J. Phys. Chem. 1995,99, 9479-9484
9479
Analysis of Li Promotion of Methanol Synthesis on Pd/SiO2 Using Isotopic Transient Kinetics Sturla Vadat and James G. Goodwin, Jr.* Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261 Received: November 4, 1994; In Final Form: April 3, 1995@
Li promotion is known to greatly enhance the rate of methanol synthesis on Pd. The mechanism of Li promotion of PdSi02 at 483-563 K and 1.8 atm was studied using isotopic transient kinetic analysis (ITKA) with carbon tracing to decouple the effects of Li on the methanol-producing sites during the initial period (at 10 min) of reaction. ITKA results indicate that the concentration of active surface intermediates leading to CH3OH increased upon Li promotion, indicating that the number of active sites increased or that the coverage of active sites by surface intermediates increased. This increase in concentration of active intermediates was not due to any effect of the Li on the Pd dispersion. On the other hand, a measure of the average intrinsic k for methanol formation (&OH) approximated using the reciprocal of the surface residence time of MeOH intermediates was lower for the Li-promoted than the unpromoted catalyst. Because of the greater importance of the increase in the concentration of surface intermediates, the overall effect was that the Li-promoted catalyst was more active for methanol synthesis.
Introduction
Numerous studies of methanol formation on Pd catalysts have been done in the past two decades since it was shown that Pd is very active and selective under certain conditions.' It is, thus, well known that CO and H2 adsorb nondissociatively and dissociatively, respectively, on Pd. For this reason methanol can be selectively synthesized over Pd catalysts. Compared to the catalysts used in commercial processes (CdZnOICr203 or Cu/ZnO/A1203), Pd catalysts have the advantage of thermal stability. Higher selectivity and activity for methanol synthesis have been obtained for Pd by promotion with group IA and IIA oxides.2-8 Kikuzono et aL6 have studied alkali-promoted Pd catalysts at atmospheric pressure. They found that the Li-promoted Pd catalyst exhibited remarkably high activity for methanol synthesis, comparable with the Na-promoted catalyst, whereas the activities of K-, Rb-, and Cs-doped catalysts decreased drastically in that order. In a study of Li-promoted Pd/SiOz catalysts, Kelly et aL2 concluded that Li+ ions affect sintering of the silica support and promote activity by increasing TOF. They suggested that the increase in TOF may, in part, be the result of a more efficient use of the Pd surface via the exclusion of small pores. The most active Pd/SiO2 catalysts were found to be comparable in activity to a commercial methanol catalyst. However, conclusions about how Pd is affected by promoters are complicated by the fact that the presence of even minute quantities of impurities in the support may play a significant role in determining methanol synthesis activity of a c a t a l y ~ t . ~ - I ~ In addition, numerous hypotheses have been put forth as whether or not the active site for methanol synthesis is Pdo, Pd+, or silica itself.6.9.I 1 - 12 To develop a better understanding of how Li promotion affects methanol synthesis on Pd, CO hydrogenation over Lipromoted and unpromoted Pd/SiO2 was studied using isotopic transient kinetic analysis (ITKA). ITKA, developed in large + Present address: Department of Industrial Chemistry, Norwegian Institute of Technology, University of Trondheim, N-7034 Trondheim, Norway. * To whom all correspondence should be addressed. Abstract published in Advance ACS Abstracts, May 15, 1995. @
TABLE 1: Properties of Unpromoted and Li-Promoted 5 wt % Pd/Si0215 Pd average particle size catalyst Pd LiPd
COi,,
@moVg)
105 140
(A)
CO chem"
XRD
42 42
154 108
a Based on CO chemisorption on the base, unpromoted catalyst assuming COi,,iPd, = 1.
part by Happel et al.I3 and Biloen et al.,I4permits the monitoring of important kinetic parameters and the calculation of the concentration of active surface intermediates under reaction conditions. Experimental Section
A Si02 (Davison grade 59)mpported Pd catalyst was prepared by incipient wetness using an aqueous solution of PdCl2 in order to produce a 5 wt % Pd/SiOz catalyst. The silica was analyzed by ICP and was found to contain the following impurities: 580 ppm Na, 580 ppm Ca, 190 ppm Mg, 280 ppm Al, 130 ppm Fe, and 35 ppm S.I5 The impregnated catalyst was dried for 5 h at 363 K, calcined for 4 h at 673 K, and then reduced for 5 h at 673 K after ramping at 2 Wmin. A portion of the reduced base catalyst was then modified by impregnating with an aqueous solution of LiNO3 to get a Li-promoted catalyst with the desired (Li/F'd)atom,cratio of 1. This was dried for 5 h and then reduced for 5 h at 673 K. The base and promoted catalysts are referred to in the subsequent discussion as Pd and LiPd in order to simplify notation. The promoted catalyst was prepared from the reduced base catalyst in order to minimize the effect of different metal particle size distributions. The promoter is identified in this text as Li; however, Li was actually present on the catalyst in compound form as Lif. The exact compound form of Li varies from pretreatment to reaction. During CO hydrogenation and in the presence of H20, it probably exists as LiOH and/or Li2CO3. Static CO chemisorption was carried out on these catalysts at 298 K. The results are summarized in Table 1. Irreversible CO chemisorption was used to determine the amount of exposed Pd in the base, unpromoted catalyst and to calculate the average
0022-3654/95/2099-9479$09.00/0 0 1995 American Chemical Society
Vada and Goodwin
9480 J. Phys. Chem., Vol. 99, No. 23, 1995
TABLE 2: CO Hydrogenation Characteristics of the Unpromoted Pd CatalysP
0 Equil. c o n v . Exp. conv. (LiPd) v E x p . conv. (Pd)
reaction temperature (K) reaction Darameters
483
493
COconv(%) 0.13 RM~OH 0.07 lumol/(n s)) se1ectivity;o 93.8 CH3OH (%)b selectivityto 6.2 CH4 (Yo)b
503
513
523
533
v
543
553
563
0.18 0.25 0.31 0.30 0.30 0.25 0.20 0.18 0.10 0.13 0.17 0.16 0.16 0.13 0.09 0.07
v
1.8
95.1 95.8 96.4 96.0 93.5 89.5 79.7 66.0 4.9
4.2
3.6
4.0
5.8
9.1 17.4 28.5
H2/CO = 11.2/4.6(mL/min), total pressure = 1.8 atm, 55.0 mg of catalyst. Selectivity given in terms of percent CO converted to product. 460
TABLE 3: CO Hydrogenation Characteristics of Li-Promoted LiPd CatalysP reaction temperature (K) reaction parameters
483
493
COconv(%) 0.22 0.30 RM~OH 0.12 0.17 OcmoY(g s)) selectivity to 96.7 97.6 CH30H ( 7 ~ ) ~ selectivity to 3.3 2.4 CH4
503
513
523
533
543
553
563
0.33 0.37 0.35 0.30 0.22 0.16 0.12 0.18 0.20 0.18 0.16 0.11 0.08 0.06 97.6 97.8 97.3 95.7 93.4 88.5 79.6 2.4
2.2
2.7
4.3
6.6 11.5 20.4
fl H2/CO = 11.2/4.6 (mL/min),total pressure = 1.8 atm, 56.9 mg of catalyst. * Selectivity given in terms of percent CO converted to product.
0.25 0 LiPd
V Pd 0.20
1
I
0.15 -
0.10
-
0.05
1
480
500
520
540
560 580
600
Reaction Temperature ( K ) Figure 2. Variation with temperature of equilibrium conversion and experimental conversion on Pd and on LiPd.
by H2 bracketing for 20 min in between each reaction temperature. The product gases were analyzed using a Varian 3700 gas chromatograph equipped with FID and a 6 ft, 60-80 mesh Poropak Q column. Isotopic transient kinetic analysis (ITKA) was also performed at these reaction conditions. However, the reaction was utilized only in the temperature range from 483 to 503 K in order to avoid reaction equilibrium. The isotopic switch (from I2COto I3CO) was done immediately after the measurement of the reaction rate by gas chromatography. A trace amount of Ar was mixed in the I2CO flow in order to obtain the gas-phase holdup. The concentrations of Ar, I2CO, I3CO, and I3CH3OH were continuously monitored by a mass spectrometer (MS) during the switch. The MS was a Leybold-Inficon Auditor 2 , which was equipped with a high speed data acquisition system and controlled by an IBM-PC 386.
Results
480
500
520
540
560
Reaction Temperature (K) Figure 1. Variation of methanol formation with temperature for Pd and LiPd. metal particle size (assuming COi,,/Pd, = 1). X-ray diffraction line broadening using Cu K a radiation was also used to estimate average Pd particle size. These results are given in Table 1. As expected, calculations based on XRD line broadening gave a larger value for the average Pd particle size since Pd particles e50 8, could not be detected. Recent TEM results for these catalysts (average Pd particle size of 47 %.) confirm the CO chemisorption results (42 A) for the unpromoted Pd catalyst.I7 In addition, the TEM results indicate no change in the Pd particle size distribution upon Li promotion at the level used in this study. CO hydrogenation was carried out in a differential fixedbed glass microreactor. The catalysts (50-60 mg) were rereduced prior to reaction in flowing H2 at 673 K for 5 h after ramping at 2 Wmin. Reaction was performed in the temperature range from 483 to 563 K. The reactant stream consisted of H2 (1 1.2 cm3/min) and CO (4.6 cm3/min), and the total pressure was 1.8 atm. Reaction rate and selectivity were determined after 10 min of reaction. Initial catalyst behavior was guaranteed
The catalysts were studied under initial reaction conditions (at 10 min) in order to have the catalysts in essentially their initial state at the various conditions of temperature and PM~OH. This permits one to focus on the primary effects of Li promotion without the complications of secondary effects such as carbon deposition. In a parallel reaction studyI5 at higher pressures where the catalysts used here were part of a larger study of the effect of Li loading on long-term properties of PdSiOz, induction periods of ca. 1 h were observed for all the catalysts in the approach to steady-state reaction during which there were order-of-magnitude increases in activity. The promoting effect of Li on overall activity, while much greater after the induction period, was similar in direction to that found during initial reaction. The initial reaction characteristics of the unpromoted and Lipromoted catalysts are given in Tables 2 and 3. The selectivity to methanol was >95% for temperatures lower than 533 K. The selectivity is defined as percent of CO converted to product. The selectivity to methane was considerable at temperatures higher than 543 K. The rate for methanol formation decreased for temperatures >523 K. This trend was shown by both unpromoted and Li-promoted Pd catalysts (Figure 1). The reason why the rate decreases with increasing temperature (>523 K) is because of equilibrium conversion limitations on methanol synthesis. The calculated equilibrium conversion to methanol together with the experimental measured conversion to methanol over Pd and LiPd is given in Figure 2. It can be seen that the equilibrium conversion is strongly dependent on temperature at this low pressure (1.8 atm). The apparent exceeding of
Li Promotion of Pd
J. Phys. Chem., Vol. 99, No. 23, 1995 9481 I
-1.5, -2.0
1 .o
}
0.8 0.9
I\
Ar
_-_
13
co
0.7 0.6 h
0.5 LL
0.4
0.3
0.2 0.1
-5.0
0.0 1.7
1.6
1.9
2.0
0
2.1
100
200
300
400
500
600
Time (sec)
Figure 3. Arrhenius plots for rate of methanol formation and rate of methane formation on unpromoted Pd.
equilibrium shown in Figure 2 is solely due to the degree of accuracy in calculating equilibrium conversion using tabulated thermodynamic properties and neglecting side reactions. Thus, one can easily conclude that equilibrium conversion to methanol was attained for all temperatures ’530 K. Arrhenius plots for both methanol and methane formation on the unpromoted catalyst are displayed in Figure 3. The apparent activation energy for methanol formation was determined for the temperature range from 483 to 503 K, where equilibrium did not limit. The apparent activation energy for methane formation was determined for the temperature range from 543 to 563 K. The apparent activation energies for methanol and for methane formation over the unpromoted Pd catalyst were found to be 63 & 3 and 101 f 5 kJ/mol, respectively. For the LiPd catalyst the activation energies for methanol and for methane formation were found to be 69 f 7 and 93 f 8 kJ/mol, respectively. These apparent activation energies are in good agreement with what other researchers have found for similar catalysts and indicate the lack of mass and heat transfer limitations on the rates measured.’ The rate results can be put into the form of TOF’s using the results in Tables 1-3. However, due to modification of chemisorption properties, TOF can be subject to significant misinterpretation in promoter studies. Since a better measure of site activity is available from ITKA and is discussed in detail below, TOF’s will not be directly reported or discussed here. Isotopic transient kinetic analysis (ITKA) was used to investigate reaction on the Pd and LiPd catalysts in more detail in order to develop a better understanding of how Li promotion affects surface reaction. A typical normalized set of isotopic transients is displayed in Figure 4. The shape of the methanol transient indicates that the Pd surface was heterogeneous kinetically. This is typical for CO hydrogenation catalysts and has been discussed at length in other papers.’* All ITKA results reported were measured at temperatures 1503 K in order to avoid complications in interpretation due to approach to reaction equilibrium. The average surface residence time for the carbon in the intermediates leading to methanol, ZM~OH. is given by the area between the CH30H and the Ar transients. The concentration of surface intermediates leading to methanol formation, N M ~ OisH determined , by *MeOH
= ‘MeOHRMeOH
where R M ~ OisHthe overall rate of methanol formation. While some methanol formed decomposed back to the reactants due to the reversibility of the methanol synthesis reaction, especially at higher temperatures, the value of N M ~ Oreported H refers to
Figure 4. Isotopic Transients of Ar,I3CO, and I3CH3OHcorresponding to a switch from I3CO to I2CO (catalyst LiPd, T = 513 K, P = 1.8 atm, HdCO = 2.4). 2o
E
i
LiPd
the surface concentration of intermediates which led to methanol elution from the reactor. The fraction of the Pd surface (in the unpromoted catalyst) covered by active methanol intermediates (ca. 0.06-0.10) is similar to what is usually seen for CO hydrogenation (methanation) on Ru (18) where reaction reversibility does not play a role. The transients for CO were not used to calculate surface residence times and coverages for adsorbed CO due to the where FCOis the flow following reasons. Since NCO= ZCOFCO, rate of CO, it is a product of a large number (Fco)with a very small number (z co < 1 s) having a potentially significant error (&50%). In addition, in order to maximize detection of small quantities of MeOH, the mass spectrometer was set so that CO detection was not optimum. However, it is to be expected that the Pd surface was covered significantly by CO under reaction conditions, as is the case for most CO hydrogenation catalysts. The variations in N M ~ OasHa function of reaction temperature for both Pd and LiPd are shown in Figure 5. NMeOH increased with increasing reaction temperature for both catalysts, but NM~OH for the LiPd catalyst was greater at all temperatures. The surface residence time of methanol intermediates, Z M ~ O H ,as a function of temperature for methanol synthesis on the Pd and LiPd catalysts is shown in Figure 6. It was found that Z M ~ O H decreased with increasing reaction temperature for both catalysts, although Z M ~ O Hfor LiPd was greater at all temperatures.
Vada and Goodwin
9482 J. Phys. Chem., Vol. 99, No. 23, 1995
I
20,
120
t
LiPd V Pd
60 480
490
500
510
60
I
1
1
I
,
I
I
80
100
120
140
160
180
200
PMeOH
(Pa)
R e a c t i o n T e m p e r a t u r e (K)
Figure 6. Variation in average residence time for methanol intermediates with temperature for Pd and LiPd.
Figure 7. Comparison of surface concentration of methanol intermediates on Pd and LiPd at similar Ph.le~"( T = 483-513 K).
Discussion
130
At the lower temperatures only very small amounts of methane were formed, but at 543 K methane formation started to be more pronounced. The selectivity to methanol was higher for the Li-promoted catalyst at all temperatures. The Lipromoted catalyst also showed higher selectivity to methanol compared to the base Pd catalyst at 7 atm and 473 K.I5 The differences in rates found at lower temperatures is in good agreement with what other researchers have found for Lipromoted Pd catalyst^.^.^.'^ Since the apparent activation energy for methanol synthesis on Pd and LiPd was found to be 63 and 69 kJ/mol in the lower temperature range, one can conclude on the basis of a comparison with literature values that the reaction was kinetically limited at those temperatures. The derivation of Eappfrom Arrhenius plots is based on the following. The rate of reaction is govemed by the rate of the RDS which, for surface reaction controlling as is the case for MeOH synthesis, has the form
120
where k = k, exp( -Eapp/RT) and [*A] and [*B] represent surface concentrations of adsorbed reactants or intermediates. Taking the natural log of the rate equation yields
+ {Ink, + ln[*Al[*BI}
ln(r) = -(Eapp/R)(l/q
and a plot of ln(r) vs 1/T yields a straight line whose slope is (-EapdR), assuming {In ko ln[*A][*B]} is constant and the reverse reaction is insignificant. Obviously, problems arise if the latter assumptions are not met. Considering the fact that the concentrations of MeOH surface intermediates change significantly with temperature under differential and low temperature (483-503 K) reaction conditions away from reaction equilibrium (Figures 5 and 7), the assumption that {In ln[*A][*B]) is constant does not absolutely hold. Thus, Eapp determined from overall reaction data is in fact overestimated since N M ~ Oincreased H with temperature. Because of readsorption, the surface concentration of active intermediates, N M ~ O could H , be a function of partial pressure of methanol, provided methanol is able to readsorb on nonactive/ less active sites or is able to adsorb to higher coverages on the active sites. In order to examine the effect of the methanol partial pressure on this quantity, the results for N M ~ Ofor H the
+
+
u
110
100
(0
v
r
90
ul
P
80
-'\
O PLiPd d
70 60 -0 50
1
I
6
8
10
,
I
I
12
14
16
18
WHSV (h-')
Figure 8. Variation of residence time for methanol with WHSV at constant catalyst amount for Pd and LiPd (T = 493 K).
Pd and LiPd catalysts are plotted in Figure 7 as a function of the partial pressure of methanol in the reactor effluent. The results indicate that N M ~ Owas H indeed a function of the partial pressure of methanol. However, consideration of both Figures 5 and 7 together clearly indicates that the effect of the Li promoter was to increase N M ~ O relative H to that for the unpromoted catalyst after effects of temperature and methanol partial pressure are taken into account. This was definitively shown by carrying out measurements on the catalysts using different WHSV's at 493 K in order to produce a similar partial pressure of methanol. For P M ~ O H = 43 Pa, N M ~ O was H determined to be 6.9 pmoVg for Pd and 9.9 pmollg for LiPd. t is related to the time for surface reaction and, hence, surface activity. However, if readsorption of reaction products (such as methanol in this case) occurs on the catalyst surface, then rmeasured is composed of t,,,, the time for surface reaction, and &ad., the time spent in a readsorbed state, and one is more limited in calculating intrinsic surface activity. One way to find out whether readsorption happens or not is to determine the effect of space velocity at constant temperature and catalyst amount on tmeasured. A plot of such data for the Pd and LiPd catalysts is shown in Figure 8. This figure shows that readsorption of methanol occurred to some extent, since Z M ~ O H decreased with increasing WHSV at constant temperature (503 K). To obtain a truer value of Z M ~ O H ,a high WHSV should be used. However, increasing WHSV does not affect readsorption in the catalyst pores. Consequently, because of this latter fact, the difficulty of MS detection of MeOH at low conversions (high
J. Phys. Chem., Vol. 99, No. 23, 1995 9483
Li Promotion of Pd
-4'2
3
-4.4
.
-4.6
.
-4.8
.
as rate = K NH NcHx0
r
I
VI
v fir
P b=
\ c V
-
-5.0 1.95
2.00
I/T
Figure 9. Arrhenius plots for
2.05
2.10
(IO-~K-')
~ / T M ~ Ofor H
Pd and LiPd.
WHSV), and the added expense of operating at high isotopically-labeled CO flows, a lower WHSV than optimum was used for the experiments with the realization that all 7 ~ ~ 0detected ~ ' s would be affected to some degree by readsorption. However, on the basis of the results of experiments such as those shown in Figure 8, we can conclude that the measured values of ZMeOH were at most 20-40% higher due to readsorption than the actual z of reaction. Thus, values of Z M ~ O Hshould be considered to be upper limits for the z of reaction. However, considering the results shown in Figures 6 and 8, it is clear that the average surface residence time for methanol intermediates was always less for Pd than LiPd. This is seen directly by comparing results at different WHSV's presented in Figure 8 but at similar methanol partial pressures. For example, Z M ~ O Hwas 69 s for = 42 Pa) while it was 99 s for LiPd (PMeOH = 45 Pd (PM~OH Pa). For an irreversible, pseudo-first-order surface reaction where ki is a measure of the specific rate of a site and Ni is the concentration of active intermediates rate = kCJi In the absence of readsorption,
ki = zi- 1 However, if readsorption occurs, then zi must be considered to be the upper limit of the surface residence time for reaction, .,z, Hence, l/zi can be considered to be the minimum value of the activity of the intermediate on a site (ki). Figure 9 shows Arrhenius plots for ~ / Z M ~ O for H both Pd and LiPd. As discussed above, ~ / Z M ~has O Hto be considered to be a lower limit of kintr;it is possible to conclude that the intrinsic activity of LiPd is clearly lower than that of the unpromoted Pd. The apparent activation energies calculated from plotting l / t ~ (26 ~ okJ/mol ~ for Pd and 31 kJ/mol for LiPd) are much lower than the apparent activation energies calculated from the overall rate of methanol formation at the lowest temperatures. The reason is that, when the activation energy is calculated from overall rate, it is assumed that the amount of surface intermediates is not changing with changing temperature. Since this was not the case, the apparent activation energy calculated from the overall rate is overestimated since N M ~ Oalso H increased with temperature. The reaction mechanism for methanol synthesis can be expressed as (following only the C-containing molecule) CO(g) s?: *CO
* *CH,O * *CH,OH t CH,OH(g)
The true forward rate expression can probably be best expressed
and kMeOH or l / MeOH ~ actually contains the surface concentration of H. If this varies with temperature at these conditions, it should make In( 1/z) nonlinear with UT. Since In( l/z) is linear, it is highly likely that N H did not vary significantly with T in the range 483-503 K at PH = 1 . 2 8 atm. Unfortunately, it is difficult to measure NH directly by ITKA because of isotope effects during H Z D Zswitching. As shown in Figure 1, the rate of methanol formation was higher for LiPd than Pd at lower temperatures. Kikuzono et aL6 concluded that the Li promoter aids in the stabilization of reactive intermediates. The ITKA results from this work support this hypothesis since the amount of surface intermediates which led to methanol formation increased upon Li promotion (Figure 5 ) . This is true even when one considers the influence of partial pressure of methanol (Figure 7). On the other hand, Kelly et a1.* have suggested that Li+ ions promote activity by increasing turnover frequency (TOF). They concluded that this effect, in part, may be related to sintering of the support which could result in more efficient utilization of the Pd. Our results do not support this hypothesis since the average pseudo-first-order rate constant ~ M ~ O(related H to ~ / Z M ~ O His) lower for LiPd. This parameter is closely related to the intrinsic TOF of a site, although it does contain some dependence probably on hydrogen surface concentration. Where available, however, it is usually a better approximation of true site activity than the traditional TOF calculated based on chemisorption.
Conclusions As has previously been observed, the addition of Li to Pd/ Si02 was found to increase the selectivity and the activity for methanol formation during CO hydrogenation. The results presented in this paper show for the first time how Li brings about the increase in the overall rate of methanol formation by decoupling surface kinetic and concentration effects. While the measurements were complicated by the reversibility of the reaction and effects of methanol partial pressure and temperature on the surface coverage and average residence time of intermediates, careful comparisons were able to lead to conclusions about the relative effect of Li on the surface parameters. The measured surface parameters reported should be considered to be qualitative due to their dependence on multiple variables. It can be concluded that the concentration of active intermediates leading to CH30H increased upon Li promotion (either as a result of an increase in number of active sites or an increase in the coverage by surface intermediates of the active sites). This increase in surface coverage of active methanol intermediates paralleled the increase in CO chemisorption at 298 K but was not related to any change in Pd dispersion. The average pseudo-first-order rate constant for methanol formation (kMeOH), approximated by ~ / Z M ~ O Hon , the other hand, decreased upon Li promotion. However, due to the larger increase in active surface intermediates, the overall effect was that Li promotion of Pd increased the rate of methanol synthesis. Exactly how Li promotion modifies the surface catalysis in order to increase the number of methanol intermediates is difficult to conclude at this time. Certainly, the results reported here are consistent with the suggestion of Kikuzono et aL6 that Li aids in stabilizing the surface reaction intermediates. Another hypothesis posited is that Li acts to create new reaction sites (Li+ or Pd+) for MeOH synthesis while at the same time
Vada and Goodwin
9484 J. Phys. Chem., Vol. 99, No. 23, 1995
blocking Pdo sites where hydrogen can be activated.I6 However, it is impossible at this point to do more than speculate. Finally, the results indicate that the overall apparent activation energies usually determined for methanol synthesis on Pd are significantly overestimated due to the increase in surface intermediates with temperature in the kinetically-limited range (483-503 K in this study at 1.8 atm). Acknowledgment. The authors thank A. M. Kazi for the preparation and initial characterization of the Pd catalysts. S. Vada thanks the Norwegian Institute of Technology and the Norwegian Council for Scientific and Industrial Research for funding his stay at the University of Pittsburgh. We also thank National Science Foundation (Grant CTS-9 102960) for financial support. References and Notes (1) Poutsma, M. L.; Elek, L. F.; Ibarbia, P. A,; Risch, A. P.; Rabo, J. A. J. Catal. 1978, 52, 157. (2) Kelly, K. P.; Tatsumi, T.; Uematsu, T.; Driscoll, D. J.; Lunsford, J. H. J. Catal. 1986, 101, 396.
(3) Hahm, H. S. Appl. Catal. 1990, 65, 1. (4) Lietz, G.; Nimz, M.; Volter, J.; Lazar, K.; Guczi, L. Appl. Caral. 1988, 45, 71. (5) Rieck, J. S.; Bell, A. T. J. Catal. 1986, 100, 305. (6) Kikuzono, Y.; Kagami, S.; Naito, S.; Onish, T.; Tamaru, K. Faraday Discuss. Chem. SOC.1982, 72, 135. (7) Hicks, R. F.; Bell, A. T. J. Card 1985, 91, 104. (8) Deligianni, H.; Mieville, R. L.; Pen, J. B. J. Catai. 1985, 95,465. (9) van der Lee, G.; Ponec, V. Catal. Rev.-Sci. Eng. 1987, 29, 183. (10) Ponec, V.; Nonneman, L. E. Y. In Natural Gas Conversion; Holmen, A., et al., Eds.; Stud. Surf: Sci. Caral. 1991, 61, 225-234. (11) Driessen, J( M.; Poels, E. K.; Hindermann, J. P.; Ponec, V. J. Cam[. 1983, 82, 26. (12) Pitchon, V.; Praliaud, H.; Martin, G. A. In Natural Gas Conversion; Holmen, A,, et al., Eds.; Stud. Surf: Sci. Catal. 1991, 61, 265-271. (13) Happel, J.; Suzuki, I.; Kokayeff, P.; Fthenakis, V. J. Catal. 1980, 65, 59. (14) Biloen, P.; Helle, J. N.; van den Berg, F. G. A.; Sachtler, W. M. H. J. Catal. 1983, 81, 450. (15) Kazi, A. M. Ph.D. Thesis, University of Pittsburgh, 1994. (16) Ponec, V. Private communication, 1994. (17) Kazi, A. M.; Chen, B.; Goodwin, J. G., Jr.; Marcelin, G.; Rodriguez, N. M.; Baker, R. T. K. Submitted for publication. (18) Hoost, T. E.; Goodwin, J. G., Jr. J. Card 1992, 1.37, 22. JP5'42993R
