Tailored Supported Nickel Nanoparticle Catalysts for Dry Reforming of Methane via a Molecular Approach

Dry reforming of methane (DRM) has become a very appealing reaction in the last decades because it allows a valorization of un-wanted greenhouse gases (CO2, CH4) toward syn-gas production. It is often use in industry as complementary process of steam reforming of methane (SRM) process in order to tune effectively the product composition (H2/CO), which ease the syn-gas conversion to synthetic fuel. Supported nickel nanoparticles (Ni NPs) catalysts show high DRM activity in comparison to most metals and due to the very high abundance of Ni are cost-effectively the best system for DRM. However, they suffer from severe deactivation upon reaction, caused by the dramatic loss of active sites through substantial formation of carbon on the NP surface as well as by sintering. Thus, this PhD thesis has aimed at improving and understanding supported Ni NP catalysts through Surface Organometallic Chemistry and inorganic colloidal NP synthesis. A central role of these different works is placed on structure evolution upon reaction using operando XAS techniques. Control reduction of Ni(II) formate precursor specifically adsorbed at alumina surface yields to 2 nm Ni(0) NP, which have the capability to efficiently reduce the rate of coke formation while doubling the initial activity compared to catalysts prepared by conventional methods. However, such catalyst still dramatically deactivates via migration of Ni in the alumina lattice as evidenced by Operando XANES. However, the stabilization of the 2 nm NP can be achieved by suppressing the formation of a solid solution between Ni and Al2O3 through the use of (Mg,Al,O) binary oxide supports. In addition, the stability of these small size Ni NP supported on a periclase material could be further improved by doping the nickel centre with iron (Fe) metal using a colloidal NiFe bimetallic synthesis. Tuning the Ni/Fe ratio has allowed to maximize the specific activity of the Ni(0) active site while retaining the iron capability to nearly supress coke formation. Operando XAS show the segregated structure of the two metal during DRM, in which FeO oxidizes carbon deposit. The decrease of Ni(0) ensembles was also investigating by using a phosphorus as promoter. NixP were first synthesized via a colloidal approach and then deposited on silica to provide 1-2 nm nickel phosphide NP with a Ni/P ratio of 3. Despite a low activity compared to its pure Ni(0) counterpart, the nickel phosphide nanoparticle show enhanced stability with neither coke formation nor sintering. Moreover, these Ni-based catalysts show an altered selectivity by supressing WGS thus favouring H2/CO ratio, suggesting the presence of an alternative active site. Thus, the combination of advanced spectroscopy techniques with a molecular approach towards the synthesis of Ni-based NPs permit to find general trends to form high performing DRM catalysts: 1) small size Ni(0) NPs (< 4 nm) provides catalysts with superior activity and stability due to the increase of Ni surface area and a lower rate of carbon formation, 2) designing the support is necessary to stabilize the small size NP from sintering while mitigating the migration of Ni inside the support matrix. The use of (Mg,Al,O) binary oxide material is a good alternative because of their thermal stability and high surface area, 3) the incorporation of adequate quantity of oxiphilic Fe promotor in proximity of small size Ni(0) NPs allow to nearly supress the formation of carbon, 4) the site isolation of the Ni center using phosphorus dopant permit both enhanced stability and selectivity

Metal(II) Formates (M = Fe, Co, Ni, and Cu) Stabilized by Tetramethylethylenediamine (tmeda): Convenient Molecular Precursors for the Synthesis of Supported Nanoparticles

ISSN:0018-019XISSN:1522-267

Bi-functional Ru/Ca3Al2O6–CaO catalyst-CO2 sorbent for the production of high purity hydrogen via sorption-enhanced steam methane reforming

Sorption-enhanced steam methane reforming (SE-SMR) combines steam methane reforming and a CO2 abstraction reaction to yield high purity hydrogen. In this work, we report on the development of a bi-functional catalyst–sorbent containing Ru as the reforming catalyst and CaO as the solid CO2 sorbent via a citrate sol–gel route. The material contains CaO, a structural stabilizer (Ca3Al2O6) and Ru nanoparticles (∼5 nm, 3 wt%) that are formed upon reduction in H2. This new material was found to outperform significantly the benchmarks Ni–CaO and Ru/limestone in terms of yield of high-purity hydrogen and coke resistance. Using highly active Ru nanoparticles for the SMR allowed to maximize the weight fraction of the CO2 sorbent CaO, hence increasing significantly the CO2 capture capacity of the material. This favorable characteristic of the material led to an appreciably extended pre-breakthrough duration. In addition, we demonstrate that the material developed was very stable over multiple SE-SMR/regeneration cycles. The excellent cyclic stability is ascribed to the presence of Ca3Al2O6 that stabilized effectively the porous structure of the material against sintering.ISSN:2044-4753ISSN:2044-476

In Situ XRD and Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy Unravel the Deactivation Mechanism of CaO-Based, Ca3Al2O6-Stabilized CO2 Sorbents

ISSN:0897-475

Contrasting the Role of Ni/Al₂O₃ Interfaces in Water–Gas Shift and Dry Reforming of Methane

Transition metal nanoparticles (NPs) are typically supported on oxides to ensure their stability, which may result in modification of the original NP catalyst reactivity. In a number of cases, this is related to the formation of NP/support interface sites that play a role in catalysis. The metal/support interface effect verified experimentally is commonly ascribed to stronger reactants adsorption or their facile activation on such sites compared to bare NPs, as indicated by DFT-derived potential energy surfaces (PESs). However, the relevance of specific reaction elementary steps to the overall reaction rate depends on the preferred reaction pathways at reaction conditions, which usually cannot be inferred based solely on PES. Hereby, we use a multiscale (DFT/microkinetic) modeling approach and experiments to investigate the reactivity of the Ni/Al₂O₃ interface toward water–gas shift (WGS) and dry reforming of methane (DRM), two key industrial reactions with common elementary steps and intermediates, but held at significantly different temperatures: 300 vs 650 °C, respectively. Our model shows that despite the more energetically favorable reaction pathways provided by the Ni/Al₂O₃ interface, such sites may or may not impact the overall reaction rate depending on reaction conditions: the metal/support interface provides the active site for WGS reaction, acting as a reservoir for oxygenated species, while all Ni surface atoms are active for DRM. This is in contrast to what PESs alone indicate. The different active site requirement for WGS and DRM is confirmed by the experimental evaluation of the activity of a series of Al₂O₃-supported Ni NP catalysts with different NP sizes (2–16 nm) toward both reactions

Molecularly Tailored Nickel Precursor and Support Yield a Stable Methane Dry Reforming Catalyst with Superior Metal Utilization

Syngas production via the dry reforming of methane (DRM) is a highly endothermic process conducted under harsh conditions; hence, the main difficulty resides in generating stable catalysts. This can, in principle, be achieved by reducing coke formation, sintering, and loss of metal through diffusion in the support. [{Ni­(μ²-OCHO)­(OCHO)­(tmeda)}₂(μ²-OH₂)] (tmeda = tetramethylethylenediamine), readily synthesized and soluble in a broad range of solvents, was developed as a molecular precursor to form 2 nm Ni(0) nanoparticles on alumina, the commonly used support in DRM. While such small nanoparticles prevent coke deposition and increase the initial activity, <i>operando</i> X-ray Absorption Near-Edge Structure (XANES) spectroscopy confirms that deactivation largely occurs through the migration of Ni into the support. However, we show that Ni loss into the support can be mitigated through the Mg-doping of alumina, thereby increasing significantly the stability for DRM. The superior performance of our catalytic system is a direct consequence of the molecular design of the metal precursor and the support, resulting in a maximization of the amount of accessible metallic nickel in the form of small nanoparticles while preventing coke deposition

Molecularly Tailored Nickel Precursor and Support Yield a Stable Methane Dry Reforming Catalyst with Superior Metal Utilization

Syngas production via the dry reforming of methane (DRM) is a highly endothermic process conducted under harsh conditions; hence, the main difficulty resides in generating stable catalysts. This can, in principle, be achieved by reducing coke formation, sintering, and loss of metal through diffusion in the support. [{Ni­(μ²-OCHO)­(OCHO)­(tmeda)}₂(μ²-OH₂)] (tmeda = tetramethylethylenediamine), readily synthesized and soluble in a broad range of solvents, was developed as a molecular precursor to form 2 nm Ni(0) nanoparticles on alumina, the commonly used support in DRM. While such small nanoparticles prevent coke deposition and increase the initial activity, <i>operando</i> X-ray Absorption Near-Edge Structure (XANES) spectroscopy confirms that deactivation largely occurs through the migration of Ni into the support. However, we show that Ni loss into the support can be mitigated through the Mg-doping of alumina, thereby increasing significantly the stability for DRM. The superior performance of our catalytic system is a direct consequence of the molecular design of the metal precursor and the support, resulting in a maximization of the amount of accessible metallic nickel in the form of small nanoparticles while preventing coke deposition

Low Temperature Wet Conformal Nickel Silicide Deposition for Transistor Technology through an Organometallic Approach

The race for performance of integrated circuits is nowadays facing a downscale limitation. To overpass this nanoscale limit, modern transistors with complex geometries have flourished, allowing higher performance and energy efficiency. Accompanying this breakthrough, challenges toward high-performance devices have emerged on each significant step, such as the inhomogeneous coverage issue and thermal induced short circuit issue of metal silicide formation. In this respect, we developed a two-step organometallic approach for nickel silicide formation under near-ambient temperature. Transmission electron and atomic force microscopy show the formation of a homogeneous and conformal layer of NiSix on pristine silicon surface. Post-treatment decreases the carbon content to a level similar to what is found for the original wafer (similar to 6%). X-ray photoelectron spectroscopy also reveals an increasing ratio of Si content in the layer after annealing, which is shown to be NiSi2 according to X-ray absorption spectroscopy investigation on a Si nanoparticle model. I-V characteristic fitting reveals that this NiSi2 layer exhibits a competitive Schottky barrier height of 0.41 eV and series resistance of 8.5 Omega thus opening an alternative low-temperature route for metal silicide formation on advanced devices

<i>In Situ</i> XRD and Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy Unravel the Deactivation Mechanism of CaO-Based, Ca₃Al₂O₆‑Stabilized CO₂ Sorbents

CaO is an effective high temperature CO₂ sorbent that, however, suffers from a loss of its CO₂ absorption capacity upon cycling due to sintering. The cyclic CO₂ uptake of CaO-based sorbents is improved by Ca₃Al₂O₆ as a structural stabilizer. Nonetheless, the initially rather stable CO₂ uptake of Ca₃Al₂O₆-stabilized CaO yet starts to decay after around 10 cycles of CO₂ capture and sorbent regeneration, albeit at a significantly reduced rate compared to the unmodified reference material. Here, we show by a combined use of <i>in situ</i> XRD together with textural and morphological characterization techniques (SEM, STEM, and N₂ physisorption) and solid-state ²⁷Al NMR (in particular dynamic nuclear polarization surface enhanced NMR spectroscopy, DNP SENS) how microscopic changes trigger the sudden onset of deactivation of Ca₃Al₂O₆-stabilized CaO. After a certain number of CO₂ capture and regeneration cycles (approximately 10), Ca₃Al₂O₆ transformed into Ca₁₂Al₁₄O₃₃, followed by Al₂O₃ segregation and enrichment at the surface in the form of small nanoparticles. Al₂O₃ in such a form is not able to stabilize effectively the initially highly porous structure against thermal sintering, leading in turn to a reduced CO₂ uptake