Phase transformation and activity of cadmium sulfide photocatalysts for hydrogen production from water: Role of adsorbed ammonia on cadmium sulfate precursor

Adsorption-desorption of ammonia on cadmium sulfate (3CdSO(4). 8H(2)O) has been studied. Its role in formation of cubic phase of CdS when cadmium sulf ate is reacted with H2S has been investigated in detail using X-ray diffrac tion, electron diffraction, infrared spectroscopy, temperature-programmed d esorption of NH3, and thermogravimetric analysis. Three distinct; acidic si tes on cadmium sulfate where ammonia can get chemisorbed have been reported . The heats of adsorption for the three sites are 24.3, 26.9, and 42.6 kcal mol(-1), respectively. The origin of surface acidity is related to the for mation of transition intermediates during the dehydration of 3CdSO(4). 8H(2 )O. Both Bronsted and Lewis acidities have been observed. The reaction of a mmonia preadsorbed cadmium sulfate with H2S gas leads not only to the forma tion of larger crystallites but also to the cubic phase of CdS which has lo wer photocatalytic activity for hydrogen production from water. Ammonia can be completely desorbed from the surface of cadmium sulfate if it is heated above 600 K.