(73b) A Density Functional Theory Investigation of Methane Activation on a Palladium Oxide Catalyst

Methane combustion is a very active
area of current research since natural gas is expected to be used more and more
as a source of energy in the future.  Methane is also attractive because it
emits the least amount of CO₂ per unit energy of all the
hydrocarbons.  Many researchers have found palladium based catalysts to be the
most promising for the methane combustion reaction.  However, many aspects of
this reaction, such as the reaction mechanism and the activation of the C-H
bond, are not yet well understood.  This research focuses on using density
functional theory (DFT) to help answer these questions.

We have conducted periodic DFT calculations,
utilizing the VASP software, to study the adsorption of 11 different species on
two PdO surfaces (100 and 110) with and without oxygen defects.  The 11 species
were chosen as likely surface intermediates involved in the actual reaction
mechanism.  These adsorption calculations are then used as starting points for
examining different reaction pathways and performing transition state analyses.

The first reaction pathway studied
is perhaps the most important from an experimental point of view, namely, the
activation of methane.  The cleavage of the first C-H bond is considered to be
the rate limiting step in the methane combustion process.  Three possible
pathways on the (100) surface have been discovered for the activation step
through the use of constrained optimization calculations.  Two of the pathways
gave similar activation barriers of about 30 kcal/mol, which is well within the
range seen by various experimental researchers. The third pathway predicts a
barrier of 23 kcal/mol which is significantly lower than the other two.  This pathway
occurs with oxygen vacancies on the surface.  However, further thermodynamic
calculations showed that under reaction conditions that oxygen vacancies rarely
exist.

Nudged elastic band (NEB) calculations were performed in order to fine tune the constrained optimization results
and determine transition state geometries.  Attempts at such calculations
failed due to convergence problems.  To overcome this difficultly, we have
developed a cluster model of the PdO(100) surface in order to utilize the more
robust algorithms contained within the Gaussian 03 software package.  The
results from the cluster model are then compared with those from the periodic
model to validate the use of a cluster.