Characterization of Li-Doped MgO Catalysts for Oxidative Coupling of Methane by Means of Mg K-Edge XANES

Aritani, Hirofumi; Yamada, Hiroyuki; Nishio, Takashi; Shiono, and Takeshi; Imamura, Seiichiro; Kudo, Masataka; Hasegawa, Sadao; and, Tsunehiro Tanaka; Yoshida, Satohiro

doi:10.1021/jp000291y

Cited by 43 publications

(29 citation statements)

References 43 publications

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“…It has been claimed, even with the smaller Li + ions, that oxygen defects prevail in the near-surface and not in the bulk phase. 40,41 We, therefore, propose that the K + / Na + are located at the surface and not in the "bulk" of the MgAlO x . Our tentative mechanism is that K + plus substitutes a Mg 2+ , and additional oxygen vacancies at the surface are created.…”

Section: Resultsmentioning

confidence: 92%

On the Influence and Role of Alkali Metals on Supported and Unsupported Activated Hydrotalcites for CO₂ Sorption

Meis

Bitter

Jong

2010

Ind. Eng. Chem. Res.

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To increase the CO 2 capture capacity of hydrotalcites, the influence of alkali (K, Na) metal carbonate loading of activated supported and unsupported hydrotalcites (HT act ) on their CO 2 capture properties was investigated. The alkali-loaded supported hydrotalcites adsorb at 523 K, depending on the alkali metal (Na or K) and the preparation method, 1.7-2.2 mmol CO 2 g -1 HT which exceeds the capacity of unloaded supported HT (1.3 mmol CO 2 g -1 HT ) and K-loaded unsupported HT (∼0.3 mmol.g -1 ). The key for the increase in capacity in alkali-loaded HT is a close contact at least at a mesoscopic level between HT and alkali metal carbonate. The alkali metal carbonate could be successfully introduced either by impregnation of a K 2 CO 3 solution on assynthesized HT or by leaving residual K (or Na), from the synthesis, in the final material. The latter method is advocated since it omits a washing step after precipitation. The increase in capture capacity for alkali loaded HTs points to a higher concentration of defects (low-coordination oxygen sites) on the surface of the activated alkali loaded HTs compared to the unloaded HT. We propose that the higher concentration of adsorption sites is caused by the presence of Na + /K + on the surface of Mg(Al)O x .

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Section: Resultsmentioning

confidence: 92%

On the Influence and Role of Alkali Metals on Supported and Unsupported Activated Hydrotalcites for CO₂ Sorption

Meis

Bitter

Jong

2010

Ind. Eng. Chem. Res.

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“…Differences in the relative intensities of A, B and C were attributed by Wong et al (1994) to variations in the number of coordinating OH ligands in various materials. In brucite (Mg(OH) 2 ) for example, where Mg2 + is octahedrally coordinated by OH groups, octahedra share adjacent edges to form layer structures, giving a XANES spectrum (Yoshida et al 1995;Aritani et al 2000) very similar to enstatite. Although silicates also contain Si-O tetrahedra with H, O or OH at their vertices the similarity of the XANES for brucite and enstatite suggests silicate Mg K-edge XANES features are due to the existence of a larger structure containing ordered octahedral Mg extending well beyond the first coordination shell.…”

Section: Mg K-edge Xanesmentioning

confidence: 99%

Structural signatures of medium-range order in annealed laboratory silicates

Thompson¹

2008

A&A

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Context. Silicate dust grains exist in a wide range of astronomical environments and understanding the effect of these on grain structure is of great interest, particularly the effect of thermal annealing on amorphous silicates as a possible route to the formation of crystalline grains. Although laboratory simulations have largely focussed on IR-spectroscopic measurements, since these relate directly to observational data, laboratory synchrotron techniques such as X-ray absorption spectroscopy (XAS) and X-ray scattering are becoming increasingly routine in the analysis of recovered materials. With the increasing prospect of performing astronomical XAS observations, there is much to be gained from applying these techniques to laboratory analogues. Aims. Diagnostic markers for medium-range order and the effect of thermal annealing on these in amorphous silicates of differing Mg content are characterised using synchrotron X-ray methods. Methods. Three synthetic amorphous silicates with high, medium and low Mg:Si ratios were annealed at varying temperatures up to 1300 K. X-ray Absorption Near Edge Structure (XANES) spectra at the Si and Mg K-shell absorption edges and X-ray scattering patterns for low values of the X-ray scattering wavevector were recorded for these along with comparative data for commercially produced amorphous SiO 2 and mineral samples of forsterite, enstatite and quartz. Results. XANES features due to short-and medium-range structure are identified at both Mg and Si edges and a new temperature dependent behaviour observed in the medium-range structure surrounding Mg and Si. Based on changes to the morphological details of the XANES spectra, the medium-range structure changes between an enstatite-like and forsterite-like coordination with increasing temperature and appears to correlate directly with Mg content. Low wavevector X-ray scattering features were also found to be diagnostic of the type of medium-range structural ordering. However, these features depend on whether the relative arrangement of clusters of medium-range structure exhibit semi-periodic ordering over the longer-range, which can vary with annealing.

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“…37 As well as studies of catalytic activity, [38][39][40][41][42][43] fundamental studies have been focused on effects of Li dopants on the electronic, optical, and magnetic properties of MgO. [44][45][46][47][48][49][50] In the recent density-functional theory ͑DFT͒ study by Scanlon et al, 50 doping of Li in the top layer of MgO͑100͒ was shown to create an unoccupied state with O 2p valence character which assists catalytic hydrogen abstraction.…”

Section: Introductionmentioning

confidence: 99%

Calculations of Li adsorption and diffusion on MgO(100) in comparison to Ca

Henkelman

2010

Phys. Rev. B

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Density-functional theory calculations are used to study the adsorption and growth of small Li clusters on MgO͑100͒. The binding of Li is found to be similar to that of Ca in many respects. Monomers bind to oxygen sites on the terrace and diffuse at room temperature to defect sites. A range of defect sites will bind monomers with increasing energies from oxygen vacancies, steps, kinks, divacancies, and magnesium vacancies, as well as to peroxo species on step edges. The binding strength correlates with the amount of Bader charge transferred from the adsorbing metal to the substrate. Small clusters are found to be highly mobile on the surface at room temperature. In two cases, we have found qualitative differences in the binding energies of Ca and Li which lead to different growth modes. First, Ca binds less strongly to charged oxygen vacancy defects ͑F + centers͒ than it does on the terrace whereas Li forms a bond with the electron in the vacancy, and is trapped at room temperature. Second, the gas phase Ca dimer distance is longer than the O-O distance on the MgO surface so that epitaxial Ca islands are strained whereas Li is not. These two differences have a profound effect on the growth of clusters. At room temperature, a Ca monomer will not trap at defect sites occupied by another Ca monomer. Thus, Ca atoms diffuse on the terrace until the defects are saturated before nucleating threedimensional islands. On the other hand, two-dimensional epitaxial Li island nucleate at Li-bound defects. These calculations explain the lower initial heat of adsorption of Li and higher surface coverage as compared to Ca in adsorption microcalorimetry measurements by Farmer et al. ͓J. Am. Chem. Soc. 131, 3098 ͑2009͔͒.

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Characterization of Li-Doped MgO Catalysts for Oxidative Coupling of Methane by Means of Mg K-Edge XANES

Cited by 43 publications

References 43 publications

On the Influence and Role of Alkali Metals on Supported and Unsupported Activated Hydrotalcites for CO₂ Sorption

On the Influence and Role of Alkali Metals on Supported and Unsupported Activated Hydrotalcites for CO₂ Sorption

Structural signatures of medium-range order in annealed laboratory silicates

Calculations of Li adsorption and diffusion on MgO(100) in comparison to Ca

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Characterization of Li-Doped MgO Catalysts for Oxidative Coupling of Methane by Means of Mg K-Edge XANES

Cited by 43 publications

References 43 publications

On the Influence and Role of Alkali Metals on Supported and Unsupported Activated Hydrotalcites for CO2 Sorption

On the Influence and Role of Alkali Metals on Supported and Unsupported Activated Hydrotalcites for CO2 Sorption

Structural signatures of medium-range order in annealed laboratory silicates

Calculations of Li adsorption and diffusion on MgO(100) in comparison to Ca

Contact Info

Product

Resources

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On the Influence and Role of Alkali Metals on Supported and Unsupported Activated Hydrotalcites for CO₂ Sorption

On the Influence and Role of Alkali Metals on Supported and Unsupported Activated Hydrotalcites for CO₂ Sorption