TiO2/Na-HZSM-5 nano-composite photocatalyst: Reversible adsorption by acid sites promotes photocatalytic decomposition of methyl orange

TiO2/Na-HZSM-5 nano-composite photocatalysts were prepared by dispersing TiO2 onto the external surface of Na+-modified nano-ZSM-5 zeolite using a sol–gel process. Samples were characterized by XRF, XPS, HRTEM, XRD, DRUV–vis, NH3-TPD, FT-IR, and the adsorption and photodegradation of methyl orange (MO) in aqueous solution. Results show the modification of support by Na+ does not affect the loading, dispersion and structure of loaded TiO2, but reduces the acidity of the nano-zeolitic support by preferentially eliminating stronger acid sites. The MO adsorption mainly takes place on the external surface of the supports. The strong and medium strong acid sites of the supports are the adsorption sites. MO molecules adsorbed by strong acid sites are difficult to desorb, whereas the adsorption of MO by medium strong acid sites is reversible. Generally, the MO photodegradation activity of TiO2/Na-HZSM-5 nano-composites is better than that of bare TiO2. However, it changes remarkably with the Na+ content of the supports, giving the maximum value at 1.1 wt.% Na+ when the MO adsorption by the support is almost completely reversible. The enhancement of the photodegradation activity of TiO2/Na-HZSM-5 nano-composites is attributed to the reversible adsorption of MO by the medium strong acid sites on the nano-zeolitic supports.