THE ADSORPTION AND DECOMPOSITION OF FORMIC-ACID ON CLEAN AND OXYGEN-DOSED PD(110)

The interaction of formic acid (HCOOH) with Pd(110) was studied using TPD and XPS. At low doses, at an adsorption temperature of 140 K, HCOOH decomposed to yield CO, CO2, H-2 and H2O during TPD. CO and H-2 were evolved in desorption-limited peaks at 470 and 320 K, respectively. The CO and H2O desorptions were saturated at very low doses indicating only a small degree of formate dehydration in such a transient experiment. The CO2 and H-2 evolution continued to increase with dose and the CO2 evolved in a decomposition-limited peak at 237 K at low doses which broadened to lower temperature with increasing coverage. This dehydrogenation of HCOOH was of first order with an activation energy of 40 kJ mol-1. At doses above 0.5 L molecularly chemisorbed HCOOH was observed to desorb at 210 K and at still higher doses physisorbed multilayers were present and desorbed at 180 K. XPS confirmed the presence of CO above 250 K and formate below this temperature. When oxygen was pre-dosed the formate was significantly stabilized and its coverage increased, the CO desorption was removed and H-2 was produced at higher temperatures at the expense of the H-2 peak at 320 K.