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The role of chlorine promoters in catalytic ethylene epoxidation over the Ag(110) surface
28
Applicatwn!,
of Surface Science I Y ( 19X4) 2X 42 North-Holland.
ETHY L,ENE
THE ROLE OF CHLORINE PROMOTERS IN CATALYTIC EPOXIDATION OVER THE Ag( 110) SURFACE Charles
T. CAMPBELL
C‘hermstr~
Drcvsron,
and Mark T. PAFFETT
1.m Alumos
Notwnul
Luhorutor~,
Received 4 October 19X3: accepted for pubhcation
We have recently shown that the kinetics mimic
Ag(ll0)
identically
Amsterdam
the results
Lo\ A lunw~.
NCM, .M~r\,c o X7.G5,
C’S,
1X June lYX4
and selectwiry of ethylene cpoxldation
over unpromoted,
supported
Ag catalysts.
IOO-fold enhancement of the specific activity (per surface Ag atom) on Ag( 110)
over clean
except for a
[ 1.21. In
thi\ report.
we will discuss results in which we have modeled the role of chlorine promoters in this reaction h! combining ultrahigh vacuum surface analysis (XPS. AES. LEED. sure
TDS)
before and after high-prer-
100 torr) kinetic measurements. In this way, we were able to correlate the reactwn rate and
(-
selectivity
not only
atomically
adsorbed oxygen and chlorine.
with
temperature
and reactant pressures. In industrial
but also with
catalysis.
the coverages of
trace amounts of chlorinated
organica are added to the feed stream to promote selectiwty with, however, some decrease in the reaction rate. We have obtained these same results chlorine.
The
on Ag( 1 IO) by predoaing atomically adsorbed
major effect occurs between the ~(2 X 1)
structures,
suggesting an ensemble rather
completely
suppress
the rate of dissociative
OL adsorption
coverage of atomic oxygen under reaction conditions. utilize
molecularly
absorbed
(~9,~ = 0.5) and
the c(4x 2)
than electronic effect. Chlorine
oxygen as the oxidizing
and consequently
the steady-state
Both ethylene oxide and CO, agent. Atomically
(NY,, = 0.75)
coverage, ahove 0.3
absorbed
production oxygen and
sites for ethylene adsorption.
chlorine play a similar role in the reaction by creating Ag”’
1. Introduction The selective ethylene oxide):
oxidation
of ethylene
/
\ ++02+ H
is a billion-dollar-per-year class of kinetically-controlled,
epoxide
(also
known
as
“\ /“\/”
“\&” H
to ethylene
Ag
Y-1
H
H
industry. It is the simplest example of the whole selective catalytic oxidation reactions, and is
therefore of considerable fundamental interest. A common industrial process uses a silver catalyst supported on a-Al,O, of - 1 m’ gg ’ specific surface area. A major engineering problem involves the undesirable side-reaction leading to 037%5963/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
C. T Campbell, M. T. Paffetr / Chlorine promoters and ethylene epoxidation ouer Ag(l IO)
29
CO, and water, the thermodynamically preferred products. Chlorinated hydrocarbons in trace amounts are typically added to the reactant feed in order to improve the catalysts’s selectivity to ethylene oxide (EtO) [3310]. This is, however, associated with considerable loss of activity. A number of researchers have tried to study various aspects of the mechanism by which chlorine promotes catalyst selectivity [3-151. It is assumed that the chlorine acts as chloride ions adsorbed on the Ag surface, but otherwise very little is known of the role of these chlorine promoters. Chlorine seems to inhibit the dissociative adsorption of oxygen [8,10-121, and to enhance ethylene adsorption [7-lo]. We have recently studied the kinetics and the mechanism of this reaction over a clean Ag(ll0) single crystal surface by using a technique which involves rapid transfer of the catalyst sample between an ultrahigh vacuum (UHV) chamber for surface analysis and a high-pressure catalytic microreactor for kinetic measurements [1,2]. We showed that the dependences of the steady-state reaction rates and selectivity upon temperature and reactant pressures were virtually identical for clean Ag(ll0) and for high-surface-area, supported, unpromoted Ag catalysts. The Ag(ll0) surface is therefore an excellent kinetic model of real-world catalysts. However, the absolute reaction rate (per surface Ag atom) was some lOO-fold higher on Ag(ll0). We further demonstrated a technique for quantitatively measuring the coverage of atomically absorbed oxygen (B,,) existing on the surface under steady-state, high pressure reaction conditions [l]. By measuring the reaction rates and selectivity as a function of temperature, reactant pressures and u/so as a function of r9,, we were able to provide considerable new insight into the controversial reaction mechanism [l]. In the present study we have utilized this technique to correlate steady-state reaction rates and selectivity with the coverage of atomically adsorbed oxygen (9,) and chlorine (Cl,). We have observed the effects of these species upon rates of various steps in the reaction mechanism, and have gained a more detailed insight into the role of chlorine promoters in Ag catalysts for ethylene epoxidation.
2. Experimental The Ag(ll0) preparation technique is described in another paper [16]. Details of the apparatus and technique are presented in a related paper [l]. In short, the Ag(ll0) sample was cleaned by sputtering and annealing (800 K) in ultrahigh vacuum (UHV), and its cleanliness and order were proven by Auger electron spectroscopy ‘(AES) and low-energy electron diffraction (LEED). The clean sample was transferred into an evacuated microreactor attached directly to the UHV chamber, pressurized with the reaction mixture (- 100 Torr) and heated (440-610 K) until a steady-steady epoxidation rate (4 min) was
30
C. T. Campbell,
M. T Paffett / Chlorrne promoters rend ethylene epu.x&tion
over Ag( I IO)
established. (A constant rate of Et0 production was actually established in -c 20 s, and maintained over time periods during which > 2000 molecules of ethylene per surface Ag atom were converted to product. Conversions were about 3%. See ref. [l].) Then the sample was rapidly (17 s) transferred at reaction temperature back into UHV for surface analysis (AES, LEED, TDS. XPS). The coverage of 0, was quantitatively measured by flash heating the sample immediately after transfer and measuring the area under the 600 K 0, thermal desorption peak mass-spectrometrically [l]. After transfer, the reaction mixture (still in the batch microreactor) was analyzed by gas chromatography for the amount (rates) of Et0 and CO, produced. As discussed in a related paper [l], above 480 K the surface contained only atomically adsorbed oxygen and maintained a good LEED pattern after reaction. Below 480 K other adsorbed reactants and products were observed. The sides and back of the crystal were passivated to reaction by a mixed Si. Cu. Ti oxide/carbide film which built up during the early stages of high-pressure reaction attempts. Coverages (6) are defined relative to the number of Ag surface atoms (0 = 1 is 8.5 X lOi cm-‘).
3. Results 3.1. Chlorine adsorption In order to study the effect of chlorine upon the reaction rates and selectivity, chlorine was first atomically adsorbed on the clean surface and its coverage and LEED structure were characterized with UHV surface analysis. Then the sample was transferred into the microreactor where the rates were measured at that chlorine pre-coverage. In these studies, adsorbed chlorine was deposited in two ways. For one, small exposures of Cl, gas were performed in UHV. For the other, the sample was allowed to react under epoxidation conditions for extended periods in the microreactor. Adsorbed chlorine built up slowly on the surface in the microreactor, presumably due to the volatilization of chlorine from the viton seals by the corrosive action of the Et0 product [l]. After the reaction, the sample was heated to about 700 K in UHV to remove other adsorbed impurities. Atomically adsorbed chlorine generated in either fashion had identical effects on the reaction rates and selectivity and gave the same LEED patterns and AES spectra. Fig. 1 shows the two LEED patterns we observed for adsorbed chlorine: a p(2 X 1) pattern characteristic of one-half monolayer of chlorine (tic, = 0.5) and a c(4 X 2) pattern corresponding to &., = 0.75. These patterns have been observed previously on Ag(ll0) after deposition of Cl, using dichloroethane in UHV [12,13]. The p(2 X l)-Cl structure is probably quite similar to the structure proposed for the p(2 x 1)-O pattern on Ag(llO), which consists of an
C. T Campbell, M. T. Pafftt / Chlorine promoters and ethylene epoxidation over Ag(1 IO)
oxygen surface with a planes The
31
adatom in every second two-fold site along each [110] channel on the [17]. The c(4 X 2)-Cl structure has been interpreted as a mixed layer packing of silver and chlorine ions similar to that of the AgCl(ll1) [13]. chlorine coverage was assumed to be proportional to the ratio of the
Fig. 1. Low-energy electron diffraction patterns for chlorine structures observed on Ag(ll0). (a) The p(2 x 1) structure. obtained by dosing Cl, gas in the vacuum chamber (near normal. 95 eV incident beam). (b) The ~(4x2) structure, obtained by Cl, dosing (Near normal, 99 eV incident beam). (c) The c(4X2) structure, obtained by chlorine buildup in the microreactor, immediately after a reaction of the types in figs. 2 and 3 (incident beam - 6% off normal, 86 eV; the (h + A, k) spots were never detected for the ~(4x2) structure). (d) Schematic representation of LEED patterns: (0) p(1 X 1) substrate spots, ( + ) p(2 x 1) extra spots, and (w) c(4 X 2) extra spots. The (0. 0) spot is shown.
32
M. T. Paffett/ Chlorine promoters and
C.T. Campbd
ethylene
epoxtdation
ouer
A,y(l
I())
chlorine-to-silver AES peak-to-peak heights (1,.,/I,,) and normalized to the ratio for the c(4 x 2) pattern (SC, = 0.75). which was the saturation coverage. Fig. 2 indicates the coverage ranges over which we observed the above LEED patterns. The patterns were still sharp and unchanged after the steady-state reaction measurements, as indicated in fig. lc. Consistent with previous observations [13,18,19], the atomically absorbed chlorine desorbed at around 800 K. We found it necessary to sputter the surface to remove the last traces of chlorine from the AES spectrum without heating to > 800 K. The sticking probability of Cl, gas at room temperature was large (> 0.2) also in agreement with previous results [14,18,20]. Chlorine adsorption on Ag surfaces shows a large work function increase (- + 1.7 eV [14,20]) and one can therefore assume considerable negative charge on the chlorine adatoms.
<2
n
A
*
.
Fig. 2. The effect of chlorine coverage upon the rates of Et0 and CO, production. upon the selectivity and upon the atomic oxygen coverage for steady-state conditions at PE, = 4.1 Torr, PO, = 150 Torr and T = 490 K over Ag(ll0). Also shown are the coverage regions over which the chlorine LEED patterns were observed.
C. T. Campbell, A4. T Pa&t
3.2. Steudy-state
/ Chlorine promoters
and ethylene epoxidation
ouer Ag(I IO)
33
reaction kinetics
Figs. 2 and 3 show the effects of chlorine adatom precoverage upon the steady-state rates of Et0 and CO, production and upon the selectivity at 490 K, at an ethylene pressure (Pr,) of 4.1 Torr, and at oxygen pressures (PO,) of 150 and 8.2 Torr, respectively. The specific reaction rates are expressed in terms of the turn-over number (TON), i.e., the number of molecules produced per surface Ag atom (site) per second. (We assumed lOI surface Ag atoms on our sample [l], independent of Cl coverage.) The selectivity for ethylene conversion into ethylene oxide is given by SE,o = TONE,o/(TONtlto + $TON<.,,J since no other products were observed. The chlorine coverage increased about 4% of a monolayer during the course of each kinetic measurement (4 min), so that 8,-, used here was taken as the average of the initial and final values. The regions over which the p(2 X 1) and c(4 X 2) chlorine LEED patterns were observed after reaction are indicated. Also shown in fig. 2 is the coverage of atomically adsorbed oxygen (0,)) under the steady-state reaction conditions, determined as described above by thermal desorption immediately after reaction. 1.6
- 80%
’
IA\.
*,,..j
““ii”
4
0
kP(2
co2
x “_I
7 v) 1.2-o-
‘;
-0.
aI C In
B <
\
.
P
‘\A ‘0 \
n
\ \ ‘0
0.8-
&
I
0
I 0.2
a
P
\ P\/
\
0
6o
/*
I
I 0.4
‘A..‘0
“A’
I
0.6
il
0
0.8
QCl Fig. 3. The effect of chlorine upon the rates of Et0 and CO, production and upon the selectivity for steady-state conditions at PEl = 4.1 Tom, P0, = 8.2 Tom and T = 490 K over Ag(ll0).
34
C. T. Campbell, M. T. Paffett / Chlorrne promoters and eth.vlene epoxidatmn over Ag(I IO)
The most obvious conclusion in figs. 2 and 3 is that atomically adsorbed chlorine on Ag(ll0) gives the same qualitative effects as known for chlorinated feed additives on high-surface-area supported Ag catalysts: the selectivity is improved at the cost of loss in activity [3-lo]. This confirms that chlorine promoters actually function as chlorine adatoms bonded directly to the Ag surface, and that a support material (Al,O,, SiO,) is unnecessary in achieving the promoter effect. Interestingly, the maximum selectivity we have observed (- 85%) is close to that usually reported for a well-promoted Ag catalyst [9.21]. It is clear that the enhanced selectivity results because Cl;, inhibits CO? production more strongly than Et0 production. A more careful inspection of figs. 2 and 3 reveals that the major effect of enhanced selectivity and decreased activity occurs between the p(2 x I)-Cl (8c, = 0.5) and the c(4 x 2)-Cl (Q., = 0.75) LEED structures. This suggests a most interesting type of ensemble effect. Significantly, the coverage of atomically adsorbed oxygen is completely attenuated by fl,, = 0.25. while the rate of Et0 production has hardly been affected. We should point out that the conditions in figs. 2 and 3 mandate completely different rate-limiting steps (or perhaps more properly, reactant limitations) on the clean surface. In fig. 2 (at P,, = 4.1 Torr and PO, = 150 Torr) we have shown that the rates of both Et0 and CO1 production are practically zero order in Po, and first order in P,, [l], and therefore limited by the supply of ethylene to the surface. On the other hand, in fig. 3 (at Pk, = 4.1 Torr and PO, = 8.2 Torr) the reaction rates are first order in PO, (and probably zero order in PE,) on the clean surface [l], and therefore limited by the supply of oxygen. In both cases, the rate of Et0 production is practically first order in 0,, on the clean surface (&., = 0) [l] (see also fig. 7). 3.3. Dissociative
oxygen adsorption
The effect of chlorine precoverage upon the rate of dissociative oxygen adsorption on Ag(ll0) is shown in fig. 4 at 477 K. These data show the area of the 0, thermal desorption peak at 600 K measured mass spectrometrically after a given 0, exposure at 477 K. The 0, desorption peak at 600 K has been studied previously [16,22], and it reflects the associative recombination of adsorbed oxygen atoms. Its area is a measure of the amount of 0, accumulated during the exposure. For each 0,, and each exposure, the adsorption area obtained was normalized to the amount obtained on the clean surface. The relative sticking probability shown was taken as the average of these normalized adsorption areas for exposures of 0.2, 0.5, 2 and 5 X 10-j mbar s, which gave coverages (0,) between 0.02 and 0.2 on clean Ag(ll0) [16]. One can see from fig. 4 that the dissociative adsorption of 0, on Ag(ll0) is completely suppressed already at &., - 0.3. This type of effect has been observed previously on supported Ag [ll] and Ag powder [lo], where it was found that
C. T. Campbell, M. T Paffeett/ Chlorine promoters and ethylene epoxidation over Ag(l IO)
35
&., z 0.25 suppressed the rate of oxygen adsorption by more than ten-fold. On the other hand, such marked effects on oxygen adsorption were not observed on Ag(331) [14]. The saturation coverage of 0, on Ag(ll0) was shown to be suppressed almost linearly with 8,, down to zero at 8c, = 0.5 [12]. Interestingly, we found that the shape and temperature of the oxygen thermal desorption peak did not change strongly with chlorine precoverage, suggesting that the adsorbed oxygen accumulates in regions of the surface relatively free of Cl,. This is similar to a previous model of separate islands of 0, and Cl, [12]. By comparing fig. 4 with 0, in fig. 2, one can see that the strong effect of adsorbed chlorine upon the dissociative adsorption rate of 0, is almost directly reflected in the steady-state coverage of 0, under epoxidation reaction conditions. We suggest that the effect of Cl, upon oxygen absorption relates to chlorine’s role as an electronegative adsorbate in withdrawing electron density from the Ag surface atoms. We note that an electron donor such as Na, on the other hand, markedly enhances 0, adsorption [23]. 3.4. Ethylene adsorption The effect of chlorine upon the adsorption of ethylene on Ag(ll0) is shown by the thermal desorption spectra (TDS) in fig. 5. In each case, the surface Ic, /&
x 1GG
Fig. 4. The effect of chlorine coverage upon the rate of dissociative oxygen adsorption at 477 K. The rate is averaged over 0, exposures of 0.2, 0.5, 2 and 5 X lo- m3mbar s.
Oil
A&110)
36
C. T. Campbell,
M. T Paffett
/
Chlorme
promoters
und ethylene
eppoxidutmn
over Ag( 110)
containing the designated chlorine precoverage was exposed to lo- ’ mbar s of ethylene at 134 K and then rapidly heated while monitoring the mass spectrometer signal for desorbing ethylene (m/e = 27). This exposure was sufficient to almost saturate the clean surface. The maximum chlorine coverages in sequences (a) and (b) were obtained by chlorine buildup in the microreactor (see above), but the lower coverages in each sequence were obtained by flashing these surfaces to > 700 K to partially remove the chlorine adlayer. Ethylene is only weakly adsorbed on the clean surface, and desorbs molecularly at about 155 K in a single peak. Already at the lowest chlorine coverages peak appears. With ina second, higher temperature ( - 195 K) desorption creasing coverage up to e,., z 0.5. the two peaks shift to higher temperature, but the lower temperature peak intensity is suppressed at the expense of the growing higher-temperature peak. The peak temperature and the area of the higher-temperature desorption peak maximizes at 0,., = 0.4, where the p(2 x l), ,,// I I I I I I I I /
;: :: :: ;:
CpH4/C2H4/Ag(l
:: :: I:
IO) + Cl
T od =134K p = 12 K/s
I
”
/I
130
190
250
I
“130
TEMPERATURE Fig. 5. Thermal desorption ethylene on Ag(ll0) at 134 signal at a line-of-sight mass 12 K SC’. Series (a) and (b) (see text).
I
I
I
1
190
/
I
I
I
250
(K 1
spectra of ethylene obtained from an exposure of lO_’ mbar s of K containing various coverages of adsorbed chlorine. The m/e = 27 spectrometer was monitored while ramping the temperature at about are for slightly different methods of obtaining the chlorine coverage
C. T Campbell, M. T. Paffeetr / Chlorine promoters and ethylene epoxidation ouer Ag(l IO)
37
Cl LEED pattern is first observed. The peak for f$, = 0.31 is noticeably broader in sequence (b), presumably due to a slightly different chlorine distribution on the surface resulting from the different means of obtaining that chlorine coverage. The highest desorption peak temperature (at &., a 0.4) is about 220 K. If we assume first order desorption and a pre-exponential factor of lOI s-‘, this reflects a maximum heat of adsorption for ethylene of 12.8 kcal mall’. This is roughly a 44% increase over the value of 8.9 kcal mall’ for the clean surface! As dc, increases from 0.5 to 0.75 the reverse trend is observed: the desorption peak shifts back to lower temperatures, ending up at about 195 K. In no case was any significant evidence for decomposition of ethylene observed. It has been noted previously that adsorbed chlorine enhances ethylene adsorption on Ag [7,10]. We can now attribute this effect to a strongly increased heat of adsorption. We propose that chlorine acts to create Ag+ (or Ag’+) sites on the surface by electron withdrawal, and that these electron-deficient Ag atoms more favorably accept a-electron donation by the ethylene molecule in creating a dative sigma bond, analogous to the effect in organometallic chemistry [24]. A similar model has been used to explain the enhancement in ethylene adsorption caused by oxygen adatoms [7,25-271. The discrete nature and size increase in the Cl-related TDS peak in fig. 5a suggests a rather localized (first-nearest-neighbor) electronic effect at its origin. The
I 30
190
250
TEMPERATURE Fig. 6. Thermal desorption ethylene
on Ag(ll0)
(K)
spectra of ethylene obtained
at 134 K containing
various coverages
from an exposure of atomically
of 10m6 mbar s of
adsorbed
oxygen.
38
C. T. Cumphell,
M. T. PaJJetr / Chlorrne pron~o~er.s md ethylene epo.udotwn
wer A,q(I IO)
smooth shift in this peak’s temperature with 0,., indicates. however, a somewhat longer-range electronic effect. In fig. 6 we show TDS for ethylene from Ag(l10) containing various precoverages of atomic oxygen, t$,, deposited at 480 K. In each case the surface was exposed to IO-’ mbar s of ethylene at 134 K. Oxygen generates a new desorption peak for ethylene at about 185 K. reflecting a heat of adsorption of about 10.7 kcal mol ‘. compared to X.9 kcal mol ’ on clean Ag(ll0). This is in good agreement with previous results [27-281. It has been stated that ethylene does not adsorbed on a surface saturated in oxygen [26,28]. We see no strong evidence for this in fig. 6, although the high coverage adsorption experiment here was least trustworthy. This is because of the very large 0, exposure [16,22] required to achieve 0,) = 0.5, which necessarily degrades the vacuum and renders such low-temperature adsorption experiments extremely difficult.
4. Discussion The role of chlorine promoters in Ag catalysts for ethylene epoxidation can clearly be modeled by using controlled chlorine adatom coverages on a Ag(ll0) single crystal. We have presented data in this paper sufficient only to reveal the major features of the mechanism of promotion. A more detailed study of the effects of temperature and reactant pressures upon the rates and selectivity over a well-promoted surface (&., - 0.65) would promise to provide much more detailed insights. Our data in fig. 2 provide very interesting new evidence concerning the reaction mechanism itself. Earlier mechanistic models have often proposed 0, as the oxygen species actually removed from the surface in either CO, production or both Et0 and CO, production [4,7-9,11.15,25.26,29]. It is difficult to accept such models entirely when one notes in fig. 2 the precipitous decline and virtual disappearance of 0,, over a region in chlorine coverage during which the rates of Et0 and CO1 production decline only some 10 and 20%, respectively. In fig. 3, one can again expect t$, to disappear for 0,., = 0.3 (according to fig. 4). In this case the rates of Et0 and CO, production decline only some 15 and 35%, respectively, although the rates are known [l] to be first order in PC,,_ These data strongly suggest an alternative mechanism, proposed previously and supported by significant other evidence [4,7,11,15.2G29], whereby both Et0 and CO, can be produced from a rate-limiting step which involves molecularly adsorbed oxygen (O,,.,) as the oxidizing agent. Unfortunately, this well-known species is too weakly adsorbed to remain on the surface after evacuation above room temperature [1.16]. In a recent study of the mechanism on clean Ag(llO), we provided significant evidence that 9, is nevertheless quite necessary in the rate-limiting step
C. T Campbell, M. T. Pa&t
/ Chlorine promoters and ethylene epoxidation
ouer Ag(I IO)
39
[l]. Some of these data are reproduced in fig. 7. The rates of Et0 and CO, production and the steady-steady oxygen coverage (0,) were measured as a function of PO, at P,, = 4.1 Torr and 490 K. The data are plotted to show directly the effect of 19, upon the rates. Note that the conditions at 0,, = 0 in figs. 2 and 3 are encompassed in fig. 7. It is clear in fig. 7 that on the clean surface (in the absence of Cl,) the rate-limiting step in both CO, and Et0 production somehow requires adsorbed atomic oxygen. Fig. 2, however, clearly unnecessary. We can shows that in the presence of Cl,, 0, is virtually rationalize these two apparently disparate observations simply by realizing that the role played by 0, can be replaced by Cl,. We further postulate that this relates to their nature, both as highly electronegative adsorbates, in creating From electron-deficient Ags+ sites for the reaction complex or intermediates. the obvious beneficial effects of Cl, and 0, upon molecular ethylene adsorption (figs. 5 and 6), we conclude that these Agsi sites are necessary in ethylene adsorption. Oxygen and chlorine caused 20-40% increases in the heat of adsorption of ethylene, which will not only increase its steady-state coverage under reaction conditions, but will also possibly render the adsorbed ethylene molecule more favorably bonded for the reaction event with O,.,.
Fig. 7. A reproduction of the data from ref. [l] showing the effect of 8, upon the steady-state rates of Et0 and CO, production for PE, = 4.1 Torr over clean Ag(ll0) at 490 K. The atomic oxygen coverages were achieved by varying PO, as shown. In the original paper, we pointed out that 0, resided on the surface in p(2 X 1) islands of local coverage 8, = 0.5.
40
C. T. Cumphell,
M. T.
Pajfeil / Chlorine
promorrr.~ und erhslene epox~dumn
owr Ag( I/ 0)
In our previous study [l] we proposed this type of mechanism to explain the data of fig. 7. We further proposed, however, a second potential pathway to CO, which directly involved atomically adsorbed oxygen as the oxidizing agent. This reaction was assumed to predominate CO? production in its steeply sloping region of fig. 7 (below I$, = 0.02). Our data of figs. 2 and 3 are consistent with this model. When 0,, had been completely suppressed ( &, = 0.3). the rate of COz production declined by about 35% at P,, = 8.2 Torr and by about 10% at P,, = 150 Torr. According to our model, that portion of CO, production falling below the sharp break in its slope in fig. 7 (at 0,) = 0.02) is attributable to this second path to CO>. This involves some 50% of the rate at PO2 = 8.2 Torr, and 25% at PO2 = 150 Torr. These values are at least qualitatively consistent with the rate declines observed upon the replacement of O;, by Cl;, mentioned above. By this model, the small increases in selectivity effected by chlorine coverages up to 0.3 can then be attributed to the suppression of this second pathway to CO, production which directly involves 0,. The Et0 and CO, production rates are inhibited most strongly by Cl, for coverages beyond that at which the p(2 x I)-Cl structure is first observed. It is tempting to correlate this with the fact that the heat of adsorption of ethylene starts to decrease for 0,., beyond the p(2 x 1 )-Cl coverage (fig. 5). In fact. for the c(4 x 2)-Cl overlayer model that has been proposed [13], it would be sterically difficult to squeeze an ethylene molecule between the chlorine adatoms so as to overlap its T-orbital with Ag-atom orbitals. We have previously proposed that Et0 and part of the CO, are produced through a common intermediate step involving ethylene (adsorbed on Ag”+ sites) and molecularly adsorbed oxygen [l]. The changes in selectivity for 0,., z 0.5 would then reflect changes in the relative rates of the branches leading to Et0 and CO, ufter this step. One possibility is that the branch leading to CO, production requires a larger ensemble of Ag”’ sites unmasked by Cl adatoms than does the Et0 branch. An ensemble rather than electronic effect is certainly suggested by the high coverages over which these changes occur, since mutual depolarization is thought to decrease the ionicity of the Cl adatom in this coverage range [14,20]. Although we now have a reasonably good picture of the chlorine adlayer ensemble under optimum promotion conditions (~9,.,E 0.65), just how this ensemble determines selectivity must await future study. While comparing ensemble to electronic factors, we should point out that, in general, electronegative additives decrease activity and increase selectivity in this reaction, whereas electropositive additives can increase activity but decrease selectivity [3-7, 30-311. This indicates that electronic factors are certainly important in the role of surface additives in this reaction. Indeed, we suggested that the effects of CI, upon the dissociative adsorption rate of 0, (fig. 4) and the heat of ethylene adsorption (fig. 5) are determined by electronic factors, which presumably could be accomplished qualitatively with several
C. T. Campbell, M. T. Paffett / Chlorine promoters
and ethylene epoxidation
over Ag(l IO)
41
electronegative adsorbates. Nevertheless, an ensemble effect seems to play an equally important role, determining the activity and selectivity for chlorine coverages above a half monolayer. It seems essential, however, that this ensemble be generated by an array of electronegative, rather than electropositive, adatoms. The effects of chlorine upon ethylene adsorption and dissociative oxygen adsorption have been addressed to some extent in this paper. More detailed studies on these systems would be of interest. In addition, studies of the effects of Cl, upon the adsorption/desorption behavior of molecular O,, Et0 and CO, may prove valuable in understanding the promoter effect.
5. Conclusions The role of chlorine promoters in Ag catalysts for ethylene epoxidation can be accurately modeled with atomically absorbed chlorine on Ag(ll0). Chlorine inhibits both Et0 and CO, production, but CO, production more stronglyso that an overall promotion in selectivity (up to 85%) results. The major influence develops between the p(2 X l)-Cl structure at 8c, = 0.5 and the c(4 X 2)-Cl structure at I&.,= 0.75. This suggests an ensemble effect. The variation of the steady-state rates, selectivity, and particularly coverage of 0, with t?,., indicate that Cl, replaces 0, in the reaction mechanism by creating Ags+ sites for enhanced ethylene adsorption. Adsorbed molecular 0, seems to be the major oxidizing agent in epoxidation.
References [l] C.T. Campbell and M.T. Paffett, Surface Sci. 139 (1984) 396. [2] C.T. Campbell. in: Proc. 1983 Am. Vacuum Sot. Symp., Boston [J. Vacuum Sci. Tech&. A2 (1984) 10241. [3] J.T. Kummer. J. Phys. Chem. 60 (1956) 666. [4] H.H. Voge and C.R. Adams, Advan. Catalysis 17 (1967) 151. [5] V.E. Ostrovskii, N.V. Kul’kova, V.L. Lopatin and M.I. Temkin, Kinetika Kataliz 3 (1962) 160. [6] L.Y. Margolis, E.K. Enikeev, O.V. Isaev, A.V. Krylova and M.Y. Kuchnerov, Kinetika Kataliz 3 (1962) 153. [7] X.E. Verykios, F.P. Stein and R.W. Coughlin. Catalysis Rev.-Sci. Eng. 22 (1980) 197. [S] P.A. Kilty and W.M.H. Sachtler, Catalysis Rev.-Sci. Eng. 10 (1974) 1. [9] N. Giordano, J.C.J. Bart and R. Maggiore, Z. Physik. Chem. (NF) 127 (1981) 109. [lo] R.G. Meisenheimer and J.N. Wilson, J. Catalysis 1 (1962) 151. [ll] P.A. Kilty, N.C. Rol and W.M.H. Sachtler. in: Proc. 5th Intern. Congr. on Catalysis, 1973, Vol. 2, p. 929. [12] G. Rovida, F. Pratesi and E. Ferroni, J. Catalysis 41 (1976) 140. [13] G. Rovida and F. Pratesi, Surface Sci. 51 (1975) 270. 114) R.A. Marbrow and R.M. Lambert, Surface Sci. 71 (1978) 107.
42
[15] [16] [17] [18] [19] [20] [21] 122) [23] [24] [25] [26] [27] [28] [29] [30] [31]
C. T. Cumpbell,
M. T. Puff&t / Chlorine promoters und eth.vlene epoxidtitmn
over Agll IO)
R.A. Van Santen. J. Moolhuysen and W.M.H. Sachtler, J. Catalysis 65 (1980) 478. C.T. Campbell and M.T. Paffett, Surface Sci. 143 (1984) 517. W. H&land, F. Iberl, E. Taglauer and D. Menzel, Surface Sci. 53 (1975) 383. Y.Y. Tu and J.M. Blakely. Surface Sci. 85 (1979) 276. Y.Y. Tu and J.M. Blakely, J. Vacuum Sci. Technol. 15 (1978) 563. P.J. Goddard and R.M. Lambert, Surface SCI. 67 (1977) 180. J.C. ZomerdiJk and M.W. Hail, Catalysis Rev.-&i. Eng. 23 (1981) 163. M.A. Barteau and R.J. Madix. in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 4. Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam. 1982) ch. 4. R.A. Marbrow and R.M. Lambert, Surface Sci. 61 (1976) 329. J.E. Huheey. Inorganic Chemistry (Harper and Row, New York, 1972) pp. 484-486. E.L. Force and A.T. Bell, J. Catalysis 38 (1975) 440. E.L. Force and A.T. Bell, J. Catalysis 40 (1975) 356. C. Backx, C.P.M. de Groot and P. Biloen, Appl. Surface Sci. 6 (1980) 256. M. Akimoto. K. lchikawa and E. Echigoya. J. Catalysts 76 (19X2) 333. W.M.H. Sachtler, C. Backx and R.A. van Santen. Catalysis Rev.-L%. Eng. 23 (1981) 127. H.T. Spath and K. Torkar. J. Catalysis 26 (1972) 163. H.T. Spath. in: Proc. 5th Intern. Congr. on Catalysis, 1973. Vol. 2, p. 945.
Applicatwn!,
of Surface Science I Y ( 19X4) 2X 42 North-Holland.
ETHY L,ENE
THE ROLE OF CHLORINE PROMOTERS IN CATALYTIC EPOXIDATION OVER THE Ag( 110) SURFACE Charles
T. CAMPBELL
C‘hermstr~
Drcvsron,
and Mark T. PAFFETT
1.m Alumos
Notwnul
Luhorutor~,
Received 4 October 19X3: accepted for pubhcation
We have recently shown that the kinetics mimic
Ag(ll0)
identically
Amsterdam
the results
Lo\ A lunw~.
NCM, .M~r\,c o X7.G5,
C’S,
1X June lYX4
and selectwiry of ethylene cpoxldation
over unpromoted,
supported
Ag catalysts.
IOO-fold enhancement of the specific activity (per surface Ag atom) on Ag( 110)
over clean
except for a
[ 1.21. In
thi\ report.
we will discuss results in which we have modeled the role of chlorine promoters in this reaction h! combining ultrahigh vacuum surface analysis (XPS. AES. LEED. sure
TDS)
before and after high-prer-
100 torr) kinetic measurements. In this way, we were able to correlate the reactwn rate and
(-
selectivity
not only
atomically
adsorbed oxygen and chlorine.
with
temperature
and reactant pressures. In industrial
but also with
catalysis.
the coverages of
trace amounts of chlorinated
organica are added to the feed stream to promote selectiwty with, however, some decrease in the reaction rate. We have obtained these same results chlorine.
The
on Ag( 1 IO) by predoaing atomically adsorbed
major effect occurs between the ~(2 X 1)
structures,
suggesting an ensemble rather
completely
suppress
the rate of dissociative
OL adsorption
coverage of atomic oxygen under reaction conditions. utilize
molecularly
absorbed
(~9,~ = 0.5) and
the c(4x 2)
than electronic effect. Chlorine
oxygen as the oxidizing
and consequently
the steady-state
Both ethylene oxide and CO, agent. Atomically
(NY,, = 0.75)
coverage, ahove 0.3
absorbed
production oxygen and
sites for ethylene adsorption.
chlorine play a similar role in the reaction by creating Ag”’
1. Introduction The selective ethylene oxide):
oxidation
of ethylene
/
\ ++02+ H
is a billion-dollar-per-year class of kinetically-controlled,
epoxide
(also
known
as
“\ /“\/”
“\&” H
to ethylene
Ag
Y-1
H
H
industry. It is the simplest example of the whole selective catalytic oxidation reactions, and is
therefore of considerable fundamental interest. A common industrial process uses a silver catalyst supported on a-Al,O, of - 1 m’ gg ’ specific surface area. A major engineering problem involves the undesirable side-reaction leading to 037%5963/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
C. T Campbell, M. T. Paffetr / Chlorine promoters and ethylene epoxidation ouer Ag(l IO)
29
CO, and water, the thermodynamically preferred products. Chlorinated hydrocarbons in trace amounts are typically added to the reactant feed in order to improve the catalysts’s selectivity to ethylene oxide (EtO) [3310]. This is, however, associated with considerable loss of activity. A number of researchers have tried to study various aspects of the mechanism by which chlorine promotes catalyst selectivity [3-151. It is assumed that the chlorine acts as chloride ions adsorbed on the Ag surface, but otherwise very little is known of the role of these chlorine promoters. Chlorine seems to inhibit the dissociative adsorption of oxygen [8,10-121, and to enhance ethylene adsorption [7-lo]. We have recently studied the kinetics and the mechanism of this reaction over a clean Ag(ll0) single crystal surface by using a technique which involves rapid transfer of the catalyst sample between an ultrahigh vacuum (UHV) chamber for surface analysis and a high-pressure catalytic microreactor for kinetic measurements [1,2]. We showed that the dependences of the steady-state reaction rates and selectivity upon temperature and reactant pressures were virtually identical for clean Ag(ll0) and for high-surface-area, supported, unpromoted Ag catalysts. The Ag(ll0) surface is therefore an excellent kinetic model of real-world catalysts. However, the absolute reaction rate (per surface Ag atom) was some lOO-fold higher on Ag(ll0). We further demonstrated a technique for quantitatively measuring the coverage of atomically absorbed oxygen (B,,) existing on the surface under steady-state, high pressure reaction conditions [l]. By measuring the reaction rates and selectivity as a function of temperature, reactant pressures and u/so as a function of r9,, we were able to provide considerable new insight into the controversial reaction mechanism [l]. In the present study we have utilized this technique to correlate steady-state reaction rates and selectivity with the coverage of atomically adsorbed oxygen (9,) and chlorine (Cl,). We have observed the effects of these species upon rates of various steps in the reaction mechanism, and have gained a more detailed insight into the role of chlorine promoters in Ag catalysts for ethylene epoxidation.
2. Experimental The Ag(ll0) preparation technique is described in another paper [16]. Details of the apparatus and technique are presented in a related paper [l]. In short, the Ag(ll0) sample was cleaned by sputtering and annealing (800 K) in ultrahigh vacuum (UHV), and its cleanliness and order were proven by Auger electron spectroscopy ‘(AES) and low-energy electron diffraction (LEED). The clean sample was transferred into an evacuated microreactor attached directly to the UHV chamber, pressurized with the reaction mixture (- 100 Torr) and heated (440-610 K) until a steady-steady epoxidation rate (4 min) was
30
C. T. Campbell,
M. T Paffett / Chlorrne promoters rend ethylene epu.x&tion
over Ag( I IO)
established. (A constant rate of Et0 production was actually established in -c 20 s, and maintained over time periods during which > 2000 molecules of ethylene per surface Ag atom were converted to product. Conversions were about 3%. See ref. [l].) Then the sample was rapidly (17 s) transferred at reaction temperature back into UHV for surface analysis (AES, LEED, TDS. XPS). The coverage of 0, was quantitatively measured by flash heating the sample immediately after transfer and measuring the area under the 600 K 0, thermal desorption peak mass-spectrometrically [l]. After transfer, the reaction mixture (still in the batch microreactor) was analyzed by gas chromatography for the amount (rates) of Et0 and CO, produced. As discussed in a related paper [l], above 480 K the surface contained only atomically adsorbed oxygen and maintained a good LEED pattern after reaction. Below 480 K other adsorbed reactants and products were observed. The sides and back of the crystal were passivated to reaction by a mixed Si. Cu. Ti oxide/carbide film which built up during the early stages of high-pressure reaction attempts. Coverages (6) are defined relative to the number of Ag surface atoms (0 = 1 is 8.5 X lOi cm-‘).
3. Results 3.1. Chlorine adsorption In order to study the effect of chlorine upon the reaction rates and selectivity, chlorine was first atomically adsorbed on the clean surface and its coverage and LEED structure were characterized with UHV surface analysis. Then the sample was transferred into the microreactor where the rates were measured at that chlorine pre-coverage. In these studies, adsorbed chlorine was deposited in two ways. For one, small exposures of Cl, gas were performed in UHV. For the other, the sample was allowed to react under epoxidation conditions for extended periods in the microreactor. Adsorbed chlorine built up slowly on the surface in the microreactor, presumably due to the volatilization of chlorine from the viton seals by the corrosive action of the Et0 product [l]. After the reaction, the sample was heated to about 700 K in UHV to remove other adsorbed impurities. Atomically adsorbed chlorine generated in either fashion had identical effects on the reaction rates and selectivity and gave the same LEED patterns and AES spectra. Fig. 1 shows the two LEED patterns we observed for adsorbed chlorine: a p(2 X 1) pattern characteristic of one-half monolayer of chlorine (tic, = 0.5) and a c(4 X 2) pattern corresponding to &., = 0.75. These patterns have been observed previously on Ag(ll0) after deposition of Cl, using dichloroethane in UHV [12,13]. The p(2 X l)-Cl structure is probably quite similar to the structure proposed for the p(2 x 1)-O pattern on Ag(llO), which consists of an
C. T Campbell, M. T. Pafftt / Chlorine promoters and ethylene epoxidation over Ag(1 IO)
oxygen surface with a planes The
31
adatom in every second two-fold site along each [110] channel on the [17]. The c(4 X 2)-Cl structure has been interpreted as a mixed layer packing of silver and chlorine ions similar to that of the AgCl(ll1) [13]. chlorine coverage was assumed to be proportional to the ratio of the
Fig. 1. Low-energy electron diffraction patterns for chlorine structures observed on Ag(ll0). (a) The p(2 x 1) structure. obtained by dosing Cl, gas in the vacuum chamber (near normal. 95 eV incident beam). (b) The ~(4x2) structure, obtained by Cl, dosing (Near normal, 99 eV incident beam). (c) The c(4X2) structure, obtained by chlorine buildup in the microreactor, immediately after a reaction of the types in figs. 2 and 3 (incident beam - 6% off normal, 86 eV; the (h + A, k) spots were never detected for the ~(4x2) structure). (d) Schematic representation of LEED patterns: (0) p(1 X 1) substrate spots, ( + ) p(2 x 1) extra spots, and (w) c(4 X 2) extra spots. The (0. 0) spot is shown.
32
M. T. Paffett/ Chlorine promoters and
C.T. Campbd
ethylene
epoxtdation
ouer
A,y(l
I())
chlorine-to-silver AES peak-to-peak heights (1,.,/I,,) and normalized to the ratio for the c(4 x 2) pattern (SC, = 0.75). which was the saturation coverage. Fig. 2 indicates the coverage ranges over which we observed the above LEED patterns. The patterns were still sharp and unchanged after the steady-state reaction measurements, as indicated in fig. lc. Consistent with previous observations [13,18,19], the atomically absorbed chlorine desorbed at around 800 K. We found it necessary to sputter the surface to remove the last traces of chlorine from the AES spectrum without heating to > 800 K. The sticking probability of Cl, gas at room temperature was large (> 0.2) also in agreement with previous results [14,18,20]. Chlorine adsorption on Ag surfaces shows a large work function increase (- + 1.7 eV [14,20]) and one can therefore assume considerable negative charge on the chlorine adatoms.
<2
n
A
*
.
Fig. 2. The effect of chlorine coverage upon the rates of Et0 and CO, production. upon the selectivity and upon the atomic oxygen coverage for steady-state conditions at PE, = 4.1 Torr, PO, = 150 Torr and T = 490 K over Ag(ll0). Also shown are the coverage regions over which the chlorine LEED patterns were observed.
C. T. Campbell, A4. T Pa&t
3.2. Steudy-state
/ Chlorine promoters
and ethylene epoxidation
ouer Ag(I IO)
33
reaction kinetics
Figs. 2 and 3 show the effects of chlorine adatom precoverage upon the steady-state rates of Et0 and CO, production and upon the selectivity at 490 K, at an ethylene pressure (Pr,) of 4.1 Torr, and at oxygen pressures (PO,) of 150 and 8.2 Torr, respectively. The specific reaction rates are expressed in terms of the turn-over number (TON), i.e., the number of molecules produced per surface Ag atom (site) per second. (We assumed lOI surface Ag atoms on our sample [l], independent of Cl coverage.) The selectivity for ethylene conversion into ethylene oxide is given by SE,o = TONE,o/(TONtlto + $TON<.,,J since no other products were observed. The chlorine coverage increased about 4% of a monolayer during the course of each kinetic measurement (4 min), so that 8,-, used here was taken as the average of the initial and final values. The regions over which the p(2 X 1) and c(4 X 2) chlorine LEED patterns were observed after reaction are indicated. Also shown in fig. 2 is the coverage of atomically adsorbed oxygen (0,)) under the steady-state reaction conditions, determined as described above by thermal desorption immediately after reaction. 1.6
- 80%
’
IA\.
*,,..j
““ii”
4
0
kP(2
co2
x “_I
7 v) 1.2-o-
‘;
-0.
aI C In
B <
\
.
P
‘\A ‘0 \
n
\ \ ‘0
0.8-
&
I
0
I 0.2
a
P
\ P\/
\
0
6o
/*
I
I 0.4
‘A..‘0
“A’
I
0.6
il
0
0.8
QCl Fig. 3. The effect of chlorine upon the rates of Et0 and CO, production and upon the selectivity for steady-state conditions at PEl = 4.1 Tom, P0, = 8.2 Tom and T = 490 K over Ag(ll0).
34
C. T. Campbell, M. T. Paffett / Chlorrne promoters and eth.vlene epoxidatmn over Ag(I IO)
The most obvious conclusion in figs. 2 and 3 is that atomically adsorbed chlorine on Ag(ll0) gives the same qualitative effects as known for chlorinated feed additives on high-surface-area supported Ag catalysts: the selectivity is improved at the cost of loss in activity [3-lo]. This confirms that chlorine promoters actually function as chlorine adatoms bonded directly to the Ag surface, and that a support material (Al,O,, SiO,) is unnecessary in achieving the promoter effect. Interestingly, the maximum selectivity we have observed (- 85%) is close to that usually reported for a well-promoted Ag catalyst [9.21]. It is clear that the enhanced selectivity results because Cl;, inhibits CO? production more strongly than Et0 production. A more careful inspection of figs. 2 and 3 reveals that the major effect of enhanced selectivity and decreased activity occurs between the p(2 x I)-Cl (8c, = 0.5) and the c(4 x 2)-Cl (Q., = 0.75) LEED structures. This suggests a most interesting type of ensemble effect. Significantly, the coverage of atomically adsorbed oxygen is completely attenuated by fl,, = 0.25. while the rate of Et0 production has hardly been affected. We should point out that the conditions in figs. 2 and 3 mandate completely different rate-limiting steps (or perhaps more properly, reactant limitations) on the clean surface. In fig. 2 (at P,, = 4.1 Torr and PO, = 150 Torr) we have shown that the rates of both Et0 and CO1 production are practically zero order in Po, and first order in P,, [l], and therefore limited by the supply of ethylene to the surface. On the other hand, in fig. 3 (at Pk, = 4.1 Torr and PO, = 8.2 Torr) the reaction rates are first order in PO, (and probably zero order in PE,) on the clean surface [l], and therefore limited by the supply of oxygen. In both cases, the rate of Et0 production is practically first order in 0,, on the clean surface (&., = 0) [l] (see also fig. 7). 3.3. Dissociative
oxygen adsorption
The effect of chlorine precoverage upon the rate of dissociative oxygen adsorption on Ag(ll0) is shown in fig. 4 at 477 K. These data show the area of the 0, thermal desorption peak at 600 K measured mass spectrometrically after a given 0, exposure at 477 K. The 0, desorption peak at 600 K has been studied previously [16,22], and it reflects the associative recombination of adsorbed oxygen atoms. Its area is a measure of the amount of 0, accumulated during the exposure. For each 0,, and each exposure, the adsorption area obtained was normalized to the amount obtained on the clean surface. The relative sticking probability shown was taken as the average of these normalized adsorption areas for exposures of 0.2, 0.5, 2 and 5 X 10-j mbar s, which gave coverages (0,) between 0.02 and 0.2 on clean Ag(ll0) [16]. One can see from fig. 4 that the dissociative adsorption of 0, on Ag(ll0) is completely suppressed already at &., - 0.3. This type of effect has been observed previously on supported Ag [ll] and Ag powder [lo], where it was found that
C. T. Campbell, M. T Paffeett/ Chlorine promoters and ethylene epoxidation over Ag(l IO)
35
&., z 0.25 suppressed the rate of oxygen adsorption by more than ten-fold. On the other hand, such marked effects on oxygen adsorption were not observed on Ag(331) [14]. The saturation coverage of 0, on Ag(ll0) was shown to be suppressed almost linearly with 8,, down to zero at 8c, = 0.5 [12]. Interestingly, we found that the shape and temperature of the oxygen thermal desorption peak did not change strongly with chlorine precoverage, suggesting that the adsorbed oxygen accumulates in regions of the surface relatively free of Cl,. This is similar to a previous model of separate islands of 0, and Cl, [12]. By comparing fig. 4 with 0, in fig. 2, one can see that the strong effect of adsorbed chlorine upon the dissociative adsorption rate of 0, is almost directly reflected in the steady-state coverage of 0, under epoxidation reaction conditions. We suggest that the effect of Cl, upon oxygen absorption relates to chlorine’s role as an electronegative adsorbate in withdrawing electron density from the Ag surface atoms. We note that an electron donor such as Na, on the other hand, markedly enhances 0, adsorption [23]. 3.4. Ethylene adsorption The effect of chlorine upon the adsorption of ethylene on Ag(ll0) is shown by the thermal desorption spectra (TDS) in fig. 5. In each case, the surface Ic, /&
x 1GG
Fig. 4. The effect of chlorine coverage upon the rate of dissociative oxygen adsorption at 477 K. The rate is averaged over 0, exposures of 0.2, 0.5, 2 and 5 X lo- m3mbar s.
Oil
A&110)
36
C. T. Campbell,
M. T Paffett
/
Chlorme
promoters
und ethylene
eppoxidutmn
over Ag( 110)
containing the designated chlorine precoverage was exposed to lo- ’ mbar s of ethylene at 134 K and then rapidly heated while monitoring the mass spectrometer signal for desorbing ethylene (m/e = 27). This exposure was sufficient to almost saturate the clean surface. The maximum chlorine coverages in sequences (a) and (b) were obtained by chlorine buildup in the microreactor (see above), but the lower coverages in each sequence were obtained by flashing these surfaces to > 700 K to partially remove the chlorine adlayer. Ethylene is only weakly adsorbed on the clean surface, and desorbs molecularly at about 155 K in a single peak. Already at the lowest chlorine coverages peak appears. With ina second, higher temperature ( - 195 K) desorption creasing coverage up to e,., z 0.5. the two peaks shift to higher temperature, but the lower temperature peak intensity is suppressed at the expense of the growing higher-temperature peak. The peak temperature and the area of the higher-temperature desorption peak maximizes at 0,., = 0.4, where the p(2 x l), ,,// I I I I I I I I /
;: :: :: ;:
CpH4/C2H4/Ag(l
:: :: I:
IO) + Cl
T od =134K p = 12 K/s
I
”
/I
130
190
250
I
“130
TEMPERATURE Fig. 5. Thermal desorption ethylene on Ag(ll0) at 134 signal at a line-of-sight mass 12 K SC’. Series (a) and (b) (see text).
I
I
I
1
190
/
I
I
I
250
(K 1
spectra of ethylene obtained from an exposure of lO_’ mbar s of K containing various coverages of adsorbed chlorine. The m/e = 27 spectrometer was monitored while ramping the temperature at about are for slightly different methods of obtaining the chlorine coverage
C. T Campbell, M. T. Paffeetr / Chlorine promoters and ethylene epoxidation ouer Ag(l IO)
37
Cl LEED pattern is first observed. The peak for f$, = 0.31 is noticeably broader in sequence (b), presumably due to a slightly different chlorine distribution on the surface resulting from the different means of obtaining that chlorine coverage. The highest desorption peak temperature (at &., a 0.4) is about 220 K. If we assume first order desorption and a pre-exponential factor of lOI s-‘, this reflects a maximum heat of adsorption for ethylene of 12.8 kcal mall’. This is roughly a 44% increase over the value of 8.9 kcal mall’ for the clean surface! As dc, increases from 0.5 to 0.75 the reverse trend is observed: the desorption peak shifts back to lower temperatures, ending up at about 195 K. In no case was any significant evidence for decomposition of ethylene observed. It has been noted previously that adsorbed chlorine enhances ethylene adsorption on Ag [7,10]. We can now attribute this effect to a strongly increased heat of adsorption. We propose that chlorine acts to create Ag+ (or Ag’+) sites on the surface by electron withdrawal, and that these electron-deficient Ag atoms more favorably accept a-electron donation by the ethylene molecule in creating a dative sigma bond, analogous to the effect in organometallic chemistry [24]. A similar model has been used to explain the enhancement in ethylene adsorption caused by oxygen adatoms [7,25-271. The discrete nature and size increase in the Cl-related TDS peak in fig. 5a suggests a rather localized (first-nearest-neighbor) electronic effect at its origin. The
I 30
190
250
TEMPERATURE Fig. 6. Thermal desorption ethylene
on Ag(ll0)
(K)
spectra of ethylene obtained
at 134 K containing
various coverages
from an exposure of atomically
of 10m6 mbar s of
adsorbed
oxygen.
38
C. T. Cumphell,
M. T. PaJJetr / Chlorrne pron~o~er.s md ethylene epo.udotwn
wer A,q(I IO)
smooth shift in this peak’s temperature with 0,., indicates. however, a somewhat longer-range electronic effect. In fig. 6 we show TDS for ethylene from Ag(l10) containing various precoverages of atomic oxygen, t$,, deposited at 480 K. In each case the surface was exposed to IO-’ mbar s of ethylene at 134 K. Oxygen generates a new desorption peak for ethylene at about 185 K. reflecting a heat of adsorption of about 10.7 kcal mol ‘. compared to X.9 kcal mol ’ on clean Ag(ll0). This is in good agreement with previous results [27-281. It has been stated that ethylene does not adsorbed on a surface saturated in oxygen [26,28]. We see no strong evidence for this in fig. 6, although the high coverage adsorption experiment here was least trustworthy. This is because of the very large 0, exposure [16,22] required to achieve 0,) = 0.5, which necessarily degrades the vacuum and renders such low-temperature adsorption experiments extremely difficult.
4. Discussion The role of chlorine promoters in Ag catalysts for ethylene epoxidation can clearly be modeled by using controlled chlorine adatom coverages on a Ag(ll0) single crystal. We have presented data in this paper sufficient only to reveal the major features of the mechanism of promotion. A more detailed study of the effects of temperature and reactant pressures upon the rates and selectivity over a well-promoted surface (&., - 0.65) would promise to provide much more detailed insights. Our data in fig. 2 provide very interesting new evidence concerning the reaction mechanism itself. Earlier mechanistic models have often proposed 0, as the oxygen species actually removed from the surface in either CO, production or both Et0 and CO, production [4,7-9,11.15,25.26,29]. It is difficult to accept such models entirely when one notes in fig. 2 the precipitous decline and virtual disappearance of 0,, over a region in chlorine coverage during which the rates of Et0 and CO1 production decline only some 10 and 20%, respectively. In fig. 3, one can again expect t$, to disappear for 0,., = 0.3 (according to fig. 4). In this case the rates of Et0 and CO, production decline only some 15 and 35%, respectively, although the rates are known [l] to be first order in PC,,_ These data strongly suggest an alternative mechanism, proposed previously and supported by significant other evidence [4,7,11,15.2G29], whereby both Et0 and CO, can be produced from a rate-limiting step which involves molecularly adsorbed oxygen (O,,.,) as the oxidizing agent. Unfortunately, this well-known species is too weakly adsorbed to remain on the surface after evacuation above room temperature [1.16]. In a recent study of the mechanism on clean Ag(llO), we provided significant evidence that 9, is nevertheless quite necessary in the rate-limiting step
C. T Campbell, M. T. Pa&t
/ Chlorine promoters and ethylene epoxidation
ouer Ag(I IO)
39
[l]. Some of these data are reproduced in fig. 7. The rates of Et0 and CO, production and the steady-steady oxygen coverage (0,) were measured as a function of PO, at P,, = 4.1 Torr and 490 K. The data are plotted to show directly the effect of 19, upon the rates. Note that the conditions at 0,, = 0 in figs. 2 and 3 are encompassed in fig. 7. It is clear in fig. 7 that on the clean surface (in the absence of Cl,) the rate-limiting step in both CO, and Et0 production somehow requires adsorbed atomic oxygen. Fig. 2, however, clearly unnecessary. We can shows that in the presence of Cl,, 0, is virtually rationalize these two apparently disparate observations simply by realizing that the role played by 0, can be replaced by Cl,. We further postulate that this relates to their nature, both as highly electronegative adsorbates, in creating From electron-deficient Ags+ sites for the reaction complex or intermediates. the obvious beneficial effects of Cl, and 0, upon molecular ethylene adsorption (figs. 5 and 6), we conclude that these Agsi sites are necessary in ethylene adsorption. Oxygen and chlorine caused 20-40% increases in the heat of adsorption of ethylene, which will not only increase its steady-state coverage under reaction conditions, but will also possibly render the adsorbed ethylene molecule more favorably bonded for the reaction event with O,.,.
Fig. 7. A reproduction of the data from ref. [l] showing the effect of 8, upon the steady-state rates of Et0 and CO, production for PE, = 4.1 Torr over clean Ag(ll0) at 490 K. The atomic oxygen coverages were achieved by varying PO, as shown. In the original paper, we pointed out that 0, resided on the surface in p(2 X 1) islands of local coverage 8, = 0.5.
40
C. T. Cumphell,
M. T.
Pajfeil / Chlorine
promorrr.~ und erhslene epox~dumn
owr Ag( I/ 0)
In our previous study [l] we proposed this type of mechanism to explain the data of fig. 7. We further proposed, however, a second potential pathway to CO, which directly involved atomically adsorbed oxygen as the oxidizing agent. This reaction was assumed to predominate CO? production in its steeply sloping region of fig. 7 (below I$, = 0.02). Our data of figs. 2 and 3 are consistent with this model. When 0,, had been completely suppressed ( &, = 0.3). the rate of COz production declined by about 35% at P,, = 8.2 Torr and by about 10% at P,, = 150 Torr. According to our model, that portion of CO, production falling below the sharp break in its slope in fig. 7 (at 0,) = 0.02) is attributable to this second path to CO>. This involves some 50% of the rate at PO2 = 8.2 Torr, and 25% at PO2 = 150 Torr. These values are at least qualitatively consistent with the rate declines observed upon the replacement of O;, by Cl;, mentioned above. By this model, the small increases in selectivity effected by chlorine coverages up to 0.3 can then be attributed to the suppression of this second pathway to CO, production which directly involves 0,. The Et0 and CO, production rates are inhibited most strongly by Cl, for coverages beyond that at which the p(2 x I)-Cl structure is first observed. It is tempting to correlate this with the fact that the heat of adsorption of ethylene starts to decrease for 0,., beyond the p(2 x 1 )-Cl coverage (fig. 5). In fact. for the c(4 x 2)-Cl overlayer model that has been proposed [13], it would be sterically difficult to squeeze an ethylene molecule between the chlorine adatoms so as to overlap its T-orbital with Ag-atom orbitals. We have previously proposed that Et0 and part of the CO, are produced through a common intermediate step involving ethylene (adsorbed on Ag”+ sites) and molecularly adsorbed oxygen [l]. The changes in selectivity for 0,., z 0.5 would then reflect changes in the relative rates of the branches leading to Et0 and CO, ufter this step. One possibility is that the branch leading to CO, production requires a larger ensemble of Ag”’ sites unmasked by Cl adatoms than does the Et0 branch. An ensemble rather than electronic effect is certainly suggested by the high coverages over which these changes occur, since mutual depolarization is thought to decrease the ionicity of the Cl adatom in this coverage range [14,20]. Although we now have a reasonably good picture of the chlorine adlayer ensemble under optimum promotion conditions (~9,.,E 0.65), just how this ensemble determines selectivity must await future study. While comparing ensemble to electronic factors, we should point out that, in general, electronegative additives decrease activity and increase selectivity in this reaction, whereas electropositive additives can increase activity but decrease selectivity [3-7, 30-311. This indicates that electronic factors are certainly important in the role of surface additives in this reaction. Indeed, we suggested that the effects of CI, upon the dissociative adsorption rate of 0, (fig. 4) and the heat of ethylene adsorption (fig. 5) are determined by electronic factors, which presumably could be accomplished qualitatively with several
C. T. Campbell, M. T. Paffett / Chlorine promoters
and ethylene epoxidation
over Ag(l IO)
41
electronegative adsorbates. Nevertheless, an ensemble effect seems to play an equally important role, determining the activity and selectivity for chlorine coverages above a half monolayer. It seems essential, however, that this ensemble be generated by an array of electronegative, rather than electropositive, adatoms. The effects of chlorine upon ethylene adsorption and dissociative oxygen adsorption have been addressed to some extent in this paper. More detailed studies on these systems would be of interest. In addition, studies of the effects of Cl, upon the adsorption/desorption behavior of molecular O,, Et0 and CO, may prove valuable in understanding the promoter effect.
5. Conclusions The role of chlorine promoters in Ag catalysts for ethylene epoxidation can be accurately modeled with atomically absorbed chlorine on Ag(ll0). Chlorine inhibits both Et0 and CO, production, but CO, production more stronglyso that an overall promotion in selectivity (up to 85%) results. The major influence develops between the p(2 X l)-Cl structure at 8c, = 0.5 and the c(4 X 2)-Cl structure at I&.,= 0.75. This suggests an ensemble effect. The variation of the steady-state rates, selectivity, and particularly coverage of 0, with t?,., indicate that Cl, replaces 0, in the reaction mechanism by creating Ags+ sites for enhanced ethylene adsorption. Adsorbed molecular 0, seems to be the major oxidizing agent in epoxidation.
References [l] C.T. Campbell and M.T. Paffett, Surface Sci. 139 (1984) 396. [2] C.T. Campbell. in: Proc. 1983 Am. Vacuum Sot. Symp., Boston [J. Vacuum Sci. Tech&. A2 (1984) 10241. [3] J.T. Kummer. J. Phys. Chem. 60 (1956) 666. [4] H.H. Voge and C.R. Adams, Advan. Catalysis 17 (1967) 151. [5] V.E. Ostrovskii, N.V. Kul’kova, V.L. Lopatin and M.I. Temkin, Kinetika Kataliz 3 (1962) 160. [6] L.Y. Margolis, E.K. Enikeev, O.V. Isaev, A.V. Krylova and M.Y. Kuchnerov, Kinetika Kataliz 3 (1962) 153. [7] X.E. Verykios, F.P. Stein and R.W. Coughlin. Catalysis Rev.-Sci. Eng. 22 (1980) 197. [S] P.A. Kilty and W.M.H. Sachtler, Catalysis Rev.-Sci. Eng. 10 (1974) 1. [9] N. Giordano, J.C.J. Bart and R. Maggiore, Z. Physik. Chem. (NF) 127 (1981) 109. [lo] R.G. Meisenheimer and J.N. Wilson, J. Catalysis 1 (1962) 151. [ll] P.A. Kilty, N.C. Rol and W.M.H. Sachtler. in: Proc. 5th Intern. Congr. on Catalysis, 1973, Vol. 2, p. 929. [12] G. Rovida, F. Pratesi and E. Ferroni, J. Catalysis 41 (1976) 140. [13] G. Rovida and F. Pratesi, Surface Sci. 51 (1975) 270. 114) R.A. Marbrow and R.M. Lambert, Surface Sci. 71 (1978) 107.
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[15] [16] [17] [18] [19] [20] [21] 122) [23] [24] [25] [26] [27] [28] [29] [30] [31]
C. T. Cumpbell,
M. T. Puff&t / Chlorine promoters und eth.vlene epoxidtitmn
over Agll IO)
R.A. Van Santen. J. Moolhuysen and W.M.H. Sachtler, J. Catalysis 65 (1980) 478. C.T. Campbell and M.T. Paffett, Surface Sci. 143 (1984) 517. W. H&land, F. Iberl, E. Taglauer and D. Menzel, Surface Sci. 53 (1975) 383. Y.Y. Tu and J.M. Blakely. Surface Sci. 85 (1979) 276. Y.Y. Tu and J.M. Blakely, J. Vacuum Sci. Technol. 15 (1978) 563. P.J. Goddard and R.M. Lambert, Surface SCI. 67 (1977) 180. J.C. ZomerdiJk and M.W. Hail, Catalysis Rev.-&i. Eng. 23 (1981) 163. M.A. Barteau and R.J. Madix. in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 4. Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam. 1982) ch. 4. R.A. Marbrow and R.M. Lambert, Surface Sci. 61 (1976) 329. J.E. Huheey. Inorganic Chemistry (Harper and Row, New York, 1972) pp. 484-486. E.L. Force and A.T. Bell, J. Catalysis 38 (1975) 440. E.L. Force and A.T. Bell, J. Catalysis 40 (1975) 356. C. Backx, C.P.M. de Groot and P. Biloen, Appl. Surface Sci. 6 (1980) 256. M. Akimoto. K. lchikawa and E. Echigoya. J. Catalysts 76 (19X2) 333. W.M.H. Sachtler, C. Backx and R.A. van Santen. Catalysis Rev.-L%. Eng. 23 (1981) 127. H.T. Spath and K. Torkar. J. Catalysis 26 (1972) 163. H.T. Spath. in: Proc. 5th Intern. Congr. on Catalysis, 1973. Vol. 2, p. 945.