J. Phys. Chem. 1984, 88, 2183-2185 from the different mechanisms of the autocatalytic reaction sequences"J3 and from the far higher reaction rate constant of the rate-determining step in the case of the front wave.14 Higher intensities of positive electric fields cause qualitatively different effects in the two cases. Front waves are stopped when E is between 5 and 8 V/cm and a steady concentration inhomogeneity (similar to the Turing structure) arises in the reactor at the same time. Higher field intensities ( E > 8 V/cm) cause a reversible annihilation of the front wave. When the field is switched off the wave is regenerated a t the position of its annihilation and propagates with the velocity vo. Both effects are documented in Figure 2 which depicts the time dependence of the front wave position. Two waves-right (R) and left (L)-propagate from opposite ends of the capillary (Le., in opposite directions). In the first section ( I = 1, E = 0), both waves propagate with a velocity vo. In the second section ( I = 2, E = 8 V/cm), the right wave is accelerated (it moves toward the positive electrode), while the left wave (moving toward the negative electrode) is stopped and a zone with different concentrations of reaction components (denoted by a dark rectangle in Figure 2a or as SIIin Figure 2b) is formed at the same position. When the electric field is switched off ( I = 3, E = 0) a front wave separates from the boundary of the stagnant zone and moves in the original direction with the velocity vW The stagnant zone does not change its position. The right wave also propagates with a velocity vg. In the fourth section, the electric field with intensity E = 10 V/cm is switched on again. The right wave is again accelerated while the left wave is annihilated. When the field is switched off again ( I = 5 ) , the left wave reappears; after a while it takes on its normal form and propagates in the original direction with velocity vo. After the right and left waves collide, mutual annihilation of the waves occurs and the final nonhomogeneous stationary state (dissipative structure DS) is established in the reactor. A schematic concentration profile of this final state a t the time t3 is shown in Figure 2b. Detailed study of pulse waves revealed the existence of another importnat nonlinear effect at higher intensities of positive electric (14) SevEikovi,H. Ph.D. Thesis, Prague Institute of Chemical Technology, 1983.
2183
/
/
.7[v/crn]
0
Figure 3. Schematic picture of pulse wave splitting in an electric field: P, pulse wave (high concentration of [FeSt]); SI,homogeneous reaction medium (low concentration of [Fe"]).
fields-a splitting of the pulse wave. At E 10-20 V/cm an unsymmetric leading center (pacemaker) arises behind the original pulse wave. The pacemaker sends out waves in a direction opposite to the original direction of wave propagation. The period of the wave initiation in this pacemaker is shorter than the corresponding period of pulse wave generation in the reacting medium at E = 0. The refractiveness of this reacting medium is thus dependent on the applied field intensity! The spatial-temporal characteristics of the wave splitting is schematically shown in Figure 3. The process of the wave splitting can be used for the controlled generation of the pulse wavesS8 At E = 20 V/cm only a single pulse is split off the original wave which is annihilated at the same time; this can be also interpreted as a reversal of the direction of wave propagation. Irreversible pulse wave annihilation occurs at E > 20 V/cm. A homogeneous stationary state is established in the system; a new supercritical perturbation is necessary to evoke the formation of a new pulse wave. A detailed discussion of the effects of an electric field on the two types of waves based on the chemistry of the reactions and the mechanism of wave propagation will appear elsewhere.8,12
Electron Spin Resonance Spectra of Matrix- Isolated Mixed Copper-Silver Pentamers' J. A. Howard,* R. Sutcliffe? National Research Council of Canada, Ottawa, Ontario, Canada K I A OR9
and B. Mile Department of Chemistry and Biochemistry, Liverpool Polytechnic, Liverpool, England L3 3AF (Received: February 17, 1984)
The neutral mixed group 1B metal atom pentamers CuAg, and Cu2Ag3isolated in cyclohexane have been prepared in a rotating cryostat at 77 K and their ESR spectra recorded. These spectra indicate that these two pentamers have essentially isotropic A tensors and anisotropic g tensors. They both have a trigonal bipyramidal array of metal atoms and CuAg, has one Cu and one Ag atom and CuzAg3has two Cu atoms which bear most of the unpaired s spin population. Electronic ground states cannot be unambiguously assigned to these heteronuclear pentamers although 'A, in C3, and 2A2)1in D j hare favored for CuAg, and Cu2Ag3,respectively.
Introduction
In two previous publication^^*^ from these laboratories electron spin resonance spectroscopic (ESR) evidence has been presented for the production of the ligand-free homonuclear neutral five atom f
NRCC Research Associate.
0022-3654/84/2088-2183$01.50/0
clusters Cu? and AgS3produced by condensing 63Cuand Io7Ag atoms on the cold surface of a rotating cryostat" and subjecting (1) (a) Issued as NRCC No. 23112. (b) Cryochemical Studies, Part 12. For part 11, see Howard, J. A,; Sutcliffe, R.; Tse, J.; Mile, B.Organometallics,
in press. (2) Howard, J. A.; Sutcliffe, R.; Tse, J.; Mile, B. Chem. Phys. Lett. 1983, 94, 561-4.
Published 1984 American Chemical Society
2184
The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 TABLE I: ESR Parameters for 63Cu/’07AePentamers
,,0,,
B
i 1
I l l I I1 I I 1 I 1 JB
I
Letters
pentamer cu< Ag; (I) Ag5 (11) CuAg, (I)
factor 2.055 2.085 2.002 2.060
CuAg4 (11)
1.994
JA
-
CuzAg3 ( I ) 2.050 CuzAg3 (11) 1.994
-
’V 59157MHz
3200G
400G
I
, Ill I !I
I,,
U
I
I
,
I
I
]A
B
Figure 2. The ESR spectrum given by codeposited 63Cuand Io7Agobtained under conditions similar to that shown in Figure 1 at a lower ratio of Ag to Cu. A and B again refer to Cu2Ag3and CuAg.,. the deposit to photolysis before entrapment in an inert hydrocarbon matrix. We have also demonstrated that cocondensing Cu and Ag atoms in the absence of light gives the heteronuclear triatomic cluster C U A ~ C UIn . ~the present paper we report the results of similar cocondensation experiments with Cu and Ag in the presence of photolysis which give products that have been tentatively identified as the heteronuclear pentamers CuAg4 and Cu2Ag3* Experimental Section The rotating cryostat4v5and furnace used to vaporize the metals have been described previously.6 In the present work two furnaces were used to codeposit lo7Agand 63Cuobtained from Oak Ridge National Laboratory, TN. The deposit was photolyzed, after the second metal had been laid down and before the inert matrix jet, with light from a 250-W extrahigh-pressure mercury lamp which had passed through a Corning 0-52 filter (A > 320 nm). Deposits were scraped from the surface of the drum of the cryostat into an ESR tube at 77 K and examined by ESR spectroscopy (Varian E-4 spectrometer). The microwave frequency of the spectrometer was measured with a Systron-Donner Model 6016 frequency counter and the magnetic field with a Varian E-500 N M R gaussmeter. ESR parameters were calculated from exact solutions of the spin Hamiltonian with computer programs provided by Drs. J. R. Morton and K. F. Preston (NRCC). Results and Discussion The ESR spectra obtained from codeposited lo7Ag and 63Cu atoms in cyclohexane under the influence of light are shown in Figures 1 and 2. Figure 1 gives the spectrum resulting from lo7Ag (24.2 mg)8 bombarded with 63Cu(2.0 mg)8 followed by photolysis and isolation in cyclohexane. The line labeled C is the AM, = h l , MI = -3/2 ESR transition from isolated 63Cuatoms and the one labeled D is the MI = -3/2 ESR transition from a Cu monoligand 7r complex which is probably CU[C&] produced from (3) Howard, J. A.; Sutcliffe, R.; Mile, B. J. Phys. Chem. 1983, 87, 2268-7 1. (4) Bennett, J. E.; Thomas, A. Proc. R. SOC.London, Ser. A 1964, 280, 123-38. (5) Bennett, J. E.; Mile, B.; Thomas, A,; Ward, B. Adu. Phys. Org. Chem. 1970, 8, 1-77. ( 6 ) Buck, A. J.; Mile, B.; Howard, J. A. J . Am. Chem. SOC.1983, 105, 3381-7. (7) Howard, J. A.; Sutcliffe, R.; Mile, B. J. Am. Chem. SOC.1983, 105, 1394.
G 608 -201 -212 565 -227 579 -234 619 635
PM
0.28 0.31
(2) 15
PM
(1)
PM
0.007
5.5
0.008
30 11
0.014 0.016
0.26 0.35 0.29
a trace of benzene in the cyclohexane.6 The multiplet of lines labeled A can best be analyzed in terms of a species containing two equivalent Cu atoms ( I = 3/2) with acu(2) = 619 G, 1777 MHz, and g = 2.050. Of the expected 16 lines for such a species 11 can be seen to occur a t the fields expected from an exact solution of the Breit-Rabi equation. This spectrum shows no resolvable superhyperfine interaction and very little A anisotropy (Al, = 619 G, 1777 MHz, A, = 635 G, 1772 MHz) although there is resolvable g anisotropy (gll = 1.994, g, = 2.050). A second more intense almost isotropic spectrum labeled B was also formed in this system and it can be analyzed in terms of a quartet of doublets with a quartet interaction of 560 G (1614 MHz) and a doublet interaction of 227 G (654 MHz). Clearly this species has one Cu and one Ag atom which bear most of the unpaired s spin population; g, for this species was calculated to be 2.060. There are weak absorptions which are assigned to parallel features, with the parameters gll = 1.994, All(Ag)= -234 G (-653 MHz), and All(Cu)= 579 G (1613 MHz). As with the spectrum from A this spectrum showed no superhyperfine interactions from other metal atoms in the cluster. The ESR spectrum from 63Cu(4.2 mg)8 bombarded with lo7Ag (1 8.4 mg),* photolyzed, and covered with cyclohexane is shown in Figure 2. This spectrum shows all the features that are found in Figure 1. The transitions from 63Cuatoms and Cu[C6H6] are, however, much less intense and spectrum A is much more prominent than spectrum B. The species responsible for A would, therefore, appear to be preferentially formed at higher ratios of Cu to Ag. In the absence of superhyperfine interactions the unambiguous identification of the species responsible for spectra A and B is impossible. However, the fact that they have two atoms which bear most of the unpaired s spin population and are observed under experimental conditions which give high concentrations of the neutral homonuclear pentamers Cu5 and Ag, from the individual metals provides strong evidence that they are neutral heteronuclear m = 5. pentamers with the empirical formula Cu,Ag, with n The absence of resolved superhyperfine interactions suggests that the “invisible” atoms are Ag and not Cu because Cus has wellresolved splitting from three copper atoms with small hyperfine interactions whereas the analogous atoms in Ag5 give poorly resolved superhyperfine interactions. Species A and B would then be Cu2Ag3 and CuAg,, respectively. This assignment, while somewhat tentative, is consistent with the fact that A and B are formed when high ratios of Ag to Cu are codeposited and the ratio of A to B increases when the ratio of Cu to Ag is increased. It should be noted at this point that species A and B are not the principal pentamers produced in these experiments. The center of the spectrum is dominated by a triplet centered at g 2.08 from species with most of the unpaired spin population on two Ag atoms. Since there is again no resolvable superhyperfine interactions associated with these lines they originate from Ag, superimposed with Ag5-,Cu, where n = 1, 2, or 3 with the appropriate array of atoms. The ESR parameters for group 1B pentamers are given in Table I along with the unpaired s spin populations estimated from the calculated values of the hyperfine interaction for unit spinpopdation in the s orbitals of 63Cu and 107Ag.9 The g factors for
+
-
(8) Total weight deposited in -0.5 h.
J . Phys. Chem. 1984,88. 2185-2195 the homo- and heteronuclear pentamers all lie in the range 2.03-2.08 and are significantly larger than the free spin value. This large and positive Ag has been attributed to spin-orbit interaction between the ground state and a neighboring filled level which introduces p and/or d orbital character into the singly occupied molecular ~ r b i t a l . ~ , ~ The unpaired s spin populations on the atoms bearing most of the unpaired s spin population are all similar although it does appear that silver bears a larger spin population than copper. From the measured anisotropic parameters the calculated unpaired s spin populations are reasonable whereas the p and/or d values appear to be essentially zero. It should, however, be realized that the errors involved in measuring these parameters, because of the broadness of the absorptions and the number which can be measured, are probably of the order of k 5 G. These errors are sufficiently large that while they have little effect on the calculated unpaired s spin populations they make evaluation of the p and/or d contribution less meaningful because only small changes are required in the hyperfine splittings to make substantial differences to the estimated populations. We have previously concluded that CuS2and Agj3 have a distorted trigonal-bipyramidal structure with a 2Bzin C, electronic ground state, i.e., most of the unpaired s spin population resides on the terminal equatorial atoms. The ESR parameters listed in Table I are consistent with similar structures and electronic ground states for the homo- and heteronuclear pentamers although (9) Morton, J. R.; Preston, K. F. J . Magn. Reson. 1978, 30, 577-82.
2185
the undistorted trigonal-bipyramidal structures for CuAg, and CuzAg3have C,, symmetry and do not need to distort to lift the degeneracy of a 2E ground state as is the case with the homo, structure indicates a *B2ground state nuclear pentamers. A C for CuzAg3whereas a ground state cannot be assigned to CuAg4 with this symmetry. We have previously noted3 that a ZB1ground state with most of the unpaired s spin population on the axial atoms of a trigonal bipyramid cannot be discounted for Ag, (and Cu,). CuAg, with the copper atom and unique silver atom occupying the axial positions, Le., a ,Al (C3,) ground state and CuzAg3with the two Cu atoms occupying the axial positions, Le., a ,A2/1 (D3h) ground state, are, therefore, possible structures for the mixed pentamers. These structures are in fact more reasonable than C, structures because the smaller Cu atom might be expected to prefer an axial position in a close-packed structure. Unfortunately, electronegativity and thermodynamic arguments are inconclusive in determining the distribution of atoms in heternuclear trigonal-bipyramidal clusters because Cu and Ag have similar electronegativities and bond strengths are not known with the required accuracy. Finally, although we cannot unambiguously assign electronic ground states to CuAg, and Cu2Ag3they have a trigonal-bipyramidal structure. Furthermore, the results presented in this paper demonstrate that bimetallic clusters of a reasonable size can be produced by simple orbital mixing processes and identified by ESR spectroscopy using our techniques. Acknowledgment. J.A.H. and B.M. thank NATO for a collaborative research grant.
FEATURE ARTICLE Structure Sensitlvlty of Hydrocarbon Synthesls from CO and H, M. Boudart* and Mark A. McDonald Department of Chemical Engineering, Stanford University, Stanford, California 94305 (Received: September 29, 1983)
Hydrocarbon synthesis from CO-HZ mixtures on group 8 metals is discussed from the viewpoint of structure sensitivity. On the most active metals which dissociate CO and produce a broad distribution of hydrocarbons under reaction conditions, the synthesis seems to proceed at the highest rate on large ensembles of surface atoms, pointing to structure sensitivity. However, for the less-activegroup 8 metals, which do not dissociate CO readily and do not produce substantial amounts of hydrocarbons heavier than methane under reaction conditions, no clear trends can be discerned. Hydrocarbon synthesis on these catalysts can be structure sensitive under some conditions, but the effects of structure are quite different from those observed for the most active catalysts. Thus, our review suggests that both the nature of the catalyst surface and the mechanism of methane synthesis vary substantially among group 8 metals, so that no general conclusion can be made about the structure sensitivity of methane synthesis.
I. Introduction The catalytic synthesis of hydrocarbons from CO and Hz has a long history. Eighty years ago Sabatier f i s t made methane from C O and Hz, a catalytic reaction now called methanation.’J Catalytic production of liquid hydrocarbons and other organic molecules from a CO-H, synthesis gas now called syngas has been carried out since the 1920’~.~3, This is the so-called Fischer~
(1) Sabatier, P.;Senderens, J. B. Compt. Rend. 1902, 134, 512. (2) Sabatier, P.; Senderens, J. B. J . SOC.Chem. Ind. 1902, 31, 504.
Tropsch (FT) synthesis, commercialized in Germany 50 years ago and currently used in South Africa to make liquid fuels from coal., The name FT synthesis will be used in this work even though we shall be concerned mostly with work at low conversion and atmospheric pressure, conditions that are remote from those of commercial operation. (3) Fischer, F.; Tropsch, H. Brennst. Chem. 1923, 4, 276. (4) Fischer, F.; Tropsch, H. Brennst. Chem. 1926, 7 , 97. (5) Dry, M. E. In ‘Catalysis, Science and Technology”, Anderson, J. R., Boudart, M., Eds.; Springer Verlag: Heidelberg, 1981; Vol. 1, p 159.
0022-365418412088-2185$01.50/0 0 1984 American Chemical Society
