Huilei Zhao

CO2 photoreduction with H2O vapor by porous MgO–TiO2 microspheres: effects of surface MgO dispersion and CO2 adsorption–desorption dynamics

Photocatalytic reduction of CO2 with H2O vapor for CO production at a temperature of 150 °C was studied using porous MgO–TiO2 microspheres as the photocatalysts with the benefits of improved CO2 adsorption by incorporating MgO and enhanced products/intermediates desorption at a higher temperature. The MgO–TiO2 microspheres were fabricated by two methods: (1) a one-step spray pyrolysis method using TiO2 (P25) nanoparticles dispersed in Mg(NO3)2 solution as the precursors (Mg/Ti-SP), and (2) spray pyrolysis synthesis of pure TiO2 (P25) microspheres first and then wet-impregnation with MgO (Mg/Ti-WI). The two material synthesis methods led to different MgO dispersion on the TiO2 surface. For Mg/Ti-SP, the strong aggregation of MgO nanoparticles caused a rough surface of the MgO–TiO2 microsphere; while for Mg/Ti-WI, MgO was more uniformly deposited leading to a much smoother surface of the microsphere. The surface dispersion of MgO was found to significantly affect the performance of MgO–TiO2 in CO2 photoreduction. At the same MgO concentration, Mg/Ti-SP had more than two times higher activity than Mg/Ti-WI, and most importantly, little deactivation of the catalyst was observed on Mg/Ti-SP while Mg/Ti-WI started to deactivate after 1 to 2 h when the reactor was operating in a continuous flow mode. The ease of photo-induced electron transfer to the catalyst surface may have contributed to the superb activity of Mg/Ti-SP samples. The optimum MgO concentration was found to be 5% for both types of materials. Besides the dispersion of MgO, we also found that the CO2 adsorption–desorption dynamics strongly influenced the CO2 photoreduction. The results from in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) revealed two advantages of Mg/Ti-SP over Mg/Ti-WI: (1) more abundant bicarbonates (important intermediates for CO production) on the surface and (2) easier desorption/transformation of intermediates.

Synthesis of novel MgAl layered double oxide grafted TiO2 cuboids and their photocatalytic activity on CO2 reduction with water vapor

A series of magnesium/aluminum (MgAl) layered double oxide (LDO) grafted TiO2 cuboids (MgAl-LDO/TiO2) with various molar ratios of (Mg + Al) to Ti were synthesized by a combination of hydrothermal and coprecipitation methods, in which the growth of MgAl-LDO platelets was controlled. The MgAl-LDO/TiO2 composite materials were used for photocatalytic CO2 reduction with water vapor under UV light irradiation in a continuous-flow reactor. CO was found to be the main product from CO2. At near room temperature (e.g., 50 °C), MgAl-LDO/TiO2 did not significantly enhance CO2 reduction compared with pure TiO2 cuboids. At a moderately elevated reaction temperature (e.g., 150 °C), the MgAl-LDO/TiO2 sample with an optimum 10 wt.% MgAl-LDO loading demonstrated CO2 reduction activity five times higher than that of bare TiO2 cuboids. The photo-induced electrons on TiO2 may migrate to the MgAl-LDO/TiO2 interfacial sites to promote CO2 reduction. Findings in this work may lead to a new area of hybrid adsorbent/photocatalyst materials that are capable of sequential CO2 capture and photocatalytic conversion.

Visible-Light-Driven Photocatalytic Degradation of Organic Water Pollutants Promoted by Sulfite Addition

Solar-driven heterogeneous photocatalysis has been widely studied as a promising technique for degradation of organic pollutants in wastewater. Herein, we have developed a sulfite-enhanced visible-light-driven photodegradation process using BiOBr/methyl orange (MO) as the model photocatalyst/pollutant system. We found that the degradation rate of MO was greatly enhanced by sulfite, and the enhancement increased with the concentration of sulfite. The degradation rate constant was improved by 29 times in the presence of 20 mM sulfite. Studies using hole scavengers suggest that sulfite radicals generated by the reactions of sulfite (sulfite anions or bisulfite anions) with holes or hydroxyl radicals are the active species for MO photodegradation using BiOBr under visible light. In addition to the BiOBr/MO system, the sulfite-assisted photocatalysis approach has been successfully demonstrated in BiOBr/rhodamine B (RhB), BiOBr/phenol, BiOI/MO, and Bi2O3/MO systems under visible light irradiation, as well as in TiO2/MO system under simulated sunlight irradiation. The developed method implies the potential of introducing external active species to improve photodegradation of organic pollutants and the beneficial use of air pollutants for the removal of water pollutants since sulfite is a waste from flue gas desulfurization process.

A Novel Photo-Thermo-Chemical Approach for Enhanced CO2 Reforming of Methane

We report a new approach for photo‐thermochemical CO2 (dry) reforming of methane (PTC‐DRM) to produce syngas by using concentrated sunlight as the energy input. A unique catalyst, Pt‐supported Si‐modified CeO2, was designed for this novel PTC‐DRM reaction by integrating photocatalytic and thermocatalytic effects, for which Pt was the thermocatalytic DRM catalyst and co‐catalyst for photocatalysis, CeO2 was the DRM support and semiconductor photocatalyst, and Si was the promoter and stabilizer. Under irradiation of the equivalent of 30 suns at 600 °C in 30 h, the production rates of H2 and CO reached stable levels of 90 and 154 mmol g−1 h−1, respectively, which were five and two times higher than those obtained in the dark at the same temperature. The significantly enhanced catalytic performance and stability under solar irradiation, resulting from synergy between the photocatalytic and thermocatalytic effects, demonstrates the feasibility of a new direction in low‐carbon fuel production from sunlight.

Response to Comment on “Visible-Light-Driven Photocatalytic Degradation of Organic Water Pollutants Promoted by Sulfite Addition”

9 Solar-driven heterogeneous photocatalysis has been widely studied as a promising technique for 10 degradation of organic pollutants in wastewater. Herein, we have developed a sulfite-enhanced 11 visible-light-driven photodegradation process using BiOBr/methyl orange (MO) as the model 12 photocatalyst/pollutant system. We found that the degradation rate of MO was greatly enhanced 13 by sulfite, and the enhancement increased with the concentration of sulfite. The degradation rate 14 constant was improved by twenty-nine times in the presence of 20 mM sulfite. Studies using hole 15 scavengers suggest that sulfite radicals generated by the reactions of sulfite (sulfite anions or 16 bisulfite anions) with holes or hydroxyl radicals are the active species for MO photodegradation 17 using BiOBr under visible light. In addition to the BiOBr/MO system, the sulfite-assisted 18 photocatalysis approach has been successfully demonstrated in BiOBr/rhodamine B (RhB), 19 BiOBr/phenol, BiOI/MO, and Bi2O3/MO systems under visible light irradiation, as well as in 20 TiO2/MO system under simulated sunlight irradiation. The developed method implies the 21 potential of introducing external active species to improve photodegradation of organic 22 Page 1 of 25 ACS Paragon Plus Environment Environmental Science & Technology