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MgO catalysts. Surface characterization by IR spectra of adsorbed CO
CatalysisToday, 17 ( 1993) 449458 Elsevier Science Publishers B.V., Amsterdam
449
CATTOD 217
High loading Ni/MgO catalysts. Surface characterization by IR spectra of adsorbed CO G. Martraa, F. Arenab, M. Bariccoa, S. Coluccia”, L. Marchesea and A. Parmalianab ‘Dipartimento di Chimica Inorganica. Chimica Fisica e Chimica dei Materiali &ll’Universit& di Torino, via P. Giuria 7, 10125 Torino (Italy) bDipartimentodi Chimica Industriale dell’Universithdi Messina, Salita Sperone c.p. 29, 98166 S. Agata. Messina (Italy)
Abstract
The influence of calcination and reduction conditions on the surface properties of high loading Nil MgO catalysts ( 18 wt% Ni) was studied. IR spectra of adsorbed CO show that the increase of the calcination temperature decreases the reducibilty of the Ni phase as diffusion of Niz+ into the bulk of MgO particles is enhanced. CO adsorption ( 10 Torr) produces various carbonyls (mono- and polycarbonyls, linear and bridged), the distribution of the different types depending on pretreatment conditions. Disruption of metal Ni particles by CO attack on edge and comer sites produces tetra-carbony1 species subsequently stabilized on Mg*+O’- sites. Disproportionation of CO is also observed, monitored by the appearance of bands due to carbonate-like species as a consequence of the adsorption of CO2 on the basic matrix. The activity for the Boudouard reaction decreases upon pretreatments at the highest temperature as the smoothing of metal particles reduces the concentration of Ni sites in the lowest coordination, which promote CO dissociation.
1. INTRODUCTION
Due to the great catalytic interest for Ni-supported systems, and namely for Ni dispersed on oxides, attention has been devoted since many years to their structural characterization [ l-41, showing that surface activity is largely determined by size and morphology of the metal nickel particles. Such parameters depend on a number of factors, the main ones being related to metal loading and pretreatment conditions. Calcination and reduction ambient and temperature play a key role. Ni/MgO systems offer unique opportunities in characterization studies, due to the complete solubility of the precursor NiO in MgO and the consequent Correspondence to: Dr. G. Mar&a, Dipartimento di Chimica Inorganica, Chirnica Fisica e Chimica dei Materiali dell’Universitil di Torino, via P. Giuria 7, 10125 Torino, Italy.
0920-586 l/93/$06.00
0 1993 Elsevier Science Publishers B.V. All rights reserved.
450
G. Martra et al./Catal. Today 17 (1993) 449-458
possibility of tailoring the exposed metal phase by migration, in proper conditions, of nickel from or to the surface of magnesium oxide particles [ 561. Detailed information is available on surface structure of low loading ( l-5 wt% Ni ) solid solutions [ 5 1, which are very useful as model systems but quite different from potentially working catalysts. Recently, experimental work has been devoted to high loading Ni/MgO ( lo-20 wt% Ni) samples, exploring the factors affecting the structural and morphological properties of such systems [ 7 1. The related surface properties and reactivity were investigated by examining CH., steam reforming as a test reaction [ 8 1. Studying the effect of the pretreatment temperature it was concluded that [ 7-81: a) metal surface area and catalytic activity are controlled by the reduction temperature, according to a volcano-shape relationship; b) high metal dispersion negatively affects both activity and stability of the catalysts, suggesting a structure sensitive character of the methane steam reforming reaction; c) high calcination temperature and low Ni loading promote unreducible solid solutions, reducing suitability of Ni/MgO as real catalyst; d) formation of carbon is favoured on small Ni particles, having a high frequency of rough planes. Such evidence encouraged more detailed studies of the evolution of the structure of the catalysts upon progressive changes of the relevant pretreatment parameters and, specifically, of the various families of sites exposed on Ni particles produced in different conditions. Carbon monoxide is a very sensitive probe molecule for metal sites and infrared spectra of adsorbed species derived from CO are rich in information. 2. EXPERIMENTAL
The supported catalyst ( 18 wt% Ni) was prepared by incipent wetness of MgO smoke (UBE Ind. Ltd., Japan, 34 m2g-‘) with nickel acetylacetonate solutions. After drying at 120°C the powder was calcined in air at 400,600 and 800°C. These samples will be referred to as MPF-st, MPF-6 and MPF-8 respectively [ 7 1. Self supporting pellets of the catalysts were placed into IR cells permanently connected to a conventional vacuum line (residual pi 1.0~ low6 Torr, 1 Torr = 133 Nm-2) which allowed all calcination and reduction pretreatment and adsorption-desorption experiments to be carried out in situ. Each sample underwent reduction at progressively increasing temperature (25O”C, 4OO”C, 600°C and SOO’C) in 100 Torr Hz. At each reduction stage the sample was briefly outgassed ( 10 minutes) at the same temperature of reduction, cooled to room temperature and, after running the background spectrum, contacted with 10 Torr CO. The spectra of adsorbed species were run immediately after CO admission and after contact for 60
G. Martra et al./CataI. Today 17 (1993) 449-458
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minutes. IR spectra were run at beam temperature with Bruker IFS48 instrument (256 accumulated scans, resolution 4 cm- l) and are reported in absorbance scale after subtracting the background spectrum of the pellet. High purity gases (Matheson) were used with no further purification except liquid nitrogen trapping. 3. RESULTS
IR spectra of CO adsorbed on metal systems in the 2200- 1800 cm-’ range strongly depend on pressure. Low CO pressure (0.5 Torr ) produces very simple spectra essentially due to linear and bridged monocarbonyls, whereas high pressure ( 10 Torr) produces complex spectra due to polycarbonyls. In previous reports [ 8 ] attention was focused on monocarbonyls and very little was said on polycarbonyls which are considered here. Moreover, bands appear also in the low frequency region ( 1800- 1200 cm- ’ ) . The two ranges will be described and discussed separately. 3.1.2200-18OOcm-‘range Figures l-3 show the dependence of the CO spectra on the following parameters: a) calcination temperature, b) reduction temperature and c) time of contact. As the relevant spectra are extremely rich in bands and shoulders, it would be unproductive to list all of them. Attention will be focused on the main overall aspects of the complex spectral evolution upon changing the pretreatment conditions. CO adsorbed on MPF-st. Fig. 1 refers to the sample calcined at 4OO”C, taken as the standard temperature [ 7-8 ] and then reduced at increasing temperatures. Spectra obtained immediately after CO admission (sect. A) are compared with spectra after 1 hour contact (sect. B ) . The relevant results may be summarized as follows: - significant CO adsorption is observed only after reduction at T, 3 250°C; - the overall intensity of the spectra increases as the reduction temperature increases up to 600°C; a band at 2060 cm-’ is dominant in curves a,b, whereas peaks at 1930, 1975 and 2075 cm-’ are the most intense after reduction at 600’ C (curve c); other definite peaks are observed at 20 10 and 1910 cm-‘; - the intensity decreases when the reduction temperature is further increased up to 800°C (curve d); two broad absorptions with maxima at 2050 and 1975 cm- ’ are now the dominant components. The spectra strongly depend on time of contact, becoming more intense and complex (sect. B); CO adsorption on the sample reduced at 800” C seems to increase less with time (curves d-d’ ) than in previous stages of reduction
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Fig. 1. Sect. A. IR spectra of CO (CO pressure: 10 Tot-r) adsorbed on a Ni/MgO catalyst (MPFst) calcined at 400°C and then reduced in H2 at: a) 250°C; b) 400°C; c) 600°C; d) 800°C. Sect. B. a’, b’, c’, and d’ as a, b, c, and d respectively after 1 hour contact with CO.
Q
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Fig. 2. Sect. A. IR spectra of CO (CO pressure: 10 Torr) adsorbed on a Ni/MgO catalyst (MPF6) calcined at 600°C and then reduced at: a) 400°C; b) 600°C; c) 800°C. Sect. B. a’, b’, and c’ as a, b, and c respectively after 1 hour contact with CO.
(curves a-a’, b-b’ and c-c’ ); a weak band at 2 13 1 cm- ’ is evident after 1 hour of contact of CO with the sample reduced at 600’ C. CO adsorbed on MPF-6. Curves in Fig. 2 show that: - significant CO adsorption on the sample pre-calcined at 600°C starts after reduction at a temperature higher (400°C) than in the case of MPF-st (250°C); - the spectra are very complex, with much the same components observed in
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b
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Fig. 3. Sect.A. IR spectra of CO (CO pressure: 10 Torr) adsorbed on a Ni/MgO catalyst calcined at 800°C and then reduced at: a) 600°C; b) 800°C. Sect. B. a’ and b’ as a and b respectively after 1 hour contact with CO.
Fig. 1, though comparison of the spectra corresponding to the same reduction temperature in Fig. 1 and 2 indicates that the overall intensity is lower in the latter case; - the intensity increases as reduction temperature increases from 400°C up to 600’ C (Fig. 2, curves a,b) and then slightly decreases after reduction at 800°C (curve c); - also in this case prolonged contact with CO (Fig. 2, B) enhances the intensity. CO adsorbed on MPF-8. Fig. 3 shows that: - significant CO adsorption is observed only after reduction at even higher temperature (600” C, curve a); - as compared with Fig. 1 and 2 the spectra immediately after admission of CO (Fig. 3, A) are weaker but still very complex, and various components appear sharper than in Fig. 1 and 2; - the effect of time of contact (sect. B) is comparatively much larger than for MPF-st and MPF-6 catalysts, though the overall intensity is lower in curves a’ and b’ of Fig. 3 than in the parallel curves in Fig. 1 and 2; - both in the spectra immediately after admission of CO (section A) and after 1 hour of contact (section B) the bands at higher frequency are more intense than those at D< 2000 cm- l. All bands described above are very sensitive to the pressure of CO onto the sample; some are favoured by the presence of excess CO, other ones become more intense upon outgassing CO. 3.2.1800-1200
cm-’ range
Following CO adsorption, bands are also observed at low frequencies (Fig. 4). Only their dependence on the reduction temperature of the MPF-st sam-
454
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I
moo
lab0 Wavenumbers
“[xz,
1
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Fig. 4. IR spectra of carbonate-like groups produced by CO disproportionation (CO pressure: 10 Torr) on Ni/MgO catalyst (MPF-st) calcined at 400°C and then reduced in Hz at: a) 250°C; bf 400°C; c) 600°C and d) 800°C.
ple is shown here, for the sake of brevity. Such bands are complex and may be grouped in two sets: a) bands at 1450-l 500 cm - ‘: they are weak and show the same behaviour of bands at high frequency, as their intensity increases with time of contact, decreases upon pumping off CO at room temperature and is restored by re-a~ission of CO, b) bands at 1600-1750 cm-’ and 1250-1350 cm-‘: they are not related to bands at high frequencies, as confirmed by the fact that they are not affected by outgassing at room temperature and depend to a lesser extent on time of contact with CO. Curves a-d show that their intensity increases with the reduction temperature up to 600°C and decreases when the sample is reduced at 800” C. 4. DISCUSSION
Since the pioneering results by Eischens and Pliskin [ 91, infrared spectra of adsorbed CO have been firmly established as extremely sensitive probes of the structure of transition metal surfaces [ 10 1. Due to its sensitivity to oxidation number and coordination of metal cations and atoms, CO is p~i~ularly suitable for investigating nature, shapes and dimension of faces exposed on the microparticles in finely dispersed systems [ lo]. Vast information has been accumulated on Ni supported on a number of oxides, most classically SiOt [ 9- 15 1, though, due to the increasing interest for basic promoters, large attention has also been devoted to Ni/MgO catalysts [ 13,16,17 1. CO adsorbed on unsupported microcrystalline NiO was also investigated as a spectroscopic reference system [ 18,19 1. On such bases, convincing conclusions have been reached both for the as-
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signment of IR bands associated with adsorbed CO and for the mechanisms leading to surface carbonyl species. CO adsorbed on various NY+ cations was also observed [ 17-201, but this is unlikely to contribute significantly to the spectra shown here. In fact, all bands increase with the reduction temperature and, therefore, with the fraction ofNi” [7,8]. Though finger-print ranges for the different carbonyl structures cannot be defined unambiguously, it is broadly accepted [ 9- 17 ] that linear species absorb at P 2 2000 cm- 1 and bridged species at P < 2000 cm- ‘. On the basis of the above mentioned massive literature, the following band assignments may be proposed. 4.1. Band assignments of carbonylicspecies Bands at P > 2000 cm- ‘. The extreme richness of components in this region indicates that a great number of different linear carbonyl species can be produced, the relative population being strongly dependent on pretreatment and adsorption conditions. The spectra obtained immediately after admission of CO show several broad and overlapped components (Fig. 1A, 2A, 3A) typical of monocarbonylic and subcarbonylic species, which may be mono- and polynuclear. The high frequency region is dominated by a band at 2060-2050 cm-’ which is ascribable to linear monocarbonyls formed on the faces of metal particles. Most significantly, these spectra depend on time as shown by the complex evolution of the various components observed comparing sections A and B of Fig. 1-3. The spectra after one hour contact with CO (sections B) show sharper components, suggesting that better defined mononuclear carbonyls are progressively produced by disruption of Ni particles [ 8 1. The new species are similar to well known molecular carbonyls. In fact, two peaks at x 2080 cm- ’ and 2020 cm- ’ show that chemisorbed nickel tetracarbonyl is produced and stabilized as shown in the following scheme [ 17,211:
The weak band at 2 13 1 cm- l, evident in Fig. 1, c’ and in Fig. 2, b’ is likely due to Ni tetracarbonyls with structure different from that shown in the scheme, as described elsewhere [ 221. Bands due to subcarbonyls are present also after prolonged contact, overlapped with those of the newly formed molecular species.
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Bands at 1950-2000 cm- ‘. This is a region where the bands of linear and bridged carbonyls overlap. Desorption experiments show that bands of polycarbonyls species, whose intensity decreases upon outgassing and is readily restored by re-adsorption of CO, also contribute to these absorptions. The tricarbonylic entity of adsorbed Ni ( CO)4, for example, exhibit bands at 1990 and 1975 cm-’ [ 17,211. Bands at 1800- 1950 cm- ‘. Bridged carbonyls Ni,CO (x = 2,3 ) absorb in this region. It is generally accepted that the larger the number of Ni atoms involved, the lower the stretching frequency of the bonded CO [ lo,15 1. Bands at 1450-l 500 cm- ‘. These bands are assigned to the asymmetric stretching vibrations of the carboxylate-like group anchoring the tetracarbony1 species to the MgO surface as shown in the scheme [ 17,2 11, and follow the destiny of their partners at high frequencies in the Ni (CO) 3entity. In fact, their intensities a) increase with time, b) decrease upon decarbonylation and c) are restored by CO readmission [ 17,2 11. Bands of carbonate-like species. Absorptions at 1750- 1600 cm- ’ and 13001400 cm- ’ (Fig. 4) are due to carbonate-like species stabilized on the basic MgO matrix. It was shown that such species are slowly produced when CO is allowed on oxidized nickel supported catalysts [ 171. In the present case, where CO is sent on catalysts pretreated in Hz they are formed by Boudouard reaction on metal Ni” sites: 2co-Co2
+c
as previously proposed [ 8 1. The results in Fig. 4 confirm this hypothesis, showing that the intensity of the carbonate bands, and hence the activity for the disproportionation reaction, increases with the amount of Ni” as the reduction temperature goes up to 600 ’ C (Fig. 4, a-c). The intensity of the bands formed on the sample pre-reduced at the highest temperature (Fig. 4, d) is lower for reasons discussed in the next section. 4.2. Dependence of surface species on surface structure It was shown by catalytic measurements, HRTEM and other surface analyses that the evolution of the structure of the catalyst is determined by the overlap of a number of effects [ 78 ] : a) diffusion of Ni2+ into the bulk of MgO increases as the calcination temperature increases; the system moves more and more towards solid solution and the overall reducibility decreases [ 13,17,18 ] ; b) the metal fraction on NiO/MgO systems increases with the temperature of reduction [ 7-8 ] ;
G. Martra et al./Catal. Today I7 (I 993) 449-458
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c) sintering of the metal phase is favoured at the highest temperatures of reduction, and this increases particle size and progressively enhances surface smoothness [ 8,15,17 1; as a consequence of the annihilation of steps, terraces, kinks, etc., the relative population of Ni sites in low coordination on comers and edges decreases. The spectroscopic results shown here follow the trends described above and well agree with structural data described in detail in ref. 8: a) the overall intensities of the spectra decrease from Fig. 1 to Fig. 3 because increasing calcination temperature promotes diffusion of nickel into the bulk and reduces the actual amount of Ni2+ available for subsequent reduction on the surface of MgO particles [ 8,17 ] ; b) in the experiments in Fig. 1 and Fig. 2 the intensities of the spectra increase as the reduction temperature increases up to 600 oC together with the progressively increasing amount of Ni”; the intensity decreases in the last step of reduction at 800°C because of the competing and winning effect of sintering which, even in the presence of larger fractions of Ni’, decreases the effective surface area of the metal phase [ 8 ] ; the bands at low frequency (D< 2000 cm- l) become more important as cl reduction temperature increases because bridged carbonyls are favoured by increasing sizes of metal particles [ 8,12- 17 ] ; d) the spectra after reduction at 800°C tend to be simpler; this indicates lower surface heterogeneity due to the enhanced regularity of the metal particles caused by sintering [ 7,8,15 1; the spectra tend to become simpler also upon increasing calcination teme) perature (Fig. 3 ) , possibly reflecting the shift towards more homogeneous solid solutions [ 1,17,23], which decreases the reducible amount of nickel on the surface; tetracarbonyls are essentially formed by CO attack to step and comer sites of small particles; f ) the intensity of carbonate bands decreases when the sample is reduced at 800°C because the smoothing of metal particles reduces the fraction of sites in low coordination on steps and comers able to dissociate CO, which is one step of the disproportionation reaction [ 8 1.
ACKNOWLEDGEMENTS
Financial support by MURST and CNR and fruitful discussion with Prof. A. Zecchina are gratefully acknowledged.
REFERENCES 1
G.C. Bond and S.P. Sarsam, Appl. Catal., 38 (1988)
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G. Martra et al./Catal. Today 17 (1993) 449-458 C.H. Bartholomew and W.L. Sorensen, J. Catal., 8 1 ( 1983 ) 13 1. S. Narayanan and K. Uma, J. Chem. Sot. Faraday Trans. 1,8 1 ( 1985) 2733. H. Tamagawa, K. Gyama and T. Yamaguchi, J. Chem. Sot. Faraday Trans. 1,83 (1987) 3189. J.G. Highfield, A. Bossi and F.S. Stone, in ‘Preparation of Catalysts III’ (G. Poncelet, P. Grange andP.A. Jacobs, eds.), Elsevier, Amsterdam, The Netherlands, 1983, p. 181. N. Takezawa, H. Terenuma, M. Shimokawabe and H. Kobayashi, Appl. Catal., 23 ( 1986) 291. F. Arena, B.A. Horrel, D.L. Cocke, A. Parmaliana and N. Giordano, J. Catal., 132 ( 1991) 58. A. Parmaliana, F. Arena, F. Frusteri, S. Coluccia, L. Marchese, G. Martra and A.L. Chuvilin, J. Catal., 141 (1993) 34. R.P. Eischens and W.A. Pliskin, Advances in Catalysis, Academic Press, New York, 1958, vol. 10, p. 1. N. Sheppard and T.T. Nguyen, in ‘Advances in Infrared and Raman Spectroscopy’ (P.J. Clarke and R.E. Hester eds. ), Wiley, New York, vol. 4, p. 67. G. Blyholder and D.O. Bowen, J. Chem. Phys, 66 (1962) 1288. M. Primet, J.A. Dalmon and G.A. Martin, J. Catal., 46 (1977) 25. C.H. Rochester and R.J. Terrell, J. Chem. Sot. Faraday Trans. 1,73 (1977) 609. C.H. Bartholomew and R.B. Pannell, J. Catal., 65 ( 1980) 390. D.G. Blackmond and E.I. Ko, J. Catal., 96 ( 1985) 210; M.V. Ciocco and D.G. Blackmond, Appl. Catal., 44 (1988) 105. A. Chiorino, E. Garrone, G. Ghiotti , E. Guglieminotti and A. Zecchina, in ‘Proc. 7ti Int. Cong. Catal.’ (T. Seiyama and K. Tanabe eds. ) , Elsevier, Amsterdam, 198 1, p. 136. A. Zecchina, G. Spoto, S. Coluccia and E. Guglielminotti, J. Chem. Sot. Faraday Trans. 1, 80 (1984) 1875; ib., 80 (1984) 1891. E. Escalona Platero, S. Coluccia and A. Zecchina, Langmuir, 1 ( 1985) 407; Surf. Sci., 171 (1986) 465. M.W. Roberts and R. St.C. Smart, Surf. Sci, 100 ( 1980) 590; J. Chem. Sot. Faraday Trans. 1,80 (1984) 2957. M. Kermarec, D. Delafosse and M. Che, J. Chem. Sot. Chem. Comm., ( 1983) 411. E. Guglielminotti, A. Zecchina, F. Boccuzzi, in ‘Growth and Properties of Metal Clusters’ (J. Bourdon ed.), Elsevier, Amsterdam, The Netherlands, 1980, p. 165. G. Martra, S. Coluccia and L. Marchese, submitted to Catalysis Letters. A.P. Hagan, M.G. Lofthouse, ES. Stone and M.A. Trevethan, in ‘Preparation of Catalysts II’ (B. Delmon, P. Grange, P.A. Jacobs and G. Poncclet eds. ), Elsevier, Amsterdam, The Netherlands, 1979, p. 4 17.
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High loading Ni/MgO catalysts. Surface characterization by IR spectra of adsorbed CO G. Martraa, F. Arenab, M. Bariccoa, S. Coluccia”, L. Marchesea and A. Parmalianab ‘Dipartimento di Chimica Inorganica. Chimica Fisica e Chimica dei Materiali &ll’Universit& di Torino, via P. Giuria 7, 10125 Torino (Italy) bDipartimentodi Chimica Industriale dell’Universithdi Messina, Salita Sperone c.p. 29, 98166 S. Agata. Messina (Italy)
Abstract
The influence of calcination and reduction conditions on the surface properties of high loading Nil MgO catalysts ( 18 wt% Ni) was studied. IR spectra of adsorbed CO show that the increase of the calcination temperature decreases the reducibilty of the Ni phase as diffusion of Niz+ into the bulk of MgO particles is enhanced. CO adsorption ( 10 Torr) produces various carbonyls (mono- and polycarbonyls, linear and bridged), the distribution of the different types depending on pretreatment conditions. Disruption of metal Ni particles by CO attack on edge and comer sites produces tetra-carbony1 species subsequently stabilized on Mg*+O’- sites. Disproportionation of CO is also observed, monitored by the appearance of bands due to carbonate-like species as a consequence of the adsorption of CO2 on the basic matrix. The activity for the Boudouard reaction decreases upon pretreatments at the highest temperature as the smoothing of metal particles reduces the concentration of Ni sites in the lowest coordination, which promote CO dissociation.
1. INTRODUCTION
Due to the great catalytic interest for Ni-supported systems, and namely for Ni dispersed on oxides, attention has been devoted since many years to their structural characterization [ l-41, showing that surface activity is largely determined by size and morphology of the metal nickel particles. Such parameters depend on a number of factors, the main ones being related to metal loading and pretreatment conditions. Calcination and reduction ambient and temperature play a key role. Ni/MgO systems offer unique opportunities in characterization studies, due to the complete solubility of the precursor NiO in MgO and the consequent Correspondence to: Dr. G. Mar&a, Dipartimento di Chimica Inorganica, Chirnica Fisica e Chimica dei Materiali dell’Universitil di Torino, via P. Giuria 7, 10125 Torino, Italy.
0920-586 l/93/$06.00
0 1993 Elsevier Science Publishers B.V. All rights reserved.
450
G. Martra et al./Catal. Today 17 (1993) 449-458
possibility of tailoring the exposed metal phase by migration, in proper conditions, of nickel from or to the surface of magnesium oxide particles [ 561. Detailed information is available on surface structure of low loading ( l-5 wt% Ni ) solid solutions [ 5 1, which are very useful as model systems but quite different from potentially working catalysts. Recently, experimental work has been devoted to high loading Ni/MgO ( lo-20 wt% Ni) samples, exploring the factors affecting the structural and morphological properties of such systems [ 7 1. The related surface properties and reactivity were investigated by examining CH., steam reforming as a test reaction [ 8 1. Studying the effect of the pretreatment temperature it was concluded that [ 7-81: a) metal surface area and catalytic activity are controlled by the reduction temperature, according to a volcano-shape relationship; b) high metal dispersion negatively affects both activity and stability of the catalysts, suggesting a structure sensitive character of the methane steam reforming reaction; c) high calcination temperature and low Ni loading promote unreducible solid solutions, reducing suitability of Ni/MgO as real catalyst; d) formation of carbon is favoured on small Ni particles, having a high frequency of rough planes. Such evidence encouraged more detailed studies of the evolution of the structure of the catalysts upon progressive changes of the relevant pretreatment parameters and, specifically, of the various families of sites exposed on Ni particles produced in different conditions. Carbon monoxide is a very sensitive probe molecule for metal sites and infrared spectra of adsorbed species derived from CO are rich in information. 2. EXPERIMENTAL
The supported catalyst ( 18 wt% Ni) was prepared by incipent wetness of MgO smoke (UBE Ind. Ltd., Japan, 34 m2g-‘) with nickel acetylacetonate solutions. After drying at 120°C the powder was calcined in air at 400,600 and 800°C. These samples will be referred to as MPF-st, MPF-6 and MPF-8 respectively [ 7 1. Self supporting pellets of the catalysts were placed into IR cells permanently connected to a conventional vacuum line (residual pi 1.0~ low6 Torr, 1 Torr = 133 Nm-2) which allowed all calcination and reduction pretreatment and adsorption-desorption experiments to be carried out in situ. Each sample underwent reduction at progressively increasing temperature (25O”C, 4OO”C, 600°C and SOO’C) in 100 Torr Hz. At each reduction stage the sample was briefly outgassed ( 10 minutes) at the same temperature of reduction, cooled to room temperature and, after running the background spectrum, contacted with 10 Torr CO. The spectra of adsorbed species were run immediately after CO admission and after contact for 60
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minutes. IR spectra were run at beam temperature with Bruker IFS48 instrument (256 accumulated scans, resolution 4 cm- l) and are reported in absorbance scale after subtracting the background spectrum of the pellet. High purity gases (Matheson) were used with no further purification except liquid nitrogen trapping. 3. RESULTS
IR spectra of CO adsorbed on metal systems in the 2200- 1800 cm-’ range strongly depend on pressure. Low CO pressure (0.5 Torr ) produces very simple spectra essentially due to linear and bridged monocarbonyls, whereas high pressure ( 10 Torr) produces complex spectra due to polycarbonyls. In previous reports [ 8 ] attention was focused on monocarbonyls and very little was said on polycarbonyls which are considered here. Moreover, bands appear also in the low frequency region ( 1800- 1200 cm- ’ ) . The two ranges will be described and discussed separately. 3.1.2200-18OOcm-‘range Figures l-3 show the dependence of the CO spectra on the following parameters: a) calcination temperature, b) reduction temperature and c) time of contact. As the relevant spectra are extremely rich in bands and shoulders, it would be unproductive to list all of them. Attention will be focused on the main overall aspects of the complex spectral evolution upon changing the pretreatment conditions. CO adsorbed on MPF-st. Fig. 1 refers to the sample calcined at 4OO”C, taken as the standard temperature [ 7-8 ] and then reduced at increasing temperatures. Spectra obtained immediately after CO admission (sect. A) are compared with spectra after 1 hour contact (sect. B ) . The relevant results may be summarized as follows: - significant CO adsorption is observed only after reduction at T, 3 250°C; - the overall intensity of the spectra increases as the reduction temperature increases up to 600°C; a band at 2060 cm-’ is dominant in curves a,b, whereas peaks at 1930, 1975 and 2075 cm-’ are the most intense after reduction at 600’ C (curve c); other definite peaks are observed at 20 10 and 1910 cm-‘; - the intensity decreases when the reduction temperature is further increased up to 800°C (curve d); two broad absorptions with maxima at 2050 and 1975 cm- ’ are now the dominant components. The spectra strongly depend on time of contact, becoming more intense and complex (sect. B); CO adsorption on the sample reduced at 800” C seems to increase less with time (curves d-d’ ) than in previous stages of reduction
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I
lo’ Wavenumberr
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Fig. 1. Sect. A. IR spectra of CO (CO pressure: 10 Tot-r) adsorbed on a Ni/MgO catalyst (MPFst) calcined at 400°C and then reduced in H2 at: a) 250°C; b) 400°C; c) 600°C; d) 800°C. Sect. B. a’, b’, c’, and d’ as a, b, c, and d respectively after 1 hour contact with CO.
Q
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)
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2:
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Fig. 2. Sect. A. IR spectra of CO (CO pressure: 10 Torr) adsorbed on a Ni/MgO catalyst (MPF6) calcined at 600°C and then reduced at: a) 400°C; b) 600°C; c) 800°C. Sect. B. a’, b’, and c’ as a, b, and c respectively after 1 hour contact with CO.
(curves a-a’, b-b’ and c-c’ ); a weak band at 2 13 1 cm- ’ is evident after 1 hour of contact of CO with the sample reduced at 600’ C. CO adsorbed on MPF-6. Curves in Fig. 2 show that: - significant CO adsorption on the sample pre-calcined at 600°C starts after reduction at a temperature higher (400°C) than in the case of MPF-st (250°C); - the spectra are very complex, with much the same components observed in
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b
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-,
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cm
Fig. 3. Sect.A. IR spectra of CO (CO pressure: 10 Torr) adsorbed on a Ni/MgO catalyst calcined at 800°C and then reduced at: a) 600°C; b) 800°C. Sect. B. a’ and b’ as a and b respectively after 1 hour contact with CO.
Fig. 1, though comparison of the spectra corresponding to the same reduction temperature in Fig. 1 and 2 indicates that the overall intensity is lower in the latter case; - the intensity increases as reduction temperature increases from 400°C up to 600’ C (Fig. 2, curves a,b) and then slightly decreases after reduction at 800°C (curve c); - also in this case prolonged contact with CO (Fig. 2, B) enhances the intensity. CO adsorbed on MPF-8. Fig. 3 shows that: - significant CO adsorption is observed only after reduction at even higher temperature (600” C, curve a); - as compared with Fig. 1 and 2 the spectra immediately after admission of CO (Fig. 3, A) are weaker but still very complex, and various components appear sharper than in Fig. 1 and 2; - the effect of time of contact (sect. B) is comparatively much larger than for MPF-st and MPF-6 catalysts, though the overall intensity is lower in curves a’ and b’ of Fig. 3 than in the parallel curves in Fig. 1 and 2; - both in the spectra immediately after admission of CO (section A) and after 1 hour of contact (section B) the bands at higher frequency are more intense than those at D< 2000 cm- l. All bands described above are very sensitive to the pressure of CO onto the sample; some are favoured by the presence of excess CO, other ones become more intense upon outgassing CO. 3.2.1800-1200
cm-’ range
Following CO adsorption, bands are also observed at low frequencies (Fig. 4). Only their dependence on the reduction temperature of the MPF-st sam-
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I
moo
lab0 Wavenumbers
“[xz,
1
cm
Fig. 4. IR spectra of carbonate-like groups produced by CO disproportionation (CO pressure: 10 Torr) on Ni/MgO catalyst (MPF-st) calcined at 400°C and then reduced in Hz at: a) 250°C; bf 400°C; c) 600°C and d) 800°C.
ple is shown here, for the sake of brevity. Such bands are complex and may be grouped in two sets: a) bands at 1450-l 500 cm - ‘: they are weak and show the same behaviour of bands at high frequency, as their intensity increases with time of contact, decreases upon pumping off CO at room temperature and is restored by re-a~ission of CO, b) bands at 1600-1750 cm-’ and 1250-1350 cm-‘: they are not related to bands at high frequencies, as confirmed by the fact that they are not affected by outgassing at room temperature and depend to a lesser extent on time of contact with CO. Curves a-d show that their intensity increases with the reduction temperature up to 600°C and decreases when the sample is reduced at 800” C. 4. DISCUSSION
Since the pioneering results by Eischens and Pliskin [ 91, infrared spectra of adsorbed CO have been firmly established as extremely sensitive probes of the structure of transition metal surfaces [ 10 1. Due to its sensitivity to oxidation number and coordination of metal cations and atoms, CO is p~i~ularly suitable for investigating nature, shapes and dimension of faces exposed on the microparticles in finely dispersed systems [ lo]. Vast information has been accumulated on Ni supported on a number of oxides, most classically SiOt [ 9- 15 1, though, due to the increasing interest for basic promoters, large attention has also been devoted to Ni/MgO catalysts [ 13,16,17 1. CO adsorbed on unsupported microcrystalline NiO was also investigated as a spectroscopic reference system [ 18,19 1. On such bases, convincing conclusions have been reached both for the as-
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signment of IR bands associated with adsorbed CO and for the mechanisms leading to surface carbonyl species. CO adsorbed on various NY+ cations was also observed [ 17-201, but this is unlikely to contribute significantly to the spectra shown here. In fact, all bands increase with the reduction temperature and, therefore, with the fraction ofNi” [7,8]. Though finger-print ranges for the different carbonyl structures cannot be defined unambiguously, it is broadly accepted [ 9- 17 ] that linear species absorb at P 2 2000 cm- 1 and bridged species at P < 2000 cm- ‘. On the basis of the above mentioned massive literature, the following band assignments may be proposed. 4.1. Band assignments of carbonylicspecies Bands at P > 2000 cm- ‘. The extreme richness of components in this region indicates that a great number of different linear carbonyl species can be produced, the relative population being strongly dependent on pretreatment and adsorption conditions. The spectra obtained immediately after admission of CO show several broad and overlapped components (Fig. 1A, 2A, 3A) typical of monocarbonylic and subcarbonylic species, which may be mono- and polynuclear. The high frequency region is dominated by a band at 2060-2050 cm-’ which is ascribable to linear monocarbonyls formed on the faces of metal particles. Most significantly, these spectra depend on time as shown by the complex evolution of the various components observed comparing sections A and B of Fig. 1-3. The spectra after one hour contact with CO (sections B) show sharper components, suggesting that better defined mononuclear carbonyls are progressively produced by disruption of Ni particles [ 8 1. The new species are similar to well known molecular carbonyls. In fact, two peaks at x 2080 cm- ’ and 2020 cm- ’ show that chemisorbed nickel tetracarbonyl is produced and stabilized as shown in the following scheme [ 17,211:
The weak band at 2 13 1 cm- l, evident in Fig. 1, c’ and in Fig. 2, b’ is likely due to Ni tetracarbonyls with structure different from that shown in the scheme, as described elsewhere [ 221. Bands due to subcarbonyls are present also after prolonged contact, overlapped with those of the newly formed molecular species.
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Bands at 1950-2000 cm- ‘. This is a region where the bands of linear and bridged carbonyls overlap. Desorption experiments show that bands of polycarbonyls species, whose intensity decreases upon outgassing and is readily restored by re-adsorption of CO, also contribute to these absorptions. The tricarbonylic entity of adsorbed Ni ( CO)4, for example, exhibit bands at 1990 and 1975 cm-’ [ 17,211. Bands at 1800- 1950 cm- ‘. Bridged carbonyls Ni,CO (x = 2,3 ) absorb in this region. It is generally accepted that the larger the number of Ni atoms involved, the lower the stretching frequency of the bonded CO [ lo,15 1. Bands at 1450-l 500 cm- ‘. These bands are assigned to the asymmetric stretching vibrations of the carboxylate-like group anchoring the tetracarbony1 species to the MgO surface as shown in the scheme [ 17,2 11, and follow the destiny of their partners at high frequencies in the Ni (CO) 3entity. In fact, their intensities a) increase with time, b) decrease upon decarbonylation and c) are restored by CO readmission [ 17,2 11. Bands of carbonate-like species. Absorptions at 1750- 1600 cm- ’ and 13001400 cm- ’ (Fig. 4) are due to carbonate-like species stabilized on the basic MgO matrix. It was shown that such species are slowly produced when CO is allowed on oxidized nickel supported catalysts [ 171. In the present case, where CO is sent on catalysts pretreated in Hz they are formed by Boudouard reaction on metal Ni” sites: 2co-Co2
+c
as previously proposed [ 8 1. The results in Fig. 4 confirm this hypothesis, showing that the intensity of the carbonate bands, and hence the activity for the disproportionation reaction, increases with the amount of Ni” as the reduction temperature goes up to 600 ’ C (Fig. 4, a-c). The intensity of the bands formed on the sample pre-reduced at the highest temperature (Fig. 4, d) is lower for reasons discussed in the next section. 4.2. Dependence of surface species on surface structure It was shown by catalytic measurements, HRTEM and other surface analyses that the evolution of the structure of the catalyst is determined by the overlap of a number of effects [ 78 ] : a) diffusion of Ni2+ into the bulk of MgO increases as the calcination temperature increases; the system moves more and more towards solid solution and the overall reducibility decreases [ 13,17,18 ] ; b) the metal fraction on NiO/MgO systems increases with the temperature of reduction [ 7-8 ] ;
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c) sintering of the metal phase is favoured at the highest temperatures of reduction, and this increases particle size and progressively enhances surface smoothness [ 8,15,17 1; as a consequence of the annihilation of steps, terraces, kinks, etc., the relative population of Ni sites in low coordination on comers and edges decreases. The spectroscopic results shown here follow the trends described above and well agree with structural data described in detail in ref. 8: a) the overall intensities of the spectra decrease from Fig. 1 to Fig. 3 because increasing calcination temperature promotes diffusion of nickel into the bulk and reduces the actual amount of Ni2+ available for subsequent reduction on the surface of MgO particles [ 8,17 ] ; b) in the experiments in Fig. 1 and Fig. 2 the intensities of the spectra increase as the reduction temperature increases up to 600 oC together with the progressively increasing amount of Ni”; the intensity decreases in the last step of reduction at 800°C because of the competing and winning effect of sintering which, even in the presence of larger fractions of Ni’, decreases the effective surface area of the metal phase [ 8 ] ; the bands at low frequency (D< 2000 cm- l) become more important as cl reduction temperature increases because bridged carbonyls are favoured by increasing sizes of metal particles [ 8,12- 17 ] ; d) the spectra after reduction at 800°C tend to be simpler; this indicates lower surface heterogeneity due to the enhanced regularity of the metal particles caused by sintering [ 7,8,15 1; the spectra tend to become simpler also upon increasing calcination teme) perature (Fig. 3 ) , possibly reflecting the shift towards more homogeneous solid solutions [ 1,17,23], which decreases the reducible amount of nickel on the surface; tetracarbonyls are essentially formed by CO attack to step and comer sites of small particles; f ) the intensity of carbonate bands decreases when the sample is reduced at 800°C because the smoothing of metal particles reduces the fraction of sites in low coordination on steps and comers able to dissociate CO, which is one step of the disproportionation reaction [ 8 1.
ACKNOWLEDGEMENTS
Financial support by MURST and CNR and fruitful discussion with Prof. A. Zecchina are gratefully acknowledged.
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