Homogeneous Catalysis of Gas-Phase Atom Recombination. Recombination of H Atoms at High Temperatures

Abstract

An analysis is made of the effect of a catalyst C on the rate of recombination of atoms X when C forms a relatively long-lived complex with X. Labeling the energy-rich, nascent complex CX*, the kinetic sequence is X+C⇋ lim baCX*,CX*+M⇋ lim dcCX+M,CX+X→ lim eC+X2. Using steady states for CX* and CX, the rate of recombination assumes three interesting forms in different limiting cases: (1) Rate ∞ (C) (X)2; (2) rate ∞ (C) (M) (X); (3) rate=ka (C) (X). Case 1 corresponds to a weak complex at high pressures; Case 2 has not been reported. It corresponds to the case of a not-too-weak complex forming at very low total pressures. The last case, 3, corresponds to a very strong complex with a long lifetime. It is shown that this is an easily realized case, and that catalytic efficiencies for it can be very high. Applying the scheme to H-atom recombination in the temperature range 1500–2500°K and calculating the lifetime of CX* from RRK theory, it is shown that CH3≡(C) may be a very effective catalyst. At 3 mm Hg pressure of (H) and 100 mm Hg of (H2) adding 1 mm Hg of CH4 which forms CH3 could produce a rate of recombination 1000-fold faster than the normal termolecular reaction. Under the same conditions NH3 is about 10-fold slower, C2H2 about 30-fold slower, and H2O about 500-fold slower than CH4.