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J . Phys. Chem. 1992, 96, 7088-7092
(53) Mezei, M.; Beveridge, D. L. J . Chem. Phys. 1981, 74, 622. (54) Molina, M. J., unpublished results. (55) Mazzega, E.; del Pennino, U.; Loria, A,; Mantovani, S. J . Chem. Phys. 1976.64, 1028. (56) Gilpin, J . Colloid Sci. 1980, 77, 435.
(57) Golecki, I.; Jaccard, C. J . Phys. C 1978, 1 1 , 4229. (58) Kroes, G. J., unpublished results. (59) Eisenberg, D.; Kauzman, W. The Sfrucrureand Properties of Water, Clarendon: Oxford, 1969. (60) Mizuno, Y.; Hanafusa, N . J . Phys. C1, Suppl. 3 1987, 48, 5 1 1 .
Infrared Spectroscopy above Atmospheric Pressure. Support Effects on the Adsorption of Dinitrogen on Rhodium J. P. Wey, C. G. Worley, W. C. Neely, and S. D. Worley* Department of Chemistry, Auburn University, Auburn, Alabama 36849 (Received: March 20, 1992; In Final Form: May 18, 1992)
High-pressure infrared spectroscopy has been used to study the interaction of N2 with supported Rh films as a function of temperature and pressure. Adsorption equilibrium constants for N2 on Rh/X (X = Ti02,A1203,SO2)at 298 K were 0.376, 0.206, and 0.193 atm-I, respectively. The heat of adsorption for N2/Rh/Ti02 extrapolated to zero coverage was -10.2 kcal/mol. A sample film of Rh/Ti02 pretreated so as to approach SMSI conditions adsorbed N2 more tenaciously, with an adsorption equilibrium constant at 298 K of 0.541 atm-I. This work demonstrates the utility of high-pressure infrared measurements in obtaining useful surface thermodynamic data.
Introduction Recent work in these laboratories has focused on infrared studies of species adsorbed on supported transition metals in the presence of gases a t high pressure (1-13 atm). A new high-pressure infrared cell reactor was used in conjunction with transmission FTIR spectroscopy to investigate the interactions of Rh/Al2O3 films with high-pressure dinitrogen,’V2dihydr~gen,~ dioxygen: and carbon m ~ n o x i d e .A~ primary limitation of such high-pressure studies is competitive adsorption of small amounts of impurity species such as CO present in the gases employed which adsorb strongly to the supported metal and/or have high oscillator strengths.l However, it was shown that this limitation could be circumvented by utilizing a Rh/A1203 catalytic converter in the high-pressure manifold.2 The work led to infrared observable N2/Rh/A1203 and H/Rh/A1203 surface species a t ambient temperature. In the current study the work has been extended to the interaction of N2with Rh as a function of support material, pressure, and temperature. Langmuir isotherms have been determined which show that Ti02 promotes a much more significant interaction of N2 with Rh than do A1203 and Si02. Appropriate adsorption equilibrium constants a t 298 K have been measured for the three types of supported Rh films, and in the case of Rh/Ti02, variable-temperature studies have provided the enthalpy of adsorption for N2/Rh/Ti02. Finally, unusual behavior for N2/Rh/Ti02 was noted as the prereduction temperature was increased toward the limit generally attributed to the SMSI onExperimental Section Infrared-transparent supported Rh films (2.2 wt % Rh) were prepared by spraying slumes of RhC13.3H20 (Johnson Matthey), a support material (Ti02, Degussa titanium dioxide P25, 50 m2 g-I; Al2O3, Degussa aluminum oxide C, 100 m2g-I; Si02,Degussa Aerosil 200, 200 m2 g-l), spectroscopic grade acetone, and distilled4eionized water onto a 25-mm CaF2 IR window held at 353 K. Evaporation of the volatile components provided thin supported catalyst films (4.4 mg cm-2) firmly adhered to the window. The sample windows were then mounted in a high-pressure infrared cell reactor capable of operation in the pressure and temperature The cell ranges of 104-104 Torr and 100-600 K, re~pectively.’,~ reactor was then evacuated to 10” Torr, and the sample films were held at 373 K at this pressure for 1 h. All samples were calcined in 100 Torr of O2at 553 K for 30 min and then subjected *Author to whom correspondence should be addressed.
0022-3654/92/2096-7088$03.00/0
to reduction cycles of 10, 5, 10, and 20 min with 100 Torr of H2 at 473 K. After evacuation a t 10” Torr for 1 h at 298 K, the catalyst films were exposed to increasing pressures of N2 at 298 K with the FTIR spectra monitored as a function of pressure following 10-min equilibration time a t each pressure. This procedure worked well for Rh/Ti02 and Rh/A1203, but not for Rh/Si02,for which no band corresponding to N2/Rh/Si02 could be detected at any N2 pressure. It was found that a more stringent reduction process was needed for the Rh/Si02 films; in this case, the samples were exposed to 6500 Torr of H2 a t 298 K for 13 h, followed by heating to 473 K for 2 h at that pressure of H2, and then evacuation at 298 K for 1 h at 10” Torr before exposure to N2. The gases used in this study were purchased from Air Products (N2, Research Grade, 99.9995%;H2, Ultra High Purity, 99.995%;02,Research Grade, 99.996%). The N2 and H2 were further subjected to a catalytic converter in the high-pressure manifold containing Rh/A1203held at 373 K and trapping at 158 and 77 K, respectively, to remove all traces of CO and C02from the gas stream^.^,^ In the case of Rh/Ti02 variable-temperature experiments were also performed. In these experiments a 2.2% Rh/Ti02 film, following preoxidation and prereduction as described above, was exposed to 8000 Torr of N2 at 301 K. After 1-h equilibration, the equilibrium pressure was recorded, and the infrared spectrum was obtained. The process was then repeated at six higher temperatures in the range 3 12.5-361 K, allowing 5-min equilibration at each new temperature. Then the sample was returned to 301 K, and the process was repeated a t a starting pressure of 7000 Torr. Similar data were obtained at starting pressures of 6000, 5000,4000, 3000, 2000, and 1000 Torr. From all of these data adsorption isotherms as a function of temperature were generated, which were used in constructing Clausius-Clapeyron plots, which in turn were employed to determine heats of adsorption as a function of N 2 coverage. A few experiments were performed in which a 2.2%.Rh/Ti02 film was reduced vigorously using 4000 Torr of H2 at 583 K for 9 h followed by an additional 12 h at 503 K and evacuation at 10” Torr at 298 K for 3 h. Then N2 was introduced at pressures ranging from ca. 300 to 9400 Torr at 298 K with FTIR data being collected as usual. Equilibration times were 10 min at each new pressure. Identical experiments were performed for a film containing T i 0 2 alone. All IR spectra were obtained by using an IBM 32 Fourier transform spectrometer operated at 2-cm-l resolution. Generally, 500 scans accumulated over a period of 7.5 min were generated 0 1992 American Chemical Society
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Figure 1. Infrared spectra for the interactionof N2 with a 2.2% Rh/Ti02film (4.4 mg cm-2)at 298 K and pressures of (a) 1062, (b) 2026, (c) 3003, (d) 4021, (e) 5016 (f) 6044, (g) 7008, and (h) 8003 Torr. Inset shows a Langmuir isotherm computed from the integrated areas of the absorption
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Figure 2. Infrared spectra for the interaction of N2with a 2.2% Rh/A1203film (4.4 mg cm-') at 298 K and pressures of (a) 1007, (b) 2010, (c) 3015, (d) 4000, (e) 5000, (f) 6020, (8) 7012, and (h) 8027 Torr. Inset shows Langmuir isotherm.
for each spectrum. The spectra shown in the figures represent differences between the spectra for the sample films exposed to Nz and those at lo4 Torr. An MKS Baratron capacitance manometer was used to measure N2 pressures.
ResulQ and Discussion The infrared spectra corresponding to N2 adsorption on Rh/ Ti02, Rh/AI2O3, and Rh/Si02 at ambient temperature as a function of N2 pressure are shown in Figures 1-3. In each case two bands are detected in the 2324-2333- and 2252-2255-cm-I regions. It is well-established that these two bands may be assigned to the N-N stretching mode for N2 physisorbed on the support and chemisorbed on Rh, respectively.1*2,8 In Figure 4 the spectra corresponding to the N2/Rh interaction on the three supports for 8000 Torr) are displayed the highest pressure of N2 studied (a. on the same absorbance scale. It is evident from Figure 4 and from Figure 5, in which the integrated areas of N2/Rh bands as a function of N2 pressure are displayed, that the N2/Rh band intensities were clearly dependent upon the nature of the support, with the order being Ti02 > AI203 > S O 2 . In all cases the
adsorption was reversible, as the band intensities declined upon pressure decrease and vanished immediately upon evacuation. From this data Langmuir adsorption isotherms were determined? The surface coverages 0 were estimated from the ratios Ap/Apmr where A, represent integrated areas of the 2050-cm-I bands at the various N2 pressures and A, represent the integrated areas of the bands at the maximum pressure generally utilized (ca.8000 Torr). The assumption must be made here that the extinction coefficient for the N-N stretch is not dependent upon coveragea8 It was found that the integrated area of the N2/Rh/Ti02 band only increased by 3.5% upon raising the N2 pressure to 9500 Torr, so it was assumed that the 8000-Torr pressure represented a reasonable P,,, in that the pressure manometer used in these experiments only functions up to lo4 Torr, and safety considerations make operation of our high-pressure cell near this highpressure limit at bit perilous. From the Langmuir adsorption isotherms (see insets of Figures 1-3), equilibrium constants for the adsorption of N2 on supported Rh at 298 K were determined. They were as follows: N2/ Rh/Ti02, 0.376 atm-I; N2/Rh/AlZO3,0.206 atm-I; and N2/
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Figure 3. Infrared spectra for the interaction of N2 with a 2.2%Rh/Si02 film (4.4 mg (d) 6016, (e) 7012, and (f) 8085 Torr. Inset shows Langmuir isotherm.
at 298 K and pressures of (a) 2006, (b) 3016, (c) 4017,
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Figure 4. Infrared spectra for the interaction of N2 with a 2.2% film (4.4 mg c d ) of (a) Rh/Ti02 at 8003 Torr, (b) Rh/Al2O3at 8027 Torr, and (c) Rh/Si02 at 8085 Torr at 298 K plotted on the same absorbancescale.
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Figure 6. Adsorption isotherms for N2on a 2.2%Rh/Ti02 film (4.4 mg
prereduced under mild conditions, as a function of temperature utilizing integrated Rh/N2 infrared band areas as an expression of coverage.
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Figure 5. Adsorption of N2 on (A) Rh Ti02, (B) Rh/A120,, and (C) Rh/Si02 films (2.2% Rh, 4.4 mg cm- ) at 298 K as computed from integrated areas of the RhN2infrared absorption band as a function of
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Rh/Si02, 0.193 atm-I. Thus, the interaction of N2 with Rh/Ti02 is clearly more pronounced than that for Rh/A1203or Rh/Si02, even for Rh/TiOZprereduced at only 473 K, at which temperature the SMSI effect should not be a A curious observation was that the N2/Rh/A1203 infrared band absorption intensities were quite a bit larger than those for N2/Rh/Si02 at comparable N2 pressures; yet, the adsorption equilibrium constants measured for the two catalyst films from the IR data were almost the same. A possible explanation for this apparent anomaly is that the oscillator strength for the N-N
vibrational mode may be significantly higher for the NZ/Rh/Al2O3 species than for the N2/Rh/Si02 species due to a subtle difference in electronic interaction of the two supports with the metal. A second possible explanation for the observations is that the dispersions of the two catalysts may have been different. As mentioned earlier in the Experimental Section, the Rh/Si02 film was necessarily subjected to a much more stringent reduction process than was Rh/Al2O3 in order to produce a sample which provided a detectable 2255-cm-I infrared band. Thus, sintering could have occurred producing a catalyst of low dispersion which contained fewer available adsorption sites for Nz than did the Rh/A1203 film which was only subjected to mild reduction conditions. Unfortunately, we were not able to measure the dispersions of the two catalysts following their reduction processes. A third possible explanation for the weak infrared absorption detected for Nz/Rh/Si02 relative to N2/Rh/A1203 is that a portion of the potential Rh adsorption sites were encapsulated by SiOzcausing a reduction in adsorption, such as has been reported for Rh films on oxidized and Ni/TiOz.l2 The studies for Rh/Ti02 were extended to temperature variations in order that the isosteric heat of adsorption for Nz/ Rh/Ti02 could be measured. The general procedure and its utilization have been d i s c u s s e d recently."J4 Adsorption isotherms for N2/Rh/Ti02 at several temperatures measured from integrated areas of the IR absorption band near 2250 cm-l are shown in Figure 6. From these data (e.g., at integrated band area of 0.200, ~ can be obtained as a function of temperature, etc.), In P N values
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1 / T x l o 3 (R') Figure 7. ClausiusClapeyron plots for the data shown in Figure 6.
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Figure 8. Heat of adsorption for N2on 2.2% Rh/Ti02, prereduced under mild conditions, as a function of Nz coverage.
Clausius-Clapeyron plots were generated as shown in Figure 7 which provided heats of adsorption for each set of data. The solid lines shown in Figures 6 and 7 are empirical fits of the data rather than theoretically generated. Some curvature was noted in the Clausius-Clapeyron plots for the data corresponding to integrated band areas of 0.100.200. This was probably due to small errors in integration of the absorption bands at low surface coverages. The higher surface coverages provided increased linearity. The measured heats of adsorption were dependent upon coverage ranging from -9.52 kcal/mol at integrated band area of 0.100 to -6.28 kcal/mol at integrated band area of 0.350. A plot of heat of adsorption versus the integrated areas of the 2250-cm-'
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band which were proportional to coverage is displayed in Figure 8. From Figure 8 the heat of adsorption for N2/Rh/TiOz extrapolated to zero coverage was -10.2 kcal/mol. We did not determine the heats of adsorption for N2/Rh/A1203and N2/ Rh/Si02; the value at zero coverage for Nz/Rh/A1203obtained from low-pressure, low-temperature data by Wang and Yatess was -2.24 kcal/mol. The current value for Nz/Rh/Ti02 is thus reasonable given the obvious stronger interaction of N z with Rh/TiOz than with Rh/A1203 as observed in this study. The value is considerably lower than that for CO/Rh (ca. -30 kcal/mol),8 as expected, given the ease with which the N2 can be desorbed from Rh/Ti02 relative to CO from Rh/TiOz. It should be noted that a value of -2.6 kcal/mol has been reported for the heat of adsorption for N2/Ti02 for a sample which was preoxidized at 725 K, but not prereduced.I5 This AHHadS was obtained from infrared data accumulated at low temperatures (
