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Ferromagnetic resonance investigation of dispersed Ni catalysts: epitaxial and textural effects from the support
Volume 52, number 3
CHEMICAL PHYSICS LETTERS
15 December 1977
FERROMAGNETIC RESONANCE INVESTIGATION OF DISPERSED Ni CATALYSTS: EPITAXIAL AND TEXTURAL EFFECTS FROM THE SUPPORT Eric G. DEROUANE
*, Anthony
J. SIMOENS
Facultd Universitairesde Namur, Laboratoire B-SOOO-Namur. Belgium
de Catalyse,
and Jacques C. VGDRINE Instittct de Recherches
sur h Catalyse,
F-69626- Villeurbanne.
France
Received 7 April 1977
Catalysts consisting of dispersed Ni particles supported on silica and alumina, with sizes ranging from 6 to 20 nm, have been studied by ferromagnetic resonance. For the Ni on A1203 catalyst, a textural promotion effect is shown to be present and it is attributed to the possible presence of NiA1204. The FMR data confirm the epitaxial growth of Ni on SiOl when Ni antigorite is reduced and show that some anisotropy is still present after sintering of the catalyst at about 1200 IS.
Many results from films and single crystals studies show that different crystallographic faces of a given metal can have varying catalytic activities [ 1,2] and the same effect has been proposed more recently for supported and divided metal catalysts [3]. In the latter case, oriented Ni crystallites with either (111) or (I 10) planes parallel to the Si02 support were obtained by reduction of a synthetic antigorite of Ni, as a result of the epitaxial growth of the Ni particles on the support, and the catalytic activity for the hydrogenation of ethylene was correlated with the type of surface crystallographic planes. Another support effect, in catalysis, is the so-called promote: effect. Although its existence ls known since about four decades for the iron synthetic ammonia catalyst, it is only very recently that alumina has been proved, by MGssbauer spectroscopy, to act as a textural promoter [4]. Al203 is present in the iron as small Inclusions consisting of FeAl20, in the incompletely reduced catalyst and of Al203 itself after complete reduction. Promoter effects are also found for catalysts consisting of Fe, Co, or Ni which are used for the *To whom correspondence should be sent.
industrial conversion of natural gas or methane-rich gases into synthesis gas or hydrogen [51- Here too, activating agents are usually metals or compounds thereof, of which the oxides are not easily reducible (K20, MgQ A1203 , ___),and much work still has to be done to ascertain the role of these promoters. The present contribution is a continuation of our previous work on dispersed Ni catalysts as studied by ferromagnetic resonance absorption (FMR) [6] and it demonstrates how FMR can be used successfully to gather relevant information on the shape of supported metal crystallites and possible textural promoter effects from the support. It is obviously restricted to ferro- and/or ferri-magnetic compounds. For this purpose, two different systems have been investigated. First is a Ni on 77-A1203 catalyst containing 13 wt.% Ni of which the metal surface area measured by chemisorption is 6 m2 g-l, corresponding to an average crystal size of 14.6 rum in rather good agreement with the size distribution obtained by electron microscopy (6-20 nm). Prior to the FMR measurements, this catalyst was heated under H2 flow at 1130 K for 3 h before being desorbed and sealed off when the vacuum reached 2 X 10m6 torr. This sample 549
Volume
52, number
3
CHEMICAL PHYSKS
LETTERS
15 December 1977
I
3
Fig. 1. Transmission electron micrograph of the Ni-SiOa-A 1273 I(. The number average crystallite size is 13.8 nm.
catalyst after reduction at 953 K and sintering under vacuum at 1173-
Volume 52, number 3
CHEMICAL PHYSICS LETTERS
be referred to as Ni-7pA1203. A Ni-rich catalyst, on SiO, support, was prepared as described previously [3], the syntheric antigorite of Ni @recursor) being compressed to 1 t cm-2 to achieve a partial orientation of the sheet-like crystalites. After desorption at 953 K, the sample is reduced at the same temperature under flowing l-l, during 5 h. It is sealed off under a vacuum of 2 X IOm6 torr and partially sintered at 1173-1273 K. It contains 45 wt.% of Ni of which the number average crystallite size is 13.8 nm, with a major distribution of sizes in the range 6-20 run. Contrarily to what is observed after evacuation and reduction at 973 K (preferential orientation and well-defined hexagonal shape [7] ), the sizes and forms of the Ni particles seems now heterogeneous as shown by transmission electron microscopy (see fig. 1, to be compared to fig. 8 of ref. [7]) and as a result of the higher temperature sintering of the Ni crystallites. This catalyst will be referred to as Ni-SiO,-A. Apart from the nature of the support and the Ni content, both catalysts seem then rather similar if we restrict ourselves will
15 December 1977
perature, B = T’T, (with T, = 631 K as for bulk Ni), in both cases. Xt is seen that the Ni-SiOa-A catalyst shows the same Curie temperature as bulk Ni while the alumina supported catalyst has a Curie temperature shifted to lower values by about 35 K. In view of our former conclusions [6] and of the similarity of both systems, the Curie temperature (T,) shift cannot be attributed to a particle size effect. However, static magnetic measurements indicate that while complete reduction is achieved for SiOz supported Ni at temperztures above 920 K, it is not the case for Ni dispersed on Al,O, _Indeed, in the latter case, typical reduction degree values at 930, 1030, and 1130 K are respectively equal to 90,94, and 97% IS] _Therefore,
to the above data. Fig. 2 shows the variation of the reduced FMR signal intensity, I/IO (f being the intensity at temperature T and f, being the intensity at the lowest measurement temperature), as a function of the reduced tem-
Fig. 2. Variation of the reduced FMR signal intensity (ilr,) as a function of the reduced temperature (T./T& = 6, Tc = 631 K) for the Ni-n-Al203 loo)and the Ni-SiOz-A catalysts (or).
Fig. 3. First derivative FMR spectra of Ni-SiO2 -A at 180, 298, and 480 K for two orientations (i.e., parafiel and perpendicular to the pellet) of the static magnetic field.
551
Volume 52, number 3
CHEMICAL PHYSICS LETTERS
as in our previous work [6], we have to correlate the 7” shift with incomplete reduction of Ni. We believe that, when the Al,O, impregnated with a Ni salt is progressively heated and reduced, some NiA1204 is formed which cannot be reduced at temperatures below about 1250 IL The latter can be present as small indusions in the Ni CrystatIites, after reduction at 1130 K, and accounts for the T, shift. It is tempting, in view of these conclusions, to put forward the idea that as for the Fe synthetic ammonia catalyst, AI,O, is also playing in the present case the role of a textural promoter_ Further info~ation on the Ni-SiOz-A catalyst can also be obtained by looking at the angular dependence of the position of the FMR signal. Namely, one important question is whether or not sintering at 11731273 K completely erases any preferential orientation or shape of the Ni particles as they were observed after reduction at 953 IL Electron microscopy is of no help as it shows rather spherical particles (fig. I). Two extreme orientations of the reduced Ni antigorite peilet can be defined with respect to the external static magnetic field, Ho. In the first one, & is paralfel to the original C-axis and perpendicular to the
(A, B) atomic planes of antigorite. It is then perpendicular to the silica sheets and flat Ni crystallites obtained after reduction at 953 K and perpendicular to the pellet. That orientation will be referred to as HL. The other extreme orientation corresponds to Ho parallel to the original (A, B) plane of antigorite, i.e., parallel to the pellet and it is designated by HE. Fig, 3 shows the first derivative FMR spectra of the Ni-SiO*-A catalyst at various temperatures and for these two orientations of the external magnetic field Ho. Fig 4 represents the angular dependence of the position of the FhIR signal when the pellet is turned around over 360”. The indicated magnetic field value corresponds to the point at which the signal cuts the baseline. It is clear from these data that the FMR signal shifts regularly by about 200 Oe when going progressively from any perpendicular to parallel orientation and that the magnitude of this shift does not depend much on temperature beIow 500 K. That is a clear-cut case of shape anisotropy as it shouId be expected for compressed ellipsoids when their revolution axes are progressively tilted with respect to the external magnetic field, because of the angular dependence of the demagnetizing field [9] .
Fig. 4. Angulardependenceof the position of the FMK signal of the Ni-SiOz-A catalyst.
552
15 December 1937
Volume 52, number 3
CHEMICAL PHYSICS LETTERS
Therefore, we must conclude that even after sintering the Ni crystallites still retain some preferential orientation and shape, a conclusion that could not be inferred from the electron microscopy observations. The FMR spectra also show other anisotropy effects which depend on temperature as possibly due to magnetocrystalline and/or magnetostriction effects. As a conclusion, the present work shows very clearly that FMR can prove to be a very powerful technique in the characterization of metal-support interactions. -Textural promotion results in Curie temperature lowering which is evidenced by plotting the FMR signal intensity as a function of temperature while structural (epitaxial) relationships between the metal particles and the support can be observed as a result of variations in the average demagnetizing field value, provided that partial orientation of the individual (ferromagnetic) crystallites can be achieved and defined with respect to the external static magnetic field. These conclusions prove to be even more general when it is realized that FRM spectra of diluted ferromagnetic materials are very easily obtained using conventional EPR spectrometers, sych as an X-band spectrometer used in the present study. E.G.D. would like to thank Haldor Topple A/S for supplying and partially characterizing the Ni on alumina
15 December 1977
catalyst. J-C-V. acknowledges financial support from the Facult& Universitaires de Namur during his stay in Belgium. Thanks are also due to G.A. Martin (CNRS, Villeurbanne) for most interesting discussions and to Y. Houbion for his technical help.
References [ 1] G. Ertl, Surface Sci. 7 (1967) 309. [2] G. Dalmai-Imelik and J.C. Bertolini, Compt. Rend. Acad. Sci (Paris) 270 C (1970) 1079. [3] G. Dahnai-Imelik, C. Leclercq, J. Massardier, A. MaubertFranc0 and A.Zalhout, Japan. J. Appl. Phys. Suppl. 2, Pt. 2 (1974) 489. [4] H. Topsge, J-A. Dumesic and M. Boudart, J_ Catal. 28 (1973) 477. [S] J.R. Rostrup-Nielsen, Steam reforming catalysts (Teknisk Forlag A/S, Copenhagen, 1975) pp. 27-28. [6] E.G. Derouane, A.J. Simoens,C. Colin, G.A. Martin, J-A. D&non and J-C. VGdrine, J. Catal., submitted for publication. [7] G. Dalmai-Imelik, C. Leclcrcq and A. Maubert-Muguet, J. Solid State Chem. 16 (1976) 129. [8] G.A. Martin, N. Cephalan, Ph. de Montgolfier and B. Imelik, J. Chim. Phys. (1973) 1422; G-A. Martin, private communication. [9] A. Herpin, Thiorie du mamnktisme (Presses Universitaires de France, Paris, 1968) pp. 25-27; ch. XI-C.
553
CHEMICAL PHYSICS LETTERS
15 December 1977
FERROMAGNETIC RESONANCE INVESTIGATION OF DISPERSED Ni CATALYSTS: EPITAXIAL AND TEXTURAL EFFECTS FROM THE SUPPORT Eric G. DEROUANE
*, Anthony
J. SIMOENS
Facultd Universitairesde Namur, Laboratoire B-SOOO-Namur. Belgium
de Catalyse,
and Jacques C. VGDRINE Instittct de Recherches
sur h Catalyse,
F-69626- Villeurbanne.
France
Received 7 April 1977
Catalysts consisting of dispersed Ni particles supported on silica and alumina, with sizes ranging from 6 to 20 nm, have been studied by ferromagnetic resonance. For the Ni on A1203 catalyst, a textural promotion effect is shown to be present and it is attributed to the possible presence of NiA1204. The FMR data confirm the epitaxial growth of Ni on SiOl when Ni antigorite is reduced and show that some anisotropy is still present after sintering of the catalyst at about 1200 IS.
Many results from films and single crystals studies show that different crystallographic faces of a given metal can have varying catalytic activities [ 1,2] and the same effect has been proposed more recently for supported and divided metal catalysts [3]. In the latter case, oriented Ni crystallites with either (111) or (I 10) planes parallel to the Si02 support were obtained by reduction of a synthetic antigorite of Ni, as a result of the epitaxial growth of the Ni particles on the support, and the catalytic activity for the hydrogenation of ethylene was correlated with the type of surface crystallographic planes. Another support effect, in catalysis, is the so-called promote: effect. Although its existence ls known since about four decades for the iron synthetic ammonia catalyst, it is only very recently that alumina has been proved, by MGssbauer spectroscopy, to act as a textural promoter [4]. Al203 is present in the iron as small Inclusions consisting of FeAl20, in the incompletely reduced catalyst and of Al203 itself after complete reduction. Promoter effects are also found for catalysts consisting of Fe, Co, or Ni which are used for the *To whom correspondence should be sent.
industrial conversion of natural gas or methane-rich gases into synthesis gas or hydrogen [51- Here too, activating agents are usually metals or compounds thereof, of which the oxides are not easily reducible (K20, MgQ A1203 , ___),and much work still has to be done to ascertain the role of these promoters. The present contribution is a continuation of our previous work on dispersed Ni catalysts as studied by ferromagnetic resonance absorption (FMR) [6] and it demonstrates how FMR can be used successfully to gather relevant information on the shape of supported metal crystallites and possible textural promoter effects from the support. It is obviously restricted to ferro- and/or ferri-magnetic compounds. For this purpose, two different systems have been investigated. First is a Ni on 77-A1203 catalyst containing 13 wt.% Ni of which the metal surface area measured by chemisorption is 6 m2 g-l, corresponding to an average crystal size of 14.6 rum in rather good agreement with the size distribution obtained by electron microscopy (6-20 nm). Prior to the FMR measurements, this catalyst was heated under H2 flow at 1130 K for 3 h before being desorbed and sealed off when the vacuum reached 2 X 10m6 torr. This sample 549
Volume
52, number
3
CHEMICAL PHYSKS
LETTERS
15 December 1977
I
3
Fig. 1. Transmission electron micrograph of the Ni-SiOa-A 1273 I(. The number average crystallite size is 13.8 nm.
catalyst after reduction at 953 K and sintering under vacuum at 1173-
Volume 52, number 3
CHEMICAL PHYSICS LETTERS
be referred to as Ni-7pA1203. A Ni-rich catalyst, on SiO, support, was prepared as described previously [3], the syntheric antigorite of Ni @recursor) being compressed to 1 t cm-2 to achieve a partial orientation of the sheet-like crystalites. After desorption at 953 K, the sample is reduced at the same temperature under flowing l-l, during 5 h. It is sealed off under a vacuum of 2 X IOm6 torr and partially sintered at 1173-1273 K. It contains 45 wt.% of Ni of which the number average crystallite size is 13.8 nm, with a major distribution of sizes in the range 6-20 run. Contrarily to what is observed after evacuation and reduction at 973 K (preferential orientation and well-defined hexagonal shape [7] ), the sizes and forms of the Ni particles seems now heterogeneous as shown by transmission electron microscopy (see fig. 1, to be compared to fig. 8 of ref. [7]) and as a result of the higher temperature sintering of the Ni crystallites. This catalyst will be referred to as Ni-SiO,-A. Apart from the nature of the support and the Ni content, both catalysts seem then rather similar if we restrict ourselves will
15 December 1977
perature, B = T’T, (with T, = 631 K as for bulk Ni), in both cases. Xt is seen that the Ni-SiOa-A catalyst shows the same Curie temperature as bulk Ni while the alumina supported catalyst has a Curie temperature shifted to lower values by about 35 K. In view of our former conclusions [6] and of the similarity of both systems, the Curie temperature (T,) shift cannot be attributed to a particle size effect. However, static magnetic measurements indicate that while complete reduction is achieved for SiOz supported Ni at temperztures above 920 K, it is not the case for Ni dispersed on Al,O, _Indeed, in the latter case, typical reduction degree values at 930, 1030, and 1130 K are respectively equal to 90,94, and 97% IS] _Therefore,
to the above data. Fig. 2 shows the variation of the reduced FMR signal intensity, I/IO (f being the intensity at temperature T and f, being the intensity at the lowest measurement temperature), as a function of the reduced tem-
Fig. 2. Variation of the reduced FMR signal intensity (ilr,) as a function of the reduced temperature (T./T& = 6, Tc = 631 K) for the Ni-n-Al203 loo)and the Ni-SiOz-A catalysts (or).
Fig. 3. First derivative FMR spectra of Ni-SiO2 -A at 180, 298, and 480 K for two orientations (i.e., parafiel and perpendicular to the pellet) of the static magnetic field.
551
Volume 52, number 3
CHEMICAL PHYSICS LETTERS
as in our previous work [6], we have to correlate the 7” shift with incomplete reduction of Ni. We believe that, when the Al,O, impregnated with a Ni salt is progressively heated and reduced, some NiA1204 is formed which cannot be reduced at temperatures below about 1250 IL The latter can be present as small indusions in the Ni CrystatIites, after reduction at 1130 K, and accounts for the T, shift. It is tempting, in view of these conclusions, to put forward the idea that as for the Fe synthetic ammonia catalyst, AI,O, is also playing in the present case the role of a textural promoter_ Further info~ation on the Ni-SiOz-A catalyst can also be obtained by looking at the angular dependence of the position of the FMR signal. Namely, one important question is whether or not sintering at 11731273 K completely erases any preferential orientation or shape of the Ni particles as they were observed after reduction at 953 IL Electron microscopy is of no help as it shows rather spherical particles (fig. I). Two extreme orientations of the reduced Ni antigorite peilet can be defined with respect to the external static magnetic field, Ho. In the first one, & is paralfel to the original C-axis and perpendicular to the
(A, B) atomic planes of antigorite. It is then perpendicular to the silica sheets and flat Ni crystallites obtained after reduction at 953 K and perpendicular to the pellet. That orientation will be referred to as HL. The other extreme orientation corresponds to Ho parallel to the original (A, B) plane of antigorite, i.e., parallel to the pellet and it is designated by HE. Fig, 3 shows the first derivative FMR spectra of the Ni-SiO*-A catalyst at various temperatures and for these two orientations of the external magnetic field Ho. Fig 4 represents the angular dependence of the position of the FhIR signal when the pellet is turned around over 360”. The indicated magnetic field value corresponds to the point at which the signal cuts the baseline. It is clear from these data that the FMR signal shifts regularly by about 200 Oe when going progressively from any perpendicular to parallel orientation and that the magnitude of this shift does not depend much on temperature beIow 500 K. That is a clear-cut case of shape anisotropy as it shouId be expected for compressed ellipsoids when their revolution axes are progressively tilted with respect to the external magnetic field, because of the angular dependence of the demagnetizing field [9] .
Fig. 4. Angulardependenceof the position of the FMK signal of the Ni-SiOz-A catalyst.
552
15 December 1937
Volume 52, number 3
CHEMICAL PHYSICS LETTERS
Therefore, we must conclude that even after sintering the Ni crystallites still retain some preferential orientation and shape, a conclusion that could not be inferred from the electron microscopy observations. The FMR spectra also show other anisotropy effects which depend on temperature as possibly due to magnetocrystalline and/or magnetostriction effects. As a conclusion, the present work shows very clearly that FMR can prove to be a very powerful technique in the characterization of metal-support interactions. -Textural promotion results in Curie temperature lowering which is evidenced by plotting the FMR signal intensity as a function of temperature while structural (epitaxial) relationships between the metal particles and the support can be observed as a result of variations in the average demagnetizing field value, provided that partial orientation of the individual (ferromagnetic) crystallites can be achieved and defined with respect to the external static magnetic field. These conclusions prove to be even more general when it is realized that FRM spectra of diluted ferromagnetic materials are very easily obtained using conventional EPR spectrometers, sych as an X-band spectrometer used in the present study. E.G.D. would like to thank Haldor Topple A/S for supplying and partially characterizing the Ni on alumina
15 December 1977
catalyst. J-C-V. acknowledges financial support from the Facult& Universitaires de Namur during his stay in Belgium. Thanks are also due to G.A. Martin (CNRS, Villeurbanne) for most interesting discussions and to Y. Houbion for his technical help.
References [ 1] G. Ertl, Surface Sci. 7 (1967) 309. [2] G. Dalmai-Imelik and J.C. Bertolini, Compt. Rend. Acad. Sci (Paris) 270 C (1970) 1079. [3] G. Dahnai-Imelik, C. Leclercq, J. Massardier, A. MaubertFranc0 and A.Zalhout, Japan. J. Appl. Phys. Suppl. 2, Pt. 2 (1974) 489. [4] H. Topsge, J-A. Dumesic and M. Boudart, J_ Catal. 28 (1973) 477. [S] J.R. Rostrup-Nielsen, Steam reforming catalysts (Teknisk Forlag A/S, Copenhagen, 1975) pp. 27-28. [6] E.G. Derouane, A.J. Simoens,C. Colin, G.A. Martin, J-A. D&non and J-C. VGdrine, J. Catal., submitted for publication. [7] G. Dalmai-Imelik, C. Leclcrcq and A. Maubert-Muguet, J. Solid State Chem. 16 (1976) 129. [8] G.A. Martin, N. Cephalan, Ph. de Montgolfier and B. Imelik, J. Chim. Phys. (1973) 1422; G-A. Martin, private communication. [9] A. Herpin, Thiorie du mamnktisme (Presses Universitaires de France, Paris, 1968) pp. 25-27; ch. XI-C.
553