Low Temperature SCR Catalyst Research Articles

Effect of water vapor on low temperature SCR performances over Cu and Mn-based catalysts: A comparison study

In this study, the commonly used Cu or Mn-based low-temperature SCR catalysts were employed to investigate their different reaction behaviors in the presence of high-content water vapor. Experimental results reveal that CuCeTi sample possesses superior water resistance at low temperature compared with MnCeTi catalyst. Upon the introduction of water vapor, both catalysts exhibit a quick loss in deNOx efficiency, while that is more pronounced on MnCeTi sample. In addition, unlike CuCeTi sample, MnCeTi catalyst also shows a gradual deactivation tendency after initial quick activity loss. Characterization and simulation results indicate that H2O is more easily adsorbed and dissociated on MnCeTi catalyst, showing stronger suppression on NH3 adsorption, causing more serious initial deactivation. Furthermore, more abundant hydroxyl groups derived from dissociative adsorption of water on MnCeTi catalyst will lead to more NH4NO3 deposition and the decrease in redox capacity. This is the main reason of gradual deactivation of MnCeTi catalyst at high-content water vapor. Such findings could pave a new way for development of highly efficient SCR catalysts with good water resistance for real application.

Regeneration of alkali metal K deactivation in low-temperature manganese-based SCR catalyst

Regeneration of alkali metal K deactivation in low-temperature manganese-based SCR catalyst

Combined Experimental and Density Functional Theory Study on the Mechanism of the Selective Catalytic Reduction of NO with NH3 over Metal-Free Carbon-Based Catalysts.

Metal-free carbon-based catalysts are attracting much attention in the low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR). However, the mechanism of the NH3-SCR reaction on carbon-based catalysts is still controversial, which severely limits the development of carbon-based SCR catalysts. Herein, we successfully reconstructed carbon-based catalysts through oxidation treatment with nitric acid, thereby enhancing their low-temperature activity in NH3-SCR. Combining experimental results and density functional theory (DFT) calculations, we proposed a previously unreported NH3-SCR reaction mechanism over carbon-based catalysts. We demonstrated that C-OH and C-O-C groups not only effectively activate NH3 but also remarkedly promote the decomposition of intermediate NH2NO. This study enhances the understanding of the NH3-SCR mechanism on carbon-based catalysts and paves the way to develop low-temperature metal-free SCR catalysts.

Strong metal oxide-zeolite interactions during selective catalytic reduction of nitrogen oxides

In response to the stricter EU VII emission standards and the "150 ℃ challenge", selective catalytic reduction by ammonia (NH3-SCR) catalysts for motor vehicles are required to achieve high NO conversion below 200 °C. Compounding metal oxides with zeolites is an important strategy to design the low-temperature SCR catalysts. Here, we original prepared Cu-SSZ-13 @ MnGdOx (Cu-Z @ MGO), which achieved over 90% NO conversion and 95% N2 selectivity at 150 ℃. It has been demonstrated that a uniform mesoporous loaded layer of MGO grows on Cu-Z, and a recrystallization zone appears at the MGO-Cu-Z interface. We discover that the excellent low-temperature SCR activity derives from the strong metal oxide-zeolite interaction (SMZI) effects. The SMZI effects cause the anchor and high dispersion of MGO on the surface of Cu-Z. Driven by the SMZI effects, the Mn3+/Mn4+ redox cycle ensures the low and medium temperature-SCR activity and the Cu2+/Cu+ redox cycle guarantees the medium and high temperature-SCR activity. The introduction of MGO improves the reaction activity of -NH2 species adsorbed at Mn sites at 150 ℃, achieving a cycle of reduction and oxidation reactions at low temperatures. This strategy of inducing SMZI effects of metal oxides and zeolites paves a way for development of high-performance catalysts.

Highly efficient MnCe/Ti catalyst for NH3-SCR of NO from sintering flue gas at low temperature: CO tolerance and reaction mechanism assessed by in situ DRIFTS

The effect of metal doping (Ce, Fe, and Co) on CO tolerance of Mn/Ti catalyst used for eliminating NO from sintering flue gas has been investigated. The best MnCe/Ti catalyst achieves a NOx conversion of 80 % from 125 to 250 °C at a GHSV of 80,000 h−1. NOx conversion dropped by only 4 % below 200 °C and N2 selectivity slightly increased after introduction of CO, indicating strong resistance of MnCe/Ti catalyst to CO. XPS results suggest that CO oxidation does not alter surface content of Mn4+ and Ce3+ sites of used MnCe/Ti catalyst while it reduces the surface adsorbed oxygen of used MnCe/Ti catalyst because surface adsorbed oxygen consumed by CO oxidation cannot be readily replenished owing to the insufficient capacity in oxygen and electron transport. This is primarily accountable for the slight decrease in NOx conversion. Furthermore, NH3 can completely restrain CO adsorption on MnCe/Ti catalyst while CO cannot hinder the adsorption of NH3 and NO on the catalyst because the interaction between CO and MnCe/Ti catalyst significantly weakened in the presence of NH3, leading to the strong CO tolerance of MnCe/Ti. On MnCe/Ti Eley-Rideal mechanism governs the NH3-SCR involving gaseous NO and adsorbed NH3, while Mars-van Krevelen mechanism describes the oxidation of CO, indicating its reaction with gaseous CO and surface adsorbed oxygen. The findings in this paper can provide modification strategy for low-temperature SCR catalysts which are expected to be applied for sintering flue gas.

Sulfur and Water Resistance of Carbon-Based Catalysts for Low-Temperature Selective Catalytic Reduction of NOx: A Review

Low-temperature NH3-SCR is an efficient technology for NOx removal from flue gas. The carbon-based catalyst designed by using porous carbon material with great specific surface area and interconnected pores as the support to load the active components shows excellent NH3-SCR performance and has a broad application prospect. However, overcoming the poor resistance of H2O and SO2 poisoning for carbon-based catalysts remains a great challenge. Notably, reviews on the sulfur and water resistance of carbon-based low-temperature NH3-SCR catalysts have not been previously reported to the best of our knowledge. This review introduces the reaction mechanism of the NH3-SCR process and the poisoning mechanism of SO2 and H2O to carbon-based catalysts. Strategies to improve the SO2 and H2O resistance of carbon-based catalysts in recent years are summarized through the effect of support, modification, structure control, preparation methods and reaction conditions. Perspective for the further development of carbon-based catalysts in NOx low-temperature SCR is proposed. This study provides a new insight and guidance into the design of low-temperature SCR catalysts resistant to SO2 and H2O in the future.

Synthesis of MnO(OH) nanorods via hydrothermal method as efficient low-temperature SCR catalysts

We conducted a hydrothermal reaction using lactic acid as a reducing agent and potassium permanganate. This reaction successfully produced MnO(OH) with a one-dimensional nanorod structure, and its XRD spectrum perfectly matched the standard reference. Despite the excellent low-temperature ammonia selective catalytic reduction (NH3-SCR) performance observed in manganese-based catalysts, there was no reported performance data for any type of MnOOH in NH3-SCR prior to our research. Our test results now reveal that MnO(OH) nanorods, synthesized using this method, consistently maintain an impressive 98.8 % NO conversion rate even at the low temperature of 80 °C. This finding is of significant importance for the advancement of low-temperature NH3-SCR catalysts.

Sm-MnOx/TiO2-{001} with preferentially exposed anatase {001} facet for selective catalytic reduction of NO with NH3

TiO2-{001} with preferentially exposed {001} supported SmMnOx was prepared for NOx reduction with NH3. The optimal TiMn30Sm10-{001} demonstrated remarkable low temperature catalytic activity (T50 % = 71 °C). Besides, TiMn30Sm10-{001} achieved 80 % NO conversion under a wide temperature window (140–280 °C) under gas hourly space velocity (GHSV) of 50000 h−1, along with excellent catalytic stability (> 85 % NO conversion for 24 h at 200 °C) in both dry and humid conditions. It also showed enhanced N2 selectivity with T80 % range from 60 to 160 °C. The outstanding catalytic performance of TiMn30Sm10-{001} was attributed to the combined efforts of high concentration of Mn3+, abundant surface adsorbed oxygen, and more acid sites due to the addition of SmOx and facets effect of TiO2-{001}. In situ infrared fourier transform spectroscopy analysis revealed that the NO reduction over TiMn30Sm10-{001} obeyed Langmuir-Hinshelwood mechanism. This work provides a rational strategy for designing facet-engineered catalysts for low-temperature SCR.

Poisoning mechanism of alkali metal on Cu–Fe2O3 catalyst for selective catalytic reduction of NOx with NH3

The poisoning mechanism of Na on Cu–Fe2O3 solid solution catalyst was studied by a series of characterization methods, such as X-ray diffraction (XRD), temperature-programmed reduction by hydrogen (H2-TPR), X-ray photoelectron spectroscopy (XPS), Temperature-programmed desorption (TPD) and in-situ Diffuse Reflectance Infrared Fourier Transform spectroscopy (in-situ DRIFT). The characterization results manifest that one part of Na ions can enter into the crystal lattice of Fe2O3, and replace the original Cu2+ in the crystal lattice of Fe2O3. The replacement results in the electrophilicity of Fe3+ species, thereby restraining the generation and adsorption of NO2, nitrate and nitrite species and reducing Lewis acid sites. In addition, the displaced Cu2+ ions form CuO species and lead to excessive oxidation of NH3 to NOx. In addition, Na2O covers the adsorption sites for NH3, NO2, nitrate and nitrite species. According to in-situ DRIFTS results, the reaction still proceeds through both “fast SCR” and Langmuir-Hinshelwood (L-H) reaction mechanism after sodium poisoning, but the reaction rate reduces obviously. This work reveals the influence and poisoning mechanism of alkali metal on the solid solution catalyst, establishes an alkali metal poisoning model, and offers a new way to develop low-temperature SCR catalysts with high alkali resistance.

Polyol-Mediated Synthesis of V2O5-WO3/TiO2 Catalysts for Low-Temperature Selective Catalytic Reduction with Ammonia.

We demonstrated highly efficient selective catalytic reduction catalysts by adopting the polyol process, and the prepared catalysts exhibited a high nitrogen oxide (NOX) removal efficiency of 96% at 250 °C. The V2O5 and WO3 catalyst nanoparticles prepared using the polyol process were smaller (~10 nm) than those prepared using the impregnation method (~20 nm), and the small catalyst size enabled an increase in surface area and catalytic acid sites. The NOX removal efficiencies at temperatures between 200 and 250 °C were enhanced by approximately 30% compared to those of the catalysts prepared using the conventional impregnation method. The NH3-temperature-programmed desorption and H2-temperature-programmed reduction results confirmed that the polyol process produced more surface acid sites at low temperatures and enhanced the redox ability. The in situ Fourier-transform infrared spectra further elucidated the fast absorption of NH3 and its reduction with NO and O2 on the prepared catalyst surfaces. This study provides an effective approach to synthesizing efficient low-temperature SCR catalysts and may contribute to further studies related to other catalytic systems.

Poisoning mechanism of KCl, K2O and SO2 on Mn-Ce/CuX catalyst for low-temperature SCR of NO with NH3

In this study, a series of K species or/and SO2 poisoned Mn-Ce doped CuX (MCCX) catalysts were prepared from waste blast furnace slag (BFS), and the influence mechanism of K species or/and SO2 poisoning on the catalysts in low-temperature NH3-SCR process was investigated. The results demonstrated that MCCX-KCl catalyst had poorer NO conversion than MCCX-K2O below 200 °C, and the co-existence of K species and SO2 considerably inhibited NO conversion. Aggregation of the Cu active component might also be facilitated by co-poisoning with K species and SO2. Furthermore, K species and SO2 co-poisoned catalysts showed fewer isolated Cu2+ species and Mn4+ species than that of K-poisoned catalysts, indicating a decrease in active sites after the addition of SO2. The co-effect of K2O/KCl and SO2 species impaired the redox capacity and decreased the surface acidity than K-poisoned catalysts to a greater extent. Nevertheless, K-poisoning had little effect on the surface intermediates during SCR reaction process, and K-poisoned catalysts followed both Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanism. However, MCCX-KCl-SO2 and MCCX-K2O-SO2 catalysts contained fewer nitrate species, indicating that the co-effect of K species and SO2 impeded the reaction between NO + O2 species and pre-NH3 species, thereby resulting in poor low-temperature NH3-SCR activity.

A kinetic study on mercury oxidation by HCl over typical Mn-based SCR catalysts

A kinetic approach was adopted to analyze the reaction mechanism of elemental mercury (Hg0) oxidation by HCl over three Mn-based low-temperature SCR catalysts, MnOx/TiO2, Fe-MnOx/TiO2 and CeMnO3. Experimental results validated that Hg0 oxidation efficiencies of the catalysts were promoted by HCl at different temperatures. The kinetic models for the heterogeneous Hg0 oxidation were established and verified based on the experimental data. The verification results demonstrated that Hg0 oxidation over the Mn-based catalysts follows Langmuir-Hinshelwood mechanism. The kinetic parameters of reaction rate constant and HCl adsorptionconstant were calculated from the model fitting equations. The reaction rate constant was raised with the increase of temperature, while the HCl adsorptionconstant presented the opposite trend. The Mn-based catalysts with the reaction rate constant of 84.17–335.77 s−1 showed the advantage over some other catalysts such as commercial V2O5-(WO3)/TiO2 and CeO2/TiO2. The kinetic parameters were further improved in the presence of O2. The apparent activation energy for Hg0 oxidation over the catalysts that was derived from the kinetic parameters was 4.13–12.53 kJ/mol, which was much lower than that of the other approaches. The advantageous kinetic parameters and apparent activation energy were favorable for the Mn-based catalysts to act as low-temperature SCR catalyst for synergistic Hg0 removal. The kinetic study results were of fundamental significance for designing the reactor according to the known flue gas conditions, used catalyst and required Hg0 oxidation efficiency.

Efficient MnOx-CeO2/Ti-bearing blast furnace slag catalyst for NH3-SCR of NO at low temperature: Study of support treating and Mn/Ce ratio

Exploring novel low-temperature SCR catalyst supports with excellent properties and low cost is a significant challenge. Ti-bearing blast furnace slag (Ti-BFS) presented great potential in this field due to high contents of Ti, Si and Al elements. In this work, Ti-BFS was pretreated and optimized by hydrochloric solution, and suitable treatment conditions were obtained. Ti and Si were well enriched and other components (especially Ca) were mostly leached, while the modified slag exhibited well-developed porosity and high surface area. An efficient MnOx-CeO2/Ti-bearing blast furnace slag catalyst (Mn-Ce/ATS) can be developed by loading appropriate MnOx-CeO2 ingredients on this support. 4Mn-Ce/ATS catalyst performed better SCR performance because of its excellent NH3 adsorption capacity and reactivity, as well as more alkali sites. Cerium was beneficial to bond NH3 species and manganese was advantageous to adsorb NO. Although SO2 resistance of this catalyst was relatively undesired, its H2O resistance was perfect. This work can provide a guidance on realizing waste Ti-bearing blast furnace slag utilization and reducing the denitrification cost.

Different lead species deactivation on Mn-Ce activated carbon supported catalyst for low-temperature SCR of NO with NH3: Comparison of PbCl2, Pb (NO3)2 and PbSO4

Due to the accumulation of heavy metal compounds produced by the sintering process in steel industry, the catalysts used for low-temperature selective catalytic reduction of NO with NH3 (NH3-SCR) might be seriously deactivated. In this work, the deactivation effect of PbCl2, Pb(NO3)2, and PbSO4 on Mn-Ce activated carbon supported catalyst for low-temperature NH3-SCR of NO was investigated and compared. Poisoned catalysts were provided by impregnating fresh catalysts with Pb(NO3)2, PbSO4 and PbCl2 aqueous solutions, respectively. Deactivation could be observed on the poisoned samples, and the deactivation degree was following PbCl2 > PbSO4 > Pb(NO3)2. The catalytic activities of all samples were tested, and the physicochemical properties of fresh and poisoned catalysts were assessed. PbCl2 caused the most severe deactivation of the catalyst, owing to its poor redox property and surface acidity. Cl- could also react with Mn active sites to form -O-Mn-Cl bonds, resulting in additional acid sites, although these newly generated sites were not reactive in NH3-SCR reaction process. PbSO4 exhibited moderate poisoning effect due to the addition of SO42-, which created new Brønsted acid sites, facilitating the NH3 adsorption and NO reduction. Pb(NO3)2 had the least poisoning impact on the catalyst due to the NO3–, promoting the NH3 activation. The in situ DRIFTS results revealed that NH3-SCR reaction over all samples was governed by Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanism, and did not change due to the lead poisoning. Finally, a possible mechanistic model for different lead salts poisoning over Mn-Ce/AC catalyst was proposed.

Influence of SO3 on the MnOx/TiO2 SCR catalyst for elemental mercury removal and the function of Fe modification

Elemental mercury (Hg0) is a highly hazardous pollutant of coal combustion. The low-temperature SCR catalyst of MnOx/TiO2 can efficiently remove Hg0 in coal-burning flue gas. Considering its sulfur sensitivity, the effect of SO3 on the catalytic efficiency of MnOx/TiO2 and Fe modified MnOx/TiO2 for Hg0 removal was investigated comprehensively for the first time. Characterizations of Hg-TPD and XPS were conducted to explore the catalytic mechanisms of Hg0 removal processes under different conditions. Hg0 removal efficiency of MnOx/TiO2 was inhibited irreversibly from 92% to approximately 60% with the addition of 50 ppm SO3 at 150 ℃, which resulted from the transformation of Mn4+ and chemisorbed oxygen to MnSO4. The existence of H2O would intensify the inhibitory effect. The inhibition almost disappeared and even converted to promotion as the temperature increased to 250 ℃ and above. Fe modification on MnOx/TiO2 improved the Hg0 removal performance in the presence of SO3. The addition of SO3 caused only a slight inhibition of 1.9% on Hg0 removal efficiency of Fe modified MnOx/TiO2 in simulated coal-fired flue gas, and the efficiency maintained good stability during a 12 h experimental period. This work would be conducive to the future application of MnOx/TiO2 for synergistic Hg0 removal.

Insights into co-doping effect of Sm and Fe on anti-Pb poisoning of Mn-Ce/AC catalyst for low-temperature SCR of NO with NH3

A novel anti-Pb poisoning Sm and Fe co-doped Mn-Ce/AC catalyst was prepared by impregnation method for low-temperature NH3-SCR of NO reaction. Maintaining the excellent low-temperature activity of Mn-Ce/AC catalyst, Sm and Fe co-doping remarkably enhanced the catalyst’s Pb-resistance with NO conversion up to 77%. Sm and Fe co-doped poisoning-catalysts exhibited large specific surface area and pore volume, and the crystallinity of the MnOx and CeOx reduced. The redox properties and surface acidity of the lead poisoned SmFeMnCe/AC catalyst were effectively retained, which could promote NO oxidation and NH3 activation, further benefiting the SCR reaction. In situ DRTFTS results indicated that the low-temperature NH3-SCR mechanism of all poisoned catalysts followed both L-H mechanism and E-R mechanism, and the L-H mechanism was dominated. On the one hand, the extraordinary lead resistance of Sm weakened the damage of lead species on the physicochemical properties of the catalyst; on the other hand, the high oxidation properties and surface acidity of Fe enhanced the catalytic activity of the poisoned catalyst. Finally, a possible synergistic mechanism model of Sm and Fe co-doping on Mn-Ce/AC catalyst against Pb-poisoning was proposed.

Deactivation mechanisms of MnOx-CeO2/Ti-bearing blast furnace slag low-temperature SCR catalyst by PbO and PbCl2

Pb poisoning of MnOx-CeO2/Ti-bearing blast furnace slag SCR catalyst (MC) at low temperature is a tough challenge for NOx emission control in sintering plants. In this work, the poisoning behaviors of PbO and PbCl2 on MC catalyst were explored, and their deactivation mechanisms were also clarified. Results indicated that PbCl2 presented more severe poisonousness than PbO. The addition of PbO obstructed favorable electron conversions between Mn and Ce atoms, consuming more oxygen vacancies and active oxygen species, consequently hindering the redox cycle. Apart from Pb poisoning effect, newly generated HCl over PbCl2-doped catalyst could preferentially adsorb on CeO2 to form inactive Cl− bonds and NH4Cl deposition, further limiting the conversion of Ce4+ to Ce3+ and reducing surface acid sites. Therefore, MC catalyst was deactivated more severely by the double poisoning cycles (Pb + HCl). This work paves a way for the development of high-performance Pb-tolerant low-temperature SCR catalyst, as well as Pb-deactivated catalyst regeneration technology.

Understanding the dual-acting of iron and sulfur dioxide over Mn-Fe/AC catalysts for low-temperature SCR of NO

Poor N2 selectivity and low SO2-tolerant are dominated weaknesses for low-temperature selective catalytic reduction of NO with NH3 (NH3-SCR) catalysts, which must be overcome for practical applications. Herein, a Mn-Fe mixed catalyst supported on active carbon (Mn-Fe/AC) for low-temperature NH3-SCR of NO was prepared, and the influence of SO2 on this process was also studied. Both iron and sulfur dioxide exhibited a dual-acting over Mn-Fe/AC catalyst for NH3-SCR of NO. Sulfur dioxide had two effects when SO2 was introduced in the feed gas under SCR conditions, including inhibiting the low temperature activity of the catalysts and improving N2 selectivity which exceeded 90% during the whole testing temperature. The introduction of SO2 could promote the formation of NH2NO which was the main intermediate and then decomposed into N2. Fe species in Mn-Fe/AC catalyst could improve N2 selectivity. In the presence of SO2, it showed considerable SO2 resistance compared to Mn/AC catalyst. On the one hand, iron species improved the selectivity through forming new active sites promoting the formation of NH2NO and suppressing the free NO3- generation. On the other hand, iron existing in Mn-Fe/AC catalyst could decrease the amount of sulfur species adsorbent and SO2 oxidation, thereby exhibiting better SO2 resistance.

Promoting mechanism of SO2 resistance performance by anatase TiO2 {0 0 1} facets on Mn-Ce/TiO2 catalysts during NH3-SCR reaction

Anatase TiO2 with {001} facets (TiO2-NS) exposed and anatase TiO2 with {101} facets (TiO2-NP) exposed as carrier were used to prepare Mn-Ce/TiO2-NS and Mn-Ce/TiO2-NP catalysts, separately. The Mn-Ce/TiO2-NS catalyst still had a high specific surface area after the SO2 resistance test, which was attributed to the inhibition of the formation of ammonium bisulfate and ammonium sulfate. Additionally, anatase TiO2 {001} surface could effectively inhibit the sulfation of active ingredients, which was owing to the five-coordinated titanium atoms (Ti5c) and two-coordinated oxygen atoms on the anatase TiO2 {001} facets could preferentially react with SO2 to avoid deactivation of active sites after the poisoning experiment. Thus, the Mn-Ce/TiO2-NS catalyst had excellent resistance to SO2 in the poisoning test, indicating that it has great application prospects as a low-temperature SCR catalyst.

Insights into Co-Doping Effect of Sm and Fe on Anti-Pb Poisoning of Mn-Ce/Ac Catalyst for Low-Temperature Scr of No with Nh3

Insights into Co-Doping Effect of Sm and Fe on Anti-Pb Poisoning of Mn-Ce/Ac Catalyst for Low-Temperature Scr of No with Nh3