Mechanism for the decomposition of iron pentacarbonyl on Pt(111): evidence for iron tetracarbonyl and iron tricarbonyl intermediates

Abstract

The surface chemistry of iron pentacarbonyl on Pt(111) was studied using reflection-absorption infrared spectroscopy (RAIRS). A small amount of molecular adsorption is observed below half saturation at 110 K., but about 70% of the iron pentacarbonyl decomposes upon chemisorption to yield atomic iron and coadsorbed carbon monoxide. Deposition at 275 K, on the other hand, results in total decomposition and in a steady state growth of iron films on the surface; only CO bonded directly to platinum atoms is seen by RAIRS. The molecular adsorption of iron pentacarbonyl is proposed to occur with a concomitant change of geometry from a trigonal bipyramid to a square pyramid as suggested by the high frequency value observed for the CO mode, around 2060 cm −1. Annealing the surface to 185 K after exposures at 120 K results in desorption of the condensed multilayer and in decarbonylation of the remaining molecules to form iron tetracarbonyl, which displays a peak in the RAIRS spectrum at 2035 cm −1. Further heating to 240 K yields two new bands at 1980 and 1935 cm −1, which we assign to Fe(CO) 3. Both Fe(CO) 4 and Fe(CO) 3 desorb as iron pentacarbonyl after recombination with CO at 195 and 265 K, respectively.