Influence of the thermal treatment on the physicochemical properties and photocatalytic degradation of 4-chlorophenol in aqueous solutions with tungstophosphoric acid-modified mesoporous titania

Abstract

Materials based on titania directly modified with tungstophosphoric acid were prepared using titanium isopropoxide as titania precursor and urea as a low-cost pore-forming agent. The obtained mesoporous solids presented an average pore diameter higher than 3.1 nm. The specific surface area decreased when the TPA amount and the calcination temperature increased. All the modified solids only showed the anatase structure of titania by X-ray diffraction, while for the unmodified sample this structure was transformed into the rutile phase from 600 ◦C on. The crystallite size increased with the calcination temperature,though the TPA-modified samples presented lower values when the TPA amount was increased. The 31P nuclear magnetic resonance studies showed that the Keggin structure of the tungstophosphate anion was partially transformed when it was thermally treated up to 600 ◦C, though structure disruption took place at higher temperature. The band gap values estimated from the UV–vis-diffuse reflectance spectra did not show important variation, though they slightly decreased with the TPA content. The 4-chlorophenol degradation was studied in liquid phase, with an air flow continuously bubbled. The degradation behavior of the catalysts as a function of time depended on the TPA amount and the thermal treatment temperature, the samples containing 30% TPA calcined at 500–600 ◦C and 20% TPA calcined at 600 ◦C being more effective. The apparent reaction constant, estimated assuming a pseudo-first-order kinetics, followed the same trend. The catalysts can be reused at least three times without an important decrease in the degradation and mineralization degrees.