An <i>in situ</i> infrared cell capable of studying reactions over heterogeneous catalysts in the temperature range 77 to 773 K has been designed. In particular, the adsorption of formic acid on a model Cu/SiO<sub>2</sub> methanol synthesis catalyst was investigated. Exposure of a reduced copper surface to formic acid at 300 K resulted in the formation of both formic acid molecules, which were ligated to the copper catalyst, and chemisorbed bidentate copper formate species. Under temperature-programming conditions, the bidentate species displayed a maximum rate of desorption at 433 K, which correlates to a desorption activation energy of 120 kJ mol<sup>-1</sup>. In contrast, on the reoxidized catalyst, unidentate formate species were preferentially formed. These exhibited a maximum rate of desorption at a temperature of 408 K, and a desorption activation energy of 113 kJ mol<sup>-1</sup>. A mechanism was postulated to explain this behavior, and evidence was presented to show that useful kinetic data can be obtained for desorption from a catalyst in the form of a pressed disk.

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