J. Phys. Chem. 1981, 85, 3832-3835
3832
Proton Adsorption-Desorption Kinetics on Iron Oxides in Aqueous Suspensions, Using the Pressure-Jump Method' R. D. Ashmian,' M. Sasaki, T. Yasunaga," Department of Chemistry, Faculty of Science, Hiroshima University, Hiroshima 730, Japan
and 2. A. Schelly' Department of Chemistry, The University of Texas at Arlington, Arlington, Texas 76019 (Received: May 7, 1981; In Flnal Form: August 3, 198 1)
Single relaxations were observed in aqueous suspensions of y-Fez03(hematite) and Fe304(magnetite) by using the pressure-jump relaxation method with conductometric detection. No relaxation could be observed in goethite (FeOOH) suspension. The reciprocal relaxation times obtained for both the hematite and magnetite suspensions show the same parabolic dependence on pH, displaying a minimum at pH 3.4. The relaxation can be interpreted in terms of proton adsorption-desorption at the surface of the iron oxide particles, preceded by the rapid reequilibration of their ionic atmospheres. For all samples, t potential, surface potential, and adsorption isotherms were determined. The statically obtained values of the acidity constants Kal of the surface sites can be used to predict the relaxation amplitudes. At I = 2 X and 25 "C,the proton adsorption and desorption rate constants were determined to be kaht = 2.4 X lo6 mol-l dm3s-l and kimt = 1.6 X 10-1 s-l for hematite, respectively, and kaint = 1.4 X lo6 mol-l dm3 s-l and kdint = 3.4 X lo-' s-l for magnetite, respectively.
Introduction Adsorption of anions and cations from aqueous solution by suspended metal oxides have been a topic of interest among soil scientists, geochemists, and colloid chemists. It has been well-known that the amphoteric properties of the hydroxyl groups existing on the surface of oxide particles play an important role in the adsorption and are characterized by two acidity constants, Kal and Kaz,as a function of the surface potential created by the adsorbed Recently, the interaction of the surface groups with specifically adsorbed counterions has also been investigated to clarify the structure of the electrical double layer surrounding the particles.'-9 The equilibrium properties of iron oxide (such as hematite (y-Fe203), magnetite (Fe304), and geothite (FeOOH)) suspensions have been extensively investigated?l0 and their slow dynamic behaviors have been studied as we11.11J2 In the present work, we focus on the elucidation of the rapid kinetics of the adsorption-desorption processes in iron oxide suspensions which take place on (1) Abstracted in part from R. D. Astumian's thesis to be submitted to The University of Texas at Arlington. (2) Japanese Ministry of Education Research Scholar. Department of Chemistry, The University of Texas at Arlington, Arlington, TX 76019. (3) J. A. Davis, R. 0. James, and J. 0. Leckie, J. Colloid Interface Sci., 63, 480 (1978). They proposed a site binding model in which bound species are considered to be located at certain sites. (4) R. J. Atkinson, A. M. Posner, and J. P. Quirk, J . Phys. Chem., 71, 550 (1967). (5) G. A. Parks and P. L. de Bruyn, J . Phys. Chem., 66,967 (1962). (6) G. A. Parks, Chem. Reu., 65, 177 (1965). (7) R. 0. James, R. T. Stigtlich, and T. W. Healy, Faraday Discuss. Chern. SOC., 59,143 (1975). (8) G. R. Wiese, R. 0.James, and T. W. Healy, Discuss. Faraday SOC., 62, 302 (1971). (9) R. J. Hunter and H. J. L. Wright, J. Colloid Interface Sci., 37,564 (1971). (10) A. Breeuwsma and J. Lyklema, J. Colloid Interface Sci., 43,437 (1973). (11)G. Y. Onoda and P. L. de Bruyn, Surf. Sei., 4, 48 (1966). (12) Y. G. Berube, G. Y. Onoda, and P. L. de Bruyn, Surf. Sci., 8,448 (1967).
the subsecond time scale. Because of the high rate of the reactions involved, the pressure-jump relaxation method13 has been employed. This technique has recently been successfully used also in the investigation of other metal oxide dispersions.14-16 In addition to the kinetic data obtained, we also report results of static measurements of adsorption density, surface charge, and t potential on these systems. Numerical values of the rate constants of the elementary surface reactions involved are reported, and the adsorption-desorption mechanism is discussed.
Experimental Section Chemicals and Sample Preparation. Hematite was prepared by refluxing Fe(N03)3in aqueous solution, according to the method of Parks and DeBruyna6 Both geothite and magnetite were supplied by the Toda Co. All three samples were exhaustively electrodialyzed until the conductivities were approximately the same as that of distilled water. Subsequently, their identities were confirmed by X-ray diffraction. In the hematite sample, no trace of goethite was found. After electrodialysis, the samples were dispersed in water by ultrasonication. Both goethite and hematite formed very stable suspensions, with no sign of appreciable sedimentation over a period of 1h. However, some settling was observable in the magnetite suspension after 15 min. Thus, extra care was taken in the experiments with magnetite, making certain that after 2-3 min the sample in the pressure-jump cell was replaced with freshly ultrasonicated sample. All samples were allowed to equilibrate for 24 h after the addition of the salt and acid. The ionic strengths of (13) M. Eigen and L. DeMaeyer in "Technique of Organic Chemistry", Vol. 8. Part 2. S. L. Friess. E. S. Lewis. and A. Weissbercrer. Eds.,. Interscience, New York, 1963. (14) M. Ashida, M. Sasaki, H. Kan,T. Yasunaga, K. Hachiya, and T. Inoue, J. Colloid Interface Scz., 67, 219 (1978). (15) K. Hachiya, M. Ashida, M. Sasaki, H. Kan, T. Inoue, and T. Yasunaga, J. Phys. Chem., 83, 1866 (1979). (16) T. Ikeda, M. Sasaki, R. D. Astumian, and T. Yasunaga, Bull. Chem. SOC.Jpn., in press.
0022-365418112085-3832$01.2510 0 1981 American Chemical Society
The Journal of Physical Chemistty, Vol. 85, No. 25, 7981 3833
Proton Adsorption-Desorption Kinetics
all samples were adjusted to 2 X M with tetramethylammonium perchlorate, where the contribution of the added acid to the ionic strength was neglected. Perchloric acid and tetramethylammonium perchlorate were reagent-grade chemicals and were used without further purification. The pressure-jump as well as the static experiments were carried out on samples of 30 g dm-3 particle concentration [PI for hematite and magnetite, and [PI = 20 g dm-3 for goethite (due to the high viscosity of the latter suspension). In the experiments where turbidity detection was used to determine whether sedimentation contributed to the relaxation, the [PI’S were 1 g dm-3. Experiments. The pressure-jump apparatus with conductometric detection has been described previ0us1y.l~It has a time constant of 100 1 s at a bursting pressure of 200 atm. Acid-base titration at various different ionic strengths was used to determine the pH,,, (the “pH of zero point charge”; i.e., where the total charge on the particle is zero) of hematite (8.4),goethite (8.2), and magnetite (7.1). The { potentials were determined by microelectrophoresis.17
Theoretical Background Following the approach of Davis et if the counterions in solution are not specifically adsorbed (Le., the energy of interaction is only Coulombic), one can write the adsorption-desorption and counterion binding reactions a t the amphoteric surface hydroxyl group, FeOH, as the following stoichiometric equations: in the pH range below the pH,,, FeOH2+ FeOH,+
+ H+
(1)
FeOH2+-A-
(2)
FeOH
- + + + A-
KPU , iOO
in the pH range above pH,, FeOH FeO-
K&
C+
FeO-
KC&h
7. E
52v, 5.0
It.‘ 2.5
0
(3)
FeO--C+
(4) where Kal and Ka2are the acidity constants, and Kanionand Kcation are the equilibrium constants for the counterion binding reactions 2 and 4,respectively. Under the present experimental conditions pH < pH,; thus, reactions 3 and 4 can be neglected because the concentration of FeO- is vanishingly small. In the case of adsorption of ions onto a charged surface, the approach of the ions is either hindered or facilitated, and Kal and Ka2can be expressed as functions of the electrostatic potential a t the planes of adsorbed H+ and A-, respectively:
where \ko and \kd are the electrostatic potentials. The superscript “int” denotes intrinsic; the subscripts “s” and “d” refer to the surface plane and the plane of adsorbed counterion, respectively. k is the Boltzmann constant, and (17) A. N. Dolzhenkova, B. A. Gevorkyan, G. V. Vishnyakova, Obogashch. Rud (Leningrad),18, 31 (1973).
I
4
PH Flgure 1. pH dependence of the reciprocal relaxation times 7-‘ obtained in the acidic hematite (0) and magnetite (0)aqueous suspensions at I = 2 X lo3 and 25 OC. The solid line is a best-fit parabolic curve for the data.
T is the absolute temperature. In general, the counterion binding reaction 2 is extremely rapid. Hence, with the electrostatic effect taken into consideration, the reciprocal value of the slower (the measurable one) of the two relaxation times 7-l expected for the coupled system 1-2 is given by eq 7, where k, and ([FeOH] kdint
= kaintex.( H+
I
3
+ [H+]) +
(eqo) Kanion + [ F ~ O H ~ + I eXp 2kT Kanion+ [FeOH2+]+ [A-]
-%)(
[FeOH] + [H+] +
Kanion + [FeOH2+I KalKanion + [FeOH2+] [A-]
+
)
(7)
kd are the proton adsorption and desorption rate constants, respectively. The brackets indicate equilibrium concentrations.
Results and Discussion Application of pressure to the acidic suspensions promotes dissociation in eq 1 and 2. These shifts are reversed in the reequilibration process, as evidenced by the decrease of conductance during the relaxation. Since no relaxations are present in acid-free suspensions, in HCIOl solution with no particles present, or in the supernatant solution of the acidic suspensions obtained by centrifugation, the single relaxations observed in the acidic suspensions may be attributed to proton adsorption-desorption on the suspended particles. Control experiments using turbidity detection confirmed that sedimentation did not contribute to the relaxations observed. The amplitudes of the relaxation signals for the three suspensions decrease in the order of magnetite, hematite, and goethite. Because of the small amplitudes observed in the hematite system, the relaxations had to be signal averaged 4-20 times. The amplitudes are vanishingly small for goethite, and no reproducible relaxations could be obtained even after averaging 25 relaxation curves. A possible reason for this will be given later. The pH dependence of the reciprocal relaxation times 7-l for the hematite and magnetite suspensions are shown
3834
Astumian et ai.
The Journal of Physical Chemistry, Vol. 85, No. 25, 198 1
I
I
I
2 CFeOGI , 10-3rnd 1
0
0.5 [H'l
,W3N
1.o
1.5
Figure 4. pK,, as a function of the amount of adsorbed proton in the acidic hematite (0),magnetite (O), and goethite (8)aqueous susand 25 O C . pensions at I = 2 X
Figure 2. Proton adsorption isotherms in the acidic hematite (0), magnetite (O), and goethite (8)aqueous suspensions at I = 2 X and 25 OC.
0
1
2
3 CH+l&
4
5
6
, lo-"
Figure 5. { potentials as a function of the added acid concentration in the acidic hematite (0),magnetite (O), and goethite (8)aqueous and 25 OC. suspensions at I = 2 X
0
1 [H'I-'
3
2 , lo4 N-'
TABLE 11: Intrinsic Rate Constants of Proton Adsorption-Desorption in Various Aqueous Metal Oxide Suspensions at 25 "C
Figure 3. Langmuir plots of the adsorption isotherms shown in Figure
2.
metal oxide
TABLE I: Equilibrium Parameters of Proton Adsorption-Desorption in Various Aqueous Metal Oxide Suspensions at 25 "C metal oxide
[PI, gdm-3
Fe30, a-FeOOH TiO,
30 30 20 9.9
p%m
pet
ref
8.4 7.1 8.2 5.8
6.2 5.6 6.0 4.1
present work present work present work 14
in Figure 1. Data for both systems fall on the same parabolic curve. To obtain the equilibrium parameters needed in eq 7, we carried out an extensive study of the static properties of the suspensions. The proton adsorption isotherms obtained (Figure 2) indicate that H+ is more strongly adsorbed on goethite than on either hematite or magnetite. Langmuir plots of the adsorption isotherms (Figure 3) show that also the amount of proton adsorbed at saturation is the largest in goethite. The acidity quotients pKa1 calculated as a function of the FeOH2+concentration are shown in Figure 4. The intrinsic values of the acidity quotients, pFzt, were obtained through extrapolation a t [FeOH2+] 0 and are listed together with the pH,, values in Table I. In the pH ranges of our kinetic experiments, Kal's for the three suspensions decrease in the order of
-
10 ' k p , mol-' 10-3J dm3 s-l kdint,s-' 2.0
Fe304 TiO, silica-alumina
2.0 0.4 5.5
2.4 1.4 6.2 0.29
0.16
0.34 13 46
ref present work present work 14 19
magnetite, hematite, and goethite. The surface potential q,,can be evaluated by using eq 8.3 As can be seen from Figure 4, the dependence of \ko e q o / k T = 2 . 3 0 3 ( ~ K 2-~pK,J (8) on [FeOH2+]is very similar for hematite and magnetite, and is less pronounced for goethite than for the other two oxides. Now, let us consider again the signal amplitudes observed in the three systems. In general, upon applying a perturbation to a chemical equilibrium, the shift of the equilibrium (relaxation amplitude) is the larger the more similar the equilibrium populations of the species involved. Systems with very small or very large equilibrium constants, therefore, are relatively insensitive to perturbation. From this point of view, in comparing the pKal's prevalent during the kinetic studies of several different oxide suspensions (pKal magnetite > hematite. Thus, relaxations can be observed in the approximate range of 2 5 pKal 5 4. The potentials determined in the hematite, magnetite, and goethite suspensions (shown in Figure 5) are very similar, indicating that specific surface properties have little effect on the electrostatic potential at the slipping plane of the double layer in these systems. The value of for the perchlorate ion is not known, but numerical values of the constant for various other anions have been reported3 to be 50-100. Since 7-l in eq 7 i s quite insensitive to the value of in this range, F20n = 50 was used in the preparation of the plot 7-l vs. the concentration term in eq 7. The result is shown in Figure 6, where the values of \ko were calculated by the use of eq 8, and was used for \kd. The experimental data fall on straight lines passing through the origin. The values of kptcan be obtained from the slopes, and kPtcan be calculated from the values of FAt and k,i"t. For comparison, the rate constants obtained for the iron oxides are listed in Table I1 together with results available for some other oxides. Silica-alumina, being a mixed oxide, clearly represents a special case. However, for pure metal oxides, it seems that differences in the acid-base properties of the surface hydroxyl sites are mainly reflected in the intrinsic proton desorption rate constants, since the specific adsorption rates are quite similar in all cases studied.
8
r
I
6
Til
Fzion
0 4 T
P
r
2
0
2
4
Figure 6. 7-l vs. the concentration term in eq 3 at 25 hematite: (0)magnetite.
OC:
(0)
for magnetite, 3.9-4.5 for hematite, 4.3-5.0 for goethite, and 4.4-6.0 for y-Al2O3l5),it is not surprising that no relaxations could be observed in the Si02, goethite, and y-Al,03 suspensions. In the other systems, the relative
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work, and to the Robert A. Welch Foundation for additional support. R.D.A. thanks the Japanese Ministry of Education for a research scholarship.
Adsorption and Bonding of Butane and Pentane on the P t ( l l 1 ) Crystal Surfaces. Effects of Oxygen Treatments and Deuterium Preadsorption M. Salmeront and 0. A. Somorjal" Materials and Molecular Research Division, Lawrence Berkeley Laboratory. Department of Chemistty, University of California, Berkeley, California 94720 (Received: May 19, 198 1; In Final Form: August 3, 198 1)
The adsorption of C4H10 and C5H12 on Pt(ll1) was studied by thermal desorption spectroscopy. Both hydrocarbons show a first-order desorption process with peak temperatures of 166 and 195 K. Kinetic parameters s-l and E = 8.2 f 1.2 kcal mol-l for C4H10; v = 1011.6*1.3 s-l and E = 10.2 obtained were as follows: v = 1011.0*1.7 f 1.0 kcal mol-l for C5H12.Multilayers of both hydrocarbons are formed at 110 K and high exposures. The presence of subsurface oxygen introduces new adsorption sites and decreases the exposure needed for multilayer formation. Preadsorbed deuterium interacts repulsively with the adsorbed hydrocarbons,lowering its desorption temperature.
Introduction The adsorption characteristics (heats of adsorption, monolayer structure, surface sensitivity, and bonding) of alkanes on metal surfaces are an important area of investigation for many reasons. Alkanes are often reactants in hydrocarbon conversion reactions leading to the pro-
'
Instituto de Fisica del Estado Solido, Universidad Autonoma de Madrid, Cantoblanco, Madrid, Spain.
duction of olefins and cyclic hydrocarbons. They are also major constituents of lubricants, adhesives, and other surface-active agents where the molecular scale bonding properties to the solid surface determine their utility. The use of single crystals for adsorption studies permits precise control of the surface structure which can markedly influence the nature of the chemical bonds between the organic adsorbates and the solid substrates as demonstrated by many recent studies.l
0022-3654/8112085-3835$01.2510 0 1981 American Chemical Society
