Reaction Mechanism of Platinum Dimer Cation With Ammonia Based on the Relativistic Density Functional Study

摘要

The gas-phase reactions between Pt_2~+and NH3 have been investigated using the relativistic density functional approach(ZORA-PW91/TZ2P).The quartet and doublet potential energy surfaces of Pt_2~++NH3 have been explored.The minimum energy reaction path proceeds through the following steps:Pt_2~+(~4sum_u)+NH3->q-1->d-2->d-3->d-4->d-Pt2NH++H2.In the whole reaction pathway,the step of d-2->d-3 is the rate-determining step with a energy barrier of 36.1 kcal/mol,and exoergicity of the whole reaction is 12.0 kcal/mol.When Pt2NH+reacts with NH3 again,there are two rival reaction paths in the doublet state.One is degradation of NH_4~+and another is loss of H2.In the case of degradation of NH_4~+,the activation energy is only 3.4 kcal/mol,and the overall reaction is exothermic by 8.9 kcal/mol.Thus,this reaction is favored both thermodynamically and kinetically.However,in the case of loss of H2,the rate-determining step's energy barrier is 64.3 kcal/mol and the overall reaction is endothermic by 8.5 kcal/mol,so it is difficult to take place.Predicted relative energies and barriers along the suggested reaction paths are in reasonable agreement with experimental observations.