Langmuir 1991, 7, 2574-2579
2574
Hydrogen-Carbon Monoxide Interactions on Metal Surfaces at High Hydrogen Coverages John L. Gland* Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055
Daniel A. Fischer Exxon PRT, NSLS, Brookhaven National Laboratory, Upton, New York 11973
Deborah Holmes Parker Department of Chemistry, University of California a t Irvine, Irvine, California 92717
Shikong Shen Lanzhou Institute of Chemical Physics, The Chinese Academy of Science, Lanzhou, Ghonzhu, The People's Republic of China Received April 10,1991.I n Final Form: September 20, 1991 Carbon monoxide displacement has been observed for hydrogen pressures above 0.01 Torr on both the Pt(ll1) and Ni(100)surfaces. Displacement occurs well below the usual thermal desorption temperatures. 'Crowding" or repulsive interactions between large coverages of coadsorbed atomic hydrogen and carbon monoxide are consistent with the displacement energeticsand kinetics observed. Fluorescence yield near edge spectroscopy (FYNES), a new ultrasoft X-ray method, has been used as an in situ probe of CO surface coverages. Both isothermal and temperature programmed FYNES experiments have been used to characterize hydrogen-induceddisplacement of CO. On the Pt(ll1)surfacein situ isothermaldisplacement experiments in the temperature range 318-348 K clearly indicate that CO displacement is a first-order process with an activation energy of about 11kcal/mol in the presence 0.2 Torr of hydrogen. Activation energies for CO desorption decrease with increasing CO coverage from the 30 kcal/mol value measured in the limit of zero coverage;thus isothermaldesorptionin this same temperature range cannot be described by a simple first-order exponential process. Temperature-programmed FYNES experiments on the Pt(111)surface indicate that at a temperature of 168 K even for hydrogen pressures below 0.2 Torr where CO displacement by hydrogen is not complete, hydrogen removes the (3lI2X 31/2)compression structure observed for the saturated monolayer (0.67 ML) and induces the formation of the well-known (2 X 2) structure (0.5ML) with reduced CO-CO repulsive interactions. On the Ni(100) surface in situ isothermal displacement experiments in the 278-330 K temperature range indicate that CO is rapidly displaced by hydrogen pressures above 1.0 X 10-3 Torr. The displacement rate can be fit by two sequential first-order processes with activation energies of 7 kcal/mol for high CO coverages and 10 kcal/mol for lower CO coverages. These in situ soft X-ray displacement experiments have been independently confirmed by a series of mass spectrometric ex situ isothermal displacement experiments for both the Pt(ll1) and Ni(100) surfaces. Taken together these displacement experiments suggest that hydrogen-induced displacement processes may be general and clearly may play an important role in the transient behavior of catalytic processes run at 'high" pressure. These displacement experiments also highlight the importance of understanding the chemistry of coadsorbed overlayers at extremely high coverages. In situ characterization of coadsorbed and surface reaction systems clearly offers an important general approach for discovering new surface phenomena which occur at high coverage and characterizing and establishing the primary mechanisms which govern these new phenomena.
Introduction The adsorption and interactions of CO and H2 on transition-metal surfaces have received wide spread attention due to the fundamental importance of these molecules as model adsorbates, the importance of catalytic hydrocarbon synthesis based on catalytic reactions of these molecules, and their extensive use to characterize catalytic surfaces. A brief review of hydrogen and carbon monoxide adsorption and coadsorption studies on the Pt(ll1) and "00) surfaces is presented in the first section of this paper. Recent in situ displacement studies on the Pt(111)surface are summarized and in situ studies on the Ni(100) surface are presented. A series of ex situ temperature programmed displacement (TPD) experiments on both the Pt(ll1) and Ni(100) surfaces which confirm the in situ displacement results are summarized. The
primary conclusions and mechanistic proposals from this work are summarized in the final section of this paper. Hydrogen adsorption alone on platinum surfaces has been investigated in detail by several groups.'-3 Hydrogen adsorbs dissociatively on the Pt(ll1) surface and has a saturation coverage of 0.80 monolayer (ML).2 The activation energy for hydrogen desorption is 19 kcal/mol in the limit of zero coverage.' CO adsorption of Pt(ll1)has also been studied e~tensively.~*CO is adsorbed molec(1) Poelaema,B.; Mechterscheimer, G.;Comsa, G . Surf. Sci. 1981,111, 519. (2) Chrietmann, K.; Ertl, G.;Pignet, T. Surf. Sci. 1976, 54, 365. (3) Norton, P. R.; Richards, P. J. Surf. Sci. 1974, 44, 129. (4)Ertl, G.; Neumann, M.; Streit, K. M. Surf. Sci. 1977,64, 393-410. (5) Krebs, H. J.; Luth. H. Appl. Phy8. 1977, 13, 147. (6) McCabe, R. W.; Schmidt, L. D. Surf. Sci. 1977,66,101. (7) Poelaema, B.; Palmer, R.; Comsa, G. Surf. Sci. 1984, 136, 1. (8)Norton, P. R.; Goodale, J. W.; Selkirk, E. B. Surf. Sci. 1979,83,189.
0143-1463/91/24O7-2514$02.50/0 0 1991 American Chemical Society
H-CO Interactions on Metal Surfaces
Langmuir, Val. 7, No. 11, 1991 2575
CO/Pt(1 1 1)
Figure 1. Transient isothermal experiments performed on Pt(ll1) at 318, 328, 338, and 348 K. The left panel shows isothermal desorption CO performed in vacuum. The right panel shows isothermal desorption (displacement)of CO in the presence of 0.2 Torr hydrogen. The dotted lines in both panels are single exponential fits of the form Bt = KO+ K1 exp(-t&). ularly on Pt(ll1) with an activation energy for desorption of 31 kcal/mol at low coverage4-' which decreases sharply above Bco = 0.50 ML where compression of the adlayer begins and repulsive interactions become important.4 Coadsorption of CO and Hz on the Pt(ll1) surface has also been studied particularly because of the repulsive nature of the interactions between these adsorbed species and because of the tendency for these coadsorbed species to form islandsSg Bernasek et a1.10 performed thermal energy atom scattering (TEAS) experiments on a Pt(ll1) surface precovered with 8co = 0.22 ML which showed that CO forms islands having a local density of 8co = 0.50 ML on the hydrogen saturated surface a t 180 K. This CO islanding induced by coadsorbed hydrogen has also been studied by infrared reflection absorption spectroscopy (IRAS)," which confirms the results of Bernasek et al.1° and suggests an optimum temperature of 150 K for the growth of large CO islands. Direct repulsive interactions in the range of 4-6 (kcal/mol)/mol have been observed between the coadsorbed species.ll Carbon monoxide and hydrogen have also been studied extensively on the Ni(100)1z-17surface. CO is adsorbed with a heat of adsorption of 30 kcal/mo115on the Ni(100) surface. Increasing repulsive interactions between adsorbed CO molecules results in decreasing heats of adsorption with increasing CO coverage. Hydrogen has a heat of adsorption of 23 kcal/mol on the Ni(100) surface.16J7 The interaction between coadsorbed hydrogen CO and CO is quite complex on the Ni(100) ~urface.~~J8Jg adsorbed on a hydrogen presaturated surface desorbs at about 210 K, about 100 K lower than CO desorption from the clean surfaces; this suggests that a weakened CO-Ni (9) Thrush, K. A.; White, J. M. Appl. Surf. Sci. 1985, 24, 157. (10) Bernasek, S.L.; Lenz, K.; Poelsema, B.; Comsa, G. Surf. Sci. 1987, 183, L319. ( 1 1 ) Hoge, D.; Tiishaus, M.; Bradshaw, A. M. Surf. Sei. 1988, 207, L935. (12) Tracy, J. C. J. Chem. Phys. 1971,56, 2736. (13) Sanderson, S. Solid State Commun. 1977,21, 75. (14) Mitchell, G. E.;Gland, J. L.;White, J. M. Surf. Sci. 1983,131,167. (15) Christman, K.; Schober,O.; Ertl, G.;Neumann, M. J. Chem.Phys. 1974,60,4528. (16) Froitzheim, H.; Hopster, H.; Ibach, H.; Lehwald, S. Appl. Phys. 1977, 13, 147. (17) Lapujoulade, J.; Neil, K. S. Surf. Sci. 1973, 35,288. (18) Goodman, D. W.; Yates, J. T.; Madey, T. E. Surf. Sci. 1980, 93, L135. (19) Koel, B. E.; Peebles, D. E.;White, J. M. Surf.Sci. l981,107,L367.
interaction can occur in the presence of a large amount of coadsorbed hydrogen.
In Situ Studies of Carbon Monoxide Displacement Fluorescence yield near edge spectroscopy (FYNES) was used as an in situ monitor of the CO coverage both with and without hydrogen. FYNES is a photon-in/photonout method which allows near-edge X-ray absorption fine structure (NEXAFS) spectra to be obtained in situ under high pressures of reactive gases.20 We independently verified that the CO ?r intensity on both the Pt(ll1) and Ni(100) surfaces is proportional to CO coverage. The in situ capability of FYNES to monitor the ?r* resonance of chemisorbed CO as a function of time a t various crystal temperatures and pressures of hydrogen was used in order to obtain the surface CO concentration as a function of time in the presence of substantial hydrogen overpressures.
Isothermal Displacement Experiments on the Pt(ll1) Surface A series of in situ isothermal displacement experiments in the 318-348 K temperature range are shown in the right panel of Figure 1 which indicate that for hydrogen Torr essentially all of the pressures greater than 2 X CO can be removed from the surface with a constant activation energy of about 11kcal/mol. In the absence of hydrogen, CO desorption activation energies in the 15-20 kcal/mol range are expected for coverages of 0.64 ML. The activation energy should increase rapidly as CO desorbs because of the well-established increase in the desorption activation energy with decreasing coverage. Thus, as illustrated in the left panel of Figure 1,isothermal desorption cannot be described by a simple first-order exponential process. These experimental results clearly indicate that substantial interactions between coadsorbed CO and hydrogen are modifying the removal mechanism for adsorbed CO in the presence of hydrogen pressures. The results illustrated in Figure 1clearly show that for all temperatures, the rate of CO removal is greater in the (20) Gland, J. L. Surface Kinetics with Near-Edge X-ray Absorption Fine Structure, Chemistry and Physics of Solid Surfaces VII; Vaneelow, R., Howe, R. F., Eds.; Ser. Surf. Sci. 10 (Chem. Phys. Solid. Surf. 7), 1988; pp 221-242.
Gland et al.
2576 Langmuir, Vol. 7, No. 11, 1991
Isothermal Displacement 0.2 Ton Hydrogen
TP FYNES CO Displacement Pt(ll1)
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.
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.
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00.
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Figure 2. An Arrhenius plot for the rate constants determined from the exponentialfits shown in Figure 1. The slope gives an activation energy of 10.9 kcal/mol. presence of hydrogen than in vacuum and also that essentially all the CO can be removed in the presence of hydrogen. Each displacement and desorption experiment in Figure 1 has been fit to a single exponential function of the form et = KO+ K1 exp(-Kzt), the integrated rate expression for a first-order reaction. The hydrogen displacement data are fit nicely by this integrated rate equation. The value of KOwas fixed at 0.03 ML (the CO coverage remaining at infinite time measured from the experiment performed in the presence of hydrogen. K1 is the initial coverage, and K2 is the energy-dependent Arrhenius term. The fits for the vacuum desorption data in the right panel are quite poor, because the activation energy of desorption of CO from Pt(ll1) is a strong function of ~overage.~ Additionally, isothermal desorption in vacuum at a particular temperature only removes a small amount of CO because of the increase in the activation energy as the coverage drops. Essentially all of the chemisorbed CO can be displaced in the presence of hydrogen because the activation energy remains constant until very small CO coverages are reached (-3% ML). Figure 2 shows the Arrhenius plot for the rate constants determined from the exponential fits to the displacement data shown in Figure 1. An activation energy of 10.9 kcal/ mol is obtained for the isothermal displacement of CO by 0.2Torr hydrogen. A mechanistic proposal consistent with these kinetic results is presented followinga brief summary of the temperature-programmed displacement experiments.
Temperature Programmed Displacement Experiments on the Pt(ll1) Surface Figure 3 shows in situ TP FYNES data for CO on the Pt(ll1) surface in vacuum and for a series of increasing hydrogen pressures (0.001, 0.01, and 0.1 Torr). The vacuum experiment shows a sharp drop in the CO coverage with the maximum desorption rate occurring at 399 K. This corresponds to the thermal desorption of CO from the clean Pt(ll1) surface.14 When the hydrogen pressure is increased to 0.01 Torr, a new low-temperature CO displacement channel is observed with a maximum rate a t 168K. Under these experimental conditions (0.01 Torr H2,0.5 K / s heating rate), one-fourth of the saturated CO monolayer is lost through low-temperature displacement, lowering the surface CO coverage from OcO = 0.64 ML to 0.5 ML. As the temperature increases further, the remaining CO is displaced from the surface with a maximum rate at 386 K, 13 K lower than for the vacuum
400
600
I
8M)
Temperature (K)
Figure 3. A series of temperature programmed fluorescence yield near edge spectroscopy (TP FYNES) experiments illustrating the effect of hydrogen pressure on CO removal from the Pt(ll1)surface. Note the new CO removal process which begins at 130 K for hydrogen pressure above 10-*Torr. desorption case. When the hydrogen pressure is increased to 0.1 Torr, the thermal conductivity of the room temperature hydrogen gas causes the temperature of the sample to increase to 200 K. Thus, the displacement of 0.14 ML of CO occurs during H2 introduction and the starting coverage is 8co = 0.5 ML. This curve for 0.1 Torr Hz exhibits an inflection point a t 386 K, as in the 0.01 Torr hydrogen case. The activation energy for this hydrogen-induced CO displacement can be estimated using the method of Redheadsz1 Assuming a preexponential factor of 1013s-l and a reaction order of 1,we estimate that the activation energy for this process is 10.6 kcal/mol. The amount of CO displaced, from 8co = 0.64 ML to 0.5 ML, corresponds to the amount present in the compressed layer. Thus, it appears that the low-temperature displacement under the conditions of these TP FYNES experiments corresponds to the removal of the CO compression structure.
Carbon Monoxide Displacement Mechanism on the Pt(ll1) Surface Hydrogen-induced displacement of carbon monoxide from the Pt(ll1)surface is consistent with simple energetic arguments. The heat desorption of CO is strongly dependent on the surface coverage.' Hydrogen will displace more weakly bound CO under conditions where repulsive interactions in the adsorbed layer (CO(,)-CO(a), CO(,)H(a))reduce the desorption energy for CO to a value smaller than the heat of adsorption for hydrogen. Based on the predominance of nearest neighbor interactions, and using the 4-6 kcal/mol value for the CO-H repulsion energy estimated by Bernasek et al.lo in vacuum coadsorption studies, the lowering of the activation energy for desorption (displacement) from 26 to 11kcal/mol suggests that two to three hydrogen atoms are involved in the displacement of each CO molecule. We propose that under isothermal conditions, hydrogen overpressures are inducing formation of CO islands with local coverages near the saturation value of 0.64 coverage because of repulsive interactions in the coadsorbed overlayer. Thus as displacement proceeds, the coverage of hydrogen increases to maintain the densely packed CO islands which contain CO molecules with a heat of adsorption of about 11kcal/ mol. In the absence of hydrogen, desorption of a small amount of CO increases the heat of adsorption, rapidly ~~
(21)Redhead, P. A. Vacuum 1962,12,203.
H-CO Interactions on Metal Surfaces
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I
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Figure 4. A set of in situ displacement experiments for the Ni(100) surface at 0.1 Torr hydrogen pressure. These surface
transients were determined with fluorescence yield near edge spectroscopy as a real-time monitor of CO displacement. causing a rapid decrease in the desorption rate. Temperature programmed FYNES experiments on the Pt(111)surface indicate that a t a temperature of 168 K even for hydrogen pressures below 0.2 Torr where CO displacement by hydrogen is not complete, hydrogen removes the (3Il2 X 3Il2) compression structure observed for the saturated monolayer (0.67 ML) and induces the formation of the well-known (2 X 2) structure (0.5 ML) with reduced CO-CO repulsive interactions. This also corresponds to the temperature for the hydrogen-induced onset of CO mobility reported by Lenz et al.22 On the basis of several structural studies in agreement with the results presented here, a CO coverage of 0.5 ML seems to have special stability in the presence of hydrogen. Lower coverages coalesce to form 0.5 ML local coverages in the presence of hydrogen,IOand higher coverages are removed at 130 K to form an overlayer containing 0.50 ML of CO. Taken together these data strongly support our contention that the low temperature displacement process on Pt(ll1) is caused by destabilization of the compressed CO monolayer in the presence of hydrogen pressures above 0.01 Torr.
In Situ Studies of Carbon Monoxide Displacement on the Ni(100) Surface Hydrogen-induced displacement of CO from the Ni(100) surtace has been characterized using a series of isothermal transient FYNES experiments similar to those discussed in the previous section for the Pt(ll1) surface. As shown in Figure 4 chemisorbed CO is rapidly displaced in the presence of hydrogen in the 288-308 K temperature range. On the Ni(100) surface, displacement begins well below the temperature where substantial desorption begins even for hydrogen pressures as low as lo+ Torr. As indicated by the nonlinear behavior of the log coverage versus time plots, the displacement kinetics are also more complexontheNi(100) surfacethanonthePt(ll1)surface. The displacement rate can be modeled accurately by two sequential first-order displacement processes which operate in the high and low CO coverage regimes. This model for the displacement rate seems physically reasonable since CO forms a compression structure at high CO coverage with substantial CO-CO repulsions, leading to decreased desorption energies at high coverage. An Arrhenius plot based on this simple sequential model is shown in Figure 5. Displacement in the high coverage regime occurs with an activation energy of 7 kcal/mol, while displacement of CO for coverages below 0.4 ML results in an activation (22) Lenz, K.; Poelsema,B.;Bernaaek,S. L.;Comsa,G. Surf. Sci. 1987, 183, L319.
s t
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Figure 5. An Arrhenius plot for displacement of CO from the
Ni(100)surfaceat0.1Torr,0.013Torr,and0.0013Torrhydrogen pressure. Two sequential first-order reactions have been used to model the displacement reaction for high and low CO coverage. energy of 11 kcal/mol. Thermal desorption activation energies are in the 25-30 kcal/mol range for the Ni(100) surface. The displacement rate was approximately half order in hydrogen pessure over the to 10-1 Torr pressure range suggesting that adsorbed atomic hydrogen is in equilibrium with gas phase hydrogen over this pressure and temperature range. This result is consistent with the expected result that adsorbed atomic hydrogen is the primary hydrogen-containing surface species involved in hydrogen-induced displacement of adsorbed CO.
Ex Situ Temperature Programmed Studies of Carbon Monoxide Displacement The data presented in Figure 6 illustrate the key aspects of a typical TPD displacement experiment. After the Ni(100) surface was exposed to 2 langmuirs of Cl80 at 309 K the reference C180 spectrum (0 time) shown in the left panel was obtained. After a second 2-langmuir exposure Torr of at 309 K, the surface was exposed to 1.6 X hydrogen for 600 s. During the hydrogen exposure 65 %5 of the C180 was removed from the surface as shown by the second spectrum in the left panel. This spectrum was taken after a 4-min evacuation period to remove the gasphase hydrogen from the system. Relative coverageswere determined by comparing the integrated area with the reference spectrum obtained from a saturated surface at the same temperature. The Auger spectrum in the right panel illustrates that the surface was initially clean. After displacement, the surface remains clean. Only undisplaced CO and hydrogen remain on the surface. The CO desorption spectra from a series of displacement experiments at 309 K are shown in the left panel of Figure 6. The rate of displacement was determined by running a series of displacement reactions for different time periods and plotting the CO coverage as a function of displacement time. The right panel presents all the data from several series of displacement experiments at 309, 320, and 330 K. The thermal activation energies estimated from are about 8 f 2 kcal/mol in the high coverage region (the
2518 Longmuir, Vol. 7, No.11, 1991
Gland et al.
Ni(1OO) 2L CO" T. = 309K Dispbnwnt 1.6 x 10') torr H2309K 1
1
TDS Displacement
1
0.9
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-
-
a
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0.0
I
I
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Time (wc)
Temperature (K)
Figure 6. A representative series of CO TDS spectra following displacement of adsorbed CO by 1.6 X Torr hydrogen for the indicated time periods. The right panel presents the logarithm of CO coverage versus displacement time. The two linear regions observed suggest that CO displacements is first order in CO coverage. PI (111) Dlepiecrmnt and Derorptlon at 328K 6L Cola Ta-328K HI Displacement 2x10-* tow 1
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/Ill 400
a500
Figure 7. A representative series of CO TPD spectra from the Pt(ll1) surface illustrating CO isothermal desorption at 328 K (lower right panel) and CO displacementby hydrogen at 328 K (lower left panel). The rates of isothermal desorption and displacement are compared directly in the semilog plot in the upper panel. initial rate) and 12 f 1kcal/mol in the low coverage region (less than 0.3 to 0.4 monolayers of CO).23
Conclusions Carbon monoxide displacement has been observed for hydrogen pressures above 0.01 Torr on both Pt(ll1) and Ni(100)surfaces. Displacement occurs well below the usual (23) Shen, S.; Zaera, F.; Fischer, D. A.; Gland, J. L. J . Chem. Phys.
1988,89,590.
thermal desorption temperatures. "Crowding" or repulsive interactions between large coveragesof coadsorbed atomic hydrogen and carbon monoxide are consistent with the displacement energetics and kinetics observed. Previous coadsorption studies for lower hydrogen coverages adsorbed at low temperature have established that coadsorbed overlayers of CO and adsorbed atomic hydrogen form segregated islands on the Pt(ll1)surface, while mixed overlayers are formed on the Ni(100) surface. It is interesting to speculate that the requirement for higher hydrogen pressures to initiate CO displacement on the Pt(ll1)surface compared to the Ni(100) surface may be related to the segregation of the coadsorbed CO and hydrogen. On the Pt(ll1)surface in situ isothermal displacement experiments in the temperature range 318-348 K clearly indicate that CO displacement is a first-order process with an activation energy of 10.9 kcal/mol in the presence 0.2 Torr of hydrogen. In the absence of hydrogen, isothermal desorption cannot be modeled by a simple first-order exponential process, reflecting the increase in the desorption activation energy with decreasing coverage. Temperature programmed FYNES experiments on the Pt(ll1) surface indicate that even for hydrogen pressures below 0.2 Torr where CO removal is not complete, hydrogen displaces the CO coverage from 0.67ML for the saturated compressed monolayer to 0.50 ML for the well-known (2 X 2) structure with reduced CO-CO repulsive interactions. On the Ni(100) surface in situ isothermal displacement experiments in the 278-330 K temperature range indicate that CO is rapidly displaced by hydrogen pressures above 1.0 X 10-3 Torr. The displacement rate can be fit by two sequential first-order processes with an activation energies of 7 kcal/mol for high CO coverages and 10 kcal/mol for lower CO coverages. In order to independently confirm these in situ synchrotron based displacement experiments, a series of interrupted isothermal displacement experiments were also performed on both the Pt(ll1) and Ni(100)surfaces. The amount of CO remaining on the surface after a specific displacement time was determined using
H-CO Interactions on Metal Surfaces carefully calibrated integrated temperature programmed desorption results. All of these experimentalresults clearly indicate that there are substantial interactions between coadsorbed CO and hydrogen, which modify the removal mechanism for adsorbed CO in the presence of hydrogen. These experiments emphasize the importance of understanding the chemistry of coadsorbbd overlayers in the presence of reactive atmospheres. These results illustrate the importanceof in situ methods capable of performing kinetic experiments on wellcharacterized adsorbed monolayers on single crystal
Langmuir, Vol. 7, No. 11, 1991 2579 surfaces. High coverages of coadsorbed species resulting from high pressures of the coadsorbate substantially modify surface coverages of chemisorbed species, may affect desorption activation energies, and may provide new kinetic pathways available for adsorbed species. These observations play an important role in understanding surface reactions which occur at high pressure. Registry No. Hg,1333-74-0; CO,630-08-0;Pt,7440-06-4; Ni, 7440-02-0.
