Transition Metal Ions Assisted Trimerization of Polyisocyanurate and Their Pyrolyzed Derivatives as Synergistic Electrocatalysts for Oxidation of Alcohols

Abstract

We demonstrate the trimerization of polyisocyanurate nanoporous foams by reacting aromatic isocyanate in the presence of transition metal salts (NiCl2·2H2O and CoCl2·6H2O) as catalysts. The metal ions were found to accelerate the gelation of the polyisocyanurate networks. The nickel–cobalt–polyisocyanurate (Ni-Co-PIR-RT) hybrid monoliths were pyrolyzed under inert atmosphere at varying temperatures (from 600 to 1000 °C) to yield oxide and metallic forms of Ni and Co with significant carbon content. Pyrolysis of Ni-Co-PIR-RT at 600 °C (Ni-Co-PIR-600) resulted in the complete reduction of Ni and Co ions to metallic Ni and Co, whereas samples pyrolyzed at 800 °C (Ni-Co-PIR-800) resulted in the partial transformation of the metallic Ni and Co to its respective oxides. The complete transformation of metallic Co to CoO was obtained at 1000 °C (Ni-Co-PIR-1000) with a significant amount of metallic Ni and NiO. The Ni-PIR-1000 sample displayed minimal activity toward methanol oxidation reaction (MOR) with an onset potential and maximum current density of 1.44 V (vs reversible hydrogen electrode) and 0.6 mA/cm2 respectively, whereas the Co-PIR-1000 sample displayed oxygen evolution reaction (OER) at an onset potential of 1.33 V with no evident activity toward MOR in 0.1 M KOH with 0.2 M CH3OH. On the other hand, under the same conditions Ni-Co-PIR-1000 displayed 5 manifold increase in current density and reduction in onset potential by 100 mV toward MOR (with an onset potential of 1.34 V and current density 4 mA/cm2) relative to Ni-PIR-1000. The synergistic and superior activity of Ni-Co-PIR-1000 can be attributed to the combined presence of Ni, NiO, and CoO. Here, (1) Ni and NiO act as the catalysts for methanol oxidation by electrochemically transforming to NiOOH (active centers for methanol oxidation) via Ni(OH)2, (2) the presence of CoO facilitates the electrochemical generation of atomic oxygen ([O]) (an intermediate in oxygen evolution reaction and a well-known oxidizing agent) by transforming to CoOOH, the [O] formed here may be responsible for the formation of NiOOH by chemically reacting with Ni(OH)2 resulting in increased activity toward MOR.