A Review of Mn-Based Catalysts for Abating NOx and CO in Low-Temperature Flue Gas: Performance and Mechanisms

Mn-based catalysts have attracted significant attention in the field of catalytic research, particularly in NOx catalytic reductions and CO catalytic oxidation, owing to their good catalytic activity at low temperatures. In this review, we summarize the recent progress of Mn-based catalysts for the removal of NOx and CO. The effects of crystallinity, valence states, morphology, and active component dispersion on the catalytic performance of Mn-based catalysts are thoroughly reviewed. This review delves into the reaction mechanisms of Mn-based catalysts for NOx reduction, CO oxidation, and the simultaneous removal of NOx and CO. Finally, according to the catalytic performance of Mn-based catalysts and the challenges faced, a possible perspective and direction for Mn-based catalysts for abating NOx and CO is proposed. And we expect that this review can serve as a reference for the catalytic treatment of NOx and CO in future studies and applications.

Spectroscopy and photochemistry of copper nitrate clusters

The investigation of copper nitrate cluster anions Cu(ii)n(NO3)2n+1-, n ≤ 4, in the gas phase using ultraviolet/visible/near-infrared (UV/vis/NIR) spectroscopy provides detailed insight into the electronic structure of the copper salt and its intriguing photochemistry. In the experimentally studied region up to 5.5 eV, the spectra of copper(ii) nitrate exhibit a 3d-3d band in the vis/NIR and well-separated bands in the UV. The latter bands originate from Ligand-to-Metal Charge Transfer (LMCT) as well as n-π* transitions in the nitrate ligands. The clusters predominantly decompose by loss of neutral copper nitrate in the electronic ground state after internal conversion or via the photochemical loss of a neutral NO3 ligand after a LMCT. These two decomposition channels are in direct competition on the ground state potential energy surface for the smallest copper nitrate cluster, Cu(ii)(NO3)3-. Here, copper nitrate evaporation is thermochemically less favorable. Population of π* orbitals in the nitrate ligands may lead to N-O bond photolysis. This is observed in the UV region with a small quantum efficiency, with photochemical loss of either nitrogen dioxide or an oxygen atom.

Decomposition of Copper Formate Clusters: Insight into Elementary Steps of Calcination and Carbon Dioxide Activation

The decomposition of copper formate clusters is investigated in the gas phase by infrared multiple photon dissociation of Cu(II) n (HCO2)2n+1 -, n≤8. In combination with quantum chemical calculations and reactivity measurements using oxygen, elementary steps of the decomposition of copper formate are characterized, which play a key role during calcination as well as for the function of copper hydride based catalysts. The decomposition of larger clusters (n >2) takes place exclusively by the sequential loss of neutral copper formate units Cu(II)(HCO2)2 or Cu(II)2(HCO2)4, leading to clusters with n=1 or n=2. Only for these small clusters, redox reactions are observed as discussed in detail previously, including the formation of formic acid or loss of hydrogen atoms, leading to a variety of Cu(I) complexes. The stoichiometric monovalent copper formate clusters Cu(I) m (HCO2) m+1 -, (m=1,2) decompose exclusively by decarboxylation, leading towards copper hydrides in oxidation state +I. Copper oxide centers are obtained via reactions of molecular oxygen with copper hydride centers, species containing carbon dioxide radical anions as ligands or a Cu(0) center. However, stoichiometric copper(I) and copper(II) formate Cu(I)(HCO2)2 - and Cu(II)(HCO2)3 -, respectively, is unreactive towards oxygen.

Choice of precipitant and calcination temperature of precursor for synthesis of NiCo2O4 for control of CO-CH4 emissions from CNG vehicles

Compressed natural gas (CNG) is most appropriate an alternative of conventional fuel for automobiles. However, emissions of carbon-monoxide and methane from such vehicles adversely affect human health and environment. Consequently, to abate emissions from CNG vehicles, development of highly efficient and inexpensive catalysts is necessary. Thus, the present work attempts to scan the effects of precipitants (Na₂CO₃, KOH and urea) for nickel cobaltite (NiCo₂O₄) catalysts prepared by co-precipitation from nitrate solutions and calcined in a lean CO-air mixture at 400°C. The catalysts were used for oxidation of a mixture of CO and CH₄ (1:1). The catalysts were characterized by X-ray diffractometer, Brunauer-Emmett-Teller surface-area, X-ray photoelectron spectroscopy; temperature programmed reduction and Scanning electron microscopy coupled with Energy-Dispersive X-Ray Spectroscopy. The Na₂CO₃ was adjudged as the best precipitant for production of catalyst, which completely oxidized CO-CH₄ mixture at the lowest temperature (T₁₀₀=350°C). Whereas, for catalyst prepared using urea, T₁₀₀=362°C. On the other hand the conversion of CO-CH₄ mixture over the catalyst synthesized by KOH limited to 97% even beyond 400°C. Further, the effect of higher calcination temperatures of 500 and 600°C was examined for the best catalyst. The total oxidation of the mixture was attained at higher temperatures of 375 and 410°C over catalysts calcined at 500 and 600°C respectively. Thus, the best precipitant established was Na₂CO₃ and the optimum calcination temperature of 400°C was found to synthesize the NiCo₂O₄ catalyst for the best performance in CO-CH₄ oxidation.

Synthesis of K–Pd doped NiCo2O4−δ by reactive calcination route for oxidation of CO–CH4 emissions from CNG vehicles

The present study is devoted to formulating a doped spinel catalyst by a novel route of calcination for the oxidation of the CO–CH₄ mixture emitted from compressed natural gas (CNG) vehicles.

Preparation and application of effective different catalysts for simultaneous control of diesel soot and NOX emissions: An overview

Soot particulates and nitrogen oxides (NO_X) from diesel engine exhaust have been causing serious problems to human health and the global environment.

Reflectometry-Ellipsometry Reveals Thickness, Growth Rate, and Phase Composition in Oxidation of Copper

The oxidation of copper is a complicated process. Copper oxide develops two stable phases at room temperature and standard pressure (RTSP): cuprous oxide (Cu2O) and cupric oxide (CuO). Both phases have different optical and electrical characteristics that make them interesting for applications such as solar cells or resistive switching devices. For a given application, it is necessary to selectively control oxide thickness and cupric/cuprous oxide phase volume fraction. The thickness and composition of a copper oxide film growing on the surface of copper widely depend on the characteristics of as-deposited copper. In this Research Article, two samples, copper films prepared by two different deposition techniques, electron-beam evaporation and sputtering, were studied. As the core part of the study, the formation of the oxidized copper was analyzed routinely over a period of 253 days using spectroscopic polarized reflectometry-spectroscopic ellipsometry (RE). An effective medium approximation (EMA) model was used to fit the RE data. The RE measurements were complemented and validated by using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and X-ray diffraction (XRD). Our results show that the two samples oxidized under identical laboratory ambient conditions (RTSP, 87% average relative humidity) developed unique oxide films following an inverse-logarithmic growth rate with thickness and composition different from each other over time. Discussion is focused on the ability of RE to simultaneously extract thickness (i.e., growth rate) and composition of copper oxide films and on plausible physical mechanisms responsible for unique oxidation habits observed in the two copper samples. It appears that extended surface characteristics (i.e., surface roughness and grain boundaries) and preferential crystalline orientation of as-deposited polycrystalline copper films control the growth kinetics of the copper oxide film. Analysis based on a noncontact and nondestructive measurement, such as RE, to extract key material parameters is beneficial for conveniently understanding the oxidation process that would ultimately enable copper oxide-based devices at manufacturing scales.

Enhanced visible photocatalytic activity of Cu2O nanocrystal/titanate nanobelt heterojunctions by a self-assembly process

Photodegradation curves of Cu₂O/titanate heterojunctions under visible light irradiation.