J. Phys. Chem. 1993,97, 5128-5131
5128
Determination of Thermodynamic Parameters for the Dissociative Adsorption of Dihydrogen on Rh/A1203 T.H.Fang, J. P. Wey, W. C. Neely, and S. D. Worley’ Department of Chemistry, Auburn University, Auburn University, Alabama 36849-531 2 Received: December 4, 1992; In Final Form: February 23, I993
High-pressure infrared spectroscopy has been used to study the interaction of H2 with Rh/A1203 catalyst films as a function of temperature and pressure. The interaction process was found to be dissociative and fit Langmuir adsorption isotherms. Two dissociative Langmuir models were employed in fitting the data. A model in which reversible dissociation of H1 on Rh, together with reversible H spillover to react with hydroxyl groups on the A1203 support was included, provided a A&& of -7.7 kcal mol-’ for the process. This low enthalpy of adsorption is indicative of weak Rh-H bonding and may explain why Rh functions well as a hydrogenation catalyst. This work demonstrates the utility of Fourier transform infrared spectroscopy in obtaining thermodynamic data for supported catalysts.
Introduction It has been proposed that two forms of adsorbed hydrogen may exist on supported transition metals used to catalyze hydrogenation reactions.1-7A strongly bound “irreversible”form of hydrogen has been suggested as an unreactive species which may be adsorbed on a metal cluster site and which has been reported to exhibit an infrared band at 950 cm-1 for Pt/MgOl but to be infrared inactive or masked by support bands for supported Rh.2 On the other hand, a weakly bound ”reversible” form of hydrogen, which is quite likely to be an active participant during catalytic hydrogenation reactions, has been reported to exhibit infrared bands in the 1800-2200-~m-~ region for Pt/ MgOl and Ni/A1203.3 Recently, work in these laboratories, in which high pressures of dihydrogen (100-8000 Torr) were employed in a new infraredcell reactor, led to the observation of an infrared band at 2013 cm-’ which increased in intensity with increasing pressure while vanishing upon evacuation. This band was assigned to the Rh-H stretching vibrational mode for Rh/A1~03.~ An infrared band at 1618 cm-1, which increased in intensity with dihydrogen pressure concomitantly with the 201 3-cm-1 band and declined in intensity upon evacuation, was assigned to the bending mode for water produced upon the support as a result of spillover of hydrogen atoms from Rh and subsequent reaction with OH groups on A1203.8Deuterium-labelingstudiesconfirmedtheseassignmenk8 In the current work, the infrared studies of dihydrogen interactions with Rh/A1203 have been extended to include temperature variations so that adsorption/desorption equilibrium constants and enthalpies and entropies of adsorption for the H2/ Rh/A1203 system could be determined. A similar study has been performed for supported N2/Rh, although in this case the adsorption was nondissociative.9
Experimental Section Supported IR-transparent Rh/A1203 films (2.2 wt % Rh) on 25-mm CaF2 IR windows were prepared by spraying slurries of RhC13.3H20(Johnson Mathey), A1203(Degussa Aluminumoxid C, 100 m2 g-I), spectroscopic grade acetone, and distilled, deionized water onto the windows held at 353 K. The solvents evaporated rapidly, leaving a film of RhCls.3H20/A1203 containing 4.4 mg/cm-2. The window containing the catalyst film was then mounted into the center of an infrared-cell reactor capable of operation in the 100400 K temperature and
* Author to whom correspondence should be addressed.
10-6-104-Torr pressure regimes, which has been described previously.I0 Each sample was evacuated at 10-6Torr overnight, oxidized by 100-Torr samples of 02 at 523 K for two 15-min periods, and then reduced with 100-Torr samples of H2 at 473 K for cycles (pressure/evacuation) of 10,5,10, and 20 min. The O2 and H2 employed in these experiments were obtained from Air Products with stated purities of 99.996% and 99.99556, respectively. The 0 2 was used without further purification, but the H2 was passed through a catalytic converter installed in the high-pressure manifold and a trap held at 77 K to remove all traces of CO and COZimpurities.8 This step was essential in preventing the adsorption of tightIy bound CO on the Rh/A1203 films, which would cause interfering infrared bands in the 2000-cm-1 region.8 Following the reduction cycles, the cell was evacuated for 2 h at 10-6 Torr and 298 K. Then 8000 Torr of H2 was introduced into the cell and allowed to contact the Rh/ A1203film overnight at 298 K. The equilibrium constant data were obtained from the integrated areas of the 2013- and 1618-cm-1 bands as a function oftemperature and pressure. Starting at elevated H2 pressure (ca. 8000 Torr), infrared spectra were obtained at sample temperatures in the range 300-340 K, in increasing 5-deg increments, with 5 min of equilibration time allowed at each new temperature. Then the HZpressure was reduced in increments in the range 8000-200 Torr, with infrared spectra obtained at each of the same temperatures as noted above for the 8000-Torr experiment. From these pressure data, equilibrium constants as a function of temperature were determined, and the van’t Hoff equation was used to calculate the enthalpies and entropies of adsorption for the H-Rh/A1203 system. The IR spectra were obtained by using an IBM 32 Fourier transform spectrometer operated at 2-cm-’ resolution, with 500 scans being accumulated over 7.5 min for each spectrum. Each spectrum represents a difference between the Rh/A1203 film exposed to H2 and that at 10-6 Torr. An MKS Baratron capacitance manometer was used to measure H2 pressures.
Results and Discussion Figure 1 shows the infrared spectra in the 1400-2300-~m-~ region for a typical experimental run in which the H2 pressure was decreased from ca. 8000 Torr (peak a in Figure 1) to ca. 600 Torr (peak c in Figure 1) at 298 K. Only three such spectra have been shown in the figure for clarity of presentation, although at least six pressures in the range 8000-200 Torr were employed in obtaining the equilibrium constant data at each temperature studied. It is evident that the Rh-H band at 2013 cm-1 and the
0022-3654/93/2097-5 128%04.00/0 0 1993 American Chemical Society
Dissociative Adsorption of Dihydrogen on Rh/A1203
The Journal of Physical Chemistry, VO~. 97, No. 19, 1993 5129 1.2
I
31
0.008
0.6
1MO
Iwo
1100
ll00
t
1400
WAyunrWSnl
Figure 1. Infrared spectra for the interaction of H2 with a prereduced 2.2% Rh/A120, film (4.4 mg cm-2) at 298 K and decreasing pressures of (a) 7986, (b) 2500, and (c) 602 Torr. The equilibration time was 30 min at each new pressure.
I
I T
pwo
eooo
ibm
IWO
I
0.2 0
2
4
6
8
10
12
Hydrogen P r e s s u r e ( a t m ) Figure 3. Integrated area of the 2013-cm-' infrared band as a function of temperature and pressure. 1.4
,
1400
WAVENUMBER
Figure 2, Infrared spectra for the interaction of H2 with a prereduced 2.2% Rh/A1203 film (4.4 mg cm-2) at (a) 300 K and 1996 Torr, (b) 320 K and 2027 Torr, and (c) 340 K and 2070 Torr. The equilibration time was 5 min at each new temperature.
H20/A1203band at 1618 cm-1 both declined in intensity concomitantly as the H2 pressure was reduced. This was not reported to be the case in our previous work, as the Rh-H band declined more rapidly upon evacuation than did the H20/A1203 band.8 We attribute this observation to insufficientequilibration time in the previous work (only a few seconds at each new pressure). In the current work, at least 30 min of equilibration time at each new pressure was allowed before collecting spectra as a function of temperature. The temperature changes employed (5-deg increments) were much less dramatic than the pressure changes, such that 5-min equilibration periods were sufficient for each new temperature. The interaction of H2 with Rh/A1203 was reversible, as the 2013- and 1618-cm-I bands grew and declined in intensity in direct proportion to pressure changes. Bands at 3688 and 3741 cm-i corresponding to OH on the A1203 changed in intensity inversely with the 2013- and 1618-cm-I bands; these OH bands also behaved reversibly as the pressure of HZwas altered (see Figure 2 in ref 8). These data thus indicate that the interaction of H2 with Rh/A1203 is dissociative and reversible. Yates and co-workers had previously reported that the adsorption of hydrogen on Rh( 111) was dissociative, following Langmuir ((1 -e)*) kinetics at 175 K.11 Figure 2 shows that the behavior of the Rh-H and H20/A1203bands was as expected when the temperature was increased from 300 to 340 K at nearly constant pressure (1996 to 2070 Torr). Figures 3 and 4 show the integrated areas for the 20 13-cm-I Rh-H band and the 1618-cm-' H20/A1203 band, respectively, as a function of H2 pressure. The surface coverages for the Rh-H species (ORh-H) and theH200nthesupport (eH,0)wereestimated from the ratios Ap/ApmaX, where A, values represent integrated areasofthe2013- and 1618-cm-1bands,respectively,at thevarious
0
2
4
6
8
10
Hydrogen P r e s s u r e ( a t m ) Figure 4. Integrated area of the 1618-cm-' infrared band as a function of temperature and pressure.
H2 pressures, and Apma,values represent the integrated areas of the bands at the maximum pressure utilized (ca. 8000 Torr) at 298 K. The assumption must be made here that the extinction coefficientsfor the Rh-H and HzO/A1203vibrational modes are not dependent uponcoverage.l2 Although from thedata in Figures 3 and 4 it does not appear that saturation coverage was completely reached at a pressure of 8000 Torr, only a slight increase in integrated band areas ( < l a ) was noted upon increasing the pressure to 9000 Torr. We view pressures significantly above 8000 Torr as pushing the safety limits of the cell reactor, thus, 8000 Torr was employed as P,,,,,,in this study. The coverage data were used to obtain adsorption equilibrium constants at each temperature and could be further employed to estimate the heat of adsorption and entropy of adsorption for the H2/Rh/A1203 system. Thedata were fit to two dissociativeLangmuir isothermmodels. In the first (model I), which is analogous to the model employed by Primet and co-workers for dissociative adsorption of H2 on Pt/Al2O3,I3only the dissociative adsorption of H2 on Rh was considered (eq 1); i.e., the subsequent spillover of an H atom to
Hz(g) + 2Rh + 2Rh-H
(1)
the support to react with the OH groups on A1203 was not considered. The equilibrium constant K1for this model can be WrittenBSP'H,[eRh-H(l -eRh-H)-'I2SUCh that aplotof [eRh-H(1 - 8Rh_H)-'l2versus PH,should provide a straight line with slope equal to K,at each temperature. Figure 5 shows that a straight
Fang et al.
5130 The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 4,
7 ,
Model I Slope = K I 6 -
m
Model I1 Slope =
K4
/
V 310 K V 315 K 0 320 K
V 310 K
5 X
NA 4 a I r (
2
3 -
v
2 -
I r
0 0.0
I
01
0.5
0.0
1.5
1.0
2.0
2.5
3.0
1.0
3.0
Figure 7. Adsorption isotherms as a function of temperature for the model I1 dissociative Langmuir adsorption (see text).
1.o
absolute temp, K
model 1" Klb
K2b
305 310 315 320 325 330 335
1.95 1.63 1.51 1.29 1.05 0.83 0.64
3.70 3.20 2.60 2.40 1.88 1.53 1.15
0.5
sl E:
r (
0.0
a
-0.5
3.0
3.1
3.2
3.3
+ 2Rh * 2Rh-H
+ OH/Al,O,
+ Rh
+ H,O/Al,O,
+ Rh + OH/Al,03 * Rh-H + H,0/Al,03
(2)
(3) (4)
Here, K4 = PIH2[eRb-HeHzO(1- eRh-H)-'(l - eH20)-I]* A plot of [e(i-8)-']Rh-H[e(1-e)-']H20verSuSPHz(Figure7) produced a straight line at each temperature for the pressure regime 20& 2000 Torr with slope q u a l to K4. As for model I, there was some curvature at pressures above 3000 Torr, although not nearly as significant in this case (e.g., the value of [e(l - e)-']Rh-H[e(l - e)-']HzOat 335 K and 4000 Torr was 1.85), so these data were again not included in the evaluation of the equilibrium constants.
Model I1
Slope = Ks
0.
1.2
a I
H,(g)
K4'
1.11 0.99 0.75 0.62 0.51 0.40 0.30
h
4
line fit was obtained for each temperature at pressures between 200 and 2000 Torr. However, significant curvature was noted at hydrogen pressures of 3000 Torr and above (e.g., the value of [eRh-H( 1- e ~ h - ~ ) -at' ]335 ~ K and 4000 Torr Was 3-46),SO these data were not included in the subsequent evaluation of m a d s and Asads. From the KI values derived from Figure 5 (see Table I), a van't Hoff plot was constructed (Figure 6). A straight line fit of the data provided a m a d s value of -7.1 f 0.1 kcal mol-I from the slope and an entropy of adsorption of -22 f 1 cal mol-' degl from the intercept. In model 11, the following equilibria were considered:
K3 0.30 0.3 1 0.29 0.26 0.27 0.26 0.26
1.4
3.4
(I/T) x 1 0 3 ~ Figure 6. A van't Hoff plot of the data utilizing model I dissociative Langmuir adsorption (see text).
model IIa
See text. b In units of atm-1. 1.8
H2(g)
2.5
7
Model 1
Rh-H
2.0
TABLE I: Adsorption uilibrium Constants for the Interaction of H2 with Rb A I 2 0 3 Derived from Infrared Data and Two Dissociative Langmuir Models
1.5
-1.0 2.9
1.5
Hydrogen Pressure ( a t m )
Hydrogen Pressure ( a t m ) Figure 5. Adsorption isotherms for the Rh-H species as a function of temwrature assuming model I dissociative Langmuir adsorption (see textj.
0.5
1.0
2 w
0.8
0.8
0.5
1.0
1.5
2.0
2.5
(e/l-e)Rh-H
Figure 8. Determination of K , for the model I1 dissociative Langmuii adsorption (see text).
Fromeq3,K3= [ w ( l - e ) ] ~ h - ~ [ e -e)-']~~oSUChthataplot (l of [e(1 - e)-*]Hzo versus [e(1 - e p 1 R h - H provided a reasonably straight line (Figure 8) with slope equal to & for each temperature. From the relationship K4 = K2K3, values of K2 were obtained which represent dissociative Langmuir adsorption equilibrium constants corrected for the H atom spillover process in eq 3. A van't Hoff plot of the K2 data (Figure 9) provided a AHa&of -7.7 f 0.1 kcal mol-l and an entropy of adsorption of -23 f 1 cal mol-' degl . Thus, utilizing the data corrected for H atom spillover and reaction with the OH/A1203 sites, only small differences in M a d s and &&!+s for the dissociative adsorption of H2 on Rh/ A1203 were noted when compared to model I. From Table I, it can be seen that the equilibrium constant K3 for the spillover reaction changed very little over the temperature range 305-335 K, while K2 for the dissociation step declined markedly as the temperature was increased. A van? Hoff plot of In K4versus T I
Dissociative Adsorption of Dihydrogen on Rh/A1203
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 5131 from the intercept of Figure 9, which contained data from a rather narrow range of temperatures extrapolated over a large temperature range. We estimate its accuracy at ca. f1 cal mol-' deg-1 and suggest that its value is reasonable for monatomic H interacting with Rh in the coverage range 0.4-0.7,15 but we are not able to draw definite conclusions concerning the geometric structure, number of degrees of translational entropy lost upon adsorption, etc., from the data.
-0.5
-1.0
I1
Conclusion Theenthalpy of adsorption of hydrogenon Rh/A1203 has been measured to be ca. -7.7 kcal mol-I. This low is indicative of weak Rh-H bonding, and the reversible species produced is likely present and an active participant during catalytic hydrogenation reactions over supported Rh. This work illustrates the utility of Fourier transform infrared spectroscopy in obtaining thermodynamic data at rather high pressures of reactant gases over supported catalysts.
I
2.9
3.0
3.1
3.2
3.3
3.4
(I/T) x 103 K Figure 9. A van't Hoff plot of the data utilizing model I1 dissociative Langmuir adsorption (see text).
provided AH and hs values for the overall dissociative reaction process of -8.9 f 0.1 kcal mol-' and -29 f 1 cal mol-' deg-I, resbectivelv. ?he entialpy of adsorption of -7.7 kcal mol-' measured in this work for H?/Rh/AbO2 over the range of coverages 0.4-0.7 was lower than ;hat i-15.0 kcal mol-') reported by h i m e t and coworkers'3 for the H2/Pt/A1203 system at in the range 0.4-0.6. A similar infrared van't Hoff analysis was utilized in the platinum work in which a dissociative Langmuir model was used to fit the data.I3 Thus, it would appear that hydrogen is more weakly bound by Rh/A1203 than by Pt/Al2O3; this observation was not unexpected given that the Pt-H stretching that for Rh-H is lower at 2013 is 2120 cm-'913 cm-I. The relativebond strengthsinferred froma simpleharmonic oscillator approximation using these frequencies differ by 11.4% On the other-hand, a thermodynamicargument utilizing the bond dissociation energy of dihydrogen gas (104 kcal mol-I) leads to the conclusion that the bond energy for Pt-H should be only 3.8%higher than that for Rh-H.I4 Thediscrepancy between the two estimations is probably due to the anharmonicity of the metal-H potential functions, which are unknown. It should be noted that the entropy of adsorption value (-23 cal mol-' deg-1) reported in this work for H*/Rh/Al203 is probably of lesser accuracy than the m a d s value because it was derived
Acknowledgment. We thank the Strategic Defense Initiative Organization's Office of Innovative Science and Technology through Contract N60921-9 1-C-0078 with the Naval Surface Warfare Center for support of this work. References and Notes (1) Candy, J. P.; Fouilloux, P.; Primet, M. Surf.Sci. 1978, 72, 167. (2) Delgass, W. N.; Haller, G. L.; Kellerman, R.; Lunsford, J. H. Spectroscopy in Heterogeneous Catalysis;Academic Press: New York, 1979;
pp 34, 60. (3) Nakata, T. J . Chem. Phys. 1976, 65, 487. (4) Pliskin, W. A.; Eischens, R. P. Z . Phys. Chem. 1960, 24, 11. (5) Dixon, L. T.; Barth, R.; Gryder, J. W. J. Catal. 1975, 37, 368. (6) Bozon-Verduraz,F.;Contour, J.; Pannetier, G. C. R. Acad. Sci. (Paris) 1969, 269, 1436. (7) See: Szilagi, T. Infrared Spectroscopy of Adsorbed Hydrogen. In Hydrogen EIfects in Catalysis; Pad, Z . , Menon, P. G., Eds.; Decker: New 1988; p 183 and references quoted therein. (8) Wey, J. P.; Neely, W. C.; Worley, S . D. J . Phys. Chem. 1991, 95, 8881.
(9) Wey, J. P.; Worley, C. G.; Neely, W. C.; Worley, S.D. J. Phys. Chem. 1992, 96, 7088. (IO) Wey, J. P.; Neely, W. C.; Worley. S.D. J . Am. Chem. Soc. 1991, lZ39 2919. (1 1) Yates, J. T.; Thiel, P. A.; Weinberg, W. H. Surf.Sci. 1979,84,427. (12) Wang, H. P.; Yates, J. T. J . Phys. Chem. 1984,88, 852. (13) Primet, M.; Basset, J. M.; Mathieu, M. V. J . Chem. Soc., Faraday Trans*I 19749 703 293. (14) We thank a reviewer for this suggestion. ( 15) Clark, A. The Theory of Adsorption and Catalysis;Academic Press: New York, 1970; pp 38-44.
