AN INFRARED STUDY OF THE ADSORPTION OF AMMONIA ON POROUS VYCOR GLASS

Abstract

The infrared spectrum of ammonia adsorbed on porous glass at 20 °C and 150 °C has been studied in the region 1450–4000 cm−1. No absorption band due to the asymmetric bending mode of ammonia was observed but in the NH stretching region, bands occurred at 3280 cm−1, 3320 cm−1, 3365 cm−1, and 3400 cm−1. The bands at 3320 cm−1 and 3400 cm−1 were easily removed by evacuation and are due to ammonia molecules hydrogen bonded through the nitrogen atom to surface hydroxyl groups. The bands at 3280 cm−1 and 3365 cm−1 were not removed by evacuation even at 150 °C and are due to ammonia molecules held to surface Lewis acid sites by the nitrogen lone-pair electrons. The site for this adsorption is not a surface hydroxyl group. These results are further evidence for the existence of the two adsorption sites proposed by Folman and Yates. Deuteration of the surface OH groups was easily accomplished with D2O vapor at 300 °C and the rate of hydrogen exchange between adsorbed ammonia molecules and surface OD groups was found to be rapid.