J. Phys. Chem. 1994,98, 13631-13635
13631
Hydrogen Adsorption on Rhodium Particles Supported on Strontium Titanate As Followed by 'H NMR and Microcalorimetry JosC M. Rojo,* Juan P. Belzunegui, and Jesus Sanz Instituto Ciencia de Materiales, CSIC, c/Serrano 115 bis, 28006 Madrid, Spain
JosC M. Guil Instituto de Quimica Fisica "Rocasolano ", CSIC, c/Serrano 119, 28006 Madrid, Spain Received: June 11, 1994; In Final Form: September 28, 1994@
Hydrogen adsorption on supported rhodium particles has been studied by 'H NMR, volumetry, and microcalorimetry techniques. The amounts of hydrogen adsorbed on the metal and support have been evaluated. There is a good correlation between the shift of the NMR line assigned to hydrogen adsorbed on the metal and the mean integral heat involved in this adsorption. This permits an estimation of differential shifts corresponding to each dose. From analysis of differential NMR shift and differential adsorption heat, it is deduced that the hydrogen-metal interaction becomes weaker and less energetic with adsorption. This effect is discussed on the basis of the heterogeneity of the adsorption sites and a modification of the electronic properties at the metal surface.
Introduction The dissociative adsorption of hydrogen on group VIII metals has been studied for years in relation to several topics:'-" arrangement of adsorbed atoms, reconstruction of the metal surface, variation of the adsorption enthalpy with surface coverage, mobility of the adsorbed species with temperature, etc. To increase the exposed metal surface, samples consisting of small metal particles (4 = 1-10 nm) of F't, Rh, Ir, etc., supported on different oxides, Si02, Al2O3, TiO2, CeO2, etc., are usually prepared. In general, the catalysts are characterized by volumetric and calorimetric techniques, but they cannot distinguish between hydrogen adsorbed on the metal and that transferred to the support. An estimation of both contributions has been recently obtained by 'H NMR spectroscopy,12-22which has permitted in several cases a correct determination of metal dispersion. Furthermore, hydrogen-metal interactions can be studied through the shift of the NMR line assigned to hydrogen adsorbed on the metal. Two types of hydrogen and three types of deuterium species interacting with metal particles of rhodium have been identified in M i 0 2 and WSiO2 sample^.^'^^^^^^ In addition, a modification of the metal surface has been detected by lg5Pt NMR spectroscopy when hydrogen is adsorbed on W A1203 sample^.^^-^^ In this work, we have combined 'H NMR, adsorption volumetry, and microcalorimetry techniques to study hydrogen adsorption on a WSrTiO3 sample. This system was chosen for two reasons: (i) hydrogen adsorption on the metal is not appreciably affected by metal-support interactions (SMSI effect),29 and (ii) the amount of hydrogen spilt over to the support is much less than that observed when other more reducible oxides such as Ti02 or CeO2 are u ~ e d . ~ O , ~ '
Experimental Section SrTiO3 powders were obtained by thermal decomposition in air at 823 K of Sr(Ti(C204)3).32 The calcined material was treated with dilute HCl to eliminate surface carbonates and then @
Abstract published in Advance ACS Absrrucrs, November 15, 1994.
0022-365419412098-13631$04.5010
washed with water to remove residual chlorine. Only a perovskite phase, which had a specific surface area SBET= 54 m2/g, was detected by X-ray diffraction. This oxide was impregnated with a RhC13-3H20 aqueous solution by the incipient wetness method, then dried in air at 383 K, and reduced under flowing H2 in two steps: 473 K (2 h) and 773 K (3 h). The catalyst was stored in air at room temperature. The final metal load was 2.5% by weight. Metal dispersion determined by the Hd02 titration method gave a HlRh ratio of 0.13. The sample was treated thermally in Pyrex tubular cells provided with high vacuum stopcocks. Prior to hydrogen adsorption experiments, the WSrTiOs sample was reduced in flowing H2 at 773 K (1 h) and oxidized in flowing 0 2 at 673 K (1 h), then reduced in static hydrogen atmosphere (150 Torr; 1 Torr = 133.32 Pa) at 473 K (1 h), and outgassed at 573 K (3 h) to clean metal particles. H2 was adsorbed at room temperature by successive doses at increasing pressures. Transmission electron micrographs were obtained on a Jeol 2000FX electron microscope, working at 200 kV and equipped with a f25" goniometer. The WSrTi03 sample was crushed in an agate mortar, suspended in acetone, and transferred to carbon-coated copper grids. A heat-flow microcalorimeter of the Tian-Calvet type (Model BT, Setaram) was used to determine differential heats of hydrogen adsorption. For this purpose, the calorimeter cells form part of a conventional volumetric apparatus, which allows the simultaneous determination of volumetric and calorimetric isotherms. A capacitance manometer (Baratron 310, MKS) was used. Dead volumes were carefully calibrated either by mercury weighings or by helium expansions. Reproducibility of the amounts adsorbed was always better than 0.2 pmol. The limit of detection of the microcalorimeter is about 0.2 mJ or 2 pW. The correction for the heat involved in gas compression was previously determined with helium; typical correction values of 25 mJ were calculated for the maximum pressure increment used. Experiments were made at 298 K. 'H NMR spectra were recorded at room temperature with a SXP 41100 Bruker spectrometer (75 MHz). H2 was adsorbed by keeping the NMR cell connected to a volumetric apparatus. The spectra were taken after n12 pulse excitations (4 p s ) ; the 0 1994 American Chemical Society
Rojo et al.
13632 J. Phys. Chem., Vol. 98, No. 51, 1994 TABLE 1: Distribution of Rhodium Particle Size As Deduced from TEMa size, A freq, % 10-25 25-50 50-75 75-100 100-150 150-200 200-300
A
A
8.5 22.6 29.5 18.7 11.1 6.6 2.9
The sample population was 600.
200
0 d ( ppm)
-200
200
0
A
-200
(ppm)
Figure 2. 'H NMR spectra of the Rh/SrTiO3 sample (a) exposed to H2 at increasing pressures of X 0 (b), 1.2 (c), 8 (d), 70 (e), and 150 (f) Torr in an accumulative way, and then outgassed at room temperature (g) and 423 K (h). 1
0
8
16 24 32 n(umol Hlg-cat)
40
Figure 1. Top: Isotherms of hydrogen adsorbed on the WSrTi03 sample at 298 K. Curve a was obtained in the fist adsorption experiment. After that, the sample was outgassed at room temperature and then exposed to H2 in a new adsorption experiment (curve b). Curve c was calculated from subtraction between isotherms a and b for pH2 > 1 Torr. Bottom: Differential heats measured in the adsorption (0) and readsorption (H) experiments. intervals between successive accumulations (1-2 s) were chosen to avoid saturation effects. The number of accumulations (1OOO2000) was selected to obtain a signal-to-noise ratio greater than 20. Chemical shift values are given relative to TMS (MeSi); the estimated mean error is 2 ppm. The intensities of the NMR lines were determined by comparing their integrated intensities with that of a known external mica specimen; the errors in intensities were lower than 10%.
Results Supported rhodium particles in the range 10-300 A were observed in transmission electron micrographs. The particle size distributionis given in Table 1. From these data, an average diameter of 77 A and a standard deviation of 50 were deduced. Volumetric isotherms corresponding to hydrogen adsorption on Rh/SrTiOs are given in Figure 1 (top). A first isotherm (curve a) was measured; after that, the sample was outgassed at room temperature for 15 min, and a second readsorption isotherm (curve b) was obtained. Isotherm a shows a significant increase under negligible pressures, a clear knee, and then a small increase that becomes almost linear in its final part. A total amount of 42 pmol of Wg of catalyst was adsorbed at 230 Torr. The shape of isotherm b is similar to isotherm a but the amount of hydrogen adsorbed at pressures close to zero and the slope in the final part are lower. The hydrogen retained after
the outgassing treatment (20.6 pmol of Wg of catalyst) is estimated by subtraction of isotherm b from isotherm a (curve c). In addition, for pH2 > 5 Torr, a small increase of curve c is observed, indicating that the amount of hydrogen adsorbed at these pressures in isotherm a is slightly higher than in isotherm b. Differential adsorption heat (qd),corresponding to successive doses, vs the amount of hydrogen adsorbed (n) is shown in Figure 1 (bottom). qad decreases progressively as n increases (open squares), but the variation is greater in the range 30-36 pmol of Wg of catalyst. Above 36 pmol of Wg of catalyst, qd attains a constant value at ~7 Wmol of H. After the adsorption experiment, the sample was outgassed at room temperature and hydrogen adsorbed again. Data of the readsorption experiment (closed squares) are plotted by shifting the origin in the horizontal axis to 20.6 pmol of Wg of catalyst, which corresponds to the amount of hydrogen irreversibly adsorbed. The qd values are in the range 30-2 kJ/mol of H, reproducing partly the first curve. However, the pleateau at high n values is not observed in this case. 'H N M R spectra of the Rh/SrTiOs sample exposed to H2 at increasing pressures and room temperature are given in Figure 2. Two lines are observed: line A centered at the resonance frequency mainly assigned to OH groups located on the support, and line B shifted upfield due to hydrogen adsorbed on the metal.17929Before hydrogen adsorption, the 'H NMR spectrum only displayed line A. As hydrogen pressure was raised, lines A and B increased in intensity, and the separation of line B from the resonance frequency decreased. Outgassing of the sample at room temperature decreased the intensity of both lines and shifted line B toward positions of low HZpressures. Line B disappeared from the spectrum when the sample was outgassed at 423 K. These results are plotted as a function of Hz pressure in Figure 3. At very low pressures, close to zero, the intensity (ZA) of line A is not appreciably modified, while the intensity (ZB)of line B increases significantly, and the shift (A) of this line in absolute value decreases abruptly. At higher pressures two effects are observed: (i) ZA increases slightly, and (ii) ZB and A follow their respective tendencies but with smoother variations. After the adsorption experiment, outgassing of the sample at room temperature decreases the intensity of lines A and B (see marks labeled vac. in Figure 3). Once the sample was outgassed, it was again exposed to H2. The ZB and A values fit well to the curves obtained in the first adsorption run. On the contrary, the initial value of ZA is higher than that corresponding
Hydrogen Adsorption of Rhodium Particles
J. Phys. Chem., Vol. 98, No. 51, I994 13633
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Figure 3. Top: Intensity (ZA) of line A vs Hz pressure. Bottom: Intensity (ZB) and shift (A) of line B (0,0 ) vs Hz pressure. The ZA and ZB values corresponding to the sample outgassed after the adsorption experiment are also included (vac.). Full symbols correspond to the readsorption experiment. to the first curve, and the increase of ZA with H:! pressure is smaller than in the first adsorption experiment. To compare the amounts of hydrogen adsorbed deduced from the IH NMR spectra with those determined directly by volumetric measurements, ZB ZA - ZAO vs n is plotted in Figure 4. ZAO is the intensity of line A before hydrogen adsorption, due to hydroxyl groups of the support surface. ZA and ZB are the intensities of lines A and B after each adsorption dose. So ZB ZA - ZAO gives the total amount of hydrogen adsorbed on the catalyst. The plot has been made by taking pairs of ZB ZA ZAO and n values at the same pressure from Figures 3 and 1, respectively. A good linear dependence through the origin is observed and an equivalence of 0.47 pmol of H per arbitrary unit of intensity is calculated. On these basis, a WRh ratio of 0.13 is deduced by extrapolating at pH2 0 the ZB curve of Figure 3. The value coincides with that obtained by the titration method, proving that no significant fraction of adsorbed hydrogen on the metal remains undetected by ‘H NMR spectroscopy.
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20 30 40 50 n(1mol H) Figure 4. Hydrogen uptake deduced from the ‘HN M R spectra (ZB + ZA - ZAO) vs that obtained by volumetric measurements (n). ZB and ZA are intensities of lines B and A, respectively. IAOis the intensity of line A before hydrogen adsorption experiment. 10
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Discussion From the observed dependence of ZB and ZA with H2 pressure (Figure 3), the relative contribution of hydrogen adsorption on the metal and support has been analyzed. At very low pressures (pH2 0), only ZB increases, indicating that hydrogen is preferentially adsorbed on the metal. At intermediate pressures (0-60 Torr), a simultaneous adsorption on the metal and support takes place, as deduced from the similar increase of ZB and ZA parameters. Above 60 Torr, ZA increases quicker than ZB due to an adsorption more important on the support. These results can be used to explain the calorimetric data (Figure 1). We ascribe the region in which qd changes from 40 to 26 kJ/mol of hydrogen atoms (region I) to hydrogen only adsorbed on the metal. The value of qad for the lowest coverage agrees reasonably well with those deduced from desorption experim e n t ~ on ~ , ~polycrystalline rhodium and the (111) face of rhodium (69.5-77.5 kJ/mol of H2 molecules) and from IR measurement^^^ in Rh/A1:!03 catalysts (32.1 kJ/mol H). The qd values between 26 and 7 kJ/mol of H (region 11) correspond to hydrogen adsorption on both metal and support. Finally, we
ascribe the plateau observed at the end of the calorimetric curve (region In) to hydrogen adsorption on the support. When the sample was outgassed at room temperature and H2 is readsorbed, the curve qd vs n (closed squares) did not show regions I and 111, and region I1 of the first curve was only partly reproduced (Figure 1). Based on the NMR results, the differences between the two calorimetric curves can be explained. Region I is not observed in the readsorption curve since hydrogen, associated with this region, remains adsorbed on the metal after the outgassing treatment, as deduced from ZB data. Region I1 of the readsorption curve is different from that of the first run because hydrogen was not appreciably adsorbed on the support. The absence of a plateau in the final part of the readsorption curve (region 111) suggests that the amount of hydrogen spilt over to the support in the second adsorption run is much smaller than in the first one. This is c o n f i i e d by the small increase of ZA vs pH2 observed in the second run (Figure 3, top). Therefore, the qd curve of the readsorption experiment can be mostly associated with reversibly adsorbed hydrogen on the metal. To analyze the hydrogen-metal interaction, variation of the shift (A) of line B vs the amount of hydrogen adsorbed on the metal, deduced from the intensity (ZB)of this line, is given in Figure 5 , top. In this figure, the amounts of adsorbed hydrogen have been calculated by using the equivalence 0.47 pmol of Wintensity arbitrary unit. A decreases continuously although its variation below 22 pmol of Wg of catalyst is less pronounced than above that value. These two regions correspond to hydrogen adsorption at pressures close to zero and in the range 0-190 Torr, respectively. In the case of the Rh/TiO:! system, the A vs ZB plot showed17 a plateau for relative low ZB values and then an asymptotic decrease of A going to zero. The constant value of A (-130 ppm) was ascribed to an adsorption type on the metal surface. However, the observed variation of A could not be explained by assuming only an exchange model between two hydrogen species with shifts -130 and 0 ppm, and a modification of the first adsorption type as adsorption progresses was proposed. In our WSrTiO3 sample, the absence of plateaus in the A curve does not support again the existence of two well-differentiated adsorption sites on the metal surface. On the basis of hydrogen adsorption on metal sites with different shifts, line B should broaden as adsorption progresses. On the other hand, it is known that an increase in mobility of adsorbed atoms averages magnetic interactions and narrows the NMR line.34,35 To analyze both effects, full width at halfmaximum of line B (&2) as a function of hydrogen adsorbed on the metal (ZB)has been plotted in Figure 5 , bottom. For
Rojo et al.
13634 J. Phys. Chem., Vol. 98, No. 51, 1994 200
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Figure 5. Plots of shift (top) and full width at half-maximum (bottom) of line B vs amount of hydrogen adsorbed on the metal (ZB). Values of 6112 obtained at 36 MHz (0)are also included.
increasing IB, ~3112 increases, reaches a maximum, and then decreases. The increase at low coverages can be due either to an increase in the H-H dipolar interactions or to differences in line shifts. To determine which of the two interactions is responsible for the observed increase of 8112, 'H NMR spectra were recorded at two frequencies, 75 and 36 MHz. 81/2 obtained at 36 MHz shows the same trend observed at 75 MHz, although with lower values. Because dipolar interactions (measured in frequency) do not depend on the external magnetic field sed,^^,^^ the lower values indicate that the line width is affected by shift dispersions. However, the d1,2 values are not strictly proportional to the Larmor frequency, indicating an appreciable contribution of the hydrogen-hydrogen dipolar interaction to the line width. The observed decrease of 61/2 at higher coverages can be associated with an increase in hydrogen mobility on the surface that cancelates shift and dipolar interactions, in agreement with other reported results.24 Exchange between the adsorbed species with different shifts leads to a single line, as observed in the NMR spectra, with a mean shift that changes with the amount of hydrogen adsorbed. From these results, it is clear that heterogeneity in the adsorption sites and the mobility of the adsorbed species affect the adsorption of hydrogen on supported metal particles. This heterogeneity introduces energetic differences as deduced from the calorimetric curves, and a comparison between NMR shift and calorimetric data could give additional information about the adsorption process. However, a direct comparison is not possible because (i) the shift of line B is a weighted mean value of shifts corresponding to hydrogen atoms adsorbed in different adsorption sites of the metal, Le., A = ZiAiniEini and (ii) the differential heat corresponds to hydrogen adsorbed on the metal and support in each dose. To solve the last problem, differential heats relative to hydrogen adsorbed on the metal must only be considered. For that, we construct a differential calorimetric curve by connecting that obtained in the first adsorption experiment with the curve of the readsorption run at the common qad value of 28.7 kJ/mol of H. To overcome the first impediment mean integral adsorption heats, defined as Q = Ciqa,jni/Cni, have been calculated. For comparison, mean
20 30 40 n (Umol Hlg -cat) Figure 6. Variation of mean integral heat (Q) vs hydrogen uptake (n). The Q values were calculated by using two differential calorimetric curves (Figure 1, bottom): that obtained in the frst adsorption experiment (open circles) and that assembled by connecting the curves of the adsorption and readsorption experiments at the common qd value of 28.7 !d/mol of H (closed circles). 200
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200
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Figure 7. (a) Plot of shift (A)of line B vs mean integral heat (Q). Pairs of A and Q values are taken from Figures 5 and 6 for the same
amount of adsorbed hydrogen. Open and closed circles correspond to the adsorption and readsorption experiments, respectively. (b) Plots of differential shift (Ai)and differential adsorption heat (qd)vs amount of hydrogen adsorbed on the metal. Values of Ai and qd are represented as closed triangles and squares, respectively. integral heats have been also calculated from the differential curve corresponding to the first adsorption experiment in which hydrogen was adsorbed on the metal and support. Plots of mean integral heats (Q), deduced from the two mentioned qd curves, vs the amount of adsorbed hydrogen (n) are shown in Figure 6. In both cases, Q decreases for increasing n, but the decrease is higher in the assembled curve, i.e., when only adsorption on the metal is considered (closed circles). Pairs of A and Q values taken from Figures 5 and 6 for the same amount of hydrogen adsorbed are plotted in Figure 7a. A depends linearly on Q (closed circles) when the Q values were obtained from the assembled qad curve. However, when the Q values were calculated from the qad curve of the first adsorption experiment, two dependencies are observed (open circles). The linear dependence (range 30-40 kJ/mol of H) corresponds to hydrogen adsorbed on the metal and the other (25-30 kJ/mol of H) to simultaneous adsorption on the metal and support. Therefore, for a given A, the difference between Q values of the two plots must be ascribed to the contribution of hydrogen adsorbed on the support. From this analysis, it is deduced that the shift of line B is proportional to the mean integral heat corresponding to hydrogen adsorption on the metal. The A and Q quantities exhibit their highest values at low metal coverages, and they decrease with adsorption extent. In the limit when the hydrogen-metal interaction is very weak (A a 0), the associated adsorption heat (Q) approaches zero. Moreover, the good correlation between
Hydrogen Adsorption of Rhodium Particles
A and Q should imply the existence of a straight forward dependence between individual shifts (Ai) and differential adsorption heats (qd) corresponding to each separate dose. Differential Ai values have been calculated by assuming a A = ZiAinilCini expression in which Ai does not change with adsorption extent. For that, an ni constant interval of 2 pmol of Wg of catalyst has been used. Plots of Ai and qad as a function of hydrogen adsorbed on the metal are shown in Figure 7b. As expected, the two parameters follow similar dependences. Moreover, we observed two regimes of Ai and qd that we interpret as follow. For coverages below 20 pmol of Wg of catalyst, the small variation of Ai and qd is ascribed to adsorption on different sites of the metal surface. This heterogeneity could be modulated by the particle size distribution. Above 20 pmol of Wg of catalyst, the important decrease of both parameters would require the existence of adsorption sites with Ai and qad, taking values in the ranges -(130-0) ppm and 27-0 kJ/mol, respectively. The abrupt variation of Ai and qad in this regime suggests a strong modification of the electronic properties of the metal rather than such broad heterogeneity in metal adsorption sites. Conclusions From the analysis of hydrogen adsorption on a FWSrTiOs sample by 'H NMR, volumetry, and microcalorimetry techniques, it can be concluded that hydrogen is first adsorbed on the metal at HZpressures close to zero, then on the metal and support at intermediate pressures (0-60 Torr), and finally on the support at high pressures (above 60 Torr). After adsorption, outgassing at room temperature removes partly hydrogen adsorbed on the metal and support. Hydrogen readsorbs mostly on the metal, which permits a determination of differential heats corresponding to hydrogen adsorption on the metal at intermediate pressures (0-60 Torr). A correlation between the shift of the NMR line and mean integral heat, both associated with hydrogen adsorption on the metal, has been obtained. Moreover, differential shifts corresponding to each adsorption dose have been estimated. The hydrogen-metal interaction becomes weaker and less energetic as HZ adsorption progresses. We interpret this modification in terms of the heterogeneity of metal adsorption sites and a change in electronic properties of the metal particles. The observed weakening is accompanied by an increase in mobility of adsorbed hydrogen.
Acknowledgment. Financial support of this work under contracts from CICYT and DGICYT (MAT91-1080 and PB870327) and the European Community (SCI-CT91-0704) is gratefully acknowledged. We thank Dr. P. Herrero for electron microscopy measurements. References and Notes (1) Castner, D. G.; Sexton, B. A.; Somojai, G. A. Sui$ Sci. 1978, 71, 5 19-540.
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