摘要
Although metal oxide-zeolite hybrid materials have long been known to achieve enhanced catalytic activity and selectivity in NO_(x)removal reactions through the inter-particle diffusion of intermediate species,their subsequent reaction mechanism on acid sites is still unclear and requires investigation.In this study,the distribution of Brønsted/Lewis acid sites in the hybrid materials was precisely adjusted by introducing potassium ions,which not only selectively bind to Brønsted acid sites but also potentially affect the formation and diffusion of activated NO species.Systematic in situ diffuse reflectance infrared Fourier transform spectroscopy analyses coupled with selective catalytic reduction of NO_(x)with NH_(3)(NH_(3)-SCR)reaction demonstrate that the Lewis acid sites over MnO_(x)are more active for NO reduction but have lower selectivity to N_(2)than Brønsted acids sites.Brønsted acid sites primarily produce N_(2),whereas Lewis acid sites primarily produce N_(2)O,contributing to unfavorable N_(2)selectivity.The Brønsted acid sites present in Y zeolite,which are stronger than those on MnO_(x),accelerate the NH_(3)-SCR reaction in which the nitrite/nitrate species diffused from the MnO_(x)particles rapidly convert into the N_(2).Therefore,it is important to design the catalyst so that the activated NO species formed in MnO_(x)diffuse to and are selectively decomposed on the Brønsted acid sites of H-Y zeolite rather than that of MnO_(x)particle.For the physically mixed H-MnO_(x)+H-Y sample,the abundant Brønsted/Lewis acid sites in H-MnO_(x)give rise to significant consumption of activated NO species before their inter-particle diffusion,thereby hindering the enhancement of the synergistic effects.Furthermore,we found that the intercalated K+in K-MnO_(x)has an unexpected favorable role in the NO reduction rate,probably owing to faster diffusion of the activated NO species on K-MnO_(x)than H-MnO_(x).This study will help to design promising metal oxide-zeolite hybrid catalysts by identifying the role of the acid sites in two different constituents.
尽管金属氧化物-沸石杂化材料通过中间物种的颗粒间扩散在氮氧化物去除反应中增强了催化活性和选择性,但它们在酸性位点上的后续反应机理尚不清楚,需要进一步深入研究本文通过引入钾离子精确调节了杂化材料中Bronsted/Lewis酸位点的分布,钾离子不仅选择性地与Bronsted酸位点结合,而且可能影响活化NO物种的形成和扩散.原位漫反射红外傅里叶变换光谱和NH选择性催化还原NO_(x)(NH_(3)-SCR)反应结果表明,MnO_(x)上的Lewis酸位点对NO还原的活性更高,但对N_(2)的选择性低于Bronsted酸位点.MnO_(x)上Bronsted酸位点主要产生N_(2),而Lewis酸位点主要形成了N_(2)O,使得N,选择性较低.Y沸石上Bronsted酸比MnO_(x)上的更强,从而加速了NH_(3)-SCR反应,其中从MnO_(x)颗粒扩散来的亚硝酸盐/硝酸盐物种迅速转化为N_(2).因此,设计催化剂非常重要的是使MnO,上形成的活性NO物种扩散到H-Y沸石的Bronsted酸位,而不是扩散到MnO_(x)的Bronsted酸位并选择性分解。对于物理混合的H-MnO_(x)+H-Y样品,H-MnO_(x)中丰富的Bronsted/Lewis酸位点在颗粒间扩散前会导致活性NO物种的大量消耗,从而阻碍了协同效应的增强.此外,研究发现K-MnO_(x)中嵌入的K对NO还原速率具有意想不到的促进作用,这可能是由于活化的NO物种在K-MnO_(x)上的扩散速度比H-MnO_(x)快.综上,本文通过确定酸位点在两种不同成分中的作用,对设计高效的金属氧化物-沸石杂化催化剂提供参考.
