The effect of TiO2 doping on the catalytic properties of nano-Pd/SnO2 catalysts during the reduction of nitrate

The monometallic catalysts Pd/TiO2–SnO2 and Pd/SnO2 prepared by impregnation method were investigated for their ability to catalyze the reduction of nitrate with formic acid as the reducing agent and were characterized by X-ray diffraction (XRD), BET surface area, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Pd and TiO2 were in high dispersion on the surface of the SnO2 support when the mol fraction of TiO2 was less than 15%. The SnO2 crystal lattice was partially replaced by titanium ions, causing SnO2 lattice distortion and resulting in the increase of oxygen vacancies as catalytic active sites. The best catalytic properties of the formic acid–Pd/TiO2–SnO2 system were achieved at a mol fraction of 10% TiO2. Compared with the Pd/SnO2 catalyst, the catalytic activity of Pd/TiO2–SnO2 increased by 48% from 2.08 mg/(min gcata) to 4.00 mg/(min gcata), while the NH4+ concentration decreased from 13.0 mg/L to 5.2 mg/L. The catalytic activity, selectivity and stability of the Pd/TiO2–SnO2 catalyst were superior to those of Pd/SnO2 with the same Pd loading and formic acid concentration. The results of the XPS analysis indicated that the catalysts Pd/SnO2 and Pd/TiO2–SnO2 were of high oxygen vacancy. Pd/TiO2–SnO2 has better catalytic properties than Pd/SnO2 may attribute to the increased amount of oxygen vacancy after TiO2 doping.

Graphical abstractThe XRD spectra of Pd/TiO2–SnO2 at different mol fraction of TiO2 shows when the TiO2 mol fraction exceeded 10%, the characteristic peaks of TiO2 appeared at the diffraction angle 2θ 48.03° and 75.04°, suggesting that TiO2 had exceeded the maximum dispersion content on the surface of SnO2 and was partly deposited on the surface of SnO2 in the form of crystalline TiO2.The catalytic activity of the doped Pd/TiO2–SnO2 catalysts was higher than that of the simple Pd/SnO2 catalyst. Lattice distortion caused by introduced titanium ions which increase the redox active center oxygen vacancies is helpful for the improvement of the catalytic activity of Pd/TiO2–SnO2 composite catalysts.Figure optionsDownload full-size imageDownload as PowerPoint slide