Catalytic behavior of size-controlled palladium nanoparticles in the hydrodechlorination of 4-chlorophenol in aqueous phase

• Published in: Journal of catalysis 2012-09, Vol.293, p.85-93
• Main Authors: Baeza, J.A., Calvo, L., Gilarranz, M.A., Mohedano, A.F., Casas, J.A., Rodriguez, J.J.
• Format: Article
• Language: English
• Subjects: 4-Chlorophenol; Aqueous chemistry; Catalysis; Catalysts; Chemical synthesis; Chemistry; Colloidal nanoparticles; Colloidal state and disperse state; Exact sciences and technology; General and physical chemistry; Hydrodechlorination; Nanoparticles; Palladium; Particle size; Physical and chemical studies. Granulometry. Electrokinetic phenomena; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2012.06.009

Not only the particle size, but also the synthesis conditions are relevant for activity: (■) methanol and (○) ethanol series. [Display omitted] ► The load of PVP, the alcohol used and its concentration affect to the particle size. ► Decreasing particle size increases catalytic activity. ► The larger particles showed a more active surface in the case of the methanol series. ► Apparent activation energy values within the range of 100-118 kJ/mol were obtained. Unsupported Pd nanoparticles of controlled size were tested as catalyst in liquid-phase hydrodechlorination (303–323K, 1atm) using 4-chlorophenol (4-CP) as target compound. The Pd nanoparticles were synthesized by chemical reduction, using ethanol and methanol as reducing agents and poly(N-vinyl-2-pyrrolidone) (PVP) as capping agent. The size of the nanoparticles and the Pdn+/Pd0 ratio decreased with increasing alcohol concentration and PVP/Pd ratio, both being lower for ethanol medium. High 4-CP conversion values (80–100%) were achieved at low Pd concentration (2.45×10−3g/L). Phenol was the only reaction by-product detected in contrast to the previous results with supported Pd catalysts, where the active phase–support interaction in 4-CP HDC led to obtain also cyclohexanone and cyclohexanol as by-products in equivalent experimental conditions. The smaller nanoparticles showed higher activity due to the higher available surface (m2/gcat). Thus, the smaller nanoparticles synthesized in ethanol medium ranged between 2.7 and 2.8nm and yielded activity values between 16.7 and 39.1mmol/gcatmin, whereas the smaller particles obtained in methanol medium were in the 3.1–4.2nm range and exhibited activity values of 20.1–25.7mmol/gcatmin. However, the large nanoparticles exhibited higher activity per unit of catalyst surface, for example, 0.34–0.43mmol/minm2 in the case of those synthesized in methanol medium. On the other hand, higher activity was observed for the nanoparticles synthesized in methanol medium when equivalent nanoparticle size was compared. Activation energy values around 100kJ/mol were obtained for Pd nanoparticles of different characteristics, significantly higher than the value reported for supported Pd catalysts.