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Langmuir 1990, 6, 970-973
Registry No. 22BPY, 366-18-7; 24BPY, 581-47-5; 33BPY, 581-46-4;44BPY, 553-26-4; BPPY, 1008-89-5;3PPY, 1008-88-4; 4PPY, 939-23-1;26DPPY, 3558-69-8;BMPY, 109-06-8;26DMPY,
108-48-5;22BPDC, 482-05-3; 44BPDC, 787-70-2; 44DC, 681338-3; 4M4C, 103946-54-9; 55DC, 1802-30-8; 4455TC, 12522788-5; Pt(lll), 7440-06-4.
Pyridine Adsorption on Polycrystalline Platinum Studied by the Radioactive-Labeling Method E. K. Krauskopf, L. M. Rice-Jackson, and A. Wieckowski* University of Illinois, Urbana-Champaign, 1209 W. California St., Urbana, Illinois 61801 Received October 9, 1989. In Final Form: December 5, 1989 The adsorption of pyridine on a polycrystalline platinum electrode was measured by using a radioactive-labeling technique. The potential dependence and concentration dependence of adsorption were measured in 0.1 M HC104, and two exchange experiments were performed. The potential dependence revealed behavior typical for a strongly adsorbed organic species; a broad plateau of maximum adsorption occurs in the double-layer region with decreased adsorption toward potential extremes. No exchange of surface species with added unlabeled solution pyridine occurred at 0.2 V (vs Ag/AgCl, 1 M Cl-1, but slow exchange was observed at -0.2 V over about 2 h; slow hydrogenation is the most likely cause of the loss of adsorbate at this potential. Two differentconcentration dependence profiles were observed depending on the treatment of the platinum electrode surface; a clean surface gave consistently higher packing densities as a function of pyridine concentration than one that had been pretreated at 104 M. The observed packing densities correspond to horizontally oriented pyridine, formed at low concentrations, and vertically oriented pyridine, formed at high concentrations.
Introduction The adsorption of aromatic compounds on metal surfaces is interesting to study because of the multiple orientations the molecules can adopt with respect to the substrate. Pyridine, in particular, is of interest because it has two dominant sites for the formation of a surface bond: the nitrogen atom in the ring or the entire ring system itself. For example, for gas-phase dosed pyridine at low coverages on Ag(lll), the aromatic *-cloud interacts strongly with the metal surface, and the pyridine molecule is adsorbed with the ring plane parallel to the surfaceel A t higher exposures, pyridine becomes a-bonded through the nitrogen lone pair and stands up from the surface a t an inclined angle.’ The molecular orientation of a wide range of adsorbed aromatic compounds has been studied by Hubbard and c o - w ~ r k e r s . ~They - ~ used a thin-layer electrochemical (TLE) cell to measure the loss of electroactive analyte from a small volume of solution due to adsorption by a platinum electrode. This method is limited to electroactive species, so pyridine was not studied. For hydro(1)Demuth, J. E.;Christmann, K.; Sanda, P. N. Chem. Phys. Lett. 1980,76,201. (2)Soriaga, M.P.; Hubbard, A. T. J. Am. Chem. SOC.1982,104,2735. (3)Soriaga, M.P.; Wilson, P. H.; Hubbard, A. T. J.Electroanal. Chem. 1982,142,317. (4)Soriaga, M.P.; Stickney, J. L.; Hubbard, A. T. J. Mol. Catal. 1983,21,211. ( 5 ) Soriaga, M. P.; Hubbard, A. T. J . Electroanal. Chem. 1984,167, 79. (6) Soriaga, M.P.; Stickney, J. L.; Hubbard, A. T. J . Electroanal. Chem. 1983,144,207. (7)Soriaga, M.P.; Hubbard, A. T. J. Phys. Chem. 1984,88,1089. (8)Soriaga, M. P.; Hubbard, A. T. J. Electroanal. Chem. 1984,167, 79. (9) White, J. H.; Soriaga, M. P.; Hubbard, A. T. J.Electroanal. Chem. 1984,177,89.
0743-763190/2406-0970$02.50/0
quinone, two plateaus were observed in the packing density versus solution concentration profile, corresponding to a flat orientation (low packing density) a t low concentrations and a vertical orientation (high packing density) a t high concentration^.^ The shape of the transition between the two orientations depended on whether the electrode was clean or “predosed”; the presence of flat molecules already on the surface inhibited the transition to a more tightly packed vertical state. Kuwana and co-workers developed a related technique, the long optical path length thin-layer cell method, which can measure the adsorption of nonelectroactive compounds as long as there is measurable solution absorbance.1° With this method, Kuwana’s group studied both hydroquinone-related compounds and pyridine-related c o m p ~ u n d s . l l - ~The ~ pyridine work was focused on the potential dependence of adsorption, and their study to 1.5 X was limited to a concentration range of M, over which only a slight increase in coverage was reported.13 The vertical orientation of adsorption was assumed, and the predosing effect was not addressed. The objective of our experiments was to see if the predosing effect identified by Hubbard would also be observed for pyridine. In contrast to Hubbard‘s and Kuwana’s work, our radiotracer technique is not based on measuring the loss of adsorbate from a small volume but rather is based on measuring the surface signal directly. In this paper, we discuss our results of t h e adsorption of [2,614C]pyridineon polycrystalline platinum, which includes characterizing the potential dependence of adsorption, (10)Zak, J.; Porter, M. D.; Kuwana, T. Anal. Chem. 1983,55, 2219. (11)Gui, Y.-P.; Porter, M. D.; Kuwana, T. Anal. Chem. 1985,57, 1474. (12)Gui, Y.-P.; Kuwana, T. Langmuir 1986,2,471. (13)Gui, Y.-P.; Kuwana, T. J. Electroanal. Chem. 1987,222,321.
0 1990 American Chemical Society
Pyridine Adsorption on Polycrystalline Platinum
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Figure 1. Two measurement positions in a radiotracer experiment. (A) Electrode lifted up for adsorhate equilibration and bulk counting rate measurement. (B)Electrode pressed down against scintillator for surface counting rate measurement. evaluating the strength of adsorption through exchange experiments, and measuring the concentration dependence for clean and predosed show our that the predosing effect applies to pyridine, with flat pyridine preventing the formation of the more tightly packed vertical formation, which is seen at high concentrations on a clean surface. Experimental Section The radiotracer apparatus, in which a glass scintillator detector is coupled to a photomultiplier tube ccounting system, has been described previously.1' The working electrode was a polycrystalline disk electrode (Engelbard, 11-mm diameter), and it was polished with diamond paste to give a resulting roughness factor of 1.4, as determined by integrating the anodic peak of the voltammogram due to hydrogen desorption and assuming one hydrogen atom per platinum site. A platinum wire counter electrode and Ag/AgCI (1 M CI-) reference electrode completed the cell, and all potentials are reported with respect to this reference electrode. The electrolyte was 0.1 M HCI04, prepared in Millipore water and deaerated with N2. Radioactive 12.6-'4Clovridine was obtained from Amersham and diluted to anactivjiiof 8.31 mCi/mmol. l h e counting time for each~dai point was SO 8. and each reported counting rate consists of the average of three consecutive data pointu. T u briefly review the method. there are twu measurement positims of the electrode with respect to the glass scintillatur detector at the bottom of the cell (Figure I ) . When the elcctrode is lifted up above the scintillator, to a height of ahout 1-2 mm. the counting rate of the bulk pyridine in solution is measured. In this position. the potential can he changed, and mol. ecules from the hulk may adsorb to or desorb from the elec. trode surface as a result. In the second position, the electrode is pressed down against the scintillator, to a height of about 1-3 Nm. In this position, the signal is composed of the counting rates from the adsurbate and from the thin layer of trapped solution. Because pyridine is stronglyadsorbed, the solution from which it adsorbed can he rinsed out and replaced with clean electrolyte solution without losing any adsorhate in the process, The cell geometry allows for the electrode to he rinsed while maintaining full potential control. The electrode is covered by solution at all times; the adsorbate is not exposed to air during the rinsing process. The surface signal measured at this point will he due only the adsorbed pyridine. The packing density is then calculated as follows:
r = (lo-3N~*N~C)/[N,,,~~~ exp(-rrx)l (1) In the above equation, r is the packing density, expressed in
molecules/cm2; Nab is the net adsorhate counting rate; NA is Avogadro's constant, 6.02 X loz3;C is the bulk concentration in mol/L; Nb,ik is the net bulk solution counting rate; rr is the adsorption coefficient for carbon-14 in water, 300 cm-1;16.16 R is the roughness factor of the electrode surface, 1.4; j b is the
(14) Krauskopf, E. K.; Chan, K.:Wieckowski, A. J. Phys. Ckem. 1987, 91, 2321. (15) Lereh. P.Helu. Phys. Acta 1953.26.663.
-0.2
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Figure 2. Voltammogram recorded at 50 mV/s for polycrystalline Pt in the meniscus position (0.950 cm2 geometric area, R = 1.4). Solid line: 0.1 M HCIOI alone. Dashed lines: pyridine was adsorbed from a 10-3 M pyridine solution at 0.2 V. The SohtiOn W a s replaced hy clean electrolyte, and the first two scans were recorded, initially going negative from 0.2 V. backscattering factor for platinum, 1.86;" and x is the gap distance in cm, 1.9 X lo4 cm. The expression, exp(-w), is equivalent to the squeezing efficiency, 0.944, as described previo~slv.'~
Results and Discussion Voltammetry. After the electrode was dosed with the pyridine solution at 0.2 V, the cell was thoroughly rinsed out with 0.1 M HC104, taking care to keep the electrode face submerged in solution at all times to maintain potential control. The electrode was then pulled up to the top of the cell so that a meniscus formed at the electrode/ solution interface. In this way, a voltammogram of the polished working face alone could he recorded. A representative voltammogram of the adsorbate from 10-3 M [2,6-14C]pyridineis shown in Figure 2. The first two s w s are shown tonether with the scan of the clean electrode surface for reference. The first scan, recorded going toward negative potentials from an initial point of 0.2 V, shows the typical teatures of a strongly adsorbed organic material: distortion of the and hydrogen regions, Repeated cycling slowly the adsorbate from [he surfare as it is repeatedly oxidized and reduced, Potential Dependence. An aliquot of [2,6-"C]pyridine W a s injected into 10 mL of deaerated electrolyte so that a final concentration of lo-' M was achieved. The potential dependence began at 0.2 V and proceeded by 100-mV steps negative to -0.2 V, then positive to 1.2 V. At each potential, the electrode was lifted for equilihration and lowered for measurement until a steady surface signal was observed. This generally took on theorder of 3-5 min. From the voltammogram, we would predict that the coverage would be lower toward both extremes of the potential range, as oxidation or reduction occurs, and higher in the middle, in the double-layer region. T h e potential dependence of adsorption indeed follows this predicted profile, as shown in Figure 3. The observed hysteresis in the hydrogen region is probably due to slow hydrogenation which occurs here. Not shown are the data from M pyridine, which follows the same trend. At this higher concentration, the background signal from the solution itself begins to interfere with observation of the surface signal, so it hecomes more difficult to see small changes in packing. (16)Suttle, A. D., Jr.; Libby, W. F. Am!. Chem. 1955,27,921. (17) Zumwalt, L. R.Absolute Beto Countrw Usiw End- Window Geiger- Muellrr Counters ond Erperimmtol Ihta on Bero.Poniele Scotterm# W e m , AECC-567, I . 3 . Atomic Energy Commission, Technical Information Service. Oak Ridp,~.TN
972 Langmuir, Vol. 6, No. 5, 1990
Krauskopf et al. 6000
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Figure 3. Potential dependence of adsorption for 10-4 M [2,6Wlpyridine in 0.1 M HC104, beginning at 0.2 V, going stepwise to -0.2 V and then going to 1.2 V.
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Figure 4. Exchange at 0.2 V (double-layer region). After the electrode surface was dosed with 104 M [2,6-14C]pyridine,the solution was replaced by clean 0.1 M HC104: (M) bulk (background) counting rate when electrode is lifted after replacing the electrolyte; ( 0 )surface counting rate in squeeze position. The arrow indicates when 0.012 M unlabeled pyridine was added.
Exchange. To evaluate the strength of the interaction between pyridine and the surface, two exchange experiments were performed in which unlabeled pyridine was added to the cell to see if the cold molecules would replace the hot ones already adsorbed on the electrode surface. This was done a t two potentials, in the double-layer region at 0.2 V, and in the hydrogen region a t -0.2 V. In both cases, after equilibrium packing was established, the bulk pyridine solution was removed from the cell and replaced by clean electrolyte. With the electrode pressed down so exposure would not be immediate, an aliquot of unlabeled pyridine was injected, yielding a cell concentration of 0.012 M. The electrode was moved up for exposure to the cold pyridine during which time the bulk counting rate of the cell was recorded as a means to monitor the background counting rate during the experiment. (This background counting rate includes the dark count, the signal from radioactive material adsorbed directly onto the glass scintillator, and the signal from any residual bulk material still in solution after rinsing.) Next, the electrode was pressed down for a surface measurement to see how much loss had occurred during the previous 50 s of exposure (the duration of the background count). The electrode was repeatedly exposed and lowered to track the progress of the exchange with time. The results from the exchange experiment a t 0.2 V are shown in Figure 4. The time axis represents counting interval time (a series of 50-s intervals), not the actual time elapsed; the time it took to change the electrode's position between measurements, which was small with respect to the counting interval, is not included. The
Figure 5. Exchange at -0.2 V (hydrogen region). After the electrode surface was dosed with 10-3 M [2,6-'4C]pyridine, the solution was replaced by clean 0.1 M HC104: (m) bulk (background) counting rate when electrode is lifted after replacing the electrolyte; ( 0 )surface counting rate in squeeze position. The arrow indicates when 0.012 M unlabeled pyridine was added.
steady surface signal observed initially shows that there is no significant exchange with the pyridine-free electrolyte solution. At the arrow, upon addition of cold pyridine, still no change was observed. After taking a set of alternating surface and background measurements, the electrode was exposed for an additional 15-min time period of undisturbed time in the lifted position. No decrease in the surface counting rate occurred, confirming that in the double-layer region pyridine is strongly bound to the platinum surface and will not be displaced by pyridine molecules in the bulk solution. At -0.2 V, toward the onset of hydrogen evolution, the situation is quite different. After a stable coverage had been established with the hot pyridine, the switch to cold pyridine in solution resulted in the slow loss of radiolabeled pyridine from the surface over about 2 h (Figure 5). In contrast to the situation in the double-layer region, in this potential window, the surface bond weakens and desorption occurs, freeing up sites for cold pyridine to occupy. These data provide indirect evidence that the hydrogenation and subsequent desorption of pyridine may be occurring a t this potential. This is in agreement with Kuwana and co-workers, who used hydrogen consumption calculations and spectral changes to indicate that pyridine is probably being hydrogenated to a ring-saturated compound.13 Concentration Dependence. The concentration dependence experiments were done in two ways. (1) In the predosed case, the electrode was dosed with lo4 M [2,6-14C]pyridine. Higher bulk concentrations of pyridine were achieved by simply injecting more pyridine into the cell containing the previous solution. The potential was held fixed at 0.2 V so that the adsorbate layer formed from the start was not disturbed. (2) With the clean electrode experiments, in between the different concentrations, the pyridine solution was rinsed out of the cell, and the potential was cycled between the limits of oxygen and hydrogen evolution several times as the cell was repeatedly rinsed with clean electrolyte until a clean scan could be obtained. (The roughness factor did not change more than 5% throughout the experiments.) After this cleaning step, a new concentration of pyridine was introduced for adsorption a t 0.2 V. Two concentration dependence profiles were obtained, one for the predosed electrode (Figure 6, lower curve) and one for the clean electrode (Figure 6, upper curve). These results are qualitatively similar to the results obtained for hydroquinone by Hubbard et al.,' except
Langmuir, Vol. 6, No. 5, 1990 973
Pyridine Adsorption on Polycrystalline Platinum
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Figure 6. Concentrationdependence of [2,6-'4C]pyridine adsorp tion at 0.2 V: (A)packing density observed for a clean electrode at each concentration; (m) packing density observed for an electrode that was predosed with lo* M [2,6J4C]pyridine. Cumulative additions were made to this initial concentration to reach higher ones.
that in our case the two curves do not meet a t a common plateau above M. The data show that the less densely packed adsorbate which is formed a t 1 0 4 M is preserved with very little change in the value of r even as concentrations as high as 10-3 M are introduced into the cell. This shows that this adsorbate formation is relatively stable as long as the potential is held fixed a t 0.2 V. This predosed state, assumed to be horizontally oriented pyridine, prevents the formation of the more densely packed (vertically oriented) state that is observed when the clean electrode surface is exposed directly to higher pyridine concentrations. At a concentration of M, for example, the clean electrode allowed 3.1 X 1014 molecules/cm2 to be adsorbed, while for that same concentration, the predosed electrode only allowed 1.7 X 1014 molecules/cm2 to be adsorbed. For comparison, theoretical packing densities for pyridine are 4.38 X 1014 molecules/cm2 for the vertical orientation (nitrogen attached to the metal surface, with the ring plane perpendicular to the surface) and 2.31 X 1014 molecules/cm2 for the flat orientation (ring plane parallel to the metal surface).18 The theoretical calculation is based on the assumption that an adsorbed molecule fits inside a box, the dimensions of which are determined by geometrically accounting for individual bond lengths and atomic van der Waals radii.2*3*8The area taken up by a flat or a vertically oriented molecule is calculated from this model. The reciprocal of the area per molecule gives the packing density, I?, the number of molecules per unit area a t full coverage. It should be noted that Kuwana and co-workers found a packing density of 0.684 nmol/cm2 (4.12 X 1014 molecules/cm2) for 1.4 X loe4M pyridine,l3 which is close to the theoretical packing for vertical pyridine and is thus in disagreement with our results. However, our surfaces, although mirror-polished, were not perfectly smooth; a consistent roughness factor of 1.4 was found. As reported by Hubbard et al., even small deviations from ideal smoothness can affect the chemisorption of aromatic molecules, resulting in differences in packing densities at individual bulk concentrations.9 Since our measurements were not extended above 1 mM concentration, it is possible (18) Stern, D. A.; Leguren-Davidson,L.; Frank, D. G.; Gui, J. Y.; Lint C.-H.; Lu, F.; Salaita, G. N.; Walton, N.; Zapien, D. C.; Hubbard, A. T. J. Am. Chem. SOC.1989,111,877.
that saturation packing of the surface with pyridine molecules, as reported by Kuwana et al., could take place a t higher concentrations. The assumption in the model of a perfectly vertical mode of attachment through only the nitrogen, with no contribution from any of the ring carbons, is rather simplistic, to the extent t h a t tilted species are being neg1e~ted.l~ Indeed, gas-phase results of pyridine adsorption on Pt(ll1) and P t ( l l 0 ) indicate that different types of bonding can occur with the surface, and a purely veran tical species has never been ~ b s e r v e d . ~ O Instead, -~~ a-pyridyl species is observed, where both the nitrogen and one adjacent carbon atom appear to be attached directly to the metal surface. Moreover, the ring plane is not observed to be perpendicular to the surface but is instead tilted, forming an angle of about 74' with the surface, measured by using NEXAFS.20 Platinum(ll1) dosed with aqueous pyridine and studied by Auger electron spectroscopy (AES) also revealed coverages corresponding to a tilted species.18 The concentration dependence for pyridine on Pt(ll1) showed virtually no change in r over 4 orders of magnitude, when dosed electrochemically and evaluated by AES,18 indicating that no flat species or reorientation was observed. On the (110) single crystal face, however, a compression phase change was observed by HREELS as the pyridine exposure increased. At low coverages, nearly complete hydrogen-deuterium exchange occurred, while a t high coverages only partial exchange occurred, evidence that the transition was between a flat to a more vertically oriented species.22 Our results for solutionphase pyridine adsorbed on polycrystalline platinum thus more closely match the behavior observed on Pt(ll0) than on Pt(ll1).
Conclusions Using an in situ radiotracer technique, we have demonstrated that pyridine displays the same sort of behavior as hydroquinone and related compounds, as documented by Hubbard and co-workers, in that it forms a stable flat adsorbate array a t low concentrations that does not allow vertical reorientation to occur a t higher bulk concentrations. A vertically oriented molecular arrangement is achieved if the electrode surface is cleaned prior to exposure, although the observed packing density is lower than the packing found by Kuwana et al. and by theoretical calculation. Pyridine is strongly chemisorbed, as demonstrated by the absence of exchange a t 0.2 V, but possible hydrogenation removes it from the surface as evidenced by the slow exchange observed a t -0.2 V. The potential dependence of adsorption appears as expected based on its voltammetry in solution. Acknowledgment. This work was supported by the Air Force Office of Scientific Research (AFOSR-8903681, Dow Chemical, and the University of Illinois. E.K.K. would like to thank Du Pont for support in the form of a fellowship. Registry No. Pt,7440-06-4;pyridine, 110-86-1. (19) Gland, J. L.; Somorjai, G. A. Surf. Sci. 1973, 38, 157.
(20) Johnson, A. L.; Muetterties, E. L.; Stohr, J.; Seth, F. J. Phys. Chem. 1986.89.4071. (21) Gra&i&, V. H.; Muetterties, E. L. J.Phys. Chem. 1986,90,5900. (22) Surman, M.; Bare, S. R.; Hofmann, P.; King, D. A. Surf. Sci. 1987. 179.243.
(23) Connolly, M.; Somers, J.; Bridge, M. E.; Lloyd, D. R. Surf. Sci. 1987,185,559.
