Enhanced Ammonia Sorption by Copper and Zinc Molybdates with Triclinic Structure

Ammonia gas sorption by nanocrystalline cobalt, nickel, copper, and zinc molybdates with various polymorphic modifications has been studied. The samples of monoclinic cobalt, nickel, and zinc molybdates were obtained by citrate and hydrothermal synthesis methods, while triclinic copper and zinc molybdates were obtained by coprecipitation and ultrasonic synthesis. Using X-ray diffraction, low-temperature nitrogen adsorption, and scanning electron microscopy, it was established that the samples possess close microstructure parameters. Temperature-programmed desorption revealed that NH3 desorption from the monoclinic phases of CoMoO4, NiMoO4, and β-ZnMoO4 proceeds at temperatures of 70–600°C and corresponds to fractions of the monolayer of adsorbed ammonia on surface acid sites. Triclinic modifications of α-ZnMoO4 and α-CuMoO4 sorb higher amounts of ammonia (0.3–0.5 mol NH3 per mol of oxide) and desorb it on heating to 260°C. X-ray diffraction analysis and infrared spectroscopy revealed that ammonia molecules in the adducts are bound to copper(II) and zinc cations. It has been concluded that the crystal structure of molybdates plays a key role in ammonia adsorption. The results can be used in the development of sorbents, sensors, catalysts, and photocatalysts.