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Ammonia induced preferential nitriding of molybdenum (111) planes
Surface Science 93 (1980) L143-L146 0 North-Holland Publishing Company
SURFACE SCIENCE LETTERS AMMONIA INDUCED PREFERENTIAL
NITRIDING OF MOLYBDENUM
(111) PLANES G. BERGERET and M. ABON Institut de Recherches sur la Catalyse, CNRS, 2, Gvenue Einstein, F-69626 Villeurbanne Ckdex, France Received 24 October 1979
Field emission patterns show that preferential nitride formation on (111) MO planes results of the interaction with NHa at 750 K. This observation should be related to the high activity of the (111) planes towards catalytic NH3 synthesis or decomposition.
In previous works [ 11, relevant to the mechanism of catalytic decomposition and synthesis of NH3, the interaction of NH3 with MO has been investigated by Field Emission Microscopy (FEM) [2], Thermal Desorption Mass Spectrometry (TDMS) [3] and Electron Stimulated Desorption [4]. The chemisorption of NHs on MO at 200 K lowers the work function @ of the different planes by amounts ranging from -2.25 eV on the (110) plane to -1.55 eV on the (111) plane. These work function changes A$ have been interpreted [5,6] as a result of a molecular chemisorption by partial transfer of the NHa lonepair electrons into the metal. The crystal face specificity towards NH3 chemisorption may be ascribed to variations in ionic contribution to the bond [5]. At 200 K, the NH3 molecule would be the least strongly bonded and the least stable on the (111) plane where the smallest A$ has been measured. It has been actually checked [2] that NH3 decompose at a lower temperature on the (111) plane. This decomposition leaves a nitrogen residue. When the temperature of a MO wire is increased at 600 K during NHa adsorption, TDMS spectra revealed the formation of a new nitrogen (n-N*) and a new hydrogen (n-H*) state which desorb about simultaneously [3]. The v-N2 and v-Ha uptakes increase with ammonia exposure at an equal rate and go through a maximum at an adsorption temperature of 750 K [3]. Peng and Dawson [7] reported the formation of similar Nz and Hz states as a result of NHa interaction with W at 700 K. However, the uptake of nitrogen is much greater on MO than on W and can be as high as 32 X lOI N atoms cm-’ for a 6 X lo4 L NH, exposure at 750 K. Such a large nitrogen amount points clearly to the formation of a bulk state on MO which could be a nitride precursor with incorporation of some hydrogen, Kiperman L143
L144
G. Bergeret,
M. Ahon /Ammonia
induced nitriding of Mofl II)
a loi\\ 211 ?0 \
211*--,
IIP
’
*
,
__w2i
, ‘\
~,OlO
i 121
Fig. 1. MO field emission patterns; (a) clean, and (b)-(d) 750 K,(b) 3 x lo2 L,(c) 1.4 x lo4 L, (d) 3 x lo8 L.
after
different
NH3 exposures
at
and Temkin [8], Hillis et al. [9], Tsuchiya and Osaki [lo] demonstrated that nitride formation results from NH3 interaction with MO at high temperature. More recently, Kawai et al. [l l] showed that the KLL Auger spectrum of surface nitrogen coming from interaction of NH3 with MO at 723 K strongly supports the formation of a nitride. In the present study, FEM has been used in an attempt to see whether nitride formation might display some crystal face specificity. Fig. 1 shows the changes in a MO field emission pattern obtained for increasing
G. Bergeret,
M. Abon f Ammonia
induced nitriding of Mo(ll1)
L145
NH3 exposures on a MO tip at 750 K. Differences between patterns (a) and (b) result from the presence on the tip of a nitrogen surface layer coming from NHa decomposition [1,2]. A very similar pattern with bright (100) planes has been observed [l] following NH3 adsorption at 200 K and heating the MO tip in the 650-950 K temperature range. A larger NH, exposure (1.4 X lo4 L, pattern c) leads to drastic changes. The electron emission is no more smooth but comes from very bright spots located on the (111) planes. For a still larger NH3 exposure (3 X lo8 L, pattern d), these bright spots are more numerous and can be seen, not only on the (111) planes, but also on step regions such as the circular edges of the (110) and (100) planes. As we have recalled above, nitride formation results from the interaction of NH3 with MO at about 750 K [3,7-l 11. The bright spots in the field emission patterns may therefore be ascribed to the progressive formation of nitride crystallites which build up first on the (111) planes and then on step regions. This would mean a preferential nitride formation on the least closely packed regions. The build-up of nitride crystallites leads to a change in the tip geometry with local electric field enhancement and consequent high electronic emission. These changes in the tip geometry explain that reliable work function measurements are no more possible as we have checked. The preferential nitride formation on the (111) MO planes may have some bearings on the catalytic reactions of decomposition or synthesis of NHa. Body centered cubic metals such as Fe, MO, W are efficient catalysts for these reactions which would be structure sensitive [ 121. Brill et al. [13] found that nitrogen is preferentially adsorbed on the (111) Fe plane and that the (100) and (110) planes of the same metal are transformed into (111) plane at 673 K. The synthesis of NH3 on Fe would be more rapid on (111) planes than on other planes [ 141. Dumesic et al. [ 151 reported a nitrogen induced surface reconstruction of iron leading to the creation of so-called C7 sites which exist on the (111) plane. These C7 sites (i.e. surface atoms with seven nearest neighbours) would be more active than others in NH3 synthesis. LGffler and Schmidt [ 161 found an extensive facetting of iron by reaction with NH3 leading to a “surface nitride”. Bozso et al. [ 171 claimed the formation of surface nitrides on Fe(l1 l), whereas such species were not detected on Fe(lOO). McAllister and Hansen [ 181 reported that the rate of decomposition of NH3 on the (111) W plane is higher than on (100) and (110) planes. To sum up, it turns out from studies on Fe [12-171, MO [7-lo] and W [7,18], first, that the catalytic reactions of decomposition or synthesis of NH3 on these metals occur more easily on the (111) planes, and second that the same (111) planes can be readily covered by a nitride or at least a surface nitride. It is also of interest to note that the temperature required for nitride formation on molybdenum (750 K) lies in the temperature range where NH, synthesis or decomposition reactions occurs [ 8-101. The field emission observations reported here therefore give further experi-
G‘. Bergeret, M. Ahon / Animonia induced nitriding of MO (1 II)
L146
mental evidence to the statement that the high activity of the (111) planes of Fe, W and MO towards catalytic NH3 synthesis or decomposition must be related to preferential
nitride
formation
on these planes.
References [l] G. Bergeret, Thesis No. 78-11, Univ. Lyon, France (1978). [2] M. Abon, G. Bergeret and B. Tardy, Surface Sci. 68 (1977) 305. [3] G. Bergeret, B. Tardy and M. Abon, in: Proc. 7th Intern. Vacuum [4] [5] [6] (71 [8] [9] [lo] [Ill 1121 [13] [14] [15] [16] [17] [IS]
Congr. and 3rd Intern. Conf. on Solid Surfaces, Vienna, 1977, p. 1089. M. Abon, B. Tardy and G. Bergeret, Ned. Tijdschr. Vacuumtech. 2-4 (1978) 133. P.T. Dawson and R.S. Hansen, J. Chem. Phys. 48 (1968) 623. M. Wilf and M. Folman, Faraday Trans. I, 72 (1976) 1165. Y.K. Peng and P.T. Dawson, J. Chem. Phys. 54 (1971) 950. S. Kiperman and M.I. Temkin, Acta Physicochim. URSS 21 (1946) 267. M.R. Hillis, C. Kemball and M.W. Roberts, Trans. Faraday Sot. 62 (1966) 3570. S. Tsuchiya and A. Osaki, Bull. Chem. Sot. Japan 42 (1969) 344. T. Kawai, K. Kunimori, T. Kondow, T. Onishi and K. Tamaru, Japan. J. Appl. Phys. Suppl. 2, Pt 2 (1974) 513. J.A. Dumesic, H. Tops@, S. Khammouma and M. Boudart, J. Catalysis 37 (1975) 503. R. Brill, E.-L. Richter and E. Ruth, Angew. Chem. Intern. Ed. Engl. 6 (1967) 882. R. Brill and J. Kurzidim, Colloq. Intern. CNRS 187 (1969) 99. J.A. Dumesic, H. Topsbe and M. Boudart, J. Catalysis 37 (1975) 513. D.G. Liiffler and L.D. Schmidt, J. Catalysis 44 (1976) 244. F. Bozso, G. Ertl, M. Grunze and M. Weiss, J. Catalysis, 49 (1977) 18. J. McAllister and R.S. Hansen, J. Chem. Phys. 59 (1973) 414.
SURFACE SCIENCE LETTERS AMMONIA INDUCED PREFERENTIAL
NITRIDING OF MOLYBDENUM
(111) PLANES G. BERGERET and M. ABON Institut de Recherches sur la Catalyse, CNRS, 2, Gvenue Einstein, F-69626 Villeurbanne Ckdex, France Received 24 October 1979
Field emission patterns show that preferential nitride formation on (111) MO planes results of the interaction with NHa at 750 K. This observation should be related to the high activity of the (111) planes towards catalytic NH3 synthesis or decomposition.
In previous works [ 11, relevant to the mechanism of catalytic decomposition and synthesis of NH3, the interaction of NH3 with MO has been investigated by Field Emission Microscopy (FEM) [2], Thermal Desorption Mass Spectrometry (TDMS) [3] and Electron Stimulated Desorption [4]. The chemisorption of NHs on MO at 200 K lowers the work function @ of the different planes by amounts ranging from -2.25 eV on the (110) plane to -1.55 eV on the (111) plane. These work function changes A$ have been interpreted [5,6] as a result of a molecular chemisorption by partial transfer of the NHa lonepair electrons into the metal. The crystal face specificity towards NH3 chemisorption may be ascribed to variations in ionic contribution to the bond [5]. At 200 K, the NH3 molecule would be the least strongly bonded and the least stable on the (111) plane where the smallest A$ has been measured. It has been actually checked [2] that NH3 decompose at a lower temperature on the (111) plane. This decomposition leaves a nitrogen residue. When the temperature of a MO wire is increased at 600 K during NHa adsorption, TDMS spectra revealed the formation of a new nitrogen (n-N*) and a new hydrogen (n-H*) state which desorb about simultaneously [3]. The v-N2 and v-Ha uptakes increase with ammonia exposure at an equal rate and go through a maximum at an adsorption temperature of 750 K [3]. Peng and Dawson [7] reported the formation of similar Nz and Hz states as a result of NHa interaction with W at 700 K. However, the uptake of nitrogen is much greater on MO than on W and can be as high as 32 X lOI N atoms cm-’ for a 6 X lo4 L NH, exposure at 750 K. Such a large nitrogen amount points clearly to the formation of a bulk state on MO which could be a nitride precursor with incorporation of some hydrogen, Kiperman L143
L144
G. Bergeret,
M. Ahon /Ammonia
induced nitriding of Mofl II)
a loi\\ 211 ?0 \
211*--,
IIP
’
*
,
__w2i
, ‘\
~,OlO
i 121
Fig. 1. MO field emission patterns; (a) clean, and (b)-(d) 750 K,(b) 3 x lo2 L,(c) 1.4 x lo4 L, (d) 3 x lo8 L.
after
different
NH3 exposures
at
and Temkin [8], Hillis et al. [9], Tsuchiya and Osaki [lo] demonstrated that nitride formation results from NH3 interaction with MO at high temperature. More recently, Kawai et al. [l l] showed that the KLL Auger spectrum of surface nitrogen coming from interaction of NH3 with MO at 723 K strongly supports the formation of a nitride. In the present study, FEM has been used in an attempt to see whether nitride formation might display some crystal face specificity. Fig. 1 shows the changes in a MO field emission pattern obtained for increasing
G. Bergeret,
M. Abon f Ammonia
induced nitriding of Mo(ll1)
L145
NH3 exposures on a MO tip at 750 K. Differences between patterns (a) and (b) result from the presence on the tip of a nitrogen surface layer coming from NHa decomposition [1,2]. A very similar pattern with bright (100) planes has been observed [l] following NH3 adsorption at 200 K and heating the MO tip in the 650-950 K temperature range. A larger NH, exposure (1.4 X lo4 L, pattern c) leads to drastic changes. The electron emission is no more smooth but comes from very bright spots located on the (111) planes. For a still larger NH3 exposure (3 X lo8 L, pattern d), these bright spots are more numerous and can be seen, not only on the (111) planes, but also on step regions such as the circular edges of the (110) and (100) planes. As we have recalled above, nitride formation results from the interaction of NH3 with MO at about 750 K [3,7-l 11. The bright spots in the field emission patterns may therefore be ascribed to the progressive formation of nitride crystallites which build up first on the (111) planes and then on step regions. This would mean a preferential nitride formation on the least closely packed regions. The build-up of nitride crystallites leads to a change in the tip geometry with local electric field enhancement and consequent high electronic emission. These changes in the tip geometry explain that reliable work function measurements are no more possible as we have checked. The preferential nitride formation on the (111) MO planes may have some bearings on the catalytic reactions of decomposition or synthesis of NHa. Body centered cubic metals such as Fe, MO, W are efficient catalysts for these reactions which would be structure sensitive [ 121. Brill et al. [13] found that nitrogen is preferentially adsorbed on the (111) Fe plane and that the (100) and (110) planes of the same metal are transformed into (111) plane at 673 K. The synthesis of NH3 on Fe would be more rapid on (111) planes than on other planes [ 141. Dumesic et al. [ 151 reported a nitrogen induced surface reconstruction of iron leading to the creation of so-called C7 sites which exist on the (111) plane. These C7 sites (i.e. surface atoms with seven nearest neighbours) would be more active than others in NH3 synthesis. LGffler and Schmidt [ 161 found an extensive facetting of iron by reaction with NH3 leading to a “surface nitride”. Bozso et al. [ 171 claimed the formation of surface nitrides on Fe(l1 l), whereas such species were not detected on Fe(lOO). McAllister and Hansen [ 181 reported that the rate of decomposition of NH3 on the (111) W plane is higher than on (100) and (110) planes. To sum up, it turns out from studies on Fe [12-171, MO [7-lo] and W [7,18], first, that the catalytic reactions of decomposition or synthesis of NH3 on these metals occur more easily on the (111) planes, and second that the same (111) planes can be readily covered by a nitride or at least a surface nitride. It is also of interest to note that the temperature required for nitride formation on molybdenum (750 K) lies in the temperature range where NH, synthesis or decomposition reactions occurs [ 8-101. The field emission observations reported here therefore give further experi-
G‘. Bergeret, M. Ahon / Animonia induced nitriding of MO (1 II)
L146
mental evidence to the statement that the high activity of the (111) planes of Fe, W and MO towards catalytic NH3 synthesis or decomposition must be related to preferential
nitride
formation
on these planes.
References [l] G. Bergeret, Thesis No. 78-11, Univ. Lyon, France (1978). [2] M. Abon, G. Bergeret and B. Tardy, Surface Sci. 68 (1977) 305. [3] G. Bergeret, B. Tardy and M. Abon, in: Proc. 7th Intern. Vacuum [4] [5] [6] (71 [8] [9] [lo] [Ill 1121 [13] [14] [15] [16] [17] [IS]
Congr. and 3rd Intern. Conf. on Solid Surfaces, Vienna, 1977, p. 1089. M. Abon, B. Tardy and G. Bergeret, Ned. Tijdschr. Vacuumtech. 2-4 (1978) 133. P.T. Dawson and R.S. Hansen, J. Chem. Phys. 48 (1968) 623. M. Wilf and M. Folman, Faraday Trans. I, 72 (1976) 1165. Y.K. Peng and P.T. Dawson, J. Chem. Phys. 54 (1971) 950. S. Kiperman and M.I. Temkin, Acta Physicochim. URSS 21 (1946) 267. M.R. Hillis, C. Kemball and M.W. Roberts, Trans. Faraday Sot. 62 (1966) 3570. S. Tsuchiya and A. Osaki, Bull. Chem. Sot. Japan 42 (1969) 344. T. Kawai, K. Kunimori, T. Kondow, T. Onishi and K. Tamaru, Japan. J. Appl. Phys. Suppl. 2, Pt 2 (1974) 513. J.A. Dumesic, H. Tops@, S. Khammouma and M. Boudart, J. Catalysis 37 (1975) 503. R. Brill, E.-L. Richter and E. Ruth, Angew. Chem. Intern. Ed. Engl. 6 (1967) 882. R. Brill and J. Kurzidim, Colloq. Intern. CNRS 187 (1969) 99. J.A. Dumesic, H. Topsbe and M. Boudart, J. Catalysis 37 (1975) 513. D.G. Liiffler and L.D. Schmidt, J. Catalysis 44 (1976) 244. F. Bozso, G. Ertl, M. Grunze and M. Weiss, J. Catalysis, 49 (1977) 18. J. McAllister and R.S. Hansen, J. Chem. Phys. 59 (1973) 414.