Hydrocarbon rearrangements on Ni surfaces
The chemistry of hydrocarbons on nickel has been studied for several decades because of its technological significance and a scientific interest in some of the questions it poses. Nickel is used as the primary catalyst in the steam reforming process which converts methane and water into synthesis gas (carbon monoxide and hydrogen) which is then used in important industrial processes such as the Haber Bosch synthesis of ammonia and the Fischer-Tropsch formation of higher hydrocarbons. Recently nickel has also been used extensively to catalyze the formation and growth of carbon nanotubes from hydrocarbon feedstock.
To elucidate the catalytic conversion of hydrocarbons by Ni-surfaces in steam reforming catalysts, fuel cell anodes and carbon nanotube manufacturing, we have performed periodic DFT (PBE ) calculations to compute the binding energies and structures for all significant CHx and C2Hx species on the nickel(111) surface. We have then used our DFT to parameterize a ReaxFF potential to describe the chemistry of hydrocarbons in the presence of Ni.
From our DFT studies we see that CHx species prefer binding to three-fold sites, with bond energies proportional to the number of bonds the carbon atom can form to the surface after rehybridizing to form a tetrahedral orbital configuration. Replacing hydrogen atoms with bulkier methyl groups in CHx species leads to a preference for binding sites with lower nickel coordination numbers, thus CH2Me prefers an on-top site and CMe2 a bridge site while CH3 and CH2 prefer three-fold sites.
For C2Hx species we find that whenever the two carbons are not already fully coordinated then both carbons bind at two sites simultaneously. The nature of these sites depends primarily on steric effects, as determined by the number of substituents on each carbon and the orientation of each carbon's hybridized orbitals. Higher coordinated species prefer to bind to lower coordination sites. Thus for C-CH2 the terminal carbon binds to a three-fold site, while the internal carbon binds to an on-top site. We find that HC-CH retains one pi bond, to bond perpendicular to the surface, and uses C sp2 orbitals to bind to three-fold sites. For Hi2C-CH2 we find that C sp3 orbitals bind to on-top sites. These results are in good agreement with the available experimental data on the thermodynamic stability of small hydrocarbon species following the dissociation of methane on nickel(111), and explain the dissociation pathway observed for methane and other hydrocarbons on nickel(111) surfaces.
We are now using the ReaxFF potential we've fit to this data to explore more complex aspects of hydrocarbon chemistry on Ni surfaces and particles. For example, dynamics starting with methyl groups bound to Ni111 either with or without a step show that the presence of a step greatly enhances the rate at which C-H bonds are cleaved and CH3 is converted to bare C atoms.
Personnel: Jonathan Mueller
This project is directed Dr. Adri van Duin.
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