Acetonitrile Adsorption and Adhesion Energies Onto the Pt(111) Surface by Calorimetry

Acetonitrile is a common solvent for heterogeneous catalysis and electrochemical applications, as well as the simplest example of an organic nitrile. Understanding the energetics and nature of its interactions with Pt(111) is of interest for a wide array of applications, especially for estimating the effect of acetonitrile as a solvent on the adsorption energies of catalytic and electrocatalytic reaction intermediates on Pt-group metals. Here, the heat of adsorption of molecular acetonitrile on clean Pt(111) was measured by single-crystal adsorption calorimetry (SCAC) as a function of coverage, and from that, the adhesion energy of liquid acetonitrile to Pt(111) was estimated. At 180 K, the differential heat of adsorption is initially 82.9 kJ/mol, decreasing to 63.2 kJ/mol by a saturation coverage of 0.25 ML. The integral (average) heat of adsorption at 180 K is 74.3 kJ/mol at a saturation coverage of 0.25 ML. At 100 K, the heat of adsorption is 84.5 kJ/mol initially, decreases to similar to 45 kJ/mol after the completion of the first layer (0.35 ML), and finally reaches a constant multilayer heat of 43.4 kJ/mol at coverages higher than 0.7 ML. (Errors bars on all heats are +/- 4%.) The saturation coverage is higher at 100 K than 180 K, attributed to the formation of a less stable adsorbate configuration at 100 K due to lower surface mobility. Using the heats of adsorption at 100 K up to bulklike multilayer coverages, we estimate the adhesion energy for liquid acetonitrile to Pt(111) to be 0.198 J/m(2), which can be used to estimate the effect of acetonitrile as a solvent on the adsorption energies of catalytic reactants relative to the gas phase. This adhesion energy is considerably lower than those for water, benzene, and phenol on Pt(111) but slightly greater than those for methanol and formic acid.