Effect of hydrothermal treatment on properties of Ni–Al layered double hydroxides and related mixed oxides (original) (raw)
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Bulgarian Chemical Communications
The effect of nickel amount on the structure, reducibility and activity of the mixed metal oxides generated by controlled thermal treatment of co-precipitated Ni-Al layered double hydroxides as catalyst precursors for CO2 removal by methanation reaction, was examined by variation of the Ni2+/Al3+ molar ratio (Ni2+/Al3+ = 0.5, 1.5, and 3.0), the reduction and reaction temperatures. The powder X-ray diffraction of the thermally treated samples (200−1000 °C) established the formation of nano-sized NiO- and spinel NiAl2O4-like phases in different proportion and degree of crystallinity. The calcination temperature of 500 °C was selected as most convenient one because of the high dispersion of the mixed oxide phases predetermining the high dispersion of the metallic nickel. It was revealed that after preliminary ‛in situ’ reduction at 400, 450, 530 and 600 °C, all studied catalysts hydrogenated CO2 effectively at reaction temperatures from 400 to 280 °C and space velocities between 3000 a...
Journal of Catalysis, 1992
A series of nickel-aluminum mixed hydroxides was prepared by coprecipitation and their structure and composition was examined by X-ray diffraction and thermal analysis. The homogeneity of the Ni(II) and AI(III) cations in the brucite-type layers has been verified by EXAFS spectroscopy. The hydrotalcite-type precipitates were calcined and the composition of the resulting mixed oxides investigated. The selective dissolution of a Ni-doped alumina phase has been performed in concentrated NaOH solution. The alumina phase significantly contributes to the surface area of the mixed oxides but is suggested to play a minor role for the thermal stability of nickel oxide. A model for the nickel-aluminum mixed oxides obtained from hydrotalcite decomposition is proposed to involve the formation of nickel oxide and Ni-doped alumina phases in strong interaction with partially decomposed precursor layers or with a spinal-type phase at the NiO-A1203 interface. The latter phase is thought to be responsible for the thermal stabilization of the nickel oxide phase.
Effect of Synthesis Method on Properties of Layered Double Hydroxides Containing Ni(III
Crystals, 2021
Unstable oxidation state +3 of nickel can be stabilized in the structure of layered double hydroxides, the resulting crystallinity and properties being dependent on the synthesis method. Three different wet chemical methods (co-precipitation at variable pH, co-precipitation followed by hydrothermal treatment, co-precipitation with microwave treatment) were used to synthesize Mg/Ni-Al layered double hydroxides containing triply charged nickel cations. Lattice parameters of the samples synthesized by various methods were found to differ from each other by about 1.5%. The most crystallized sample was obtained by hydrothermal synthesis. The oxidation state of nickel in the LDH samples was confirmed by XPS. TEM mapping gave evidence of the uniform distribution of nickel in all the samples. The LDHs' reduction with hydrogen and thermal transformations of the phase composition and morphology of LDHs were studied in detail. The properties of the samples synthesized by the different methods were shown to be quite similar.
Microporous and Mesoporous Materials, 2012
Supported nickel catalysts were obtained by exchange of Mg/Al Layered Double Hydroxides (LDHs) compensated with NO À 3 ions with negatively charged Ni complexes, followed by thermal reduction. With this aim, suspensions of Ni complexes were prepared by controlled hydroxylation of Ni 2+ cations in the presence of citrate (obtaining ½NiðC 6 H 5 O 7 ÞðOHÞ 2yÀ y species) or chloride (obtaining [NiCl 4 ] 2À species) complexing ions. For comparative purposes, other two supported Ni catalysts were prepared starting from Mg/Al LDHs compensated with Cl À ions and thereafter exchanged with ½NiðC 6 H 5 O 7 ÞðOHÞ 2yÀ y species (nominal degree of exchange: 100% and 20%).
Catalysis Today, 2008
The preparation of three different types of mixed nickel oxides is described. These systems include: (i) the perovskite LaNiO3 oxide, (ii) a mixed-oxide derived from a hydrotalcite Ni–Al (64:38) precursor, and (iii) the spinel-type NiAl2O4 oxide. These systems were prepared with the aim of studying the activation procedure that develops small nickel nanoparticles deposited on a La2O3 or Al2O3 substrate active in H2 production through catalytic decomposition of CH4. Different preparation procedures have been applied to each precursors (i)–(iii). Perovskite-type oxide LaNiO3 was prepared by the sol–gel methodology (citrates method). Mixed oxide derived from hydrotalcite was obtained by co-precipitation using urea as a basic agent. NiAl2O4 spinel synthesis was performed by the ceramic method. The three oxide-type materials were characterized by XRD, BET specific area, TPR and XPS. Characterization results showed that the preparation methods used allow formation of highly crystalline and homogeneous oxide precursors. After activation, the oxide precursors showed a high activity in the decomposition reaction of CH4. The catalysts derived from hydrotalcite mixed oxide showed the highest activity with CH4 conversions reaching 50% at 500 °C.
The Journal of Physical Chemistry B, 2005
A structural study of the thermal evolution of Ni 0.69 Cr 0.31 (OH) 2 (CO 3 ) 0.155 ‚nH 2 O into NiO and tetragonal NiCr 2 O 4 is reported. The characteristic structural parameters of the two coexisting crystalline phases, as well as their relative abundance, were determined by Rietveld refinement of powder x-ray diffraction (PXRD) patterns. The results of the simulations allowed us to elucidate the mechanism of the demixing process of the oxides. It is demonstrated that nucleation of a metastable nickel chromite within the common oxygen framework of the parent Cr III -doped bunsenite is the initial step of the cationic redistribution. The role that trivalent cations play in the segregation of crystalline spinels is also discussed.
Applied Catalysis B: Environmental, 2009
A series of catalysts was prepared by thermal treatment (500 8C) of the coprecipitated Co-Mn-Al layered double hydroxide (Co:Mn:Al molar ratio of 4:1:1) doped with various amount of potassium (0-3 wt%). Obtained spinel-like mixed oxides were characterized by powder X-ray diffraction, X-ray photoelectron spectroscopy, BET surface area measurements, temperature-programmed H 2 reduction, and temperature-programmed CO 2 and NH 3 desorption. The prepared catalysts were tested for N 2 O decomposition to determine the effect of K addition on the catalytic activity in the presence of O 2 , NO x and H 2 O. An optimum K content of ca. 0.9-1.6 wt% was found to achieve a high catalytic activity in the presence of O 2 and H 2 O, while the non-modified Co-Mn-Al mixed oxide was the most active catalyst in the presence of O 2 and NO x. The addition of potassium to the Co-Mn-Al mixed oxide apparently results in a modification of both electronic properties of active metals and acid-base function of the catalyst surface.
Electrochemistry
The Ni-Al layered double hydroxide materials with high crystallinity were synthesized by the room-temperature liquid process and applied as oxygen evolution reaction (OER) catalysts. It is interesting to note that the OER activity was found to be relatively high in comparison with the Ni-based materials. In addition, the structural analyses and the effects of interlayer anion specie on the OER activity were also evaluated. It was found that the changes in the interlayer anion species from F − to OH − resulted in the enhancement of the OER activity. The maintained chemical compositions and crystallinity after the electrochemical measurements were also observed through the X-ray analyses. The present work provides the insights about the possibility of the well-defined structures as the electrodes which were prepared by the unique liquid phase process.
Catalytic performance of Ni-Al layered double hydroxides in CO purification processes
Russian Journal of Physical Chemistry A, 2013
Ni-Al layered double hydroxides with Ni 2+ /Al 3+ molar ratios of 1.5 and 3.0 have been synthesized by co precipitation and studied as catalyst precursors for purification of CO containing gas mixtures by means of CO oxidation to CO 2 and conversion of CO by water vapor (water-gas shift reaction). The influence of the alkali additives (K + ions) on the water gas shift activity has been also examined. It was established that the catalytic activity of both reactions increases with the temperature and the nickel content. Hypothetic schemes are proposed about activation of the catalysts in the WGSR and CO oxidation including redox Ni 2+ ↔ Ni 3+ transition on the catalyst surface. The activity in WGSR is positively affected by the presence of potassium promoter, depending on its amount. The sample with higher nickel loading is the most effective catalyst as for CO oxidation as well as for WGSR at intermediate temperatures after potassium promotion.