Lithium as a Modifier for Morphology and Defect Structure of Porous Magnesium Oxide Materials Prepared by Gel Combustion Synthesis

Abstract

Defect rich MgO nanocrystals arranged in a hierarchic three‐dimensional pore network were synthesized by using gel combustion synthesis (GCS). By adding Li to the combustion precursor, Li‐induced changes in the morphology and defect structure of MgO could be studied systematically. At low Li loadings (up to 1 wt %), the three‐dimensional pore network was resistant to temperatures up to 800 °C, even though the primary MgO nanoparticles had changed their morphology from on average 8 nm size {100} terminated nanocubes to up to 250 nm large complex polyhedral, exposing more and more {111} facets. At higher Li loadings, the primary MgO particles grow even further, to up to 500 nm, causing the three‐dimensional pore network to collapse. After describing the GCS method, detailed structural characterizations of all of the materials synthesized were conducted by means of XRD, BET and pore size analysis, and electron microscopy. IR and thermogravimetric mass spectroscopy (TG‐MS) in combination with XRD were used to investigate the formation and decomposition of carbonate species during synthesis and calcination. Diffuse reflectance UV/Vis (DR‐UV/Vis) spectroscopy was used to characterize surface defects, such as low coordinated O2− ions at edges, corners, and kinks of the MgO surface. Bulk defects were studied by using electron paramagnetic resonance (EPR) spectroscopy. Morphology and defect concentration of the Li/MgO materials were found to be strongly dependent on the fuel‐to‐oxidizer ratio used in the combustion synthesis, the Li concentration, and the calcination atmosphere.

Structure sensitivity of the oxidative activation of methane over MgO model catalysts: II. Nature of active sites and reaction mechanism

“…Computational studies indicate that CO coordination via carbon only in form of a bridged complex is more likely [14]. Trionfetti et al observed this band also after CO adsorption on Li/MgO catalysts, but with increasing Li content, the contribution of the step sites decreases [15], which is in agreement of the ''ironing'' effect of alkali additives [16].…”

Li/MgO Catalysts Doped with Alio‐valent Ions. Part I: Structure, Composition, and Catalytic Properties

“…in Li/MgO single crystals and proposed as the active center of Li/MgO in the OCM by Lunsford . But it is doubtful whether it is really the active center of Li/MgO, and if this defect is stable under OCM conditions, as it was only found after calcination under O 2 …”

Zentren der Methanaktivierung auf Oberflächen von Lithium‐dotiertem MgO

“…[10,11] Andererseits konnten diese Zentren in Li-dotierten MgO-Einkristallen nachgewiesen werden, [10] die durch Lichtbogenschmelzen von MgO/ Li 2 CO 3 hergestellt worden waren. Sorgfältige Charakterisierung durch unterschiedliche Methoden [11,12] [15] und 90 kJ mol À1 (Bildung von C 2 -Kohlenwasserstoffen). [16] In mikrokinetische Simulationen wurden deutlich hçheren Barrieren verwendet, 147 kJ mol À1 .…”

“…[7] Bei kürzlich durchgeführten ENDOR-Experimenten (Elektron-Kern-Doppelresonanz) gelang für keinen der Katalysatoren, die einer OCM-Reaktion ausgesetzt waren, der Nachweis von Li + OC À -Zentren. [10,11] Andererseits konnten diese Zentren in Li-dotierten MgO-Einkristallen nachgewiesen werden, [10] die durch Lichtbogenschmelzen von MgO/ Li 2 CO 3 hergestellt worden waren. Sorgfältige Charakterisierung durch unterschiedliche Methoden [11,12] ergab, dass die Anwesenheit von Li zu einer solchen Umstrukturierung der MgO-Oberfläche führt, dass im Unterschied zu reinem MgO hochindizierte Flächen bzw.…”

“…[17] In einer neueren Arbeit wird über die Instabilität von Li-dotierten MgO-Katalysatoren unter Reaktionsbedingungen berichtet. [18] [10][11][12] Für weitere kinetische Experimente wurde ein reiner MgO-Modellkatalysator durch Sol-Gel-Synthese gewonnen und mittels Transmissionselektronenmikroskopie (TEM) charakterisiert. Die Aktivität in der Anfangsphase der OCM-Reaktion unterschied sich wesentlich von der im stationären Zustand, womit substantielle morphologische Veränderungen verbunden waren.…”