Chemical energy accommodation at catalyst surfaces. Flow reactor studies of the association of nitrogen atoms on metals at high temperatures

Abstract

The fate of the energy release in highly exoergic surface-catalysed chemical reactions is of considerable fundamental interest and influences catalyst volatilization/sintering, the aerodynamic heating of hypersonic glide vehicles subject to bombardment by atomic nitrogen and atomic oxygen, etc. To provide the first available high temperature data (T > 800 K) on what fraction (β) of the equilibrium (bond) dissociation energy is delivered to the catalyst per atom association event, a coaxial filament flow reactor (CFFR) has been developed, well-suited to both precise atom mass balance and isothermal calorimetric measurements. Experimental results for the chemical energy accommodation (CEA) coefficient β, and the corresponding N-atom recombination probabilities, γ, are presented for the metals Pt, Ir, Rh, Pd, Co, W and Re at temperatures up to 2600 K. Catalyst energy deposition can be an order of magnitude less than the equilibrium reaction energy. However, since this is not true at all surface temperatures, simple rankings of β-values for metals (at, say, room temperature) or correlations based only on one or two relevant system parameters (e.g. bulk Debye temperature) are of limited application. Alternatively, for N/Re, N/W a Langmuir-type mass-action analysis of the operative elementary steps, combined with a simple postulate (viz. Rideal-produced molecules leave excited whereas Langmuir—Hinshelwood (LH) produced molecules do not) provides a semi-quantitative understanding of β-trends in terms of the adatom binding energy, the equilibrium bond dissociation energy of the product molecule and an elementary Rideal reaction probability. However, high β-values can be observed at low temperatures if the system admits LH-reaction, or Rideal-formed excited molecules are rapidly quenched prior to desorption. We postulate that low catalyst energy deposition occurs at high temperatures (N/Pt, N/Ir) if LH-reaction occurs prior to the complete accommodation of the reactant (atom) chemisorption energy.