The modification of Pd core–silica shell catalysts by functional molecules (KBr, CTAB, SC) and their application to the direct synthesis of hydrogen peroxide from hydrogen and oxygen

Abstract

Pd core–silica shell catalysts (Pd_X@SiO₂, X = KBr, CTAB, SC) were prepared with and without the addition of three different functional molecules, potassium bromide (KBr), cetyltrimethyl ammonium bromide (CTAB) and sodium citrate dihydrate (SC), by the reverse micelle method, and then characterized by TEM, XRD, EDS, XPS and N₂ adsorption/desorption. Results showed that the addition of functional molecules decreased the Pd particle size of the catalysts, and Br⁻ of KBr and CTAB were selectively adsorbed on the surface of Pd particles. Among the prepared catalysts, the catalyst Pd_KBr@SiO₂ had the highest H₂O₂ selectivity (98%) and productivity (716 mmol g_Pd⁻¹ h⁻¹). Compared with the catalyst Pd@SiO₂ prepared without any additional functional molecules, whose H₂O₂ selectivity and productivity were 26% and 396 mmol g_Pd⁻¹ h⁻¹, respectively, the H₂O₂ selectivity and productivity of the catalyst Pd_KBr@SiO₂ were 277% and 81% higher, respectively, than those of the catalyst Pd@SiO₂. The reason for this was that the Br⁻ of KBr can be easily selectively adsorbed on the coordinatively unsaturated sites of the Pd particle surface, which could decrease the energy of the Pd surface and restrain side reactions by inhibiting the O–O bond cleavage of O₂ and H₂O₂. And the catalyst Pd_KBr@SiO₂ has better stability and recyclability than the conventional supported catalyst Pd/SiO₂. In contrast, the catalyst Pd_SC@SiO₂ had the lowest H₂O₂ selectivity (23%) and productivity (395 mmol g_Pd⁻¹ h⁻¹) because the catalyst Pd_SC@SiO₂ prepared with the functional molecule SC and without Br⁻ had the smallest Pd particle size, which can lead to more coordinatively unsaturated sites and increase side reactions.