Non-noble electrocatalysts for O2 reduction:: How does heat treatment affect their activity and structure?: Part I.: Model for carbon black gasification by NH3:: Parametric calibration and electrochemical validation

In part I of this paper, a model for the gasification of pristine carbon blacks by NH3 is developed. Here, it is applied to investigate non-noble catalysts for the O-2 reduction reaction (ORR) in fuel cells. These catalysts are produced by pyrolyzing a furnace black, loaded with 0.2 wt % Fe, in NH3. The model predicts (i) the gasification rate of an initially pore-free particle of furnace black and (ii) the internal porous network thereby created. The model assumes two rate constants for the gasification of carbon black: one for the graphitic crystallites and one for the disordered matrix phase. The model is fitted to experimental time-evolutions of (i) weight loss of carbon black and (ii) specific surface area measured during the synthesis of non-noble catalysts for the ORR. The fittings yield a rate constant that is ten times larger for the gasification of the disordered phase than that for the graphitic phase. For a 42 nm diameter particle, the model predicts that (i) the gas-carbon reaction, and therefore the internal porous network, occurs in an outer shell of thickness <= 8 nm and (ii) the particle shrinks beyond 45% weight loss. Experimentally, the micropore area created during the heat treatment controls the activity for the ORR of such catalysts. The model is able to reproduce the experimental micropore area if the fraction of disordered carbon of the pristine carbon black particle is assumed larger in the core (35%) than in the periphery (20%). Thus, in agreement with experimentals the model tells that Fe/N/C sites for the ORR are created when the micropore area increases (0 to 30-35 wt % loss) and destroyed when it decreases (weight loss > 40%).