Langmuir 1986,2, 101-105
/ Hz C H2C
'
\ / CH2
Hz C
/
\
o=c / CH2
\0 H
Ge Figure 10. Orientation of the stearic acid molecule in the first monolayer.
are oriented so that their ab planes lie on the germanium surface (the xy plane),15 showing a tendency that the a crystal axis aligns parallel to the y axis and the c axis makes an angle of about 30° (in the case of the C form) with the z axis. These features are schematically shown in Figure 9. The ring dimer of stearic acid is formed through two hydrogen bonds between the even-numbered monolayer and the succeeding odd-numbered monolayer. Cause of the Disappearance of the C=O Stretching Band in the First Monolayer. As was mentioned above, stearic acid in the first monolayer deposited on the germanium plate does not react with the substrate and consists of free acid of the cis configuration. Thus the absence of the C=O stretching band in this monolayer cannot be easily understood in the usual sense. Although the cause of this phenomenon has not been established, a possible explanation may be given by the image field (35) Lennard-Jones, J. E. Trans. Faraday Soc. 1932,28, 334. (36) King, F.; Van D u p e , R. P.; Schatz, G. C. J . Chem. Phys. 1978, 69, 4412.
101
acting at a very short-range distance from the germanium surface. The image field at semiconductor surfaces has been pointed out by LuthBag and theoretically treated by Sheka et al.@ If we take into account the previous findings for stearic acid in the first monolayer that the hydrocarbon chains are oriented approximately perpendicular to the germanium surface and that the molecule occurs as the cis configuration for the C = O and C,-C, bonds, stearic acid should be placed on the germanium surface as shown in Figure 10, where the C=O bond lies almost parallel to the surface. In this case, the induced molecular and image dipoles are out of phase and canceled at the interface. This may markedly diminish the intensity of the C=O stretching band in the first monolayer. In the present case, the image field may be partly screened by the thin overlayer of the oxide. Thus, owing to this screening effect and the short-range nature of the image field at semiconductor surfaces,35-40it may be plausible that only vibrations with the transition moment parallel and locating very close to the surface exhibit marked diminutions of intensity, while those at comparatively long distances from the surface, such as the CH2 stretching vibrations, do not show perceptible intensity reductions. In the case of calcium stearate, the image field would be further screened by the calcium ion between the substrate and carboxylate ion, and this seems to be the cause of the antisymmetric COOstretching band observed in the 1-monolayer film of stearate ion, though its intensity is relatively weaker than that of the 11-monolayer film shown in Figure 4.
Acknowledgment. This research was partly supported by the Grant-in-Aid on Special Project Research for Organic Thin Films for Information Conversion from the Ministry of Education, Science and Culture, Japan. Registry No. Germanium, 7440-56-4; stearic acid, 57-11-4. (37) H o h , P.; Pritchard, J. 'Vibrational Spedroscopies for Adsorbed Species"; Bell, A. T., Hair,M. L., Eds.; American Chemical Society: Washington, DC, 1980; p 51. (38)M t h , H.; "FeatkBrperprobleme/Advancesin Solid State Physics"; Treusch, J., Ed.; Vieweg: Braunschweig, 1981; Vol. 21. (39) Leth, H.Surf. Sci. 1983, 126, 126. (40) Sheka, D. I.; Voskoboiiikov, A. M. Sou. Phys.-Solid State (Engl. Transl.) 1983,25,92.
Improved Calorimetric Method To Investigate Adsorption Processes from Solutions onto Solid Surfaces S. Partyka,* M. Lindheimer, S. Zaini, E. Keh, and B. Brun Laboratoire "Physico-Chimie des Systkmes Polyphasb" LA. 330, U.S.T.L.,Place E. Bataillon, 34060 Montpellier, Cedex, France Received June 11, 1985. In Final Form: September 18,1985 An improved calorimetric device intended for the measurements of both differential heats of adsorption and dilution is described. The formalism related to the actual calorimetric experiment is developed. This makes possible a new treatment of adsorption thermodynamic data. Differential molar enthalpy of adsorption (as well &s integral molar enthalpy of adsorption) is thus related to fraction surface coverage. The procedure is applied to the adsorption of a poly(ethy1ene glycol) octylphenyl ether (Triton X-100) onto silica gel and activated carbon.
Introduction Adsorption mechanism studies are relatively few despite their practical involvement in many industrial processes. Several questions dealing with adsorption mechanisms
remain to be answered. These concern the interactions of organic molecules with a solid surface at low concentrations: the strong increase of adsorption in the intermediate concentration range (S curve) and also the for-
0743-7463/86/2402-OlO1$01.50/00 1986 American Chemical Society
102 Langmuir, Vol. 2, No. I , 1986 h'
~
h o r i z o n t a l adjustment
Partyka et al. . driven magnet
iron piece PMP
frame tube
-
steel tube
alaes reservoir coni:aining the solution Calorimeter
block
calorimetric cell
thermocouples
Figure 1. Calvet microcalorimeter with ita new device.
mation of the definite structure of adsorbed molecules near the saturation plateau. In a recent bibliographical review' concerning the adsorption of surfactants from aqueous solutions, Clunie and Ingram note a pressing need to develop calorimetric and spectroscopic methods in the adsorption field. In the current work, the application of a new calorimetric method to the study of adsorption of nonionic surfactants onto silica gel and activated carbon has been tried. Few calorimetric works have been published in the field of adsorption from Until recently, the more important calorimetric investigations of adsorption from solution (batch and flow calorimeters) do not give fully satisfactory data. The major deficiency of all commercially available batch calorimeters is the lack of a continuous and effective stirring for solid su~pensions.~~'J' Flow calorimeters (LKB16Micr~scal,~ and recently Rouquerol's calorimeter9 do not agitate the solids; they give good results for larger particles (small surface area). They provide good qualitative indications of thermal effects due to adsorption or desorption; but it is difficult to estimate the correction term due to the dilution of the solution injected through the solid suspension. On the other hand, the base line displacement due to flow resistance variations through the sample bed (especially for small particles) during and after calorimetric measurements hampers the precise determination of the integral heat of adsorption. In our first adsorption calorimetric experiment, the Calvet differential (1) Clunie, J. S.; Ingram, T. B. In "Adsorption from Solution at the Solid-Liquid Interface"; Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: London, 1983, p 105. (2) Groszek, A. J. ASLE Trans. 1970, 13, 278. (3) Morimoto, T.; Naono, H. Bull. Chem. SOC.Jpn. 1972, 45, 700. (4) Corkill, J. M.; Goodman, J. F.; Tate, J. R. Trans. Faraday SOC. 1966, 62, 939. (5) Killmann, E.; Eckart, R. Macromol. Chem. 1976, 144, 45. (6) Berg, R. L.; Noll, L.; Good, W. D. ACS Symp. Ser. 1979, No. 91. (7) Kern, H.; Piechocki, A.; Brauer, U.; Findenegg, G. H. B o g . Colloid Polym. Sci. 1979, 65, 118. (8) Bocquenet, Y.;Siffert, B. J. Chim. Phys. 1980, 77, 295. (9) Everett, D. H. Isr. J. Chem. 1975, 14, 267. (10) Klimenko, N. A.; Polyakov, V. E.; Permilovskaya, A. A. Kolloidn. Zh. 1979, 41, 1081. (11) Rouquerol, J.; Partyka, S. J. Chem. Tech. Biotechnol. 1981,31, 584. (12) Denoyele, R.; Rouquerol, J.; Rouquerol, F. In "Adsorption from Solution"; Rochester, C., Ed.; Academic Press: London, 1982; p 226. See also: Rouquerol, J. Pure Appl. Chem. 1985,57, 67. A recent review of calorimetric devices, which appeared after this manuscript was submitted.
water suspension of particules
Figure. 2. Device for introduction of reactants and for agitation.
calorimeter was fitted with a special device enabling the study of solid-liquid interfaces.l' This device was sensitive and allowed the determination of integral heats of adsorption for all levels of surface coverage (0-1);however, deficiencies in the stirring and mechanical settings were restrictive for both heavy particles and diluted solutions. After several attempts, a new device for stirring and introducing a solution directly into the calorimetric cell was constructed and fitted to the Calvet microcalorimeter. In our experiment using this device, the adsorption of a nonionic surfactant onto silica gel monitored by calorimetry is given with quantitative analysis. The variation of the differential molar heat of adsorption as a function of the surface coverage degree provides new insights on the mechanism of interfacial processes.
Apparatus Description of the Calorimetric Device. For the standard Calvet micro~alorimeter~~ (Figure 1)with 35-mm diameter cells, a device for stirring and a glass reservoir (15 cm3) are incorporated within the 100 cm3 cells (Figure 2). The choice of a flexible transmission between the calorimeter outside and the internal device of the agitator allows us to decrease the mechanical noise, which usually interferes strongly on the quality of the base line, and therefore to work at the highest sensitivity of the microcalorimeter (15 pV full scale). The device contains three independant parts: (i) The insertion flexible pipe used to introduce the surfactant solution runs along the frame tube (h) down to the kel-F plug (j). A precision syringe or a peristaltic pump (c), connected to the surfactant solution reservoir (k) by the insertion pipe, monitors the volume of the liquid introduced. Whether inside or outside the calorimeteric cell (n) the reservoir (k) is always in the calorimetric block. (ii) A thin steel tube (i) headed by a rectangular iron piece (0 is guided down the frame tube to the helicoid glass stirrer (p). The rotation of the iron piece uses magnetic connection through a micromotor-driven magnet (e). (13) Calvet, E.; Prat, H. "Recent Progress in Microcalorimetry"; Pergamon Press: New York, 1963.
Langmuir, Vol. 2, No. 1, 1986 103
Calorimetric Method To Investigate Adsorption Processes Thus, mechanical noises generated by the motors (d) do not propagate to the frame tube. (iii) The third part consists of bearers which allow the adjustement of the magnetic connections. Lateral and vertical displacements of each motor are obtained by double adjustment (b,b') on the two supporting arms (a) disconnected from the calorimeter body. Experimental Procedure. The adsorbent and adsorbat are respectively put into the calorimetric cell (n) and the glass reservoir (k). When the thermal equilibrium is reached, the experiment is carried out in the following way: a volume (about 0.5 cm3) of surfactant solution, which contains a definite quantity of molecules, is injected by the "Gilson" pump. One part of the surfactant molecules introduced into the suspension (9) is adsorbed and the other one remains in water, and so a thermal effect can be recorded. The equilibrium state characterized by the return to the base line is reached quickly if suitable stirring is used. Then, the next injection is operated and so on. Each new injection allows us to attain a higher coverage degree of the adsorbent and is followed by a consistent effect. To calculate the exact quantity of molecules adsorbed in relation to the quantity introduced during the injection, the adsorption isotherm must be known: this allows the determination of differential molar adsorption enthalpies. These values become more accurate as the injected quantities become smaller. However, the experiments must induce measurable thermal effects. A calorimetric experiment to determine the differential molar heats of adsorption and the whole range of coverage (about 20 points) lasts about 1 day.
Determination of the Adsorbed Quantity under the Conditions of Calorimetric Experiment The calorimetric cell is initially filled with m g of solid and Mo of pure water. Co is the surfactant concentration of the injected solution (in mol kg-') and d is the flow (in g min-'). Now, we call r the quantity of surfactant adsorbed on the solid in mole of surfactant per gram of adsorbent and C, the equilibrium concentration of the surfactant after injection. The aim is to follow the partition between the molecules adsorbed onto the particles and those remaining in the supernatent in each injection. The injected quantity equals td in grams, containing Cotd X mol of surfactant and the final mass of solution is Mo ( t d ) - (rmM,), where M , corresponds to the molecular weight of the surfactant. The number of molecules in the supernatent is
+
C,(Mo + t d - I'mM,)
X
Thus Cotd
X
= rm
+ C,(Mo + t d - I'mM,)
X
and I' = [Cotd X
- C,(Mo
+ td) X
10-3]/m(l - C a w X 10-3)
For low C, values, the curve r = f(t,C,) corresponding to a given t value is linear; its intersection with the experimental isotherm I' = f(C,) leads to the determination of the r and C, values at the equilibrium (Figure 3). When the number of molecules adsorbed and the corresponding thermal effect are determined, the curve of adsorption enthalpy can be obtained at progressive coverage degrees. If the successive injections are small enough, a curve of differential molar enthalpy of adsorption can be established.
Figure 3. Graphical determination of the adsorbed quantity of
solute.
Thermodynamic Description of an Experiment nT mol of surfactant diluted in n, mol of water (initial concentration Co) are injected into the suspension in the calorimetric cell content. This content includes n; mol of water and a mass m of solid particles with a surface S. The enthalpy of adsorption AH is related to this mixture: (nT
+ n,) + ( S + n;)
1
(nT
+ n,) + [ ( S+ cwa) + (n', - cwa)l
initial concentration = Co (where 6," is the number of water molecules adsorbed onto the surface S )
LiH
( S + nTa + ewa')
+ (nTS+ n, + n&-
6,"')
final concentration = C, where +s is the number of surfactant molecules remaining in solution, nTathe one adsorbed onto the surface S ; cwa corresponds to the number of water molecules adsorbed in pure water and ewa' has the same significance but after adsorption of nTamol of surfactant. For this process, we can consider that the nT mol are diluted from Co to C, concentration. Only %a mol arising from this diluted state are adsorbed onto the solid surface. In these conditions, the coverage degree of the adsorbent takes the value 8. Therefore
AH= nT(Ahc, - Ahc,)
+ nTa(Ah8- Ahc,) + (e,'
- cWa)Ah,
Ahc is the dilution molar enthalpy from a reference solution, Ah8 the adsorption molar enthalpy from the same reference solution, and Ah, a molar enthalpy of water displacement depending on the 8 value. The adsorption isotherm allows us to calculate the nTs and nTavalues, whereas the calorimetric measurements permit the determination of the A H and the difference Ahc, - Ahc,. Therefore, it is possible to reach the term AH - nT(Ahce - Ahc,). It will be considered as the apparent adsorption term compared to the true one nTa( A h 8- Ahc,). Indeed, it is impossible to estimate the term (cwa' - cWa)Ah,,concerning the displacement of a part of adsorbed water molecules by the surfactant molecules. T o reach the apparent differential molar enthalpy of adsorption, it is necessary to derive in function of nTa.A good approximation is obtained by calculating the term A [ A H - nT(Ahc, - A h C o ) ] / A n Tfor a small AnTa values.
Products and Experimental Techniques The nonionic alkylphenol surfactant C8H1,CsH4(OCH2CHz)e,loOH(TX100) was supplied by Rhom and Haas, France. It is polydisperse and contains less than 0.5% impurities.
104 Langmuir, Vol. 2, No. 1, 1986
Partyka et al. .
I
T7---' 1
Figure 4. Calorimetric recording and conditions of adsorption of the alkylphenol oxyethylenicsurfactant, TX 100,onto silica gel (spherosil XOB 015). Adsorption temperature = 35 "C; mass of silica gel = 1 g; initial water volume for the suspension = 35 g; concentration of the surfactant solution Ci = 12 g kg-'; flow rate d = 0.4 g mi&. Injection times for the point 1:0.5 min; 2:l min; 3:l min; 4 to 11:2 min.
1 1,
t 1
2
1
IO3 c ,
(mol.kg-I)
Figure 5. Heat of dilution of C8H17C6H4(0CH2CH2),,loOH vs. the final concentration. Initial concentration of surfactant = 1.92 X mol kg-'; t = 35 "C.
The silica gel adsorbent is a macroporous specimen SPHEROSIL XOB 015 manufactured by Rhone Poulenc. The mea-
surements of the surface area of this adsorbent as determined by the B.E.T. (N2= 16.2 A) and Harkins and Jura methods14are 24.6 and 25 m2g-l, respectively. The latter value has been used in our calculations. The adsorption isotherm is drawn by measuring the surfactant concentration before and after adsorption. The value of the concentrations is determined by UV spectroscopy (Uvikon 810) (276 nm). Calorimetric technique has been described above. A calorimetric experiment needs about 1 g of adsorbent in suspension in 35 g of water. As in the dilution and adsorption calorimetric experiments,the initial concentration Cois nearly equal to 12 g kg-'. The rotation speed of the stirrer for this experiment is about 180 rpm. The isotherms and the calorimetric measurements of dilution and adsorption have been made at 35 "C.
Results and Discussion Figure 4 shows typical recordings of calorimetric experiments of adsorption. The different thermograms correspond to the successive injections of surfactant solution (C,) in the low compartment of the calorimetric cell where the silica gel particles are correctly suspended. If we follow the previous equation relative to AH, we add the successive enthalpies, each sum corresponding to a 0 value. Each one is due to adsorption of surfactant molecules onto the solid surface and also to the heat of dilution of the injected surfactant solution. This last term arises essentially from the demicellization. In Figure 5, the enthalpies (14)
Partyka, S.; Rouquerol, F.; Rouquerol, J. J.Colloid Interface Sci.
1979, 68, 21.
Figure 6. Differential molar enthalpy of adsorption of CBH17C6H4(OCH2CH2)9,100H onto silica gel vs. the coverage degree.
of dilution per mole of TX 100 are plotted as a function of the concentration. In this case, the initial concentration Co is in mol Kg-'. It is important to underline that for TX100, the dilution process is exothermic like the global enthalpic effect AH. Therefore, the micellization is an endothermic process. If we cast the water displacement aside, we can determine the apparent differential molar heats of adsorption Ahadsdiffor the mean equilibrium concentrations C, or the corresponding coverage degrees 0. This last variable is the better one, because it does not depend on the nature of the surfactant: it is a normalized variable. Figure 6 shows the dependence of Ahadson 8 (0 = r/r, where rmax is the plateau value of the isotherm). This diagram is typical for nonionic surfactants adsorbed onto silica gel. Until 0 is nearly equal to 0.25, we can note an exothermic effect. This indicates a direct interaction between the oxyethylenic chain of the Triton and the solid surface. Then values are endothermic. This effect is similar the Madsdif to the micelle formation in the bulk s ~ l u t i o n . ' ~Indeed, for 0 values larger than 0.7, heats of adsorption are approximately equal in absolute value to the dilution ones. By virtue of comparison, we have studied the adsorption of the same surfactant onto another surface, an activated (15) Partyka, S.; Zaini, S.; Lindheimer, M.; Brun, B. Colloid Surf. 1984, 12, 255.
Langmuir 1986,2, 105-108
105
factant and the surface, which is microporous, partially hydrophobic, and with plenty of various chemical functions.16 The microscopic description of the adsorption mechanism is very difficult to understand and an attempt to explain it will be made in a future work. From comparison of the calorimetric results of the same surfactant molecules onto chemically different solid surfaces, we can do more adequate analysis of adsorption processes and mechanism.
1 0
e---0.25
0.5
0.75
. I
Conclusion The microcalorimetric device described here allows the determination of differential and integral molar heats of adsorption in all the range of coverage. Alternatively, it can also be used to obtain heats of dilution or heats of reaction in liquid phases. A very versatile unit is obtained by associating this device to the Calvet calorimeter. The high sensitivity, the large calorimetric cell (io0mL), and the variable stirring speed cope with a wide range of pH, physicochemical applications.
*
e
Figure 7. Differential molar heat of adsorption of C8Hl7C8H4(OCH2CH2)s,loOH onto activated carbon (CECA 1240) vs. the
coverage degree.
carbon. Figure 7 shows the differential molar heats of adsorption w. 8. The enthalpic behavior is different from the previous one, since Ahadsdifvalues are always exothermic, suggesting direct interactions between the sur-
(16) Mattson, J. S.; Marc, H. B. In 'Activated Carbon, Surface Chemistry and Adsorption from Solution"; Dekker: New York, 1971; p 11.
Hydrogenation of Carbon Monoxide over Model Rhenium Catalysts: Additive Effects and a Comparison with Iron E. L. Garfunkel, J. Parmeter, B. M. Naasz, and G. A. Somorjai* Department of Chemistry, University of California, and Materials and Molecular Research Division, Lawrence Berkeley Laboratory, Berkeley, California 94720 Received March 6,1985. In Final Form: September 12, 1985 Small area (approximately 1 cm2)rhenium and iron foil catalysts have been studied for CO hydrogenation reactions. Rhenium produces primarily methane and showed lower activity than iron. The addition of submonolayer amounts of alkali decreased the overall rate of reaction and caused a selectivity change toward longer chain hydrocarbons on both metal surfaces. Oxidation of the surface usually caused a higher selectivity toward methane and a decreased rate of carbon buildup, but the overall rate of methanation remained relatively constant. The hydrogenation of carbon or CH, fragments appears to be the rate-determining step in the reaction.
Introduction The hydrogenation of carbon monoxide to produce hydrocarbons at a high rate and selectivity is under intensive study in many laboratories. Many different transition metals and transition-metal compounds have been identified as good catalysts to produce C1 molecules (methane, methanol),' high molecular weight liquid or oxygenated molecules (acetaldehyde and higher alcohol^).^ Often promotion by alkali yields increased molecular weight products and a lower concentration of methane?6 while transition-metal oxide catalysts produced more of the oxygenated specie^.^^^*' Rhenium has received relatively little attention as a catalyst in comparison with other transition metals. Nevertheless, rhenium has recently been shown to be a very active catalyst for ammonia synthesis.8 CO and N2 (1) Somorjai, G. A. Catal. Reu.-Sci. Eng. 1981,21, 189. (2) Bell, A. T. Catal. Rev-Sei. Eng. 1981,21, 203. (3)Watson, P.R,Somorjai, G. A. J. Catal. 1982, 74, 282. (4) Anderson, R. B. J. Catal. 1965,4, 56. (5) Weaner, D. A.; Ccener, F. P.; Bonzel, H. D. Langmuir 1985, I, 478. (6) Vannice, M. A. Catal. Reo-Sci. Eng. 1976, 14, 153. (7) Natta, G.In "Catalysis"; Emmett, P. G., Ed, R a i i o l d New York, 1955; Vol. 3, 5. (8)Spencer, N. D.; Somorjai, G. A. J. Catal. 1982, 78, 142.
I
bond scission is thought to be a prerequisite for both ammonia synthesis (N2/H2)and CO hydrogenation (CO/H2). Since iron is known to be active in both reactions and rhenium is active for the ammonia synthesis, it can be inferred that rhenium might also display good catalytic behavior for CO hydrogenation. In one survey study, promoted rhenium oxides on silica support were reported to have high selectivity for alcohol produ~tion.~ The purpose of this report is to explore the catalytic activity of rhenium metal foil for the hydrogenation of CO when clean and in the presence of alkali and oxygen and to compare its activity and selectivity with that of iron.
Experimental Section All work was performed in a combined ultrahigh-vacuum-
high-pressure catalysis chamber shown schematically in Figure 1. In its UHV mode, the chamber was equipped with a double-pass CMA for Auger electron spectroscopy (AES),a UTI masa spectrometer for thermal desorption spectroscopy (TDS),an alkali deposition gun, an argon ion gun, and a molecular doser. While still positioned in the main chamber, the sample could be enclosed in a special high-pressure cell to allow reactions to be performed (9) Tsunda Chern. Lett. 1981, 819.
0743-I463I86 I24Q2-QlQ5~Q1.5Q IO 0 1986 American Chemical Societv
