E. García-gonzález - Academia.edu (original) (raw)
Papers by E. García-gonzález
Catalysis Science & Technology, 2019
The control of Nb incorporation in W–Nb–O system leads to tailored Brönsted/Lewis acid properties... more The control of Nb incorporation in W–Nb–O system leads to tailored Brönsted/Lewis acid properties for the selective transformation of either glycerol or aqueous mixtures of short chain oxygenates into value added products.
Journal of Solid State Chemistry, 1996
Applied Catalysis A: General, 2006
Oxidative dehydrogenation of ethane to ethylene was investigated over a series of MoO 3 added V 2... more Oxidative dehydrogenation of ethane to ethylene was investigated over a series of MoO 3 added V 2 O 5-Al 2 O 3 catalysts. The catalysts were characterized by BET, XRD, Laser-Raman and FT-IR spectroscopies and TPR technique. Catalytic tests were carried out in a fixed bed stainless steel reactor in the temperature range from 450 to 600°C. Results revealed that the loading of molybdophosphoric acid (MPA) and the method of preparation had a clear influence on the catalytic performance. Among all, 10 wt.% MPA/V 2 O 5-Al 2 O 3 solid was found to possess superior activity and selectivity (X-C 2 H 6~3 5% and S-C 2 H 4~6 5%). Formation of Mo-V mixed oxide phases on Al 2 O 3 appeared to be responsible for this improved performance. This best catalyst also exhibited good long-term stability over a period of ca. 36 h.
Applied Catalysis A: General
Unsupported and KIT-6 supported tungsten oxide-based bronzes have been prepared (with different N... more Unsupported and KIT-6 supported tungsten oxide-based bronzes have been prepared (with different Nb/W ratios and different amount of KIT-6), characterized and tested in the selective transformation of oxygenated compounds (such as ethanol, propanal, hydroxyacetone and acetic acid) present in aqueous fractions derived from lignocellulosic biomass processing. The catalysts are active and selective in both consecutive condensation and ketonization of oxygenated compounds. Consequently, intermediates such as 2-methyl-2-pentenal (2M2P), C5-C8 and C9-C10+ hydrocarbon fractions can be selectively achieved depending on both the catalyst composition and the reaction conditions.
Dalton Transactions, 2016
The intricate nanostructure of La0.5−xLi0.5−xSr2xTiO3 Li-ion conductors has been elucidated. Adva... more The intricate nanostructure of La0.5−xLi0.5−xSr2xTiO3 Li-ion conductors has been elucidated. Advanced transmission electron microscopy has allowed investigation where average structure models cannot account for the changeable local atomic arrangements detected.
ChemInform, 2003
Molybdenum Molybdenum I 5600 The Nature and Structure of a New MoVTeO Crystalline Phase.-The new ... more Molybdenum Molybdenum I 5600 The Nature and Structure of a New MoVTeO Crystalline Phase.-The new phase Te 0.33 (Mo,V)O 3.33 is obtained by evaporation of an aqueous slurry comprising (NH 4) 6 Mo 7 O 24 , Te(OH) 6 , and VOSO 4 , followed by calcination at 600°C for 2 h in N 2. The samples are characterized by powder XRD, SAED, HREM, and UV/VIS spectroscopy. The XRD pattern is indexed on the basis of a hexagonal cell. However, HREM reveals the crystals as being formed by structural domains of an orthorhombically distorted unit cell derived from the K 0.13-0.33 WO 3 structure. The results constitute the first step for elucidating the role of this new orthorhombic phase in the effectiveness of Mo-V-Te-Nb mixed oxide catalysts.-(GARCIA-GONZALEZ*, E.; LOPEZ NIETO,
Journal of Solid State Chemistry, 1996
phase (1 : 2 : 3), where two octahedral layers alternate with The microstructural characterizatio... more phase (1 : 2 : 3), where two octahedral layers alternate with The microstructural characterization of La 1/3 Sr 2/3 FeO 3؊y a tetrahedral layer along the b axis, the stacking sequence (y ؍ 0.15 and y ؍ 0.33) materials has been studied by means being. .. OOT. .. (T, tetrahedron) (6), which in the followof electron diffraction and high-resolution electron microscopy. ing we will call the G phase. This is the only ordered phase A new unit cell of parameters 2a c ͙2 ؋ 3a c ؋ ͙2 c has been isolated for the cationic ratio 1 : 2 : 3, La : Ca : Fe. Anionic observed for the highly reduced material La 1/3 Sr 2/3 FeO 2.67 (or compositions higher than O 8 led to microdomain forma-LaSr 2 Fe 3 O 8) as a consequence of the anionic vacancy order. tion (7). Most of the rare earth elements lead to the G This cell seems to avoid any ordered distribution of Sr and phase in the Ca system (8). When Ca is replaced by Ba La in the A sublattice, which is only possible for a random in the above 1 : 2 : 3 phase, two different situations distribution of oxygen vacancies (y ؍ 0.15).
Inorganic Chemistry, 2009
The thermal stability of nanostructured Ce(0.5)Zr(0.5)O(2) powders prepared by the Pechini method... more The thermal stability of nanostructured Ce(0.5)Zr(0.5)O(2) powders prepared by the Pechini method was studied on the nanometric scale by X-ray diffraction (XRD), energy-dispersive spectrometry (EDS), transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), and Raman techniques. Obtained results demonstrate that amorphous powders coming from the thermal decomposition of the precursor transform into the stable crystalline state through one highly disordered and metastable intermediate. This is a new example of successive reactions controlled by Ostwald's rule in inorganic systems. At low calcination temperatures, the combination of Raman spectroscopy, high-resolution electron microscopy, and EDS nanoanalysis showed the formation from the precursor powder of a disordered pseudocubic phase. At 900 degrees C, metastable T' and stable T and C phases were detected in XRD patterns. As increasing temperature, crystallites growth and proportions of stable T and C phases increased at the expense of the T' phase, which completely disappeared at 1300 degrees C. In analyzed samples, the Raman technique and (crystal by crystal) EDS nanoanalyses were used to detect local phase inhomogeneity. Compositions and relative percentages of phases were investigated by XRD Rietveld analysis and discussed in terms of phase diagrams previously reported.
Inorganic Chemistry, 2011
An exhaustive temperature dependent structural and dielectric study of the tetragonal tungsten br... more An exhaustive temperature dependent structural and dielectric study of the tetragonal tungsten bronze-type Sr 2 NaNb 5 O 15 (SNN) compound has been performed in the 300−100 K temperature range, by combining X-ray, neutron diffraction, and transmission electron microscopy with dielectric measurements, in order to clarify the structural effects responsible for the observed low temperature dielectric properties. Interestingly, a relevant second anomaly in the dielectric constant, in addition to the ferroelectric (FE) to paraelectric (PE) transition at T C = 518 K is found at T ≈ 240 K, revealing a relaxor-like behavior of the material at low temperature. This phenomenon has been previously observed in FE perovskite-type phases and referred to as the re-entrant phenomenon. However, FE polarization tends to vanish below this low temperature dielectric anomaly and this fact is not expected for a classical relaxor-ferroelectric phase. Although there is no structural transition from RT to 100 K, there is a change in the elastic properties of the material in the considered temperature range and the intense anomaly at ∼240 K could be associated to a smeared-out phase transition to a frustrated FE/ ferroelastic (FEL) low temperature state in correlation with subtle structural effects.
Vanadium free WO3 and vanadium-containing W0.92V0.08O3-x oxides with the HTB structure have been ... more Vanadium free WO3 and vanadium-containing W0.92V0.08O3-x oxides with the HTB structure have been prepared by hydrothermal synthesis. The addition of vanadium rises up the stability range of the hexagonal phase which became a stable polymorph up to 600o C. The structural characterization performed revealed the presence of a dense population of polyhedral nanocavities on the crystals surface distributed along particular crystallographic directions. Nanocavities are formed during the annealing process leading to catalyst activation and the process has been followed by TG/DTA-MS, HRTEM and HAADF STEM. The topotactic relationship between the as synthesized product and the desired heat-treated final product is proposed as the origin of the surface nanostructure. Samples calcined at different temperatures have been tested as active catalysts in the partial oxidation of H2S and the results provide experimental evidence about the positive influence of the nanocavities in the catalytic activi...
ECS Meeting Abstracts, 2021
Journal of Solid State Chemistry - J SOLID STATE CHEM, 1993
The microstructural characterization of LnBa[sub 2]Fe[sub 3]O[sub 8] materials (Ln = Dy, Ho) has ... more The microstructural characterization of LnBa[sub 2]Fe[sub 3]O[sub 8] materials (Ln = Dy, Ho) has been carried out by means of electron diffraction and high resolution electron microscopy. The results obtained lead to a lattice with tetragonal symmetry and parameters a[sub c] x a[sub c] x 3a[sub c] (a[sub c] being the perovskite subcell parameter) and P4/mmm as possible space group. A structural model in which two square pyramids [FeO[sub 5]] and one octahedron [FeO[sub 6]] alternate along the c axis is proposed. 23 refs., 5 figs.
Chemistry of Materials, 2009
... 10. Lente, MH; Moreira, EN; Garcia, D.; Eiras, JA; Neves, PP; Doriguetto, AC; Mastelaro, VR; ... more ... 10. Lente, MH; Moreira, EN; Garcia, D.; Eiras, JA; Neves, PP; Doriguetto, AC; Mastelaro, VR; Mascarenhas, YP Phys. Rev. B 2006, 73054106 [CrossRef]. 11. Long, X.; Ye, Z.-G. J. Appl. Phys. 2007, 101, 124101 [CrossRef]. 12. ...
Chemistry of Materials, 1998
A new ordering scheme in cationic-deficient perovskite-like phases of the Ba n (Mo,Nb) n-δ O 3n-x... more A new ordering scheme in cationic-deficient perovskite-like phases of the Ba n (Mo,Nb) n-δ O 3n-x system has been described for the composition Ba 3 MoNbO 8.5 , which has an oxygen stoichiometry between those of the 9R polytype and the palmierite structure. An electron diffraction and high-resolution electron microscopy study shows that the oxygen defect with respect to the 9R polytype (hhc) 3 leads to an ordered distribution of octahedra and tetrahedra in a 3:2 ratio in the cubic layers. The flexibility of the (hhc) 3 stacking sequence allows the formation of a modulated structure through an ordered shearing mechanism along c by introducing mixed BaO 2.6 cubic layers in the 9R polytype.
Chemistry of Materials, 2007
ABSTRACT The structural features of the ferroelectric compound Sr2NaNb5O15 have been carefully st... more ABSTRACT The structural features of the ferroelectric compound Sr2NaNb5O15 have been carefully studied. A detailed investigation has been performed by means of room temperature (RT) and high temperature (HT) powder X-ray diffraction (XRD), selected area electron diffraction (SAED), high resolution electron microscopy (HREM), and impedance spectroscopy. Comparison is done with the barium homologue compound and the two RT ferroelectric phases can be structurally distinguished. The use of electron diffraction in combination with HREM has revealed the formation of a superlattice structure of tetragonal tungsten bronze (TTB) at RT. While in Ba2NaNb5O15 the incommensurate nature of this RT phase seems to be doubtless, the Sr2NaNb5O15 ferroelectric phase displays a commensurate superstructure of the TTB structure-type. The RT XRD pattern can be indexed in the Im2a space group. The transition to the paraelectric phase occurs at 518 K, and the material can be indexed in the centrosymmetric P4/mbm space group. From the Rietveld refinement of this HT phase, disordered distribution of Na and Sr in the A positions of the structure seems to occur, as opposite to the barium compound where barium atoms show clear preference for the largest A2 sites.
Catalysis Science & Technology, 2019
The control of Nb incorporation in W–Nb–O system leads to tailored Brönsted/Lewis acid properties... more The control of Nb incorporation in W–Nb–O system leads to tailored Brönsted/Lewis acid properties for the selective transformation of either glycerol or aqueous mixtures of short chain oxygenates into value added products.
Journal of Solid State Chemistry, 1996
Applied Catalysis A: General, 2006
Oxidative dehydrogenation of ethane to ethylene was investigated over a series of MoO 3 added V 2... more Oxidative dehydrogenation of ethane to ethylene was investigated over a series of MoO 3 added V 2 O 5-Al 2 O 3 catalysts. The catalysts were characterized by BET, XRD, Laser-Raman and FT-IR spectroscopies and TPR technique. Catalytic tests were carried out in a fixed bed stainless steel reactor in the temperature range from 450 to 600°C. Results revealed that the loading of molybdophosphoric acid (MPA) and the method of preparation had a clear influence on the catalytic performance. Among all, 10 wt.% MPA/V 2 O 5-Al 2 O 3 solid was found to possess superior activity and selectivity (X-C 2 H 6~3 5% and S-C 2 H 4~6 5%). Formation of Mo-V mixed oxide phases on Al 2 O 3 appeared to be responsible for this improved performance. This best catalyst also exhibited good long-term stability over a period of ca. 36 h.
Applied Catalysis A: General
Unsupported and KIT-6 supported tungsten oxide-based bronzes have been prepared (with different N... more Unsupported and KIT-6 supported tungsten oxide-based bronzes have been prepared (with different Nb/W ratios and different amount of KIT-6), characterized and tested in the selective transformation of oxygenated compounds (such as ethanol, propanal, hydroxyacetone and acetic acid) present in aqueous fractions derived from lignocellulosic biomass processing. The catalysts are active and selective in both consecutive condensation and ketonization of oxygenated compounds. Consequently, intermediates such as 2-methyl-2-pentenal (2M2P), C5-C8 and C9-C10+ hydrocarbon fractions can be selectively achieved depending on both the catalyst composition and the reaction conditions.
Dalton Transactions, 2016
The intricate nanostructure of La0.5−xLi0.5−xSr2xTiO3 Li-ion conductors has been elucidated. Adva... more The intricate nanostructure of La0.5−xLi0.5−xSr2xTiO3 Li-ion conductors has been elucidated. Advanced transmission electron microscopy has allowed investigation where average structure models cannot account for the changeable local atomic arrangements detected.
ChemInform, 2003
Molybdenum Molybdenum I 5600 The Nature and Structure of a New MoVTeO Crystalline Phase.-The new ... more Molybdenum Molybdenum I 5600 The Nature and Structure of a New MoVTeO Crystalline Phase.-The new phase Te 0.33 (Mo,V)O 3.33 is obtained by evaporation of an aqueous slurry comprising (NH 4) 6 Mo 7 O 24 , Te(OH) 6 , and VOSO 4 , followed by calcination at 600°C for 2 h in N 2. The samples are characterized by powder XRD, SAED, HREM, and UV/VIS spectroscopy. The XRD pattern is indexed on the basis of a hexagonal cell. However, HREM reveals the crystals as being formed by structural domains of an orthorhombically distorted unit cell derived from the K 0.13-0.33 WO 3 structure. The results constitute the first step for elucidating the role of this new orthorhombic phase in the effectiveness of Mo-V-Te-Nb mixed oxide catalysts.-(GARCIA-GONZALEZ*, E.; LOPEZ NIETO,
Journal of Solid State Chemistry, 1996
phase (1 : 2 : 3), where two octahedral layers alternate with The microstructural characterizatio... more phase (1 : 2 : 3), where two octahedral layers alternate with The microstructural characterization of La 1/3 Sr 2/3 FeO 3؊y a tetrahedral layer along the b axis, the stacking sequence (y ؍ 0.15 and y ؍ 0.33) materials has been studied by means being. .. OOT. .. (T, tetrahedron) (6), which in the followof electron diffraction and high-resolution electron microscopy. ing we will call the G phase. This is the only ordered phase A new unit cell of parameters 2a c ͙2 ؋ 3a c ؋ ͙2 c has been isolated for the cationic ratio 1 : 2 : 3, La : Ca : Fe. Anionic observed for the highly reduced material La 1/3 Sr 2/3 FeO 2.67 (or compositions higher than O 8 led to microdomain forma-LaSr 2 Fe 3 O 8) as a consequence of the anionic vacancy order. tion (7). Most of the rare earth elements lead to the G This cell seems to avoid any ordered distribution of Sr and phase in the Ca system (8). When Ca is replaced by Ba La in the A sublattice, which is only possible for a random in the above 1 : 2 : 3 phase, two different situations distribution of oxygen vacancies (y ؍ 0.15).
Inorganic Chemistry, 2009
The thermal stability of nanostructured Ce(0.5)Zr(0.5)O(2) powders prepared by the Pechini method... more The thermal stability of nanostructured Ce(0.5)Zr(0.5)O(2) powders prepared by the Pechini method was studied on the nanometric scale by X-ray diffraction (XRD), energy-dispersive spectrometry (EDS), transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), and Raman techniques. Obtained results demonstrate that amorphous powders coming from the thermal decomposition of the precursor transform into the stable crystalline state through one highly disordered and metastable intermediate. This is a new example of successive reactions controlled by Ostwald's rule in inorganic systems. At low calcination temperatures, the combination of Raman spectroscopy, high-resolution electron microscopy, and EDS nanoanalysis showed the formation from the precursor powder of a disordered pseudocubic phase. At 900 degrees C, metastable T' and stable T and C phases were detected in XRD patterns. As increasing temperature, crystallites growth and proportions of stable T and C phases increased at the expense of the T' phase, which completely disappeared at 1300 degrees C. In analyzed samples, the Raman technique and (crystal by crystal) EDS nanoanalyses were used to detect local phase inhomogeneity. Compositions and relative percentages of phases were investigated by XRD Rietveld analysis and discussed in terms of phase diagrams previously reported.
Inorganic Chemistry, 2011
An exhaustive temperature dependent structural and dielectric study of the tetragonal tungsten br... more An exhaustive temperature dependent structural and dielectric study of the tetragonal tungsten bronze-type Sr 2 NaNb 5 O 15 (SNN) compound has been performed in the 300−100 K temperature range, by combining X-ray, neutron diffraction, and transmission electron microscopy with dielectric measurements, in order to clarify the structural effects responsible for the observed low temperature dielectric properties. Interestingly, a relevant second anomaly in the dielectric constant, in addition to the ferroelectric (FE) to paraelectric (PE) transition at T C = 518 K is found at T ≈ 240 K, revealing a relaxor-like behavior of the material at low temperature. This phenomenon has been previously observed in FE perovskite-type phases and referred to as the re-entrant phenomenon. However, FE polarization tends to vanish below this low temperature dielectric anomaly and this fact is not expected for a classical relaxor-ferroelectric phase. Although there is no structural transition from RT to 100 K, there is a change in the elastic properties of the material in the considered temperature range and the intense anomaly at ∼240 K could be associated to a smeared-out phase transition to a frustrated FE/ ferroelastic (FEL) low temperature state in correlation with subtle structural effects.
Vanadium free WO3 and vanadium-containing W0.92V0.08O3-x oxides with the HTB structure have been ... more Vanadium free WO3 and vanadium-containing W0.92V0.08O3-x oxides with the HTB structure have been prepared by hydrothermal synthesis. The addition of vanadium rises up the stability range of the hexagonal phase which became a stable polymorph up to 600o C. The structural characterization performed revealed the presence of a dense population of polyhedral nanocavities on the crystals surface distributed along particular crystallographic directions. Nanocavities are formed during the annealing process leading to catalyst activation and the process has been followed by TG/DTA-MS, HRTEM and HAADF STEM. The topotactic relationship between the as synthesized product and the desired heat-treated final product is proposed as the origin of the surface nanostructure. Samples calcined at different temperatures have been tested as active catalysts in the partial oxidation of H2S and the results provide experimental evidence about the positive influence of the nanocavities in the catalytic activi...
ECS Meeting Abstracts, 2021
Journal of Solid State Chemistry - J SOLID STATE CHEM, 1993
The microstructural characterization of LnBa[sub 2]Fe[sub 3]O[sub 8] materials (Ln = Dy, Ho) has ... more The microstructural characterization of LnBa[sub 2]Fe[sub 3]O[sub 8] materials (Ln = Dy, Ho) has been carried out by means of electron diffraction and high resolution electron microscopy. The results obtained lead to a lattice with tetragonal symmetry and parameters a[sub c] x a[sub c] x 3a[sub c] (a[sub c] being the perovskite subcell parameter) and P4/mmm as possible space group. A structural model in which two square pyramids [FeO[sub 5]] and one octahedron [FeO[sub 6]] alternate along the c axis is proposed. 23 refs., 5 figs.
Chemistry of Materials, 2009
... 10. Lente, MH; Moreira, EN; Garcia, D.; Eiras, JA; Neves, PP; Doriguetto, AC; Mastelaro, VR; ... more ... 10. Lente, MH; Moreira, EN; Garcia, D.; Eiras, JA; Neves, PP; Doriguetto, AC; Mastelaro, VR; Mascarenhas, YP Phys. Rev. B 2006, 73054106 [CrossRef]. 11. Long, X.; Ye, Z.-G. J. Appl. Phys. 2007, 101, 124101 [CrossRef]. 12. ...
Chemistry of Materials, 1998
A new ordering scheme in cationic-deficient perovskite-like phases of the Ba n (Mo,Nb) n-δ O 3n-x... more A new ordering scheme in cationic-deficient perovskite-like phases of the Ba n (Mo,Nb) n-δ O 3n-x system has been described for the composition Ba 3 MoNbO 8.5 , which has an oxygen stoichiometry between those of the 9R polytype and the palmierite structure. An electron diffraction and high-resolution electron microscopy study shows that the oxygen defect with respect to the 9R polytype (hhc) 3 leads to an ordered distribution of octahedra and tetrahedra in a 3:2 ratio in the cubic layers. The flexibility of the (hhc) 3 stacking sequence allows the formation of a modulated structure through an ordered shearing mechanism along c by introducing mixed BaO 2.6 cubic layers in the 9R polytype.
Chemistry of Materials, 2007
ABSTRACT The structural features of the ferroelectric compound Sr2NaNb5O15 have been carefully st... more ABSTRACT The structural features of the ferroelectric compound Sr2NaNb5O15 have been carefully studied. A detailed investigation has been performed by means of room temperature (RT) and high temperature (HT) powder X-ray diffraction (XRD), selected area electron diffraction (SAED), high resolution electron microscopy (HREM), and impedance spectroscopy. Comparison is done with the barium homologue compound and the two RT ferroelectric phases can be structurally distinguished. The use of electron diffraction in combination with HREM has revealed the formation of a superlattice structure of tetragonal tungsten bronze (TTB) at RT. While in Ba2NaNb5O15 the incommensurate nature of this RT phase seems to be doubtless, the Sr2NaNb5O15 ferroelectric phase displays a commensurate superstructure of the TTB structure-type. The RT XRD pattern can be indexed in the Im2a space group. The transition to the paraelectric phase occurs at 518 K, and the material can be indexed in the centrosymmetric P4/mbm space group. From the Rietveld refinement of this HT phase, disordered distribution of Na and Sr in the A positions of the structure seems to occur, as opposite to the barium compound where barium atoms show clear preference for the largest A2 sites.