Preparation Of W-doped PdAu Alloy Nanocatalysts And Their Performance For Direct Synthesis Of Hydrogen Peroxide

As a class of green chemicals,hydrogen peroxide（H₂O₂）is extensively utilized in chemical synthesis,papermaking and textile,waste water treatment as well as other industries.The current industrial method of producing H₂O₂ is the anthraquinone（AQ）method,which is an indirect method of continuous hydrogenation and oxidation of AQ.However,the anthraquinone method has disadvantages such as complex synthesis process,high production cost and large pollution.The direct synthesis of H₂O₂（DSHP）from hydrogen and oxygen has received a lot of interest in recent years because of its unique benefits such like flexibility,energy efficiency and non-polluting to the environment.And in this process,Pdexhibits better catalytic activity than other transition metals.However,Pd-based catalysts also catalyze the generation of H₂O as a by-product,resulting in low H₂O₂ selectivity,which limits the further industrial application of the DSHP process.The addition of a second or third metal component has been reported as a common strategy to enhance the performance of Pd-based catalysts.In this paper,the catalytic performance of PdAu alloy nanocatalysts in DSHP was improved by introducing the non-precious metal W into them,and the promotion mechanism of W additives in PdAu alloy nanocatalysts was analysed.The specific studies in this paper are as follows:（1）The W-doped PdAu alloy nanocatalysts were synthesized by changing the loading order of W and PdAu using the impregnation method,namely PdAu-W/TiO₂（loading W before PdAu）,W-PdAu/TiO₂（loading W after PdAu）and Pa Au W/TiO₂（loading W and PdAu at the same time）,and it was determined that loading W before PdAu on the carrier was the optimal loading order.The H₂O₂ productivity and selectivity of the PdAu-W/TiO₂catalyst were 662.6 mol _H2O2 kg^-1_cat h^-1 and 85.5%,respectively,which were superior to those of the PdAu/TiO₂ catalyst(450.2 mol _H2O2 kg^-1_cat h^-1 and73.9%).The performance of the three different loading order catalysts was ranked in order of magnitude:PdAu-W/TiO₂>W-PdAu/TiO₂>PdAu W/TiO₂.Further characterisation showed that the loading order influenced the degree of interaction between W and PdAu in the catalysts,with the stronger the interaction,the better their ability to catalyse H₂O₂ production.Thus,the strong W-induced interaction with PdAu was a key to improving the catalytic selectivity of H₂O₂,with the WO_x species partially encapsulating the PdAu particles and inducing the exposure of surface PdO,which was favourable for H₂O₂ production.（2）A set of PdAu-W/TiO₂ catalysts with different W contents were prepared by varying the W loading（1,2,3,4 and 5 wt%）.The H₂O₂productivity,H₂ conversion and H₂O₂ selectivity showed a volcano-type trend with increasing W content,and the catalytic performance of PdAu-3W/TiO₂was best when the W loading was 3 wt%.However,further increasing the W content would lead to complete encapsulation of the PdAu active site by WO_xand inhibit the synthesis of H₂O₂.（3）Three groups of PdAu and PdAu-W catalysts with different carriers were prepared by varying the type of carrier（TiO₂,Al₂O₃ and Si O₂）.On the catalysts with Al₂O₃ and Si O₂ as carriers,the promotion of W was not obvious,and the characterisation results indicated that the strong W-induced interactions were dependent on the type of carrier.