AI,O,

J . Phys. Chem. 1985, 89, 5840-5845

5840

activation energy converted in translational energy is given by26a TPS = T Ps + T Ps + T Ps =

that the reason for this behavior is the low value of the vibrational frequencies in the transition state of this step, 7. It appears as well that the high frequencies in the reactant structure favor a fast rate.23 Thus, in reactions where rearrangements occur before groups are lost, Le., in multiple-step processes, it is difficult to perform any RRKM calculation giving reliable information without having an accurate knowledge of the potential surface, and the related parameters. Kinetic Energy Release. In a unimolecular dissociation, the kinetic energy release (KZR) may have two contribution^:^^ Te, the KER due to a potential energy barrier (reverse activation energy), and T*,the KER associated with the nonfixed energy, or excess energy. Baer et aI.l3 have obtained at a photon energy of 13.6 eV (3.8 eV above the products 4) an average kinetic energy release of 0.74 eV, which corresponds to 19.5% of the total energy. A statistical distribution of the rvailable energy among the vibrational, rotational, and translational degrees of freedom can be calculated by25

X

Downloaded by NANYANG TECHNOLOGICAL UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: December 1, 1985 | doi: 10.1021/j100272a053

+ k T ( R - 1 ) / 2 + C h v , [ e x p ( h v , / k T )- 11-'

+ (UyPSURC)2 + (24:s

RC

1

(4)

where PS refers to three independent products separation motions, and R C refers to the direction of the reaction coordinate in the transition-state geometry. The uRCvector corresponds to the mass-weighted atomic displacements vector associated with the imaginary frequency of the transition state. By this technique, the calculated relation between F and the reverse activation energy is 47%, at 13.6 eV; the available energy for the products 4 is 2.41 eV, where 0.51 eV is the reverse activation energy and 1.90 eV is the nonfixed energy. The translational energy coming from the nonfixed energy can be estimated by partitioning the energy among all degrees of freedom.26b The three translational degrees would mean a fraction 3/N, N being the number of internal degrees of freedom of the reactant. In this way, the global translational energy is 0.41 eV, 17.1% of the available energy, which agrees well with the experimental relation, 19.5%. The absolute value is lower than the experimental value, but it is probably due to the fact that our reverse activation energy is too low, since the relative energy of the products seems to be overOn the other hand, in this reaction the proportion of reverse activation energy converted in kinetic energy calculated by the Derrick method is probably exaggerated. The two error factors compensate, giving almost the experimental result. In summary, the M I N D 0 / 3 method gives a good description of the reaction C6H@H+' C5H6+' C o . The RRKM/QET rate constants and the kinetic energy release can be calculated from the information provided by the theoretical study, leading to the conclusion that the determining step is the isomerization from the phenol ion to the keto form, owing to both energetic and vibrational factors. It can be inferred that, in theoretical studies, attention has to be paid to both aspects, since cases exist where the kinetic analysis may invert the predictions made from the potential energy surface. Nevertheless, the results presented here should not be extrapolated to other similar radical cations without cautions. A theoretical analysis in other keto-enol isomerizations and fragmentations are now under way.

(3)

,=l

where R and {v,]are the number of rotational degrees of freedom and the vibrational frequencies of the products, respectively. 'The translational energy is just k T , where T i s calculated from eq 3 from an available energy E above products 4. The statistical distribution obtained in this way gives 5.3% of the available energy. The result is in agreement with Baer's calculation, with assumed vibrational frequencies for the products (CSH6+'+ c o ) . This value is much lower than the experimental one (19.5%), showing that in reaction 1 kinetic energy release is not distributed statiscally. The reproduction of the KER would be important since it would be a test of the transition-state and the product structures. Recently, an approximate method to determine the KER in terms of the transition-state reaction coordinate has been developed by Derrick et In this method, the proportion of the reverse

-

(23) To obtain insight into the role of the vibrational frequencies in the rate constant, we are carrying out a study on the system C6H5X+./C6DSX+' C6H5+/C6D5+ X , where the relative rates will be mainly determined by vibrational effects. (24) R. G. Cooks, J. H. Beynon, R. M. Caprioli, and G. R. Lester, "Metastable Ions", Elsevier, Amsterdam, 1973. (25) C. E. Klots, J . Chem. Phys., 64, 4269 (1976). (26) (a) J. R. Christie, P. J. Derrick, and G. J. Richard, J . Chem. Sac., Faraday Trans. 2, 74, 304 (1978); (b) N . W. Cole, G. J. Richard, J. R. Christie, and P. J. Derrick, Org. Mass Spectram.. 1, 337 (1979); (c) Ibid.. J . Am. Chem. SOC.,100, 2904 (1978).

-

z

(U,PSURC)Z

n

E = kT

Y

+

+

Acknowledgment. Computer time was provided by the C.P.D. of the Ministerio de Educaci6n y Ciencia, made available through the terminal of the Centre de Cglcul de la Universitat PolitEcnica de Catalunya and by the Centre de Cglcul of the Universitat de Barcelona. Registry No. C6H,0H+', 40932-22-7.

FT-IR Study of Nitric Oxide Chemisorbed on Rh/AI,O, Jim Liang,* H. P. Wang, and L. D. Spicer*+ Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (Received: May 20, 1985) Chemisorption of NO on Rh/AI2O3surfaces has been examined by FT-IR. The spectra are assigned to two forms of Rh(N0) as well as the Rh(N0)2 species. Apparent interconversion of the linear nitrosyl and dinitrosyl complexes is readily observed at room temperature. The dinitrosyl complex is characterized both by an invariant ratio of 1743- and 1825-cm-' asymmetric and symmetric stretch bands with coverage and by isotopic data in 15N0 and the mixed 14N0 and 15N0systems. Force constants for NO stretching motions and for NO/NO ligand interactions on Rh(N0)2 have been used to successfully calculate the experimentally observed spectrum for the mixed isotope, dinitrosyl species. Thermal desorption data and displacement of adsorbed NO with CO are also reported. Introduction The characterization of molecules chemisorbed on transition metals by vibrational spectroscopy has recently attracted renewed +Present address: Departments of Biochemistry and Radiology, Duke University, Durham, NC 27710.

0022-3654/85/2089-5840$01 SO10

attention due largely to the availability of FT-IR and computerized grating IR. By use of these techniques difference spectra for surfaces with and without adsorbed gas can be obtained with high sensitivity. These spectra typically reveal even very weak features and in addition permit one to follow the progress of reactions "in situ" using multiple 0 1985 American Chemical Society

The Journal of Physical Chemistry, Vol. 89, No. 26, 1985 5841

Nitric Oxide Chemisorbed on Rh/A1203

,

IR Call for Spectroscopic Studies of Adsorbed Species

+k-

TEMPERATURE REGULATION (BOK -7WK ) MOLECULAR DOSING

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RmLOlutlon = 4 crn-’

TC FEEDTHRU

\

-

T O PUMP

U W

0

2

a m a

am Downloaded by NANYANG TECHNOLOGICAL UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: December 1, 1985 | doi: 10.1021/j100272a053

a

/\

SAMPLE

!

4

Figure 1. High-vacuum cell for infrared spectroscopy of adsorbed species.

In this laboratory a systematic FT-IR study of bonding associated with molecular adsorption on rhodium metal and supported rhodium has been initiated. Rhodium has been widely used in catalytic applications, and adsorption of small molecules on rhodium has attracted considerable interest. Adsorption of molecules like C0,l4 N2,5and on supported rhodium has been studied extensively in the past partially due to their importance in catalytic treatment of automotive exhaust streams. Particular attention has been given N O where three forms of chemisorbed NO on Rh/A1203are classified: Rh-NOd+, Rh/NO, and Rh-NO*-. However, Lunsford and his co-workers13 have shown that a gem-dinitrosyl complex, Rh(N0)2, exists in the adsorbed NO/Rh-Y zeolite system. Hyde et a1.I0 subsequently assigned 1830- and 1740-cm-’ bands to R h ( N 0 ) 2 in the study of NO/Rh/A1203, but this assignment requires further proof to be firmly established. In order to conclusively demonstrate the existence of R h ( N 0 ) 2 and to characterize NO adsorption at low surface coverage on supported rhodium, results of FT-IR studies of N O chemisorption on Rh/A1203 are reported here.

Experimental Section The preparation of supported Rh samples has been described by Yang et al.I4 and Yates et al.’ Briefly, a mixture of Rh”’C13.3H20 dissolved in HzO, high surface area A1203(Degussa A1203-C, 100 m2 g-l), and acetone (spectroscopic grade) was sprayed with an atomizer onto a CaF2 disk (32 mm in diameter) maintained at 350 K to flash evaporate solvents. Following reduction in H2(g) and evacuation of the cell, the sample was placed (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. Vaughn, J . Chem. Phys., 70, 1255 (1979). (3) J. T. Yates, Jr., T. M. Duncan, and R. W. Vaughan, J . Chem. Phys., 71, 3908 (1979). (4) R. R. Cavanagh and J. T. Yates, Jr., J . Chem. Phys., 74,4150 (1981). (5) H. P. Wang and J. T. Yates, Jr., J . Phys. Chem., 88, 852 (1984). (6) P. Gelin, A. R. Siedle, and J. T. Yates, Jr., J . Phys. Chem., 88, 2978 (1984). (7) H. Arai and H. Tominaga, J . Catal., 43, 131 (1976). (8) F. Solymosi and J. Sarkany, Appl. Sur!. Sci., 3, 68 (1979). (9) B. J. Savatsky and A. T. Bell, ACSSymp. Ser., No.178, 105 (1982). (10) E. A. Hyde, R. Rudham, and C. H. Rochester, J. Chem. SOC., Faraday Trans. I, 80, 531 (1984). (11) V. Rives-Arnau and G. Munuera, Appl. Surf. Sci., 6, 122 (1980). (12) J. C. Conesa, M. T. Sainz, J. Soria, G. Munuera, V. Rives-Arnau, and A. Munoz, J . Mol. Catal., 17, 231 (1982). (13) T. Iizuka and J. H. Lunsford, J. Mol. Caral., 8, 391 (1980). (14) A. C. Yang and C. W. Garland, J . Phys. Chem., 61, 1504 (1957).

21 )O

lsbo

16m I700 1600 WAVENUMBERS (crn-ll

I&

1400

Figure 2. Infrared spectra for I4NO adsorbed on Rh/AI,O, as a function of increasing I4NO coverage.

in the FT-IR spectrometer. The final “density” of Rh on the to 8.7 X A1203support was 8.3 X g/(cm2 of disk) at the 2.2 wt 7% loading employed here. The IR cell’5 with a CaF2 window used in this study is shown in Figure 1. A dosing tube is pointed toward the center of the sample disk from an oblique angle, thus allowing the dosing gas to interact with the rhodium surface directly and evenly. Nitric oxide with a stated purity of 99.0% from Matheson was further purified by trapto-trap distillation. Matheson O2(99.98%) and C O (99.99%) were used directly from cylinders as was H2 (99.999%) obtained from Airco Co. The RhCl3.3H20was purchased from Pressure Chemical Co., Pittsburgh, PA. Infrared spectra were recorded with a Nicolet 7199 series Fourier transform IR spectrometer in the 4000-400-cm-’ range at 4-cm-I resolution. A KBr beam splitter and MCT detector were used, and each sample was scanned 400 times. Measurement capability below the absorbance level is achieved with good signal-to-noise ratios and resolution by using the multiple scan data accumulation method. The subtraction technique incorporated in the computerized data-processing package was used throughout this study. All the spectra reported here are difference spectra which are obtained by subtracting the IR spectra with and without a given amount of adsorbate. The catalysis cell was securely mounted and remained stationary throughout the course of data acquisition, thus minimizing artifacts in the subtraction technique. Pressure was monitored with MKS Baratron capacitance manometers covering a range from to lo3 torr.

Results and Discussion Spectra of Chemisorbed NO. We have studied N O adsorption at 301 K on a reduced, A1203 supported Rh surface over the to 5 X torr. By incrementally pressure range of 3 X increasing the pressure and carefully examining the spectral changes as the coverage is increased, the development of characteristic infrared bands and consequently the chemistry which (15) The design of this cell was kindly provided by Prof. J. T. Yates, Jr., Department of Chemistry, University of Pittsburgh, Pittsburgh, PA.

5842

4

Liang et al.

The Journal of Physical Chemistry, Vol. 89, No. 26, 1985

I

T=300K

i

a i

I

, I

, 0

10

I

I

30

20

50

40

60

torr)

P,,(Io-~


Figure 3. Peak absorbance ratio of the Rh(NO), doublet during adsorption of I4NO on Rh/AI,O,. TABLE I: Infrared Frequencies and Calculated Bond Angles for Dinitrosyl Species Adsorbed on Various Supports

dinitrosyl species Rh(NO),/Y zeolite Rh(NO),/Y zeolite Mo(N0)2/A1203 Rh(N0)2/AI,O, Cr"'(NO),/SiO, Fe(NO),/Y zeolite

cm-'

LN-M-N, deg 94"

1860

cm-' 1780

1848

1771

1817 1825

104 120

1875

1713 1743 1745

1917

1815

145"

wSym,

waaym,

96*

135.2

ref 13 16

18 present work 19 22

"Calculated from the published spectra 00

produces the spectra could be monitored. It is well-known that nitric oxide dissociates on and oxidizes rhodium surfaces at room temperature and low coverages. Thus, the rhodium surfaces studied here are expected to be similar to preoxidized ones in their adsorptive properties. Figure 2 shows that as very small quantities of N O gas were added prior to successive measurements, four bands began to grow at 1912, 1825, 1743, and 1648 cm-I. The two broad bands centered at 1912 and 1648 cm-' develop first and can be assigned to Rh-N06+ and Rh-N=O (bent NO) species, respecitvely, in agreement with the results of Arai et a].? At higher coverage, another pronounced spectral feature is a doublet with components at 1825 and 1743 cm-I. This doublet increases in intensity during the entire course of adsorption without shifting frequency and has been attributed to Rh(NO), by Hyde et a1.I0 Earlier investigators,l6 however, did not detect these features, and thus confirmation of the assignment is important. To be consistent with a dinitrosyl species, an invariant ratio of the doublet peaks should exist over the pressure range below site saturation. Figure 3 shows that this is true in the data represented in Figure 2. This invariant ratio confirms that the doublet bands at 1743 and 1825 cm-I represent the dinitrosyl species, and these bands can be assigned as the asymmetric and symmetric stretching modes, respectively.17 A dinitrosyl species has also been reported in the study of N O adsorbed on molybdena-alumina,'s on chromia-~ilica,"-~' on Fe-Y zeolites,22 and on Rh-Y ze01ites.I~ The constant ratio of ~3 between the two peaks also indicates that the angle between the N O groups is approximately 120'. In (16) H . Arai. Ind. Eng. Chem. Prod. Res. Deu., 19, 507 (1980). (17) J. R. Pearce, D. E. Sherwood, M. B. Hall, and J. H. Lunsford, J . Phys. Chem., 84, 3215 (1980). (18) J. Valyon and W. K. Hall, J . Catal., 84, 216 (1983). (19) E. L. Kuger, R. J. Kodes, and J. W. Gryder, J . Catal., 36, 142 (1975). (20) A. Zecchina, E. Garrone, C. Morterra, and S . Coluccia, J . Phys. Chem., 79,978 (1975). (21) G. Ghiotti, E. Garrone, G . D. Gatta, B. Fubini, and E. Giamello, J . Card., 80, 249 (1983). (22) K. Segawa, Y. Chen, J. E. Kubsh, W. N. Delgass, J. A. Dumesic, and W. K. Hall. J . Card., 76, 112 (1982).

Figure 4. FT-IR spectra of isotopically substituted adsorbed NO species on Rh/A1203at room temperature: (a) I4NO;(b) 14N0+ ISNO(1:l

ratio); (c) ISNO. Table I we compare our result with related dinitrosyl species adsorbed on different supported metal surfaces to illustrate consistency of this finding with literature reports. In order to further confirm the existence of Rh(N0)2, studies of isotopically substituted'N0 adsorbed on Rh were carried out. Identical Rh/AI2O3 surfaces were exposed to saturation coverage of 1 5 N 0 and a 1:l ratio of I4NO and I5NO, respectively. The spectra for these surface species are shown in Figure 4 along with the spectrum of adsorbed I4NO from Figure 2. It can readily be observed that the bands at 1825 and 1743 cm-' from I4NO are shifted to 1784 and 1698 cm-I in the presence of pure I5NO. The 1:1 mixed system provides conclusive evidence for the dinitrosyl moiety. If a mononitrosyl species were responsible for the 1743 cm-' peak, then the IR spectrum for the 1:l mixture of I4NO and 15N0 would simply be a linear and equal combination of these two bands at 1743 and 1698 cm-]. On the other hand, if a dinitrosyl species were responsible for the band at 1743 cm-' in the system ''NO/Rh/A1203, then three bands should be expected in this region for the 1:l mixture of I4NO and ISNO, Le., a band at 1743 cm-' due to Rh(I4N0)*, a band at 1698 cm-I due to Rh(15NO)z,and an intense band between 1743 and 1698 cm-I due to Rh(l4NO)(ISNO). Moreover, statistically the relative intensities for the three peaks should be in the ratio 1:2:1. The middle spectrum of Figure 4 shows this result with an intense peak at 1722 cm-I located between 1743 and 1698 cm-I and with two incompletely resolved smaller bands at 1743 and 1698 ~ m - ' . ~ ~ This confirms the assignment of the band at 1743 cm-I in the I4NO adsorption to a dinitrosyl species. Indeed, because the intensities of the bands at 1825 and 1743 cm-' are linearly correlated over (23) Since the observed half-width of the band centered at 1722 cm-' is 70 cm-l, clear resolution of the triplet in Figure 4b is not likely. Deconvolution can be used to verify the observation, but a more definitive method which we are currently pursuing is to study the process with 15N180-labeledspecies at reduced temperature.

The Journal of Physical Chemistry, Vol. 89, No. 26, 1985 5843

Nitric Oxide Chemisorbed on Rh/A1203 TABLE 11: infrared Frequencies and Force Constants for Isotopically Substituted Dinitrosyl Species Adsorbed on Rh/Al,OB Surfaces

species

vSymrcm-I vaSym, cm-'

Rh('4N0)2 Rh(lSN0)2 Rh('4N0)(15NO) (obsd) Rh(14N0)('5NO)

1825 1784 1807 1807

1743 1698 1722 1719

k , , dyn cm-l

10" k2

1.402 1.385

0.064 0.068

1.395

0.066

(calcd) I

I

I

I

I

1743

a W

0

z


a

m

Lz

0

v,

m

a

21 10

I900

1800 1700 1600 WAVENUMBERS

1500

1400

Figure 5. Development of infrared spectra for I4NO adsorbed on Rh/ Al2O3at low temperature as a function of pressure: (a) PN0 = 2.8 X lo-' torr; (c) PNO = 8.4 X torr; (d) PNO = torr; (b) PNo L: 5.5 X 85.9 X torr.

the full coverage range, the band at 1825 cm-' can also be assigned to the same dinitrosyl species. Additional proof of the existence of the dinitrosyl species comes from an analysis of the normal modes of the isotopically substituted Rh(N0)2, analogous to that carried out for the R h ( C 0 ) 2 speci e ~ . The ~ ~ frequencies , ~ ~ can be obtained by solving the secular equation

where pi = reciprocal mass of the ith atom (1 = l6O; 2 = I4N; 3 = I6O; 4 = I5Nfor Rh('4NO)('5NO) species), v = observed frequencies in units of s-l, k l = force constant for N-0 stretch, and k2 = force constant for N O / N O coupling. For the Rh('4N0)2 and R h ( 1 5 N 0 ) 2species by use of the observed asymmetric and symmetric frequencies, values of the k , and k2 were calculated and are tabulated in Table 11. As can be seen, the agreement between k l and k2 for these two independent measurements is good. Based on these values for kl and k2, vam and vSymfor Rh('4NO)('5NO) were calculated to compare with the observed frequencies. Results indicate excellent agreement between predicted and measured spectral frequencies based on assignment as a dinitrosyl surface species. Adsorption of N O on Rh/A1203at low temperature (161 K) was also studied. The spectra observed are shown in Figure 5 where all three surface species, Rh-NO*+, Rh-N=O (bent NO), and Rh(N0)2, are clearly represented, and the ratio of intensities for the dinitrosyl peaks at 1743 and 1825 cm-' is between 3 and 4. It should be noted there appears to be no physisorption of NO on Rh/A1203at this temperature. There are two differences as one compares the spectra in Figure 5 of N O adsorption at low temperature with those spectra in (24) H. Knozinger, E. W. Thornton, and M. Wolf J . Chem. SOC.,Faraday Trans. 1 , 75, 1888 (1979). (25) J. T. Yates, Jr., and K. Kolasinski, J . Chem. Phys., 79, 1026 (1983).

-

Figure 2 measured at room temperature. First, the Rh-NO*+ band at 19 12 cm-l in Figure 5 appears to be suppressed, and second, the development of the dinitrosyl doublet at 1825 and 1743 cm-' in Figure 5 is expedited at low coverage relative to the same features at equivalent N O coverage in Figure 2a-c. Several factors may account for the first difference. Kinetic control via differences in activation energies for the adsorption process for the different species is possible. From the relative intensities of the peaks in Figure 5, one could conclude that the activation energies for the linear and bent species are higher than for the dinitrosyl species, and the overall order would be &,(linear) > ,!?,,,(bent) > E,,,(dinitrosyl). Since the band at 1912 cm-I is suppressed at 161 K, the activation energy for this adsorption process, ,!?,,,(linear), may be greater than k T at this temperature. Another plausible explanation for suppression of the 1912-cm-' band at the lower temperature is that dissociation for NO is much slower at 161 K and the preoxidized surface cannot readily form. The third possible explanation for the observed 1912-cm-' peak intensities at the two temperatures comes from metal-support interaction properties and their effect on bonding at the surface. Our results26in fact show that the 1912-cm-' band can be used as a probe to gauge the interaction between metal and support. The second difference is illustrated by comparing the spectrum of Figure 5c with that in Figure 2c where the NO pressure is close to 8.0 X torr in both cases. Figure 5c shows a 1743-cm-' absorbance of 0.09A whereas Figure 2c shows only 0.01A. We believe this ninefold difference in absorbance cannot be accounted for simply by sample thickness. Comparable intensities for this doublet are found in Figures 5c and 2b, leading to the conclusion that lowering the temperature of adsorption is equivalent to increasing the pressure of the adsorbate gas at room temperature?' Thermal Desorption of N O and the Apparent Interconversion ofRh(NO), and Rh(N0). Thermal desorption of N O on Rh/ A1203was also studied in this important system by heating a fully covered surface and observing the spectra as a function of temperature. Results indicate clearly that NO could be reversibly adsorbed and desorbed. The adsorbed species are very stable under vacuum at room temperature as evidenced by no detectable changes in the infrared spectra over extended periods. When the surface is warmed from 301 to 469 K, the 1743- and 1825-cm-I peaks disappear and the 1912-cm-' peak gains intensity as illustrated in Figure 6A. Over this range in temperature it should be noted that the vibration frequencies of the components of the R h ( N 0 ) 2 doublet are invariant at all stages of desorption. When the system is heated over 469 K, the 1912-cm-' peak also starts to diminish as shown in Figure 6B. Upon reaching 573 K, a complete desorption spectrum of the NO/Rh/A120, is observed, as seen in Figure 6C which is a mirror image of the adsorption spectrum of N O on Rh/AI2O3at room temperature. This textbook example of image formation clearly indicates that no additional surface reactions occur during the entire course of thermal desorption. It should be noted that during the development of the NO adsorption spectrum, as the pressure was increased to above torr, the 1 8 2 5 , 1743-, and -1640-cm-I bands gained intensity while the 1912-cm-' band did not. Upon detailed investigation of this spectrum, one finds that the intensity of the 1912-cm-' band in fact actually decreases as shown in Figure 7. A likely explanation is that some of the Rh-N06+ species is interconverted to form a dinitrosyl complex via

As illustrated in Figure 6, however, upon desorption of the dinitrosyl species, R h ( N 0 ) is formed. While this apparent backreaction of eq 2 could occur directly, it is more likely to originate from a two-step process involving liberation of N O gas and adsorption of the gas on the oxidized Rh sites present. This result also indicates that the bond strength of the Rh-NO*+ species is ~

~~~

( 2 6 ) J. Liang, H. P. Wang, and L. D. Spicer, manuscript in preparation. (27) R. J. Madix, Surf. Sci., 89, 540 (1979).

5844

Liang et al.

The Journal of Physical Chemistry, Vol. 89, No. 26, 1985

r

I

I

I

I

1

I

I\

Tz300K

Resolution = 4 cm-'

4k-

I\

A A = 0,0°7

~


t

0

i

-

i\

I900

I800 IiOO 1600 WAVENUMBERS (cm-9

I500

1400

Figure 7. Difference spectrum obtained from spectrum 2h minus spectrum 2e shown in Figure 2.

I

1

2OW

1900

,v,

le00

1700

C IS00 ,

1500 ,

WAVENUMBERS

Figure 6. Spectral changes during desorption of I4NO species. (A) Desorption involving interconversion of surface species Rh(NO)z and R h ( N 0 ) at different temperatures: (a) 379, (b) 414, and (c) 469 K. (B) Desorption of all surface species at different temperatures: (d) 469, (e) 553, and (0 573 K (C) Mirror images of adsorption and desorption.

greater than that of the Rh(NO)2 species. If the bond of RhNO*+ were weaker than that of the R h ( N 0 ) 2 species, the consumption of R h ( N 0 ) 2 would not result in the net increase in intensity of the 1912-cm-' band as shown in Figure 6A. Figure 8 shows the desorption spectra of the 1:l mixture of 14N0and 15N0. This study further verifies the existence of the dinitrosyl species on rhodium and its interconversion to R h ( N 0 ) by clearly showing that the broad triplet at 1722 cm-' is converted to both Rh-I4NO and Rh-15N0 characteristic adsorptions at 1913 and 1874 cm-', respectively. Interaction of CO with Adsorbed NO on RhIA120,. Carbon monoxide adsorption on supported rhodium surfaces has been clearly documented t o produce Rh'(CO), by Y a t e s and his cow o r k e r ~ .In ~ ~an effort to gain insight into the relative stabilities and displacement kinetics of adsorbed CO and NO on rhodium, the ligand-exchange process was studied. Ten torr of CO gas was introduced into the sample cell at room temperature after the rhodium surface had been saturated by N O adsorption. Essentially complete displacement of adsorbed N O species was observed within 10 min as shown in the spectrum of Figure 9. Since desorption of NO species is relatively slow at ambient temperatures, active displacement must occur in this process rather than sequential site occupation. After complete C O exchange for adsorbed N O species was accomplished, the sample cell was evacuated thoroughly and NO(g) was readmitted to study the reverse process. The spectra under these conditions clearly indicate that NO(g) does not

-008 2000

1900

le00 1700 1600 WAVENUMBERS(cm")

1500

Figure 8. IR difference spectrum showing interconversion of the isotopically substituted surface species Rh(NO)z and R h ( N 0 ) .

I

T-300 K

a w z 0

a a m

wa

m

2

3

2100

2000

1900 I800 1700 WAVENUMBERS (cm-')

1600

1500

I

Figure 9. Interaction of CO(g) with the adsorbed I4NO on Rh/AI,O,.

J. Phys. Chem. 1985, 89, 5845-5849 completely displace the Rh(C0)2, thus indicating that the dicarbonyl species is more stable than the dinitrosyl species on this supported rhodium surface. Figure 9 also indicates very little detectable linear CO or bridged CO on rhodium crystallite sites (Rh,), and in fact, the major surface species is Rh'(C0)2. This suggests that Rh' sites are the predominant sites represented on the surface.


ConcIusion The chemisorption of N O on A1203 supported Rh has been studied using FT-IR spectroscopy. Based on the data presented, the following conclusions are reached: (1) The N O species on the surfaces are linear NO, bent NO, and gem-dinitrosyl complexes, and each is distinguishable on Rh/AI2O3 surfaces by FT-IR. (2) An invariant ratio for the doublet IR band at 1825 and 1743 cm-' and studies of isotopic N O confirm the existence of Rh(NO)z species. (3) There is no evidence for interaction of

5845

the dinitrosyl complexes with neighboring N O molecules as coverage is increased, and the characteristic doublet represents stretching modes for this species. (4) Adsorption and desorption studies clearly show that as the dinitrosyl species is formed, the concentration of linear species is reduced and as the dinitrosyl species is desorbed, the linear R h ( N 0 ) is formed. Desorption studies also show the ligand bond of the Rh-N06+ is stronger than those in either R h ( N 0 ) 2 or Rh-N=O (bent NO). (5) Rapid displacement of CO with NO(ads) occurs for all of the adsorbed N O species. Acknowledgment. We acknowlege helpful comments and suggestions by Dr. Max Matheson and one of the referees. Support for this research by the United States Department of Energy under Contract No. DE-AC02-76ER02190 is gratefully acknowledged. 'Registry No. NO, 10102-43-9; Rh, 7440-16-6.

+ CH3CI(H20),

An MO Study of S,2 Reactions in Hydrated Gas Clusters: (H,O),OHHOCH3 CI( n m)H2O

+ + +

+

Katsuhisa Ohta and Keiji Morokuma* The Institute for Molecular Science, Myodaiji, Okazaki 444, Japan (Received: June 21, 1985)

-

Potential energy surfaces are calculated with the ab initio MO method for the SN2reaction (H,O),OH- + CH3C1(H,0), HOCH, + C1- + ( n + m)H20, where the reactants are complexed with up to two water molecules. When the hydroxide ion is solvated by water molecules, the reaction takes place through the first step of reactant complex formation, followed by inversion of the methyl group. The migration of water molecules from the hydroxide side to the chloride side is not involved in the rate-determiningprocess. In the case of (n,m) = ( 2 , O ) the transition state for methyl inversion has an energy comparable to that of the reactants.

Introduction

(H,O),CI-

Chemical reactions in gas-phase clusters are attracting considerable attention, as they provide information filling a wide gap between reactions in the gas phase and in solution. sN2 reactions of type X- CH3Y XCH, Y- have been studied extensively in both solution and the gas phase.'-, The rate constant in the gas phase has been found to be up to lozotimes faster than in solution. To explain this large difference, Brauman and cc-workers have suggested for the gas-phase reaction a double well potential curve, which gradually changes to a single barrier upon stepwize solvation of reactants.' Bohme and co-workers actually have investigated the sN2 reaction in gas-phase hydrated clusters by the flowing afterglow For the reaction

+

OH-(H20),

-

+

+ CH3C1

-

CH30H

+ C1- + n H 2 0

(1)

they have reported the kinetics for the hydration number from n = 0 to 3.3c The rate becomes slower in clusters for n = 1, 2, and 3 by ca. 1.6, 500, and 1000 times, respectively, than that for n = 0. A drastic decrease in rate upon going from n = 1 to 2 has been interpreted as an indication that the overall barrier has become positive at n = 2. In order to provide information concerning the potential energy surfaces in hydrated clusters, we have previously carried out ab initio MO calculations for the following symmetric SN2 reaction^:^ (1) Olmstead, W. N.; Brauman, J. I. J . Am. Chem. SOC.1977, 99, 4219. Pallerite, M. J.; Brauman, J. I. J . Am. Chem. SOC.1980, 102, 5993. (2) Tanaka, K.; Mackay, G. I.; Payzant, J. D.; Bohme, D. K. Can. J . Chem. 1976, 54, 1643. (3) (a) Mackay, G. I.; Bohme, D. K. J. Am. Chem. SOC.1978, 100, 327. (b) Bohme, D. K.; Mackay, G. I. J . Am. Chem. SOC.1981, 103, 978. (c) Bohme, D. K.; Raksit, A. B. J . Am. Chem. SOC.1984, 106, 3447.

+ CH3Cl-

ClCH3

+ Cl-(H,O),

n = 0, 1, 2 (2)

- -

Our findings, shown in Figure 1, can be summarized as follows. (i) The most favorable reaction path for n = 1 is reactants reactant complex transition state for CH3 inversion transfer of H 2 0 from the left (the newly formed CH3C1 side) to the right product complex products. (the newly formed C1- side) Since the system is symmetric, the process by which H 2 0 transfer takes place before CH, inversion is equally favorable. The path of simultaneous C H 3 inversion and H 2 0 transfer is both energetically and entropically unfavorable. (ii) For n = 2 the most favorable path is reactants reactant complex transfer of one H 2 0molecule from the left (the C1side) to the right (the CH3Cl side) CH, inversion transfer of the other HzO molecule from left (the newly formed CH3C1 side) to the right (the newly formed C1- side). Having one water molecule on each chlorine atom is the best way to stabilize the intrinsically symmetric transition state for CH, inversion of reaction 2 . (iii) The transfer of H 2 0from one side to the other takes place with little or no barrier, via an intermediate having a bent C1C - C l configuration. Henchman et al. have analyzed the product ions for the reaction

-

-

-

OH-(H,O),

-

+ CH3Br

-

CH30H

-

+ Br- + n H 2 0

(3)

and found the product bromide ion is actually unsolvated, probably (4) Morokuma, K. J . Am. Chem. SOC.1982,104, 3732. Morokuma, K.; Kato, S.;Kitaura, K.; Obara, S.;Ohta, K.; Hanamura, M. In 'New Horizons of Quantum Chemistry", Lowdin, P. O., Pullman, B., Eds.; Reidel: Dordrecht, 1982; p 221.

0022-3654/85 /2089-5845%01.50/0 0 1985 American Chemical Societv

AI,O,

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