Catalytic Hydrogenation of Acetylene on Ni(111) - American Chemical

11480

J. Phys. Chem. B 2001, 105, 11480-11492

Catalytic Hydrogenation of Acetylene on Ni(111) by Surface-Bound H and Bulk H K. L. Haug, T. Bu1 rgi, M. Gostein, T. R. Trautman, and S. T. Ceyer* Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 ReceiVed: May 22, 2001; In Final Form: August 31, 2001

The reactions of hydrogen atoms adsorbed on a Ni(111) surface (surface-bound H) and hydrogen atoms just below the surface (bulk H) with coadsorbed acetylene are probed under ultrahigh vacuum conditions. Bulk H is observed to react with acetylene upon emerging onto the surface at 180 K. Gas-phase hydrogenation products, ethylene and ethane, are produced as well as an adsorbed species, ethylidyne. Ethylidyne is identified by high-resolution electron energy loss spectroscopy. Surface-bound H reacts with adsorbed acetylene above 250 K to produce a single product, adsorbed ethylidyne. No gas-phase hydrogenation products, such as ethylene or ethane, are observed. The reaction of surface-bound H is extremely slow, with a rate constant determined from measurements of the initial reaction rate to be in the range of 10-5-10-3 (ML s)-1 for a temperature range of 250-280 K. The activation energy for the rate-determining step, which is shown to be the addition of the first surface-bound H to acetylene to form an adsorbed vinyl species, increases from 9 to 17 kcal/mol as the total coverage decreases from 0.92 to 0.74 ML. The reaction rate cannot be described by a simple first-order dependence on the coverage of either reactant, indicating the presence of strong interactions between reactants. Measurements of the equilibrium constant reveal strong interactions between the reactant surface H and the product ethylidyne, possibly resulting in island formation. Mechanisms for the formation of ethylidyne by the reactions of both surface-bound and bulk H are proposed, as well as mechanisms for the formation of ethylene and ethane by bulk H. The different product distributions resulting from the reaction of acetylene with the two forms of hydrogen are discussed in terms of the large energy difference between bulk and surface-bound H.

I. Introduction The catalytic hydrogenation of unsaturated hydrocarbons over transition metal catalysts, particularly Ni, is a commercial process in the production of fine chemicals and pharmaceuticals and in the hydrogenation of unsaturated fats and nitriles. As a result, the interactions of the simplest unsaturated hydrocarbons, ethylene and acetylene, with hydrogen on single-crystal transition metal surfaces have been studied for a very long time. In fact, an attractive and straightforward mechanism for ethylene hydrogenation, known as the Horiuti-Polanyi mechanism, has long been considered the operable mechanism.1 The mechanism depicts the carbon-carbon bond of C2H4 to be parallel to the surface so that C2H4 forms two bonds to the surface through the two carbon atoms. Eventually, a coadsorbed H atom migrates up to C2H4 and reacts, forming a “half-hydrogenated” species which then reacts with a second adsorbed H to form volatile ethane. Recently, the universality of this mechanism has been called into question because a coadsorbed layer of C2H4 and H on Ni(111), formed by exposure to 10-4 Torr of C2H4 and H2 in an UHV environment, does not react to form ethane,2-9 even though C2H4 adsorbs with the geometry depicted by the HoriutiPolanyi mechanism.10-12 Instead, hydrogen atoms that are initially beneath the surface, called bulk H, are observed to be the reactive species in hydrogenating C2H4 to C2H6.2 Bulk H reacts with the C2H4 as it emerges onto the surface. Hydrogen adsorbed on the surface, called surface-bound or surface H, is unreactive for the hydrogenation of ethylene. The rapid reactivity of the bulk H is rationalized in terms of its direction of approach to C2H4. As the bulk H emerges, it * Author to whom correspondence should be addressed.

approaches the adsorbed C2H4 from underneath and from a direction in which the rehybridized π orbitals of C2H4 are oriented. This approach, in contrast to that of a surface-bound H atom, maximizes the overlap between the reacting hydrogen atom and the π orbitals and minimizes the Pauli repulsion between the reacting hydrogen atom the hydrogen atoms of C2H4. Indeed, the approach of a H atom from above and below the C2H4 molecular plane is the minimum energy path along the potential energy surface for hydrogen addition to C2H4 in the gas phase.13 But in addition to the lower barrier for hydrogenation that is encountered by a bulk H as compared to that encountered by a surface-bound H, the emerging bulk H atom can be as much as 24 kcal/mol more energetic than a surface-bound H atom.14,15 Its greater potential energy is the result of its weaker binding to the Ni atoms in the bulk and the necessity for it to surmount the barrier at the bulk-surface interface. The relative potential energies are discussed in more detail below, but the fact that a bulk H atom can be 24 kcal/ mol more energetic than a surface-bound H atom means that there will be reaction channels open to bulk H that are closed to surface-bound H. The interaction of bulk H and surface-bound H with C2H2 adsorbed on Ni(111) is an interesting test case of both the importance of a direct approach of the hydrogen to the π orbitals of an adsorbed, unsaturated hydrocarbon and the role of the different energetics of the two kinds of hydrogen.16 Acetylene has two mutually perpendicular π orbitals. Like adsorbed ethylene, its C-C bond is parallel to the surface17 and one of acetylene’s rehybridized π orbitals is directed toward the metal.18 Thus, bulk H atoms are easily accessible to this π orbital as they approach C2H2 from below upon emerging from the bulk.

10.1021/jp0119734 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/27/2001

Catalytic Hydrogenation of Acetylene on Ni(111) Unlike adsorbed ethylene, C2H2 has a second rehybridized π orbital that lies approximately parallel to the surface.18 This latter π orbital is accessible to surface-bound H atoms. Therefore, if the direction of approach of the H atom to the π orbitals is a factor in the hydrogenation activity and if the surface H is sufficiently energetic to overcome the barrier to the reaction from this direction of approach, then surface-bound H should be reactive with adsorbed C2H2. Indeed, the reaction of adsorbed C2H2 with surface-bound H to form ethylidyne, CCH3, has been noted previously on Pt,19,20 Rh,21 and Pd22,23 (111) surfaces. In addition, a coadsorbed layer of C2H2 and surface-bound H on Pd(111) has been observed to react to form gas-phase C2H4.24 It therefore seems reasonable to expect reactivity of a surfacebound H with C2H2, in contrast to its unreactivity with C2H4. Given that the energetics of the bulk H atom and the surfacebound H atom are so different, it also seems reasonable to expect the products of the reaction of surface-bound H to be distinct from the products of the reaction of bulk H. These expectations are borne out in the results of the experiments presented here.16 Section III describes the reaction, initiated by raising the crystal temperature, of a layer of C2H2 and surface-bound H coadsorbed on Ni(111). The formation of gas-phase products is explored by monitoring the partial pressures of the desorbing products with a quadrupole mass spectrometer while the formation of adsorbed products is probed using a vibrational spectroscopy, high-resolution electron energy loss spectroscopy. Once the products are identified, the reaction is studied quantitatively through measurements of the equilibrium constant as well as the reaction rate. The rate-determining step of the reaction is identified and a reaction mechanism is proposed. The reaction of bulk H with adsorbed C2H2, discussed in Section IV, is similarly explored, once the synthesis of bulk H in the absence of surface-bound H has been effected. Section V compares the reaction of bulk H and surface H with C2H2 and discusses their different mechanisms and, consequently, their different product distributions in terms of the vastly different energetics of the two kinds of hydrogen atoms. II. Experimental Section A. Apparatus and Reactants. The apparatus, which consists of a triply differentially pumped, molecular beam source precisely coupled to a UHV vacuum chamber, has been described in detail.25-28 The Ni(111) crystal, oriented to within 0.2°, is mounted on the end of a liquid N2 cryostat that is attached to a manipulator in the UHV chamber. The chamber is equipped with a cylindrical mirror analyzer for Auger spectroscopy, a quadrupole mass spectrometer, and a highresolution electron energy loss spectrometer. The elastically scattered, 6.5 eV electron beam in the EEL spectrometer typically has a full width at half-maximum (fwhm) of 37-48 cm-1 and an intensity of (1-4) × 105 counts/s. Spectra are measured with a channel width of 16 cm-1. Measurements of spectra in the off-specular direction are achieved by rotation of the crystal with respect to the incident electron beam and the analyzer. The crystal is heated to temperatures above 500 K by electron bombardment and radiative heating of the backside of the crystal.29 Temperatures up to 500 K are attained solely by radiative heating. Temperatures between 120 and 280 K are achieved either by flowing cold N2 gas through the cryostat, by a combination of radiative heating and the flow of cold N2 gas, or by radiative heating in the presence of liquid N2 in the cryostat. Care is taken in these experiments to ensure that no gas-phase hydrogen atoms (or ions) are produced by thermal

J. Phys. Chem. B, Vol. 105, No. 46, 2001 11481 dissociation of H2 on and then H atom desorption from the crystal heating filament. The equilibrium constants and rates are measured with temperature stabilities of (0.5 K. The temperature is measured with a chromel-constantan thermocouple spot-welded to the crystal. Gases and gas mixtures are handled in a bakeable manifold. The C2H2 (99.6% purity from MG Industries), C2D2 (99% purity from Cambridge Isotope Laboratories), C2H4 (99.95% purity from MG Industries), C2D4 (98% purity from Cambridge Isotope Laboratories), are mixed with Ar and are introduced into the UHV chamber as a beam. Ethylene and C2D2 are used as received without further purification. The C2H2 is passed through a dry ice/acetone trap to remove the acetone that is present to stabilize the high pressure of C2H2 in the cylinder. Hydrogen (99.9999% purity from MG Industries) and deuterium (99.8% purity from Cambridge Isotope Laboratories) are passed through a liquid N2 trap. B. Procedures for Reactions of Surface-Bound H. In the experiments that probe the identity of the adsorbed product, namely ethylidyne, the deuterated species, C2D2 and D2, are used as the reactants. Unlike the spectrum of adsorbed CCH3 and H, the most intense feature in the CCD3 product vibrational spectrum, the C-C stretching mode at 987 cm-1, is clearly resolved in the deuterated system. This clear separation from other features allows an unambiguous measurement of its intensity and hence a quantitative determination of the CCD3 coverage. The error bars shown on the features of all spectra are approximately (7% of the absolute intensity. This uncertainty represents the 90% confidence limit resulting from six measurements of the intensity of the same loss feature. Since an absolute intensity measurement is sensitively dependent on the crystal position and the tuning of the spectrometer, each measurement was made after removing the crystal from the spectrometer and then replacing it. Between each measurement, the spectrometer was retuned minimally to maximize the intensity. In some experiments shown here, adsorbed C2D2 is prepared by heating 0.25 ML of adsorbed C2D4 to 270 K at a rate of 2 K/s. Adsorbed C2D4 decomposes cleanly to form adsorbed D and C2D2.3-6,30 In other experiments, the adsorbed C2D2 is prepared by exposing the Ni surface to a beam of 2% C2D2 seeded in Ar. The results are independent of the method of preparation of the adsorbed C2D2. The initial C2D2 coverages, ΘiC2D2, are determined from measurements of the absolute carbon coverage. The carbon coverage is measured by comparing the ratio of the C(272 eV) to Ni(848 eV) Auger signals to that measured from a (2 × 2) ordered layer of C2D4 (0.5 ML of C) at 80 K,31 as observed by electron diffraction. The precision of ΘiC2D2, determined from the standard deviation of a linear least-squares fit to measurements of the C/Ni ratio of Auger signals vs C2D2 exposure, is (2.5%. The surface-bound D is prepared by exposure of the crystal at 220 K to between 5400 and 18 000 L of D2, by increasing the D2 partial pressure in the vacuum chamber. The initial D coverage, ΘiD, is based on a measurement of the integrated partial pressure of D2 measured in a thermal desorption trace. The integrated partial pressure is calibrated against that measured from the recombination and desorption of 1 ML of adsorbed D. The calibrated monolayer of D, which is the saturation coverage at 80 K, is prepared by exposure of the Ni(111) crystal to gas-phase D atoms. This exposure produces bulk D, as described in the next section, as well as surface-bound D. The bulk D is then thermally desorbed by heating the crystal to about

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270 K. The deuterium that remains is exactly 1 ML of surfacebound D.14 However, the D2 partial pressure arising from a thermal desorption trace of the reactant layer of coadsorbed D and C2D2, labeled PiD2/D+C2D2, is not solely related to the initial D coverage. It has a contribution from the dissociation of C2D2. This contribution, labeled PiD2/C2D2, is determined by integrating the D2 partial pressure resulting from the thermal decomposition of a neat layer of C2D2 at a given coverage. The initial D coverage is proportional to

ΘiD ∝ PiD2/D+C2D2 - PiD2/C2D2

(1)

The precision of ΘiD based on the standard deviation of at least 7 measurements of the initial D coverage is (5%. C. Procedures for Reactions of Bulk H. Bulk H is synthesized by exposure of Ni(111) at 120 K to gas-phase H atoms.14,32-34 The H atoms are produced by dissociating H2 on a hot tungsten filament placed 0.25 in. in front of the crystal. The filament is heated resistively to about 1800 K while the H2 partial pressure in the chamber is held at 5 × 10-6 Torr during the H atom exposure. The H atom flux is estimated as 3.7 × 1014 H atoms/cm2 s.35 Bulk H is known to emerge onto the surface, recombine, and desorb between 170 and 220 K as the crystal temperature is raised at 2 K/s.14 The amount of bulk H is measured in equivalent ML by comparison of its integrated partial pressure measured in a thermal desorption trace to that from 1 ML of surface-bound H. III. Results, Analysis, and Discussion A. Reaction of Surface-Bound D with C2D2. 1. Adsorbed Reaction Product. a. Identification. The reactivity of adsorbed C2D2 and surface-bound D is explored using the following procedure. A coadsorbed layer of 0.17 ML of C2D2 and 0.5 ML of surface-bound D is prepared on Ni(111) at 80 K. The vibrational spectrum of this coadsorbed layer is the bottom spectrum shown in Figure 1a. The features at 460, 540, 640, 890, 1090, and 1190 cm-1 are assigned to the C-Ni symmetric stretch, the symmetric and antisymmetric C-D out-of-plane bend, the symmetric and antisymmetric C-D in-plane bend, and the C-C stretch modes of C2D2, respectively.26,36,37 The features labeled in italics at 700 and 850 cm-1 are the antisymmetric and symmetric D-Ni stretch modes, respectively.14 The feature at 267 cm-1 arises from the longitudinal S2 phonon mode that becomes observably dipole active when an adsorbate is present on a Ni(111) surface.38 The frequencies of either adsorbate’s modes are unaffected by the presence of the other coadsorbate. This coadsorbed layer is heated at a rate of 2 K/s to a temperature indicated in Figure 1, and then cooled at ∼2 K/s to 80 K, the temperature at which the spectra in Figure 1 are measured. Quenching to 80 K does not change the relative coverages of reactants, as discussed below. In the spectrum labeled 300 K, a new mode at 987 cm-1 is present. The intensity of this mode increases after raising the temperature momentarily to 320 and to 340 K but then decreases after momentary temperature increases beyond 340 K. After raising the temperature momentarily to 400 K, the mode at 987 cm-1, the symmetric D-Ni stretch mode at 850 cm-1, as well as the antisymmetric D-Ni stretch mode at 700 cm-1, which is not shown in Figure 1, are no longer visible. The recombination and desorption of D2 and the disappearance of the new mode at 987 cm-1 occur simultaneously between 380 and 400 K. The spectrum labeled as 400 K is that of C2D2, as will be seen more

Figure 1. (a) Vibrational spectra of 0.17 ML C2D2 and 0.5 ML surface D heated at 2 K/s to temperatures indicated and then quenched to 80 K. Spectra measured at 80 K and 6° off specular. (b) Vibrational spectrum of 0.17 ML C2D2 and 0.5 ML surface D heated at 2 K/s to 280 K, maintained at 280 K for 30 min. Spectrum measured at 280 K and 10° off specular. Frequencies of surface D modes in italics.

convincingly below. Although these experiments clearly show that a new species has been formed by the reaction of C2D2 with surface D, the amount of the product is low, and hence the spectrum remains dominated by the vibrational features of C2D2 and D. In an attempt to increase the amount of product, a coadsorbed layer of 0.17 ML of C2D2 and 0.5 ML of surface-bound D is heated at 2 K/s to 280 K, but then is held at 280 K for 30 min before measuring the spectrum at 280 K. The resulting spectrum is shown in Figure 1b. Indeed, the intensity of the feature at 987 cm-1 is greater than in the previous experiment where the temperature reached 280 K momentarily, but the spectrum is still dominated by the C2D2 and D modes. Annealing for longer times does not substantially increase the intensity. The necessity to heat the reaction for 30 min is an indication of the very low reaction rate. The low reaction rate is the reason for the failure to observe the reaction between adsorbed C2D2 and surface D in an earlier study.6 To investigate the possibility that surface-bound D is acting like a limiting reagent, and thereby limiting the amount of product, additional deuterium was made available during the reaction via the following procedure. A spectrum of 0.17 ML of C2D2, measured at 280 K, is shown at the bottom of Figure 2. This layer at 280 K is then exposed to 150 L of D2 and annealed at 280 K for 30 min and then a spectrum is measured at 280 K. This sequence of steps is repeated several times, and the corresponding spectra are shown in Figure 2. The new mode at 987 cm-1 continues to grow with increasing D2 exposure while the C2D2 features almost vanish. Figure 3a shows the top

Catalytic Hydrogenation of Acetylene on Ni(111)

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Figure 2. Vibrational spectra of 0.17 ML C2D2 (bottom spectrum) as a function of D2 exposure of this layer at 280 K. Spectra measured after annealing at 280 K for 30 min. Spectra measured at 280 K and 10° off specular.

Figure 3. (a) Vibrational spectra of CCD3 resulting from reaction at 280 K of 0.17 ML C2D2 and 0.5 ML surface D during exposure to 10350 L D2 (top spectrum of Figure 2). Spectrum measured at 280 K and 10° off specular. (b) Vibrational spectrum of coadsorbed CCD3 and surface D from ref 34, measured at 80 K and 6° off specular. Frequencies of surface D and C2D2 modes in italics.

spectrum from Figure 2 resulting from a total D2 exposure of 10 350 L and compares it to the known spectrum of CCD3 coadsorbed with surface D35 in Figure 3b. The product can now be readily identified as ethylidyne, CCD3, by the features at 436, 836, 987, 1152, 2079, 2201, and 2285 cm-1 which correspond to the antisymmetric C-Ni stretch, CD3 bend, symmetric CD3 deformation, C-C stretch, C-H symmetric and antisymmetric stretch, and the C-C stretch overtone modes, respectively. Two other modes at 640 and 700 cm-1, marked in italics, are the antisymmetric C-D out-of-plane bend mode of C2D2,36,37 and the antisymmetric stretch mode of D-Ni.14 The similarity of these two spectra provides clear evidence that CCD3 is the product of the reaction of adsorbed C2D2 and surface-bound D,

C2D2 + D f CCD3

(2)

Using a similar procedure, this reaction is also observed to proceed at temperatures as low as 250 K. Below 250 K, the reaction is too slow to be observable. Above 290 K, the slow recombination and desorption of D2 makes it impossible to push the reaction, by maintaining a high coverage of surface-bound D, far to the product side.

Figure 4. (a) Vibrational spectrum of coadsorbed CCD3 and surface D, prepared as described in text. (b) Spectrum measured after heating layer, whose spectrum is shown in part (a), at 2 K/s to 330 K, quenching to 280 K, annealing at 280 K for 30 min, and then quenching to 80 K. (c) Spectrum measured after heating layer, whose spectrum is shown in part (b), at 280 K for an additional 30 min, and then quenching to 80 K. All spectra measured at 80 K and 10° off specular.

The importance of a high coverage of D to push the reaction far to the product side is also apparent in the following experiment. A spectrum of a layer of mostly CCD3 and surfacebound D, prepared by exposure of 0.17 ML of C2D2 at 280 K to three cycles of 12 000 L of D2 followed by a 30 min anneal at 280 K, is shown in Figure 4a. This layer is heated at 2 K/s to 330 K in order to desorb a small amount of surface D. The amount of surface D desorbed is between 0.03 and 0.08 ML, as determined by integration of the D2 partial pressure during the temperature ramp. The crystal temperature is then lowered at 2 K/s to 280 K, maintained there for 30 min, and then quenched to 80 K. The spectrum of the resulting layer is shown in Figure 4b. The decrease in the intensity of the 987 cm-1 mode of CCD3 and the increase in intensity of the 640 cm-1 mode of C2D2 relative to that in Figure 4a indicates that the reverse reaction, CCD3 f C2D2 + D, has taken place, resulting in a decrease of the amount of CCD3 and an increase in the amount of C2D2. The spectrum measured after annealing this layer at 280 K for an additional 30 min and then quenching it to 80 K is shown in Figure 4c. It reveals that little, if any, CCD3 remains. The reverse reaction has converted all of the CCD3 back to C2D2 and surface D. The ability to push this reaction toward products by increasing the D coverage and to reverse the reaction toward reactants by decreasing the D coverage signals evidence for an equilibrium process. However, there are two peculiarities. First, if eq 2 is a correct description of the reaction, then under the conditions of the experiments in Figure 1, where the initial coverages of C2D2 and surface-bound D are 0.17 and 0.5 ML, respectively, surfacebound D should not have been a limiting reagent. The reaction should have proceeded completely to products, consuming all of the C2D2 present. Second, the reaction is not reversible with temperature. That is, once a layer of CCD3 is formed at 280 K, such as those layers whose spectra are shown in Figure 2, lowering the temperature to 270 or 260 K and annealing the layer at these temperatures for over 120 min does not result in any decrease in the amount of CCD3. The failure to observe the reverse reaction at lower temperatures is not a result of its immeasurably slow rate. The reverse reaction at 260 K is observable in this time frame upon removal of some surfacebound D. Clearly, the D coverage plays what appears to be an overwhelmingly critical role in this reaction. Its critical role signals that this reaction may not be described by a simple equilibrium.


Haug et al.

Figure 6. Ratio of equilibrium coverages, [CCD3]eq/[C2D2]eq, vs equilibrium D coverage, [D]eq, in ML. Error bars described in text. Figure 5. Intensity of 640 cm-1 mode of C2D2 (b), 987 cm-1 of CCD3 (×), and sum of these intensities (9) vs D2 exposure. Coverage of hydrocarbon plotted on right axis. Intensities from spectra in Figure 2. Error bars are shown only on sum for clarity and are explained in Experimental Section.

b. Equilibrium Constant. Experiments were carried out to investigate whether the reaction shown in eq 2 could be described by the simple equilibrium constant,

K)

[CCD3]eq

(3)

[C2D2]eq[D]eq

where [CCD3]eq, [C2D2]eq, and [D]eq represent the coverages in ML at equilibrium. The equilibrium coverage of C2D2 is determined from the intensity of the antisymmetric C-D bend at 640 cm-1, the most intense feature of the C2D2 spectrum. Its intensity is calibrated for coverage by correlating it with the ratio of the C/Ni Auger signals. As discussed in the Experimental Section, the ratio of the C/Ni Auger signals has been calibrated for C2D2 coverage. The intensity of this C2D2 mode is found to be linear with coverage over the range of 0.02-0.2 ML. The equilibrium coverage of CCD3 is determined from the intensity of the CCD3 symmetric deformation mode at 987 cm-1, the most intense feature of its spectrum. Its intensity is calibrated for coverage by anticorrelating it with the intensity of the 640 cm-1 mode of C2D2. Since there is no loss of carbon from the surface during the reaction, as established by measuring the C Auger signal before and after the reaction, the amount of C2D2 that reacts must equal the amount of CCD3 that is formed. The intensities of the 640 cm-1 mode of C2D2 and the 987 cm-1 mode of CCD3 during the course of a reaction are plotted in Figure 5. These data are taken from the spectra shown in Figure 2. It is clear that their intensities are anticorrelated and that the sum of their intensities is essentially a constant as the reaction proceeds, as shown in Figure 5. Since the intensity of the 640 cm-1 mode of C2D2 is linear with coverage, the intensity of the 987 cm-1 mode of CCD3 must also be linear with coverage, given that the sum of their intensities is a constant during the extent of the reaction. Therefore, the sum of the C2D2 and CCD3 coverages is always equal to the initial C2D2 coverage. The equilibrium coverages are then given by

( (

[C2D2]eq ) ΘiC2D2

[CCD3]eq ) ΘiC2D2

I(C2D2)

I(C2D2) + I(CCD3) I(CCD3)

) )

I(C2D2) + I(CCD3)

is the intensity of the 987 cm-1 mode, and ΘiC2D2 is the initial coverage of C2D2. Since one surface-bound D atom is consumed for each CCD3 produced and since no D2 desorbs during the experiment, the equilibrium D coverage is the initial D coverage, ΘiD, minus the equilibrium CCD3 coverage, or

[D]eq ) ΘiD - [CCD3]eq

The equilibrium measurements are carried out by first exposing the crystal to a known coverage of C2D2. The C2D2covered surface at 220 K is then exposed to D2 until a saturation coverage of D is achieved. No CCD3 is formed during the D2 exposure. The initial C2D2 coverages, ΘiC2D2, that were investigated range from 0.03 to 0.19 ML, while the corresponding initial D coverages, ΘiD, range from 0.94 to 0.21 ML. The surface is fully saturated at the beginning of the reaction. No empty sites are available. The crystal with this well-defined coadsorbate layer is then heated to 280 K and held at that temperature for 60 min to attain equilibrium. The crystal is then cooled to 80 K at which temperature an EEL spectrum is measured. No change in the ratio of products and reactants is observed if the crystal is held at 280 K for a longer time, confirming that equilibrium is reached. The results of the equilibrium coverage measurements are plotted as [CCD3]eq/[C2D2]eq vs [D]eq, where all coverages are in ML, as shown in Figure 6. The error bars are determined by propagating the error determined for each coverage or loss intensity measurement, as described in the Experimental Section. If this equilibrium reaction obeys the equilibrium constant expressed in eq 3 above, then the data should lie on a line through the origin, the slope of which will be the value of Keq. It is clear that this system does not obey this expression for the equilibrium constant, which is based on the assumption that the system is ideal, with no interactions between adsorbates. However, the importance of a high D coverage for pushing the reaction to products is very clear from the data plotted in Figures 4-6. The data suggest that there may be a stabilizing interaction between surface D and CCD3. In an attempt to model this interaction within the limits of an ideal system, the reaction is written as C2D2 + 2D T CCD3*D, where CCD3*D represents a weakly bound complex as a result of the stabilizing influence of D on CCD3. The corresponding equilibrium constant is

K) (4)

where I(C2D2) is the intensity of the 640 cm-1 mode, I(CCD3)

(5)

[CCD3*D]eq [C2D2]eq[D]2eq

(6)

The equilibrium concentration of [CCD3*D]eq is taken as that of [CCD3]eq. A plot of [CCD3*D]eq/[C2D2]eq versus [D]eq2, not shown here, is not a straight line, indicating that if there is an

Catalytic Hydrogenation of Acetylene on Ni(111) interaction between CCD3 and D, then this representation of it is too simplistic. The presence of very strong interactions between adsorbates can lead to separation of adsorbates into islands of one kind of adsorbate and islands of another. If islanding is occurring in the present overlayer, then the simple equilibrium expression of eq 3 will not hold because inherent in the derivation of eq 3 is the assumption that the reactants are randomly distributed. The behavior of the plot of [CCD3]eq/[C2D2]eq versus [D]eq in Figure 6 may suggest island formation and in particular suggests a transition between a randomly distributed layer of reactants and products and island formation. That is, the range of [D]eq from 0.2 to 0.8 ML may represent a well-mixed layer where the coverage of the product, [CCD3]eq, is relatively low. Above 0.8 ML, an ordering transition may occur where the product CCD3 is stabilized by the formation of somewhat ordered islands of a mixture of coadsorbed CCD3 and surface D. The increased stability of adsorbed CCD3 in the presence of high surface D coverage results in a large increase of its coverage. The C2D2 reactant may remain around the edges of islands comprised of a mixture of CCD3 and D and react only with the surface D that is at the edges. Such a scenario is made more realistic by the observation of island formation in a similar system, the decomposition of C2H4 to CCH3 on Pt(111).39 Scanning tunneling microscope images recorded during the conversion of C2H4 to CCH3 show an inhomogeneous layer of adsorbates. The images are interpreted as evidence for the decomposition of C2H4 taking place at the edges of CCH3 islands. Measurement of the kinetics of this same reaction are successfully reproduced by a Monte Carlo calculation which in turn predict the observed patchy and nonuniform adsorbate layer.40 Island formation has also been observed during the trimerization of C2H2 adsorbed on Pd(111) using scanning tunneling microscopy.41 These images are interpreted as evidence for the C2H2 molecules trimerizing to benzene at the edge of benzene islands. Unfortunately, no similar quantitative studies of the extent of ethylidyne formation as a function of the acetylene and adsorbed hydrogen coverage have been carried out on other metals. On Ni,42 Pd,22,23 and Rh,21 some ethylidyne formation has been reported upon adsorption of C2H2 in the absence of surface-bound H, but the presence of surface-bound H is observed to greatly increase the amount of CCH3 formed. On Pt,19 coadsorption of surface-bound H with C2H2 is necessary for the formation of CCH3. c. Rate Measurements. The rate of the reaction in eq 2 is sufficiently slow for the initial rate of CCD3 formation to be measured. The procedure for the rate measurement is as follows. A known coverage of C2D2 is adsorbed on the crystal at 80 K. The remaining surface sites are filled with surface-bound D by exposure of the crystal at 220 K to 18 000 L of D2. Control experiments show that no CCD3 is formed and no C2D2 is desorbed during exposure to D2. The adsorbed layer is heated at ∼1 K/s to the reaction temperature, between 250 and 280 K, for a specific time, and cooled at ∼2 K/s to 80 K. As discussed earlier, the reverse reaction does not occur as the temperature is lowered. A vibrational spectrum is measured to obtain the absolute coverage of CCD3 from the intensity of its 987 cm-1 mode. The crystal is then heated to the reaction temperature for an additional time increment, cooled to 80 K, and then another vibrational spectrum is measured to monitor the progress of the reaction. Figures 7 and 8 show slices of a series of spectra as a function of reaction time measured for three temperatures for initial C2D2 coverages of 0.06 and 0.17 ML, respectively.


Figure 7. Vibrational spectra measured as a function of time of the reaction of C2D2 with surface D at 280, 270, and 260 K. Initial coverages are 0.06 ML C2D2 and 0.86 ML surface D. Dotted line marks frequency of the 987 cm-1 mode of the CCD3 product. Spectra measured at 80 K and 10° off specular.

Figure 8. Vibrational spectra measured as a function of time of the reaction of C2D2 with surface D at 280, 270, and 260 K. Initial coverages are 0.17 ML C2D2 and 0.57 ML surface D. Dotted line marks frequency of the 987 cm-1 mode of the CCD3 product. Spectra measured at 80 K and 10° off specular.

In all, a series of such spectra were measured for 5 initial C2D2 coverages, 0.06, 0.09, 0.105, 0.12, and 0.17 ML. The CCD3 coverage, [CCD3], is plotted versus the reaction time as shown in Figures 9a and 10a for initial C2D2 coverages of 0.06 and 0.17 ML, respectively. The CCD3 coverage increases rapidly at short times, and then levels off at longer times. These data are fit to the expression

[CCD3] ) ΘiC2D2[1 - e-(Θi

D-Θ C2D2)kt i

]

(7)

where k is the rate constant of the reaction in eq 2. This expression is the time dependence of CCD3 product formation by a second-order reaction, D + C2D2, in the absence of a significant rate for the reverse reaction and in the limit that ΘiD . [CCD3]. This latter approximation is reasonable because the initial D coverage is always at least 16 times as large as [CCD3] in these experiments. It follows from eq 7 that the rate of CCD3 formation is given by

d[CCD3] D C2D2 ) ΘiC2D2(ΘiD - ΘiC2D2)ke-(Θi -Θi )kt dt

(8)

The initial rate of the reaction, which is the rate at t ) 0,


Haug et al.

TABLE 1: Initial Rates and Rate Constants versus Surface Temperature and Activation Energies and Pre-exponential Factors for Five Initial Acetylene and Deuterium coverages initial coverage [ML] ΘiC2D2

ΘiD

Θi

T [K]

initial rate [ML/s]

rate constant, k [ML‚s]-1

Ea [kcal/mol]

A [ML‚s]-1

10-6

16.7 ( 0.7

2.6 × 109(0.5

0.17

0.57

0.74

260 270 280

2.4 ( 0.6 × 8.6 ( 0.8 × 10-6 2.4 ( 0.2 × 10-5

2.5 ( 0.6 × 8.9 ( 0.8 × 10-5 2.5 ( 0.2 × 10-5

0.82

1.0 ( 0.1 × 10-5 2.8 ( 0.4 × 10-5 7.9 ( 0.3 × 10-5

1.2 ( 0.1 × 10-4 3.3 ( 0.4 × 10-4 9.4 ( 0.4 × 10-4

4.3 × 108(0.3

0.70

260 270 280

14.9 ( 0.4

0.12

0.85

2.0 ( 0.1 × 10-5 7.4 ( 0.9 × 10-5 1.8 ( 0.2 × 10-4

2.6 ( 0.1 × 10-4 9.5 ( 0.1 × 10-4 2.3 ( 0.3 × 10-3

1.5 × 105(1

0.74

250 265 280

10 ( 0.8

0.105

0.87

2.5 ( 0.3 × 10-5 5.3 ( 0.6 × 10-5 1.3 ( 0.3 × 10-4 1.8 ( 0.3 × 10-4

3.6 ( 0.4 × 10-4 7.5 ( 0.9 × 10-4 1.9 ( 0.4 × 10-3 2.6 ( 0.4 × 10-3

5.2 × 104(1

0.78

250 260 270 280

9.3 ( 0.9

0.09

0.92

1.5 ( 0.1 × 10-5 2.5 ( 0.3 × 10-5 5.1 ( 0.3 × 10-5

2.9 ( 0.2 × 10-4 4.9 ( 0.6 × 10-4 9.9 ( 0.6 × 10-4

7 × 103(1

0.86

260 270 280

8.8 ( 1.0

0.06

is given by

(

)

d[CCD3] dt

t)0

) ΘiC2D2(ΘiD - ΘiC2D2)k

(9)

The initial rate is equivalent to the slope of the plots in Figures 9a and 10a at t ) 0. The value of the rate constant k is evaluated from these slopes using the expression

(

)

d[CCD3] dt t)0 k) C2D2 D Θi Θi

(10)

where the approximation ΘiD . ΘiC2D2 is made. The validity of this approximation is discussed below. The initial rates and rate constants at an initial C2D2 coverage and temperature are evaluated according to eqs 9 and 10 and are listed in Table 1. The error bar on the initial rate represents the standard deviation from a nonlinear Levenberg-Marquardt fit to the data.43 The error bar on the rate constant is the propagated error of eq 10. Arrhenius plots of some of these rate data are shown in Figures 9b and 10b. The activation energy for this reaction, Ea, and the preexponential factor, A, are evaluated from these plots and are listed in Table 1 for given values of ΘiD and ΘiC2D2 and for the total initial coverage Θi ) ΘiD + ΘiC2D2. The error bars on Ea and A represent the standard deviation of a linear least-squares fit to the data. The approximation ΘiD > ΘiC2D2 introduces an error of less than 0.6% in the value of Ea for ΘiD ) 0.57 and ΘiC2D2 ) 0.17, the initial conditions for which this approximation is most severe. Before discussing the trends in Ea and the A factor, it should be made clear that the rate-limiting step of the reaction, C2D2 + D f CCD3, is the addition of the first D atom to C2D2 to form a vinyl species, CDCD2. Hence, the value of Ea is the activation energy of this addition step. The evidence for the identification of this step as the rate-limiting one comes from a careful study of the dissociation of C2D4 on Ni(111), as described in the next section. As can be seen from Table 1, the value of Ea decreases as the total coverage increases. This trend likely arises as a result of the weaker binding of the reactants as the coverage is increased. The binding energy of D is known to decrease with coverage while the binding energy of C2D2 likely decreases with coverage by analogy to the trend in the binding energy of

10-5

ethylene with coverage. The weaker binding leads to a higher potential energy of the reactants and hence a smaller value of Ea. This analysis assumes that the binding energy of the transition state is not affected by the coverage or that the binding energy of the transition state decreases less than the binding energy of the reactants as the coverage is increased. There are no studies that measure the kinetics of this C-D bond formation on other metals, so no data are available for comparison. However, in the reaction of CO + O on Pt, one of the few bimolecular surface reactions whose kinetics have been carefully studied, a decrease in Ea with total coverage has been observed and was similarly attributed to the weakening of the reactant binding as the coverage increased.44,45 The preexponential factor A is also observed to decrease as the total coverage is increased. This trend implies that the entropy change between the reactants and transition state decreases as the total coverage increases. That is, as the coverage increases, the degree of constraint on the reactants and transition state becomes more similar. The A factor also decreases as the coverage increases in the reaction of CO + O on Pt. The decrease in the preexponential factor does not compensate for the decrease in the activation energy so as to make the rate constant independent of total coverage, as can be seen in Table 1. Rather, the rate constant first increases with total coverage, reaches a maximum value at about Θi ) 0.85-0.87 ML, and then decreases with coverage. Correspondingly, the reaction rate does not exhibit a simple first-order dependence in the coverage of each reactant. Instead, the reaction rate is a maximum at about ΘiD ) 0.74-0.78 ML and ΘiC2D2 ) 0.105-0.09 ML. It is clear that the reaction of D + C2D2 is not a simple one that can be described by a well-mixed and homogeneous layer of noninteracting adsorbates. Clearly, the interactions between the reactant adsorbates as well as the transition state and the product adsorbate are strong. They are likely strong enough to support island formation, as described above in the discussion of the equilibrium measurements. The observed maximum in the reaction rate at Θi ) 0.85-0.87 ML may reflect this kind of inhomogeneity in the reaction layer. d. Mechanism for CCD3 Formation. The initial step of this reaction, the addition of D to C2D2 to form a vinyl species, CDCD2, has been discussed as the rate-determining step in the previous section. The evidence for the identification of this step as the rate-limiting one comes from a careful study of the dissociation of C2D4 on Ni(111). It is known that C2D4 slowly


Figure 9. (a) Coverage of CCD3 product vs reaction time at 3 temperatures. Initial coverages are 0.06 ML C2D2 and 0.86 ML surface D. (b) Ln rate constant, k, as determined from eq 10, vs inverse temperature. Error bars discussed in text. Activation energy, Ea, determined from plot also shown.

Figure 10. (a) Coverage of CCD3 product vs reaction time at 3 temperatures. Initial coverages are 0.17 ML C2D2 and 0.57 ML surface D. (b) Ln rate constant, k, as determined from eq 10, vs inverse temperature. Error bars discussed in text. Activation energy, Ea, determined from plot also shown.

decomposes to C2D2 between 210 and 240 K. It is reasonable to expect that the C-D bonds are broken sequentially so that CDCD2 is an intermediate in this decomposition. If the subsequent steps in the conversion of the CDCD2 intermediate to CCD3 in the presence of a high surface D coverage are fast steps at temperatures between 210 and 240 K, then the dissociation of C2D4 in the presence of a high D coverage should result in the formation of CCD3. This test is carried out in the following manner. A saturated surface of 0.25 ML of C2D4 is slowly heated at a rate of 0.12 K/s from 80 to 210 K and then held at temperatures between 210 and 220 K for 10 min. During this heating, the surface is continually exposed to D2 from an elevated background pressure of 2 × 10-7 Torr of D2. The reaction is then quenched by lowering the temperature rapidly to 80 K, the temperature at which the vibrational spectrum shown in Figure 11 is measured. The spectrum is largely that of C2D2, as can be seen by comparison to Figure 1. However, a new feature, not assignable to C2D2 or D, is apparent at 987 cm-1. This feature occurs at exactly the frequency of the symmetric CD3 deformation mode of CCD3, the most intense CCD3 mode. Therefore, the presence of CCD3 is consistent with the picture of a CDCD2 species as an intermediate in this reaction. As the C2D4 molecules slowly decompose, short-lived CDCD2 species are formed. In the presence of a high coverage of surface-bound D, a small number of these CDCD2 molecules convert to CCD3. The constant D2 flux is necessary for the formation of CCD3. The identical experiment carried out in the absence of D2 shows no evidence


Figure 11. Vibrational spectra of predominantly C2D2 measured after annealing 0.25 ML C2D4 at 220 K with and without an ambient pressure of D2. See text for procedure. Dotted line marks frequency of the 987 cm-1 mode of the CCD3 product. Spectra measured at 80 K and 6° off specular.

for the feature at 987 cm-1. An additional control experiment is carried out to demonstrate that the reaction of D with C2D2 is not responsible for the appearance of CCD3. Adsorbed C2D2 at 220 K is exposed to a constant flux of 2 × 10-5 Torr D2 for 10 min, followed by a 30 min anneal at 220 K. The exposure to D2 and the anneal are carried out three times. The resulting vibrational spectrum shows no evidence for the symmetric CD3 deformation mode of CCD3 at 987 cm-1. The observation that the short-lived CDCD2 intermediate forms CCD3 in the presence of a high coverage of D at 220 K confirms that the steps in the conversion of CDCD2 to CCD3 occur readily and quickly at 220 K. These same steps are expected to occur even faster at 260-280 K, the temperatures at which the reaction of D with C2D2 is observed to form CCD3. Therefore, it is concluded that the rate-determining step of the D + C2D2 reaction at 260-280 K is indeed the addition of the first D atom to C2D2 to form CDCD2 and that the measured activation energy is that of this rate-determining step. Once the vinyl species, CDCD2, is formed, the two possible routes for the subsequent steps to CCD3 formation are D atom migration,

D(s) + C2D2 f CDCD2 f CCD3

(11)

or addition of a second D atom to form ethylidene, CDCD3, followed by C-D bond cleavage to form CCD3,

2D(s) + C2D2 f CDCD2 + D(s) f CDCD3 f CCD3 + D(s) (12) There is no definitive evidence for either of these pathways. However, a couple of observations suggest D atom migration, eq 11, is not operable. As discussed above, C2D4 decomposes to CDCD2 and then to C2D2 between 210 and 240 K. If D atom migration were taking place, then it is reasonable to expect that some of the CDCD2 would convert to CCD3. However, it is very clear from the vibrational spectra that C2D4 cleanly dissociates to C2D2. In contrast, when C2D4 dissociates in the presence of coadsorbed D, then CCD3 formation is apparent, as shown in Figure 11. The necessity for coadsorbed D suggests that a second D atom adds to the vinyl species to form ethylidene, which dissociates quickly to form CCD3, as shown in eq 12. The caveat to this conclusion is that the coadsorbed D may be necessary to stabilize the CCD3 species. If this is the


Figure 12. Partial pressures at m/e ) 2, 26, 27, and 30 measured while heating 0.13 ML C2H2 coadsorbed with 0.7 ML surface H at 2 K/s.

case, then D atom migration may be occurring in the absence of coadsorbed D, but the absence of its stabilizing interaction with CCD3 may result in the rapid dissociation of CCD3. With this caveat in mind, the mechanism shown in eq 12 is slightly favored as the operable mechanism. 2. Search for Gas-Phase Reaction Products. The production of gas-phase ethylene and ethane from the reaction of surface H with adsorbed C2H2 is probed in the following experiment. A layer of 0.13 ML of C2H2 is prepared by exposure of the crystal, held at 80 K, to a beam of 2% C2H2 in Ar. The diameter of the beam at the crystal is smaller than the crystal diameter so that no C2H2 is adsorbed on the crystal edges. The remaining sites are then saturated with surface-bound H by exposure of the crystal at 80 K to 9000 L of H2. The resulting surface H coverage is 0.7 ML. The crystal is then heated at a rate of 2 K/s while the partial pressures at m/e ) 2, 26, 27, and 30 are monitored by a quadrupole mass spectrometer. The partial pressures are plotted in Figure 12. The recombination and desorption of hydrogen (m/e ) 2) occurs between 300 and 450 K, but neither ethylene (m/e ) 27) nor ethane (m/e ) 30), two products that might be expected from C2H2 hydrogenation, are observed. Ethylene is monitored at m/e ) 27 to minimize interference with the dissociative ionization of the parent ethane molecule at m/e ) 28. No desorption of C2H2 (m/e ) 26) is observed. The intensity of the carbon Auger signal is observed to be identical, within experimental error, to that measured before the reaction, indicating that no carbon-containing species has desorbed. A range of ratios of C2H2 to surface H coverage were explored, from 0.03 to 0.21 ML of C2H2 with the corresponding 0.95 to 0.46 ML of surface H, but no gas-phase hydrogenation products were observed. However, CCH3 is produced during the temperature ramp. The spectra shown in Figure 1, measured after the same temperature ramp for slightly different C2H2 and surface H coverages, clearly show evidence of CCH3 formation at temperatures of 280 K and higher. Therefore, the absence of C2H4 production results from the failure of a surface-bound H to add rapidly enough to CHCH2, as shown in eq 13:

The reaction in eq 13 is too slow to compete effectively against the reaction of a second surface H to produce CHCH3, which subsequently dissociates to CCH3 and H(s), as shown in eq 12. It is also possible that the addition of a surface-bound H atom to CHCH2 is too slow to compete effectively against the dissociation of CHCH2. This possibility is considered viable because C2H4 does dissociate rapidly above 250 K to form C2H2, and therefore, this dissociation is indicated in eq 13.

Haug et al.

Figure 13. Vibrational spectra of (a) an equivalent of 3.7 ML bulk H and 0.25 ML C2H2, and (b) 0.25 ML C2H2. Spectra measured at 80 K and at 6° and 8° off specular, respectively. The bulk H mode at 800 cm-1 labeled in italics.

B. Reaction of Bulk H With C2H2. 1. Gas-Phase Products. The gas-phase products of the reaction of bulk H with C2H2 are explored in the following manner. The Ni(111) crystal, held at 120 K, is exposed to approximately 2.2 × 1017 H atoms/ cm2. This H atom exposure results in 3.7 ML of bulk H as well as 1 ML of surface-bound H. The surface H is removed by collision-induced recombinative desorption induced by the impacts of Xe atoms incident with 144 kcal/mol of translational energy. They are generated by the adiabatic expansion of a mixture of 0.25% Xe in He through a nozzle held at 1000 K. The crystal, held at 80 K, is exposed for 30 min. to the resulting beam, incident at 40° from the normal. Collision-induced recombinative desorption removes only the surface-bound H, leaving the bulk H unperturbed, as demonstrated previously.32-34 A layer of 0.25 ML of C2H2 is then prepared by exposure of the crystal, held at 80 K, to a beam of 2% C2H2 in Ar. A vibrational spectrum of the resulting system is shown in Figure 13a. For comparison, a spectrum of 0.25 ML of C2H2 in the absence of bulk H or surface-bound H is also shown in Figure 13b. Comparison of the two spectra reveals that the frequencies of the vibrational modes of adsorbed C2H2 are unchanged by the presence of bulk H, verifying that bulk H does not modify the C2H2-Ni interaction. The features at 480, 690, 860, 1080, 1220, and 1370 cm-1 are the C-Ni symmetric stretch, the symmetric and antisymmetric C-H out-of-plane bend, the symmetric C-H in-plane bend, the C-C stretch, and the antisymmetric C-H in-plane bend modes, respectively.19,26 The feature at 800 cm-1 in the spectrum in Figure 13a confirms the presence of bulk H.14 The absence of the vibrational modes of surface-bound H at 950 and 1150 cm-1 confirms the efficacy of the Xe beam in sweeping the surface clean of surface H.14 With the reactants so synthesized, the crystal is then heated at a rate of 2 K/s while the partial pressures at m/e ) 2, 26, 27, and 30 are measured. The resulting partial pressures are plotted in Figure 14a. The recombination and desorption of hydrogen (m/e ) 2) between 170 and 250 K arises from hydrogen emerging from the bulk, while that occurring between 300 and 450 K arises from surface-bound H. Note that although no hydrogen is initially adsorbed on the surface, the surface does become covered with hydrogen once the bulk H emerges onto the surface. This is the origin of most of the hydrogen signal observed between 300 and 450 K. The feature at 100 K in the m/e ) 26 trace arises from acetylene that has been physisorbed onto the crystal support rods during exposure of the crystal to the C2H2 beam. The beam diameter that was used in the measurements shown here was slightly larger than that of the crystal. This feature is absent in experiments that employ a beam


Figure 14. (a) Partial pressures at m/e ) 2, 26, 27, and 30 measured while heating 0.25 ML C2H2 and an equivalent of 1.8 ML bulk H at 2 K/s. Feature at 100 K in m/e ) 26 trace arises from desorption of C2H2 physisorbed onto the cryostat. See text. (b) Partial pressures from (a) deconvoluted for dissociative ionization of multiple masses. See text. Resulting signals proportional to acetylene, ethylene, and ethane.

whose diameter is smaller than that of the crystal.46 The most important observation in Figure 14a is the rapid desorption of m/e ) 26, 27, and 30 at 180 K, the temperature at which bulk H emerges onto the surface. The signal at m/e ) 30 represents solely the desorption of the parent ethane molecule. The signal at m/e ) 27 is a sum of the contributions from the dissociative ionization of the parent ethylene molecule at mass 28, the dissociative ionization of the parent ethane molecule at mass 30 and the 13C contribution from 13C2H2. The signal at m/e ) 26 is the sum of contributions from the parent acetylene molecule at mass 26 and from the dissociative ionizations of the parent ethylene and ethane molecules. The signals at m/e ) 27 and 26 are deconvoluted to represent solely the contributions from ethylene and acetylene, respectively, using the dissociative ionization probabilities of ethane, ethylene, and acetylene. These probabilities are measured using mass spectrometer settings that are identical to those used to measure the thermal desorption traces.46 The deconvoluted signals at masses 27 and 26, as well as the signal at mass 30, representing ethane, are then normalized for the different ionization cross sections of ethane,47-49 ethylene,49-51 and acetylene,50 using established literature values. The resulting signals are plotted versus temperature in Figure 14b. Note that once the fragmentation contributions are properly subtracted from the mass 26 signal in Figure 14b, the absence of C2H2 desorption at 180 K is clearly apparent. However, the hydrogenation products ethylene and ethane are clearly produced at the temperature at which the bulk H emerges onto the surface in the presence of adsorbed C2H2. Integration of the ethylene and ethane signals in Figure 14b over temperature reveals that approximately equal amounts of ethylene and ethane are formed. Carbon is observed by Auger spectroscopy to remain on the surface after this reaction is carried out. Measurement of the carbon Auger signal shows that approximately 30% of the initial 0.25 ML of C2 hydrocarbon remains, implying that 0.18 ML of C2H2 has been hydrogenated to about 0.09 ML of ethylene and 0.09 ML of ethane. An insufficient number of measurements of the ethylene and ethane yields were made as a function of the C2H2 coverage and the equivalent coverage of bulk H for a statistical analysis to be carried out. However, in general, the ethane yield increases and the amount of carbon remaining of the surface decreases as the equivalent coverage of bulk H increases. 2. Adsorbed Products. In addition to the gas-phase products ethylene and ethane, hydrogenation of C2H2 by bulk H also yields an adsorbed product, ethylidyne. An overlayer of 0.22


Figure 15. (a) Vibrational spectrum resulting from heating 0.22 ML C2H2 and an equivalent of 3.7 ML bulk H, prepared at 120 K, to 220 K at 2 K/s and then quenching to 80 K. Spectrum measured at 80 K and 5° off specular. (b) Vibrational spectrum of coadsorbed CCH3 and surface H from ref 34, measured at 80 K and 10° off specular. Frequencies of surface H and C2H2 modes in italics.

ML of C2H2 is prepared on the Ni(111) crystal containing about 3.7 ML of bulk H as described in the previous section. The crystal is then heated from 80 to 220 K at a rate of 2 K/s and then quenched rapidly to 80 K. The vibrational spectrum of the resulting overlayer is shown in Figure 15a. The features at 457, 1025, 1129, 1336, 1410, 2883, and 2940 cm-1 are the Ni-C antisymmetric stretch, CH3 rock, C-C stretch, CH3 symmetric and antisymmetric deformation, and the C-H symmetric and antisymmetric stretch modes, respectively, of CCH3.35 The known spectrum of CCH3 is shown in Figure 15b for comparison.35 Adsorbed H is present in both spectra, as evidenced by the antisymmetric Ni-H mode14 at 950 cm-1. In Figure 15a, the feature 860 cm-1, which is the antisymmetric C-H outof-plane bend mode36,37 of C2H2, indicates that some unreacted C2H2 remains on the surface. The ratio of the intensities of the 1129 cm-1 feature of CCH3 to that of the 860 cm-1 feature of C2H2 is about 4. Since the intensities of these modes are known to be linear with the coverage of these species, the coverage of the CCH3 product is estimated to be four times as large as that of C2H2. This ratio is consistent with the ratios observed for the equilibrium reaction of surface-bound H with C2H2 to form CCH3. 3. Mechanism. The reaction of bulk H with C2H2 produces both ethylene and ethane as gas-phase products as well as adsorbed ethylidyne. For a crystal heating rate of 2 K/s, the reaction begins at the temperature at which bulk H begins to emerge onto the surface, 180 K, and is complete at about 200 K, the temperature by which the C2H2 reactant is consumed. The reaction is viewed as the addition of a bulk H atom, as it moves out from the bulk, to the adsorbed C2H2 molecule to form a vinyl species, CHCH2, as shown in eq 14:

H(b) + C2H2 f CHCH2

(14)

Less clear is the identity of the second H atom to react with CHCH2 to form gas-phase C2H4. Certainly, a bulk H atom is expected to react readily with the CH end of the vinyl species to form C2H4, in analogy to the rapid reaction of an emerging bulk H with adsorbed CH3 to form volatile CH4.34 It is likely that the reaction of surface-bound H with CHCH2 does not happen fast enough to compete successfully with the reaction of bulk H with CHCH2, because the potential energy of surfacebound H is considerably less than that of bulk H. The energetic difference between the two kinds of H atoms is discussed in more detail in the next section.


Haug et al.

While some of the resulting C2H4 desorbs immediately into the gas phase, about half of it, under the conditions of Figure 14b, remains bound and is further hydrogenated to ethane. The first H atom to add to C2H4 is a bulk H atom to form CH2CH3, as shown in eq 15:

H(b) + C2H4 f CH2CH3

(15)

Surface-bound H atoms are observed not to react with C2H4, nor is exchange between C2H4 and surface-bound D observed, except in the temperature range at which C2H4 is known to dissociate to C2H2. The identity of the second H atom to add to CH2CH3 to form gas-phase ethane is not known. Certainly, a bulk H atom is expected to add readily to CH2CH3 to form C2H6, just as an emerging bulk H reacts rapidly with adsorbed CH3 to form methane.34 But the addition of a surface-bound H atom to the CH2CH3 species to form C2H6 is also possible. There is no evidence for or against the role of surface-bound H in hydrogenating an adsorbed ethyl radical. The initial step in the formation of the adsorbed CCH3 product is likely identical to that in the formation of the gas-phase products and is represented in eq 14. In fact, the vinyl intermediate, CHCH2, is believed to be the critical intermediate in the formation of CCH3. This conclusion is based on the absence of CCH3 formation in the reaction of bulk H with C2H4,2 which is a reaction in which a CHCH2 intermediate cannot be formed. That is, at 180 K, the temperature at which bulk H reacts with C2H4, there is little, if any, dissociation of C2H4 to form CHCH2. The absence of this CHCH2 intermediate apparently precludes the formation of CCH3. There are three possible mechanisms for the conversion of CHCH2 to CCH3. One mechanism, H atom migration, is represented in eq 11. Arguments presented above describe the evidence against this process at 220 K. That the reaction of bulk H with C2H2 to form CCH3 takes place at a slightly lower temperature, 180 K, makes H atom migration even more unlikely. Another possible mechanism involves the reaction of surface-bound H with CHCH2 to form CHCH3 that then dissociates to form CCH3, as shown in eq 12. This mechanism is possible even in a reaction in which no surface H is initially present because a bulk H that does not react immediately with an adsorbate upon emerging onto the surface becomes a surfacebound H. A third mechanism, similar to that in eq 12 but involving bulk H, is shown in eq 16:

2H(b) + C2H2 f CHCH2 + H(b) f CHCH3 f CCH3 + H(s) (16) The evidence for and against either mechanism in eqs 12 and 16 is not strong. It is clear that the addition of a surface-bound H to CHCH2 to form CHCH3 and the subsequent dissociation of CHCH3 to CCH3, as shown in eq 12, can occur at 220 K, because CCH3 production is observed as C2H4 dissociates at 220 K in the presence of a high H coverage, as discussed above. However, it is unknown whether the mechanism shown in eq 12 can occur at 180 K and in particular, whether it can compete effectively with the reaction of a bulk H with CHCH2. The reaction with bulk H is expected to occur faster than with surface H, because the emerging bulk H atom is about 24 kcal/mol more energetic than a surface-bound H atom. The difference in the energetics of these two kinds of H atoms is a critical element in their unique reactivity. This subject is discussed in the next section. IV. Comparison of Reactions of H(s) and H(b) with C2H2 The rate-determining step in the reaction of surface-bound H with C2H2 has been determined to be the addition of surface

H to C2H2 to form adsorbed vinyl, CHCH2. The activation energy for this step ranges between 9 and 17 kcal/mol, depending on the initial coverages of the two reactants. The sole product of this reaction is adsorbed CCH3. No gas-phase products, such as C2H4 or C2H6, are observed. The mechanism is summarized in eq 17:

The rate-determining step in the reaction of bulk H with C2H2 is the emergence of the bulk H from below to above the surface, which starts at about 180 K. Once the emerging bulk H encounters an adsorbed C2H2, the reaction to produce ethylene, ethane, and adsorbed CCH3 takes place rapidly and is complete at about 200 K. The mechanism is summarized in eq 18:

While both the surface-bound H and the bulk H approach a rehybridized π orbital of the C2H2, the barrier to reaction is expected to be different because of the different degrees of interaction of each orbital with the Ni surface. Unfortunately, an angle-resolved photoemission study, which would distinguish the electronic structure of the two orbitals on the basis of their different orientations in space, has not been carried out for C2H2 on Ni(111). However, it has been for C2H2 on Ni(110).18 Although the surface structures are not identical, some observations are relevant to the interaction of C2H2 with Ni(111) in general. The two πu orbitals of the gas-phase C2H2 molecule both are shifted substantially below the substrate 3d band upon adsorption, indicating the strong involvement of both orbitals in the binding of C2H2 to Ni. However, these orbitals are not degenerate in the adsorbed molecule. Rather, the πu orbital derived from the pz orbitals perpendicular to the surface is shifted lower in energy by 1.5-2.5 eV as compared to the πu orbital derived from the px orbitals parallel to the surface. The larger energy shift of the pz derived orbital implies its greater role in the interaction of C2H2 with Ni as compared to the px derived orbital. Therefore, it might be expected that the barrier to reaction of C2H2 with a bulk H, which approaches C2H2 from the direction in which the pz derived orbital is oriented, will be higher than the reaction with a surface-bound H. The surfacebound H approaches C2H2 from the direction of orientation of the px derived orbital. However, it is likely that the difference between the barriers to reaction of a bulk H versus a surface-bound H pale in comparison to the relative potential energy differences of the two H atoms and the source of the energy to overcome the barrier. In the case of surface-bound H, the energy is supplied thermally and the reaction begins at 250 K. In the case of bulk H, the reaction occurs at 180 K because it is enabled by the high potential energy of a bulk H atom relative to a surfacebound H atom. Its high energy is a consequence of two factors. First, its weaker binding to the bulk Ni atoms than to the surface Ni atoms results in the bulk H atom being about 15 kcal more energetic than a H atom bound to the surface as shown in Figure


Figure 16. Potential energy diagram for hydrogen-Ni system. Points to the left of vertical axis represent a H atom beneath surface. Points to the right represent a H atom or a H2 molecule at or away from surface. See text for the origins of the energies shown. Energies given in kcal/mol.

16. This value is approximated as the sum of the enthalpy of solution of a H atom in Ni, which has a value of about 4 kcal/ mol,52 and the binding energy of a surface-bound H atom, 11 kcal/mol.53 Second, in order for a bulk H atom to participate in chemistry at the surface, it has to emerge onto the surface by surmounting the barrier of 9 kcal/mol at the bulk-surface interface. This barrier is approximated as the activation energy for diffusion of a H atom in the bulk of Ni metal.54 An emerging bulk H atom at the top of this barrier is therefore about 24 kcal/ mol more energetic than a H atom bound to the surface. This high potential energy of a bulk H atom can facilitate a reaction that might not otherwise occur for a surface H atom. Recent computational studies55,56 of the reaction of bulk H with an adsorbed methyl radical34 also discuss the critical role of the high potential energy of bulk H relative to that of surface-bound H. Of course, it is important to recognize that the emerging bulk H is transiently energetic. If the emerging bulk H does not encounter an adsorbate, it dissipates its energy to the solid and ultimately equilibrates with it and becomes a surface-bound H. It is also possible that some of the energy of the emerging bulk H will have already dissipated by the time it reacts with the adsorbate. The reaction could take place at any time during the dissipation of the bulk H’s energy, as long as the remaining energy is sufficient to overcome the barrier to reaction with the adsorbate. In the present case, a bulk H atom emerges onto the surface at 180 K, and, in so emerging, approaches the underside of an adsorbed C2H2 molecule and reacts with it to form a vinyl species, CHCH2. The barrier to this insertion reaction as well as the actual amount of energy remaining in the emerging bulk H at the time of reaction are unknown. However, it is clear that the energy possessed by the bulk H atom is sufficient to overcome the barrier to CHCH2 formation. That is, the bulk H reacts before it has time to equilibrate with the surface and become a surface-bound H. In principle, it is also possible for there to be some lateral motion of this emerging bulk H atom as it dissipates its energy. It is even possible for the laterally moving bulk H atom to encounter and react with the px derived πu orbital of C2H2 via a side-on approach. The available experimental data do not eliminate this possibility. What distinguishes this laterally moving bulk H from surface H is its energy. The laterally moving bulk H is transiently but sufficiently energetic to

J. Phys. Chem. B, Vol. 105, No. 46, 2001 11491 overcome the barrier to reaction with C2H2 via a side-on approach at 180 K whereas surface H is insufficiently energetic to react at this temperature. Once the vinyl species, CHCH2, is formed, the routes followed by the reaction involving only surface-bound H and the reaction involving bulk H are decidedly different. Clearly, a surface-bound H atom does not add rapidly enough to CHCH2 to form gas-phase ethylene before either CHCH2 dissociates to C2H2 or reacts with a surface-bound H to form CHCH3, as shown in eq 17. In contrast, a bulk H atom does add rapidly enough to form gas-phase ethylene before CHCH2 dissociates to C2H2 or reacts with either a bulk H or a surface H to form CHCH3, as shown in eq 18. While the different directions of approach of a surface-bound and a bulk H to the C2H2 species certainly may affect the reaction rates, the likely more important source of the different addition rates are the relative energetics of the two H atoms. If the emerging bulk H atom encounters adsorbed CHCH2, it can use this 24 kcal/mol of energy to overcome any barrier that may exist to reaction with adsorbed CHCH2, and hence add rapidly enough to form ethylene before other pathways occur to produce CCH3 from CHCH2. In contrast, surface-bound H is not sufficiently energetic to react fast enough with CHCH2 to form ethylene and therefore to compete effectively with other pathways. It is the different rates of addition of surface-bound H and bulk H to CHCH2 that lead to the distinct product distributions for hydrogenation of C2H2 by the two different H atoms. The distinct product distributions emphasize the different energetics of the two H atoms. But these results are also significant for their demonstration that hydrogenation of unsaturated hydrocarbons by bulk H is the lower energy pathway! That is, hydrogenation of C2H2 by bulk H is complete at 180 K while the rate of hydrogenation by surface H is just beginning to be measurable at 250 K. Therefore, in high-pressure environments where bulk H is available, the dominant reaction pathway is likely hydrogenation by bulk H over hydrogenation by surface-bound H. V. Summary The reaction of surface-bound H with adsorbed C2H2 on Ni(111) produces a single product, adsorbed ethylidyne, CCH3. This reaction does not produce ethylene or ethane, products that normally are expected in a hydrogenation reaction of an unsaturated C2 hydrocarbon. The reaction mechanism for CCH3 formation is summarized in eq 17. The rate-determining step is the first step, the addition of a surface-bound H to C2H2 to form a vinyl species, CHCH2. The activation energy for this step ranges from 17 to 9 kcal/mol as the total coverage increases from 0.74 to 0.92 ML. The reaction is extremely slow and is observed to proceed with a measurable rate between 250 and 280 K. The slow recombination and desorption of hydrogen precludes rate measurements above 280 K. The initial rates range from about 1 × 10-6 to 2 × 10-4 ML/s, depending on coverage. The rate does not exhibit a simple first-order dependence on the coverage of either reactant. Rather, the rate, and consequently the rate constant, is a maximum at an initial coverage of 0.09 ML of C2H2 and 0.78 ML of H, indicating the strong interactions between the reactants. Measurements of the equilibrium constants also indicate that this system is highly nonideal. They suggest that strong interactions also exist between the surfacebound H reactant and the CCH3 product, possibly leading to an island of C2H2 and one of a mixture of CCH3 and surface H. The reaction of bulk H with adsorbed C2H2 produces three products, gas-phase ethylene and ethane and adsorbed ethyl-

11492 J. Phys. Chem. B, Vol. 105, No. 46, 2001 idyne, CCH3. The reaction is rate limited by the emergence of bulk H onto the surface, which begins at about 180 K for a heating rate of 2 K/s. At 200 K, the reaction is complete. The reaction mechanism is summarized in eq 18. The reactivity of both surface-bound H and bulk H with acetylene demonstrates the importance of a direct approach to the rehybridized π orbitals of the unsaturated hydrocarbon. The different product distributions observed for the two kinds of hydrogen are attributed to the large energetic differences between the two forms of hydrogen. Because an emerging bulk H atom can be as much as 24 kcal/mol more energetic than a surface-bound H atom, it will have reaction channels open to it that are closed to a surface-bound H atom. As expected, the chemistry of bulk H is found to be unique from that of surfacebound H. Acknowledgment. DoE (DE-FG02-89ER14035) supports this work. K.L.H. acknowledges the Corning Foundation for partial support. References and Notes (1) Horiuti, J.; Polanyi, M. Trans. Faraday Soc. 1934, 30, 1164. (2) Daley, S. P.; Utz, A. L.; Trautman, T. R.; Ceyer, S. T. J. Am. Chem. Soc. 1994, 116, 6001. (3) Demuth, J. E.; Eastman, D. E. Phys. ReV. Lett. 1974, 32, 1132. (4) Demuth, J. E. Surf. Sci. 1978, 76, L603. (5) Klimesch, P.; Henzler, M. Surf. Sci. 1979, 90, 57. (6) Lehwald, S.; Ibach, H. Surf. Sci. 1979, 89, 425. (7) Benninghoven, A.; Beckmann, P.; Greifendorf, D.; Schemmer, M. Appl. Surf. Sci. 1980, 6, 288. (8) Hasse, W.; Gu¨nter, H.-L.; Henzler, M. Surf. Sci. 1983, 126, 479. (9) Dalmai-Imelik, G.; Massardier, J. In Proceedings of the Sixth International Congress on Catalysis; Bond, G. C., Wells, P. B., Tompkins, F. C., Eds.; Chemical Society: London, 1977; pp 90-100. (10) Bertolini, J. C.; Rousseau, J. Surf. Sci. 1979, 83, 531. (11) Hammer, L.; Hertlein, T.; Mu¨ller, K. Surf. Sci. 1986, 178, 693. (12) Bao, S.; Hofmann, Ph.; Schindler, K.-M.; Fritzsche, V.; Bradshaw, A. M.; Woodruff, D. P.; Casado, C.; Asensio, M. C. Surf. Sci. 1995, 323, 19. (13) Schlegel, H. B. J. Phys. Chem. 1982, 86, 4878. (14) Johnson, A. D.; Maynard, K. J.; Daley, S. P.; Yang, Q. Y.; Ceyer, S. T. Phys. ReV. Lett. 1991, 67, 927. (15) Ceyer, S. T. Acc. Chem. Res. 2001, 34, 737. (16) Haug, K. L.; Bu¨rgi, T.; Trautman, T.; Ceyer, S. T. J. Am. Chem. Soc. 1998, 120, 8885. (17) Bao, S.; Hofmann, Ph.; Schindler, K.-M.; Fritzsche, V.; Bradshaw, A. M.; Woodruff, D. P.; Casado, C.; Asensio, M. C. Surf. Sci. 1994, 307, 722. (18) Weinelt, M.; Huber, W.; Zebisch, P.; Steinruck, H. P.; Ulbricht, P.; Birkenheuer, U.; Boettger, J. C.; Rosch, N. J. Chem. Phys. 1995, 102, 9709. (19) Ibach, H.; Lehwald, S. J. Vac. Sci. Technol. 1978, 15, 407. (20) Kesmodel, L. L.; Dubois, L. H.; Somorjai, G. A. J. Chem. Phys. 1979, 70, 2180. (21) Dubois, L. H.; Castner, D. G.; Somorjai, G. A. J. Chem. Phys. 1980, 72, 5234. (22) Kesmodel, L. L.; Gates, J. A. J. Electron Spectrosc. Relat. Phenom. 1983, 29, 307.

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Catalytic Hydrogenation of Acetylene on Ni(111) - American Chemical

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