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Dissociation of carbon monoxide on stepped nickel surfaces
Surface Science 118( 1982) L28 1-L285 North-Holland Publishing Company
SURFACE
SCIENCE
L281
LETTERS
DISSOCIATION SURFACES
OF CARBON
MONOXIDE
ON STEPPED
Z. MURAYAMA
*, I. KOJIMA
**, E. MIYAZAKI
NICKEL
and 1. YASUMORI
Deparrment of Chemistry, Tokyo Institute of Technology, Ookaynma, Meguro-ku, Tokyo 152, Japan Received
27 January
1982
Field emission microscopy with photometric measurements has been applied to investigate the adsorption of carbon monoxide on various crystal planes of a nickel field emitter. Upon heating the CO-covered surface, the work function drastically decreased during desorptjon of CO molecules into the gas phase and exhibited almost the value of the clean surface at 450 K. However, part of the CO molecules adsorbed on the stepped planes such as (510) and (310) were found to dissociate upon heating at 450-470 K, which was accompanied by an increase of the work function of -0.2 eV.
The adsorption of carbon monoxide on nickel has extensively been investigated by using various surface techniques. The adsorption states of CO are of importance for the catalytic reactions such as methanation, Fischer-Tropsch synthesis, and CO oxidation. It is generally supported that room temperature exposure leads to molecular adsorption on various low index planes of nickel, but recently, several investigators reported that CO dissociates on the surface with structural defects such as steps and kinks [1,2]. Further, Erley et al. observed a rather low stretching frequency of 1520 cm-’ for CO adsorbed on the stepped Ni, indicating a weakening of the C-O bond [3]. However, it has not been established on which oriented planes CO molecules are more likely to dissociate and at which temperature they can decompose. Field emission microscopy (FEM) has the advantage of providing information on the properties of various crystal planes existing on the spherical surface of the field emitter under identical experimental conditions. Especially combining this technique with a probe-hole measurement, more detailed information about active sites on which a surface reaction takes place may be obtained. However, so far no available FEM results have been reported on the CO/Ni FEM measurement of adsorptios system. We describe here a photometric ,
* Present address: Chemical Research Laboratory, Takasago Heavy Industries Ltd., Shinhama, Arai-cho, Takasago, Hyogo ** To whom correspondence should be addressed.
0039-6028/82/0000-OOOO/$O2.75
Technical Institute, Pref. 676, Japan.
0 1982 North-Holland
Mitsubishi
L282
Z. Muruyama et al. / Dissociation of CO on stepped NI surfaces
individual regions of the tip for CO adsorption on Ni, indicating that CO can dissociate on the stepped planes around 470 K. A glass FEM tube with a flat fluorescent screen was employed for the present study [4]. The photometric measurements were carried out as follows: The light from the small area of the fluorescent screen was permitted to fall on a photoelectron multiplier tube, Hamamatsu TV Co., R647, passing through an aperture of 1 mm diameter and double screens with light-holes. The output current was amplified and recorded as a function of the applied voltage. The work function of a certain plane, +, is computed from +-7i2 = $i/’ + bl/, log( iO/i), where b is constant, i is the output current for the adsorbate-covered surface and i, is the current for the clean surface with the work function of & when the voltage, YO,is applied to the tip 151. Experimental details will be described elsewhere [6]. Figs. la and lb show the disposition of the crystal planes and the field emission pattern for a clean nickel emitter with the central (211) plane, respectively. When the clean emitter was exposed to carbon monoxide of 5 X lo-” Torr for 10 min at 295 K, the work function averaged over the surface increased by 1.2 eV and the emission pattern showed some additional
Fig. 1. Field emission patterns of nickel: (a) disposition of p!anes of the fee metal emitter; (b) (21 I)-oriented clean nickel emitter; (c) after adsorption of CO at 295 K; (d) heating at 450 K; (e) 690 K, and 780 K.
2. Murayama et al. / Dissociation
of CO on stepped Ni surfaces
L283
emission anisotropy as given by fig. lc. The value 1.2 eV agrees with the previously reported value for Ni surfaces with adsorbed CO and it is established that the value is a characteristic one due to molecularly adsorbed CO species on Ni. After evacuating the molecules remaining in the gas phase, the CO-covered surface was heated at various temperatures for 20 s each. The emission patterns after heating are given in figs. Id-lf. The resulting changes in work function of several crystal planes are shown in fig. 2, where the light-pass aperture was located at the (lOO), (5 lo), (310) and (110) planes. The work function, $I, rapidly decreased with heating to 375 K in the low index planes such as (110) and (lOO), and finally recovered the values close to those for the clean surface above 400 K. The temperature necessary for restoring the original work function is higher on the (100) plane than on the (110) plane. This indicates that molecularly adsorbed CO species were almost completely desorbed by heating the flat surface, which is in good agreement with the previous results reported by others [7-91. On the other hand, the relatively rough surfaces consisting of terraces and steps or kinks exhibited different behavior in +-change with heating: In the case of the (510) and (310) planes, as shown in fig. 2, $I greatly decreased with heating up to 390 K indicating desorption of a part of the CO molecules, but subsequent heating to 450-470 K caused an increase in $ by about 0.2 eV. This increase can be associated with the formation of adsorbed oxygen atoms due to the C-O bond rupture: The presence of oxygen atoms on the emitter is consistent with the pattern with locally enhanced emission after heating the CO-covered surface up to 690 K (fig. le). It is generally known that the heat treatment of the surface with adsorbed oxygen brings about some local
-0.4
-
temperature/ K
Fig. 2. Changes in work function, A+, of the CO-covered surface with heating: (510) plane; (- - -) (310) plane; (-.-.-) (110) plane. plane; ( -)
(. * . . .) (100)
L2X4
Z. Murayama et al. / Dissociation of CO on stepped Ni surfaces
rearrangement of the substrate surface leading to the formation of definitely oriented oxide layers and that these layers enhance the local field and largely increase electron emission [lo]. Therefore, the ring-shaped emission surrounding the (100) plane, which appeared at 590 K (fig. le), is attributed to the local formation of an oxide layer. This may also be responsible for the apparent +-decrease to a negative level between 600-800 K on the (310) plane (fig. 2). The partly oxidized layer thus produced was rather stable up to 770 K, however, it decomposed on heating to higher temperatures, which was accompanied with some contraction of the bright ring toward the (100) plane (fig. 1f). The trace of the oxide layer disappeared completely at 850 K and the emission pattern resembled that of the clean surface. At this temperature, + also recovered the corresponding values of the original clean surfaces. These results strongly suggest that the oxygen atoms recombine with carbon atoms to desorb as CO molecules at 770-850 K. By means of ultra-violet photoelectron spectroscopy, Eastman et al. found that CO adsorbs as a molecular state at room temperature and decomposes by heating up to 450 K on a sputter-damaged Ni(ll1) surface [l]. Erley et al. reported that thermal desorption of CO on the stepped Ni(S)[S( 111) X (1 IO)] surface yielded two different peaks of desorption: the a-peak which is characteristic of molecularly adsorbed CO species on the smooth Ni( 111) surface, and the &-peak around 820 K which is specific to the stepped surface [2]. The latter peak was assigned to the associative CO desorption. Consequently, they concluded that steps lower the activation energy for CO decomposition but increase the activation energy for associative desorption [2]. The present measurements show that CO dissociates on the (100) terrace-plus-step sites at 450-470 K. This temperature is well in accord with the result obtained on the sputter-damaged surface. As shown in fig. 2, the larger G-increase on the (510) plane than on the (310) plane suggests that there is an optimum width of (100) terraces for CO dissociation. The desorption temperature of &-species, 820 K, observed on the stepped surface by Erley et al. [2] is well correlated with the present temperature.range, 770-850 K, around which $ changed and finally recovered to the original values on the stepped planes of the emitter. These considerations lead to the conclusion that CO adsorbed on Ni surfaces is possible to decompose on the clean surface with steps or kinks at around 470 K and that associative desorption of CO occurs around 770-850 K restoring the clean surface. Furthermore, the results obtained here suggest that steps and kinks play an important role in the reactions including CO dissociation processes such as methanation and Fischer-Tropsch synthesis.
References [l] D.E. Eastman, J.E. Demuth and J.M. Baker, J. Vacuum 12) W. Erley and H. Wagner, Surface Sci. 74 (1978) 333.
Sci. Technol.
11 (1974) 273.
Z. Murayama ei al. / Dissociarion oj CO on stepped Ni surfaces
[3] [4] [5] [6] [7] [8] [9]
[lo]
L285
W. Erley, H. Ibach, S. Lehwald and H. Wagner, Surface Sci. 83 (1979) 585. I. Kojima, E. Miyazaki and 1. Yasumori, Appl. Surface Sci. 6 (1980) 93. A.J. Becker, Solid State Phys. 7 (1958) 379. To be published. J.C. Tracy, J. Chem. Phys. 56 (1972) 2736. K. Christmann, 0. Schober and G. Ertl, J. Chem. Phys. 60 (1974) 4719. J.C. Bertolini and B. Tardy, Surface Sci. 102 (1981) 131. R. Comer, in: Field Emission and Field Ionization (Harvard University Press, Cambridge, MA, 1961).
SURFACE
SCIENCE
L281
LETTERS
DISSOCIATION SURFACES
OF CARBON
MONOXIDE
ON STEPPED
Z. MURAYAMA
*, I. KOJIMA
**, E. MIYAZAKI
NICKEL
and 1. YASUMORI
Deparrment of Chemistry, Tokyo Institute of Technology, Ookaynma, Meguro-ku, Tokyo 152, Japan Received
27 January
1982
Field emission microscopy with photometric measurements has been applied to investigate the adsorption of carbon monoxide on various crystal planes of a nickel field emitter. Upon heating the CO-covered surface, the work function drastically decreased during desorptjon of CO molecules into the gas phase and exhibited almost the value of the clean surface at 450 K. However, part of the CO molecules adsorbed on the stepped planes such as (510) and (310) were found to dissociate upon heating at 450-470 K, which was accompanied by an increase of the work function of -0.2 eV.
The adsorption of carbon monoxide on nickel has extensively been investigated by using various surface techniques. The adsorption states of CO are of importance for the catalytic reactions such as methanation, Fischer-Tropsch synthesis, and CO oxidation. It is generally supported that room temperature exposure leads to molecular adsorption on various low index planes of nickel, but recently, several investigators reported that CO dissociates on the surface with structural defects such as steps and kinks [1,2]. Further, Erley et al. observed a rather low stretching frequency of 1520 cm-’ for CO adsorbed on the stepped Ni, indicating a weakening of the C-O bond [3]. However, it has not been established on which oriented planes CO molecules are more likely to dissociate and at which temperature they can decompose. Field emission microscopy (FEM) has the advantage of providing information on the properties of various crystal planes existing on the spherical surface of the field emitter under identical experimental conditions. Especially combining this technique with a probe-hole measurement, more detailed information about active sites on which a surface reaction takes place may be obtained. However, so far no available FEM results have been reported on the CO/Ni FEM measurement of adsorptios system. We describe here a photometric ,
* Present address: Chemical Research Laboratory, Takasago Heavy Industries Ltd., Shinhama, Arai-cho, Takasago, Hyogo ** To whom correspondence should be addressed.
0039-6028/82/0000-OOOO/$O2.75
Technical Institute, Pref. 676, Japan.
0 1982 North-Holland
Mitsubishi
L282
Z. Muruyama et al. / Dissociation of CO on stepped NI surfaces
individual regions of the tip for CO adsorption on Ni, indicating that CO can dissociate on the stepped planes around 470 K. A glass FEM tube with a flat fluorescent screen was employed for the present study [4]. The photometric measurements were carried out as follows: The light from the small area of the fluorescent screen was permitted to fall on a photoelectron multiplier tube, Hamamatsu TV Co., R647, passing through an aperture of 1 mm diameter and double screens with light-holes. The output current was amplified and recorded as a function of the applied voltage. The work function of a certain plane, +, is computed from +-7i2 = $i/’ + bl/, log( iO/i), where b is constant, i is the output current for the adsorbate-covered surface and i, is the current for the clean surface with the work function of & when the voltage, YO,is applied to the tip 151. Experimental details will be described elsewhere [6]. Figs. la and lb show the disposition of the crystal planes and the field emission pattern for a clean nickel emitter with the central (211) plane, respectively. When the clean emitter was exposed to carbon monoxide of 5 X lo-” Torr for 10 min at 295 K, the work function averaged over the surface increased by 1.2 eV and the emission pattern showed some additional
Fig. 1. Field emission patterns of nickel: (a) disposition of p!anes of the fee metal emitter; (b) (21 I)-oriented clean nickel emitter; (c) after adsorption of CO at 295 K; (d) heating at 450 K; (e) 690 K, and 780 K.
2. Murayama et al. / Dissociation
of CO on stepped Ni surfaces
L283
emission anisotropy as given by fig. lc. The value 1.2 eV agrees with the previously reported value for Ni surfaces with adsorbed CO and it is established that the value is a characteristic one due to molecularly adsorbed CO species on Ni. After evacuating the molecules remaining in the gas phase, the CO-covered surface was heated at various temperatures for 20 s each. The emission patterns after heating are given in figs. Id-lf. The resulting changes in work function of several crystal planes are shown in fig. 2, where the light-pass aperture was located at the (lOO), (5 lo), (310) and (110) planes. The work function, $I, rapidly decreased with heating to 375 K in the low index planes such as (110) and (lOO), and finally recovered the values close to those for the clean surface above 400 K. The temperature necessary for restoring the original work function is higher on the (100) plane than on the (110) plane. This indicates that molecularly adsorbed CO species were almost completely desorbed by heating the flat surface, which is in good agreement with the previous results reported by others [7-91. On the other hand, the relatively rough surfaces consisting of terraces and steps or kinks exhibited different behavior in +-change with heating: In the case of the (510) and (310) planes, as shown in fig. 2, $I greatly decreased with heating up to 390 K indicating desorption of a part of the CO molecules, but subsequent heating to 450-470 K caused an increase in $ by about 0.2 eV. This increase can be associated with the formation of adsorbed oxygen atoms due to the C-O bond rupture: The presence of oxygen atoms on the emitter is consistent with the pattern with locally enhanced emission after heating the CO-covered surface up to 690 K (fig. le). It is generally known that the heat treatment of the surface with adsorbed oxygen brings about some local
-0.4
-
temperature/ K
Fig. 2. Changes in work function, A+, of the CO-covered surface with heating: (510) plane; (- - -) (310) plane; (-.-.-) (110) plane. plane; ( -)
(. * . . .) (100)
L2X4
Z. Murayama et al. / Dissociation of CO on stepped Ni surfaces
rearrangement of the substrate surface leading to the formation of definitely oriented oxide layers and that these layers enhance the local field and largely increase electron emission [lo]. Therefore, the ring-shaped emission surrounding the (100) plane, which appeared at 590 K (fig. le), is attributed to the local formation of an oxide layer. This may also be responsible for the apparent +-decrease to a negative level between 600-800 K on the (310) plane (fig. 2). The partly oxidized layer thus produced was rather stable up to 770 K, however, it decomposed on heating to higher temperatures, which was accompanied with some contraction of the bright ring toward the (100) plane (fig. 1f). The trace of the oxide layer disappeared completely at 850 K and the emission pattern resembled that of the clean surface. At this temperature, + also recovered the corresponding values of the original clean surfaces. These results strongly suggest that the oxygen atoms recombine with carbon atoms to desorb as CO molecules at 770-850 K. By means of ultra-violet photoelectron spectroscopy, Eastman et al. found that CO adsorbs as a molecular state at room temperature and decomposes by heating up to 450 K on a sputter-damaged Ni(ll1) surface [l]. Erley et al. reported that thermal desorption of CO on the stepped Ni(S)[S( 111) X (1 IO)] surface yielded two different peaks of desorption: the a-peak which is characteristic of molecularly adsorbed CO species on the smooth Ni( 111) surface, and the &-peak around 820 K which is specific to the stepped surface [2]. The latter peak was assigned to the associative CO desorption. Consequently, they concluded that steps lower the activation energy for CO decomposition but increase the activation energy for associative desorption [2]. The present measurements show that CO dissociates on the (100) terrace-plus-step sites at 450-470 K. This temperature is well in accord with the result obtained on the sputter-damaged surface. As shown in fig. 2, the larger G-increase on the (510) plane than on the (310) plane suggests that there is an optimum width of (100) terraces for CO dissociation. The desorption temperature of &-species, 820 K, observed on the stepped surface by Erley et al. [2] is well correlated with the present temperature.range, 770-850 K, around which $ changed and finally recovered to the original values on the stepped planes of the emitter. These considerations lead to the conclusion that CO adsorbed on Ni surfaces is possible to decompose on the clean surface with steps or kinks at around 470 K and that associative desorption of CO occurs around 770-850 K restoring the clean surface. Furthermore, the results obtained here suggest that steps and kinks play an important role in the reactions including CO dissociation processes such as methanation and Fischer-Tropsch synthesis.
References [l] D.E. Eastman, J.E. Demuth and J.M. Baker, J. Vacuum 12) W. Erley and H. Wagner, Surface Sci. 74 (1978) 333.
Sci. Technol.
11 (1974) 273.
Z. Murayama ei al. / Dissociarion oj CO on stepped Ni surfaces
[3] [4] [5] [6] [7] [8] [9]
[lo]
L285
W. Erley, H. Ibach, S. Lehwald and H. Wagner, Surface Sci. 83 (1979) 585. I. Kojima, E. Miyazaki and 1. Yasumori, Appl. Surface Sci. 6 (1980) 93. A.J. Becker, Solid State Phys. 7 (1958) 379. To be published. J.C. Tracy, J. Chem. Phys. 56 (1972) 2736. K. Christmann, 0. Schober and G. Ertl, J. Chem. Phys. 60 (1974) 4719. J.C. Bertolini and B. Tardy, Surface Sci. 102 (1981) 131. R. Comer, in: Field Emission and Field Ionization (Harvard University Press, Cambridge, MA, 1961).