Influence of synthesis method on leaching of the Cr-TiO2 catalyst for visible light liquid phase photocatalysis and their stability

•Cr-TiO2 nanoparticles have been synthesized by various methods.•Degradation of 4-chlorophenol under visible light (400–800 nm) conditions.•Cr+6 had significant influence in the photocatalytic activity of the catalyst.•Photocatalytic activity decreased for Cr/TiO2 whereas Si/Ti/Cr is stable.•Leaching of the Cr into the liquid phase for pH ranging from 3 to 11 was studied.

Cr-TiO2 nanoparticles have been synthesized by using flame spray pyrolysis (FSP), co-precipitation, and sol–gel synthesis techniques. Liquid phase photocatalytic activity of the synthesized catalysts by various methods were studied for photocatalytic degradation of 4-chlorophenol under visible light (400–800 nm) conditions. It was found that the FSP Cr-TiO2 has superior photocatalytic activity to other synthesis methods. XRD patterns showed that the anatase phase was higher for the sol–gel and co-precipitation made catalyst. Our TPR results indicated higher presence of the Cr+6 of the FSP made catalysts in comparison to the other ones, which had significant influence in the activity of the catalyst. All the three catalysts showed the decline in the activity which was a consequence of the decrease of Cr+6 content of the catalyst along with the forming agglomerates in the spent and reactivated catalyst. The leaching of the Cr into the liquid phase for pH ranging from 3 to 11 was studied of the FSP made catalyst and was found to be minimal with respect to other synthesis methods. For this reason there should be careful consideration for use of Cr containing catalysts for practical water treatment. We developed Si/Ti/Cr catalyst by FSP method which stabilizes the photocatalytic activity of the catalyst. The BET surface area of the FSP Si/Ti/Cr catalyst was stable even with the reactivated catalyst. The developed catalyst also demonstrated an advantage of low levels of metal leaching during the catalytic reaction thus avoiding secondary metal contamination to the treated wastewater. We found higher Cr(VI) concentration in the reactivated materials which is a significant factor for the improvement in the photocatalytic activity of the materials. We have observed the improvement in the activity with the addition of Si by FSP method as a support and it also caused the improvement in the surface area and structural stability.

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