Langmuir 1988,4,637-646 the persistence of the velocities of vapor molecules.
Conclusions We have measured the evaporation rates of stationary single droplets by suspending them individually in an electrodynamic balance under precisely controlled conditions. By altering the total pressure of the surrounding gas, we have obtained the evaporation rates of the same droplets over a wide range of pressures (i.e., 100 Torr 1 P 1 10-6 Torr).The ratio of the evaporation rate at a given pressure to that a t high vacuum provides a direct measurement of the flux ratio J / Jk,thus eliminating the uncertainties associated with the estimation of flux Jk in the kinetic regime. The results show that as the pressure decreases the evaporation rate increases to a maximum, beyond which a further decrease of pressure causes no change in the evaporation rate. The experimental evaporation rate data have been presented in terms of the flux ratio J / J k as a function of the Knudsen number Kn. For Kn I3.8, the
637
data are in excellent agreement with the presently extended theories (Sahni,21Loyalka22 and Monchick, and Blackmore') on the evaporation of droplets. However, for Kn 2 3.8, the theories fail to predict the observed evaporation rates. The theories predict a slower approach of to the kinetic limit of evaporation. The experimental data show that for Kn 1 6.0 the evaporation behavior can be correctly described by the free molecular theory. The present study suggests that there is a need either for modifications of the existing theories or for the development of a new model for the correct description of evaporation behavior near the kinetic regime.
Acknowledgment. We are grateful to the National Science Foundatin, Brown and Williamson Tobacco Corp., Tennessee Eastman Co., and Dow Corning Corp. for their generous support under the Presidential Young Investigator Award (Grant No. CPE-8351190). We thank Prof. C. B. Richardson of the University of Arkansas for his technical advice.
Characterization of Hydroquinone and Related Compounds Adsorbed at Pt(ll1) from Aqueous Solutions: Electron Energy-Loss Spectroscopy, Auger Spectroscopy, Low-Energy Electron Diffraction, and Cyclic Voltammetry Frank Lu, Ghaleb N. Salaita, Laarni Laguren-Davidson, Donald A. Stern, Edna Wellner, Douglas G. Frank, Nikola Batina, Donald C. Zapien, Nicholas Walton, and Arthur T. Hubbard" Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 Received October 2, 1987. In Final Form: December 9, 1987 Adsorption of hydroquinone and a series of related compounds from aqueous solutions at well-defined Pt(111)single-crystalsurfaces has been studied: hydroquinone (HQ), benzoquinone (BQ),phenol (PL), perdeuteriophenol (PDPL; phenol-d6),tetrafluorohydroquinone (TFHQ), and 2,5-dihydroxy-4-methylbenzyl mercaptan (DMBM). Packing densities (moles adsorbed per unit area) were measured for each compound by quantitativeAuger electron spectroscopy. Packing densities of HQ, BQ, PL, PDPL, and TFHQ adsorbed from millimolar solutions indicated adsorption with the ring parallel to the Pt(ll1) surface; in contrast, DMBM was adsorbed with the ring pendant from the surface. Vibrational spectra of the adsorbed layers formed from these compounds were obtained by electron energy-lossspectroscpy (EELS) and were compared with the infrared spectra of the parent compounds in KBr. The EELS and IR spectra were closely similar except that the phenolic hydrogens of HQ, PL, PDPL, and TFHQ are removed during adsorption. EELS bands of polar groups such as OH are not broadened to the same extent as in the IR spectra of the solid compounds, evidently due to less intermolecular bonding among such groups at the surface. LEED observations revealed that HQ, BQ, PL, and PDPL were present as a Pt(111)(3X3)ordered layer at packing densities near 0.1 molecule/surface Pt atom, slightly below saturation in the horizontal orientation. All other compounds and conditions evidently did not produce long-range ordering with respect to the Pt(ll1) surface. Adsorbate orientation and mode of surface bonding exert a profound influence on the product distribution of electrocatalytic oxidation of these well-characterized adsorbed organic intermediates, on the basis of cyclic voltammetry and potential-step chronocoulometry experiments. These results also demonstrate that evacuation does not alter the composition or electrochemical properties of the chemisorbed layer formed from solution.
Introduction The orientations of adsorbed aromatic molecules a t annealed polycrystalline Pt electrodes have been found to depend upon a number of variables, including adsorbed molecular structure, adsorbate concentration, electrode potential, nature of the electrolyte anion, adsorbate chirality, temperature, order of addition of the reagents, 0743-7463/88/2404-0637$01.50/0
surface roughness, nature of the solvent, and concentrations of competing adsorbates. Recent reviews are available,lV2 and subsequent work is found in ref 3. Adsorbate (1) Hubbard, A. T.; Stickney, 3. L.; Soriaga, M. P.; Chia, V. K. F.; Rosasco, S. D.; Schardt, B. C.; Solomun,T.; Song, D.; White, J. H.; Wieckowski, A. J.EZectroanaZ. Chem. 1984, 168,43.
1988 American Chemical Society
Lu et al.
638 Langmuir, Vol. 4,No. 3, 1988
D
C W
P
h
W
v
5
B
n
IC
n
IIRCREASED ;SEHsIII"IT*
,
I .,I
A Pt
'' tt 100
v
,Pt Pt, 200
Pt ,
Pt ' ,
I
300
,
I
400
,
, 500
,
1
,
The present work employed well-defined P t ( l l 1 ) surfaces rather than the electrochemically cycled polycrystalline Pt surfaces of our earlier work. Six compounds related to hydroquinone and benzoquinone were studied. These were chosen to maximize the interpretive value of the resulting data. Vibrational spectra of the adsorbed layers were obtained by means of high-resolution electron energy-loss spectroscopy (EELS) under ultrahigh vacuum (UHV). Adsorption isotherms were measured by means of quantitative Auger electron spectroscopy combined with electron-scattering formulas to extend the range of applicability to all adsorbates including those which do not display ready electrochemical reactivity. This work has demonstrated that evacuation does not alter the electrochemical propeties of chemisorbed aromatic layers formed with the electrode in contact with solution. Accordingly, surface spectroscopies in UHV (LEED, Auger, EELS,and others) are directly applicable to characterization of these chemisorbed species. EELS was found to give straightforward spectra affording high sensitivity to hydrocarbons (about 0.01 monolayer) and moderate resolution (80cm-l). Useful new information has been obtained as to adsorbate structure and mode of attachment to the surface. Such studies lead to a firm experimental basis for identification of adsorbed electrochemical species such as reactants, intermediates,products, solvents, electrolytes, and impurities. There do not appear to have been any previous studies of the subject compounds by EELS or Auger spectroscopy.
600
KINETIC ENERGY (eV) Figure 1. Auger spectra: A, clean P t ( l l 1 ) ; B, P t ( l l 1 ) treated with HQ (1.0 mM); C, P t ( l l 1 ) treated with TFHQ (0.7 mM); D, P t ( l l 1 ) treated with DMBM (0.7 mM). Experimental conditions: the incident beam was lo-' A at 2000 eV, normal to the surface; modulation amplitude was 5 V peak-to-peak, 10 mM KF adjusted to pH 4 with HF; electrode potential 0.20 V vs Ag/AgC1(1 M KCl reference).
orientation and mode of surface bonding are matters of fundamental and practical importance because of their influence on the nature of electrocatalytic oxidation and reduction product.^.^ (2)Hubbard, A. T.; Chia, V. K. F.; Frank, D. G.; Katekaru, J. Y.; Rosasco, S. D.; Salaita, G. N.; Schardt, B. C.; Song, D.; Soriaga, M. P.; Stern, D. A.; Stickney, J. L.; White, J. H.; Vieira, K. L.; Wieckowski, A.; Zapien, D. C. In New Dimensions in Chemical Analysis; Shapiro, B. L., Ed.; Texas A&M University Press: College Station, TX, 1985;p 135. (3)(a)Soriaga, M.P.; Binamira-Soriaga,E.; Hubbard, A. T.; Benziger, J. B.; Pang, K. W. P. Znorg. Chem. 1985,24,65.(b) Soriaga, M.P.; White, J. H.; Chia, V. K. F.; Song, D.; Arrhenius, P. 0.; Hubbard, A. T. Inorg. Chem. 1985,24,73. (c) Pang, K.W. P.; Benziger, J. B.; Soriaga, M. P.; Hubbard, A. T. J. Phys. Chem. 1984,4853.(d) White, J. H.; Soriaga, M. P.; Hubbard, A. T. J. Electroanal. Chem. 1984,177,89. (e) Soriaga, M. P.; Song,D.; Hubbard, A. T. J.Phys. Chem. 1985,89,285. (f) Soriaga, M. P.; Song, D.; Zapien, D. C.; Hubbard, A. T. Langmuir 1985,I, 123. (9) White, J. H.; Soriaga, M. P.; Hubbard, A. T. J.Phys. Chem. 1985,89, 3226. (h) White, J. H.; Soriaga, M. P.; Hubbard, A. T. J. Electroanal. Chem. 1985,185,331.(i) Benziger, J. B.; Pascal, F. A.; Bernasek, S. L.; Soriaga, M. P.; Hubbard, A. T. J. Electroanal. Chem. 1985,198,65.(i) Song, D.; Soriaga, M. P.; Vieira, K. L.; Zapien, D. C.; Hubbard, A. T. Phys. Chem. 1985,89,3999.(k) Song, D.; Soriaga, M. P.; Hubbard, A. T. Electroanal. Chem. 1986,201, 153. (1) Song, D.; Soriaga, M. P.; Hubbard, A. T. Langmuir 1986,2,20. (m)Song, D.; Soriaga, M. P.; Hubbard, A. T. J. Electroanal. Chem. 1985,193,255.(n) Chia, V. K. F.; Soriaga, M. P.; Hubbard, A. T. J.Phys. Chem. 1987,91,78. ( 0 ) Song, D.;Soriaga, M. P.; Hubbard, A. T. J. Electrochem. SOC.1987,134,874. (4) (a) Soriaga, M. P.; Stickney, J. L.; Hubbard, A. T. J.Molec. Catal. 1983,21,211. (b) Soriaga, M. P.; Stickney, J. L.; Hubbard, A. T. J. Electroanal. Chem. 1983,144,207. (c) Soriaga, M.P.; Hubbard, A. T. J. Phys. Chem. 1984,88, 1758. (d) Soriaga, M.P.; Hubbard, A. T. J. Electroanal. Chem. 1983, 159, 101. (e) Vieira, K. L.; Zapien, D. C.; Soriaga, M. P.; Hubbard, A. T.; Low, K. P.; Anderson, S. E. Anal. Chem. 1986,58,2964.(f) Chia, V. K. F.; White, J. H.; Soriaga, M. P.; Hubbard, A. T. J. Electroanal. Chem. 1987,217,121.
Experimental Section The general procedures employed were as described in ref 5. Vacuum systems were constructed for these studies for examination of the electrode surface by surface-sensitive spectroscopy and diffraction under ultrahigh vacuum (UHV) and by electrochemical methods (voltammetry, coulometry) at atmospheric pressure without contamination or alteration of structure. The Pt(ll1) single-crystal surface employed for this work was oriented6 (5)(a) Hubbard, A. T.; Stickney, J. L.; Rosasco, S. D.; Soriaga, M. P.; Song, D. J. Electroanal. Chem. 1983,150,165. (b) Stickney, J. L.; Rosaaco, S. D.; Song, D.; Soriaga, M. P.; Hubbard A. T. Surf. Sci. 1983,130, 326. (c) Stickney, J. L.; Hubbard, A. T. J. Electrochem. SOC.1984,131, 260. (d) Stickney, J. L.; Rosasco, S. D.; Schardt, B. C.; Hubbard, A. T. J.Phys. Chem. 1984,88,251.(e) Wieckoweki,A,; Rosasco, S. D.; Schardt, B. C.; Stickney, 3. L.; Hubbard, A. T. Inorg. Chem. 1984,23,565. (f) Wieckowski, A.; Schardt, B. C.; Rosasco, S. D.; Stickney, J. L.; Hubbard, A. T. Surf. Sci. 1984,146,115.(9) Solomun, T.; Schardt, B. C.; Rosasco, S. D.; Wieckowski, A.; Stickney, J. L.; Hubbard, A. T. J. Electroanal. Chem. 1984,176,309.(h) Stickney, J. L.; Rosasco, S. D.; Salaita, G. N.; Hubbard, A. T. Langmuir 1985,1,66.(i) Rosasco, S.D.; Stickney, J. L.; Salaita, G. N.; Frank, D. G.; Katekaru, J. Y.; Schardt, B. C.; Soriaga, M. P.; Stem, D. A.; Hubbard, A. T. J. Electroanal. Chem. 1985,188,95.(i) Frank, D. G.; Katekaru, J. Y.; Rosasco, S. D.; Salaita, G. N.; Schardt, B. C.; Soriaga,M. P.; Stem, D. A.; Stickney, J. L.; Hubbard, A. T. Langmuir 1985,1,587.(k) Schardt, B. C.; Stickney, J. L.; Stern, D. A.; Frank, D. G.; Katekaru, J. Y.; Rosasco, S. D.; Salaita, G. N.; Soriaga, M. P.; Hubbard, A. T. Inorg. Chem. 1985,24,1419. (1) Salaita, G.N.; Stern, D. A.; Lu, F.; Baltruschat, H.; Schardt, B. C.; Stickney, J. L.; Soriaga, M. P.; Frank, D. G.; Hubbard, A. T. Langmuir 1986,2,20. (m)Stern, D. A,; Baltruschat, H.; Martinez, M.; Stickney, J. L.; Song, D.; Lewis, S. K.; Frank, D. G.; Hubbard, A. T. J. Electroanal. Chem. 1987,217,101.(n) Stickney, J. L.; Stern, D. A.; Schardt, B. C.; Zapien, D. C.; Wieckowski, A.; Hubbard, A. T. J.Electroanal. Chem. 1986,213,293. ( 0 ) Stickney, J. L.; Schardt, B. C.; Stern, D. A.; Wieckowski, A.; Hubbard, A. T. J. Electrochem. SOC.1986,133,648. (p) Schardt, B. C.; Stickney, J. L.; Stern, D. A.; Wieckowski, A.; Zapien, D. C.; Hubbard, A. T. Langmuir 1987,3,239.(4)Schardt, B. C.; Stickney, J. L.; Stern, D. A.; Wieckowski, A,; Zapien, D. C.; Hubbard, A. T. Surf. Sci. 1986, 175,520. (r) Baltruschat, H.; Martinez, M.; Lewis, S. K.; Lu, F.; Song,D.; Stern, D. A.; Datta, A.; Hubbard, A. T. J. Electroanal. Chem. 1987,217,111.(s) Lu, F.; Salaita, G. N.; Baltruschat, H.; Hubbard, A. T. J.Electroawl. Chem. 1987,222,305. (t) Baltruachat, H.; Lu, F.; Song, D.; Lewis, S. K.; Zapien, D. C.; Frank, D. G.; Salaita, G. N.; Hubbard, A. T. J.Electroanal. Chem., in press. (u) Salaita, G. N.; Lu, F.; Laguren-Davidson, L.; Hubbard, A. T. J. Electroanal. Chem., in press. (v) Laguren-Davidson, L.; Lu, F.; Salaita, G. N.; Hubbard, A. T. Langmuir, in press.
Langmuir, Vol. 4, No. 3, 1988 639
Characterization of Hydroquinone a n d Related Compounds and polished' such that all faces were crystallographically equivalent. All six faces were cleaned simultaneously by bombardment with Ar+ ions (4pA/cm2 at 500 eV) and annealed by resistance heating (about lo00 K), under UHV. After characterization by means of low-energy electron diffraction (LEED) and Auger s p e c t " p y , the crystal was isolated in an argon-filled antechamber for immersion into buffered aqueous electrolyte solutions containing the subject adsorbates. Electrode potentials and currents were measured and controlled by means of threeelectrode electrochemical circuitry based upon operational amplifiers. The electrochemical cell was constructed of Pyrex glass and Teflon double-wall tubing; spaces between the tubing walls were purged with argon as a necessary further protection against intrusion of air through the tubing walls into the cell atmosphere and solution. The electrochemical cell, equipped with a reference electrode and a Pt counterelectrode, was introduced into the antechamber when needed by means of a bellows assembly and a gate valve. Potentials are referred to a Ag/AgCl reference electrode prepared with 1 M KCl. Solutions employed for the adsorption measurements contained 10 mM KF adjusted to pH 4 with HF. The supporting electrolyte for voltammetric/coulometric experiments contained 10 mM trifluoroacetic acid electrolyte (TFA) because TFA afforded a lower pH (2.0)than was accesible in dilute fluoride (about pH 4). Solution temperature was 23 1 "C. Prior to use of such solutions, experiments involving immersion of the Pt(ll1) surface into the fluoride and TFA electrolytes followed by Auger spectroscopy were carried out to verify that the water evaporated upon evacuation without leaving a chemisorbed layer or residue, apart from the electrolyte species themselves. Solutions were prepared from water pyrolytically distilled in pure oxygen through a Pt gauze catalyst (800 "C). Studies dealing with adsorption profiles at Pt electrodes require exceptionally clean techniques at all stages of the experimentation (combined with direct verification of surface cleanliness by a method such as Auger spectroscopyand surface structure as by LEED) because at the low end of the concentration range adsorption is not sufficiently strong to guard the surface against accidental contamination. All but one of the adsorbates studied in the present work were obtained from Aldrich Chemical Co. (Milwaukee,WI) as reagent preparations and were used as received: hydroquinone (HQ), benzoquinone (BQ), phenol (PL), perdeuteriophenol (PDPL), and tetrafluorohydroquinone (TFHQ).
*
OH
6 HQ
OH
00
6
5
4
-LOG
3
2
1
0
C (M)
Figure 2. Packing densities of HQ/BQ at Pt(ll1): (0) based upon Ic/I"pt (eq 8-11 and 15); ( 0 ) based upon Ipt/Iop,(eq 12-14 and 16). Experimental conditions as in Figure 1. made from trifluoroacetic acid (TFA, Aldrich) on potassium fluoride (Aesar, Johnson Matthey, Inc., Seabrook, NH) adjusted to pH 4 with hydrogen fluoride (Fischer Scientific, Pittsburgh, PA). Packing densities, r (moles of adsorbed atoms or molecules per area), were measured by two independent procedures. In the first procedure, Auger signals, I., due to each element x were measured from which the packing densities, r., were calculated. The initial calculation in each w e neglected scattering of Auger electrons within the layer:
where Is/ZoR)was the intensity of the derivative Auger signal due to element x normalized by the derivative Auger signal at 161 eV due to the clean Pt(ll1) surface, Figure 1 and Table I. Numerical values of B, were determined by means of calibration experiments using a molecular layer of known packing density containing the appropriate element. For nonflat adsorbed molecules (that is, packing densities from eq 1which suggest a layer thickness greater than one atom) it is necessary to allow for scattering of Auger electrons within the layer:
where Li is the fraction of atoms or type x located in level i (i = 1is adjacent to the solid surface, and N is the outermost layer). The scattering factor of the ith carbon atom in the layer is (3)
D
BQ -
HS
T F H Q -DMBM 2,5-Dihydroxy-4-methylbenzyl mercaptan (DMBM) was synthesized according to published procedures.8 Electrolytes were (6) Wood, E. A. Crystal Orientation Manual; Columbia University Press: New York, 1963. (7) Samuels, L. E. Metallographic Polishing by Mechanical Metho&, Pittman: London, 1963. (8) Fields, D. L.; Miller, J. B.; Reynolds, D. D. J.Org. Chem. 1965,30,
"""_.
7
RW3
(9) (a) Schoeffel, J. A; Hubbard, A. T. Anal. Chem. 1977,49,2330. (b) Garwood, G. A., Jr.; Hubbard, A. T. Surf. Sci. 1982,118, 223. (c) Katekaru, J. y.;Hershberger, J.; Garwood, G. A., Jr.; Hubbard, A. T. Surf. Sci. 1982,121, 396. (d) Chia, V. K. F.; Stickney, J. L.; Soriaga, M. P.; Roeasco, S. D.; Salaita, G. N.; Hubbard, A. T.; Benziger, J. B.; Pang, K. W. P. J. E l e c t r o a d . Chem. 1984,163,407. (e) Frank, D. G.; LagurenDavidmn, L.; Lu, F.; Salaita, G. N.; Hubbard,A T. Anal. Chem.,in press. (10) Becker, E. D.;Charney,E.; Anno, T. J. Chem. Phys. 1965,42,942.
where Mi is the number of atoms located on the path from the emitting carbon atom to the detector. The quantity fc = 0.70 was the attenuation of the Pt signal at 235 eV by the close-packed layer of horizontally oriented HQ at Pt(ll1). In the second procedure, the Auger signal at 161 )V due to the Pt substrate was measured before (I'd and after (Idapplication of an adsorbed layer. The ratio Ipt/Zopt wm employed to calculate the packing density (atoms/cm2) by means of electron-scattering formulas. Specifically,the initial calculation in each case treated the layer as being one atom is thickness:
I ~ ~ = /1 -I JIKxr ~ ~
(4)
where J1is the number of non-hydrogen atoms in the molecule. Auger spectra obtained for a variety of monoatomic adsorbed layers have revealed that K is equal to 0.160 cm2/nmol for at(11) Wilson, H. W. Spectrochim. Acta 1974, 30, 2141. (12) Soriaga, M. P.; Hubbard, A. T. J.Am. Chem. SOC.1982,104,3397. (13) (a) Pouchert, C. J. The Aldrich Library of FTIR Spectra; Aldrich Chemical Co., Inc.: Milwaukee, 1985. (b) Standard Spectra Collection; Sadtler Research Laboratories, Inc.: Philadelphia, PA, 1980. (14) Bist, H. D.; Brand, J. C. D.; Williams, D. R. J. Mol. Spectrosc. 1967, 24, 402. (15) Dyer, J. R. Applications of Absorption Spectroscopy of Organic Compounds; Prentice-Hall: Englewood Cliffs, NJ, 1965. (16) Stern, D. A,; Wellner, E.; Salaita, G. N.; Laguren-Davidson, L.; Lu, F.; Batina, N.; Frank, D. G.; Hubbard, A. T. J. Am. Chem. SOC.,in press.
Lu et al.
640 Langmuir, Vol. 4,No. 3, 1988 Table I. Auger and Electrochemical Data for Adsorbed Molecules at Pt(l11)" normalized Auger peak heights -log
c
ZC/ZOP,
ZO/I" Pt
ZRIZOPt
ZF/Z"Pt
ZS/Z"Pt
coulometric charge density (Qox - %')/A, pC/cm2
HQ
6.52 6.00 5.52 5.00 4.52 4.00 3.52 3.00 2.52 2.00 1.52 1.00 0.70 0.52
0.149 0.572 0.610 0.625 0.640 0.663 0.671 0.664 0.675 0.736 0.763 0.853 0.867 0.978
0.066 0.168 0.256 0.263 0.283 0.279 0.278 0.285 0.312 0.326 0.292 0.339 0.366 0.432
0.831 0.685 0.648 0.645 0.641 0.623 0.611 0.613 0.582 0.602 0.559 0.547 0.523 0.499
3.16
0.518
0.220
0.694
3.52
0.717
0.244
0.677
3.16
0.768
0.212
0.712
3.16
0.316
0.120
0.676
3.16
2.117b
0.541b
0.455b
154 649 653 715 691 737 684 772
BQ PL 810
PDPL TFHQ 0.058
DMBM 3.996b
A at 2000 eV, incident normal to the "Experimental conditions: concentration of adsorbate in units of mol/L. Auger beam waa surface; modulation amplitude was 5 V peak-to-peak. Auger peak heights for the coated surface were normalized by the peak height of the Pt peak at 161 eV for the clean Pt(ll1) surface. Supporting electrolyte was 10 mM KF adjusted to pH 4 with HF. Electrode potential was 0.2 V vs a Ag/AgCl (1 M KCl) reference. Coulometric charge, Q, was measured as described in the narrative. bAuger peak heights normalized by the positive lobe of the 235-eV Pt peak. tenuation of the Pt Auger signal at 161 eV by light elements such as C, N, and 0. The sulfur Auger signal overlaps with Pt at 161 eV, requiring use of the positive lobe of the Pt signal at 235 eV (carbon interferes with the negative lobe); in this case (sulfur compounds, 235-eV Pt signal) Ks = 0.219 cm2/nmol. For some adsorbates the packing density found from eq 2 exceeds what is possible for a layer having monoatomic thickness. In those instances a nonlinear equation applies, and consideration of molecular orientation in the layer is required: ~ ~ / =z (1 o- JIKxr) ~ ~ ... (1 - J N K x r ) (5) where Ji represents the number of non-hydrogen atoms per molecule located at the ith level of the adsorbed layer and N is the outermost layer, as before. Conversion from atoms per area to molecules per area was based upon molecular formulas, as usual; hydrogen atoms were averaged in with the other elements rather than being treated separately. For convenience, r values were sometimes converted to 8, the packing density expressed in molecules adsorbed per surface Pt atom, as defined by eq 6: B = r/rpt (6) The results of application of these two alternative approaches (I, in eq 1or ZPt/ZoRin eq 4)were in excellent agreement, Figure 2 and Table 11. Overlap between C (268 eV) and Pt(235 eV) signals is minimized by employing the larger and readily distinguishable negative (high energy) lobe of the C derivative spectrum. Spectra of sulfur compounds are normalized by the Pt signal at 235 eV rather than 161 eV to avoid the influence of overlap with the S signal at 149 eV. The sulfur signal (149 eV) is separated from the Pt signal (161 eV) by substracting the digitized spectrum of the clean Pt surface after normalization for attenuation due to the adsorbed layer (based on attenuation of the A signal at 235 eV). Electron energy-loss spectra (EELS) were obtained by means of a Kesmodel EELS spectrometer (Bloomington, IN). Beam current was approximately A at 4 eV. The spectrometer was operated at a solution of approximately 0.01 eV (80 cm-') in these experiments.
Infrared spectra of solid compounds were obtained by using a Perkin-Elmer Model 1420 dispersive instrument operated at 4-cm-' resolution.
Results and Discussion 1. Hydroquinone (HQ) and Benzoquinone (BQ). Auger spectra of the adsorbed layers formed from HQ and related compounds appear in Figure 1. These and similar spectra yielded the data in Table I, which were converted to packing densities by two independent methods as described in the Experimental Section. Packing densities of C and 0 were calculatedfrom the C and 0 Auger signals, ICand Io,by means of eq 1for packing densities up to the plateau and otherwise by means of eq 7-10: vrc =
- Bc hPrc + f C / 2 - hprC/vprC)
IC/IOPt BC(1/2
with fc = 0.70 and Bc = 0.377 cm2/nmol hrC
= (l - vpC/vprC)hprC r C
=
hrC
+ vrC
FO = (Io/IOpt)/Bo
(7) (8) (9)
(10)
where Ic/Iopt and 10/Iopt are Auger peak height ratios of carbon (268 eV) and oxygen (532 eV) to the clean Pt signal (161 eV) and h r C and vI'c are the packing densities of C due to the horizontally and vertically oriented HQ/BQ componenta of the adsorbed layer, respectively. hprc and wI'Care theoretical limiting plateau values, calculated from covalent and van der Waals radii tabulated by Pauling,l' for horizontal and vertical orientations shown in parts A (17) Pauling, L. C. The Nature of' the Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1960.
Characterization of Hydroquinone and Related Compounds
Langmuir, Vol. 4, No. 3, 1988 641
Table 11. Packing Densities and Oxidation Factors at Pt(ll1)" packing density, nmol/cm2 from elemental Auger signals -log
c
rC
r0
rF
r
rs
from Pt Auger signal attenuation
oxidation factor nox (Box- Qb)/FAr, e-/molecule
0.132 0.246 0.275 0.277 0.280 0.295 0.307 0.304 0.348 0.320 0.380 0.397 0.430 0.464
24.2
r
HQ
6.52 6.00 5.52 5.00 4.52 4.00 3.52 3.00 2.52 2.00 1.52 1.00 0.70 0.52
0.395 1.518 1.619 1.662 1.698 1.759 1.780 1.762 1.810 2.039 2.141 2.479 2.531 2.948
0.139 0.353 0.538 0.553 0.595 0.586 0.584 0.599 0.655 0.685 0.613 0.712 0.769 0.908
3.16
1.374
0.462
0.066 0.253 0.270 0.277 0.283 0.294 0.299 0.295 0.302 0.340 0.357 0.413 0.422 0.491 BQ
0.229
24.9 23.9 2j.8 23.7 21.4 16.8 16.3
0.239
PL
3.30
1.902
0.513
3.16
2.037
0.445
3.16
0.838
0.252
3.16
2.982
0.560
0.318
0.288
26.4
PDPL
0.340
0.257
TFHQ
0.62
0.140
0.169
DMBM
0.35
0.373
0.386
Experimental conditions: molecular packing density, r, from elemental Auger signals was employed to calculate no, (oxidation factor); other conditions were as in Table I.
and B of Figure 5, respectively: h rc = 1.76 and wI'C = 3.47 nmol/cm2. rcis the number oPmoles of w b o n atoms per unit area of the layer, and Bo = 0.476 cm2/nmol. Alternatively, packing densities were calculated from the ratio of Pt signals for the coated,IR, and clean, IOp, surface at 161 eV by means of eq 4 up to the plateau and eq 11-13 otherwise:
vr= +A$(h h r
- Ipt/IopJ/(Ah - A,)
(11)
= hpr(1 - vr/vpr)
(12)
r = h r + vr
(13)
where Ah = (1 - 8Kchpr)and A, = (1 - !XcJ')2(1 4Kc I'). The attenuation coefficient (Kc = 0.16 cm2/ nm07 was evaluated from data for the Pt(111)(3X3)-BQ layer (see below). hI'and (mol/cm2) are the packing densities of the horizontal and vertical components of the HQ/BQ layer, hprand ,.J are the theoretical limiting (plateau) values calculated form covalent and van der Waals radii," and r is the total packing density of all elements in the layer. Molecular packing densities, r, were obtained from the elemental values by dividing by the number of atoms of the given type per molecule:
r = rc/6
(14)
Both methods of packing density measurement gave similar results, Figure 2 and Table 11. Likewise, rc and ro gave virtually identical absorption profiles, r. A packing density plateau was observed, r = 0.29 nmmol/ cm2,followed by an upturn at HQ concentrations greater than 1 mM. The experimental plateau value is in good agreement with the theoretical packing density of 0.293 nmol/cm2 (56.6 A2/molecule) for a horizontally oriented, close-packed BQ layer, on the basis of covalent and van der Waals radii."
Electron energy-loss (EELS) spectra of adsorbed layers formed in HQ or BQ solutions at Pt(ll1)were very similar, as shown in Figure 3. Also shown in Figure 3 are the locations and relative stengths of the mid-infrared absorption bands of HQ and BQ. The EELS spectra contain all of the bands expected for HQ and BQ, except that the 0-H (3260 em-') and the C=O (ca. 1750 cm-') bands are absent. Interpretation of the EELS spectra in line with accepted assignments for the E t bands of HQ (adsorbate symmetry, D%) and BQ (Kw)loJ1led to the EELS assignments given in Table 111. Evidently, the hydrogens of the phenolic groups in HQ are lost during chemisorption, in agreement with earlier findings based upon thin-layer electrochemistry,14J2and form the horizontally oriented phenoxide form: PH
?-SURFACE
Pt surface
bH
t Z H + t 2e-
SURFACE
The similarity of EELS spectra starting from HQ or BQ solutions suggests formation of a similar adsorbed species starting from either adsorbate. That is, the adsorbed species formed from HQ or BQ at low concentrations is intermediate between HQ and BQ. Pt surface
0
4
Q-S U R FACE
0
(16)
0-SURFACE
LEED patterns observed at packing densities slightly lower than the plateau value were (3x3) as shown in Figure 4A. On the basis of this (3x3) symmetry and the presence of 1molecule/unit mesh, the ideal packing density would be r = 0.277 nmol/cm2. Accordingly, we have taken this value at the point of maximum brightness of the (3x3)
642 Langmuir, Vol. 4, No. 3, 1988 '
n
'
Lu et al. , . ' . ' I ' ' ' ' I . " '
I
2919
A . . . . . . . . . . . . . . . . . . . . .
IO00
0
1
HO BANDS
1
BO BANDS
i50H2'l
""
.
1
. . . . , . . . . , . . . . , . . . . , . . .1 . 2000
3000
4 000
ENERGY LOSS ( c m - I )
0
IO00
2000
3000
4000
ENERGY LOSS ( c m - I )
Table 111. Assignments of EELS Bands for Adsorbed Diphenols peak concn, frequency, symcompd" mM cm-' metry mecies descrDtion* 0.1 2979 Dul BZu,BsuPh-H stretch 1611 B3, CC stretch 1496 B1, CC stretch 1247 Bk Ph-0 stretch 905 B1, Ph-H bend B~ ring bend 785 B1, ring bend 506 PL 10 3020 Czv A1,B2 Ph-H stretch 1620 A1,B2 CC stretch 1480 A1,Bz CC stretch 1370 Bz CC stretch 1259 Al Ph-0 stretch 1130 Bz Ph-H bend 981 B1 ring bend, Ph-H bend 890 B1 Ph-H bend B1 Ph-H bend 787 Bz,Bl ring bend, CC 637 twist 450 Bz ring bend 2258 PDPL Czu Al,BP Ph-D stretch 1677 Al,Bz CC stretch 1460 A1,B2 CC stretch 1255 Al Ph-0 stretch 834 Bz Ph-D bend 650 B1 ring bend, CC twist 575 B1 Ph-D bend 450 Bz ring bend TFHQ 1.0 1695 Dul B3, CC stretch 1155 Bzu Ph-0 stretch 1036 Bzu,BBuPh-F stretch 775 Bsu Ph-F bend 627 Bzu ring bend, CC twist DMBM 0.1 3568 Cl A 0-H stretch
(metal
3374 2981 1600 1411 1194 1021 865 710 646 425 ENERGY L O S S ( c m - I )
Figure 3. Electron energy-loss spectra at Pt(ll1): A, HQ (1.0 mM); B,E@ (1.0 mM); CyHQ (0.50M).Experimental conditions: supporting electrode, 10 mM KF adjusted to pH 4 with HF; temperature 23 1 O C ; beam energy, 4 eV; beam current, 0.15 nA; incidence and detection angles, 62O from surface normal.
*
LEED pattern (0.01 mM HQ) as the calibration point for both types of electron spectroscopic measurement of packing density. It follows that the plateau value is r = 0.29 nmol/cmz. On the basis of the (3x3) LEED symmetry Auger intensities and EELS spectra described above, we propose the structure shown in Figure 5A. When the HQ concentration was less than lo4 Mymass transport limitations to formation of the adsorbed layer were severe. This condition detracts somewhat from the credibility of such measurements, although the results (Figure 2 and Tables I and 11) were reproducible and appear reasonable. An explanation for the increase in packing density at HQ concentrations above 1mM is provided by studies of HQ/BQ adsorption at electrochemically cycled polycrys-
0-H stretch (ortho) C-H, 0-H stretches CC stretch (0) CC stretch Ph-0 stretch Ph-H bend C-H bend ring bend C-S stretch ring bend
a HQ, hydroquinone; BQ, benzoquinone; PL, phenol; PDPL, perdeuteriophenol; TFHQ, tetrafluorohydroquinone; DMBM, 2,5dihydroxy-4-methylbenzylmercaptan. * Ph = phenyl.
talline Pt thin-layer electrodes.12 At concentrations above 1 mM a transition from horizontal to vertical (2,3q2 edgewise) orientation was indicated, Figure 5B. Evidently, the same transition occurs a t the Pt(ll1) surface but somewhat less abruptly than at the polycrystalline surface. This interpretation is supported by the EELS spectrum of the layer formed at high concentration of HQ, Figure 3C,where the aromatic ring bending modes and phenylhydrogen bending modes increased due to less immediate interaction with the surface. Experiments were performed in which the Pt(ll1) surface containing a layer of adsorbed HQ was transferred to pure electrolyte, after which the surface and adsorbed layer were oxidized at constant potential. Referring to the voltammograms in Figure 6A, the potential at which oxidation was carried out was chosen such that the process proceeded rapidly to completion (1.0 V vs Ag/AgCl). The surface was then reduced (0.10 V), followed by coulometric
Langmuir, Vol. 4. No.3, 1988 643
Characterization of Hydroquinone and Related Compounds
r I ~
i i
~
i
Figure 4. LEED pattern for HQ (A, left) and PL (B,right) at Pt(ll1). Experimental conditions: beam energy, 54 eV for HQ and 60 eV for PL; 0.012 mM HQ (A) or 0.010 m M PL (B)in 10 mM KF adjusted to pH 4 with H F electrode potential 0.20 V. oxidation of the resulting bare surface to determine the background charge in the absence of an adsorbed layer. The coulometric charges, Q, and QG,respectively (dwect electronic integration of the current), were measured after the chargetime curves had become parallel, Figure 6B. Combining these data with the packing densities, r (nmol/cm2),yielded the average number of electrons to oxidatively desorb an adsorbed molecule: n, = (Q, - Qb')/(FAr) (17) The results are shown in Figure 7 and Table IIL n, began to decrease at the same HQ concentration a t which the packing density began to increase (1mM). At the packing density plateau, no. (24 f 0.5 electrons/molecule) corresponded to complete oxidation of adsorbed HQ to CO,:
;)
0-0
>
,I!
/I +
ion20
+s c o 2 +
pen*+
we-
(18)
The decrease in n, above 1mM HQ closely followed that expected for oxidation of the vertically oriented fraction of the adsorbed layer to maleic acid and COP,as shown in eq 1 9
";%/
o~
+ 2coZ +
izn++ me-
2. Phenol (PL). Packing densities and vibrational spectra were obtained for PL, Tables I and I1 and Figure 8.4. That PL and HQ/BQ have analegous adsorbed states is evident from the similarity of their packing densities. The packing density of PL at 0.3 mM, r = 0.3 nmol/cm2, agrees with the calculated limiting horizontal packing density of PL based upon covalent and van der Waals radii, 0.312 nmol/cm2.I7 Packing density equations 1,4, and 14 apply to PL as for BQ/HQ, except that Ah is A, = (1- mer)
(20)
As for HQ, there was no detectable OH stretch in the EELS spectrum adsorbed at PL concentrations less than 1mM, Figure SA, indicating loss of the phenolic hydrogen upon adsorption in the horizontal orientation. The PL EELS spectrum is otherwise very similar to the IR spectrum of a PL melt.13 By analogy with the accepted IR assignments," the EELS bands of PL (C,) are assignable as given in Table 111. Evidently, since PL lacks a quinone-like stable higher oxidation state, the redox process of PL does not result in loss of aromaticity:
(19)
At 0.2 M, for example, the layer consists of 61% vertical species, based on rc,which corresponds to a theoretical n, of 16.7 compared to the observed value of 17.6. Hydroxylation of the Pt surface accompanies oxidation of the adsorbed layer thus preventing subsequent adsorption and further oxidation of desorbed fragments." In order to verify that HQ is not removed from the Pt(ll1) surface by evacuation, n, measurements were made by modification of the above procedure in which the coated surface was placed under UHV for about 1h prior t o measurement of Q , and Qd, Figure 6A. The resulting n, values did not differ detectably from those obtained without evacuation, indicating stability of the adsorbed layer in vacuum.
In actuality, bonding to the surface probably results in pairing of the odd electron with electron density of the metal. Measurement of n, at a PL concentration of 0.3 mM yielded a value of 26.4 eledrons/molecule. This result supports the idea that the adsorbate is horizontally oriented and partially dehydrogenated a
LEED patterns observed for PL adsorbed a t concentrations near 0.01 mM were Pt(111)(3X3)-PL, Figure 4B, similar to those for HQ/BQ. A model structure consistent with the symmetry, packing density, and vibrational as-
644 Langmuir, Vol. 4, No. 3, 1988
Lu et al.
A
I
=_
I
I
I
I
I
I
I
12
Ln
Z
W
0
0
L
0.0 0.2 0.4 0.6 0.8 1.0 1.2 POTENTIAL,
B
$
6
0
v I
I
I
VOLT
1
I
,
VS.
Ag/AgCI
1
1
1
1
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.-. 3 0 . Y 3 0 iil
i
0 2
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I
I
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1
2070
\
10 Z
0
-
(L
IO
F
-
0 W J
-
4
-
-
x 0
I
1
0'
; w p I / / / / / / / /
/ / /
Figure 5. Structure of HQ adsorbed at Pt(ll1): A, Pt(ll1)(3X3)-HQ, B = '/e (l? = 0.276 nmol/cm2); B, vertically oriented HQ at Pt(ll1) (0.276 < r G 0.578 nmol/cm2)); C, P t ( l l l ) ( 3 ~ 3)-PL, 0 = l/g (r = 0.276 nmol/cm2).
signmenta is shown in Figure 5C. EELS spectra were obtained for perdeuteriophenol (PDPL), Figure 8B and Table 111. As expected, there was virtually no C-H stretch a t 3020 cm-l, and the expected C-O stretch was present at 2258 cm-l. CC stretches and CO stretch were unchanged in frequency, as expected, CD bending was shifted as indicated in Table 111, and ring
modes remained fixed. EELS spectra were also obtained for samples in which H/D exchange at the phenolic OH had been allowed to occur in solution prior to adsorption: OH
b
D
00
OH
"@:+ H
excess D 2 0
H
e
+
H H
(24)
Langmuir, Vol. 4, No. 3,1988 645
Characterization of Hydroquinone and Related Compounds
C
,
50kHz
"
"
I
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~
Ilj
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1
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4000
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LOSS ( c m - I 1
-
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0 . M
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-
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w -
D
a:
w . t-
z
t-
3
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0
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3000
4000
E N E R G Y LOSS ( c m - 1 1
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 IO00 2000 3000 ENERGY L O S S ( c m - I )
I 4000
Figure 8. Electron energy-loss spectra at Pt(ll1): A, phenol (PL), 10 mM; B, perdeuteriophenol (PDPL), 10 mM; C, tetrafluorohydroquinone (TFHQ), 10 mM; D, 2,5-dihydroxy-4-methylbenzylmercaptan, 0.1 mM. Experimental conditions as in Figure 3.
The spectra confirmed the absence of OH in the adsorbed material; that is, adsorption of PDPL from H20, eq 23, gave the same EELS spectrum as PDPL adsorbed from DzO.Furthermore, PL adsorbed from D20,eq 24,yielded EELS spectra identical with those of ordinary PL. While performing the PDPL experiments, we noticed that in a preliminary experiment hydrocarbon contamination resulted in a detectable C-H stretch peak near 3000 cm-' in the EELS spectrum. This is because PL is susceptible to competition from a wide variety of chemisorbable hydrocarbons and hydrocarbon derivatives,ls which have strong C-H vibrational bands. Such hydrocarbons are abundant in the laboratory and in nature. The presence/absence of C-H peaks in the EELS spectrum of adsorbed PDPL is persuasive evidence as to whether the reagents and apparatus are sufficiently clean to yield the intended absorbed layer. Therefore, we recommend that others attempting to obtain EELS spectra of organic materials adsorbed at electrode surfaces obtain spectra of adsorbed PDPL os a procedural test before publishing spectra of other adsorbed compounds. 3. Tetrafluorohydroquinone (TFHQ). Vibrational spectra and packing densities were obtained for TFHQ adsorbed from a 0.7 mM solution, Figure 8C and Tables I and 11. The packing density of TFHQ was calculated by using eq 1, 4, and 14 but with J1 = 12 for eq 4. The observed packing density for TFHQ was about 0.15 nmol/cm2, which is half that of HQ at the same concen-
(18) Stickney, J. L.; Soriaga, M. P.; Hubbard, A. T.; Anderson, S. E. J. Electroanal. Chem. 1981, 125, 73.
tration, Table 111. The theoretical limiting packing densities expected for the horizontally oriented molecules of TFHQ and HQ based on covalent and van der Waals radii are 0.26 nmol/cm2 (63.3 A2/molecule) and 0.29 nmol/cm2 (57.3A2/molecule),respectively. Evidently, the difference in the packing densities is not due solely to the 11% difference in theoretical area but is also due to significant differences in the kinetics or strength of adsorption. This effect was also observed in earlier work on annealed polycrystalline thin-layer cells, where halogeqation of the aromatic ring diminished its activity for the surface, resulting in less adsorption and smaller packing densities.lg The EELS spectrum of TFHQ contains all of the I R bands observed for TFHQ in a KBr pellet (Figure 8C) except for the 0-H stretch at 3390 cm-', which was completely absent. Evidently, the phenolic hydrogens are lost upon adsorption as for HQ: OH
0-SURFACE
dH
SURFACE
The frequencies of the bands were blue-shifted compared to the bulk mid-infrared bands (Figure 8C) apparently due to the high electronegativity of the fluorine atoms and the ability of the surface to donate electrons upon adsorption. Because TFHQ is more weakly adsorbed than HQ and contains no C-H bonds, adsorption of TFHQ is also a very sensitive procedural test for interfering organic contami(19)Soriaga,M. P.; Hubbard, A. T. J.Am. Chem. SOC.1982,104,2735.
Langmuir 1988,4, 646-653
646
nants as is PDPL adsorption. 4. 2,5-Dihydroxy-4-methylbenzylMercaptan (DMBM).Studies by means of polycrystalline Pt thinlayer electrodes have demonstrated that thiophenol derivatives bind to Pt surfaces through the sulfur atom with the aromatic ring perpendicular to the surface.'12 In order to compare the behavior of horizontally oriented adsorbed phenols such as HQ and PL with analogous vertically oriented adsorbates, studies of DMBM were included in this work. Auger and EELS spectra of DMBM appear in Figures 1D and 8D. Packing densities were obtained from C, 0, and S Auger signals by means of eq 26: r c = (Ic/Iopt)/[Bc(1/2
+ 7fc/16 + fc2/16)]
(26)
where IoR was measured at the positive lobe of the Pt signal at 235 eV to minimize the effect of overlap with other peaks in the spectrum: Bc = 0.848 cm2/nmol and fc = 0.70. Molecular packing density, I', was also determined from attenuation of the positive lobe of the Pt Auger signal at 235 eV:
Ipt/Iopt = (1 - Ksr)(i - 5 ~ ~ 1 3 2
(27)
where Kc = 0.153 and Ks = 0.219 cm2/nmol. Packing densities of 0 and S were obtained from Auger signals due to 0 and S:
r o = (Io/Iopt)/[Bo(l/2
-k
fo/4
+ fo2/4)I
rs = (Is/Iopt)/(Bsfs)
nmol/cm2, compared with a theoretical packing density from covalent and van der Waals radii" of 0.399 nmol/cm2 (41.7 A2/molecule). The small peak at 1574 cm-' is an aromatic CC stretch. Although the S-H band is not particularly strong in the IR spectra of solid thiophenols,16 the absence of this band from the EELS spectrum is evidence that the mercaptan hydrogen is removed as a result of the adsorption process:
""a -1 Horn;; ;, 0
t
H2iH
H' + e-
(30)
H28
7577-7Adsorbed DMBM is stable in contact with solution and vacuum: the electrode potential was constant at open circuit, at least on the time scale of the measurements (about 1 h). Furthermore, when the layer was removed from solution, evacuated under UHV for about 1 h, and then transferred from vacuum back into solution, a positive-going scan from the open-circuit potential produced the usual voltammetric peak indicative of adsorbed DMBM?
(28) (29)
where Bo = 1.27 cm2/nmol, Bs = 18.6 cm2/nmol, fo = 0.70, and fs = 0.62. The molecular packing density of DMBM from eq 26 is 0.373 nmol/cm2 and from eq 27 is 0.386
Acknowledgment. Acknowledgment is made to the Air Force Office of Scientific Research for support of this work. Registry No. HQ, 123-31-9; BQ, 106-51-4;PL, 108-95-2; TFHQ, 771-63-1; DMBM, 81753-11-9.
Mossbauer Spectroscopic Studies of Ferrisilicates Zhu Yixiang,? Luis M. Aparicio, and J. A. Dumesic* Department of Chemical Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706 Received September 10, 1987. In Final Form: December 11,1987 Crystalline and amorphous ferrisilicates were characterized by using Mossbauer spectroscopy to probe the coordination of iron cations in these materials. The trivalent iron originally present in the ferrisilicates could be reduced to divalent iron by treatment with Hz at 700 K. No metallic iron was observed after the reduction pretreatment,except in samples with iron loadings greater than about 5 wt %. In the trivalent and divalent states, coordinatively unsaturated iron accessible to adsorbate gases could be distinguished from coordinatively saturated iron. The fraction of iron found in coordinatively unsaturated sites was found to decrease with iron loading. Nevertheless, it was possible to prepare a sample in which approximately half of the iron was present in low coordination sites at a total iron loading of 3.6 wt %.
Introduction The study of interactions between dispersed metals and supports of metal catalysts is an important area of heterogeneous catalysis. Strong interactions can also be expected, however, between transition-metal oxides and oxidic supports, due to the structural and chemical similarity between these materials. One example of such an interaction has been found for iron oxide supported on silica (Fe/SiOz). Whereas bulk iron oxide is reduced to
* Author to whom correspondence should be addressed. 'Present address: Department of Chemistry, Xiamen University, Xiamen, Fujian, China.
metallic iron by high-temperature treatment with H2, supported iron cannot be reduced below the divalent state by the same treatment.l Supported iron oxide has also been found to be a less active catalyst for water-gas shift as compared to bulk magnetite2 Infrared and Mbssbauer spectra collected after adsorption of NO on Fe/Si02 were interpreted in terms of a strong interaction between ferrous cations and the support, leading to the stabilization of iron cations in sites of low c~ordination.~ (1) Berry, F. J. Adv. Inorg. Chem. Radiochem. 1978,21, 255. (2) Rethwisch, D. G.; Dumesic, J. A. J. Catal. 1986, 101, 35. (3) Yuen, S.; Chen, Y.; Kubsh, J. E.; Dumesic, J. A.; Topme, N.; Topme, H. J. Phys. Chem. 1982,86, 3022.
0743-7463f 88f 2404-0646$01.50f 0 0 1988 American Chemical Society
