Residual NH3 Research Articles

Electron-induced surface chemistry: Synthesis of NHx fragments on Ag(111)

Electron-induced decomposition of chemisorbed NH3, dosed on Ag(111), at 115 K has been studied using x-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). There is no thermally activated dissociation during dosing or TPD. The N(1s) XPS peak is at 400.1 eV binding energy (BE) and, in TPD, there is a sharp peak at 130 K with a trailing edge toward higher temperatures. Following low energy (50 eV) electron irradiation the 130 K TPD peak area drops sharply while the XPS N(1s) peak area drops much less. This indicates that, while some N-containing species are ejected during electron irradiation, most of the initial N is retained in some form. After irradiating with 5×1016 e cm−2, the N(1s) BE shifts lower and the peak can be fit with two peaks, one corresponding to residual NH3, and a second to NH2 (398.9–399.1 eV BE). Increasing the fluence to 1×1017 e cm−2, shifts the N(1s) profile to even lower BE; two peaks assigned to NH2 and NH (398.1 eV BE) give an acceptable fit.

In situ measurement of ammonia concentration in industrial combustion systems

In the case of noncatalytic reduction of NOx by ammonia, urea or other nitrogen containing compounds, a fast responding ammonia detection system is needed to control the NH3-addition, since the reaction of any residual NH3 with sulfur oxide also present in the flue gas leads to the formation of solid ammonium hydrogen sulfate. This can cause severe damage to plants, especially deposition at heat exchanger surfaces. A laser monitoring system has therefore been developed to measure in situ concentrations of ammonia at temperatures up to 600° C. The NH3 concentration is determined by measuring absorption of 13CO2 emission lines from a tunable 13CO2 waveguide laser. By varying the resonator length, the laser is frequency modulated between two neighboring emission lines. One line coincides with a strong ammonia absorption line while the other is used as a reference. By this referencing the system is autocalibrated continuously. The high spectral resolution of the laser supresses interferences by other species. A detection limit of 1 ppm could be reached at a time constant of 10 s. The system was built up for use in industrial environments and has been applied successfully in several power plants, the chemical industry and waste incinerators.