Development of a ReaxFF Reactive Force Field for the Pt-Ni Alloy Catalyst.

Abstract

We developed the ReaxFF force field for Pt/Ni/C/H/O interactions, specifically targeted for heterogeneous catalysis application of the Pt-Ni alloy. The force field is trained using the DFT data for equations of state of Pt3Ni, PtNi3 and PtNi alloys, the surface energy of the PtxNi1-x(111) (x = 0.67-0.83), and binding energies of various atomic and molecular species (O, H, C, CH, CH2, CH3, CO, OH, and H2O) on these surfaces. The ReaxFF force field shows a Pt surface segregation at x ≥ 0.67 for the (111) surface and x ≥ 0.62 for the (100) surface in vacuum. In addition, from the investigation of the preferential alloy component of the adsorbates, it is expected that H and CH3 on the alloy surface to induce a segregation of Pt whereas the oxidation intermediates and products such as C, O, OH, H2O, CO, CH, and CH2 are found to induce Ni segregation. The relative order of binding strengths among adsorbates is a function of alloy composition and the force field is trained to describe the trend observed in DFT calculations, namely, H2 < H2O < CH3 ≈ O2 ≈ CO < OH < CH2 < C ≈ CH on Pt8Ni4, H2 < H2O < CO ≈ O2 < CH3 < OH < CH2 < CH < C on Pt9Ni3, and H2 < H2O < O2 < CO < CH3 < OH < CH2 < C ≈ CH on Pt10Ni2. Using this force field, we performed the grand-canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations for a Pt3Ni slab and a truncated cuboctahedral nanoparticle terminated by (111) and (100) faces, to examine the surface segregation trend under different gas environments. It is found that Pt segregates to the alloy surface when the surface is exposed to vacuum and/or H2 environment while Ni segregates under the O2 environment. These results suggest that the Pt/Ni alloy force field can be successfully used for the preparation of Pt-Ni nanobimetallic catalysts structure using GCMC and run MD simulations to investigate its role and the catalytic chemistry in catalytic oxidation, dehydrogenation and coupling reactions. The current Pt/Ni force field still is found to have difficulties in describing the observed segregation trend in Ni-rich alloy compositions (x < 0.6), suggesting the need for additional force field training and evaluation for its application to describe the characteristics and chemistry of Ni-rich alloys.