(560jg) Effect of Pd Precursors on Pd/TiO2 Catalysts Prepared by Different Methods and Their Catalytic Activity in Hydrogenation of Maleic Acid

The size, shape, and distribution of metal clusters on TiO₂ with strong metal-support interaction are generally controlled by the preparation methods, the nature of palladium precursors, pH values, etc.^1,2 Herein, a series of 5 wt% Pd/TiO₂ catalysts were prepared by varying two parameters; the types of palladium precursors and preparation methods. The catalytic performances were evaluated via hydrogenation of maleic acid (MA). The reaction was carried out under temperature=90 ℃, pressure of H₂=3 bar, reaction time=60 min, and stirring rate=700 rpm. We prepared the Pd/TiO₂ catalysts using four different precursors such as H₂PdCl₄ (H), K₂PdCl₄ (K), Na₂PdCl₄ (N), and Pd(NH₄)Cl₂∙H₂O (PA) by different methods. We also investigated the effect of preparation methods (i.e. chemical reduction (CR), incipient wetness impregnation (IWI), and deposition-precipitation (DP)) on the Pd particles of the Pd/TiO₂ catalysts. The prepared catalysts were denoted as follows: Pd/TiO₂_A_B (A: Pd precursor, B: preparation method). The XRD, FT-IR, N₂ physisorption, CO chemisorption, UV-vis spectroscopy, and XPS were employed to characterize the physicochemical properties of prepared catalysts. The CO chemisorption results showed that Pd/TiO₂_K_DP had the highest Pd dispersion of 12.4%. K₂PdCl₄ (K) in the aqueous solution formed rapidly [Pd(OH)₂]_n, which could be uniformly precipitated the hydroxyl groups on TiO₂ support.³ However, Pd/TiO₂_PA_CR had the lowest Pd dispersion due to the self-reduction and agglomeration of Pd by interaction with the NH₃ ligands, causing lower Pd dispersion.⁴ From the XPS results, Pd/Ti atomic ratio of Pd/TiO₂_K_DP was the highest value of 0.079, while Pd/Ti atomic ratio of Pd/TiO₂_PA_CR was lower than 1.97 times. It was found to be in accordance with the CO chemisorption results. The catalytic activities were evaluated via MA hydrogenation over prepared Pd/TiO₂ catalysts. The MA conversion of almost 40% was achieved over Pd/TiO₂_K_CR. Although Pd/TiO₂_K_CR had similar Pd dispersion to Pd/TiO₂_K_DP, the maleic acid conversion was different. From above XPS results, we suggested that the Pd²⁺/Pd⁰ ratio influenced the catalytic activity of maleic acid hydrogenation.⁵

Keywords: Hydrogenation, Maleic acid, Palladium catalyst, Pd/TiO₂

References

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