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The design and manufacture of highly efficient nanocatalysts for the oxygen reduction reaction (ORR) is key to achieve the massive use of proton exchange membrane fuel cells. Up to date, Pt nanocatalysts are widely used for the ORR, but they have various disadvantages such as high cost, limited activity and partial stability. Therefore, different strategies have been implemented to eliminate or reduce the use of Pt in the nanocatalysts for the ORR. Among these, Pt-free metal nanocatalysts have received considerable relevance due to their good catalytic activity and slightly lower cost with respect to Pt. Consequently, nowadays, there are outstanding advances in the design of novel Pt-free metal nanocatalysts for the ORR. In this direction, combining experimental findings and theoretical insights is a low-cost methodology-in terms of both computational cost and laboratory resources-for the design of Pt-free metal nanocatalysts for the ORR in acid media. Therefore, coupled experimental and theoretical investigations are revised and discussed in detail in this review article.
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Detecting and monitoring air-polluting gases such as carbon monoxide (CO), nitrogen oxides (NOx), and sulfur oxides (SOx) are critical, as these gases are toxic and harm the ecosystem and the human health. Therefore, it is necessary to design high-performance gas sensors for toxic gas detection. In this sense, graphene-based materials are promising for use as toxic gas sensors. In addition to experimental investigations, first-principle methods have enabled graphene-based sensor design to progress by leaps and bounds. This review presents a detailed analysis of graphene-based toxic gas sensors by using first-principle methods. The modifications made to graphene, such as decorated, defective, and doped to improve the detection of NOx, SOx, and CO toxic gases are revised and analyzed. In general, graphene decorated with transition metals, defective graphene, and doped graphene have a higher sensibility toward the toxic gases than pristine graphene. This review shows the relevance of using first-principle studies for the design of novel and efficient toxic gas sensors. The theoretical results obtained to date can greatly help experimental groups to design novel and efficient graphene-based toxic gas sensors.
RESUMO
Due to the thermodynamic conditions prevailing at very shallow depths of calcite stone oil fields, molecular hydrogen has been reported to be released from hydrocarbon or heavy oil located on the surface of the calcite stone. Since this region is physically inaccessible, there is a need to realize modeling and simulation of the hydrogen adsorption and storage process under reservoir conditions. Motivated by the previous problem, in this work, based on recent reports of hydrogen production from oil fields, we present a theoretical methodology to describe the process of hydrogen adsorption on naturally fractured and carbonated (limestone (CaCO3)) reservoirs and to quantify their storage capacity. Firstly, the calcite rock model was optimized inside a simulation cell containing a vacuum layer, for which energy optimization techniques based on density functional theory were used. Subsequently, using ab initio methods also based on DFT, calcite rock was characterized obtaining structural, electronic, vibrational, thermodynamic properties, and Mulliken population analysis of CaCO3. Finally, molecular dynamics simulations were performed in order to simulate the adsorption process and obtain percentages of hydrogen adsorption on (110) surface of the (2 × 2) CaCO3 supercell, for N = 3, 5, 10 hydrogen molecules. The molecular dynamics simulations showed that the surface of CaCO3 rock has hydrogen capacity of only 0.42 mass %.