表面增强拉曼光谱原位捕获Pt-NiO界面水煤气变换反应中的碳酸盐中间物种

Direct identification of the carbonate intermediate during water-gas shift reaction at Pt-NiO interfaces using surface-enhanced Raman spectroscopy

水煤气变换反应(WGSR)是制备高纯氢的重要反应之一,一直是人们的研究热点.以Pt为代表的贵金属催化剂,在低温条件下表现出优异的WGSR活性.其中,Pt可还原性氧化物界面往往被认为是水煤气变换反应最高效的活性位点.然而,由于缺乏直接的光谱证据,该界面处的水煤气变换反应分子机理仍然存在争议.本文通过制备具有三元核壳结构的Au@Pt@NiO纳米结构,在具有高表面增强拉曼效应的Au纳米颗粒表面构建了丰富的Pt-NiO界面,成功实现了Pt-NiO界面处WGSR过程及其关键中间物种的原位表面增强拉曼光谱(SERS)研究.通过控制镍前驱体的量,结合透射电镜和元素面扫描表征,制备了一系列具有不同NiO壳层厚度的Au@Pt@NiO纳米结构.以CO作为探针分子,利用原位SERS表征,当镍前驱体添加量为0.05 mL时,可以同时得到Pt-C以及Ni-O的拉曼信号,说明此时NiO是以岛状形式沉积于Au@Pt表面,从而构筑出丰富的Pt-NiO界面.原位SERS测试结果表明,当将此Au@Pt@NiO纳米粒子置于WGSR气氛时,随着反应温度的升高,在1065 cm^(-1)处出现了碳酸根物种的拉曼信号.而当将Au@Pt@NiO纳米粒子置于单独Ar,CO和气态水的气氛条件下时,均未观察到与新物种相关的拉曼信号产生.进一步研究结果表明,碳酸根物种的拉曼信号强度随着反应温度的上升呈现先增加后减少的趋势,且在130 ℃便已出现(此时并未测到WGSR活性),并在200 ℃显示出最大值(WGSR的起燃温度).同时,在气氛切换实验结果表明,若体系中先通入CO/Ar,再切换成H_(2)O/Ar时,可在Au@Pt@NiO表面观察到碳酸盐物种的生成;反之,若先通入H_(2)O/Ar,再切换为CO/Ar时,则无碳酸盐物种.说明碳酸盐物种是通过吸附的CO与气态水反应生成的,并且在催化剂表面可以稳定吸附,需要在更高温度才可分解,故碳酸盐的分解可能是WGSR中一个较为缓慢的步骤.此外,当将Au@Pt和Au@NiO置于WGSR气氛条件下进行对照实验,并没有观察到碳酸盐物种生成.这表明Pt-NiO界面可促进碳酸盐中间物种的形成,从而导致该界面比纯Pt表面具有更高的水煤气变换反应活性.本文研究结果加深了对WGSR机理的认识,有望用于指导高效WGSR催化剂的设计.

Noble metal-reducible oxide interfaces have been regarded as one of the most active sites for water-gas shift reaction.However,the molecular reaction mechanism of water-gas shift reaction at these interfaces still remains unclear.Herein,water-gas shift reaction at Pt-NiO interfaces has been in-situ explored using surface-enhanced Raman spectroscopy by construction of Au@Pt@NiO nanostructures.Direct Raman spectroscopic evidence demonstrates that water-gas shift reaction at Pt-NiO interfaces proceeds via an associative mechanism with the carbonate species as a key intermediate.The carbonate species is generated through the reaction of adsorbed CO with gaseous water,and its decomposition is a slow step in water-gas shift reaction.Moreover,the Pt-NiO interfaces would promote the formation of this carbonate intermediate,thus leading to a higher activity compared with pure Pt.This spectral information deepens the fundamental understanding of the reaction mechanism of water-gas shift reaction,which would promote the design of more efficient catalysts.