1-D Reversible Phase of Chemisorbed N2 on Stepped Pt Surfaces

Feb 21, 2001 - Alain J. Phares , David W. Grumbine , Jr. and Francis J. Wunderlich. Langmuir 2009 25 (2), 944-951. Abstract | Full Text HTML | PDF | P...
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J. Phys. Chem. B 2001, 105, 3733-3740

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1-D Reversible Phase of Chemisorbed N2 on Stepped Pt Surfaces† Tykhon S. Zubkov, C. Emil Tripa, and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260 ReceiVed: August 29, 2000

The behavior of the N2 phase adsorbed exclusively on the steps of the Pt(335) and Pt(779) surfaces was studied at 88 K under N2 gas flux by reflection-absorption infrared spectroscopy and molecular beam adsorption experiments. Under equilibrium N2 pressure, the saturated 1-D chemisorbed layer is able to be additionally reversibly populated (by 2-20%). Under these conditions, fast isotopic exchange between 14N2 and 15N2 occurs. No chemisorption is detected on the terraces. The reversibly chemisorbed N2 species is spectrally indistinguishable from the irreversibly preadsorbed N2 (N-N frequency 2236-2230 cm-1) and therefore must be chemisorbed on vacant Pt step sites. The infrared feature undergoes a frequency decrease with increasing N2 coverage, which is a result of two opposite effects: a vibrational coupling shift (upward) and a dominant static shift (downward). The proposed kinetic model involving terrace- and step-bound precursors explains the experimentally observed Langmuirian adsorption isotherm for the reversible N2 phase. This is the first reported reversible precursor-mediated 1-D chemisorption, which obeys the same “pseudoLangmuirian behavior” as is known for its 2-D analogues.

I. Introduction The dynamic behavior of a solid surface in catalysis or during chemical vapor deposition depends on the morphology of the surface, especially on the presence of defects (steps, kinks, vacancies, etc.). Due to the lower coordination number of atoms at the surface defects, these sites are usually preferentially populated and the adsorbates there may have structure and reactivity which differ from those on flat terraces. The 1-D arrangement of the low-coordinated atoms that exists on various stepped surfaces may, therefore, act as a template for 1-D structuring of the adsorbates. Both atomic and molecular species have been shown to decorate surface steps and rows. 1-D structures are reported for K on Si,1 Cs on sapphire,2 and Xe on various Pt crystal faces.3-5 For the chemisorption of molecules, the site selectivity varies from preferential chemisorption on step sites followed by chemisorption on terraces (CO on Pt(335)6) to exclusive chemisorption on steps (N2 on Pt(335)7,8). The reduction of the dimensionality of adsorbed layers for 2-D to 1-D is of some theoretical interest, especially under equilibrium with the gas phase. Existing studies are mostly theoretical and use Monte Carlo simulations,9-11 cluster approximation modeling,9 or the van der Waals mean field corrections to the Langmuir model.12 In one of these works,11 however, the calculation results are compared to the experimental adsorption isotherms for the 1-D Xe phase on rutile TiO2 (110). To our knowledge, no other experimental facts are reported about the properties of 1-D adlayers under equilibrium conditions. The 1-D phase we studied is formed by N2 on Pt(335) and Pt(779) crystal faces. The Pt(335) surface is composed of fouratom-wide (111) terraces interrupted by (100) monatomic steps. The Pt(779) surface has a similar structure with the terraces being twice as wide and exhibiting 1/2 the step density of Pt(335). Recently we showed that molecular nitrogen chemisorbs †

Part of the special issue “John T. Yates, Jr. Festschrift”.

on such stepped Pt surfaces.7,8 The IR-active molecular adsorbate is bound in an end-on configuration located exclusively at the edge atoms of the steps. The adsorption occurs with a constant sticking coefficient, which was explained by the coexistence of several precursors, which convert to the chemisorbed state.13 The saturation coverages were found to be (1.51 ( 0.09) × 1014 cm-2 for Pt(335) and (1.01 ( 0.16) × 1014 cm-2 for Pt(779). This corresponds to a coverage of 0.38 N2/Pt step atom or 0.52 N2/Pt step atom, respectively, or roughly to the occupation of every other Pt step atom at most. Here we present additional studies of this system under a flux of N2. Under the effective pressure, the N2 population on the Pt steps reversibly increases as a result of the dynamical equilibrium with the gas phase. This reversible fraction of the adsorbed N2 molecules obeys the Langmuir adsorption isotherm. We show that the adsorption occurs via step- and terrace-bound precursors. The proposed kinetic model explains the Langmuirian isotherm. From the infrared spectroscopic data, we conclude that all N2 molecules in the densely populated 1-D layer are essentially equivalent. The proposed adsorption model is that the incoming N2 molecules adsorb on the empty Pt step atoms, squeezing between the previously irreversibly adsorbed molecules. II. Experiment All the reported experiments were performed in a stainless steel ultrahigh vacuum (UHV) chamber with a base pressure below 1.0 × 10-10 Torr, equipped with an unshielded quadrupole mass spectrometer (QMS), a Fourier transform infrared (FTIR) spectrometer, and an absolutely calibrated molecular beam doser. The sample was a Pt single crystal with two stepped surfaces, (335) and (779), on opposite faces. The surfaces were prepared and characterized as described in a previous paper.8 The crystal was connected to a liquid N2 reservoir by two W wires, through which heating and cooling to 88 K were achieved. 14N of natural isotopic abundance and 99.9999% purity was 2 used. The information about the N2 partial pressure in adsorption

10.1021/jp003118a CCC: $20.00 © 2001 American Chemical Society Published on Web 02/21/2001

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Figure 1. Detection of a reversible, weakly adsorbed phase of N2 on the Pt(335) and Pt(779) surfaces at 88 K. The N2 partial pressure is monitored with a mass spectrometer. After N2(g) is admitted to the chamber through a molecular beam effusion doser and reaches a steady pressure, the crystal is rapidly translated into the beam. The front surface starts to adsorb, and the N2 pressure drops. As the surface reaches saturation, the pressure returns to its initial level. When the crystal is translated out of the N2, beam a short desorption pulse occurs, indicating that a minor fraction of N2 was weakly bound and adsorbed reversibly under the experimental conditions. The signal is baseline corrected.

experiments was obtained by monitoring the 28 amu signal with the UTI 100C quadrupole mass spectrometer. The signals at 14 and 12 amu were also monitored to distinguish between N2 and CO (data not presented), and it was shown that CO constitutes less than 5% of the mass 28 signal during adsorption experiments. The reflection-adsorption infrared (RAIR) measurements were performed with a Mattson-Cygnus FTIR spectrometer upgraded to Galaxy 7020 with a double-beam optical bench described elsewhere.14 The IR radiation was p-polarized. RAIR spectra were averaged over 2000 scans with 2 cm-1 resolution. For a better estimation of the IR peak positions, a cubic spline connection of the data points was used (Microcal Origin 3.5). For more controlled N2 exposure and a minimal background pressure rise, we used a capillary array effusive doser,15 which produced the N2 molecular beam. For our crystal geometry, the fraction of the beam intercepted by the crystal was calculated to be 0.16.16 Our adsorption experiments described in detail elsewhere13 followed the principle of the King and Wells method of determining the sticking coefficient and coverage using a molecular beam of a known flux.17 As the N2 beam is admitted to the chamber, the N2 pressure immediately rises to a level higher than the initial base pressure. When the flow of N2 molecules becomes balanced by the pumping speed, the pressure reaches a steady equilibrium value. As the crystal is translated into the beam, its front surface starts to adsorb and the pressure drops (see Figure 1). The magnitude of the pressure drop is determined by the fraction of gas adsorbed from the beam, which involves the sticking coefficient and the fraction of the beam intercepted by the crystal. The former can be calculated using the formalism of Madey.18 The pressure will return to its initial steady level after the surface is saturated. When the crystal is rapidly withdrawn from the beam, the signal deviates in the opposite way: the pressure temporarily increases and eventually returns to its steady value after the desorption has been completed. For such adsorption or desorption features, the areas

Zubkov et al.

Figure 2. Reversible adsorption-desorption behavior of the weakly bound phase of N2 on the Pt(335) and Pt(779) surfaces at 88 K. After the surface is saturated in N2, subsequent translation of the crystal in and out of the N2 beam of constant flux is accompanied by reversible adsorption and desorption features, respectively. The signal is baseline corrected. The ratio of the amount of the reversible N2 phase on the two crystal faces is shown in the frame (averaged over five different N2 fluxes).

under the resulting curves are proportional to the total amount of gas adsorbed or desorbed, respectively. The steady-state baseline pressure is not exactly constant because the pumping speed of the chamber walls decreases as the gas is being admitted to the chamber. To ensure a correct analysis of the adsorption or desorption features, the baseline under them was determined by fitting the data points before and after the respective adsorption or desorption feature, and the experimental curve was corrected by subtraction. The maximum molecular beam flux was 1.2 × 1012 cm-2 s-1, which set an upper limit to our adsorption experiments by the King and Wells method. Since our infrared bench is located at a different level from the beam doser, we also utilized a randomized N2 flux from system dosing through a leak valve, achieving much higher fluxes than in the beam experiments (up to 3 × 1015 cm-2 s-1). The systematic error of the gas flux values is 1.5% using either method. III. Results A. Adsorption Measurements. The N2 adsorption uptake curves for Pt(335) and Pt(779) exhibit similar behavior, as shown in Figure 1. After the Pt crystal is translated into the N2 beam, the partial pressure of nitrogen in the chamber immediately drops. It remains at a constant level over an extended period of time. This shows that the N2 sticking coefficient remains constant over a wide range of N2 coverage. As the surface approaches saturation, the sticking coefficient gradually drops to zero, the adsorption process stops, and the N2 pressure curve merges with the baseline. This adsorption pattern has been described and analyzed in detail in the previous part of our study.13 The main focus of this paper is the second feature shown in Figure 1. It is observed when the crystal is translated out of the beam after being saturated with N2. A certain amount of nitrogen desorbs. This portion of the adsorbed nitrogen was bound weakly enough that it was able to exist on the Pt surface only in equilibrium with the incident flux of N2 molecules. Removal of the flux reversed the equilibrium toward desorption. On the

Reversible Phase of N2 on Pt Surfaces

Figure 3. Strongly and weakly adsorbed N2 phases on Pt(335) and Pt(779) at 88 K. Coverage vs N2 exposure and flux, respectively. The uptake data for the stronger bound irreversible phase are taken from ref 13. The ratio of the irreversible N2 saturation coverages on the two crystal faces is shown in the frame. Coverages of the weakly bound reversibly adsorbed species were obtained by integrating the adsorption parts of the curves such as those shown in Figure 2 (see the text for details).

basis of the area under the curve, we can define the amount of such reversibly adsorbed N2, which is a small fraction of the total amount of nitrogen adsorbed. The majority of N2(a) remains on the Pt surface without desorption. Hereafter, we designate the strongly and weakly bound types of nitrogen as irreversibly adsorbed and reversibly adsorbed, respectively, and depict their coverages as σirr and σrev. The reversible behavior occurs at a temperature of 88 K and a N2 background pressure of (1-6) × 10-10 Torr during exposure to the beam. Figure 2 shows that cycles of reversible adsorption and desorption can be achieved repeatedly by moving the presaturated Pt crystal in and out of the N2 beam. During these cycles, the 12 amu trace was also monitored and demonstrated to be constant, showing that CO is not involved in the phenomenon. We also demonstrated by control experiments that the effect is not an artifact of the crystal translation. From the reversible adsorption features such as shown in Figure 2, the sticking coefficient was determined at different N2 fluxes. At the very first moment of the adsorption, it reaches 0.38-0.63 on Pt(335) and 0.23-0.49 on Pt(779). A striking correlation exists between the amount of the reversible nitrogen phase and the step densities on the two surfaces. The Pt(335) stepped surface was found to reversibly adsorb almost twice as much nitrogen as the Pt(779) surface in accordance with the factor of 2 density ratio for step sites on the two surfaces. The observation is valid at five different N2 fluxes (from 8 × 1010 to 1.2 × 1012 cm-2 s-1). The average ratio of the reversible coverages on the two surfaces is 1.9 ( 0.3. The two stages of N2 adsorption are shown in Figure 3. For the first stage (irreversible adsorption, left-hand panel), the coverage vs exposure data are taken from our previous study.13 For both Pt stepped surfaces, the coverage rises linearly at the beginning due to a constant sticking coefficient. At saturation, the ratio of coverages on the two crystal surfaces becomes 1.49 ( 0.25. The second, reversible stage of adsorption can occur only under a N2 flux, and therefore, coverage is presented as a

J. Phys. Chem. B, Vol. 105, No. 18, 2001 3735

Figure 4. Amount of reversibly adsorbed N2 phase as a function of the average beam flux. The data are fit with the Langmuirian isotherm equation and shown as solid curves. The maximum possible coverage of the reversible phase σrev max and the effective constant K are the fitting parameters. The shaded areas represent (1σ of errors of the data fits.

Figure 5. N2(a) RAIR band frequency as a function of N2 exposure (irreversible adsorption) or of N2 flux (reversible adsorption) at 88 K. Examples of typical RAIR spectra are provided in the insets as evidence of a single IR band throughout the range of N2 coverages.

function of N2 flux (right-hand panel). The coverages were obtained by integration of areas under the adsorption spikes shown in Figure 2. The absolute coverage scale was established using data from previous work13 by correlating the integrated areas to the corresponding coverages. A magnified picture of the reversible adsorption stage is shown in Figure 4. The aforementioned coverage ratio of about 2 for the two surfaces is easy to see here. For both crystal faces the adsorption isotherms are of the Langmuir type. The data were fit with a Langmuirian equation shown in the frame, and the resulting fit parameters are presented. The margins of their systematic deviation ((1σ) are depicted as the shaded areas. The maximal possible coverage, σrev max, given by the fit is also about twice as much for Pt(335) as for Pt(779) (5.6 × 1013 vs 2.9 × 1013 cm-2), and the effective constant K is almost the same for both surfaces (8.0 × 10-13 vs 8.3 × 10-13 cm-2 s-1). B. Reflection-Absorption Infrared Measurements. The absorbance and frequency of the N2 infrared band were followed during the two stages of adsorption.

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Figure 6. RAIR integrated absorbance of N2 on Pt(335) and Pt(779) at 88 K as a function of N2 exposure (irreversible adsorption) and N2 beam flux (reversible adsorption). Solid lines are fits to the experimental data.

Figure 7. N2 coverage and integrated IR absorbance vs N2 exposure and flux (for irreversible and reversible stages, respectively). The plot is a combination of Figures 3 and 6. The dots represent coverage data, and the triangles represent IR integrated absorbances. For the irreversible adsorption stage, coverage and IR absorbances are both accurately fitted with the same curve. Fitting curves for the reversible stage are separate and taken from Figure 4 (coverage) and Figure 6 (IR integrated absorbances).

The position of the IR band maximum shifts to lower frequencies as the N2 coverage increases. As can be seen from Figure 5, a frequency decrease with increasing coverage is observed in both stages. Initially, in the limit of zero coverage, the adsorbed nitrogen exhibits its single IR band at 2236 cm-1 on Pt(335) and 2236.5 cm-1 on Pt(779). As the coverage increases and irreversible adsorption approaches saturation, the IR signal shifts to a steady value of 2231.4 or 2234.0 cm-1, respectively. When the reversible adsorption begins under a N2 flux, no additional IR bands appear. The existing band continues to shift downward in frequency, reaching 2229.5 cm-1 on Pt(335) and 2232.3 cm-1 on Pt(779) at a N2 flux of 3 × 1015 cm-2 s-1. Figure 6 shows how the integrated absorbance of the N2 IR band changes during the adsorption processes. The irreversible stage shows an almost linear rise in the absorbance that reaches a steady value as the coverage reaches saturation. This value is

Zubkov et al.

Figure 8. Isotopic substitution of the irreversibly adsorbed N2 on Pt(335) under ambient pressure of N2. RAIR spectra of N2(a). (a) The surface saturated with 14N2 at 88 K is then exposed to a 15N2 ambient pressure of 3 × 10-7 mbar (random flux of 8.8 × 1013 cm-2 s-1) for 50 s. (b) The isotopes are added in the reversed order. The inset shows the 14N2 and 15N2 IR frequencies vs the estimated composition of the saturated N2 adlayer. The uncertainty of the peak positions of the lowintensity bands is depicted by the error bars.

0.045 cm-1 for Pt(335) and 0.03 cm-1 for Pt(779). As the reversible N2 phase is produced under the N2 flux, the integrated absorbance keeps growing, but this growth is much less pronounced. It can be fit very approximately by the logarithmic function of the flux, which however is not based on any model. For comparison purposes, the infrared absorbance data (from Figure 6) are superimposed on the corresponding coverage data (from Figure 3). The combined plot is shown in Figure 7. There is a striking correlation between the two sets of data for the irreversible strongly bound N2 phase. The integrated IR absorbance (triangles) and the N2 coverage (dots) data fall exactly onto the same curve, meaning that any coupling effect between the N2 oscillators influencing the infrared absorbance as the coverage rises is absent to within the accuracy of the measurements. The picture changes drastically with the onset of the reversible N2 adsorption process. For the reversible phase, the IR absorbance (triangles) increases much less compared to the coverage (dots). At a N2 flux of 1.2 × 1012 cm-2 s-1, the coverage on each of the two Pt crystal faces increases by 13-18% as found in the molecular beam experiments. However, the estimated increase of the integrated IR absorbance is less than 3% at this flux. Besides the increase of N2 coverage, another effect takes place under the incident N2 flux. Using isotopic labeling, we were able observe extensive exchange of N2 on the surface by IR. 14N was irreversibly adsorbed on the Pt(335) surface at 88 K, 2 and this saturated surface was exposed to 15N2 (exposure of 4.4 × 1015 cm-2). Almost complete isotopic exchange occurred, as shown in Figure 8a. The 14N2 phase (2232 cm-1) was almost displaced, and the 15N2 phase (2158 cm-1) appeared instead. Reversing the order of the isotopes led to the same phenomenon (Figure 8b). From the integrated IR absorbancies, we roughly estimate that the degree of the isotopic exchange is 70-85%. We can also use the presented data to plot semiquantitatively the NtN infrared frequencies as functions of the surface phase composition (inset in Figure 8).

Reversible Phase of N2 on Pt Surfaces IV. Discussion All aspects of the reversible N2 phase behavior will be discussed by comparison with the irreversible N2 adsorption process, which was studied separately.8,13 A. Location of the Reversible N2 Phase. The dramatic difference between the properties of atomic steps and terraces was addressed in our previous studies on irreversible N2 adsorption.8,13 We showed that this more strongly bound N2 species is adsorbed exclusively on the steps of the Pt(335) and Pt(779) surfaces. Here we conclude that the reversible N2 phase is also located on the step sites. At various N2 fluxes, the Pt(335) surface was shown to reversibly adsorb twice as much nitrogen as the Pt(779) surface (Figures 2 and 4), which perfectly correlates with a factor of 2 in the density of atomic steps. Terraces have to be excluded from consideration since the Pt(335) surface, in comparison with the Pt(779) surface, has fewer terrace sites, yet exhibits a much higher adsorption capacity. This is in agreement with previous DFT calculations8 for the Pt(112) surface, which has (100) steps and close-packed (111) terraces similar to those of the Pt(335) surface, except that terraces are one atom shorter. It was shown that on such a highly corrugated surface it was impossible to have stable molecular N2 species elsewhere but on the step edges. Recently the adsorption of N2 on the close-packed Pt(111) face was reported,19,20 but this N2 species is mainly physisorbed and exists only below 60 K. The idea of exclusive adsorption on step sites also agrees with our RAIR results (see Figure 5). First, the IR band of the step-bound N2 remains the only spectral feature throughout both stages of N2 adsorption. No new IR bands develop. Second, as the nitrogen coverage on the steps increases during the irreversible adsorption stage, its IR frequency experiences a shift to lower frequencies. It keeps shifting in the same direction as the reversible stage of N2 adsorption progresses. This observation strongly indicates that further accumulation of nitrogen occurs on the steps during the adsorption of the reversible N2 phase. The origin of the red shift is discussed later. B. Nature of the Reversible N2 Phase. A weak reversible adsorption under a molecular beam has been observed before in similar King-Wells-type experiments, namely, in lowtemperature studies of Pt(111) under a beam of ethane,21 ethylene,22 or CO.23 All these works report characteristic desorption spikes as observed by us for N2 (Figure 2), and explain them by the existence of a weakly adsorbed species that is populated only under an adsorbate flux and depopulated in a vacuum. However, given the chemical differences of the molecules studied, the interpretation of the origin of such weakly bound phases also differs. The key points are (1) whether such a phase forms a different adlayer on top of the strongly bound phase and, if not, (2) whether the two kinds of species are qualitatively different. In the study of ethane on the Pt(111) surface, the reversible adsorption is attributed to the formation of a physisorbed, liquidlike second layer of ethane.21 For CO adsorbed on Pt(111), the effect is viewed as a dynamical extra population of the empty Pt sites within the same chemisorbed layer, which makes it a more densely packed layer with new CO molecules indistinguishable from the preadsorbed ones.23 Such a conclusion confirms previous studies,24 in which this phenomenon was reported for CO on Pt(111). For ethylene adsorption on the Pt(111) surface, a similar model was proposed along with an alternative one, which considers strongly and weakly held species as chemically different (σ- and π-bound) but still coadsorbed in the same single layer.22

J. Phys. Chem. B, Vol. 105, No. 18, 2001 3737 We will discuss the applicability of such models to the 1-D adsorption of N2 studied here, starting with the possibility of N2 physisorbed overlayers. It was shown19 that multilayers of N2 on a close-packed Pt surface desorb at 28 K. One can visualize another structure in which a 1-D chemisorbed adlayer on the step edge facilitates 1-D physisorption along and next to itself, directly on the Pt surface (for example, in the step trough). The recent findings that N2 can physisorb on Pt(111) next to isolated chemisorbed N2 molecules19 could favor such a model. The second model involving an increased coverage of the step-bound chemisorbed layer appears to be more applicable to our system. The irreversible chemisorbed N2 phase on Pt step sites consists of N2 molecules in an atop configuration.8,13 They occupy about half of the step edge atoms at saturation. According to DFT calculations for N2 on Pt step sites,8 the new molecules should adsorb the same way, in an atop configuration. This results in an increase in the steric N2-N2 repulsion and therefore a decrease of the binding energy per molecule. This is clearly confirmed by the same calculations. If the degree of destabilization is significant, it can lead to an appreciable desorption rate of the N2 molecules from such crowded areas even at temperatures as low as 88 K. However, the highcoverage molecular arrangement can be maintained in dynamical equilibrium with the constant supply of oncoming molecules. Removal of the N2 flux will lead to desorption of the extra N2, which is exactly what we detect. There is an important prediction in this model. As the new N2 molecule adsorbs on an empty Pt step site between the two occupied sites, it becomes chemically equivalent to the neighboring molecules and indistinguishable from them. This perfectly agrees with our observation of a single IR band throughout the entire range of coverages studied (Figure 5). On the other hand, the energetic equivalence of the N2 molecules in the compressed zone makes them equal candidates for spontaneous desorption. After a number of elementary events of reversible adsorption and desorption, all the nitrogen phase must be completely refreshed, being substituted with new N2 molecules. The observation of almost total isotopic substitution for the adsorbed N2 (Figure 8) confirms the above idea of denser packing for the 1-D adlayer under dynamical equilibrium conditions. Since the Pt(335) surface in comparison with the Pt(779) surface should have twice as many steps, the former should adsorb twice as much nitrogen as the latter. Here we must mention that for the stronger (irreversible) adsorption the observed ratio was close to 1.49.13 The fractional population of the Pt(335) steps was lower than on the Pt(779) surface, and this was attributed to a stronger intermolecular repulsion across the steps of the Pt(335) surface, compared to step-step N2 repulsion across the wider Pt(779) terraces. However, for the reversible N2 phase, we have a ratio of 1.9, which raises the question of why the repulsions are no longer important under high-coverage conditions. It is likely that for the compressed 1-D chains the strongest interaction is between the adjacent molecules on the same atomic step edge. This repulsion mainly determines the thermodynamics of the 1-D layer and the achieved extra coverage. The interaction across the steps becomes a secondary factor. C. IR Investigation of the Adsorbed N2. 1. Origin of N2 Frequency Shifts with CoVerage. As shown in Figure 5, the N2 infrared band peak exhibits a significant red shift with increasing N2 coverage. This is unusual behavior for oscillating dipoles on a surface. With increasing coverage, the vibrational coupling

3738 J. Phys. Chem. B, Vol. 105, No. 18, 2001 should shift the internal modes to higher frequencies.25 The chemical bonding change and electrostatic interaction with the metal substrate can also affect these frequencies. The common way to distinguish between the dynamic vibrational coupling and the static effects is to attenuate the former by using different isotopic varieties of the adsorbate.25 When the same total N2 coverage is maintained, the static interactions are invariant and the observed effects have to be attributed to dynamic coupling. The inset in Figure 8 corresponds to this situation. As the fraction of 14N2 in the saturated adlayer increases, its frequency shifts upward by 5-15 cm-1. The same applies to 15N2 as its fraction grows. This demonstrates that even though the decreased dimensionality of the 1-D adlayer makes the vibrational coupling less efficient, the dynamic coupling shifts do occur and they occur in the correct directions to higher frequencies. Yet the frequency of the N2 vibration decreases with the N2 coverage. Thus, it appears that the coupling shift is dominated by a larger negative static shift. The estimated magnitude of the latter is 10-20 cm-1 (in the coverage range of the irreversibly adsorbed N2). Either electrostatic or chemical effects could be the origin of this negative shift. N2 adsorption on corrugated Pt surfaces was reported to produce a work function decrease.26 Recent calculations for the atop N2 on the stepped Pt(112) surface also predicted a decrease of the work function,8 which implies charge transfer from the molecule to the surface. As the charge accumulates on the surface, the electrostatic interaction can tune the frequency of the oscillator. This Stark effect is frequently observed in electrochemical systems. For step-bound CO on Pt(557), the negative potentials on the Pt electrode induced a downward frequency shift.27 As we see, the sign of the effect is consistent with our results. On the other hand, increased electron density on the Pt surface might facilitate π-back-donation to the N2 molecule, which would also soften the internal mode. It is difficult to discriminate between the two effects, especially since they could be very much interconnected. 2. IR Absorbance as a Function of the N2 CoVerage. For the irreversible N2 phase, the absorbance is proportional to N2 coverage; for the reversible phase, only a small IR absorbance is observed. The suggested chemical equivalence of the irreversibly and reversibly adsorbed N2 raises the question of why these two phases exhibit quantitatively different IR activity. The perfect proportionality of the integrated IR absorbance to the coverage of the irreversible N2 species (see Figure 7) means that the IR absorption coefficient remains constant as the phase builds up and it has the same value on both Pt(335) and Pt(779) surfaces. Thus, at this stage, all the N2 molecules must be oriented the same way parallel to each other (probably close to normal to the macroscopic plane on both stepped surfaces, giving similar extinction coefficients). The DFT calculations predict this.8 As the reversible N2 phase develops, the average extinction coefficient drops. If all the N2 admolecules remain chemically equivalent, it is suggested that in the compressed zone of the 1-D layer the N2 molecules become equally less IR active. For some chemical systems, the infrared band intensity not only stops increasing with coverage, but even starts decreasing as the coverage goes up.25 This phenomenon is generally attributed to the depolarization of static and dynamic dipole moments of admolecules at higher coverage. We can hardly speculate about the static dipole decrease. On one hand, as the N2 fractional coverage on the steps increases from 0.5 to 1, the calculated work function change shifts from -0.28 to -0.09 eV/N2.8 This

Zubkov et al. implies a smaller molecule-to-surface charge transfer. On the other hand, we do not know exactly how the decreased charge transfer affects the N2 intramolecular dipole moment. The dynamic dipole moment is indeed expected to drop with the coverage. In the compressed adlayer, the N2 molecules come into close proximity to each other and their electronic polarizability leads to dielectric screening.25 Another possible explanation is based on the lateral forces that may cause the adjacent molecules to tilt apart. As the new N2 molecule binds to a vacant Pt step atom between two preadsorbed molecules, the admolecules become affected by a strong steric repulsion. As a result, they orient away from each other in a zigzag structure along the step edge. A phenomenon like this has been observed in ESDIAD studies of CO on a stepped Pt(112) surface.28 Compression of the CO adlayer on the steps first caused formation of close pairs of CO molecules tilted away from each other along the step direction. Further compression produced longer multiplets of adjacent molecules with the inner molecules of each multiplet tilted in both directions perpendicular to the steps. Tilting from the surface normal will lead to a mutual partial cancellation of the dipoles, which subsequently decreases IR absorption. This hypothesis involving N2 tilting at high coverages on step sites contradicts the results of previous calculations,8 which predict no tilting of N2 on the Pt steps up to the highest possible coverage. D. Kinetic Model of the Reversible N2 Adsorption. Direct adsorption on steps from the gas phase does not explain our observations. As the reversible adsorption starts under the molecular beam, the sticking coefficient of nitrogen can reach 0.62 for Pt(335) and 0.49 for Pt(779). Such high values are inconsistent with the fraction of unoccupied step sites on the whole surface (20% or 7%, respectively). The high sticking coefficient compared to the empty chemisorption site density indicates the involvement of a precursor in the adsorption kinetics.29 Moreover, these high sticking coefficients exceed the total fraction of step sites on the surface (33% or 14%, respectively). On this basis, we conclude that the terraces do participate in the trapping of nitrogen. Therefore, the process must involve precursors on the terrace sites and, even more probably, at the step sites.13 Since both strongly and weakly bound N2 species are chemically equivalent, it is reasonable to apply here the same adsorption scheme that was derived for the irreversible N2 phase.13 The model excludes the possibility of direct chemisorption from the gas phase and introduces several intermediate species: a terrace-bound precursor and step-bound intrinsic and extrinsic precursors. For the simplification of the formalism, the two kinds of step precursors were set indistinguishable, which made their coverage independent of the coverage of the final chemisorbed phase. Here we will use the model in its full reversibility limit. The proposed kinetic model can be described by the following scheme: where G depicts the gas phase, T* is

the precursor on the terraces, S* is the precursor on the steps, S is the chemisorbed phase on the step atoms, and ki are rate constants. The precursors are formed by direct adsorption from the gas phase and interconvert into each other. The step precursor can undergo reversible chemisorption. The following approximations are made: (1) The absolute steady-state coverage of the terrace-bound precursor, σT*, is

Reversible Phase of N2 on Pt Surfaces

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expected to be very small. The remaining number of unoccupied terrace sites is NT - σT* ≈ NT, where NT stands for the density of terrace sites. (2) As the precursors on the steps are indistinguishable, they can exist on top of chemisorbed species as well as on vacant step sites. Therefore, their absolute coverage σS* depends only on the density of the step Pt atoms NS and is also expected to constitute a small fraction of NS. The remaining number of sites available for the step precursors is NS - σS* ≈ NS. (3) Lateral interactions are neglected for each of the participants. (4) The surface sites for chemisorption are assumed equivalent. The final 1-D chemisorbed phase is composed of two portions of N2 moleculessthose irreversibly and reversibly adsorbed (σ ) σirr + σrev). Since σirr is constant, it is ignored in the derivations. The reversible N2 coverage σrev is the new variable. Solution of the conjugated rate equations for steady-state conditions (as the system reaches equilibrium under the beam flux) gives the following expression for the amount of the reversibly chemisorbed N2:

KF σrev ) σrev max 1 + KF

(1)

where σrev max represents the maximal possible coverage of the reversible N2 and

K)

k-1k2 + k1k3NT + k2k3NS k4 k-1k-2 + k-1k-3NT + k-2k3NS k-4

The derived adsorption isotherm has a Langmuirian form, in full agreement with the experimental observation (Figure 4). The applicability of the Langmuirian isotherm implies that the effective probability of chemisorption is proportional to the rev fraction of unoccupied chemisorption sites (σrev max - σ ), as if direct adsorption occurs from the gas phase. The presence of precursors does not eliminate this dependence. This can be understood in the following way. Close to saturation, the adsorption sites become scattered widely across the surface such that the precursors have a chance to reach these sites before desorption only if the adsorbed precursor is present in the near vicinity of the empty site. This causes the aforementioned proportionality and corresponds to a “pseudo-Langmuirian” limit. In eq 1, the coverage of the reversible phase on the step sites (σrev) does not strongly depend on the density of the step atoms (NS), which may seem surprising. However, an indirect dependence of σrev on NS is embedded in σrev max that must be roughly proportional to the density of the step atoms NS. Therefore, doubling of the step density should lead to doubling of σrev max as well as σrev at any flux F, which is what we observed comparing Pt(335) to Pt(779). The values of σrev max obtained from extrapolating the Langmuirian fits (see Figure 4) to highest coverages are somewhat lower than expected. At the maximum possible coverage, σrev max, only up to about 30% of all the empty Pt step atoms would be occupied by the reversible N2 phase at 88 K. We were unable to extract rate constants for the individual steps. The profile analysis of the adsorption and desorption traces shown in Figure 2 did not provide more insight given the complexity of the mechanism. It was shown that, in the case of negligible lateral interactions and equivalence of all chemisorption sites, the Langmuirian isotherm is expected for any precursor-involving model.29 We already mentioned studies of the reversibly chemisorbed CO

or C2H4 on Pt(111), which are also believed to form via a precursor-mediated mechanism.22,23 Both 2-D systems exhibited an adsorption isotherm of the Langmuir form. To our knowledge, the presented work is the first one reporting precursormediated reversible 1-D chemisorption. The fact that the 1-D system obeys the same kinetics as its 2-D counterparts demonstrates the universality of this adsorption kinetics concept and is important for future theoretical predictions. V. Conclusions In this study we have shown the following: (1) The saturated 1-D chemisorbed layer of N2 on the stepped Pt(335) and Pt(779) surfaces can be additionally populated at 88 K under a N2 molecular flux. This produces a reversibly adsorbed portion of N2. (2) The isotopic exchange of all N2 species is easily accomplished under these conditions. (3) The amount of the reversibly adsorbed N2 at various gas fluxes is proportional to the step density: for Pt(335) it is almost twice (1.9 times) as much as for Pt(779). (4) The reversible N2 phase is indistinguishable from the irreversible one in the IR spectrum. In the whole range of coverages, the adsorbed N2 exhibits a single IR band (22362230 cm-1). (5) The N2 infrared feature undergoes a shift to lower frequencies with increasing N2 coverage. This is a superposition of the two opposite effects: a vibrational coupling shift (to higher frequencies) and a larger static shift (to lower frequencies). (6) For the irreversibly adsorbed N2, the IR integrated absorbance is proportional to the coverage. For the reversible N2 phase, the proportionality fails, indicating a significant decrease of the average extinction coefficient for crowded N2 species on the step sites. This is attributed to depolarization of the dynamic dipole moment and/or to tilting of the molecules away from each other at high coverage. (7) High initial sticking probabilities for the reversible N2 adsorption (0.23-0.63) indicate that the adsorption involves a precursor on the terraces. (8) The experimentally observed Langmuirian adsorption isotherm for the reversible N2 phase is explained by the proposed kinetic model that includes terrace- and step-bound precursors. Thus, the first reported reversible precursor-mediated 1-D chemisorption was demonstrated to obey the same “pseudoLangmuirian behavior” as known for its 2-D analogues. Acknowledgment. We thank the Department of Energy, Office of Basic Energy Sciences, for support of this work. References and Notes (1) Soukiassian, P.; Kubby, J. A.; Mangat, P.; Hurych, Z.; Schirm, K. M. Phys. ReV. B 1992, 46, 13471. (2) Bonch-Bruevich, A. M.; Vartanyan, T. A.; Maksimov, Yu. N.; Przibel’ski, S. G. J. Exp. Theor. Phys. 1997, 85, 200. (3) Weiss, P. S.; Eigler, D. M. Phys. ReV. Lett. 1992, 69, 2240. (4) Trischberger, P.; et al. Surf. Sci. 1997, 377-379, 155. (5) Siddiqui, H. R.; Chen, P. J.; Guo, X.; Yates, J. T., Jr. J. Chem. Phys. 1990, 92, 7690. (6) Hayden, B. E.; Kretzschmar, K.; Bradshaw, A. M. Surf. Sci. 1985, 149, 394. (7) Tripa, C. E.; Yates, J. T., Jr. ReV. Roum. Chim. 1999, 44, 1035. (8) Tripa, C. E.; Zubkov, T. S.; Yates, J. T., Jr.; Mavrikakis, M.; Nørskov, J. K. J. Chem. Phys. 1999, 111, 8651. (9) Ramirez-Pastor, A. J.; Bulnes, F. M.; Riccardo, J. L. Surf. Sci. 1999, 426, 48. (10) Bojan, M. B.; Steele, W. A. Mol. Phys. 1998, 95, 431. (11) Rittner, F.; Paschek, D.; Boddenberg, B. Langmuir 1995, 11, 3097. (12) Bakaev, V. A.; Steele, W. A. J. Chem. Phys. 1993, 98, 9922.

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