Reversible transformation between terrace and step sites of Pt nanoparticles on titanium under CO and O_(2) environments

二氧化钛负载的铂纳米颗粒在CO和O_(2)环境下表面平台位点和台阶位点的可逆转变

Understanding the dynamic evolution of active sites of supported metal catalysts during catalysis is fundamentally important for improving its performance,which attracts tremendous research interests in the past decades.There are two main surficial structures for metal catalysts:terrace sites and step sites,which exhibit catalytic activity discrepancy during catalysis.Herein,by using in situ transmission electron microscopy and in situ Fourier transform infrared spectroscopy(FTIR),the transformation between surface terrace and step sites of Pt-TiO_(2) catalysts was studied under CO and O_(2) environments.We found that the{111}step sites tend to form at{111}terrace under O_(2) environment,while these step sites prefer to transform into terrace under CO environment at elevated temperature.Meanwhile,quantitative ratios of terrace/step sites were obtained by in situ FTIR.It was found that this transformation between terrace sites and step sites was reversible during gas treatment cycling of CO and O_(2).The selective adsorption of O_(2) and CO species at different sites,which stabilized the step/terrace sites,was found to serve as the driving force for active sites transition by density functional theory calculations.Inspired by the in situ results,an enhanced catalytic activity of Pt-TiO_(2) catalysts was successfully achieved through tuning surface-active sites by gas treatments.

在催化反应环境下,金属催化剂表面结构会随着温度和气氛环境的变化而改变,从而对其催化性能产生至关重要的影响.因此,原位研究金属催化剂表面结构在反应条件下的重构过程与机制,对调控金属催化剂表面结构和提高其催化性能都有重要意义.负载在TiO_(2)载体上的Pt纳米催化剂在能源环境领域具有重要应用,因而其表面结构在气氛环境下的动态演变行为引起了研究者的广泛关注.Pt纳米颗粒表面主要有两种结构:配位数为8–9的平台位点和配位数小于7的台阶位点.研究表明,这两种位点存在巨大的催化活性差异,但由于缺乏催化反应环境下原子级别的动态信息,Pt纳米颗粒表面平台位点和台阶位点的相互转变过程和转变机制仍然有待研究.本文利用原位透射电子显微学和原位红外技术,在原子尺度研究了还原性(CO)和氧化性(O_(2))气氛环境下Pt纳米颗粒表面平台位点和台阶位点的可逆转变过程.利用原位透射电镜中的气体样品杆技术,将Pt-TiO_(2)纳米催化剂封装在芯片纳米反应器中,在电镜中一个大气压环境下原位观察Pt纳米颗粒表面结构的变化过程.以CO分子作为探针分子,利用其在Pt纳米颗粒不同位点的饱和吸附定量分析不同环境下平台位点和台阶位点的比例.结果表明,在O_(2)环境下,{111}表面上的平台位点会部分转变为台阶位点,而这些台阶位点在CO环境下又会部分转变成平台位点.通过CO和O_(2)环境反复循环处理,发现这一转变过程是随着气体环境变化而可逆变换的.此外,在对Pt-TiO_(2)催化剂进行在不同气氛中非原位处理之后,利用电镜对Pt纳米颗粒的表面结构进行统计分析,也发现了相同的转变现象.进一步利用第一性原理计算,发现这一转变过程是由于O_(2)会选择性吸附在台阶位点,从而稳定台阶位点并使其扩展.而CO可以与Pt表面吸附的O物种反应,从而释放台阶位点上选择性吸附的氧,促使台阶位点转变成平台位点.同时,也对平台位点和台阶位点的活性差异进行了研究,发现Pt-TiO_(2)催化剂在303 K下催化CO氧化反应时,单位位点催化活性(TOF)与Pt纳米颗粒表面平台位点占比正相关,说明平台位点是Pt-TiO_(2)催化剂在此反应条件下的活性位点.结合理论计算发现,O_(2)在台阶位点的选择性吸附甚至会对该位点具有毒化作用,从而使Pt-TiO_(2)催化剂整体活性下降.本文揭示了Pt纳米颗粒表面平台位点和台阶位点在CO和O_(2)环境下的可逆转变机制,并进一步阐释了该转变过程对其催化CO氧化反应活性的影响.基于本文原位研究,可实现对催化剂表面结构的调控和优化:通过CO气氛环境处理,使其暴露更多的平台位点,从而提高其催化活性.