Alexey Fedorov

Last Name

Fedorov

First Name

Alexey

ORCID

0000-0001-9814-6726

Organisational unit

03865 - Müller, Christoph R. / Müller, Christoph R.

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Publications 1 - 10 of 88

Redox dynamics and surface structures of an active palladium catalyst during methane oxidation
Yue, Shengnan; Praveen, C.S.; Klyushin, Alexander; et al. (2024)
Nature Communications
Catalysts based on palladium are among the most effective in the complete oxidation of methane. Despite extensive studies and notable advances, the nature of their catalytically active species and conceivable structural dynamics remains only partially understood. Here, we combine operando transmission electron microscopy (TEM) with near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory (DFT) calculations to investigate the active state and catalytic function of Pd nanoparticles (NPs) under methane oxidation conditions. We show that the particle size, phase composition and dynamics respond appreciably to changes in the gas-phase chemical potential. In combination with mass spectrometry (MS) conducted simultaneously with in situ observations, we uncover that the catalytically active state exhibits phase coexistence and oscillatory phase transitions between Pd and PdO. Aided by DFT calculations, we provide a rationale for the observed redox dynamics and demonstrate that the emergence of catalytic activity is related to the dynamic interplay between coexisting phases, with the resulting strained PdO having more favorable energetics for methane oxidation.
Nature of GaOx Shells Grown on Silica by Atomic Layer Deposition
Chen, Zixuan; Zimmerli, Nora K.; Zubair, Muhammad; et al. (2023)
Chemistry of Materials
Gallia-based shells with a thickness varying from a submonolayer to ca. 2.5 nm were prepared by atomic layer deposition (ALD) using trimethylgallium, ozone, and partially dehydroxylated silica, followed by calcination at 500 degrees C. Insight into the atomic-scale structure of these shells was obtained by highfield Ga-71 solid-state nuclear magnetic resonance (NMR) experiments and the modeling of X-ray differential pair distribution function data, complemented by Ga K-edge X-ray absorption spectroscopy and Si-29 dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) studies. When applying one ALD cycle, the grown submonolayer contains mostly tetracoordinate Ga sites with Si atoms in the second coordination sphere (Ga-[4](Si)) and, according to N-15 DNP SENS using pyridine as the probe molecule, both strong Lewis acid sites (LAS) and strong Bronsted acid sites (BAS), consistent with the formation of gallosilicate Ga-O-Si and Ga-mu(2)-OH-Si species. The shells obtained using five and ten ALD cycles display characteristics of amorphous gallia (GaOx), i.e., an increased relative fraction of pentacoordinate sites (Ga-[5](Ga)), the presence of mild LAS, and a decreased relative abundance of strong BAS. The prepared Ga1-, Ga5-, and Ga10-SiO2-500 materials catalyze the dehydrogenation of isobutane to isobutene, and their catalytic performance correlates with the relative abundance and strength of LAS and BAS, viz., Ga1-SiO2-500, a material with a higher relative fraction of strong LAS, is more active and stable compared to Ga5- and Ga10SiO(2-500). In contrast, related ALD-derived Al1-, Al5-, and Al10-SiO2-500 materials do not catalyze the dehydrogenation of isobutane and this correlates with the lack of strong LAS in these materials that instead feature abundant strong BAS formed via the atomicscale mixing of Al sites with silica, leading to Al-mu(2)-OH-Si sites. Our results suggest that Ga-[4]( Si) sites provide strong Lewis acidity and drive the dehydrogenation activity, while the appearance of Ga-[5]( Ga) sites with mild Lewis activity is associated with catalyst deactivation through coking. Overall, the atomic-level insights into the structure of the GaOx-based materials prepared in this work provide a guide to design active Ga-based catalysts by a rational tailoring of Lewis and Bronsted acidity (nature, strength, and abundance).
Molybdenum Carbonitride MXene in Catalysis of Ammonia Synthesis and Decomposition
Kountoupi, Evgenia; Piankova, Diana; Agrachev , Mikhail; et al. (2025)
H2 pretreatment of Mo2(C,N)Tx increases the NH3 productivity by 4-fold relative to unpretreated Mo2(C,N)Tx or the bulk β‑Mo2N. Bulk β‑Mo2N outperforms Mo2(C,N)Tx in the NH3 decomposition, irrespective of the H2 pretreatment. Diffusion limitations in the narrow 2D pores of multilayered Mo2(C,N)Tx inhibit the utilization of the vast MXene surface area.
Bulk and surface transformations of Ga2O3 nanoparticle catalysts for propane dehydrogenation induced by a H2 treatment
Castro-Fernández, Pedro; Mance, Deni; Liu, Chong; et al. (2022)
Journal of Catalysis
Three γ/β-Ga2O3 nanoparticle catalysts that differ in the relative ratio of γ-Ga2O3 to β-Ga2O3 were prepared to evaluate the effect of H2 treatment (500 °C, 2 h) on the coordination environment of bulk and surface Ga sites, Lewis acidity and catalytic activity in propane dehydrogenation (PDH). Independent of the H2 treatment, the initial PDH activity of the γ/β-Ga2O3 catalysts increases with the fraction of the β-Ga2O3 phase. This is explained by the presence of weak Lewis acid sites (LAS) in β-Ga2O3 while such sites are absent in γ-Ga2O3. Treatment with H2 increases the catalytic activity of all three γ/β-Ga2O3 catalysts but for different reasons. For catalysts with higher fractions of β-Ga2O3, H2 treatment increases further the relative abundance of weak LAS, likely by generating coordinatively unsaturated Ga sites (such as tricoordinated Ga sites nearby oxygen vacancies). In contrast, H2 treatment of a catalyst containing a predominant fraction of γ-Ga2O3 phase induces disorder in the sub-surface structure of the nanoparticle, that is, it forms gallium and oxygen vacancies in the bulk and favors migration of gallium, and likely also of oxygen, to the surface. This induces a surface reconstruction that notably increases the fraction of strong LAS (and proportionally decreases the fraction of medium LAS), while creating no weak LAS in γ-Ga2O3-H2. Therefore, the increase in the catalytic activity of H2-treated γ-Ga2O3 is explained by the higher density of surface Ga sites in γ-Ga2O3-H2 relative to calcined γ-Ga2O3. H2-treated catalysts that contain a higher relative amount of weak LAS also feature a higher relative abundance of gallium hydride species associated with a low frequency FTIR band at ca. 1931–1939 cm−1, that is, weak LAS likely give weakly-bound hydrides in β-Ga2O3. Our results highlight that weak LAS in unsupported Ga2O3 catalysts are more active in PDH than mild or strong LAS.
The Key RuV=O Intermediate of Site-Isolated Mononuclear Water Oxidation Catalyst Detected by in Situ X-ray Absorption Spectroscopy
Lebedev, Dmitry; Pineda-Galvan, Yuliana; Tokimaru, Yuki; et al. (2018)
Journal of the American Chemical Society
Ultrafast electron-phonon relaxations in 2D carbide Ti3C2Tx (MXene): A case study by attosecond transient absorption spectroscopy
Neb, Sergej; Shin, Dongbin; Schumacher, Zeno; et al. (2022)
8th International Conference on Attosecond Science and Technology (ATTO VIII). Conference Handbook
Single-Atom-Substituted Mo2CTx:Fe-Layered Carbide for Selective Oxygen Reduction to Hydrogen Peroxide: Tracking the Evolution of the MXene Phase
Kuznetsov, Denis; Chen, Zixuan; Abdala, Paula Macarena; et al. (2021)
Journal of the American Chemical Society
This work critically assesses the electrocatalytic activity, stability, and nature of the active phase of a two-dimensional molybdenum carbide (MXene) with single-atomic iron sites, Mo2CTx:Fe (Tx are surface terminating groups O, OH, and F), in the catalysis of the oxygen reduction reaction (ORR). X-ray absorption spectroscopy unequivocally confirmed that the iron single sites were incorporated into the Mo2CTx structure by substituting Mo atoms in the molybdenum carbide lattice with no other detectable Fe-containing phases. Mo2CTx:Fe, the first two-dimensional carbide with isolated iron sites, demonstrates a high catalytic activity and selectivity in the oxygen reduction to hydrogen peroxide. However, an analysis of the electrode material after the catalytic tests revealed that Mo2CTx:Fe transformed in situ into a graphitic carbon framework with dispersed iron oxyhydroxide (ferrihydrite, Fh) species (Fh/C), which are the actual active species. This experimental observation and the results obtained for the titanium and vanadium 2D carbides challenge previous studies that discuss the activity of the native MXene phases in oxygen electrocatalysis. Our work showcases the role of 2D metal carbides as precursors for active carbon-based (electro)catalysts and, more fundamentally, highlights the intrinsic evolution pathways of MXenes in electrocatalysis.
Molybdenum carbide and oxycarbide from carbon-supported MoO3 nanosheets: phase evolution and DRM catalytic activity assessed by TEM and in situ XANES/XRD methods
Kurlov, Alexey; Huang, Xing; Deeva, Evgeniya B.; et al. (2020)
Nanoscale
Molybdenum carbide (β-Mo2C) supported on carbon spheres was prepared via a carbothermal hydrogen reduction (CHR) method from delaminated nanosheets of molybdenum(vi) oxide (d-MoO3/C). The carburization process was followed by combined in situ XANES/XRD analysis revealing the formation of molybdenum oxycarbide Mo2CxOy as an intermediate phase during the transformation of d-MoO3/C to β-Mo2C/C. It was found that Mo2CxOy could not be completely carburized to β-Mo2C under a He atmosphere at 750 °C, instead a reduction in H2 is required. The β-Mo2C/C obtained showed activity and stability for the dry reforming of methane at 800 °C and 8 bar. In situ XANES/XRD evaluation of the catalyst under DRM reaction conditions combined with high resolution TEM analysis revealed the evolution of β-Mo2C/C to Mo2CxOy/C. Notably, the gradual oxidation of β-Mo2C/C to Mo2CxOy/C correlates directly with the increased activity of the competing reverse water gas shift reaction.
Structure-Activity Study of ALD-Made Ga-Si-Al2O3 Catalysts for Propane Dehydrogenation: Elucidating the Role of SiOx
Yarar, Melis; Chen, Zixuan; Abdala, Paula Macarena; et al. (2024)
Probing the molecular character of periodic mesoporous organosilicates: Via photoluminescence of Lewis acid-base adducts
Thiel, Indre; Fedorov, Alexey; Verel, René; et al. (2016)
Physical Chemistry Chemical Physics
Photoluminescence decay was used as a structure-sensitive method to compare the distribution of emitting sites in periodic mesoporous organosilicates (PMOs) to their respective molecular analogs. The observed close similarity of PL decays confirms the molecular nature of PMOs and high homogeneity of emitting sites.

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Alexey Fedorov

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