Hydrogen (H
) represents a promising avenue for reducing carbon emissions in energy systems. However, achieving its widespread adoption requires more effective and scalable synthesis methods. Herein, ...we investigated the isothermal carburization process of the MoO
catalyst. This reaction was carried out at a constant temperature of 700 °C in a 60% CO/He stream, with hold reaction times varying (60-min, 90-min, and 120-min). This investigation was conducted using a micro-reactor Autochem with the aim of enhancing the yield of H
. The study focused on evaluating the chemical reduction and carburization behavior of the MoO
catalyst through X-ray diffraction (XRD), transmission electron microscopy (TEM), and CHNS elemental analysis. The XRD analysis revealed the formation of carbides, Mo
C, and MoO
, serving as active sites for subsequent H
production in the thermochemical water splitting (TWS) process. The carburization at a 60-min hold time exhibited enhanced H
production, generating approximately ~ 6.60 µmol of H
with a yield of up to ~ 32.90% and a conversion rate of ~ 54.83%. This finding emphasizes the essential role played by the formation of carbides, particularly Mo
C, in the carburization process, contributing significantly to the facilitation of H
production. These carbides serve as exceptionally active catalytic sites that actively promote the generation of hydrogen. This study underscores that the optimized duration of catalyst exposure is a key factor influencing the successful carburization of MoO
catalysts. This emphasizes how important carbide species are to increasing H
efficiency. Additionally, it is noted that carbon formation on the MoO
active sites can act as a potential poison to the catalysts, leading to rapid deactivation after prolonged exposure to the CO precursor.
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•Carburization Raney Fe investigated by in situ and temperature-programmed methods.•CO dissociation on metallic Fe proceeds at sub-ambient temperatures.•H2 accelerates O removal: O ...removal via H2O is intrinsically faster than via CO2.•A higher H2/CO ratio leads to a higher C coverage and a higher carburization rate.•Fe-carbide phase formation strongly related to kinetic driving force (C coverage)
The formation of Fe-carbide phases is relevant to the synthesis of Fischer-Tropsch synthesis catalysts. We investigated the carburization of Raney Fe as a model catalyst using spectroscopic and temperature-programmed techniques. IR spectroscopy shows that CO dissociation already occurs at −150 °C, while C diffusion into metallic Fe requires much higher temperature (~180 °C). The carburization rate increases with increasing H2/CO ratio, which can be attributed to the lower overall barrier for O removal as H2O as compared to CO2. O removal frees vacancies that are needed for CO dissociation. The resulting higher C coverage increases the driving force for Fe-carbide formation. A higher driving force leads to predominant formation of the more carbon-rich ε(́)-carbide, while χ-Fe5C2 is formed at lower H2/CO ratio. The removal of surface O appears to be the rate-limiting step under all conditions. Initially, most of deposited C is used for Fe-carbide formation with a small contribution to hydrocarbons formation.
•Sample thickness did not affect oxidation kinetics, products, or the carburized microstructure for exposures below 1250 h.•Mechanisms for oxidation behavior and a recent model for non steady-state ...carburization kinetics were validated.•The effect of CO2 pressure on the carburization profile was correctly predicted by modeling.
The influence of 9Cr-1Mo sample thickness on oxidation and carburization kinetics was investigated. State of the art characterization and modeling techniques were employed to investigate the oxidation and carburization behavior. Within the tested exposure times, the sample thickness did not affect oxidation kinetics, products, or the carburized microstructure. The carburization profile measured by various techniques were in good agreement. From these measurements, a quantitative carburization kinetic law is given for 9Cr-1Mo steels allowing the prediction of carbon concentration profile and carbon concentration at the surface for any exposure time and sample thickness.
The Ti-6Al-7 Nb alloy and its carbide possess a wide range of engineering applications, therefore, it is utmost required to fabricate high-quality carburized layers on the alloy surface. In this ...study, the carburization of Ti-6Al-7 Nb alloy was conducted using molten salts (including Carbon Nano Tubes, LiCl, KCl, and KF) in a planetary ball mill, followed by placement in an alumina tube furnace under a nitrogen atmosphere at 1050 °C for various durations. Several characterization techniques were employed to analyze the results, including X-ray diffraction (XRD), field emission electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The XRD results reveal that as the carburization duration increased, the alloy achieved complete carburization, forming a 120 µm thick layer of TiC0.3N0.7. After 24 h of carburization, the crystallite size of TiC0.3N0.7 increased, and the micro-strain decreased, indicating improved structural quality. The morphology of the carburized layer at shorter durations exhibits micro-cracks and defects due to incomplete carburization, where carbon (C) and nitrogen (N) could not effectively occupy in the grain boundaries of alloy. After 24 h, an agglomerated, cauliflower-like layer of TiC0.3N0.7 formed, enhancing the alloy's engineering properties. XPS confirmed the presence of carbon and nitrogen in the carburized sample, which contributed to the formation of the TiC0.3N0.7 layer on the alloy surface. AFM analysis supported the SEM findings, revealing broad islands with microgrooves on the carburized layer. These features indicate a thick and well-formed carburized layer, confirming the successful carburization of Ti-6Al-7 Nb. Overall, the study demonstrates that a 24-h carburization process at 1050 °C in molten salts under a nitrogen atmosphere effectively produces a high-quality, thick, and adherent TiC0.3N0.7 layer on Ti-6Al-7 Nb alloy, significantly enhancing its engineering properties.
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•Carburization of Ti-6Al-7 Nb was successfully achieved after 24 h at 1050 °C.•Low duration of carburization leads to form cracks in the carburized layer.•C and N diffuse into grain boundary during carburization and form TiC0.3N0.7 layer.•TiC0.3N0.7 form a homogenous carburized layer after 12 and 24 h of carburization.
•Water vapour slowed the corrosion of alloys with 5–15 wt% Cr.•Water vapour accelerated the thickening rate of chromia at 800 °C.•Growth mechanism of chromia layer was changed in the presence of ...water vapour.•The critical Cr content required for chromia maintenance was increased in wet CO2.
The corrosion behaviour of binary Ni-Cr alloys with 5–30 wt% Cr was studied in Ar-20CO2-20H2O at 700 and 800 °C. Samples were electropolished before exposure to avoid the effect of surface cold work on alloy diffusion. At 700 °C, all alloys underwent both outer NiO formation and internal oxidation. At 800 °C, the alloys containing 5–20 wt% Cr developed non-protective scales, presenting high weight gain kinetics. However, the Ni-25Cr and Ni-30Cr alloys formed a continuous chromia layer, resulting in much reduced corrosion rates. The effects of water vapour on NiO growth, chromia formation and carburization are discussed.
The corrosion and carburization behaviour of four austenitic stainless steels were evaluated in CO2 and CO2+impurities (CH4, CO and O2) at 600 °C. Exposure to CO2, CO2+CH4, and CO2+CO environments ...showed poor corrosion resistance, especially in 316 grades with the spallation of Cr2O3 layer and breakaway corrosion. Conversely, materials showed improved corrosion resistance in CO2+O2 environment with thin Cr2O3 layer. TEM analysis of 316LN revealed carbon-accumulated layer at the Cr2O3–matrix interface for highly corrosive environments. The degradation of Cr2O3 adherency is correlated with the carbon-accumulated layer, resulting from the high carbon activity in CO2, CO2+CH4, and CO2+CO environments.
•Stainless steels were tested in CO2, CO2+CH4, CO2+CO, and CO2+O2 at 600 °C.•Poor corrosion resistance was observed in CO2, CO2+CH4 and CO2+CO environments.•Carbon-accumulated layer observed at Cr2O3–matrix interface in those environments.•Carbon-accumulated layer affects the adhesion between Cr2O3 layer and matrix.•O2 impurity prevents carbon-accumulation and provides better corrosion resistance.
•We examine structure diversity of MoCx at various preparation conditions.•Carbon deposition phenomenon was studied by Raman spectroscopy.•The carburization degree of Mo2C increased with ...carburization temperature.•We explore structure–property relationship on Mo2C in CO hydrogenation.
Molybdenum carbides were prepared under different carburization conditions. The role of carburization protocol was studied while their CO hydrogenating performances were evaluated in a fixed-bed reactor at 280°C, 3.1MPa, and H2/CO=2.0. The structure of the carbides mainly depended on the type and concentration of carbon source while the crystallite size depended on carbon source and temperature. The surface area, morphology and surface carbon deposition phenomenon of the carbides were sensitive to heating rate and holding time other than the above factors. Though the bulk structure was the same at a carburization temperature range from 630 to 760°C, the carburization degree of the carbides was changing continuously. This diversity led the bond strength of molecular adsorbed CO weaken at a higher carburization temperature while the adsorptive strength of H2 was hardly changed. Both the adsorptive quantities of CO and H2 followed the same trend, with the highest amount on the carbide prepared at 630°C. The activities of the catalysts correlated well with their adsorptive quantities of CO and H2, and the product selectivity was related to their hydrogenation capacity.
Aiming at corrosion issues of structural materials in supercritical carbon dioxide Brayton systems, the corrosion behaviors of T91 and 316 were studied under S-CO2 at 500 ℃, 600℃and 20 MPa. The ...corrosion kinetics conform to the parabolic law. The corrosion products of T91 are mainly composed of outer columnar Fe3O4 and inner Fe-Cr spinel, which had porous structure and no protective effect for further corrosion. Thin and dense Cr2O3 formed on the 316 surface after 500℃corrosion.While large numbers of Fe-oxide nodules formed on surface of Cr-rich scale at 600 ℃. The corrosion mechanism was discussed.
•The corrosion behavior and mechanism of T91 and 316 exposed to supercritical carbon dioxide at 500 ℃, 600 ℃ and 20 MPa up to 1000 h was studied.•The morphology, structure and composition of oxide layers and carburization behavior of T91 and 316 were compared and analyzed.•The corrosion mechanism of T91 and 316 were discussed.
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•SS310 and Alloy 740 were exposed to supercritical CO2 with 100 ppm H2O/O2 impurities.•Direct mass gains of two alloys followed parabolic or near-parabolic oxidation law.•Chromia was ...the dominant corrosion product on two alloys in all environments.•H2O enhanced general + localized oxidation while O2 had an opposite effect on two alloys.•H2O made SS310 more prone to carburization while Alloy 740 was rarely affected.
This study investigated corrosion performance of SS310 and Alloy 740 in supercritical CO2 with the impurity of 100 ppm H2O or O2 at 600 °C and 30 MPa for up to 1000 h. The addition of 100 ppm H2O remarkably enhanced general and localized oxidations while the presence of O2 led to the opposite tendency. Alloy 740 exhibited better resistance to carburization compared to SS310, possibly due to the higher solubility of carbides in Ni-based alloys than those in Fe-based alloys. Both alloys exhibit near parabolic oxidation behavior and acceptable corrosion resistance in the supercritical CO2 environments.
The surface modification of the equiatomic CoCrFeNi high-entropy alloy has been studied via solid carburization at 920 °C for 10 h. Microstructure, hardness and wear resistance were investigated by ...X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), microhardness and nanoindentation tester. Microstructure and phase analysis indicated that two types of nano-size carbide precipitates (M7C3 and M23C6) formed in the surface. The orientation relationship between the M7C3 precipitate and FCC matrix were (1(_)100)C1//(11(_)1)M and 0001C1//1(_)12M. The M23C6 precipitate possessed a cube-cube orientation relationship with the FCC matrix. Vickers hardness and nanoindentation testing results indicated that the carburized surface performed significant improvement of about 100% in hardness compared with the matrix. The strengthening mechanisms including solid-solution strengthening and precipitate strengthening were studied. Moreover, nano-scratch experiments have been conducted on the cross section of the carburized sample under a constant load mode to investigate the friction and wear behavior. The surface profile and depth of scratched tracks were examined using scanning probe microscopy (SPM) technology. The wear and deformation mechanisms at different distances from the surface have been discussed. The calculated values of the wear volume, wear rate, and wear resistance coefficient reveal that the carbide precipitates can improve the wear resistance of the carburized surface.
•The hard carbide coating was fabricated through solid carburization for CoCrFeNi HEA.•The surface hardness and wear resistance were enhanced significantly.•The carburized layer containing M7C3 and M23C6 carbide precipitates was detected.•The precipitation and solution hardening mechanism was studied.
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