摘要
A series of SnO2‐based catalysts modified by Mn, Zr, Ti and Pb oxides with a Sn/M （M=Mn, Zr, Ti and Pb） molar ratio of 9/1 were prepared by a co‐precipitation method and used for CH4 and CO oxidation. The Mn3＋, Zr4＋, Ti4＋and Pb4＋cations are incorporated into the lattice of tetragonal rutile SnO2 to form a solid solution structure. As a consequence, the surface area and thermal stability of the catalysts are improved. Moreover, the oxygen species of the modified catalysts become easier to be reduced. Therefore, the oxidation activity over the catalysts was improved, except for the one modified by Pb oxide. Manganese oxide demonstrates the best promotional effects for SnO2. Using an X‐ray diffraction extrapolation method, the lattice capacity of SnO2 for Mn2O3 was 0.135 g Mn2O3/g SnO2, which indicates that to form stable solid solution, only 21%Sn4＋cations in the lattice can be maximally replaced by Mn3＋. If the amount of Mn3＋cations is over the capacity, Mn2O3 will be formed, which is not favorable for the activity of the catalysts. The Sn rich samples with only Sn‐Mn solid solution phase show higher activity than the ones with excess Mn2O3 species.
CH_4和CO是两种主要的温室效应气体和空气污染物,催化氧化是最有效的消除CH_4和CO的方法.研发不含贵金属的金属氧化物催化剂或者减少催化剂中贵金属用量为该领域研究热点.SnO_2是一种重要的宽禁带n型半导体材料,广泛应用于气敏器件、锂离子电池以及光电设备.SnO_2表面富含活泼的缺位氧且具有良好的热稳定性,因此其在催化方面的性能近年来逐渐受到人们关注.在过去的5年中,本团队深入研究了SnO_2材料在空气污染治理和绿色能源生产等领域的应用及其催化性质.发现通过其它阳离子如Fe^(3+),Cr^(3+),Ta^(5+),Ce^(4+)和Nb^(5+)等的掺杂,替换晶格中部分Sn^(4+),形成金红石型SnO_2固溶体结构,显著提高了催化剂氧物种的流动性、活性和催化剂本身的热稳定性.固溶体材料是一类重要的催化剂,受到广泛关注.一个典型的例子是铈锆固溶体,其作为储氧材料已广泛应用于汽车尾气净化器.形成固溶体结构后,氧化铈的储氧能力和热稳定性得到显著提高.为有效形成固溶体,两个阳离子需要具有相似的离子半径和电负性.以往,人们基于结构中金属阳离子和氧阴离子的离子半径提出了容忍因子的判别方法,以此来判断固溶体是否能有效形成及所生成固溶体的稳定性.我们在前期工作中,以Sn-Nb固溶体为例,提出了简单的X射线衍射(XRD)外推法来计算固溶体晶格容量,即形成稳定固溶体时客体阳离子取代主体晶格阳离子的最大值.作为延续工作,本文采用共沉淀法制备了一系列Sn/M(M=Mn,Zr,Ti,Pb)摩尔比为9/1的SnO_2基催化剂,并用于CH_4和CO催化氧化.结果表明,Mn^(3+),Zr^(4+),Ti^(4+)和Pb^(4+)均可以掺杂进四方金红石型SnO_2晶格中,形成稳定的固溶体结构.其中Sn-Mn-O固溶体表现出最高活性.为了深入研究Mn_2O_3在SnO_2中的晶格容量及最优催化剂配比,采用共沉淀法制备了一系列不同Sn/Mn摩尔比的样品,采用XRD,N_2-BET,H_2-TPR,SEM和XPS等手段对其物理化学性能进行了表征,并考察了对CH_4的催化氧化性能.通过XRD外推法测定了Mn^(3+)离子在SnO_2中的晶格容量为0.135g Mn_2O_3/g SnO_2,相当于Sn/Mn摩尔比为79/21.这表明形成稳定的固溶体后,SnO_2晶格中最多只有21%Sn^(4+)可以被Mn^(3+)替代;当Mn^(3+)含量超过晶格容量时,过量的Mn^(3+)在催化剂表面形成Mn_2O_3,对催化剂活性不利.类似于Sn-Nb-O固溶体,在Sn-Mn-O催化剂体系中亦观察到明显的晶格容量效应.纯相的Sn-Mn-O固溶体比含过量Mn_2O_3晶相的Sn-Mn-O催化剂具有更高活性.
基金
supported by the National Natural Science Foundation of China (21263015,21567016 and 21503106)
the Education Department Foundation of Jiangxi Province (KJLD14005 and GJJ150016)
the Natural Science Foundation of Jiangxi Province (20142BAB213013 and 20151BBE50006),which are greatly acknowledged by the authors~~
