Interactive network of the dehydrogenation of alkanes, alkenes and alkynes – surface carbon hydrogenative coupling on Ru(111)

Abstract

To understand the reaction mechanisms of the dehydrogenation and retrosynthesis of alkanes, the consecutive dissociation of methane, ethane, ethene and ethyne, as well as propane, propene and propyne, on the fcc Ru(111) surface has been investigated using periodic density functional theory computations (rPBE). Methane dissociation has the energy minimum path of → → → CH* → C*. Although ethane dissociation does not have ethene and ethyne as intermediates, they have the same final surface species with the minimum energy paths for ethane [ → → CH₃CH* → CH₃C* → → HC*C* → HC* + C*], ethene [ → → → HC*C* → HC* + C*] and ethyne [CH*CH* → HC*C* → HC* + C*]. Propane dissociation has the competitive routes of n-propyl [ → CH₃CH₂CH* → CH₃CH₂C* → CH₃CH*C* → CH₃C*C* → CH₃C* + C* →→ HC* + C*] and isopropyl with propyne as an intermediate [CH₃CH*CH₃ → CH₃C*CH₃ → → CH₃C*CH* → CH₃C*C* →→ HC* + C*], and the n-propyl route has propene as an intermediate for dissociation [ → CH₃C*CH₂*/CH₃CH*CH* → CH₃CH*C*/CH₃C*CH* → CH₃C*C* →→ HC* + C*]. In these reactions, the most stable surface intermediates are HC*, CH₃C* and CH₃CH₂C* as homologs, as found experimentally on other metal surfaces. Our results rationalized the experimentally observed interconversion between + H* and CH₃C* as well as surface HC*C* and CH₃C*C* as key intermediates for the first C–C bond dissociation [HC*C* → HC* + C*; CH₃C*C* → CH₃C* + C*]. On the basis of surface C* and H₂ gas, the retrosynthesis of methane, ethane and propane has increasing apparent barriers of 1.08, 1.51 and 1.66 eV, respectively, at 490 K and 1 atm H₂ and 0.83, 1.14 and 1.15 eV, respectively, at 19.7 atm H₂. Surface carbon coverage changes the formation of alkanes from endergonic to exergonic. This pressure- and coverage-dependency is very important for understanding the reaction mechanism and selectivity. Surface alkynyl groups should be the intermediates for C–C coupling. The computed vibrational frequencies of CH*, CH₃C*, CH₂C*, HC*C* and agree with the experiments. The comparison in adsorption energies and reaction barriers and energies shows that the fcc Ru(111) surface is more active than the hcp Ru(0001) surface despite their very similar surface structures.