88
Ind. Eng. Chem. Fundam.
D = dimensionless diffusivity of hydrogen, D = Db/Deff DA = diffusivity of nitrobenzoic acid in water, mz/s Db= diffusivity of hydrogen in the bulk, mz/s Deff= effective diffusivity of hydrogen at the wall, m2/s E, = the activation energy of the reaction, J/mol H e = Henry's constant, m3 bar/mol k = dimensionless constant defined by eq 18 k,,, k,, = mass transfer coefficients, m/s ko = a rate constant, (m3of fluid /m3 of cat. s) k , = rate constant, s-l L, = length of a gas plug, m Le = length of a liquid plug, m N1,N 2 = mass transfer fluxes, mol/m2 s P = total pressure, bar PH2= partial pressure of hydrogen, bar Q, = gas flow rate, m3/s Qe = liquid flow rate, m3/s r l , r2 = rates of hydrogen reaction during the passage of the gas and liquid plug, respectively, through the channel, mol of H/kg of cat. s = mean value of the hydrogen reaction rate, mol of H/kg of cat. s R = the gas constant Sh = Sherwood number t = time, s u = u = variable, u = z2 x = variable wall thickness, m
1984,2 3 , 88-96
X = half wall thickness, m y = dimensionless hydrogen concentration, y = c / c * z = dimensionless wall thickness, z = x / X Greek Letters q =
effectiveness factor
rl = time constant for the gas plug, s 72 $J
= time constant for the liquid plug, s
= Thiele modulus
Registry No. Palladium, 7440-05-3; nitrobenzoic acid, 27178-83-2; aminobenzoic acid, 1321-11-5.
Literature Cited Andersson, 0 . AlChE J . 1982, 2 8 , 333. Bonon, R.; Cosserat, D.; Charpentler, J. C. Chem. Eng. J . 1980, 2 0 , 87. de Vos. R.; Hatziantoniou, V.; SchijGn, N.-H. Chem. Eng. Sci. 1982, 3 7 , 1719. Hatziantoniou, V.; Anhrsson, 0 . I d . Eng. Chem. Fundem. 1982, 21, 451. SanerfieM, C. N.; Ozel, F. Ind. Eng. Chem. Fundam. 1977, 16, 81. Skeggs, L. J. Am. J . Clin. Pathol. 1957, 2 8 , 311. Taylor, G. I. J . FlUMMech. 1981, 161. Viliadsen, J.; Michelsen, L. M. "Solution of Differential Equation Models by Polynomial Approximation": Prentlce-Hall Inc.: Englewood Cllffs, NJ, 1978; pp 135-1751, Votruba, J.; Mikui, 0.;Nguen Khue; HlavBEek, V.; SkiivBnek, J. Chem. Eng. Sci. 1975, 30, 201.
Received for review October 25, 1982 Revised manuscript received August 29, 1983 Accepted October 7, 1983
Adsorption and Catalytic Behavior of Palladium Dispersed on Rare Earth Oxides Mlchael D. Mitchell and M. Albert Vannlce' Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802
Palladium was dispersed on La2O3, CeO,, Pre0,,, Nd2O3, Sm203,and E u ~ O and ~ these catalysts were characterized by XRD, H2,CO, and O2 adsorption, H, absorptlon, and the H2titration of adsorbed oxygen. Results indicated that large fractions of the Pd surfaces were blocked by the oxide support. Turnover frequencies (TOF) for both CH, and C 0 2formation varied by an order of magnitude, and CO, formation appeared to be a consequence of reaction on the rare earth (RE) oxide surfaces. For both reactions a similar pattern existed, and activity maxima at Pr or Nd occurred when plotted vs. atomic number, which strongly inferred that the support plays a direct role in determining the catalytic properties of these WIRE oxide systems. Oxygenate formation seemed to be favored in these catalysts, and only methanol was formed over Pd/Nd203,the one catalyst studied at higher pressures.
Introduction Supported metal catalysts are among the most important materials in heterogeneous catalysis, and the support is usually a high surface area porous material considered to be inert, excluding dual functionality. Metals of importance in catalysis, such as palladium, are typically very expensive and maximum catalytic utilization is obtained by dispersing the metal on a support, thereby obtaining very small crystallites and enhancing the specific metal surface area. Maintenance of this high metal dispersion under process conditions is another key function of the support since separation of the metal particles inhibits agglomeration, which decreases the metal surface area. Although the support is often thought to have little effect on the adsorption and catalytic properties of the metal present on its surface, evidence has been found in recent years which shows that the support can be important in determining the behavior of certain dispersed metal
systems. Such possibilities had first been proposed over two decades ago based upon bulk properties of metals and semiconductors (Schwab et al., 1959; Szabo and Solymosi, 1960). More recently, direct bonding between metal and support has been proposed by Tauster et al. based on their studies of Strong Metal-Support Interaction (SMSI) behavior for noble metals supported on titanium dioxide (Tauster et al., 1978). Low-temperature reduction near 473 K produced well-dispersed metal particles which exhibited normal chemisorption behavior, whereas reduction at 773 K suppressed hydrogen and carbon monoxide adsorption to near zero. They postulated that the SMSI effect was a consequence of localized bond formation between the noble metal and titanium cations or titanium atoms, the latter involving the formation of surface intermetallic compounds. Later, Tauster and Fung studied Ir dispersed on nine oxides belonging to groups IIA, IIB, IVB, VB, and they found
0196-431318411023-0088$01.50/0 0 1984 American
Chemical Society
Ind. Eng. Chem. Fundam., Vol. 23, No. 1, 1984
evidence for a correlation between SMSI behavior and bulk reducibility among these transition metal oxides, as evidenced by suppression of H2 chemisorption (Tauster and Fung, 1978). Baker et al. (1979a,b) used TEM to investigate the structure and growth characteristics of platinum particles under SMSI conditions, and they found that with TiOzdispersed Pt a high temperature reduction (973-1073 K) produced small Pt particles that were predominantly thin hexagons of uniform thickness indicating "raft-like" morphology on a lower oxide of titania, Ti407. Later, these same authors reported that the SMSI state could be destroyed by a high-temperature oxidation of Pt/TiOz, which resulted in hemispherical Pt crystallites which had normal chemisorption properties after reduction at 473 K (Baker et al., 1979a,b). Electron diffraction analysis demonstrated that the Ti407support had also undergone a structural change, reverting to TiOz. Subsequent theoretical studies by Horsley (1979) favored a model invoking electron transfer from Ti3+cations of Pt atoms. Recent XPS studies have supported this model (Kao et al., 1980; Kao et al., 1982) thereby providing evidence for modification of metallic properties by the support. Perhaps more importantly from a catalytic perspective, Vannice and co-workers have shown that significant activity enhancement for CO hydrogenation occurs for titania-supported Ni (Vannice and Garten, 1979b, 1980a), Pt (Vannice et al., 1980), and certain other group VI11 metals (Wang et al., 1981; Vannice, 1982). Marked changes in selectivity were also observed for Ni/Ti02 (Vannice and Garten, 1979b, 1980a) and Ru/TiOz (Vannice and Garten, 1980b). Palladium has become especially interesting because at higher pressure, under favorable conditions, it can be a selective catalyst for methanol synthesis (Poutsma et al., 1978),whereas at higher temperatures and lower pressures it produces only methane (Vannice and Garten, 1979a). Since the first report by Rabo and co-workers of methanol formation over Pd (Poutsma et al., 1978), this metal has received much attention (Vannice and Garten, 1979a; Ichikawa and Shikakura, 1980; Ichikawa, 1982; Poels et al., 1981; Ryndin et al., 1981; Fajula et al., 1982; Doering et al., 1982; Palazov et al., 1982). It has been quite clearly shown that the selectivity to methanol is extremely dependent upon the support; however, the reason for this selectivity switch is not yet clear although various explanations have been proposed (Poels et al., 1981; Ryndin et al., 1981; Fajula et al., 1982). This investigation of Pd dispersed on rare earth (RE) oxides was initiated for several reasons. First, a systematic study of the RE oxides as catalyst supports has not been conducted, and in view of the possibility of support effects, such an investigation seemed to be warranted. In this study a series of Pd catalysts was prepared which consisted of Pd supported on La2O3, Ce02,Pr6OI1,NdzO3, SmzO3 and Euz03-lanthanide Re oxides which exhibit smoothly varying periodic trends in surface basicity and cationic If-electron configuration. Second, several of these oxides, CeOz,Pr6011,Sm203,and Euz03,possess lower oxidation states, similar to Ti02, while La203and Ndz03 do not exhibit bulk reduction. This allows a test for the importance of bulk reducibility of the support. In addition, since no d orbitals are available for bonding in these RE oxides, the importance of d orbitals in SMSI could be assessed. In particular, this study was concerned with how trends in basicity, electronic configuration, and free energy of reduction of the supports might correlate with catalytic behavior. Finally, Pd was chosen not only because it is
89
Table I. Pd Loadings (Wt 7%) and Support Surface Areas initial N, final N, wt 9% BET area, BET area, cat. Pd m2/g m'/g 0.84 12.6 Pd/ La 2 0 3 0.77 Pd/CeO, 0.74 15.9 same P d P , 0, 0.70 0.46 10.1 Pd/ Nd ,0 23.9 same 0.81 Pd/Sm,03 0.77 2.8 8.4 Pd/Eu,O 0.61 37.1 same
susceptible to SMSI behavior, but also because of its intriguing selectivity dependence on the support in CO hydrogenation. These catalysts were characterized by using Hz, CO, and O2chemisorption, H2-02titration, and XRD, and CO hydrogenation was used as a probe reaction to study the kinetics of both CHI and C02 formation. Experimental Methods Materials and Catalyst Preparation. The rare earth (RE) oxide support materials used in this study were lanthanum oxide, Laz03,99.9% from Molycorp, Inc., along with cerium oxide, CeOz, 99.9%, praseodymium oxide, Pr6OI1,99.9%, neodymium oxide, Nd203,99.9%, samarium oxide, Sm203, 99.9%, and europium oxide, Eu203,99.9%, from Sigma Chemical Co. The initial oxide phases reported by the manufacturer were initially confirmed using XRD and are listed in Table I. A procedure similar to that used by Rosynek and Magnuson (1977) was used to increase the surface area of the lanthanum, praseodymium, and samarium oxides: the initial oxides were hydrolyzed in an excess of distilled, deionized (DD) water for 16 h at 80 "C, filtered, and dried overnight in air at 120 "C. Rosynek and Magnuson (1977) have shown that this procedure results in complete conversion to the trihydroxide, RE(OH)3,and yields a product having a considerably larger surface area and smaller particle size than the original starting material. These higher surface area trihydroxides were then heated in 2 SCFH helium (Airco, 99.99%) or 2 SCFH air (Airco, Grade Zero 0.1) at 450-500 "C for 3 h in a Fischer Isotemp tube furnace. This heating procedure ensured complete conversion to the original oxide and increased the surface area by factors of 4 to 25 times, as shown by the initial and final Nz BET areas in Table I. All the catalysts listed in Table I1 were prepared by an incipient wetness technique. The amount of DD water needed to fill the pore volume of each support material was determined experimentally by adding water dropwise to a known weight of support and stirring thoroughly after each drop. The support maintained a powdery texture until the pore volume was filled, at which point the next drop would produce a slurry in the beaker. The required volumes of water needed for each support were 0.84 cm3/g for lanthanum oxide, 0.39 cm3/g for cerium oxide, 0.23 cm3/g for praseodymium oxide, 0.47 cm3/g for neodymium oxide, 0.32 cm3/g for samarium oxide and 0.47 cm3/g for europium oxide. Palladium chloride (PdClZfrom Ventron Corporation) was used as the metal salt in the catalyst preparations. The amount of PdClz needed to give a nominal 1wt % Pd loading was dissolved in concentrated hydrochloric acid, evaporated twice to near dryness, and then diluted with just enough DD water to fill the pore volume of the support. This solution was then added dropwise to the dried support powder with thorough stirring after each addition, and these impregnated catalysts were then dried in air at 120 "C for 16 h, bottled, and stored in a desiccator. The gases used in the chemisorption and kinetic experiments, Hz(Airco, 99.999%), CO (Matheson, 99.99%), He (Airco, 99.9999%), and Oz (Airco, 99.993%), were pu-
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Table 11. Oxide Phases Identified after Pretreatment Procedures Pd catalysts __
__._______l_-_l-_
starting support material
LTR chemisorption only
Eu20,
-
-
-
___
___
HTR chemisorption only
blank supports
_____
_ _ i _ _ _ _ _ _ _ _ I _ _
LTR used sample
EuOOH(?)
HTR used sample
LTR chemisorption
HTR chemisorption
LTR used
HTR used
-
Dashes indicate analysis not determined.
rified further before use. Hz was passed through a Deoxo unit (Engelhard Ind.), a 5-A molecular sieve trap, and an Oxy-trap (Alltech Associates). The He was passed through a Drierite molecular sieve gas purifier (Alltech Associates) followed by an Oxy-trap. Both the H, and He were supplied to the chemisorption system through external lines. CO was further purified by passage first through a 5-A molecular sieve trap to remove water and carbonyls and then through a dry iceacetone bath prior to storage in 5 L gas bulbs attached to the adsorption system. Oxygen was purified further by passage through a dry ice-acetone bath before being stored in a 5-L gas bulb attached to the adsorption system. Determination of Pd Loadings. The palladium content of every catalyst was determined by flame emission spectroscopy (FES) analysis which was performed by the Mineral Constitution Laboratory at The Pennsylvania State University with a Spectraspan I11 A emission spectrometer. The spectrometer was calibrated with a series of standards containing various known concentrations of palladium dissolved in hydrochloric acid, and the plot of intensity vs. concentration was linear (Mitchell, 1982). With the exception of Ce0,-supported Pd, the catalyst samples completely dissolved in a dilute HC1 solution; however, the good agreement between FES and neutron activation analysis shows that all the Pd was leached from the CeOP The palladium weight loadings were determined in two different runs by FES, and they agreed to within &0.02%. The average values are listed in Table I. Palladium weight loadings of the fresh, unreduced catalysts were also determined by neutron activation analysis (NAA) when no interference from the support occurred, i.e., for Pd/Ce02 and Pd/NdzOs, and excellent agreement was found with the Pd/Ce02 catalyst. The NAA results for Pd/Nd203were about 10% higher, but they are less accurate because the overlap of Pd and Nd decay rates could not be completely resolved. X-ray Diffration (XRD). XRD experiments were performed on a Rigaku Geigerflex Model 4036-A1 diffractometer using Cu Ka radiation. The Pd(ll1) peak was used for line broadening calculations, and samples were typically scanned in the 28 range of 10 to 100' at a scanning speed of 4 O m i d , while a slower scan of 1' min-' was utilized for 28 from 42 to 55'. XRD measurements were performed on both the pure RE oxides and the reduced catalysts using powder samples packed into an aluminum holder backed by a glass slide. Metal crystallite sizes, when Pd peaks were visible, were calculated from the line width of the Pd(l11) peak by use of the Schemer equation d, = KX/P cos eB with Warren's correction for instrumental broadening, which was determined to be 0.18O at a 28 value of 40.2' (Mitchell, 1982).
Adsorption Measurements. Chemisorption of hydrogen, carbon monoxide, and oxygen at 300 K, and the hydrogen titration of chemisorbed oxygen at 373 K were performed in a glass high-vacuum system. The system utilized an Edwards Model E02 oil diffusion pump with liquid nitrogen traps located on both sides, which provided an ultimate vacuum of lo4 torr. A Texas Instruments Model 145 precision pressure gage was used to measure gas pressure during isotherm measurements. Details have been given elsewhere (Palmer and Vannice, 1980). Two samples of each catalyst were used in the chemisorption studies: one sample was given a low-temperature reduction (LTR) at 448 K while the other sample was given a high-temperature reduction (HTR) at 773 K following the two pretreatment procedures used by Tauster et al. (1978). The sequence of adsorption experiments on a given sample was as follows: (1)pretreatment; (2) Hz chemisorption; (3) evacuation at room temperature for 30 min; (4) H, back-sorption; (5) repeat pretreatment in ambient (nonflowing) H,; (6) 0, chemisorption; (7) evacuation for 30 min; (8) heat to 373 K for Hz titration of chemisorbed oxygen; (9) determination of He dead volume at 373 K; (10) pretreatment; (11)first CO isotherm; (12) second CO isotherm after 2-min evacuation at 300 K; (13) determination of He dead volume at 300 K. A sample of each RE oxide was subjected to identical pretreatment and chemisorption procedures so that blank uptakes could be determined on the support in the absence of Pd. In addition, each Pd catalyst was further characterized by Hzand CO chemisorption after its use in the kinetic studies using the same pretreatment employed with the fresh catalyst. The chemisorption procedure used for the Pd/CeO, catalystswas slightly different from that used for the other catalysts because only one Pd/Ce02 sample was studied, and the procedure used was as follows: (1) LTR pretreatment; (2) LTR chemisorption; (3) HTR pretreatment; (4) HTR chemisorption; (5) HTR kinetic studies; (6) HTR pretreatment; (7) HTR chemisorption. Each chemisorption isotherm consisted of five or six data points typically covering a pressure range 50 to 300 torr (1 torr = 0.133 kPa) with all measurements made in the direction of increasing pressure. Thirty minutes was allowed for equilibration of the initial data point although pressure was usually stabilized after 20 min. The remaining data points were recorded after allowing 5 min for equilibration at each successive pressure. A set of representative adsorption isotherms for one catalyst system in Figure 1, while all isotherms are shown elsewhere (Mitchell, 1982). The method of Benson, Hwang, and Boudart was used to determine both hydrogen chemisorbed on the Pd surface and that absorbed to form the bulk p-hydride phase (Benson et al., 1973). The first isotherm represents the total H uptake-both adsorbed and absorbed hydrogen-
Ind. Eng. Chem. Fundam., Vol. 23, No. 1, 1984 91 20
I -0-0-
0 -
-0-0-.-e-
16
O-
/.-.
0-0-
o/o-
.
0 -
-, .
-0
18
14
O
'
$10
O
.-1 I
-0
a --.,
/o-
/o
,
. , O
100
200 Pressure [torr)
Figure 1. (a) Adsorption at 300 K on Pd/Nd203after LTR pretreatment: (0) 1st H2 isotherm; ( 0 )2nd H2 isotherm; (A)1st CO isotherm; (A)2nd CO isotherm; (v)O2 isotherm; (0)H2titration. (b)Adsorption at 300 K on Pd/Ndz03after HTR pretreatment: (0) 1st Hzisotherm; ( 0 )2nd H2isotherm; (A)1st CO isotherm; (A)2nd CO isotherm; (v)O2 isotherm; (0)H2 titration.
and evacuation for 30 min at room temperature completely destroys the @-hydridephase (Aben 1968); therefore, the second (backsorption) isotherm represents the amount of hydrogen absorbed into bulk palladium since hydrogen chemisorbed on the palladium surface cannot be pumped off at room temperature (Benson et al., 1973). The irreversible chemisorption on the palladium surface was obtained by the difference between the two isotherms at 250 torr. The technique of Yates and Sinfelt (1967) to measure CO chemisorption involves the measurement of two isotherms. Evacuation of the sample at 300 K for 2 min after the first isotherm removes only the weakly bound CO, and the difference between the two isotherms at 100 torr was chosen to represent the irreversible CO chemisorption on the palladium surface. Oxygen adsorption at 300 K and hydrogen titration at 100 "C were performed using the method of Benson et al. (1973). The linear high-pressure region of the isotherms was extrapolated to the zero-pressure intercept to determine the irreversible oxygen uptake on the catalyst. The cell was evacuated at room temperature for 30 min before heating to 373 K under dynamic vacuum as previous work has shown that no 0,desorbs during this step (Boudart and Hwang, 1975). Hydrogen titration of the adsorbed oxygen was measured at 373 K because the P-phase hydride phase does not form at hydrogen pressures below 350 torr (Aben, 1968). Kinetic Measurements. Kinetic studies were performed in a steady-state, plug flow microreactor operated at 760 torr total pressure. Catalyst charges were typically 0.5-1.0 g, with sample weights and space velocities such that CO conversions were kept below 5% to eliminate heat and mass transfer effects. Calculations based on the Weisz criteria showed the absence of any diffusional effects
(Weisz, 1957). Gas feed rates were measured with Hastings-Raydist mass flowmeters and the effluent from the reactor was analyzed using a Perkin-Elmer Sigma 3 gas chromatograph with Chromosorb 102 columns and subambient temperature programming capabilities. Peak areas of the separated components in the exit gas were determined by a Perkin-Elmer Sigma 10 data analyzer. To achieve stable operation in the microreactor, the H2/C0 feed stream was flowed over the catalyst for 20 min at 24 cm3min-l before a sample was taken for analysis. The CO flow was then stopped and pure H, was flowed for 20 min to help maintain a clean metal surface. This bracketing technique with Hz has been found to be effective in eliminating complications due to catalyst deactivation. All catalyst samples used in the microreactor had been previously characterized by chemisorption techniques with the exception of the (LTR) 0.74 % Pd/CeOP catalyst. The pretreatment procedure used in the kinetic studies was identical with that used in the chemisorption characterization of the catalysts. After pretreatment, the catalyst sample was brought to reaction temperature, usually 523 K, and held for 30 min. The temperature of the reactor was then increased about 10 K after the sample had been taken for GC analysis. Additional activity data were obtained during a descending temperature run to check for deactivation. The temperature range used in the kinetic studies was normally 523 to 573 K. An identical study was conducted on each pure RE oxide.
Results X-ray Diffraction (XRD). X-ray diffraction studies of the initial oxide starting materials confirmed the presence of the oxide phases listed in Table I. Several catalysts were examined by XRD after the chemisorption experiments, but prior to kinetic studies, while all of the catalysts were examined by XRD after use in the kinetic studies. No distinct Pd diffraction peaks were observed for any catalyst sample, regardless of the pretreatment procedure; however, for the Pd/La203 and Pd/Ndz03 catalysts, a large diffraction peak from the RE oxide overlapped any Pd peak. In order to ensure that palladium could be detected at the weight loadings used in this study, a physical mixture of 0.7% Pd black-Ce0, was prepared and analyzed by XRD. A distinct Pd (111)diffraction peak was observed a t 28 = 40.15", and the calculated crystallite size was 470 A, in good agreement with that expected for Pd black. Although no large Pd crystallites could be detected, the XRD powder patterns were useful because they revealed changes that occurred in some of the RE oxide supports. Because an aqueous technique was used during preparation, each of the initial oxide materials in Table I was at least partially converted to either the hydroxide, RE(OH)3, or the oxyhydroxide,RE(OOH), phase. As the catalyst was subjected to the various high-temperature treatments, including the kinetic study, the support material dehydrated and underwent structural changes, and the oxide phases that were identified after some of the chemisorption and kinetic experiments are listed in Table 11. Chemisorption, Table I11 presents initial gas uptakes on the fresh catalysts while Table IV shows the final uptakes on the used catalysts after completion of the kinetic studies. To determine if any irreversible adsorption occurred on the support, experiments were performed on the pure RE oxides, and these results are presented in Table V. Initially, measurements were made on the pure oxides to correct for any irreversible CO adsorption on the support; however, this procedure could not always be used with the catalyst systems in this study because CO uptakes
92
fnd. Eng. Chem. Fundam., Vol. 23, No. 1, 1984
Table 111. Initial Adsorption Measurements on Fresh Catalysts LTR pretreatment __________I__ _ _ I _
--__l-_-_____ _ l I _ _ _ _
chemisorption, pmol g-l
--
cat.
H2
co
0,
H, (titer)
H/Pdt
CO/Pdt
Pd/La, 0, Pd/ CeO Pd/Pr,O,, Pd/Nd,O, Pd/Sm,03 Pd/Eu,O,
2.0 2.7 2.8 2.7 3.7 2.5
6.0 5.0 4.2 2.5 5.7 8.5
7.7 36.0 5 .O 3.6 7.5 19.2
18.2 63.0 16.0 9.8 22.8 38.1
0.055 0.081 0.084 0.072 0.101 0.087
0.083 0.075 0.063 0.033 0.078 0.148
cryst. a size, nm 21 14 13 16
11 13
HTR pretreatment ~~
a
~
~~
~
chemisorption, pmol g-l
-cat.
H2
co
Pd/La,O, Pd/CeO, Pd/Pr6Ol1 Pd/Nd,O, Pd/ Sm , Pd/Eu,O,
1.6 1.6 0.7 1.5 3.6 2.6
3.8 1.2 0.9 1.7 2.5 3.7
~~~
0,
H, (titer)
H/Pd,
CO/Pdt
cryst. a size, nm
6.2 83.0 19.0 3.0 4.5 4.3
14.2 67.0 66.0 8.5 17.0 10.3
0.044 0.048 0.021 0.040 0.098 0.091
0.053 0.018 0.013 0.023 0.034 0.065
26 24 54 28 12 12
_ I -
Based on H/Pdt ratios.
Table IV. __
Final Adsorption Measurements on Used Catalysts
_______________I__
LTR pretreatment
-
chemisorption, pmol g-' ___I______
cat.
_-
HTR pretreatment
i _ l _ _ _ _ _
co
H,
cryst. size, nm
chemisorption, pmol g-l H,
cryst. size, nm
co
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ I _ _
Pd/LazO 3 PdiCeO, Pd/Pr601L Pd/Nd ,O, Pd/Sm,O, Pd/Eu,O ,
2.5
4.0
1.3 2.2 1.2 1.6
16
3.6 2.9 1.7 3.7
29 19 34 20
Table V. Initial Irreversible Gas Uptakes on Oxide Supports (pmol g-') LTR HTR I _ _ _ -
H? H, CO 0, titer La,O, CeO, Pr6011 Nd,O, Sm,O, Eu,O,
0 0 0 0 0 0
0 0 0 0 0 0
1.8 7 0 0 0 0
0 0 0 0 0 0
CO
0,
H, titer
2.6 42 127
0.6 0 0.7
0
0 10 1.2 0 4.7
0
5.0
H, 0 0 0 0
0
0
0
0
0
0
on the pure support material after HTR were sometimes greater than the irreversible uptakes on the supported Pd catalysts. Similar complications were found for O2 adsorption on Ce02and Pr6OI1after the HTR pretreatment. For this reason the values reported in Table I11 are the
1.6 0.3 0.3 1.5 1.4 2.1
-____I__
26 120
1.0
110
0.6 1.3 1.6 3.2
28 30 16
actual gas uptakes, with no correction for adsorption on the support. The ratios of adsorbate to total Pd atoms, Pd,, are also listed in Tables I11 and IV. The hydrogen titer values represent Hz uptakes on the oxygen-covered catalysts. Pd crystallite sizes were calculated using the relation d, (nm) = 1.13/D where d, is the surface-weighted average crystallite diameter, and D is the fraction exposed (dispersion) (Wang et al., 1981). Kinetics. Activities per gram of catalyst and specific activities in the form of turnover frequencies (TOF) for CHI and COz formation are listed in Tables VI and VI1 based upon Pd surface sites determined by hydrogen or CO chemisorption. Activation energies for formation of CHI and COz were derived from Arrhenius plots such as
Table VI. Effect of Pretreatment of Methanation Activities and Turnover Frequenciesn LTR activity, activity, ,vCH4, s-1 x 103 (pmol/s.g of cat.) ___-(pmol/s.g of cat.) cat. x 103, rCH4 b C x 103, rCH, Pd/ La ,0 31 6.2 7.8 22 Pd/CeO, 45 5.1 11.4 6.0 Pd/Pr,O,, 41 15.8 Pd/Nd,O , 11.9 2.7 4.1 21.0 28.7 12.0 16.9 15.2 Pd/Sm,O N.D.d 5 Pd/Eu, 0, N.D. 6 _____
4.7
___I__-_________I_ _ _ _ I
_-__-I_____
T = 548 K ; P = 101 kPa; H,/CO = 3. Based on irreversible H 2 uptake on used catalyst. uptake o n used catalyst. Not detectable.
__HTR
_
-
N c H ~S ,- I x
I
b
6.9 8.8 10.0
7.0 5.4 1.4 1.7
_
io3 C
4.7 5.3 10.0 16.2 9.5 1.6 1.9
Based on irreversible CO
_
~
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Ind. Eng. Chem. Fundam., Vol. 23, No. 1, 1984
Table VII. Effect of Pretreatment o n Activities and Turnover Frequencies for CO, Formationa HTR LTR activity , N,,,, activity , N ~ ~ , , S x- 103 ~ (pmo1ls.g of cat.) (pmo1,'s.g of cat.) x i o 3 , reo, b cat. x i o 3 , rcoz b C -____ 32 10.0 Pd/ La 0 98 19.6 24.5 10.1 16.8 Pd/CeO, 10 10.6 17.7 Pd/Pr.O.. 58 22.3 16.1 28.0 9.3 Pd/Ni,O; 57 13.0 19.7 19.1 6.8 Pd/Sm,O 69 28.8 40.6 3.8 0.9 Pd/Eu ,0 25 7.8 6.8 7.5 1.8 22 6.9 5.9 T = 548 K;P = kPa; H, /CO = 3. on used catalyst.
Based on irreversible H, uptake on used catalyst.
Table VIII. Activation Energies for CH, and CO, Formation (kcal/mol) _ _ _ LTR HTR cat. Pd/La,O, Pd/CeO, Pd/Pr,O,, Pd/Nd,03 Pd/Sm,O, Pd/Eu,O,
Ec 0 2
ECH4
32.4 * 27.4 * 28.3 * 19.9 f 23.5 i
0.9 0.5 1.1 1.8 2.2
25.0 ?: 0.4 13.3 i 0.5 19.2 i 0.6 29.3 i 0.6 12.1 i 2.0 21.6 * 1.2
ECH4
25.5 * 1.2 22.2 i 0.8 30.4 i 2.6 21.6 i 0.7 25.2i 1.7 24.6 i 0.5
s-1 x 103 C
6.8 10.1 17.7 21.5 11.9 1.2 2.3
Based on irreversible CO uptake
~ -
E , 0, 19.2 i 1.0 13.7 * 1.4 13.4 * 1.2 18.4 * 0.3 20.8 k 1.5 20.4 * 1.0
those shown in Figure 2, and the results are given in Table VIII. Agreement between the ascending and descending temperature branches of the plots was quite good, which indicated that poisoning, sintering, and deactivation were not significant during the reaction studies. Identical kinetic studies were performed on the pure support materials, and between 523 and 573 K the only detectable product was a trace amount of COPfor all oxides except Eu203. Over Eu203,trace amounts of COPwere detected in the effluent stream only when the reaction temperature was above 573 K. Discussion The chemisorption experiments were conducted not only to determine the number of Pd surface atoms, Pd,, from which dispersions (fractions exposed), average crystallite sizes, and turnover frequencies (molecule.s.Pd site) could be calculated, but also to measure hydride formation and to see if O2 adsorption and subsequent H2 titration could reveal any information about changes in the Pd and the support. An examination of the H2and CO uptakes in Tables I11 and IV allows two conclusions: first, both uptakes are always small producing low apparent dispersions as indicated by the H/Pd, and CO/Pd, ratios, and second (consideringthat the CO/Pd, adsorption stoichiometry can vary between 0.5 and 1)the two measurements are almost always in reasonable agreement despite the uncertainty that exists regarding CO uptakes after the HTR because of the significant adsorption that can occur on the support in some cases (Table V). In contrast, O2 adsorption was always larger than either H2 or CO adsorption, and was sometimes markedly higher. This is attributed to additional irreversible oxygen adsorption on the RE oxide, and the following reasons support this conclusion. First, in some cases the O/Pdt ratio is greater than 1;second, larger O2 blanks can occur on some pure oxides after an HTR treatment; third, all the adsorbed oxygen frequently does not react with H2,giving lower than predicted titer values, which should be three times the O2 uptake (Benson et al., 1973);finally, Pd is a good catalyst for H2 dissociation and can induce spillover which can reduce the oxide in the vicinity of the Pd crystallites. The large H2 titer values
100
-i
3%
Zt" ? L
4"
lo
t I73
\A
"\ I77
1.81
I85
1.89
{ x IO'tK-') Figure 2. Rates over Pd/Laz03after the LTR pretreatment: (A) CHI (ascending 7');(4CHI (descending 71; ( 0 )COz (descending T); (0) COz (ascending 2').
in some cases, such as Pd/Ce02, show that some of the oxygen associated with the support can react with hydrogen at 100 "C. These experiments indicate that RE oxide-supported metals may be more difficult to characterize by adsorption than metals on typical supports. However, H2 chemisorption appears applicable and is clearly preferred because no correction for the support is needed. In addition, CO uptakes should be useful and need no correction after the LTR treatment. Regardless, there is an obvious descrepancy between the Pd crystallite sizes estimated from the chemisorption measurements and the XRD results. The lack of detection of a Pd peak in every supported catalyst indicates crystallites that are ca. 4 nm or smaller, and the resolution of a distinct Pd (111)peak in a physical mixture of 0.7% Pd black-Ce02 shows that the Pd loading is sufficiently high to allow detection of the 11-120 nm crystallites calculated to exist from the H2 chemisorption values. A clue to a reconciliation of these results is provided by Table 11. These lanthanide oxides hydrolyze easily (Rosynek and Magnuson, 1977), and the aqueous impregnation procedure produces hydroxide or oxyhydroxide phases which can still remain after the LTR kinetic studies, in some cases. During the HTR treatment, the supports dehydrate and reform into the initial bulk oxide. Substantial surface restructuring should accompany this process, and it is very likely that the Pd crystallites become partially (or totally) covered with the support material during this process. Therefore, we believe that much higher Pd dispersions
94
Ind. Eng. Chem. Fundam., Vol. 23, No. 1, 1984
Table IX. Pd Crystallite Size, D, Determined from H Solubility, mra and Percent, f, of Pd Surface Covered by Support LTR ______-__-_ fresh -____._I
a
cat.
m
Pd/ La, 0 , PdICeO, Pd/Pr,O,, Pd/Nd,O, Pd/Sm,O, Pd/Eu,O,
0.44 0.38 0.29 0.47 0.19 0.27
D,nm 5.6 3.8 2.4 6.6 1.7 2.3
f 72 68 83 52 85 83
m
HTR I -
used
0.25
D,nm 1.6
0.17 0.49 0.12 0.26
1.7 9.4 1.5 2.3
fresh
f 90. 94 55 96 89
m 0.52 0.25 0.12 0.44 0.22 0.46
D,nm 10 2.1 1.5 5.7 1.9 7.1
used
f
D,nm 10 2.8 1.5 16 1.7 8.7
m 0.52 0.32 0.13 0.54 0.19 0.49
60 91 97 50 84 43
f 60 98 99 43 94 44
Defined as Hbulk/Pdt.
actually exist, and that substantial surface contamination occurs on small Pd crystallites thereby blocking a large fraction of the Pd, sites, decreasing adsorption, and indicating low apparent dispersions. To test this hypothesis, advantage was taken of the additional information available in the H2sorption-backsorption technique used by Benson et al. (1973) and Boudart and Hwang (1975). Because bulk absorption can be distinguished from surface adsorption, Pd dispersion can be plotted as a function of hydrogen solubility (Benson et al., 1973). Assuming that normal hydride formation occurs in the Pd/RE oxide systems, the H solubilities were determined, the Pd dispersions and crystallite sizes were calculated from the relationship shown by Benson et al. (1973), and the values are shown in Table IX. These results indicate that the Pd is relatively well dispersed and the Pd crystallites are typically too small to be detected by XRD. In the two cases where Pd peaks might be most easily observed, overlapping peaks from the La203and Nd203supports prohibit their detection. From the crystallite sizes in Table IX, the fraction of the Pd surface covered by the support can be estimated, and these values are quite high, as indicated in Table IX. Complete encapsulation of some Pd particles cannot be completely discounted; however, no hydride formation would occur in these particles and this assumption results in larger estimated crystallite sizes which should be detected by XRD if they existed (Mitchell, 1982). In these Pd/RE oxide catalysts, the HTR pretreatment usually resulted in a decrease in initial CO and H2 adsorption, compared to uptakes after the LTR pretreatment, as shown in Figure 3. The adsorption behavior on the used samples showed a similar trend, also shown in Figure 3, but these samples had seen temperatures near 300 "C in the reactor. Regardless, with the exception of the Pd/Pr6011catalyst, the marked decreases that occur on titania-supported Pd after HTR (Tauster et al., 1978 Wang et al., 1981) were not observed because uptakes after LTR were already quite low. As reduced chemisorption has been one of the most apparent manifestations of SMSI behavior, it is difficult to conclude that SMSI exists in these systems based on adsorption behavior alone, although the intimate contact between the support and the Pd surface as a consequence of our surface coverage hypothesis complicates this analysis. The only explanation invoking SMSI consistent with the adsorption measurements is that substantial SMSI behavior can be induced in these catalysts during the LTR treatment, and there are currently no additional data to support this hypothesis. The kinetic parameters were carefully determined for both CH4 and C02 formation. Although rates of C02 formation at atmospheric pressure are usually quite low over typical Pd catalysts, they were surprisingly high over these Pd/RE oxide catalysts and were usually higher than the rates of CHI formation, as indicated in Tables VI and VII. In the case of Eu20,-supported Pd, substantial C 0 2
t
0
La
Ce
PI
Nd
Sm
Eu
b
/
0 A
1
Lo
Ce
Pr
Nd
Sm
Eu
Figure 3. (a) Effect of pretreatment on H2chemisorption: (A)fresh used sample. (b) Effect of pretreatment on CO chemisample; (0) sorption: (A)fresh sample; (0) used sample.
formation occurred after LTR while no detectable methanation occurred. Because of these results, COz formation cannot be attributed to a subsequent water gas shift reaction on the Pd alone. Although CO disproportionation on the Pd could contribute to the C02 reaction, it is not a satisfying explanation because CO does not dissociate readily on Pd at these temperatures (Rabo et al., 1978), and in addition, these catalysts showed no deactivation. These RE oxides can easily form surface carbonates (Rosynek and Magnuson, 1977)and at this time a major COz formation route is attributed to the support surface via decomposition of surface carbonates to produce COz. A dual functional role of the catalyst system may be possible here because only very small amounts of C 0 2 were observed over the pure oxides under reaction conditions, and such a scheme has been proposed for the water gas shift reaction over supported metals (Grenoble et al., 1981). With one exception-the LTR Pd/Nd203 catalyst-activation energies for C02 formation were always lower than those for methanation, and the values are in the range found at higher pressures by Bell and coworkers (Ryndin et al., 1981). Because of the low available Pd surface areas, methanation rates per gram of catalyst were low compared to other Pd catalysts (Wang et al., 1981), but on a turnover frequency (TOF) basis, the rates were very comparable to Pd supported on A1203and silica-alumina. However, the two most active catalysts, Pd/Pr6011and Pd/Nd203,had CH, TOF values between those found on Al2O3-supported Pd and TiOp-supported Pd, the latter values being the
Ind. Eng. Chem. Fundam., Vol. 23, No. 1, 1984 95
20
t 2o
La
Ca
Pr
Nd
Sm
t
Eu
Figure 4. Formation of C02 over RE oxide-supported palladium after HTR pretreatment (H2/C0 = 3): ( 0 )based on H(ad)on used sample; (A)baaed on co(,d)on used sample.
highest observed for this metal. The activation energies for methanation were quite consistent with those reported in the literature (Wang et al., 1981; Ryndin et al., 1981; Fajula et al., 1982) the were usually most similar to values found over Pd/SiOz and Pd/TiOz catalysts. The HTR pretreatment increased methanation activity for only two catalysts, Pd/Nd203and Pd/EuzOa, but no trend existed for changes in the TOF as a consequence of pretreatment (Mitchell, 1982). The HTR step always reduced the rate of COz formation, providing additional support for the argument that this reaction occurs on the surface of the RE oxide support because the HTR would facilitate removal of surface carbonate and hydroxyl groups. Despite the strong possibility that a large portion of the Pd surface appears to be covered by the RE oxide after either pretreatment, the specific activities of available Pd surface atoms can still be determined by using the chemisorption values. When normalized to available Pd surface sites, an order of magnitude variation in TOF occurred in this family of catalysts for both CHI and COz formation. Although no trend existed for activation energies alone, a particularly intriguing pattern emerged regarding specific activity, as shown in Figures 4 and 5. When plotted vs. position of the oxide in the periodic table, after HTR an activity maximum for both reactions exists in the same position-Pr601, or Nd203-depending on the choice of H(ad)or CO(*d)to count surface sites. The pattern is independent of choice of adsorbate, however, This cannot be attributed to a crystallite size effect because none exists for Pd over this range (Wang et al., 1981). A nearly identical plot was found for the rate of catalytic oxidation of nitric oxide over these same RE oxides (with no metal), and these authors also found similar patterns existed for variations in the conductivity and the dielectric constant (Matsuda et al., 1975). In addition, Sinha (1976) has correlated the orbital angular quantum number, L, with many different thermodynamic and physical properties and has shown that these plots invariably appear as an “inverted W”, with minima occurring at empty, half-filled, and filled f orbitals. A plot of L VS. atomic number of the RE produced a plot extremely similar to those in Figures 4 and 5; however, it is difficult to relate L directly to chemical and physical properties, unfortunately. The indication that an “inverted W” correlation exists for specific activity over the Pd/RE oxide systems is quite interesting because it implies a direct influence of the support on catalytic behavior. Although a support effect on COz formation might be understandable because this
La
Ca
Pr
Nd
Sm
Eu
Figure 5. Turnover frequency for methanation over RE oxidesupported palladium after HTR pretreatment (H2/C0 = 3): (0) based on H/Pd, for used sample; (A) based on CO/Pdt for used sample.
reaction is associated with the RE oxide surface, it is much more difficult to explain the role of the support in the methanation reaction, There are several possibilities: (1) metal-support interactions involving electron transfer, either localized (Tauster et al., 1981) or via bulk properties (Schwab et al., 1959; Szabo and Solymosi, 1960); (2) stabilization Pd+ ions (Poels et al., 1981); (3) special active sites at the metal-support interface, particularly for CO adsorption, and (4)dual functional catalysis (highly unlikely for methanation, however). At this time, although arguments invoking some form of electron transfer seem very possible, a lack of data which systematically describe the electronic properties of these RE oxides prohibits a search for correlations between these parameters and catalytic activity. The third explanation is attractive because of the support effect and the intimate metal-support contact presumed to exist because of surface coverage by the support. The acid-base properties of the support could then play a role (Ichikawa, 1982). This could allow the formation of special active sites at the Pd-support interface which facilitate rupture of the C-0 bond, thereby affecting methanation activity. At higher pressure, another type of site at the interface could enhance oxygenate formation. Further studies are needed to determine the state of both Pd and the support in these catalysts. Bell and co-workers have found that the support has a major influence on the selectivity of Pd for methanol, and they related this to the acid-base properties of the support (Ryndin et al., 1981). However, similar silica supports have been found to have markedly different selectivities, and alternative explanations have been proposed (Poels et al., 1981; Fajula et al., 1982). Although the kinetic studies here were conducted at only 1 atm, these Pd/RE oxides give evidence of being efficient catalysts for oxygenate formation. For example, the Pd/Ndz03catalyst gave yields of dimethyl ether plus methanol (the two elute together on Chromosorb 102) which paralleled, but were somewhat higher than, the calculated equilibrium yields of dimethyl ether and methanol over the temperature range studied (500-550 K). Confirming that these yields were equilibrium limited, recent studies in this laboratory have found that this Pd/Ndz03 catalyst produced essentially 100% methanol at 523 K, 1.5 MPa (215 psia), and a Hz/CO ratio of 3. These results agree with previous work (Poels et al., 1981).
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Ind. Eng. Chem. Fundam., Vol. 23, No. 1, 1984
The SMSI behavior envisoned by the Exxon workers has been explained by localized bonding between the metal atoms and the support and has been directly related to the reducibility of the support and the availability of d orbitals from the cations in the support (Tauster et al., 1981). Neither the suppression of adsorption (Figure 3) nor the activity pattern correlated with bulk reducibility as CeOz, Pr6OI1,Smz03 and Euz03 have lower oxidation states whereas La203and Ndz03are refractory. This observation plus the unavailability of unfiied d orbitals from these RE cations argues against SMSI behavior like that proposed for the metal/TiOz systems. However, SMSI behavior cannot be completely discounted for at least two reasons: first, these RE oxides may undergo significant reduction at 448 K, at least at the surface, thereby inducing suppressed chemisorption even at low temperatures, and second, some filling of the 5d orbitals may occur before the 4-f orbitals are filled (Remy, 1956). These RE oxides clearly require more investigation to explain the intriguing catalytic properties found in this study-in particular, the activity maximum corresponding to the first half of an "inverted W" plot frequently found with RE oxides. Studies are underway in this laboratory involving a similar examination of the second half of the lanthanide RE oxides, a study of nonaqueous preparative techniques to avoid hydrolysis of the support and to eliminate surface contamination of the Pd, and an investigation of catalytic behavior of Pd/RE oxide catalysts at higher pressures. Answers to the questions which have been found to exist with these catalysts should enhance our understanding of metal-support effects and may well lead to new catalyst systems. Acknowledgment This research was sponsored by a grant from Molycorp, Inc. Registry No. Pd, 7440-05-3; L%03, 1312-81-8; CeOz, 1306-38-3; Pr6011,12037-29-5; Nd,O,, 1313-97-9; Sm,03, 12060-58-1; Eu,O,, 1308-96-9.
Literature Cited Aben, P. C. J. Catal. 1988, 10, 224. Baker, R. T. K.; Prestrldge, E. B.; Garten, R. L. J. Catal. 1979a, 56, 300. Baker, R. T. K.; Prestrldge. E. B.; Garten, R. L. J. Catal. 1979b, 59, 293. Benson. J. E.; Hwang, H. S.; Boudart, M. J. Catal. 1973,30, 146. Boudart, M.; Hwang, H. S. J. Catal. 1975,39, 44. Doering, D. L.; Poppa, H.; Dickinson, J. T. J. Catal. 1982, 73, 110. Fajula, F.; Anthony, R. G.;Lunsford, J. H. J. Catal. 1982, 73, 237. Grenoble, D. C.; Estadt, M. M.; Oiiis, D. F. J. Catal. 1981,67, 90. Horsiey, J. A. J. Am. Chem. SOC. 1979, 101,2870. Ichikawa, M. Chemtech Nov 1982,674. Ichikawa, M.; Shikakura, K. R o c . 7th Int. Congr. Catal. Tokyo, 1980, Paper C 817. Kao, C-C.; Tsai, S-C.; Bahl, M. K.; Chung, Y-W. Surf. Sci. 1980, 95, 1. Kao, CC.; Tsai, S-C.; Chung, Y-W. J. Catal. 1982, 7 3 , 136. Matsuda, Y.; Nishibe, S.;Takasu, Y. React. Kinet. Catal. Lett. 1975,2 , 207. Mitchell, M. D. MS Thesis, The Pennsylvania State University, University Park, PA, 1982. Palazov, A.; Kadinor, G.; Bonev, Ch.; Shopov, D. J. Catal. 1982, 74, 44. Palmer, M. 8.; Vannice, M. A. J. Chem. Techno/. Biotechnol. 1980, 3 0 , 205. Poeis, E. K.; Van Broekhoren, E. H.; Van Barneveid, W. A. A,; Ponec, V. React. Klnet. Catal. Lett. 1981, 78, 223. Poutsma, M. L.; Eiek, L. F.; Ibarbia, P. A.; Risch, A. P.: Rabo, J. A. J. Catal. 1978,52, 157. Rabo, J. A.; Risch, A. P.; Poutsma, M. L. J. Catal. 1978,53, 295. Remy, H. "Treatise in Inorganic Chemistry", Kieinberg, J., Ed.; Elsevier: Amsterdam, 1956. Rosynek, M. P.; Magnuson, D. T. J. Catal. 1977,4 6 , 402. Ryndin, Y. A.; Hicks, R. F.; Bell, A. T. J. Catal. 1981, 70, 287. Schwab, G. M.; Block, J.; Muller, W.; Schultze, D. Angew. Chem. 1959, 7, 101. Sinha, S.P. Struct. Bonding 1978,3 0 , 1. Szabo, 2 . G.; Solymosi, F. Act. Deux. Congr. Int. Catal. faris 1980,1627. Tauster, S. J.; Fung, S. C. J. Cats/. 1978,55, 29. Tauster, S. J.; Fung, S. C.; Baker, R. T. K.; Horsiey, J. A. Science 1981, 211, 1121. Tauster, S. J.; Fung, S. C.; Garten, R. L. J. Am. Chem. SOC. 1978, 100. 170. Vannice, M. A. J. Catal. 1982, 74, 199. Vannice, M. A.; Garten, R . L. Ind. Eng. Chem. prod. Res. Dev. 1979a, 78, 186. Vannice, M. A.; Garten, R. L. J. Catal. 1979b,56, 236. Vannice, M. A.; Garten, R. L. J. Catal. 198Oa,66, 242. Vannice, M. A.; Garten, R. L. J. Catal. ISSOb,63, 255. Vannice, M. A.; Moon, S. H.; Twu, C. C. frepr. Dlv. fetr. Chem. Am. Chem. SOC.1980,25, 303. Wang, S. Y.; Moon, S. H.; Vannice, M. A. J. Catal. 1981, 7 7 , 167. Weisz, P. B. Z . fhys. Chem. (Frankfurt am Maln) 1957, 7 1 , 1. Yates, D. J. C.; Sinfeit, J. H. J. Catal. 1987,8 , 348.
Received for review February 28, 1983 Accepted October 28, 1983
