Application of MnOx/HNTs catalysts in low-temperature NO reduction with NH3

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.

Low-Temperature NH3-SCR over Hierarchical MnO x Supported on Montmorillonite Prepared by Different Methods

Hierarchical MnO _x pillared or supported on montmorillonite were prepared by three methods, i.e., impregnation (IM), chemical precipitation (CP), and in situ deposition (SP). The catalysts were characterized by low-temperature N₂ adsorption (BET), XRD, XPS, SEM, TEM, H₂-TPR, NH₃-TPD, NO-TPD, TPSR, in situ DRIFTS, and evaluation of catalytic performance for NH₃-SCR. The best catalytic performance was obtained for catalysts prepared by SP in terms of activity and selectivity, obtaining >90% NO conversion with >95% selectivity to N₂ in 100-300 °C and GHSV of 70,000 h^-1. Compared to IM and CP, SP greatly simplified catalyst preparation, resulting in higher BET surface areas; a spongy pore structure; more highly dispersed, pillared MnO _x species; and higher density of acid sites distributed on catalysts surface, which all contributed to its superior performance for NH₃-SCR. The activity for low-temperature NH₃-SCR of manganese catalysts could be widely tailored by preparation methods.

Clay nanotube-metal core/shell catalysts for hydroprocesses

Catalytic hydroprocesses play a significant role in oil refining and petrochemistry. The tailored design of new metal nanosystems and optimization of their support, composition, and structure is a prospective strategy for enhancing the efficiency of catalysts. Mesoporous support impacts the active component by binding it to the surface, which leads to the formation of tiny highly dispersed catalytic particles stabilized from aggregation and with minimized leaching. The structural and acidic properties of the support are crucial and determine the size and dispersion of the active metal phase. Currently, research efforts are shifted toward the design of nanoscale porous materials, where homogeneous catalysts are displaced by heterogeneous. Ceramic materials, such as 50 nm diameter natural halloysite nanotubes, are of special interest for this. Much attention to halloysite clay is due to its tubular structure with a hollow 10-15 nm diameter internal cavity, textural characteristics, and different chemical compositions of the outer/inner surfaces, allowing selective nanotube modification. Loading halloysite with metal particles or placing them outside the tubes provides stable and efficient mesocatalysts. The low cost of this abundant nanoclay makes it a good choice for the scaled-up architectural design of core-shell catalysts, containing active metal sites (Au, Ag, Pt, Ru, Co, Mo, Fe₂O₃, CdS, CdZnS, Cu-Ni) located inside or outside the tubular template. These alumosilicate nanotubes are environment-friendly and are available in thousands of tons. Herein, we summarized the advances of halloysite-based composite materials for hydroprocesses, focusing on the selective binding of metal particles. We analyze the tubes' morphology adjustments and size selection, the physicochemical properties of pristine and modified halloysite (e.g., acid-etched or silanized), the methods of metal clusters formation, and their localization. We indicate prospective routes for the architectural design of stable and efficient nanocatalysts based on this safe and natural clay material.

Cation Doping Approach for Nanotubular Hydrosilicates Curvature Control and Related Applications

The past two decades have been marked by an increased interest in the synthesis and the properties of geoinspired hydrosilicate nanoscrolls and nanotubes. The present review considers three main representatives of this group: halloysite, imogolite and chrysotile. These hydrosilicates have the ability of spontaneous curling (scrolling) due to a number of crystal structure features, including the size and chemical composition differences between the sheets, (or the void in the gibbsite sheet and SiO2 tetrahedron, in the case of imogolite). Mineral nanoscrolls and nanotubes consist of the most abundant elements, like magnesium, aluminium and silicon, accompanied by uncontrollable amounts of impurities (other elements and phases), which hinder their high technology applications. The development of a synthetic approach makes it possible to not only to overcome the purity issues, but also to enhance the chemical composition of the nanotubular particles by controllable cation doping. The first part of the review covers some principles of the cation doping approach and proposes joint criteria for the semiquantitative prediction of morphological changes that occur. The second part focuses on some doping-related properties and applications, such as morphological control, uptake and release, magnetic and mechanical properties, and catalysis.

Mn/Ce Oxides Decorated Polyphenylene Sulfide Needle-Punching Fibrous Felts for Dust Removal and Denitration Application

Development of a novel filter material is urgently required for replacing the high-cost flue gas purification technology in the simultaneous removal of both fine dust and Nitrogen oxides (NO_x). In this study; polyphenylene sulfide (PPS) needle-punching fibrous felts (NPFF) were employed as the filter material to remove the fine dust; and in the meanwhile; Mn and Ce oxides were loaded onto the PPS NPFF as the catalyst for selective catalytic reduction of NO_x with NH₃. Two different pretreatment methods; i.e., sodium alginate (SA) deposition and plasma treatment; were employed to modify the PPS NPFF before the traditional impregnation and thermal treatment processes during the catalyst loading. The results showed that these two pretreatment methods both afforded the PPS NPFF with the enhanced loading rate and stability of Mn/Ce oxides compared to those without any pretreatments; which were significantly beneficial for the denitration application. Moreover; we found that both SA deposition and plasma pre-treated samples presented excellent dust-removal properties; and the filtration efficiency could reach 100% when the particle size of the fine particulates was above 4 μm. This study demonstrated that our Mn/Ce oxides decorated PPS NPFF have great potential to be applied in the fuel gas purification field; due to their stable structure; handling convenience; and excellent filtration efficiency; as well as high denitration performance.

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.