Photocatalytic performance of HMS doped with chromium or vanadium for hydrogen production in aqueous formic acid solution

A series of transition metal doped photocatalysts, denoted as M(x)-HMS (here, M represents Cr or V, x represents the molar ratio of M/Si in the feed reagents and HMS represents hexagonal mesoporous silica molecular sieves), were synthesized by adjusting the value of x. The HMS materials were characterized and analyzed by X-ray fluorescence spectroscopy (XRF), N-2 adsorption-desorption isotherms, X-ray diffraction patterns (XRD), ultraviolet-visible diffuse reflectance spectra (UV-vis), high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDXS). The photocatalytic activity was investigated using high pressure mercury lamp as the light source and the decomposition of formic acid to produce hydrogen as probe-reaction. Interestingly, the rate of hydrogen production for both Cr(x)-HMS and V(x)-HMS shows bimodal character which coincidentally corresponds to the x value of 0.01 and 0.05, and it was interpreted in terms of the components and structures of M(x)-HMS.