Efficiently photoelectrocatalyze CO2 to methanol using Ru(II)-pyridyl complex covalently bonded on TiO2 nanotube arrays

Aiming at improving the efficiency of photoelectrocatalytic (PEC) CO 2 conversion to methanol, a heterogeneous photoelectrocatalyst was prepared by covalently binding a Ru(II) metal-organic complex (Ru-Py) containing exposed pyridyl on the periodic TiO 2 nanotube arrays (TNTAs). PEC characterization indicated that the Ru-Py/TNTAs photocathode exhibited excellent PEC CO 2 reduction activity in aqueous solution. The initial CO 2 reduction potential on Ru-Py/TNTAs photocathode reached −0.4   V (vs. NHE) under simulated solar irradiation. The cathodic photocurrent response of Ru-Py/TNTAs in CO 2 -satuarated aqueous solution was about 1.79 mAcm −2 , which was 2.4 times that of TNTAs . It may be caused by the enhanced light absorption in the visible region originated from Ru-Py. Meanwhile, Ru-Py played an important role in accelerating the separation of photoinduced electron-hole pairs evidenced by the photoluminescence spectra. PEC CO 2 reduction test showed that 84.8   μmol methanol was produced with 63.9% faraday efficiency, 62.6 turnover number (TON) and 45% selectivity at −0.9   V under 8   h irradiation. Furthermore, the non-covalent bonded system only produced 41.3   μmol methanol indicating that the carboxylic covalent linking displayed excellent electron transfer efficiency and methanolization performance. The mechanism investigation revealed that the exposed pyridyl provided the active sites in forming pyridiniumformate intermediates confirmed by the differential UV–vis absorption ( ΔA ) spectrum, which was of great significance in the efficient CO 2 methanolization. The probable reaction pathway for catalytic CO 2 reduction to methanol was also proposed combining the possible intermediates detected by gas chromatography mass spectrometry (GCMS).