Pt-Cu合金催化剂上甲烷无氧偶联的机理与微观动力学研究:从单原子位点到单团簇位点

Mechanistic and microkinetic study of nonoxidative coupling of methane on Pt‐Cu alloy catalysts:From single‐atom sites to single‐cluster sites

随着天然气和页岩气资源的大量发现,作为主要成分的甲烷,其转化为高值化学品是一条具有开发潜力的路径.在各种甲烷转化途径中,甲烷无氧直接转化具有碳原子利用率高和二氧化碳排放少的优点,更具应用前景.然而,甲烷的无氧转化仍然面临反应温度高、C_(2)烃选择性低和催化剂易积碳失活的难题.因此,大量的研究集中在催化剂研发上,期望通过选择性地打破C-H键,并且催化C-C偶联,实现高效活化甲烷、高选择性生成C_(2)烃,近期研究发现,被Cu等金属隔离的Pt位点,被金属氧化物分散的Pt位点,都有利于甲烷无氧偶联生成C_(2)烃.特别是Pt-Cu单原子合金催化剂,其中分散的Pt单原子不仅.具有较高的打破C-H键活性,而且能够抑制甲烷深度脱氢,具有很好的抗积碳性能.虽然单原子Pt具有很好的甲烷活化性能,但进一步催化C_(2)烃生成的反应过程并不清楚,同时单分散的Pt团簇也可能存在于Pt-Cu合金表面,而关于它们催化甲烷无氧偶联的机制也缺乏研究和认识.本文在Cu(111)表面建立Pt_(1),Pt_(2)和Pt_(3)位点(分别标记为Pt_(1)©Cu(111),Pt_(2)©Cu(111)和Pt_(3)©Cu(111)),采用密度泛函理论计算与微观动力学模拟相结合的方法,研究甲烷无氧偶联的催化反应机理与反应性能,评估并比较单原子与单团簇的催化反应性能.通过对甲烷分解的基元反应计算发现,CH_(4),CH_(3)和CH_(2)的脱氢反应分别在Cu(111)上的Pt_(1),Pt_(2)和Pt_(3)位点上最有利.然而,相应的CHx(x=3,2,1)物种直接偶联形成C_(2)H_(6),C_(2)H_(4)和C_(2)H_(2)的反应,分别在Cu(111)上的Pt_(3),Pt_(1)和Pt_(2)位点上最有利.三种Pt位点独特的反应趋势,主要源于Pt位点与CHx物种不同的结合能力.反应条件下甲烷无氧偶联的微观动力学模拟表明,Pt_(1)©Cu(111)催化甲烷转化的活性最高,而加入少量的氢气可以显著提高乙烯的选择性,750 K时最高选择性可达96.2%.在Pt_(1)©Cu(111)表面,Pt位点主要负责C-H键裂解,Cu位点是C-C偶联的活性中心.通过密度泛函理论计算,发现Pt_(1)©Cu(111)在反应条件下结构稳定.综上,本文揭示了Pt单原子位点(SASs)和Pt单团簇位点(SCSs)上的甲烷无氧偶联反应机制,并预测Pt SASs在甲烷无氧偶联中比Pt SCSs更有优势,为高效甲烷无氧偶联催化剂制备提供一定借鉴.

Desirable catalysts possessing the ability to selectively break C–H bond and controllably catalyze C–C bond formation are highly demanded for the nonoxidative coupling of methane(NOCM).Herein,a series of Pt‐Cu alloy catalysts including Pt_(1)©Cu(111),Pt_(2)©Cu(111)and Pt_(3)©Cu(111)are deliberately designed and systematically studied for NOCM.Density functional theory calculations reveal that the Pt_(1),Pt_(2),and Pt_(3)sites on Cu(111)can selectively break the C–H bond to generate CH_(3),CH_(2),and CH species,respectively.However,direct coupling of corresponding CHx(x=3,2,1)to form C_(2)H_(6),C_(2)H_(4),and C_(2)H_(2)are favorable on Pt_(3),Pt_(1),and Pt_(2)sites on Cu(111),respectively.The different reactivity trends of the three Pt sites mainly originate from the varying bonding abilities of CHx species at the Pt sites.Microkinetic modeling manifests that the Pt_(1)©Cu(111)is the most active for methane dissociation(TOF=2.98 s^(-1)at 1000 K)and can selectively convert methane into ethylene with the highest selectivity up to 96.2%at 750 K.Moreover,Pt_(1)©Cu(111)also shows superb stability under reaction conditions.Overall,our studies not only provide a comprehensive understanding of the reaction mechanism of NOCM on Pt single‐atom sites(SASs)and Pt single‐cluster sites(SCSs)but also predict that Pt SASs are advantageous over Pt SCSs for NOCM.