In situ observation of the exchange reaction of formate with molecular formic acid on Ni(110)

The exchange reaction of adsorbed formate on Ni(110) with formic acid in ga s phase was studied by the time-resolved infrared reflection absorption spe ctroscopy (TR-IRAS). It was found that the pre-adsorbed deuterated formate (DCOO(a)) was changed to HCOO(a) in the presence of HCOOD(g) at the tempera ture region from 240 to 300 K. In such a temperature region, the formate wa s stable on the surface in vacuum and the decomposition of formate was not observed. It was suggested that the pre-adsorbed DCOO(a) was desorbed as DC OOD accepting deuterium atom from HCOOD. The activation energy of the excha nge reaction was estimated to be 23 +/- 2 kJ mol(-1). The reaction order wi th respect to formic acid pressure was 0.3 +/- 0.1, while that as to the co verage of DCOO(a), was unity. Taking into account all the experimental resu lts, the exchange of pre-adsorbed formate with post-exposed formic acid is suggested to proceed via forming a hydrogen-bonded intermediate complex on the formate's site, because all the sites were already occupied by formate. The small activation energy of the exchange reaction is discussed on the b asis of 'adsorption-assisted desorption'. We propose that when desorption a nd adsorption are coupled, the activation energy of the exchange reaction s hould be described as a sum of the potential energy profiles of a desorbing and an adsorbing molecule. (C) 1999 Elsevier Science B.V. All rights reser ved.