Assessing the effect of acid and alkali treatment on a halloysite-based catalyst for dry reforming of methane

Dry reforming of methane (DRM) has recently received wide attention owing to its outstanding performance in the reduction and conversion of CH4 and CO2 to syngas (H2 and CO). From an industrial perspective, nickel (Ni)-supported catalysts have been deemed among the most suitable catalysts for DRM owing to their low cost and high activity compared to noble metals. However, a downside of nickel catalysts is their high susceptibility to deactivation due to coke formation and sintering at high temperatures. Using appropriate supports and preparation methods plays a major role in improving the activity and stability of Ni-supported catalysts. Halloysite nanotubes (HNTs) are largely utilized in catalysis as a support for Ni owing to their abundance, low cost, and ease of preparation. The treatment of HNTs (chemical or physical) prior to doping with Ni is considered a suitable method for increasing the overall performance of the catalyst. In this study, the surface of HNTs was activated with acids (HNO3 and H2SO4) and alkalis (NaOH and Na2CO3 + NaNO3) prior to Ni doping to assess the effects of support treatment on the stability, activity, and longevity of the catalyst. Nickel catalysts on raw HNT, acid-treated HNT, and alkali-treated HNT supports were prepared via wet impregnation. A detailed characterization of the catalysts was conducted using X-ray diffraction (XRD), BET surface area analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), solid-state nuclear magnetic resonance (ssNMR), H2-temperature programmed reduction, (H2-TPR), CO2-temperature programmed desorption (CO2-TPD), and Ni-dispersion via H2-pulse chemisorption. Our results reveal a clear alteration in the structure of HNTs after treatment, while elemental mapping shows a uniform distribution of Ni throughout all the different supports. Moreover, the supports treated with a molten salt method resulted in the overall highest CO2 and CH4 conversion among the studied catalysts and exhibited high stability over 24 hours testing.

Synergetic Effect of Fe2O3 Doped-CeO2 Nanocomposites Prepared via Different Techniques on Photocatalytic Desulfurization of Heavy Gas Oil

The photocatalytic performances of three Fe2O3–CeO2 nanocomposites were investigated toward the sulfur removal from a petroleum heavy gas oil (HGO) sample. The three composites were prepared by three different routes namely; auto-combustion, post-precipitation and precipitation. The physio-chemical features and optical properties of the presented composites were determined via proper analytical techniques. Formation of Fe2O3–CeO2 solid solution in all the prepared composites was verified via XRD analysis. These composites were then employed in photo-desulfurization of HGO and their activities were investigated at several operating conditions. The highest photocatalytic desulfurization exploit (91.5%) could be detected for the composite which was prepared via auto-combustion technique, denoted as (Fe20Ce80)ac. This maximum percentage of sulfur removal could be obtained under visible light irradiation at the following optimum operating conditions: 15 g/L (as photocatalyst dose), time of 6 h and 2:1 of H2O2 to oil ratio. The subsequent implementation of a solvent extraction step using N-methyl pyrrolidone was needed to attain the deepest desulfurization of HGO. The efficiencies of the presented composites against the process of sulfur removal were discussed in spot of their textural and optical characteristics as well as the available oxygen vacancies through their lattices structures.

Room temperature blooming of CeO2 3D nanoflowers under sonication and catalytic efficacy towards CO conversion

Carbon monoxide (CO), being a highly toxic gas, bears hazardous effects on human health and contributes majorly to environmental pollution. It is mostly produced by automobile exhausts and incomplete combustion of carbon-containing substances. Thus, the development of catalysts for CO conversion is highly imperative and has always gained interest for real field applications. Besides the high oxygen storage capacity and facile transitions between oxidation states, the huge abundance of cerium on earth makes CeO2 a low-cost and highly effective alternative to noble metal catalysts for CO oxidation. The present work delineates the room temperature synthesis of flower-shaped 3D CeO2 nanostructures using a sonication-assisted simple synthesis method within 2 hours under the pivotal importance of a structure-directing agent, polyvinylpyrrolidone (PVP). The bifunctional contributions of PVP as a surfactant and as a capping agent are discussed with a plausible mechanism. The method leading to the formation of hierarchical CeO2 nanoflowers provides an appreciable surface area of 132.69 cm2 g-1. The morphological and structural characterizations of the catalyst were thoroughly investigated using FESEM, TEM, XRD, UV-visible spectroscopy, photoluminescence spectroscopy, FTIR spectroscopy and X-ray photoelectron spectroscopy. The structural efficacies of flower-like CeO2 nanostructures have also been correlated to the narrowing of the band gap and the generation of the corresponding oxygen vacancies, resulting in surface catalytic properties towards 80% conversion of CO.

Hollow Micro/Nanostructured Ceria-Based Materials: Synthetic Strategies and Versatile Applications

Hollow micro/nanostructured CeO₂ -based materials (HMNCMs) have triggered intensive attention as a result of their unique structural traits, which arise from their hollowness and the fascinating physicochemical properties of CeO₂ . This attention has led to widespread applications with improved performance. Herein, a comprehensive overview of methodologies applied for the synthesis of various hollow structures, such as hollow spheres, nanotubes, nanoboxes, and multishelled hollow spheres, is provided. The synthetic strategies toward CeO₂ hollow structures are classified into three major categories: 1) well-established template-assisted (hard-, soft-, and in situ template) methods; 2) newly emerging self-template approaches, including selective etching, Ostwald ripening, the Kirkendall effect, galvanic replacement, etc.; 3) bottom-up self-organized formation synthesis (namely, oriented attachment and self-deformation). Their underlying mechanisms are concisely described and discussed in detail, the differences and similarities of which are compared transversely and longitudinally. Niche applications of HMNCMs in a wide range of fields including catalysis, energy conversion and storage, sensors, absorbents, photoluminescence, and biomedicines are reviewed. Finally, an outlook of future opportunities and challenges in the synthesis and application of CeO₂ -based hollow structures is also presented.

Facet-Dependent Photodegradation of Methylene Blue Using Pristine CeO2 Nanostructures

This work comprises the shape- and facet-dependent catalytic efficacies of different morphologies of CeO₂, namely, hexagonal, rectangular, and square. The formation of different shapes of CeO₂ is controlled using polyvinyl pyrrolidone as a surfactant. The surface reactivity of formation of differently exposed CeO₂ facets is thoroughly investigated using UV-visible, photoluminescence, Raman, and X-ray photoelectron spectroscopies. A correlation between the growth of a surface-reactive facet and the corresponding oxygen vacancies is also established. Considering the tremendous contamination, caused by the textile effluents, the present study articulates the facet-dependent photocatalytic activities of pristine CeO₂ for complete degradation of methylene blue within 175 min. The observed degradation time deploying pristine CeO₂ as a catalyst is the shortest to be reported in the literature to our best knowledge.

Highly coke resistant Mg-Ni/Al2O3 catalyst prepared via a novel magnesiothermic reduction for methane reforming catalysis with CO2: the unique role of Al-Ni intermetallics

Addition of alkaline promoters is considered to be an effective way to improve the coking resistance of the metal/support composite catalysts for dry reforming of methane (DRM). The traditional metal/promoter/support composites for DRM catalysis are generally obtained from alkaline species impregnation and then high temperature H2 reduction. This two-step process leads to a random distribution of metal-promoter interaction. We herein report a novel magnesiothermic method to reduce Ni from spinel precursor and introduce alkaline Mg(ii) into the composite at the same time, which also gratifies the interaction between the promoter and metal nanoparticles (NPs). The reaction paths to Mg reduction are proposed. The as prepared catalysts show good activity and outstanding coking resistance in DRM. The Ni-Al intermetallics in the catalyst were found for the first time to play an important role in coking resistance as they can be in situ transformed into Ni nanoparticles and MgAl2O4 with strong metal-support interaction during the DRM.

Facile one-step preparation of ordered mesoporous Ni–M–Al (M = K, Mg, Y, and Ce) oxide catalysts for methane dry reforming

Excellent DRM performances of a Ni–Y–Al oxide catalyst is related to the comprehensively advantageous textural properties, such as high Ni dispersion, large surface area, and small Ni nanoparticles inlaid in the honeycomb-like mesoporous skeleton.

Homogenous electrochemical water oxidation by a nickel(ii) complex based on a macrocyclic N-heterocyclic carbene/pyridine hybrid ligand

Electrochemical water oxidation catalyzed by a homogeneous Ni–NHC/pyridine complex demonstrated electrolyte-dependent catalytic performances. The catalyst displayed a stable catalytic current of oxygen evolution in long-term bulk electrolysis.

Coke-resistant defect-confined Ni-based nanosheet-like catalysts derived from halloysites for CO2 reforming of methane

In this study, halloysites, one of the most abundant clays, with hollow nanotube features were reconstructed by selectively etching silica from the outermost layer of the halloysites associated with unzipping the nanotubes to nanosheets via ball milling, and then, nickel nanoparticles were confined by the resulting defects in the nanosheets to boost charge transfer by a wet impregnation method. The obtained materials were developed as coke-resistant defect-confined Ni-based nanosheet-like catalysts for CO2 reforming of methane (CRM) for the first time. The as-prepared catalyst exhibited good coke and sintering resistance performance in CRM, and especially, there was almost no loss of activity even after a 20 h stability test due to the strong interaction between the Ni nanoparticles and the support. The present investigations may provide a new pathway for the design and application of highly coke-resistant CRM catalysts.

The application of a nickel(ii) Schiff base complex in water oxidation: the importance of nanosized materials

The role of Ni oxide in the electrocatalytic water oxidation of a nickel(ii) Schiff base (N,N′-bis (salicylidene) ethylenediamino nickel(ii)) is investigated.

Sintering resistant Ni nanoparticles exclusively confined within SiO2 nanotubes for CH4 dry reforming

Ni nanoparticles are exclusively confined within the channels of SiO₂ nanotubes (NTs) using the Ni phyllosilicate@SiO₂ nanocomposite as a precursor where Ni phyllosilicate will in situ decompose into Ni nanoparticles within SiO₂ shell NTs, exhibiting good sintering and carbon resistance for CO₂ reforming of CH₄ reaction.