Research On Direct Synthesis Of H₂O₂from H₂and O₂Over Pd/TiO₂Catalysts

Hydrogen peroxide (H2O2) is widely used in many fields. It is now produced through anthraquinone auto-oxidation (AO) process, which has many drawbacks including high investigation, complicate processes, pollutions and so on. Synthesis of H2O2directly from H2and O2over a catalyst is a very promising way, which considered to be a substitution of the AO process. The key point is designation and preparation of a high performance catalyst. In this paper, the investigation of TiO2, SiO2, A12O3, MCM-41supports shows that1wt%Pd/TiO2catalyst has a very high H2O2selectivity (60.9%). Catalysts calcination temperature has very strong influence on the Pd leaching, and could enhance the stablity of the catalysts. The Pd leaching the least under a calcination temperature of673K. Pd loading amount affects the catalytic performance significantly, H2conversion increase with Pd loading amount, while H2O2selectivity and turnover frequency (TOF) decrease. The results show H2O2concentrtion would decrease through catalytic hydrogenation, and the rate increases with the Pd loading amount increased. TEM images show that Pd dispersed uniformly on the TiO2support, and the average size of Pd is2.3nm. X-ray photoelectron spectroscopy (XPS), Extended X-ray absorption fine structure (EXAFS), O2-Temperature progammed desorption (O2-TPD) demonstrate the Pd8+formation on the surface of the catalyst. CO-Diffusion reflection infrared Fourier Transform spectra (CO-DRIFTS) prove the statement that Pd on the Pd/TiO2catalyst could be oxidized when contacted with O2. CO-DRIFTS of catalysts with different Pd loading amount reveal the difference of Pd arrangement on the surface of the catalysts. These evidences indicate the Pd8+formed during the exposure in air could isolate the continious Pd ensembles, which affects the adsorption and dissciation of the reagent gas and also desorption of the product. This isolation leads to the increase of H2O2selectivity. Based on this result, N-doped TiO2is synthesized to modify the catalysts. The catalytic performance changes with the doping process.1%Pd/Nc-TiO2catalyst, which is made from TiO2pretreated with N2, exhibits a much higher H2O2selectivity (74.9%). XPS shows there’s more Pd8+on the surface of1%Pd/Nc-TiO2catalyst. The results provides a new strategy and method for the controllable synthesis of catalyst for H2O2direct formation.