(582bc) A DFT Study of CO Adsorption and Coverage on Co7Pd6 13 Atom Cluster | AIChE

(582bc) A DFT Study of CO Adsorption and Coverage on Co7Pd6 13 Atom Cluster

Authors 

Gundamaraju, A. - Presenter, Clemson University
With the new shale gas revolution in United States methane is found in abundance. This abundance of methane is demanding for new technological developments for converting methane to other forms of energy. Methane can be converted to syngas which is a mixture of CO and H2. Syngas can be further converted to liquid fuel alternative Ethanol. In the current research we are interested in catalytic conversion of syngas to ethanol. First and foremost, step in the ethanol conversion process is CO adsorption on Catalytic surface.

In this study, CO adsorption and coverage is studied on 13 atom bimetallic clusters of cobalt and palladium as catalyst. Atomic orbital approach is used to study CO adsorption on the cluster. Atomic simulation software Jaguar is used. Two hybrid exchange correlation functionals B3LYP and MO6 are considered. Number of CO molecules on the surface is varied from 1 to 6. Three adsorption sites atop, bridge and threefold are considered.

CO adsorbs molecularly on both cobalt and palladium further CO preferentially binds on palladium than cobalt. On the palladium surface initially when only one molecule of CO is adsorbed CO prefers to adsorb on bridge site and as number of molecules of CO on surface increases beyond 3 adsorption site preference shifts from bridge to atop. This can further be explained by shift in HOMO from palladium surface to Cobalt surface. Adsorption energy of CO is calculated as -1.38 ev using M06 and -1.32 using B3LYP. The difference in the values can be attributed to B3LYP not including dispersion effects in this case CO back donation of electrons to metal surface. Vibrational frequency of CO is also determined. When CO adsorbs on atop site vibrational frequency is in between 2015-2065, on bridge values are between 1920-2000, and on threefold site it is in the range 2040-2100. Further as surface coverage of CO increases adsorption energy decreases, also vibrational frequency shifts to a higher value. Bond lengths between palladium metal and carbon atom are underestimated by B3LYP whereas bond length between carbon and oxygen is overestimated. This further explains that B3LYP does not include dispersion effects in CO bonding.

Calculated values for adsorption energy, bond lengths and vibrational frequency are in very good agreement with experimental values.

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