Photoinduced Generation of CO2- Anion Radical on K-Promoted Rh(111) Surface

Abstract

Photoinduced formation of the CO[sub 2][sup [minus]] anion radical was investigated Rh(111) surfaces that were clean or covered with hydrogen, iodine, or potassium. The methods used were high-resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy. Illumination of CO[sub 2] adsorbed on a clean Rh(111) 0surface with a 100-W Hg are lamp produced no HREEL spectral signals indicative of the formation of CO[sub 2][sup [minus]]. The same was observed for the hydrogen- and iodine-promoted surfaces. However, in the presence of potassium adatoms, which lowered the work function of the metal by 3.5-4.0 eV, illumination of the adsorbed CO[sub 2] induced or enhanced formation of the CO[sub 2][sup [minus]] radical even at 90 K. New loss features attributed to the asymmetric stretch, to the symmetric stretch, and to the bending mode of the bent CO[sub 2][sup [minus]] appeared at 1610, 1320, and 830 cm[sup [minus]1] in the HREEL spectrum of Rh(111). The presence of the species CO[sub 2][sup [minus]] on the surface led to an enhancement of the amount of physisorbed CO[sub 2], as indicated by significantly higher losses at 2350 and 640 cm[sup [minus]1]. This suggests the coupling between neutral and charged CO[sub 2][sup [minus]] to yield a clustermore » compound (CO[sub 2][center dot]CO[sub 2][sup [minus]]), which is known to exist in the gas phase. 22 refs., 7 figs., 1 tab.« less