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氮氧化物(NO_x)是主要的大气污染物之一,给环境和人类健康带来巨大危害.NH_3选择性催化还原(NH_3-SCR)技术是处理氮氧化物最有效的方法之一,现已被广泛用于氮氧化物的处理,其中SCR催化剂是该技术的核心.锰基催化剂具有相对较好的低温催化活性,而其中MnO_2的催化活性最为突出.二氧化锰可以形成多种晶型,如α,β,δ和γ.晶相结构可显著影响催化活性,不同晶型的MnO_2具有不同的催化性能,其在电化学催化和CO催化方面已经得到广泛应用和研究.然而对于不同晶型MnO_2的NH_3低温选择催化还原NO_x的催化活性和催化机理的研究特别少.α-,β-,γ-和δ-MnO_2具有不同的晶型尺寸,并在气体吸附、气体扩散和催化反应过程中表现出较大差异.因此,有必要揭示不同MnO_2晶型表面物理化学性质与SCR催化活性之间的关系.本文成功制备出四种不同晶型的纳米MnO_2(α,β,δ和γ型)催化剂,并测试了其NH_3低温选择催化活性.此外,对催化剂进行了XRD,SEM,Raman,TG,BET,NH_3-TPD,XPS和H_2-TPR等一系列测试表征,探究了导致不同晶型MnO_2的NH_3催化还原NO_x差异的主要原因.结果表明,四种不同晶型纳米MnO_2的NH_3-SCR催化活性顺序为γ-MnO_2>α-MnO_2>δ-MnO_2>β-MnO_2.γ-MnO_2和α-MnO_2上的NO_x转化率在140–200°C范围内可达90%以上,但β-MnO_2在200°C只有40%的NO_x转化率.分析表明,γ-MnO_2和α-MnO_2的纳米线形貌具有较好的分散性,导致它们的比表面积较高,这为活性位点提供了更多的分散空间,有利于活性气体分子的吸附和催化过程的进行.H_2-TPR和NH_3-TPD结果显示,γ-MnO_2和α-MnO_2比其他两种晶型的MnO_2催化剂具有较好的可还原性和更多更强的酸性位点.XPS分析结果表明,在γ-MnO_2和α-MnO_2表面存在更多的化学吸附氧.基于上述这些有利的物理化学性质,γ-MnO_2和α-MnO_2催化剂显示出较好的低温NH_3-SCR活性.尽管δ-MnO_2的比表面积较小,但是NH_3-TPD结果显示其层状形貌的纳米晶体表面分散有较多的酸性位点,从而有效提升了其催化效果,同时其较好的氧化还原性能也有助于催化性能的改善.
Nitrogen oxides (NO x) are one of the major air pollutants and cause great harm to environment and human health.NH_3 selective catalytic reduction (NH_3-SCR) is one of the most effective methods to treat nitrogen oxides, Which is widely used in the treatment of nitrogen oxides, of which SCR catalyst is the core of the technology.Many-based catalysts have relatively good low-temperature catalytic activity, of which MnO 2 catalytic activity is the most prominent.Many manganese dioxide can form a variety of crystal forms, Such as α, β, δ and γ. The crystalline structure of MnO 2 has different catalytic properties, and its catalytic activity has been widely studied and applied in electrochemical catalysis and CO catalysis. However, The catalytic activity and catalytic mechanism of MnO_2 for NH_3 selective reduction of NO_x are studied very little.α-, β-, γ- and δ-MnO_2 have different crystal sizes, and in the process of gas adsorption, gas diffusion and catalysis Therefore, it is necessary to reveal the relationship between the physical and chemical properties of different MnO 2 crystal surfaces and the SCR catalytic activity.In this paper, four kinds of MnO 2 (α, β, δ and γ) Catalyst, and In addition, a series of tests were carried out to characterize the catalytic activity of NH_3, such as XRD, SEM, Raman, TG, BET, NH_3-TPD, XPS and H_2- The main reason for the reduction of NO_x is that the order of catalytic activity of NH_3-SCR for the four different MnO_2 nanocrystals is γ-MnO 2> α-MnO 2> δ -MnO 2> β -MnO 2 .γ-MnO 2 andα-MnO 2 The conversion of NO_x can reach more than 90% in the range of 140-200 ° C, but only 40% of the NO_x conversion of β-MnO_2 at 200 ° C. The results show that the nanowire morphology of γ-MnO_2 and α-MnO_2 Good dispersibility, resulting in their higher specific surface area, which provides more space for the active site dispersion, is conducive to the adsorption of active gas molecules and the catalytic process.H_2-TPR and NH_3-TPD results show that γ MnO 2 and α-MnO 2 have better reducibility and more acidic sites than the other two MnO 2 catalysts.The results of XPS analysis show that more MnO 2 and α-MnO 2 on the surface of γ-MnO 2 and α-MnO 2 Chemically adsorbed oxygen Based on these favorable physicochemical properties described above, the γ-MnO 2 and α-MnO 2 catalysts show better low-temperature NH 3-SCR Although the specific surface area of δ-MnO 2 is small, NH 3 -TPD results show that more acidic sites are dispersed on the nanocrystalline surface of the layered morphology, which effectively enhances the catalytic effect, while its better oxidation The reduction performance also contributes to the improvement of the catalytic performance.