Removal of humic acids from synthetic wastewater by rice husk ash and modified rice husk ash

Abstract

In this study, titanium dioxide was loaded with chromium together with either platinum or rhodium in attempt to improve its photocatalytic activity for water splitting. Titanium dioxide was synthesized via a sol-gel method and the metals were loaded by incipient wetness impregnation technique. The amount of chromium added was varied between 0 to 1 % (w/w), while the amount of the second metals (either Pt or Rh) added was fixed at 1 % (w/w). The catalysts were characterized by UV-visible diffuse reflectance spectroscopy, X-ray diffraction (XRD), nitrogen physisorption, transmission electron microscopy (TEM), and photoluminescence. Addition of chromium up to 0.05 % (w/w) enhanced the photocatalytic activity because chromium prevented recombination of photogenerated electrons and holes in titanium dioxide. However, when the amount of chromium added exceeded 0.05% (w/w), the photocatalytic activity decreased owing to overtrapping of charge carriers by chromium. Photoluminescence measurement also suggested that titanium dioxide loaded with 0.05 % (w/w) Cr, of which photoluminescence signal was the lowest, had the slowest recombination of charge carriers. Addition of chromium together with platinum or rhodium further increased the photocatalytic activity for water splitting. The catalysts that possessed the highest activity were titanium dioxide that was loaded with 0.01 % (w/w) Cr and 1 % (w/w) Pt or Rh. The optimal loading of the metals could be explained by the similar reason above. Furthermore, a synergy between chromium and the other metal was observed. The amount of hydrogen gas produced from photocatalytic water splitting over titanium dioxide loaded with chromium together with either platinum or rhodium was greater than the sum of the amount of hydrogen gas produced from the reaction over either metal alone.