Oxidation catalysis on metal particles : a theoretical study

• Main Author: Thomas, Liam
• Published: Cardiff University 2016
• Subjects: 541; QD Chemistry
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.716003

Oxygen adsorption and subsequent activation on unsupported gold, palladium and gold/ palladium particles were investigated using computational methods. For all particles studied the route to dissociation is via adsorption on (100) facets followed by dissociation into adjacent (111) facets, this indicates particle morphology plays a significant role in the activity of the particles towards oxygen. Calculated reaction barriers confirm 0.40 eV ,0.04 eV and 0.12 eV is required to dissociate O2 on Au38, Pd38 and Au6Pd32 respectively. Further studies conclude it is not feasible to dissociate more than one molecular oxygen on Au38 – this is due to a calculated secondary adsorption energy being significantly weaker than the 0.5 eV required to dissociate the second species (the secondary adsorption energy is the calculated adsorption energy of the second O2 molecule on a cluster with a pre-adsorbed species) This result is in sharp contrast to Pd38 and Au6Pd32 where full adsorption and dissociation of O2 occurs until particle saturation. Hydroxymethylfurfural – metal particle interaction is found to occur via the furan ring – (100)/ (111) junction. Adsorption energies are found to be greatest on sub nanometre Au13 and Pd13. For larger nanometre sized particles (Au38, Pd38 & Au6Pd32) the interaction with hydroxymethylfurfural can be up to 75 kJ mol-1 weaker than sub nanometre counterparts. Nudged elastic band calculations indicate a barrier to O-H activation in hydroxymethylfurfural to be 106 kJ mol-1 and 137 kJ mol-1 for Pd13 and Au13 respectively. The presence of pre adsorbed and dissociated oxygen destabilises the interaction with HMF however it can lower O-H activation barriers in the case of Pd13. The presence of a Pd10 particle on the surface of (0001) Fe2O was found to lower surface oxygen vacancy defect energies by as much as 1.12 eV. This defect energy can be reduced further by substituting the Pd atoms with Au. If Au10 is supported the oxygen vacancy defect energies are dramatically reduced (1.44 eV) even at defect site distances greater than 3 Å from the particle.