- Email: [email protected]
Hydrogenation of some organic compounds in acidic aqueous solutions on noble metal catalysts
Blectrochhttica Acta, 1970, Vol. IS. pp. 999 to 1012. Pcrmt~on Press. Printed in Northern Ireland
HYDROGENATION OF SOME ORGANIC COMPOUNDS IN ACIDIC AQUEOUS SOLUTIONS ON NOBLE METAL CATALYSTS* Z. TAKEHARAt Max-Planclc-Institut fiir physikalische Chemie, Gbttingen, Bundesrepublik Deutschland Abstract-The mechanism of the hydrogenation of quinone, ally1 alcohol and vinyl acetate in acidic aqueous solutions on Pt and Pd-Ag as catalysts has been investigated. The hydrogenation of quinone proceeds predominantly by an electrochemical mechanism involving consecutive anodic and cathodic reactions with zero net current at the metal/solution interface. In contrast, the hydrogenations of ally1 alcohol and vinyl acetate proceed predominantly by an non-electrochemical mechanism, ie consecutive reactions between adsorbed hydrogen atoms, adsorbed organic compounds, and intermediates take place in the same way as during the catalytic hydrogenation of a gaseous compound. R&mn&-Etude du m&car&me de l’hydrogenation de la quinone, de l’allyl alcool et de l’aa5tate de vinyle en solutions aqueuses acides, avec Pt et Pd-Ag comme catalyseurs. L’hydrogenation de la quinone provient principalement d’un m&canisme electrochimique impliquant con&utivement des reactions anodique et cathodique 21courant nul, a l’interface m&al/solution. Au contraire, les hydrogenations de l’allyl alcool et de l’ac&ate de vinyle d6coulent essentiellement d’un mecanisme non4ectrochimique, c’est & dire de reactions consecutives entre les atomes d’hydrogene adsorb& les composes organiques adsorb& et les intermtiiaires qui se forment de la m6me man&e durant l’hydrogenation catalytique d’un compose gazeux. Zusammenfassung--Der Mechanismus der Wasserstoffanlagerung an Chinon, Allylalkohol und Vinylacetat in saurer wlssriger Li5sung mit Pt und Pd-Ag als Katalysatoren wurde untersucht. Die Anlagerung an Cbinon spielt sich vorwiegend nach einem elektrochemischen Mechanismus mit konsekutiven anodischen und kathodischen Reaktionen ohne Stromfluss an der Phasengmnze Metall/LBsung. Im Gegensatz dazu l&uft die Anlagerung an Allylalkohol und Vinylacetat vorwiegend nach einem rein chemischen Mechanismus ab, d.h. konsekutive Reaktionen zwischen adsorbierten Wasserstoffatomen, adsorbiertem organ&hem Stoff und Zwischenprodukten finden auf die gleiche Weise statt wie bci der katalytischen Wasserstoffanlagerung in der Gasphase. INTRODUCTION
PRFNIOUSauthors1-3 have discussed whether the hydrogenation of an organic compound A on a noble metal as catalyst in an aqueous solution occurs by anodic ionization of hydrogen and consecutive cathodic reduction of A and/or by a sequence of a non-electrochemical reaction involving adsorbed hydrogen atoms and adsorbed A. However, definite conclusions have not yet been reached. For clarification, Wagnera has suggested experimental methods which are especially promising in order to decide whether an electrochemical or a non-electrochemical mechanism prevails In this paper, investigations on the hydrogenation of for a particular compound. quinone, ally1 alcohol and vinyl acetate are reported. SYMBOLS
We shall use E, electrode potential DSthe standard hydrogen electrode (she), E*, electrode potential of the back-side of a foil bi-electrode USthe she, &t, Est*, values of E and E* under steady-state conditions of catalytic hydrogenation without polarizing current, * Manuscript received 10 April 1969. t Present address: Department of Industrial Chemistry, Faculty of Engineering, Kyoto University,
Kyoto,
Japan.
999
2. TAKEHARA
loo0
E eq’9 equilibrium E”eq 9 equilibrium
potential for the electrode reaction Hz $G 2 H+ + 2 e-, potential for the electrode reaction AH, ---t A + 2 II+ + 2 e-,
r”, E - Eeq”, S, area of electrode used as catalyst, i, I/S, cd of polarization in the anodic direction, i’, partial cd for the process H, -+ 2 H+ + 2 e-, i”, partial cd for the process AH, -+ A + 2 H+ + 2 e-, i,(j), exchange cd of reaction j, axj, charge-transfer coefficient in the anodic direction of reaction j , a CI apparent charge-transfer coefficient of the cathodic reduction of quinone, +!UXp~rate of formation of AH, in mol per unit time at surface area S, rate of hydrogen consumption, -fin*, st, subscript to indicate steady-state conditions without polarizing current (i = 0),
(I), (II),
subscripts to indicate the non-electrochemical chemical mechanism, F, Faraday constant, R, universal gas constant, T, absolute temperature. I. HYDROGENATION
OF
mechanism
and the electro-
QUINONE
Experimental technique
Anodic and cathodic polarization characteristics and the rate of hydrogen consumption in acidic solutions of the quinone-hydroquinone system on a rotating disk of smooth Pt as catalyst at 25°C were measured with the apparatus shown in Fig. 1. During the experiments, the pressure of hydrogen or argon above the solution was kept at 1 atm after hydrogen or argon had been bubbled through the solution. The rotating disk electrode (apparent surface area 28.3 cm2, thickness O-5 mm) was pretreated as follows. The electrode was immersed in 1 N KOH at 5O”C, and then in a solution of K,CrsO, in concentrated HzSOp at 50°C, and subsequently the electrode was subjected to anodic and cathodic polarization in O-1 N H,SO, at 25°C. The counter- and the reference electrode consisted also of Pt and were in the same solution, but free of organic compounds (see Fig. 1). The solutions were prepared from quinone, hydroquinone and acidic aqueous solutions of reagent grade. Hydrogen and argon were purified by passing through alkaline pyrogallol solution. Steady-state currents were measured at potentials provided by a Wenking potentiostat in order to obtain potential/cd curves. The potentials of a Pt electrode in acidic solutions containing quinone and hydroquinone without and with hydrogen (Ees” - E,,’ and E,, - E,,‘) were measured us an electrode in the acidic solutions free of quinone and hydroquinone under a hydrogen pressure of 1 atm. The rates of hydrogen consumption at the potentials of & and Eeq” were also measured : where the rates were small, the hydrogen-volume change above the solution was measured by means of the horizontal capillary with silicone oil shown in Fig. 1, and where the rates were large, the hydrogen-pressure change above the solution was measured with a water manometer. In the latter case, both the disk electrode and the counter-electrode were immersed in the same cylindrical vessel and the volume of hydrogen evolved at the counterelectrode was subtracted.
Hydrogenationof organiccompounds in acidic aqueous solutionson noble metals
1001
II
Motor
Silicone oi I H20
Capillary /
I
‘d
FIG. 1. Apparatusfor naeasuring hydrogenationrates and polarizationcharacteristics. a, Pt disk electrode; b, Pt counter-electrode; c, Pt-H,-H+ referenceelectrode; d, apparatusfor measuringhydrogenconsumption.
Results Concentrations of quinone and hydroquinone were varied in the range O-02 M to 0401 M in 0.1 N H,S04. The potentials of a Pt electrode in these solutions under argon were measured zw an electrode in O-1 N H&O, under a hydrogen pressure&of 1 atm. These potentials satisfied the Nernst equation, E eqIt - Eeq' = O-699 + O-030 log
[Quinone] V at 25°C. [Hydroquinone]
(1)
Figure 2 shows anodic and cathodic polarization curves in a solution O-1 N H,SO, + O-015 M quinone + 0.015 M hydroquinone under a hydrogen pressure of 1 atm, the cathodic polarization curve in the same solution under argon, and the anodic polarization curve in O-1 N H,SO, free from quinone and hydroquinone under a hydrogen pressure of 1 atm on a Pt disk rotated at 500 rpm. The values of Ees" - E,,,E,,- Eeq',(U/di)E_,t, and the rates of hydrogen consumption which were measured at various rotational speeds on a Pt disk at open circuit (E = Est)and at the potential J?Z,~ * are listed in Table 1. These values in various 9
2. TAKEHARA
1002
I
0
2 6 -_
-I
s”
-2
-3
Fro. 2. Anodic and cathodic polarization curves for the quinonehydroquinone system in a 0.1 N H,SO, solution on a rotatingdisk &&rode (500 rpm) at 25°C. a, anodic pohuizationcurvein a solution01 N HISO* + 0.015 M quinone + 0.015 M hydroquinoneandpnI = 1 atm; b, cathodic polarizationcurve in the same solution as that of a; c, anodtc polarizationcurve in 8 61 N HISO, solution at put = 1 atm; d, cathodicpolarizationcurvein a solution01 N HtSOl + O*OlS M q&one + 0,015 M hydroquinoneunder argon; e, anodic polarization curve for the ionization of hydrogen, determinedfrom curvesa, b and d. TABIZ 1. VALUW OFEep”- Gt , Et - &a’, (dJ$d&-~~,, ANDTHERATesOF HYDROGEN CONSUMPTION, MEASURED AT VARIOUS ROTATIONAL SPEEDS ON A Pt DISK AT OPEN CIRCUIT
Rotational speed of Pt disk revfmin
11:* 150 300 500 700 1E
IL,,” - Eea’= O-699V
~~~~ _ ~~~ V
Est - Eea’ V
(dEldi)B-~8t V cmg/A
0.018 0.059
0681 0.640 0.635 0.624 0613 0.605
240 150 133 99 74 65
::E 0086 0.094 0.098 0.106
O-601
0.593
Z
Rate of hydrogenconsumption mol/cm’/s at open circuit 037 1.65 190 2.75 3.30 3.80 467 5.11
x x x x x x x x
10-O 10-O 10-O lo-@ 10-o 10-B 10-O 10-n
at E = &a* 160 x lo-’ 2.36 x lo-’ 3.19 x 10-s -
* Convection in the solution was provided by a stirrerat the bottom of the vessel rotated by magneticcoupling.
Hydrogenation of organic compounds in acidic aqueous solutions on noble metals TABLE2. SUMPTION
VALUES OF I&” - EBt, E,,t AT OPEN CIRCUIT, MSASURED
0.015 M HYDR~QWINONE
0.1 N HCl 0.1 N HpSOl + 0.1 N Kl3r
ACXDIC
0699 V
V
01 N H,SO
IN VARIOUS
ON A ROTATED Pt DISK (500 IXW/min)
E eIl#-
Solution
- Eep’, (dE/di)+,,,
Gt
0.086 0118 0.102
E.t - EM’ V
(Wdih-~,,
AND TEIERATESOF HY’DRO~XN CXC+ SOLUTIONS at&
V cma/A
0.613 0.581 0.597
1003
=
WITW
0.015 M
QUINONE
AND
1 atm AT 25°C. Eeq” - &’
=
Rate of hydrogenconsumption (mol/cm9 . s)
1:: 257
3.30 x 10-g 2.15 x 1O-m 1.05 x 10-s
acidic aqueous solutions at the rotational speed of 500 rev/min are compared in Table 2. With increasing the rotational speeds of the disk, the values of E,, - Eeq'change in the direction of negative potential and the rate of hydrogen consumption at open circuit, ie the hydrogenation rate of quinone; increases, and that of hydrogen consumption at the potential Eecl” also increases. The hydrogenation rate is affected by the nature of the anion. The rate is decreased by the addition of Cl- and even more by the addition of Br. In the case of cathodic polarization in acidic solutions containing quinone and hydroquinone under argon, there were observed limiting diffusion currents (depending on the rotational speed) at potentials considerably more negative than Eeb (see Fig. 2). Charge-transfer coefficients of the cathodic reaction obtained from the Tafel plot, ao, are nearly equal to 0.54, in accord with Vetter. 6 The exchange cds depend on the nature of the anions (l-07 x 10-l mA/cm2 in 0-f N H,SOd, 4.8 x 10F2 mA/cm2 in O-1 N HCl, and 1.9 x 10s2 mA/cma in a solution containing 0.1 N H,SO, + O-1 N KBr)
,
Discussion Investigations on cathodic reduction of quinone to hydroquinone by Vetteld have shown that at low pH (pH < 5) the following consecutive processes occur:. A (aq) -
A (ad),
A (ad) + H+ (aq) - AH+ (ad), AH+ (ad) + e- -+ AH (ad),
(3) (4)
AH (ad) + H+ (aq) - AH,+ (ad), AH,+ (ad) + e- -+ AH, (ad),
(5) (6) (7)
AH2 (ad) -
AH2 (a@,
where A is quinone and AH, is hydroquinone. According to Vetter6 the dependence of the cd i” on the overpotential charge-transfer
(2)
q” for
control is represented by (8)
I
.I#
=
In the case of cathodic reduction of quinone, the reaction is considered to be chargetransfer-controlled at E SWI& according to the experimental results.
2. TAKEHARA
1004
Anodic oxidation of hydrogen in acidic aqueous solutions is considered to proceed as follows, H, (aq) -
H, (ad),
(9)
H, (ad) + 2 H (ad),
(10)
2 H (ad) + 2 H+ (aq) + 2 e-.
(11)
If Iq”F/RTI < 1, ie, Eeqn - Est < RT/F and the change in the rate of anodic ionization of hydrogen with potential is small in comparison to that of cathodic reduction of quinone, the rate of formation of hydroquinone in mol per unit time per unit surface area due to an electrochemical mechanism involving these consecutive cathodic and anodic reactions with zero net current at the metal/solution interface is represented by the formula deduced by Wagner,*
If E,/
-
Est >
RT/F,
(8) for the cathodic current i” becomes -
exp -(l
.” c=a -2i,(4)
1
=
exp
-2i,(4)
Hence, fiAnS(II)/S at a non-polarized
CQ)$:
1
(1
-E)
1 .
(13)
platinum foil (E = IT,,) is
(14) or in view of (13) fiAEp(ll>
s
&(4) w 7 . exp
(1 -
a$ &
(EBq” -
Ed
1.
(15)
Assuming the same dependence of i” on E in solutions without and with hydrogen, one may use values of i” , i,,, and a4 observed in solutions free of hydrogen in order to calculate fiAn,(II)/s from (14) or (15). The total cd is i = i’ + i”. If the change in the rate of anodic ionization of hydrogen is relatively small in comparison to that of cathodic reduction of quinone, between EBt and EeqN, one has
Taking the logarithm of both sides of (13) and differentiating with respect to E, one obtains in view of (16) =
(--i*JE,Bat
(1
(17)
Hydrogenation
Substitution
1005
of organic compounds in acidic aqueous solutions on noble metals
of (17) in (14) yields fi&(II) s
=
RT 2(1 - oc,)F
1 ,
dE
(18)
( di >B-E&=0 where, in the case of the quinone-hydroquinone system in acidic aqueous solutions, (1 - CQ) = 0.54. The rate of hydrogen consumption at open circuit (E = &) does not differ greatly from that at the potential Eeqn (see cohmns 5 and 6 in Table 1). Therefore, if the electrochemical mechanism prevails the variation of the rate of anodic ionization of hydrogen is relatively small in comparison to that of the cathodic reduction of quinone in the potential range between & and Ees”_ At the rotational speed of 0 rev/min (Table l), the potential difference, Eeq” - Est, is relatively small than RT/F. At speeds higher than 110 rpm (Tables 1 and 2), the differences are much greater than RT/F. Thus, in the former case, fiaHs(II)/S may be calculated from (12) and in the latter case from (15) or (18). ti~n~(II)/S calculated from (12), (15) and (18) is given in TABLE AT
3.
VALUES
VARIOUS
OF THE
ROTATIONAL
RATE
+
Rotational speed of Pt disk rev/min
OF HYDROOENATION
SPEEDS
OF QUINONE,
ON A pt DISK IN A SOLUTION M HYDROQUINONE AT pB, =
0415
1
Rate of hydrogenation
Eea”- E,t V
Observed value at open circuit
0.018 0.059 OX%4 0.075 0.086 0,094 0.098 0.106
ot
110 150 z 700 900 1100
0.37 1.65 1.90 2.75 3-30 3.80 4.67 5-11
x x x x x x x x
Calculated value from (12)
10-e 1O-8 10-Q 1O-9 10-O 1O-9 1O-s 10-D
DETERMINED O-1 N HtSOl atm AT 25°C
1-93 2.08 2.70 3.41 4.03 4.39 5.18
METHODS QUTNONE
0.015 M
of quinone (mol/cm* . s)
Calculated value from (15)
0.39 x 10-n -
BY VARIOUS
+
x x x x x x x
10-s 10-O lb* 10-g 10-D 10-e 10-g
Calculated value from (18) 1.64 1.85 2.49 3.33 3.79 4.57 5.45
x x x x x x x
1O-g 10-O 1O-s 10-O 10-O lo-” 10-e
From* i’ at E = Eat 0.47 2.66 3.11 4-25 5.28 6.95 8-35 9.55
x x x x x x x x
IO-’ lo-* lo-# lo-’ lo-@ 1O-s lo-“ 10-a
* Determined from the partial anodic cd of hydrogen in a 0.1 N H,SO, solution at pEa = 1 atm (eg, from curve c in Fig. 2). t Convection in the solution was provided by a stirrer at the bottom of the vessel rotated by magnetic coupling. TABLE 4. VALUES OF VARIOUS
ACIDIC
THE RATE OF HYDROGENATION SOLUTIONS WITH 0.015 M QUINONE
DISK (500 rpm)
OF QUINONE, AND
AT pHI
=
0.015
DBTERhfINED BY VARIOUS MElT-IODS IN HYDROQUINONE ON A ROTATED Pt
bf
1 atm AT 25°C
Rate of hydrogen consumption Solution
O-1 N HsSOI 0.1 N HCI o-1 N HISO, +O*l N KBr
Eeq” - E.t V
(mol/cma/s)
Observed value at open circuit
Calculated value from (12)
Calculated value from (15)
Calculated value from (18)
0.086 0.118
3.30 x 10-O 2-15 x 10-O
-
3.41 x lo-* 3.00 x 10-s
3.33 x lo-’ 5.28 x 10-O 2.12 x 10-O 457 x 10-s
0.102
1.05 x
-
0.80
10-o
l Determined from the partial anodic (eg, from curve c in Fig. 2).
x
10-O
0.96
x
10-O
From* i at E = Eat
3.11 x 1O-s
cd of hydrogen in a O-1 N HISOl solution atpHa = 1 atm
2.
1006
TAKEHARA
columns 4,5 and 6 in Tables 3 and 4. In the comparison of columns 3 and 4 or 3 and 6, the values in columns 3 and 4 are nearly equal at the rotational speed of 0 rpm and the values in columns 3 and 6 for each rotational speed are nearly equal at speeds higher than 110 revlmin. It must be noticed, however, that these values are not exactly equal. This may be due to the fact that change in the rate of anodic ionization of hydrogen in the potential range between & and Eeq” is not completely nil. Since the contributions to the rate of hydrogenation of quinone, fiAH,/SIIr calculated from (12), (15) and (18), due to the electrochemical mechanism (II), agree approximately with the experimental values of the over-all rate, it is concluded that the eIectrochemical mechanism prevails. In (12) only electrochemical data obtained for a quinone-hydroquinone solution with hydrogen are used. In (15), dE/di is taken from measurements in a solution with hydrogen but the value (1 - a4) is taken from measurements in a solution without hydrogen. In contrast, for the evaluation of (15), only electrochemical data for a quinone-hydroquinone solution without hydrogen are used. Therefore, use of (15) assumes that the cd/potential curve for the cathodic reduction of quinone, and especiahy the exchange cd is not affected by the presence of hydrogen. Seemingly, this assumption is justified, since rates calculated from (15) and (18) agree within the limits of experimental errors. In addition, one may calculate hydrogenation rates by letting jlAHp/(S>,, equal to i’/2F at E = E,, where i’ is the anodic cd for the reaction H, (aq) = 2 H+ + 2 ein a HaSO*-H, solution free from quinone and hydroquinone (column 7 in Table 3 and 4). This calculation assumes that the anodic oxidation of hydrogen is not affected by the presence of the organic substances. This assumption, however, is not justified, since values listed in column 7 of Tables 3 and 4 are substantially greater than the experimental rates listed in column 3. Seemingly, the presence of the organic substances impedes the anodic oxidation of hydrogen, probably because of preferential adsorption of the organic substances. It is noteworthy that under the conditions of this research the anodic oxidation of hydrogen is not exclusively controlled by diffusion in spite of the pronounced dependence on the rotational speed of the platinum disk. Table 5 shows that the presence of Cl- and Br ions decreases the rate of hydrogenation of quinone. This indicates preferential adsorption of Cl- and Br- anions on platinum. The foregoing observations show that the electrochemical mechanism prevails. For a further check, the rate of formation of hydroquinone has been determined TABLE
5. VALUES
WHILE
E =
QUINONE
OF T’HE RATE OF HYDROGEN
CONSUMPTION
AND
THJ! EQUIVALENT
E-Q” WAS ENFORCED IN A SOLUTION 0.1. N HzSOc + 0.015 M QUINONE AND VALUES OF THE RATE OF HYDROGENATION AT P=P = 1 atm AT 25T, CALCULATED FROM (19)
speed of Pt disk rev/min
OF THE TOTAL +
0.015 M
Rotational
150 300 500
CD
HYDROOF OUINONE CAL-
mol/cmp Js
mol/cm”/s
mol/cmg/s
1.60 x 1O-s 2.36 x IO-* 3.19 x 1o-s
1.56 x 10-O 2.29 x 10-a 3.21 x 10-O
0.04 x 10-m 0.07 x 10-O -002 x 10-e
Hydrogenation of organiccompounds in acidicaqueoussolutionson noblemetals
1007
at an enforced potent&l E = Eeqn where electrochemical reduction of quinone is
suppressed. The rate of formation of hydroquinone is generally equal to the difference of hydrogen consumption and the current equivalent for anodic oxidation,
i (19) s S 2F’ The terms on the right hand side of (19) for E = I&” as determined experimentally are nearly equal (Table 5, columns 2 and 3). Accordingly, (fiA&/S) at E = _Eecl” are virtually zero (Table 5, column 4). Thus most of the hydrogen approaching the anode is oxidized anodically and virtually no hydrogen reacts with quinone according to the non-electrochemical mechanism.
‘--a _ --‘Ha
II.
HYDROGENATION
OF
ALLYL ALCOHOL
AND VINYL ACETATE
Preliminary tests The hydrogenation of ally1 alcohol and vinyl acetate, which have a carboncarbon double bond and are water-soluble, has also been investigated, CH,: CHCH,OH + H, + CH,CH,CH,OH, CH,: CHCOOCH, + H2 -
(20)
CH,CH,COOCH,.
(21)
Potentials in a O-1 M ally1 alcohol + O-1 M propyl alcohol solution and a O-1 M vinyl acetate + O-1 M ethyl acetate solution with 0.1 N H,S04, under argon on Pt, were not stable and changed with time. The electrochemical equilibrium of these systems is not readily established. The values of these systems under a hydrogen pressure of 1 atm on a Pt disk rotated at 500 rev/min at 25°C were measured with the apparatus shown in Fig. 1. Steady-state potentials were found to be comparatively stable (I& - Eecl’= O-053V for the ally1 alcohol and propyl alcohol system and O-065V for the vinyl acetate and ethyl acetate system). Upon using estimated values for Eeqfl, the following relation holds EBt - I&’ < I&” - Es,. Cw Under these conditions, it is necessary to measure the potentials B and E* without current and with current in the double cell suggested by Wagner,* reference electrode
Compartment (A) [H+ bq), A (aq) AH, (aq), H, @qN
(c)
electrode as cataIyst (E) Pt or Pd-Ag(+H)
Compartment (B) [H+ (aq)l
US)
reference electrode @) (ED)
t--E--J?zG-*
Experimental
f-E*
-
ED-
technique
The apparatus shown in Fig. 1 was used with the rotating disk bi-electrode shown in Fig. 3 in place of the working electrode in Fig. 1. A thin Pt foil (apparent surface area 21.2 cm2, thickness 0.01 mm) or a thin Pd-Ag foil (apparent surface area 21.2 cm2, thickness O-03 mm, atomic ratio of Pd: Ag 77 :23) was used as the bi-electrode. The measurements with Pt foil were carried out at 50°C and those with Pd-Ag foil at 25°C. The solution in compartment A was 0.1 N &SO* + O-1 M ally1 alcohol + O-1 M propyl alcohol or O-1 N H2SOp + O-1 M vinyl acetate + 0.1 M ethyl acetate. The solution at the back of the foil in compartment B was a HCl solution with the same hydrogen-ion activity as that in 0.1 N HPISOd(0.052 N HCl at 50°C and
Z. TAICEIWU
1008
bad
Plastic
L l?IG.
3.
Pt or Pd-Ag foil disk electrode
Rotating disk K-electrode.
0.064 N-HCl at 25°C). Reference electrode C was a Pt, H, electrode in O-1 N H,SO,; reference electrode D was a Ag/AgCl electrode. The activity of hydrogen atoms is assumed to be the same on both sides of the foil. At 25°C this condition may be readily established with a Pd-Ag foil, but not with a Pt foil. According to Gileadi et ~1,~ the diffusion rate of hydrogen atoms in Pt foil is sufficiently high at 5O”C, so 50°C was used for Pt. Use of a Pd-Ag alloy with an atomic ratio 77 : 23 has the advantage of a rather high hydrogen solubility within the range of the a phase without occurrence of the /I phase, which is found in the binary system Pd-H under a hydrogen pressure of 1 atme Thus discontinuous changes in volume upon dissolution of hydrogen and also hysteresis phenomena are avoided.
Hydrogenation of organic compounds in acidic aqueous solutions on noble metals
1009
The Pt foil was purified by the same method as for the experiments with quindne. In the case of the Pd-Ag foil immersion in acidic perchromate solution was omitted. Solutions of H,SO, and HCl were purified by pre-electrolysis and then saturated with hydrogen or argon. H,SC4 solution was introduced in compartment A and then after the equilibrium state of the electrode under a hydrogen pressure of 1 atm was reached, chemical grade organic compounds were added. Hydrogen and argon were purified by the same method as for the experiments with quinone. If the solution in compartment A was 0.1 N HsSO, saturated with hydrogen without organic compounds, potentials observed after one day at both sides of the Pt and the Pd-Ag rotating disk foil electrodes were equal to the equilibrium potential of a hydrogen electrode, Ees’. At such electrodes, equilibrium is readily established when the partial pressure of hydrogen is changed. Furthermore, upon changing polarization of the front side of the foil electrode, steady-state potentials were reached within 15-20 min. Further, the front side of the bi-electrode was polarized by applying a pre-determined pd tls the reference electrode C. The resulting current between the b&electrode and the counter-electrode and the resulting potential of the back side against the reference electrode D were measured. The corresponding pds E - Eeq’ and E* Eeq’ for a rotational speed of 250 rpm are shown in Fig. 4. Values of the back-side potential were obtained only for relatively small polarizing currents since at higher currents steady values of E* were not reached within 30 min. In addition, cds and rates of hydrogen consumption as a function of the front-side electrode potential E - E,,’ were measured on a Pt electrode shown in Fig. 1 and are shown in Fig. 5.
-0.41
0
I 0.02
E-E;
I
I
OG4
O-96
01
E*-Ee;,
I
I oa
V
0
0
I oa2
E-E&or
I
I
I 0.04
O-08
0.06
&"-E,',
, V
FIG,4. Anodic and cathodic polarization curves for the allyl-alcohol-propyl-alcohol and the vinyl-acetate-ethyl-acetate system in O-1 N HISO solution on a rotating disk bi-electrode (250 rev/mm) at 50°C. (A) O-1 .M ally1 alcohol + 0.1 M prop 1 alcohol (B) O-1 M vinyl acetate + O-1 M ethy r acetate a, cd as a function of potential of the front side (i/E curve); b, cd as a function of potential of the back side (i/E* curve).
0
Z. TAKEIURA
05
0.05
0
E-E:,,
0.
E-EA,
v
005 V
040
FIG. 5. Current density corresponding to the rate of hydrogen consumption as a function of potential and polarization curves in a O-1 N HISO4 solution with and without organic compounds at PHI - 1 atm on a rotating Pt disk electrode (500 rpm) at 25’C. (A) @1 M ally1 alcohol + O-1 M propyl alcohol (B) O-1 M vinyl acetate + O-1 M ethyl acetate a, polarization curve in a O-1 N E&SO, solution with organic compounds at pa, = 1 atm; b, polarization curve in a O-1 N H&O1 solution at PHI = 1 atm; c, cd corresponding to the rate of hydrogen consumption; d,cd corresponding to therateof hydrogenation, determined from curves a and c.
Numerical values for further evaluation are listed in Tables 6 and 7. Discussion
The general features of the dependence of the front side potential E and the back side potential E* on the cd applied for polarization have been discussed by Wagner.* In the present research, the following experimental results are noteworthy. (1) The steady-state potentials E,, and EBt* do not differ greatly. Values of ES, ES,* are listed in column 7 of Table 6. (2) Relatively low applied cds are required in order to enforce a state where E = E* (sde Fig. 4). Accordingly, the shift of the front side potential for reaching E = E* is relatively small. Thus one has the conditions considered in Wagner’s Fig. 2a.4 If the state E = E* is enforced, the driving force for ionization of hydrogen vanishes and, therefore, the observed rate of hydrogen consumption is exclusively due to hydrogenation of the organic compound by the non-electrochemical mechanism, Hs (aq) - Hs (ad), H, (ad) + 2 H (ad), A (aq) A (ad) + H (ad) AH (ad) + H (ad) AH, (ad) -
(23) (24)
A (ad), AH (ad),
(25)
AH, (ad),
(27)
AH, (a&
(28)
(26)
where A is ally1 alcohol or vinyl acetate and AH, is propyl alcohol or ethyl acetate.
i
i
0*062 0.111 0,075 0.128
50 25 50 25
Pd-Ag
Pt
Pd-Ag
V
Est - Ees’
Pt
Electrode
Temperature “C
0.110
0.063
0*098
0.058
V
Eat* - EM,’
0.096
0.050
0.080
DO54
(E - Ee,3,,1
0.018
0.012
0.013
0004
V
Eet - Emt*
0.014
0.013
0.018
O%kl
V
Ent* - (EL*
HaSOdCONTAININGORGANIC
Pd-Ag
Pt
Pd-Ag
0.1 M propyl alcohol
0.1 M vinyl acetate +
0.1 M ethyl acetate
Pt
0.1 M ally1 alcohol +
I
Electrode
organic compound
25
50
25
50
Temperature “C
1.5 x 10-10
7.11 x 10-a
1*7 x 10-10
5.86 x 10-s
(- kf*hE* s mol/cma/s
1.4 x 10-10
685 x 1O-e
2.0 x 10-10
5~86x 10-O
-Qr,cE4u S mol/cm*/s
1.4 x 10-10
6.85 x 10-D
l-2 x 10-10
6.64 x 10-O
(-~&*keq’ S mol/cm*/s
6.2 x 10-l’
3.3 x lo-‘0
4.2 x lo-la
1.7 x 10-10
( - il2%-~~ mol/cma/s
4.2 x 10-l’
1.46 x 1O-s
3.3 x 10-11
1.53 x 10-o
(-i/2F)B_lep~ mol/cm”/s
RATESOFHYDROGEN CXINXJMPTIONATOPENCIRCUIT(E= ht),E=E* AND E= E eq’, ANDTHE EQUIVALEF OF THE CD (-i/2F) AT E = E* AND E= Eeq’ IN A SOLUTION O.lNHBSOz CONTAINING ORGANIC COMPOUNDS ON A ROTATED FOIL RI-ELECTRODE(250rev/min) AT~H%= 1 atm
TABLE 7.
0.1 M ally1 alcohol + O-1M propyl alcohol 0.1 M vinyl acetate + 0.1 M ethyl acetate
Organic compounds
A SOLUTIONOF0'1N coMPouNDs ON A ROTATED FOIL BI-ELECTRODE(25Orev/min) AT pa, = latm
TABLE6. VALUES OF Eat - Eeq’, Eet* - Eeq’, (E - Eep’)E_E*, .?$t- Eat* AND &t* - (I?&_~*IN
e
1 & 0 B
F nE. B 8
8 sv)
$
B 6 8 g
i w e
Q.
B Z 0 u s
&i
J
1012
z. TAKEHARA
According to columns 4 and 5 of Table 7, the rates of hydrogen consumption at open-circuit conditions (E = E,J do not differ greatly from rates observed when E = E* was enforced, ie, when the electrochemical mechanism was suppressed. Thus again, the non-electrochemical mechanism prevails for the hydrogenation of ally1 alcohol and vinyl acetate. Values of the rate of hydrogen consumption at an enforced potential E = Ees' listed in column 6 of Table 7 are about equal to those for open-circuit conditions in column 5. This also supports the conclusion of prevalence of the non-electrochemical mechanism. The deviation of values in columns 5 and 6 for the hydrogenation of ally1 alcohol on the Pd-Ag foil cannot be explained. Hydrogen consumption at E = E* on the Pt foil is much greater than that on the Pd-Ag foil (column 4, Table 7). Thus the catalytic activity of a Pt foil for hydrogenation by the non-electrochemical mechanism is much greater than that of a Pd-Ag foil. Since E,, - Eet*is greater than zero (column 7, Table 6), the ionization of hydrogen by (11) proceeds at finite rate and the hydrogenation due to the electrochemical mechanism also proceeds at open circuit, (E = Esb),as pointed out by Sokol’skaya and Sokol’skii,” although only as a side reaction. The difference between the cd corresponding to the rate of hydrogen consumption per unit area and observed cd at the potential of E is considered to be the cd corresponding to the hydrogenation rate of an organic compound at this potential. These rates at various potentials are shown by dotted line in Fig. 5. These rates increase upon changing the potential in the negative direction. This may be due to a higher activity of hydrogen atoms at more negative potentials.4 CONCLUDING
REMARKS
The hydrogenation of an organic compound on a noble metal in acidic aqueous solutions is considered to proceed according to various reaction mechanisms. In the case of the quinone and hydroquinone system where an electrochemical equilibrium is readily established, the hydrogenation proceeds predominantly by the electroIn the case of the allyl-alcohol-propyl-alcohol system and the chemical mechanism. vinyl-acetate-ethyl-acetate system, where an electrochemical equilibrium is not readily established because the reaction rates are sluggish, the hydrogenation proceeds predominantly by the non-electrochemical mechanism. Further measurements of the rate of hydrogenation at a non-polarized and a polarized electrode as a function of the concentrations of reactants, products and hydrogen ions are needed in order to decide the rate-determining step for each mechanism. Acknowledgement-The author is indebted to Professor Dr. C. Wagner of the Max-Planck-Institut fiir physikahsche Chemie for many helpful suggestions and discussions. He gratefully acknowledges receiving a scholarship from the Max-Planck-Institut ftir physikalische Chemie, Giittingen, Germany during his stay as a research fellow in 1966-1967. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
F. BECK and H. GERISCHER, 2. Elektrochem. 65,504 (1961). F. BECK, Z. Elektrochem. 69, 199 (1965). A. M. SOKOL'SICAYA and D. V. SOKOL'SKII,Kinetika i Kafaliz 6, 658 (1965). C. WAGNER, Electrochem. Acta, 15, 987 (1970). K. J. VETI-ER,Z. Eiektrochem. 56,797 (1952). K. J. VETTER,Z. Naturf. 7a, 32X (1952); Sa, 823 (1953). E. GLLEADI,M. A. FULLENWIDER and J. O’M. BOCKRIS,J. electrochem. Sot. 113, 926 (1966). F. A. Lswrs, Z%e PaNadium-Hydrogen System, pp. 75, 144. Academic Press, London (1967).
HYDROGENATION OF SOME ORGANIC COMPOUNDS IN ACIDIC AQUEOUS SOLUTIONS ON NOBLE METAL CATALYSTS* Z. TAKEHARAt Max-Planclc-Institut fiir physikalische Chemie, Gbttingen, Bundesrepublik Deutschland Abstract-The mechanism of the hydrogenation of quinone, ally1 alcohol and vinyl acetate in acidic aqueous solutions on Pt and Pd-Ag as catalysts has been investigated. The hydrogenation of quinone proceeds predominantly by an electrochemical mechanism involving consecutive anodic and cathodic reactions with zero net current at the metal/solution interface. In contrast, the hydrogenations of ally1 alcohol and vinyl acetate proceed predominantly by an non-electrochemical mechanism, ie consecutive reactions between adsorbed hydrogen atoms, adsorbed organic compounds, and intermediates take place in the same way as during the catalytic hydrogenation of a gaseous compound. R&mn&-Etude du m&car&me de l’hydrogenation de la quinone, de l’allyl alcool et de l’aa5tate de vinyle en solutions aqueuses acides, avec Pt et Pd-Ag comme catalyseurs. L’hydrogenation de la quinone provient principalement d’un m&canisme electrochimique impliquant con&utivement des reactions anodique et cathodique 21courant nul, a l’interface m&al/solution. Au contraire, les hydrogenations de l’allyl alcool et de l’ac&ate de vinyle d6coulent essentiellement d’un mecanisme non4ectrochimique, c’est & dire de reactions consecutives entre les atomes d’hydrogene adsorb& les composes organiques adsorb& et les intermtiiaires qui se forment de la m6me man&e durant l’hydrogenation catalytique d’un compose gazeux. Zusammenfassung--Der Mechanismus der Wasserstoffanlagerung an Chinon, Allylalkohol und Vinylacetat in saurer wlssriger Li5sung mit Pt und Pd-Ag als Katalysatoren wurde untersucht. Die Anlagerung an Cbinon spielt sich vorwiegend nach einem elektrochemischen Mechanismus mit konsekutiven anodischen und kathodischen Reaktionen ohne Stromfluss an der Phasengmnze Metall/LBsung. Im Gegensatz dazu l&uft die Anlagerung an Allylalkohol und Vinylacetat vorwiegend nach einem rein chemischen Mechanismus ab, d.h. konsekutive Reaktionen zwischen adsorbierten Wasserstoffatomen, adsorbiertem organ&hem Stoff und Zwischenprodukten finden auf die gleiche Weise statt wie bci der katalytischen Wasserstoffanlagerung in der Gasphase. INTRODUCTION
PRFNIOUSauthors1-3 have discussed whether the hydrogenation of an organic compound A on a noble metal as catalyst in an aqueous solution occurs by anodic ionization of hydrogen and consecutive cathodic reduction of A and/or by a sequence of a non-electrochemical reaction involving adsorbed hydrogen atoms and adsorbed A. However, definite conclusions have not yet been reached. For clarification, Wagnera has suggested experimental methods which are especially promising in order to decide whether an electrochemical or a non-electrochemical mechanism prevails In this paper, investigations on the hydrogenation of for a particular compound. quinone, ally1 alcohol and vinyl acetate are reported. SYMBOLS
We shall use E, electrode potential DSthe standard hydrogen electrode (she), E*, electrode potential of the back-side of a foil bi-electrode USthe she, &t, Est*, values of E and E* under steady-state conditions of catalytic hydrogenation without polarizing current, * Manuscript received 10 April 1969. t Present address: Department of Industrial Chemistry, Faculty of Engineering, Kyoto University,
Kyoto,
Japan.
999
2. TAKEHARA
loo0
E eq’9 equilibrium E”eq 9 equilibrium
potential for the electrode reaction Hz $G 2 H+ + 2 e-, potential for the electrode reaction AH, ---t A + 2 II+ + 2 e-,
r”, E - Eeq”, S, area of electrode used as catalyst, i, I/S, cd of polarization in the anodic direction, i’, partial cd for the process H, -+ 2 H+ + 2 e-, i”, partial cd for the process AH, -+ A + 2 H+ + 2 e-, i,(j), exchange cd of reaction j, axj, charge-transfer coefficient in the anodic direction of reaction j , a CI apparent charge-transfer coefficient of the cathodic reduction of quinone, +!UXp~rate of formation of AH, in mol per unit time at surface area S, rate of hydrogen consumption, -fin*, st, subscript to indicate steady-state conditions without polarizing current (i = 0),
(I), (II),
subscripts to indicate the non-electrochemical chemical mechanism, F, Faraday constant, R, universal gas constant, T, absolute temperature. I. HYDROGENATION
OF
mechanism
and the electro-
QUINONE
Experimental technique
Anodic and cathodic polarization characteristics and the rate of hydrogen consumption in acidic solutions of the quinone-hydroquinone system on a rotating disk of smooth Pt as catalyst at 25°C were measured with the apparatus shown in Fig. 1. During the experiments, the pressure of hydrogen or argon above the solution was kept at 1 atm after hydrogen or argon had been bubbled through the solution. The rotating disk electrode (apparent surface area 28.3 cm2, thickness O-5 mm) was pretreated as follows. The electrode was immersed in 1 N KOH at 5O”C, and then in a solution of K,CrsO, in concentrated HzSOp at 50°C, and subsequently the electrode was subjected to anodic and cathodic polarization in O-1 N H,SO, at 25°C. The counter- and the reference electrode consisted also of Pt and were in the same solution, but free of organic compounds (see Fig. 1). The solutions were prepared from quinone, hydroquinone and acidic aqueous solutions of reagent grade. Hydrogen and argon were purified by passing through alkaline pyrogallol solution. Steady-state currents were measured at potentials provided by a Wenking potentiostat in order to obtain potential/cd curves. The potentials of a Pt electrode in acidic solutions containing quinone and hydroquinone without and with hydrogen (Ees” - E,,’ and E,, - E,,‘) were measured us an electrode in the acidic solutions free of quinone and hydroquinone under a hydrogen pressure of 1 atm. The rates of hydrogen consumption at the potentials of & and Eeq” were also measured : where the rates were small, the hydrogen-volume change above the solution was measured by means of the horizontal capillary with silicone oil shown in Fig. 1, and where the rates were large, the hydrogen-pressure change above the solution was measured with a water manometer. In the latter case, both the disk electrode and the counter-electrode were immersed in the same cylindrical vessel and the volume of hydrogen evolved at the counterelectrode was subtracted.
Hydrogenationof organiccompounds in acidic aqueous solutionson noble metals
1001
II
Motor
Silicone oi I H20
Capillary /
I
‘d
FIG. 1. Apparatusfor naeasuring hydrogenationrates and polarizationcharacteristics. a, Pt disk electrode; b, Pt counter-electrode; c, Pt-H,-H+ referenceelectrode; d, apparatusfor measuringhydrogenconsumption.
Results Concentrations of quinone and hydroquinone were varied in the range O-02 M to 0401 M in 0.1 N H,S04. The potentials of a Pt electrode in these solutions under argon were measured zw an electrode in O-1 N H&O, under a hydrogen pressure&of 1 atm. These potentials satisfied the Nernst equation, E eqIt - Eeq' = O-699 + O-030 log
[Quinone] V at 25°C. [Hydroquinone]
(1)
Figure 2 shows anodic and cathodic polarization curves in a solution O-1 N H,SO, + O-015 M quinone + 0.015 M hydroquinone under a hydrogen pressure of 1 atm, the cathodic polarization curve in the same solution under argon, and the anodic polarization curve in O-1 N H,SO, free from quinone and hydroquinone under a hydrogen pressure of 1 atm on a Pt disk rotated at 500 rpm. The values of Ees" - E,,,E,,- Eeq',(U/di)E_,t, and the rates of hydrogen consumption which were measured at various rotational speeds on a Pt disk at open circuit (E = Est)and at the potential J?Z,~ * are listed in Table 1. These values in various 9
2. TAKEHARA
1002
I
0
2 6 -_
-I
s”
-2
-3
Fro. 2. Anodic and cathodic polarization curves for the quinonehydroquinone system in a 0.1 N H,SO, solution on a rotatingdisk &&rode (500 rpm) at 25°C. a, anodic pohuizationcurvein a solution01 N HISO* + 0.015 M quinone + 0.015 M hydroquinoneandpnI = 1 atm; b, cathodic polarizationcurve in the same solution as that of a; c, anodtc polarizationcurve in 8 61 N HISO, solution at put = 1 atm; d, cathodicpolarizationcurvein a solution01 N HtSOl + O*OlS M q&one + 0,015 M hydroquinoneunder argon; e, anodic polarization curve for the ionization of hydrogen, determinedfrom curvesa, b and d. TABIZ 1. VALUW OFEep”- Gt , Et - &a’, (dJ$d&-~~,, ANDTHERATesOF HYDROGEN CONSUMPTION, MEASURED AT VARIOUS ROTATIONAL SPEEDS ON A Pt DISK AT OPEN CIRCUIT
Rotational speed of Pt disk revfmin
11:* 150 300 500 700 1E
IL,,” - Eea’= O-699V
~~~~ _ ~~~ V
Est - Eea’ V
(dEldi)B-~8t V cmg/A
0.018 0.059
0681 0.640 0.635 0.624 0613 0.605
240 150 133 99 74 65
::E 0086 0.094 0.098 0.106
O-601
0.593
Z
Rate of hydrogenconsumption mol/cm’/s at open circuit 037 1.65 190 2.75 3.30 3.80 467 5.11
x x x x x x x x
10-O 10-O 10-O lo-@ 10-o 10-B 10-O 10-n
at E = &a* 160 x lo-’ 2.36 x lo-’ 3.19 x 10-s -
* Convection in the solution was provided by a stirrerat the bottom of the vessel rotated by magneticcoupling.
Hydrogenation of organic compounds in acidic aqueous solutions on noble metals TABLE2. SUMPTION
VALUES OF I&” - EBt, E,,t AT OPEN CIRCUIT, MSASURED
0.015 M HYDR~QWINONE
0.1 N HCl 0.1 N HpSOl + 0.1 N Kl3r
ACXDIC
0699 V
V
01 N H,SO
IN VARIOUS
ON A ROTATED Pt DISK (500 IXW/min)
E eIl#-
Solution
- Eep’, (dE/di)+,,,
Gt
0.086 0118 0.102
E.t - EM’ V
(Wdih-~,,
AND TEIERATESOF HY’DRO~XN CXC+ SOLUTIONS at&
V cma/A
0.613 0.581 0.597
1003
=
WITW
0.015 M
QUINONE
AND
1 atm AT 25°C. Eeq” - &’
=
Rate of hydrogenconsumption (mol/cm9 . s)
1:: 257
3.30 x 10-g 2.15 x 1O-m 1.05 x 10-s
acidic aqueous solutions at the rotational speed of 500 rev/min are compared in Table 2. With increasing the rotational speeds of the disk, the values of E,, - Eeq'change in the direction of negative potential and the rate of hydrogen consumption at open circuit, ie the hydrogenation rate of quinone; increases, and that of hydrogen consumption at the potential Eecl” also increases. The hydrogenation rate is affected by the nature of the anion. The rate is decreased by the addition of Cl- and even more by the addition of Br. In the case of cathodic polarization in acidic solutions containing quinone and hydroquinone under argon, there were observed limiting diffusion currents (depending on the rotational speed) at potentials considerably more negative than Eeb (see Fig. 2). Charge-transfer coefficients of the cathodic reaction obtained from the Tafel plot, ao, are nearly equal to 0.54, in accord with Vetter. 6 The exchange cds depend on the nature of the anions (l-07 x 10-l mA/cm2 in 0-f N H,SOd, 4.8 x 10F2 mA/cm2 in O-1 N HCl, and 1.9 x 10s2 mA/cma in a solution containing 0.1 N H,SO, + O-1 N KBr)
,
Discussion Investigations on cathodic reduction of quinone to hydroquinone by Vetteld have shown that at low pH (pH < 5) the following consecutive processes occur:. A (aq) -
A (ad),
A (ad) + H+ (aq) - AH+ (ad), AH+ (ad) + e- -+ AH (ad),
(3) (4)
AH (ad) + H+ (aq) - AH,+ (ad), AH,+ (ad) + e- -+ AH, (ad),
(5) (6) (7)
AH2 (ad) -
AH2 (a@,
where A is quinone and AH, is hydroquinone. According to Vetter6 the dependence of the cd i” on the overpotential charge-transfer
(2)
q” for
control is represented by (8)
I
.I#
=
In the case of cathodic reduction of quinone, the reaction is considered to be chargetransfer-controlled at E SWI& according to the experimental results.
2. TAKEHARA
1004
Anodic oxidation of hydrogen in acidic aqueous solutions is considered to proceed as follows, H, (aq) -
H, (ad),
(9)
H, (ad) + 2 H (ad),
(10)
2 H (ad) + 2 H+ (aq) + 2 e-.
(11)
If Iq”F/RTI < 1, ie, Eeqn - Est < RT/F and the change in the rate of anodic ionization of hydrogen with potential is small in comparison to that of cathodic reduction of quinone, the rate of formation of hydroquinone in mol per unit time per unit surface area due to an electrochemical mechanism involving these consecutive cathodic and anodic reactions with zero net current at the metal/solution interface is represented by the formula deduced by Wagner,*
If E,/
-
Est >
RT/F,
(8) for the cathodic current i” becomes -
exp -(l
.” c=a -2i,(4)
1
=
exp
-2i,(4)
Hence, fiAnS(II)/S at a non-polarized
CQ)$:
1
(1
-E)
1 .
(13)
platinum foil (E = IT,,) is
(14) or in view of (13) fiAEp(ll>
s
&(4) w 7 . exp
(1 -
a$ &
(EBq” -
Ed
1.
(15)
Assuming the same dependence of i” on E in solutions without and with hydrogen, one may use values of i” , i,,, and a4 observed in solutions free of hydrogen in order to calculate fiAn,(II)/s from (14) or (15). The total cd is i = i’ + i”. If the change in the rate of anodic ionization of hydrogen is relatively small in comparison to that of cathodic reduction of quinone, between EBt and EeqN, one has
Taking the logarithm of both sides of (13) and differentiating with respect to E, one obtains in view of (16) =
(--i*JE,Bat
(1
(17)
Hydrogenation
Substitution
1005
of organic compounds in acidic aqueous solutions on noble metals
of (17) in (14) yields fi&(II) s
=
RT 2(1 - oc,)F
1 ,
dE
(18)
( di >B-E&=0 where, in the case of the quinone-hydroquinone system in acidic aqueous solutions, (1 - CQ) = 0.54. The rate of hydrogen consumption at open circuit (E = &) does not differ greatly from that at the potential Eeqn (see cohmns 5 and 6 in Table 1). Therefore, if the electrochemical mechanism prevails the variation of the rate of anodic ionization of hydrogen is relatively small in comparison to that of the cathodic reduction of quinone in the potential range between & and Ees”_ At the rotational speed of 0 rev/min (Table l), the potential difference, Eeq” - Est, is relatively small than RT/F. At speeds higher than 110 rpm (Tables 1 and 2), the differences are much greater than RT/F. Thus, in the former case, fiaHs(II)/S may be calculated from (12) and in the latter case from (15) or (18). ti~n~(II)/S calculated from (12), (15) and (18) is given in TABLE AT
3.
VALUES
VARIOUS
OF THE
ROTATIONAL
RATE
+
Rotational speed of Pt disk rev/min
OF HYDROOENATION
SPEEDS
OF QUINONE,
ON A pt DISK IN A SOLUTION M HYDROQUINONE AT pB, =
0415
1
Rate of hydrogenation
Eea”- E,t V
Observed value at open circuit
0.018 0.059 OX%4 0.075 0.086 0,094 0.098 0.106
ot
110 150 z 700 900 1100
0.37 1.65 1.90 2.75 3-30 3.80 4.67 5-11
x x x x x x x x
Calculated value from (12)
10-e 1O-8 10-Q 1O-9 10-O 1O-9 1O-s 10-D
DETERMINED O-1 N HtSOl atm AT 25°C
1-93 2.08 2.70 3.41 4.03 4.39 5.18
METHODS QUTNONE
0.015 M
of quinone (mol/cm* . s)
Calculated value from (15)
0.39 x 10-n -
BY VARIOUS
+
x x x x x x x
10-s 10-O lb* 10-g 10-D 10-e 10-g
Calculated value from (18) 1.64 1.85 2.49 3.33 3.79 4.57 5.45
x x x x x x x
1O-g 10-O 1O-s 10-O 10-O lo-” 10-e
From* i’ at E = Eat 0.47 2.66 3.11 4-25 5.28 6.95 8-35 9.55
x x x x x x x x
IO-’ lo-* lo-# lo-’ lo-@ 1O-s lo-“ 10-a
* Determined from the partial anodic cd of hydrogen in a 0.1 N H,SO, solution at pEa = 1 atm (eg, from curve c in Fig. 2). t Convection in the solution was provided by a stirrer at the bottom of the vessel rotated by magnetic coupling. TABLE 4. VALUES OF VARIOUS
ACIDIC
THE RATE OF HYDROGENATION SOLUTIONS WITH 0.015 M QUINONE
DISK (500 rpm)
OF QUINONE, AND
AT pHI
=
0.015
DBTERhfINED BY VARIOUS MElT-IODS IN HYDROQUINONE ON A ROTATED Pt
bf
1 atm AT 25°C
Rate of hydrogen consumption Solution
O-1 N HsSOI 0.1 N HCI o-1 N HISO, +O*l N KBr
Eeq” - E.t V
(mol/cma/s)
Observed value at open circuit
Calculated value from (12)
Calculated value from (15)
Calculated value from (18)
0.086 0.118
3.30 x 10-O 2-15 x 10-O
-
3.41 x lo-* 3.00 x 10-s
3.33 x lo-’ 5.28 x 10-O 2.12 x 10-O 457 x 10-s
0.102
1.05 x
-
0.80
10-o
l Determined from the partial anodic (eg, from curve c in Fig. 2).
x
10-O
0.96
x
10-O
From* i at E = Eat
3.11 x 1O-s
cd of hydrogen in a O-1 N HISOl solution atpHa = 1 atm
2.
1006
TAKEHARA
columns 4,5 and 6 in Tables 3 and 4. In the comparison of columns 3 and 4 or 3 and 6, the values in columns 3 and 4 are nearly equal at the rotational speed of 0 rpm and the values in columns 3 and 6 for each rotational speed are nearly equal at speeds higher than 110 revlmin. It must be noticed, however, that these values are not exactly equal. This may be due to the fact that change in the rate of anodic ionization of hydrogen in the potential range between & and Eeq” is not completely nil. Since the contributions to the rate of hydrogenation of quinone, fiAH,/SIIr calculated from (12), (15) and (18), due to the electrochemical mechanism (II), agree approximately with the experimental values of the over-all rate, it is concluded that the eIectrochemical mechanism prevails. In (12) only electrochemical data obtained for a quinone-hydroquinone solution with hydrogen are used. In (15), dE/di is taken from measurements in a solution with hydrogen but the value (1 - a4) is taken from measurements in a solution without hydrogen. In contrast, for the evaluation of (15), only electrochemical data for a quinone-hydroquinone solution without hydrogen are used. Therefore, use of (15) assumes that the cd/potential curve for the cathodic reduction of quinone, and especiahy the exchange cd is not affected by the presence of hydrogen. Seemingly, this assumption is justified, since rates calculated from (15) and (18) agree within the limits of experimental errors. In addition, one may calculate hydrogenation rates by letting jlAHp/(S>,, equal to i’/2F at E = E,, where i’ is the anodic cd for the reaction H, (aq) = 2 H+ + 2 ein a HaSO*-H, solution free from quinone and hydroquinone (column 7 in Table 3 and 4). This calculation assumes that the anodic oxidation of hydrogen is not affected by the presence of the organic substances. This assumption, however, is not justified, since values listed in column 7 of Tables 3 and 4 are substantially greater than the experimental rates listed in column 3. Seemingly, the presence of the organic substances impedes the anodic oxidation of hydrogen, probably because of preferential adsorption of the organic substances. It is noteworthy that under the conditions of this research the anodic oxidation of hydrogen is not exclusively controlled by diffusion in spite of the pronounced dependence on the rotational speed of the platinum disk. Table 5 shows that the presence of Cl- and Br ions decreases the rate of hydrogenation of quinone. This indicates preferential adsorption of Cl- and Br- anions on platinum. The foregoing observations show that the electrochemical mechanism prevails. For a further check, the rate of formation of hydroquinone has been determined TABLE
5. VALUES
WHILE
E =
QUINONE
OF T’HE RATE OF HYDROGEN
CONSUMPTION
AND
THJ! EQUIVALENT
E-Q” WAS ENFORCED IN A SOLUTION 0.1. N HzSOc + 0.015 M QUINONE AND VALUES OF THE RATE OF HYDROGENATION AT P=P = 1 atm AT 25T, CALCULATED FROM (19)
speed of Pt disk rev/min
OF THE TOTAL +
0.015 M
Rotational
150 300 500
CD
HYDROOF OUINONE CAL-
mol/cmp Js
mol/cm”/s
mol/cmg/s
1.60 x 1O-s 2.36 x IO-* 3.19 x 1o-s
1.56 x 10-O 2.29 x 10-a 3.21 x 10-O
0.04 x 10-m 0.07 x 10-O -002 x 10-e
Hydrogenation of organiccompounds in acidicaqueoussolutionson noblemetals
1007
at an enforced potent&l E = Eeqn where electrochemical reduction of quinone is
suppressed. The rate of formation of hydroquinone is generally equal to the difference of hydrogen consumption and the current equivalent for anodic oxidation,
i (19) s S 2F’ The terms on the right hand side of (19) for E = I&” as determined experimentally are nearly equal (Table 5, columns 2 and 3). Accordingly, (fiA&/S) at E = _Eecl” are virtually zero (Table 5, column 4). Thus most of the hydrogen approaching the anode is oxidized anodically and virtually no hydrogen reacts with quinone according to the non-electrochemical mechanism.
‘--a _ --‘Ha
II.
HYDROGENATION
OF
ALLYL ALCOHOL
AND VINYL ACETATE
Preliminary tests The hydrogenation of ally1 alcohol and vinyl acetate, which have a carboncarbon double bond and are water-soluble, has also been investigated, CH,: CHCH,OH + H, + CH,CH,CH,OH, CH,: CHCOOCH, + H2 -
(20)
CH,CH,COOCH,.
(21)
Potentials in a O-1 M ally1 alcohol + O-1 M propyl alcohol solution and a O-1 M vinyl acetate + O-1 M ethyl acetate solution with 0.1 N H,S04, under argon on Pt, were not stable and changed with time. The electrochemical equilibrium of these systems is not readily established. The values of these systems under a hydrogen pressure of 1 atm on a Pt disk rotated at 500 rev/min at 25°C were measured with the apparatus shown in Fig. 1. Steady-state potentials were found to be comparatively stable (I& - Eecl’= O-053V for the ally1 alcohol and propyl alcohol system and O-065V for the vinyl acetate and ethyl acetate system). Upon using estimated values for Eeqfl, the following relation holds EBt - I&’ < I&” - Es,. Cw Under these conditions, it is necessary to measure the potentials B and E* without current and with current in the double cell suggested by Wagner,* reference electrode
Compartment (A) [H+ bq), A (aq) AH, (aq), H, @qN
(c)
electrode as cataIyst (E) Pt or Pd-Ag(+H)
Compartment (B) [H+ (aq)l
US)
reference electrode @) (ED)
t--E--J?zG-*
Experimental
f-E*
-
ED-
technique
The apparatus shown in Fig. 1 was used with the rotating disk bi-electrode shown in Fig. 3 in place of the working electrode in Fig. 1. A thin Pt foil (apparent surface area 21.2 cm2, thickness 0.01 mm) or a thin Pd-Ag foil (apparent surface area 21.2 cm2, thickness O-03 mm, atomic ratio of Pd: Ag 77 :23) was used as the bi-electrode. The measurements with Pt foil were carried out at 50°C and those with Pd-Ag foil at 25°C. The solution in compartment A was 0.1 N &SO* + O-1 M ally1 alcohol + O-1 M propyl alcohol or O-1 N H2SOp + O-1 M vinyl acetate + 0.1 M ethyl acetate. The solution at the back of the foil in compartment B was a HCl solution with the same hydrogen-ion activity as that in 0.1 N HPISOd(0.052 N HCl at 50°C and
Z. TAICEIWU
1008
bad
Plastic
L l?IG.
3.
Pt or Pd-Ag foil disk electrode
Rotating disk K-electrode.
0.064 N-HCl at 25°C). Reference electrode C was a Pt, H, electrode in O-1 N H,SO,; reference electrode D was a Ag/AgCl electrode. The activity of hydrogen atoms is assumed to be the same on both sides of the foil. At 25°C this condition may be readily established with a Pd-Ag foil, but not with a Pt foil. According to Gileadi et ~1,~ the diffusion rate of hydrogen atoms in Pt foil is sufficiently high at 5O”C, so 50°C was used for Pt. Use of a Pd-Ag alloy with an atomic ratio 77 : 23 has the advantage of a rather high hydrogen solubility within the range of the a phase without occurrence of the /I phase, which is found in the binary system Pd-H under a hydrogen pressure of 1 atme Thus discontinuous changes in volume upon dissolution of hydrogen and also hysteresis phenomena are avoided.
Hydrogenation of organic compounds in acidic aqueous solutions on noble metals
1009
The Pt foil was purified by the same method as for the experiments with quindne. In the case of the Pd-Ag foil immersion in acidic perchromate solution was omitted. Solutions of H,SO, and HCl were purified by pre-electrolysis and then saturated with hydrogen or argon. H,SC4 solution was introduced in compartment A and then after the equilibrium state of the electrode under a hydrogen pressure of 1 atm was reached, chemical grade organic compounds were added. Hydrogen and argon were purified by the same method as for the experiments with quinone. If the solution in compartment A was 0.1 N HsSO, saturated with hydrogen without organic compounds, potentials observed after one day at both sides of the Pt and the Pd-Ag rotating disk foil electrodes were equal to the equilibrium potential of a hydrogen electrode, Ees’. At such electrodes, equilibrium is readily established when the partial pressure of hydrogen is changed. Furthermore, upon changing polarization of the front side of the foil electrode, steady-state potentials were reached within 15-20 min. Further, the front side of the bi-electrode was polarized by applying a pre-determined pd tls the reference electrode C. The resulting current between the b&electrode and the counter-electrode and the resulting potential of the back side against the reference electrode D were measured. The corresponding pds E - Eeq’ and E* Eeq’ for a rotational speed of 250 rpm are shown in Fig. 4. Values of the back-side potential were obtained only for relatively small polarizing currents since at higher currents steady values of E* were not reached within 30 min. In addition, cds and rates of hydrogen consumption as a function of the front-side electrode potential E - E,,’ were measured on a Pt electrode shown in Fig. 1 and are shown in Fig. 5.
-0.41
0
I 0.02
E-E;
I
I
OG4
O-96
01
E*-Ee;,
I
I oa
V
0
0
I oa2
E-E&or
I
I
I 0.04
O-08
0.06
&"-E,',
, V
FIG,4. Anodic and cathodic polarization curves for the allyl-alcohol-propyl-alcohol and the vinyl-acetate-ethyl-acetate system in O-1 N HISO solution on a rotating disk bi-electrode (250 rev/mm) at 50°C. (A) O-1 .M ally1 alcohol + 0.1 M prop 1 alcohol (B) O-1 M vinyl acetate + O-1 M ethy r acetate a, cd as a function of potential of the front side (i/E curve); b, cd as a function of potential of the back side (i/E* curve).
0
Z. TAKEIURA
05
0.05
0
E-E:,,
0.
E-EA,
v
005 V
040
FIG. 5. Current density corresponding to the rate of hydrogen consumption as a function of potential and polarization curves in a O-1 N HISO4 solution with and without organic compounds at PHI - 1 atm on a rotating Pt disk electrode (500 rpm) at 25’C. (A) @1 M ally1 alcohol + O-1 M propyl alcohol (B) O-1 M vinyl acetate + O-1 M ethyl acetate a, polarization curve in a O-1 N E&SO, solution with organic compounds at pa, = 1 atm; b, polarization curve in a O-1 N H&O1 solution at PHI = 1 atm; c, cd corresponding to the rate of hydrogen consumption; d,cd corresponding to therateof hydrogenation, determined from curves a and c.
Numerical values for further evaluation are listed in Tables 6 and 7. Discussion
The general features of the dependence of the front side potential E and the back side potential E* on the cd applied for polarization have been discussed by Wagner.* In the present research, the following experimental results are noteworthy. (1) The steady-state potentials E,, and EBt* do not differ greatly. Values of ES, ES,* are listed in column 7 of Table 6. (2) Relatively low applied cds are required in order to enforce a state where E = E* (sde Fig. 4). Accordingly, the shift of the front side potential for reaching E = E* is relatively small. Thus one has the conditions considered in Wagner’s Fig. 2a.4 If the state E = E* is enforced, the driving force for ionization of hydrogen vanishes and, therefore, the observed rate of hydrogen consumption is exclusively due to hydrogenation of the organic compound by the non-electrochemical mechanism, Hs (aq) - Hs (ad), H, (ad) + 2 H (ad), A (aq) A (ad) + H (ad) AH (ad) + H (ad) AH, (ad) -
(23) (24)
A (ad), AH (ad),
(25)
AH, (ad),
(27)
AH, (a&
(28)
(26)
where A is ally1 alcohol or vinyl acetate and AH, is propyl alcohol or ethyl acetate.
i
i
0*062 0.111 0,075 0.128
50 25 50 25
Pd-Ag
Pt
Pd-Ag
V
Est - Ees’
Pt
Electrode
Temperature “C
0.110
0.063
0*098
0.058
V
Eat* - EM,’
0.096
0.050
0.080
DO54
(E - Ee,3,,1
0.018
0.012
0.013
0004
V
Eet - Emt*
0.014
0.013
0.018
O%kl
V
Ent* - (EL*
HaSOdCONTAININGORGANIC
Pd-Ag
Pt
Pd-Ag
0.1 M propyl alcohol
0.1 M vinyl acetate +
0.1 M ethyl acetate
Pt
0.1 M ally1 alcohol +
I
Electrode
organic compound
25
50
25
50
Temperature “C
1.5 x 10-10
7.11 x 10-a
1*7 x 10-10
5.86 x 10-s
(- kf*hE* s mol/cma/s
1.4 x 10-10
685 x 1O-e
2.0 x 10-10
5~86x 10-O
-Qr,cE4u S mol/cm*/s
1.4 x 10-10
6.85 x 10-D
l-2 x 10-10
6.64 x 10-O
(-~&*keq’ S mol/cm*/s
6.2 x 10-l’
3.3 x lo-‘0
4.2 x lo-la
1.7 x 10-10
( - il2%-~~ mol/cma/s
4.2 x 10-l’
1.46 x 1O-s
3.3 x 10-11
1.53 x 10-o
(-i/2F)B_lep~ mol/cm”/s
RATESOFHYDROGEN CXINXJMPTIONATOPENCIRCUIT(E= ht),E=E* AND E= E eq’, ANDTHE EQUIVALEF OF THE CD (-i/2F) AT E = E* AND E= Eeq’ IN A SOLUTION O.lNHBSOz CONTAINING ORGANIC COMPOUNDS ON A ROTATED FOIL RI-ELECTRODE(250rev/min) AT~H%= 1 atm
TABLE 7.
0.1 M ally1 alcohol + O-1M propyl alcohol 0.1 M vinyl acetate + 0.1 M ethyl acetate
Organic compounds
A SOLUTIONOF0'1N coMPouNDs ON A ROTATED FOIL BI-ELECTRODE(25Orev/min) AT pa, = latm
TABLE6. VALUES OF Eat - Eeq’, Eet* - Eeq’, (E - Eep’)E_E*, .?$t- Eat* AND &t* - (I?&_~*IN
e
1 & 0 B
F nE. B 8
8 sv)
$
B 6 8 g
i w e
Q.
B Z 0 u s
&i
J
1012
z. TAKEHARA
According to columns 4 and 5 of Table 7, the rates of hydrogen consumption at open-circuit conditions (E = E,J do not differ greatly from rates observed when E = E* was enforced, ie, when the electrochemical mechanism was suppressed. Thus again, the non-electrochemical mechanism prevails for the hydrogenation of ally1 alcohol and vinyl acetate. Values of the rate of hydrogen consumption at an enforced potential E = Ees' listed in column 6 of Table 7 are about equal to those for open-circuit conditions in column 5. This also supports the conclusion of prevalence of the non-electrochemical mechanism. The deviation of values in columns 5 and 6 for the hydrogenation of ally1 alcohol on the Pd-Ag foil cannot be explained. Hydrogen consumption at E = E* on the Pt foil is much greater than that on the Pd-Ag foil (column 4, Table 7). Thus the catalytic activity of a Pt foil for hydrogenation by the non-electrochemical mechanism is much greater than that of a Pd-Ag foil. Since E,, - Eet*is greater than zero (column 7, Table 6), the ionization of hydrogen by (11) proceeds at finite rate and the hydrogenation due to the electrochemical mechanism also proceeds at open circuit, (E = Esb),as pointed out by Sokol’skaya and Sokol’skii,” although only as a side reaction. The difference between the cd corresponding to the rate of hydrogen consumption per unit area and observed cd at the potential of E is considered to be the cd corresponding to the hydrogenation rate of an organic compound at this potential. These rates at various potentials are shown by dotted line in Fig. 5. These rates increase upon changing the potential in the negative direction. This may be due to a higher activity of hydrogen atoms at more negative potentials.4 CONCLUDING
REMARKS
The hydrogenation of an organic compound on a noble metal in acidic aqueous solutions is considered to proceed according to various reaction mechanisms. In the case of the quinone and hydroquinone system where an electrochemical equilibrium is readily established, the hydrogenation proceeds predominantly by the electroIn the case of the allyl-alcohol-propyl-alcohol system and the chemical mechanism. vinyl-acetate-ethyl-acetate system, where an electrochemical equilibrium is not readily established because the reaction rates are sluggish, the hydrogenation proceeds predominantly by the non-electrochemical mechanism. Further measurements of the rate of hydrogenation at a non-polarized and a polarized electrode as a function of the concentrations of reactants, products and hydrogen ions are needed in order to decide the rate-determining step for each mechanism. Acknowledgement-The author is indebted to Professor Dr. C. Wagner of the Max-Planck-Institut fiir physikahsche Chemie for many helpful suggestions and discussions. He gratefully acknowledges receiving a scholarship from the Max-Planck-Institut ftir physikalische Chemie, Giittingen, Germany during his stay as a research fellow in 1966-1967. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
F. BECK and H. GERISCHER, 2. Elektrochem. 65,504 (1961). F. BECK, Z. Elektrochem. 69, 199 (1965). A. M. SOKOL'SICAYA and D. V. SOKOL'SKII,Kinetika i Kafaliz 6, 658 (1965). C. WAGNER, Electrochem. Acta, 15, 987 (1970). K. J. VETI-ER,Z. Eiektrochem. 56,797 (1952). K. J. VETTER,Z. Naturf. 7a, 32X (1952); Sa, 823 (1953). E. GLLEADI,M. A. FULLENWIDER and J. O’M. BOCKRIS,J. electrochem. Sot. 113, 926 (1966). F. A. Lswrs, Z%e PaNadium-Hydrogen System, pp. 75, 144. Academic Press, London (1967).