Metastability of palladium carbide nanoparticles during hydrogen release from liquid organic hydrogen carriers

Abstract

Efficient hydrogen release from liquid organic hydrogen carriers (LOHCs) requires a high level of control over the catalytic properties of supported noble metal nanoparticles. Here, the formation of carbon-containing phases under operation conditions has a direct influence on the activity and selectivity of the catalyst. We studied the formation and stability of carbide phases using well-defined Pd/α-Al₂O₃(0001) model catalysts during dehydrogenation of a model LOHC, methylcyclohexane, in a flow reactor by in situ high-energy grazing incidence X-ray diffraction. The phase composition of supported Pd nanoparticles was investigated as a function of particle size and reaction conditions. Under operating conditions, we detected the formation of a Pd_xC phase followed by its conversion to Pd₆C. The dynamic stability of the Pd₆C phase results from the balance between uptake and release of carbon by the supported Pd nanoparticles in combination with the thermodynamically favorable growth of carbon deposits in the form of graphene. For small Pd nanoparticles (6 nm), the Pd₆C phase is dynamically stable under low flow rate of reactants. At the high reactant flow, the Pd₆C phase decomposes shortly after its formation due to the growth of graphene. Structural analysis of larger Pd nanoparticles (15 nm) reveals the formation and simultaneous presence of two types of carbides, Pd_xC and Pd₆C. Formation and decomposition of Pd₆C proceeds via a Pd_xC phase. After an incubation period, growth of graphene triggers the decomposition of carbides. The process is accompanied by segregation of carbon from the bulk of the nanoparticles to the graphene phase. Notably, nucleation of graphene is more favorable on bigger Pd nanoparticles. Our studies demonstrate that metastability of palladium carbides associated with dynamic formation and decomposition of the Pd₆C and Pd_xC phases is an intrinsic phenomenon in LOHC dehydrogenation on Pd-based catalysts and strongly depends on particle size and reaction conditions.