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Fe-modified MgAl2O4 makes a surprisingly active catalyst support, likely linked to a structural effect of the Fe incorporation. Two catalyst supports, MgAl2O4 and MgFeAlO4, have been studied in fre...

How green is an electrified methanol production process? Up to 43% greenhouse gas emission curbing is possible when renewable electricity is utilized to drive a novel plasma-assisted dry methane reforming-based process. The sustainability of novel electrified processes for methanol production, including plasma-assisted and electrically heated thermocatalytic dry methane reforming based processes, is assessed. Conceptual process design is applied to obtain the life cycle inventory data to perform the ex-ante life cycle assessment, with a focus on the climate change impacts expressed in kg CO2-eq. per kg(MeOH). The plasma-assisted technology results in lower greenhouse gas emissions than the conventional thermocatalytic counterpart, when the plasma reactor itself is powered by renewable (solar or wind) electricity. This also holds for most of the environmental indicators; only a few trade-offs on (eco)toxicity, particulate matter and mineral resource indicators were found, due to the impact from wind turbine construction. For a fully electrified modus operandi, i.e. when all unit operations are electrified by renewable sources, both the plasma-assisted and thermocatalytic technologies result in low climate change impacts, in the range of 0.6-0.7 kg CO2-eq. per kg(MeOH). This is comparable to the climate change impact of CO2-based methanol production utilizing electrolytic H-2. Finally, it is estimated that up to 43% CO2 abatement may be possible by replacing the state-of-the-art (natural gas steam reforming-based) methanol production process with electrified alternatives running on renewable electricity.

Catalyst deactivation due to carbon deposition is a major issue in all reforming technologies. Because of the significant economic cost of catalyst replacement, catalyst regeneration is increasingly attracting attention. The regeneration mechanism of Ni catalysts, with respect to carbon removal, was investigated on support materials prepared by one-pot synthesis. The supports were classified based on their redox functionality: Al2O3, MgAl2O4 show no redox properties in contrast to MgFe0.09Al1.91O4 and CeZrO2 that have redox functionality. A Temporal Analysis of Products (TAP) setup was used to investigate the isothermal regeneration mechanism of Ni catalysts at 993 K by O2. Different mechanisms were distinguished depending on the redox functionality of the support material. Two consecutive steps occur on the support that have no redox properties (Al2O3 and MgAl2O4): metallic Ni is oxidized to form NiO (oxidation step), resulting in an initial local temperature increase of 50–60 K in total, enabling metal particle migration to carbon that was initially separated from the metal and subsequent oxidation through NiO lattice oxygen (reduction step). On the other hand, the mechanism of carbon removal by O2 from Ni catalysts on supports with redox properties does not require particle migration. Two parallel contributions are proposed: 1) Ni metal is oxidized to form NiO, where after lattice oxygen of NiO is used for the oxidation of carbon that is deposited upon the metals, 2) carbon oxidation through lattice oxygen that is provided by the support. No dependency of the carbon gasification mechanism on the exposed fraction of the metal (particle size in the nanoscale) or on the structure of the deposited carbon was concluded.

In this study we report a simple preparation technique for Ni/γ-Al2O3 catalysts for the dry reforming of methane (DRM) at 800 °C to produce CO and H2 (synthesis gas). Hard-templating with low and high surface area activated carbon was applied. The produced synthesis gas exhibited a low product ratio of H2:CO [0.04–0.12], due to reverse water-gas shift. After 75 h time on stream (TOS) minimal deactivation of the catalyst could be observed. A rather unusual activity evolution was found involving a sequence of minimum-maximum-plateau. A scheme was suggested, explaining the activity evolution based on the Ni-nanoparticle positioning from being bare or encapsulated by Al2O3. The Al2O3 shell cracks and undergoes restructuring during reaction making more active sites available for the reaction. Superior metal dispersion was achieved with average nickel nanoparticle size at 4.9 ± 1.3 nm. The sintering mechanism was also investigated. Surprisingly, hollow nickel nanoparticles were observed at 25 h TOS due to the nanoscale Kirkendall effect. This diffusion phenomenon between the core, Ni0, and the outer shell, NiO, (Ni2+) lead to pronounced structural and morphological changes of the catalyst.

We report a MgFexAl2-xO4 synthetic spinel, where x varies from 0 to 0.26, as support for Ni-based catalysts, offering stability and carbon control under various conditions of methane reforming. By incorporation of Fe into a magnesium aluminate spine!, a support is created with redox functionality and high thermal stability, as concluded from temporal analysis of products (TAP) experiments and redox cycling, respectively. A diffusion coefficient of 3 x 10(-17) m(2) s(-1) was estimated for lattice oxygen at 993 K from TAP experiments. X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) modeling identified that the incorporation of iron occurs as Fe3+ in the octahedral sites of the spinel lattice, replacing aluminum. Simulation of the X-ray absorption near edge structure (XANES) spectrum of the reduced support showed that 60 +/- 10% of iron was reduced from 3+ to 2+ at 1073 K, while there was no formation of metallic iron. A series of Ni/MgFexAl2-xO4 catalysts, where x varies from 0 to 0.26, was synthesized and reduced, yielding a supported Ni-Fe alloy. The evolution of the catalyst structure during H-2 temperature-programmed reduction (TPR) and CO2 temperature-programmed oxidation (TPO) was examined using time-resolved in situ XRD and XANES. During reforming, iron in both the support and alloy keeps control of carbon accumulation, as confirmed by O-2-TPO on the spent catalysts. By fine tuning the amount of Fe in MgFexAl2-xO4, a supported alloy was obtained with a Ni/Fe molar ratio of similar to 10, which was active for reforming and stable. By comparison of the performance of Ni-based catalysts with Fe either incorporated into or deposited onto the support, the location of Fe within the support proved crucial for the stability and carbon mitigation under reforming conditions.

The evolution of the constituents of an 8 wt%Ni-5 wt%Fe/MgAl2O4 catalyst for dry reforming of methane (DRM) is monitored by in situ quick X-ray absorption spectroscopy (QXAS) and Fe-57 Mossbauer spectroscopy. In as prepared state, Fe is present as NiFe2O4 at the surface and as MgFex3+Al2-xO4 within the support, whereas Ni is mainly present as NiO. During H-2-TPR, NiFe2O4 and NiO form an alloy from 500 degrees C on and (MgFex+Al2-xO4)-Al-3 is partially reduced to MgFex2+Al2-xO4, such that Ni-Fe alloy, MgFex2+Al2-xO4 and MgFex3+Al2-xO4 are the prevalent phases in the reduced catalyst. During DRM, dominantly oxidizing environments (CH4/CO2 = 1/2, 1/1.5) lead to formation of FeOx nanoparticles at the surface of the Ni-Fe alloy, thereby affecting the DRM activity, and possibly to some reincorporation of Fe into the support. For CH4/CO2 = 1/1, no significant changes occur in the catalyst's activated state, as a consequence of reduction by CH4 dissociation species counteracting oxidation by CO2. However, Mossbauer analysis detects continued extraction of Fe from the support, sustaining ongoing NiFe alloying.

Fe–Ni catalysts present high activity in dry reforming of methane, with high carbon resistance, but suffer from deactivation via sintering and Fe segregation. Enhanced control of the stability and activity of Fe–Ni/MgAl 2 O 4 was achieved by means of Pd addition. The evolution of the catalyst structure during H 2 Temperature Programmed Reduction (TPR) and CO 2 Temperature Programmed Oxidation (TPO) was investigated using time-resolved in situ X-ray diffraction (XRD). During reduction of Fe–Ni–Pd supported on MgAl 2 O 4 , a core shell alloy forms at the surface, where Fe–Ni is in the core and Fe–Ni–Pd in the shell. A 0.2 wt% Pd loading or Ni:Pd molar ratio as high as 75:1 showed the best performance in terms of both activity and stability of the catalyst at 1023 K and total pressure of 101.3 kPa. Experimental results and DFT calculations showed that Pd addition to bimetallic Fe–Ni reduces the tendency of Fe to segregate to the surface of the alloy particles under methane dry reforming (DRM) conditions, due to the formation of a thin Fe–Ni–Pd surface layer. The latter acts as a barrier for Fe segregation from the core. Segregation of Fe from the trimetallic shell still occurs, but to a lesser extent as the Fe concentration is lower. This Ni:Pd molar ratio is capable of controlling the carbon formation and hence ensure high catalyst activity of 24.8 mmol s −1 g metals −1 after 21 h time-on-stream.

A bifunctional 9 wt.%NiO-16 wt.%Fe 2 O 3 /MgAl 2 O 4 material was prepared for CO 2 conversion to CO by auto-thermal catalyst-assisted chemical looping. This process is designed to maximize CO 2 conversion. The generation of CO from CO 2 was investigated between 873 K and 1023 K. The high endothermicity of methane dry reforming and the material deactivation by coke deposition were avoided by the simultaneous feeding of CH 4 , CO 2 and O 2 in a 1:1:0.5 molar ratio during the reduction half-cycle of chemical looping. In this half-cycle, interaction of Ni with Fe leads to Ni-Fe alloy formation. The resulting Ni-based catalyst converts CH 4 + CO 2 + O 2 into a mixture of CO and H 2 , which both reduce Fe 3 O 4 , producing CO 2 and H 2 O. In the CO 2 re-oxidation half-cycle, CO is produced and the Ni-Fe alloy decomposes into Ni and Fe 3 O 4 . The reduction capacity ( R c ) of the gas mixture strongly depends on the ratio R c between reducing and oxidizing gases. Based on thermodynamic calculations, high conversion of Fe 3 O 4 to reduced state can be reached if R c > 2 and T > 873 K. During prolonged auto-thermal chemical looping at 1023 K, the 9 wt.%NiO–16 wt.%Fe 2 O 3 /MgAl 2 O 4 suffers from deactivation in the first five cycles, after which a more stable operation is established. Based on TEM measurements, sintering was found to be the main cause for the initial decrease of CO production.

Carbon species removal was studied from a Fe–Ni catalyst supported on MgAl2O4 after methane dry reforming at 1023 K, atmospheric pressure and a CH4/CO2 molar ratio of 1:1. The deactivated and regenerated catalysts were characterized using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and energy-dispersive X-ray spectroscopy (EDX)-STEM mapping. The catalyst regeneration was studied by CO2 and O2 temperature programmed oxidation (TPO) and by operando time-resolved X-ray diffraction (XRD). A transient response technique, Temporal analysis of products (TAP), was applied to investigate the isothermal carbon species gasification. Two different types of carbon species, graphitic and amorphous, were present after reaction. CO2 oxidation could remove part of the carbon species, although EDX-STEM mapping showed the presence of carbon species located far from active metals phase even after CO2–TPO at 1123 K. Carbon species removal by CO2 involves two contributions: (1) the dissociation of CO2 over Ni followed by the oxidation of carbon species by surface oxygen; (2) Fe oxidation by CO2 and subsequent carbon species oxidation by Fe oxide lattice oxygen. The oxidation of carbon species by O2 was identified from temperature programmed and isothermal experiments as a process including two processes: (1) oxidation of surface carbon by lattice oxygen and (2) particles migration to carbon species deposited far from active metals and subsequent oxidation through lattice oxygen of the iron and/or nickel oxides. The contribution of oxygen spillover in carbon gasification was considered to be negligible.

The addition of Rh, in low concentrations ( During reduction of the Ni-Rh catalyst, two types of Ni-Rh alloy were formed. The Rh-rich Ni alloy remained stable up to 1123 K under CO2 oxidation. The catalyst with a Ni:Rh molar ratio of 41 showed the best performance in terms of both activity and stability, in presence of 7 ppm H2S as a contaminant, at 1173 K and total pressure of 111.3 kPa, reaching 0.24 molCH4·s−1 ·kgmetals−1 after 42.5 h time-on-stream. Regeneration of the catalysts was performed by removing H2S from the feed stream. The catalyst regeneration ability depended on the formation of a Ni-Rh alloy and hence on the Ni:Rh molar ratio. According to density functional theory calculations on the adsorption and dissociation of H2S on Ni and NiRh (111) surfaces, the Ni-Rh alloy inhibited H2S decomposition in contrast to monometallic Ni.

The increasing energy consumption creates the need to investigate new routes for the utilization of the available resources, like natural gas and biogas, to produce fuels and chemicals. Natural gas, which primarily consists of methane (CH4), has an actual market price of 1.9 USD/GJ. This makes CH4 one of the most affordable carbon feedstocks in the world. Direct conversion of CH4 into chemicals results in low yields due to the high C―H bond dissociation energy (436 kJ/mol). As a result, CH4 is mainly converted indirectly, by reforming it to syngas, a mixture of H2 and CO, as an intermediate step. Syngas is a key building block with many downstream applications, as it is used in a number of synthesis processes of a wide range of chemicals and fuels. Most of the produced syngas is used for the synthesis of ammonia for fertilizers and for hydrogen production, which is exploited in refining processes, while the “gas-to-liquid” routes (i.e., Fischer-Tropsch) for fuel production account for ~ 8% consumption.

A series of bimetallic Fe-Ni/MgAl2O4 catalysts with Fe/Ni ratios between 0 and 1.5 have been examined for methane dry reforming at 923–1073 K, atmospheric pressure, and a CH4/CO2 ratio of 1. The evolution of the catalyst structure during H2 temperature-programmed reduction (TPR), CO2 temperature-programmed oxidation (TPO), and dry reforming is examined using time-resolved in situ X-ray diffraction (XRD). During H2-TPR up to 973 K, Fe2O3 and NiO are reduced to Fe and Ni. Higher temperatures lead to Fe-Ni alloy formation. The alloy remains stable up to 900 K under CO2-TPO and is decomposed to Ni and Fe3O4 at higher temperatures. The Fe-Ni alloy is the active phase while Fe partially segregates from the alloy forming FeOx during dry reforming. This is beneficial as it reduces the surface carbon accumulation through interaction with FeOx lattice oxygen, producing CO. Alternate CH4 and CO2 pulse experiments over Ni, Fe, and Ni-Fe samples showed that dry reforming over Fe-Ni catalysts can follow a Mars–van Krev...

The role of iron in view of its further utilization in chemical processes is presented, based on current knowledge of its properties. The addition of iron to a catalyst provides redox functionality, enhancing its resistance to carbon deposition. FeOx species can be formed in the presence of an oxidizing agent, such as CO2, H2O or O2, during reaction, which can further react via a redox mechanism with the carbon deposits. This can be exploited in the synthesis of active and stable catalysts for several processes, such as syngas and chemicals production, catalytic oxidation in exhaust converters, etc. Iron is considered an important promoter or co-catalyst, due to its high availability and low toxicity that can enhance the overall catalytic performance. However, its operation is more subtle and diverse than first sight reveals. Hence, iron and its oxides start to become a hot topic for more scientists and their findings are most promising. The scope of this article is to provide a review on iron/iron-oxide containing catalytic systems, including experimental and theoretical evidence, highlighting their properties mainly in view of syngas production, chemical looping, methane decomposition for carbon nanotubes production and propane dehydrogenation, over the last decade. The main focus goes to Fe-containing nano-alloys and specifically to the Fe–Ni nano-alloy, which is a very versatile material.