Hydrogen production from aqueous phase reforming (APR) has been carried out on a large variety of different supports by many research groups. We have shown that carbon nanotubes (CNT), including both single-walled carbon nanotube (SWNT) and multi-walled carbon nanotube (MWNT), work as good catalyst supports for platinum (Pt) in APR, however, different metal-support interactions greatly affect the performance of the Pt/CNT catalysts. Previously we studied the effect of surface oxygen containing groups on the APR activity of Pt/CNT catalysts, and found that the competitive adsorption between water and polyol onto CNT is the key factor that governs the conversion of APR. In this study, different aspects of metal-support interactions are explored and correlated with APR activity.
Several factors affect the metal-support interaction in Pt/CNT catalysts. Larger particles behave more like bulk Pt and are less affected by the support, while the electronic structure of smaller particles is more affected by the CNT support. The particle size also affects the selectivity and TOF of APR. We have discovered that the binding energy of the CNT supported Pt particles are different than bulk Pt, and this binding energy change depends on the particle size, the radius of curvature of the CNT, as well as the location of particles inside/outside the CNT channel. The effects of particle size, CNT radius of curvature and particle location on APR activity have also been studied. The catalyst preparation method and the metal precursor also play important roles. As incipient wetness impregnation is the most commonly used method in research labs, which gives Pt particles with good dispersion, some methods such as solution reduction or mechanical mixing, give larger particles, but with less metal-support interaction, while other methods such as ion exchange give the catalyst more interaction with the support. Larger particles lead to larger turnover frequency, but not necessarily higher catalyst mass time yield. An optimal particle size is required for the best utilization of the precious metal.