Energetics of Adsorbed Formate and Formic Acid on Cu(111) by Calorimetry

Abstract

The heats of adsorption of formic acid molecularly adsorbed on clean Cu(111) and dissociatively adsorbed on oxygen-predosed Cu(111) were measured by adsorption calorimetry. The dissociative adsorption of formic acid on oxygen-predosed Cu(111) at 240 K produces adsorbed bidentate formate and gaseous water. For this, the differential heat of adsorption is initially 99 kJ/mol, decreasing to 80.4 kJ/mol by ∼0.20 ML. This is (to our knowledge) the only experimental measurement of the energy of any molecular fragment bonded to any clean and well-ordered Cu single-crystal surface, making these results important benchmarks for validating the energy accuracy of computational models for a wide range of adsorbates on Cu catalysts. The integral (average) heat of adsorption at 240 K and 0.20 ML is 89.7 kJ/mol. From this, the heat of formation of bidentate formate and its bond enthalpy to Cu(111) were determined to be approximately −465 and 335 kJ/mol, respectively. Corresponding values were estimated for monodentate formate on Cu(111), giving −437 and 307 kJ/mol. Comparing these values to previous calorimetric results on Pt(111) and Ni(111) shows that the bond enthalpies and enthalpies of formation of formate on Cu(111) are similar to those on Ni(111) but ∼75 kJ/mol stronger than on Pt(111). Comparing these to reported DFT calculations shows that DFT systematically underestimated the stability of formate on these metals by ∼50 kJ/mol. The differential heat of molecular adsorption of formic acid on clean Cu(111) at 120 K is initially 81 kJ/mol, dropping to ∼70 kJ/mol by ∼0.50 ML, before decreasing to a multilayer energy of 64.3 kJ/mol. Using this heat of adsorption through bulk-like multilayer coverages, we estimate the adhesion energy for liquid formic acid to Cu(111) to be 0.27 J/m2.

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