Langmuir 2002, 18, 4005-4009
4005
Kinetics of Hydrogen Adsorption and Desorption on Silica-Supported Pt, Rh, and Ru Catalysts Studied by Solid State 1H NMR N. Savargaonkar,†,‡ D. Uner,*,§ M. Pruski,‡ and T. S. King†,‡,| Department of Chemical Engineering, Iowa State University, Ames, Iowa 50011, Ames Laboratory, Ames, Iowa 50011, and Department of Chemical Engineering, Middle East Technical University, Ankara 06531, Turkey Received December 12, 2001. In Final Form: February 28, 2002 The kinetics of adsorption and desorption of hydrogen on the multifaceted surfaces of silica-supported Pt, Rh, and Ru catalysts were studied by means of solid state 1H NMR. Experiments with selective inversion of 1H magnetization and the formalism developed by Engelke et al. [J. Chem. Phys. 1994, 101 (9), 7262] were used to extract the adsorption and desorption rate constants and the apparent sticking coefficient. The sticking coefficients of hydrogen measured at 333 K at a surface coverage of 0.4 over 5% Pt/SiO2, 4% Ru/SiO2, and 3% Rh/SiO2 were determined to be 0.014, 0.12, and 0.52, respectively. The corresponding activation energies for desorption were 66, 43, and 79 kJ/mol, respectively.
1. Introduction The importance of hydrogen as a reactant for practical and fundamental studies in catalysis is widely recognized. For example, commercial processes such as hydrocracking and hydrotreating of petroleum feedstocks involve hydrogenation over supported catalysts.1 Most of these catalytic reactions involve chemisorption of hydrogen at some stage of the reaction. In addition, hydrogenolysis and isomerization when used in model reaction studies produce valuable information for structural catalyst characterization.2,3 It is now established that the availability and the mobility of hydrogen play an important role in determining the selectivity of the catalyst.4-8 Therefore, it is important to develop the experimental methods that will enable the determination of the kinetic parameters for hydrogen adsorption and desorption on supported metals under conditions relevant in catalytic processes. There is a wealth of experimental data regarding the hydrogen binding energy and sticking coefficients on single-crystal surfaces of Pt, Ru, and Rh.9-18 Most of these data, which are summarized in Table 1, were collected on * Corresponding author. Telephone: +90-312-210-4383. Fax: +90-312-210-1264. E-mail:
[email protected]. † Iowa State Univeristy. ‡ Ames Laboratory. § Middle East Technical University. | Current address: College of Engineering, Kansas State University, Manhattan, KS 66506-5201. (1) Satterfield, C. N. Heterogeneous Catalysts in Industrial Practice; McGraw-Hill Inc.: New York, 1991. (2) Boudart, M.; Djega-Mariadassou, G. Kinetics of Heterogeneous Catalytic Reactions; Princeton University Press: Princeton, NJ, 1984. (3) Sinfelt, J. H. Bimetallic Catalysts; John Wiley and Sons: New York, 1983. (4) Uner, D. O.; Pruski, M.; King, T. S. Top. Catal. 1995, 2, 59-69. (5) Kiskinova, M. M. Poisoning and Promotion in Catalysis based on Surface Science Concepts and Experiments; Studies in Surface Science and Catalysis 70; Elsevier: Amsterdam, 1992. (6) van der Klink, J. J. Adv. Catal. 2000, 44, 1-117. (7) Uner, D. O. Ind. Eng. Chem. Res. 1998, 37, 2239-2245. (8) Savargaonkar, N.; Narayan, R. L.; Pruski, M.; Uner, D. O.; King, T. S. J. Catal. 1998, 178, 26-33. (9) Goodman, D. W.; Madey, T. E.; Ono, M.; Yates, J. T., Jr. J. Catal. 1977, 50, 279-290. (10) Shimizu, H.; Christmann, K.; Ertl, G. J. Catal. 1980, 61, 412429.
Table 1. Sticking Coefficients of Hydrogen on Various Crystalline Surfaces of Ru, Rh, and Pt surface
temp (K)
Ru(0001) Ru(1010) Rh(111)
200 100-500 150
Rh(110)
85
Pt(111)
150
Pt(100) Pt(211) Pt(110) Pt filament
189 350 273 298
hydrogen coverage
sticking coeff
0.0 0.0 0.0 0.4 0.0 0.4 0.0 0.4 0.0 0.0 0.0 0.0 0.0 0.4 0.0 0.4 0.4 0.0 0.0
0.25 1.0 0.55-0.65 0.28 1.0 0.88 0.06 0.00 0.016 0.07 0.14 0.33 0.13 0.017 0.06 0.4 0.0 0.0045 0.1-0.16
ref 10 11 12, 13 14 12 15 15 15 15 16 16 16 17 17 18 17
well-defined crystal planes under cryogenic temperatures and low pressures. Although the sticking coefficient itself is a temperature-independent adsorption parameter, the surfaces gain higher mobility and change configuration at higher temperatures, which influences the kinetics of adsorption. Furthermore, the sticking coefficient depends very strongly on the surface density and the coordination of adsorbent atoms. Supported metal catalysts possess a combination of low index planes and lower coordination edge and corner sites as well as defectlike sites. A portal model of chemisorption developed previously described rapid adsorption and dissociation at the low coordination (11) Lauth, G.; Schwarz, E.; Christmann, K. J. Chem. Phys. 1989, 91 (6), 3729-3743. (12) Seebauer, E. G.; Kong, A. C. F.; Schmidt, L. D. Appl. Surf. Sci. 1988, 31, 163-172. (13) Yates, J. T., Jr.; Thiel, P. A.; Weinberg, W. H. Surf. Sci. 1979, 84, 427-439. (14) Ehsasi, M.; Christmann, K. Surf. Sci. 1988, 194, 172-198. (15) Lu, K. E.; Rye, R. R. Surf. Sci. 1974, 45, 677-695. (16) Norton, P. R.; Richards, P. J. Surf. Sci. 1974, 41, 293-311. (17) Lisowski, W. Appl. Surf. Sci. 1988, 31, 451-459. (18) Procop, M.; Volter, J. Surf. Sci. 1972, 33, 69-81.
10.1021/la0157263 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/12/2002
4006
Langmuir, Vol. 18, No. 10, 2002
Savargaonkar et al.
Table 2. Kinetic Parameters for Hydrogen Chemisorption on Silica Supported Rh, Ru, and Pt Surfaces at Hydrogen Coverage of 0.4 dispersion (%) temp (K) kex (s-1) kd (s-1) ka (Pa-1 s-1) S at 350 K Edes (kJ/mol)
3% Rh/SiO2
4% Ru/SiO2
5% Pt/SiO2
60 333 4.0 × 102 1.0 × 103 4.2 × 105 0.52 79
20 296 2.0 × 102 5.0 × 102 6.3 × 105 0.12 43
38 333 8.0 2.0 × 101 2.3 × 105 0.014 66
edge and corner sites, followed by migration and chemisorption on low index planes.19,20 To gain further insight into the adsorption processes over multifaceted surfaces of metals, we performed a detailed study of the adsorption/desorption kinetic parameters and sticking coefficients of hydrogen on silicasupported Pt, Ru, and Rh. The dynamics of hydrogen on these metals was studied by means of solid-state NMR experiments with selective excitation of 1H nuclei. Using a multisite exchange model of Engelke et al.,21 values of activation energy and adsorption/desorption rate constants for hydrogen desorption and sticking coefficients of hydrogen on Pt and Rh surfaces were obtained, as described in the subsequent sections. 2. Methods 2.1. Catalyst Preparation. Platinum, rhodium, and ruthenium catalysts supported on silica were prepared by incipient wetness impregnation method using Pt(NH3)4(NO3)2, Rh(NO3)3‚2H2O, and Ru(NO)(NO3)3 as precursors. All reagents were obtained from Johnson Matthey, AESAR. Appropriate amounts of the metal salts were dissolved in distilled water, and a measured amount of support was added to the solution. The resulting slurry was dried at room temperature for 20 h and then at 393 K for 8 h. The catalysts were reduced in flowing hydrogen at 673 K and subsequently washed with hot distilled water to remove residual water-soluble impurities and reduced again in the NMR probe.22 Selective hydrogen chemisorption was used to measure the dispersion of the catalysts according to a method developed by Uner et al.,23 and these dispersions are listed in Table 2. 2.2. NMR Experiments. The NMR experiments were performed at 250 MHz on a home-built spectrometer equipped with an in situ NMR probe built at the Ames Laboratory. The probe was connected to a vacuum/dosing manifold, which allowed for an easy control of hydrogen pressure during the measurements. All spectra were recorded at a temperature of 333 ( 1 K for Pt and Rh catalysts and at 296 ( 1 K for Ru catalyst. The activation energy measurements were carried out between 298 and 400 K. A dwell time of 5 µs was used and the number of scans varied from 3600 to 7200 with a repetition time of 0.5 s. Selective inversion of the 1H magnetization (NMR hole burning) was done using a DANTE sequence consisting of 30 short pulses.21 A pulse separation of 10 µs was chosen, resulting in total duration of 300 µs for the DANTE sequence and a spectral hole width of 3.3 kHz. After a recovery period of 10 µs, a final 90° pulse was applied prior to the detection of the free induction decay. (19) VanderWiel, D. P.; Pruski M.; King, T. S. J. Catal. 1999, 188, 186-202. (20) Kumar, N.; King, T. S.; Vigil, R. D. Chem. Eng. Sci. 2000, 55, 4973-4979. (21) Engelke, F.; Vincent, R.; King, T. S.; Pruski, M. J. Chem. Phys. 1994, 101(9), 7262-7272. (22) Wu, X.; Gerstein, B. C.; King, T. S. J. Catal. 1992, 135, 68-80. (23) Uner, D. O.; Pruski, M.; King, T. S. J. Catal. 1995, 156, 60-64.
2.3. The Multisite Exchange Model. The average mobility of hydrogen on metal particles was characterized by the NMR exchange parameter kex measured by using the DANTE sequence. The effect of motion on the frequency-selective inversion of spin magnetization was incorporated into a multisite exchange model, as described in detail elsewhere.21 The model provided a consistent interpretation of NMR data by relating the exchange parameter kex to desorption, gas-phase diffusion, and dissociative readsorption of hydrogen on the surface of another metal particle. We note that the “real” process for hydrogen motion was modeled as a second-order process with kex treated as a pseudo-first-order rate constant.21 This method has been later successfully used to obtain the exchange parameters as well as activation energies for desorption over a series of monometallic and bimetallic surfaces.8,24 It can also be shown that the exchange parameter kex may be used to determine the sticking coefficient. According to ref 21, kex is related to the apparent rate constant for desorption kd by the relation N
kex )
σi′kdi ∑ i)1
(1)
where
σ i′ )
(1 - νi)σi N-1
and νi )
kaisi2
(2)
N
kaisi ∑ i)1
2
In eq 2, si and kai correspond respectively to the fraction of vacant sites and adsorption rate constant for site i, σi is the hydrogen density in sites i relative to the total adsorption surface area, and N denotes the total number of magnetically inequivalent sites. By assuming that the adsorption and desorption rate constants, the hydrogen coverage, and the fraction of vacant sites are uniform over the entire metal surface, these equations can be simplified to νi ) 1/N (and hence σi′ ) σi/N). In this case, the relationship between kex and kd simplifies to
kex ) kdθH
(3)
where θH corresponds to the surface coverage of hydrogen (H/Msurface). One should note that, after the aforementioned approximations, θH and σi possess similar meaning. At adsorption-desorption stationary state, the desorption rate constant is related to the adsorption rate constant by
kaPH2(1 - θH)2 ) kdθH2
(4)
Thus, the desorption and adsorption rate constants can be determined provided the exchange parameter and hydrogen coverage are known from the experiment. On the other hand, the apparent sticking coefficient of a gas, defined as the ratio of the fraction of the total number of impinging molecules that stick on the surface, can be written as the ratio of the rate of adsorption to the rate of impingement: (24) Savargaonkar, N.; PhD Dissertation, Iowa State University, Ames, IA, 1996.
Hydrogen Adsorption and Desorption on Catalysts
S)
dNa/dt dNf/dt
Langmuir, Vol. 18, No. 10, 2002 4007
(5)
Note that the apparent sticking coefficient defined in eq 5 refers to the entire surface of the metal particles although hydrogen adsorption might be occurring mainly on the defectlike sites. The rate of adsorption can be calculated once the exchange parameter is known:
dNa dθ ) Ns ) NskaPH2(1 - θH)2 dt dt
(6)
where Ns is the number of the exposed surface atoms. The rate of impingement of hydrogen molecules can be calculated as a product of the total flux, I (molecules/ (cm2‚s)), and the available surface area of the catalyst, A (cm2):25
dNf ) IA dt
(7)
For a randomly oriented impingement, as in the case of our adsorption experiment, the flux can be calculated by
I)
vj Fg 4
(8)
where vj is the average velocity of the impinging molecules, given by
vj )
(8RT πM )
1/2
(9)
and Fg is the molar gas density. By substituting the flux from eq 8, velocity from eq 9, and Fg from the ideal gas law, one obtains the molar impingement rate as
PH2A dNf ) dt x2πMRT
(10)
where A is the surface area of catalyst sample, M is the molar mass of impinging gas particle, T is the surface temperature in kelvin, and Ns is the number of exposed surface metal atoms. Thus, knowing the experimental variables such as the gas pressure, the exposed surface area, the number of exposed surface atoms, and the surface temperature, one can determine the average sticking coefficient of the gas on the surface of supported metals using the adsorption rate constants obtained from the NMR experiments. 3. Results Typical experimental and simulated 1H NMR spectra obtained using selective excitation for the Rh catalyst at various temperatures and a coverage (H/Rhs) of 0.4 are given in Figure 1. The resonance observed upfield (at 0 kHz) represents hydrogen adsorbed on the rhodium particles. It is well separated from the strong downfield line associated with hydrogen on silica, mainly the OH groups. For each temperature studied, the exchange parameter kex was determined by comparing the experi(25) Masel, R. I. Principles of Adsorption and Reaction on Solid Surfaces; Wiley: New York, 1996. (26) Christmann, K.; Behm, R. J.; Ertl, G.; Van Hove, M. A.; Weinberg, W. H. J. Chem. Phys. 1979, 70, 4168-4184. (27) Rendulic, K. D.; Winkler, A.; Steinruck, H. P. Surf. Sci. 1987, 185, 469-478.
mental spectra with the simulations performed using the formalism described above.21 To determine the activation energies of desorption, the exchange parameters measured at different temperatures and at constant hydrogen coverage were fitted to the Arrhenius equation
kex ) k0 exp(-Edes/RT)
(11)
where Edes is the activation energy of desorption and k0 is the preexponential factor. The Arrhenius plots for Pt/ SiO2, Rh/SiO2 and Ru/SiO2 catalysts are presented in Figure 2. The sticking coefficients obtained for these catalysts are plotted in Figure 3. For the Ru/SiO2 catalyst, the sticking coefficient was measured at 296 K as a function of coverage. The single-crystal data reported for Pt, Rh and Ru in refs 10, 12, and 15 are also plotted on this same graph. The desorption and adsorption rate constants, the apparent sticking coefficients, and the activation energies of hydrogen desorption are collected in Table 2. It is seen that ka, kd, and S determined in this work are much higher for Rh and Ru catalysts than those for the Pt catalyst. The activation energy Edes for hydrogen desorption decreases in the order Rh > Pt > Ru (see Table 2). 4. Discussion 4.1. Sticking Coefficients. The sticking coefficients of hydrogen over multifaceted surfaces of Pt/SiO2, Ru/ SiO2, and Rh/SiO2 were determined based on the exchange parameter kex, as described in section 2.3. At 333 K, the sticking coefficients of hydrogen at a surface coverage of 0.4 were found to be 0.52 and 0.014 on Rh and Pt catalysts, respectively, whereas the value of 0.12 was measured on the Ru catalyst at 296 K. It is not surprising that these values are somewhat higher than those measured on the single crystals of Ru and Rh (see Table 1 and Figure 3). It is well-known that the sticking process is a strong function of gas and surface temperatures and, most importantly, the surface structure.25-27 The effect of the presence of defects on the sticking probability of oxygen over Pt has been discussed in detail in ref 25. Christmann et al.26 and Rendulic et al.27 have demonstrated that the concentration of surface defects on the metal surface governs the trapping and subsequent dissociation of an H2 molecule. For example, the roughness of crystallographically “open” surfaces provides very efficient channels for such process. Therefore, on such crystal planes a higher sticking coefficient is expected than that measured on low index planes. The catalysts used in our study have high dispersions (60% for Rh, 20% for Ru, and 38% for Pt) with a relatively high fraction of the defectlike edge and corner sites serving as adsorption centers or portals20 for facile adsorption, which further increases the sticking probability of hydrogen. The results obtained in this study for Pt/SiO2 catalysts are in good agreement with the literature data for hydrogen sticking coefficients measured on single crystals. There could be enough defectlike sites on the surfaces of Pt single crystals which facilitated the adsorption processes. It is important to note that the present measurements were done at room temperature and above (298-333 K), while the single-crystal studies used temperatures ranging from 150 to 270 K (Figure 3). The effect of temperature on the sticking coefficients is complex. The process of sticking involves an energy transfer between the colliding molecule and the surface and is controlled by the ability of the surface to make bonds with the adsorbate. If the sticking process involves a passage through a precursor state, the net energy content, i.e., the temperature, becomes even more important. In a precursor-mediated
4008
Langmuir, Vol. 18, No. 10, 2002
Savargaonkar et al.
Figure 1. Left: Experimental NMR spectra of 3% Rh/SiO2 catalyst obtained at different temperatures using selective inversion of a narrow portion of the H/Rh resonance at 0 kHz (with the corresponding shift of ∼120 ppm with respect to TMS). Right: Simulated spectra that provide the best fit and the corresponding exchange parameter kex.
Figure 2. Arrhenius plots for 3% Rh/SiO2, 4% Ru/SiO2, and 5% Pt/SiO2 catalysts.
Figure 3. Sticking coefficients of hydrogen on supported and single-crystal Pt, Rh, and Ru surfaces as a function of hydrogen coverage.
process the sticking coefficient may be determined by the number of jumps/hops of the adatoms on the surface before it can desorb.28 The number of such jumps and the
probability of desorption increase with temperature, which, in turn, lowers the sticking probability of the precursor state. 4.2. Activation Energies of Desorption. In this work, the activation energies of hydrogen desorption from Rh, Pt, and Ru surfaces were found to be 79, 66, and 43 kJ/ mol, respectively, at a hydrogen coverage of 0.4. These results are in relatively good agreement with the existing literature. The reported activation energies for hydrogen desorption on thin films of Rh, Ru, and Pt are 78, 73, and 69 kJ/mol, respectively.29 The desorption activation energy of hydrogen over Ru/SiO2 measured in this work is in good agreement with the data reported by Engelke et al.21 as 52 ( 5 kJ/mol via NMR spectroscopy21 and the heat of adsorption data reported by Savergoankar et al.8 as 55 ( 5 kJ/mol at 0.5 H/surface Ru stoichiometry. Lauth et al.11 also reported the desorption activation energy of H from Ru(101 h 0) surface to be ∼60 kJ/mol at a coverage of 0.5. The preexponential factors are determined as 2.5 × 1015 s-1 for Rh, 1.9 × 1010 s-1 for Ru, and 4.5 × 1011 s-1 for Pt. The preexponential term contains information about the transition state entropy changes, and hence the mobility of the adsorbate. Thus, it is fair to conclude that the surface mobility of hydrogen decreases in the order of Rh > Pt > Ru. 4.3. Implications for Catalysis. The results of this study indicate that the exchange parameter of hydrogen on Pt was approximately 2 orders of magnitude lower than that on Ru or Rh. This can be qualitatively correlated with the differences in the catalytic activity of these metal catalysts for various reactions. For example, Rh, Ru, and Ir show higher activity for hydrogenolysis whereas Pt is more active for skeletal isomerization.30-33 Clearly, the rates of hydrogen adsorption and desorption described here are much faster than the rates of these catalytic reactions. However, the surface coverage of the intermediates can alter the overall rates as well as the selectivities (28) Tompkins, F. C. Chemisorption of Gases on Metals; Academic Press: London, 1978. (29) Sinfelt, J. H. Catal. Rev. 1969, 3, 175-205. (30) Seebauer, E. G.; Allen, C. E. Prog. Surf. Sci. 1995, 49, 265-330. (31) Sinfelt, J. H.; Yates, D. J. C. J. Catal. 1967, 8, 82-90. (32) Oliver, J. A.; Kemball, C. Proc. R. Soc. London, A: Mater. 1990, 429, 17-43. (33) Smale, M.; King, T. S. J. Catal. 1990, 125, 335-352.
Hydrogen Adsorption and Desorption on Catalysts
of the reactions7 even in the situations where these intermediates form at much faster rates than the slowest step of the overall reaction network. The exchange parameter measured by NMR spectroscopy can be an indicator of the pseudoequilibrium constant of hydrogen adsorption as demonstrated in previous studies.7,34 As such, the exchange parameter can be used as a measure of the relative surface coverage of hydrogen over different metals. The desorption activation energies of hydrogen on three different metal surfaces measured in this work are not sufficient to explain the differences in the catalytic activities of these metals in the aforementioned reactions, suggesting that factors other than the simple energetics of hydrogen adsorption are at play. Our results consistently point in the direction of the mobility of adsorbates, in this case hydrogen. Thus, the mobility of hydrogen involving adsorption and desorption processes dictates not only the concentration of hydrogen on the surface but also its reactivity and the selectivity of a catalyst. 5. Conclusions In this study, solid-state NMR spectroscopy was used to determine the sticking coefficient and other kinetic parameters of hydrogen adsorption/desorption over supported metals under conditions similar to those of the real catalyst studies. The method involved measuring 1H NMR spectra selective inversion of magnetization and obtaining an exchange parameter from a pesudo-firstorder approximation model based on the Bloch equations. The sticking coefficient of hydrogen was thus obtained from the exchange parameters and the physical param(34) Uner, D. O.; Savargoankar, N.; Pruski, M.; King, T. S. Stud. Surf. Sci. Catal. 1997, 109, 315-324.
Langmuir, Vol. 18, No. 10, 2002 4009
eters of the system such as the coverage, temperature, and gas pressure. The sticking coefficients on supported metal catalysts measured using this method were systematically higher, except for Pt, than the corresponding values reported in the literature for hydrogen on singlecrystal surfaces, suggesting that the defectlike sites are more active for dissociative hydrogen adsorption on these metal surfaces. The desorption activation energies measured in this work were comparable to those reported in the literature for the single crystals. The method used here offers an alternative for measurement of adsorption rate data under conditions similar to those of the real catalyst studies. Acknowledgment. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, under Contract No. W-7405-ENG-82. Nomenclature kai: adsorption rate constant for site i kd: desorption rate constant, s-1 kex: exchange parameter, s-1 M: molecular weight, g/(g-mol) N: total number of the magnetically inequivalent sites Na: number of adsorbed molecules Nf: number of impinging molecules Ns: number of exposed surface atoms R: ideal law gas constant, J/(K‚g-mol) si: fraction of vacant sites, dimensionless S: sticking coefficient, dimensionless T: temperature, K θH: hydrogen coverage, dimensionless LA0157263
