Catalytic Consequences of Supported Pd Catalysts on Dehydrogenative H₂ Evolution from 2-[(<i>n</i>-Methylcyclohexyl)methyl]piperidine as the Liquid Organic Hydrogen Carrier

Abstract

Herein, nanoporous Al2O3, CeO2, TiO2, ZrO2, and SnO2 were used as the supports for Pd nanoparticles, and effects of surface characteristics on catalytic performances for dehydrogenation of 2-[(n-methylcyclohexyl)methyl]piperidine (H12-MBP) as the H2-rich liquid organic hydrogen carrier were investigated. The H2 yield, dehydrogenation rate, product selectivity, and recyclability of the supported Pd catalysts depended on the metal oxide support and Pd loading. The H2 yield and reaction rate of the Al2O3 supporting 5 wt % Pd with a mean size of 5.76 nm were the highest (75.8%) and the fastest (k1 = 0.076 min–1), respectively, of all the catalysts. CeO2 exhibited the highest reducibility and the best supporting ability for Pd nanoparticles, which thus dispersed Pd with the smallest mean size of 3.45 nm. Although this catalyst exhibited a lower H2 yield (67.1%) and a slower reaction rate (k1 = 0.030 min–1) than Al2O3, it showed the best recyclability without a significant loss of activity during four consecutive runs, which could be attributed to the strong metal–support interaction of Pd to the surface of CeO2. The H2 yield and the dehydrogenation rate were systematically correlated with the surface characteristics of the metal oxides, such as acidity, adsorption affinity (adsorption energy), and charge transfer value of H12-MBP, which were determined via combined experimental and theoretical studies.