Paper SS1-TuM2
Measurement of the C-Pt Bond Energy for Adsorbed Methyl on Pt(111) by Methyl Iodide’s Dissociative Adsorption Energy

Tuesday, November 1, 2011, 8:20 am, Room 109

Knowing the bond energies of molecular fragments adsorbed to surfaces is an important ingredient in the fundamental understanding needed for rational design of heterogeneous catalysts. This study utilizes the ultrahigh vacuum technique of single crystal adsorption calorimetry to study the energetics of adsorbed methyl (CH_3,ad) on Pt(111), using measurements of the dissociative adsorption energy of methyl iodide (CH₃I) on Pt(111). At temperatures above 250 K, adsorbed CH₃I on Pt(111) is known to undergo C-I bond scission to form CH_3,ad and adsorbed iodine (I_ad). At 270 K, the rate of C-I scission is fast relative to the 100 ms time of calorimetric measurement, so that it provides the enthalpy for the net, two-step dissociative adsorption reaction:

CH₃I_g → CH₃I_g → CH_3,ad + I_ad. (1)

The integral heat of Reaction (1) is -224 kJ/mol at a coverage of 0.05 ML. Through the use of a simple thermodynamic cycle and other known adsorption energies, this provides a heat of formation of CH_3,ad of -63 kJ/mol and a Pt—CH₃ bond energy of 209 kJ/mol.

Mass spectrometer measurements taken during experiments at 300 K and 320 K indicate that methane is produced. Previous studies indicate that at these temperatures, adsorbed methyl both decomposes to form adsorbed methylene (CH_2,ad) and hydrogen (H_ad) and reacts with H_ad to form methane according to the reactions :

CH_3,ad → CH_2ad + H_ad (2),

CH_3,ad + H_ad → CH_4,g (3)

where CH_4,g is gas phase methane. The microcalorimetric measurements performed at 300 K and 320 K, along with information on the rates of Reactions (2) and (3) available in the literature, allowed for the decoupling of Reactions (1)-(3) and an analysis of the energetics of all three reactions as a function of surface coverage, and the heat of formation of CH_2,ad.