Influence of hydrogen and various carbon monoxide concentrations on reduction behavior of iron oxide at low temperature

The aims of this study are to produce Fe₃O₄ from Fe₂O₃ using hydrogen (H₂) and carbon monoxide (CO) gases by focusing on the influence of these gases on reduction of Fe₂O₃ to Fe₃O₄ at low temperature (below 500 °C). Low reduction temperature behavior was investigated by using temperature programmed reduction (TPR) with the presence of 20% H₂/N₂, 10% CO/N₂, 20% CO/N₂ and 40% CO/N₂. The TPR results indicated that the first reduction peak of Fe₂O₃ at low temperature appeared faster in CO atmosphere compared to H₂. Furthermore, reducibility of first stage reduction could be improved when increasing CO concentration and reduction rate were followed the sequence as: 40% CO > 20% CO > 10% CO > 10% H₂. All reduction peaks were shifted to higher temperature when the CO concentration was reduced. Although, initial reduction by H₂ occurred slower (first peak appeared at higher temperature, 465 °C) compared to CO, however, it showed better reduction with Fe₂O₃ fully reduced to Fe at temperature below 800 °C. Meanwhile, complete reduction happened at temperature above 800 °C in 10% and 20% CO/N₂. Thermodynamic calculation revealed that CO acts as a better reducer than H₂ as the enthalpy change of reaction (ΔH_r) is more exothermic than H₂ and the change in Gibbs free energy (ΔG) at 500 °C is directed to more spontaneous reaction in converting Fe₂O₃ to Fe₃O₄. Therefore, formation of magnetite at low temperature was thermodynamically more favorable in CO compared to H₂ atmosphere. XRD analysis explained the formation of smaller crystallite size of magnetite by H₂ whereas reduction of CO concentration from 40, 20 to 10% enhanced the growth of highly crystalline magnetite (31.3, 35.5 and 39.9 nm respectively). All reductants were successfully transformed Fe₂O₃ → Fe₃O₄ at the first reduction peak except for 10% CO/N₂ as there was a weak crystalline peak due to remaining unreduced Fe₂O₃. Overall, less energy consumption needed in reducing Fe₂O₃ to Fe₃O₄ by CO. This proved that CO was enhanced the formation of magnetite, encouraged formation of highly crystalline magnetite in more concentrated CO, considered better reducing agent than H₂ and these are valid at lower temperature.     

Studies on influence of hydrogen and carbon monoxide concentration on reduction progression behavior of molybdenum oxide catalyst

Temperature programmed reduction (TPR) analysis was applied to investigate the chemical reduction progression behavior of molybdenum oxide (MoO₃) catalyst. The composition and morphology of the reduced phases were characterized by X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS), and field emission scanning electron microscopy (FE-SEM). The reduction progression of MoO₃ catalyst was attained with different reductant types and concentration (10% H₂/N₂, 10% and 20% CO/N₂ (%, v/v)). Two different modes of reduction process were applied. The first approach of reduction involved non-isothermal mode reduction up to 700 °C, while the second approach of reduction involved the isothermal mode reduction for 60 min at 700 °C. Hydrogen temperature programmed reduction (H₂-TPR) results showed the reduction progression of three-stage reduction of MoO₃ (Mo⁶⁺ → Mo⁵⁺ → Mo⁴⁺ → Mo⁰) with Mo⁵⁺ and Mo⁴⁺. XRD analysis confirmed the formation of Mo₄O₁₁ phase as an intermediate phase followed by MoO₂ phase. After 60 min of isothermal reduction, peaks of metallic molybdenum (Mo) appeared. Whereas, FESEM analysis showed porous crater-like structure on the surface cracks of MoO₂ layer which led to the growth of Mo phase. Meanwhile, the reduction of MoO₃ catalyst in 10% carbon monoxide (CO) showed the formation of unstable intermediate phase of Mo₉O₂₆ at the early stage of reduction. Furthermore, by increasing 20% CO led to the carburization of MoO₂ phase, resulted in the formation of Mo₂C rather than the formation of metallic Mo, as confirmed by XPS analysis. Therefore, the presented study shows that hydrogen gave better reducibility due to smaller molecular size, which contributed to high diffusion rate and achieved deeper penetration into the MoO₃ catalyst compared to carbon monoxide reductant. Hence, the reduction of MoO₃ in carbon monoxide atmosphere promoted the formation of Mo₂C which was in agreement with the thermodynamic assessment.     

Studies on reduction of chromium doped iron oxide catalyst using hydrogen and various concentration of carbon monoxide

Chemical reduction behaviour of 3% chromium doped (Cr–Fe₂O₃) and undoped iron oxides (Fe₂O₃) were investigated by using temperature programmed reduction (TPR). The reduced phases were characterized by X-ray diffraction spectroscopy (XRD). The reduction processes were achieved with 10% H₂ in nitrogen (%, v/v), 10% and 20% of carbon monoxide (CO) in nitrogen (%, v/v). In hydrogen atmosphere, the TPR results indicate that the reduction of Cr–Fe₂O₃ and Fe₂O₃ proceed in three steps (Fe₂O₃ → Fe₃O₄ → FeO → Fe) with Fe₃O₄ and FeO as intermediate states. A complete reduction to metallic iron for both samples occurred at 900 °C. As for CO reductant, the profiles show the reduction of Fe₂O₃ also proceeded in three steps with a complete reduction occurs at 900 °C in 10% CO with no sign of carbide formation. Nevertheless, a 20% CO was able to reduce the completely at lower temperature at 700 °C and there is a formation of iron carbide at 500 °C but the carbide disappeared as the reduction temperature increase. Meanwhile in 10% CO atmosphere, Cr–Fe₂O₃ shows a better reducibility compared to Fe₂O₃ with a complete reduction at 700 °C, which is 200 °C lower than Fe₂O₃. A Cr dopant in the Fe₂O₃ can lead to formation of various forms of iron carbides such as F₂C, Fe₅C₂, Fe₂₃C₆ and Fe₃C as the CO concentration was increased to 20%. The transformation profile of iron phases during carburization follows the following forms, Fe₂O₃ → Fe₃O₄ → iron carbides (Fe_xC). The XRD pattern shows the diffraction peaks of Cr–Fe₂O₃ are more intense with improved crystallinity for the characteristic peaks of Fe₂O₃ compare to undoped Fe₂O₃. No visible sign of chromium particles peaks in the XRD spectrum that indicates the Cr particles loaded onto the iron oxide are well dispersed. The uniform dispersion with no sign of sintering effects of the Cr dopant on the Fe₂O₃ was confirmed by FESEM. The study shows that Cr dopant gives a better reducibility of iron oxide but promotes the formation of carbides in an excess CO concentration.     

Metal oxide hydrogen,oxygen, and carbon monoxide sensors for hydrogen setups and cells

Use of hydrogen, oxygen, and carbon oxide semiconductor sensors made of metal oxides allows controlling electronically the content of these gases in operation of many hydrogen setups, cells and devices. Present review-paper gives a general idea of achievements in this field.     

Physical and chemical behaviour of tungsten oxide in the presence of nickel additive under hydrogen and carbon monoxide atmospheres

The physical and chemical behaviour of bulk tungsten oxide (WO₃) and Ni doped tungsten oxide (15% Ni/WO₃) were examined by performing a temperature-programmed reduction (TPR) technique. The chemical composition, morphology, and surface composition of both samples before and after reduced were analysed by X-ray diffraction (XRD), scanning electron microscopy (FESEM), and X-ray photoelectron spectroscopy (XPS) analysis. The XRD pattern of calcined Ni doped tungsten oxide powder comprised of WO₃ and nickel tungstate (NiWO₄) phases. The reduction behaviour was investigated by a non-isothermal reduction up to 900 °C achieved under (10 and 20% v/v) hydrogen in nitrogen (H₂ in N₂) and (20 and 40% v/v) carbon monoxide in nitrogen (CO in N₂) atmospheres. The H₂-TPR were indicated the reduction of bulk WO₃ and 15% NiWO₃ proceed in three steps (WO₃ → WO₂ → WO₂ + W) and (WO₃ → WO₂ → W + Ni₄W) respectively under 20% H₂. Whereas, the reduction of 15% WO₃ under 40% CO involves of two following stages: (i) low temperature (<800 °C) transformation of WO₃ → WO_2.72 → WO₂ and, (ii) high temperature (>800 °C) transformation of WO₂ → W → WC. Furthermore, NiWO₄ alloy phase was transformed according to the sequence NiWO₄ → Ni + WO_2.72 → Ni + WO₂ → Ni + W → Ni₄W + W at temperature >700 °C and >800 °C in H₂ and CO atmospheres, respectively. It can be concluded that the reduction behaviour of WO₃ is matched with the thermodynamic data. In addition, the reduction under H₂ is more favourable and have better reducibility compared to the CO gas. It is due to the small molecule size and molecule mass of H₂ that encourages the diffusion of H₂ molecule into the internal surface of the catalyst compared to CO. Moreover, Ni additive had improved the WO₃ reducibility and enhancing the CO adsorption and promotes the formation of tungsten carbide (WC) by carburisation reaction. Besides, the formation of Ni during the reduction of 15% Ni/WO₃ under CO reductant catalysed the Boudouard reaction to occur, which disproportionated the carbon monoxide to carbon dioxide and carbon (CO → CO₂ + C).     

Synthesis of Pt nano catalyst in the presence of carbon monoxide: Superior activity towards hydrogen evolution reaction

The effect of carbon monoxide (CO) on the reduction of Pt ion to metallic Pt is studied. The modified GC electrode with platinum metal synthesized in the presence of CO shows excellent activity for hydrogen evolution reaction (HER). Despite the decrease in the loading of platinum (4.5 × 10⁻⁴ mg cm⁻²) a substantial increase in its electrocatalytic activity towards HER is observed in a sulfuric acid environment. The observed electrocatalytic activity is comparable to available commercial catalysts like Pt/C. Tafel slope was obtained to be 34 mV.dec^−1, and the overpotential was acquired to be 31 mV at the mass activity of 10 mA mg⁻¹ were observed which was very close to kinetic parameters of Pt/C catalyst.     

Kinetic interplay between hydrogen and carbon monoxide in syngas-fueled catalytic micro-combustors

The combined oxidation of hydrogen and carbon monoxide over platinum in micro-combustors at catalytic surface temperatures below 600 K was studied numerically, using a two-dimensional computational fluid dynamics (CFD) model with detailed heterogeneous and homogeneous reaction mechanisms and multicomponent transport. Simulations were performed at different surface temperatures and feed compositions to study the kinetic interplay between hydrogen and carbon monoxide. A sensitivity analysis of the heterogeneous reaction mechanism was performed to identify the rate-controlling steps. Finally, possible mechanisms for the observed behavior were discussed. It was shown that there is significant kinetic interplay between hydrogen and carbon monoxide. Carbon monoxide significantly inhibits the catalytic oxidation of hydrogen. In contrast, the presence of hydrogen was found to promote the catalytic oxidation of carbon monoxide, with the largest effect shown for the small addition of hydrogen, then this effect progressively decreases with the further increase of hydrogen concentration. Accordingly, the apparent reaction order with respect to hydrogen changes from positive to negative, then to zero. The promoting effect of hydrogen can be attributed to the carboxyl pathway, which is crucial to describe the process.     

The inhibitory effect of carbon monoxide contained in hydrogen gas environment on hydrogen-accelerated fatigue crack growth and its loading frequency dependency

The effect of carbon monoxide (CO) contained in H₂ gas as an impurity on the hydrogen-accelerated fatigue crack growth of A333 pipe steel was studied in association with loading frequency dependency. The addition of CO to H₂ gas inhibited the accelerated fatigue crack growth due to the hydrogen. The inhibitory effect was affected by the CO content in the H₂ gas, loading frequency, and crack growth rate. Based on these results, it was revealed that the inhibitory effect of CO was governed by both competition between the rate of fresh surface creation by the crack growth and the rate of coverage of the surface by CO and time for hydrogen diffusion in the material to the crack tip with reduced hydrogen entry by CO.     

A new approach to the manufacturing of elemental boron from boron oxide by carbon monoxide

Elemental boron is one of the most valuable high-tech boron products and it has highest energy density 14 kcal/g in the world for this type of product. With the rapid advancements in technology in recent years, a demand has grown for a light materials with functionality and excellent properties such as high hardness, high melting point, high strength, high chemical resistance and nuclear characteristics that can be used in the fields of aerospace, aviation, automotive and solar cells. In this study boron oxide was reduced using carbon monoxide via a batch system to produce elemental boron. To determine the most suitable conditions for the reduction reaction different temperatures and different CO/B₂O₃ mol ratio parameters were studied. As a result of thermodynamic calculations for the most efficient parameters for reaction temperature was 140–210 °C and the CO/B₂O₃ mol ratio being studied was 3/1 and 2/1 for the batch system. Boron oxide reduction was performed by carbon monoxide gas with the pressure set at 10 bar. Characterization of the product was carried out by using X-Ray Diffractometer (XRD), Fourier Transform Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) at optimum temperature and mol ratio (140 °C and 3/1). Boron phase was seen in both XRD and FT-IR analysis. Also, SEM analysis was performed in order to observe morphological structure of elemental boron.     

Alumina supported nano-platinum on copper nanoparticles prepared via galvanic displacement reaction for preferential carbon monoxide oxidation in presence of hydrogen

Improved catalytic centres with a minimum mass-loading of expensive platinum (Pt) have been anticipated for various catalytic applications, for instance preferential oxidation (PROX) of carbon monoxide (CO) in the presence of Hydrogen. Here, we report the synthesis of nano-Pt on the surface of copper (Cu) nanoparticles (NPs) supported on γ-Al₂O₃ (Pt_n(Cu)/γ-Al₂O₃) via galvanic displacement reaction (GDR) for the catalytic CO-PROX reaction. Pt_n(Cu)/γ-Al₂O₃ showed much improved CO-PROX performance compared to that of the as-synthesized Pt_l(Cu)/γ-Al₂O₃ catalyst. Importantly, no significant conversion of hydrogen at a lower temperature range (<200 °C) is observed during the CO-PROX reaction which is one of the essential prerequisites for the CO-PROX reaction. Moreover, Pt_n(Cu)/γ-Al₂O₃ showed the durable, long-term catalytic CO-PROX performance for 120 h. These results infer that realization of nano-Pt on the surface of the Cu NPs holds the promise as the catalytic centres with the minimum mass-loading of Pt for the CO-PROX reaction.     

Periodically regenerating diesel particulate filter with a hydrogen/carbon monoxide mixture addition

This investigation analyses the effect of introducing a H₂/CO mixture, upstream of a diesel particulate filter (DPF), in an attempt to support the regeneration process. The introduction of the mixture was achieved via various periodic strategies in an attempt to reduce the volume of mixture required while still maintaining proficient regeneration qualities. In addition to this, the effect of space velocity and engine load on the regeneration process was also investigated. The experimental data showed that the mixture addition supported the regeneration process by increasing the filter temperature via an exothermic reaction. The most beneficial spraying strategy introduced the mixture to the DPF every 20 s, with each injection event lasting for a period of 10 s. This strategy required 50% less mixture volume than the constant spray strategy but still induced similar regeneration capabilities. In addition to this, it was noted that a decrease in space velocity reduced the rate of temperature increase but improved the peak filter temperature resulting in increased regeneration proficiency. It was also noted that a reduction in engine load reduced the mean filter temperature but overall had minimal effect on the regeneration process.     

Analysis of the effects of reformate (hydrogen/carbon monoxide) as an assistive fuel on the performance and emissions of used canola-oil biodiesel

Evolving technology and a reoccurring energy crisis creates a continued investigation into the search for sustainable and clean-burning renewable fuels. One possibility is hydrogen that has many desirable qualities such as a low flammability limit promoting ultra-lean combustion, high laminar flame speed for increased thermal efficiency and low emissions. However, past research discovered certain limiting factors in its use such as pre-ignition in spark ignition engines and inability to work as a singular fuel in compression ignition engines. To offset these issues, this work documents manifold injection of a hydrogen/carbon monoxide mixture in a dual-fuel methodology with biodiesel. While carbon monoxide does degrade some of the desirable properties of hydrogen, it acts partially like a diluent to restrict pre-ignition. The result of this mixture addition allows the engine to maintain power while reducing biodiesel fuel consumption with a minimal NO_x emissions increase.     

Plasma-chemical reduction of iron oxide photoanodes for efficient solar hydrogen production

We demonstrate the effect of hydrogen plasma treatment on hematite films as a simple and effective strategy for modifying the existing substrate to improve significantly the band edge positions and photoelectrochemical (PEC) performance. Plasma treated hematite films were consist of mixed phases (Fe₃O₄:α-Fe₂O₃) which was confirmed by XPS and Raman analysis, treated films also showed higher absorption cross-section and were found to be a promising photoelectrode material. The treated samples showed enhance photocurrent densities with maximum of 3.5 mA/cm² at 1.8 V/RHE and the photocurrent onset potentials were shifted from 1.68 V_RHE (untreated) to 1.28 V_RHE (treated). Hydrogen plasma treatment under non-equilibrium conditions induced a valence dynamics among Fe centers in the sub-surface region that was sustained by the incorporation of hydrogen in the hematite lattice as supported by the density functional theory calculations.     

Silicon monoxide assisted synthesis of Ru modified carbon nanocomposites as high mass activity electrocatalysts for hydrogen evolution

Extremely low content of Ruthenium (Ru) nanoparticles were loaded on the carbon black (Ru/C) via reducing Ru ions with silicon monoxide. The obtained Ru/C nanocomposites exhibit an exciting electrochemical catalytic activity for hydrogen evolution reaction (HER) in the oxygen-free 0.5 M H₂SO₄ medium. The optical one (Ru/C-2) with a low Ru amount of 2.34% shows higher activity than previously reported Ru-based catalysts. The overpotential at 10 mA cm⁻² is 114 mV and the Tafel slope is 67 mV·dec⁻¹. Ru/C-2 catalyst also has good stability. The overpotential that afford the current density of 10 mA cm⁻² of 20 wt% Pt/C increased 92 mV while that of Ru/C-2 only increased 50 mV after a 30,000 s chronopotentiometry test. Furthermore, the mass activity of Ru/C-2 catalyst is even better than that of the commercial 20 wt% Pt/C when the overpotential is larger than 0.18 V. This silicon monoxide-mediated strategy may open a new way for the fabrication of high performance electrocatalysts.     

Efficiently generating COx-free hydrogen by mechanochemical reaction between alkali hydrides and carbon dioxide

This article aims to investigate the mechanochemical hydrogen desorption reactions of alkali hydrides (XH: X = Li, Na, or K) with carbon dioxide. The result of this investigation shows that CO₂ can be efficiently reduced by XH (X = Na or K), generating CO_x-free hydrogen under room-temperature mechanical ball milling condition in the absence of a catalyst. The mole percentage and production rate of hydrogen in the gaseous product, which can reach 98.72% and 95.37%, respectively, depend on the milling time, rotation rate of milling and the mole ratio of XH/CO₂. During the mechanochemical reactions, carbon dioxide is fast and wholly consumed by XH, producing element carbon, alkali carbonates, and H₂. This work establishes a new, simple and efficient means for the room-temperature preparation of CO_x-free hydrogen and the elimination of CO_x contaminant in hydrogen.     

Room temperature hydrogen gas sensing properties of mono dispersed platinum nanoparticles on graphene-like carbon-wrapped carbon nanotubes

We present platinum nanoparticles dispersed wrinkled graphene-like carbon-wrapped carbon nanotubes (Pt/GCNTs) as a room temperature chemiresistive hydrogen gas sensor. Pt nanoparticles are decorated over GCNTs surface using poly (sodium 4-styrene sulfonate) (PSS) functionalization, followed by ethylene glycol reduction method. The highly defective wrinkled graphene-like surface of GCNTs provides large surface area and PSS functionalization provides stable immobilization of mono dispersed Pt nanoparticles on the carbon surface. A simple and inexpensive drop cast technique is used to fabricate the thick film sensor of the material. Hydrogen resistive gas sensing properties of Pt/GCNTs are studied at different gas concentrations, temperatures and Pt wt. % loadings. Pt/GCNTs sensor shows optimal sensitivity at room temperature with stable and reproducible response towards hydrogen. The sensor with 2 wt. % of Pt showed maximum sensitivity that is three fold higher than Pt decorated carbon nanotubes (Pt/CNTs) with the same Pt wt. % loading. The present study shows potential to explore novel H2 sensors^.     

Intercritical annealing temperature dependence of hydrogen embrittlement behavior of cold-rolled Al-containing medium-Mn steel

The present investigation attempts to evaluate the influence of intercritical annealing temperature (T_IA) on the hydrogen embrittlement (HE) of a cold-rolled Al-containing medium-Mn steel (Fe-0.2C-4.88Mn-3.11Al-0.62Si) by using electrochemical hydrogen-charging, slow strain rate tensile test and scanning electron microscope. The results show that an excellent combination of strength and ductility (the product of ultimate tensile strength and total elongation) up to ∼53 GPa·% was obtained for the specimen intercritically annealed at an intermediate temperature of 730 °C, whereas the HE index increases significantly with an increase in T_IA up to 850 °C. Being different from the typical dimple ductile fracture for the uncharged specimen, the hydrogen-charged specimen exhibits a mixed brittle interface decohesion and ductile intragranular fracture mode in the crack initiation region and the brittle fracture fraction increases with increasing T_IA. Both the stability and amount of austenite play a critical role in governing the HE behavior of TRIP-assisted medium-Mn steel. Thus, it is suggested that suitable T_IA should be explored to guarantee the safety service of automotive parts made of this type of steel in addition to acquiring excellent mechanical properties.     

Maximizing the production of hydrogen and carbon nanotubes: Effect of Ni and reaction temperature

With the objective of maximizing hydrogen and CNTs production, the catalytic cracking of naphtha has been carried out at progressive reaction temperatures i.e. from 600 to 750 °C. The ZSM-5 and nickel impregnated ZSM-5 were used as catalysts for cracking purpose in fluidization mode. The catalyst analysis imparted that impregnation of metallic nickel induces a strong adhesion on MFI structure of ZSM-5 associated with an enhancement in textural properties and acid density. In addition, the results disclose that the incorporation of nickel on ZSM-5 leads to increment in stability of catalyst which in turn pushes the yields of H₂, CNTs and conversion to greater values of 3.29%, 4.84% and 90%, respectively. The as-grown carbon structures over the catalyst surface were found to be multiwall carbon nanotubes confirmed by Raman spectra and TGA analysis where they exhibited high quality (I_D/I_G = 0.65) and purity, respectively, at 750 °C.     

Effect of carbon deposition by carbon monoxide disproportionation on electrochemical characteristics at low temperature operation for solid oxide fuel cells

The deterioration by carbon deposition was evaluated for electrolyte- and anode-supported solid oxide fuel cells (SOFCs) in comparison with carbon monoxide disproportionation and methane cracking. The polarization resistance of the nickel-yttria stabilized zirconia (Ni-YSZ) anode increased with a rise in CO concentration in H₂-CO-CO₂ mixture for the electrolyte-supported cells at 923 K. The resistance, however, did not change against CO concentration for the anode-supported cells. In a methane fuel with a steam/carbon (S/C) ratio of 0.1, the cell performance decreased for both of the cells at 1073 K. A large amount of agglomerated amorphous carbon was deposited from the anode surface to the interface between the anode and the electrolyte after power generation at S/C = 0.1 in methane fuel. On the other hand, the crystalline graphite was deposited only at the anode surface for the anode-supported cell after power generation in CO-CO₂ mixture. These results suggest that the reaction rate of CO disproportionation is faster than that of methane cracking. The deposited carbon near the anode/electrolyte interface caused the increase in the polarization resistance.