Quantitative analysis of ammonia adsorption in Ag/AgI-coated hollow waveguide by mid-infrared laser absorption spectroscopy

Abstract

The adsorption effect of ammonia gas molecules on the inner surface of silver and silver iodide-coated hollow waveguides (Ag/AgI-HWGs) was studied by using a direct absorption spectrometry system with a 9.56-µm quantum cascade laser as the light source. The dynamic changes in ammonia concentration in the Ag/AgI-HWG during the process of NH3 influx and N2 purge were measured with a high time resolution. The adsorption and desorption processes of ammonia were quantitatively analyzed. The results indicate that the density of ammonia molecules adsorbed on the inner wall surface of the Ag/AgI-HWG was 1015 molecules/cm2, characterized as multilayer adsorption, and the corresponding F value (a dimensionless number commonly used to characterize influence of the adsorption effect) was about 5.89. The adsorption capacity increased with the pressure and decreased with the temperature, which is consistent with Le Chatelier's principle. The differential heats of adsorption were deduced to be 7.6 and 6.1 kJ/mol for monolayer and multilayer adsorption, respectively, and the Brunauer-Emmett-Teller model was established and agreed well with our measurement results. The adsorption kinetic process also fit well to both the pseudo-first-order and pseudo-second-order kinetic model. This study fills in the gap in the research of Ag/AgI-HWGs adsorption and is meaningful for accurate gas detection using HWG-based spectroscopy systems.