852
J . Phys. Chem. 1984,88, 852-856
Infrared Spectroscopic Study of N, Chemisorption on Rhodium Surfaces H. P. Wang and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: May 12, 1983; In Final Form: October 12, 1983)
The adsorption of N2by alumina-supported Rh surfaces has been studied by high-sensitivity infrared transmission methods. For I4N2on Rh, a single N-N absorption band at 2257 cm-' is observed at high N2 pressure (218 torr) and at temperatures below about 240 K. This chemisorbed N2 species exhibits an adsorption energy of about 2.2 kcal mol-', based on infrared studies of the equilibrium situation. Physisorbed N2 on A1203was also detected below 200 K by its infrared spectrum. Its stretching frequency (2331 cm-') is identical with that of N,(g), indicating a very weak electronic interaction. The adsorption energy for this physisorbed species is about 1.6 kcal mol-'. This work is an example of the usefulness of high-sensitivity IR techniques, coupled with multiple-scan data acquisition methods, for studying weakly absorbing surface species in detail. The work also illustrates the utility of IR cells designed for working at cryogenic sample temperatures and capable of studying support and metal adsorption simultaneously.
Introduction Rh is a very useful transition metal and is widely used in catalytic applications. Chemisorbed CO on supported Rh has been studied extensively in the past, and the infrared results are briefly reviewed in previous It is generally agreed that, for Rh supported on A1203,Rh crystallite sites which adsorb both linear and bridged C O molecules coexist with the isolated Rh sites (probably Rh' sites) which adsorb two COSper Rh atom, giving a characteristic doublet in the infrared spectrum. N, is isoelectronic with CO and has similar molecular orbitak6 Despite the similarity between the two molecules, the infrared study of the adsorption of N2on supported metals has been largely neglected. However, following the pioneering work of Eischens and Jacknow in which N2 adsorption was investigated on Si0,-supported Ni by the infrared method,' it has been established that N z is not strongly adsorbed by the supported group 8 transition metals, such as Ni/Si02,7-9Pd/A1203,9Pt/Si02>10 Rh/ Si02," and Ru/Al2O3.I2 In the systems investigated, it was found that either low temperature and/or high pressure of N2 must be employed to maintain a measurable surface ~ o v e r a g e . ~ It has been generally suggested that the chemisorbed N 2 molecule interacts with the metal surface through an end-on configuration,l-1°J2but also a lying-down configuration is discussed in the 1iterat~re.I~The infrared spectrum of adsorbed N 2 on supported Rh has not been extensively studied; the work of Borod'ko and Lyutov on Rh/Si02 was unable to determine the structure of the N 2 adsorbate molecule." The aim of the work reported here is to explore for N2 the extension of the analogy to the CO-Rh-Al203 system, and to more thoroughly investigate the interaction of N 2 with Rh/A1203 by using enhanced sensitivity I R methods to be described. Specifically, we examine the adsorption of the three isotopic species of N2 on Rh/AI2O3 and make an estimate of the heat of adsorption (1) J. T. Yates, Jr., T. M. Duncan, S.D. Worley, and R. W. Vaughan, J . Chem. Phys., 70, 1219 (1979). (2) J. T. Yates, Jr., S.D. Worley, T. M. Duncan, and R. W. Vaughan, J. Ckem. Phys., 70, 1225 (1979). (3) J. T. Yates, Jr., T. M. Duncan, and R. W. Vaughan, J . Chem. Phys., 71, 3908 (1979). (4) J. T. Yates, Jr., and R. R. Cavanagh, (Proceedings of 4th International
Conference on Solid Surfaces, Supplement a la Revue, Le Vide) J. French Vacuum SOC.,750 (1980). (5) R. R. Cavanagh and J. T. Yates, Jr., J. Chem. Phys., 74,4150 (1981). (6) W. L. Jorgensen and L. Salem, "The Organic Chemist's Book of Orbitals", Academic Press, New York, 1973. (7) R. P. Eischens and J. Jacknow, Proc. I n f . Congr. Card., 3rd. 627 (1965). (8) T. Nakata and S.Matsushita, J . Catal., 4, 631 (1965). (9) R. VanHardeveld and A. VanMonfwrt, Surf. Sci., 4, 396 (1966). (10) T. A. Egerton and N. Sheppard, J . Chem. Soc., Faraday Trans. I , 70. 1357 - - - (1974). (11) Y;. G.'Borod'ko and V. S.Lyutov, Kinet. Karal., 12, 238 (1971). (12) K. Aika, H. Midorikawa, and A. Ozaki, J . Caral., 78, 147 (1982). (13) J. Chatt, J. R.Dilworth, and R. L. Richards, Chem. Reu., 78, 589 (1978).
-.
0022-3654/84/2088-0852$0 1.50/0
of N 2 on Rh. We also examine the physical adsorption of N, on A1203using infrared spectroscopic methods. Experimental Section A double-beam ratio recording infrared spectrometer (Perkin-Elmer Model 580B) was used in this work. The spectrometer is equipped for fully computerized data treatment. Measurement capability below the absorbance level can be achieved with good signal-to-noise and resolution by using multiple-scan data accumulation-average methods [e+, noise/signal E 5% for signal of order 0.01 absorbance unit (A); accumulated scanning time = 4.0 s/cm-' at resolution = 5.3 cm-' in this work]. All spectra in this work have been treated with a 19-point smoothing function. With these advantages, we may study very weak infrared absorption bands. To faciliate this objective, a purging procedure as well as a reference cell were used to decrease but not completely eliminate the atmospheric C 0 2 gas interference within the spectrometer optical path. The infrared cell used in these experiments is shown in Figure 1. It affords good temperature regulation between 80 and 310 K which is attained by the use of cooled nitrogen gas and control of its flow rate through the copper sample support assembly. The sample is mounted in a copper ring within the cell body. The preparation of supported Rh on Al,03 has been described previ~usly.~ Briefly, a mixture of Rh1"C13 dissolved in H,O, high area AlZO3, and acetone is sprayed with an atomizer onto a CaF2 sample plate maintained at 350 K to flash evaporate solvents. Here we have modified the spraying procedure by masking one side of the CaF2 plate with a metal baffle while one half of the CaF2 plate is sprayed with the Rh"' preparation. In the reverse procedure, a suspension of the Rh-free A1203mixture was sprayed onto the other side of the CaFz plate (see Figure 1). Following reduction in H2(g), and evacuation of the cell, the sample was placed in the IR spectrometer. A moveable cell holder was used to position the sample in the infrared light beam in order to transmit through one side or the other of the divided sample. This "half-plate" design ensures identical experimental treatment conditions for both Rh/A1203 and pure A120,. This permits not only the comparison of the infrared spectrum of N2adsorption on the Rh surface and on the A1203 support but also a proper background fitting for small absorbance bands obtained in this case. The A1203-supportedRh was produced by hydrogen reduction The final "density" at 450 K by a procedure described previ~usly.~ of Rh on the A1203support was 0.26 X g/cm2 at the 2.2 wt 7% loading employed here. Total "densities" (Rh + A1203)ranged g/cm2. The A1203"density" on to 11.8 X from 9.9 X the control side of the sample plate was typically 9.4 X g/cm2. The reduced Rh sample was prepared and studied in an ultrahigh vacuum stainless steel infrared cell having CaF2 windows (Figure 1). A grease-free bakeable stainless steel ultrahigh vacuum system described previously5equipped with a liquid nitrogen cooled $3 1984 American Chemical Society
IR Study of N z Chemisorption on Rh
The Journal of Physical Chemistry, Vol. 88, No. 5 , 1984 853
U H V CELL FOR INFRARED SPECTROSCOPY
PHYSISORPTION
OF ADSORBED SPECIES
r
I
OF
N, ON Rh/AI,O,
I
I
0002=A
i -M IC Resolution = 5 3crn-l
iCoF2 i
Gas Handlina s y stern-
Windows
+
OMS
105
Figure 1. Ultrahigh vacuum cell for infrared spectroscopy of adsorbed
species. ,
ADSORPTION
OF
,
'
'
'
I
44.4
"
j
N2 ON Rh/A1203
6.4
0.85
,nl __
,lo>,-
1
4000
,
,
,
,
1
,
3000
,
,
,
1
1
I-. ,
1
,
,_____ 1 , , 2000
,
1
1
1500
,
I
1200
Wovenumber ( c m - ' 1
Figure 2. Broad-scan IR spectrum showing N2(ads)on Rh/A1,03. The dashed spectrum for full coverage CO on Rh is shown for comparison with the intensity of impurity CO present prior to N2 adsorption. This comparison indicates that chemisorbed impurity CO is present at a small surface concentration on the Rh.
2350
2300
2250
2200
Wovenumber (cm-' 1 Figure 3. Infrared spectra for physisorbed N2 on Rh/A120,.
Results Adsorption of N 2 on Rh/AI2O3and A1203. Representative infrared spectra showing the comparison of the spectra of full coverage CO on 2.2% Rh/A1203at 309 K and the adsorption of I4N2to full coverage at low temperature ( T = 90 K) and high pressure (P = 218 torr) are shown in Figure 2. It can be seen in comparison to CO(ads) that the relative intensity of the adsorbed I4N2infrared absorbance is very small; the observed 14N2 absorbance is below 0.02 for the fully covered surface. The spectral feature due to impurity CO(ads) on our Rh samples is
of very low intensity compared to that observed for a full monolayer of chemisorbed CO. On this basis, we believe that only a small fraction of Rh sites are occupied by the adsorbed impurity CO, and that these impurity effects were not of importance in this study of N2(ads). It should be noted that the A1203contains significant quantities of surface hydroxyl groups, as expected. In order to distinguish the nature of the adsorption of N2 on Rh sites from the nature of the molecules on the A1203support, the sample was cooled to 90 K and a series of comparative experiments were conducted on pure A1203and on Rh on A1203. The use of a divided sample ensures that the treatment of the A1203and the Rh/A1203 sample were identical. Figure 3 and Figure 4 show the spectral development at 90 K as the pressure of I4N2was increased. The single band at 2331 cm-l for both A1203and Rh/AlZ0, is assigned to I4N2physical adsorption on A1,0,. The frequency observed is the same as the stretching frequency of I4N2gas.l5 A typical isotherm for l4N2physical adsorption on the A1203support is shown in the upper corner of Figure 4. Each point corresponds to the absorbance at 2331 cm-' taken from the labeled infrared spectra a-f in Figure 4. It was found that saturation 14N2coverage, based on IR intensities, was achieved at 90 K for I4N2 pressures above -44 torr. The broad band at 2257 cm-' observed only on Rh is due to chemisorbed I4N2on Rh. The presence of chemisorbed I4N2on Rh is confirmed by the high wavenumber shift (Au = -74 cm-') from the N, gas-phase stretching vibrational frequency and also by the absence of this absorption band on pure A1203. All three N2isotopic species have been studied on the Rh/A1203 surface at three levels of Rh loading on A1203(IO, 2.2, and 0.5 wt%). A summary of results obtained in these studies is shown
(14) E.K. Plyler, A. Danti, L. R. Blaine, and E. D. Tidwell, J . Res. Nail. Bur. Stand., Secr. A , G4, 29 (1960).
( 1 5 ) H. S.Szymanski, 'Raman Spectroscopy", Vol. 2, Plenum Press, New York/London, 1970.
zeolite pump and an ion pump was employed. Careful bakeout procedures were used to eliminate hydrocarbon impurities that tend to be present on unbaked walls of stainless steel apparatus. In these experiments, following bakeout, the background pressure was below IO-* torr; the major residual gases in the system at this pressure were CO and H2, as measured with a quadrupole mass spectrometer. The IR spectrometer wavenumber scale was calibrated with the CO(g) absorption s p e c t r ~ m . ' ~The I4NlSN(99% enriched) and lsNI5N (96% enriched) gas were obtained from Prochem, Inc. and the Matheson research gas grade CO (99.99%), N2(99.995%), and O2 (99.99%), all in glass breakseal flasks, were used without further purification. Hydrogen gas for reduction was 99.9995% pure (obtained from Matheson Co.) and was dispensed into our system from a compressed gas cylinder.
The Journal of Physical Chemistry, Vol. 88, No. 5, '984
854
CHEMISORPTION
OF
Wang and Yates CHEMISORPTION OF
Nz ON Rh/AlzO,
N,
ON Rh/Al,O,
I
2350
2300 2250 2200 Wovenumber (cm-' )
Figure 4. Infrared spectra for chemisorbed and physisorbed N2 on
Rh/A1203. 2350 N E
2300
N V I B R A T I O N A L FREQUENCY OF ADSORBED N, I S O T O P E S I
I
I
2250
2200
2150
Wovenumber ( c m - l )
Figure 6. Temperature dependence of physisorption and chemisorption of N2 on Rh/AI2O3.
I
TEMPERATURE DEPENDENCE
OF ABSORBANCE FOR
N, on Rh/AI,O,
PHYSISORBED
[corrected for background s h i f t )
-
;
IO %
2 2% 2 2% 2 290 5%
; +
0 5 %
I
I ,,
2300
2250
I , 2200
Wovenumber ( c m - l )
Figure 5. Comparison of infrared spectrum of isotopic N2 species physisorbed and chemisorbed on Rh/AI20, in multiple experiments.
-
0 -. I
I
1
/
1
1
I
90
113
136
155
173
189
204
Temperoture ( K )
~
x ,O-'
1
I
1
I
1
219'
233
245
260
273
-
Figure 7. Temperaturedependence of absorbance for physisorbed N2on
in Figure 5 , along with estimates of the error possible in wavenumber measurements for the weakly absorbing species. No chemisorbed N2 was observed on the 0.5% Rh surfaces. This observation suggests that the chemisorbed species are only formed on metallic Rh sites which become dominant only in higher loadings.5 In addition, by comparison of relative intensities of the chemisorbed and physisorbed N2 for the three loadings, it is apparent that this ratio (chemisorbed/physisorbed) increases for increased Rh loading, as the surface area of the metallic Rh sites increases. Temperature Effect on N 2 Adsorption. Figure 6 shows the spectral development for 14N2adsorption at low temperature ( T I189 K). It can be seen that the physically adsorbed I4N2band at 2331 cm-' develops significantly only at lower temperature,
Rh/AI2O3(corrected for background shift). whereas the chemisorbed 14N2band is present at appreciable intensity under all conditions shown. A second type of experiment was carried out at a fixed wavenumber of 2331 or 2256 cm-'. The sample was first cooled to 90 K and then slowly warmed in the presence of N2gas while the infrared spectrum was continuously recorded. The temperature dependences of absorbance for adsorbed I4N2on Rh/AI2O3at Y = 2331 cm-' and at Y = 2256 cm-' are shown in Figure 7 and Figure 8, respectively. Data for both experiments were corrected for a very reproducible temperature-dependent background intensity shift corresponding approximately to the magnitude of the effect shown in Figures 7 and 8. This reversible background shift
IR Study of N2 Chemisorption on Rh ,
,
,
I
,
,
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984 855
,
,
,
I
,
I
,
INFLUENCE
'
TEMPERATURE DEPENDENCE O F ABSORBANCE FOR
N e o n Rh/AI,O,
CHEMISORBED
2036
for bockground s h i f t )
(corrected
O F N, A D S O R P ~ I O N
ON THE I R S P E C T R U M O F L O W COVERAGE CO ON Rh/A1,03
T=90K
I 0.01 = A
d IC Resolution
I
90
113
I
1
1
155
136
173
1
I
IE9
204
Temperature ( K )
1
219
.I
233
I
1
245
260
=5
3cm-l
273
\
Figure 8. Temperature dependence of absorbance for chemisorbed N2 on Rh/A120, (corrected for background shift). INFRARED SPECTRUM OF N p ADSORBED ON OXIDIZED Rh/AI,O,
I " " " 2200
2100
2000
1900
1800
Wovenumber ( c m - ' )
Figure 10. Influence of N, adsorption on the IR spectrum of low coverage CO on Rh/AI20,. Chemisorbed
PN2(Torr)
T(K) -
218
90
218
148
218
183
218
20 I
The Interaction of N2 and Low Coverage CO. Figure 10 shows the interaction of adsorbed 14N2with a low coverage of chemisorbed CO on Rh/A120,. As the pressure of N2 was increased at 90 K, the bands due to the linear and bridged-CO molecules on Rh shift to lower frequency by a small amount.
Discussion Physical Adsorption of N 2 on A1203. The free nitrogen molecule is infrared inactive because of its symmetry. However, a band corresponding to the vibration of 14N2does occur in the Raman spectrum at 2331 cm-1.15Perturbation of the molecule by the electric field at the surface permits the appearance of an infrared absorption band. The weak absorption band observed in this work at 233 1 cm-l is assigned to I4N2physically adsorbed on A1203. These species begin to appear at 2331 cm-l when the sample is cooled to -200 K in the presence of N, at 218 torr. If the extinction coefficient, cN2, of physically adsorbed N2 on A1203is assumed to be independent of N2 coverage, a thermodynamic estimate of the energetics of adsorption can be made. The lack of a coverage dependence of the N=N stretching frequency suggests that intermolecular interactions are small, and that this assumption is good for physically adsorbed N2on A1203. Therefore, for the adsorption state change N2(gas)
2350
2300
2250
2200
+ site
N2(ad, ON,), &Z
(1)
Wavenumber (crn-l) Figure 9. Adsorption of N2 on oxidized Rh/AI20,.
was thoroughly investigated by studies made at wavenumbers near the N2absorption bands and may be due to temperature-dependent light-scattering effects. Chemisorption of N2 on Oxidized Rh. Both the chemisorbed and physically adsorbed N2 on the Rh/AI20, were completely torr at 309 K. O2 was desorbed by pumping down to 2 X admitted at 309 K for 5 min to a pressure of 103 torr to oxidize the Rh surface. Upon readmitting 218 torr of I4N2(g) to the evacuated cell two bands due to chemisorbed 14N2were produced upon cooling. These bands are shifted to higher wavenumbers in comparison to the case for unoxidized Rh. Figure 9 shows a series of the infrared spectra for N 2 adsorbed on oxidized Rh/ A120, at different temperatures; the two new bands were formed at 2303 and 2270 cm-I, and the chemisorbed N, band at 2257 cm-' is essentially absent.
Assuming
Kq
=
AN,/AN~'"~~ -
AH = -R d(ln Kq)/d(l/7')
(4)
(5)
The absorbance of N2 physically adsorbed on A1203is shown as a function of temperature in Figure 7 . The measured value of AI? N -1.61 kcal/mol is in satisfactory agreement with earlier results,16 where a value of AH E -2 kcal/mol was determined (16) S. Brunauer, P. H. Emmett, and E. Teller, J . A m . Chem. Soc., 60, 309 (1938).
856
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984
by volumetric adsorption methods. The curve of the plot (inset, Figure 7) on the left-hand end could be due to a systematic error that appears at low N2 coverages or to site heterogeneity on A1203. Chemisorption of N 2 on Rh. An estimate of the heat of N 2 chemisorption on Rh is shown in Figure 8. The measured value of A H -2.24 kcal/mol is much smaller than the heat of adsorption for CO on Rh (-30 kcal/mol), despite the significant frequency shift for N2(ads). These data (low heat, large frequency shift) suggest that N 2 is very weakly chemisorbed on Rh. Similarly, smaller shifts from the free molecule vibrational frequency have been observed for carbonyl complexes than for N 2 complexes,” despite stronger M-CO bonding. The comparison of this result for N 2 chemisorption on Rh with the work of Eischens and Jacknow’ concerned with N 2 chemisorption on Ni is enlightening. For the N,/Ni case, a stronger form of chemisorption is involved in which a terminal structure is postulated, Ni-NEN. For this species, the decrease (from gas-phase N2) in stretching frequency is 129 cm-I, compared to an observed decrease of 74 cm-’ for I4N2/Rh. Most importantly, for the more strongly bound Ni-N2 species, the extinction coefficient is very high, whereas in the N,/Rh case a low extinction coefficient is found, assuming equal saturation coverages in both cases. Thus, the lower frequency shift, the lower heat of adsorption, and the lower extinction coefficient for N2/Rh compared to N,/Ni are consistant with a different type of chemisorption bonding in the two cases. Possibly the involvement of different levels of charge transfer is the major distinguishing factor in the comparison of Rh and Ni interactions with N2. In the system investigated, the observed shifted band position for the chemisorption of N 2 on oxidized Rh (see Figure 9) would suggest that N, is more likely chemisorbed on Rh crystallite sites than on the isolated Rh’ site. Rh’ sites are extensively present according to CO chemisorption measurements on the oxidized Rh,5 and oxidation primarily modifies the crystalline Rh sites. These results are also consistent with intensity measurements of the chemisorbed species on Rh preparations at different Rh loadings. Considerations on the Molecular Orientation of Chemisorbed N 2 on Rh. The IR spectroscopic examination using three isotopic N 2 molecules was unable to distinguish the mode of bonding of N 2 chemisorbed on the Rh metal surface due to the high bandwidths exhibited by adsorbed N2. Whether the broad band of chemisorbed N 2 corresponds to the sequential occupation of distinct binding states and/or to changing interactions within a homt:geneous adlayer is not clear. It is likely that lifetime broadening is also a dominant factor here. Both the end-on and lying-down configurations have been considered. The lying-down
I
end - o n
lying down
species are in principle not accessible to IR spectroscopy. The restriction on an extended metal surface is that only those vibrations will be infrared active that have a component of their oscillating dipole moment perpendicular to the metal surface.Isa This restriction may be relaxed for small metal particles.1sb Thus, in the case of N2 chemisorbed on Rh, we are dealing with an infrared intensity which is low compared to that found for CO. This result is consistent with either a “lying-down” N 2 species or more likely with a linear N 2 species of low inherent extinction coefficient. The low heat of adsorption of chemisorbed N 2 on Rh combined with the shift of vN2 by 74 cm-’ (compared to N, on Ni where AvN2 = 129 cm-’) is also consistent with either form of chemisorbed N2. N 2 Physisorption on A120,. The observation of essentially no shift between the gas-phase N 2 frequency and the physisorbedphase N2 frequency for all three N2 isotopes suggests a rather weak
Wang and Yates and nonspecific form of bonding for physisorbed N2 It is probable that physisorbed N2 on Al,O, is not structurally oriented. This would be consistent with the absence of a shift from the gas-phase N2 stretching frequency for physisorbed N2 on Al2O3. This general force field is responsible for the observation of IR intensity for the molecule and produces a finite value of dw/dr for the species, Chemisorption of N2 on Oxidized Rh. The stretching frequency of chemisorbed N 2 on the oxidized Rh surface is observed to shift (-50 cm-’) to higher frequency (v = 2303 cm-.’) compared to chemisorbed N2 on Rh. It is likely that the back-donation from metals to the orbitals of N 2 is significantly lowered by an “electron-withdrawing” oxygen ligand effect. The stretching frequency of the chemisorbed N, then rises to a higher value. Similarly, an infrared matrix isolation study of Pt(O,)(N,) has been reported.Ig The stretching frequency of chemisorbed N, shifted to higher frequency by -60 cm-’ due to oxygen complexation. In addition to the band at v = 2303 cm-I, a weaker band at v = 2270 cm-’ is also observed. These observations of shifted N, bands on heavily oxidized Rh surfaces suggest that the original chemisorbed N, species interact with metallic Rh sites to give the 2257-cm-’ species rather than with Rh’ sites that may also be present in these preparations before oxidation by 02(g). Influence of N2 Adsorption on the Spectrum of Low Coverage CO. The adsorbed N 2 on Rh/Al20, at low temperature causes the stretching vibrational frequency of the chemisorbed C O on Rh to shift to lower frequency by a small amount as the pressure of N 2 is increased (Figure 10). We cannot determine the exact origin of this effect. It may be due to at least two possibilities: (1) The physical adsorption of N, around chemisorbed C O species may produce a “matrix effect”, in which induced dynamic dipole effects in the matrix cause small downward shifts in frequency of the C O oscillators. Such effects are definitely present when CO condenses around chemisorbed CO species.20 (2) Inductive effects between chemisorbed N2 and chemisorbed C O on Rh may exist. Here electron-withdrawing effects of chemisorbed N 2 on chemisorbed CO would be invoked. Summary of Results. The following conclusions have been reached regarding the interaction of N, with Rh supported on A1203. 1. A weakly bound molecular N 2 state is adsorbed on Rh with a heat of adsorption of the order of 2.2 kcal mol-’. For 14N14N an N s N stretching frequency of 2257 cm-’ is observed. 2. The mode of bonding of the N 2 species to Rh cannot be ascertained on the basis of studies of the N-N stretching frequency for the three isotopic species studied. Either end-on or lying-down configurations are possible. 3. The N, is adsorbed on metallic Rh sites rather than Rh’ sites. 4. N 2 is weakly chemisorbed by Rh and, in comparison to N2/Ni, exhibits a much lower extinction coefficient in the infrared. 5 . Physisorbed I4N2on A1203was also investigated in this work. Its frequency (2331 cm-’) is identical with the gas-phase frequency, suggesting very weak electronic interaction. The heat of adsorption of this species is 1.6 kcal mol-’, in agreement with the work of others. 6. The work reported here illustrates how high-sensitivity IR spectroscopy can be employed, along with cryogenic cooling, to detect and study weakly bound adsorbed species which exhibit low intensities in the IR.
-
Acknowledgment. The authors acknowledge with pleasure the support of this work by the 3M Science Research Laboratory and 3M Central Research Laboratories in a research grant, as well as useful discussions with Dr. Allen Sidle of the 3M Laboratories. Registry No. Nitrogen, 7727-37-9; rhodium, 7440-16-6; alumina, 1344-28-1.
(17) A. D. Allen, R. 0. Harris, B. R. Loescher, J. R. Stevens, and R. N. Whiteley, Chem. Reu., 73, 11 (1973). (18) (a) H. A. Pearce and N. Sheppard, Surf. Sci., 59, 205 (1976); (b) R. G. Greenler, D. R. Snider, D. Witt, and R. S. Sorbello, ibid., 118, 415 ( 1982).
(19) G. A. Ozin and W. E. Klotzbucher, J . Am. Chem. Soc., 97, 3965 (1975). (20) J. T. Yates, Jr., and G. A. Haller, to be submitted for publication.
