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Field emission study of ammonia adsorption and catalytic decomposition on individual molybdenum planes
Surface Science 68 (1977) 305-311 0 North-Holland Publishing Company
FIELD EMISSION STUDY OF AMMONIA ADSORPTION AND CATALYTIC DECOMPOSITION ON INDIVIDUAL MOLYBDENUM PLANES M. ABON, G. BERGERET and B. TARDY Institut de Recherches
sur la Catalyse, 69626
Villeurbanne Cbdex, France
A probe-hole field emission microscope was used to investigate the crystallographic specificity of ammonia adsorption at 200 and 300 K on (llO), (loo), (211) and (111) molybdenum crystal planes. Chemisorbed NH3 causes a large work function decrease, especially at 200 K in agreement with an associative adsorption model which can also explain that this decrease is more important on the crystal planes of highest work function (At 200 K, A@= -2.25 eV on Mo(ll0) compared to A$ = -1.55 eV on MO (111). The decomposition of NH3 was followed by measuring the work function changes for stepwise heating of the MO tip covered with NH3 at 200 K. On the four studied planes NH3 decomposition and Hz desorption are completed at about 400 K. A@ changes above 400 K depend on the crystal plane and have been related to two different nitrogen surface states. No inactive plane towards NH3 adsorption and decomposition has been found but the noted crystallographic anisotropy in this low pressure study is relevant to the structure sensitive character of the NH3 decomposition and synthesis reactions.
1. Introduction It is frequently assumed that catalytic synthesis or decomposition of ammonia are structure sensitive reactions. According to Boudart et al. [ 11, the (111) plane of iron would be particularly active in NH3 synthesis. McAllister and Hansen [2] have reported that the rate of NH, decomposition is higher on the (111) plane than on the (100) and (110) planes of tungsten. It has also been suggested that the (100) plane of W should be the most active [3,4] and the (110) plane the least [5,6] in the adsorption and decomposition of NH3. A crystal plane specificity in the adsorption of NH, may be therefore expected. In the present work, the chemisorption and subsequent thermal decomposition of NH3 has been investigated on the (1 lo), (loo), (211) and (111) molybdenum planes by “probe-hole” field emission microscopy (FEM). MO has the same (bee) structure as Fe and W, and is also a very efficient catalyst for the decomposition and synthesis of NH, [7-lo]. 2. Experimental The fully bakeable “probe-hole” field emission microscope used in this work has been described previously [ 1 l-l 31. Changes in the average or local work function 305
306
M. Abon et al. /Ammonia
adsorption on molybdenum
planes
of the MO crystal during NH, adsorption or decomposition were determined from measurements of the total or local field emission current-voltage characteristics using the Fowler-Nordheim (FN) equation [14,15]. A base pressure of lo-” Torr was obtained using an ion pump and a titanium sublimator. Anhydrous high purity “NH3 was admitted from a supply bottle via a metal leak valve. The purity of this gas was checked using a quadrupole mass spectrometer mounted in the vicinity of the microscope. Decomposition of *‘NH3 gave 30N2 so that contamination by 28C0 could be detected and was found to be low.
3. Ammonia planes
adsorption at 200 and 300K
on (llO),
(lOO), (211) and (111) MO
At 200 K and in high vacuum there should not be any physical NH3 adsorption [ 161. The tip was cleaned by heating at 1800 K and cooled in vacuum [17,18] before the introduction of 10v6 Torr NH, for 2 min. To eliminate NH3 decomposition on hot filaments, the ionisation gauge and the mass spectrometer were turned off during the experiment. Measurements were made after NH3 pumping. 3.1. Results After NH3 adsorption, the field emission pattern shows a bright emission around the (100) planes as previously described for NH3 on W by Dawson and Hansen
1161. Results are summarized in table 1. Relative work functions #o of clean MO single-crystal planes have been determined in a previous work [12]. This table shows that NH3 adsorption results in a large work function decrease especially at 200 K, in good agreement with related works on W [4,5,16,19]. NH3 adsorption causes also a decrease in the FN pre-exponential term, A log A being between -1 and -2 for total emission and individual planes.
Table 1 Relative clean MO work function 90, atom densities adsorption at 200 K and 300 K at full coverage Total emission Go (eV) no (1014 atoms/cm2) A@ (eV) - 200 K A@ (eV) - 300 K A@ (200 K)/A@ (300 K)
4.20 -1.80 -1.0 1.8
no and work function
(110)
(100)
5.00 14.3 ~ 2.25 _ 1.2 1.9
4.45 10.1 - 1.95 _ 1.1 1.8
changes
(211)
4.60 8.28 -1.90 -1.0 1.9
A@ by NH3
(111)
4.20 5.86 -1.55 -0.9 1.7
M. Abon et al. /Ammonia
adsorption on molybdenum
planes
307
3.2. Ammonia adsorption model At 200 K, total dissociation of NH3 would give H and N adsorbed species which is incompatible with the observed work function decrease [20]. Partial dissociation leading to NH or NH2 may be considered, but according to Gutman et al. [21], these species on MO would be electron acceptors and would therefore increase the work function. An associative chemisorption model with bonding by partial transfer of the NH3 lone-pair electrons into the vacant orbitals of the metal seems the most likely interpretation [4,5,16]. However, the bonding would be mainly covalent, in contrast to ionic potassium adsorption which leads to an increase of log A [22] whereas the observed decrease is usual for covalent type gas chemisorption on metals [ 16,201. Additional experimental evidence for the presence even at 300 K of a molecular NH3 adsorbed complex comes from recent spectroscopic studies [23,24]. However there is a sharp reduction in the work function decrease at 300 K compared to 200 K (table 1) which may be explained by the presence on the surface at 300 K of products coming from the decomposition of a fraction of the NH, adsorbed molecules [ 16,241. 3.3 Crystal plane specificity Dawson and Hansen [ 161 demonstrated that NH3 decomposition occurs before surface diffusion on W. Then NH3 migration should not explain that no inactive plane towards NH3 adsorption was found in this study. The nearly constant value of the ratio A4 (200 K)/A@ (300 K) (table 1) gives some support to the statement that NH3 surface migration on MO is also unsignificant up to 300 K and shows that the same kind of crystal plane specificity exists at 200 and 300 K. Owing to strong repulsion between dipoles and to steric reasons [25], the NH3 population should not depend very much on crystal plane. The anisotropy in A$ may be therefore tentatively ascribed mainly to differences in dipole moment p. High A$ would correspond to high 1-1and then to strong bonding. NH, would be more strongly held on Mo(ll0) than on Mo(ll1).
4. Work function changes with temperature At 200 K, the tip was first fully covered with NH3. After 2 h pumping, the tip was stepwise heated for 60 set at increasing temperatures. NH3 is very difficult to pump and owing to re-adsorption @‘NH3 in the 10e9 Torr range), A@ data (fig. 1) were obtained using the “single point” method [15]. Results are in general agreement with FEM studies of NH3 decomposition on W by Dawson and Hansen [16] and Wilf and Folman [ 191. In these studies, the microscope itself was immersed in liquid helium [ 161 or liquid nitrogen [ 191. It is suggested that NH3 re-adsorption which occured in the present work where cryogenic pumping was not used, might
308
M. Abon et al. /Ammonia
200
Fig. 1. Work function
400
600
200
400
600
changes
for stepwise
adsorption on molybdenum
800
600
heating
for totalemission and for (llO), (loo), (2ll)and
account plane.
for the differences
1000
1000
planes
1200
1200
of the MO tip covered
with NH3 at 200 K
(111) planes.
which concern mainly the total emission and the (100)
4. I. Workfunction changes in the 200-400
K temperature range
The important rise in A@ which occurs between 200 and 400 K (fig. 1) indicates NH3 decomposition followed by Hz desorption, in agreement with thermal desorption results [26]. A near simultaneous decomposition is observed on all planes
M. Abon et al. /Ammonia adsorption on molybdenum planes
309
studied though decomposition is easier on (111) plane where A$I = 0 at 350 K instead of N 400 K on the other planes. NH, surface migration is likely to be unsignificant as stated before and then cannot account for the very similar behaviour of the different planes. The observed broad temperature range of NHs decomposition might be explained by a large variation in the activation energy of decomposition with coverage. Wilf and Folman [19] observed the same A@ rise though somewhat sharper on some planes such as the W(211). It is considered that the differences with our results are not great enough to suggest that NH3 re-adsorption can explain the gradual character of the rise in A@ 4.2. Work function changes above 400 K Nitrogen is more electronegative than MO and its presence explains the positive h@ maximum observed at 600-700 K on all planes studied. Above 700 K, nitrogen migration followed by desorption at = 1000 K might account for the observed evolution of the curves. However, a negative A@ minimum is observed only on the (100) plane at 950 K. Thermal desorption experiments [26] showed that NH, readsorption can provide an interpretation to the FEM results. In addition to the high temperature P-nitrogen peak, a low temperature nitrogen peak with a greater total nitrogen coverage results of NHa re-adsorption on a MO filament first covered with NH3 at 240 K and then heated up to 750 K. On W, this low temperature nitrogen state has been designated “X-nitrogen” by Matsushita and Hansen [25,27] and ‘%-nitrogen” by Peng and Dawson [28]. The (0 + X)-nitrogen would be electronegative [25,33] and its gradual formation by NH3 re-adsorption above 350 K would explain the rise in A@ and the positive A@ maximum at 600-700 K on all the planes. Above 700 K, this (0 t X)-structure would be progressively destroyed with a partial desorption of nitrogen giving near 950 K a /l-nitrogen state. This transition would account for the decrease in A$ above 700 K and for the negative minimum only observed on the (100) plane at 950 K. It is well known that P-nitrogen decreases the work function of W (100) [30-321. General agreement is found with the results of Wilf and Folman [19] with the main exception of the W(100) plane where a positive A+ maximum was not observed. As stated before, NH3 re-adsorption on W(100) at 78 K was probably very limited and NHs decomposition directly leaves above 400 K the electropositive p2 state which lowers A@ by = 0.9 eV [ 191 instead of 0.4 eV on Mo(l00) (fig. 1). On further exposure to NHs and heating to 800 K a less strongly-bonded /3,-state which raises A$ by 0.15 eV on W(100) has been observed [ 191. It is not clear whether one may relate this &-state to the X-nitrogen state. Wilf and Folman found also this &-state even when molecular nitrogen is adsorbed on W(100) [31], but it has been demonstrated [25,28] that the X-state (or b-state) cannot result of non-activated molecular nitrogen interaction with W. Thermal desorption experiments [26] supported the formation of the X-nitrogen state in good agreement with related works on MO [27] and W [25,28].
310
M. Abon et al. /Ammonia
adsorption on molvbdenum
planes
5. Conclusion NH3 chemisorption on MO leads to a large work function decrease especially at 200 K. A# results support molecular NHs adsorption at 200 K whereas at 300 K a fraction of the NH3 layer may be decomposed. The observed crystal face specificity has been tentatively related to dipole moments variations. The dipole associated with the NH3 adsorbed molecule would be the greatest on high work function planes. This would mean that NH3 is more strongly adsorbed on the (110) plane than on other planes and especially on (11 I) plane. it is also worth stressing that no inactive plane towards NH3 adsorption has been found. NH3 decomposition followed by Ha desorption may explain the Atg rise between 200 and 400 K. Above this temperature, NH3 interaction with the ~-nitrogen deposit left on the surface would give on all the crystal planes an electronegative (p f X)-structure (A$ > 0) of a greater nitrogen surface coverage than the P-layer. Above 700 K, partial nitrogen desorption leaves a @-nitrogen layer: A# >0 for all the crystal planes with the exception of the (100) where A~#J = -0.4 eV. Results have been compared with related works especially with the study of NH3 on W by Wilf and Folman [ 19 1. This low pressure study on MO crystal planes of high cleanliness showed that the only species left on the surface above 400 K would be nitrogen and this observation may be related with h&h pressure kinetic studies which usually favoured adsorption-desorption of nitrogen as rate limiting step in the catalytic synthesis or decomposition of NH3 [9,36]. The strong crystal face specificity towards Na adsorption [30,343 gives the impression that the (100) plane may be the most active in NH3 synthesis or decomposition reaction. This work shows that NH3 may be indeed adsorbed and decomposed on the (100) plane as on other planes but NH3 decomposition would be easier on the (111) plane.
References [l f [2] [3] [4] IS] [6]
J.A. Dumesic, H. Topsde and M. Boudart, 5. Catalysis 37 (1975) 513. J. McAllister and R.S. Hansen, J. Chem. Phys. 59 (1973) 414. P.T. Dawson and R.S. Hansen, J. Chem. Phys. 45 (1966) 3148. P.J. Estrup and J. Anderson, J. Chem. Phys. 49 (1968) 523. J.W. May, R.J. Szostak and L.H. Cermer, Surface Sci. 15 (1969) 37. R.C.A. Contaminard, R.C. Cosser and F.C. Tompkins, in: Adsorptjon-Desorpt~on Phenomena, Ed. F. Ricca (Academic Press, London, 1972) p. 29 1. [7] S. Kiperman and M.I. Temkin, Acta Physicochim. USSR 2i (1946) 267. [8] M.R. Hillis, C. Kcmball and M.W. Roberts, Trans Faraday Sot. 62 (1966) 3570. [9] W.G. Frankenbur~, in: Catalysis HI, Ed. Emmett (Reinhold, New York, 1955), p. 18.5. [lo] S. Tsuchiya and A. Ozaki, Bull. Chem. Sot. Japan 42 (1969) 344. [ 111 G. Bergeret, M. Abon and S.J. Teichner, J. Chim. Phys. 10 (1974) 1299. [ 121 G. Bergeret, M. Abon, B. Tardy and S.J. Teichner, J. Vacuum Sci. Technol. 11 (1974) 1193. [ 131 G. Bergeret, M. Abon, B. Tardy and S.J. Teichner, Vide 30 (A) (1975) 104.
M. Abon et al. /Ammonia
adsorption on molybdenum
planes
311
[ 141 R. Gomer, Field Emission and Field Ionization (Harvard Univ. Press, Cambridge, MA, 1961). [ 151 L. Schmidt and R. Gomer, J. Chem. Phys. 45 (1966) 1605. [16] P.T. Dawson and R.S. Hansen, J. Chem. Phys. 48 (1968) 623. [ 171 K. Matsushita and R.S. Hansen, J. Chem. Phys. 51 (1969) 472. [ 181 P.T. Dawson and Y.K. Peng, .I. Chem. Phys. 52 (1970) 1014. [ 191 M. Wilf and M.Folman, Faraday Trans I, 72 (1976) 1165. [ZO] M. Abon and S.J. Teichner, Nuovo Cimento Suppl. 5 (1967) 521. [21] E.E. Gutman, LA. Myasnikov and E.V. Bol’shun, Russian J. Phys. Chem. 49 (1975) 24. [22] L. Schmidt and R. Gomer, J. Chem. Phys. 42 (1965) 3573. (231 T. Kawai, K. Kunimori, T. Kondow and K. Tamaru, Japan. J. Appl. Phys. Suppl. 2 (1974) 513. [24] W.F. Egelhoff, J.W. Linnett and D.L. Perry, Faraday Trans. I, Faraday Discuss. Chem. Sot. 60 (1975) 127. [25] K. Matsushita and R.S. Hansen, J. Chem. Phys. 52 (1970) 4877. [26] G. Bergeret, B. Tardy and M.Abon, to be published. [27] K. Matsushita and R.S. Hansen, J. Chem. Phys. 54 (1971) 2278. [28] Y.K. Peng and P.T. Dawson, J. Chem. Phys. 54 (1971) 950. [29] A. Ignatiev, F. Jona, D.W. Jepsen and P.M. Marcus, Surface Sci. 49 (1975) 189. [30] D.L. Adams and L.H. Germer, Surface Sci. 27 (1971) 21. [31] M. Wilf and M. Folman, Surface Sci. 52 (1975) 10. [32] T.A. Delchar and G. Ehrlich, J. Chem. Phys. 42 (1965) 2686. [33] K. Matsushita and R.S. Hansen, J. Chem. Phys. 52 (1970) 3619. [34] S.P. Singh-Boparai, M. Bowker and D.A. King, Surface Sci. 53 (1975) 55. [35] P.T. Dawson, J. Catalysis 33 (1974) 47. [ 361 G.C. Bond, Catalysis by Metals (Academic Press, New York, 1962) p. 374.
FIELD EMISSION STUDY OF AMMONIA ADSORPTION AND CATALYTIC DECOMPOSITION ON INDIVIDUAL MOLYBDENUM PLANES M. ABON, G. BERGERET and B. TARDY Institut de Recherches
sur la Catalyse, 69626
Villeurbanne Cbdex, France
A probe-hole field emission microscope was used to investigate the crystallographic specificity of ammonia adsorption at 200 and 300 K on (llO), (loo), (211) and (111) molybdenum crystal planes. Chemisorbed NH3 causes a large work function decrease, especially at 200 K in agreement with an associative adsorption model which can also explain that this decrease is more important on the crystal planes of highest work function (At 200 K, A@= -2.25 eV on Mo(ll0) compared to A$ = -1.55 eV on MO (111). The decomposition of NH3 was followed by measuring the work function changes for stepwise heating of the MO tip covered with NH3 at 200 K. On the four studied planes NH3 decomposition and Hz desorption are completed at about 400 K. A@ changes above 400 K depend on the crystal plane and have been related to two different nitrogen surface states. No inactive plane towards NH3 adsorption and decomposition has been found but the noted crystallographic anisotropy in this low pressure study is relevant to the structure sensitive character of the NH3 decomposition and synthesis reactions.
1. Introduction It is frequently assumed that catalytic synthesis or decomposition of ammonia are structure sensitive reactions. According to Boudart et al. [ 11, the (111) plane of iron would be particularly active in NH3 synthesis. McAllister and Hansen [2] have reported that the rate of NH, decomposition is higher on the (111) plane than on the (100) and (110) planes of tungsten. It has also been suggested that the (100) plane of W should be the most active [3,4] and the (110) plane the least [5,6] in the adsorption and decomposition of NH3. A crystal plane specificity in the adsorption of NH, may be therefore expected. In the present work, the chemisorption and subsequent thermal decomposition of NH3 has been investigated on the (1 lo), (loo), (211) and (111) molybdenum planes by “probe-hole” field emission microscopy (FEM). MO has the same (bee) structure as Fe and W, and is also a very efficient catalyst for the decomposition and synthesis of NH, [7-lo]. 2. Experimental The fully bakeable “probe-hole” field emission microscope used in this work has been described previously [ 1 l-l 31. Changes in the average or local work function 305
306
M. Abon et al. /Ammonia
adsorption on molybdenum
planes
of the MO crystal during NH, adsorption or decomposition were determined from measurements of the total or local field emission current-voltage characteristics using the Fowler-Nordheim (FN) equation [14,15]. A base pressure of lo-” Torr was obtained using an ion pump and a titanium sublimator. Anhydrous high purity “NH3 was admitted from a supply bottle via a metal leak valve. The purity of this gas was checked using a quadrupole mass spectrometer mounted in the vicinity of the microscope. Decomposition of *‘NH3 gave 30N2 so that contamination by 28C0 could be detected and was found to be low.
3. Ammonia planes
adsorption at 200 and 300K
on (llO),
(lOO), (211) and (111) MO
At 200 K and in high vacuum there should not be any physical NH3 adsorption [ 161. The tip was cleaned by heating at 1800 K and cooled in vacuum [17,18] before the introduction of 10v6 Torr NH, for 2 min. To eliminate NH3 decomposition on hot filaments, the ionisation gauge and the mass spectrometer were turned off during the experiment. Measurements were made after NH3 pumping. 3.1. Results After NH3 adsorption, the field emission pattern shows a bright emission around the (100) planes as previously described for NH3 on W by Dawson and Hansen
1161. Results are summarized in table 1. Relative work functions #o of clean MO single-crystal planes have been determined in a previous work [12]. This table shows that NH3 adsorption results in a large work function decrease especially at 200 K, in good agreement with related works on W [4,5,16,19]. NH3 adsorption causes also a decrease in the FN pre-exponential term, A log A being between -1 and -2 for total emission and individual planes.
Table 1 Relative clean MO work function 90, atom densities adsorption at 200 K and 300 K at full coverage Total emission Go (eV) no (1014 atoms/cm2) A@ (eV) - 200 K A@ (eV) - 300 K A@ (200 K)/A@ (300 K)
4.20 -1.80 -1.0 1.8
no and work function
(110)
(100)
5.00 14.3 ~ 2.25 _ 1.2 1.9
4.45 10.1 - 1.95 _ 1.1 1.8
changes
(211)
4.60 8.28 -1.90 -1.0 1.9
A@ by NH3
(111)
4.20 5.86 -1.55 -0.9 1.7
M. Abon et al. /Ammonia
adsorption on molybdenum
planes
307
3.2. Ammonia adsorption model At 200 K, total dissociation of NH3 would give H and N adsorbed species which is incompatible with the observed work function decrease [20]. Partial dissociation leading to NH or NH2 may be considered, but according to Gutman et al. [21], these species on MO would be electron acceptors and would therefore increase the work function. An associative chemisorption model with bonding by partial transfer of the NH3 lone-pair electrons into the vacant orbitals of the metal seems the most likely interpretation [4,5,16]. However, the bonding would be mainly covalent, in contrast to ionic potassium adsorption which leads to an increase of log A [22] whereas the observed decrease is usual for covalent type gas chemisorption on metals [ 16,201. Additional experimental evidence for the presence even at 300 K of a molecular NH3 adsorbed complex comes from recent spectroscopic studies [23,24]. However there is a sharp reduction in the work function decrease at 300 K compared to 200 K (table 1) which may be explained by the presence on the surface at 300 K of products coming from the decomposition of a fraction of the NH, adsorbed molecules [ 16,241. 3.3 Crystal plane specificity Dawson and Hansen [ 161 demonstrated that NH3 decomposition occurs before surface diffusion on W. Then NH3 migration should not explain that no inactive plane towards NH3 adsorption was found in this study. The nearly constant value of the ratio A4 (200 K)/A@ (300 K) (table 1) gives some support to the statement that NH3 surface migration on MO is also unsignificant up to 300 K and shows that the same kind of crystal plane specificity exists at 200 and 300 K. Owing to strong repulsion between dipoles and to steric reasons [25], the NH3 population should not depend very much on crystal plane. The anisotropy in A$ may be therefore tentatively ascribed mainly to differences in dipole moment p. High A$ would correspond to high 1-1and then to strong bonding. NH, would be more strongly held on Mo(ll0) than on Mo(ll1).
4. Work function changes with temperature At 200 K, the tip was first fully covered with NH3. After 2 h pumping, the tip was stepwise heated for 60 set at increasing temperatures. NH3 is very difficult to pump and owing to re-adsorption @‘NH3 in the 10e9 Torr range), A@ data (fig. 1) were obtained using the “single point” method [15]. Results are in general agreement with FEM studies of NH3 decomposition on W by Dawson and Hansen [16] and Wilf and Folman [ 191. In these studies, the microscope itself was immersed in liquid helium [ 161 or liquid nitrogen [ 191. It is suggested that NH3 re-adsorption which occured in the present work where cryogenic pumping was not used, might
308
M. Abon et al. /Ammonia
200
Fig. 1. Work function
400
600
200
400
600
changes
for stepwise
adsorption on molybdenum
800
600
heating
for totalemission and for (llO), (loo), (2ll)and
account plane.
for the differences
1000
1000
planes
1200
1200
of the MO tip covered
with NH3 at 200 K
(111) planes.
which concern mainly the total emission and the (100)
4. I. Workfunction changes in the 200-400
K temperature range
The important rise in A@ which occurs between 200 and 400 K (fig. 1) indicates NH3 decomposition followed by Hz desorption, in agreement with thermal desorption results [26]. A near simultaneous decomposition is observed on all planes
M. Abon et al. /Ammonia adsorption on molybdenum planes
309
studied though decomposition is easier on (111) plane where A$I = 0 at 350 K instead of N 400 K on the other planes. NH, surface migration is likely to be unsignificant as stated before and then cannot account for the very similar behaviour of the different planes. The observed broad temperature range of NHs decomposition might be explained by a large variation in the activation energy of decomposition with coverage. Wilf and Folman [19] observed the same A@ rise though somewhat sharper on some planes such as the W(211). It is considered that the differences with our results are not great enough to suggest that NH3 re-adsorption can explain the gradual character of the rise in A@ 4.2. Work function changes above 400 K Nitrogen is more electronegative than MO and its presence explains the positive h@ maximum observed at 600-700 K on all planes studied. Above 700 K, nitrogen migration followed by desorption at = 1000 K might account for the observed evolution of the curves. However, a negative A@ minimum is observed only on the (100) plane at 950 K. Thermal desorption experiments [26] showed that NH, readsorption can provide an interpretation to the FEM results. In addition to the high temperature P-nitrogen peak, a low temperature nitrogen peak with a greater total nitrogen coverage results of NHa re-adsorption on a MO filament first covered with NH3 at 240 K and then heated up to 750 K. On W, this low temperature nitrogen state has been designated “X-nitrogen” by Matsushita and Hansen [25,27] and ‘%-nitrogen” by Peng and Dawson [28]. The (0 + X)-nitrogen would be electronegative [25,33] and its gradual formation by NH3 re-adsorption above 350 K would explain the rise in A@ and the positive A@ maximum at 600-700 K on all the planes. Above 700 K, this (0 t X)-structure would be progressively destroyed with a partial desorption of nitrogen giving near 950 K a /l-nitrogen state. This transition would account for the decrease in A$ above 700 K and for the negative minimum only observed on the (100) plane at 950 K. It is well known that P-nitrogen decreases the work function of W (100) [30-321. General agreement is found with the results of Wilf and Folman [19] with the main exception of the W(100) plane where a positive A+ maximum was not observed. As stated before, NH3 re-adsorption on W(100) at 78 K was probably very limited and NHs decomposition directly leaves above 400 K the electropositive p2 state which lowers A@ by = 0.9 eV [ 191 instead of 0.4 eV on Mo(l00) (fig. 1). On further exposure to NHs and heating to 800 K a less strongly-bonded /3,-state which raises A$ by 0.15 eV on W(100) has been observed [ 191. It is not clear whether one may relate this &-state to the X-nitrogen state. Wilf and Folman found also this &-state even when molecular nitrogen is adsorbed on W(100) [31], but it has been demonstrated [25,28] that the X-state (or b-state) cannot result of non-activated molecular nitrogen interaction with W. Thermal desorption experiments [26] supported the formation of the X-nitrogen state in good agreement with related works on MO [27] and W [25,28].
310
M. Abon et al. /Ammonia
adsorption on molvbdenum
planes
5. Conclusion NH3 chemisorption on MO leads to a large work function decrease especially at 200 K. A# results support molecular NHs adsorption at 200 K whereas at 300 K a fraction of the NH3 layer may be decomposed. The observed crystal face specificity has been tentatively related to dipole moments variations. The dipole associated with the NH3 adsorbed molecule would be the greatest on high work function planes. This would mean that NH3 is more strongly adsorbed on the (110) plane than on other planes and especially on (11 I) plane. it is also worth stressing that no inactive plane towards NH3 adsorption has been found. NH3 decomposition followed by Ha desorption may explain the Atg rise between 200 and 400 K. Above this temperature, NH3 interaction with the ~-nitrogen deposit left on the surface would give on all the crystal planes an electronegative (p f X)-structure (A$ > 0) of a greater nitrogen surface coverage than the P-layer. Above 700 K, partial nitrogen desorption leaves a @-nitrogen layer: A# >0 for all the crystal planes with the exception of the (100) where A~#J = -0.4 eV. Results have been compared with related works especially with the study of NH3 on W by Wilf and Folman [ 19 1. This low pressure study on MO crystal planes of high cleanliness showed that the only species left on the surface above 400 K would be nitrogen and this observation may be related with h&h pressure kinetic studies which usually favoured adsorption-desorption of nitrogen as rate limiting step in the catalytic synthesis or decomposition of NH3 [9,36]. The strong crystal face specificity towards Na adsorption [30,343 gives the impression that the (100) plane may be the most active in NH3 synthesis or decomposition reaction. This work shows that NH3 may be indeed adsorbed and decomposed on the (100) plane as on other planes but NH3 decomposition would be easier on the (111) plane.
References [l f [2] [3] [4] IS] [6]
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