An infrared study of support effects on the interaction of dihydrogen and dinitrogen with supported rhodium films

ELSEVIER

Colloids and Surfaces A: Physicochemicaland EngineeringAspects 105 (1995) 79-86

COLLOIDS AND A SURFACES

An infrared study of support effects on the interaction of dihydrogen and dinitrogen with supported rhodium films Z. Pei, T.H. Fang, S.D. Worley * Department of Chemistry, Auburn University, Auburn, AL36849, USA Received 14 February 1995; accepted 30 June 1995

Abstract A substantial support effect exists for the interaction of N 2 and H 2 with supported Rh. Infrared spectroscopy was used to show that the interaction of N2 with Rh/TiO2 (average AH= -5.0 kcal mol a) was stronger than that of N2 with Rh/AI203 (average AH = -2.2 kcal tool 1). It was also demonstrated that H2 dissociates to a greater extent over Rh/TiO2 than over Rh/A1203 to produce H20 on the support. Low coverages of Rh-H (O =0.1-0.2) on the supported Rh caused an enhanced interaction of N2 with Rh for both supports which was evident in enhancement of the intensity of the N-N stretching mode for the Rh-N2 surface species. Both weakly bound and strongly bound forms of hydrogen were thought to be involved in the enhanced N 2 interaction with Rh. The interaction mechanism probably involves an electronic effect transmitted between H and N z through the Rh metal centers, causing increased surface coverage of Rh-N2 and possibly an increase in the N-N vibrational mode extinction coefficient.

Keywords: Aluminum oxide; Dihydrogen; Dinitrogen; Infrared spectroscopy; Supported rhodium; Titanium dioxide

1. Introduction Recent work in these laboratories has suggested that transmission Fourier transform infrared spectroscopy is an excellent analytical technique for determining the thermodynamic parameters A H and AS of adsorption for the interaction of small molecules with supported transition-metal films. In this endeavor the integrated areas of infrared bands corresponding to surface species such as Rh-N2 and R h - H are used to estimate the surface coverages of those species; the surface coverages are then employed to calculate interaction equilibrium constants as a function of temperature, which can, in turn, be used to calculate A H and * Corresponding author. 0927-7757/95/$09.50 © 1995Elsevier ScienceB.V. All rights reserved SSDI 0927-7757(95)03300-9

AS of adsorption with the van't Hoff equation [1,2]. Thus far, studies have been confined to the interaction of dinitrogen or dihydrogen, at pressures of up to 8000Torr and at near-ambient temperatures, with supported Rh films [ 1-5]. For this work a specially designed high-pressure infrared-cell reactor has been employed; the design of the reactor and parameters of its performance have been discussed elsewhere [-3,4]. In the current work we report the extension of our high-pressure studies to the support effects on the interaction of N 2 and Hz with Rh films, and, in particular, to the support effects on co-adsorbed R h - N 2 and Rh-H. It will be demonstrated that low coverages of R h - H cause dramatic enhancements of the intensity of the N - N stretching infrared band for R ~ N 2 on both A1203 and TiO2 supports.

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Z. Pei et al./Colloids SurJaces A: Physicochem. Eng. Aspects 105 (1995) 79-86

2. Experimental Films of supported Rh were prepared by spraying a slurry of RhC13"3H20 (Johnson Mathey), A1203 (Degussa Alumninumoxid C, 100 m 2 g 1) or TiO 2 (Degussa Titandioxid P25, 50 m 2 g 1), spectroscopic grade acetone, and distilled, deionized water onto a 25 mm CaF2 infrared window held at a temperature of 353 K. Following evaporation of the solvents, the window contained approximately 4.4 mg cm -1 of supported RhC13.3H20 at a loading of 2.2% Rh (w/w). The window was then positioned in a high-pressure infrared cell reactor [3], which was attached to a stainless steel highvacuum/high-pressure manifold, and the reactor evacuated at 10 6Torr for 6 h at 298 K. The sample was exposed to cycles of 02 at 100 Torr and 563 K for 30 min, H 2 at 100 Torr and 473 K for 5, 10, 10, and 20 min, and then evacuated at 298 K for 4 h. Past experience had demonstrated that such sample pretreatment produced infraredtransparent Rh/A1203 or Rh/TiO 2 films of high quality. Temperature and pressure ranges of 298-350 K and 0-8000 Tort, respectively, were employed for determination of adsorption thermodynamic parameters. At each new.pressure of N z o r H 2 the supported Rh film was exposed to the gas at 298 K for 2 h and then heated to various prescribed temperatures at each of which a 5 min equilibration time was allowed before infrared data were collected. In some experiments N 2 and H2 were allowed to interact with the supported Rh film simultaneously; in these, a 15 rain equilibration time was allowed for each new condition before accumulation of infrared data. The equilibration times stated above are sufficient to produce no further changes in the infrared spectra. The N2 and H2 used in these experiments were obtained from Air Products with stated purities of 99.9995% and 99.995%, respectively. Catalytic converters containing Rh supported on AlzO3 (5% w/w) held at 373 K were used in the high pressure manifold to purify the gases further. Small quantities of CO in a high-pressure gas cause interference in the experimentation due to the high affinity of supported Rh for CO. The catalytic converters serve to remove the CO by oxidation to CO2,

which is then trapped at low temperature (77 K for H2; 158 K for N 2 ) . The infrared spectra were obtained with an IBM 32 FTIR spectrometer operated at 2 cm -1 resolution with 500 scans being accumulated over 7.5 min for each spectrum. The spectrum for the pretreated supported Rh film served as the reference. Pressure measurements were made with an MKS Baratron capacitance manometer.

3. Results and discussion IR spectra corresponding to the interaction of N2 at approximately 8000 Torr with 2.2% (w/w) Rh/AI203 and Rh/TiO2 at 298 K are shown in Fig. la and lb, respectively. Wang and Yates had reported the infrared spectrum of N 2 interacting with Rh/A1203 at temperatures and pressures in the regimes of 90-273 K and of up to 218 Torr, respectively [6]. They assigned an intense band at 2258 cm -1 to the N - N stretching mode for a Rh N 2 surface species and a weak band at 2331 cm -1 to physisorbed N2 on A1203. Work in these laboratories at high pressures and 298 K verified these assignments [-1,3,4] for Rh/AI203, although the frequency of the band for the R h - N 2 species in the current work occurred at 2248 cm-1 when the supported Rh films had been exposed t0 N 2 alone. The data in Fig. 1 indicate either that the concentration of Rh-N2 on Rh/TiO2 is greater than that for Rh/AI203, or that the extinction coefficient for the N - N stretching mode band is greater for Rh/TiO2, or, possibly, that both phenomena are operable. In any case, there is obviously a significant support effect on the R h - N 2 surface sites, for the integrated area of the 2248 c m - 1 band for R h / T i O / i s larger than that for Rh/AI203 by a factor of 1.23 . On the other hand, the band for physisorbed N 2 on the support at 2331 cm -~ does not seem to be significantly affected by the nature of the support material. For both supports the interaction of N 2 with Rh is reversible and non-dissociative, as evidenced by the fact that both bands vanish immediately upon evacuation and reappear upon subsequent exposure to N 2 with no change in frequency or intensity at a given pressure.

Z. Pei et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 79-86

81

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I 2400

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I

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I

I-I

2P00

WAVENUMBERS

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I-I 2000

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( c m -1)

Fig. 1. Infrared spectra for the interaction o f N 2 at 8000 Torr with prereduced 2.2% (a) Rh/A120 3 and (b) Rh/TiO2 films (4.4 mg crn -2) at 298 K.

In contrast, the infrared spectra shown in Fig. 2 for the interaction of H 2 with supported Rh display the opposite effect, namely the band at 2013 cm-1 has a larger integrated area when A1203 is the support compared with TiO2 as support. Prior work in these laboratories [5] has demonstrated that the 2013 cm -1 band may be assigned to the stretching mode for a R h - H surface species. The band at 1618cm -1 corresponds to the bending mode for water adsorbed on the support. These bands also cycle with evacuation/re-exposure cycles, although longer equilibration times are required than for the R h - N 2 species. We have postulated that H2 dissociates on Rh to produce R h - H and that some H spills over to the support to react with O H groups to produce H 2 0 [ 5 ] . The fact that the water band at 1618 cm -1 is much more intense for the TiO 2 support than for the A1203 support, while the opposite is true for the R h - H band at 2013 cm-1, probably indicates that the spillover process is controlled by the support interaction with Rh. A second possibility is that

the extinction coefficients for the bands are affected by the support-Rh interaction in opposite ways; this seems less probable than an explanation based upon different coverages caused by the two supports. From the infrared data for the Rh-N2 and R h - H surface species, surface coverages can be obtained which may be used to determine adsorption equilibrium constants as a function of temperature and then A H and AS of adsorption. Fig. 3 and Fig. 4 illustrate how this is accomplished for the N2/Rh/TiO 2 system. The expression for the equilibrium interaction of N 2 with the Rh sites is shown in Eq. (1) below: Nz(g ) + Rh ~ R h - N 2

( 1)

The equilibrium constant is given by K = O(1 - O ) - 1 p - 1 , where O is the surface coverage of N 2 o n Rh sites, 1 - O is the fraction of unoccupied sites, and P is the pressure of N 2. If it is assumed that the N - N stretching mode extinction coefficient is independent of pressure, then O at

Z. Pei et aL/Colloids Surfaces A." Physicochem. Eng. Aspects 105 (1995) 79-86

82

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Fig. 2. Infrared spectra for the interaction of H 2 at 8000 Torr with prereduced 2.2% (a) Rh/AI203 and (b) Rh/TiO 2 films (4.4 mg cm 2) at 298 K.

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Fig. 4. Van't Hoff plot for the interaction of N 2 with 2.2% Rh/TiO2.

any pressure may be determined from the integrated area of the 2248 c m - t band, A, by dividing that area by the integrated area of the band at infinite pressure, Areax. A plot of A t versus P

can be used to determine

Amaxfrom Eq. (2):

A -1 = K 1Amlxp-l+ ama -1x

(2)

so that a plot of O-1 versus P-1 is linear with

Z. Pei et al./Colloids SurJdces A: Physicochem. Eng. Aspects 105 (1995) 79-86

83

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slope K-1. Fig. 3 shows a series of such plots at various temperatures. It is noticeable that linearity is poor at the highest temperature (334 K) and that the lines for the higher temperatures have intercepts slightly greater than O - 1 = 1. This is indicative of lower infrared transmission leading to lower signal/noise ratios and to greater errors in band integration at the higher temperatures. Nevertheless, a plot of In K versus T -1 is linear over the entire temperature range (Fig. 4), and the van't Hoff equation can be used to determine AH and AS of adsorption: lnK= -R-IT-1AH+R-1AS

(3)

The values obtained from Fig. 4 were A H = - 5 . 0 + 0 . 1 kcal mol ~ and A S = - 2 3 . 7 _ + 1.0 cal mol-1 K 1. The estimated error for the AS value is higher than that for AH since data over a narrow temperature range must be used in extrapolating to give the intercept of the van't Hoff plot. These values for AH and AS for N2/Rh/Ti02, which are average values over the coverage range O = 0.2-0.7, can be compared to those of - 2 . 2 kcal mo1-1

cm

2) at 298 K and

and - l l . 9 c a l mol 1 K -1, respectively, for N2/Rh/AI203 determined in the same manner [7]. Wang and Yates had previously reported a AH of - 2.24 kcal m o l - 1 from their low-temperature/lowpressure data for the N2/Rh/Alz03 system [-6]. Thus it is evident that the interaction of N 2 with Rh/TiO2 is stronger than that of N 2 with Rh/AI203. This could be seen both in increased surface coverage and in increased extinction coefficient in causing the N-intensity trend observed in Fig. 1. It should be noted that both catalysts were treated under the same conditions at temperatures well below those which normally induce TiO2 to become an SMSI (strong metal support interaction) support [8,9]. Fig. 5 and Fig. 6 show the infrared spectra produced upon exposure of Rh/AI203 and Rh/TiO2, respectively, t o N 2 at 1000 Torr and then to H2 at increasing pressures up to 1000 Torr. The expected result was that the R h - H surface species would displace the Rh-N2 species since earlier infrared studies in these laboratories had led to an average AH value for Rh-H/A1203 in the coverage range

Z. Pei et aL /Colloids Surfaces A." Physicochem. Eng. Aspects 105 (1995) 79-86

84

-]-0.001

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Fig. 6. Infrared spectra for the interaction of N 2 at 1000 Torr with a prereduced 2.2% Rh/TiO2 film (4.4 mg cm -2) at 298 K and increasing pressures of H z at (a) 0, (b) 100, (c) 200, (d) 300, (e) 500, and (f) 1000 Tort.

0 = 0 . 4 - 0 . 7 of --7.7 kcal mo1-1. Although the spectra clearly indicate that the 2013 cm -~ band for R h - H grew continuously as the pressure of Hz was increased, the band for the R h - N 2 also increased in intensity until a pressure of 200 Torr of H2 was attained; at 300Torr of H 2 , and at higher pressures, the Rh N 2 band declined in intensity as expected. The same effect was observed for both supports. The maximum factor for the enhancement of intensity for Rh/AI203 was 2.5 and occurred at a R h - H coverage of about O = 0.2; the corresponding factor for maximum intensity enhancement for Rh/TiO2 was 1.9, occurring at a R h - H O of about 0.1. The latter Rh H O was difficult to determine because of the low intensity of the 2013 cm -~ band for Rh-H/TiO2. The same type of observation resulted when increased pressures (of 3000, 4000, and 8000 Torr) of N 2 w e r e used, although higher pressures of HE were required to achieve the maximum enhancement as the N 2 pressure was increased. Nevertheless, the maximum enhancement of intensity remained constant at 2.5 for Rh/AI203 and 1.9 for Rh/TiO2, and the ORh_H

at which the Rh-N2 band intensity became a maximum remained constant at 0.2 and 0.1, respectively. Furthermore, the same results were obtained when H2 was introduced first, followed by increasing pressures of N 2. Although the frequency of the R h - H band remained essentially constant at 2013 cm -1 for both supports during these experiments, the R h - N 2 frequency did shift from 2248 cm-1 when no H2 was present to 2258 cm-1 when the maximum intensity enhancement occurred; it then remained at 2258 cm-~ a s N 2 was displaced from the surface by the R h - H species. The AH of adsorption of the Rh-N2 species in the presence of H2 at a pressure of 50 Torr was also measured for both supports. In the case of Rh/AI20 3 the value became - 4 . 6 kcal mol-~ (compared with - 2 . 2 kcal mo1-1 with no H 2 present); for Rh/TiO2 the corresponding value was - 6 . 0 kcal mol ~ ( - 5 . 0 kcal mol -a with no H2 present). Thus, in both cases, the presence of H 2 c a u s e s an enhanced interaction of N 2 with Rh. The intensity enhancement of the R h - N 2 band caused by the presence of H 2 is a very intriguing

z. Pei et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 79-86

phenomenon which could have a variety of explanations. The increased R h - N 2 band intensity could be due to either increased surface coverage by N z, or to an increased oscillator strength for the N - N vibrational mode, or to a combination of these effects. The enhanced interaction of N 2 with Rh, as evidenced by the change in A H of adsorption, could logically fit either of these postulates. Several other experiments have been performed in an effort to explain the observed intensity enhancement. In one, deuterium was used under the same conditions as H2. Exactly the same results were obtained for the R h - N 2 band intensity enhancement. This suggests that dipole coupling of the N - N and R h - H (Rh-D) oscillators is not a factor because the magnitude of the effect should be frequency dependent ( R h - H absorbs at 2013 cm -1, R h - D at 1441 cm 1 [5]). In a second, Rh/A1203 was predosed with H 2 0 at 10 -5 Torr to see if the H 2 0 forming on the supports during H 2 decomposition on Rh affected the R h - N 2 band intensity. In this case the R h - N 2 band declined in intensity relative to the support containing no H20, but the intensity enhancement was again observed when H 2 w a s subsequently added to a pressure of 8000 Torr. Thus the presence of H 2 0 on the support is not the cause of intensity enhancement. In a third experiment a very small amount of H 2 (1 Torr) was used. At this pressure the 2013 cm -~ band due to the Rh H species was not observed. Yet a small intensity enhancement (32%) of the 2248 cm -1 Rh N 2 band did occur, and the band at 1618 cm-1 due to water on the support was observed at a low intensity [7]. We believe that this observation is consistent with the formation of a "strongly bound" form of hydrogen which is not infrared active (or its band occurs at frequency below the support and CaF 2 cutoffs) which affects the R-Nz band intensity and frequency in similar manner to the weakly bound R h - H species with infrared absorption at 2013cm 1. Such a strongly bound form of hydrogen has been postulated for various supported transition metals by others [ 10,11 ], and we believe that our experiments have provided some evidence for its existence. However, the "strongly bound" form of hydrogen has generally been termed "irreversible hydrogen". This does not

85

appear to be the case for supported Rh because evacuation of the cell reactor followed by re-exposure to N 2 causes the R h - N 2 band to revert to 2248 cm-x and the intensity which it possessed before exposure to 1 Torr of H 2. Our current thinking concerning these observations is that there is an electronic interaction mechanism between H and N 2 which is transmitted through the Rh metal centers in a manner similar to the "through-bond" interaction mechanism commonly invoked in organic chemistry. Both strongly bound and weakly bound forms of hydrogen may be involved in this interaction mechanism. It is the cause of the intensity enhancement and blue frequency shift, probably via increased surface coverage of N 2 and an increased extinction coefficient for the N - N stretching mode.

4. Conclusion This work has once again demonstrated the utility of infrared spectroscopy in studying the interaction of small molecules with supported transition metals. It has illustrated the rather dramatic changes which may occur when different support materials are used, and has reviewed the use of integrated band intensities in determining thermodynamic data. The interaction of N 2 with Rh/TiO2 has been shown to be stronger than that of N2 with Rh/AI203, and the presence of H 2 causes an enhanced interaction of N2 with both catalysts.

Ackn6wledgements The authors thank Professors M.A. Vannice and J.T. Yates for the helpful suggestions concerning this work which were made at the symposium at the American Chemical Society Meeting in Washington, D. C. honoring Dr. Robert Eischens. We would also acknowledge the magnificent pioneering work concerning infrared studies of surfaces performed over the years by Dr. Eischens. Finally, we thank the Department of Energy through the Consortium for Fossil Fuel Liquefaction Science contract number DE-FC-22-93PC-93053 for partial support of this work.

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Z. Pei et al./Colloids Surfaces A: Ph),sicochem. Eng. Aapects 105 (1995) 79-86

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S.D. Worley, S.D. Worley, S.D. Worley, Phys. Chem., Phys. Chem.,

[6] H.P. Wang and J.T. Yates, J. Phys. Chem., 88 (1984) 852. [7] Z. Pei, T.H. Fang and S,D. Worley, J. Phys. Chem., 99 (1995) 3663. [8] S.J. Tauster, S.C. Fung and R.L. Garten, J. Am. Chem. Soc., 100 (1978) 170. [9] S.J. Tauster and S.C. Fung, J. Catal., 55 (1978) 29. [10J J.P. Candy, P. Fouilloux and M. Primet~ Surf. Sci., 72 (1978) 167. [11] W.N. Delgass, G.L. Haller, R. Kellerman and J.H. Lunsford, Spectroscopy in Heterogeneous Catalysis, Academic Press, New York, 1979, pp. 34, 60.