Research on SCR of NO with CO over the Cu0.1La0.1Ce0.8O mixed-oxide catalysts: Effect of the grinding

CuCeOx/CuO Catalyst Derived from the Layered Double Hydroxide Precursor: Catalytic Performance in NO Reduction with CO in the Presence of Water and Oxygen

Valencies of metal species and lattice defects, such as oxygen vacancies, play a pivotal role in metal oxide-catalyzed reactions. Herein, we report a promising synthetic strategy for preparing CuO-supported CuCeO_x catalysts (CuCeO_x/CuO) by calcination of a hydrotalcite precursor [Cu₆Ce₂(OH)₁₆]CO₃·nH₂O. The structural and chemical properties of catalysts were characterized by XRD, ICP-AES, TEM, TPR, NH₃-TPD, XPS, Raman spectroscopy, and N₂ adsorption, which revealed that the thermal pretreatment in an oxidative atmosphere caused segregation and reconstitution processes of the precursor, resulting in a mesoporous catalyst consisting of well-dispersed CuO-supported CuCeO_x clusters of 1.8-3.2 nm in size with a high population of oxygen vacancies. The as-prepared catalyst shows excellent catalytic performance in the reduction of NO by CO in the absence as well as in the presence of water and oxygen. This behavior is attributed to its high oxygen defect concentration facilitating the interplay of the redox equilibria between Cu²⁺ and reduced copper species (Cu⁺/Cu⁰) and (Ce⁴⁺/Ce³⁺). The high surface population of oxygen vacancies and in situ-generated metallic copper species have been evidenced by Raman spectroscopy and X-ray photoelectron spectroscopy. The layered double hydroxide-derived CuCeO_x/CuO also showed good water tolerance and long-term stability. In situ infrared spectroscopy investigations indicated that adsorbed hyponitrite species are the main reaction intermediates of the NO conversion as also corroborated by theoretical simulations.

Rationally tailored redox ability of Sn/γ-Al2O3 with Ag for enhancing the selective catalytic reduction of NO x with propene

The development of excellent selective catalytic reduction (SCR) catalysts with hydrocarbons for lean-burn diesel engines is of great significance, and a range of novel catalysts loaded with Sn and Ag were studied in this work. It was found that the synergistic effects of Sn and Ag enabled the 1Sn5Ag/γ-Al₂O₃ (1 wt% Sn and 5wt% Ag) to exhibit superior C₃H₆-SCR performance. The de-NO _x efficiency was maintained above 80% between 336 and 448 °C. The characterization results showed that the presence of AgCl crystallites in the 1Sn5Ag/γ-Al₂O₃ catalyst helped its redox ability maintain an appropriate level, which suppressed the over-oxidation of C₃H₆. Besides, the number of surface adsorbed oxygen (O_α) and hydroxyl groups (O_γ) were enriched, and their reactivity was greatly enhanced due to the coexistence of Ag and Sn. The ratio of Ag⁰/Ag⁺ was increased to 3.68 due to the electron transfer effects, much higher than that of Ag/γ-Al₂O₃ (2.15). Lewis acid sites dominated the C₃H₆-SCR reaction over the 1Sn5Ag/γ-Al₂O₃ catalyst. The synergistic effects of Sn and Ag facilitated the formation of intermediates such as acetates, enolic species, and nitrates, and inhibited the deep oxidation of C₃H₆ into CO₂, and the C₃H₆-SCR mechanism was carefully proposed.

Promotion effect of Ni doping on the oxygen resistance property of Fe/CeO2 catalyst for CO-SCR reaction: Activity test and mechanism investigation

Catalytic reduction of NO using CO, which is usually present in the flue gas of the iron and steel industry, is considered as an economical and eco-friendly de-NO_x method (CO-SCR). However, the oxygen present in the flue gas will significantly inhibit the CO-SCR activity of the catalyst, thereby limiting its industrial application. Herein, catalysts based on iron and cerium oxides were prepared and modified with different metals to investigate their performance for the CO-SCR reaction in the presence of oxygen. The results show that the Fe/CeO₂ catalyst can reach 99% NO conversion at 200 °C, but its activity decreased dramatically to 42.7% when the oxygen is present (0.5 vol%). By contrast, the NO conversion of Ni-doped Fe/CeO₂ catalyst demonstrated significant enhanced oxygen resistance and could achieve 92% even at 150 °C in the presence of 0.5 vol% oxygen. Characterization techniques such as N₂ adsorption, XRD, SEM/TEM, XPS, H₂-TPR, and in situ DRIFT were employed to investigate the mechanism of the improved oxygen resistance property of Ni-doped catalyst. The results show that the doped Ni can interact with Fe species, increases the BET surface area of the catalyst and generates more surface oxygen vacancies (SOV) and surface synergetic oxygen vacancy (SSOV) in CO-SCR reaction, thereby improving the redox performance of the catalyst. In situ DRIFT results show that the better redox performance of NiFe/CeO₂ catalyst is conducive to the conversion of adsorbed NO_x species to the reactive intermediate NO₂^- species during the reaction. Meanwhile, the enhanced SOV/SSOV in the NiFe/CeO₂ catalyst can remain active in the presence of oxygen. Therefore, the NiFe/CeO₂ catalyst exhibits a promising catalytic activity in CO-SCR reaction when oxygen is present.

Mechanism of Zn salt-induced deactivation of a Cu/activated carbon catalyst for low-temperature denitration via CO-SCR

In the process of industrial flue gas denitration, the presence of heavy metals, especially Zn salts, is known to lead to the deactivation of the denitration catalysts. However, the specific mechanism of the catalyst deactivation remains unclear. In this paper, the mechanism of the ZnCl₂- and ZnSO₄-induced deactivation of low-temperature denitration catalysts in the carbon oxide (CO) selective catalytic reduction (CO-SCR) reaction was investigated using a Cu/activated carbon (AC) catalyst, in which HNO₃/AC was used as the carrier. Cu/AC, ZnCl₂–Cu/AC, and ZnSO₄–Cu/AC catalysts were prepared by the incipient wetness impregnation method. The physicochemical properties of the catalyst were examined via the Brunauer–Emmett–Teller method, X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy analyses, which proved the mechanism of catalyst denitrification and enabled the elucidation of the toxicity mechanism of the Zn salts on the Cu/AC catalyst for CO-SCR denitration at low temperatures. The results show that Zn doping reduces the physical adsorption of CO and NO and decreases the concentration of Cu²⁺ and chemisorbed oxygen (O_β), leading to the reduction of active sites and oxygen vacancies, thus inhibiting the denitration reaction. Moreover, ZnCl₂ is more toxic than ZnSO₄ because Cl⁻ not only occupies oxygen vacancies but also inhibits O_β migration. In contrast, SO₄²⁻ increases the surface acidity and promotes O_β supplementation. This study can provide a reference for the development of CO-SCR denitration catalysts with high resistance to Zn salt poisoning.

Zn slats compete with CO and NO for the active sites. Cl⁻ not only occupies oxygen vacancies but also inhibits the O_β migration. SO₄²⁻ increases the surface acidity and promotes the O_β supplementation, which inhibits toxicity.

CuCeOx/VMT powder and monolithic catalyst for CO-selective catalytic reduction of NO with CO

The reaction path of a CuCe/VMT(M) catalyst in the CO-SCR reaction at N2O low temperature was found.

A promising catalytic solution of NO reduction by CO using g-C3N4/TiO2: A DFT study

The direct catalytic reduction of nitric oxide (NO) by carbon monoxide (CO) to form harmless N₂ and CO₂ is an ideal strategy to simultaneously remove both these hazardous gases. To investigate the feasibility of using graphitic carbon nitride/titanium dioxide (g-C₃N₄/TiO₂) to catalyze the NO reduction by CO, we systematically explore the effect of the interfacial coupling between g-C₃N₄ and TiO₂ on the photo-induced carrier separation, the light absorption, and the surface reaction for the NO reduction by using density functional theory. The g-C₃N₄/TiO₂ is predicted to have a better photocatalytic activity for NO reduction than g-C₃N₄, due to the enhanced light absorption intensity and the accelerated separation of the photo-excited electron-hole pairs. By comparing the reaction routes on g-C₃N₄/TiO₂ and g-C₃N₄, the results indicate that the introduction of TiO₂ can keep the surface reaction process intact with the NO dissociation (N₂O formation) being the rate-determining (crucial) step. Moreover, TiO₂ can facilitate the desorption of NO reduction products, avoiding the deactivation of g-C₃N₄. This work shows that the composition of TiO₂ into g-C₃N₄ provides a promising catalyst in NO reduction by CO.

NO reduction with CO over a highly dispersed Mn/TiO2catalyst at low temperature: a combined experimental and theoretical study

A highly dispersed Mn/TiO₂catalyst, which has high efficiency for NO conversion with CO and almost completed N₂selectivity at a low-temperature range (350-550 K), was investigated using experimental and DFT theoretical calculation. The characterization results illustrated that the catalyst assembled with nanoparticles and the Mn doping into the TiO₂surface lattice led to the formation of Mn-O-Ti configuration, which enhanced the dispersion of Mn on the body of TiO₂. The DFT study mapped out the complete catalytic cycle, including reactants adsorption, oxygen vacancy generation, N₂O intermediates formation, N₂formation in Eley-Rideal (ER), Langmuir-Hinshelwood, and termolecular Eley-Rideal mechanisms. With thermodynamic and kinetic analysis combined with experimental results, the ER reaction process was considered to be the fundamental mechanism over the highly dispersed Mn/TiO₂catalyst. The calculation results indicated that N₂O was a significant intermediate. However, the rapid N₂O reduction process led to high N₂selectivity. The rate-limiting step was the deoxygenation step of NO-MnO_v/TiO₂from N-O bond scission. The active site Mn-O_vpair embedded in Mn/TiO₂was responsible not only for the formation of N-Mn/TiO₂in the ER-1 step but also for the N₂O deoxygenation process to make the final product N₂in the ER-2 step. The synergetic effect between Mn 3d electron and the oxygen vacancy of TiO₂were responsible for the catalytic activity of Mn/TiO₂.

Promotional Effect of Manganese on Selective Catalytic Reduction of NO by CO in the Presence of Excess O2 over M@La–Fe/AC (M = Mn, Ce) Catalyst

The catalytic performance of a series of La-Fe/AC catalysts was studied for the selective catalytic reduction (SCR) of NO by CO. With the increase in La content, the Fe2+/Fe3+ ratio and amount of surface oxygen vacancies (SOV) in the catalysts increased; thus the catalytic activity improved. Incorporating the promoters to La3-Fe1/active carbon (AC) catalyst could affect the catalyst activity by changing the electronic structure. The increase in Fe2+/Fe3+ ratio after the promoter addition is possibly due to the extra synergistic interaction of M (Mn and Ce) and Fe through the redox equilibrium of M3+ + Fe3+ ↔ M4+ + Fe2+. This phenomenon could have improved the redox cycle, enhanced the SOV formation, facilitated NO decomposition, and accelerated the CO-SCR process. The presence of O2 enhanced the formation of the C(O) complex and improved the activation of the metal site. Mn@La3-Fe1/AC catalyst revealed an excellent NO conversion of 93.8% at 400 °C in the presence of 10% oxygen. The high catalytic performance of MnOx and double exchange behavior of Mn3+ and Mn4+ can increase the number of SOV and improve the catalytic redox properties.

Technologies for the nitrogen oxides reduction from flue gas: A review

The required energy of the global industry is mostly generated from fossil fuel sources, such as natural gas, gasoline, diesel, oil, and coal. Nitrogen oxides are one of the main air pollutants that are produced from the combustion of fossil fuels in stationary and mobile sources. Development of new technologies to decrease the NO_x emission from exhaust gases is essential due to the harmful effect of NO_x on the environment and human health. Compared with pre-combustion and combustion methods (with <50% NO_x removal efficiency), the post-combustion methods with higher efficiency (above 80%) have attracted more attention in NO_x elimination. This review describes the currently used technologies of NO_x abatement. Different available post-combustion methods of NO_x removal, including selective catalytic reduction (using different types of reducing reagents, including ammonia, hydrogen, hydrocarbons, and carbon monoxide), selective noncatalytic reduction, wet scrubbing, adsorption, electron beam, nonthermal plasma, and electrochemical reduction of NO_x, are discussed.

A Critical Review of Recent Progress and Perspective in Practical Denitration Application

Nitrogen oxides (NOx) represent one of the main sources of haze and pollution of the atmosphere as well as the causes of photochemical smog and acid rain. Furthermore, it poses a serious threat to human health. With the increasing emission of NOx, it is urgent to control NOx. According to the different mechanisms of NOx removal methods, this paper elaborated on the adsorption method represented by activated carbon adsorption, analyzed the oxidation method represented by Fenton oxidation, discussed the reduction method represented by selective catalytic reduction, and summarized the plasma method represented by plasma-modified catalyst to remove NOx. At the same time, the current research status and existing problems of different NOx removal technologies were revealed and the future development prospects were forecasted.

Mn-Modified CuO, CuFe2O4, and γ-Fe2O3 Three-Phase Strong Synergistic Coexistence Catalyst System for NO Reduction by CO with a Wider Active Window

A series of samples with the precursor's molar ratio of {KMn₈O₁₆}/{CuFe₂O₄} = 0, 0.008, 0.010, 0.016, and 0.020 were successfully synthesized for selective catalytic reduction of NO by CO. The physicochemical properties of all samples were studied in detail by combining the means of X-ray photoelectron spectroscopy, H₂-temperature-programmed reduction, scanning electron microscopy mapping, X-ray diffraction (XRD), N₂ physisorption (Brunauer-Emmett-Teller), NO + CO model reaction, and in situ Fourier transform infrared spectroscopy techniques. The results show that three phases of γ-Fe₂O₃, CuFe₂O₄, and CuO, which have strong synergistic interaction, coexist in this catalyst system, and different phases play a leading role in different temperature ranges. Mn species are highly dispersed in the three-phase coexisting system in the form of Mn²⁺, Mn³⁺, and Mn⁴⁺. Because of the strong interaction between Mn²⁺ and Fe species, a small amount of Cu²⁺ precipitates from CuFe₂O₄ and grows along the CuO(110) plane, which has better catalytic performance. Mn³⁺ can inhibit the conversion of γ-Fe₂O₃ to α-Fe₂O₃ at high temperature and then increases the high-temperature activity. The synergistic effect between Mn⁴⁺ and the surfaces of three phases generates active oxygen species Cu²⁺-O-Mn⁴⁺ and Mn⁴⁺-O-Fe³⁺, which can be more easily reduced to some synergistic oxygen vacancies during the reaction. Furthermore, the formed synergistic oxygen vacancies can promote the dissociation of NO and are also propitious to the transfer of oxygen species. All of these factors make the appropriate manganese-modified three-phase coexisting system have better catalytic activity than the manganese-free catalyst, making NO conversion rate reach 100% at around 250 °C and maintain to 1000 °C. Combining comprehensive analysis of various characterization results and in situ infrared as well as XRD results in the equilibrium state, a new possible NO + CO model reaction mechanism was temporarily proposed to further understand the catalytic processes.