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The adsorption and reaction of CO and O2 on PdAu alloy wires
Surface Science Ill (1981) 325-343 North-Holland Publishing Company
THE ADSORPTION WIRES
325
AND REACTION OF CO AND O2 ON Pd-Au
ALLOY
D.D. ELEY and P.B. MOORE * Dep~rt~~~t of Chemistry, ~rliv~rsity of ~~tt~ng~~~rn~University Park, ~ottir~gh~~l NG7 2RD, UK
Received 16 October 1980; accepted for publication 6 July 1981
Tempera~re programmed desorption (TPD) of CO and 02 on PdAu alloy wires has been studied. The heat of adsorption, sticking coefficient and maximum coverage of CO were recorded for Pd, 83 Pd 17 Au, 60 Pd 40 Au. For Pd and Pd-rich alloys the heat of adsorption remained fairly constant but the maximum coverage fell markedly from 0.42 for Pd to less than 0.05 for bulk palladium atom fraction yPd Q 0.83. The heat of adsorption, sticking coefficient and maximum coverage of 02 were investigated for pure Pd. A very limited adsorption was recorded on 83 Pd 17 Au and none on the more Au-rich alloys. The adsorption data are used to discuss the CO + 02 reaction. Activation energy and frequency factor are estimated on Pd, for the TPD conditions used here. Earlier rate constants (0.2 Torr, 150°C) for CO + 02 on PdAu as a function of Au content correlates with the maximum coverage of chemisorbed CO, which in turn is correlated with the probability of finding a Pdgtr ensemble in the surface. Modern results on the d-band structure of the PdAu alloys suggest that the Pdg ensemble, i.e. a surface Pd atom without an Au atom in its coordination shell, would tend to optimise both the donor and acceptor actions of the Pd atoms involved in chemisorbing CO.
1. Introduction The carbon monoxide-oxygen reaction on Pd-Au alloy polycrystalline wires was investigated by Daglish and Eley [ 11 as part of a general programme of reactions on these alloys 12-41. The apparent activation energy had a high value, 118. 167 kJ mole-’ on Pd and the Pd-rich alloys, declining abruptly at about 40 at% Pd to a value around zero. It was natural to relate this result to the’ disappearance of “holes in the d band” which also occurs at 48 at% Pd, on the rigid-band theory generally accepted at that time [2]. However, there were problems apparent even in the original presentation. For example, the fastest reactions occur on the Pd wires, where the kinetics (rate aP02P& seemed the most likely law) clearly point to inhibition by strongly adsorbed CO displacing adsorbed 02, with a corresponding * Department of Chemistry, Ellison Building, Newcastle Polytechnic, Upon-Tyne, NE1 3ST, UK.
0039-6028~81~0000-0000/$02.50
0 1981 North-Holland
Ellison Place, Newcastle-
326
D.D. Eley, P.B. Moore /Adsorption
and reaction of CO and 0,
OHPd-Au
augmentation of the activation energy by twice the adsorption energy of the CO. The further elucidation of this problem requires a study of the temperature programmed desorption of both gases from the alloys, which forms a major part of the present paper. There have been many studies of CO oxidation on Pd in recent years, incidentally, with the balance of evidence favouring P& rather than P& kinetics [5], implying CO inhibition of single site, rather than 2 site oxygen chemisorption. Equally important the concept of a common d-band for the alloys has attracted criticism from the results of photoelectron spectroscopy which favours, for Pd-Ag [6] and Pd-Au [7], two separated d-bands for the two component metals. A further view regards the catalytic activity as determined by the separate surface atoms, with neighbours exerting electronic or “ligand” effects, and associates adsorption and reactivity with particular Pd,Au, surface atom ensembles [8,9]. This has led us to the work described in the present paper. Auger analysis has already established that the presently used PdAu wires, outgassed by heating in vacua, show the surface to have an average monolayer enrichment factor, referred to Pd, of only 0.6, so that there is only a small surface enrichment in Au [lo].
2. Experimental Experiments were carried out in a stainless steel vacuum system (base pressure 4 X lo-” Torr) which was equipped with a 4 grid retarding field electron energy analyser and a normal incidence electron gun for Auger electron spectroscopy. The Pd-Au alloys, in the form of 32 swg wires, were spot welded between oxidised iron supports and were heated by passage of a controlled electric current. The temperature-resistance characteristics of the alloy were determined and were used to mea-
Fig. 1. Schematic diagram of the wire temperature controller.
D.D. Eley, P.B. Moore /Adsorption
and reaction of CO and 02 on Pd-Au
327
sure and control sample temperatures. Fig. 1 shows the essential features of the temperature controller. A voltage divider compares voltage drop across the sample and across a standard 1 S’2resistor in series with the sample. The sample resistance thus calculated is compared in a differential amplifier with a reference voltage equal to the desired sample resistance. The amplifier output is used to control the current through the sample and hence the sample temperature. A resistance control of better than 10m3 n is readily achieved with sample resistances, R, in the range 0.3 < R < 10 a at currents up to 10 A. Temperature programmed desorption is achieved by substituting a ramped voltage for the fixed reference voltage. All desorption spectra were recorded with a Bayard-Alpert ion gauge and direct plotting of the logarithmic out-put was used to accentuate the shape of desorption peaks. The species desorbed were analysed using a quadrupole mass spectrometer scanning the mass range lo-50 AMU at 1 s intervals during desorption. This arrangement was preferred to scanning a single mass number because of its greater reproducibility and the unambiguous identification of most peaks resulted. Gases employed were “Research Grade” (BOC) and were admitted through a variable leak valve. For exposures less than 3 L (1 L = 10S6 Torr s) exposure times were typically 100 s whereas exposure times were progressively increased for higher doses but pressures were not allowed to exceed 10e6 Torr. Hydrogen used in the cleaning of 60% Pd-Au was purified by passage through a palladium-silver thimble. Prior to insertion into the UHV system, all samples were degreased in chloroform, ether, acetone and rinsed in deionised water. After bakeout (200°C) and annealing at 1000 K for 6 h in a vacuum better than 5 X lo-” Torr, the samples were heavily contaminated with sulphur, carbon and oxygen. The impurities, carbon and sulphur, were removed from Pd and 83% Pd-Au by oxidation at 1000 K in 10m6 Torr O2 for 16 h and the surface oxide formed was decomposed by annealing at the same temperature for a further 8 h. A final 1.5 min of oxidation and annealing produced a surface which appeared uncontaminated (as judged by AES) and did not recontaminate during prolonged heating at red heat. As a result of such treatments, 60% Pd-Au became covered in a stable oxide which could not be thermally decomposed. A reduction was effected by heating to 1100 K in 10e2 Torr H,. The formation of similar stable oxide surfaces was observed for 26% Pd-Au and li% Pd-Au but not for pure Au.
3. Results 3.1. Chemisorption of CO on Pd-Au
alloys
The thermal desorption spectra of CO from Pd, 83% Pd-Au, 60% Pd-Au and Au are shown in figs. 2-5. In all cases, the desorption maxima do not shift appreciably with change in fractional surface coverage, 0, although a low temperature
spectra
spectra
Fig. 3. Desorption
@6L
500
/ K
I
600
70
for CO from an 83% Pd-177r’
for CO from a Pd wire initially
Temperature
Fig. 2, Desorption
20-
25-
30-
35-
30-
15-
o-
200
600 400 500 Temperature/K
700
of gas shown.
of gas shown (1 L = 1 !I@ Torr s).
300
at 180 I; or 298 K to the quantities
at 298 K to the quantities Au wire exposed
exposed
3.5 -
40-
800
D.D. Eley, P.B. Moore /Adsorption
and reaction of CO and O2 on Pd-Au
m
I
0
e
I
R 6
I
I
Lp
In
0
1 0
I
0 6oL
X JJO$@
z I
cu
aJfKSaJd
I 0
330
D.D. Hey. P.B. Moore /Adsorption
and reaction of CO and 02 on Pd-Au
shoulder is clearly visible for Pd (fig. 2). Thus, it is concluded that the desorption processes are first order and that the desorption activation energy Ed is essentially independent of 0 over the ranges studied. The saturation coverage of CO on Pd and 83% Pd was measured by rapidly desorbing the gas (flash rate fl> 100 K s-r) into a closed vessel. Corrections were applied for pumping of the gas by the ion gauge and adsorption on the vessel walls. The maximum coverage of CO on Pd was found to be (4.84 * 0.14) X 1014 molecules/cm’ averaged over the whole length of the wire sample. If we assume [ 1 l] that there are 1.16 X 10” surface Pd atoms per square centimetre of wire, a value of emax = 0.42 * 0.012 is obtained. However a substantial error in 8,,, arises from the temperature gradients present along the wire during the desorption. Thus, we will assume that, in keeping with indications from LEED studies [5], the maximum coverage at the temperatures of our experiments was emax = 0 5. Coverages 0 < 8 max were calculated from the relative integrated areas under the desorption curves calculated using Redhead’s eq. (3) [12] and our measured pumping speeds. Table 1 shows the maximum coverages recorded for alloys relative to palladium (e,,, = 0.5). The desorption energies (table 1) were calculated from the temperature corresponding to the maximum in the desorption spectra assuming first order kinetics [ 121. It is apparent that the total quantity of CO adsorbed at 295 K falls rapidly from 0 = 0.5 for Pd to 0.04 for 83% Pd-Au and to approximately 1.2 X 10e3 for 50% Pd-Au and Au itself. By contrast, the activation energy for desorption decreases only 23.9 kJ mol-’ from 126.7 for Pd to 102.8 kJ mol-’ for 60% Pd-Au. The sticking coefficient, S, is the ratio of the number of molecules adsorbed by the sample to the number incident on the sample surface, per unit time. This was
Table 1 Adsorption At % Pd
of carbon Adsorption temperature
monoxide
on palladium-gold
alloys
Number of peaks
Desorption
Ed (kJ mol-l)
1+sa S 1 1+s 1 1 1 1 1 1
1 ? 1 1 ? ? 1 1 ? ?
126.7 96.1 111.6 83.6 102.8 115.0 67.3 67.6 72.4 101.2
e max
Sticking coefficent (initial)
(K) 100 83 60
0
298 180,298 223 303 380 143 173 218 293
a S: shoulder. b Assumed value.
0.5
b
0.91 * 0.15
4.02 x 1O-2 6.30 x 1O-3 1.19 x 10-3 <10-4
2.96 1.62 3.96 1.14
x 1O-2 x 1O-2 x 1O-3 x 10-s
0.19 f 0.02
S
D.D. Eley, P.B. Moore /Adsorption
0
0.5
1.0
1.5 Exposure/
and reaction of CO and O2 on Pd-Au
2.0 Langmuir
Fig. 6. Relative surface coverage @/b~,,,~~ versus CO exposure Au (o), (iii) 60% Pd-40% Au wires (0).
2.5
331
30
for (i) Pd (o), (ii) 83% Pd-17%
calculated for Pd and 83% Pd-Au and is shown in table 1. It was found that CO has a sticking coefficient close to unity for 0 < 0.32 and thereafter the sticking coefficient falls rapidly and is accompanied by the appearance of a shoulder in the desorption spectrum (fig. 2) corresponding to a lower energy bonding state. The low sticking coefficient recorded for 83% Pd-Au reflects the low surface coverage for the system. An indication of the reactivity of the CO adsorption sites in the surversus exposure (fig. 6). This suggests that, face is obtained by plotting e/6,,, because S - 1 for pure palladium, that the ratio of incident molecules to adsorption sites per unit area exceeds unity implying that such sites are ensembles of molecules on which chemisorption occurs via a mobile precursor state on surrounding atoms. Examination of the desorption plots of CO adsorbed on 60% Au at low temperatures (223 K) in fig. 4 reveals that for low exposures, there is a distinct shoulder on the desorption trace which disappears at higher coverages. The higher energy state corresponds to Ed = 100 kJ mol-’ whereas the lower one corresponds to Ed = 84 kJ mol-’ , the former figures corresponding well with the desorption energy of 102.8 kJ mol-’ measured for adsorption at room temperatures. Thus, it is apparent that at lower temperatures, a distinct form of chemisorption occurs which eventually far outweighs the less populated higher energy state as evidenced by the disappearance of the shoulder at high exposure. The desorption traces for gold suggest that the process is first order. From the position of the desorption maximum, the desorption energy Ed is estimated to be 67.3 kJ mol-’ . The coverage increases markedly as the adsorption temperature is lowered, this is expected because the desorption peak occurs at approximately 290-310 K. This desorption energy is markedly lower than that measured for Pd and broadly corresponds to the lower binding energy state recorded for 60% Au. The maximum coverages recorded are still low - typically only a few per cent, nevertheless, saturation still occurs at low exposures -0.4 L. The low coverages
332
D.D. Hey, P.B. Moore /Adsorption
alld reaction ofC0
and 02 on Pd-Au
may arise from the reversibility of a CO chemisorption on gold since, prior to gas desorption, the system was evacuated to typically 3 X lo-” Torr (4 X 10m8 Pa). During the evacuation, equilibrium between the gas phase and surface coverage may have been partially reestablished leading to low surface coverages. In conclusion, it is apparent that as generally reported CO strongly adsorbs on palladium with a coverage -0.5 and that under such conditions, the adsorption is characterised by at least two distinct binding states. However, for 83% Pd-Au adsorption occurs only to a limited extent despite its apparent strength (Ed = 111.6 kJ mol-‘). For 60% Pd-Au and Au, only a vestige of the strong adsorption remains, and a weaker adsorption appears at lower temperatures. 3.2. Oxygen desorption studies Temperature programmed desorption traces for palladium are shown in fig. 7. The adsorption was carried out at 490 K to minimise interference from the residual CO within the system whilst remaining below the temperature (520 K) at which a surface oxide forms on Pd(ll1) faces [ 131. It can be seen that after 2.7 L exposure to oxygen the system approached saturation and large exposures gave only moderate increases in uptake (fig. 9). Saturation corresponds to (3.67 + 0.04) X 1014 oxygen atoms per square centimetre and is equal to a fractional coverage 0 of 0.3 1. At low coverages oxygen was desorbed at 808 K and this temperature (T,,,) progressively decreased to 758 K at saturation. It may be expected that oxygen desorption follows second order kinetics and accordingly a plot of log,(neT&,,) where no is the initial coverage, was found to be linear. From this versus 1IT,,, plot, a desorption energy of 177 f 20 kJ mol-’ was calculated for oxygen and second order kinetics were inferred. A plot of coverage vs exposure to oxygen was linear for 0 < 19< 0.15 and a sticking coefficient of 0.36 f 0.04 was calculated (fig. 9). Desorption traces for oxygen adsorbed at room temperature were strikingly different and sometimes difficult to reproduce. They were characterised by the appearance of an additional peak at low exposures (<3 L) which had a Tmax value of 415 K (fig. 8). This peak arose from the desorption of CO? into the system. It is well established that CO* does not strongly adsorb on palladium, thus it is concluded that the peak arises from reaction between residual CO and oxygen and that the appearance of the peak at 415 K is governed by the activation energy for CO oxidation catalysed by palladium. It may be envisaged that the reaction proceeds by either a Langmuir-Hinshelwood (I) or Rideal-Eley (II) mechanism, c&d, co
+ Oads -+ co2 + Oads + co2
.
3
(I) (11)
If the residual pressure in our system (<4 X lo- lo Torr) during the experiment was due solely to CO then throughout the duration of the process 1.6 X 10” mole-
D.D. Eley, P.B. Moore /Adsorption
and reaction of CO and 0,
on Pd-Au
333
A 270L 14OL
500
600
700 800 Temperature/K
900
Fig. 7. Desorption spectra for 02 from a Pd wire initially exposed at 490 K to the quantities of gas indicated.
cules/cm2 would strike the Pd surface. However, at least 2.10 X 1013 molecules/ cm’ of CO1 were released during the desorption and clearly reaction (II) could not lead to the observed quantities of CO*. Assuming a sticking coefficient of unity for CO, the quantity of CO needed to lead to such a CO2 peak could be adsorbed at the base pressure of the system in 186 s or approximately half the time between successive desorption experiments. Thus, it appears that CO? is formed from CO in the residual gas co-adsorbing with oxygen. Above 415 K, the activation energy is overcome and the reaction proceeds via a Langmuir-Hinshelwood mechanism. At increased oxygen exposures, a third peak with a desorption maximum at ca. 500 K becomes increasingly important and at exposures in excess of 100 L became comparable with the O2 peak itself. This peak was due to CO desorption and at exposures >300 L became the predominant peak whilst the oxygen peak at 758 K
7
,
600
9
0.25
2
4
6 8 Exposure,/
10 12 Langmuir
14
16
Pd wire initially exposed at 298 K to 3 L of oxygen when the partial pressure of CO in the system is less
800
Fig. 9. Surface coverage of oxygen versus 02 exposure for a Pd wire at 490 K.
from a
Temperature/K
I
400
r?
co2
Fig. 8. Desorption spectra for 02 than 4 X lo-” Torr.
x L
p
0
8
9
D.D. Eley,
P.3. Moore /Adsorption
apld reactim of CO and 02 on
Pd-Au
335
became progressively reduced. At these high exposures, the COa peak is merged with the low temperature shoulder on the CO peak. The particular doses at which the CO peak became dominant were variable, lending further support to the argument that such effects were merely due to residual CO. Thus, it is concluded that a relatively small amount of oxygen (0 - 0.31) adsorbs on Pd and that CO initially adsorbs onto unoccupied sites. It is uncertain from our studies whether, at higher CO coverages, oxygen is displaced or whether excess CO reacts with oxygen atoms to produce COa leading to an apparent reduction in the size of the oxygen peak at 758 K. Studies by Ertl and co-workers have shown that over periods of time, CO will displace Oads and it seems probable that such a process is occurring here [ 141. The measured uptake of oxygen by 60% Pd-Au and pure Au was minimal. These materials did not adsorb oxygen (19< 2 X 10b4) and after large exposures to oxygen only the desorption of quantities of CO were recorded. The desorbing gas was unambiguously identified as mass 28 and the quantities of gas, their desorption energies and the desorption peak shapes were strictly comparable with the desorption spectra recorded after CO adsorption on these materials. Thus it was concluded that oxygen adsorption on Au at 142,173,218,293 K and on 60% Pd-Au at 223, 303 and 380 K was virtually non-existent and that 00~ < 2 X 10e4 in all instances. A small uptake of oxygen was recorded for 83% Pd-Au at 278 and 180 K after prolonged exposure (-250 L). Under such conditions a single desorption peak with a Max of 470 K was recorded. This peak comprised both oxygen and carbon monoxide in the approximate ratio 1 : 3 and the quantities of CO evolved were comparable with those recorded during pure CO adsorption studies. It is interesting to note in view of the low activity of 83% Pd-Au as a catalyst for CO oxidation that the desorbed gases were CO and OZ, not COa. 3.3. Calculation ofact~yation energy and ore-exponential factor for the La~~~~ir~i~~~elwoo~ reaction The activation
energy and pre-exponential
factor for the reaction
CO,& f O&s -+ CO2 , can be calculated from the CO2 desorption peak produced by the reaction (fig. 8). The rate of production of COZ, r (molecules cm-’ s-i) is given by: r = kBoOco = Ai30dco exp(-E,/RT)
,
0)
where 80 and t3c~ are the surface coverages of 0 and CO atoms (mole~~es~cm’). If the wire temperature T is increased linearly at a rate fl (K s-r) from a temperature Te, such that T=To+/3t, then a maximum
(2) in the desorption
trace, TmaX,will occur at a temperature
given by
336
D.D. Eley, P.B. Moore /Adsorption
and reaction of CO and 0,
on Pd-Au
If Gl,, is the pressure increase (in Torr) at the point of maximum rate then, following
the treatment
of temperature
programmed
desorption
given by Redhead
1121, (3) where S = pumping speed, K = 3.27 X 10’ 9 molecules Torr-’ l-r, n = sample area. Similarly, it may be shown that for second order kinetics, the coverage of CO at T max will be BcO = $3& where i3& is the initial CO coverage. However, the curves obtained had a shape typical of a first order desorption process indicating that 0: > t$, giving pseudo first order kinetics. Here, at Tmax, OCO= t&/2.718. e0 may be calculated from the initial surface coverages es, 0& using the equation B0 = e”, @, + ecO. The initial surface coverage of CO, et,, is given by
e;,
=-I_ q-P& akT V
dt )
where V = volume of system, and where the integration peak only. Similarly
is carried out over the CO2
Using this equation, a value of E, = 73.4 kJ mol-’ may be calculated. Substitution of this value back into eq. (1) allows a value of A equal to 3.88 X lo-’ cm-* molecule-’ s-l to be calculated. Recent studies of CO and 0, adsorption on Pd( 111) surfaces have indicated that separate domains of CO,d and Oad may be present and that reaction could occur at domain boundaries [S]. Changes in the LEED patterns of oxygen covered Pd(ll1) surfaces were recorded after exposure to 0.6 L carbon monoxide [15] which would correspond to 0 - 0.15 if the CO sticking coefficient remains essentially unaltered. In our experiments, B0 - 0.25 and ecO - 0.05, thus it seems unlikely that compression of any oxygen domains occurred.
4. Discussion 4.1. Electronic structure of the PdAu alloys PdAg and PdAu both form a continuous series of homogeneous, random fee alloys, the band theory of which has recently been revised following the results of optical, UPS and XPS spectroscopies. In PdAg there are separate d-bands for Pd and Ag, the Pd d-band near the Fermi level and the Ag d-band lower by some 2.5 eV. The situation is similar for PdAu, except the d-band separation is less (there is some
D.D. Eley, P.B. Moore /Adsorption and reaction of CO and O2 011Pd-Au
337
overlap) [ 171. As Ag or Au atoms are added to pure Pd, the Pd d-band simultaneously narrows and shifts downwards, so that the 0.36 holes in the Pd d-band per Pd atom [18] decrease linearly to zero at the 52 at% Au or Ag concentration, where classical measurements have established the onset of diamagnetization [2] (often given on average as 60 at% noble metal). Above this concentration of Ag or Au, the Pd delectrons are localised in the so called virtual bound states [ 19,201 any paramagnetic effect of which is outweighed by the bulk diamagnetism of the alloys. In summary, although holes remain in the d-shells of the Pd atoms at all concentrations of Au or Ag, at 52 at% Au or Ag, the Pd collective d-band has narrowed to the point of disappearance, to be replaced by localised levels [2 11. Although in the case of Ag and Pd Norris and Myers [21] have postulated some electron transfer Ag + Pd, in practice the coherent potential approximation [22] which successfully predicts separate Pd and Ag d-bands [23] assumes no such transfer. In the past the catalyst chemist has looked for correlations with holes, or densities of states at the Fermi level in a single rigid PdAu-band ]2], but must now transfer his attention to the single Pd d-band. We consider successively CO chemisorption, O2 chemisorption, and the CO t O2 reaction, on Pd, Au and finally PdAu alloys. 4.1.1. Clzemisorption ofcarbon monoxide Our desorption activation energy Ed = 127 kJ mol-’ for a Pd wire, may be comPd wire [ 141, and indi~dual latpared with 134 kJ mol-’ , also for a polycryst~line tice plane values of 142 (11 l), 153 (loo), 167 (1 lo), 149 (210) and 146 kJ mol-’ (311) [5]. It seems probable that the wires exhibit an appreciable fraction of (111) plane, where LEED points to CO chemisorbed over interstices between 3 Pd atoms on this face, with a possible change above surface fraction 0 = 0.33 to bridge bonding to 2 Pd atoms [24]. This might relate to our sticking coefficient, as may be inferred from fig. 6, which maintains its initial value of 0.91 (table 1) up to B = = 0.7, when it falls to 0.07, corresponding to adsorption of the 0.35, i.e. e/e,,, lower energy species which is also manifest as a shoulder on the TPD spectrum. For CO on Au the maximum 8 values were achieved rapidly, i.e. within 1.3 to 4 L, at all temperatures, and were very small, viz. 0.011 at 293 K and 0.029 at 143 and 173 K, the heat values at these two temperatures being, respectively, Ed = 101.2 kJ mol-’ and _Ed= 67.3 kJ mol-’ . The heat values may be compared with literature values of 56 [25] and 47.3 kJ mol-’ [26] for polycrystalline Au, and 58 kJ mol-’ falling with coverage to 16 kJ mol-’ on Au(100) [27]. The rapidity of adsorption suggests a mobile precursor state leading to a final surface defect state, possibly at a grain boundary. Our average surface enrichment factor for the alloy wires ilO] allows us to calculate vacuum surface atom fractions X”’ in equilibrium with bulk atom fractions XA” viz. X9 = 0.17 in equilibrium with X”” = 0.25, and Xi” = 0.40 with XA” = 0.53: in fig. 10 the linear change in logloec~ from pure Pd to X‘$” = 0.4, would be little changed by using vacuum surface values XA”. The Boo then remains (apparently) unchanged from Xi” = 0.4 to pure Au. There is a corresponding but much less marked change in the Ed value, which suggests that while
338
D.D. Hey, P.B. Moore /Adsorption
r
20
and reaction of CO and O2 on Pd-Au
w”
10 1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
I.0
XAU B
Fig. 10. A comparison of the maximum CO coverage ( -), the rate constant at 423 K relative to pure Pd (- - - - - -), the activation energy (- . - . -) and preexponential factor (. . . . . .) for CO oxidation, and the CO desorption tion of bulk atom fraction X6”.
energy (- - -) for palladium gold alloys, as a func-
alloying Au with Pd does lower the site binding energy for CO, there is also a strong decrease in the number density of sites suitable for CO adsorption. We shall now enquire whether a particular Pd,Au, ensemble in the surface is responsible for CO adsorption. First, we note that if a CO is linearly bonded to a single Pd atom on a (111) or (100) face say, and this Pd atom is “liganded” to its next neighbours in the sub-surface layer, then this will contribute a lo-ensemble on the (111) face and a 9ensemble on the (100) face. Turning to multicentre sites, there are two 3 coordinates sites on (11 l), one of which is a 3ensemble, the other a 4ensemble of 3 surface atoms and 1 immediately sub-surface. On the (100) face there is a single multicentre site comprising a square pyramid of 5 metal atoms, which becomes a 9-ensemble if we include sub-surface nearest neighbours. So we conclude that if experiments point to ensemble sizes of 8-10 atoms, then ligand effects involving nearest neighbours are probably involved. Dowden [8] has calculated a probability Pk,,, for a binary alloy AB, of finding a surface atom A part of an ensemble of n atoms of which at least m are of one particular type (A), given by m=n-1
Pnm=
c x* (nil )(X*)m(XB)n-m-l m
)
D.D. Eley, P.B. Moore /Adsorption
0
I
I
I
0.1
0.2
0.3
I
0.4
and reaction of CO and O2 on Pd-Au
I
05 XAU
I
0.6
I
0.7
I
0.8
339
I
0.9
I.0
Fig. 11. A plot of log1 oP& (ensemble population) for 5, 8,9, 10 palladium atom ensembles as a function of surface atom fraction X Au, together with the maximum CO coverage (0) at 298 K.
where XA is the surface mole fraction of A. This equation has been used to evaluate the curves of fig. 11, and the experimental points are seen to lie on the Pb,* curve. If we now take the case where all n atoms of the ensemble are of type A, e.g. Pd, then we substitute m = n - 1 and the above equation simplifies to P&1
= (X”)” .
This leads to kA
log,P&r
= CA .
Thus the gradient of log$‘A,._i versus XA will yield a value of n, the total number of atoms in the ensemble. In general, P’,, is not measured directly, but rather the amounts of gas chemisorbed or the rate of a catalytic reaction. For chemisorption it is reasonable to assume that Omax a Pk,,, and hence across a series of alloys a plot of log,O,,, versus XA will have a gradient n/XA. By measuring the slope of this curve (fig. 11) at
340
D.D. Elev, P.B. Moore /Adsorption
arld reactiotl of CO arld 0,
on Pd-Au
several points including XA = 1, we calculate that the ensemble size responsible for CO adsorption is 9 + 1 Pd atoms. The size of the ensemble involved and the relatively small change of Ed from pure Pd to 60% Pd-Au makes the assumption implicit in the calculation, i.e. AHa, the heat of adsorption of CO, is independent of 0. seem valid. This effectively means the active site is a Pd atom which does not contain an Au atom in its coordination shell. This effect might arise in either of two ways: (a) The Blyholder model for CO adsorbed on transition metals involves two “donor” links 0 = C ++Pd (see Batra and Bagus [28] for a recent discussion for Ni). If a transfer of electrons occurs from Au to Pd then this will weaken the C -+ Pd component. We have noted the evidence [21] for electron transfer Ag to Pd already. (b) The contraction in the Pd d-band will lower the energy of the metal d-electrons and weaken the C +- Pd component. Both of these effects, of which (b) is probably the more important, should lower Ed for CO on the alloys; as observed, with a corresponding logarithmic effect on 0,-o. In the past changes in the IR frequency of CO adsorbed on PdAg alloys have been explained in terms of an Ag + Pd electron transfer 1291, but more recently this effect on PtCu alloys has been explained in terms of a dilution effect reducing the lateral coupling of CO frequencies [30]. So while (b) seems more probable, there is a need for quantitative calculations to settle the matter. 4.1.2. Chemisorption of oxygen Our calculated maximum coverage of 4.08 X 1014 atoms cm-* for the dissociative chemisorption of O2 on a Pd wire corresponds to B0 = 0.35, and may be compared with LEED values for single crystal planes of 0.25 (loo), 0.33 (111) and 0.5 (110) [14,31]. This again is easily reconciled with the notion of a preponderance of (111) planes in the Pd wire surface, although other explanations are, of course, possible. The activation energy for adsorption must be small, since an exposure of only 3 L at 490 K nearly saturates all the sites. Therefore the heat of adsorption of O2 on polycrystalline Pd must approximate to the activation energy for desorption Ed, given by TPD (assuming second-order desorption) as 177 f 20 kJ mol-’ . Our earlier result of 00 < 2 X 1Oe4, i.e. no measureable adsorption for O2 on Au for P < 3 X low3 Pa and T = 1000 down to 300 K [32] may now be extended down to 131 K. A bond energy calculation [33] gives a heat of chemisorption per mole 02, as 420 kJ mol-’ for 0 = atoms and 125 kJ mol-’ for -O-Omolecules. It therefore seems likely that a high activation energy restricts the adsorption of oxygen molecules on Au, and the demonstrated effect of traces of calcium in promoting adsorption of oxygen [34] must work by lowering the activation energy for adsorption. While 83 Pd 17 Au shows a small oxygen adsorption, none at all was detected on 60 Pd 40 Au. However, there must be some tendency for adsorption, since we have noted that Au-rich alloys such as 1.5 Pd 98.5 Au form stable surface oxides at
D.D. Hey, P.B. Moore /Adsorption
and reactiotl of CO and 02 OIZPd-AU
341
1000 K, although pure Au does not do so. Presumably, a few Pd atoms tend to lower the activation energy for O2 chemisorption on Au, perhaps on special defect sites on Au, small in number. Alternatively, it may be an impurity such as Si in the Pd which is active in this way, as recently found for Si in Pt [35]. 4.1.3. The carbon monoxide-oxygen reaction The value of apparent activation energy we estimated of E, = 73.4 kJ mol-’ for co,d,
+ Oads + co2
,
on our polycrystalline Pd wire, is lower than the value estimated by Ertl and co-workers for Pd(ll1) as 105 kJ mol -’ by a similar meihod [36]. Our value might be lowered by simultaneous desorption effects of the reactants, or it might reflect a structural difference in the surfaces. We now use our chemisorption data to discuss the earlier kinetic study on PdAu over 368-400 K (Pd) and 473-673 K (Au) and the markedly higher pressures of 10m2 to 1 Torr. These results, which are summarised in fig. 10 show that the velocity constant falls by 3 X lo3 over pure Pd to X$” = 0.4, i.e. X*” = 0.52, and then rises again by a factor 10 to pure Au. The apparent activation energy E, rises from pure Pd somewhat and then drops sharply over Xi” = 0.4-0.6 to near zero, the apparent frequency factor A following a similar path. In fact, Daglish and Eley found a compensation law held, logrd = yE, + /3 (unpublished). The Pd-rich kinetics PO,/P& pointed to a mechanism in which adsorbed CO is displacing adsorbed O2 and a rate-determining step of oxygen adsorption on two adjacent empty sites in an almost complete adsorbed layer of CO. The Au-rich kinetics,po,Paco on the other hand pointed to a reaction of O2 and CO adsorbed side by side on a very few (cf. low A factor) defect sites in Au or in the Au rich alloy. The true activation energies for the two mechanisms appear to be quite different, 93 kJ mol-r on Pd and 0 kJ mol-’ on Au. Schwab and Gossner [37] found 92 kJ mol-’ for Pd and Po2/PCo kinetics, while Cant and Frederichson [38] found E, = 0 kJ mol-r for Au sponge, and kinetics approaching P”&&, at pressures of 100 Torr. While it is clear that on Pd and Pd-rich alloys the reaction rate is inhibited by strong CO adsorption, and the apparent activation energy therefore exceeds the true activation energy by a term dependent on the heat of adsorption of CO at the high coverages concerned, it does not seem possible to formulate this quantitatively on the basis of our present data. The low pressure data point to adsorption of CO and O2 in separate domains, but simple formulations of this model will not lead to Po,/Pco (or Po,/P~~) kinetics. It does however, seem reasonable to conclude that the catalytic activity of the Pd-rich PdAu alloys for CO t O2 is primarily determined by the extent of adsorption of the gas CO, occurring on Pdg+r ensembles. On the Au-rich alloys a different mechanism involving a small number of Au defect sites takes over. It remains the summarise the possible involvement of an electronic factor in the ensemble effect.
342
D.D. Hey, P.B. Moore
/Adsorption
and reactiorz
of CO
and O2 on Pd--Au
4.1.4. Ensembles and the electronic factor If we conclude that the determining factor in CO chemisorption and catalysis on Pd-rich Pd-Au alloys is a Pdali ensemble, we explain the way the three loga~thmjc plots, of BcO, KrsO and F&,s decrease with Au content to a point at Xf’i” = 0.40. If Norris and Myers PdAg conclusions apply to PdAu, as seems reasonable, the holes in the d-band and density of states. respectively 0.36 and 2.0 for pure Pd, decrease linearly to zero at Xn*” = 0.52. If the heat of adsorption of CO was decided by either of these quantities, then loglOS~~ would also decrease linearly, but to Xg” = 0.52, not to Xi” = 0.40 found in practice. This difference is too small for a hard and fast conclusion, but does point to ensemble size rather than the collective d-band behaviour as the determining factor. However, we have noted the possible importance of the donor link OC -+ Pd, and this would require two holes in the d-band per Pd atom. Since each Pd atom in pure Pd has 0.36 holes, two holes require the collective action of six Pd atoms, which would certainly be available in a Pdp+r ensemble. Finally, we note the possible differences in behaviour between PdAu and the closely analogous PdAg system, both of which yield a complete range of random FCC alloys. Because of the surface tensions of the pure metals, Pd 1430 erg cm-‘, Au 1343 erg cmm2 and Ag 1060 erg cme2, there is to be expected considerably greater enrichment of Ag, than Au, in the corresponding alloy surface [393. This will certainly change geometric factors in the present sense. Because of the smaller separation of Pd and Au d-bands, than Pd and Ag, the electronic factors will also be altered, in ways not too easy to predict at present.
Acknowledgements We thank the SRC for a Research Fellowhip cfI3M) and Professor C. Kemball and Dr. D. Whan for the alloy wires used in this research.
References [ lf A.G. Daglish and D.D. Eley, in: Actes 2ihme Con@. Intern. de Catalyse. Vol. 2 (Technip, Paris, 1961) p. 1615. [2] D.D. Eley, J. Res. Inst. Catalysis, Hokkaido Univ. 16 (1968) 101. [3] E.G. Allison and G.C. Bond, Catalysis Rev. 7 (1972) 233. [4] R.L. Moss and L. Whalley, Advan. Catalysis 22 (1972) 115. [S] T. Engel and G. Ertl, Advan. Catalysis 28 (1979) 1. [6] S. Hiifner, G.K. Werthe~ and J.H. Werwick, Phys, Rev. B8 (1973) 4511. [7] J.A. Nicholson, J.D. Riley, R.C.G. Leckey, J.G. Jenkin and J, Liesgang, J. Electron Spectrosc. 15 (1979) 95. [S] D.A. Dowden, in: Proc. 5th Intern. Congr. on Catalysis, 1972, Vol. 1, p. 621. (91 W.M.H. Sachtler and R.A. Santen, Advan. Catalysis 26 (1977) 69. [ 101 D.D. Eley and P.B. Moore, KS Faraday I, 76 (1980) 1388. [I l] D.D. Eley and E.J. Pearson, JCS Faraday I, 74 (1978) 223.
D.D. Eley. P.B. Moore /Adsorption
and reaction of CO and 02 on Pd-Au
[ 121 [ 131 [ 141 [15] [16] [ 171 [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [ 321 [33] [34] [35]
P.A. Redhead, Vacuum 12 (1962) 203. H. Conrad, G. Ertl, J. Kttppers and E.E. Latta, Surface Sci. 65 (1977) 245. G. Ertl and J. Koch, in: Proc. 5th Intern Congr. on Catalysis, 1972, Vol. 2, p. 969. H. Conrad, G. Ertl and J. Ktippers, Surface Sci. 76 (1978) 323. S. Htifner, G.K. Wertheim and J.H. Werwick, Solid State Commun. 17 (1975) 1585. A. Karlsson, H.P. Myers and L. Wallden, Solid State Commun. 5 (1967) 971. J.J. Vieullmin and M.G. Priestley, Phys. Rev. Letters 14 (1965) 307. J. Friedel, Nuovo Cimento, Suppl. 7 (1958) 287. P.W. Anderson, Phys. Rev. 124 (1961) 41. C. Norris and H.P. Myers, J. Phys. F (Metal Phys.) 1 (1971) 62. P. Kirkpatrick, B. Velicky and H. Ehrenreich, Phys. Rev.Bl (1970) 3750. G.M. Stocks, Intern. J. Quantum Chem. 5 (1971) 533. A.M. Bradshaw and F.M. Hoffman, Surface Sci. 72 (1978) 573. M.L. Kottle, R.G. Greenler and F.C. Tompkins, Surface Sci. 32 (1972) 231. N.A. Surplice and W. Brearley, Surface Sci. 52 (1975) 62. G. McElhiney and J. Pritchard, Surface Sci. 60 (1976) 397. I.P. Batra and P.S. Bagus, Solid State Commun. 16 (1975) 1097. M. Primet, M.V. Mathieu and W.M.H. Sachtler, J. Catalysis 44 (1976) 324. F.J.C.M. Toolenaar, D. Reinalda and V. Ponec, J. Catalysis 64 (1980) 110. G. Ertl and P. Rau, Surface Sci. 15 (1969) 443. D.D. Eley and P.B. Moore, Surface Sci. 76 (1978) L599. D.D. Eley, Disc. Faraday Sot. 8 (1950) 34. P. Ligare, L. Hilaire, M. Sotto and G. Maire, Surface Sci. 91 (1980) 175. H.P. Bonzel, A.M. Franken and G. Pirug, Surface Sci. 104 (1981) 625.
[36] [37] [38] [39]
T. Engel and G. Ertl, J. Chem. Phys. 69 (1978) 1267. G.M. Schwab and K. Gossner, Z. Physik. Chem. (NF) 16 (1958) N.W. Cant and P.W. Frederickson, J. Catalysis 37 (1975) 531. B.J. Wood and H. Wise, Surface Sci. 52 (1975) 151.
39.
343
THE ADSORPTION WIRES
325
AND REACTION OF CO AND O2 ON Pd-Au
ALLOY
D.D. ELEY and P.B. MOORE * Dep~rt~~~t of Chemistry, ~rliv~rsity of ~~tt~ng~~~rn~University Park, ~ottir~gh~~l NG7 2RD, UK
Received 16 October 1980; accepted for publication 6 July 1981
Tempera~re programmed desorption (TPD) of CO and 02 on PdAu alloy wires has been studied. The heat of adsorption, sticking coefficient and maximum coverage of CO were recorded for Pd, 83 Pd 17 Au, 60 Pd 40 Au. For Pd and Pd-rich alloys the heat of adsorption remained fairly constant but the maximum coverage fell markedly from 0.42 for Pd to less than 0.05 for bulk palladium atom fraction yPd Q 0.83. The heat of adsorption, sticking coefficient and maximum coverage of 02 were investigated for pure Pd. A very limited adsorption was recorded on 83 Pd 17 Au and none on the more Au-rich alloys. The adsorption data are used to discuss the CO + 02 reaction. Activation energy and frequency factor are estimated on Pd, for the TPD conditions used here. Earlier rate constants (0.2 Torr, 150°C) for CO + 02 on PdAu as a function of Au content correlates with the maximum coverage of chemisorbed CO, which in turn is correlated with the probability of finding a Pdgtr ensemble in the surface. Modern results on the d-band structure of the PdAu alloys suggest that the Pdg ensemble, i.e. a surface Pd atom without an Au atom in its coordination shell, would tend to optimise both the donor and acceptor actions of the Pd atoms involved in chemisorbing CO.
1. Introduction The carbon monoxide-oxygen reaction on Pd-Au alloy polycrystalline wires was investigated by Daglish and Eley [ 11 as part of a general programme of reactions on these alloys 12-41. The apparent activation energy had a high value, 118. 167 kJ mole-’ on Pd and the Pd-rich alloys, declining abruptly at about 40 at% Pd to a value around zero. It was natural to relate this result to the’ disappearance of “holes in the d band” which also occurs at 48 at% Pd, on the rigid-band theory generally accepted at that time [2]. However, there were problems apparent even in the original presentation. For example, the fastest reactions occur on the Pd wires, where the kinetics (rate aP02P& seemed the most likely law) clearly point to inhibition by strongly adsorbed CO displacing adsorbed 02, with a corresponding * Department of Chemistry, Ellison Building, Newcastle Polytechnic, Upon-Tyne, NE1 3ST, UK.
0039-6028~81~0000-0000/$02.50
0 1981 North-Holland
Ellison Place, Newcastle-
326
D.D. Eley, P.B. Moore /Adsorption
and reaction of CO and 0,
OHPd-Au
augmentation of the activation energy by twice the adsorption energy of the CO. The further elucidation of this problem requires a study of the temperature programmed desorption of both gases from the alloys, which forms a major part of the present paper. There have been many studies of CO oxidation on Pd in recent years, incidentally, with the balance of evidence favouring P& rather than P& kinetics [5], implying CO inhibition of single site, rather than 2 site oxygen chemisorption. Equally important the concept of a common d-band for the alloys has attracted criticism from the results of photoelectron spectroscopy which favours, for Pd-Ag [6] and Pd-Au [7], two separated d-bands for the two component metals. A further view regards the catalytic activity as determined by the separate surface atoms, with neighbours exerting electronic or “ligand” effects, and associates adsorption and reactivity with particular Pd,Au, surface atom ensembles [8,9]. This has led us to the work described in the present paper. Auger analysis has already established that the presently used PdAu wires, outgassed by heating in vacua, show the surface to have an average monolayer enrichment factor, referred to Pd, of only 0.6, so that there is only a small surface enrichment in Au [lo].
2. Experimental Experiments were carried out in a stainless steel vacuum system (base pressure 4 X lo-” Torr) which was equipped with a 4 grid retarding field electron energy analyser and a normal incidence electron gun for Auger electron spectroscopy. The Pd-Au alloys, in the form of 32 swg wires, were spot welded between oxidised iron supports and were heated by passage of a controlled electric current. The temperature-resistance characteristics of the alloy were determined and were used to mea-
Fig. 1. Schematic diagram of the wire temperature controller.
D.D. Eley, P.B. Moore /Adsorption
and reaction of CO and 02 on Pd-Au
327
sure and control sample temperatures. Fig. 1 shows the essential features of the temperature controller. A voltage divider compares voltage drop across the sample and across a standard 1 S’2resistor in series with the sample. The sample resistance thus calculated is compared in a differential amplifier with a reference voltage equal to the desired sample resistance. The amplifier output is used to control the current through the sample and hence the sample temperature. A resistance control of better than 10m3 n is readily achieved with sample resistances, R, in the range 0.3 < R < 10 a at currents up to 10 A. Temperature programmed desorption is achieved by substituting a ramped voltage for the fixed reference voltage. All desorption spectra were recorded with a Bayard-Alpert ion gauge and direct plotting of the logarithmic out-put was used to accentuate the shape of desorption peaks. The species desorbed were analysed using a quadrupole mass spectrometer scanning the mass range lo-50 AMU at 1 s intervals during desorption. This arrangement was preferred to scanning a single mass number because of its greater reproducibility and the unambiguous identification of most peaks resulted. Gases employed were “Research Grade” (BOC) and were admitted through a variable leak valve. For exposures less than 3 L (1 L = 10S6 Torr s) exposure times were typically 100 s whereas exposure times were progressively increased for higher doses but pressures were not allowed to exceed 10e6 Torr. Hydrogen used in the cleaning of 60% Pd-Au was purified by passage through a palladium-silver thimble. Prior to insertion into the UHV system, all samples were degreased in chloroform, ether, acetone and rinsed in deionised water. After bakeout (200°C) and annealing at 1000 K for 6 h in a vacuum better than 5 X lo-” Torr, the samples were heavily contaminated with sulphur, carbon and oxygen. The impurities, carbon and sulphur, were removed from Pd and 83% Pd-Au by oxidation at 1000 K in 10m6 Torr O2 for 16 h and the surface oxide formed was decomposed by annealing at the same temperature for a further 8 h. A final 1.5 min of oxidation and annealing produced a surface which appeared uncontaminated (as judged by AES) and did not recontaminate during prolonged heating at red heat. As a result of such treatments, 60% Pd-Au became covered in a stable oxide which could not be thermally decomposed. A reduction was effected by heating to 1100 K in 10e2 Torr H,. The formation of similar stable oxide surfaces was observed for 26% Pd-Au and li% Pd-Au but not for pure Au.
3. Results 3.1. Chemisorption of CO on Pd-Au
alloys
The thermal desorption spectra of CO from Pd, 83% Pd-Au, 60% Pd-Au and Au are shown in figs. 2-5. In all cases, the desorption maxima do not shift appreciably with change in fractional surface coverage, 0, although a low temperature
spectra
spectra
Fig. 3. Desorption
@6L
500
/ K
I
600
70
for CO from an 83% Pd-177r’
for CO from a Pd wire initially
Temperature
Fig. 2, Desorption
20-
25-
30-
35-
30-
15-
o-
200
600 400 500 Temperature/K
700
of gas shown.
of gas shown (1 L = 1 !I@ Torr s).
300
at 180 I; or 298 K to the quantities
at 298 K to the quantities Au wire exposed
exposed
3.5 -
40-
800
D.D. Eley, P.B. Moore /Adsorption
and reaction of CO and O2 on Pd-Au
m
I
0
e
I
R 6
I
I
Lp
In
0
1 0
I
0 6oL
X JJO$@
z I
cu
aJfKSaJd
I 0
330
D.D. Hey. P.B. Moore /Adsorption
and reaction of CO and 02 on Pd-Au
shoulder is clearly visible for Pd (fig. 2). Thus, it is concluded that the desorption processes are first order and that the desorption activation energy Ed is essentially independent of 0 over the ranges studied. The saturation coverage of CO on Pd and 83% Pd was measured by rapidly desorbing the gas (flash rate fl> 100 K s-r) into a closed vessel. Corrections were applied for pumping of the gas by the ion gauge and adsorption on the vessel walls. The maximum coverage of CO on Pd was found to be (4.84 * 0.14) X 1014 molecules/cm’ averaged over the whole length of the wire sample. If we assume [ 1 l] that there are 1.16 X 10” surface Pd atoms per square centimetre of wire, a value of emax = 0.42 * 0.012 is obtained. However a substantial error in 8,,, arises from the temperature gradients present along the wire during the desorption. Thus, we will assume that, in keeping with indications from LEED studies [5], the maximum coverage at the temperatures of our experiments was emax = 0 5. Coverages 0 < 8 max were calculated from the relative integrated areas under the desorption curves calculated using Redhead’s eq. (3) [12] and our measured pumping speeds. Table 1 shows the maximum coverages recorded for alloys relative to palladium (e,,, = 0.5). The desorption energies (table 1) were calculated from the temperature corresponding to the maximum in the desorption spectra assuming first order kinetics [ 121. It is apparent that the total quantity of CO adsorbed at 295 K falls rapidly from 0 = 0.5 for Pd to 0.04 for 83% Pd-Au and to approximately 1.2 X 10e3 for 50% Pd-Au and Au itself. By contrast, the activation energy for desorption decreases only 23.9 kJ mol-’ from 126.7 for Pd to 102.8 kJ mol-’ for 60% Pd-Au. The sticking coefficient, S, is the ratio of the number of molecules adsorbed by the sample to the number incident on the sample surface, per unit time. This was
Table 1 Adsorption At % Pd
of carbon Adsorption temperature
monoxide
on palladium-gold
alloys
Number of peaks
Desorption
Ed (kJ mol-l)
1+sa S 1 1+s 1 1 1 1 1 1
1 ? 1 1 ? ? 1 1 ? ?
126.7 96.1 111.6 83.6 102.8 115.0 67.3 67.6 72.4 101.2
e max
Sticking coefficent (initial)
(K) 100 83 60
0
298 180,298 223 303 380 143 173 218 293
a S: shoulder. b Assumed value.
0.5
b
0.91 * 0.15
4.02 x 1O-2 6.30 x 1O-3 1.19 x 10-3 <10-4
2.96 1.62 3.96 1.14
x 1O-2 x 1O-2 x 1O-3 x 10-s
0.19 f 0.02
S
D.D. Eley, P.B. Moore /Adsorption
0
0.5
1.0
1.5 Exposure/
and reaction of CO and O2 on Pd-Au
2.0 Langmuir
Fig. 6. Relative surface coverage @/b~,,,~~ versus CO exposure Au (o), (iii) 60% Pd-40% Au wires (0).
2.5
331
30
for (i) Pd (o), (ii) 83% Pd-17%
calculated for Pd and 83% Pd-Au and is shown in table 1. It was found that CO has a sticking coefficient close to unity for 0 < 0.32 and thereafter the sticking coefficient falls rapidly and is accompanied by the appearance of a shoulder in the desorption spectrum (fig. 2) corresponding to a lower energy bonding state. The low sticking coefficient recorded for 83% Pd-Au reflects the low surface coverage for the system. An indication of the reactivity of the CO adsorption sites in the surversus exposure (fig. 6). This suggests that, face is obtained by plotting e/6,,, because S - 1 for pure palladium, that the ratio of incident molecules to adsorption sites per unit area exceeds unity implying that such sites are ensembles of molecules on which chemisorption occurs via a mobile precursor state on surrounding atoms. Examination of the desorption plots of CO adsorbed on 60% Au at low temperatures (223 K) in fig. 4 reveals that for low exposures, there is a distinct shoulder on the desorption trace which disappears at higher coverages. The higher energy state corresponds to Ed = 100 kJ mol-’ whereas the lower one corresponds to Ed = 84 kJ mol-’ , the former figures corresponding well with the desorption energy of 102.8 kJ mol-’ measured for adsorption at room temperatures. Thus, it is apparent that at lower temperatures, a distinct form of chemisorption occurs which eventually far outweighs the less populated higher energy state as evidenced by the disappearance of the shoulder at high exposure. The desorption traces for gold suggest that the process is first order. From the position of the desorption maximum, the desorption energy Ed is estimated to be 67.3 kJ mol-’ . The coverage increases markedly as the adsorption temperature is lowered, this is expected because the desorption peak occurs at approximately 290-310 K. This desorption energy is markedly lower than that measured for Pd and broadly corresponds to the lower binding energy state recorded for 60% Au. The maximum coverages recorded are still low - typically only a few per cent, nevertheless, saturation still occurs at low exposures -0.4 L. The low coverages
332
D.D. Hey, P.B. Moore /Adsorption
alld reaction ofC0
and 02 on Pd-Au
may arise from the reversibility of a CO chemisorption on gold since, prior to gas desorption, the system was evacuated to typically 3 X lo-” Torr (4 X 10m8 Pa). During the evacuation, equilibrium between the gas phase and surface coverage may have been partially reestablished leading to low surface coverages. In conclusion, it is apparent that as generally reported CO strongly adsorbs on palladium with a coverage -0.5 and that under such conditions, the adsorption is characterised by at least two distinct binding states. However, for 83% Pd-Au adsorption occurs only to a limited extent despite its apparent strength (Ed = 111.6 kJ mol-‘). For 60% Pd-Au and Au, only a vestige of the strong adsorption remains, and a weaker adsorption appears at lower temperatures. 3.2. Oxygen desorption studies Temperature programmed desorption traces for palladium are shown in fig. 7. The adsorption was carried out at 490 K to minimise interference from the residual CO within the system whilst remaining below the temperature (520 K) at which a surface oxide forms on Pd(ll1) faces [ 131. It can be seen that after 2.7 L exposure to oxygen the system approached saturation and large exposures gave only moderate increases in uptake (fig. 9). Saturation corresponds to (3.67 + 0.04) X 1014 oxygen atoms per square centimetre and is equal to a fractional coverage 0 of 0.3 1. At low coverages oxygen was desorbed at 808 K and this temperature (T,,,) progressively decreased to 758 K at saturation. It may be expected that oxygen desorption follows second order kinetics and accordingly a plot of log,(neT&,,) where no is the initial coverage, was found to be linear. From this versus 1IT,,, plot, a desorption energy of 177 f 20 kJ mol-’ was calculated for oxygen and second order kinetics were inferred. A plot of coverage vs exposure to oxygen was linear for 0 < 19< 0.15 and a sticking coefficient of 0.36 f 0.04 was calculated (fig. 9). Desorption traces for oxygen adsorbed at room temperature were strikingly different and sometimes difficult to reproduce. They were characterised by the appearance of an additional peak at low exposures (<3 L) which had a Tmax value of 415 K (fig. 8). This peak arose from the desorption of CO? into the system. It is well established that CO* does not strongly adsorb on palladium, thus it is concluded that the peak arises from reaction between residual CO and oxygen and that the appearance of the peak at 415 K is governed by the activation energy for CO oxidation catalysed by palladium. It may be envisaged that the reaction proceeds by either a Langmuir-Hinshelwood (I) or Rideal-Eley (II) mechanism, c&d, co
+ Oads -+ co2 + Oads + co2
.
3
(I) (11)
If the residual pressure in our system (<4 X lo- lo Torr) during the experiment was due solely to CO then throughout the duration of the process 1.6 X 10” mole-
D.D. Eley, P.B. Moore /Adsorption
and reaction of CO and 0,
on Pd-Au
333
A 270L 14OL
500
600
700 800 Temperature/K
900
Fig. 7. Desorption spectra for 02 from a Pd wire initially exposed at 490 K to the quantities of gas indicated.
cules/cm2 would strike the Pd surface. However, at least 2.10 X 1013 molecules/ cm’ of CO1 were released during the desorption and clearly reaction (II) could not lead to the observed quantities of CO*. Assuming a sticking coefficient of unity for CO, the quantity of CO needed to lead to such a CO2 peak could be adsorbed at the base pressure of the system in 186 s or approximately half the time between successive desorption experiments. Thus, it appears that CO? is formed from CO in the residual gas co-adsorbing with oxygen. Above 415 K, the activation energy is overcome and the reaction proceeds via a Langmuir-Hinshelwood mechanism. At increased oxygen exposures, a third peak with a desorption maximum at ca. 500 K becomes increasingly important and at exposures in excess of 100 L became comparable with the O2 peak itself. This peak was due to CO desorption and at exposures >300 L became the predominant peak whilst the oxygen peak at 758 K
7
,
600
9
0.25
2
4
6 8 Exposure,/
10 12 Langmuir
14
16
Pd wire initially exposed at 298 K to 3 L of oxygen when the partial pressure of CO in the system is less
800
Fig. 9. Surface coverage of oxygen versus 02 exposure for a Pd wire at 490 K.
from a
Temperature/K
I
400
r?
co2
Fig. 8. Desorption spectra for 02 than 4 X lo-” Torr.
x L
p
0
8
9
D.D. Eley,
P.3. Moore /Adsorption
apld reactim of CO and 02 on
Pd-Au
335
became progressively reduced. At these high exposures, the COa peak is merged with the low temperature shoulder on the CO peak. The particular doses at which the CO peak became dominant were variable, lending further support to the argument that such effects were merely due to residual CO. Thus, it is concluded that a relatively small amount of oxygen (0 - 0.31) adsorbs on Pd and that CO initially adsorbs onto unoccupied sites. It is uncertain from our studies whether, at higher CO coverages, oxygen is displaced or whether excess CO reacts with oxygen atoms to produce COa leading to an apparent reduction in the size of the oxygen peak at 758 K. Studies by Ertl and co-workers have shown that over periods of time, CO will displace Oads and it seems probable that such a process is occurring here [ 141. The measured uptake of oxygen by 60% Pd-Au and pure Au was minimal. These materials did not adsorb oxygen (19< 2 X 10b4) and after large exposures to oxygen only the desorption of quantities of CO were recorded. The desorbing gas was unambiguously identified as mass 28 and the quantities of gas, their desorption energies and the desorption peak shapes were strictly comparable with the desorption spectra recorded after CO adsorption on these materials. Thus it was concluded that oxygen adsorption on Au at 142,173,218,293 K and on 60% Pd-Au at 223, 303 and 380 K was virtually non-existent and that 00~ < 2 X 10e4 in all instances. A small uptake of oxygen was recorded for 83% Pd-Au at 278 and 180 K after prolonged exposure (-250 L). Under such conditions a single desorption peak with a Max of 470 K was recorded. This peak comprised both oxygen and carbon monoxide in the approximate ratio 1 : 3 and the quantities of CO evolved were comparable with those recorded during pure CO adsorption studies. It is interesting to note in view of the low activity of 83% Pd-Au as a catalyst for CO oxidation that the desorbed gases were CO and OZ, not COa. 3.3. Calculation ofact~yation energy and ore-exponential factor for the La~~~~ir~i~~~elwoo~ reaction The activation
energy and pre-exponential
factor for the reaction
CO,& f O&s -+ CO2 , can be calculated from the CO2 desorption peak produced by the reaction (fig. 8). The rate of production of COZ, r (molecules cm-’ s-i) is given by: r = kBoOco = Ai30dco exp(-E,/RT)
,
0)
where 80 and t3c~ are the surface coverages of 0 and CO atoms (mole~~es~cm’). If the wire temperature T is increased linearly at a rate fl (K s-r) from a temperature Te, such that T=To+/3t, then a maximum
(2) in the desorption
trace, TmaX,will occur at a temperature
given by
336
D.D. Eley, P.B. Moore /Adsorption
and reaction of CO and 0,
on Pd-Au
If Gl,, is the pressure increase (in Torr) at the point of maximum rate then, following
the treatment
of temperature
programmed
desorption
given by Redhead
1121, (3) where S = pumping speed, K = 3.27 X 10’ 9 molecules Torr-’ l-r, n = sample area. Similarly, it may be shown that for second order kinetics, the coverage of CO at T max will be BcO = $3& where i3& is the initial CO coverage. However, the curves obtained had a shape typical of a first order desorption process indicating that 0: > t$, giving pseudo first order kinetics. Here, at Tmax, OCO= t&/2.718. e0 may be calculated from the initial surface coverages es, 0& using the equation B0 = e”, @, + ecO. The initial surface coverage of CO, et,, is given by
e;,
=-I_ q-P& akT V
dt )
where V = volume of system, and where the integration peak only. Similarly
is carried out over the CO2
Using this equation, a value of E, = 73.4 kJ mol-’ may be calculated. Substitution of this value back into eq. (1) allows a value of A equal to 3.88 X lo-’ cm-* molecule-’ s-l to be calculated. Recent studies of CO and 0, adsorption on Pd( 111) surfaces have indicated that separate domains of CO,d and Oad may be present and that reaction could occur at domain boundaries [S]. Changes in the LEED patterns of oxygen covered Pd(ll1) surfaces were recorded after exposure to 0.6 L carbon monoxide [15] which would correspond to 0 - 0.15 if the CO sticking coefficient remains essentially unaltered. In our experiments, B0 - 0.25 and ecO - 0.05, thus it seems unlikely that compression of any oxygen domains occurred.
4. Discussion 4.1. Electronic structure of the PdAu alloys PdAg and PdAu both form a continuous series of homogeneous, random fee alloys, the band theory of which has recently been revised following the results of optical, UPS and XPS spectroscopies. In PdAg there are separate d-bands for Pd and Ag, the Pd d-band near the Fermi level and the Ag d-band lower by some 2.5 eV. The situation is similar for PdAu, except the d-band separation is less (there is some
D.D. Eley, P.B. Moore /Adsorption and reaction of CO and O2 011Pd-Au
337
overlap) [ 171. As Ag or Au atoms are added to pure Pd, the Pd d-band simultaneously narrows and shifts downwards, so that the 0.36 holes in the Pd d-band per Pd atom [18] decrease linearly to zero at the 52 at% Au or Ag concentration, where classical measurements have established the onset of diamagnetization [2] (often given on average as 60 at% noble metal). Above this concentration of Ag or Au, the Pd delectrons are localised in the so called virtual bound states [ 19,201 any paramagnetic effect of which is outweighed by the bulk diamagnetism of the alloys. In summary, although holes remain in the d-shells of the Pd atoms at all concentrations of Au or Ag, at 52 at% Au or Ag, the Pd collective d-band has narrowed to the point of disappearance, to be replaced by localised levels [2 11. Although in the case of Ag and Pd Norris and Myers [21] have postulated some electron transfer Ag + Pd, in practice the coherent potential approximation [22] which successfully predicts separate Pd and Ag d-bands [23] assumes no such transfer. In the past the catalyst chemist has looked for correlations with holes, or densities of states at the Fermi level in a single rigid PdAu-band ]2], but must now transfer his attention to the single Pd d-band. We consider successively CO chemisorption, O2 chemisorption, and the CO t O2 reaction, on Pd, Au and finally PdAu alloys. 4.1.1. Clzemisorption ofcarbon monoxide Our desorption activation energy Ed = 127 kJ mol-’ for a Pd wire, may be comPd wire [ 141, and indi~dual latpared with 134 kJ mol-’ , also for a polycryst~line tice plane values of 142 (11 l), 153 (loo), 167 (1 lo), 149 (210) and 146 kJ mol-’ (311) [5]. It seems probable that the wires exhibit an appreciable fraction of (111) plane, where LEED points to CO chemisorbed over interstices between 3 Pd atoms on this face, with a possible change above surface fraction 0 = 0.33 to bridge bonding to 2 Pd atoms [24]. This might relate to our sticking coefficient, as may be inferred from fig. 6, which maintains its initial value of 0.91 (table 1) up to B = = 0.7, when it falls to 0.07, corresponding to adsorption of the 0.35, i.e. e/e,,, lower energy species which is also manifest as a shoulder on the TPD spectrum. For CO on Au the maximum 8 values were achieved rapidly, i.e. within 1.3 to 4 L, at all temperatures, and were very small, viz. 0.011 at 293 K and 0.029 at 143 and 173 K, the heat values at these two temperatures being, respectively, Ed = 101.2 kJ mol-’ and _Ed= 67.3 kJ mol-’ . The heat values may be compared with literature values of 56 [25] and 47.3 kJ mol-’ [26] for polycrystalline Au, and 58 kJ mol-’ falling with coverage to 16 kJ mol-’ on Au(100) [27]. The rapidity of adsorption suggests a mobile precursor state leading to a final surface defect state, possibly at a grain boundary. Our average surface enrichment factor for the alloy wires ilO] allows us to calculate vacuum surface atom fractions X”’ in equilibrium with bulk atom fractions XA” viz. X9 = 0.17 in equilibrium with X”” = 0.25, and Xi” = 0.40 with XA” = 0.53: in fig. 10 the linear change in logloec~ from pure Pd to X‘$” = 0.4, would be little changed by using vacuum surface values XA”. The Boo then remains (apparently) unchanged from Xi” = 0.4 to pure Au. There is a corresponding but much less marked change in the Ed value, which suggests that while
338
D.D. Hey, P.B. Moore /Adsorption
r
20
and reaction of CO and O2 on Pd-Au
w”
10 1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
I.0
XAU B
Fig. 10. A comparison of the maximum CO coverage ( -), the rate constant at 423 K relative to pure Pd (- - - - - -), the activation energy (- . - . -) and preexponential factor (. . . . . .) for CO oxidation, and the CO desorption tion of bulk atom fraction X6”.
energy (- - -) for palladium gold alloys, as a func-
alloying Au with Pd does lower the site binding energy for CO, there is also a strong decrease in the number density of sites suitable for CO adsorption. We shall now enquire whether a particular Pd,Au, ensemble in the surface is responsible for CO adsorption. First, we note that if a CO is linearly bonded to a single Pd atom on a (111) or (100) face say, and this Pd atom is “liganded” to its next neighbours in the sub-surface layer, then this will contribute a lo-ensemble on the (111) face and a 9ensemble on the (100) face. Turning to multicentre sites, there are two 3 coordinates sites on (11 l), one of which is a 3ensemble, the other a 4ensemble of 3 surface atoms and 1 immediately sub-surface. On the (100) face there is a single multicentre site comprising a square pyramid of 5 metal atoms, which becomes a 9-ensemble if we include sub-surface nearest neighbours. So we conclude that if experiments point to ensemble sizes of 8-10 atoms, then ligand effects involving nearest neighbours are probably involved. Dowden [8] has calculated a probability Pk,,, for a binary alloy AB, of finding a surface atom A part of an ensemble of n atoms of which at least m are of one particular type (A), given by m=n-1
Pnm=
c x* (nil )(X*)m(XB)n-m-l m
)
D.D. Eley, P.B. Moore /Adsorption
0
I
I
I
0.1
0.2
0.3
I
0.4
and reaction of CO and O2 on Pd-Au
I
05 XAU
I
0.6
I
0.7
I
0.8
339
I
0.9
I.0
Fig. 11. A plot of log1 oP& (ensemble population) for 5, 8,9, 10 palladium atom ensembles as a function of surface atom fraction X Au, together with the maximum CO coverage (0) at 298 K.
where XA is the surface mole fraction of A. This equation has been used to evaluate the curves of fig. 11, and the experimental points are seen to lie on the Pb,* curve. If we now take the case where all n atoms of the ensemble are of type A, e.g. Pd, then we substitute m = n - 1 and the above equation simplifies to P&1
= (X”)” .
This leads to kA
log,P&r
= CA .
Thus the gradient of log$‘A,._i versus XA will yield a value of n, the total number of atoms in the ensemble. In general, P’,, is not measured directly, but rather the amounts of gas chemisorbed or the rate of a catalytic reaction. For chemisorption it is reasonable to assume that Omax a Pk,,, and hence across a series of alloys a plot of log,O,,, versus XA will have a gradient n/XA. By measuring the slope of this curve (fig. 11) at
340
D.D. Elev, P.B. Moore /Adsorption
arld reactiotl of CO arld 0,
on Pd-Au
several points including XA = 1, we calculate that the ensemble size responsible for CO adsorption is 9 + 1 Pd atoms. The size of the ensemble involved and the relatively small change of Ed from pure Pd to 60% Pd-Au makes the assumption implicit in the calculation, i.e. AHa, the heat of adsorption of CO, is independent of 0. seem valid. This effectively means the active site is a Pd atom which does not contain an Au atom in its coordination shell. This effect might arise in either of two ways: (a) The Blyholder model for CO adsorbed on transition metals involves two “donor” links 0 = C ++Pd (see Batra and Bagus [28] for a recent discussion for Ni). If a transfer of electrons occurs from Au to Pd then this will weaken the C -+ Pd component. We have noted the evidence [21] for electron transfer Ag to Pd already. (b) The contraction in the Pd d-band will lower the energy of the metal d-electrons and weaken the C +- Pd component. Both of these effects, of which (b) is probably the more important, should lower Ed for CO on the alloys; as observed, with a corresponding logarithmic effect on 0,-o. In the past changes in the IR frequency of CO adsorbed on PdAg alloys have been explained in terms of an Ag + Pd electron transfer 1291, but more recently this effect on PtCu alloys has been explained in terms of a dilution effect reducing the lateral coupling of CO frequencies [30]. So while (b) seems more probable, there is a need for quantitative calculations to settle the matter. 4.1.2. Chemisorption of oxygen Our calculated maximum coverage of 4.08 X 1014 atoms cm-* for the dissociative chemisorption of O2 on a Pd wire corresponds to B0 = 0.35, and may be compared with LEED values for single crystal planes of 0.25 (loo), 0.33 (111) and 0.5 (110) [14,31]. This again is easily reconciled with the notion of a preponderance of (111) planes in the Pd wire surface, although other explanations are, of course, possible. The activation energy for adsorption must be small, since an exposure of only 3 L at 490 K nearly saturates all the sites. Therefore the heat of adsorption of O2 on polycrystalline Pd must approximate to the activation energy for desorption Ed, given by TPD (assuming second-order desorption) as 177 f 20 kJ mol-’ . Our earlier result of 00 < 2 X 1Oe4, i.e. no measureable adsorption for O2 on Au for P < 3 X low3 Pa and T = 1000 down to 300 K [32] may now be extended down to 131 K. A bond energy calculation [33] gives a heat of chemisorption per mole 02, as 420 kJ mol-’ for 0 = atoms and 125 kJ mol-’ for -O-Omolecules. It therefore seems likely that a high activation energy restricts the adsorption of oxygen molecules on Au, and the demonstrated effect of traces of calcium in promoting adsorption of oxygen [34] must work by lowering the activation energy for adsorption. While 83 Pd 17 Au shows a small oxygen adsorption, none at all was detected on 60 Pd 40 Au. However, there must be some tendency for adsorption, since we have noted that Au-rich alloys such as 1.5 Pd 98.5 Au form stable surface oxides at
D.D. Hey, P.B. Moore /Adsorption
and reactiotl of CO and 02 OIZPd-AU
341
1000 K, although pure Au does not do so. Presumably, a few Pd atoms tend to lower the activation energy for O2 chemisorption on Au, perhaps on special defect sites on Au, small in number. Alternatively, it may be an impurity such as Si in the Pd which is active in this way, as recently found for Si in Pt [35]. 4.1.3. The carbon monoxide-oxygen reaction The value of apparent activation energy we estimated of E, = 73.4 kJ mol-’ for co,d,
+ Oads + co2
,
on our polycrystalline Pd wire, is lower than the value estimated by Ertl and co-workers for Pd(ll1) as 105 kJ mol -’ by a similar meihod [36]. Our value might be lowered by simultaneous desorption effects of the reactants, or it might reflect a structural difference in the surfaces. We now use our chemisorption data to discuss the earlier kinetic study on PdAu over 368-400 K (Pd) and 473-673 K (Au) and the markedly higher pressures of 10m2 to 1 Torr. These results, which are summarised in fig. 10 show that the velocity constant falls by 3 X lo3 over pure Pd to X$” = 0.4, i.e. X*” = 0.52, and then rises again by a factor 10 to pure Au. The apparent activation energy E, rises from pure Pd somewhat and then drops sharply over Xi” = 0.4-0.6 to near zero, the apparent frequency factor A following a similar path. In fact, Daglish and Eley found a compensation law held, logrd = yE, + /3 (unpublished). The Pd-rich kinetics PO,/P& pointed to a mechanism in which adsorbed CO is displacing adsorbed O2 and a rate-determining step of oxygen adsorption on two adjacent empty sites in an almost complete adsorbed layer of CO. The Au-rich kinetics,po,Paco on the other hand pointed to a reaction of O2 and CO adsorbed side by side on a very few (cf. low A factor) defect sites in Au or in the Au rich alloy. The true activation energies for the two mechanisms appear to be quite different, 93 kJ mol-r on Pd and 0 kJ mol-’ on Au. Schwab and Gossner [37] found 92 kJ mol-’ for Pd and Po2/PCo kinetics, while Cant and Frederichson [38] found E, = 0 kJ mol-r for Au sponge, and kinetics approaching P”&&, at pressures of 100 Torr. While it is clear that on Pd and Pd-rich alloys the reaction rate is inhibited by strong CO adsorption, and the apparent activation energy therefore exceeds the true activation energy by a term dependent on the heat of adsorption of CO at the high coverages concerned, it does not seem possible to formulate this quantitatively on the basis of our present data. The low pressure data point to adsorption of CO and O2 in separate domains, but simple formulations of this model will not lead to Po,/Pco (or Po,/P~~) kinetics. It does however, seem reasonable to conclude that the catalytic activity of the Pd-rich PdAu alloys for CO t O2 is primarily determined by the extent of adsorption of the gas CO, occurring on Pdg+r ensembles. On the Au-rich alloys a different mechanism involving a small number of Au defect sites takes over. It remains the summarise the possible involvement of an electronic factor in the ensemble effect.
342
D.D. Hey, P.B. Moore
/Adsorption
and reactiorz
of CO
and O2 on Pd--Au
4.1.4. Ensembles and the electronic factor If we conclude that the determining factor in CO chemisorption and catalysis on Pd-rich Pd-Au alloys is a Pdali ensemble, we explain the way the three loga~thmjc plots, of BcO, KrsO and F&,s decrease with Au content to a point at Xf’i” = 0.40. If Norris and Myers PdAg conclusions apply to PdAu, as seems reasonable, the holes in the d-band and density of states. respectively 0.36 and 2.0 for pure Pd, decrease linearly to zero at Xn*” = 0.52. If the heat of adsorption of CO was decided by either of these quantities, then loglOS~~ would also decrease linearly, but to Xg” = 0.52, not to Xi” = 0.40 found in practice. This difference is too small for a hard and fast conclusion, but does point to ensemble size rather than the collective d-band behaviour as the determining factor. However, we have noted the possible importance of the donor link OC -+ Pd, and this would require two holes in the d-band per Pd atom. Since each Pd atom in pure Pd has 0.36 holes, two holes require the collective action of six Pd atoms, which would certainly be available in a Pdp+r ensemble. Finally, we note the possible differences in behaviour between PdAu and the closely analogous PdAg system, both of which yield a complete range of random FCC alloys. Because of the surface tensions of the pure metals, Pd 1430 erg cm-‘, Au 1343 erg cmm2 and Ag 1060 erg cme2, there is to be expected considerably greater enrichment of Ag, than Au, in the corresponding alloy surface [393. This will certainly change geometric factors in the present sense. Because of the smaller separation of Pd and Au d-bands, than Pd and Ag, the electronic factors will also be altered, in ways not too easy to predict at present.
Acknowledgements We thank the SRC for a Research Fellowhip cfI3M) and Professor C. Kemball and Dr. D. Whan for the alloy wires used in this research.
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39.
343