Metalphthalocyanine frameworks grown on TiO2 nanotubes for synergistically and efficiently electrocatalyzing urea production from CO2 and nitrate

Electrocatalytic synthesis of urea from CO2 and NO3- under ambient conditions provides an appealing alternative to the traditional energy-intensive urea synthetic protocol. Highly active and selective electrocatalysts for efficient urea production are therefore urgently desired owing to the unsatisfactory performance of the thus far reported catalysts. Herein, a phthalocyanine-based (Pc-based) covalent organic framework (COF), namely CoPc-COF, fabricated from the nucleophilic substitution reaction of hexadecafluorophthalocyaninato cobalt with octahydroxylphthalocyanine cobalt, in situ grew on the surface of multilayered TiO2 nanotubes (NTs), generating the CoPc-COF@TiO2 NTs composite. Powder X-ray diffraction analysis in combination with electron microscopy measurements discloses the uniform coating of crystalline CoPc-COF on the multilayered TiO2 NTs in CoPc-COF@TiO2 NTs. Remarkably, electrochemical tests reveal the superior electrocatalytic activity of CoPc-COF@TiO2 NTs towards urea production from CO2 and NO3- with a record-high yield of 1,205 mu g h(-1) cm(-2) and an outstanding Faraday efficiency of 49% at -0.6 V versus reversible hydrogen electrode due to the significant synergistic catalysis effect. In situ attenuated total reflection infrared spectroscopic investigation and theoretical calculations unveil the efficient C-N coupling reaction between *CO intermediate derived from CO2 on CoPc moieties and *NH2 intermediate formed from NO3- on TiO2 NTs during the urea formation process over CoPc-COF@TiO2 NTs. This work should be helpful towards designing and fabricating high-performance electrocatalysts for sustainable synthesis of urea through efficient synergistic effect of multiactive centers