Interaction of ammonia with intrazeolitic silver ions: Development of an ammonia sensor

This paper examines the interaction of ammonia with Ag+-exchanged zeolite Y (Ag-Y) over a range of temperatures (250–450 °C). Impedance spectroscopy over the range of 0.5–107 Hz indicates that the Ag+ motion within the zeolite is facilitated in the presence of ammonia. Infrared spectroscopy and X-ray photoelectron spectroscopy indicate that Ag+NH3 bonds are being formed. Temperature programmed desorption coupled with impedance spectroscopy show that the NH3 bonding to Ag+ is disrupted around 300 °C. However, in the presence of excess NH3, a fraction of the Ag+NH3 bond survives up to 450 °C. Control experiments show that there is no measurable autoreduction of Ag+, and therefore, proton-mediated change of impedance in the presence of NH3 is not considered relevant. The change in mobility of the Ag+ in the presence of NH3 has been exploited to design an impedance-based NH3 sensor at a fixed frequency of 1000 Hz in the temperature range of 275–350 °C, with the optimal sensor response at 300 °C. No cross-sensitivity was observed for O2, CO, CO2 and propane. Nitric oxide showed minor interference. Water, however, did show an interference with a decrease in baseline impedance with increasing water. Thus, for practical applications of this sensor, the water content in the gas stream will need to be controlled to a constant value.