Plasma-Induced Chemisorption of Nitrogen and Its Chemical Reactivity on Al₂O₃ and Pt/Al₂O₃ Surfaces

Abstract

Nitrogen, one of the most chemically inactive molecules, does not adsorb on A1₂O₃ and Pt/Al₂O₃ catalysts at room temperature. However, once it was excited by plasma discharge using radio-frequency (13.56 MHZ), activated adsorption of nitrogen occurred on Pt/Al₂O₃ and Al₂O₃ at room temperature. The characterization of chemisorbed N₂ and its chemical reactivity with hydrogen have been studied by temperature programmed desorption (TPD) and infrared spectroscopy (IR). The adsorbed nitrogen on Pt/Al₂O₃ or Al₂O₃ was desorbed below 300°C by TPD, and IR spectra of the adsorbed nitrogen on Pt/Al₂O₃ were observed at ca . 2260 and 2230 cm^-1, which were assigned to N-N stretching vibration. Since almost the same absorption bands were observed with N₂ on Al₂O₃, it was considered that N₂(a) was chemisorbed mainly on the Al₂O₃ support . The H₂ exposure experiment at room temperature showed that the infrared bands of N₂(a) on the Al₂O₃ support (without Pt) did not change, but the intensity of the spectrum of N₂ on the Pt/Al₂O₃ catalyst decreased significantly with time. Moreover, NH₃ was detected in the liquid N₂ trap by GC after the TPD run. It was proposed that the N₂(a) reacted with the hydrogen spilt-over from the Pt surface to form other species like NH_x(a) (X=1∼3) even at room temperature.