Ni-alumina dry reforming catalysts: Atomic layer deposition and the issue of Ni aluminate

Formation of a Porous Crystalline Mg1-xAl2Oy Overlayer on Metal Catalysts via Controlled Solid-State Reactions for High-temperature Stable Catalysis

Catalyst deactivation by sintering and coking is a long-standing issue in metal-catalyzed harsh high-temperature hydrocarbon reactions. Ultrathin oxide coatings of metal nanocatalysts have recently appeared attractive to address this issue, while the porosity of the overlayer is difficult to control to preserve the accessibility of embedded metal nanoparticles, thus often leading to a large decrease in activity. Here, we report that a nanometer-thick alumina coating of MgAl2O4-supported metal catalysts followed by high-temperature reduction can transform a nonporous amorphous alumina overlayer into a porous Mg1-xAl2Oy crystalline spinel structure with a pore size of 2-3 nm and weakened acidity. The high porosity stems from the restrained Mg migration from the MgAl2O4 support to the alumina overlayer through solid-state reactions at high temperatures. The resulting Ni/MgAl2O4 and Pt/MgAl2O4 catalysts with a porous crystalline Mg1-xAl2Oy overlayer achieved remarkably high stability while preserving much higher activity than the corresponding alumina-coated Ni and Pt catalysts on MgO and Al2O3 supports in the reactions of dry reforming of methane and propane dehydrogenation, respectively.

Improved Selectivity and Stability in Methane Dry Reforming by Atomic Layer Deposition on Ni-CeO2-ZrO2/Al2O3 Catalysts

Ni can be used as a catalyst for dry reforming of methane (DRM), replacing more expensive and less abundant noble metal catalysts (Pt, Pd, and Rh) with little sacrifice in activity. Ni catalysts deactivate quickly under realistic DRM conditions. Rare earth oxides such as CeO2, or as CeO2-ZrO2-Al2O3 (CZA), are supports that improve both the activity and stability of Ni DRM systems due to their redox activity. However, redox-active supports can also enhance the undesired reverse water gas shift (RWGS) reaction, reducing the hydrogen selectivity. In this work, Ni on CZA was coated with an ultrathin Al2O3 overlayer using atomic layer deposition (ALD) to study the effects of the overlayer on catalyst activity, stability, and H2/CO ratio. A low-conversion screening method revealed improved DRM activity and lower coking rate upon the addition of the Al2O3 ALD overcoat, and improvements were subsequently confirmed in a high-conversion reactor at long times onstream. The overcoated samples gave an H2/CO ratio of ∼1 at high conversion, much greater than uncoated catalysts, and no evidence of deactivation. Characterization of used (but still active) catalysts using several techniques suggests that active Ni is in formal oxidation state >0, Ni-Ce-Al is most likely present as a mixed oxide at the surface, and a nominal thickness of 0.5 nm for the Al2O3 overcoat is optimal.

Atomic/molecular layer deposition strategies for enhanced CO2 capture, utilisation and storage materials

Elevated levels of carbon dioxide (CO2) in the atmosphere and the diminishing reserves of fossil fuels have raised profound concerns regarding the resulting consequences of global climate change and the future supply of energy. Hence, the reduction and transformation of CO2 not only mitigates environmental pollution but also generates value-added chemicals, providing a dual remedy to address both energy and environmental challenges. Despite notable advancements, the low conversion efficiency of CO2 remains a major obstacle, largely attributed to its inert chemical nature. It is imperative to engineer catalysts/materials that exhibit high conversion efficiency, selectivity, and stability for CO2 transformation. With unparalleled precision at the atomic level, atomic layer deposition (ALD) and molecular layer deposition (MLD) methods utilize various strategies, including ultrathin modification, overcoating, interlayer coating, area-selective deposition, template-assisted deposition, and sacrificial-layer-assisted deposition, to synthesize numerous novel metal-based materials with diverse structures. These materials, functioning as active materials, passive materials or modifiers, have contributed to the enhancement of catalytic activity, selectivity, and stability, effectively addressing the challenges linked to CO2 transformation. Herein, this review focuses on ALD and MLD's role in fabricating materials for electro-, photo-, photoelectro-, and thermal catalytic CO2 reduction, CO2 capture and separation, and electrochemical CO2 sensing. Significant emphasis is dedicated to the ALD and MLD designed materials, their crucial role in enhancing performance, and exploring the relationship between their structures and catalytic activities for CO2 transformation. Finally, this comprehensive review presents the summary, challenges and prospects for ALD and MLD-designed materials for CO2 transformation.

Structural insight into an atomic layer deposition (ALD) grown Al2O3 layer on Ni/SiO2: impact on catalytic activity and stability in dry reforming of methane

The development of stable Ni-based dry reforming of methane (DRM) catalysts is a key challenge owing to the high operating temperatures of the process and the propensity of Ni for promoting carbon deposition. In this work, Al2O3-coated Ni/SiO2 catalysts have been developed by employing atomic layer deposition (ALD). The structure of the catalyst at each individual preparation step was characterized in detail through a combination of in situ XAS-XRD, ex situ 27Al NMR and Raman spectroscopy. Specifically, in the calcination step, the ALD-grown Al2O3 layer reacts with the SiO2 support and Ni, forming aluminosilicate and NiAl2O4. The Al2O3-coated Ni/SiO2 catalyst exhibits an improved stability for DRM when compared to the benchmark Ni/SiO2 and Ni/Al2O3 catalysts. In situ XAS-XRD during DRM together with ex situ Raman spectroscopy and TEM of the spent catalysts confirm that the ALD-grown Al2O3 layer suppresses the sintering of Ni, in turn reducing also coke formation significantly. In addition, the formation of an amorphous aluminosilicate phase by the reaction of the ALD-grown Al2O3 layer with the SiO2 support inhibited catalysts deactivation via NiAl2O4 formation, in contrast to the reference Ni/Al2O3 system. The in-depth structural characterization of the catalysts provided an insight into the structural dynamics of the ALD-grown Al2O3 layer, which reacts both with the support and the active metal, allowing to rationalize the high stability of the catalyst under the harsh DRM conditions.

Robust nanosheet-assembled Al2O3-supported Ni catalysts for the dry reforming of methane: the effect of nickel content on the catalytic performance and carbon formation

Nanosheet-assembled Al2O3 for loading Ni were successfully prepared. Enhancing Ni loading decreases the Ni dispersion and the interaction between Ni and support. 5%-Ni/(NA-Al2O3) catalyst possesses an excellent activity and coke resistance for dry reforming of methane.

Constructing Ni-based confinement catalysts with advanced performances toward the CO2 reforming of CH4: state-of-the-art review and perspectives

The concept of Ni-based confinement catalysts has been proposed and developed to address the challenge of the thermal sintering of metallic Ni active sites during CRM by the space and/or lattice confinement effects.

Reforming of methane with carbon dioxide over cerium oxide promoted nickel nanoparticles deposited on 4-channel hollow fibers by atomic layer deposition

CeO₂ can significantly enhance the catalytic performance of Ni/Al₂O₃ catalysts prepared by atomic layer deposition for dry reforming of methane.