In situ IR and temperature programmed desorption-mass spectrometry study of NO absorption and decomposition by silica supported 12-tungstophosphoric acid

Purification Technologies for NOx Removal from Flue Gas: A Review

Nitrogen oxide (NOx) is a major gaseous pollutant in flue gases from power plants, industrial processes, and waste incineration that can have adverse impacts on the environment and human health. Many denitrification (de-NOx) technologies have been developed to reduce NOx emissions in the past several decades. This paper provides a review of the recent literature on NOx post-combustion purification methods with different reagents. From the perspective of changes in the valence of nitrogen (N), purification technologies against NOx in flue gas are classified into three approaches: oxidation, reduction, and adsorption/absorption. The removal processes, mechanisms, and influencing factors of each method are systematically reviewed. In addition, the main challenges and potential breakthroughs of each method are discussed in detail and possible directions for future research activities are proposed. This review provides a fundamental and systematic understanding of the mechanisms of denitrification from flue gas and can help researchers select high-performance and cost-effective methods.

Germanium-based polyoxometalates for the adsorption-decomposition of NOx

A series of germanium-based Keggin-type polyoxometalates (POMs), including H₄GeW₁₂O₄₀ (HGeW), H₅GeW₁₁VO₄₀ (HGeWV), H₅GeMo₁₁VO₄₀ (HGeMoV) and H₅GeW₉Mo₂VO₄₀ HGeWMoV), were first synthesized and utilized as catalysts for removal of NOx. The fourier transform infrared spectroscopy (FT-IR) and temperature-programmed desorption-mass spectroscopy (TPD-MS) characterizations were carried out to investigate the adsorption-desorption behavior and decomposition mechanism of NOx. The adsorption experiments revealed that HGeW had the highest NOx adsorption capacity up to 16.2 mg NOx per gram catalyst. Two infrared characteristic bands appeared at 2210 and 1851 cm^-1 after NOx adsorption, while the latter firstly discovered over NOx-absorbed POMs could be assigned to nitrosyl radical (NO·). The regeneration of HGeW could be realized via reducing the temperature in wet atmosphere to desorb NOx. TPD-MS was conducted to investigate the decomposition behavior of NOx over HGeW and tungstophosphoric acid (HPW). O₂ was firstly detected among the decomposition products, besides N₂ and N₂O. According to conservation calculation based on N mass, NOx removal rate of 81.5 % and N₂ selectivity of 68.3 % could be achieved for HGeW. While the NOx conversion rate and N₂ selectivity for HPW reached 54.1 % and 53.4 %, respectively.

Surface acidity enhancement of CeO2 catalysts via modification with a heteropoly acid for the selective catalytic reduction of NO with ammonia

The surface acidity enhanced CeO₂ catalysts exhibited a SCR reaction path that both mechanisms existed but E-R dominated.

CARBON NANOTUBES AND NANO-Ce–Zr OXIDES SUPPORTED H3PW12O40 FOR EFFECTIVE ADSORPTION–DECOMPOSITION OF NOx

Surface-modified carbon nanotubes (CNTs) and nano- Ce–Zr mixed oxides (CZO) were prepared and employed initially as supports of H3PW12O40 (HPW) for NO x adsorption–decomposition. Both CNTs and nano-CZO are favorable supports for HPW. After loading with HPW, the NO x adsorption efficiency increases, especially for HPW/CZO in which the highest adsorption efficiency can achieve 98% at the HPW loading of 70%, much higher than that of single HPW. NO x adsorption efficiency can be influenced considerably by catalyst preparing conditions, in particularly, ethyl alcohol is superior to water as solvent for HPW loading onto CNTs; the –OH containing CNTs shows better promotion effect on the adsorption of NO x than that containing –COOH when using absolute ethyl alcohol as solvent; mechanical grinding method is superior to incipient impregnation method in loading HPW onto the support of CZO. For both catalysts of HPW/CNTs and HPW/CZO, with the increase of HPW loading, the NO x adsorption efficiency tends to reach a peak value before dropping down. Heated from 150°C to 450°C at a rate of 50°C/min, the adsorbed NO was found to decompose into N2 , O2 and N2O , and yields of N2 being 21.8% and 27.3%, respectively for HPW/CNTs and HPW/CZO were obtained.

Pollution by nitrogen oxides: an approach to NO(x) abatement by using sorbing catalytic materials

This article summarises the abatement of NO(x) pollution by using sorbing catalytic materials with special relevance to the challenge presented in fixed installations sources. A general vision of the origins of the different pollutants, with emphasis on nitrogen oxides formation, is presented as introduction. The impact of NO(x) pollution comprises additionally a quick view of its toxicity and environmental effects. Actual solutions are presented especially the case of the selective catalytic reduction (SCR) process with its advantages and difficulties. The new concepts for NO(x) abatement are also analysed. In such a way, updated information on solid sorbents for NO(x) removal is provided by including metal oxides, spinelles, perovskites, double-layered cuprates, zeolites, carbonaceous materials, heteropolyacids (HPAs), and supported heteropolyacids. The possibility of reducing those sorbed NO(x) is also underlined. Sorption mechanisms are analysed and clarified by emphasising convergence and disagreement points.

Removal of NOx from lean exhaust gas by storage/reduction on H3PW12O40 x 6H2O supported on Ce(x)Zr4-xO8

We have demonstrated the interest of using Zr-Ce mixed oxides as supports for the system H3PW12O40 x 6H2O (HPW) and Pt for the storage (absorption into HPW and adsorption on Zr-Ce support)-reduction of NOx. Zr-Ce oxides adsorb NOx (mainly NO2) as nitrates, which are desorbed thermally as NO2 and NO (12 mg of NOx x g(-1)). On (HPW-Pt)/Zr-Ce system, NO and NO2 are stored equimolarly by substitution of water molecules of the HPW structure. The Zr/Ce support molar ratio has an important influence on the quantity of NOx stored with a maximum for the system Zr/Ce = 0.5. The presence of Pt has no influence on storage capacity but rather on its efficiency, and it is essential when reducing. Pt strongly accelerates both NOx desorption and reduction processes. Reduction of NOx occurs with H2, CO, or an H2/CO mixture. With H2, NO2 is reduced to NO. With CO, it seems reasonable that the mechanism of NO2 reduction concerns its reaction with CO to produce CO2 and NO. Addition of H2 to CO accelerates the reduction process. However, the difference between NOx stored and desorbed implies also the presence of another reduction step. This could be related to an NO interaction with an active site which produces its decomposition with nitrogen as product. The catalytic system proposed is highly selective since no N2O formation was detected.