The Journal of
Physical Chemistry
0 Copyright, 1985, by the American Chemical Society
VOLUME 89, NUMBER 3
JANUARY 31,1985
LETTERS Surface-Enhanced Raman Scattering as a Means of Studying Competitive Adsorption onto Silver Colloids Susanne M. Heard, Franz Grieser,* Colin G . Barraclough, Department of Physical Chemistry, University of Melbourne, Parkville, Victoria, 3052, Australia
and John V. Sanders Commonwealth Scientific and Industrial Research Organisation, Division of Materials Science, University of Melbourne, Parkville, Victoria, 3052, Australia (Received: June 19, 1984; In Final Form: November 19, 1984)
The competitive adsorption of pyridine with citrate present on Carey Lea silver sols has been observed by surface-enhanced Raman scattering. A strong influence of C1- on this competitive adsorption is seen. The use of polyvinylpyrrolidone to stabilize colloidal aggregates is reported and competitive adsorption in this system is compared to that in nonstabilized sols.
Introduction Even though the origins of the surface-enhanced Raman scattering (SERS) phenomenon have still to be clearly defined, SERS nonetheless provides a powerful means for studying the in situ adsorption of a range of molecules onto an ever increasing number of metal substrates. SERS has been used to identify the orientation of certain adsorbates on metal surfaces,' as well as providing information on surface packing of the adsorbate.2 In the present letter we report on the use of SERS in identifying competitive adsorption of citrate and pyridine onto silver colloids.
contained in glass capillary tubes with scattered light collected at 90° to the incident beam. A 4-cm-' band-pass and laser powers (514.5-nm argon ion) of 100-200 mW were routinely employed. The preparation of the sols and procedures used for the addition of adsorbates have been given in detail e l s e ~ h e r e . ~The ? ~ concentration of the silver sols used in the present study was generally M in silver. Electron micrographs were obtained with a JEOL JEM-1 OOCX transmission electron microscope. Stabilization of the sols was achieved by the addition of small aliquots of polyvinylpyrrolidone (PVP) solution to the prepared sol? The final PVP concentration was of the order of 0.2% (w/v).
Experimental Section Raman spectra were obtained with a Spex Ramalog spectrometer, equipped with a data acquisition facility. Samples were
Results and Discussion The preparation of the two Carey Lea sols used in the present study has been detailed in a previous communi~ation.~The
(1) (a) Allen, C. S.; Van Duyne, R. P.Chem. Phys. Lett. 1979, 63,455. (b) Bunding K. A.; Lombard, J. R.; Birke, R. L. Chem. Phys. 1980,49,53. ( 2 ) Heard, S. M. Ph.D. Thesis, University of Melbourne, 1984.
(3) Heard, S. M.; Grieser, F.; Barraclough, C. G.; Sander, J. V. J . Colloid Interface Sci. 1983, 93, 545. (4) Heard, S. M.; Grieser, F.; Barraclough, C. G . Chem. Phys. Lett. 1983, 95, 154.
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390 The Journal of Physical Chemistry, Vol. 89, No. 3, 1985
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~ i g i o rI. Raman spectrum of citrate: (a) 40% aqueous solution, as traces remaining on C a y Lea sols; (b) Ag(C.L.)-r; (c) Ag(C.L.)-y (160 mW, photon counts = 1 X Id counts/full scale, X = 514.5 nm).
differences in the two sols result from using diffmnt salt solutions in the reflocculation and redispersion (washing) steps that follow the initial Ag+ reduction. A yellow sol (colloidal Ag particles are dispersed) is formed from citrate washings, and a red sol (some aggregation of the colloidal Ag particles exists) is formed when NaN03 is used in the washings. A close inspection of the Raman spectrum of the yellow sol (Ag(C.L.)-y) showed the presence of citrate bands, although these were extremely weak (the citrate concentration in solution was estimated to be lbs-lW M). In contrast, the Raman spectrum of the red sol (Ag(C.L.)-r) showed the presence of strong citrate bands, although no nitrate features, indicating that citrate had adsorbed on the silver particles during the initial preparation and that subsequent washings had not totally displaced the surface adsorbed species. Figure 1 compares the Raman spectrum of aqueous (40%) citrate solution with the spectra of two primary sols. If one considers the method of preparation of the two sols, the difference in the intensities of the citrate bands observed is unlikely to be due to a lower citrate coverage of the Ag(C.L)-y particles. The difference in intensity most likely stems from the difference in the degree of aggregation of the colloidal particles, which we and others have noted previou~ly.~Electron microscopy pictures of the two sols show that the yellow sol consists predominantly of single particles, whereas the red sol is composed of aggregates consisting of a large number of silver particles as seen in Figure 2a. Addition of pyridine to these sols (yellow or red) made very little, if any, change to the SERS spectrum-the citrate bands remained dominant (seeFigures 3 and 4). Parts a and b of Figure 3 show spectra of yellow and red Carey Lea sols, respectively, before pyridine addition. Against these are compared the spectra (in the region 900-1100 an-')containing the strongest pyridine signals for the same sols after pyridine addition. As can be seen, any pyridine features are weak and superimposed on the stronger (5) Siiman, 0.;Bumm, L. A.; Callaghan, R.; Blatchford, C. G.; Kcrker,
M.1.Phys. Chem. 1983,87,1014. (6) Creighton, J. A.; Alvam, M.S.;Weitz, D. A.; Gamff, S.; Kim, M. W.J . Phys. Chem. 1983,87,4193.
F'igme 2 Electton micrographsof (a) Ag(C.L) red sol and (b) the same sol with 5 X lo-* M NaCl added. This sol was yellow with an absorption maximum at 400 nm.
citrate bands. This implies that under the conditions used citrate adsorbs far stronger on silver than does pyridine. Addition of NaCl (up to ca. 5 X 10-4 M)to the two Carey Lea sols either before or after adsorbate addition produced striking effects on both the extinction and Raman spectra seen from the sol. In the yellow Carey Lea sol, the chloride appeared to assist adsorption of the amine, manifested by the further enhancement of the SERS intensity (seeFigure 4b). In some cases, a previously barely observable or inobservable surface spectrum was observed with the addition of NaCl. The usual particle aggregation pattern followed adsorption of the amine, as seen by a change in color of the soL3 For the red Carey Lea sol, however, unusual aggregation behavior was observed. When chloridewas added, the dark red sol became yellow, that is, a reversal of the usual aggregation trend. This yellowing even took place on sols to which pyridine had already been added. Furthermore,where the Ag(C.L.)-r/pyridine system had previously only shown citrate bands in the Raman spectrum, strongly enhanced pyridine bands were then seen, while the citrate became indetectable (see Figures 3 and 4). Figure 3c shows the radical change in the surface spectrum produced by NaCl addition to the sol of Figure 3b. (Intermediate citrate desorption/pyridine absorption could sometimes be seen as shown
The Journal of Physical Chemistry, Vol. 89, No. 3, 1985 391
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Figure 5. Surface spectra (960-1060 cm-I) for 0.005 M pyridine on a Ag(C.L.) red sol in the presence of polymeric stabilizer, PVP (0.2%),and NaCl (lo4 M) (160 mW, photon counts = 1 X 10' counts/full scale, X = 514.5 nm).
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Figure 3. SERS spectra of pyridine on Carey Lea sols: (a) Ag(C.L.)-y sol only, (a') with 5 X lo-' M pyridine added (breathing band region); (b) Ag(C.L.)-r sol only, (b') with 0.005 M pyridine added; (c) same as (b') after 5 X lo4 M NaCl was added (sol was yellow) (160 mW, photon counts = 3 X 10' counts/full scale, X = 514.5 nm).
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Figure 4. SERS spectra of pyridine on Carey Lea sols with and without NaC1: (a and b) 0.005 M pyridine on Ag(C.L.)-y with (a) no NaCl and (b) lo4 M NaCI; (c-e) 0.005 M pyridine on Ag(C.L.)-r with (c) no NaCI, (d) preaddition of lo4 M NaC1, and (e) post-pyridine addition of lo4 M NaCl (160 mW, photon counts = 3 X lo3 counts/full scale, X = 514.5 nm).
in Figure 4d depending somewhat on the NaCl concentration used, age of the sol, and on the order pyridine and NaCl were added to the sol. The Figure 4b spectrum reveals several Raman bands which are not identifiable with either citrate or pyridine bands shown in spectra c and e of Figures 4, respectively. It is possible that the bands are due to citrate decomposition products5 which also partake in competitive adsorption on the silver surface.) If NaCl solution was added prior to pyridine addition, the initially red sol became yellow, but subsequent pyridine addition usually brought about the same aggregation changes seen for the Ag(C.L.)-y sol or a Ag(NaBH4) sol3 (this is a silver sol prepared by the reduction of Ag* with NaBH4), with the exception being when very low ( G l X lo4 M) pyridine concentrations were used. Both pre- and postaddition of C1- resulted in the introduction of
strong pyridine Raman bands to the spectrum of the sol. The change of color to yellow upon chloride addition to the red sol indicates dispersion of the large aggregates present in the sol. This was also supported by electron micrographs of the sols before and after chloride addition (see Figure 2b). In light of what we have found with other Ag sols, the observation of a strong SERS spectrum from an unaggregated sol provides an unusual contrast. We shall return to this point in attempting to explain the reasons for the enhanced pyridine Raman spectra. From the observations listed above, there are two factors which appear to contribute to the enhancement recorded. The first involves the influence of C1- in enhancing the total adsorption of pyridine onto the colloid surface. Prior to NaCl addition the surface was covered with citrate, as indicated by the Raman spectrum. The chloride appears to either remove the citrate or increase the pyridine/silver affinity beyond that for the citrate. (Enhanced adsorption using C1- was also suggested for pyridine in Ag(NaBH4) sols, although not to the extent found with Carey Lea sols2). The second reason for enhancement from these sols is possibly related to the Ag density of Carey Lea sols. In these preparations, the Ag concentration used was 1 or 2 orders of magnitude higher than in other sol preparations reported A yellow sol is, as stated before, predominantly composed of dispersed single colloidal particles. There are, however, a small but finite collection of colloidal silver aggregates of varying particle numbers. This can be seen in Figure 2b. If we use the premise that SERS is only associated with the cluster fraction of the sol, then a much higher concentration of Ag in a yellow sol is required before SERS becomes observable. This was further investigated by varying the silver concentration of the sols. When the spectra from a series of Ag(C.L.) red sols at different dilutions were recorded, all sols (containing the same amount of pyridine) became yellow after C1- addition, but only the most dense sols showed enhanced pyridine signals. That is, when the concentration of Ag in the Carey Lea sols was comparable to the other Ag sols reported previously: no enhanced pyridine signals were observed. At their lower Ag concentration SERS was only observed if particle aggregation was extensive. One further series of experiments was carried out using polyvinylpyrrolidone (PVP) in its capacity as a colloid ~ t a b i l i z e r . ~ When C1- was added to a pyridine-containing Ag(C.L.)-r sol which had been stabilized with PVP (the PVP had been added after the pyridine), only a small optical change in the sol occurred. The sol did not turn yellow but instead appeared brownish. The Raman spectrum of this system showed, weakly, both pyridine and citrate. The following adsorption scheme, using the data of Figure 5 , appeared to have taken place: (i) For the sol only, the surface was covered with citrate (a). (ii) When pyridine was added, citrate coverage persisted and pyridine remained in solution (b). (iii) When PVP was added, the polymer effectively coated the citrate-covered particle aggregates. Pyridine still remained largely
392
J. Phys. Chem. 1985, 89, 392-394
in solution (c). (iv) C1- addition brought about a small degree of dispersion, but complete dispersion was prevented by PVP stabilization of the aggregates. Limited adsorption of pyridine and removal of citrate took place (d). (v) The same sol without PVP dispersed when C1- was added. Pyridine replaced the citrate on the surface (e). What is immediately striking in this set of experiments is that the enhancement of the pyridine spectrum from a silver-clustercontaining sol (a red sol) (Figure 5d) is considerably weaker than the enhancement for a yellow sol (Figure 5e) of comparable silver colloid concentration. This can, in one sense, be taken to imply that extensive aggregate formation is not a necessity for observing SERS, in contrast to what was stated earlier. The apparent anomaly can, however, be rationalized if two additional observations are considered. The first is that pyridine is not strongly adsorbed in the sol of Figure 5d as in the sol of Figure 5 e F i g u r e 5d still shows the presence of citrate not unlike Figure 4d. The second is that the structure (morphology) of the cluster can have ~ have some bearing on the overall enhancement ~ b t a i n e d .We noticed that in comparing the SERS intensities of pyridine from a series of differently prepared Ag sols that considerable variation
exists as does the shape of the particle cluster seen by electron microscopy, i.e. strings or bunched aggregates. We have not, however, been able to find a direct correlation between the two. The changes in the aggregation associated with the competitive adsorption onto the Ag(C.L.)-y colloids are consistent with the electrophoresis results reported previ~usly.~ To explain all the events occurring in the Ag(C.L.)-r system is, to say the least, difficult. Even with this restriction in explaining the observations in full, the present results indicate that SERS can be used to study competitive adsorption onto colloidal metal substrates and also the control of adsorption by certain adsorbates, such as shown here, C1-.
Acknowledgment. S.M.H. acknowledges receipt from a Commonwealth Postgraduate Research Award, and F.G. the support of a Queen Elizabeth I1 Research Fellowship. This work was supported in part by the Australian Research Grants Scheme from the Commonwealth of Australia. We also indebted to Professor T. W. Healy for his constant support and encouragement. Registry No. PVP, 9003-39-8; Ag, 7440-22-4; pyridine, 110-86-1; citrate, 11-92-9.
An Infrared Study of Isotopic Exchange during Methanation over Supported Rhodium Catalysts: An Inverse Spillover Effect M. A. Henderson and S. D. Worley* Department of Chemistry, Auburn University, Auburn University, Alabama 36849 (Received: August 8, 1984; In Final Form: October 2, 1984) Infrared spectroscopy has been employed to study the reduction of carbon dioxide on supported catalyst films. The investigation included isotopic labeling using D2 as the reduction gas. Isotopic exchange was observed for both C02/D2 and CH4/D2 mixtures. The mechanism of this isotopic exchange involves migration of hydrogen from the support to the Rh sites, an "inverse spillover effect". A key intermediate in the dissociation of C 0 2on the supported Rh films was a carbonyl hydride species.
Introduction A substantial amount of effort in these laboratories has been devoted recently to the study of the hydrogenation of carbon dioxide over supported rhodium catalysts. Of considerable interest and importance in this reaction is its likely first step, the dissociation of C 0 2 on the supported Rh. The dissociation of C02on Rh, or lack thereof, is a controversial topic at this time, with Somorjai and co-workers favoring a high dissociation probability (10-l) at 300 K,] while Weinberg, and Goodman and co-workers3 suggest a low probability and lo-" at 300 K, respectively). Very recently Solymosi and Kiss4 have concluded that the dissociation probability at 300 K is low on clean Rh( 1 1 1) but enhanced by the presence of small amounts of hydrogen gas or boron impurity. Henderson and Worley have reached a similar conclusion for supported Rh catalyst films5 Part of the preliminary evidence concerning C 0 2 dissociation on Rh/Ti02 and Rh/A1203 obtained in these laboratories5 was derived from isotopic labeling (1) B. A. Sexton and G. A. Somorjai, J. Catal., 46, 167 (1977); D. G. Castner, B. A. Sexton, and G. A. Somorjai, Surf.Sci., 71,519 (1978); D. G. Castner and G. A. Somorjai, Surf. Sci., 83, 60 (1979); L. H. Dubois and G. A. Somorjai, Surf.Sci., 88, L13 (1979); L. H. Dubois and G. A. Somorjai, Surf.Sci., 91, 514 (1980); L. H. Dubois and G. A. Somorjai, Surf.Sci., 128, L231 (1983); see also the numerous references quoted therein. (2) W. H.Weinberg, Surf.Sci., 128, L224 (1983). (3) D. W. Goodman, D. E. Peebles, and J. M. White, Surf.Sci., 140, L239 (1984). (4) F. Solymosi and J. Kiss, Surf.Sci., in press; Chem. Phys. Left.,in press. ( 5 ) M. A. Henderson and S. D. Worley, Surf.Sci., in press.
0022-3654/85/2089-0392$01 .50/0
studies. The purpose of this Letter is to present those data in more detail and to examine the hydrogen spillover phenomena for supported Rh catalyst films using infrared spectroscopy as a probe.
Experimental Section The supported Rh catalysts used in this study were prepared at thin films on CaF, infrared windows by spraying slurries of RhC13-3H20, support material, water, and acetone onto the windows under mild heating to remove the solvents. Complete details concerning this method of sample preparation and subsequent reduction procedures have been presented previously concerning infrared studies of CQ adsorbed on supported Rh6 and the hydrogenation of CO over supported Rh.' The support materials were alumina (Degussa Aluminum Oxide C, 100 mz g-]) and titania (Degussa Titanium Dioxide P25, 50 m2 g-l). The gases employed (H2, D2, C 0 2 , CH4) were of the highest purity obtainable from Matheson; the H,, D,, and CH, were passed through liquid nitrogen traps before use. The Pyrex infrared cell used in this investigation was similar to those employed in our earlier studies6*'except that it was designed such that it could be heated to ca. 523 K in situ in a Perkin-Elmer Model 983 infrared (6) C. A. Rice, S. D. Worley, C. W. Curtis, J. A. Guin, and A. R. Tarrer, J. Chem. Phys., 74,6487 (1981); S . D. Worley, C. A. Rice, G. A. Mattson, C. W. Curtis, J. A. Guin, and A. R. Tarrer, J. Chem. Phys., 76, 20 (1982); S. D. Worley, C. A. Rice, G. A. Mattson, C. W. Curtis, J. A. Guin, and A. R. Tarrer, J . Phys. Chem., 86, 2714 (1982). (7) S.D. Worley, G. A. Mattson, and R.Caudill, J . Phys. Chem., 87, 1671 (1983).
0 1985 American Chemical Society
