Fang Zongtang, Vasiliu Monica, Peterson Kirk A, Dixon David A
Department of Chemistry and Biochemistry , The University of Alabama , Shelby Hall , Box 870336, Tuscaloosa , Alabama 35487-0336 , United States.
Department of Chemistry , Washington State University , Pullman Washington 99164-4630 United States.
J Phys Chem A. 2018 May 3;122(17):4338-4349. doi: 10.1021/acs.jpca.7b12634. Epub 2018 Apr 18.
Hydrogen adsorption on small group 4 metal oxide clusters for both the singlet and the first excited triplet states have been investigated by density functional theory and correlated molecular orbital theory at the coupled cluster CCSD(T) level. The reaction starts with hydrogen physisorption on a metal center followed by formation of metal hydride/hydroxides due to splitting H into H and H. The hydrogen physisorption energies are predicted to be -1 to -8 kcal/mol for the singlet and -1 to -26 kcal/mol for the triplet, respectively. The formation of metal hydride/hydroxides does not involve redox processes. Chemisorption leading to formation of metal hydride/hydroxides is exothermic by -10 to -50 kcal/mol for the singlet, and exothermic by up to -60 kcal/mol for the triplet. The predicted energy barriers are less than 20 kcal/mol. Formation of metal dihydroxides from the metal hydride/hydroxides is generally endothermic for the monomer and dimer and is exothermic for the trimer and tetramer. Formation of the dihydroxide is a proton coupled electron transfer (PCET) process. The singlet energy barriers for the H→ H transfer process are predicted to be 35-60 kcal/mol, in comparison to triplet energy barriers of less than 15 kcal/mol for the H → H transfer process. For trimers and tetramers, there exist two different pathways: the first is a direct pathway with PCET to a terminal oxygen and the second is a two-step pathway with initial formation of a bridge OH group followed by a proton transfer to generate a terminal OH group. For the singlet, the two-step pathway is preferred for M = Ti and the direct pathway is more favorable for M = Zr and Hf. The two-step pathway is always preferred for the triplet as one-electron transfer is always more likely than two-electron transfer in the direct pathway.
采用密度泛函理论和耦合簇CCSD(T)水平的相关分子轨道理论，研究了氢在第4族小金属氧化物团簇上的单重态和第一激发三重态吸附情况。反应起始于氢在金属中心的物理吸附，随后由于H分裂为H和H而形成金属氢化物/氢氧化物。预测单重态的氢物理吸附能为-1至-8千卡/摩尔，三重态为-1至-26千卡/摩尔。金属氢化物/氢氧化物的形成不涉及氧化还原过程。导致形成金属氢化物/氢氧化物的化学吸附对于单重态放热-10至-50千卡/摩尔，对于三重态放热高达-60千卡/摩尔。预测的能垒小于20千卡/摩尔。由金属氢化物/氢氧化物形成金属二氢氧化物对于单体和二聚体通常是吸热的，而对于三聚体和四聚体是放热的。二氢氧化物的形成是一个质子耦合电子转移（PCET）过程。预测H→H转移过程的单重态能垒为35 - 60千卡/摩尔，相比之下H→H转移过程的三重态能垒小于15千卡/摩尔。对于三聚体和四聚体，存在两种不同的途径：第一种是通过PCET直接转移到末端氧的途径，第二种是两步途径，首先形成桥连OH基团，然后通过质子转移生成末端OH基团。对于单重态，当M = Ti时，两步途径更有利，而当M = Zr和Hf时，直接途径更有利。对于三重态，两步途径总是更有利，因为在直接途径中，单电子转移总是比双电子转移更有可能发生。
