1758
J. Phys. Chem. 1984, 88, 1758-1761
rangement of homocubane proceeds via the rupture of one of the bonds between a diagonal-plane carbon and C8 or C9, e.g., the c4-c8 bond.40,41These are exactly the bonds that we predict to be the most reactive toward electrophiles, after rotation of the diagonal-plane C-H.
Acknowledgment. We greatly appreciate the support of this work by the U S . Army Research Office. Registry No. Cubane, 277-10-1; homocubane, 452-61-9; bishomocubane, 5603-27-0.
Influence of Temperature on the Electrocatalytic Oxidation of Aromatic Compounds Adsorbed on Platinum Manuel P. Soriaga* and Arthur T. Hubbard* Department of Chemistry, Unioersity of California, Santa Barbara, California 93106 (Received: June 30, 1983)
The effect of temperature on the electrocatalytic oxidation of aromatic compounds (1,4-dihydroxybenzene and 1,4-dihydroxynaphthalene) adsorbed on smooth polycrystalline platinum in aqueous solutions has been investigated. Adsorption occurred spontaneously when the clean platinum surface was immersed into aqueous solutions of the aromatic compounds. Analytical measurements were made by using thin-layer electrochemical methods. As the temperature was raised from 5 to 65 OC,the extent of oxidation of species bound in the edgewise orientation was increased considerably, in contrast to that of species attached in the flat orientation, which was nearly constant. The oxidation data suggest that C 0 2 is the principal product from flat-adsorbed species at or above room temperature but that the product distribution from edge-oriented intermediates is a sensitive function of temperature.
Introduction Adsorbed-molecule orientation and its influence on surface chemical reactivity are of fundamental interest in heterogeneous catalysis. A systematic study of nearly 50 organic compounds'-8 irreversibly adsorbed from aqueous solutions onto smooth polycrystalline platinum9J0electrodes has revealed that, at low concentrations (@ < 0.1 mM) and in the absence of competing surfactants, simple aromatic compounds are chemisorbed with the phenyl ring parallel to the surface.' However, these flat orientations are converted irreversibly to vertical orientations at higher concentration^.^*^ Such reorientations are also brought about by coadsorption of iodine.*J1 The edgewise orientation is a sensitive function of temperature, presumably because of temperature-dependent librational motions within the close-packed oriented monolayer.6 Other factors such as solvent and electrode surface structure may affect orientation, also. The expectation that orientation influences surface reactivity has been confirmed by recent ~ o r k . ~ , The ' , ~ present article provides an extension of (1) M. P. Soriaga and A. T. Hubbard, J . Am. Chem. SOC.,104, 2735 (1 982). (2) M. P. Soriaga and A. T. Hubbard, J . Am. Chem. Soc., 104, 2742 ( 1982). (3) M. P. Soriaga and A. T. Hubbard, J . Am. Chem. Soc., 104, 3937 (1982). (4) M. P. Soriaga, P. H. Wilson, A. T. Hubbard, and C. S. Benton, J . Electroanal. Chem., 142, 317 (1982). (5) V. K. F. Chia, M. P. Soriaga, A. T. Hubbard, and S. E. Anderson, J . Phys. Chem., in press. (6) M. P. Soriaga, J. H. White, and A. T. Hubbard, J . Phys. Chem., 87, 3048 (1983). (7)'M. P.Soriaga, J. L. Stickney, and A. T. Hubbard, J. Mol. Catal., 21, 211 (1983). (8) M. 'P. Soriaga, J. L. Stickney, and A. T. Hubbard, J . Electroanal. Chem., 144, 207 (1983). (9) A. T. Hubbard, J . Vac. Sei. Technol., 17, 49 (1980). (IO) A. T. Hubbard, Ace. Chem. Res., 13, 177 (1980). (11) J. Y. Katekaru, G. A. Garwood, J. F. Hershberger, and A. T. Hubbard, Surf. Sci., 121, 396 (1982).
the above studies and describes results on the effect of temperature on the electrochemical oxidation of aromatic compounds [ 1,4dihydroxybenzene (hydroquinone, HQ) and 1,4-dihydroxynaphthalene (naphthohydrcquinone, NHQ)] adsorbed in specific orientations at smooth polycrystalline p l a t i n ~ m ~electrodes. .'~ It is worthwhile to mention that the chemisorbed-aromatic reorientation phenomena observed at Pt electrode-electrolyte interfaces are somewhat similar to findings obtained for other systems under very different conditions. For example, under ultrahigh vacuum, exposure of clean Pt single crystals to benzene at low dosages invariably resulted in flat-oriented species;l2-I4but, at longer and higher dosages, reorientation of adsorbed benzene has been reported to occur.12 Infrared studies of benzene adsorbed on Pt dispersed in alumina have provided evidence for flat-oriented and vertically oriented chemisorbed species.l5 Likewise, occurrence of perpendicularly adsorbed benzene on transition metals was indicated by results of deuterium exchange, magnetic measurements, and 14C-labeled studies.16 Irreversible, concentration-induced changes in the packing densities of benzene, phenol, and naphthalene adsorbed on Pt were also observed by using radiotracer methods." Among organometallic complexes, cases exist in which changes in mode of binding (Le., changes in ligand orientation) accompany changes in the number of attaching ligands (Le., changes in ligand packing density): when more than two cyclopentadienyl (Cp) ligands per metal center are present, different states of bonding of the Cp ligands may r e ~ u l t . ' ~ . ' ~ (12) J. L. Gland and G. A. Somorjai, Adu. Colloid Interface Sei., 5, 205 (1976). (13) F. P. Netzer and J. A. D. Matthew, Solid State Commun., 29, 209 (1979). (14) S. Lehwald, H. Ibach, and J. E. Demuth, Surf. Sci., 78, 577 (1978). (15) D. M. Haaland, Surf. Sci., 111, 555 (1981). (16) R. B. Moyes and P. B. Wells, Adu. Catal., 23, 121 (1973). (17) V. E. Kazarinov, A. N. Frumkin, E. A. Ponomarenko, and N. V. Andreev, Elektrokhimiya, 11, 860 (1975). (18) J. L. Calderon, F. A. Cotton, B. G. DeBoer, and J. Takats, J . A m . Chem. Soc., 93, 3592 (1971).
0022-3654/84/2088-1758$01.50/00 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 9, 1984 1759
Aromatic Compounds Adsorbed on Platinum Measurement of Adsorbed Amounts Precise measurement of adsorbed amounts is based on thin-layer Packing density I' was deelectrochemical techniques. termined by use of eq l where Q was the full quinone/diphenol 1-8v20,21
0 0 . 7 0 ~A
electrolytic charge, Qb the charge in the absence of dissolved aromatic, Q1 the electrolytic charge observed after a single filling of the thin-layer cavity, and Qlbthe background charge measured under similar conditions. F was the Faraday and A the area of the electrode. The packing density is related to the average area, u, occupied by an adsorbed molecule by u = (NAr)-l,where N A is Avogadro's number. The predominant adsorbate orientations have been deduced via the classical m e t h ~ d ~of~comparing ,*~ the observed u with those calculated for various possible orientations; molecular area calculation^^^^ were based on covalent and van der Waals distances tabulated by P a ~ l i n g . This ~ ~ method proved highly consistent for all oriented-adsorbed aromatic compounds thus far ~tudied.~
Hydroquinone
99
B
040 035
Determination of the Stoichiometry and Extent of Electrocatalytic Oxidation In the present study, electrocatalytic oxidation was characterized mainly by no,, the number of electrons transferred during oxidative desorption of the chemisorbed aroma ti^.^^^^* Since the number of electrons involved in an electrode reaction reflects the stoichiometry of such a reaction, noxprovides a measure of the effective stoichiometry of the electrocatalytic oxidation reaction typified by eq 2. Clearly, changes in nox indicate changes in the oxidation aromaticad,
electrochemical oxidation Pt
products
+ no,e-
(2)
product distribution. Complete oxidation of an organic molecule converts all its carbon atoms to CO,. Hence, electrocatalytic oxidation may also be described in terms of A,, the extent of oxidative desorption of aromatic, defined with respect to complete conversion to CO,: io,
=
~ox/~ox,co2
(3)
A 25
(19) R. D. Rogers, R. V. Bynum, and J. L. Atwood, J . Am. Chem. Soc., 100, 5238 (1978). (20) A. T. Hubbard in "Critical Reviews of Analytical Chemistry", Vol. 3, L. Meites, Ed., Chemical Rubber Publishing Co., Cleveland, OH, 1973, p 201. (21) C. N. Lai and A. T. Hubbard, Inorg. Chem., 11,2081 (1972). (22) I. Langmuir, Proc. R.Soc. London, Ser. A , 170, 1 (1939). (23) N. K. Adam, "The Physics and Chemistry of Surfaces", Oxford University Press, London, 1941. (24) L. C. Pauling, "The Nature of the Chemical Bond", Cornell University Press, New York, 1960.
30
-LOG CIM) Figure 1. (A) Packing density, F (nmol cm-2), and (B) number of electrons, nox,for oxidative desorption of adsorbed hydroquinone, as a function of concentration at various temperatures. The relative standard deviations in nonand r were f3% from 5 to 45 " C and &6% at 65 "C. The solid lines interconnect experimental points and do not represent any theoretical curve.
material. The surface oxide reduction peak (0.54 V, Figure 3B) was the same whether the electrode was clean or precoated with aromatic, demonstrating that the Pt surface was oxidized to the same extent in either case. To ensure that desorbed products did not contribute to Qb,, experiments were done in which the thinlayer cell was rinsed several times during the coulometric measurement. Values of noxwere extracted from the data by using Faraday's law in the form of eq 4. Pox E
where no, is the measured number of electrons transferred during oxidation, and nox,co2is the number of electrons required to oxidize the adsorbate completely to C 0 2 . Determination of nox is based upon precise measurements of F (see above) and the electrolytic charge Qoxfor oxidative desorption of the adsorbed material. Qoxwas determined as follows: (i) the clean Pt thin-layer electrode was exposed for 180 s to a surfactant solution for which r had been measured separately; (ii) excess dissolved material was then removed by rinsing the thin-layer cavity with pure supporting electrolyte; and (iii) the total electrolytic charge QL, for the oxidative process was determined by potential-step coulometry, starting from the potential at which adsorption was done to a potential just below that for oxygen evolution, 1.18 V [Ag/AgCl (M C1-) reference] in 1 M HClO4. The background charge was determined by an identical procedure except for omission of the adsorbed material. QLXand Qox,bwere evaluated where the charge-time curves became parallel, indicating cessation of oxidation of the organic
40
50
Qbx
-
Qox,b
= noxFAr
(4)
It is important to stress that the term no, refers to the irreversible oxidation of the chemisorbed species, not to the reversible quinone/diphenol reaction of the unadsorbed aromatic. Experimental Section The preparation of electrodes, electrolytes, and surfactants has been described p r e v i o ~ s l y . ~The - ~ adsorption temperature was controlled by immersing the H cells in a thermostated water bath. The temperature was varied at intervals from 0 to 65 O C , as determined by a thermometer placed in the reference compartment of the H cell. The temperature fluctuated less than f0.5 "C below 50 OC and f l OC above 50 OC. Adsorption was carried out in aqueousZS1 M HClO, at controlled potential: 0.200 V below 45 "C; 0.100 V above 45 OC. A waiting period of 180 s was allowed for adsorption. However, no significant changes were noted when adsorption was carried out at open circuit or when the adsorption time was varied from 120 to 300 se4 In between experimental trials, the electrode was cleaned by electrochemical oxidation in 1 M HC104 at 1.2 V and reduction at -0.2 V.I Linear-potential sweep voltammetry and potential-step coulometry were done by using a conventional multipurpose electrochemical circuit based on operational am(25) B. E. Conway, H. Angerstein-Kozlowska, W. B. A. Sharp, and E. E. Criddle, Anal. Chem., 45, 1331 (1973). (26) B. B. Damaskin and V. E. Kazarinov in "Comprehensive Treatise of Electrochemistry", J. O'M. Bockris, B. E. Conway, and E. Yeager, Eds., Plenum Press, New York, 1980, p 353. (27) T. Smith, J. Colloid Interface Sci., 23, 27 (1967).
1760 The Journal of Physical Chemistry, Vol. 88, No. 9, 1984
Soriaga and Hubbard
F
0.70
POTENTIAL, VOLT
0.2
E. AgCl
0.4 0.6 0.8
1.0
1.2
ip-Napht ho hydroquinone
0.60
0.40
’
0.30
0.2 0
. ......
, , , .>
401
7
35 e
+ 2
-g
% co
V
20
65OC .45
0 35
15
ti5 5.0
4.0
3.0
-LOG C(M) Figure 2. (A) Packing density, I? (nmol cm-2), and (B) number of
electrons, nox, for oxidative desorption of adsorbed 1,4-dihydroxynaphthalene as a function of concentration at various temperatures. All other experimental conditions were as in Figure 2. plifiers, as described previously; 2o the electrolytic charge Q was measured by means of an electronic integrator and four-place digital voltmeter.
Results and Discussion Packing density-vs.-concentrationcurves for HQ and NHQ at various temperatures are shown in Figures 1A and 2A, respectively. These results have been discussed in detail elsewhere; observations relevant to the present purpose are as follows: (i) the (irreversible) concentration-induced transitions from low to high coverages are attributable to (irreversible) changes in the orientation of the adsorbed species from flat (lower plateau) to edgewise (upper plateau) structures; (ii) the transitions are most sharply defined at the lowest temperature studied, 5 OC; (iii) as the temperature is increased, the packing densities at low concentrations increase, while at high concentrations, the packing densities decrease; (iv) at intermediate temperatures (25 < T < 65 “C), an additional plateau appears; and (v) transitions in packing density are barely noticeable at 65 OC, the highest temperature studied. Figure 3 shows thin-layer current-potential curves for the irreversible electrochemical oxidation of HQ adsorbed on Pt electrodes at ambient temperatures. The solid curve in Figure 3A was obtained when both dissolved and $-adsorbed forms were present; the dot-dash curve is for &adsorbed material only. The two curves are identical above 0.5 V, indicating that oxidation of benzoquinone (the only species in solution above 0.5 V) in the unadsorbed state is negligible under the present conditions. Evidently, under the present conditions, adsorption is a prerequisite to the irreversible oxidation of benzoquinone at potentials below 1.18 V. This is not unexpected in view of the fact that anodic oxidation of bulk aromatic compounds usually requires potentials at which an aqueous solvent starts to decompose (>1.2 V).28 The area under the dot-dash curve yields the total charge QL,, whereas the area under the dotted (clean Pt) curve gives the background charge Qox,b. Current-potential curves for hydroquinone adsorbed in the flat (dashed curve) and edgewise (solid curve) orientations 3*436
(28) S . D. Ross, M . Finkelstein, and E. J. Rudd, “Anodic Oxidations”, Academic Press, New York, 1975.
0.2 0.4 0.6 0.8 1.0 1.2 . Figure 3. Thin-layer current-potential curves for electrocatalytic oxidation of hydroquinone irreversibly adsorbed at a polycrystallineplatinum electrode at 25 O C . A: (-) in the presence of dissolved hydroquinone (0.1 mM); adsorbed hydroquinone only; clean electrode. B: (---) $-orientation; (-) q2-orientation,first anodic oxidation cycle; $-orientation, second cycle after two rinses at 1.18 V; clean electrode. Thin-layer volume, V = 4.08 pL; electrode surface area, A = 1.18 cm2;rate of potential sweep, r = 2.00 mV s“; T = 25 O C . (.-e)
(e..)
(.e.)
(.-a)
are shown in Figure 3B. It is evident that the anodic peak is larger for the q2-orientation. It is also apparent that the anodic peak potential ,Ep, a measure of the electron-transfer kinetics for irreversible reactions,29 is not strongly affected by changes in orientations [,Ep(q6)- ,Ep(v2)< 20 mv]. Two other features in Figure 3B are important: Firstly, as previously noted, the surface-oxide reduction peak (0.54 V) was the same regardless of whether the electrode was clean or precoated with ($- or q2-oriented)aromatic, which demonstrates that the Pt electrode was ultimately oxidized to the same extent regardless of surface pretreatment. Secondly, when the thin-layer cavity was rinsed (at 1.18 V) after the first oxidative scan of an aromatic-coated electrode, the subsequent current-potential curve (dotted line) closely resembled that of a clean electrode (dot-dash curve), indicating that essentially complete desorption occurs during the first electrochemical oxidation cycle (that is, chemisorption of oxidation products does not take place on the oxidecoated Pt surface). NHQ gave results similar to those in Figure 3. Figure 4 shows thin-layer current-potential curves for the anodic oxidation of q2-adsorbed NHQ at selected temperatures. It is is strongly dependent on temperature, important to note that &Ep and the shift to less-positive potentials indicates that the rate of electrochemical oxidation increases with temperature; this is as expected since an increase in temperature commonly leads to an increase in the rates of activated processes. The marked shifts in ,Ep with temperature suggest that the oxidation reactions have substantial activation energies. Figure 4 also shows that, for a (29) A. T. Hubbard, J . Electroanal. Chem., 22, 165 (1969).
The Journal of Physical Chemistry, Vol. 88, No. 9, 1984 1761
Aromatic Compounds Adsorbed on Platinum
TABLE I : Extent of Electrocatalytic Oxidation of Aromatic Compounds Chemisorbed on Pt and Effect of Temperature
postulated predominant onentation=
compd
1,4-dihydroxybenzene(HQ)
flat (@) edgewise (2,3-q2)
flat ( q " )
1,4-dihydroxynaphthalene (NHQ)
edgewise (2,3-q2) a
A,,
b
5 "C
25 "C
35 "c
45 "C
65 "C
0.80 0.53 0.70 0.34
0.94 0.62 0.87 0.42
0.95 0.68 0.90 0.52
0.95 0.85 0.93 0.64
0.98 0.96 0.96 0.91
llat-oriented species were obtained by adsorption a t C o = 0.08mM; edge-oriented species, a t C o = 2 mM. 0.2
0.6
1.0
nox/nox,~~,.
Evidence was presented elsewhere that p-diphenols are adsorbed oxidatively on Pt probably in the form of quinone^.^ For this reason one would expect for chemisorbed H Q and N H Q to be 24 and 42, respectively. Using these numbers, we have cn tabulated values of the parameter A, for both compounds for the W flat and edgewise orientations at selected temperatures; these are OI W given in Table I. The results show the following: (i) at a given a temperature, the extent of electrocatalytic oxidation is greater for I a the flat orientation than for the $-orientation; (ii) as the tem0 a perature is raised, the extent of oxidation is increased regardless 0 of initial orientation; (iii) above 5 OC, values are close to unity, 2 which suggests that COz is the principal product from oxidation of flat-adsorbed species; and (iv) at T 2 65 "C, q2-oriented inIZ termediates are oxidized essentially completely to COz, also based w upon the fact that A, values are near unity. The oxidation efa ficiency of flat-adsorbed species at T 5 5 OC is unexpectedly low; 3 V the possibility exists that, because the rate Qf organic oxidation at T I5 "C is retarded (as evidenced by the large positiue shift in ,Ep at this temperature, Figure 4), reorientation of part of the adsorbed layer occurs due to surface-oxide formation and that subsequent oxidation of this reoriented species falls short of total conversion to COz. P O T E N T I A L , VOLTS VS. A g C l The variations in noxand ,X with orientation may be explained Figure 4. Thin-layer current-potential curves for electrocatalytic oxiin terms of only those carbons directly bound to the Pt electrode dation of v2-1,4-naphthohydroquinoneadsorbed on Pt at various tembeing converted to COz.7,8For example, in the flat orientation peratures: 5, 25, (---) 45, (-) 65 O C . The two bottom of HQ, all six carbons are bound to the surface and all are oxidized curves are for clean Pt. All other experimental conditions were as in completely to COz (Table I). In the rigid edgewise orientation, Figure 3. on the other hand, only two carbons are in intimate contact with the electrode; hence, only those two carbons are expected to be clean Pt electrode (i) the potential a t which surface oxide starts converted to CO,. The oxidation data show that the extent of to form is not strongly influenced by temperature, and (ii) the oxidation of the unattached carbon atoms is a sensitive function background current increases only slightly with temperature. HQ gave results similar to those in Figure 4. of temperature. It is interesting to note that the extent of oxidation correlates with the nonrigidity of the g2-bound states; that is, the The effect of orientation on aromatic oxidation at various less rigid the v$orientation,6 the greater the extent of oxidation. temperatures can be seen from Figures 1B (HQ) and 2B (NHQ); the room-temperature results have been discussed p r e v i o u ~ l y . ~ * ~ Possible identities of the oxidation products may be inferred from Important features to be noted from these figures are as follows: nox and the initial orientation.'^^ (i) regardless of temperature, transitions in no, occur at the Acknowledgment. Acknowledgment is made to Air Force concentrations where changes in r take place; (ii) below 65 OC, Office of Scientific Research and to the donors of the Petroleum nox is higher for the flat orientation than for the &orientation; Research Fund, administered by the American Chemical Society, (iii) at 65 OC, noxbecomes essentially independent of orientation; for support of this research. and (iv) above 5 OC, no, for the flat-adsorbed layer is not as strongly influenced by temperature as no, for the $-layer; in fact, Registry No. HQ, 123-3 1-9;NHQ, 571-60-8;platinum, 7440-06-4; for HQ, v6-no, is virtually constant. carbon dioxide, 124-38-9. (-.e)
(-a-)
