4598
J. Phys. Chem. 1989, 93, 4598-4603
H,OH] species may dissociate to form a C H 2 radical which is a precursor for the formation of ethene; alternatively, the [CH,OH] may react with other methanol molecules to form surface bonded oxonium methylide as proposed by Hutchings et al.24 Indeed, although our calculations predict the abstraction of hydrogen from the methyl group of methanol, the abstraction of hydrogen may occur from an oxonium methylide intermediate, since the rate of formation of this intermediate may be faster than the dissociation of methanol. Thus, the key feature of our results is the suggestion of C-H bond weakening which may occur in (24) Hutchings, G. J.: Gottschalk, F.; Hall, M. V . M.; Hunter, Soc., Faraday Trans. 1 1987, 83, 571
R. J. Chem.
species other than methanol. Our future work will focus on the study of the adsorption behavior of ethene and dimethyl ether in ZSM-5 zeolite to improve the understanding of the subsequent steps in the reaction pathway. It is of course also possible that sorption at different sites characterized by different AI substitutions may give rise to alternative dissociation mechanisms. Future studies will examine these possibilities. The present study demonstrates, we believe, the power of the combination of modeling with quantum chemical techniques in the study of these catalysts. Acknowledgment. We thank IC1 PIC for financial support to R.V. (under the Joint Research Scheme arrangement). Registry No. MeOH, 67-56- I .
Observation of Cyanate and Isocyanate Surface Species during the Reaction of Ammonia and Carbon Monoxide over Supported Rhodium D. K. Paul, M. L. McKee, S. D. Worley,* Department of Chemistry, Auburn University, Auburn, Alabama 36849
N. W. Hoffman, Department of Chemistry, University of South Alabama, Mobile, Alabama 36688
D. H. Ash, and J. Gautney National Fertilizer Development Center, Tennessee Valley Authority, Muscle Shoals, Alabama 35660 (Received: October 7 , 1988; In Final Form: January 27, 1989)
The reaction of CO with NH3 over preoxidized Rh/Si02 has been studied by infrared spectrwcopy to detect surface intermediates. Two of the surface species having infrared bands at 2172 and 2225 cm-l have been identified as RhNCO and RhOCN, respectively. This is likely the first observation of these two surface species simultaneously on a supported transition-metal catalyst. High-quality ab initio computations have been employed to aid in the band assignments.
Introduction More efficient synthetic routes to dicyandiamide, a potent nitrification inhibitor that significantly enhances nitrogen fertilizer efficiency and has potential for reducing ground water pollution by nitrates, are being explored by the fertilizer industry. Cyanamide, which is an intermediate in the production of dicyandiamide, could be a product in the catalytic reaction of ammonia with C O or CO,. We have been exploring the CO/NH, reaction over a variety of supported transition-metal catalysts. Among our efforts have been studies of adsorbed species on these catalysts using infrared spectroscopy in work similar to that reported earlier for catalytic methanati~nl-~ and the decomposition of small organic molecule^.^ Specifically, in this investigation surface species formed during the C O / N H 3 reaction over preoxidized Rh/Si02 in an infrared-cell reactor were monitored and high-quality ab initio computations for the suspected rhodium cyanate and isocyanate species were performed. Experiment and Theory Supported Rh/Si02 films (2.2 wt % Rh) were prepared by spraying a slurry of RhC1,.3H20 (Alfa Products, Inc.), SiO,
(Cabosil M5, 200 m2 g-l, Cabot Corp.), acetone, and water onto a heated 20-mm CaF2 infrared plate. The IR plate containing the adhered RhCI3.3H2O/SiO2was placed in an IR cell reactor similar in design to that employed previously in these laborat o r i e ~ . I -A ~ chromel-alumel thermocouple was used to monitor the temperature of the film during reaction. The samples were outgassed at 298 K for 15-20 h, heated at 10” Torr for 1 h, and subjected to 5-, 5-, IO-, and 20-min cycles of exposure to 78 Torr of H2 or O2at 523 K (each cycle followed by evacuation to Torr) and 1 h of evacuation to 10” Torr at 533 K. Then the cell was dosed with a reactant gas (2:l CO/NH3, HNCO, HCN, or formamide) with the pressure measured by an MKS Baratron capacitance manometer, and the infrared spectra were monitored (IBM FTIR 44 Fourier-transform IR spectrometer operating at a resolution of 4 cm-I) as a function of temperature and/or reaction time. The HNCO gas was prepared by reaction of aqueous KOCN with 95% H3P0, at 283 K; H C N was generated from reaction of KCN with H 2 S 0 4at 298 K. Both were purified by trap-to-trap distillation. Formamide was purchased from Aldrich Chemical Co. and purified by vacuum distillation. The theoretical computations employed in this work were of the ab initio type (GAUSSIAN M).’ The 36 core electrons of Rh
( 1 ) Henderson. M. A.; Worley, S. D.
J. Phys. Chem. 1M15, 89, 1417. ( 2 ) Worley, S. D.; Mattson, G . A,; Caudill, R . J . Phys. Chem. 1983, 87,
1671. (3) Dai. C . H.; Worley, S. D. J . Phys. Chem. 1986, 90, 4219. ( 4 ) Dai, C . H . ; Worley, S. D. Langmuir 1988, 4, 326.
0022-3654/89/2093-4598$01 .Sol0 , , ,
( 5 ) GAUSSIAN 86: Frisch, M. J.; et al. Carnegie-Mellon Quantum Chemistry Publishing Unit, Carnegie-Mellon University: Pittsburgh, PA. The ECP code was obtained from the Theoretical Division, Los Alamos National Laboratory, Los Alamos, N M .
Q 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 11, I989 4599
Cyanate and Isocyanate Surface Species
2200
2400
2000
1800
1600
1400
2500.0 WAVENUMBERS(CM-’) 1275.0 Figure 1. Infrared spectra for the interaction of CO and NH, over a preoxidized 2.2% Rh/Si02 film (4.3 mg cm-2) at a total pressure of 15 Torr.
TABLE I: Band Positions and Tentative Assignments in This Work
wavenumber, cm-l 2359 2300 2270 2225 2217 2208 2174 2172
2165 2160 2150 2143 2139 21 I7
2105 2090-2 1 10 2040-2076 2024-2050 2022 1800- 1900 I668 1625 1599 1464 1405 1300
figure Ic-e; 3a,b; 5c-e le; 3a-e; 5c-f Id; 3a,d; 5b IC-f; 2d,e; 4a 4d 4b 4c Id-f; 2d,e; 3a-c; 4a; 5b-f 4b 4d 2c,d Ib-e; 5d,e la 4c 2a,c la-; 3b,c; 5b-f Id-f; 5~-f la-c; 3b,c; 5b-f 1d-f
la-f; 5b-e le,f 1b-f
le,f 1e,f le,f 1b-d
species C02(g) NCO(a)/SiO, HNCOk) OCN(a)/Rh I80CN(a)/ Rh OCISN(a)/Rh O”CN(a)/Rh NCO(a)/Rh IsNCO(a)/Rh NCI8O(a)/Rh CN(a)/Rh CO(g) CO(a)/Rh(II) N”CO(a)/Rh HCN(physisorbed) (CWa)/Rh CO(a)/Rh (CO),(a)/Rh CO(H) (a)/Rh” CO(a)/Rh2 amide/Rh NH,(a)/Rh amide/Rh amide/Rh NH,+/Si02 NH,(a)/Rh
assignment CO stretch (asym) NCO stretch (asym) NCO stretch (asym) OCN stretch (asym) I80CN stretch (asym) OCIsN stretch (asym) Oi3CNstretch (asym) NCO stretch (asym) ”NCO stretch (asym) NCI80 stretch (asym) CN stretch CO stretch CO stretch NI3CO stretch (asym) CN stretch CO stretch (sym) CO stretch CO stretch (asym) CO stretch CO stretch CO stretch* NH def (asym)c NH2 bend (asym) NH, bend (sym) NH deformationC NH deformation (sym)
@ A nadsorbed Rh carbonyl hydride, species. *For related organometallic amide band assignments, see: Balahura, R. J.; Jordan, R. B. J. Am. Chem. SOC.1970, 92, 1533. For related surface species band assignments, see: Kiselev, A. V.; Lygin, V. I. Infrared Spectra ofSurface Compounds; Wiley: New York, 1975; p 245.
were replaced by a relativistic effective core potential as generated by Hay and Wadt.6 The outer valence orbitals (5s, 4d, and 5p) were represented by a (3s3p4d) primitive Gaussian basis contracted to (2s2p2d). A 6-31+G basis was used for C, N, and 0. All geometric variables were optimized within the assumed symmetry point group, and frequencies were computed by taking finite differences of analytical first derivatives.
Results and Discussion Preliminary experiments involving the interaction of NH, with Rh/Si02 in the absence of CO were not very revealing. Only broad infrared bands at 1624 and 1430 cm-l corresponding to adsorbed NH3 and possibly NH2, respectively, were obtained. We (6) Hay, P. J.; Wadt, W.
R.J . Chem. Phys.
1984.82, 270, 299.
have observed a similar band at 1626 cm-l for the complex [Rh(NH3)SCI]C12. Following treatment of the Rh/Si02 film with hydrogen (prereduction), upon introduction of a CO/NH3 mixture and heating up to 448 K over a period of several hours, the only infrared spectral features that could be attributed to surface species were bands at 2097, 2056, 2039, 2030, and 1864 cm-I. These correspond to the usual rhodium “gem-dicarbonyl” species (2097, 2030 cm-I), linear carbonyl (2056 cm-I), carbonyl hydride (2039 cm-I), and bridged carbonyl (1864 cm-I). However, following preoxidation of Rh/SiO, the spectral developments shown in Figure 1 occurred. The data from all of the various figures are summarized in Table I. Figure l a represents the IR spectrum following exposure of the preoxidized surface to 30 Torr of CO at 298 K for 15 min and then rough evacuation. The most
4600
The Journal of Physical Chemistry, Vol. 93, No. 11, 1989
Paul et al.
473K,30 min followed b y
e evacuation at 298 K
r d
f
3 8 8 K , 30
Rh/StOp I
1
I
6.0 t o r r
I
3
/
I
I
h
I
I
I
I
I
Figure 2.
prevalent bands may be assigned to the Rh gem-dicarbonyl species (2101, 2034 cm-l) which is known to correspond to Rh(1) and to a highly oxidized Rh carbonyl species (2139 cm-I). The latter has been detected previously for preoxidized Rh/AI2O3 and can be assigned to either Rh(II1) carbonyl or to a Rh(I1) carbonyl oxide species.'-* When C O / N H 3 was introduced in a 2.1 ratio at 15 Torr and 298 K for 30 min (Figure 1b), the gem-dicarbonyl bands shifted to lower frequency and the 2139-cm-' band disappeared, indicating possible reduction of the Rh. Other possible causes of the low-wavenumber shift of the gem-dicarbonyl bands might be electron donation into the 7r* orbitals of the CO moieties and image forces caused by nearby adsorbed N H 3 ~ p e c i e s . ~ Furthermore, significant intensity distribution alteration in the gem-dicarbonyl species bands occurred (Figure 1b), probably due to a change in the OC-Rh-CO angle caused by the presence of adsorbed NH39 or to the formation of a Rh carbonyl hydride species with its band at 2022 cm-l (see below). At the same time new bands began to develop at approximately 2170 and 2225 cm-l; these new bands continued to intensify upon heating (Figure Ic,d) and remained upon cooling ( l e ) and evacuation ( I f ) at the conclusion of the experiment. Also noted as the experiment progressed were copious amounts of CO, (2359 cm-I), a small amount of H N C O (2270 cm-I) at 500 K, the presence of amide species ( 1 500-1700 cm-I), and a new RhCO surface species (2022 cm-'). The latter species most probably is a Rh carbonyl the source of H being NH3. The interesting question to be answered concerns the assignment of the 2225- and 2172-cm-' bands which are most prevalently represented in Figure le. A number of possibilities could be suggested for the two surface species causing the 2225- and 21 72-cm-l infrared bands, such as isocyanate (NCO), cyanate (OCN), and cyanide, both on Rh and on the Si02support. There have been several studies reported that are relevant to the present work. Hecker and Bell have reported the observation of IR bands at 2170 and 2300 cm-l upon reacting N O with a H 2 / C 0 mixture over 4.6% Rh/SiO,. They assigned these two bands to RhNCO and isocyanate on the Si02 support, respectively." Dictor studied the NO/CO reaction over 0.475% Rh/AI2O3; IR bands were noted at 2230, 2175, and 2 130-2 150 cm-I, which were assigned to isocyanate on the support, (7) Yang, A. C.; Garland, C. W. J. Phys. Chem. 1957, 61, 1504. (8) Rice, C. A.; Worley, S. D.; Curtis, C. W.; Guin, J. A,: Tarrer, A. R. J . Chem. Phys. 1981, 7 4 , 6487. (9) We thank a referee for suggesting these possibilities. (IO) Solymosi, F.; Erdohelyi, A.; Kocsis, M. J. Catal. 1980, 65, 428. ( 1 1 ) Hecker, W. C.; Bell, A. T.J . Coral. 1984, 85, 389.
isocyanate on Rh, and adsorbed cyanide, respectively.12 Several laboratories have reported the observation of isocyanate species for the NO/CO reaction over other transition metals, e.g., PtNCO for a Pt film (2180 cm-I),I3 PtNCO for Pt/AI2O3 (2148 cm-', ionic; 2267 cm-I, covalent),I4 and RuNCO for Ru/Si02 (2180 cm-').I5 Sheets and Blyholder noted that, for reaction of N H 3 and C O over V and Fe films, infrared bands at 2180 and 2170 cm-l could be assigned to VNCO and FeNCO, respectively.I6 Thus, there is certainly ample precedent for assigning our 2172-cm-' band to RhNCO for the reaction of C O and N H 3 over the Rh/Si02 surface. On the other hand, we are not aware of prior work on CO/NH3 or CO/NO over silica-supported transition metals that produced a surface species with a prominent IR band near 2225 cm-' as seen in Figure Ic-f. (In contrast to A1203,12 a silica support provides an isocyanate species with a band at 2300 cm-l.Il) Morrow and Cody" have reported IR bands at 2313 and 2218 cm-I observed during the decomposition of H C N over Si02 at 1073 K. They found that both bands shifted to lower frequency when H13CN or HCi5N was used, but only the 2313-cm-' band was employed. They thus shifted to lower frequency when Si1802 assigned the 231 3- and 2218-cm-l bands to isocyanate and cyanide, respectively, on Si02. Several experiments were performed to attempt to clarify the assignment for the 2225-cm-' band. First, HCN was decomposed over preoxidized 2.2% Rh/SiO, and Si02under conditions resembling those of our CO/NH3 experiment (Figure 2). For $30, alone the only IR band detected between 2000 and 2300 cm-l was one at 2105 cm-' which disappeared upon evacuation (Figure 2a,b) and can be assigned to the C N stretch for physisorbed HCN. Gas-phase H C N at the same resolution (4 cm-I) employed for our surface spectroscopic measurements provides a band centered at 2090 cm-' containing resolved rotational structure. For the Rh/Si02 surface a second band at 2150 cm-' appeared at 298 K (Figure 2c) which is probably due to a RhCN surface species. Upon further heating at 388 K (Figure 2d) the RhCN band intensified, and a new band appeared at 2218 cm-' which may correspond to the same surface state observed for the C O / N H 3 reaction. However, upon heating to 473 K followed by evacuation (12) Dictor, R. J . Catal. 1988, 109, 89. ( 1 3 ) Rasko, J.; Solymosi, F. J. Caral. 1981, 71, 219. (14) Unland, M. L. J . Phys. Chem. 1973, 77, 1952. (15) Brown, M. F.; Gonzalez, R. D. J . Catal. 1976, 44, 477. (16) Sheets, R . W.; Blyholder, G. J . Phys. Chem. 1975, 79, 1572. (17) Morrow, B. A.; Cody, 1. A. J. Chem. SOC.,Faraday Trans. 1 1975, 7, 1021.
The Journal of Physical Chemistry, Vol. 93, No. 11, 1989 4601
Cyanate and Isocyanate Surface Species
1
1
2500
1
~
1
1
2400
1
~
1
2300
'
l
1
~
2200
1
1
1
2100
~
1
1
2000
WAVENUMBERS (CM-'
Figure 3. Infrared spectra for the decomposition of HNCO over preoxidized 2.2% Rh/Si02 and S O z films (4.3 mg
1 2300
1
1
1
~
1
1
1
1
~
2200
2250
1
1
1 2150
1
~
1
1
1
1
2100
WAVENUMBERS(CM-' )
Figure 4. Infrared spectra for the interaction of isotopically labeled CO and NH3over preoxidized 2.2% Rh/Si02 films (4.3 mg c d ) following heating fo; 30 min at 500 K and then cooling to 298 K.
at 298 K, a large broad band centered at 2180 cm-' resulted (Figure 2e). We believe that the 2180-cm-' band corresponds to the C N stretch for RhNCO, and it is likely that the 2218-cm-l band refers to the C N stretch for RhOCN, Le., the cyanate as will be discussed later. In both cases the source of the oxygen atom would be the Si02support. However, the H C N experiment obviously does not provide compelling evidence for the presence or absence of a cyanate species in the C O / N H 3 reaction over Rh/SiO,. In a second experiment H N C O gas was decomposed over preoxidized Rh/Si02 and S O 2 (see Figure 3). For SiO, alone at 298 K only an intense band near 2270 cm-' corresponding to the N C stretch of H N C O gas was observed. Upon heating at 453 K, this band vanished and was replaced by one at 2304 cm-l which remained under evacuation (Figure 3d,e). The 2304-cm-I feature corresponds to the presence of the isocyanate species on SiO,. When Rh was present (Figure 3a-c), it is evident that isocyanate was produced on both Rh (2174 cm-I) and the support
(2298-2304 cm-I); it is interesting that C 0 2 gas and the Rh gem-dicarbonyl surface species were also formed upon heating. However, the infrared band at 2225 cm-' was nor evident in any of the spectra. We believe that this observation strongly indicates that the 2225-cm-' band for the CO/NH, system is not due to a RhNCO species involving a surface site different than the one causing the 2172-cm-' band; Le., two different RhNCO species are not formed. In the third and most significant set of experiments isotopic labeling was employed. Preoxidized Rh/Si02 samples were exposed to mixtures (2:l) of CO and NH3 with various combinations of 13C,lSN,and I*O employed, as well as ND,. The samples were all heated to 500 K for 30 min (after ramping the temperature up to this value over a period of 4 h) and then cooled to 298 K at which temperature the spectra shown in Figure 4 were recorded. The CO/ND3 run (not shown in Figure 4) produced bands identical with those for CO/NH3, so it can be concluded that the surface species causing both the 2172- and 2225-cm-l bands do
1
4602 The Journal of Physical Chemistry, Vol. 93, No. 1 I , I989
Paul et al.
w
0
z
6
m
a
0 m
v)
6
h
b 4 1 0 K . 5min
I
2500.0
I 2400
'
I
'
I
1
I
I
2200
I
'
I
2000
I
I
1000
WAVENUMBERS (CM-')
Figure 5. Infrared spectra for the decomposition of formamide over a preoxidized 2.2% Rh/Si02 film (4.3 mg cm-2).
not contain hydrogen atoms. On the other hand, all other isotopic substitution caused significant shifts to lower frequency for both bands. Thus, it can be concluded that the surface species which give rise to the IR bands at 21 72 and 2225 cm-l in the reaction of C O and N H 3 over Rh/Si02 must both contain C, N, and 0 atoms. We are left with the probable conclusion that the 2172and 2225-cm-' bands can best be assigned to RhNCO and RhOCN surface species, respectively. It should be noted that I80 substitution causes a larger frequency shift in the 2172-cm-' band (12 cm-I) than in the 2225-cm-l band (8 cm-I), which might be expected given that 0 is bonded to the heavy Rh atom as well as to C in the cyanate species, but only to C in the isocyanate. It is gratifying that the reverse was true for ISN substitution. Anderson and co-workers have reported the preparation of the organometallic complexes Rh(PPh3)3NC0and Rh(PPh,),OCN and their C N stretches at 2230 and 2215 cm-l, respectively.18 However, Bailey and KozakI9 have suggested that cyanates always have higher C N stretching frequencies than do corresponding isocyanates, which is consistent with our interpretation in this work. Given this conflicting data from organometallic chemistry, we decided to perform a b initio calculations for the RhNCO and RhOCN species. While it is true that such calculations can only be performed for the gas-phase species at this time, recent work in these laboratories has shown that high-quality ab initio calculations give an excellent account of the Rh carbonyl and carbonyl hydride species,20,2'so it is reasonable to assume that it may perform satisfactorily, at least qualitatively, for the Rh cyanate and isocyanate species as well. The theoretical data are given in Table 11. It can be seen that the computations predict that the Rh isocyanate should be more stable than the Rh cyanate, but only by 12.4 kcal mol-'. The antisymmetric stretching frequency for the cyanate moiety is predicted to be 40 cm-' higher than for the isocyanate. The experimental data are in reasonable accord with this prediction (cyanate higher by 53 cm-I). Furthermore, the computed isotopic shifts are in good qualitative agreement with those observed experimentally. The optimized geometries predict that the isocyanate is linear, while the cyanate should be bent, at least in the gas phase. Thus, the ab initio computations provide excellent support for our hypothesis that the 2225-cm-' (18) Anderson, S. J.; Norbury, A. H.: Songstad, .I. J . Chem. Soc., Chem. Commun. 1974, 37. (19) Bailey, R. A.: Kozak, S . L. Inorg. Nucl. Chem. 1969, 31, 689. (20) McKee, M. L.; Dai, C. H.; Worley, S. D.J . Phys. Chem. 1988, 92, 1056. (21) McKee, M. L.: Worley. S. D. J . Phys. Chem. 1988, 92, 3699.
TABLE 11: Ab Initio Calculations with Effective Core Potential for RhNCO and RLOCN'
RhNCO
Relative Energies, kcal mo1-l 0.0 RhOCN
12.4
Rh-NCO
Bond Energies, kcal mol-' 41.9 Rh-OCN
29.5
Antisymmetric Stretching Frequencies,bcm" RhOCN 2459 RhNCO 2419 Rhi80CN 2456 (-3) RhNC'*O 2413 (-6) RhOl'CN 2397 (-62) RhN"C0 2358 (-61) RhOCI5N 2432 (-27) Rh"NC0 2395 (-24) Geometries (Bond Lengths in A, Bond Angles in deg)
'Relative energies and bond energies include the effect of electron correlation at the MP3 level. For isotopically substituted species the shift in cm-' is given in parentheses.
IR band refers to RhOCN while the 2172-cm-' band refers to RhNCO for the CO/NH3 reaction over Rh/SiOz. Although the RhNCO band is generally more intense than that for RhOCN (Figure 4) as would be expected, it is evident in Figure 1 that the 2225-cm-l band grows in and reaches a constant magnitude stable to evacuation at 298 K (Figure If). On the other hand, the 2172-cm-' band diminishes upon evacuation at 298 K. The ab initio computations reasonably predict that RhNCO should be more stable than RhOCN in the gas phase. We must conclude that the surface site supporting RhOCN is exceptionally stable. We might speculate that this could result from a geometric effect in that the cyanate is predicted to be bent and thus could be interacting with the support as well as Rh. The isocyanate is predicted to be linear and thus could not be expected to interact significantly with the support. The mechanisms of formation of the rhodium cyanate and isocyanate species during the reaction of CO and NH, over preoxidized Rh/SiOZ are of interest. A reasonable proposal could be the insertion of CO into an N-H bond of ammonia to produce formamide which could subsequently decompose to NCO and OCN. To evaluate this possibility, we dosed preoxidized Rh/Si02 and SiO, with formamide and decomposed it under heating. Silica alone provided no detectable surface species with IR bands in the 2000-2300-cm-' region. However, the preoxidized Rh/Si02 surface produced the results shown in Figure 5 . One can see that upon heating the usual Rh carbonyl species were produced, as
4603
J . Phys. Chem. 1989, 93, 4603-4608 well as CO, C 0 2 , and H N C O gases, and the isocyanate species
Conclusion
on Rh (2174-2184 cm-l) and on the support (2295 cm-I). There was no band observed at 2225 cm-I, indicating theabsence of the
It has been shown that during the reaction of CO and NH3 over preoxidized Rh/Si02 several surface species result which can be detected by infrared spectroscopy. Those causing IR bands at 2172 and 2225 cm-' may be reasonably identified as RhNCO (isocyanate) and RhOCN (cyanate) on the basis of ab initio computations and various experimental deductions. As far as we are aware, this is the first detection of a Rh cyanate surface species on a supported transition-metal catalyst. The Rh isocyanate species may be produced from decomposition of a Rh formamide surface intermediate, but the Rh cyanate must result from s m e other mechanism, possibly from attack of a Rh oxide species by CN.
RhOCN species. Thus, while the RhNCO species may well result from decomposition of a formamide surface species, RhOCN must be produced by a different mechanism. We would suggest that such a mechanism could involve the attack of rhodium oxide by CN. The surface species causing the band at 2139 cm-I in Figure la, which is likely a rhodium carbonyl oxide, might thus be a reasonable precursor to the Rh cyanate species through attack on 0 by C N during the C O / N H 3 reaction. This could also reasonably explain the presence of the 2218-cm-l band noted in Figure 2d upon decomposition of HCN over preoxidized Rh/Si02. Finally, it should be reiterated that preoxidation is a necessary condition for observing the cyanate and isocyanate species over Rh/Si02 during the C O / N H 3 reaction under the conditions of our experiments. Furthermore, our preliminary results for Rh/AI2O3and Rh/TiOz, preoxidized or prereduced, show only the existence of isocyanate for the preoxidized former.
Acknowledgment. This work was partially supported by the Alabama Universities/TVA Research Consortium and the Tennessee Valley Authority National Fertilizer Development Center, Muscle Shoals, AL. We acknowledge the Auburn University Computation Center for a generous allotment of computer time.
Radiolytic Production and Properties of Ultrasmall CdS Particles David Hayes,+ Olga I. Mitit,* M. T. Nenadovit,t V. Swayambunathan,+and Dan Meisel*,+ Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439, and Boris Kidric Institute of Nuclear Chemistry, P.O. Box 522, Belgrade, Yugoslavia 11001 (Received: October 14, 1988; In Final Form: January 10, 1989)
The radiolytic production of CdS particles from solutions containing cadmium ions and a thiol (3-mercapto-1,2-propanedioI, RSH) is described. The production of the colloids is initiated by the reaction of solvated electron with the thiol to release HS-ions. The polynuclear complexes of cadmium with the thiolate form of RSH act as moderators to the growth of the particles and allow reproducible production of practically any predetermined size particles. The complexes at the surface of the particles also stabilize the particles for long periods of time. The particles are strongly fluorescent from size of ca. 8 8, in radius. Excess electrons in these particles lead to bleaching of the exciton band due to Coulomb screening effects. At higher doses, formation of cadmium atoms leads to increased absorption in the visible range.
Introduction
Ultrasmall particles have been a subject of extensive studies in recent years1 Prominent among the materials studied in this respect is CdS, whose properties in the subcolloidal size regime were shown to correlate with the evolution of the particle's dimensions. Due to the confinement of the electron and hole in the small particles the physical (e.g., band gap and lowest exciton energy2) and chemical (e.g., redox potentials3) properties are appreciably modified from their bulk values. This provides added incentive to the quest for methods to control the size and the size distribution of the particles. Among the various approaches proposed to control the size of the particles, arrested growth at low temperatures and in nonaqueous solutions,' confinement within ~ , ~ and reversed micelles$ vesicle^,^ polymer films: g l a s s e ~ ,clays: zeolites8q9were all utilized. Yet generation of colloidal particles of predetermined sizes is still rather difficult. In recent communications the utility of radiation chemical techniques to generate and study the growth process of silver halide particles was demonstrated.1° It was noted then that the technique is quite versatile and may be used to generate, study the mechanisms of, and then control the growth of other sparingly soluble salts as well. In the present report we describe the radiolytic generation and control of extremely small CdS pa,rticles. A recent report of the chemical formation of CdS colloids in the presence of thiols appeared during this study." Some of the experimental observations reported are similar to ours and the end product seems identical. We, however,
'Argonne National
Laboratory. *Boris Kidric Institute of Nuclear Chemistry.
0022-3654/89/2093-4603$0 1.50/0
emphasize the role of complexation of CdZ+ions with thiolates, both in solution and at the colloid surface, in controlling the particle size. Steady-state ?-irradiation of the aqueous thiol solutions was used to homogeneously release sulfide ions into the solution which also contained Cd2+ions. Dissociative addition of solvated electron to thiols (reaction I ) leads to the formation of a supersaturated RSH
+ eaq-
-
R'
+ HS-
(1)
solution of CdS and the growth of particles ensues. Key to the ( I ) For comprehensive reviews see: (a) Brus, L. E. J . Phys. Chem. 1986, 90, 2555. (b) Henglein, A. Top. Curr. Chem. 1988, 143, 113. (2) (a) Brus, L. E. J . Chem. Phys. 1984,80,4403. (b) Rossetti, R.: Ellison, J . L.; Gibson, J. M.; Brus, L. E. J . Chem. Phys. 1984, 80, 4464. (3) NedeljkoviC, J. M.; NenadoviC, M. T.; Mi%, 0. 1.; Nozik, A. J. J . Phys. Chem. 1986, 90, 12. (4) (a) Dannhauser, T.; ONeil, M.; Johansson, K.; Whitten, D.; McLendon, G. J . Phys. Chem. 1986,90,6074. (b) Lianos, P.; Thomas, J. K. Chem. Phys. Lett. 1986, 125, 299. (5) Watzke, H. J.; Fendler, J. H. J . Phys. Chem. 1987, 91, 854. (6) (a) Wang, Y.; Suna, A.; Mahler, W.; Kasowski, R. J . Chem. Phys. 1987, 87, 7315. (b) Mahler, W. Inorg. Chem. 1988, 27, 435. (7) (a) Ekimov, A. 1.; Efros, AI. L.; Onuschenko, A. A. Solid Sfafe Commun. 1985, 56, 921. (b) Rajh, T.; VucemiloviE, M. I.; DimitrieviE, N. M.; MiEii., 0. I.; Nozik, A. Chem. Phys. Leu. 1988, 143, 305. (8) Stramel, R. D.; Nakamura, T.; Thomas, J. K. J . Chem. Soc., Faraday Trans. I 1988, 84, 1287. (9) Wang, Y.; Herron, N . J . Phys. Chem. 1987, 91, 257. (IO) (a) Schmidt, K. H.; Patel, R.; Meisel, D. J . Am. Chem. SOC.1988, 110, 4882. (b) Hayes, D.; Schmidt, K. H.; Meisel, D. J . Phys. Chem.. in press. ( I I ) Nosaka, Y.; Yamaguchi, K.; Miyama, H.; Hayashi, H. Chem. Letf. 1988, 605.
0 1989 American Chemical Society
