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Memory effect of activated Mg-Al hydrotalcite: in situ XRD studies during decomposition and gas-phase reconstruction.
Chemistry. 2007; 13(3):870-8.C

Abstract

The thermal decomposition of Mg-Al hydrotalcite and the subsequent reconstruction of the decomposed products in the presence of water vapor (2 vol. % H(2)O in N(2)) have been investigated by in situ XRD. Thermographic analysis and temperature-programmed desorption MS results complemented the diffraction data. Valuable mechanistic and kinetic insights into these processes, which are of prime importance for optimal activation of this type of material for catalytic applications, were obtained. Hydrotalcite decomposition to the mixed oxide proceeds via formation at 423-473 K of an intermediate phase, consisting of a highly disordered, dehydrated, layered structure. The latter evolves by removal of interlayer water on heating, causing a shrinking of the interlayer space (it is up to 45 % smaller than in the as-synthesized hydrotalcite). Above 623 K, Mg(Al)O(x) oxide with the periclase structure is formed. Reversion of the intermediate dehydrated structure to hydrotalcite upon contact with water vapor is complete and very fast at room temperature. Recovery of hydrotalcite from the oxide calcined at 723 K is two orders of magnitude slower than rehydration of the intermediate layered structure and one order of magnitude slower than the typically practiced liquid-phase reconstruction. In contrast to the decomposition, the reconstruction mechanism does not involve an intermediate phase. The gas-phase rehydration and reconstruction was interrupted above 303 K. This is attributed to the poor wetting of the surface of the decomposed materials induced by hampered H(2)O adsorption above room temperature at the water vapor pressure applied. The Avrami-Erofe'ev model describes the reconstruction kinetics well.

Authors+Show Affiliations

Pérez-Ramírez J
Laboratory for Heterogeneous Catalysis, Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, 43007 Tarragona, Spain. jperez@iciq.es
Abelló S
No affiliation info available
van der Pers NM
No affiliation info available

MeSH

Aluminum HydroxideGasesMagnesium HydroxideModels, MolecularParticle SizeTemperatureTime FactorsX-Ray Diffraction

Pub Type(s)

Journal Article

Language

eng

PubMed ID

17029312

Citation

Pérez-Ramírez, Javier, et al. "Memory Effect of Activated Mg-Al Hydrotalcite: in Situ XRD Studies During Decomposition and Gas-phase Reconstruction." Chemistry (Weinheim an Der Bergstrasse, Germany), vol. 13, no. 3, 2007, pp. 870-8.
Pérez-Ramírez J, Abelló S, van der Pers NM. Memory effect of activated Mg-Al hydrotalcite: in situ XRD studies during decomposition and gas-phase reconstruction. Chemistry. 2007;13(3):870-8.
Pérez-Ramírez, J., Abelló, S., & van der Pers, N. M. (2007). Memory effect of activated Mg-Al hydrotalcite: in situ XRD studies during decomposition and gas-phase reconstruction. Chemistry (Weinheim an Der Bergstrasse, Germany), 13(3), 870-8.
Pérez-Ramírez J, Abelló S, van der Pers NM. Memory Effect of Activated Mg-Al Hydrotalcite: in Situ XRD Studies During Decomposition and Gas-phase Reconstruction. Chemistry. 2007;13(3):870-8. PubMed PMID: 17029312.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Memory effect of activated Mg-Al hydrotalcite: in situ XRD studies during decomposition and gas-phase reconstruction. AU - Pérez-Ramírez,Javier, AU - Abelló,Sònia, AU - van der Pers,Niek M, PY - 2006/10/10/pubmed PY - 2007/3/27/medline PY - 2006/10/10/entrez SP - 870 EP - 8 JF - Chemistry (Weinheim an der Bergstrasse, Germany) JO - Chemistry VL - 13 IS - 3 N2 - The thermal decomposition of Mg-Al hydrotalcite and the subsequent reconstruction of the decomposed products in the presence of water vapor (2 vol. % H(2)O in N(2)) have been investigated by in situ XRD. Thermographic analysis and temperature-programmed desorption MS results complemented the diffraction data. Valuable mechanistic and kinetic insights into these processes, which are of prime importance for optimal activation of this type of material for catalytic applications, were obtained. Hydrotalcite decomposition to the mixed oxide proceeds via formation at 423-473 K of an intermediate phase, consisting of a highly disordered, dehydrated, layered structure. The latter evolves by removal of interlayer water on heating, causing a shrinking of the interlayer space (it is up to 45 % smaller than in the as-synthesized hydrotalcite). Above 623 K, Mg(Al)O(x) oxide with the periclase structure is formed. Reversion of the intermediate dehydrated structure to hydrotalcite upon contact with water vapor is complete and very fast at room temperature. Recovery of hydrotalcite from the oxide calcined at 723 K is two orders of magnitude slower than rehydration of the intermediate layered structure and one order of magnitude slower than the typically practiced liquid-phase reconstruction. In contrast to the decomposition, the reconstruction mechanism does not involve an intermediate phase. The gas-phase rehydration and reconstruction was interrupted above 303 K. This is attributed to the poor wetting of the surface of the decomposed materials induced by hampered H(2)O adsorption above room temperature at the water vapor pressure applied. The Avrami-Erofe'ev model describes the reconstruction kinetics well. SN - 0947-6539 UR - https://www.unboundmedicine.com/medline/citation/17029312/Memory_effect_of_activated_Mg_Al_hydrotalcite:_in_situ_XRD_studies_during_decomposition_and_gas_phase_reconstruction_ L2 - https://doi.org/10.1002/chem.200600767 DB - PRIME DP - Unbound Medicine ER -