THE TEMPERATURE AND COVERAGE DEPENDENCES OF ADSORBED FORMIC-ACID AND ITS CONVERSION TO FORMATE ON PT(111)

We have studied the adsorption of HCOOH on Pt(111) at 80-100 K and its conversion to formate with increasing surface temperature. The techniques employed are thermal desorption spectroscopy and high-resolution electron energy loss spectroscopy. At very low exposures (< 0.2 langmuir), we Posit that HCOOH exists molecularly as monomers or discrete dimer pairs. As exposure increases, there is evidence for hydrogen-bonded chains with the molecular plane of HCOOH nearly parallel to the surface, At an exposure of 0.2 L, these chains resemble the solid-phase beta-polymorph as indicated by the vibrational frequencies of the OH out-of-plane bending vibration. Heating this surface causes the chains to break apart into discrete dimer pairs, followed by deprotonation to a bridging formate adspecies. The formate decomposes between 210 K and 280 K, causing CO2 desorption and leaving hydrogen adatoms. Increasing the exposure above 0.6 L causes the chains to adopt a structure similar to the denser alpha-polymorph. Heating this surface causes molecular desorption in two states. One is centered at 160 K and is dominated by gaseous HCOOH dimers. Desorption in this state leaves bridging formate and beta-polymorphic HCOOH coexistent on the surface. The other is centered at 200 K and is primarily gaseous monomers. Desorption in this state leaves only the formate adspecies. Its decomposition then proceeds as on the surface exposed to 0.2 L HCOOH.