The iron industry is the largest energy-consuming manufacturing sector in the world, emitting 4–5% of the total carbon dioxide (CO₂). The development of iron-based systems for CO₂ capture and storage could effectively contribute to reducing CO₂ emissions. A wide set of different iron oxides, such as hematite (Fe₂O₃), magnetite (Fe₃O₄), and wüstite (Fe_(1−y)O) could in fact be employed for CO₂ capture at room temperature and pressure upon an investigation of their capturing properties. In order to achieve the most functional iron oxide form for CO₂ capture, starting from Fe₂O₃, a reducing agent such as hydrogen (H₂) or carbon monoxide (CO) can be employed. In this review, we present the state-of-the-art and recent advances on the different iron oxide materials employed, as well as on their reduction reactions with H₂ and CO.

The chemical structure of coal is very composite, consisting of a heterogeneous carbonaceous matrix with variable degrees of “turbostratic” order and the inclusion and/or exclusion of mineral matter (ash). The formation of surface oxides on carbon has long been recognized as a key to understanding many chemical and physical properties of carbon materials relevant to their consolidated or emerging applications. The extent and nature of surface oxides can effectively be assessed by high-resolution X-ray photoelectron spectroscopy (XPS), which provides excellent insight into the functional nature of C-O moieties. However, the XPS analysis of ash-bearing carbons may be biased by the interfering effects of inorganics with the most relevant spectral ranges, namely the core levels O_1s and C_1s. The effect of ash components on the spectroscopic characterization of carbon is scrutinized here with reference to a sub-bituminous coal characterized by a fairly large ash content. The coal is subjected to different treatments, including devolatilization, milling, and oxidation. A synthetic carbon (Carboxen) is used as a reference sample for the correct assignment of the carbon–oxygen functionalities in the core-level XPS spectra (C_1s and O_1s) in the absence of mineral matter. On the opposite side, fly ash from an industrial coal boiler is analyzed to investigate the effects of mineral matter. It is shown that the establishment of non-uniform charging of the sample induced by ash provides a key to the interpretation of the XPS spectra of ash-bearing carbon samples. The positive charge on the surface, referred to as the charging effect, brings about a shift of the core-level binding energies towards higher values. Grinding of the samples or partial combustion emphasizes the charging effect. XPS analysis of the fly ash, where carbon is largely consumed and dispersed in the inorganic matter, confirms that charging arises from non-conductive aluminosilicates. These effects may induce remarkable changes in carbon and oxygen peak shapes and need to be accounted for to obtain correct interpretations of the XPS spectra of ash-rich carbonaceous fuels.

The valorization of industrial byproducts is an emerging practice that aims to transform waste materials generated during production processes into valuable resources. In this work, a preliminary study was carried out on the thermal conversion of an industrial solid byproduct resulting from the pickling of metal surfaces, mainly containing iron (II) oxalate. In a fixed-bed reactor, the thermal conversion was investigated as a function of the operating temperature and overall time. The starting material and the products obtained after heat treatment were characterized in detail, using numerous qualitative and semi-quantitative techniques. The aim of this research was to determine the optimal operating conditions for the transformation of the industrial byproduct into a high-quality product. By varying the operating conditions, it was found that complete conversion of iron (II) oxalate to magnetite was achieved at high temperatures (i.e., 773 K and 873 K) after one hour of treatment. The resulting product had a low degree of crystallization, which increased slightly with an increasing reaction time at a temperature of 873 K, reaching a maximum of about 11%. The magnetite obtained can be used in the future as a starting material for chemical looping processes as a chemical/energy carrier for the production of hydrogen.

In recent decades, the mitigation of carbon dioxide (CO₂) emissions represents one of the most urgent scientific and technological challenges

Hydrogen-based reduction of iron ores is a promising route toward low-carbon ironmaking, making reliable determination of the reduction degree essential. However, XRD analysis may overestimate conversion in coarse particles with radial phase gradients, as X-rays mainly probe the particle surface. This study investigates the effect of X-ray penetration depth on the XRD characterization of hydrogen-reduced magnetite particles (~100 μm radius). Two magnetite materials were reduced at 500–600 °C and analyzed before and after milling to ~20 μm. At short reaction times, both measurements gave comparable results, while at longer times non-milled particles showed significantly higher apparent Fe contents and reduction degrees. After 90 min at 600 °C, for example, the Ballast sample appeared fully reduced without milling, whereas milling revealed ~31 wt% residual magnetite and a reduction degree of ~75%. Thermogravimetric measurements confirmed the latter value. The results show that XRD can be strongly surface-weighted in coarse, heterogeneous particles, leading to overestimation of bulk conversion. Milling improves measurement representativeness, while particle size, X-ray energy and reaction pattern must be considered together.