Supercritical water syntheses of transition metal-doped CeO2 nano-catalysts for selective catalytic reduction of NO by CO: An in situ diffuse reflectance Fourier transform infrared spectroscopy study

Mechanisms for Modifying the Electronic and Spatial Distribution of the Single-Atom Ni/CeO2 Surface to Enhance CO-SCR Reactivity: Density Functional Theory Study

Modulating the intrinsic activity of heterogeneous catalysts at the atomic level is an effective strategy to improve the low-temperature CO-SCR (selective catalytic reduction) reaction activity and N2 selectivity, but it remains challenging by the experiment. In this paper, a single-atom-loaded surface generation strategy is developed to construct single-atom catalysts by density functional theory analysis, which will effectively reduce the reaction energy barriers in CO-SCR reaction. Specifically, the reaction of NO reduction by CO before and after Ni adsorption was thoroughly investigated and the reactivity was evaluated by using the CeO2 (1 1 1) surface as a carrier, with the application of density functional theory, electronic structure analysis, and transition state theory. The loading of Ni increases the energy barrier for the generation of N2O on the CeO2 (1 1 1) surface by 1.498 eV and decreases the energy barrier for the generation of N2 by 1.864 eV. This indicates that the adsorption of Ni inhibits the generation of N2O and promotes the generation of N2. After thermodynamics and kinetics analysis, the pathway of CeO2 (1 1 1)-Ot-Ni via O atoms filling O vacancies to generate N2 is a spontaneous unidirectional reaction when no nonvolumetric work is done at constant temperature and pressure. Theoretical calculations show that the modification of isolated Ni atoms on CeO2 induces electronic coupling and redistribution, which leads to the activation of neighboring O sites around Ni atoms. This study provides the strategy mechanism to enhance the activity and N2 selectivity of the low-temperature CO-SCR reaction at the atomic level and provides theoretical guidance for the theory of novel catalysts for synergistic removal of NO and CO.

Catalytic removal of gaseous pollutant NO using CO: Catalyst structure and reaction mechanism

Carbon monoxide (CO) has recently been considered an ideal reducing agent to replace NH3 in selective catalytic reduction of NOx (NH3-SCR). This shift is particularly relevant in diesel engines, coal-fired industry, the iron and steel industry, of which generate substantial amounts of CO due to incomplete combustion. Developing high-performance catalysts remain a critical challenge for commercializing this technology. The active sites on catalyst surface play a crucial role in the various microscopic reaction steps of this reaction. This work provides a comprehensive overview and insights into the reaction mechanism of active sites on transition metal- and noble metal-based catalysts, including the types of intermediates and active sites, as well as the conversion mechanism of active molecules or atoms. In addition, the effects of factors such as O2, SO2, and alkali metals, on NO reduction by CO were discussed, and the prospects for catalyst design are proposed. It is hoped to provide theoretical guidance for the rational design of efficient CO selective catalytic denitration materials based on the structure-activity relations.

Selective catalytic reduction of NO by NH3 using Mn-based catalysts supported by Ukrainian clinoptiolite and lightweight expanded clay aggregate

In this study, Mn-based multicomponent catalysts supported by two different carriers (lightweight expanded clay aggregate and the Ukrainian clinoptiolite) were prepared by electroless metal deposition method and tested for the selective catalytic reduction of NO with ammonia (NH₃-SCR de-NO). Prior to the activity test, all the catalysts prepared were characterized by inductively coupled plasma optical emission spectroscopy, field emission scanning electron microscopy (FESEM), energy dispersive X-ray mapping, X-ray photoelectron spectroscopy, H₂-TPR and NH₃-TPD techniques. The particular interest of the present study was focused on the investigation of the carrier's role in the NO catalytic reduction and the promoting effect provided by the incorporation of the small amount of Pt (0.1 wt.%) in the Mn-based catalytic layer. The results revealed that the carrier's role in the NO catalytic conversion can be considered as a factor determining the effectiveness of the conversion process. Ukrainian clinoptiolite was proved to be a more attractive carrier for the preparation of the effective SCR de-NO catalysts due to its intrinsic sorption capacity, surface acidity and the redox potential. The high NO conversion efficiency provided by the Mn-based clinoptiolite-supported catalysts can be explained by the synergistic effect between the carrier and the active species deposited. It was shown that both the Mn_97.6Cu_2.4/clinoptiolite and the Mn_97.5Co_2.5/clinoptiolite catalysts can be successfully applied as the low-temperature (100-300°C) catalysts for NH₃-SCR de-NO. When the NO removal efficiency varies in the range of 86-91%, the additional incorporation of Pt in the active layer in the amount of 0.1 wt.% can enhance the NO reduction by about 5% on average.

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.

Investigation of the NO reduction by CO reaction over oxidized and reduced NiOx/CeO2 catalysts

NO reduction by CO reaction was investigated by NiOx/CeO2 catalysts with different pretreatment conditions. Surface area, oxygen defect sites, and CeO2 crystallite size are closely related to the catalytic performance.

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.

Passive Nitrogen Oxides Removal from a Diesel-engine Exhaust Gas using a Biomass-Carbon Catalyst

Nitrogen oxides (NOx) removal from a diesel-engine exhaust gas is limited to the utilization of ammonia/urea as a reducing agent (SCR) which arose environmental concerns over the use of this chemical. Therefore, this study explored the potential of a sustainable NOx removal system by replacing ammonia with intrinsic reductants present in the exhaust gas such as hydrocarbons and carbon monoxide, and by application of cost-effective carbon-supported transitional metals catalyst. Copper-cerium catalyst supported over palm kernel shell activated carbon (Cu-Ce/PKS) was synthesized via deposition-precipitation method. The characterization shows that the catalyst has a considerably high surface area (though lower than the support). The high NOx removal by Cu-Ce/PKS in a passive catalytic reaction is attributed to the surface area provided by the carbon support, the low copper reducibility giving the low optimum operating temperature, and the synergistic effect between Cu and Ce resulting in the wide temperature window at low-temperature range. It is concluded that Cu-Ce supported over palm kernel shell activated carbon can be further developed to reduce NOx in a passive catalytic removal for a sustainable and cost-effective SCR system.  

Effects of Ni loading on the physicochemical properties of NiOx/CeO2 catalysts and catalytic activity for NO reduction by CO

The NO reduction by CO reaction was investigated using NiO_x/CeO₂ catalysts with different Ni loadings. Surface NiO_x controls the catalytic activity which was related to the molecular structure and reducibility of the catalysts.

The influence of support composition on the activity of Cu:Ce catalysts for selective catalytic reduction of NO by CO in the presence of excess oxygen

Excellent catalytic performance for NO reduction by CO in the presence of 5% O₂ over Cu1:Ce3/Al₂O₃.

Catalytic NO reduction by CO over ceria–cobalt oxide catalysts

The effectiveness of the tandem catalysts was verified by a combination of Co₉₈Ce₂ for low temperature reactions and Co₁₅Ce₈₅ for high temperature reactions.