The amorphous phase in titania and its influence on photocatalytic properties

•Amorphous phase amount is an important property of titania-based catalysts.•The proposed method allows one to measure the percentage of amorphous phase.•Titania samples containing a higher amount of the amorphous phase show a lower PCA.•The partial removal of amorphous phase leads to significant increasing of the PCA.

In the present work, we compare the photocatalytic activity (PCA) of titania samples normalized to the sample weight and specific surface area (SSA), and show a correlation between the PCA of titania catalysts with similar phase compositions and amorphous phase contents. In our studies, we used the commercial samples, P25 Evonik (formerly Degussa), Hombikat UV100, and pre-synthesized mesoporous titania. Catalysts with a significant amorphous content were annealed and processed by partial dissolution in acid and included in the analysis. PCA was measured by methyl orange (MO) photodegradation in an aqueous catalyst suspension under high-pressure Hg bulb illumination and the pH was controlled using phosphate buffer (pH 6.9). The weight percentage of the amorphous phase was determined using reference intensity ratios (RIR) for X-ray diffraction patterns of the titania samples measured with crystalline silicon additives. The reproducibility of the proposed method was demonstrated by measuring the amorphous content in mixtures of the sample and XRD-amorphous titania. The contributions of amorphous titanium oxohydroxides TiO2−0.5n(OH)n ·xH2O and water physically adsorbed to the amorphous phases were distinguished by thermogravimetric analysis. The obtained results show that the PCA of the titania samples decreases with the weight percentage of the amorphous phase, as low as 5% of the PCA of P25 in the case of ωAm(TiO2)> 25 %. It was demonstrated that the partial removal of the amorphous phase by annealing or dissolution in nitric acid leads to a significant increase of PCA.

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