Stavros-Alexandros Theofanidis

Bio: Stavros-Alexandros Theofanidis is an academic researcher from Ghent University. The author has contributed to research in topics: Catalysis & Carbon dioxide reforming. The author has an hindex of 10, co-authored 16 publications receiving 712 citations. Previous affiliations of Stavros-Alexandros Theofanidis include Aristotle University of Thessaloniki.

Enhanced Carbon-Resistant Dry Reforming Fe-Ni Catalyst: Role of Fe

Abstract: 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...

Carbon gasification from Fe–Ni catalysts after methane dry reforming

Abstract: 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.

Controlling the stability of a Fe-Ni reforming catalyst : structural organization of the active components

Abstract: 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.

CO2 conversion to CO by auto-thermal catalyst-assisted chemical looping

Abstract: 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.

Control of metal-support interactions in heterogeneous catalysts to enhance activity and selectivity

Abstract: Metal nanoparticles stabilized on a support material catalyse many major industrial reactions. Metal-support interactions in these nanomaterials can have a substantial influence on the catalysis, making metal-support interaction modulation one of the few tools able to enhance catalytic performance. This topic has received much attention in recent years, however, a systematic rationalization of the field is lacking due to the great diversity in catalysts, reactions and modification strategies. In this review, we cover and categorize the recent progress in metal-support interaction tuning strategies to enhance catalytic performance for various reactions. Furthermore, we quantify the productivity enhancements resulting from metal-support interaction control that have been achieved in C1 chemistry in recent years. Our analysis shows that up to fifteen-fold productivity enhancement has been achieved, and that metal-support interaction is most impactful for metal nanoparticles smaller than four nanometres. These findings demonstrate the importance of metal-support interaction to improve performance in catalysis. Methods to control the performance of heterogeneous catalysts are extremely relevant to the success of industrial processes. This review provides a rationalization of the effects that metal support interactions have on the reactivity of different catalytic systems, emphasizing strategies to tune such effects.

Ni-based bimetallic heterogeneous catalysts for energy and environmental applications

Abstract: Bimetallic catalysts have attracted extensive attention for a wide range of applications in energy production and environmental remediation due to their tunable chemical/physical properties. These properties are mainly governed by a number of parameters such as compositions of the bimetallic systems, their preparation method, and their morphostructure. In this regard, numerous efforts have been made to develop “designer” bimetallic catalysts with specific nanostructures and surface properties as a result of recent advances in the area of materials chemistry. The present review highlights a detailed overview of the development of nickel-based bimetallic catalysts for energy and environmental applications. Starting from a materials science perspective in order to obtain controlled morphologies and surface properties, with a focus on the fundamental understanding of these bimetallic systems to make a correlation with their catalytic behaviors, a detailed account is provided on the utilization of these systems in the catalytic reactions related to energy production and environmental remediation. We include the entire library of nickel-based bimetallic catalysts for both chemical and electrochemical processes such as catalytic reforming, dehydrogenation, hydrogenation, electrocatalysis and many other reactions.

A Review on Bimetallic Nickel‐Based Catalysts for CO2 Reforming of Methane

Abstract: In recent years, CO2 reforming of methane (dry reforming of methane, DRM) has become an attractive research area because it converts two major greenhouse gasses into syngas (CO and H2 ), which can be directly used as fuel or feedstock for the chemical industry. Ni-based catalysts have been extensively used for DRM because of its low cost and good activity. A major concern with Ni-based catalysts in DRM is severe carbon deposition leading to catalyst deactivation, and a lot of effort has been put into the design and synthesis of stable Ni catalysts with high carbon resistance. One effective and practical strategy is to introduce a second metal to obtain bimetallic Ni-based catalysts. The synergistic effect between Ni and the second metal has been shown to increase the carbon resistance of the catalyst significantly. In this review, a detailed discussion on the development of bimetallic Ni-based catalysts for DRM including nickel alloyed with noble metals (Pt, Ru, Ir etc.) and transition metals (Co, Fe, Cu) is presented. Special emphasis has been provided on the underlying principles that lead to synergistic effects and enhance catalyst performance. Finally, an outlook is presented for the future development of Ni-based bimetallic catalysts.

A review on catalyst development for dry reforming of methane to syngas: Recent advances

Abstract: The abrupt and massive deactivation of methane dry reforming catalysts especially Ni-based is still a monumental impediment towards its industrialization and commercialization for production of value-added syngas via Fischer-Tropsch process. The need for further and more critical understanding of inherent and tailored interactions of catalyst components for performance and stability enhancement during reforming reaction cannot be over-emphasized. This review provides a contemporary assessment of progresses recorded on synergistic interplay among catalyst components (active metals, support, promoters and binders) during dry reforming using state-of-the-art experimental and theoretical techniques. Advancements achieved during interplay leading to improvements in properties of existing catalysts and discovery of novel ones were stated and expatiated. Reaction pathways, catalytic activities, selection of appropriate synthesis route and metal/support deactivation via sintering or carbon deposition have over time been successfully studied and explained using information from these crucial component interactions. This perspective describes the roles of these interactions and their applications towards development of robust catalysts configurations for successful industrial applications.

Atomically dispersed nickel as coke-resistant active sites for methane dry reforming

Abstract: Dry reforming of methane (DRM) is an attractive route to utilize CO2 as a chemical feedstock with which to convert CH4 into valuable syngas and simultaneously mitigate both greenhouse gases. Ni-based DRM catalysts are promising due to their high activity and low cost, but suffer from poor stability due to coke formation which has hindered their commercialization. Herein, we report that atomically dispersed Ni single atoms, stabilized by interaction with Ce-doped hydroxyapatite, are highly active and coke-resistant catalytic sites for DRM. Experimental and computational studies reveal that isolated Ni atoms are intrinsically coke-resistant due to their unique ability to only activate the first C-H bond in CH4, thus avoiding methane deep decomposition into carbon. This discovery offers new opportunities to develop large-scale DRM processes using earth abundant catalysts.