通过调变碳纳米管大π体系与含氧官能团的共轭作用以改变碳纳米管的催化性能

选取碳纳米管(CNT)催化剂催化丙烷氧化脱氢作为模型反应,通过系统研究CNT上多种活性氧位的微观结构,以及对其电子结构的表征,发现通过调控CNT大π体系与含氧官能团之间的共轭效应可以改变CNT的催化活性.CNT体系充当一个电子存储器,通过活性位与CNT的离域效应向活性位提供或夺取电子改变反应活性.对于丙烷氧化脱氢,当电子从CNT流向官能团时,第一步C-H断键活性提高,但反应过程中生成的中间体过于稳定将导致CNT催化剂活性位被占据,从而抑制第二步C-H活化生成丙烯.反之,当电子从官能团流向CNT时,第一步C-H断键活性较低,生成的中间体较不稳定,较易生成丙烯.因此,可以通过调控CNT与官能团之间的电子共轭效应来平衡两步C-H键的活化.这些结果有助于从微观尺度上理解CNT催化剂活性的来源,并为制备高活性CNT催化剂提供理论指导.

Tuning the catalytic performance of carbon nanotubes by tuning the conjugation between the π orbitals of carbon nanotubes and the active oxygenic functional groups

Periodic density functional theory calculations were used to systematically investigate the origin of the catalytic activity of carbon nanotubes (CNTs) and ways to improve the catalytic performance of CNTs for the oxidative dehydrogenation of propane. Detailed characterizations of the geometric and electronic structure of the active oxygenic functional groups (CO groups) were presented. The results reveal that the catalytic performance of CNTs can be tuned by tuning the conjugation between CNT π orbitals and the orbitals of the active CO groups. Through conjugation, CNTs act as an electron reservoir to donate or accept electrons from the CO group. We found that CO groups with a negative charge have higher C-H bond activation activity but lead to a more stable i-propoxide intermediate, which inhibits the formation of propene, while positively charged CO groups have the opposite effect. The balance between the activities of the two C-H activation steps can be obtained by tuning the charge of the CO group. This study increases the understanding of the origin of the activity of CNT catalysts at the microscopic scale and gives guidance for the preparation of high-performance CNT catalysts.