Ind. Eng. Chem. Res. 1993,32, 1315-1322
1316
Role of Catalyst Characteristics in Electrocatalytic Hydrogenation: Reduction of Benzaldehyde and Acetophenone on Carbon Felt/Pd Electrodes Anna M. Polcaro,’ Simona Palmas, and Stella Dernini Dipartimento di Zngegneria Chimica e Materiali, Universitb di Cagliari, 09123 Piazza D’Armi Cagliari, Italy
The hydrogenation of benzaldehyde and acetophenone waa investigated at two carbon felt-supported Pd electrocatalysts, prepared by two different methods. The faradaic yield and the selectivity of the reaction were found to be greatly affected by the preparation conditions of the catalyst. A model, based on a reaction electrocatalytic mechanism, involving two parallel steps through which alcohol and hydrocarbon are generated from the reactant adsorbed on different active sites, waa performed. The kinetics was described by means of the Langmuir-Hinshelwood rate equations, and the kinetic and equilibrium parameters were determined for both electrocatalysts.
Introduction The use of electrochemical methods to produce organic compounds could represent an alternative approach to the traditional chemical synthesis: in particular, several papers have described interesting electrochemical hydrogenation reactions at Ni-Raney and Pt or Rh black cathodes (Bonnier et al., 1987;Pintauro et al., 1987;Chiba et al., 1983; Pletcher and Razaq, 1981; Casadei and Pletcher, 1988). On this kind of electrode the reaction is thought to proceed through two subsequent steps: the former involves the electrolytic decomposition of the solvent with formation of hydrogen atoms adsorbed on the catalyst surface and the latter involves the reaction between these atoms and the organicmolecules of substrate also adsorbed, to give the final reaction products (Beck, 1971;Wendt, 1984). Therefore, electrocatalytic reduction presents several aspects in common with heterogeneous catalytic hydrogenation, but it offers the advantage that the kinetic barrier to the splitting of hydrogen molecules is completely avoided, as is the mass transport of the poorly soluble hydrogen. However, it can be noticed that this electrochemical hydrogenation mechanism can occur only at electrodes on which the reaction
H+aq+ e-
-
Had
is fast enough to be favored with respect to the direct reduction of substrate X:
X,
+ e-
XThe present work is a part of the research program carried out in our laboratory concerning the electrocatalytic hydrogenation of carbonyl compounds. In particular, acetophenone (ACF) and benzaldehyde (BA) reduction are investigated here. To perform the reduction, palladium was adopted as active metal; a carbon felt support was utilized owing to its high mass-transfer coefficient (which makes the resistances due to the mass transport from and to the electrode surface negligible) and its good fluid permeability so that it is possible to operate with high liquid flow rates without excessive pressure drop in the system. Moreover, the high specific area allows sufficient productivity per unit volume of electrode. A typical protic medium constituted by an alkaline or acid hydroalcoholic solution was utilized as a solvent. In a previous work (Polcaro et al., 19931, it was already stated that on these electrodes the electrocatalytic mechanism for the hydrogenation of BA and ACF in protic
-I
a) cell b) pumps c) catholyte reservoir d) ~ o l v t reservoir e
+
----------A -
!I
f -
T
-
7
b
C
I Figure 1. Schematic view of the experimental setup.
media was verified: the influence of the kind of active metal (Ptor Pd) and the supporting electrolyte pH on the faradaic yield and reaction selectivity was examined. It was also remarked that the different morphology of the active metal deposit, resulting from different experimental electrodeposition conditions (Polcaro and Palmas, 19911, could give rise to strong variations either in total current yield of electrohydrogenation or in the yield of the single products (Polcaro et al., 1992). In the present work the study was completed by examining the electrochemical reduction of ACF and BA in both alkaline and acid ethanoVwater solutions at two different electrocatalysta obtained by depositing the same amount of palladium under different experimental conditions, and focusingthe dependence of the electrocatalyst behavior on its preparation conditions. The importance of the support and the catalyst preparation conditions in the catalytic reactions has been evidenced by several authors (Vannice, 1992; Alba et al., 1985; Polyanszky and Petro, 19901,and by Suh et al. (1992) in the particular case of liquid-phase chemical hydrogenation. It seemed interesting therefore to underline the importance of these parameters for the examined electrocatalytic reactions.
Experimental Section The experimental setup is shown in Figure 1. It consisted of an electrochemical three-electrode cell connected to an electrolyte reservoir, well mixed by means of a magnetic stirrer. The anodic circuit was independent from the cathodic one, and centrifugal pumps allowed the
0888-5SS5/93/2632-1315$04.00/0 0 1993 American Chemical Society
1316 Ind. Eng. Chem. Res., Vol. 32, No.7, 1993
Figure 2. Scanning electron microscopyof the active metal deposit (Acat).
Figure 3. Scanning electron microscopyof the active metal deposit (Bcat).
recirculation of the liquid through the system; the overall volume of recirculated catholyte was about 50 cm3and a liquid flow rate equal to 0.25 em% was usually adopted in the experimental runs. The catholyte composition in the reservoir was uniform, and the fluid was continuously purged, by bubbling nitrogen, for the Hz excess produced during the electrolysis. Constant temperature at 25 i 0.1 'C was achieved bv immersine: - the reservoirs in a thermostated bath. The two comoartments of the cell (2 X 0.5 X 1 cm) were tilled by carbo; felt strips as electrodes, separatedby an ionic exchange membrane. The electric contacts, made of thin platinum wires, generated a current flow perpendicularto theelectrolyte flow. Aconstant currentthrough the cell or a cathodic potential were superimposed by means of a potentiostat-galvanostat (EG&G 273); the potential values were measured with respect to a saturated calomel electrode (SCE). A small amount of active metal was deposited on the cathodic felt, in such a way that the conversion per pass of reactants in the bed was extremely small. Since the residence time in the reservoir, T,, is much higher (200 s) than the residence time in the cell, T, (4 s), a batch model can be used to representthe recirculating (electrochemical cell plus well-mixed tank) system. Electrocatalysts: Preparation and Morphology. Two electrmtalysts were both prepared by electrochemi d deposition of palladium from a 1V M PdCld0.1 M H&O4 solution. The first (Acat) was obtained by deposition of active metal under kinetic regime: a constant current equal to 0.05 A, lower than the limiting current, was sent through the cell and the electrolysistime was 600 s. The second (Bcat)wasprepared by potential-controlled electrolysis: the potential was set at such a value (about 0 mV vs SCE) that the deposition was under diffusion control and the electrolysis was carried out until the same Pd amount (about 10% Pd/carbon felt) was deposited on the fibers. Scanningelectron microscopies showed that, in the case of Acat, the growth of crystallization nuclei was rather uniform on the fibers and, after the selected deposition time, an average diameter of the centers equal to about 0.2pm wasachieved. WhenBcat wasprepared,thecrystal growth occurred prevalently on the nuclei more directly expoaedtotheelectrolyteflowandtheformationofclwters (average diameter ranging from 0.3 to 0.7 pm) was noticed as result of the center overlapping (Figures 2 and 3). Electrolytes. The catholyte was constituted by a 0.1 M KOH or HzSO, hydroalcoholic solution (10% HzO +
90% ethanol) of BA or ACF. The organic substrate concentration was in the range from 0.01 to 1 M. The same hydroalcoholicsolution,without carbonylcompound, was utilized as anolyte. The electrolyteswere prepared fromchemicalssupplied by Carlo Erba with purity grade greater than 99.5 %. The organic compound purity was also confirmed by gas chromatographic analysis. Experimental Procedure. Electrolysis experiments were carried out at constant current or constant potential. The trend of the reaction was followed by withdrawing samples of catholyte at regular intervals of time and analyzing the concentration of the residual reagents and the reaction products. A preliminary check to investigate the possible products from the two substrate hydrogenation was performed by analysis using high-pressure liquid chromatography (HPLC Hewlett Packard 1050) with a 25-cm reverse-phase column (Chromopack ChromospherC8; innerdiameter4.6mm). ThisHPLCanalysis of the solutions revealed that the only reaction products were benzyl alcohol and toluene from the BA reduction, and phenylethanol and ethylbenzene from the ACF reduction, and no secondary products were appreciahly detected then the analyses were usually made by gas Chromatography. APerkin Elmer 8310GC equipped with a 25-m-wide fused silica as a stationary phase (CP-Wax 52) was used operating with an He carrier gas flux of 16 mWmin at programmed temperature up to 190 O C . Product concentrations were obtained by calibrationwith prepared standard solutions, and the minimum revealed composition was about 5 X lo-' M. The GC error in the single analysis was usually estimated to he 1 % . However, it can be noticed that the scatter between the data related to a single run was generally within 5%. If the same run was performed on a newly prepared electrocatalyst, an agreement between the data within 8% on average was observed, probably due to the imperfect reproducibility of the catalyst surface characteristics. A check at the end of each run showed that if the losses due to the eventual leakage of reactant to the anolyte were taken into account, the material balance was always within 100% in the limit of the experimental errors.
Experimental Results The effect of preparation conditions of the carbon feltsupported palladium catalyst was studied by comparing the experimental results from electrochemical reduction of carbonyl compounds carried out under the same
Ind. Eng. Chem. Res., Vol. 32, No. 7,1993 1317 15
a)
1 E \
:
0
200
400
600
800
1000
o
u l0
t / s
15 1
0
10,
1
500
1000
1500
I000
500
0
1200
t / s
1500
2000
0
2500
I
500
1000
1500
2000
2500
6000
8000
I0000
t / s
t / s 20 7
E
v
3
10
t
\
\
V
V
0
0 0
2000
4000
6000
8000
10000
0
2000
4000
t / s
t / s
Figure 4. Concentration vs time curves for acid solution of BA (E])
Figure 6. Concentration vs time curves for acid solution of ACF (E]) at Acat (full symbols) and at Bcat (empty symbols) during an electrolysis at 0.2 A (a) CxO = 0.5 M (b) Cxo = 0.12 M, (c) Cxo 0.02 M. (A, phenylethanol; 0,ethylbenzene).
at Acat (full symbols) and at Bcat (empty symbole) during an electrolysis at 0.2 A (a) Cxo = 0.5 M (b) Cxo = 0.12 M, (c) Cxo 0.02 M. (A, benzyl alcohol; 0,toluene).
experimental conditions at the two prepared electrocatalysts. Figures 4 and 5 show some typical examplesof the trend of the product concentration in time during the electrolyses carried out at a constant current intensity equal to 0.2 A. The cathode potentials were -320 mV and -1100 mV vs SCE in acid and alkaline medium, respectively, both in BA and in ACF solutions. In order to compare the electrolysis behavior at different electrocatalysts, the current efficiencies q for the carbonyl compound hydrogenation, to give each reaction product, was calculated as
where ni and zi are the number of moles of product and the number of electrons involved in the reaction and F is the Faraday constant. Assuming that the volume of therecirculating electrolyte V , was unchanged in the course of the experimental run, eq 1 may be expressed as V , dCi qi = -2iF I dt When the initial substrate concentration was sufficiently elevated, as in the cases shown in Figures 4a,b, and 5a,b, the current yield was directly evaluated by introducing in eq 2 the ratio between the variation in the product concentration ACi and the related electrolysis time At. For more dilute solutions, the minimum product concentration, which could be analyzed with acceptable accuracy, was no longer negligible compared to the initial substrate concentration, so that the latter could not be assumed as unchanged during the run. In these cases, the electrolysis was carried on up to a near complete consumption of the
carbonyl compound: the initial slope of the curve that fitted the product concentration us time experimental data (Figures 4c and 5c) was used to calculate vi according to eq 2 for the initial substrate concentration. The experimental vi values for the hydrogenation in acid solution of ACF and Ba in the full examined concentration range are shown in Figure 6. Figure 7 illustrates the results in alkaline medium: lines represent the yield values, calculated, as explained below, according to the kinetic model that was performed to interpret the experimental results. The strong variation of the faradaic yield and the reaction selectivity that arises from the use of catalysts with different morphology is clearly evident for the hydrogenation of both carbonyl compounds. Further information about the behavior of the systems was obtained from controlled-potential electrolyses of very dilute solutions of carbonyl compounds. The current I flowing through the cell was recorded in time, and the amount of charge corresponding to the variation of the product concentration was evaluated by numerical integration of I vs t curves in the relative interval of time. The current yield of the reaction for CX= Cx0was determined from the initial slope of the curves representing the product concentration as a function of the amount of charge passed through the cell. In all examined cases, at the same substrate concentration, when a more negative potential value was imposed at the cathode, vi values strongly decreasing were observed, as reported in Figure 8. Discussion Previous investigations (Polcaro et al., 1993) on the hydrogenation of BA and ACF in hydroalcoholic acid solutions at these porous electrodes have already confiied that the reaction occurs by the electrocatalytic mechanism,
1318 Ind. Eng. Chem. Res.,Vol. 32, No. 7, 1993 0.5 ~
.O
benzylalcohol phenylethanol
0.40.6
0.5
0.31
q 0.4
tl
0.3
'/
I 0
0.2
0.4
0.6
1
0.8
Benzaldehyde concentration mol/dm3 0.9 ,
,
lo phenylethanol
08
b)
ethylbenzene
1 I
I 06 ;
05
-
0
0.2
0.4
0.6
0.8
1
Acetophenone concentration moYdm3 Figure 6. Faradaic yield for BA (a) and ACF (b) reduction in acid solutions as a function of substrate concentration. Lines represent the model prediction; full symbols refer to reduction products on Acat; empty symbols refer to reduction products on Bcat.
typical for low hydrogen overpotential electrodes. In fact, both during the electrolysis process and after the current interruption, a cathodic potential independent of the presence of the carbonyl compound and coherent with the value related to the hydrogen evolution reaction was observed. Moreover, the reaction products obtained at these electrodes are typical for indirect electrochemical hydrogenation: indeed, at high hydrogen overpotential electrodes (Farina et al., 1990; Honnorat and Martinet, 1983), such as Hg or Pb, at which the reduction takes place by means of direct transfer of electrons between the substrate and the electrode surface, it was shown that a carbanion is first generated and pinacols and superior alcohols are the final products. In addition the results indicated that alcohol and hydrocarbon are originated through parallel steps from the carbonyl compounds adsorbed on the catalyst surface (Polcaro and Palmas, 1993). The formation of alcohol and hydrocarbon through parallel steps was also observed during the electrochemical hydrogenation of BA and ACF at vitreous carbon-Ni supported catalysts in acid solution, and it was interpreted on the basis of the hypothesis that the two products were originated from the reactant adsorbed on different active sites, each with its own chemistry (Lain and Pletcher, 1987). Moreover, in the catalytic hydrogenation of ACF at A1203Pd supported catalysts, the formation of alcohol and
0
0.2
0.4
0.6
08
Organic substrate concentration moYdm3 Figure 7. Faradaic yield for ACF and BA reduction, in alkaline solution, as a function of substrate concentration. Lines represent the model prediction; full symbols refer to reduction producta on Acat; empty symbols refer to reduction products on Bcat.
hydrocarbon through parallel steps was also noticed, at least while the product concentration did not exceed the residual concentration of ACF in the solution. The observed behavior was explained by supposing that lowcoordination active sites (us),such as those present in steps and kinks, are needed for ethylbenzene formation, whereas phenylethanol was generated at active sites of higher coordination (uf),distributed throughout the crystal surface (Alba et al., 1985). It has also hypothesized that the fractional catalyst coverage by organic compound and by hydrogen were independent of each other: a similar assumption was already adopted to interpret the kinetic data related to the catalytic hydrogenation of the dinitrotoluene (Jansen et al., 1990). In fact, as aconsequence of the very different species involved, it seemed reasonable to assume that the active sites on which the carbonyl compound is adsorbed and activated to give the two products were substantially different from those where the atomsof adsorbed hydrogen are obtained from electrolysis. On the basis of these considerations, the mechanism of the electrohydrogenation in acid medium can be sketched as Hf,
=Had =x, Us =x,
+
e-
x + x +
Uf
/
Had
Xf
-
H2 A
where X indicates the organic substrate. Great attention was paid to eliminate mass-transfer effects and to measure the intrinsic reaction rate; moreover, owing to the low hydrogen overpotential, the electrochemical reaction of Had formation (Voher reaction) was considered at quasi-equilibrium state. Therefore, the LangmubHinshelwood rate equation was used to describe the kinetics of the two parallel reaction steps in the ACF or BA hydrogenation process. The Langmuir-Hinshelwood rate equations for the organic substrate hydrogenation to give the reaction products, alcohol and hydrocarbon, can be written as
Ind. Eng. Chem. Res., Vol. 32, No. 7, 1993 1319 0.5 1
0.4
0.3
I
phenylcthauol octhylbmzene
I
/
rl 0.2
substituting the rate equations (eqs 3,4, and 6) in eq 2 and taking into account the hydrogen mass balance expressed by eq 10. In order to correctly determine the kinetic and equilibrium parameters, the results from electrolysis of extremely dilute solutions, for which some simplifications could be made, were first analyzed. In fact, a t low organic substrate concentration, the catalyst coverage by reactant on the two kinds of sites becomes
I
P
I
1 /-
I
04
-350
-300
-250
-200
-1%
-100
/ mV
E
Figure 8. Effect on the cathode potential on the faradaicyield during the ACF reduction ((2x0 = 0.01 M)at Bcat.
where kA and kI represent the reaction rate constants per unit of liquid volume. They are functions of the true reaction rate constant k', as well as of the mass catalyst Wand the ratio between the residence time in the reactor 71c and in the reservoir 7,, as in the following relation:
6, = Kx,Cx 6, = K&, (11) Moreover, if the current efficiency for the hydrogenation of the carbonyl compound is low, the reaction rate for both alcoholand hydrocarbon production may be neglected if compared with the molecular hydrogen evolution reaction; thus, eq 10 becomes I/2FVm= k ~2 6 ~ (12) Introducing the simplified expressions for e%, Ow, and eH, given by eqs 11 and 12 respectively, in the kinetic eqs 2 and 3 and integrating, the following relationships between the reactant and product concentration and electrolysis time are obtained:
(5) where As a parallel step, the desorption process of adsorbed hydrogen to molecular H2 (Tafel reaction) has to be considered. Its kinetics may be expressed as follows (Bockris and Reddy, 1972):
~2 6 ~ (6) kH being the kinetic constant for this reaction. Taking into account for the model assumptions, the catalyst coverage can be expressed by rH = k
K&X 1 KxfCx
+
(7)
,e = 1+ K,Cx where Cx represents the organic substrate concentration and K& and &are the adsorption constants of substrate on uBand ut sites, respectively. Moreover, owing to the assumption of quasi-equilibrium state for the Volmer reaction, the catalyst coverage by hydrogen may be expressed as (Bockris and Reddy, 1972) KH[H+] eXP(-Em) F
=
€3
(9)
1 KH[H+]exp E-
(-
After a short transient following the closure of the electric circuit, the concentration of Had, which represents the intermediate product of the reaction, can be considered constant and a balance equation leads to
Therefore, the a values can be calculated from the slope of the straight lines obtained by plotting, as a function of time, the ln(Cx/Cxo) data experimentally determined during the constant current electrolysis of dilute solutions of carbonyl compound. Then, B and y parameters were obtained from the slope of the linear plot of CdCxO and CI/Cx0 vs (1- exp(-at)). The experimental values of a, 8, and y, determined in the examined systems, are listed in Table I. Data from controlled-potential electrolyses of dilute solutions were elaborated by assuming the approximations of eq 11still valid and correlating the imposed potential to the catalyst coverage 6H by eq 9. If the initial faradaic yields are combined in the form
r = 1 - 2vI - ?A VA
(15)
the r values, plotted vs OH, give a straight line. In fact, substituting in eq 15 7 values from eqs 2-4 and 9, I' can be expressed by
r e 66,
=
kH
(16)
kAKX&Xo6H
The relationship between the current efficiency for the carbonyl compound hydrogenation at constant current density I and the substrate concentration may be obtained
and it is a linear function of 6 ~ The . values of the slope 6 obtained for the examined systems are also included in Table I.
1320 Ind. Eng. Chem. Res., Vol. 32, No. 7, 1993 Table I. Experimental a,19,y, and 6 Values for the Examined Systems at Different Substrate Concentrations substrate catalvst concn a (8-1) B Y 6 Acetophenone 0.025 4.5 X 10-5 0.82 0.18 8.1 Acat 0.20 10 4.7 X 10-5 0.81 0.020 0.84 0.17 0.012 4.3 X 10-5 Bcat 0.020 2.0X lo-' 0.69 0.31 1.1 0.018 2.3 X lo-' 0.65 0.28 1.0 0.011 2.5 X lo-' 0.71 0.35 0.68 0.30 0.009 1.7 X lo-' Acat Bcat
0.020 0.015 0.018 0.020 0.018 0.012 0.009
Benzaldehyde 4.1 X 10-5 4.1 X 10-5 4.0 X 10-5 4.7 X lo-' 4.5 X lo-' 4.6 X lo-' 4.9 X lo-'
0.5
A 0.4
*
benzylslcohol
0
toluene
A/
0.3
rl 0.2
0.75 0.73 0.78 0.58 0.62 0.56 0.57
0.24 0.22 0.24 0.42 0.40 0.44 0.42
7.0 9.2 2.1 2.4
0.1
0
Table 11. Estimated Values of Kinetic and Equilibrium Parameters acidic solution alkaline solution model parameters Acat Bcat Acat Bcat Acetophenone 106 k A (mol/(s dm3)) 2.1 30.9 0.73 0.63 K m (dms/mol) 0.45 0.25 0.1 0.55 106 k1 (mol/(sdm3)l 0.69 0.88 K% (dmVmol) 0.45 6.33 lo6kH (mol/(a dm3)) 0.19 0.17 0.42 0.36 €A 0.0049 0.0042 0.0011 0.0046 €1 0.0032 0.0011 Benzaldehyde 1 0k~ (mol/(s dm3)) 3.09 9.45 1.60 3.46 KW (dmg/mol) 0.3 0.45 0.4 0.3 0.88 1.05 106 k~ (mol/(s dm3)) Kx,(dma/rnol) 0.23 1.8 1 0k~ (mol/(s dm3)) 0.13 0.19 0.40 0.21 EA 0.0019 0.0054 0.0038 0.0128 €1 0.0011 0.0038
0
0.2
0.4
0.6
0.8
benzaldehyde concentration / mol dm-3
Figure 9. Model prediction (lines) and experimental data from reduction of BA at Acat (full symbols)and at Bcat (empty symbols). Experimental data from Polcaro et al. (1992).
proposed that the role of a catalyst is to provide not only a high surface area on which the active metal can be dispersed, but also properties peculiar to the supported catalyst. In fact, surface characteristics of both support and active metal, together with synergistic effects between catalyst and support, may significantly enhance the catalytic activity (Ehrburger et al., 1976; Vannice, 1992). Other parameters that seem to be important to determine catalytic activity are the particle size (Mukerjee, 1990; Bagotzkyand Skundin, 1984)and the distributionof active metal on the support (Suhet al., 1992). The carbon felt/ Pd electrodes used in this work differ from each other in size, form, and distribution of metal particles throughout The values of kH,k ~ K x tand , k&%, calculated by means the support. Indeed, it has already been shown (Polcaro a, j3, y, and of eqs 14-16 from the average experimental and Palmas, 1991) that the metal electrodeposition 6 parameters, were introduced in the complete model conditions strongly affect the deposit morphology as well equation elaborated to represent the current efficiency as the catalytic activity toward the hydrogen reduction for organic reaction as a function of the substrate conreaction at these electrodes. The experimental results centration. The single values of kinetic and equilibrium indicated that the electrocrystallization of palladium on parameters were finally calculated by means of a nonlinear carbon felt, at the initial stage, is always controlled by regression analysis by minimizing the sum of the squared formation of thermodynamically stable nuclei, whereas, residuals between the experimental and calculated current the center growth depends on the electrode potential and yield values. Table I1 reports the final results of calculations; EA and €1, expressed by the sum C(vexp- v ~ a ~ ) ~ / Non , the fluid-dynamic conditions. If the metal deposition reaction is kinetically controlled, widely spaced and fairly give an idea of the agreement between model prediction regularly distributed, hemispherical particles are oband data for the alcohol and hydrocarbon production, served their diameters grow uniformly in time on all the respectively. fibers. If the Pd electrodeposition is carried out under The adopted model, even with the outlined assumptions, diffusion-controlled conditions, at a more cathodic elecappears to be able to describe the hydrogenation reaction trode potential, a larger number of smaller particles are of the carbonyl groups for both examined compounds at formed. During the growth, some irregular clusters, arising the two different kinds of electrocatalysts. from the overlapping of crystallization nuclei, are noticed Previous experimental data on BA hydrogenation at on the fibers. carbon felt/Pd electrodes (Polcaro et al., 1992) prepared A different step or kink density between the hemiwith the same procedure but using shorter electrolysis spherical particles, which characterizes the Acat, and the time are also fitted by this model, as can be seen in Figure clusters of crystallization centers, which mainly grow on 9. However, in this case, a reliable determination of the the Bcat fibers, could be responsible for the different model parameters is difficult since only a few, fairly selectivity exhibited by the two kinds of electrocatalysts, scattered, experimental data were available: in particular, given that, on the basis of the presented mechanism, it no runs, in the range of low substrate concentration, were could affect the ratio between the kinetic constants for carried out in order to determine a,j3, y, and 6 values. the parallel reactions. Metals of the platinum group on several supports have Different characteristics of the active metal electrodebeen widely studied in conventional catalytic and elecposit may also justify the differences noticed between the trocatalytic hydrogenation: in particular, it has been
Ind. Eng. Chem. Res., Vol. 32, No. 7, 1993 1321 results obtained in this work for BA hydrogenation and those previously observed (Polcaro et al., 1992) for the same kind of electrocatalyst. Indeed, in the last case, the electrode not only presented a smaller surface area of active metal, owing to the smaller amount of Pd deposited, which explains the lower overall current efficiency, but also the particle size was different because of the shorter electrolysis time, thus avariation in the reaction selectivitycould occur. In fact, larger differences are noticed a t Bcat for which the metal deposit particularly changes in time owing to the cluster growth. However, it can also be remarked that, in that previous work, a supporting electrolyte with a different anion was used. The different metal center distribution throughout the felt could be responsible for the higher kinetic constant obtained at Bcat compared with Acat, with the same metal amount, which has a more uniform distribution of small Pd particles on the felt fibers. A difference in the observed catalytic properties for carbon-supported Pd catalysts prepared by different methods was also evidenced (Suh et al., 1992)in fluid-phase hydrogenation of nitroaromatics. Also,in that case, a better utilization of active component was noticed for the catalyst characterized by a less uniform distribution of Pd throughout the porous support on which the growth of the crystallization nuclei located on the surface of carbon particle was favored. As can be seen from Figure 7, in alkaline solutions the total current yield of the reactions appears to be lower than in acid solution, and the formation of hydrocarbon is not observed, according to the behavior already verified at this kind of electrode (Polcaro et al., 1992) and during the ACF hydrogenation at Pt/Pt electrodes (Casadei and Pletcher, 1988). A previous study on the electrochemical hydrogenation of nitro compounds on Pt electrodes (Vassiliev et al., 1981a,b)evidenced a lower reaction rate in alkaline rather than in acid solutions: in that case a change in the reaction mechanism was hypothesized when one considered acid or alkaline solution. It was supposed that, in alkaline solution, the hydrogenation occurred by direct reduction of the organic substrate, as occurs in acid solution but at electrode with high hydrogen overpotential. In the present case, this change of mechanism seems improbable, since an analogous trend of the alcohol formation rate as a function of the potential and substrate concentration is observed in either acid or alkaline solutions. The system behavior could then be explained by the presence of alkaline metal ions that, as suggested by previous work (Pintauro and Bontha, 19911, are preferentially adsorbed on the active sites, making the interface structure hydrophilic. In turn, the average residence time of hydrophobic organic compound on the catalyst surface could be lowered. The absence of hydrocarbon in the hydrogenation products may suggest that this mechanism could be more probable on CT~sites on which the adsorption of the organic compound could be particularly hindered. By assuming the same electrocatalytic mechanism as in acid solutions, the model parameters resumed in Table I1 can be derived, and Figure 7 shows how the model prediction agrees with experimental results in the case of alkaline solutions. However, the data obtained up to now in our laboratory do not seem to be sufficient to explain completely the hydrogenation mechanism in alkaline solution, and further experiments are still going on to better define the problem. Nomenclature C = concentration, mol/dm3 E = electric potential, V
F = Faraday constant, 96 500 C/mol I = current intensity, A Ki = adsorption constant of species i, dmVmol ki = rate constant for reaction, mol/(a.dms) k'i = rate constant for reaction, mol/(s*g,) N = number of experimental data r = reaction rate, mol/(s.dms) R = gas constant, J/(mol.K) T = temperature, K t = time, s V = volume, dm3 W = mass of catalyst, g z = number of electrons involved in the reaction
Greek Letters and Symbols a = defined by eq 14a, s-1 0 = defined by eq 14b y = defined by eq 14c I' = defined by eq 15 6 = defined by eq 16 = faradaic yield of the ith component 0 = fractional surface coverage = active site T = residence time, s Sub- and Superscripts 0 = at initial time calc = calculated exp = experimental f = at high-coordination site i = ith component rc = at reactor rs = at reservoir s = at low-coordination site Abbreviations Acat = catalyst obtained at constant current ACF = acetophenone BA = benzaldehyde Bcat = catalyst obtained at constant potential X = organic substrate Literature Cited Alba, A.; Aramendia, M. A.; Borau, V.; Jimenez, C.; Marinas, J. M. Reduction of benzylideneacetone and acetophenone over Pd/ AlP04 and Pd/SiO2 catalysta. Appl. Catal. 1985, 17, 223-231. Bagotzky, V. S.; Skundin, A. M. Electrocatalysta on support-11 Comparison of platinum microdepositson inert supporta with other binary systems. Electrochim. Acta 1984,29,951-956. Beck, F. Electrochemical and catalytic hydrogenation: common features and differences. Int. Chem. Eng. 1971, 19, 1-11. Bockris, J. M.; Reddy, A. K. N. Modern electrochemistry; Plenum Press: New York, 1972; Vol. 2, Chapter 10. Bonnier, J. M.; Damon, J. P.; Masson, J. h e y nickel aa a selective catalyst for aldehyde reduction in the presence of ketones. Appl. CQtQl.1987, 30, 181-184. Casadei, M. A.; Pletcher D. The influence of conditions on the electrocatalytic hydrogenation of organicmolecules. Electrochim, Acta 1988, 33, 117-120. Chiba, T.; Okimoto, M.; Nagai, H. Electrocatalytic reduction using raney nickel. Bull. Chem. SOC.Jpn. 1983,56, 719-723. Ehrburger, P.; Mahajan, 0. P.; Walker, P. L. Carbon aa a support for catalysts. I. Effect of surface heterogeneity of carbon on dispersion of platinum. J. &tal. 1976, 43, 61-67. Farina, G.; Sondona, G.; Fornasier, R.; Marcuzzi, F. Electrochemical reduction of carbonyl compounds in aqueous media in presence of ,3 cyclo-dextrine. Reduction of acetophenone and methoxyacetophenone in alkaline solution. Electrochim. Acta 1990,35, 1149-1 155. Honnorat, H.; Martinet, P. Reduction electrochimique de composes organiques en presence de tensio-actifs. I11Etude polarografique de l'acetophenone en milieu aqueux micellaires anionique, cationique et neutre. Electrochim. Acta 1983, 28, 1703-1711.
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