-13 zeolites on the performance of NOx reduction reactions

In diesel vehicle exhaust after-treatment systems, high silica Cu-SSZ-13 has received increasing attention as a commercialized catalyst owing to its high efficiency in selective catalytic reduction reactions with NH3 (NH3-SCR). However, research on the regulation of Cu2+ species within metal active centers in the pore structure of chabazite (CHA)-type zeolite to improve catalytic performance lacks sufficient depth. Herein, SSZ-13 zeolite with various Al distributions was prepared by adjusting the proportion of N,N,N-Trimethyl-1-adamantanaminiumhydroxide (TMAdaOH) and Na in the initial gel. Moreover, three representative Cu-SSZ-13 catalysts with different Cu2+ species were synthesized via an ion exchange method. 23Na MAS NMR and NO adsorption infrared spectroscopy results revealed that with increasing Na content and decreasing TMAdaOH concentration in the initial gel, the number of paired Al in the CHA framework gradually increased. Consequently, this increased the number of Cu2+-2Z sites near six-membered ring (MR) and reduced the amount of [Cu(OH)]+-Z sites near eight-MR. The fresh Cu-SSZ-13 catalyst, with a moderate amount of Cu2+-2Z and [Cu(OH)]+-Z, exhibited optimal catalytic performance across the entire temperature window. After hydrothermal aging, Cu-SSZ-13 with a higher concentration of Cu2+-2Z species featured optimal performance among all Cu-SSZ-13 catalysts above 400 °C. These findings indicated the significance of balancing the amount of both Cu2+-2Z and [Cu(OH)]+-Z in the Cu-SSZ-13 zeolite. Additionally, the results elucidated the reaction intermediates involved in the NH3-SCR process catalyzed by Cu-SSZ-13 with various Cu species.

Designing ammonia oxidation catalysts via physical mixing of Pt/SiO2 and Cu/ZSM-5

Ammonia (NH3) is emerging as a carbon-neutral fuel alternative; however, its use poses environmental and health risks due to NH3 slip. The NH3-selective catalytic oxidation (NH3-SCO) process, which converts NH3 into N2 and H2O, offers a solution. However, it faces challenges with respect to the formation of byproducts, particularly N2O and NOx. This study focuses on designing NH3-SCO catalysts to enhance low temperature NH3 conversion efficiency while minimizing undesired byproducts. Pt/SiO2 catalysts were synthesized with adjusted Pt particle sizes to improve the NH3-SCO activity and were physically mixed with Cu/ZSM-5 to overcome the low N2 selectivity of Pt-based catalysts by utilizing the selective catalytic reduction (SCR) ability of Cu/ZSM-5. Pt1Cu3 catalyst, a physical mixture of Pt/SiO2 and Cu/ZSM-5 in a 1:3 mass ratio, was found to be the most effective, achieving complete NH3 oxidation at 250 ℃ while maintaining a high N2 selectivity of approximately 80%. Further investigation revealed that the performance of the Pt1Cu3 catalyst was significantly influenced by the arrangement and physical proximity of the Pt/SiO2 and Cu/ZSM-5 catalysts, with a closely contacted mixture showing synergistic effects that enhanced activity and selectivity. These results suggest that the Pt1Cu3 catalyst offers a promising solution for NH3 emission control by utilizing a balanced approach that combines SCO and SCR reactions.

Supercritical ion exchange: A new player in Cu-zeolite catalyst synthesis towards NH3-SCR of NOx

In this study, the effect of zeolite type, copper loading, and synthesis method/conditions on the performance of a range of Cu/Zeolites in the Standard Selective Catalytic Reduction (SCR) reaction was investigated via utilizing synthesis methods of Supercritical and Aqueous Ion Exchange (SCIE and AIE). Cu/MORS synthesized via SCIE outperformed the conventionally prepared catalyst, Cu-MORA. Cu/MORS and Cu/ZSM-5 exhibited higher NO conversion than Cu/SSZ-13 at low temperatures due to higher Cu loading. However, Cu/SSZ-13 ones surpassed the other catalysts in terms of NO conversion at temperatures above 450 °C. Selective nature of SCIE under different conditions resulted in an obvious difference between the catalyst’s performance while the SCIE temperature varied from 40 to 80 °C. The results of this study showed that synthesis method/condition affects directly the Cu speciation and catalyst performance in NH3-SCR, calling for developing new synthesis methods of Cu/Zeolites to modify the catalysts performance in after-treatment systems.

Insight into the Alkali Resistance Mechanism of FeMoTiOx Catalysts for NH3 Selective Catalytic Reduction of NO: Self-Defense Effects of MoOx for Alkali Capture.

The deactivation of selective catalytic reduction (SCR) catalysts caused by alkali metal poisoning remains an insurmountable challenge. In this study, we examined the impact of Na poisoning on the performance of Fe and Mo co-doped TiO2 (FeaMobTiOx) catalysts in the SCR reaction and revealed the related alkali resistance mechanism. On the obtained Fe1Mo2.6TiOx catalyst, the synergistic catalytic effect of uniformly dispersed FeOx and MoOx species leads to remarkable catalytic activity, with over 90% NO conversion achieved in a wide temperature range of 210-410 °C. During the Na poisoning process, Na ions predominantly adsorb on the MoOx species, which exhibit stronger alkali resistance, effectively safeguarding the FeOx species. This preferential adsorption minimizes the negative effect of Na poisoning on Fe1Mo2.6TiOx. Moreover, Na poisoning has little influence on the Eley-Rideal reaction pathway involving adsorbed NHx reacting with gaseous NOx. After Na poisoning, the Lewis acid sites were deteriorated, while the abundant Brønsted acid sites ensured sufficient NHx adsorption. As a benefit from the self-defense effects of active MoOx species for alkali capture, FeaMobTiOx exhibits exceptional alkali resistance in the SCR reaction. This research provides valuable insights for the design of highly efficient and alkali-resistant SCR catalysts.

Unravelling the Role of Zr–Laponite Supports in Enhancing the Activity of a MnNi Catalyst for the Low-Temperature NH3-Selective Catalytic Reduction Reaction

Unravelling the Role of Zr–Laponite Supports in Enhancing the Activity of a MnNi Catalyst for the Low-Temperature NH₃-Selective Catalytic Reduction Reaction

Unexpected Promotion Effect of H2O on the Selective Catalytic Reduction of NOx with NH3 over Cu-SSZ-39 Catalysts.

Water molecules commonly inhibit the selective catalytic reduction (SCR) of NOx with NH3 on most catalysts, and water resistance is a long-standing challenge for SCR technology. Herein, by combining experimental measurements and density functional theory (DFT) calculations, we found that water molecules do not inhibit and even promote the NOx conversion to some extent over the Cu-SSZ-39 zeolites, a promising SCR catalyst. Water acting as a ligand on active Cu sites and as a reactant in the SCR reaction significantly improves the O2 activation performance and reduces the overall energy barrier of the catalytic cycle. This work unveils the mechanism of the unexpected promotion effect of water on the NH3-SCR reaction over Cu-SSZ-39 and provides fundamental insight into the development of zeolite-based SCR catalysts with excellent activity and water resistance.

Investigating NH3-SCR coupled with CO oxidation reaction mechanisms on titanium nanotube-loaded CuMnFe composite metal catalysts

NOx and CO are the primary air pollutants emitted from sintering flue gas, and their effective treatment is crucial. However, achieving good N2 selectivity with the CuMnFe composite metal oxide catalyst alone is challenging due to its oxidizing nature. Hence, in this study, titanium dioxide nanotubes were employed as carriers for the catalyst. CuMnFe composite metal oxide catalysts supported on titanium dioxide nanotubes were prepared using the co-precipitation method. The optimized catalysts demonstrated excellent performance, maintaining over 90% conversion of NOx and CO at low temperatures ranging from 120 to 200 ℃. Furthermore, significant improvement in N2 selectivity was achieved using these catalysts. Characterization results revealed several positive effects of incorporating titanium dioxide nanotube carriers into the catalyst. Firstly, it efficiently reduced the mobility of adsorbed oxygen on the catalyst surface, thus decreasing its oxidation capacity. Secondly, the addition of carriers helped balance the acidity on the catalyst surface, leading to an increase in Lewis acid sites. These modifications in the catalyst's surface properties proved advantageous for the selective catalytic reduction reaction. Moreover, the mechanisms of NH3-SCR and CO oxidation over the catalysts were extensively studied using DRIFTS. The results demonstrated that the CO oxidation reaction followed the Mars-van Krevelen mechanism, with a competitive adsorption relationship observed between O2 and CO at the active sites. On the other hand, the NH3-SCR reaction primarily followed the Eley-Rideal mechanism, with minimal occurrence of the Langmuir-Hinshelwood mechanism in the SCR reaction. The presence of NH3 inhibits CO adsorption at the active site, resulting in decreased CO oxidation performance in the NH3-SCR-coupled CO oxidation reaction at low temperatures. Nevertheless, NH3-SCR can enhance NO conversion by utilizing some of the latent heat generated from the exothermic CO oxidation.

Insight into the reasons for enhanced NH3-SCR activity and SO2 tolerance of Mn-Co layered oxides

Mn-based catalysts have a fatal flaw in selective catalytic reduction (SCR) of NOx with NH3, which is poor SO2 tolerance. In this work, we successfully fabricated Mn-Co layered double oxides (LDOs) by calcining the Mn-Co layered double hydroxides (LDHs) generated by a hydrothermal method using urea solution. The obtained Mn-Co LDO presents excellent catalytic activity and SO2 tolerance in the SCR reaction of NOx with NH3. The enhanced activity can be attributed to the special surface physicochemical properties endowed by its LDO structure, such as large specific surface area, more exposed active sites, relatively high Mn4+ and Oα/(Oα + Oβ) percentage, good reducibility, and abundant acidic sites. The enhanced SO2 tolerance can be ascribed to the weak adsorption energy of SO2 on Mn-Co LDO, which makes it difficult for SO2 to interact with the active sites. Furthermore, the NH3-SCR of NOx reaction that occurs on the surface of the MnCo-LDO follows both Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms. This study shines light on the design of Mn-based de-NOx catalysts with excellent SO2 tolerance.

Copper-exchanged SUZ-4 zeolite catalysts for selective catalytic reduction of NOx

The copper-exchanged SUZ-4 zeolite with optimal compositions exhibits comparable catalytic properties to commercial Cu-SSZ-13 for the selective catalytic reduction reaction.

Enhanced SO2 resistance of NiFe/CeO2 catalyst by adding WS2 for CO-assisted selective catalytic reduction reaction

Selective catalytic reduction (SCR) is an effective NOx reduction method. Recently, research has been conducted on simultaneously removing NOx and CO using passive SCR technology that consumes CO gas that is normally generated in industrial combustion processes (designated as CO-SCR). SO2 gas in industrial flue gas can compete with NOx and CO, leading to catalyst deactivation and a shortened lifespan. Catalysts with resistance to the sulfur present in industrial processes are crucial for CO-SCR. Therefore, this study aimed to develop a catalyst resistant to the SO2 present in exhaust gas; specifically, WS2 which has excellent chemical stability, was added to a transition metal-based catalyst NiFe/CeO2 and the CO-SCR activity of the unmodified and WS2-incorporated catalysts (NiFe/CeO2 and NiFe/WS2-CeO2, respectively) were examined. NiFe/WS2-CeO2 (1 wt% WS2 content) exhibited a high NOx conversion rate of more than 80% in a sulfur atmosphere. The addition of WS2 to NiFe/CeO2 improved its CO adsorption capacity and catalytic reduction properties by 1.63 and 1.28 times, respectively, thereby promoting the catalytic performance. Fourier-transform infrared analysis revealed that NiFe/WS2-CeO2 was more resistant to sulfur poisoning than NiFe/CeO2. These findings can be leveraged to develop sulfur-resistant catalysts for eliminating NOx and CO from industrial flue gas.

Experimental study of Fe modified Mn/CeO2 catalyst for simultaneous removal of NO and toluene at low temperature

A series of Mn/CeO2 catalysts modified with different Fe contents were prepared by impregnation method and tested for their low-temperature performance for simultaneous de-nitrification and toluene removal. It was found that the Fe5Mn/CeO2 catalyst showed the best catalytic performance and the conversion efficiency of toluene reached 90% at 175 °C and NO conversion reached 90% at 95−300 °C. The physical and chemical properties of the catalysts were characterized by BET, SEM, XRD, XPS, H2-TPR, NH3-TPD and O2-TPD. XPS results showed that the increased content of Ce3+ and Mn4+ in the Fe5Mn/CeO2 catalyst promoted the formation of oxygen vacancies and unsaturated chemical bonds, providing more active sites, thus facilitating the efficient removal of NO and toluene at low temperatures. Compared with other catalysts, H2-TPR, NH3-TPD and O2-TPD indicate that Fe5Mn/CeO2 catalyst has great redox ability, stronger acidity and better oxygen migration ability. In addition, this paper explores the effects between selective catalytic reduction (NH3-SCR) and catalytic oxidation reaction of toluene over Fe5Mn/CeO2 catalyst. NH3 preferentially reacts with the active site on the catalyst to inhibit the toluene oxidation process, while NO promotes the toluene removal process. Toluene can promote the NH3-SCR process in a certain temperature range. While NO promotes the formation of NO2, NO2 effectively promotes the combination of toluene and active sites, which is conducive to the catalytic oxidation of toluene; The inhibition of toluene on the NH3-SCR process weakens with the increase of temperature. At 100 °C, the inhibition of toluene on the NH3-SCR process disappears. When the temperature exceeds 225 °C, toluene reacts with NO as a reducing agent and promotes the formation of NO2, thus promoting the NH3-SCR reaction.

Low temperature NH3 selective catalytic reduction of NOx over CeOx-biochar catalyst prepared by air oxidation

A highly efficient CeOx-BC0.6 (BC: biochar. 0.6: mass ratio of Ce/(BC + Ce)) catalyst was prepared by air oxidation for low temperature NH3 selective catalytic reduction of NOx. The catalytic performance, characterization and reaction mechanism of the CeOx-BC0.6 were investigated. The results showed that NOx conversion and N2 selectivity of CeOx-BC0.6 catalyst obtained by calcining in air were above 90 % at 150–300 °C and 100,000 h−1. The surface oxygen groups, acid sites and oxygen vacancies were generated by air oxidation treatment, which promoted the adsorption and activation of reactants. NH3 and NOx were adsorbed on the catalyst surface in the form of NH4+, NH3+, nitrite and nitrate adspecies. The redox cycles of ketonic carbonyl/phenol (CO/C–OH) pairs and Ce4+/Ce3+ pairs were responsible for the activation of NH3 and NOx. NH2NO and NH4NO2 were the main active intermediates. The reaction of NH3 selective catalytic reduction of NOx over CeOx-BC0.6 mainly followed Eley-Rideal mechanism. These results will promote the understanding of mechanism of NH3 selective catalytic reduction of NOx for biochar-based catalyst prepared by air oxidation.

Enhancement of SCR denitrification control strategy considering fluegas temperature fluctuation: Fundamental principle and performance evaluation

Nitrogen oxides (NOx) constitute a primary pollutant in coal-fired power plants, and are mitigated through the application of selective catalytic reduction (SCR). Coal-fired power plants should frequently change their output power to help the power grid to accommodate high penetration of intermittent renewable power. However, NOx emission transient overshoot often occurs in coal-fired power plants when coal-fired power plants dynamically change their output power. To address this issue, dynamic models for both coal-fired power plants and SCR denitrification were established. The study identifies fluctuations in fluegas temperature as a critical factor contributing to transient NOx overshoots. The change law of SCR reaction temperature during load cycling processes is obtained, and its prediction model based on neural network algorithm is developed. An enhanced control strategy, accounting for the temperature variations at the SCR inlet, was subsequently introduced. Performance comparisons indicate that the enhanced control strategy effectively eliminates transient NOx overshoots during load cycling processes. Specifically, the maximum NOx emissions during loading down and loading up processes under the enhanced control strategy are 48.62 and 49.44 mg m−3, representing reductions of 7.36 and 4.63 mg m−3 compared to the original control strategy. Additionally, when the enhanced control strategy is adopted, the cumulative ammonia (NH3) consumption decreases by 0.48 and 11.84 g m−3 during loading down and loading up processes, respectively.

Time-resolved in situ/operando infrared spectroscopy of Cu-supported zeolite X catalyst during NH3–SCR of NO: A modulated excitation study

Zeolite X was used as support of copper catalyst (Cu/X) for selective catalytic reduction (SCR) of nitrogen oxide (NO) with ammonia (NH3). Modulated excitation in situ/operando infrared (IR) spectroscopy and phase-sensitive detection (PSD) were applied to study the reaction intermediates that formed on the Cu/X during transient experiments. Changing the gas concentration to periodically create unsteady conditions enabled the selective extraction of the dynamic species that were directly involved in the reaction. The IR results showed that only Lewis acid sites (LX) existed on zeolite X,and Brønsted acid sites (B) and extra Lewis acid sites (LCu) were generated after Cu loading. The transient experiment results indicated that the SCR reactions on Cu/X obeys the Eley − Rideal (E − R) mechanism, in which NH3 is first adsorbed on the LX and LCu sites (LX–NH3, LCu–NH3) and then moved to the B sites (B–NH3), where it reacted with gas-phase NO to form NH2NO intermediate, which finally decomposed into N2 and H2O. Moreover, it is found that the NH3 adsorbed on LCu and B sites exhibits stronger reactivity than that on the LX sites.

Density functional theory of NO2 and N2O adsorption on the transition metal modified TiO2 surface

The geometric structure and electronic properties of transition metal M (M = Cu, Fe, Mn)-TiO2 (101) surface adsorbed by NO2 and N2O were calculated by density functional theory (DFT) and DFT + U theory. Eight models were established to explore the effect of NO2 and N2O adsorption, and the results show that transition metal adsorption has different effects on the redox reaction of NO2 and N2O. From the stability of adsorption structure, the adsorption of NO2/N2O on Mn atom is more stable, and the adsorption of N2O on Fe atom is more stable than that of NO2. The analysis of charge transfer and electronic structure show that a large number of free active electrons are formed around the Mn and Fe atoms, which are connected with the N atoms of NO2/N2O, and one N atom in N2O changes from losing electrons to gaining a small number of electrons. This means that the Mn and Fe atoms can stimulate more active electrons to exchange with NO2 and N2O, that is, they can exchange more electrons with NO2/N2O before selective catalytic reduction (SCR) reaction, which is beneficial to the succeeding catalytic reaction.

Uncovering the Dinuclear Mechanism of NO2-Involved NH3-SCR over Supported V2O5/TiO2 Catalysts.

Commercial vanadium oxide catalysts exhibit high efficiency for the selective catalytic reduction (SCR) of NO with NH3, especially in the presence of NO2 (i.e., occurrence of fast NH3-SCR). The high-activity sites and their working principle for the fast NH3-SCR reaction, however, remain elusive. Here, by combining in situ spectroscopy, isotopic labeling experiments, and density functional theory (DFT) calculations, we demonstrate that polymeric vanadyl species act as the main active sites in the fast SCR reaction because the coupling effect of the polymeric structure alters the elementary reaction step and effectively avoids the high energy barrier of the rate-determining step over monomeric vanadyl species. This study unveils the high-activity dinuclear mechanism of the NO2-involved SCR reaction over vanadia-based catalysts and provides a fundamental basis for developing high-efficiency and low V2O5-loading SCR catalysts.