N2气氛下焙烧制备的Mn基催化剂催化NOx脱除性能的提升机理:低MnOx结晶度与氧化度

在众多的氧化物类NH3-SCR催化剂体系中,Mn基氧化物催化剂因具有极高的低温(≤473 K)脱硝性能而备受关注.其主要原因可能是Mn物种具有丰富的可变价态,作为活性组分的MnOx能够提供自由电子.大量研究发现,由于不同金属元素间协同作用的存在,复合金属氧化物的催化脱硝活性普遍优于单金属氧化物类催化剂.为了抑制MnOx在锻烧过程中的烧结,提高MnOx的催化活性,一系列过渡金属氧化物,如Fe,Cu,Ni和Cr等的氧化物,被用来作为改性剂加入到MnOx催化剂中.近年来,很多研究者将稀土元素作为改性剂加入到MnOx催化剂中,并发现稀土金属氧化物的添加可以改善催化剂的活性、选择性、热稳定性及抗毒性能,是良好的添加助剂,其中对Ce的关注度颇高.而储氧性能是CeO2最重要的性质,CeO2对氧气的存储和释放可以通过Ce4+和Ce3+两种价态之间的变化实现.文献研究表明,将CeO2加入到锰氧化物材料中,能够提高锰氧化物在程序升温脱附过程中氧的脱附量,并且在低温条件下能够为锰氧化物提供氧,从而对锰氧化物的氧化态产生影响.此外,我国拥有丰厚的稀土Ce资源储备,使得锰铈复合氧化物在吸附脱除NOx方面得到广泛应用.催化剂作为选择性催化还原(SCR)工艺的核心,现阶段的研究重点主要集中于新型低温高活性催化剂的研究,如活性组分、载体组分、焙烧温度、焙烧时间及焙烧升温程序等,这表明焙烧过程对于催化剂性能的重要性.然而,在催化脱硝领域,对焙烧气氛的研究极少,但借鉴其他领域对焙烧气氛的研究,确有研究者证实焙烧气氛对材料的颗粒大小、缺陷浓度、价态及物相组成等有着显著的影响,进而影响材料的活性.我们课题组曾研究了焙烧气氛对MnOx/TiO2脱硝性能的影响,并发现惰性气氛中焙烧的催化剂表现出最佳活性,然而对于催化剂催化性能增强的原因并未深入探究.在前期研究基础上,以MnOx和CeOx为活性组分,采用浸渍法制备得到Ce-Mn/TiO2催化剂,通过X射线衍射(XRD)、氢气程序升温还原(H2-TPR)、热重(TG)、扫描电子显微镜(SEM)、氨气程序升温脱附(NH3-TPD)和X射线光电子能谱(XPS)等表征手段系统地研究了MnOx和CeOx担载于TiO2表面制成的催化剂在不同气氛(N2,空气和O2)中焙烧后的催化性能和物相结构.XRD,TG和H2-TPR测试结果表明,在N2气氛中焙烧有利于催化剂氧化度与结晶度的降低,催化剂中主要存在两种主要活性成分:大量的Mn2O3和少量的Mn3O4.SEM图揭示了在N2气氛下焙烧能够有效抑制晶粒长大,促进颗粒分散.NH3-TPD结果表明,N2气氛下焙烧的催化剂拥有更多的表面酸性位点,从而有利于反应气在催化剂表面的吸附和活化.结合XPS分析结果与脱硝活性测试结果,较低价态的MnOx以及较高的表面活性氧浓度(Oα)更有利于NH3-SCR反应的进行.不同焙烧气氛下0.20Ce-Mn/TiO2催化剂(Ce:Ti摩尔比为0.20)上NO转化率顺序如下:N2(94%)>空气(85.6%)>O2(75.6%).以上结果清晰地表明N2焙烧气氛显著提升了催化剂的脱硝活性.

Performance enhancement mechanism of Mn-based catalysts prepared under N2 for NOx removal: Evidence of the poor crystallization and oxidation of MnOx

Among multitudinous metal-oxide catalysts for the selective catalytic reduction of NOx with NH3 (NH3-SCR), Mn-based catalysts have become very popular and developed rapidly in recent years because of its superior low-temperature denitrification activity, mainly originating from mul-ti-valence of Mn. Most studies suggest that the catalytic activity of multi-component oxides is supe-rior to that of single-component catalysts owing to the synergistic effect among the metallic ele-ments in such materials, of which more attentions have been given to Ce as an additive owing to its powerful oxygen storage capacity, redox ability and its ready availability. As the core of SCR tech-nology, the research points in catalyst development at the present stage of all researchers in coun-tries mainly centralize on the optimization of active components, carriers, calcination temperature, calcination time and temperature-raising procedure, giving little thought to the effects of the calci-nation atmosphere. In the present work, Ce-modified Mn-based catalysts were prepared by a simple impregnation method. The effects of the calcination atmosphere (N2, air or O2) on the performance of the resulting materials during NH3-SCR and its causes of the differences were subsequently in-vestigated and characterized using various analytical methods. Data obtained from X-ray diffraction, thermogravimetry and temperature-programmed reduction with hydrogen show that calcination under N2 reduces both the degree of oxidation and crystallization of the MnOx. Scanning electron microscopy also demonstrates that the use of N2 inhibits the growth of grains and increases the dispersion of the catalysts. In addition, the results of temperature-programmed desorption with ammonia indicate that catalysts calcined under N2 exhibit a greater quantity of acid sites. Finally, X-ray photoelectron spectrometry and activity results demonstrate that MnOx in the lower valence states is more favorable for NH3-SCR reactions. In conclusion, catalysts calcined under N2 show superior performance during NH3-SCR for NOx removal, allowing NO conversions up to 94% at 473 K.