Photocatalytic reduction of carbon dioxide and methane using a plasmonic property of silver/titania nanoparticles

Human industrial undertakings have rendered carbon dioxide (CO2) and methane (CH4) as key greenhouse gas constituents. Greenhouse gases aggravate global warming considerably along with the greenhouse effect. Catalyzed reaction of CO2 with CH4 to produce valuable chemicals has received increasing attention from both environmental and industrial players. Photocatalysis seems to be an encouraging method of realizing green chemistry objectives through reducing the concentrations of predominant greenhouse gases, especially CO2 and CH4. One of the biggest challenges in the study of photocatalysts is to achieve new catalysts with high activity in the visible light range. Noble metal nanoparticles are known to absorb visible light extremely well due to the surface plasmon resonance (SPR) effect characterized by the strong field enhancement at the interface. Therefore, it is possible to attain chemical reactions with a significant fraction of the entire solar spectrum. In this study, immobilized silver/titania (Ag/TiO2) nanoparticles were coated on stainless steel webnet by dip-coating method to enhance the visible light plasmonic photocatalyst and reduce CO2 in the presence of CH4 under ultraviolet-visible (UV-Vis) light irradiation. The synthesized catalysts were characterized using x-ray diffraction, field emission scanning electron microscopy, energy dispersive x-ray, transmission electron microscopy, Fourier transform infrared, nitrogen adsorption-desorption isotherm, UV-Vis spectrophotometry, Raman spectrometry, temperature programmed desorption of carbon dioxide and photoluminescence analysis techniques. P25 titania nanoparticle model served as the photocatalyst due to the large surface area with CO2, CH4 and nitrogen as feeds in the batch photoreactor. Experimental results revealed that the photocatalytic activity for the conversion of CH4 and CO2 under UV-Vis light irradiation over Ag-loaded TiO2 was better than that of pure TiO2 due to the synergistic effect between light excitation and SPR enhancement. Response surface methodology was applied for analysis and optimization to achieve the highest conversion of CO2 and CH4. The optimal process parameter values were 9 h for irradiation time, 4 wt% for Ag-loaded, an equal initial ratio of CO2:CH4 and 100 mesh size. The maximum conversion of CO2 and CH4 in optimal condition was achieved at 29.05% and 34.85%, respectively. In addition, the photon energy in the UV-Vis range was high enough to excite the electron transition in Ag/TiO2 to produce some hydrocarbons and oxygenates, such as ethane (C2H6), ethylene (C2H4), acetic acid and formic acid. During the reaction, the maximum yields of C2H6 and C2H4 achieved were 1500.52 and 1050.50 μmole. gcat-1, respectively. Furthermore, the Ag/TiO2 plasmon photocatalyst exhibited great reusability with almost no change after three runs. Finally, a kinetic model was developed based on the Langmuir–Hinshelwood mechanism to model the hydrocarbon formation rates through the photocatalytic reduction of CO2 with CH4. The experimental data fit well with the kinetic model. Based on the findings, these nanostructured materials are considered promising and effective photocatalysts for conversion of CO2 and CH4 into high-value products.