Preparation of palladium catalysts using the strong electrostatic adsorption technique for stearic acid conversion via the deoxygenation process

Abstract

The strong electrostatic adsorption (SEA) technique was used to prepare palladium catalysts on a graphene support to convert stearic acid to diesel-like hydrocarbon via a deoxygenation process. The pH shifts of graphene were determined, and the point of zero charge (PZC) was obtained at pH = 4.6. With a moderately low PZC, the cation Pd precursor (i.e., [Pd(NH3)4]2+—palladium tetraammine—PdTA) was preferred. In the adsorbed conditions, PdTA and the graphene surface attained the strongest electrostatic adsorption at pH = 12 and had the maximum metal surface density around 0.6 μmol/m2. The Pd loading of 5 wt% catalysts was controlled by the initial concentration of PdTA. The Pd particle size distribution was considerably uniform and had a diameter around 2–3 nm according to transmission electron microscopy (TEM). The ring pattern from electron diffraction (ED) and the spectra from X-ray diffraction (XRD) verified that the Pd metal had a face-centered cubic (fcc) crystal structure. The deoxygenation reaction was carried out and reached 99% conversion of stearic acid using 5 wt% Pd/graphene catalysts with mass of 0.6 g. The main product was straight chain hydrocarbon called heptadecane (C17H36), suggesting a decarboxylation pathway. Moreover, the diesel-like hydrocarbon (C16–C21) attained a maximum selectivity at 85.4%.