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Adsorption and dissociation of carbon dioxide on rhodium surfaces
503
Surface Science 154 (1985) 503-523 North-Holland, Amsterdam
ADSORPTION AND DISSOCIATION RHODIUM SURFACES H.A.C.M. HENDRICKX, B.E. NIEUWENHUYS
A.P.J.M.
OF CARBON
JONGENELIS
DIOXIDE
ON
and
Gorlaeus Laboratories, State University of Leiden, P. 0. Box 9502, 2300 RA L.eiden, The Netherlands Received
16 October
1984
The adsorption and dissociation of CO, on Rh has been investigated using field emission microscopy and thermal desorption spectroscopy. In addition, the influence of hydrogen on the interaction of CO, with Rh has been examined. Two clearly distinguishable adsorption states exist, one of a chemical nature and an activation energy of desorption of 60 kJ/mol and a physically adsorbed species with an activation energy of desorption of 27 kJ/mol. The chemisorbed species desorbs to a small extent at 210 K and dissociates for the other part to CO, +Oa. The heat of adsorption of chemisorbed CO, is particularly high on the open crystal planes like (210) (320) and (531). In dissociation the stepped planes around (111) and (100) are most active. No evidence was found for dissociation on the smooth (110). (100) and (111) surfaces. These results are discussed in relation with the disagreement in the literature concerning dissociation of CO, on Rh. Our results point to surface heterogeneity as a major effect explaining the controversy. In addition some investigations on the adsorption of CO and H, on Rh starting at 78 K are reported.
1. Introduction Much recent CO, with group This interest which CO, is mention in this
effort has been directed toward the study of the interaction of VIII metals and their oxides [l-43]. is connected with the importance of the catalytic reactions in involved either as a reactant or as a reaction product. We respect the hydrogenation of CO* to methane:
CO,+4H,+CH,+2H,O and the oxidation reactions 2CO+2NO+2C0,+Nz
[lo-14,17,34-38,42,52],
(I)
of CO : [41,45,47,63],
(II)
2co+o,-,2co~
[16,19-31,39,44,48,52],
(III)
CO+H,O+COz+H,
[39,48].
(IV)
Reactions (II) and (III) are of great importance in diminishing air-pollution problems, as e.g. arising from combustion engines. Reaction (IV), the water-gas shift reaction, is of economic importance and it is applied in the industry for 0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
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et al. / Adsorption and dissociation of CO, on Rh
the production of Hz. The reverse reaction of (I) is also used in the H, production. Our interest in the interaction of CO, on Rh was provoked by its role in reaction (II) which has been a recent subject of investigation in our laboratory. In literature there is a dispute on the possibility of CO, adsorption and dissociation on Rh [1,2]. Many authors found evidence for dissociation of CO, on Rh [3-15,34,38,45]. However, several other groups could not detect either adsorption or dissociation on Rh at room temperature [2,16,48,55]. Using thermodynamic and kinetic information Weinberg [2] concluded that CO, dissociation must be negligibly small at 300 K and low pressure. Dubois and Somorjai [l] reported a sticking probability for dissociative adsorption of CO, on Rh(ll1) of about 0.1 whereas Goodman et al. [55] found that the probability of dissociative adsorption on Rh(ll1) is not greater than 1 x lo-” at 300 K. Solymosi et al. [11,12] reported that CO, on Rh/Al,O, dissociates only in the presence of hydrogen. On Rh(ll1) and a polycrystalline Rh foil dissociation was observed only in the presence of impurities like boron [56]. However, Iizuka and Tanaka [13] found evidence for dissociation of CO, on Rh/Al,O, at 300 K even in the absence of hydrogen whereas on SiO, supported Rh dissociation was not detected. Up to now no satisfactory explanation for this controversy concerning the adsorption and dissociation of CO, on Rh has been given. Some authors point to the possible role of the surface structure, such as the presence of different particle sizes and catalyst preparation [1,22]. The reason for crystal face specificity in the dissociative adsorption may be the rather large differences in adsorption enthalpies for 0 and CO on Rh single crystal surfaces. Other authors suggest that impurities on the surface or in the gas phase may be responsible for the reactivity of Rh in the dissociation of CO, reported by other groups [11,12,55,56]. In this paper we present some new experimental results obtained with field emission microscopy (FEM) and thermal desorption spectroscopy (TDS). The interaction of CO, with Rh surfaces was investigated in a wide temperature range starting at 78 K. To our opinion this low temperature is a prerequisite for research on this subject. In addition the influence of hydrogen on the adsorption of CO, was briefly examined.
2. Experimental The apparatus consisted of a bakeable stainless steel UHV system with a mbar, pumped by ion and titanium sublimation base pressure of 5 X lo-” pumps and equipped with a conventional Pyrex-glass field emission microscope. Partial and total pressures were measured using a quadrupole mass spectrometer and an ion gauge. The emitter was made by etching a 0.10 mm
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Fig. 1. Field emission pattern of a clean Rh tip and the disposition of some crystal faces.
diameter Rh wire (purity better than 99.95%) in a KOH and NaCl aqueous solution. It was spot-welded on a Rh filament (0.25 mm diameter, purity better than 99.95%) which was used for the TDS experiments. In thermal desorption experiments the filament was heated resistively with linear heating rates of 20 K/s. Temperatures were measured by recording the electrical resistance and were checked by optical pyrometry and by calorimetry. Activation energies of desorption were estimated assuming a preexponential factor of 1 x lo-r3 s-‘. The gases used had a purity better than 99.9%. Changes in work function were derived from the slopes of linear plots obtained using the Fowler-Nordheim equation and related to a work function value of 4.80 eV for clean rhodium. For further, more extensive details, see refs. [31,52,54]. For the purpose of comparison fig. 1 shows a clean (111) oriented Rh tip with the disposition of the main crystal faces on the emitter indicated. All pictures presented here were taken with an constant emission current of 2 x 1O-8 A.
3. Results
In order to facilitate the interpretation of the results in the CO, experiments we shall first describe the adsorption of CO and hydrogen on Rh at 78 K. Gross0 modo these results are analogous to those reported earlier by Gorodetskii and Nieuwenhuys [52].
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A@ leV1
3.0 0
1 2
I L
6
1 8
exposure
I 10 (n bars1
Fig. 2. Variation in work function of a Rh tip with CO exposure at 78 K
3.1.
CO adsorption
and desorption
Gorodetskii and Nieuwenhuys studied the adsorption of CO on Rh with FEM and TDS at temperatures above 300 K. Their main purpose was to examine possible CO dissociation and they calculated an upper limit on the CO dissociation rate of 1 X 10” molecules cmm2 s-‘. However, no experiments at 78 K were performed by them. In the present study some of the experiments were repeated and extended to temperatures below 300 K. During exposure the CO pressure was kept constant at about 5 x lop9 mbar. During thermal desorption the CO residual pressure was better than 8 X lo-‘” mbar. Fig. 2 shows the change of the work function A$J of the emitter as a function of the exposure to CO at 78 K. The maximum A+ observed, 1.5 eV, is significantly higher than the value found for the tip at 300 K (1.2 eV). Saturation is reached after an exposure of about 8 nbar s (= nanobar seconds). The FEM patterns observed during CO exposure at 78 K and during thermal desorption are shown in fig. 3. The pattern characteristic of a Rh tip saturated with CO at 78 K differs from that found at 300 K as described earlier by Gorodetskii and Nieuwenhuys [52]. In particular the areas around (110) and the stepped surfaces around (111) are much darker than observed at 300 K. In the first stage of adsorption rough surfaces, like (210) and (531) are darkened. Also the emission of the stepped surfaces, e.g. (221) and (223) falls, whereas the emission of the kinked crystal faces, like (432) (321) and (510) remains constant. Only near saturation the emission of the kinked surfaces around (111) and (100) decreases (see the difference between figs. 3b and 3~). Also near saturation the (110) surface and surroundings become dark. Upon comparing these patterns with those from ref. [52] observed at 300 K, it appears that fig. 3b resembles the pattern of a tip saturated at 300 K. Fig. 4a
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Fig. 3. Field emission patterns of Rh interacting with CO: (a) A+ = 0.35 eV, T= 78 K; (b) Acg = 0.50 eV, T = 78 K; (c) full coverage, T = 78 K; (d) heating, T = 390 K; (e) heating, T = 425 K; (f) tip after heating to 465 K, clean surface.
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AP fa.u.1
T (Kl Fig. 4. (a) Variation in the work function of a Rh tip covered with CO during (b) TDS of a CO covered tip; (c) TDS of a hydrogen covered tip.
thermal
desorption;
shows the variation in the work function of a CO covered tip with temperature. As the work function change is almost proportional to the surface coverage (/3) up to high values of (3, fig. 4a may also be considered as a surface coverage versus temperature plot. In fig. 4b a TD spectrum is given. The intensity of mass 28 was monitored. Desorption of species with amu 28 could be attributed exclusively to CO because no signals of amu 14 (= N,) and 44 ( = CO,) were detectable. We also did not detect any desorption of amu 32 (= 0,). Upon heating the FEM image is not effected up to 250 K. Above 350 K fig. 4b is almost identical to the one observed for CO adsorbed at 300 K. The pictures of figs. 3d and 3e are not exactly the reverse of those observed during adsorption, figs. 3a, 3b and 3c. This indicates that CO is not mobile on all the Rh crystal surfaces at 78 K. At 350 K the kinked surfaces around (111) and (100) brighten. At about 400 K the emission from (110) and similar surfaces increases and at 425 K the pattern is almost identical to that observed by Gorodetskii et al. following CO adsorption at 300 K. The pattern of the clean tip is restored around 465 K. It should be noted that the experiments of figs. 4a, 3d, 3e and 3f were done with a much lower heating rate (1 K/s) than those during the thermal desorption mass spectroscopy experiments shown in fig. 4b (about 20 K/s). 3.2. Hydrogen adsorption The sticking probability for hydrogen at 78 K is essentially unity. The work function increases by 0.3 eV for a layer saturated at 78 K. The FEM pattern is
H.A.C.M. Hendrickx et al. / Adsorption and dissociation of CO, on Rh
0.04 0
I
I
I
5
10
15
I
20
exposure
I
25
509
1
30
In bars1
Fig. 5. Solid line: variation in work function of a Rh tip with CO, exposure at 78 K. Dashed line: pre-exposure to 20 nbar s of CO, and subsequent effect on work function when hydrogen is admitted.
identical to that observed at 196 K by Gorodetskii et al. [57]. Thermal desorption of hydrogen exhibits a single desorption peak with a maximum at about 370 K at low 0 to 320 K for a saturated layer using a heating rate of 11 K/s (see fig. 4~). 3.3. Exposure to CO2 at 78 K Fig. 5 shows the change of work function versus exposure to CO,. It illustrates that the initial sticking probability is close to unity. The Rh surface is almost saturated with CO, after an exposure of about 13 nbar s. The work function has increased then by 0.6 eV. However, it continues to change following larger doses of CO,. The FEM images at this stage suggest the presence of a 2nd or even a multilayer of CO,. Figs. 6 and 7 show the FEM patterns observed during exposure for (111) and (100) oriented emitters, respectively. The sequence of patterns shows that CO, adsorption is markedly crystal face specific and very different from the adsorption of CO on Rh. After about 9 nbar s exposure the pattern does not change much anymore, although the work function still increases by 0.1 eV upon further CO, adsorption. The first noticeable changes occur at the rough surfaces, in particular around (531) and ((310)-(320)) which areas exhibit a strong decrease in emission, whereas the emission around the (llO), (533) and the ((551)-(531)) areas does not change. In a later stage (figs. 6c, 6d and 7d, 7e) the ((llO)-(221)) and ((511)-(211)) areas also darken, while then the (321) areas remain bright. The emission of the saturated tip originates largely from (510) (320) and (321)
510
H.A.C.M. Hendrickx et al. / Adsorption and dissociation of CO, on Rh
Fig. 6. Field emission patterns observed during CO, adsorption on a Rb(ll1) tip at 78 K: (a) clean surface; (b) after 1 nbar s exposure; (c) after 3 nbar s exposure; (d) after 6 nbar s exposure; (e) fully covered surface.
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511
Fig. 7. Field emission patterns observed during CO, adsorption on a Rh(100) tip at 78 K: (a) clean surface; (b) after 1 nbar s exposure; (c) after 3 nbar s exposure; (d) after 7 nbar s exposure; (e) after 25 nbar s exposure; (f) after 50 nbar s exposure.
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0.6 0.L 0.2 0.0 TfK) Fig. 8. Work function change versus temperature dashed line: pre-exposure to CO, and subsequent
of a Rh tip. Solid line: tip exposed admission of hydrogen.
whereas these areas are dark in the pattern of a clean exposures to CO, (figs. 6d and 6e) the pattern becomes characteristic feature for the formation of a multilayer.
to CO,;
tip. Following high granular which is a
3.4. Heating of the CO, adlayer In fig. 8 the work function change versus temperature is recorded. Changes in FEM pattern accompanying the heating process are shown in fig. 9. Again, both experiments were done with a low heating rate (1 K/s). Upon heating the work function rises a little until a temperature of 110 K. The change in FEM pattern in this temperature range is not significant, only the granularity in emission disappears. Above 110 K the work function decreases gradually up to a A+ value of about 0.2 eV which is reached around 400 K. Then it is almost constant up to 450 K, at which temperature it falls fast towards the value of the clean surface. The FEM patterns show that at 125 K the (110) surface and at higher temperatures also its surrounding crystal faces like (551) and (331) become intensively emitting areas. The areas around (533) (311) and (711) have become very dark at the temperature of 215 K. Then, within 30 K the pattern changes completely. The dark area around (311) extends to other stepped surfaces and simultaneously the bright spot around (110) decreases in size and intensity, and (110) itself becomes dark again. At 250 K the pattern has become identical to the pattern characteristic of CO at low coverage (see e.g. fig. 3e). Around 470 K the tip becomes clean. Fig. 10 gives a series of TD spectra taken after varying exposure. Mass 28 is attributed to CO and to CO, of which it is a 11% peak. No N, (amu 14) was detectable in our system.
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513
Although we searched extensively for it, oxygen (amu 32) could not be detected. Following an exposure of 4 nbar s the TDS curve is composed of a main desorption peak around 240 K and a smaller peak around 140 K. After an exposure of 10 nbar s the contribution of the low temperature peak is much more important than the high temperature peak . After large exposures the low temperature peak is split into two distinct peaks, one around 100 K and the other around 130 K. 3.5. Exposure
of CO_, at 300 K
TDS obtained following a 4 nbar s exposure to CO, at 300 K did not exhibit any desorption peak, neither amu 28, 32 nor 44 were detected. A very small CO (amu 28) peak around 500 K was observed after the filament had been exposed to more than 12 nbar s at 300 to 400 K. The work function increases slowly with exposure. After an exposure of 20 nbar s the work function still increases upon further exposure. The FEM pattern in this stage is similar to the pattern obtained following an exposure of about 1 nbar s CO. The pattern and the work function of a clean tip are restored upon heating the tip above 500 K. 3.6. TDS experiments
with 13C0,
Some experiments have been performed with 13C0, in order to discriminate adsorption of 13C0 following dissociation of 13C0,,and adsorption of ‘*CO, the major constituent of the residual gas. TD spectra were monitored following heating at various temperatures like 200 K, 300 K and 400 K for 60 s in the presence of 1 X lo-* mbar 13C02 and subsequent cooling in the CO, atmosphere to 78 K. The resulting TDS are consistent with those of fig. 10. The spectra show just the 45 and its 11% 29 fragmentation peak at 140 and 240 K. r3C0 or ‘*CO peaks around 500 K were not observed under these experimental conditions. 3.7. Co-adsorption
of H2 and CO, at 78 K
Fig. 5 shows the effect of hydrogen on the work function of a tip saturated with CO, at 78 K. Obviously, hydrogen is adsorbed on the tip covered with CO,. The pattern changes slightly into a “four-leaves clover” pattern. Upon heating the tip in the presence of hydrogen in the gas phase, the work function decreases in three stages, as is shown in fig. 8: first the temperature range 80-220 K after which a hydrogen-like pattern is obtained, second the 300-400 K range corresponding with desorption of hydrogen and third near 500 K when the CO-like pattern disappears. Note that the small increase in work
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H.A. C. hf. Hendrickx et al. / Adsorption and dissociation of CO, on Rh
Fig. 9. Field emission patterns observed during heating of a CO, covered tip starting at 78 K. (a) T =110 K; (b) T =125 K; (c) T = 215 K; (d) T = 220 K; (e) T = 230 K; (0 T = 250 K; (g) T = 410 K; (h) T = 470 K.
function that is observed for Rh exposed to CO, without hydrogen is absent. Figs. 11 and 12 show the effect of CO, on a Rh tip precovered with hydrogen at 78 K. The work function decreases by about 0.05 eV upon exposing the hydrogen covered Rh tip to CO, at 78 K. The FEM pattern does not change upon CO, exposure. The clean pattern is obtained around 380 K while the work function is then back to the value of the clean tip. TDS obtained after exposing a Rh tip to a mixture of hydrogen and CO, (4: 1) at 78 K did not
AP (0.u.l
I
I
100
200
300
,
I
LOO
500 T(KI
Fig. 10. TDS of a Rh tip covered with CO, at 78 K (amu 28 was recorded): (a) following exposure; (b) following 10 nbar s exposure; (c) following 40 nbar s exposure.
4 nbar s
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516
exposure
(nbars.1
Fig. 11. Work function change versus exposure of a Rh tip at 78 K. Solid line: hydrogen
dashed
line: pre-exposure
to 8 nbar s hydrogen
and subsequent
exposure
exposure;
to CO,
AQieVl 0.4 0.3 0.2 0.1 -
0.b
1
i
200
100
300
LOO T (Kl
Fig. 12. Work function exposed to hydrogen.
result
change
in the production
versus temperature
of H,O
of a tip pre-exposed
or CO peaks under
to CO, and subsequently
our experimental
condi-
tions. 4. Discussion 4.1. Adsorption
of Hz and CO
The results presented here are discussed in relation with those published earlier by Gorodetskii et al. [52,57]. The present results on hydrogen adsorp-
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511
tion at 78 K are in excellent agreement with those of Gorodetskii et al., who studied the adsorption of hydrogen at temperatures above 196 K. The FEM and TDS results show that the maximum coverage of CO at 80 K is significantly larger than that can be obtained at 300 K and the low pressures used in these studies. Furthermore, our TDS results show the existence of a low temperature adsorption state of CO on Rh, with an Ed_ = 40 kJ/mol. This adsorption state can be attributed to physically adsorbed CO. It has only a small influence on the work function (see figs. 2 and 4). Similar states have been reported for CO on Ni, Pd and Pt (for a review, see ref. [41]). The main desorption peak has a maximum at 450 K which has been found by many authors [7,8,10,15,16,19,52]. However, in our studies the peak is much broader and more symmetric than in most of the other studies. As will be discussed below this effect can be attributed to the dependence of the adsorption energy on surface structure and coverage. The larger coverage at 78 K also explains the differences on A+,,, for T = 78 K and T = 300 K. Assuming that A+ is linearly proportional to the surface coverage 8 then at 300 K the relative surface coverage is 0 = 1.2/1.5 = 0.8 of that at 78 K. This value looks quite reasonable in view of fig. 4. The pictures of fig. 3 show the different stages in CO adsorption and desorption. It should be kept in mind that the pictures were taken at constant emission current. Since CO adsorption causes a drastic decrease in emission the voltage had to be raised during the adsorption process. Since the work function changes brought about by CO adsorption on Rh at saturation are not very surface structure sensitive it may be expected that the variation in A+ and, hence, in the extent of darkening over the tip surface is related to the variation in surface coverage. Fig. 3a suggests that CO is first adsorbed on the rough surfaces, like (531) or (210). It is also adsorbed on stepped surfaces like (221) but not yet in a significant amount on the kinked crystal faces like (432) and (510). Only near saturation the kinked surfaces around (111) and (100) are covered with CO (see the differences between figs. 3b and 3~). Also near saturation the (110) surface and surroundings are covered with CO. Upon comparing the patterns with those of ref. [52] it appears that fig. 3b resembles very much the pattern obtained at 300 K. At room temperature CO adsorbs readily on rough Rh surfaces and on stepped surfaces like (311), but only at higher exposures on kinked surfaces around (ill), (100) and on (110) and similar surfaces. On heating, figs. 3 and 4 show very little effect up to 250 K. The physically adsorbed CO seems to have no significant influence on A$ and is not surface structure specific, so until this temperature hardly anything happens. This in agreement with fig. 2. Above 350 K, fig. 4 is almost identical to the one measured for CO adsorbed at 300 K. Above 350 K, CO desorption starts from the kinked surfaces around (111) and (100). At about 400 K, CO is desorbed from (110) and similar surfaces and around 465 K from the rough surfaces like (531) and (210). Hence, we may conclude that the heat adsorption decreases in the order:
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Q(531)
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= Q(320) = Q(210) = Q(221)
= Q(322)
= Q(533) > Q(432)
= Q(321) = Q(510). Unfortunately, the present results do not provide information concerning the heat of adsorption on the most densely packed surfaces, the (111) and (100) surfaces. It has been reported earlier that the heat of adsorption is lower on Rh (111) than on stepped Rh surfaces like (755) (331). On smooth surfaces like (ill), (100) and (110) the differences are very small [9,15,58]. 4.2. The adsorption of CO, The maximum work function increase of 0.6 eV at 78 K and p = 3 x 10P9 mbar, is much lower than those found for CO (1.5 eV) and 0, (0.9 eV at 196 K) adsorption. This is a first indication that CO, does not dissociate on Rh at 78 K. Furthermore, the FEM patterns are quite different from those observed during exposure to CO. The sequence of patterns observed at 78 K suggests that at low exposures CO, is first adsorbed on the rough surfaces. In this stage the areas around (110) and the stepped surfaces with (111) terraces and steps are not covered to a large extent. Then, following 3 nbar s exposure the surfaces near (110) are covered. At about 7 nbar s exposure the surfaces with (100) terraces and (111) steps become covered. Following long exposures the FEM pattern points to the formation of a 2nd or multilayer adsorption. The pressure of CO, has no significant influence on the pattern but it has some influence on the emission properties. If the CO, pressure is increased more CO, molecules are adsorbed in the multilayer and the emission decreases consequently. This CO, can, to a large extent, be pumped off at 78 K. Upon heating the tip the work function first rises up to about 0.7 eV as shown in fig. 8. The TD spectrum shown in fig. 10 has a peak at the same temperature. A similar peak at this temperature has been found by Lin, reported in ref. [l], for a Rh (111) surface. At high exposures the peak is split into two peaks (fig. 10~). The physically adsorbed CO, has an estimated Edes of 27 kJ/mol. Lin (1) found 29.4 kJ/mol. The Ed_ for the CO, bound in the multilayer can be compared with the value of 23 kJ/mol, the heat of vaporization of CO, [59]. It should be expected that desorption of this weakly bound CO, should increase the emission since the emission decreases upon its adsorption. However, in the temperature range at which it is desorbed the emission decreases (the apparent work function increases). Retooling the tip, reexposure to CO, and subsequent heating causes the expected effect: an emission increase. Hence, it appears that in the temperature range 80 to 120 K an irreversible change occurs in the adlayer. (For example an rearrangement of the adsorbed molecules). At 125 K the (110) surface and surrounding crystal faces start to emit intensively, indicating that these surfaces are denuded. This increase in emission is not accompanied by a drastic decrease in the average work function. The fact that
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519
a desorption maximum near 125 K has not been observed in TDS can be explained by remembering that a Rh filament has only very few (110) faces on its surface. After 110 K the work function decreases continuously (fig. 8). At 240 K the TD spectrum has its second CO, desorption peak. Now CO* desorbs with an Ed_ = 60 kJ/mol. In this temperature range the clean areas around (110) have extended to their surrounding stepped surfaces. By then the crystal face (533) has become very dark. Then, within 30” the picture changes completely. The dark areas (533) and other (100) surfaces with (111) steps become larger and simultaneously the (110) spot decreases. At 250 K the (110) surface itself is dark and the pattern has become identical to fig. 3e. After this stage all changes are similar to those observed for CO (compare with fig. 3) andat 470 K the surface is almost clean again. The interpretation of these results is as follows: at 215 K a large area around (110) is clean, so it emits strongly. At about the same temperature CO, starts to dissociate in CO, and 0,. The FEM pattern change points to a striking influence of surface structure on the ability of the surface to dissociate CO. Dissociation starts on the (100) surfaces with (111) steps, such as (533) and (711). Therefore, these regions become dark. The adsorbed CO is mobile on the surface at this temperature and it will travel to the clean areas around (110). In the mean time all adsorbed CO, molecules dissociate, first on the stepped surfaces. At 250 K the whole surface is covered with CO, the local CO coverages being determined by the thermodynamic equilibrium distribution over all the tip surfaces, i.e. by the variation of the heat of adsorption of CO with the surface structure. The picture resembles that of fig. 3e, i.e. a surface covered for about 50% with CO. No indication for the presence of adsorbed oxygen is obtained. Similar observations have been made by Castner et al. [9] who could also not detect adsorbed oxygen after CO, dissociation. Most probably the oxygen diffuses readily into the bulk of Rh even at this low temperature [7,1.5]. The second CO, desorption peak in the TDS around 240 K has been assigned by Dubois et al. [l] to recombination of CO, + Oa. Our FEM results do not support their view, the peak should be attributed to desorption of ~he~sorbed CO2 (Ed_ = 60 kJ/mol). Above 250 K the whole process is identical to the one described for CO adsorption. The results show that hydrogen can be adsorbed on Rh covered with CO, at 78 K. Upon heating the initial rise in work function observed for the adsorption of pure CO, is absent. No other effect of hydrogen on adsorbed CO, or of CO2 on adsorbed hydrogen has been found. If the surface is first exposed to hydrogen and then to CO,, CO2 is adsorbed on the hydrogen covered Rh surface with a very low heat of adsorption an a work function decrease. This adsorbate, may be attributed to CO, bonded to hydrogen, for example as shown in fig. 13a. No influence of hydrogen on the dissociation of CO, could be observed under our experimental conditions. The FEM results point to dissociation of CO, on certain Rh surfaces around
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520
H
,,,,,,,
m
Fig. 13. (a) CO, weakly adsorbed on a hydrogen covered surface; monodentate structure; (c) CO, bonded in the bidentate structure.
(b) CO,
bonded
in the
215 K whereas the TDS results did not provide a clear evidence for dissociation of CO,. It has been noted above that both the experimental conditions (heating rate) and’the surface structure of the samples used in the FEM and TDS experiments differ. Polycrystalline filaments equilibrated at high temperatures consist largely of (111) and (100) surfaces. The FEM results show that the surface structure has a striking influence on the dissociation of CO,. (100) surfaces with (111) steps are active in the dissociation while dissociation was not observed on most other Rh surfaces including (110) (100) and (111). Apparently, the concentration of sites on the filament surface at which CO, can readily dissociate is very low. As noted in the introduction, there is disagreement about whether or not CO, dissociation can occur on Rh. Our results point to surface heterogeneity as a major effect explaining this controversy. Certain sites present on stepped surfaces are active whereas other sites are not or less active in CO, dissociation. Finally, we shall discuss the nature of the chemisorbed CO, desorbing or dissociating in the temperature range from 80 to 210 K. The FEM results show that the surface structure has a striking influence on its heat of adsorption and, possibly, on the nature of this adsorbed complex. The positive A+ for CO, adsorption is consistent with a model, see fig. 13b, of CO, adsorption on group VIII metals by a metal-carbon bond as first suggested by Eischens and Pliskin [60] for CO, on Ni powders on the basis of IR spectroscopy. In later papers it was shown that CO, dissociates readily on Ni [40]. On Pt and Cu, however, weak molecular adsorption has been observed with only small core and valence electron level shifts [64,65]. On the basis of these results the authors concluded that CO, is physically adsorbed on Pt and Cu. In coordination complexes of CO, with Ni, Rh and Ir it has been proposed that the CO, is bonded to a metal ion M through the C atom in a monodentate or a bidentate complex [32,60], see figs. 13b and 13~. The monodentate structure with the C atom directed to the surface and with
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and dissociation of CO, on Rh
521
an OCO angle of 120” has also been proposed by Anderson [61] for CO, on a Cu(100) surface based on molecular orbital calculations. The bending would be favoured by donation from the metal to an empty CO, 27r* orbital which is sufficiently stabilized on bending on the metal surface to become occupied. However, in a later study of COz adsorbed on Pt and Cu surfaces, Ray and Anderson [26] suggested a bonding via the lone pair of 0 with the molecular bond vertically since photoemission studies indicate no energy level shifts as would be expected if CO, were bent on the surface. Our results point to a striking influence of the surface structure on both the molecular adsorption and the dissociation of CO,. This observation suggests that the nature of the adsorption complex might be quite different on the various single crystal surfaces. Let us assume, just for the sake of argument, that CO, is bonded to the surface in a monodentate structure via the C atom on top of a Rh atom (fig. I3b). On flat surfaces like (111) the m~imum density of CO, molecules bound in this way would be much smaller than for example CO molecules (10” molecules/cm*) Hydrogen can be easily adsorbed in this open layer consistent with our observations. If, on the other hand, CO, would be bonded vertically then the expected maximum surface coverage would be comparable to that of CO. Hydrogen cannot be adsorbed in such a layer, as suggested by the observed block.ing of H adsorption by CO 1621,this in contrast to our results. The observed large differences between the various surfaces, towards adsorption and dissociation might also be understood in terms of the CO, structure on the surface. For example, assuming again that CO, is adsorbed in the monodentate structure on top of a Rh atom on the (111) terrace of a stepped surface near a (111) like step then an interaction between an 0 atom and the Rh atoms at the step above the (111) terrace considered should be weak but on a (100) like step strong in view of the distance of the 0 atom to the relevant Rh atoms. This situation is illustrated in fig. 14. Similar geometrical situations consist near (111) steps and (100) terraces. A structure like that might be the first step for dissociation. The observed influence of the structure on both heat of adsorption and the dissociation of CO, might be related to this effect. It is hoped that future studies using vibration spectroscopy may provide more information concerning the nature of the CO, metal bond. Anyhow, the positive work function change found in the present study suggests a net charge transfer from the metal to the Ccl, molecule.
Fig. 14. Possible structure of CO2 near a step.
522
H.A.C. M. Hendrickx
et al. / Adsorption and dissociation of CO_, on Rh
References [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll] [12] [13] [14] [15] [16] (171 [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] (391 [40] [41] [42] [43] [44] [45] [46] [47]
L.H. Dubois and G.A. Somorjai, Surface Sci. 128 (1983) L231. W.H. Weinberg, Surface Sci. 128 (1983) L224. A.C. Collins and B.M.W. Trapnell, Trans. Faraday Sot. 53 (1957) 1476. J.T. Grant and T.W. Haas, Surface Sci. 21 (1970) 76. M. Primet, J. Chem. Sot. Faraday I, 74 (1978) 2570. R.J. Koestner, M.A. Van Hove and G.A. Somorjai, Surface Sci. 107 (1981) 439. L.H. Dubois and G.A. Somorjai, Surface Sci. 88 (1979) L13. L.H. Dubois and G.A. Somorjai, Surface Sci. 91 (1980) 514. D.G. Castner, B.A. Sexton and G.A. Somorjai, Surface Sci. 71 (1978) 519. B.A. Sexton and G.A. Somorjai, J. Catalysis 46 (1977) 167. F. Solymosi, A. Erdchelyi and M. Kocsis, J. Catalysis 65 (1980) 428. F. Solymosi and A. Erdohelyi, J. Catalysis 70 (1981) 451. T. Iizuka and Y. Tanaka, J. Catalysis 70 (1981) 449. P.R. Watson and G.A. Somorjai, J. Phys. Chem. 86 (1982) 3993. D.G. Castner and G.A. Somorjai, Surface Sci. 83 (1979) 60. C.T. Campbell and J.M. White, J. Catalysis 54 (1978) 289. F. Solymosi and A. Erdohelyi, J. Mol. Catalysis 8 (1980) 471. D. Brennan and D.O. Hayward, Phil. Trans. Roy. Sot. London A258 (1965) 375. C.T. Campbell, S.-K. Shi and J.M. White, Appl. Surface Sci. 2 (1979) 382. C.T. Campbell, S.-K. Shi and J.M. White, J. Vacuum Sci. Technol. 16 (1979) 605. CT. Campbell, S.-K. Shi and J.M. White, J. Phys. Chem. 83 (1979) 2255. K. Tanaka and J.M. White, J. Phys. Chem. 86 (1982) 3977. T. Matsushima, Surface Sci. 123 (1982) L663. J.L. Taylor, D.E. Ibbotson and W.H. Weinberg, J. Catalysis 62 (1980) 1. H.I. Lee, G. Praline and J.M. White, Surface Sci. 91 (1980) 581. N.K. Ray and B. Anderson, Surface Sci. 119 (1982) 35. S. Ladas, H. Poppa and M. Boudart, Surface Sci. 102 (1981) 151. M. Kawai, T. Onishi and K. Tamaru, Appl. Surface Sci. 8 (1981) 361. C.T. Campbell, G. Ertl, H. Kuipers and J. Segner, J. Chem. Phys. 73 (1980) 5862. J.A. Fair and R.J. Madix, J. Chem. Phys. 73 (1980) 3486. V.V. Gorodetskii, B.E. Nieuwenhuys, W.H.M. Sachtler and G.K. Boreskov, Appl. Surface Sci. 7 (1981) 355. J.C. Calabrese, T. Herskovitz and J.B. Kinney, J. Am. Chem. Sot. 105 (1983) 5914. CA. Becker, J.P. Cowin and L. Wharton, J. Chem. Phys. 67 (1977) 3394. T. lizuka, Y. Tanaka and K. Tanabe, J. Mol. Catalysis 17 (1982) 381. A. Amariglio, M. Lakhdar and H. Amariglio, J. Catalysis 81 (1983) 247. E. Zagh and J.L. Falconer, J. Catalysis 69 (1981) 1. T. Iizuka, Y. Tanaka and K. Tanabe, J. Catalysis 76 (1982) 1. F. Solymosi, A. Erdohelyi and T. Bansagi, J. Catalysis 68 (1981) 371. M. Niwa and J.H. Lunsford, J. Catalysis 75 (1982) 302. G.A. Martin, M. Primet and J.A. Dalmon, J. Catalysis 53 (1978) 321. B.E. Nieuwenhuys, Surface Sci. 126 (1983) 307. F. Solymosi, A. Erdohelyi and T. Bansagi, J. Chem. Sot. Faraday Trans. I, 77 (1981) 2645. R. van Hardeveld and F. Hartog, Advan. Catalysis 22 (1972) 75. T. Engel and G. Ertl, in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1982). Y. Tanaka, T. lizuka and K. Tanabe, J. Chem. Sot. Faraday Trans. I, 78 (1982) 2215. F. Solymosi and J. Kiss, J. Catalysis 81 (1983) 95. L.H. Dubois, P.K. Hansma and G.A. Somorjai, J. Catalysis 65 (1980) 318.
H.A.C.M. Hendrickx et al. / Adsorption and dissociation of CO, on Rh [48] [49] [SO] [51] [52] 1531 [54] [55] [56] [57] [58] [59] [60] [61] (621 [63] [64] [65]
523
A.C. Yang and C.W. Garland, J. Phys. Chem. 61 (1957) 1504. H.A.C.M. Hendrickx and B.E. Nieuwenhuys, to be published. J.T. Yates, E.D. Williams and W.H. Weinberg, Surface Sci. 91 (1980) 562. J.T. Yates, E.D. Williams and W.H. Weinberg, Surface Sci. 115 (1982) L93. V.V. Gorodetskii and B.E. Nieuwenhuys, Surface Sci. 105 (1981) 299. D. Brennan, D.O. Hayward and B.M.W. Trapnell, Proc. Roy. Sot. (London) A256 (1960) 81. H.A.C.M. Hendrickx, A. Hoek and B.E. Nieuwenhuys, Surface Sci. 135 (1983) 81. D.W. Goodman, D.E. Peebles and J.M. White, Surface Sci. 140 (1984) L239. F. Solymosi and J. Kiss, Surface Sci. 149 (1985) 17. V.V. Gorodetskii, B.E. Nieuwenhuys, W.M.H. Sachtler and G.K. Boreskov, Surface sci. 108 (1981) 225. R.A. Marbrow and R.M. Lambert, Surface Sci. 67 (1977) 489. CRC Handbook of Chemistry and Physics, 63th ed. R.P. Eischens and W.R. Pliskin, Advan. Catalysis 9 (1957) 662. A.B. Anderson, Surface Sci. 62 (1977) 119. G.A. Kok, A. Noordermeer and B.E. Nieuwenhuys, Surface Sci. 135 (1983) 65. A. Obuci, S. Naito, T. Onishi and K. Tamaru, Surface Sci. 112 (1982) 235. P.R. Norton and P.J. Richards, Surface Sci. 49 (1975) 567. P.R. Norton and R.L. Tapping, Chem. Phys. Letters 38 (1976) 207.
Surface Science 154 (1985) 503-523 North-Holland, Amsterdam
ADSORPTION AND DISSOCIATION RHODIUM SURFACES H.A.C.M. HENDRICKX, B.E. NIEUWENHUYS
A.P.J.M.
OF CARBON
JONGENELIS
DIOXIDE
ON
and
Gorlaeus Laboratories, State University of Leiden, P. 0. Box 9502, 2300 RA L.eiden, The Netherlands Received
16 October
1984
The adsorption and dissociation of CO, on Rh has been investigated using field emission microscopy and thermal desorption spectroscopy. In addition, the influence of hydrogen on the interaction of CO, with Rh has been examined. Two clearly distinguishable adsorption states exist, one of a chemical nature and an activation energy of desorption of 60 kJ/mol and a physically adsorbed species with an activation energy of desorption of 27 kJ/mol. The chemisorbed species desorbs to a small extent at 210 K and dissociates for the other part to CO, +Oa. The heat of adsorption of chemisorbed CO, is particularly high on the open crystal planes like (210) (320) and (531). In dissociation the stepped planes around (111) and (100) are most active. No evidence was found for dissociation on the smooth (110). (100) and (111) surfaces. These results are discussed in relation with the disagreement in the literature concerning dissociation of CO, on Rh. Our results point to surface heterogeneity as a major effect explaining the controversy. In addition some investigations on the adsorption of CO and H, on Rh starting at 78 K are reported.
1. Introduction Much recent CO, with group This interest which CO, is mention in this
effort has been directed toward the study of the interaction of VIII metals and their oxides [l-43]. is connected with the importance of the catalytic reactions in involved either as a reactant or as a reaction product. We respect the hydrogenation of CO* to methane:
CO,+4H,+CH,+2H,O and the oxidation reactions 2CO+2NO+2C0,+Nz
[lo-14,17,34-38,42,52],
(I)
of CO : [41,45,47,63],
(II)
2co+o,-,2co~
[16,19-31,39,44,48,52],
(III)
CO+H,O+COz+H,
[39,48].
(IV)
Reactions (II) and (III) are of great importance in diminishing air-pollution problems, as e.g. arising from combustion engines. Reaction (IV), the water-gas shift reaction, is of economic importance and it is applied in the industry for 0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
504
H.A.C.M.
Hendrickx
et al. / Adsorption and dissociation of CO, on Rh
the production of Hz. The reverse reaction of (I) is also used in the H, production. Our interest in the interaction of CO, on Rh was provoked by its role in reaction (II) which has been a recent subject of investigation in our laboratory. In literature there is a dispute on the possibility of CO, adsorption and dissociation on Rh [1,2]. Many authors found evidence for dissociation of CO, on Rh [3-15,34,38,45]. However, several other groups could not detect either adsorption or dissociation on Rh at room temperature [2,16,48,55]. Using thermodynamic and kinetic information Weinberg [2] concluded that CO, dissociation must be negligibly small at 300 K and low pressure. Dubois and Somorjai [l] reported a sticking probability for dissociative adsorption of CO, on Rh(ll1) of about 0.1 whereas Goodman et al. [55] found that the probability of dissociative adsorption on Rh(ll1) is not greater than 1 x lo-” at 300 K. Solymosi et al. [11,12] reported that CO, on Rh/Al,O, dissociates only in the presence of hydrogen. On Rh(ll1) and a polycrystalline Rh foil dissociation was observed only in the presence of impurities like boron [56]. However, Iizuka and Tanaka [13] found evidence for dissociation of CO, on Rh/Al,O, at 300 K even in the absence of hydrogen whereas on SiO, supported Rh dissociation was not detected. Up to now no satisfactory explanation for this controversy concerning the adsorption and dissociation of CO, on Rh has been given. Some authors point to the possible role of the surface structure, such as the presence of different particle sizes and catalyst preparation [1,22]. The reason for crystal face specificity in the dissociative adsorption may be the rather large differences in adsorption enthalpies for 0 and CO on Rh single crystal surfaces. Other authors suggest that impurities on the surface or in the gas phase may be responsible for the reactivity of Rh in the dissociation of CO, reported by other groups [11,12,55,56]. In this paper we present some new experimental results obtained with field emission microscopy (FEM) and thermal desorption spectroscopy (TDS). The interaction of CO, with Rh surfaces was investigated in a wide temperature range starting at 78 K. To our opinion this low temperature is a prerequisite for research on this subject. In addition the influence of hydrogen on the adsorption of CO, was briefly examined.
2. Experimental The apparatus consisted of a bakeable stainless steel UHV system with a mbar, pumped by ion and titanium sublimation base pressure of 5 X lo-” pumps and equipped with a conventional Pyrex-glass field emission microscope. Partial and total pressures were measured using a quadrupole mass spectrometer and an ion gauge. The emitter was made by etching a 0.10 mm
H.A.C. M. Hendrickx
et al. / Adsorption and dissociation of CO, on Rh
505
Fig. 1. Field emission pattern of a clean Rh tip and the disposition of some crystal faces.
diameter Rh wire (purity better than 99.95%) in a KOH and NaCl aqueous solution. It was spot-welded on a Rh filament (0.25 mm diameter, purity better than 99.95%) which was used for the TDS experiments. In thermal desorption experiments the filament was heated resistively with linear heating rates of 20 K/s. Temperatures were measured by recording the electrical resistance and were checked by optical pyrometry and by calorimetry. Activation energies of desorption were estimated assuming a preexponential factor of 1 x lo-r3 s-‘. The gases used had a purity better than 99.9%. Changes in work function were derived from the slopes of linear plots obtained using the Fowler-Nordheim equation and related to a work function value of 4.80 eV for clean rhodium. For further, more extensive details, see refs. [31,52,54]. For the purpose of comparison fig. 1 shows a clean (111) oriented Rh tip with the disposition of the main crystal faces on the emitter indicated. All pictures presented here were taken with an constant emission current of 2 x 1O-8 A.
3. Results
In order to facilitate the interpretation of the results in the CO, experiments we shall first describe the adsorption of CO and hydrogen on Rh at 78 K. Gross0 modo these results are analogous to those reported earlier by Gorodetskii and Nieuwenhuys [52].
506
H.A.C. M. Hendrickx
et al. / Adsorptwn
and dissociation of CO, on Rh
A@ leV1
3.0 0
1 2
I L
6
1 8
exposure
I 10 (n bars1
Fig. 2. Variation in work function of a Rh tip with CO exposure at 78 K
3.1.
CO adsorption
and desorption
Gorodetskii and Nieuwenhuys studied the adsorption of CO on Rh with FEM and TDS at temperatures above 300 K. Their main purpose was to examine possible CO dissociation and they calculated an upper limit on the CO dissociation rate of 1 X 10” molecules cmm2 s-‘. However, no experiments at 78 K were performed by them. In the present study some of the experiments were repeated and extended to temperatures below 300 K. During exposure the CO pressure was kept constant at about 5 x lop9 mbar. During thermal desorption the CO residual pressure was better than 8 X lo-‘” mbar. Fig. 2 shows the change of the work function A$J of the emitter as a function of the exposure to CO at 78 K. The maximum A+ observed, 1.5 eV, is significantly higher than the value found for the tip at 300 K (1.2 eV). Saturation is reached after an exposure of about 8 nbar s (= nanobar seconds). The FEM patterns observed during CO exposure at 78 K and during thermal desorption are shown in fig. 3. The pattern characteristic of a Rh tip saturated with CO at 78 K differs from that found at 300 K as described earlier by Gorodetskii and Nieuwenhuys [52]. In particular the areas around (110) and the stepped surfaces around (111) are much darker than observed at 300 K. In the first stage of adsorption rough surfaces, like (210) and (531) are darkened. Also the emission of the stepped surfaces, e.g. (221) and (223) falls, whereas the emission of the kinked crystal faces, like (432) (321) and (510) remains constant. Only near saturation the emission of the kinked surfaces around (111) and (100) decreases (see the difference between figs. 3b and 3~). Also near saturation the (110) surface and surroundings become dark. Upon comparing these patterns with those from ref. [52] observed at 300 K, it appears that fig. 3b resembles the pattern of a tip saturated at 300 K. Fig. 4a
H.A.C.M.
Hendrickx
et al. / Adsorption and dissociation of CO, on Rh
507
Fig. 3. Field emission patterns of Rh interacting with CO: (a) A+ = 0.35 eV, T= 78 K; (b) Acg = 0.50 eV, T = 78 K; (c) full coverage, T = 78 K; (d) heating, T = 390 K; (e) heating, T = 425 K; (f) tip after heating to 465 K, clean surface.
508
H.A.C.M. Hendrickx et al. / Adsorplion and dissociation of CO, on Rh
AP fa.u.1
T (Kl Fig. 4. (a) Variation in the work function of a Rh tip covered with CO during (b) TDS of a CO covered tip; (c) TDS of a hydrogen covered tip.
thermal
desorption;
shows the variation in the work function of a CO covered tip with temperature. As the work function change is almost proportional to the surface coverage (/3) up to high values of (3, fig. 4a may also be considered as a surface coverage versus temperature plot. In fig. 4b a TD spectrum is given. The intensity of mass 28 was monitored. Desorption of species with amu 28 could be attributed exclusively to CO because no signals of amu 14 (= N,) and 44 ( = CO,) were detectable. We also did not detect any desorption of amu 32 (= 0,). Upon heating the FEM image is not effected up to 250 K. Above 350 K fig. 4b is almost identical to the one observed for CO adsorbed at 300 K. The pictures of figs. 3d and 3e are not exactly the reverse of those observed during adsorption, figs. 3a, 3b and 3c. This indicates that CO is not mobile on all the Rh crystal surfaces at 78 K. At 350 K the kinked surfaces around (111) and (100) brighten. At about 400 K the emission from (110) and similar surfaces increases and at 425 K the pattern is almost identical to that observed by Gorodetskii et al. following CO adsorption at 300 K. The pattern of the clean tip is restored around 465 K. It should be noted that the experiments of figs. 4a, 3d, 3e and 3f were done with a much lower heating rate (1 K/s) than those during the thermal desorption mass spectroscopy experiments shown in fig. 4b (about 20 K/s). 3.2. Hydrogen adsorption The sticking probability for hydrogen at 78 K is essentially unity. The work function increases by 0.3 eV for a layer saturated at 78 K. The FEM pattern is
H.A.C.M. Hendrickx et al. / Adsorption and dissociation of CO, on Rh
0.04 0
I
I
I
5
10
15
I
20
exposure
I
25
509
1
30
In bars1
Fig. 5. Solid line: variation in work function of a Rh tip with CO, exposure at 78 K. Dashed line: pre-exposure to 20 nbar s of CO, and subsequent effect on work function when hydrogen is admitted.
identical to that observed at 196 K by Gorodetskii et al. [57]. Thermal desorption of hydrogen exhibits a single desorption peak with a maximum at about 370 K at low 0 to 320 K for a saturated layer using a heating rate of 11 K/s (see fig. 4~). 3.3. Exposure to CO2 at 78 K Fig. 5 shows the change of work function versus exposure to CO,. It illustrates that the initial sticking probability is close to unity. The Rh surface is almost saturated with CO, after an exposure of about 13 nbar s. The work function has increased then by 0.6 eV. However, it continues to change following larger doses of CO,. The FEM images at this stage suggest the presence of a 2nd or even a multilayer of CO,. Figs. 6 and 7 show the FEM patterns observed during exposure for (111) and (100) oriented emitters, respectively. The sequence of patterns shows that CO, adsorption is markedly crystal face specific and very different from the adsorption of CO on Rh. After about 9 nbar s exposure the pattern does not change much anymore, although the work function still increases by 0.1 eV upon further CO, adsorption. The first noticeable changes occur at the rough surfaces, in particular around (531) and ((310)-(320)) which areas exhibit a strong decrease in emission, whereas the emission around the (llO), (533) and the ((551)-(531)) areas does not change. In a later stage (figs. 6c, 6d and 7d, 7e) the ((llO)-(221)) and ((511)-(211)) areas also darken, while then the (321) areas remain bright. The emission of the saturated tip originates largely from (510) (320) and (321)
510
H.A.C.M. Hendrickx et al. / Adsorption and dissociation of CO, on Rh
Fig. 6. Field emission patterns observed during CO, adsorption on a Rb(ll1) tip at 78 K: (a) clean surface; (b) after 1 nbar s exposure; (c) after 3 nbar s exposure; (d) after 6 nbar s exposure; (e) fully covered surface.
H.A.C.M.
Hendrickx
et al. / Adsorption and dissociation of CO, on Rh
511
Fig. 7. Field emission patterns observed during CO, adsorption on a Rh(100) tip at 78 K: (a) clean surface; (b) after 1 nbar s exposure; (c) after 3 nbar s exposure; (d) after 7 nbar s exposure; (e) after 25 nbar s exposure; (f) after 50 nbar s exposure.
512
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Hendrickx
et al. / Adwption
and dissociation of CO, on Rh
0.6 0.L 0.2 0.0 TfK) Fig. 8. Work function change versus temperature dashed line: pre-exposure to CO, and subsequent
of a Rh tip. Solid line: tip exposed admission of hydrogen.
whereas these areas are dark in the pattern of a clean exposures to CO, (figs. 6d and 6e) the pattern becomes characteristic feature for the formation of a multilayer.
to CO,;
tip. Following high granular which is a
3.4. Heating of the CO, adlayer In fig. 8 the work function change versus temperature is recorded. Changes in FEM pattern accompanying the heating process are shown in fig. 9. Again, both experiments were done with a low heating rate (1 K/s). Upon heating the work function rises a little until a temperature of 110 K. The change in FEM pattern in this temperature range is not significant, only the granularity in emission disappears. Above 110 K the work function decreases gradually up to a A+ value of about 0.2 eV which is reached around 400 K. Then it is almost constant up to 450 K, at which temperature it falls fast towards the value of the clean surface. The FEM patterns show that at 125 K the (110) surface and at higher temperatures also its surrounding crystal faces like (551) and (331) become intensively emitting areas. The areas around (533) (311) and (711) have become very dark at the temperature of 215 K. Then, within 30 K the pattern changes completely. The dark area around (311) extends to other stepped surfaces and simultaneously the bright spot around (110) decreases in size and intensity, and (110) itself becomes dark again. At 250 K the pattern has become identical to the pattern characteristic of CO at low coverage (see e.g. fig. 3e). Around 470 K the tip becomes clean. Fig. 10 gives a series of TD spectra taken after varying exposure. Mass 28 is attributed to CO and to CO, of which it is a 11% peak. No N, (amu 14) was detectable in our system.
H.A.C.M. Hendrickx et al. / Adsorption and dissociation of CO2 on Rh
513
Although we searched extensively for it, oxygen (amu 32) could not be detected. Following an exposure of 4 nbar s the TDS curve is composed of a main desorption peak around 240 K and a smaller peak around 140 K. After an exposure of 10 nbar s the contribution of the low temperature peak is much more important than the high temperature peak . After large exposures the low temperature peak is split into two distinct peaks, one around 100 K and the other around 130 K. 3.5. Exposure
of CO_, at 300 K
TDS obtained following a 4 nbar s exposure to CO, at 300 K did not exhibit any desorption peak, neither amu 28, 32 nor 44 were detected. A very small CO (amu 28) peak around 500 K was observed after the filament had been exposed to more than 12 nbar s at 300 to 400 K. The work function increases slowly with exposure. After an exposure of 20 nbar s the work function still increases upon further exposure. The FEM pattern in this stage is similar to the pattern obtained following an exposure of about 1 nbar s CO. The pattern and the work function of a clean tip are restored upon heating the tip above 500 K. 3.6. TDS experiments
with 13C0,
Some experiments have been performed with 13C0, in order to discriminate adsorption of 13C0 following dissociation of 13C0,,and adsorption of ‘*CO, the major constituent of the residual gas. TD spectra were monitored following heating at various temperatures like 200 K, 300 K and 400 K for 60 s in the presence of 1 X lo-* mbar 13C02 and subsequent cooling in the CO, atmosphere to 78 K. The resulting TDS are consistent with those of fig. 10. The spectra show just the 45 and its 11% 29 fragmentation peak at 140 and 240 K. r3C0 or ‘*CO peaks around 500 K were not observed under these experimental conditions. 3.7. Co-adsorption
of H2 and CO, at 78 K
Fig. 5 shows the effect of hydrogen on the work function of a tip saturated with CO, at 78 K. Obviously, hydrogen is adsorbed on the tip covered with CO,. The pattern changes slightly into a “four-leaves clover” pattern. Upon heating the tip in the presence of hydrogen in the gas phase, the work function decreases in three stages, as is shown in fig. 8: first the temperature range 80-220 K after which a hydrogen-like pattern is obtained, second the 300-400 K range corresponding with desorption of hydrogen and third near 500 K when the CO-like pattern disappears. Note that the small increase in work
514
H.A. C. M. Hendrickx et al. / Adsorption und dissocration of CO, on Rh
H.A. C. hf. Hendrickx et al. / Adsorption and dissociation of CO, on Rh
Fig. 9. Field emission patterns observed during heating of a CO, covered tip starting at 78 K. (a) T =110 K; (b) T =125 K; (c) T = 215 K; (d) T = 220 K; (e) T = 230 K; (0 T = 250 K; (g) T = 410 K; (h) T = 470 K.
function that is observed for Rh exposed to CO, without hydrogen is absent. Figs. 11 and 12 show the effect of CO, on a Rh tip precovered with hydrogen at 78 K. The work function decreases by about 0.05 eV upon exposing the hydrogen covered Rh tip to CO, at 78 K. The FEM pattern does not change upon CO, exposure. The clean pattern is obtained around 380 K while the work function is then back to the value of the clean tip. TDS obtained after exposing a Rh tip to a mixture of hydrogen and CO, (4: 1) at 78 K did not
AP (0.u.l
I
I
100
200
300
,
I
LOO
500 T(KI
Fig. 10. TDS of a Rh tip covered with CO, at 78 K (amu 28 was recorded): (a) following exposure; (b) following 10 nbar s exposure; (c) following 40 nbar s exposure.
4 nbar s
H.A.C.M. Hendrickx et al. / Adsorption and dissociation o/ CO2 on Rh
516
exposure
(nbars.1
Fig. 11. Work function change versus exposure of a Rh tip at 78 K. Solid line: hydrogen
dashed
line: pre-exposure
to 8 nbar s hydrogen
and subsequent
exposure
exposure;
to CO,
AQieVl 0.4 0.3 0.2 0.1 -
0.b
1
i
200
100
300
LOO T (Kl
Fig. 12. Work function exposed to hydrogen.
result
change
in the production
versus temperature
of H,O
of a tip pre-exposed
or CO peaks under
to CO, and subsequently
our experimental
condi-
tions. 4. Discussion 4.1. Adsorption
of Hz and CO
The results presented here are discussed in relation with those published earlier by Gorodetskii et al. [52,57]. The present results on hydrogen adsorp-
H.A.C.M. Hendrickx et al. / Adsorption and dissociation of CO, on Rh
511
tion at 78 K are in excellent agreement with those of Gorodetskii et al., who studied the adsorption of hydrogen at temperatures above 196 K. The FEM and TDS results show that the maximum coverage of CO at 80 K is significantly larger than that can be obtained at 300 K and the low pressures used in these studies. Furthermore, our TDS results show the existence of a low temperature adsorption state of CO on Rh, with an Ed_ = 40 kJ/mol. This adsorption state can be attributed to physically adsorbed CO. It has only a small influence on the work function (see figs. 2 and 4). Similar states have been reported for CO on Ni, Pd and Pt (for a review, see ref. [41]). The main desorption peak has a maximum at 450 K which has been found by many authors [7,8,10,15,16,19,52]. However, in our studies the peak is much broader and more symmetric than in most of the other studies. As will be discussed below this effect can be attributed to the dependence of the adsorption energy on surface structure and coverage. The larger coverage at 78 K also explains the differences on A+,,, for T = 78 K and T = 300 K. Assuming that A+ is linearly proportional to the surface coverage 8 then at 300 K the relative surface coverage is 0 = 1.2/1.5 = 0.8 of that at 78 K. This value looks quite reasonable in view of fig. 4. The pictures of fig. 3 show the different stages in CO adsorption and desorption. It should be kept in mind that the pictures were taken at constant emission current. Since CO adsorption causes a drastic decrease in emission the voltage had to be raised during the adsorption process. Since the work function changes brought about by CO adsorption on Rh at saturation are not very surface structure sensitive it may be expected that the variation in A+ and, hence, in the extent of darkening over the tip surface is related to the variation in surface coverage. Fig. 3a suggests that CO is first adsorbed on the rough surfaces, like (531) or (210). It is also adsorbed on stepped surfaces like (221) but not yet in a significant amount on the kinked crystal faces like (432) and (510). Only near saturation the kinked surfaces around (111) and (100) are covered with CO (see the differences between figs. 3b and 3~). Also near saturation the (110) surface and surroundings are covered with CO. Upon comparing the patterns with those of ref. [52] it appears that fig. 3b resembles very much the pattern obtained at 300 K. At room temperature CO adsorbs readily on rough Rh surfaces and on stepped surfaces like (311), but only at higher exposures on kinked surfaces around (ill), (100) and on (110) and similar surfaces. On heating, figs. 3 and 4 show very little effect up to 250 K. The physically adsorbed CO seems to have no significant influence on A$ and is not surface structure specific, so until this temperature hardly anything happens. This in agreement with fig. 2. Above 350 K, fig. 4 is almost identical to the one measured for CO adsorbed at 300 K. Above 350 K, CO desorption starts from the kinked surfaces around (111) and (100). At about 400 K, CO is desorbed from (110) and similar surfaces and around 465 K from the rough surfaces like (531) and (210). Hence, we may conclude that the heat adsorption decreases in the order:
518
Q(531)
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= Q(320) = Q(210) = Q(221)
= Q(322)
= Q(533) > Q(432)
= Q(321) = Q(510). Unfortunately, the present results do not provide information concerning the heat of adsorption on the most densely packed surfaces, the (111) and (100) surfaces. It has been reported earlier that the heat of adsorption is lower on Rh (111) than on stepped Rh surfaces like (755) (331). On smooth surfaces like (ill), (100) and (110) the differences are very small [9,15,58]. 4.2. The adsorption of CO, The maximum work function increase of 0.6 eV at 78 K and p = 3 x 10P9 mbar, is much lower than those found for CO (1.5 eV) and 0, (0.9 eV at 196 K) adsorption. This is a first indication that CO, does not dissociate on Rh at 78 K. Furthermore, the FEM patterns are quite different from those observed during exposure to CO. The sequence of patterns observed at 78 K suggests that at low exposures CO, is first adsorbed on the rough surfaces. In this stage the areas around (110) and the stepped surfaces with (111) terraces and steps are not covered to a large extent. Then, following 3 nbar s exposure the surfaces near (110) are covered. At about 7 nbar s exposure the surfaces with (100) terraces and (111) steps become covered. Following long exposures the FEM pattern points to the formation of a 2nd or multilayer adsorption. The pressure of CO, has no significant influence on the pattern but it has some influence on the emission properties. If the CO, pressure is increased more CO, molecules are adsorbed in the multilayer and the emission decreases consequently. This CO, can, to a large extent, be pumped off at 78 K. Upon heating the tip the work function first rises up to about 0.7 eV as shown in fig. 8. The TD spectrum shown in fig. 10 has a peak at the same temperature. A similar peak at this temperature has been found by Lin, reported in ref. [l], for a Rh (111) surface. At high exposures the peak is split into two peaks (fig. 10~). The physically adsorbed CO, has an estimated Edes of 27 kJ/mol. Lin (1) found 29.4 kJ/mol. The Ed_ for the CO, bound in the multilayer can be compared with the value of 23 kJ/mol, the heat of vaporization of CO, [59]. It should be expected that desorption of this weakly bound CO, should increase the emission since the emission decreases upon its adsorption. However, in the temperature range at which it is desorbed the emission decreases (the apparent work function increases). Retooling the tip, reexposure to CO, and subsequent heating causes the expected effect: an emission increase. Hence, it appears that in the temperature range 80 to 120 K an irreversible change occurs in the adlayer. (For example an rearrangement of the adsorbed molecules). At 125 K the (110) surface and surrounding crystal faces start to emit intensively, indicating that these surfaces are denuded. This increase in emission is not accompanied by a drastic decrease in the average work function. The fact that
H.A. C.M. Hendrickx et al. / Adsorption and dissociation oj CO, on Rh
519
a desorption maximum near 125 K has not been observed in TDS can be explained by remembering that a Rh filament has only very few (110) faces on its surface. After 110 K the work function decreases continuously (fig. 8). At 240 K the TD spectrum has its second CO, desorption peak. Now CO* desorbs with an Ed_ = 60 kJ/mol. In this temperature range the clean areas around (110) have extended to their surrounding stepped surfaces. By then the crystal face (533) has become very dark. Then, within 30” the picture changes completely. The dark areas (533) and other (100) surfaces with (111) steps become larger and simultaneously the (110) spot decreases. At 250 K the (110) surface itself is dark and the pattern has become identical to fig. 3e. After this stage all changes are similar to those observed for CO (compare with fig. 3) andat 470 K the surface is almost clean again. The interpretation of these results is as follows: at 215 K a large area around (110) is clean, so it emits strongly. At about the same temperature CO, starts to dissociate in CO, and 0,. The FEM pattern change points to a striking influence of surface structure on the ability of the surface to dissociate CO. Dissociation starts on the (100) surfaces with (111) steps, such as (533) and (711). Therefore, these regions become dark. The adsorbed CO is mobile on the surface at this temperature and it will travel to the clean areas around (110). In the mean time all adsorbed CO, molecules dissociate, first on the stepped surfaces. At 250 K the whole surface is covered with CO, the local CO coverages being determined by the thermodynamic equilibrium distribution over all the tip surfaces, i.e. by the variation of the heat of adsorption of CO with the surface structure. The picture resembles that of fig. 3e, i.e. a surface covered for about 50% with CO. No indication for the presence of adsorbed oxygen is obtained. Similar observations have been made by Castner et al. [9] who could also not detect adsorbed oxygen after CO, dissociation. Most probably the oxygen diffuses readily into the bulk of Rh even at this low temperature [7,1.5]. The second CO, desorption peak in the TDS around 240 K has been assigned by Dubois et al. [l] to recombination of CO, + Oa. Our FEM results do not support their view, the peak should be attributed to desorption of ~he~sorbed CO2 (Ed_ = 60 kJ/mol). Above 250 K the whole process is identical to the one described for CO adsorption. The results show that hydrogen can be adsorbed on Rh covered with CO, at 78 K. Upon heating the initial rise in work function observed for the adsorption of pure CO, is absent. No other effect of hydrogen on adsorbed CO, or of CO2 on adsorbed hydrogen has been found. If the surface is first exposed to hydrogen and then to CO,, CO2 is adsorbed on the hydrogen covered Rh surface with a very low heat of adsorption an a work function decrease. This adsorbate, may be attributed to CO, bonded to hydrogen, for example as shown in fig. 13a. No influence of hydrogen on the dissociation of CO, could be observed under our experimental conditions. The FEM results point to dissociation of CO, on certain Rh surfaces around
H.A.C.M. Hendrickx et al. / Adsorption and dissociation of CO, on Rh
520
H
,,,,,,,
m
Fig. 13. (a) CO, weakly adsorbed on a hydrogen covered surface; monodentate structure; (c) CO, bonded in the bidentate structure.
(b) CO,
bonded
in the
215 K whereas the TDS results did not provide a clear evidence for dissociation of CO,. It has been noted above that both the experimental conditions (heating rate) and’the surface structure of the samples used in the FEM and TDS experiments differ. Polycrystalline filaments equilibrated at high temperatures consist largely of (111) and (100) surfaces. The FEM results show that the surface structure has a striking influence on the dissociation of CO,. (100) surfaces with (111) steps are active in the dissociation while dissociation was not observed on most other Rh surfaces including (110) (100) and (111). Apparently, the concentration of sites on the filament surface at which CO, can readily dissociate is very low. As noted in the introduction, there is disagreement about whether or not CO, dissociation can occur on Rh. Our results point to surface heterogeneity as a major effect explaining this controversy. Certain sites present on stepped surfaces are active whereas other sites are not or less active in CO, dissociation. Finally, we shall discuss the nature of the chemisorbed CO, desorbing or dissociating in the temperature range from 80 to 210 K. The FEM results show that the surface structure has a striking influence on its heat of adsorption and, possibly, on the nature of this adsorbed complex. The positive A+ for CO, adsorption is consistent with a model, see fig. 13b, of CO, adsorption on group VIII metals by a metal-carbon bond as first suggested by Eischens and Pliskin [60] for CO, on Ni powders on the basis of IR spectroscopy. In later papers it was shown that CO, dissociates readily on Ni [40]. On Pt and Cu, however, weak molecular adsorption has been observed with only small core and valence electron level shifts [64,65]. On the basis of these results the authors concluded that CO, is physically adsorbed on Pt and Cu. In coordination complexes of CO, with Ni, Rh and Ir it has been proposed that the CO, is bonded to a metal ion M through the C atom in a monodentate or a bidentate complex [32,60], see figs. 13b and 13~. The monodentate structure with the C atom directed to the surface and with
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and dissociation of CO, on Rh
521
an OCO angle of 120” has also been proposed by Anderson [61] for CO, on a Cu(100) surface based on molecular orbital calculations. The bending would be favoured by donation from the metal to an empty CO, 27r* orbital which is sufficiently stabilized on bending on the metal surface to become occupied. However, in a later study of COz adsorbed on Pt and Cu surfaces, Ray and Anderson [26] suggested a bonding via the lone pair of 0 with the molecular bond vertically since photoemission studies indicate no energy level shifts as would be expected if CO, were bent on the surface. Our results point to a striking influence of the surface structure on both the molecular adsorption and the dissociation of CO,. This observation suggests that the nature of the adsorption complex might be quite different on the various single crystal surfaces. Let us assume, just for the sake of argument, that CO, is bonded to the surface in a monodentate structure via the C atom on top of a Rh atom (fig. I3b). On flat surfaces like (111) the m~imum density of CO, molecules bound in this way would be much smaller than for example CO molecules (10” molecules/cm*) Hydrogen can be easily adsorbed in this open layer consistent with our observations. If, on the other hand, CO, would be bonded vertically then the expected maximum surface coverage would be comparable to that of CO. Hydrogen cannot be adsorbed in such a layer, as suggested by the observed block.ing of H adsorption by CO 1621,this in contrast to our results. The observed large differences between the various surfaces, towards adsorption and dissociation might also be understood in terms of the CO, structure on the surface. For example, assuming again that CO, is adsorbed in the monodentate structure on top of a Rh atom on the (111) terrace of a stepped surface near a (111) like step then an interaction between an 0 atom and the Rh atoms at the step above the (111) terrace considered should be weak but on a (100) like step strong in view of the distance of the 0 atom to the relevant Rh atoms. This situation is illustrated in fig. 14. Similar geometrical situations consist near (111) steps and (100) terraces. A structure like that might be the first step for dissociation. The observed influence of the structure on both heat of adsorption and the dissociation of CO, might be related to this effect. It is hoped that future studies using vibration spectroscopy may provide more information concerning the nature of the CO, metal bond. Anyhow, the positive work function change found in the present study suggests a net charge transfer from the metal to the Ccl, molecule.
Fig. 14. Possible structure of CO2 near a step.
522
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