Solar Light Activated Photocatalysts for Enhanced CO2 Conversion to Hydrocarbon Fuels

Abstract

The enormous and continuous release of anthropogenic CO2 into earth’s atmosphere have excessively increased the atmospheric CO2 level, resulting in natural carbon cycle disturbance and stemming the critical issues of global warming, climate change and environmental pollution. Amongst greenhouse gasses (GHG), CO2 is a prominent gas responsible for proliferating the greenhouse effect. An effective approach to overcome such issue of elevated atmospheric CO2 level is to capture CO2 followed by its utilization in industrial processes and/or its conversion to useful chemicals/fuels. As revealed by thermodynamic studies, CO2 is a very stable molecule, demanding additional energy to overcome the uphill barrier for its conversion to useful chemicals. In this regard, several approaches have been developed to overcome the overpotential in its conversion to useful chemicals. Various techniques include chemical conversion, thermal conversion, biological conversion, electrocatalytic conversion, photoelectrochemical conversion and photocatalytic conversion. Amongst these approaches, photocatalytic CO2 conversion/CO2 photoreduction via solar light to useful hydrocarbons and chemicals seems to be an appealing and compelling strategy, well-fitting to the objectives of renewable energy utilization and, environment and energy infrastructure in a sustainable manner. Despite of extensive research and development, the respective field still remain in its infancy and demand enormous amount of efforts for improved photocatalytic performance and product selectivity. A plenty of photocatalytic materials have been developed for improved CO2 photoreduction, amongst which Titanium dioxide/Titania (TiO2) and/or TiO2 based photocatalysts are extensively studied within the scientific society. TiO2 offers several advantages such as corrosion stability, abundant availability and low cost, though its performance is largely limited due to inadequate light absorption, mainly attributed to its wide band gap (~3.2 eV) and low quantum yield in sunlight due to surface and bulk volume charge recombination. However, despite of such critical disadvantages, TiO2 still remains a champion material in the field of photocatalysis due to its stable and commendable properties. A number of approaches have been developed to overcome the issues of limited light absorption and efficient charge separation, including doping with non-metal or noble metal co-catalysts, coupling with low band gap semiconductors, and the synthesis of carbon-based TiO2 composites. Hence, with the aim of improving the photocatalytic performance of TiO2 and TiO2 based materials, the experimental works performed and investigated in this thesis consists of three key strategies leading to enhanced light absorption and improved charge separation for TiO2 and TiO2 based materials. Mainly three experimental works are done and investigated during Ph.D. research which include approaches such as (i) foreign element doped sodium titanate nanotubes (Na+-TNT), (ii) synthesis of reduced graphene oxide (rGO) coupled TiO2 nanotube arrays, a novel heterostructured photocatalyst and (iii) development of reduced TiO2 by a newly developed approach. During past few years, TiO2 nanotubes (TNT), a one dimensional (1-D) TiO2 nanostructures have attracted a great interest among the photocatalysis research community, offering more active sites and improved charge separation by its high surface area and directional charge transport. In the first experimental approach of the thesis, an attempt was made to enhance the photocatalytic performance of sodium titanate nanotubes (Na+-TNT) by a co-doping strategy of foreign elements. Carbon and nitrogen co-doped sodium titanate nanotubes (C,N-TNT) are synthesized by designing a simple two-step process, comprising of an alkaline hydrothermal technique followed by calcining the well mixture of Na+-TNT (obtained from alkaline hydrothermal method) with varied amounts of urea (as a nitrogen and carbon dopant source). The photocatalysts are characterized using numerous experimental techniques, and investigated under simulated solar light spectrum for the photocatalytic conversion of CO2 and water vapor to methane (CH4). The C,N-TNT sample with optimum dopant concentration yields the maximum methane yield of 230.80 ppm•g-1•h-1, 2.63 times more than pure Na+-TNT. The key factors contributing to enhanced photocatalytic performance include increased light absorption, surface area and Na+ ions concentration in TNT which acts as a CO2 adsorption site and photogenerated electrons recombination centers. It is observed that higher doping of the TNT, resulted in lower photocatalytic performance which might be due to decreased surface area or increased recombination centers. Our results suggest, co-doping of nanostructured photocatalysts is an excellent pathway for improving textural and photocatalytic properties for the respective application domain. Graphene based TiO2 nanostructures have also been found to offer improved photocatalytic/photoelectrochemical properties, with graphene contents enhancing light absorption as well as promoting rapid charge transfer. With the aim of improved photocatalytic performance via enhanced light absorption and efficient charge separation, an attempt is made in second experimental work of the thesis for the synthesis of novel heterostructure comprising of reduced graphene oxide (rGO) coupled with 1-D TiO2 nanotube (TNT) arrays. A facile synthesis approach is designed resulting in a noble metal-free novel nanostructured photocatalytic material, comprising of one-dimensional arrays of TNT covered with reduced graphene oxide and TiO2 nanoparticles termed as rGO-TNTNP. The probable mechanism which might be involved in the fabrication of such novel nanostructured photocatalyst is proposed on the basis of reported literature and experimental results specifically, Raman spectra, XPS data and SEM images. The novel nanostructure exhibits significantly improved photocurrent density and photochemical activity via photocatalytic conversion of CO2 into CH4 under simulated solar light irradiation. The rGO-TNTNP produces CH4 with an evolution rate of 5.67 ppm•cm2•h-1, 4.4 times more than pure TNT sample (1.28 ppm•cm2•h-1). The improved performance appears due to the combined effect of enhanced light absorption and effective charge separation promoted by the rGO content over photocatalyst surface. The discovery of black or reduced TiO2 materials with extended light absorption and suitable band structure has offered improved photocatalytic properties. Until now a variety of methods have been reported for the synthesis of reduced TiO2 (RT), suggesting different material properties which can be manipulated by a number of process parameters. In the third and last experimental work of the thesis, the performance of RT for CO2 photoreduction with water vapor to hydrocarbons mainly CH4, is investigated under simulated solar light irradiation. The RT employed in this work is synthesized by a newly developed reduction process using dual reducing agents i.e. Mg in 5% H2/Ar. Further, to improve the charge separation efficiency, platinum (Pt) nanoparticles as co-catalyst are loaded by a photodeposition method and Pt concentration is optimized on the RT surface. With optimally photodeposited Pt nanoparticles on RT, it exhibits a stable performance and a threefold increase in CH4 production rate (1640.58 ppm•g-1•h-1 or 1.13 µmol•g-1•h-1) as compared to Pt photodeposited pure commercial nano-TiO2 (546.98 ppm•g-1•h-1, 0.38 µmol•g-1•h-1). The improved photocatalytic performance is mainly attributed to the suitable band gap with enhanced light absorption, well-aligned position of band edges against CO2/CH4 redox potential and efficient photogenerated charge separation by well-dispersed Pt nanoparticles co-catalyst having optimum size, concentration and well dispersion. ⓒ 2017 DGIST