Simulation of reduction of oxidized metal nanoparticles

Abstract

I analyze theoretically the spatio-temporal kinetics of reduction of oxidized metal nanoparticles by hydrogen (or methane). The focus is on the experimentally observed formation of metal and oxide domains separated partly by pores. The interpretation of such multiphase processes in nanoparticles at the mean-field level is hardly possible primarily due to complex geometry, and accordingly I use the lattice Monte Carlo technique in order to tackle this problem. The main conclusions drawn from the corresponding generic simulations are as follows. (i) The patterns predicted are fairly sensitive to the metal-metal and metal-oxygen interactions. With decreasing the former interaction and increasing the latter interaction, there is transition from the formation of metal aggregates and voids to the formation of a metal film around the oxide core. (ii) During the initial phase of these kinetics, the extent of reduction can roughly be described by using the power law, and the corresponding exponent is about 0.3. (iii) With decreasing the hydrogen (or methane) pressure and/or increasing the oxide nanoparticle size, as expected, the kinetics are predicted to become longer. (iv) The dependence of the patterns on the presence of the support and/or Kirkendall void in an oxide nanoparticle is shown as well.