NiAl Hydrotalcite-like Compounds Research Articles

Structure and surface properties of Ni–Al hydrotalcite-like compounds and their catalytic application in highly selective acetalization at room temperature

Oxygen vacancies improve the catalytic activity of acid sites, and their amount decreases due to the conversion of [Ni(OH)5NO3] into [Ni(OH)6] units.

Characterization and catalytic behavior of hydrotalcite-derived Ni–Al catalysts for methane decomposition

A series of Ni catalysts were prepared from Ni–Al hydrotalcite-like compounds (HTlcs) by varying the Ni/Al molar ratio (1–4) and calcination temperature (773–1173 K) of HTlcs. The catalysts were reduced with H2 at 1073 K and tested for CH4 decomposition at 773–923 K on a thermal gravimeter. Various techniques including N2 physical adsorption, XRD, H2-TPR, XPS, HAADF-STEM, TEM, and Raman were applied to characterize the catalysts and the as-produced carbon. The characterizations show that calcination of Ni–Al HTlcs leads to Ni(Al)O solid solution and minor NiO and/or NiAl2O4 spinel may be formed depending on the Ni/Al ratio and calcination temperature; upon reduction at 1073 K, most nickel species are reduced to metallic Ni. In CH4 decomposition, carbon yield shows a volcano-type dependence on the Ni content with the optimum Ni/Al ratio equal to 3. On the other hand, carbon yield is affected by the calcination temperature of the Ni3Al HTlcs to a small extent. Carbon yield is also significantly affected by the reaction temperature, which decreases remarkably with a rise of temperature to 923 K. TEM and Raman indicate that fish-bone carbon nanofibers are formed at 773–823 K, whereas multi-walled carbon nanotubes are formed at 873–923 K.

Mechanism and regeneration of sulfur-poisoned Mn-promoted calcined NiAl hydrotalcite-like compounds for C3H6-SCR of NO.

The selective catalytic reduction of NO with propene (C3H6-SCR) in the presence of SO2 was investigated over a series of Mn-promoted calcined NiAl hydrotalcite-like compounds. The obtained 5% MnNiAlO catalyst exhibits superior NO conversion efficiency (95%) at 240 °C, and excellent sulfur-poisoning resistance. The possible reaction pathways of the catalytic process were proposed according to several characterization measurements. It is demonstrated that Mn-promoted NiAlO catalysts enhance the Brønsted acid sites and surface active oxygen groups, and improve the redox properties by the redox cycle (Ni3+ + Mn2+ ↔ Ni2+ + Mn4+). Thus, the amount of the reaction intermediates is improved, and the reactivities between CxHyOz species and nitrite/nitrate species are promoted. Furthermore, in the presence of SO2, the MnNiAlO samples can give rise to minor formation of sulfate and inhibit the competitive adsorption effectively due to their nitrite/nitrate species being more abundant and stable. Finally, regeneration was studied using in situ FTIR and the water washing method showed the best performance on the regeneration of S-poisoned catalysts.

Improved NO reduction in the presence of SO2 by using Zr-promoted calcined NiAl hydrotalcite-like compounds and the regeneration of deactivated catalysts

The selective catalytic reduction of NO with propene (C3H6-SCR) was investigated over a series of Zr-promoted calcined NiAl hydrotalcite-like compounds. The obtained 5%ZrNiAlO catalyst exhibits superior NO conversion efficiency (93%) at 260 °C, and excellent sulfur-poisoning resistance. The possible reaction pathways of the catalytic process were proposed according to several characterization measurements. It is demonstrated that Zr-promoted NiAlO catalysts enhance the Brønsted acid sites and surface active oxygen groups and improve the redox ability. Meanwhile, the amount of the reaction intermediate is improved, as well as the reactivity of CxHyOz species with nitrite/nitrate species is promoted. Furthermore, in the presence of SO2, the ZrNiAlO samples can give rise to minor formation of sulfate and inhibite the competitive adsorption effectively due to their nitrite/nitrate species are more abundant and stable. In situ infrared characterization revealed the regeneration mechanism and water washing method showed best performance on the regeneration of S-poisoned catalysts.

The regulatory effect of Al atomic-scale doping in NiAlO for COS removal

Abstract In this paper, Al atomic-scale doping has been achieved by using NiAl hydrotalcite-like compounds (HTLCs) as precursors method. Regulatory effect of Al doping in NiO for carbonyl sulfide (COS) removal was studied. The samples were systematically characterized by XRD, SEM, N2 adsorption/desorption, FTIR, CO2-TPD, XPS, XAFS and In-situ DRIFTS. The results showed that the activity series for removal of COS decrease in the following sequence: Ni3Al-HTO > Ni5Al-HTO > Ni1Al-HTO. The enhanced activity after Al atomic-scale doping is attributed to the abundant surface basic sites as evidenced by results of CO2-TPD. XPS and XAFS provide evidence for the reduction of Ni2+ by Al doping in NiO, which suggest large amounts of O-vacancies present on Ni3Al-HTO. This work is useful to explore feasible routes to improve the performance of desulfurization material for the feed gas purification.

SO2 adsorption and transformation on calcined NiAl hydrotalcite-like compounds surfaces: An in situ FTIR and DFT study

Flower-like hydrotalcite-based NiAl mixed oxides with high surface area were synthesized by urea hydrolysis approach, and employed for SO2 removal. The catalyst was well characterized by TGA, ICP, XRD, SEM, TEM and N2 adsorption/desorption isotherm analyses. The calcined NiAlO showed excellent SO2 adsorption and its adsorption equilibrium was of a type I isotherm, which significantly improved the adsorption performance for low-concentration SO2. XPS and TPD methods were conducted to identify the sulfate species. Specifically, in situ Fourier transform infrared spectroscopy revealed that adsorbed SO2 molecules formed surface bidentate binuclear sulfate. Density functional theory based computations interpreted the vibrational data associated with the NiAlO surface and confirmed that sulfate species adopted a binuclear bidentate binding configuration on the surface of NiAlO catalyst.

Effect of promoter species and precursors on catalytic activity of alkali metal promoted NiAl mixed oxides for N 2O decomposition

NiAl mixed oxides with Ni/Al atomic ratio of 2.0, 2.7 and 4.1 were prepared by calcining the corresponding NiAl hydrotalcite-like compounds (NiAl-HLc), and applied in N 2O decomposition. The NiAl mixed oxide with Ni/Al atomic ratio of 2.7 was incipiently impregnated by alkali metal (Na, K, Cs) carbonate solution to prepare the promoted NiAl mixed oxide catalysts. The catalysts were characterized by XRD, ICP-AES, FTIR, BET, H 2-TPR and XPS techniques, and their catalytic activities for N 2O decomposition were tested. The effect of NiAl mixed oxides composition, alkali metal species, and potassium precursors on catalytic activity was investigated. The results show that the catalytic activity of NiAl mixed oxide impregnated by K 2CO 3 solution is much higher than Na 2CO 3 and Cs 2CO 3 modified catalysts; the catalytic activity of NiAl mixed oxides is largely enhanced by the addition of K species from the precursors of K 2CO 3, K 2C 2O 4 and CH 3COOK.

Influence of calcination temperature on the hydrolysis of carbonyl sulfide over hydrotalcite-derived Zn–Ni–Al catalyst

A novel catalyst for low temperature hydrolysis of carbonyl sulfide (COS) was prepared by thermal decomposition of Zn–Ni–Al hydrotalcite-like compounds (HTLCs). As the key factors of catalyst activity, effects of calcination temperature have been studied. The samples were carefully characterized by XRD, FTIR, SEM, CO2-TPD and N2 adsorption/desorption. Results showed that HTLCs calcined at 350 °C exhibited excellent activity due to the production of more M–O pairs which are active sites of the hydrolysis of COS. However, calcination at 500 °C led to the destruction of pore structure and reduction of active sites, ultimately led to a lower COS conversion.

Effects of Surface Features on Sulfur Dioxide Adsorption on Calcined NiAl Hydrotalcite-like Compounds

The hydrotalcite-based NiAl mixed oxides were synthesized by coprecipitation and urea hydrolysis approaches and employed for SO₂ removal. The samples were well characterized by inductively coupled plasma (ICP) elemental analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and N₂ adsorption/desorption isotherm analyses. The acid-base properties were characterized by pyridine chemisorption and CO₂ temperature-programmed desorption (TPD). The calcined NiAlO from the urea method showed excellent SO₂ adsorption and its adsorption equilibrium showed a type I isotherm, which significantly improved the adsorption performance for low-concentration SO₂. Both the physical structure and the acidic-basic sites were found to play important roles in the SO₂ adsorption process. In situ Fourier transform infrared spectroscopy (FTIR) investigation revealed that adsorbed SO₂ molecules formed surface bisulfite, sulfite, and bidentate binuclear sulfate. The mechanisms for SO₂ adsorption and transformation are discussed in detail.

Structural characterization and related properties of the stearate anions intercalated Ni–Al hydrotalcite-like compound prepared by the microwave crystallization

The stearate anions intercalated Ni–Al hydrotalcite-like compounds were prepared by a coprecipitation method coupled with the microwave-hydrothermal treatment. Their structures were characterized using FT-IR, XRD, contact angle, SEM, TEM and thermal analysis. The XRD results indicate that the interlayer anions of the microwave radiation samples have better order and stronger electrostatic interactions with hydroxyl layers than those of the conventional aging 12 h samples, the crystallinity of stearate anions intercalated Ni–Al hydrotalcite-like compounds can be rapidly enhanced in a short period of microwave treatment, and the stearate anions arrange in the form of a monolayer in the interlayer space for all samples. The morphology analysis results show that all samples consist of irregular and very thin platelets, while the crystals size of microwave treated samples was obviously smaller than that of the conventional aged samples. Based on the contact angle results, the surfaces of all samples show the hydrophobic property. This is due to the intercalation of stearate anions. The thermal analysis results point out that the sample obtained from microwave radiation 60 min possesses the best thermal stability. The rapid synthesis method explored in this paper is a promising method for studying other anionic surfactant intercalated HTLc.

Urea method for the synthesis of hydrotalcites

Mg-Al, Zn-Al and Ni-Al hydrotalcite-like compounds with high crystallinity were synthesized by using the decomposing property of urea at high temperature. The crystallinity and the regularly of the samples were much preferable to those synthesized by other methods.

Hydrotalcite-like compounds as catalysts in liquid phase organic synthesis

Abstract Ni–Al hydrotalcite-like compounds, obtained by precipitation from homogeneous solution accomplished by urea hydrolysis, have been characterised for chemical composition, BET specific surface area, X-ray diffraction and thermogravimetry. The base strength of surface sites was estimated with the method of adsorbed acid–base indicators. A sample of formula [Ni 0.73 Al 0.27 (OH) 2 ](CO 3 ) 0.135 , previously dried at 150 °C, has been used as base catalyst at 60 °C in absence of solvents in the Knoevenagel condensation of different aldehydes with malononitrile and ethylcyanoacetate. Aliphatic and aromatic aldehydes easily react in these reaction conditions and the Knoevenagel adducts were obtained in excellent yield, in short time. Dimethylmalonate, having methylene groups of very low acid strength (p K a >13) did not give the adduct with benzaldehyde because of not sufficiently high base strength of the hydrotalcite-like compound. The catalyst can be recycled several times, without loss of catalytic efficiency, and its performance has been compared with that of other heterogeneous base catalysts such as zeolite A, potassium exchanged zirconium phosphate, amberlist, modified hydrotalcites.

IR structural evidence of hydrotalcites derived oxidic forms

Ni–Al hydrotalcite-like compounds with the general formula [Ni 1− x Al x (OH) 2](CO 3) x/2 ⋅mH 2O, where 0.25≤ x≤0.66, were synthesised using coprecipitation at a constant pH, and were treated hydrothermally. The structures of the oxidic forms obtained by calcination of the hydrotalcites at 450°C and 900°C, respectively, were investigated using X-ray diffraction and, mainly, IR and UV–VIS spectroscopy. A NiO phase was identified by XRD in all calcined samples; an additional oxidic phase — the nickel spinel, NiAl 2O 4 — developed at 900°C. IR spectroscopy results gave supplementary information about the incipient, local organisation of cations in the interstices of the oxygen atoms lattice. IR spectra were different, depending on the samples' composition. In case of the HT precursors calcined at 450°C a structure like a transition alumina (γ-Al 2O 3) was found as a main oxidic phase in samples with a high Al-content; IR spectra of the samples with a high Ni content evidenced NiO as the main oxidic phase; in case of these latter samples, the formation of an oxidic structure with a spinel-type local order was identified at this temperature. This structure developed to an inverse nickel spinel oxidic phase at 900°C, as shown by the IR absorption bands. The NiO structure in the samples with a high Ni content at 450°C was found also in the oxides obtained by calcination at 900°C. The spinel-type local order was also observed by UV–VIS spectroscopy in case of the HT precursors calcined at 450°C, by the presence of both absorption bands of the tetrahedral and octahedral Ni(II) ions in the Al 2O 3 lattice and of octahedral Ni(II) ions in the NiO lattice. The same absorption bands were found also in the samples calcined at 900°C, proving that the NiAl 2O 4 spinel identified has a partial inverse structure, with the Ni(II) ions both in tetrahedral and octahedral crystalline fields. Our found structural data were in accord with the models proposed in the literature.