Promotion and inhibition of Ni(CO)₄ formation on Ni(100): A kinetic investigation coupled with ESCA measurements

The rate of formation of Ni(CO)₄ on a Ni(100) surface was investigated by continuous spectroscopic detection of the reaction product at and above room temperature, as a function of reaction time and surface composition (coverage with C, S, and O). To determine the latter, ESCA spectra of the surface were taken in the same apparatus. An initially clean surface starts with a high rate which drops off very fast to below the detection limit (of about 10¹¹ molecules cm^-2 s^-1); this is accompanied by carbon deposition, probably through CO disproportionation. After a few hours, the rate is roughly stationary and exhibits a maximum at about 390 K and an apparent activation energy of about 67 kJ/mol between room temperature and 320 K. Sulfur precoverage has a strong promoting effect; the rate at room temperature increases exponentially with S coverage for θ_s > 0.3. By S-saturation the rate maximum shifts down to 335 K, and the apparent activation energy around 300 K decreases to 8 kJ/mol. Contrary to common belief, oxygen also increases the rate for the "clean" surface, probably because of removal of C contamination; it decreases the rate for an S-precovered surface because of partial removal of S. Practical surfaces obtained at high vacuum and less stringent purity conditions lead to very complicated behavior of the rate with prehistory which can only be understood qualitatively. The results are discussed in terms of factors controlling the reaction mechanism.