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Is methanation on nickel structure sensitive?
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Is methanation on nickel structure sensitive?
In the early 1980s, turnover frequencies and activation energies for methanation were found to be very similar on Ni(100) and Ni(111) single crystals and nickelalumina catalysts and this observation has been taken as convincing evidence that the reaction can be classed as structure insensitive [see D.W. Goodman, Surf. Sci., 299/300 (1994) 837]. Other work, however, points to the opposite conclusion. It has been reported, for example, that ion bombardment of Ni(111) can increase the methanation rate by up to 400%, indicating a sensitivity to structure on, at least, the local scale of surface defects [A. Berko and H.P. Bonzel, Surf. Sci., 251/252 (1991) 1112]. The picture is complicated further by the well known fact that carbon is deposited on the surface under reaction conditions. Below ca. 600 K, this carbon takes a carbidic form which is thought to participate actively, as a whole or in part, in the catalytic process; above 700 K, it is graphitic and poisons reactivity to hydrogen. Because carbon causes nickel surfaces to reconstruct, the validity of any conclusion regarding structure sensitivity depends critically on the structural details of the carbon-nickel surface layers. Despite considerable effort over the last twenty five years, these details have remained rather elusive. The best characterised case has been Ni(100). Carbon atoms at low coverages occupy the hollow site at the centre of four Ni atoms and the latter are displaced radially, allowing C to sit low enough in the surface to increase its coordination by contacting a metal atom in the second applied catalysis A: General
layer. At higher local coverages, the Ni atoms can no longer be pushed away radially and, instead, the system undergoes a peculiar "clock" reconstruction in which the squares of Ni atoms surrounding each carbon rotate in the plane of the surface in a way that allows the adatom to maintain an increased five-fold coordination. On the close-packed Ni(111) surface, the first LEED investigation of carbon, reported in 1969, established the symmetry of the unit cell but there have been relatively few other studies. Because the cell is very large, the present capabilities of LEED are insufficient to determine the atom positions within it but these have now been revealed by the first scanning tunnelling microscope study of the (111) carbidic phase [C. Klink et al., Surf. Sci, 342 (1995) 250]. The STM images, resolving both C and Ni atoms, confirm the cell proposed in 1969 and show that, at the detailed level, Ni(111) is subject to the same clock reconstruction as Ni(100). The C-induced reconstruction on Ni(100) involves only local displacements of metal atoms in the surface layer while on Ni(111) there is a small reduction in the surface Ni atom density (with the excess atoms diffusing to steps which are present on all surfaces as defects), In contrast, recent STM and ion scattering results provide the first experimental evidence that reconstruction of the more open, "channelled" Ni(110) surface involves large scale mass transport of metal and that the carbon does not lie inthe surface layer [C. Klink et al., Surf. Sci., 360 (1996) 171]. These observations lead to a model involving carbon sandwiched between the first two Ni layers in two types of local environment, one of which is very similar to that in bulk
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nickel carbide, Ni3C. The interesting point is that the second environment is equivalent to that of the five-fold coordinated C in the clock reconstruction (except that the cluster of nickel atoms around a carbon is inverted in the plane of the surface). Instead of seeing the similarity of methanation rates over various nickel surfaces as a sign of structure insensitivity, these new results reinforce an alternative view proposed previously on the basis of LEED data. Highly dispersed catalysts are expected to expose mainly low index facets, such as (100) and (111). Hence, the fact that the C-induced reconstruction is locally identical on these two planes (and quite similar on Ni(110)) suggests that methanation, over catalysts and single crystals alike, takes place on essentially the same clock-reconstructed type surface. If this were to be indeed the case, the insensitivity of rate to the structure of clean nickel surfaces would be seen to be apparent rather than real. There is, however, another possibility. A recent microkinetic model for CO methanation on nickel indicates that the rate determining step is the hydrogenation of CH species [1. AIstrup, J. Catal., 151 (1995) 216]. It is also known that only a small part of the carbon present on the surface under reaction conditions participates directly in the reaction and that the clock reconstruction is inert for hydrogen dissociation. Thus, it is not very likely that methanation occurs on extended, well ordered clock domains and Klink et al. (op. cit.) suggest that some other type of site, present on all Ni surfaces under reaction conditions, may be involved. Whether or how such sites might relate to the clock reconstruction is, of course, unknown and so the whole
applied catalysis A: General
question of methanation structure sensitivity remains in the balance. CS. McKee
Surface Explosions On certain transition metals, an interesting phenomenon is observed during two types of temperature- programmed reaction, the oxidation of CO(ad) by NO(ad) and the decomposition of adsorbed formic or acetic acids. In each case, product desorption begins quite suddenly as the temperature reaches some particular value and is completed within a very small temperature range, giving desorption peaks up to ten times narrower than those in typical thermal desorption spectra. This "surface explosion" is taken to be indicative of an autocatalytic mechanism and was first noticed over twenty years ago in the case of formic acid decomposition on Ni(110). It was later reported for the same reaction on Ni(111), for acetate on Ni(110) and for CO/NO on Pt(100). In the last few years renewed interest in the subject has revealed further examples involving these various reactants on low-index ruthenium, rhodium and palladium surfaces and a Rh/AI203 supported catalyst. These observations have been explained in terms of a variety of reaction mechanisms, including coverage dependent activation energies, surface reconstruction, adsorbate phase transitions and vacancy requirement models. On the hexagonal close-packed Ru(0001) surface, for example, the desorption temperature of CO2 produced by formate decomposition decreases if the initial formate coverage is decreased [Y.K. Sun and W.H. Weinberg, J. Chem. Phys., 94 (1991) 4587]. This apparent change in acVolume 147 No. 1 - - 19 November 1996
Is methanation on nickel structure sensitive?
In the early 1980s, turnover frequencies and activation energies for methanation were found to be very similar on Ni(100) and Ni(111) single crystals and nickelalumina catalysts and this observation has been taken as convincing evidence that the reaction can be classed as structure insensitive [see D.W. Goodman, Surf. Sci., 299/300 (1994) 837]. Other work, however, points to the opposite conclusion. It has been reported, for example, that ion bombardment of Ni(111) can increase the methanation rate by up to 400%, indicating a sensitivity to structure on, at least, the local scale of surface defects [A. Berko and H.P. Bonzel, Surf. Sci., 251/252 (1991) 1112]. The picture is complicated further by the well known fact that carbon is deposited on the surface under reaction conditions. Below ca. 600 K, this carbon takes a carbidic form which is thought to participate actively, as a whole or in part, in the catalytic process; above 700 K, it is graphitic and poisons reactivity to hydrogen. Because carbon causes nickel surfaces to reconstruct, the validity of any conclusion regarding structure sensitivity depends critically on the structural details of the carbon-nickel surface layers. Despite considerable effort over the last twenty five years, these details have remained rather elusive. The best characterised case has been Ni(100). Carbon atoms at low coverages occupy the hollow site at the centre of four Ni atoms and the latter are displaced radially, allowing C to sit low enough in the surface to increase its coordination by contacting a metal atom in the second applied catalysis A: General
layer. At higher local coverages, the Ni atoms can no longer be pushed away radially and, instead, the system undergoes a peculiar "clock" reconstruction in which the squares of Ni atoms surrounding each carbon rotate in the plane of the surface in a way that allows the adatom to maintain an increased five-fold coordination. On the close-packed Ni(111) surface, the first LEED investigation of carbon, reported in 1969, established the symmetry of the unit cell but there have been relatively few other studies. Because the cell is very large, the present capabilities of LEED are insufficient to determine the atom positions within it but these have now been revealed by the first scanning tunnelling microscope study of the (111) carbidic phase [C. Klink et al., Surf. Sci, 342 (1995) 250]. The STM images, resolving both C and Ni atoms, confirm the cell proposed in 1969 and show that, at the detailed level, Ni(111) is subject to the same clock reconstruction as Ni(100). The C-induced reconstruction on Ni(100) involves only local displacements of metal atoms in the surface layer while on Ni(111) there is a small reduction in the surface Ni atom density (with the excess atoms diffusing to steps which are present on all surfaces as defects), In contrast, recent STM and ion scattering results provide the first experimental evidence that reconstruction of the more open, "channelled" Ni(110) surface involves large scale mass transport of metal and that the carbon does not lie inthe surface layer [C. Klink et al., Surf. Sci., 360 (1996) 171]. These observations lead to a model involving carbon sandwiched between the first two Ni layers in two types of local environment, one of which is very similar to that in bulk
Volume 147 No. 1 - - 19 November 1996
N3
nickel carbide, Ni3C. The interesting point is that the second environment is equivalent to that of the five-fold coordinated C in the clock reconstruction (except that the cluster of nickel atoms around a carbon is inverted in the plane of the surface). Instead of seeing the similarity of methanation rates over various nickel surfaces as a sign of structure insensitivity, these new results reinforce an alternative view proposed previously on the basis of LEED data. Highly dispersed catalysts are expected to expose mainly low index facets, such as (100) and (111). Hence, the fact that the C-induced reconstruction is locally identical on these two planes (and quite similar on Ni(110)) suggests that methanation, over catalysts and single crystals alike, takes place on essentially the same clock-reconstructed type surface. If this were to be indeed the case, the insensitivity of rate to the structure of clean nickel surfaces would be seen to be apparent rather than real. There is, however, another possibility. A recent microkinetic model for CO methanation on nickel indicates that the rate determining step is the hydrogenation of CH species [1. AIstrup, J. Catal., 151 (1995) 216]. It is also known that only a small part of the carbon present on the surface under reaction conditions participates directly in the reaction and that the clock reconstruction is inert for hydrogen dissociation. Thus, it is not very likely that methanation occurs on extended, well ordered clock domains and Klink et al. (op. cit.) suggest that some other type of site, present on all Ni surfaces under reaction conditions, may be involved. Whether or how such sites might relate to the clock reconstruction is, of course, unknown and so the whole
applied catalysis A: General
question of methanation structure sensitivity remains in the balance. CS. McKee
Surface Explosions On certain transition metals, an interesting phenomenon is observed during two types of temperature- programmed reaction, the oxidation of CO(ad) by NO(ad) and the decomposition of adsorbed formic or acetic acids. In each case, product desorption begins quite suddenly as the temperature reaches some particular value and is completed within a very small temperature range, giving desorption peaks up to ten times narrower than those in typical thermal desorption spectra. This "surface explosion" is taken to be indicative of an autocatalytic mechanism and was first noticed over twenty years ago in the case of formic acid decomposition on Ni(110). It was later reported for the same reaction on Ni(111), for acetate on Ni(110) and for CO/NO on Pt(100). In the last few years renewed interest in the subject has revealed further examples involving these various reactants on low-index ruthenium, rhodium and palladium surfaces and a Rh/AI203 supported catalyst. These observations have been explained in terms of a variety of reaction mechanisms, including coverage dependent activation energies, surface reconstruction, adsorbate phase transitions and vacancy requirement models. On the hexagonal close-packed Ru(0001) surface, for example, the desorption temperature of CO2 produced by formate decomposition decreases if the initial formate coverage is decreased [Y.K. Sun and W.H. Weinberg, J. Chem. Phys., 94 (1991) 4587]. This apparent change in acVolume 147 No. 1 - - 19 November 1996