Molecular beam studies of ethanol oxidation on Pd(110)
M . B o w k e r a , . , R . P . H o l r o y d a, R . G . S h a r p e % J .S . C o r n e i l l e b, S . M . F r a n c i s ~,
D . W . G o o d m a n b
a Reading Catalysis Research Centre, Department of Chemistry, University of Reading, Whiteknights Park, Reading, UK and IRC in Surface Science, University of Liverpool, Liverpool, UK
b Department of Chemistry, Texas A University, College Station, TX, USA
Received 14 April 1996; accepted for publication 22 July 1996
Abstract
The adsorption and decomposition of ethanol on Pd(110) has been studied by use of a molecular beam reactor and temperature programmed desorption. It is found that the major pathway for ethanol decomposition occurs via a surface ethoxy to a methyl group, carbon monoxide and hydrogen adatoms. The methyl groups can either produce methane (which they do with a high selectivity for adsorption below 250 K) or can further decompose (which they do with a high selectivity for adsorption above 350 K) resulting in surface carbon. If adsorption occurs above 250 K a high temperature (450 K) hydrogen peak is observed in TPD, resulting from the decomposition of stable hydrocarbon fragments. A competing pathway also exists which involves C-O bond scission of the ethoxy, probably caused by a critical ensemble of palladium atoms at steps, defects or due to a local surface reconstruction. The presence of oxygen does not significantly alter the decomposition pathway above 250 K except that water and, above 380 K, carbon dioxide are produced by reaction of the oxygen adatoms with hydrogen adatoms and adsorbed carbon monoxide respectively. Below 250 K, some ethanol can form acetate which decomposes around 400 K to produce carbon dioxide and hydrogen.
Keywords: Alcohols; Chemisorption; Low index single crystal surfaces; Molecule-solid reactions; Oxygen; Palladium; Sticking; Surface chemical reaction
1. Introduction
The ca ta ly t ic fo rma t ion of a lcohols f rom synthe- sis gas (carbon m o n o x i d e and hydrogen) is of cons iderab le fundamen ta l and indus t r ia l impor - tance. In par t icu lar , it is t hough t tha t the synthesis of e thano l via hyd rogena t i on of C O requires disso- c ia t ion of CO, subsequent hyd rogena t i on of the surface ca rbon forming a CHx species, and inser- t ion of a C O unit, and it has been p r o p o s e d tha t
this m a y proceed via a surface acetate in te rmedia te
[1 ] . Hence, an ideal e thano l synthesis ca ta lys t wou ld exhibi t weak C O dissocia t ion, thereby per- mi t t ing the fo rma t ion of a CHx species as well as d i sp lay ing act ivi ty t owards C O insert ion. Of the G r o u p VI I I metals , comple te d issoc ia t ion of C O occurs on the earl ier t rans i t ion metals whereas
noble meta ls favour molecu la r adsorp t ion . This has resul ted in numerous studies of bo th the a dso rp t i on and the decompos i t i on of var ious small cha in a lcohols on G r o u p VI I I t rans i t ion metals .
P r o m o t e d r h o d i u m cata lys ts have been shown to exhibi t the versat i le character is t ics requi red and
114 M. Bowker et al./Surface Science 370 (1997) 113-124
can achieve high selectivity to ethanol synthesis from synthesis gas [ 1,2]. Platinum and palladium, on the other hand, are commonly limited by their low ability to dissociate CO. Despite this, palla- dium is known to have excellent hydrogenation properties leading to high activity and selectivity to the production of methanol (which does not require C-O bond scission) on a number of sup- ported palladium catalysts [3-6]. Ethanol, how- ever, can be prepared on palladium surfaces if a hydrocarbon species is added to the CO and hydrogen mixture [7].
Though previous results are somewhat contra- dictory, a number of studies have shown that a small amount of C-O bond scission can occur on palladium surfaces. Indeed, strong structural dependence is expected in the ability of metals close to the CO dissociation/CO non-dissociation dividing line in the periodic table to dissociate CO [8]. Carbon-oxygen bond scission in carbon mon- oxide has been reported on a number of highly dispersed supported palladium catalysts [9-12] and even on polycrystalline palladium foil [ 13]. On single crystal surfaces C-O bond scission has been reported during methanol [ 14-18] and allyl alcohol [ 19] decomposition on Pd(111) (although in the former case this is contrary to previous studies [20,21] and has been contested [22]) and in methanol decomposition on Pt(111) [23] (again contrary to previous studies [24-26]). Wang and Masel [27-29] have also observed scission of the C-O bond in methanol on a (1 x 1)-Pt(ll0) sur- face, with a selectivity of 30% for methane forma- tion. However, this observation is also contrary to previous observations [30,31 ]. Recent studies on Pd(ll0) reported no evidence of C-O bond scis- sion in C~-C3 alcohols [32] nor following allyl alcohol adsorption [33]. One possible reason for these contrasting reports is kinetic, that is, CO bond dissociation undoubtedly takes place on all these metals, but often occurs at a low rate, perhaps at special sites, and is difficult to measure.
In this report, we have chosen to study the adsorption and decomposition of ethanol on clean and oxygen pre-covered Pd(ll0). This extends previous work in this laboratory elucidating the mechanism by which ethanol decomposition occurs on clean and oxygen pre-covered Rh(ll0)
surfaces [2,34]. Those studies showed that either ethanol oxidation [34] or the adsorption of acetic acid [35] results in the formation of a surface acetate which subsequently decomposes in an auto- catalytic manner (a "surface explosion") when it is formed in the presence of surface oxygen or carbon. The driving force of this research is therefore the study of ethanol decomposition pathways on Pd(ll0) which may increase our broad under- standing of alcohol synthesis routes on the late transition metals.
2. Experimental
Experiments were carried out using an ultra- high vacuum (UHV) system described in detail elsewhere [36]. This system operated at a base pressure of approximately 2x 10-X°Torr. The chamber was equipped for Auger electron spectro- scopy (AES) and low energy electron diffraction (LEED) as well as a quadrupole mass spectrometer used in temperature programmed desorption (TPD) and molecular beam reaction spectroscopy. The methodology associated with this well-defined thermal molecular beam has been discussed earlier [37].
The Pd(ll0) sample was mounted using tung- sten wires by which the sample could be resistively heated. It could also be cooled to temperatures as low as 170 K via a liquid nitrogen reservoir in contact with the sample mount. The sample tem- perature was monitored using a alumel/chromel thermocouple. The crystal was initially cleaned by cycles of argon ion bombardment and annealing. Subsequent cleaning cycles involved heating the crystal in oxygen at 850 K and annealing in vacuum following a well-established procedure [38-42]. The palladium (279eV) and carbon (272 eV) Auger peaks overlap so surface cleanliness was subsequently checked by the presence of the 0~ 3 peak in the carbon monoxide desorption profile [43,44]. However, since this requires the crystal to be cooled below ambient temperature, cleanli- ness was routinely confirmed by the absence of any carbon monoxide or carbon dioxide desorp- tion from the crystal either during oxygen beaming at 530 K or in TPD after oxygen adsorption at
M. Bowker et al.jSurface Science 370 (1997) 113-124 115
this temperature [45,46]. The surface order and cleanliness were also confirmed using LEED, the clean surface exhibiting a (1 x 1) LEED pattern.
High vapour pressure contaminants were removed from the 99.9% ethanol using freeze-- pump-thaw cycles. Mass spectrometry was used to verify the purity of the reactants prior to the experiments. Oxygen and ethene were used as received with a given purity of 99.6% and 99.7% respectively. Background oxygen exposures were made by backfilling the chamber using a variable leak valve while ethanol was admitted as a molecu- lar beam, which exhibits a sample spot size approx- imately 2.8 mm in diameter [ 361. Computer controlled multichannel molecular beam reaction spectroscopy and TPD (acquired at a heating rate of about 5 K s-l) monitored all possible products as well as reactant masses. The change in partial pressure as a function of time of the mass 31 signal, which is the largest cracking fragment of ethanol, can be related in a simple fashion to the sticking probability of ethanol at this temperature [36].
3. Results and discussion
3.1. Ethanol adsorption and decomposition on clean Pd(ll0)
In order to measure the temperature dependence of its sticking probability on Pd( 1 lo), ethanol was dosed onto the surface at a variety of substrate temperatures from a molecular beam. Fig. la shows the time dependence of the ethanol sticking probability at 300 K together with the products observed to desorb from the surface. The beam is allowed to impinge on the surface at t= 1 min and is cut at t= 6 mins. The sticking probability was initially 0.25 + 0.03 and then decreased monotoni- cally as the surface became covered with adsor- bates. Hydrogen and methane were produced and desorbed from the surface. It is evident from Fig. la that the desorption of hydrogen is lagged compared to the sticking probability, reaching a maximum at t=2 min.
Other products were formed but remained adsorbed on the surface. These products desorbed or dissociated into volatile products at higher
(a) 0.8 1
3 h 0.7 5 g 0.6
3m
B 0.5 e
2%
a 0.4 33 .z q 0.3
a G$ e.
.z 0.2
a
gg
0.1 t? E.
0 OG 0 2 4 6 8
Time (mins.)
(b)
Time (mins.)
Cc) 1.h l-2 i
Time (mins.)
Fig. 1. Sticking probability of ethanol and desorption of pro-
ducts as a function of time on (a) clean Pd( 110) at 300 K, (b)
oxygen pre-covered Pd( 110) at 300 K and (c) oxygen pre-cov-
ered Pd( 110) at 400 K. In all cases the beam impinges on the
crystal at t = 1 min and is cut at t= 6 min.
116 M. Bowker et af /Surface Science 370 (1997) 113-124
temperatures and can be observed using temper- ature programmed desorption (TPD). Fig. 2a shows a typical TPD subsequent to ethanol beam- ing at 300 K. This shows two hydrogen peaks: the more intense one at about 340 K and a smaller one at about 450 K, and two carbon monoxide peaks: one at about 480 K and one at about 580 K. The lower temperature hydrogen and carbon monoxide peaks coincide with peaks observed during hydrogen 1-38,47-49] and carbon monox- ide 1-50] adsorption on clean Pd(ll0) respectively and thus these peaks are desorption limited. The other two peaks will be discussed in detail below. A similar TPD profile was seen if background dosing occurred at 300 K. However, the size of the low temperature hydrogen peak was very depen- dent on the amount of time that elapsed from ethanol adsorption to the start of the temperature ramp and on the actual value of the adsorption temperature used. This is because the rate of hydrogen desorption is already significant at 300 K and increases rapidly with temperature.
Fig. 3 shows the initial ethanol sticking prob- ability as a function of surface temperature. The sticking probability decreases as the temperature increases. Below 270 K no hydrogen or methane desorbed during ethanol sticking. At higher sur- face temperatures, the amount of both products increased, peaked around 300 K and then decreased again. If adsorption of ethanol occurred above about 350 K no methane production took place although hydrogen desorption continued above this temperature.
Fig. 2b shows the TPD profile observed after background dosing of ethanol onto the clean Pd(ll0) surface at 170 K. This shows peaks of methane (287 K) and hydrogen (300 K) and two peaks of carbon monoxide (487 K and 595 K). In addition, ethanol was seen to desorb in a broad peak between 200 K and 700 K. If, instead, ethanol was beamed onto the surface, when only a small increase in background pressure is observed, this peak was much smaller and peaked between 200 K and 300 K. Thus, the background dosing TPD peak is probably mainly due to desorption of ethanol off the sample support as it heats up. This profile is in broad agreement with the work of Shekhar and Barteau who adsorbed ethanol at 120
(a) 6
..~ .~ t.'if" 4-
i.i2- III
~ o I I I I
200 300 400 500 600 700 800 9001000 Temperature (K)
(b)
.~ 2 4 i 1 8
61,,/ / ~ - C 2 H s O H (x41
/ ' / / / "~l-"-'-~'-"-'--C'~ H' (x4)
200 300 400 500 600 700 800 9001000 Temperature (K)
(c) 40"
.~.~ 3 0
t'~- ~ H20 (xl0)
c% <xlo) z z | r / / \ ~ C2HsOH (x2) '~ n I ~ k..__ CH, (xl) '
v " l I I I I I I I I
200 300 400 500 600 700 800 9001000 Temperature (K)
Fig. 2. TPD following (a) ethanol beaming to saturation onto clean Pd(ll0) at 300 K, (b) background adsorption of 2.4 L of ethanol onto clean Pd(ll0) at 170 K and (c) background adsorption of 2.4 L of ethanol onto oxygen pre-covered Pd(110) at 170 K.
M. Bowker et al./Surface Science 370 (1997) 113-124 117
1
"~ o . 8 - @ sl
0 . 6 -
e~
o~ 0 , 4 -
0 . 2 -
0
150 2 0 0
J ~ Clean J 0 0 precovered J
I I I I I I
2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 Temperature (K)
Fig. 3. Initial sticking probability of ethanol on both clean and oxygen precovered Pd(110) as a function of surface temperature.
K [32] although the high-temperature carbon monoxide peak was not reported in this earlier study. This will be discussed further below. It is of interest to note that the high temperature hydrogen TPD peak around 450 K, observed in Fig. 2a, was absent unless exposure occurred above about 250 K. This peak will also be discussed further below. The shift in the low temperature hydrogen peak observed after background dosing at 170 K to a lower temperature than that seen after adsorption at 300 K (Fig. 2a) is due to the higher coverage of hydrogen after background dosing as, unlike when adsorption occurs at 300 K, no hydrogen desorp- tion occurs during ethanol adsorption. The cover- age dependence of this peak temperature has previously been reported for the clean Pd(ll0) surface [48].
If ethanol was adsorbed onto the surface at temperatures above 250 K and the volatile pro- ducts removed by heating to 850 K, and oxygen was subsequently beamed onto the crystal at 530 K, a small amount of carbon dioxide was observed to desorb. This did not occur if the same procedure was followed without ethanol adsorption. Hence, carbon must be deposited onto the surface as a result of ethanol decomposition. The amount of carbon dioxide produced was independent of the ethanol adsorption temperature above 250 K. By comparison to the amount of carbon dioxide pro- duced by beaming carbon monoxide onto an oxygen saturated surface at 520 K 1-51] (which has a coverage of 0.5 ML [52-55]), it was estimated
that the carbon coverage was small (0.03+0.01 ML). This is similar to the observation of Shekhar and Barteau 1-32] who estimated that, after adsorp- tion at 120 K, there was 0.02 ML of carbon left on the surface. However, in our study, no surface carbon was detected by subsequent oxygen dosing if ethanol adsorption occurred below 250 K. Since Shekhar and Barteau's estimate was made by adsorbing oxygen at low temperatures and measur- ing the size of the CO2 peak in a subsequent TPD ramp, it would be very sensitive to any background adsorption of CO. Hence, we believe our findings to be more accurate.
Since hydrogen atoms recombine and desorb at relatively low temperatures (~ 340 K), the second hydrogen peak seen in TPD (at 450 K) must be due to the decomposition of hydrogenated frag- ments stable up to this temperature. Similar high temperature hydrogen TPD peaks have been observed previously on Pd(ll0). For example, a 480 K hydrogen peak was observed after the adsorption of ethene (which was assigned to the decomposition of ethynyl (CCH) species) [41,56] and hydrogen peaks at 460 K have been observed after either allyl alcohol or acrolein adsorption (which were also assigned to the decomposition of intermediate hydrocarbon fragments) [33]. Thus, it seems likely that hydrocarbon decomposition is the cause of the similar peaks seen around 450 K. Indeed, previous studies have suggested that methyl, or other hydrocarbon groups, are unusu- ally stable (to temperatures over 400 K) on the Pd(111) surface [ 17,18]. This explains the absence of both surface carbon and the 450 K hydrogen peak if ethanol is adsorbed below 250 K, since, as we have seen, most of the hydrocarbon fragments are desorbed as methane at temperatures just above this leaving very few hydrocarbon species on the surface to dissociate at higher temperatures.
The higher temperature carbon monoxide peak near 600 K has not, to the best of our knowledge, been previously reported on Pd(110). One possible explanation is that the small amount of carbon present on the surface resulting from ethanol decomposition, stabilises CO adsorption on some surface sites causing it to desorb from the surface at higher temperatures than normal. However, surface carbon is known to act as a poison and
118 M. Bowker et al. /Surface Science 370 (1997) 113-124
destabilise CO(a) [57]. Nevertheless, in order to confirm this, large quantities of ethanol were dosed onto the surface at temperatures between 300 K and 400 K and all the volatile products were removed by heating to 850 K in order to leave just carbon on the surface. Then the crystal was cooled to 170 K to adsorb carbon monoxide. Fig. 4a shows that subsequent TPD led to a carbon mon- oxide desorption profile similar to that expected from the clean surface (Fig. 4b), i.e. there was no high temperature (580 K) desorption peak. Hence, in agreement with previous work, carbon does not play a role in stabilising CO adsorption and thus in causing the second, high temperature desorption peak. This result also shows that it was not caused by any other non-volatile contaminant on the surface which might adsorb from the gas phase during ethanol adsorption. It also confirms that the surface carbon coverage due to ethanol decom- position is low, since the CO TPD profile from P d ( l l 0 ) has previously been found to be very sensitive to small coverages of carbon [43,44]. Thus, the only other possible cause of this high temperature CO peak is carbon and oxygen recom- bination. In order to check the plausibility of this explanation a controlled amount of carbon was deposited onto the surface by beaming ethene at 550 K, ethene being known to produce surface carbon [56]. The surface was then subsequently exposed to a beam of oxygen and the resulting
products monitored. Fig. 5 shows that, after adsorption of 0.10+0.01 ML of carbon, only carbon dioxide was produced if oxygen was beamed at 500 K (Fig. 5a) whereas, at 590 K, carbon monoxide was initially produced in prefer- ence to carbon dioxide, but quickly diminished and a carbon dioxide peak followed as oxygen coverage increased (Fig. 5b). This shows that the reaction between O atoms and surface carbon will occur at temperatures around 600 K under appro- priate conditions, i.e. insufficient oxygen atoms to produce carbon dioxide; when the oxygen coverage becomes high reaction of the CO with the oxygen produces CO2. This leaves the puzzling question: where do the oxygen atoms come from? The only explanation is that CO bond scission also occurs. Carbon-oxygen bond scission has not previously been observed on P d ( l l 0 ) but, as we saw in Section 1, the ability of palladium to dissociate C - O bonds is very structure dependent. Both hydrogen [ 38,47-49, 58-63 ] and carbon monoxide [42,50,64-68] are known to cause the clean Pd(110) surface to reconstruct so it is possible that the ensemble of palladium atoms required to disso- ciate C - O bonds is present due to local reconstruc- tion of the surface. Alternatively, defect or step sites might be responsible.
On the basis of these results it is possible to begin to discuss the various steps involved in ethanol decomposition. A summary is shown sche-
I I I I
200 300 400 500 600 Temperature (K)
Fig. 4. TPD following a 15 L dose of carbon monoxide at 170 K onto (a) clean Pd(110) and (b) carbon-covered Pd(110) with a surface carbon coverage of ca. 0.03 ML caused by the adsorp- tion of ethanol on-top the surface between 300 K and 400 K.
Fig. 5. Time dependence of the products desorbed from the sur- face as a result of beaming oxygen onto a Pd(ll0) crystal cov- ered with 0.10+0.01 ML of carbon at (a) 500 K and (b) 590 K.
M. Bowker et al./Surface Science 370 (1997) 113-124 119
maticaUy in Fig. 6. By analogy with previous studies on palladium and rhodium surfaces [21,32,34,69,70] the first step of ethanol decompo- sition probably involves the formation of a surface ethoxy:
CHaCH2OH(g)~-~ CHaCH2OH(a), (i)
CHaCH2OH (a)~-~CHaCH20 (a) + H(a). (2)
Since both these steps are required for chemi- sorption to occur it can be presumed that they occur at 170 K.
Subsequent reactions must entail the breaking of the carbon-carbon bond (since no product containing two carbon atoms is observed). There is no evidence for the formation of formaldehyde. Thus, it seems the ethoxy must decompose via two competing pathways, the first, occurring at about 250 K, being decomposition into a methyl group, carbon monoxide and hydrogen atoms, and the second being decomposition involving C-O bond scission into a methyl group, carbon, oxygen and
cnacrI2On(g)
1~ step (1)
CH3CH2OH(a)
1L S step(2)
CH3CH20(a)
CH3(a) + CO(a) + 2H(a)
I " " ' - - .~H step (5) step fr) ~ CH4(g)
C(a) + 3H(a)
+ H(a)
step (13)
CH3COO(a) + 2H(a)
step (4) step (14)
C(a) + CO2(a) + 3H(a)
CH3(a) + C(a) + O(a) + 2H(a)
Fig. 6. Schematic of the reaction pathways of ethanol on clean and oxygen pre-covered Pd(l l0) . The main steps identified in the text are given on the schematic.
hydrogen atoms:
CHaCH20(a) ~CH3(a) + CO(a) + 2H (a),
CH3CH20(a)~CHa(a) + C(a) + O(a) + 2H (a).
(3)
(4)
These steps are in competition, the respective areas under the peaks at 480 K and 580 K in the CO desorption profile after adsorption at 300 K (Fig. 2a) are in the ratio ~ 10:8 indicating these reactions occur with a similar probability.
The methyl species produced can recombine with surface hydrogen, produced from steps (2), (3) or (4), to produce methane. In addition, hydrogen atoms combine to release hydrogen molecules into the gas phase:
CH3(a) + H(a)~CH4(g); (5)
2n(a)~n2(g). (6)
Neither of these steps (5, or 6) occur below 250 K but, above this temperature, the two routes compete for the available surface hydrogen.
For adsorption temperatures above 250 K carbon was left on the surface after subsequent TPD so the methyl groups, since there is no dissociation of CO(a), must be capable of dissociat- ing at some temperature:
CH3(a) ~ CH2(a) + H (a),
CH2(a) ~ CH (a) + H (a),
CH(a)~C(a) + H(a).
(7a)
(7b)
(7c)
At 450 K a second hydrogen peak is seen in TPD, this being attributed to the decomposition of a hydrocarbon species:
hydrocarbon~C(a) + H(a). (8)
At 480 K carbon monoxide desorption occurs. In addition, at 580 K, O atoms produced during C-O bond dissociation combine with surface carbon to release CO molecules into the gas phase:
CO(a)~fO(g) ; (9)
C(a) + O (a)--*CO (g). (10)
In TPD, if the surface is dosed with ethanol at temperatures below 250 K and then heated, the rate of step (5) becomes much larger than the rates
120 M. Bowker et al./Surface Science 370 (1997) 113-124
of either step (6) or step (7a) as indicated by the desorption of methane before hydrogen in TPD (Fig. 2b) and the absence of any surface carbon. At higher temperatures, under isothermal condi- tions, this situation must be reversed since no methane is produced if adsorption occurs above 350 K. If the activation energy of methyl decompo- sition (step (7a)) is initially higher than the activa- tion energy of methane production (step (5)), then at higher temperatures conditions must alter in order for the selectivity of products to crossover at 350 K. This crossover can be attributed to an insufficiency in adsorbed hydrogen. As the temper- ature increases, step (6) becomes very favourable leaving the surface deficient of available hydrogen for step (5) to occur, resulting in a higher prob- ability for methyl decomposition (step (7a)). This also implies that any methyl groups which are converted to a CH2 species are then incapable of hydrogenating to methane. This CH 2 species then might further dehydrogenate (through step (7b) and step (7c)) at the adsorption temperature or might remain on the surface until higher temperatures.
This analysis also suggests that the amount of carbon left on the surface after TPD is dependent on the relative probability of methyl groups under- going step (5) (which leads to no surface carbon) and step (7a) (which leads, eventually, to the depos- ition of carbon on the surface). If adsorption occurs below about 250 K, almost all methyl groups form methane and thus very few decompose. At higher temperatures the rate of step (7a) increases relative to step (5) and hence the amount of surface carbon left per adsorbed ethanol molecule will be higher as the temperature increases. However, this effect is balanced by the lower ethanol uptake at higher temperatures and thus, as observed, the rate of surface carbon deposition is similar.
If C-O bond scission is indeed occurring on the surface then it is not clear why the oxygen pro- duced does not combine with hydrogen and desorb as water since this reaction is known to occur below 300 K. One possible explanation, although there is no direct evidence, is adsorption site sepa- ration, the carbon and oxygen atoms from C-O bond dissociation being situated away from the hydrogen atoms, resulting in a preference for a
hydrogen atom to combine with either a methyl group (step (5)) or another hydrogen atom (step (6)). This effect has previously been identified in the oxidation of methanol on Cu(ll0) [71] where it was shown that oxygen and methoxy are com- pletely phase-separated on the surface. At elevated temperatures surface diffusion may become appre- ciable, possibly breaking down the adsorption site separation model as proposed. However, at these elevated temperatures the reaction of O atoms with adsorbed hydrogen would be in competition with hydrogen atom association (step (6)), the latter being very favourable at these temperatures, reducing the likelihood of water formation.
Another possibility for the absence of water is the formation of subsurface oxygen. This process, which requires the diffusion of oxygen atoms from the first surface layer to the subsurface region, is well characterised on Pd (110) [ 72, 39 ] giving the distinctive fll desorption state in TPD. It is known to be both reversible and temperature dependent with the penetration process reportedly starting as low as 270 K and becoming more efficient with increasing temperature [39]. It would thus be possible for the O atoms produced during C-O bond scission (step (4)) to diffuse into a subsurface state, preventing reaction with adsorbed hydrogen and, hence, water formation. At elevated temper- atures the oxygen migrates back into the first surface layer, this being a slow step, reacts with surface carbon produced via either step (4) or steps (7a-c) and desorbs as CO (step (10)).
3.2. Ethanol adsorption and reaction on oxygen pre-covered Pd( llO)
For all experiments with the oxygen pre-covered surface, oxygen was beamed onto the sample until the saturation coverage associated with a c(2 x 4) LEED pattern was obtained. This corresponds to a coverage of 0.5 ML as seen previously [52-55]. Fig. 3 shows the initial ethanol sticking probability for the oxygen pre-covered surface as a function of temperature. As on the clean surface, the sticking probability decreased monotonically with temper- ature. However, at any given temperature the initial sticking probability is greater in the presence of oxygen than on the clean surface. This is due to
M. Bowker et al./Surface Science 370 (1997) 113-124 121
the removal of surface hydrogen by the adsorbed oxygen atoms driving step (2) towards dissociative adsorption. Even at high temperatures the sticking probability is higher on the clean surface, this being due to the molecular state having stronger binding on the oxygen pre-covered surface and thus a lower activation energy to dissociative adsorption.
Below 270 K, water was the only product observed to desorb during ethanol beaming onto the oxygen pre-covered surface. At higher temper- atures both hydrogen and methane also desorbed in similar quantities to those seen from the clean surface. Fig. lb shows the time dependence of the sticking probability of ethanol on the oxygen pre- covered surface at 300 K together with the time dependence of the desorption of products. Initially, the main product that desorbed into the gas phase was water, with very little hydrogen and methane produced:
H(a) + O(a)~ OH (a), (1 la)
H(a) + OH(a)~H20(g) . (1 lb)
However, since a small amount of both hydrogen and methane were produced, it is clear that methyl groups must also be present and hence step ( 1 )-(5) take place. After about 45 sec, the desorption of water stopped fairly abruptly and the rate of both hydrogen and methane desorption increased dra- matically. These effects can be easily explained by reference to the mechanisms above. During the initial sticking, methyl groups will have been cre- ated which did not react with hydrogen adatoms due to the efficiency with which such hydrogen adatoms were consumed by steps (1 la) and (11b). Thus, even once all the oxygen has been removed, the presence of an excess of methyl groups on the surface leads to efficient removal of surface hydrogen (through methane formation). This implies that the reversible step (2) is still being driven towards dissociative adsorption and thus the sticking probability is still higher than that seen on the clean surface. At this point, there is also a higher probability of forming methane, via step (5), than of forming hydrogen, via step (6), since the surface hydrogen atom coverage is low and the former step requires only one hydrogen
adatom compared to two for the latter. However, as the hydrogen atom coverage grows the relative probability of step (5) fails compared to step (6). This leads to methane production peaking prior to hydrogen production, as seen in Fig. lb. On the clean surface methyl groups do not build up, since there are no oxygen atoms present to remove hydrogen atoms, and so methane and hydrogen evolve at a similar rate.
Above 350 K, methane was no longer produced (the same threshold as that observed on the clean surface). Fig. lc shows the sticking probability and product desorption that occur at 400 K. At temper- atures above about 380 K, carbon dioxide was produced in addition to water during the initial dosing, i.e. prior to the increase in the rate of hydrogen desorption. This is presumably due to the reaction between carbon monoxide and adsorbed oxygen which is known to occur readily at temperatures above this threshold [51]:
CO(a) + O(a)~CO2(g). (12)
At these higher temperatures, and to some extent at lower adsorption temperatures, hydrogen con- tinued to desorb during beaming, as is evident in Fig. lc. This desorption being due to the continual adsorption (sticking probability remains above 0.05+0.03) and decomposition of ethanol, though there is also some background rise in the hydrogen signal.
TPD, after dosing ethanol onto an oxygen pre- covered Pd(110) surface at 300 K, produced a very similar profile to that observed after ethanol dosing onto the clean surface at the same temperature (Fig. 2a). Also, beaming oxygen at 530 K, after desorbing the volatile products by heating to 850 K, produced carbon dioxide, again revealing the presence of carbon on the surface. As on the clean surface, the amount of surface carbon seen above about 250 K (0.03 +0.01 ML) was independent of adsorption temperature.
Fig. 2c shows the TPD profile observed after background dosing of ethanol onto oxygen pre- covered Pd(110) at 170 K. This shows similar methane (290 K) and carbon monoxide (470 K and 565 K) peaks to the profile taken under the same conditions on the clean surface. However, there are a number of differences induced by the presence of oxygen. Three water peaks were pro-
122 M. Bowker et al./Surface Science 370 (1997) 113-124
duced at 225 K, 265 K and 295 K, whose magni- tude decreased as the temperature increased; two hydrogen peaks, a small one at 310 K and a larger one at 410 K, were also observed. The small relative size of the 310 K peak compared to that observed after adsorption onto the clean surface at 170 K is due to the large number of surface hydrogen atoms which desorb as water at lower temperatures. The 410 K hydrogen peak, which is absent in TPD following background dosing onto the clean surface, coincided with a carbon dioxide peak. This is not equivalent to the second hydrogen peak (at 450 K) seen in TPDs following exposure at higher temperatures on either the clean or oxygen predosed surfaces. The lack of this 450 K peak shows that, as for the adsorption of ethanol at 170 K onto the clean surface, no significant hydrocarbon fragments are left on the surface. Instead, since co-incident carbon dioxide desorp- tion is observed, it is likely that an oxyhydrocarbon intermediate species is left on the surface which decomposes to yield hydrogen and carbon dioxide. This intermediate is probably an acetate (produced via step (13)) which can also be produced directly by acetic acid adsorption on Pd(110) and is known to decompose at a temperature between 350 K and 450 K depending on the coverage of oxygen and carbon on the surface [34,35].
As mentioned above, TPDs following adsorption of ethanol onto the oxygen pre-covered surface at temperatures in excess of about 250 K are almost identical to those seen in the absence of oxygen and thus no acetate is then produced. Hence, once formed, acetate is stable till about 410 K but is not created at adsorption temperatures above 250 K. This implies that at temperatures below 250 K the ethoxy has a sufficient lifetime on the sur- face to facilitate acetate production (step (13)). However, at > 250 K the stability of the ethoxy group diminishes considerably, resulting in an inability to produce acetate. Note that the ethanol/O(ad) reaction on Rh(110) produces large amounts of acetate [34].
CH3CH20(a) + O(a)~CHaCOO(a) + 2H(a), (13)
CH3COO (a)~C~a) + ~H2(g)+ CO2(g). (14)
Thus, although the lowest temperature at which
the C O 2 peak is seen during ethanol beaming on the oxygen pre-covered surface is similar to the temperature at which the CO2 peak is seen during TPD following low temperature adsorption, the peaks are due to different mechanisms: the former due to the oxidation of carbon monoxide (step (12)) and the latter due to the decomposition of acetate (step (14)).
4. Conclusions
In this study we have elucidated the pathways for ethanol decomposition on clean and oxygen pre-covered Pd(110). The major pathway for etha- nol decomposition on Pd(110) is via the formation of a surface ethoxide which decomposes into a methyl group and adsorbed carbon monoxide and hydrogen adatoms. The methyl groups can com- bine with a surface hydrogen to produce methane (at temperatures in the range 270-350 K) or can decompose further leaving carbon on the surface. Thus, if adsorption occurs at temperatures below 250 K, methane is predominantly formed in the subsequent TPD while at higher adsorption tem- peratures the amount of methane produced (and, inversely, the amount of surface carbon left) depends upon the competition for the available hydrogen adatoms between hydrogen recombina- tion and methane production. A competing path- way also exists, probably at some critical ensemble of palladium atoms caused by steps, defects or a local surface reconstruction, in which C-O bond scission occurs as identified by a carbon and oxygen recombination CO TPD peak around 600 K.
If oxygen is present on the surface neither the major nor the minor pathway is significantly altered above 250 K except that water is produced leading to less hydrogen adatoms and a delay between the onset of sticking and the production and desorption of methane and hydrogen. In addi- tion, if adsorption occurs above 380 K, the oxygen adatoms can react with carbon monoxide to form carbon dioxide. If adsorption occurs below 250 K, the adsorbed oxygen can lead to a third decomposi- tion pathway: the formation of acetate. This then decomposes around 400 K to produce carbon
M. Bowker et al./Surface Science 370 (1997) 113-124 123
dioxide and hydrogen and leave carbon on the surface.
These studies highlight the rich and manifold reaction pathways for organic molecules reacting on palladium.
Acknowledgements
The authors are grateful to NATO for Collaborative Research Grant No. CRG 910963. R.P.H. would like to thank the EPSRC for the award of a studentship and R.G.S. for the award of a postdoctoral fellowship.
References
[1] M. Bowker, Catal. Today 15 (1992) 77. [2] W.M.H. Sachtler and M. Ichikawa, J. Phys. Chem. 90
(1986) 4752. [3] M.L. Poutsma, L.F. Elek, P.A. Ibarbia, A.P. Risch and
J.A. Rabo, J. Catal. 52 (1978) 157. [4] J.M. Driessen, E.K. Poels, J.P. Hindemann and V. Ponec,
J. Catal. 82 (1983) 26. [5] R.F. Hicks and A.T. Bell, J. Catal. 91 (1985) 104. [6] C. Sudhakar and M.A. Vannice, J. Catal. 95 (1985) 227. [7] T.L.F. Favre, G. Van der Lee and V. Ponec, J. Chem. Soc.
Chem. Commun. (1985) 230. [8] G. Brod6n, T.N. Rhodin, C. Brucker, R. Benbow and Z.
Hurych, Surf. Sci. 59 (1976) 593. [9] D.L Doering, H. Poppa and J.T. Dickinson, J. Catal. 91
(1985) 1. [10] S. Ichikawa, P. Poppa and M. Boudart, J. Catal. 97
(1986) 448. [ 11 ] V. Matolin, E. Gillet and S. Channakhone, J. Catal. 97
(1986) 448. [12] E. Gillet, S. Channakhone, V. Matolin and M. Gillet, Surf.
Sci. 152/153 (1985) 603. [13] V. Matolin, M. Rebholz and N. Kruse, Surf. Sci. 245
(1991) 233. [14] R.J. Lewis, J. Zhicheng and N. Winograd, J. Am. Chem.
Soc. 110 (1988) 4431. [15] R.J. Lewis, J. Zhicheng and N. Winograd, J. Am. Chem.
Soc. 111 (1989) 4605. [16] N. Kruse, M. Rebholz, V. Matolin, G.K. Chuah and J.H.
Block, Surf. Sci. 238 (1990) L457. [ 17] M. Rebholz, V. Matolin, R. Prins and N. Kruse, Surf. Sci.
251/252 (1991) 1117. [18] M. Rebholz and N. Kruse, J. Chem. Phys. 95 (1991) 7745. [19] J.L. Davis, PhD dissertation, University of Delaware,
1988. [20] J.A. Gates and L.L. Kesmodel, J. Catal. 83 (1983) 437.
[21] J.L. Davis and M.A. Barteau, Surf. Sci. 187 (1987) 387. [22] X. Guo, L. Hanley and J.T. Yates Jr., J. Am. Chem. Soc.
111 (1989) 3155. [23] R.J. Lewis, J. Zhicheng, N. Winograd, S. Akhter and J.M.
121 (1982) 181. [26] K.D. Rendulic and B.A. Sexton, J. Catal. 78 (1982) 126. [27] J. Wang and R.I. Masel, J. Catal. 126 (1990) 519. [28] J. Wang and R.I. Masel, J. Am. Chem. Soc. 113 (1991)
5850. [29] J. Wang and R.I. Masel, J. Vac. Sci. Technol. A 1 (1991)
1879. [30] G.E. Gdowski, J.A. Fair and R.J. Madix, Surf. Sci. 127
(1983) 541. [31] G.A. Attard, K. Chibane, H.D. Ebert and R. Parsons, Surf.
Sci. 224 (1989) 311. [32] R. Shekhar and M.A. Barteau, Catal. Lett. 31 (1995) 221. [33] R. Shekhar and M.A. Barteau, Surf. Sci. 319 (1994) 298. [34] M. Bowker and Y. Li, Catal. Lett. 10 (1991) 249. [35] N. Aas and M. Bowker, J. Chem. Soc. Faraday Trans. 89
(1993) 1249. [36] M. Bowker, P. Pudney and C. Barnes, J. Vac. Sci. Technol.
A 8 (1990) 816. [37] C. Barnes, P. Pudney, Q. Guo and M. Bowker, J. Chem.
Soc. Faraday Trans. 86 (1990) 2693. [38] J.-W. He and P.R. Norton, Surf. Sci. 195 (1988) L199. [39] M. Milun, P. Pervan, M. Vajic and K. Wandelt, Surf. Sci.
211/212 (1989) 887. [40] T. Matsushima, Surf. Sci. 217 (1989) 155. [41] M. Nishijama, J. Yoshinobu, T. Sekitani and M. Onchi,
J. Chem. Phys. 90 (1989) 5114. [42] P. Hu, L. Morales de la Garza, R. Raval and D.A. King,
Surf. Sci. 249 (1991) 1. [43] J. Goschnick, M. Wolf, M. Grunze, W.N. Unertl, J.H.
Block and J. Loboda-Cackovic, Surf. Sci. 178 (1986) 831. [44] J. Goschnick, M. Grunze, J. Loboda-Cackovic and J.H.
Block, Surf. Sci. 189/190 (1987) 137. [45] R.D. Ramsier, Q. Gao, H. Neergaard Waltenburg and J.T.
Yates Jr., J. Chem. Phys. 100 (1994) 6837. [46] T. Matsushima, K. Shobatake, Y. Ohno and K. Tabayashi,
J. Chem. Phys. 97 (1992) 273. [47] H. Conrad, G. Ertl and E.E. Latta, Surf. Sci. 41 (1974) 435. [48] R.J. Behm, V. Penka. M.G. Cattania, K. Christmann and
G. Ertl, J. Chem. Phys. 78 (1983) 7486. [49] J.-W. He, D.A. Harrington, K. Grittiths and P.R. Norton,
Surf. Sci. 198 (1988) 413. [50] J.-W. He and P.R. Norton, J. Chem. Phys. 89 (1988)1170. [51] R.G. Sharpe, Q. Guo, M. Bowker and R. Joyner, to be
published. [52] J.-W. He, U. Memmert, K. Griffiths, W.N. Lennard and
P.R. Norton, Surf. Sci. 202 (1988) L555. [53] J.-W. He and P.R. Norton, Surf. Sci. 204 (1988) 26. [54] J.-W. He, U. Memmert, K. Griffiths and P.R. Norton,
J. Chem. Phys. 90 (1989) 5082.
124 M. Bowker et al./Surface Science 370 (1997) 113-124
[55] J.-W. He, U. Memmert and P.R. Norton, J. Chem. Phys. 90 (1989) 5088.
[56] M. Nishijima, J. Yoshinobu, T. Sekitani and M. Onchi, J. Chem. Phys. 90 (1989) 5114.
[57] M. Kiskinova and D.W. Goodman, Surf. Sci. 109 (1981) L555.
[58] M.G. Cattania, V. Penka, R.J. Behm, K. Christmann and G. Ertl, Surf. Sci. 126 (1983) 382.
[59] K.H. Rieder, M. Baumberger and W. Stocker, Phys. Rev. Lett. 51 (1983) 1799.
[60] M. Jo, Y. Kuwahara, M. Onchi and M. Nishijima, Solid State Commun. 55 (1985) 639.
[61] H. Niehus, C. Hiller and G. Cosma, Surf. Sci. 173 (1986) L599.
[62] G. Kleinle, M. Stottke, V. Penka, G. Ertl, R.J. Behm and W. Moritz, Surf. Sci. 189/190 (1987) 177.
[63] M. Stottke, R.J. Behm, G. Ertl, V. Penka and W. Moritz, J. Chem. Phys. 87 (1987) 6191.
[64] H. Conrad, G. Ertl, J. Koch and E.E. Latta, Surf. Sci. 43 (1974) 462.
[65] R. Raval, S. Haq, M.A. Harrison, G. Blyholder and D.A. King, Chem. Phys. Lett. 167 (1990) 391.
[66] R. Raval, S. Haq. G. Blyholder and D.A. King, J. Electron Spectrosc. Relat. Phenom. 54/55 (1990) 629.
E67] A. Wander, P. Hu and D.A. King, Chem. Phys. Lett. 201 (1993) 393.
[68] A. Gaussmann and N. Kruse, Surf. Sci. 266 (1992) 46. [69] J.L. Davis and M.A. Barteau, Surf. Sci. 235 (1990) 235. [70] Y. Li and M. Bowker, Catal. Today 10 (1991) 421. [71] S.M. Francis, F.M. Leibsle, S. Haq, N. Xaing and M.
Bowker, Surf. Sci. 315 (1994) 284. [72] J.-W. He, U. Memmert and P.R. Norton, J. Chem. Phys
![Pd Ru , electro-catalyst in direct alkaline ethanol fuel cells · Pd xRu 1-X, electro-catalyst in direct alkaline ethanol fuel cells Alixia Farrell; Evans Monyoncho[1]; Elena A. Baranova[1]](https://static.documents.pub/doc/80x56/5acf4e2e7f8b9a71028c8173/pd-ru-electro-catalyst-in-direct-alkaline-ethanol-fuel-xru-1-x-electro-catalyst.jpg)
