Promotion effect of tungsten oxide on SCR of NO with NH3 for the V2O5–WO3/Ti0.5Sn0.5O2 catalyst: Experiments combined with DFT calculations

NO Selective Catalytic Reduction over Atom-Pair Active Sites Accelerated via In Situ NO Oxidation

Selective catalytic reduction (SCR) of NO_x with NH₃ is the most efficient technology for NO_x emissions control, but the activity of catalysts decreases exponentially with the decrease in reaction temperature, hindering the application of the technology in low-temperature SCR to treat industrial stack gases. Here, we present an industrially practicable technology to significantly enhance the SCR activity at low temperatures (<250 °C). By introducing an appropriate amount of O₃ into the simulated stack gas, we find that O₃ can stoichiometrically oxidize NO to generate NO₂, which enables NO reduction to follow the fast SCR mechanism so as to accelerate SCR at low temperatures, and, in particular, an increase in SCR rate by more than four times is observed over atom-pair V₁-W₁ active sites supported on TiO₂(001) at 200 °C. Using operando SCR tests and in situ diffuse reflectance infrared Fourier transform spectra, we reveal that the introduction of O₃ allows SCR to proceed along a NH₄NO₃-mediated Langmuir-Hinshelwood model, in which the adsorbed nitrate species speed up the re-oxidation of the catalytic sites that is the rate-limiting step of SCR, thus leading to the enhancement of activity at low temperatures. This technology could be applicable in the real stack gas conditions because O₃ exclusively oxidizes NO even in the co-presence of SO₂ and H₂O, which provides a general strategy to improve low-temperature SCR efficacy from another perspective beyond designing catalysts.

Tungsten Oxide Modified V2O5-Sb2O3/TiO2 Monolithic Catalyst: NH3-SCR Activity and Sulfur Resistance

In this study, a V2O5-Sb2O3/TiO2 monolithic catalyst was modified by introducing WO3. The WO3-modified catalyst exhibited enhanced catalytic activity in the measuring temperature range of 175–320 °C. The changes in dispersion of vanadia species were investigated by ultraviolet-visible (UV-Vis) spectroscopy and H2 temperature-programmed reduction (H2-TPR). A durability test was conducted in a wet SO2-containing atmosphere at 220 °C for 25 h. The sulfate deposition was estimated by temperature-programmed decomposition (TPDC) of sulfates, thermo-gravimetric (TG) analysis, and temperature-programmed desorption (TPD) of NH3. Isothermal SO2 oxidation and temperature-programmed surface reaction (TPSR) of NH4HSO4 with NO were performed. Based on these characterizations, effects of WO3 modification on the sulfate tolerance of the catalyst were explored.

Support promotion effect on the SO2 and K+ co-poisoning resistance of MnO2/TiO2 for NH3-SCR of NO

Mn-based catalysts are expected to be applied for removing NO_x due to its excellent low-temperature activity. However, the practical use of these catalysts is extremely restricted with the co-poisoning of alkali metal and SO₂ in the flue gas. Here the MnO₂/TiO₂ catalyst was employed to elucidate the co-poisoning mechanisms of K and SO₂ for the low temperature selective catalytic reduction (SCR) of NO. The physicochemical properties of catalysts under different toxicity conditions were studied by experiments. The adsorption of NH₃, SO₂, NO, and K on active component (MnO₂) and support (TiO₂) was studied by density functional theory. This work unravels a promotion effect of support on the alkali and sulfur resistance. The SO₂&K co-poisoning catalyst had higher SCR activity than the SO₂-poisoned and K-poisoned catalyst alone. For a single toxic condition: (1) SO₂ was preferentially bonded with the terminated O site of MnO₂ inhibiting the dehydrogenation of NH₃ and redox cycle. (2) The presence of Lewis base (K atom) on the catalyst decreased the binding energy of a Lewis base (NH₃) and hindered the adsorption of NH₃. For the synergistic effect of K and SO₂, the majority of K adsorbed on the support (TiO₂) lead to increase alkalinity, which could promote the adsorption of SO₂ on the TiO₂ and reduce the toxicity of the active component (MnO₂).

Deactivation of V2O5−WO3/TiO2 DeNOx Catalyst under Commercial Conditions in Power Production Plant

Nitrogen dioxide is one of the most dangerous air pollutants, because its high concentration in air can be directly harmful to human health. It is also responsible for photochemical smog and acid rains. One of the most commonly used techniques to tackle this problem in large combustion plants is selective catalytic reduction (SCR). Commercial SCR installations are often equipped with a V2O5−WO3/TiO2 catalyst. In power plants which utilize a solid fuel boiler, catalysts are exposed to unfavorable conditions. In the paper, factors responsible for deactivation of such a catalyst are comprehensively reviewed where different types of deactivation mechanism, like mechanical, chemical or thermal mechanisms, are separately described. The paper presents the impact of sulfur trioxide and ammonia slip on the catalyst deactivation as well as the problem of ammonium bisulfate formation. The latter is one of the crucial factors influencing the loss of catalytic activity. The majority of issues with fast catalyst deactivation occur when the catalyst work in off-design conditions, in particular in too high or too low temperatures.

A Novel One-Step Hydrothermal Preparation of Ru/SnxTi1-xO2 Diesel Oxidation Catalysts and its Low-Temperature Performance

The rutile Sn_xTi_1-xO₂ (x = 0, 0.33, 0.5, 0.67, 1) solid solution was synthesized by a one-step hydrothermal method, in which tetrabutyl titanate and Tin (IV) chloride pentahydrate were used as raw materials. A series of Ru/Sn_xTi_1-xO₂ were then prepared by the impregnation process in RuCl₃ to investigate the performance and stability of CO and C₃H₈ oxidation. These catalysts were characterized through XRD, N₂ adsorption-desorption, FT-IR, TEM, XPS, H₂-TPR, and O₂-TPD techniques. The effect of Sn/Ti molar ratio and hydrothermal condition on the low-temperature catalytic oxidized performance and stability of Ru/Sn_xTi_1-xO₂ were investigated. The results indicated that Ru/Sn_0.67Ti_0.33O₂ catalyst showed an excellent activity and stability at low temperatures. The CO conversion reached 50% at 180 °C and 90% at 240 °C. Besides, the C₃H₈ conversion reached 50% at 320 °C, the complete conversion of C₃H₈ realized at 500 °C, and no deactivation occurs after 12 h of catalytic reaction. The excellent low-temperature activity and stability of the Ru/Sn_0.67Ti_0.33O₂ were attributed to the following factors. Firstly, XRD results showed that Sn⁴⁺ was successfully introduced into the lattice of TiO₂ to replace Ti⁴⁺ forming a homogeneous solid solution (containing -Sn⁴⁺-O-Ti⁴⁺- species), which was consistent with TEM and N₂ adsorption-desorption results. The introduction of Sn could suppress the growth of anatase crystal and promote the formation of rutile phase, and this phase transition was helpful to improve the low-temperature activity of the catalysts. Secondly, TEM images showed that ultrafine Ru nanoparticles (~ 5 nm) were dispersed on Sn_0.67Ti_0.33O₂ support, suggesting that the formation of Sn_xTi_1-xO₂ solid solution was beneficial to the dispersion of Ru particles.

WO3–V2O5 Active Oxides for NOx SCR by NH3: Preparation Methods, Catalysts’ Composition, and Deactivation Mechanism—A Review

Researchers in the field of the selective catalytic reduction (SCR) of nitrogen oxides (NOx: NO, NO2, or N2O) by NH3 are still greatly challenging the optimization of low-temperature activity and selectivity, high-temperature stability, resistance to alkali metals and other poisoning agents, such as Hg, As, etc. The present study reviews the research progress, related to the latest 20 years, on WO3–V2O5-based catalysts that are expected to overcome the catalytic performances of the current SCR catalytic devices. In details, the effects of the synthesis methods, chemical composition, type of supports (metal oxides, molecular sieves, and filters), doping elements, or metal oxides added as promoters of WO3–V2O5-based catalysts and, finally, the influence of SO2 and H2O in the reaction mixture are addressed. The importance of understanding the deactivation mechanism in the presence of several poisoning agents is also emphasized, which should be taken into consideration for the design of new catalysts.

Alkali-driven active site shift of fast SCR with NH3 on V2O5–WO3/TiO2 catalyst via a novel Eley–Rideal mechanism

The heterogeneous SCR reaction obeys the well-known Eley–Rideal mechanism or Langmuir–Hinshelwood mechanism, while fast SCR over alkali-doping catalysts follows the another “E–R” mechanism with adsorbed NO₂.

Identification of the arsenic resistance on MoO3 doped CeO2/TiO2 catalyst for selective catalytic reduction of NOx with ammonia

Arsenic resistance on MoO3 doped CeO2/TiO2 catalysts for selective catalytic reduction of NOx with NH3 (NH3-SCR) is investigated. It is found that the activity loss of CeO2-MoO3/TiO2 caused by As oxide is obvious less than that of CeO2/TiO2 catalysts. The fresh and poisoned catalysts are compared and analyzed using XRD, Raman, XPS, H2-TPR and in situ DRIFTS. The results manifest that the introduction of arsenic oxide to CeO2/TiO2 catalyst not only weakens BET surface area, surface acid sites and adsorbed NOx species, but also destroy the redox circle of Ce(4+) to Ce(3+) because of interaction between Ce and As. When MoO3 is added into CeO2/TiO2 system, the main SCR reaction path are found to be changed from the reaction between coordinated NH3 and ad-NOx species to that between an amide and gaseous NO. Additionally, for CeO2-MoO3/TiO2 catalyst, As toxic effect on active sites CeO2 can be released because of stronger As-Mo interaction. Moreover, not only are the reactable Brønsted and Lewis acid sites partly restored, but the cycle of Ce(4+) to Ce(3+) can also be free to some extent.

Promotional effect of doping SnO2 into TiO2 over a CeO2/TiO2 catalyst for selective catalytic reduction of NO by NH3

The promotional effect of SnO₂ in a Ce/TiO₂ catalyst for NH₃-SCR reaction.

Direct synthesis of V–W–Ti nanoparticle catalysts for selective catalytic reduction of NO with NH3

A series of V–W–Ti nanoparticle catalysts with variable V doping amounts were directly synthesized by the sol–gel method, and their catalytic performances were tested for the selective catalytic reduction of NO with ammonia.

Insight into deactivation of commercial SCR catalyst by arsenic: an experiment and DFT study

Fresh and arsenic-poisoned V2O5–WO3/TiO2 catalysts are investigated by experiments and DFT calculations for SCR activity and the deactivation mechanism. Poisoned catalyst (1.40% of arsenic) presents lower NO conversion and more N2O formation than fresh. Stream (5%) could further decrease the activity of poisoned catalyst above 350 °C. The deactivation is not attributed to the loss of surface area or phase transformation of TiO2 at a certain arsenic content, but due to the coverage of the V2O5 cluster and the decrease in the surface acidity: the number of Lewis acid sites and the stability of Brønsted acid sites. Large amounts of surface hydroxyl induced by H2O molecules provide more unreactive As–OH groups and give rise to a further decrease in the SCR activity. N2O is mainly from NH3 unselective oxidation at high temperatures since the reducibility of catalysts and the number of surface-active oxygens are improved by As2O5. Finally, the reaction pathway seems unchanged after poisoning: NH3 adsorbed on both Lewis and Brønsted acid sites is reactive.

Ammonium adsorption on Brønsted acidic centers on low-index vanadium pentoxide surfaces

Vanadium-based catalysts are used in many technological processes, among which the removal of nitrogen oxides (NO_x) from waste gases is one of the most important. The chemical reaction responsible for this selective catalytic reaction (SCR) is based on the reduction of NO_x molecules to N₂, and a possible reductant in this case is pre-adsorbed NH₃. In this paper, NH₃ adsorption on Brønsted OH acid centers on low-index surfaces of V₂O₅ (010, 100, 001) is studied using a theoretical DFT method with a gradient-corrected functional (RPBE) in the embedded cluster approximation model. The results of the calculations show that ammonia molecules are spontaneously stabilized on all low-index surfaces of the investigated catalyst, with adsorption energies ranging from −0.34 to −2 eV. Two different mechanisms of ammonia adsorption occur: the predominant mechanism involves the transfer of a proton from a surface OH group and the stabilization of ammonia as an NH₄⁺ cation bonded to surface O atom(s), while an alternative mechanism involves the hydrogen bonding of NH₃ to a surface OH moiety. The latter binding mode is present only in cases of stabilization over a doubly coordinated O(2) center at a (100) surface. The results of the calculations indicate that a nondirectional local electrostatic interaction with ammonia approaching a surface predetermines the mode of stabilization, whereas hydrogen-bonding interactions are the main force stabilizing the adsorbed ammonia. Utilizing the geometric features of the hydrogen bonds, the overall strength of these interactions was quantified and qualitatively correlated (R = 0.93) with the magnitude of the stabilization effect (i.e., the adsorption energy).

Figure

Design strategies for development of SCR catalyst: improvement of alkali poisoning resistance and novel regeneration method

Based on the ideas of the additives modification and regeneration method update, two different strategies were designed to deal with the traditional SCR catalyst poisoned by alkali metals. First, ceria doping on the V(2)O(5)-WO(3)/TiO(2) catalyst could promote the SCR performance even reducing the V loading, which resulted in the enhancement of the catalyst's alkali poisoning resistance. Then, a novel method, electrophoresis treatment, was employed to regenerate the alkali poisoned V(2)O(5)-WO(3)/TiO(2) catalyst. This novel technique could dramatically enhance the SCR activities of the alkali poisoned catalysts by removing approximately 95% K or Na ions from the catalyst and showed less hazardous to the environment. Finally, the deactivation mechanisms by the alkali metals were extensively studied by employing both the experimental and DFT theoretical approaches. Alkali atom mainly influences the active site V species rather than W oxides. The decrease of catalyst surface acidity might directly reduce the catalytic activity, while the reducibility of catalysts could be another important factor.