A BRIEF OVERVIEW ON SINGLE-ATOM CATALYSIS: CONCEPTS AND APPLICATIONS. Catalytic processes became extremely important for the development of our society, especially after the second industrial revolution. Thus, the research for more efficient catalysts is an obstacle to overcome to achieve cheaper processes, higher yields, and selectivity of desired products. In this context, single-atom catalysis emerges as a promising alternative to unite the advantages of traditional homogeneous and heterogeneous catalysis. Catalysis by single-atoms is a bridge that unites in a single catalyst the ease of recovery and reuse (from heterogeneous catalysis) with the high exposure and uniformity of sites (from homogeneous catalysis). Thus, single-atom catalysts (SACs) and single-atom alloys (SAAs) have already found several applications in the literature, such as in hydrogenation, oxidation, conversion of biomass derivatives, electrocatalysis, and photocatalysis. However, it is essential to emphasize that it is still a field that is expanding and relatively new, with several opportunities and many barriers to surpass. In this review, concepts of homogeneous, enzymatic, and heterogeneous catalysis will be addressed, as well as fundamental aspects of single-atom catalysis, preparation methods, characterization, and current challenges.

Currently, approximately 1.4 billion tons per year of iron ore tailing wastes (IOT) are generated, mainly in Australia, Brazil, and China. This work describes the characterization and application of two typical IOT, i.e., fine and coarse wastes. The physicochemical characterization of these IOT by different techniques such as XRF (X-ray fluorescence), XRD (X-ray diffraction), Mössbauer spectroscopy, and granulometry, indicates for the fine tailing a composition of Fe2O3/FeOOH (10-55%), SiO2 (18-65%) and Al2O3 (up to 15%) with particles of 6-40 µm, whereas the coarse tailing presents 40-150 µm particles with the composition of 8-48% Fe2O3/FeOOH, 30-90% SiO2 and Al2O3 (up to 20%). The main IOT applications discussed in this review are related to civil construction (aggregates for concrete, mortar, Portland cement additives), ceramic industry, geopolymer, synthesis of new materials such as zeolites, mesoporous silica, carbon nanotubes, adsorbents, catalysts for different reactions, in batteries and in fuel cells. It was also carried out an analysis of patents related to IOT applications and the main technological and market barriers that hinder the industrial and commercial uses of these wastes.

Serpentine is a versatile mineral family rich in Mg silicate with several polymorphic phases, mainly antigorite and lizardite, all of them with similar chemical composition (Mg3Si2O5(OH)4). Their structures are generally composed of octahedral layers rich in Mg[MgO2(OH)4]6- , attached to a tetrahedral silicate [Si2O5]2- sheet. The unique physicochemical properties of serpentinites and the existence of large reserves in Brazil create new important opportunities for research and technological applications. In this work, serpentinites structures and properties are reviewed, as well as its occurrence and literature describing its applications in construction/ceramics, agriculture, as a silica source, in steel production, as an additive/filler in polymers, in the production of composites, adsorption of cations and organics contaminants, CO2 capture and catalysis. The presence of harmful chrysotile associated with serpentinites, some of its properties and uses are also described.

In this study, a nanogel was developed from a natural polymer, chitosan, to be used in the Lippia origanoides essential oil encapsulation. The encapsulation process was used in order to improve the functional capacity of this essential oil as an antioxidant. To obtain the nanogel, the chitosan was modified with caffeic acid using EDC coupling agent. The obtained material was characterized by FTIR and TGA, revealing the formation of the chitosan-caffeic acid bond. The size, morphology and essential oil amount incorporated to the nanogel were investigated by DLS, SEM and gravimetric analysis, respectively. SEM and gravimetric analysis indicated that oil were efficiently trapped inside the CS-CA matrix, with encapsulation efficiency of 42 ± 2%. The EDL studies showed that the encapsulation of essential oil in the nanogel significantly affected your structure, with a average particle size of 417.26 ± 55.96. After essential oil incorporation into the CS-CA nanogel it was also observed a increase in its antioxidant capacity in terms of elimination of DPPH and ABTS radicals, and ferric reducing antioxidant power (FRAP). The successful development of this CS-CA nanogel loaded with L. origanoides essential oil will lead to a better applicability of this essential oil as an effective antioxidant.

Solid state reaction of serpentinite Mg3Si2O5(OH)4 with NaOH was investigated to produce new NaMgSi oxide phases with basic properties for application in catalysis. X-ray powder diffraction (XRD) showed that NaOH impregnated on serpentinite, specially 20% sodium and treated at 700 ºC (20Na700), led to the formation of a new crystalline phase Na2Mg2Si2O7. This solid state reaction is discussed in terms of a first step involving Na+ diffusion and intercalation in the interlayer space of the serpentinite structure followed by a dihydroxylation and formation of the NaMgSi oxides. Temperature programmed reaction and desorption experiments with 20Na700 combined with XRD suggested a strong basicity with high CO2 uptake likely due to diffusion and interaction with the basic sites in the material structure. These materials were tested as catalysts for the transesterification of soybean oil with methanol. The obtained results are discussed in terms of a basic site Si-O-Na+ present in the in the Na2Mg2Si2O7 structure.