Modeling the effect of Cu doped TiO2 with carbon dots on CO2 methanation by H2O in a photo-thermal system

Abstract

The process of CO2 reduction by H2O is always restricted due to its non-spontaneous in thermodynamic. In this work, a photo-thermal coupled device is designed to obtain a high-achievement CO2 methanation by H2O over carbon dots (CDs) drafted Cu/TiO2 (Cu/TiO2-C). Dramatic promotion of CH4 production exhibits under UV irradiation at a higher temperature (>150 °C), but a poor photo-promotion at room temperature. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and isotopic-label temperature programmed surface reaction (TPSR) suggest that CO2 methanation process over Cu/TiO2-C can be regarded as two main spontaneous process: CO2 is firstly reduced by Cu(I) (Cu2O) to CO (CO2 + Cu2O → CO +CuO), then CO is reduced by H2O to CH4 via water-gas shift (WGS) process. Here, CDs not only act as electron storage to keep the presence of Cu(I) (from Cu(II) to Cu(I) not to Cu (0)) at a high temperature, but also capture the photo-generated holes from TiO2 induced by UV light, resulting in the cycle of Cu (I)/ Cu(II) with concomitant of electron storage and release. The synergy effect of UV light and temperature do not occur on the Cu/TiO2 sample without CDs. Here, Cu (II) is mainly reduced by H2 pretreatment to Cu (0), while the poor cycle of Cu(I)/Cu(II) was exhibited under UV irradiation. This study shows that this non-spontaneous reaction could be designed as two ongoing spontaneous processes by adding UV light, this approach may apply to other photo-thermal reactions.