(166r) Transition Metal Phthalocyanines Catalyst on Nano Scale Photothermal Support for Solar Mediated Methane to Methanol Conversion

The goal of this work is to find a suitable catalyst based on a transition metal phthalocyanines (TMPc) catalyst on photothermal support for the conversion of methane to methanol using solar energy. The conversion of greenhouse gas methane into methanol is important as methanol can be used as liquid feedstock and fuel which can be easily transported and handled. The conversion process usually requires high temperatures or pressures to ensure the partial oxidation of methane due to the very high tetrahedral C-H bond dissociation energy of methane (~4.51 eV). However, natural monooxygenase enzyme containing metal-oxo species can convert methane into methanol even at room temperature. These motivated the search of catalyst based on single-atom active sites of metal phthalocyanines (analogous to metal-oxo species of monooxygenase enzyme) deposited on the catalytic supports for methane to methanol conversion.

Here we evaluated graphene(G) and titanium nitride(TiN) nanoparticles as photothermal support for TMPc catalysts. G and TiN were chosen as substrates due to their high electron mobility, strong light absorption and photothermal properties. Moreover, titanium nitride nanomaterials are refractory plasmonic materials hence can remain stable at high reaction temperatures. Combined theoretical and experimental studies are performed to understand the electronic and material properties of G-TMPc and TiN-TMPC systems and relate that to their efficiency towards C-H bond activation. The TMPc’s chosen for study are Cobalt-Pc(CoPc) and Copper-Pc(CuPc) as theoretical calculations show these materials to have low, and sometimes negative energy barriers for the formation of Oxides on their metal cores when supported.¹ The calculations have also suggested that the oxidized metal cores have favorable energies for cleaving the C-H bond of methane. Temperature dependent diffusion reflectance FTIR spectroscopy study suggested that both G supported CoPc and TiN supported CoPc systems begin to oxidize at lower temperature than their CuPc counterparts. This agrees with theoretical findings showing a higher energy barrier in CuPc Systems (2.1 eV) in comparison to that of CoPc (0 .8 eV). It has been found that cobalt-oxo species and copper-oxo species can be formed on TiN support at room temperature and 150⁰C respectively. The temperature of the TiN support could be increased to 150⁰C using two sun intensity. Most importantly, the C-H bond breaking energy for this copper oxo-species on titanium nitride support is favorable for room temperature C-H bond breaking. The electronic interaction among the support and the TMPc molecule is an important descriptor which decides their efficiency towards both metal oxo species formation and CH bond breaking. It has been found from photon emissive electron microscopy, (PEEM) studies of graphene/CoPc molecule, the graphene acts as an electron donor. The charge transfer from graphene to metal oxo-species has favorable effect toward metal-oxo species formation energy but has less effect on C−H bond activation energy.¹

1. Filonowich, D., et al., The Journal of Physical Chemistry C (2020) 124 (8), 4502