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Pyrite is a mineral often associated with coal in coal seams and is a major source of sulfur in coal. Coal–pyrite is widely distributed, easily available, low-cost, and non-toxic, and has high light absorption coefficient. So, it shows potential for various applications. In this paper, the density-functional theory (DFT + U) is used to construct coal–pyrite with carbon doped in the sulfur and iron vacancies of pyrite. The effects of different carbon defects, different carbon doping concentrations, and different doping distributions in the same concentration on the electronic structure and optical properties of coal–pyrite were studied. The results show that the absorption coefficient and reflectivity of coal–pyrite, when its carbon atom substitutes the iron and sulfur atoms in the sulfur and iron vacancies, are significantly higher than those of the perfect pyrite, indicating that coal–pyrite has potential for application in the field of photovoltaic materials. When carbon is doped in the sulfur vacancy, this impurity state reduces the width of the forbidden band; with the increase in the doping concentration, the width of the forbidden band decreases and the visible-light absorption coefficient increases. The distribution of carbon impurities impacts the band gap but has almost no effect on the light absorption coefficient, complex dielectric function, and reflectivity, indicating that the application of coal–pyrite to photovoltaic materials should mainly consider the carbon doping concentration instead of the distribution of carbon impurities. The research results provide a theoretical reference for the application of coal–pyrite in the field of photoelectric materials. [ABSTRACT FROM AUTHOR]

Pyrite is a mineral often associated with coal in coal seams and is a major source of sulfur in coal. Coal–pyrite is widely distributed, easily available, low-cost, and non-toxic, and has high light absorption coefficient. So, it shows potential for various applications. In this paper, the density-functional theory (DFT + U) is used to construct coal–pyrite with carbon doped in the sulfur and iron vacancies of pyrite. The effects of different carbon defects, different carbon doping concentrations, and different doping distributions in the same concentration on the electronic structure and optical properties of coal–pyrite were studied. The results show that the absorption coefficient and reflectivity of coal–pyrite, when its carbon atom substitutes the iron and sulfur atoms in the sulfur and iron vacancies, are significantly higher than those of the perfect pyrite, indicating that coal–pyrite has potential for application in the field of photovoltaic materials. When carbon is doped in the sulfur vacancy, this impurity state reduces the width of the forbidden band; with the increase in the doping concentration, the width of the forbidden band decreases and the visible-light absorption coefficient increases. The distribution of carbon impurities impacts the band gap but has almost no effect on the light absorption coefficient, complex dielectric function, and reflectivity, indicating that the application of coal–pyrite to photovoltaic materials should mainly consider the carbon doping concentration instead of the distribution of carbon impurities. The research results provide a theoretical reference for the application of coal–pyrite in the field of photoelectric materials.

Kaolinite is a difficult-to-float clay existing in coal slurries which negatively impacts coal flotation. A better understanding of its adsorption behavior would improve coal-slurry processing. Carbon impurities in the kaolinite can affect this behavior. The appearance and elemental surface composition of the carbon impurities precipitated by kaolinite oxidative treatment were microscopically analyzed, and their chemical speciation and relative C and O contents were probed. The mechanisms of adsorption on the main kaolinite cleavage planes ((001) and (00 1) surfaces) for two carbon-impurity structural models, a phenolic hydroxyl unit (Ph–OH) and carbon hydroxyl unit (C–OH), were considered using density functional theory methods. The carbon impurities consisted mainly of C and O, with C present mostly as C–C, C–H, and C–O species, and O existing mainly in –OH and C–O structures. Both Ph–OH and C–OH units adsorbed stably on the kaolinite (001) and (00 1) surfaces through hydrogen bonding between the hydroxyl groups and surfaces. A strong electrostatic attraction occurred between the Ph–OH benzene ring and kaolinite surfaces, whereas the carbon ring in C–OH did not interact with either surface. Finally, for both units, adsorption on the kaolinite (001) surface was more stable than on the (00 1) surface. [ABSTRACT FROM AUTHOR]