Visible-light photocatalytic degradation of azo dyes in water by Ag3PO4: An unusual dependency between adsorption and the degradation rate on pH value

• Effects of solution pH on photodegradation of azo dyes in water by Ag3PO4 are reported.
• An exactly contrary adsorption-degradation relationship for azo dyes/Ag3PO4 is found.
• Dye adsorption decreased with increasing solution pH, while degradation rate increased with increasing solution pH.
• OH-related defects play a pivotal role in the adsorption-degradation properties of Ag3PO4.

During visible light photocatalytic degradation of Congo red (CR) in water at different pH values by Ag3PO4 catalyst, the process was found along an irregular dependency between the degradation rate and adsorption capacity. The adsorption efficiency of CR on Ag3PO4 decreased with increasing initial solution pH, whereas its degradation efficiency increased with increasing solution pH value. This phenomenon was significantly different from conventional photocatalytic properties of most catalysts for heterogeneous photocatalysis of organic compounds in aqueous solution. Three other azo dyes, i.e. Methyl orange (MO), Methyl red (MR) and Orange II, were used to further confirm such an adsorption-degradation relationship of Ag3PO4/dye at different pH value of solution. The tested three dyes behaved similar to CR, and they showed unexceptionally higher adsorption behavior and lower degradation efficiency at low pH value or vice versa at high pH value. This special adsorption-degradation relationship for azo dyes with Ag3PO4 as visible-light catalyst was correlated with the OH group-related defects present on Ag3PO4 surface as determined by FTIR spectroscopy and surface fluorination method, implying that the defects played an important role for the visible-light photocatalysis of Ag3PO4 in water solution.

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