Simple pyrolysis of urea into graphitic carbon nitride with recyclable adsorption and photocatalytic activity

The g-C(3)N(4) photocatalyst was synthesized by directly heating the low-cost melamine. The methyl orange dye (MO) was selected as a photodegrading goal to evaluate the photocatalytic activity of as-prepared g-C(3)N(4). The comparison experiments indicate that the photocatalytic activity of g-C(3)N(4) can be largely improved by the Ag loading. The strong acid radical ion (SO(4)(2-) or NO(3)(-)) can promote the degrading rate of MO for g-C(3)N(4) photocatalysis system. The MO degradation over the g-C(3)N(4) is mainly attributed to the photoreduction process induced by the photogenerated electrons. Our results clearly indicate that the metal-free g-C(3)N(4) has good performance in photodegradation of organic pollutant.

Because sunlight is diffuse and intermittent, substantial use of solar energy to meet humanity's needs will probably require energy storage in dense, transportable media via chemical bonds. Practical, cost effective technologies for conversion of sunlight directly into useful fuels do not currently exist, and will require new basic science. Photosynthesis provides a blueprint for solar energy storage in fuels. Indeed, all of the fossil-fuel-based energy consumed today derives from sunlight harvested by photosynthetic organisms. Artificial photosynthesis research applies the fundamental scientific principles of the natural process to the design of solar energy conversion systems. These constructs use different materials, and researchers tune them to produce energy efficiently and in forms useful to humans. Fuel production via natural or artificial photosynthesis requires three main components. First, antenna/reaction center complexes absorb sunlight and convert the excitation energy to electrochemical energy (redox equivalents). Then, a water oxidation complex uses this redox potential to catalyze conversion of water to hydrogen ions, electrons stored as reducing equivalents, and oxygen. A second catalytic system uses the reducing equivalents to make fuels such as carbohydrates, lipids, or hydrogen gas. In this Account, we review a few general approaches to artificial photosynthetic fuel production that may be useful for eventually overcoming the energy problem. A variety of research groups have prepared artificial reaction center molecules. These systems contain a chromophore, such as a porphyrin, covalently linked to one or more electron acceptors, such as fullerenes or quinones, and secondary electron donors. Following the excitation of the chromophore, photoinduced electron transfer generates a primary charge-separated state. Electron transfer chains spatially separate the redox equivalents and reduce electronic coupling, slowing recombination of the charge-separated state to the point that catalysts can use the stored energy for fuel production. Antenna systems, employing a variety of chromophores that absorb light throughout the visible spectrum, have been coupled to artificial reaction centers and have incorporated control and photoprotective processes borrowed from photosynthesis. Thus far, researchers have not discovered practical solar-driven catalysts for water oxidation and fuel production that are robust and use earth-abundant elements, but they have developed artificial systems that use sunlight to produce fuel in the laboratory. For example, artificial reaction centers, where electrons are injected from a dye molecule into the conduction band of nanoparticulate titanium dioxide on a transparent electrode, coupled to catalysts, such as platinum or hydrogenase enzymes, can produce hydrogen gas. Oxidizing equivalents from such reaction centers can be coupled to iridium oxide nanoparticles, which can oxidize water. This system uses sunlight to split water to oxygen and hydrogen fuel, but efficiencies are low and an external electrical potential is required. Although attempts at artificial photosynthesis fall short of the efficiencies necessary for practical application, they illustrate that solar fuel production inspired by natural photosynthesis is achievable in the laboratory. More research will be needed to identify the most promising artificial photosynthetic systems and realize their potential.

The past several decades have seen a significant rise in atmospheric carbon dioxide levels resulting from the combustion of hydrocarbon fuels. A solar energy based technology to recycle carbon dioxide into readily transportable hydrocarbon fuel (i.e., a solar fuel) would help reduce atmospheric CO2 levels and partly fulfill energy demands within the present hydrocarbon based fuel infrastructure. We review the present status of carbon dioxide conversion techniques, with particular attention to a recently developed photocatalytic process to convert carbon dioxide and water vapor into hydrocarbon fuels using sunlight.