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Recombination reactions on Ni(111)
567
Surface Science 141 (1984) 567-579 North-Holland, Amsterdam
RECOMBINATION Jay B. BENZIGER Department
REACTIONS
ON Ni( 111)
* and Richard E. PRESTON
of Chemical Engineering,
Princeton
University, Princeton,
Received 9 August 1983; accepted for publication
New Jersey 08544, USA
23 February 1984
The dissociative adsorption and recombination of CO, N,, SO, and 0, on Ni(ll1) has been studied by LEED, AES and TPD. The kinetic parameters were determined for 0+ 0, N +N, s + O, CEarbide + 0 recombination. All four reactions displayed second order kinetics with pre-exThe activation energies were E,,, = 440 kJ/mol, ponential factors of 1O-2*1 cm*/atom.s. E N+N=210 kJ/mol, Es+,=310 kJ/mol and EC,.,bbid.+O=175 kJ/mol. The kinetics of the C sraphite+0 recombination were found to be first order indicating a condensed graphite phase with reaction at the phase boundary. These kinetics were further complicated due to an apparent phase transition of the adsorbed graphite at 970 K. The TPD results were used to estimate heats of adsorption. These estimates correlated with the heats of formation of bulk compounds.
1. Introduction The recombination of two atoms adsorbed on a surface represents one of the simplest reactions catalyzed by a metal. However, except in the case of hydrogen atom recombination, there have been surprisingly few studies of the reaction kinetics for surface recombination reactions. In this paper the recombination reactions of carbon, oxygen, sulfur and nitrogen on Ni(ll1) have been examined using temperature programmed desorption (TPD). Pre-exponential factors and activation energies were determined by several different approaches. It is also shown how estimates of the heats of adsorption may be obtained from TPD experiments.
2. Experimental The experiments were carried out in a stainless steel ultrahigh vacuum system equipped with 4-grid LEED optics, quadrupole mass spectrometer, argon ion gun, gas dosing port and a rotatable sample manipulator. A Ni(l11) crystal in the form of a disc (9 mm X 0.2 mm) was heated by electron * To whom inquiries should be addressed.
0039-6028/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
568
J. B. Benriger, R.E. Preston / Recombination
reactions on Ni(l1 I)
bombardment heating from a tungsten filament located in the back of the crystal; crystal cooling to 250 K was accomplished by thermal conduction to a liquid nitrogen reservoir. The crystal temperature was monitored by a chromel-alumel thermocouple stopwelded to the back of the crystal. The crystal was cleaned initially by argon ion sputtering and annealing. Further cleaning was done by adsorption of oxygen and annealing to 1500 K. The structure and composition of the surface was verified by low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Auger spectroscopy was done using the LEED optics as a retarding field analyzer. The clean surface showed less than 0.02 monolayers (1 monolayer = 1.86 x lOI cme2) of impurities. Well defined adlayers, Ni(lll)-p(2 x 2)C, Ni(lll)-p(2 x 2)0 and Ni(lll)-p(2 x 2)S, were prepared from adsorption of ethylene, oxygen, and hydrogen sulfide; the LEED and AES results for these surfaces were correlated as calibration points for surface coverages. Nitrogen coverages were estimated by approximating the AES sensitivity for nitrogen from the calibration points for carbon and oxygen. Gas adsorption was accomplished by exposing the Ni crystal to a beam effusing from a 0.25 mm ID tube. Beam fluxes were estimated by measuring the pressure rise in the chamber for a known backing pressure and leak valve setting, which allowed the total leak rate out of the beam tube to be calculated. Assuming a cosine flux distribution from the beam tube, an effective flux at the crystal surface was calculated. Effective fluxes at the crystal surface the system equivalent to lop6 to 1 Pa were achieved while maintaining pressure below 6 x lo-’ Pa. Temperature programmed desorption (TPD) experiments were performed by adsorption at a desired temperature, allowing the crystal to cool to 300 K and subsequently heating the crystal to 1500 K at rates at 4-26 K/s by electron bombardment heating. The mass spectrometer was multiplexed with a minicomputer to record several mass fragments simultaneously. 3. Results 3.1. Carbon Carbon monoxide adsorbed molecularly on Ni(ll1) at 300 K. The TPD spectrum (fig. la) showed CO desorbed at 450 K and the Auger spectrum showed no carbon or oxygen buildup on the surface. Kinetic parameters were determined by heating rate variation and peak shape analysis to be v = 2 X lOI surface was exposed to an effective CO s-i, EA = 120 kJ/ mol. The Ni(ll1) flux of lop2 Pa for 20 s with the crystal held at temperatures from 250 to 700 K. After the exposure to CO the crystal was heated to 600 K in vacuum and an Auger spectrum was taken. The results shown in fig. 2 indicate that the rate of carbon deposition from CO dissociation became significant (i.e. detectable CO
569
i
400
600
600 TEMPERATURE
1000 ( K)
Fig. 1. Carbon oxide desorption from Ni(ll1). (a) CO desorption from CO adsorption at 300 K. (b) CO desorption from adsorbed carbide and oxide; & - 0.20, 0, = 0.25. (c) CO, desorption from adsorbed carbide and oxide; 8, = 0.20, 8, = 0.25. (d) CO desorption from adsorbed graphite and oxide; f?, = 0.40, 9, = 0.20. (e) CO, desorption from adsorbed graphite and oxide; 6, = 0.40, B* = 0.20.
dissociation from the equivalent of 20 monolayers CO exposure} at temperatures above 450 K, indicating that the rate of CO dissociation is less than the rate of CO desorption. Fig. 2 also indicates that above 600 K the form of the adsorbed carbon changes from a carbide to a graphite as the AES changed from a three peak structure characteristic of metal carbides to a single peak characteristic of graphite [1,2]. The rate of carbon and oxygen recombination was measured for both carbidic and graphitic carbon on Ni(ll1). The Ni(ll1) surface was carbided by exposure to 10m2 Pa of ethylene at 500 K for 15 s; a well ordered p(2 X 2) LEED pattern was observed. This surface was exposed to oxygen at 300 K and heated to 1200 K. Desorption of both CO and CO, were observed at 670 K, as shown in figs. lb and lc. The CO spectrum was corrected for CO1 fragmentation. Small amounts of CO desorption were observed at 450 K and 970-1050 IS, which may be ascribed to adsorbed molecular CO and adsorbed graphite respectively. The CO&CO product ratio at 670 K was found to be dependent on the adsorbed carbon-to-oxygen ratio on the surface as indicated in table 1. The data taken for the last two entries in table 1 were taken by preparing a Ni(lll)-p(2 X 2)0 surface as described below and exposing ethylene to that surface at 500 K.
570
,
t 240 ELEC’TRON
I
I
260
L
280
ENERGY
te’.‘)
Fig. 2. Auger spectra for carbon monoxide dissociation on Ni(ll1). Surface exposed to IOF Pa CO for 20 s and then flashed to 600 K in vacuum. (a) Adsorption T = 300 K; @) T= 450 K; (c) T=475K;(d)T=500K;(e)T=6001(;(f)T=700K.
The kinetics of Cfaibide+ 0 recombination were measued by two techniques. Nearly equal amounts of carbon and oxygen were adsorbed to 0.2.5 monotayers and the kinetic parameters were determined by (i) heating rate variation 131, Table
1
CO, /CO product ratio for Ccarhde + 0 recombination Carbon coverage (monolayers + 0.02)
Oxygen coverage (monolayers Ifr0.02)
co, /co product ratio ( i 0.03)
0.25 0.25 0.25 0.25 0.17 0.09
0.23 0.16 0.09 0.04 0.24 0.25
0.36 0.21 0.10 0.03 0.34 0.38
J. B. Benriger, R. E. Preston / Recombination
Table 2 Kinetic parameter
determinations
for adsorbed
carbide
reactions on Ni(l1 I)
and oxygen
s)
recombination
Method
Y (cm*/atom.
(i) /3, = 8,; heating rate variation (ii) 0, = ~9~;second order peak shape analysis (iii) f?, B e,; heating rate variation (iv) e, B e,; first order peak shape analysis (v) 6’, -SCtl,; heating rate variation (vi) e, =sze,; first order peak shape analysis
0.1
178
0.001
162
0.002
172
0.02
184
0.002
167
0.05
190
511
on Ni(lll)
E (kJ/mol)
and (ii) peak shape analysis assuming a second order reaction with 0, = 8,. Experiments were also attempted to prepare surfaces with equal amounts of carbon and oxygen at coverages less than 0.25 monolayers. We were unable to obtain satisfactory control of surface coverages to get good data; however, it was noted that the peak at 670 K shifted to higher temperature with decreasing coverage as expected. A second approach was to prepare a Ni(lll)-p(2 X 2)C surface, adsorb oxygen to less than 0.05 monolayers and analyze the kinetics as first order with f& as a constant; this was also repeated interchanging the role of oxygen and carbon. Heating rate variation and peak shape analyses were also done using the second approach. The kinetic parameters measured by six different approaches are summarized in table 2. The agreement among the different techniques is probably a good measure of the error associated with the determination of the pre-exponential factor and activation energy. The peak shape anlysis for kinetic parameters evolves from an expression for the reaction rate N(T)
= da,/dT
= ( v/p)u~ug
eeElkT,
where N is the desorption rate, u are surface coverages, v is the pre-exponential factor, /3 is the heating rate and E is the activation energy. If a, = uB then a plot of ln( N/u,) versus l/T will have a slope of -E/k and an intercept of ln(v//3). For the case a,., B us such that a, = constant, then a plot of ln( N/us) versus l/T will have a slope of -E/k and an intercept of ln(vu,/P). Auger spectra were correlated with TPD results to provide measurements for coverages and absolute rates. Graphite was deposited on the surface
572
Table 3 CO,/CO
J.B. Benz&w-, R. E. Preston / Recombmation reactions on Ni(/ 1 I)
product
ratio for Cgraphite + 0 recombination
Carbon coverage (monolayers * 0.03)
Oxygen coverage (monolayers f 0.02)
co, /co product ratio ( + 0.005)
0.34 0.31 0.35 0.35
0.20 0.14 0.07 0.03
0.082 0.065 0.048 0.029
showed a single carbon peak at 272 V characteristic of graphite. Exposure of a Ni(ll1) surface with adsorbed graphite to oxygen and subsequent heating produced a two peaked TPD spectrum as shown in fig. Id. Both CO and CO, desorption were oberved at 970 K and 1050 K. The CO peak at 670 K may be ascribed to a small amount of adsorbed carbide. The CO,/CO product ratio was observed to increase with increasing oxygen exposure to the surface as shown in table 3. The CO,/CO product ratio was Ni(lll)-C,,,,r,i,, much lower for the graphite plus oxygen reaction than observed for carbide plus oxygen. The TPD results were ambiguous as to whether the CO,/CO product ratio was the same for both the 970 and 1050 K desorption peaks. Graphite could not be adsorbed on a surface with adsorbed oxygen by cracking ethylene, suggesting the carbon first adsorbed as a carbide which reacted with oxygen before conversion to graphite occurred. The TPD results for Cgraphite+ 0 were unusual in several respects. The two desorption peaks did not shift in temperature with either carbon or oxygen coverage. The relative magnitude of the two peaks changed with carbon coverage; the peak at 970 K grew relative to the peak at 1050 K with increasing carbon coverage at a constant oxygen coverage of 13, = 0.15 monolayers. The relative separation of the two peaks made it possible to deconvolute them to obtain meaningful kinetic parameters from any of the methods discussed above. 3.2. Nitrogen Diatomic nitrogen was not observed to adsorb on Ni(ll1) at any temperature in the range 250-900 K. No N, desorption was observed subsequent to exposure of the Ni(ll1) crystal to 1 Pa N, for 100 s, and the Auger spectra showed no nitrogen independent of crystal temperature. Nitrogen was adsorbed on Ni(ll1) by decomposing hydrazine on the surface at 500 K. The Auger spectra showed adsorbed nitrogen and the TPD spectra showed N, desorption at 850 K as the only desorption product (see fig. 3). After the TPD experiment the Auger spectrum showed the surface to be clean. The kinetic
J. B. Benziger, R. E. Ptesron / Recombination
800
600
1200
1000 TEMPERATURE
reacrions on Ni(l I I)
573
1400
( K)
Fig. 3. TPD spectra foi recombination reactions on Ni(ll1). (a) N, desorption; ON = 0.20. (b) SO desorption; @s= 0.25, 8, = 0.20. (c) SO, desorption; 6s = 0.25, ~9, = 0.20. (d) 0, desorption; eo = 0.5.
parameters for N, desorption were determined from heating rate and coverage variations [3], as well as peak shape analysis; the results are summarized in table 4. A Ni(lll) surface with 0.25 monolayers adsorbed nitrogen was exposed to oxygen at 300 K. The desorption spectrum showed N, desorption at 850 K, but no significant NO desorption. The N, desorption peak showed only slight variations in peak shape and position due to the coadsorbed oxygen. A small NO desorption peak was observed at 870 K, but the magnitude of this peak was two orders of magnitude less than the N, desorption peak suggesting that it may have been due to defects,
Sulfur was adsorbed on Ni(lll) by cracking H,S on the crystal at 400 K. After exposing the Ni(ll1) crystal at 400 K to 10m3 Pa for 20 s, a well ordered Table 4 Kinetic parameters
for N + N recombination
Method
Y (cm*/atom
(i) Heating rate variation (ii) Coverage variation (iii) Peak shape analysis
0.006 0.04 0.001
s)
E (kJ/mol) 210 220 196
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J.B. Benriger, R.E. Preston / Recombination reactions on Ni(Ill)
Table 5 SO, /SO product ratio for S + 0 recombination Sulfur coverage (monolayers f 0.01)
oxygen coverage (monolayers * 0.02)
so, /so product ratio
0.25 0.25 0.25 0.17 0.06
0.22 0.14 0.08 0.24 0.25
2.3 1.7 1.1 2.7 2.9
p(2 x 2)s adlayer was detected by LEED. This adlayer could be heated to 1500 K in vacuum with no desorption products detected, nor any change in the LEED pattern or Auger spectrum. When exposed to 0, at 300 K the Ni(lll)p(2 x 2)s surface adsorbed oxygen. H,S also decomposed on a Ni(ll1) surface with adsorbed oxygen at 400 K leaving sulfur coadsorbed with the oxygen. The recombination of sulfur and oxygen occurred at ca. 1150 K yielding SO and SO, desorption products. The SO product weas corrected for SO, fragmentation. The SO,/SO product ratio was found to be dependent on the S/O ratio of adsorbed atoms as shown in table 5. The kinetic parameters for the S + 0 recombination were determined by six methods listed above for C + 0 recombination. These values are summarized in table 6. The adsorption of SO, on Ni(ll1) was also examined by AES and TPD. Exposure of the Ni(ll1) crystal at 300 K to 10m3 Pa SO, for 10 s led to adsorption of 0.1 monolayer of dissociated SO, (13, = 0.1, 8, = 0.2) as determined by AES. No SO or SO, desorption were observed to occur at temperatures below 1100 K after SO, adsorption. Changing the temperature of Table 6 Kinetic parameters for S+ 0 recombination on Ni(ll1) Method
Y (cm*/atom.
(i) 19s= 8,; heating rate variation (ii) t& = 0,; second order peak shape analysis (iii) Bs B eo; heating rate variation (iv) OSB 0,; first order peak shape analysis (v) 0, -=+z eo; heating rate variation (vi) 0s -=x0,; first order peak shape analysis
0.07
315
0.002
295
0.008
305
0.004
300
0.005
295
0.04
310
s)
E (kJ/mol)
J. B. Benriger, R. E. Preston / Recombination
reactions on Ni(lll)
575
the Ni crystal from 300 to 600 K showed no detectable difference in the amount of SO, adsorption for identical exposures. The adsorption of SO was not studied as it was not available. 3.4. Oxygen Oxygen adsorbed dissociatively on Ni(ll1) at temperatures from 250 to 1000 K. Exposures to 1O-4 Pa 0, for 20 s at 300 K led to a diffuse p(2 X 2) LEED pattern. Annealing the sufface to 1000 K caused no desorption but the LEED pattern became very sharp; the Auger spectrum showed no change due to the annealing. Equivalent oxygen exposures at 300, 500 and 800 K gave identical oxygen coverages as measured by AES. TPD results did not indicate any desorption of 0 or 0, from the Ni(lll)-p(2 x 2)0 surface up to 1475 K. At 1500 K a small increase in the 0, pressure was observed indicating some desorption. Maintaining the Ni(lll)-p-(2 x 2)0 surface at 1500 K for periods of 10-1000 s led to a decrease and eventual disappearance of the oxygen Auger peak. Shown in fig. 4 are the measured oxygen coverages determined by AES after heating the Ni(lll)-p(2 X 2)0 surface to 1500 K for a fixed period of time. The Auger data indicate that oxygen desorption or diffusion into the Ni crystal are occurring. The increase in the 0, pressure indicates desorption is the more likely event. The Auger data were used to determine an isothermal desorption rate constant for 0 + 0 recombination at 1500 K of 4.3 x lo-” cm*/atom * s. Similar experiments were performed at 1490 and 1515 K. These
*.
‘\ ‘\ ‘\ ‘\
-.
. . 0
--_. --._ ----_ 0
20
--__
40
---_
q -_
60
---____
q ----_
80
----__
II
TIME (seconds)
Fig. 4. Oxygen coverage on Ni(ll1) as a function of time at 1500 K (dashed line is for k = 8 X lo-’ S-1).
J.B. Benriger, R.E. Preston / Recombination reactions on Ni(l II)
516
data were then used to obtain kinetic parameters for 0 + 0 recombination. The values obtained were v = 10e2 * 2 cm2/atom. s, E = 440 f 50 kJ/mol. When the Ni(ll1) surface was exposed to oxygen beyond that which gave a p(2 X 2) LEED pattern the oxygen Auger signal increased and the p(2 x 2) LEED pattern became obscured. At an oxygen coverage of 0.5 monolayers the TPD spectrum showed 0, desorption became appreciable at 1450 K as indicated on fig. 3. The desorption rate maximum was not achieved by 1500 K, and holding the crystal at 1500 K led to the desorption of all the oxygen as determined by AES. Heating to 1500 K caused the oxygen in excess of 0.25 monolayers to rapidly desorb; after each experiment when the crystal was cooled immediately after heating to 1500 K a sharp p(2 x 2) LEED pattern was observed.
4. Discussion The recombination/desorption reactions of nitrogen and carbon monoxide on Ni surfaces has been observed in previous studies [4-121, but the kinetics for recombination were not accurately measured. The kinetic parameters for recombination may be understood in terms of transition state theory. For a second order surface recombination reaction the rate of reaction is r=-
Q, -kT Q,,Q,, h
( 1 Ac/kTcJAoB, e_
where Q,, is the partition function for absorbed species i, Q, i the partition function for the transition state and AC is the difference in zero point energies. A reasonable approximation for de is AC = -AH,, + Edis, where AH,, is the adsorption enthalpy and Edis is the activation barrier for dissociation. Assuming the adsorbed adatoms are mobile on the surface at conditions where recombination occurs, and that the transition state also freely translates and rotates in two dimensions then e,= Q&-h
(8r21A,+kT/h2)“2 (2vkT/h2)(
MAMe)“’
’
where I,, is the moment of inertia for AB in the transition state, and MA and MB are th> masses of A and B. This result suggests that the pre-exponential factor for all the recombination reactions studied here should be approximately 10w3. This value is the lower limit of the experimental values deThe termined here for Carbide + 0, N + N, S + 0, and 0 + 0 recombination. experimental data suggest that the transition state may have more freedom than ascribed to it above; for example, it may have a tumbling motion as well as rotating in the plane of the surface, giving a pre-exponential greater than
J. B. Benziger, R. E. Presron / Recombinarion
reactions on Ni(l I I)
577
10s3. However, the scatter of the data indicates that overinterpretation of the data must be viewed cautiously. was unique among the reactions studied The ‘graphite+ 0 recombination here as it appeared to be governed by first order kinetics. The nature of adsorbed graphitic carbon can account for this anomaly. Adsorbed adatoms, such as S, N, and 0, all exist on the surface as nearly independent entities with only weak interactions between adatoms. Carbon adsorbed in the form of carbide also behaves independent of adatoms. In the form of graphite the carbon adatoms are che~cally bound to one another and would exist in a condensed phase. the reaction between Cgraphile and 0 then occurs when an oxygen atom diffuses to the perimeter of the graphite phase. The rate of reaction is proportional to the rate with which adsorbed oxygen encounters the perimeter of the graphite phase, which should be first order in oxygen coverage and nearly independent of carbon coverage, except at very low carbon coverages. This accounts for the first order reaction kinetics observed by TPD. Another unusual feature of the Cgraphite+ 0 recombination was the two desorption peaks observed. This may be due to a phase transition of the adsorbed graphite. Eizenberg and Blakely [14] observed two phase transitions of graphite adsorbed on Ni(ll1). At 850 K the observed bulk graphite went into a solid solution on the Ni(ll1) surface leaving a surface monolayer of adsorbed graphite. At temperatures above 970 K, a second phase transition occurred and more carbon went into solution such that the bulk and surface concentrations of carbon were equal. This second phase transition would cause a decrease in the surface carbon concentration and hence reduce the rate of C sraphite+ 0 recombination, as was observed in the TPD experiment. An interesting feature in both the carbon and sulfur oxidation reactions was the evolution of dioxides coincident with the monoxides, which scaled with oxygen coverage. These results indicated that the recombination reactions form an adsorbed monoxide intermediate that has a finite surface lifetime during which it can further react. The coincidence of the product peaks indicates that the monoxide is more weakly bound than the adatom (i.e. C or S), so that the rate limiting step is the initial r~mbination. The activation energies for desorption measured here may be used to estimate adatom adsorption enthalpies. For the reaction A(a) + B(a) --* AB(a) + AB(g), the activation
energy is approximately
E =A~~(AB(g))-A~~(A(a))-A~~(B(a~)-~~i~~
If AB dissociation proceeds via molecularly only occurs when sufficient energy is available B(a), which has an activation energy E,b’is= -AH,, (AB(a)) + Edia, where AH,,(AB(a)) is the heat of adsorption
adsorbed AB then dissociation for the reaction AB(a) -+ A(a) +
of molecular
AB.
578
J.B. Benriger, R.E. Preston / Recombination
reactions on Ni(lll)
Table 7 Energetics for dissociative adsorption and recombination Molecule
A H, (AWg)) a) (kJ/mol)
02
0 0
NZ co so
Erecombinstion
Edissociation
AfffMO+
(kJ/mol)
(kJ/mol)
(kJ/mol)
440 210 175 310
-100 +6
0 > 210 15 0
A H&W)
-440 >o 190 316
a) From ref. [15].
Auger spectra were taken for fixed exposures of Ni(ll1) to O,, N,, CO and SO,. The activation energy for dissociative adsorption may be determined from temperature dependence of the coverage (0) of dissociated molecules: E&
=-
As 0, and SO, dissociative adsorption were independent of temperature, it appears that Edis = 0 for both SO and 0,. (The value for SO is assumed to be the same as for SO,.) Nitrogen dissociation was never observed, indicating that the rate of dissociation is less than the rate of recombination, suggesting that for N,. The data in fig. 2 indicate that for CO, E& = 135 + Edis ’ Erecombination 20 kJ/mol; the adsorption enthalpy for CO on Ni(ll1) has been measured to be 120 kJ/mol[16], so Edis = 15 kJ/mol for CO. The reaction energetics are summarized in table 7. These values may be used to estimate heats of adsorption of 0, N, C, and S on Ni(ll1). Temperature corrections should be made to the various enthalpies. However, uncertainty in how to compensate for surface heat capacities and the uncertainty in the experimental data make this refinement somewhat questionable. Table 8 compares estimated heats of adsorption to the heats of formation of bulk nickel carbide, nitride, oxide and sulfide. The comparison shows reasonable agreement, supporting the notion that bulk thermodynamic properties may be Table 8 Adsorbed species
Estimated heat of adsorption (kJ/mol)
Heat of formation of bulk compound at standard conditions (kJ/mol)
Carbide Nitrogen Oxygen Sulfur
+70 >o - 220 -84
+ 46 (Ni 3C) + 1 (Ni,N) - 224 (NiO) - 72 (NiS)
a) From ref. [17].
J. B. Benziger, R. E. Preston / Recombination reactions on Ni(l II)
519
used as estimates for adsorption enthalpies. The estimates obtained here also show reasonable agreement with heats of adsorption for oxygen [18,19] and sulfur [20] on nickel surfaces obtained by direct thermodynamic measurements. Clearly the thermodyna~c measurements are more accurate; however, the relative ease of obtaining estimates by TPD makes this an attractive alternative.
Acknowledgments The financial support of both the National Science Foundation and the Air Force pffice of Scientific Research are gratefully acknowledged.
References [l] T.W. Haas, J.T. Grant and G.J. Dooley, in: Proc. 2nd Intern. Symp. on Adsorption/Desorption Phenomena, Ed. F. Ricca (Academic Press, London, 1973). [2] L.E. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach and R.E. Weber, Handbook of Auger Electron Spectroscopy (Physical Electronics, 1976). [3] P.A. Redhead, Vacuum 12 (1962) 203. [4] H. Conrad, G. Ertl, J. Ktppers and E.E. Latta, Surface Sci. 50 (1975) 296. [5] M. Grunze, R.K. Driscoll, G.N. Burland, J.C.L. Cornish and J. Pritchard, Surface Sci. 89 (1979) 381. (61 G.L. Price and B.G. Baker, Surface Sci. 91 (1980) 571. [7] C.W. Seabury, T.N. Rhodin, R.J. Purtell and R.P. Merrill, Surface Sci. 93 (1980) 117. [8] W. Erley and H. Wagner, Surface Sci. 74 (1978) 333. [9] H.H. Madden and G. Ertl, Surface Sci. 35 (1973) 211. [lo] R. Sau and J.B. Hudson, Surface Sci. 102 (1981) 239. [ll] E.G. Kein, F. Labohm, O.L.J. GiJzeman and G.A. Bootsma, Surface Sci. 122 (1981) 52. [12] P.H. Holloway and J.B. Hudson, Surface Sci. 33 (1972) 56. [13] K.J. Laidler, in: Catalysis I, Ed. P.H. Emmett (Reinhold, New York, 1956). [14] M.E. Eizenberg and J.M. Blakely, Surface Sci. 82 (1979) 228. [15] S.W. Benson, in: ~e~~he~cal Kinetics (Wiley, New York, 1968). 1161 H. Conrad, G. Ertl, J. Kippers and E.E. Latta, Surface Sci. 57 (1976) 475. [17] 0. Kubaschewski, E.L. Evans and C.B. Alcock, in: Metallurgical Thermochemistry (Pergamon, New York, 1967). [18] D. Brennan, D.O. Hayward and B.M.W. Trapnell, hoc. Roy. Sot. (London) A256 (1960) 81. [19] H.J. Grabke and H. Viefhaus, Surface Sci. 112 (1981) L779. [20] J.G. McCarty and H. Wise, J. Chem. Phys. 72 (1980) 6337.
Surface Science 141 (1984) 567-579 North-Holland, Amsterdam
RECOMBINATION Jay B. BENZIGER Department
REACTIONS
ON Ni( 111)
* and Richard E. PRESTON
of Chemical Engineering,
Princeton
University, Princeton,
Received 9 August 1983; accepted for publication
New Jersey 08544, USA
23 February 1984
The dissociative adsorption and recombination of CO, N,, SO, and 0, on Ni(ll1) has been studied by LEED, AES and TPD. The kinetic parameters were determined for 0+ 0, N +N, s + O, CEarbide + 0 recombination. All four reactions displayed second order kinetics with pre-exThe activation energies were E,,, = 440 kJ/mol, ponential factors of 1O-2*1 cm*/atom.s. E N+N=210 kJ/mol, Es+,=310 kJ/mol and EC,.,bbid.+O=175 kJ/mol. The kinetics of the C sraphite+0 recombination were found to be first order indicating a condensed graphite phase with reaction at the phase boundary. These kinetics were further complicated due to an apparent phase transition of the adsorbed graphite at 970 K. The TPD results were used to estimate heats of adsorption. These estimates correlated with the heats of formation of bulk compounds.
1. Introduction The recombination of two atoms adsorbed on a surface represents one of the simplest reactions catalyzed by a metal. However, except in the case of hydrogen atom recombination, there have been surprisingly few studies of the reaction kinetics for surface recombination reactions. In this paper the recombination reactions of carbon, oxygen, sulfur and nitrogen on Ni(ll1) have been examined using temperature programmed desorption (TPD). Pre-exponential factors and activation energies were determined by several different approaches. It is also shown how estimates of the heats of adsorption may be obtained from TPD experiments.
2. Experimental The experiments were carried out in a stainless steel ultrahigh vacuum system equipped with 4-grid LEED optics, quadrupole mass spectrometer, argon ion gun, gas dosing port and a rotatable sample manipulator. A Ni(l11) crystal in the form of a disc (9 mm X 0.2 mm) was heated by electron * To whom inquiries should be addressed.
0039-6028/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
568
J. B. Benriger, R.E. Preston / Recombination
reactions on Ni(l1 I)
bombardment heating from a tungsten filament located in the back of the crystal; crystal cooling to 250 K was accomplished by thermal conduction to a liquid nitrogen reservoir. The crystal temperature was monitored by a chromel-alumel thermocouple stopwelded to the back of the crystal. The crystal was cleaned initially by argon ion sputtering and annealing. Further cleaning was done by adsorption of oxygen and annealing to 1500 K. The structure and composition of the surface was verified by low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Auger spectroscopy was done using the LEED optics as a retarding field analyzer. The clean surface showed less than 0.02 monolayers (1 monolayer = 1.86 x lOI cme2) of impurities. Well defined adlayers, Ni(lll)-p(2 x 2)C, Ni(lll)-p(2 x 2)0 and Ni(lll)-p(2 x 2)S, were prepared from adsorption of ethylene, oxygen, and hydrogen sulfide; the LEED and AES results for these surfaces were correlated as calibration points for surface coverages. Nitrogen coverages were estimated by approximating the AES sensitivity for nitrogen from the calibration points for carbon and oxygen. Gas adsorption was accomplished by exposing the Ni crystal to a beam effusing from a 0.25 mm ID tube. Beam fluxes were estimated by measuring the pressure rise in the chamber for a known backing pressure and leak valve setting, which allowed the total leak rate out of the beam tube to be calculated. Assuming a cosine flux distribution from the beam tube, an effective flux at the crystal surface was calculated. Effective fluxes at the crystal surface the system equivalent to lop6 to 1 Pa were achieved while maintaining pressure below 6 x lo-’ Pa. Temperature programmed desorption (TPD) experiments were performed by adsorption at a desired temperature, allowing the crystal to cool to 300 K and subsequently heating the crystal to 1500 K at rates at 4-26 K/s by electron bombardment heating. The mass spectrometer was multiplexed with a minicomputer to record several mass fragments simultaneously. 3. Results 3.1. Carbon Carbon monoxide adsorbed molecularly on Ni(ll1) at 300 K. The TPD spectrum (fig. la) showed CO desorbed at 450 K and the Auger spectrum showed no carbon or oxygen buildup on the surface. Kinetic parameters were determined by heating rate variation and peak shape analysis to be v = 2 X lOI surface was exposed to an effective CO s-i, EA = 120 kJ/ mol. The Ni(ll1) flux of lop2 Pa for 20 s with the crystal held at temperatures from 250 to 700 K. After the exposure to CO the crystal was heated to 600 K in vacuum and an Auger spectrum was taken. The results shown in fig. 2 indicate that the rate of carbon deposition from CO dissociation became significant (i.e. detectable CO
569
i
400
600
600 TEMPERATURE
1000 ( K)
Fig. 1. Carbon oxide desorption from Ni(ll1). (a) CO desorption from CO adsorption at 300 K. (b) CO desorption from adsorbed carbide and oxide; & - 0.20, 0, = 0.25. (c) CO, desorption from adsorbed carbide and oxide; 8, = 0.20, 8, = 0.25. (d) CO desorption from adsorbed graphite and oxide; f?, = 0.40, 9, = 0.20. (e) CO, desorption from adsorbed graphite and oxide; 6, = 0.40, B* = 0.20.
dissociation from the equivalent of 20 monolayers CO exposure} at temperatures above 450 K, indicating that the rate of CO dissociation is less than the rate of CO desorption. Fig. 2 also indicates that above 600 K the form of the adsorbed carbon changes from a carbide to a graphite as the AES changed from a three peak structure characteristic of metal carbides to a single peak characteristic of graphite [1,2]. The rate of carbon and oxygen recombination was measured for both carbidic and graphitic carbon on Ni(ll1). The Ni(ll1) surface was carbided by exposure to 10m2 Pa of ethylene at 500 K for 15 s; a well ordered p(2 X 2) LEED pattern was observed. This surface was exposed to oxygen at 300 K and heated to 1200 K. Desorption of both CO and CO, were observed at 670 K, as shown in figs. lb and lc. The CO spectrum was corrected for CO1 fragmentation. Small amounts of CO desorption were observed at 450 K and 970-1050 IS, which may be ascribed to adsorbed molecular CO and adsorbed graphite respectively. The CO&CO product ratio at 670 K was found to be dependent on the adsorbed carbon-to-oxygen ratio on the surface as indicated in table 1. The data taken for the last two entries in table 1 were taken by preparing a Ni(lll)-p(2 X 2)0 surface as described below and exposing ethylene to that surface at 500 K.
570
,
t 240 ELEC’TRON
I
I
260
L
280
ENERGY
te’.‘)
Fig. 2. Auger spectra for carbon monoxide dissociation on Ni(ll1). Surface exposed to IOF Pa CO for 20 s and then flashed to 600 K in vacuum. (a) Adsorption T = 300 K; @) T= 450 K; (c) T=475K;(d)T=500K;(e)T=6001(;(f)T=700K.
The kinetics of Cfaibide+ 0 recombination were measued by two techniques. Nearly equal amounts of carbon and oxygen were adsorbed to 0.2.5 monotayers and the kinetic parameters were determined by (i) heating rate variation 131, Table
1
CO, /CO product ratio for Ccarhde + 0 recombination Carbon coverage (monolayers + 0.02)
Oxygen coverage (monolayers Ifr0.02)
co, /co product ratio ( i 0.03)
0.25 0.25 0.25 0.25 0.17 0.09
0.23 0.16 0.09 0.04 0.24 0.25
0.36 0.21 0.10 0.03 0.34 0.38
J. B. Benriger, R. E. Preston / Recombination
Table 2 Kinetic parameter
determinations
for adsorbed
carbide
reactions on Ni(l1 I)
and oxygen
s)
recombination
Method
Y (cm*/atom.
(i) /3, = 8,; heating rate variation (ii) 0, = ~9~;second order peak shape analysis (iii) f?, B e,; heating rate variation (iv) e, B e,; first order peak shape analysis (v) 6’, -SCtl,; heating rate variation (vi) e, =sze,; first order peak shape analysis
0.1
178
0.001
162
0.002
172
0.02
184
0.002
167
0.05
190
511
on Ni(lll)
E (kJ/mol)
and (ii) peak shape analysis assuming a second order reaction with 0, = 8,. Experiments were also attempted to prepare surfaces with equal amounts of carbon and oxygen at coverages less than 0.25 monolayers. We were unable to obtain satisfactory control of surface coverages to get good data; however, it was noted that the peak at 670 K shifted to higher temperature with decreasing coverage as expected. A second approach was to prepare a Ni(lll)-p(2 X 2)C surface, adsorb oxygen to less than 0.05 monolayers and analyze the kinetics as first order with f& as a constant; this was also repeated interchanging the role of oxygen and carbon. Heating rate variation and peak shape analyses were also done using the second approach. The kinetic parameters measured by six different approaches are summarized in table 2. The agreement among the different techniques is probably a good measure of the error associated with the determination of the pre-exponential factor and activation energy. The peak shape anlysis for kinetic parameters evolves from an expression for the reaction rate N(T)
= da,/dT
= ( v/p)u~ug
eeElkT,
where N is the desorption rate, u are surface coverages, v is the pre-exponential factor, /3 is the heating rate and E is the activation energy. If a, = uB then a plot of ln( N/u,) versus l/T will have a slope of -E/k and an intercept of ln(v//3). For the case a,., B us such that a, = constant, then a plot of ln( N/us) versus l/T will have a slope of -E/k and an intercept of ln(vu,/P). Auger spectra were correlated with TPD results to provide measurements for coverages and absolute rates. Graphite was deposited on the surface
572
Table 3 CO,/CO
J.B. Benz&w-, R. E. Preston / Recombmation reactions on Ni(/ 1 I)
product
ratio for Cgraphite + 0 recombination
Carbon coverage (monolayers * 0.03)
Oxygen coverage (monolayers f 0.02)
co, /co product ratio ( + 0.005)
0.34 0.31 0.35 0.35
0.20 0.14 0.07 0.03
0.082 0.065 0.048 0.029
showed a single carbon peak at 272 V characteristic of graphite. Exposure of a Ni(ll1) surface with adsorbed graphite to oxygen and subsequent heating produced a two peaked TPD spectrum as shown in fig. Id. Both CO and CO, desorption were oberved at 970 K and 1050 K. The CO peak at 670 K may be ascribed to a small amount of adsorbed carbide. The CO,/CO product ratio was observed to increase with increasing oxygen exposure to the surface as shown in table 3. The CO,/CO product ratio was Ni(lll)-C,,,,r,i,, much lower for the graphite plus oxygen reaction than observed for carbide plus oxygen. The TPD results were ambiguous as to whether the CO,/CO product ratio was the same for both the 970 and 1050 K desorption peaks. Graphite could not be adsorbed on a surface with adsorbed oxygen by cracking ethylene, suggesting the carbon first adsorbed as a carbide which reacted with oxygen before conversion to graphite occurred. The TPD results for Cgraphite+ 0 were unusual in several respects. The two desorption peaks did not shift in temperature with either carbon or oxygen coverage. The relative magnitude of the two peaks changed with carbon coverage; the peak at 970 K grew relative to the peak at 1050 K with increasing carbon coverage at a constant oxygen coverage of 13, = 0.15 monolayers. The relative separation of the two peaks made it possible to deconvolute them to obtain meaningful kinetic parameters from any of the methods discussed above. 3.2. Nitrogen Diatomic nitrogen was not observed to adsorb on Ni(ll1) at any temperature in the range 250-900 K. No N, desorption was observed subsequent to exposure of the Ni(ll1) crystal to 1 Pa N, for 100 s, and the Auger spectra showed no nitrogen independent of crystal temperature. Nitrogen was adsorbed on Ni(ll1) by decomposing hydrazine on the surface at 500 K. The Auger spectra showed adsorbed nitrogen and the TPD spectra showed N, desorption at 850 K as the only desorption product (see fig. 3). After the TPD experiment the Auger spectrum showed the surface to be clean. The kinetic
J. B. Benziger, R. E. Ptesron / Recombination
800
600
1200
1000 TEMPERATURE
reacrions on Ni(l I I)
573
1400
( K)
Fig. 3. TPD spectra foi recombination reactions on Ni(ll1). (a) N, desorption; ON = 0.20. (b) SO desorption; @s= 0.25, 8, = 0.20. (c) SO, desorption; 6s = 0.25, ~9, = 0.20. (d) 0, desorption; eo = 0.5.
parameters for N, desorption were determined from heating rate and coverage variations [3], as well as peak shape analysis; the results are summarized in table 4. A Ni(lll) surface with 0.25 monolayers adsorbed nitrogen was exposed to oxygen at 300 K. The desorption spectrum showed N, desorption at 850 K, but no significant NO desorption. The N, desorption peak showed only slight variations in peak shape and position due to the coadsorbed oxygen. A small NO desorption peak was observed at 870 K, but the magnitude of this peak was two orders of magnitude less than the N, desorption peak suggesting that it may have been due to defects,
Sulfur was adsorbed on Ni(lll) by cracking H,S on the crystal at 400 K. After exposing the Ni(ll1) crystal at 400 K to 10m3 Pa for 20 s, a well ordered Table 4 Kinetic parameters
for N + N recombination
Method
Y (cm*/atom
(i) Heating rate variation (ii) Coverage variation (iii) Peak shape analysis
0.006 0.04 0.001
s)
E (kJ/mol) 210 220 196
574
J.B. Benriger, R.E. Preston / Recombination reactions on Ni(Ill)
Table 5 SO, /SO product ratio for S + 0 recombination Sulfur coverage (monolayers f 0.01)
oxygen coverage (monolayers * 0.02)
so, /so product ratio
0.25 0.25 0.25 0.17 0.06
0.22 0.14 0.08 0.24 0.25
2.3 1.7 1.1 2.7 2.9
p(2 x 2)s adlayer was detected by LEED. This adlayer could be heated to 1500 K in vacuum with no desorption products detected, nor any change in the LEED pattern or Auger spectrum. When exposed to 0, at 300 K the Ni(lll)p(2 x 2)s surface adsorbed oxygen. H,S also decomposed on a Ni(ll1) surface with adsorbed oxygen at 400 K leaving sulfur coadsorbed with the oxygen. The recombination of sulfur and oxygen occurred at ca. 1150 K yielding SO and SO, desorption products. The SO product weas corrected for SO, fragmentation. The SO,/SO product ratio was found to be dependent on the S/O ratio of adsorbed atoms as shown in table 5. The kinetic parameters for the S + 0 recombination were determined by six methods listed above for C + 0 recombination. These values are summarized in table 6. The adsorption of SO, on Ni(ll1) was also examined by AES and TPD. Exposure of the Ni(ll1) crystal at 300 K to 10m3 Pa SO, for 10 s led to adsorption of 0.1 monolayer of dissociated SO, (13, = 0.1, 8, = 0.2) as determined by AES. No SO or SO, desorption were observed to occur at temperatures below 1100 K after SO, adsorption. Changing the temperature of Table 6 Kinetic parameters for S+ 0 recombination on Ni(ll1) Method
Y (cm*/atom.
(i) 19s= 8,; heating rate variation (ii) t& = 0,; second order peak shape analysis (iii) Bs B eo; heating rate variation (iv) OSB 0,; first order peak shape analysis (v) 0, -=+z eo; heating rate variation (vi) 0s -=x0,; first order peak shape analysis
0.07
315
0.002
295
0.008
305
0.004
300
0.005
295
0.04
310
s)
E (kJ/mol)
J. B. Benriger, R. E. Preston / Recombination
reactions on Ni(lll)
575
the Ni crystal from 300 to 600 K showed no detectable difference in the amount of SO, adsorption for identical exposures. The adsorption of SO was not studied as it was not available. 3.4. Oxygen Oxygen adsorbed dissociatively on Ni(ll1) at temperatures from 250 to 1000 K. Exposures to 1O-4 Pa 0, for 20 s at 300 K led to a diffuse p(2 X 2) LEED pattern. Annealing the sufface to 1000 K caused no desorption but the LEED pattern became very sharp; the Auger spectrum showed no change due to the annealing. Equivalent oxygen exposures at 300, 500 and 800 K gave identical oxygen coverages as measured by AES. TPD results did not indicate any desorption of 0 or 0, from the Ni(lll)-p(2 x 2)0 surface up to 1475 K. At 1500 K a small increase in the 0, pressure was observed indicating some desorption. Maintaining the Ni(lll)-p-(2 x 2)0 surface at 1500 K for periods of 10-1000 s led to a decrease and eventual disappearance of the oxygen Auger peak. Shown in fig. 4 are the measured oxygen coverages determined by AES after heating the Ni(lll)-p(2 X 2)0 surface to 1500 K for a fixed period of time. The Auger data indicate that oxygen desorption or diffusion into the Ni crystal are occurring. The increase in the 0, pressure indicates desorption is the more likely event. The Auger data were used to determine an isothermal desorption rate constant for 0 + 0 recombination at 1500 K of 4.3 x lo-” cm*/atom * s. Similar experiments were performed at 1490 and 1515 K. These
*.
‘\ ‘\ ‘\ ‘\
-.
. . 0
--_. --._ ----_ 0
20
--__
40
---_
q -_
60
---____
q ----_
80
----__
II
TIME (seconds)
Fig. 4. Oxygen coverage on Ni(ll1) as a function of time at 1500 K (dashed line is for k = 8 X lo-’ S-1).
J.B. Benriger, R.E. Preston / Recombination reactions on Ni(l II)
516
data were then used to obtain kinetic parameters for 0 + 0 recombination. The values obtained were v = 10e2 * 2 cm2/atom. s, E = 440 f 50 kJ/mol. When the Ni(ll1) surface was exposed to oxygen beyond that which gave a p(2 X 2) LEED pattern the oxygen Auger signal increased and the p(2 x 2) LEED pattern became obscured. At an oxygen coverage of 0.5 monolayers the TPD spectrum showed 0, desorption became appreciable at 1450 K as indicated on fig. 3. The desorption rate maximum was not achieved by 1500 K, and holding the crystal at 1500 K led to the desorption of all the oxygen as determined by AES. Heating to 1500 K caused the oxygen in excess of 0.25 monolayers to rapidly desorb; after each experiment when the crystal was cooled immediately after heating to 1500 K a sharp p(2 x 2) LEED pattern was observed.
4. Discussion The recombination/desorption reactions of nitrogen and carbon monoxide on Ni surfaces has been observed in previous studies [4-121, but the kinetics for recombination were not accurately measured. The kinetic parameters for recombination may be understood in terms of transition state theory. For a second order surface recombination reaction the rate of reaction is r=-
Q, -kT Q,,Q,, h
( 1 Ac/kTcJAoB, e_
where Q,, is the partition function for absorbed species i, Q, i the partition function for the transition state and AC is the difference in zero point energies. A reasonable approximation for de is AC = -AH,, + Edis, where AH,, is the adsorption enthalpy and Edis is the activation barrier for dissociation. Assuming the adsorbed adatoms are mobile on the surface at conditions where recombination occurs, and that the transition state also freely translates and rotates in two dimensions then e,= Q&-h
(8r21A,+kT/h2)“2 (2vkT/h2)(
MAMe)“’
’
where I,, is the moment of inertia for AB in the transition state, and MA and MB are th> masses of A and B. This result suggests that the pre-exponential factor for all the recombination reactions studied here should be approximately 10w3. This value is the lower limit of the experimental values deThe termined here for Carbide + 0, N + N, S + 0, and 0 + 0 recombination. experimental data suggest that the transition state may have more freedom than ascribed to it above; for example, it may have a tumbling motion as well as rotating in the plane of the surface, giving a pre-exponential greater than
J. B. Benziger, R. E. Presron / Recombinarion
reactions on Ni(l I I)
577
10s3. However, the scatter of the data indicates that overinterpretation of the data must be viewed cautiously. was unique among the reactions studied The ‘graphite+ 0 recombination here as it appeared to be governed by first order kinetics. The nature of adsorbed graphitic carbon can account for this anomaly. Adsorbed adatoms, such as S, N, and 0, all exist on the surface as nearly independent entities with only weak interactions between adatoms. Carbon adsorbed in the form of carbide also behaves independent of adatoms. In the form of graphite the carbon adatoms are che~cally bound to one another and would exist in a condensed phase. the reaction between Cgraphile and 0 then occurs when an oxygen atom diffuses to the perimeter of the graphite phase. The rate of reaction is proportional to the rate with which adsorbed oxygen encounters the perimeter of the graphite phase, which should be first order in oxygen coverage and nearly independent of carbon coverage, except at very low carbon coverages. This accounts for the first order reaction kinetics observed by TPD. Another unusual feature of the Cgraphite+ 0 recombination was the two desorption peaks observed. This may be due to a phase transition of the adsorbed graphite. Eizenberg and Blakely [14] observed two phase transitions of graphite adsorbed on Ni(ll1). At 850 K the observed bulk graphite went into a solid solution on the Ni(ll1) surface leaving a surface monolayer of adsorbed graphite. At temperatures above 970 K, a second phase transition occurred and more carbon went into solution such that the bulk and surface concentrations of carbon were equal. This second phase transition would cause a decrease in the surface carbon concentration and hence reduce the rate of C sraphite+ 0 recombination, as was observed in the TPD experiment. An interesting feature in both the carbon and sulfur oxidation reactions was the evolution of dioxides coincident with the monoxides, which scaled with oxygen coverage. These results indicated that the recombination reactions form an adsorbed monoxide intermediate that has a finite surface lifetime during which it can further react. The coincidence of the product peaks indicates that the monoxide is more weakly bound than the adatom (i.e. C or S), so that the rate limiting step is the initial r~mbination. The activation energies for desorption measured here may be used to estimate adatom adsorption enthalpies. For the reaction A(a) + B(a) --* AB(a) + AB(g), the activation
energy is approximately
E =A~~(AB(g))-A~~(A(a))-A~~(B(a~)-~~i~~
If AB dissociation proceeds via molecularly only occurs when sufficient energy is available B(a), which has an activation energy E,b’is= -AH,, (AB(a)) + Edia, where AH,,(AB(a)) is the heat of adsorption
adsorbed AB then dissociation for the reaction AB(a) -+ A(a) +
of molecular
AB.
578
J.B. Benriger, R.E. Preston / Recombination
reactions on Ni(lll)
Table 7 Energetics for dissociative adsorption and recombination Molecule
A H, (AWg)) a) (kJ/mol)
02
0 0
NZ co so
Erecombinstion
Edissociation
AfffMO+
(kJ/mol)
(kJ/mol)
(kJ/mol)
440 210 175 310
-100 +6
0 > 210 15 0
A H&W)
-440 >o 190 316
a) From ref. [15].
Auger spectra were taken for fixed exposures of Ni(ll1) to O,, N,, CO and SO,. The activation energy for dissociative adsorption may be determined from temperature dependence of the coverage (0) of dissociated molecules: E&
=-
As 0, and SO, dissociative adsorption were independent of temperature, it appears that Edis = 0 for both SO and 0,. (The value for SO is assumed to be the same as for SO,.) Nitrogen dissociation was never observed, indicating that the rate of dissociation is less than the rate of recombination, suggesting that for N,. The data in fig. 2 indicate that for CO, E& = 135 + Edis ’ Erecombination 20 kJ/mol; the adsorption enthalpy for CO on Ni(ll1) has been measured to be 120 kJ/mol[16], so Edis = 15 kJ/mol for CO. The reaction energetics are summarized in table 7. These values may be used to estimate heats of adsorption of 0, N, C, and S on Ni(ll1). Temperature corrections should be made to the various enthalpies. However, uncertainty in how to compensate for surface heat capacities and the uncertainty in the experimental data make this refinement somewhat questionable. Table 8 compares estimated heats of adsorption to the heats of formation of bulk nickel carbide, nitride, oxide and sulfide. The comparison shows reasonable agreement, supporting the notion that bulk thermodynamic properties may be Table 8 Adsorbed species
Estimated heat of adsorption (kJ/mol)
Heat of formation of bulk compound at standard conditions (kJ/mol)
Carbide Nitrogen Oxygen Sulfur
+70 >o - 220 -84
+ 46 (Ni 3C) + 1 (Ni,N) - 224 (NiO) - 72 (NiS)
a) From ref. [17].
J. B. Benziger, R. E. Preston / Recombination reactions on Ni(l II)
519
used as estimates for adsorption enthalpies. The estimates obtained here also show reasonable agreement with heats of adsorption for oxygen [18,19] and sulfur [20] on nickel surfaces obtained by direct thermodynamic measurements. Clearly the thermodyna~c measurements are more accurate; however, the relative ease of obtaining estimates by TPD makes this an attractive alternative.
Acknowledgments The financial support of both the National Science Foundation and the Air Force pffice of Scientific Research are gratefully acknowledged.
References [l] T.W. Haas, J.T. Grant and G.J. Dooley, in: Proc. 2nd Intern. Symp. on Adsorption/Desorption Phenomena, Ed. F. Ricca (Academic Press, London, 1973). [2] L.E. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach and R.E. Weber, Handbook of Auger Electron Spectroscopy (Physical Electronics, 1976). [3] P.A. Redhead, Vacuum 12 (1962) 203. [4] H. Conrad, G. Ertl, J. Ktppers and E.E. Latta, Surface Sci. 50 (1975) 296. [5] M. Grunze, R.K. Driscoll, G.N. Burland, J.C.L. Cornish and J. Pritchard, Surface Sci. 89 (1979) 381. (61 G.L. Price and B.G. Baker, Surface Sci. 91 (1980) 571. [7] C.W. Seabury, T.N. Rhodin, R.J. Purtell and R.P. Merrill, Surface Sci. 93 (1980) 117. [8] W. Erley and H. Wagner, Surface Sci. 74 (1978) 333. [9] H.H. Madden and G. Ertl, Surface Sci. 35 (1973) 211. [lo] R. Sau and J.B. Hudson, Surface Sci. 102 (1981) 239. [ll] E.G. Kein, F. Labohm, O.L.J. GiJzeman and G.A. Bootsma, Surface Sci. 122 (1981) 52. [12] P.H. Holloway and J.B. Hudson, Surface Sci. 33 (1972) 56. [13] K.J. Laidler, in: Catalysis I, Ed. P.H. Emmett (Reinhold, New York, 1956). [14] M.E. Eizenberg and J.M. Blakely, Surface Sci. 82 (1979) 228. [15] S.W. Benson, in: ~e~~he~cal Kinetics (Wiley, New York, 1968). 1161 H. Conrad, G. Ertl, J. Kippers and E.E. Latta, Surface Sci. 57 (1976) 475. [17] 0. Kubaschewski, E.L. Evans and C.B. Alcock, in: Metallurgical Thermochemistry (Pergamon, New York, 1967). [18] D. Brennan, D.O. Hayward and B.M.W. Trapnell, hoc. Roy. Sot. (London) A256 (1960) 81. [19] H.J. Grabke and H. Viefhaus, Surface Sci. 112 (1981) L779. [20] J.G. McCarty and H. Wise, J. Chem. Phys. 72 (1980) 6337.