Ind. Eng. Chem. Res. 1988,27, 1984-1990
1984
Hydrogen Exchange and Spillover on a Palladium/Silica Catalyst Claes Niklasson Department of Chemical Reaction Engineering, Chalmers University of Technology, S-41296 Goteborg, Sweden
An extensive study performed in a continuously stirred tank reactor (CSTR) is made of the hydrogen-palladium system. Some theoretical discussion of isotopic effects and kinetic expressions is included. Spillover of hydrogen on a Pd/SiOz catalyst is examined with a step response technique. Spillover rates and amounts are measured a t different reactor temperatures (300-573 K). Severe isotope effects between hydrogen and deuterium are found. The amounts spilt over t o the carrier are found t o increase with increasing temperature, indicating an activated process. Maximum adsorption and desorption rates compared to the rate of Hz/Dz exchange support the same conclusion. The Hz/Dz exchange on the catalyst is examined thoroughly, and several kinetic rate equations are tested and compared by using a least-squares regression routine. Statistical and physicochemical reasons imply a Langmuir-Hinshelwood type of model to be the most probable one. No significant isotope effect was found in the exchange reaction. The exchange of hydrogen in the gas phase with hydrogen dissociatively adsorbed on the metal seems to proceed according t o a Bonhoeffer-Farkas mechanism. Interest in hydrogen-transition metal interaction has long been increasing due to the large growth of hydrogenation and dehydrogenation industrial processes. The Hz/Dz exchange reaction and the ortho-para hydrogen conversion are therefore two of the most examined systems in heterogeneous catalysis. As a consequence of this, kinetics and mechanism of the hydrogen-deuterium equilibration and exchange on transition metals have been discussed by many authors through the years (cf. Bond, 1962). Catalysts used in hydrogenations are primarily transition metal dispersed on inert porous supports. Palladium is one important transition metal catalyst from activity and selectivity points of view, and the palladiumhydrogen system has been studied extensively both theoretically and experimentally. A review of this particular subject and a substantial contribution to the understanding of it are given in a comprehensive book by Lewis (1967). Still, available data concerning adsorption kinetics and heat of adsorption in the literature diverge to such an extent that it is precarious to make any certain conclusions about the behavior of a palladium catalyst in a new hydrogenation process, despite the fact that some assumptions and directives given in the literature are indisputable. The adsorption of hydrogen on palladium appears to follow a dissociative mechanism (Aldag and Schmidt, 1971) and does not seem to differ appreciably from the adsorption on nickel (Christman et al., 1973). The H2/D2exchange kinetics in the palladium/hydrogen @-phaseare described (Scholten and Knovalinka, 1966) as following a simple power-law model. The total power with respect to the hydrogen isotopes in this model was found to be 0.63. The same kinetics for the a-phase of palladium are found to be somewhat different, as will be shown in the present paper. This article is a preliminary study of H2/D2exchange and other phenomena involved in the interaction between hydrogen and palladium dispersed on a porous silica support. The equipment used in these experiments is chosen to be easy to handle, to be convenient in dynamic studies (i.e., fast response), and to produce data not too complex to evaluate. Results of the study will be used in a more substantial examination of the role that hydrogen plays in the selective hydrogenation of 2-ethylhexenal to 2-ethylhexanal. Information from the kinetic expression for the H2/D2 exchange is also meant to decrease the number of parameters in the kinetic model for the same hydrogenation.
Experimental Methods The Catalyst. The catalyst Pd/SiOz was prepared by a conventional impregnation method including decomposition of palladium nitrate as precursor diluted with deionized water (pH 0.5) a t 328 K. Drying was in air a t 348 K for 8 h. Calcination was in air flow at 573 K for 12 h and thereafter for 7 h a t 473 K in a flow of hydrogen, nitrogen, and air. The final reduction was performed a t 473 K for 1 2 h in a mixture of hydrogen and nitrogen. More details about the exact procedure are given elsewhere (Smedler, 1988). The carrier delivered by Girdler (T-1571) was a porous silica carrier with a mean diameter of 5 X lW3 m and a BET surface area of 110 m2/(g of catalyst) given by the manufacturer. The metal content was 0.16%, determined by AAS (Perkin-Elmer 370), and the measured dispersion was 42.990,measured with standard hydrogen chemisorption (Chemisorb 2800). The pore-size distribution for the support as well as for the impregnated catalyst is given in Figure 1. The total pore volume for the carrier and the impregnated catalyst was 0.586 and 0.560 cm3/(g of catalyst), respectively, measured with a combination of the nitrogen condensation method and a mercury intrusion method. Equipment and Chemicals. The experimental equipment is visualized schematically in Figure 2. The reactor was a continuously stirred tank reactor (CSTR) described in a previous article (Niklasson and Smedler, 1987). The reactor was considered gradientless according to criteria developed by Weisz-Prater and Luft et al. (1973). All gases used in the experiments were of SR quality delivered by AGA (Sweden). The nitrogen was further purified from oxygen by an oxy trap (Alltech Associates) heated to 473 K to eliminate any unintentional oxidation of the catalyst during experiments. The experimental equipment, described more precisely elsewhere (Niklasson and Andersson, 1988a), was controlled by a microcomputer (ABC80). Experimental Procedure Step Response for Hydrogen and Deuterium. One method for measuring the amounts and rates of adsorption for different compounds on a metal catalyst is the method of step adsorption. The hydrogen (deuterium) step is added to a large continuous flow of nitrogen. The procedure was as follows: 1. Keep the catalyst at 623 K in nitrogen atmosphere for 12 h.
0888-588518812627- 1984$01.50/0 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 1985
x
x
X X
10-
X X X
.
*
x
X
.
X
I8
22
26
30
3L
38 I n (D,/A)
Figure 1. Relative pore-size distribution, compensated for total pore volume, for supported silica Pd/Si02 (0)and unsupported silica SiOz (x), measured with a combination of nitrogen condensation and mercury intrusion methods.
u
u
u
Figure 2. Experimental equipment for the adsorption, desorption, and H2/D2 exchange experiments. A, reactor (CSTR);B, on-off valves; C, constant flow controllers; D,switch valves; E, manometer for pressure control; F, air inlet; G, vent.
,
2. Decrease the temperature in the reactor to the experimental temperature in nitrogen atmosphere. 3. Make a hydrogen step and measure the partial pressure of hydrogen in the outflow from the reactor until it reaches its inflow value. 4. Keep the catalyst in nitrogen flow for 30 min. 5. Add a deuterium step to the nitrogen flow and measure continuously the He, D,, H,, and HD partial pressures as functions of time until the desorbing species (H, and HD) are not detectable in the mass spectrometer and the reactor inlet and outlet concentration of the adsorbing species (D,) are the same. 6. Repeat point 4. 7. Repeat points 5 and 6 again, but with hydrogen as the added flow. 8. Repeat points 4 to 7 once again. Gas flasks were prepared with mixtures of nitrogen, helium, and hydrogen or deuterium, respectively. The partial pressures of hydrogen and deuterium differed only by 2.5% (PH2. = 9.75, p ~ =, 10.01 Torr) in the two types of step experiments. The partial pressure of helium was about 7.6 Torr for both flow combinations. The response flow of the added step was 0.44 mL/s independent of composition, and the continuous flow of nitrogen was 6.59 mL/s. No pressure peak was observed in the mass spectrometer when the step was added. The pressure in the reactor was kept constant throughout the whole experimental series (833 Torr). H2/D, Exchange Experiments. The kinetics for the hydrogen-deuterium exchange are important in order to understand the hydrogen-metal interaction in heteroge-
neous catalysis. Therefore, an experimental procedure was developed and performed in computer-controlled experimental equipment. The procedure was as follows. 1. Prepare the fresh catalyst at 623 K in nitrogen flow for 1 2 h in situ. 2. Activate the catalyst with a flow mixture of H,, Dz, and Nz for 3 h at 623 K. 3. Keep the catalyst for 3 h in nitrogen flow (623 K). 4. Decrease the reactor temperature to 373 K. 5. Randomize between the nine possible flow combinations of hydrogen and deuterium and set this flow to the reactor. 6. Wait until stationary conditions prevail in the reactor (this will depend on the partial pressures, temperature, and the mean residence time). 7. Make two separate readings in the mass spectrometer for the H2, D,, and HD partial pressures, with 5 min in between. 8. Keep the catalyst in nitrogen atmosphere for 5 min. 9. Repeat points 5-7 for the rest of the possible flow combination (exclude all the old flow combinations). 10. Repeat three of the already performed experiments, randomized between the nine total flow combinations. 11. Carry out points 5-10 for the two other temperatures in randomized order. 12. Repeat points 5-9 for the temperature 373 K. The activity was measured under point 2 during 3 h. No change in activity could be detected after the first 30 min. The readings mentioned under point 7 include a filtering of the noise to avoid any perturbation peaks. The filtering procedure includes collecting about 400 readings, rearranging them in increasing order (20 X 20), and calculating the mean value and the corresponding variance. No activity decrease could be discovered throughout the whole experimental series. The temperature levels were 373,393, and 432 K. The partial pressures of the different hydrogen isotopes ranged from 7.7 to 137.3 Torr, and the total conversion of deuterium was in the interval 0.16-0.76.
Theory and Results Adsorption and Spillover Phenomena. Examination of adsorption and spillover phenomena gives useful information concerning the different possible paths for the reaction mechanism of a heterogeneous process. Furthermore, it gives additional knowledge about possible activation of the SiO, support. A recent review by Curtis Conner et al. (1984) gives an excellent survey and summary of the up-to-date knowledge in this field. As a result of these spillover measurements, a more sophisticated and substantiated model for exchange and hydrogenation mechanisms can be motivated and derived. One experimental method to achieve this insight is to exchange hydrogen in hydroxyl groups with deuterium in the gas phase, or the other way around. This exchange can be performed as step response experiments, in a CSTR, with a continuous measurement of the desorbing species as well as the adsorbing species, as functions of time (rates and amounts). Some typical experimental results from the step response experiments for hydrogen and deuterium are given in Figure 3. Several of the experiments were repeated and found to be almost completely reproducible. Curves from repeated experiments had the same shape and the measured adsorbed and desorbed amounts were within 7 %. Peaks of the desorbing species (D,, H,) of hydrogen/deuterium are sharper and reach their maximum heights earlier than the peak for HD desorption, as can be seen from the plotted curves, indicating that the reverse spillover of HD to some extent has a different desorption
1986 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 Table I. Amount of Adsorbed Hydrogen and Deuterium for Different Steps and Temperaturesa D2 for step H2 for step T,K 1 2 3 1 2 43.9 54.2 3.4 3.8 325 21.4 28.6 55.2 16.0 373 34.3 60.6 133.2 140.4 92.7 104.9 473 44.1 254.7 234.6 284.3 288.7 573 115.9 OIn mmol/(s kg of catalyst).
Table 11. Amount of Desorbed Hzand HD in D2 Step Adsomtion ExDerimentP H2 for step HD for step T. K 1 2 1 2 0.0 8.7 0.0 323 7.1 0.0 0.0 58.6 373 32.6 0.0 0.0 188.7 205.4 473 50.9 41.4 397.9 377.3 573 In mmol/(s kg of catalyst).
Table 111. Amount of Desorbed Dz and HD in Hz Step Adsorption Experiments" HD for step D2 for step T, K 2 3 2 3 323 2.1 2.0 0.1 0.4 0.2 19.8 0.5 373 14.5 4.8 3.9 99.3 115.4 473 22.0 22.8 238.3 248.3 573 OIn mmol/(s kg of catalyst).
rate compared to H2 and D2. Integrated values of the adsorbed and desorbed amounts of hydrogen isotopes in each step response experiment are given in Tables 1-111. The values given are mean values calculated from several experiments (the actual experimental time was about 1.5 h for each experiment until stationary conditions were reached). The first step of hydrogen is only a adsorption step and is therefore not included in Tables I1 and 111. The adsorbed amounts, of both adsorbates, increase with increasing temperature, which indicates that some sort of activated adsorption process is dominating. The calculated total amount of palladium metal on this catalyst was 15.04 mmol/kg of catalyst. The amount adsorbed exceeds this value by a factor of approximately 2-5 (depending on whether the adsorbing species is H2 or DJ already a t an experimental temperature below 373 K. The H/Pd value reaches a limit of about 33 at the highest temperature, and this is well above previously published data (Curtis Conner et al., 1983). The dispersion of palladium, mentioned earlier, was 42.9%, and as the ratio of H/Pd in the a-phase of the palladium-hydrogen system never exceeds 0.01 (Scholten and Knovalinka, 1966) for low-temperature and high partial pressure hydrogen isotopes, the amount of hydrogen isotopes absorbed can be disregarded from the total amount adsorbed on the dispersed metal. The amount of HD desorbed is found clearly dependent on whether the adsorbing species is a hydrogen or a deuterium molecule. The large amounts adsorbed can be explained by a spillover phenomenon where the hydrogen (deuterium) dissociatively adsorbs on the dispersed metal and then migrates to a spillover site situated on the SiO, support and thereafter via an activated surface diffusion process moves to and exchanges with the hydroxyl groups bound on the silica support. The hydrogen in the hydroxyl groups will then be exchanged with the adsorbed hydrogen atom and finally desorb, through an adsorbed species on the metal surface. A description of the different energy levels
Table IV. Maximum Adsorption Rates for Hydrogen at Different Temperatures max ads. rate, PH,, PD,, PHD, T,K steu rrmol/(s kg cat.) Torr Torr Torr 323 1 0.00 0.01 70.8 3.86 2 4.43 0.03 0.04 70.3 3 6.70 0.02 0.03 75.1 373 1 4.43 0.00 0.04 75.9 2 7.32 0.01 0.05 82.4 3 6.91 0.01 0.04 79.1 473 1 135.6 4.95 0.01 0.07 2 262.1 5.77 0.06 0.84 3 5.77 0.05 0.99 281.2 573 1 1.97 0.04 0.24 519.1 2 0.93 2.15 2.19 511.8 0.82 1.88 2.00 3 509.1 Table V. Maximum Adsorption Rates for Deuterium at Different TemDeratures max ads. rate, PH2, PD,, PHD, T. K steD umollb kn cat.) Torr Torr Torr 323 1 159.0 0.00 1.07 0.00 2 0.00 0.00 0.70 178.7 1 0.00 0.00 0.00 373 128.3 2 1.87 0.01 152.1 0.00 473 1 0.00 0.17 0.55 219.7 2 0.27 0.00 0.92 238.7 4.00 0.49 641.9 573 1 3.50 2.34 2 0.71 3.93 618.1 Table VI. Maximum Rates for Hz and HD Desorption at Different Temperatures for Dz Adsorption Step Experiments H2, bmol/ HD, bmol/ T,K step (s kg cat.) (s kg cat.) 0.0 5.0 323 2 3 0.0 7.9 373 2 0.0 10.0 3 0.0 17.9 473 2 0.0 180.0 3 8.1 208.8 573 2 298.8 354.7 3 238.7 365.7
of the different states of hydrogen atoms on the surface is given by Curtis Conner et al. (1983). An estimate of the total amount of OH groups on the SiO, part of the catalyst can be made, as 830 mmol/kg of catalyst, assuming a concentration of 5 OH groups/nm2 (Anderson, 1975). At 573 K the total amount of desorbed hydrogen is about 480 mmol of H/kg of catalyst, and the corresponding mean value for desorbed deuterium is 288 mmol of D/kg of catalyst. The amount of H adsorbed minus the amount of H desorbed in the corresponding next step for D2 (the amount of hydrogen desorbed in nitrogen flow between the steps of hydrogen and deuterium) is almost independent of temperature, but the equivalent value for deuterium increases with temperature. The adsorbed amounts of hydrogen and deuterium are approximately of the same order for steps 2 and 3 (H,) and steps 1 and 2 (Dz) (2' > 400 K), respectively, which implies that quasi-stationary phenomena reign. The first step of hydrogen includes the exchange between hydrogen in the gas phase and hydrogen in the hydroxyl groups on the catalyst carrier, showing that the amount of adsorbed hydrogen in the first step is considerably lower than in the other steps. As a comparison the maximum adsorption and desorption rates is included in the complete set of data. Tables IV-VI1 include the maximum rates of adsorption and desorption for H,, D,, and HD, respectively, calculated from the adsorption and desorption curves (Figure 3).
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 1987 Table VII. Maximum Rates for D2and HI)Desorption at Different Temperatures for H2Adsorption Step Experiments Dz,wmol/ HD,pmol/ T,K step (s kg cat.) (s kg cat.) 323 2 1.6 3.1 2.9 3 2.3 373 2 1.6 5.8 3 1.9 6.9 2 8.2 89.1 473 3 8.0 100.4 220.0 2 83.0 573 221.3 3 85.1
From these tables only qualitative reasoning is possible. The adsorption rates are quite constant until 473 K is reached. Then they increase drastically with increasing temperature. Calculated rates are the maximum spillover rates and not the rates of the normal adsorption on metal sites since adsorption on the metal sites is a much faster process. This statement is strongly supported by comparing the rates of hydrogen-deuterium exchange with the maximum rates of spillover a t corresponding temperatures and concentration levels. The rate of metal adsorption/ desorption (experimentally measured exchange rate = rate of desorption (HD) - rate of adsorption (HD)) has a minimum value corresponding to the exchange rate for H2/D2. This means that, when dealing with the H2/D2 kinetics for this catalyst, there is no need for consideration of any spillover phenomena. The surface diffusion is usually considered as being the rate-determining step in the spillover mechanism, and this explains to some extent the temperature dependency for the spillover rates (Kramer and Andre, 1979; Robell et al., 1964). No exchange with hydroxyl groups on the carrier was detected when performing the same step-response experiments only on the pure carrier itself (Niklasson and Andersson, 1988a,b). Maximal desorption rates for hydrogen and deuterium (Tables VI and VII) show significant isotope effects. The desorption rates, at temperatures above 473 K, deviate by a factor of approximately 2 in favor of hydrogen. This indicates that the major part of the deuterium also desorbs in the nitrogen atmosphere prior to the hydrogen step response. Kinetics f o r t h e H2/D2Exchange Reaction. The importance of phenomena involved in the hydrogen-metal interaction during the hydrogenations of unsaturated hydrocarbons is indisputable. The underlying mechanisms of these processes must therefore be penetrated in order to get a full description of the hydrogenation mechanism itself. One example concerning this exchange reaction is the work by Scholten and Konvalinka (1966) on the 0phase palladium-hydrogen system, describing the exchange mechanism with a relatively simple power-law model. The proposed procedure for measuring the H2/D2 exchange, in the present article, is one way of contributing to the more complete picture of the hydrogenation mechanism. The “true” rate expression for the H2/D2exchange has not been f d y elucidated or investigated, but some theories predominate in the literature. The spillover phenomenon and its effect on the exchange reaction is regarded with respect to the kinetic derivation procedure and its mechanistic implication, according to previous work. Three types of exchange mechanisms have been proposed for temperatures above room temperature: 1. Bonhoeffer a n d Farkas’ mechanism (1932) (Scheme I) includes dissociative adsorption of hydrogen and thereafter associative desorption.
Scheme I
H
!-----?
D
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Scheme I1
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2. Rideal’s mechanism (1939) (Scheme 11) assumes hydrogen to be partially adsorbed over a chemisorbed hydrogen atom situated close to a free site. 3. Eley’s mechanism (1948) (Scheme 111) postulates the same mechanism as the Rideal model except that a vacant site is not necessary for the exchange. These types of models for the exchange are very old, and no developments of their theories seem to be in preparation a t present. However, for low temperatures (T < 100 K) another exchange mechanism is proposed, which is a variant of the Rideal and Eley types (Boreskow and Vassilevitch, 1960). These mechanisms lead to several possible rate equations for the exchange, all of which have to be considered in the forthcoming statistical procedure. An exact procedure for the derivation of the different exchange rate equations (eq 1-3), originating from the three previously mentioned mechanisms, and assuming stationary conditions, is described elsewhere (Niklasson and Andersson, 1988a,b). After examination of a number of different types of models, two models remain to be more strictly examined in detail. The simple but informative power-law model is added to the selected group of models, for comparison. Model I in this group is an Eley-Rideal type of rate equation derived from the assumption of an Eley exchange mechanism (Scheme 111): model I r1 =
h+e x d - W
+ PHD) + + P H D ) I / ( ~ P ~ O ? (1 /~ +
) [ ( ~ P H ,- PHD)@PD,
(2PDz - P H D ) ( 2 P H z
[exp(-132.4/R - D J - I / R T ) P ~ ( I ~(1) /~)) where G is the weighting of the inverse temperature around the inverse of the mean temperature [G = ( l / R ) ( l / T l / T m ) ]and ptotis the total sum of the partial pressures for the different (H2, Dz, HD) isotopes in Torr (pb( = ptOt/76Oatm). Model I1 is a Langmuir-Hinshelwood type with a dependence for the enthalpy of adsorption on the square root of the partial pressures of hydrogen isotopes, derived from the assumption that a Bonhoeffer-Farkas exchange mechanism is dominating on the catalyst: model I1 r2 =
k2+[(2pHz + P H D ) (2PDz+ P H D ) /Ptot - 2 P H D I
2(1
+ [exp(-132.4/R
- A.H/RT)~,(]’/~)~
(2)
1988 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988
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Figure 3. (a, top left) Measurements of H2, D2, HD, and He after a step response (no. 2) of H2 at 473.3 K. (b, top right) Measurements of H2, D,, HD, and He after a step response (no. 2) of D2at 473.3 K. (c, bottom left) Measurements of Hz, D2,HD, and He after a step response (no. 2) of H2 at 573.5 K. (d, bottom right) Measurements of H2, D2, HD, and He after a step response (no. 2) of D2at 573.5 K. (- .-) He, (-) HD, (--) H2, (- --) D2. The reference pressure for H2, D2,and HD is the inlet pressure of (a, c) hydrogen and inlet pressure of (b, d) deuterium and for He it is the inlet pressure of helium.
where AH = AHo - ~ p ~ is t the ' / ~enthalpy of adsorption for hydrogen isotopes, a t zero surface coverage for hydrogen, corrected for a decrease with partial pressure of hydrogen isotopes. The first two models disregard any possible isotope effect in the exchange reaction, and both have three unknown parameters. Model I11 is the simple power-law model with four independent parameters to estimate: model I11 73 =
k3+
exP(-E&)PD;PH~
Table VIII. Parameters of the Three Models Calculated from the Least-Squares Regression, Describing HD Exchange on a Fresh Catalyst (Pd/SiO,). 1, Eley-Rideal; 2, Langmuir-Hinshelwood 3, Power Law 95% conf. value limit units Model 1 ss 1.3 x 10+7 klt 4.7 X lot1 2.3 X lo+' pmol/(s kg of catalyst Torr3/,) E, 1.1 x 6.6 x J/mol /w -6.1 x lot4 4.9 X lot3 J/mol
(3)
A statistical comparison between the models was made after estimation of the parameters, using a standard sumof-squares minimizing routine. The residual sum of squares and regression parameters with their individual 95% confidence limits, for the three models, are given in Table VIII. The use of a constant value for A S in the regression analysis, commonly seen in the literature, lowers the number of parameters to be estimated. The choice of a fixed value of A S is motivated by the fact that it is quite easy to estimate this parameter from theory and previously published experimental data. The value -132.4 J/(mol K) is taken from Konvalinka et al. (1981) as an estimate of the gas-phase entropy for hydrogen compared to the entropy of a localized adsorbed hydrogen atom. The entropy change of the metal was disregarded from the calculation. The gas-phase entropy for hydrogen was taken from calculations made by van Meerten (1975) on the basis of the partition functions.
ss k2+ AHo 7
ss
k3+
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a
Y
1.1 x 10+7 1.2 X
-6.8 x lot4 -6.8 X lot2
Model 2
6.3 X lo+' 1.0 X 6.9 X 10"
pmol/(s kg of catalyst Torr) J/mol J/(mol
Model 3
5.9 x 10+6 2.2 X 1.2 X lot4 6.5 X lo-' 6.4 X lo-'
3.9 X 10+O pmol/(s kg of catalyst Torr"') 6.5 X J/mol 2.6 X lo-, 2.4 X
The energy of activation for adsorption for the L-H model was a parameter that could be excluded from the model since it was found not to be significantly different from zero in the regression analysis. The L-H model also includes a pressure dependence for the enthalpy of adsorption, which is an established and frequently used procedure (Boudart and DjBga-Mariadassou, 1984; Conrad et al., 1974; van Meerten, 1975). Usually a surface coverage of hydrogen isotopes is used, but for convenience the
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 1989 Table IX. Some Typical Experimental Exchange Rates for HD on a Fresh Pd/Si02 Catalyst Pqt
PHp
PW, Torr
Torr
Torr
71.4 73.5 33.4 41.8 34.8 50.4 95.7 85.1 95.1 109.5 46.9 35.8 109.4 50.1 47.3 97.9 41.8 96.8 35.6 61.1 75.9 35.6
54.5 79.7 16.3 106.5 15.6 73.8 68.6 47.6 68.9 56.7 9.6 18.5 56.7 102.4 75.4 62.5 10.5 63.1 13.6 59.6 78.5 13.6
76.5 71.4 104.1 19.5 103.4 16.9 60.2 69.6 60.4 84.1 97.4 27.7 84.1 15.4 18.5 89.9 133.5 90.5 136.6 81.6 70.2 136.6
r, w"/
(a kg cat.) 4148.8 4458.2 1786.0 2276.8 1870.4 2647.6 5787.9 4922.3 5739.2 6864.1 2517.4 1718.8 6852.0 2716.6 2448.3 6151.5 2340.5 6031.7 1994.4 3351.5 4572.0 2000.7
T,K 373.3 373.2 373.4 373.3 373.1" 412.1 412.1 412.0 412.0'' 432.1 432.0 431.9 432.2'' 393.2 393.2 393.3 393.4 393.1" 373.Sb 373.7b 373.6b 373.5"Pb
'' Reproducibility experiments. Long-time stability test experimenta.
partial pressure of hydrogen isotopes is selected as the independent parameter. This choice lowers the number of parameters and simplifies the regression procedure. This is a well-known applied procedure in nonlinear regression analysis. For the WR model, the same procedure was tested but gave no positive result. The powers for hydrogen (a = 0.64) and deuterium (y = 0.65) in the power-law model (cf. Table VIII) indicate that the isotope effect is not significantly pronounced for the exchange on a-PdH. The results in Table VI11 also show that the exchange depends on the partial pressure of hydrogen isotopes to a greater extent in the a-PdH than in the P-PdH (cf. Scholten and Konvalinka, 1966). For the power-law model, evident residual trends in temperature and hydrogen and deuterium pressure have been found, so this model can be left out of account from now on. To discriminate between the other two models, a residual analysis was further developed. The regression parameters were all significant, as shown in Table VI11 for both models. The evaluation of individual residuals for the two models showed some differences. The residuals for the L-H model showed no trends at all, which is not a t all surprising since no significant isotope effect was found in the power-law model. The E-R model revealed small but obvious trends in temperature and the total partial pressure of hydrogen isotopes. Therefore, and because of its lower sum of squares, it was concluded that the L-H model described the data best at the present reaction conditions. The adsorption enthalpy, for the Langmuir-Hinshelwood model, ranges from 6.0 to 6.8 X lo4 J/mol, and this is of the same order as previous experimental values (Toyoshima and Somorjai, 1979). Parameter values for entropy change upon adsorption are well within the recommended limits described by Vannice et al. (1979). The calculated sticking coefficient is low but reasonable according to Boudart and DjBga-Mariadassou (1984).
Some typical experimental results for the Pd/Si02 catalyst are given by Table IX, in the same chronological order as they were obtained. As can be seen here, the reproducibility for each temperature level ranges from 0.2% to 4%. This sets the absolute limit on the extent
to which a proposed model can explain experimental results without explaining the experimental variance. The overall reproducibility was of the same order. The support @io2) itself had low activity (
