INTRODUCTION oxidation of ammonia with oxygen, while
iiaZjiiii a.iiu uuuuoi i \i j) c u o u 1 wuiiva a.
Catalyst deactivation is an important loss of catalyst activity during the decom-problem, especially in the case of large- position of nitrogen oxide over P t /Al 2 0 3 . scale production. Well-known causes of catalyst deactivation are sintering, irrevers-
Deactivation of platinum catalysts also occurs during oxidation processes in the liq-
ties in the feed, and deposition of carbonaceous material on active sites. Irreversible
et al. in oxidizing ethylene glycol (3, 4) and Dirkx et al. in oxidizing D-glucose to D-glu-
catalyst deactivation is of particular importance in the case of the application of noble m j=»tQl r"sitc»1\/ctc h p r ^ i i K P n f th**ir h i o h in i t ia l
carate (8-10). Also, patents have been published (14, 15) concerning the activity of rvlatirmm r*QtQl\/ctc Hurmo r w i r l a t i n n nrn.
costs. Platinum catalysts are often used both for
cesses in the liquid phase. The oxidation of D-gluconate (obtained
hydrogenation/dehydrogenation reactions and for oxidation reactions. Important ap-nhV.atinns of nlatirmm catalysts in the field
by oxidation of D-glucose) to D-glucarate involves a reaction intermediate, L-gulu-ronate Thft main reaction sp.rmp.nrp. is
of oxidation are the complete combustion of automotive exhaust gases (7) and the
given in Fig. 1. The compounds D-glu-conate and L-guluronate possess weak re-
oxidation ot ammonia (I, 2). Ihe oxidation of alcohols (3-6), aldehydes (6, 7), and suears (8-12) mav serve as examples of
ducing properties. Ihe overall selectivity to D-glucarate is about 50%. The remaining products are carboxvlic acids of a lower
platinum-catalyzed oxidation reactions in the liquid phase.
T-V .1 1 1
molecular weight (as D-tartrate, tartronate, glycolate, D-erythronate, and oxalate)
uuring tnese processes a sirong aeac-tivation of the platinum catalysts often occurs due to the presence of oxygen. Oster-
iormea oy c-<^ cleavage reactions on tne catalyst surface. The main product of the oxidation reaction, D-glucarate? might be of
maier et al. (2) noted a strong catalyst commercial interest on account of its ability deactivation during the low-temperature to form complexes with metal ions (16-18).
Labor gy, Ali Universi A AliTechn ology, P. 0. Box 5 MB
Eindhoven,The
therlandseptember
17, 1985; revised August 21, 1987
A study has been made of the kinetics of deactivation of a commercial Pt/C catalyst being used in an aqueous slurry for the oxidation of D-&COnate to D-glucarate at 50°C. It appears that the deactivation of the catalyst is an independent process, governed by the coverage of the platinum surface by oxygen atoms. Under steady-state conditions an exponential decay is observed. A mathematical model is presented, based on the processes occurring at the platinum surface, which describes the experimental reSUkS very well. 0 1988 Academx Press, Inc.
INTRODUCTION
Catalyst deactivation is an important problem, especially in the case of large- scale production. Well-known causes of catalyst deactivation are sintering, irrevers- ible adsorption of (by-)products or impuri- ties in the feed, and deposition of carbona- ceous material on active sites. Irreversible catalyst deactivation is of particular impor- tance in the case of the application of noble metal catalysts because of their high initial costs.
Platinum catalysts are often used both for hydrogenation/dehydrogenation reactions and for oxidation reactions. Important ap- plications of platinum catalysts in the field of oxidation are the complete combustion of automotive exhaust gases (1) and the oxidation of ammonia (I, 2). The oxidation of alcohols (3-6), aldehydes (6, 7), and sugars (8-12) may serve as examples of platinum-catalyzed oxidation reactions in the liquid phase.
During these processes a strong deac- tivation of the platinum catalysts often oc- curs due to the presence of oxygen. Oster- maier et ul. (2) noted a strong catalyst deactivation during the low-temperature
oxidation of ammonia with oxygen, while Amirnazmi and Boudart (13) also found a loss of catalyst activity during the decom- position of nitrogen oxide over Pt/AIZ03. Deactivation of platinum catalysts also oc- curs during oxidation processes in the liq- uid phase as observed for example by Khan et al. in oxidizing ethylene glycol(3, 4) and Dirkx et al. in oxidizing D-glucose to D-glu- carate (8-10). Also, patents have been pub- lished (14, 15) concerning the activity of platinum catalysts during oxidation pro- cesses in the liquid phase.
The oxidation of o-gluconate (obtained by oxidation of D-glucose) to D-glucarate involves a reaction intermediate, L-gulu- ronate. The main reaction sequence is given in Fig. 1. The compounds o-glu- conate and L-guluronate possess weak re- ducing properties. The overall selectivity to D-glucarate is about SO%. The remaining products are carboxylic acids of a lower molecular weight (as D-tartrate, tartronate, glycolate, o-erythronate, and oxalate) formed by C-C cleavage reactions on the catalyst surface. The main product of the oxidation reaction, D-glucarate, might be of commercial interest on account of its ability to form complexes with metal ions (16-18).
329 0021-9517/88 $3.00
Copyright 0 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.
riTTlf flu A A P FT AT
kept at a constant temperature. A mixture of oxygen and nitrogen is supplied to the reactor containing the aqueous catalyst *1
slurry is measured with an oxygen probe (Ingold 533 sterilizable electrode) which
D-glucos* D-gluconate L-guluronat» D-g'ucarite
F I G . 1. Reaction sequence in the oxidation of D-
displays the equivalent saturation pressure of the oxygen dissolved in the slurry. The
cose to disodium-D-glucarate. oxygen pressure in me siurry, ro 2 , is controlled by a continuous adjustment of the stirrer speed. In this way a dynamic equilib-
A possible new application is the use of num is obtained between the amount of D-glucarate as a substitute for polyphos- oxygen transferred from the gas phase to phates in detergents (79, 20). the slurry and the amount ot oxygen con-
A serious problem for the production of sumed by reaction. ^ ~i — „ i „ - , w * r ^ ^ i A The nH of the slurrv is kent at a constant JJ-giUL.cliaic un a laigc stait 13 nit iapu * j 1
deactivation of the Pt/C catalyst under the level by titration with a solution of sodium reaction conditions used. An investigation hydroxide, in order to neutralize the sugar on this subject has been started because information in the literature concerning this
acids formed during the oxidation process. Simultaneously a solution of sodium-D-glu-r-nncitp ic QHHP>H ir% tVif* c l n r n ; in Q p n n c t a n t
pnenomenon is scarce, special auenuun is given to the influence on the deactivation process of the oxygen pressure, D-elu-
& L J UUUW I V J LliV J1U1 1 J 111 W\-f 11J LUUt
proportion with the amount of alkali added (the production of 1 mole disodium-D-glu-
conate concentration, pH, and temper- carate from sodium-D-gluconate requires 1 a t u r e mole of alkali). The rate of deactivation of
EXPERIMENTAL
The catalyst used in this study was com-alkali consumption as a function of time. In this way the reaction conditions remain
mercially available 5% platinum on activated charcoal (Degussa F 196 RA/W).
constant in time except for the catalyst
Utner types or support were testea in me past but charcoal appeared to be preferable The same conclusion was drawn bv other authors oxidizing various alcohols under comparable reaction conditions (21,
A first requisite to study catalyst deac t i v o t i r m ic tr\ m a i n t a i n t h e rp.nr.tinn rondi tions at a constant level as a function of time. Batch experiments in which the deac-tivation proceeds along with conversion of D-gluconate cannot provide useful informa-
deactivation process. Therefore an apparatus has been built (Fig. 2) to study the F I G . 2. Apparatus for continuous oxidation. (1)
continuous oxidation of sodium-D-glu conate under steady-state conditions.
Keacior, muuuun vessel, \ 3 j pn iiicasuitmciiu control, (4) measurement/control of partial pressure of oxygen in the liquid, (5) feed of alkali, (6) feed of
1: -i J . - / T \ /o\ +u „ A iiv ±±±i±±u f u i t j V/A v^w^uivnv ^ SOUlum-D-glUCUnau;, V'7 pump, VA* uicnuv&iav,
reactor and the filtration vessel which are sampling system.
330 DIJKGRAAF ET AL.
D.~I"CO‘. 0-9I"CO".t* L.9YIYrm.t. D-~I"car.t*
FIG. 1. Reaction sequence in the oxidation of D-glu- case to disodium-n-glucarate.
A possible new application is the use of D-glucarate as a substitute for polyphos- phates in detergents (19, 20).
A serious problem for the production of o-glucarate on a large scale is the rapid deactivation of the Pt/C catalyst under the reaction conditions used. An investigation on this subject has been started because information in the literature concerning this phenomenon is scarce. Special attention is given to the influence on the deactivation process of the oxygen pressure, D-glu- conate concentration, pH, and temper- ature .
EXPERIMENTAL
The catalyst used in this study was com- mercially available 5% platinum on acti- vated charcoal (Degussa F 196 RA/W). Other types of support were tested in the past but charcoal appeared to be prefer- able. The same conclusion was drawn by other authors oxidizing various alcohols under comparable reaction conditions (22, 22).
A first requisite to study catalyst deac- tivation is to maintain the reaction condi- tions at a constant level as a function of time. Batch experiments in which the deac- tivation proceeds along with conversion of o-gluconate cannot provide useful informa- tion on the factors which influence the deactivation process. Therefore an appara- tus has been built (Fig. 2) to study the continuous oxidation of sodium-D-glu- conate under steady-state conditions.
The main parts of the equipment are the reactor and the filtration vessel which are
kept at a constant temperature. A mixture of oxygen and nitrogen is supplied to the reactor containing the aqueous catalyst slurry. The concentration of oxygen in this slurry is measured with an oxygen probe (Ingold 533 sterilizable electrode) which displays the equivalent saturation pressure of the oxygen dissolved in the slurry. The oxygen pressure in the slurry, PO*, is con- trolled by a continuous adjustment of the stirrer speed. In this way a dynamic equilib- rium is obtained between the amount of oxygen transferred from the gas phase to the slurry and the amount of oxygen con- sumed by reaction.
The pH of the slurry is kept at a constant level by titration with a solution of sodium hydroxide, in order to neutralize the sugar acids formed during the oxidation process. Simultaneously a solution of sodium-D-glu- conate is added to the slurry in a constant proportion with the amount of alkali added (the production of 1 mole disodium-D-glu- carate from sodium-D-gluconate requires 1 mole of alkali). The rate of deactivation of the catalyst is determined by recording the alkali consumption as a function of time. In this way the reaction conditions remain constant in time except for the catalyst
FIG. 2. Apparatus for continuous oxidation. (1) Reactor, (2) filtration vessel, (3) pH measurement/ control, (4) measurement/control of partial pressure of oxygen in the liquid, (5) feed of alkali, (6) feed of sodium-D-gluconate, (7) pump, (8) thermostat, (9) sampling system.
F»F A T T T V A T T O M O F PT ATTXTTT1U P AT AT Y55TS1 1 ' U l
concentration which slightly decreases by dilution with the solutions of alkali and sodium-D-gluconate. This is compensated •y penuuicaiiy pumping ctouui J/C UI me slurry to the filtration vessel followed by partial filtration. The resulting slurry is pumped back to the reactor. The filtrate is analyzed by high-speed liquid chromatogra-
1 1 " 1 1 1 T V f+ I S ^% \ pny as aescriDea oy LnjKgraai et at. \ZJ).
Al l experiments were performed at temperature of 50°C. a D H of 9. and catalyst concentration of 10 kg/m 3 unless mentioned otherwise. In all experiments
time Us)
FIG. 4. Activity of the catalyst as a function of time nH nH (+\ nH 7 (C)\ nH 8 ( x ) nH Q
the conversion or sodium-D-gluconate in the reactor was kept at the same low level nf nlimit 5% t<i avniH thp nn««ih1f» infliif*nr*p
of by-products on the catalyst deactivation. (0.5 mole/liter) in a slurry saturated with The reactant concentration, the partial oxygen. pressure of oxygen in the slurry, the pH, y
A + u + f , • , / RESULTS AND DISCUSSION and the temperature were varied in order to 111 v^aui att; intu miiu^iiv^ uu nit; M I I ^ U ^ S ui
the catalyst deactivation. For the determination of the rate of deac-
the deactivation of the catalyst in which the rate of reaction under constant reaction
tivation in the absence of D-gluconate a different apparatus has been used. For
conditions is plotted as a function of time. Initially there is a fast deactivation of the
mese experiments portions 01 rresn caiaiysi in water were exposed to oxygen for periods of varying length, at the same temoera-
rate of reaction is obtained. By fitting the curves presented in Fig. 3 it appears that
ture and pH as those used in the other experiments. After such a period the initial
they can be described by the formula
rate or reaction was determined by a batch R(t) — R + (R - R ex (~K t) ( I ) experiment oxidizing sodium-D-gluconate 0 0 p D
In this formula R0 and Rx stand for the initial rate of reaction and the rate of reac-tion at infinite time, respectively, and KD is the so-called deactivation constant which Hptprminps the' mte* rvf Hi=»tir,ti\7Qtir*n A .- pan
be obtained from the slope of the plot of ln[R(t) - RK] versus time.
In Fig. 4 the results are given in this way for three experiments carried out at differ-
* — I I »
o 50 mo i*n ?nn
the pH is an important parameter for the rate of reaction, and this is confirmed by
t ime (ks)
FIG. 3. Typical result of a deactivation experiment
the results in Fig. 4. However, Fig. 4 proves that the deactivation constant is t-* n ++sA 1 * i I ' n A i m r t A A ^ l ^ . r - * T T T*» Tl*^ ^ / ^ A \
using a Ft/C catalyst for the oxidation of D-gluconate i i c u u l * innuciiccu uy uic p n . 111 r a n L \ZH) (CGOZ^O = 1-0 M , p 0 l = l bar). it will be shown that the deactivation of
DEACTIVATION OF PLATINUM CATALYSTS. 1 331
concentration which slightly decreases by dilution with the solutions of alkali and sodium-D-gluconate. This is compensated by periodically pumping about 5% of the slurry to the filtration vessel followed by partial filtration. The resulting slurry is pumped back to the reactor. The filtrate is analyzed by high-speed liquid chromatogra- phy as described by Dijkgraaf et al. (23).
All experiments were performed at a temperature of WC, a pH of 9, and a catalyst concentration of 10 kg/m3 unless mentioned otherwise. In all experiments the conversion of sodium-D-gluconate in the reactor was kept at the same low level of about 5%, to avoid the possible influence of by-products on the catalyst deactivation. The reactant concentration, the partial pressure of oxygen in the slurry, the pH, and the temperature were varied in order to investigate their influence on the kinetics of the catalyst deactivation.
For the determination of the rate of deac- tivation in the absence of D-gluconate a different apparatus has been used. For these experiments portions of fresh catalyst in water were exposed to oxygen for peri- ods of varying length, at the same tempera- ture and pH as those used in the other experiments. After such a period the initial rate of reaction was determined by a batch experiment oxidizing sodium-D-gluconate
~~- 0 50 100 150 200
time (ks)
FIG. 3. Typical result of a deactivation experiment using a Pt/C catalyst for the oxidation of D-ghCOnate
(Coo,,,=, = 1.0 M, PO2 = 1 bar).
-15
: ..__ -.
ci- ‘.
:h
5 -17 .._
-19
I \+ 0 50 100
time (ks)
FIG. 4. Activity of the catalyst as a function of time and pH. (+) pH 7, (0) pH 8, (X) pH 9.
(0.5 mole/liter) in a slurry saturated with oxygen.
RESULTS AND DISCUSSION
Figure 3 illustrates a typical example of the deactivation of the catalyst in which the rate of reaction under constant reaction conditions is plotted as a function of time. Initially there is a fast deactivation of the catalyst, and after a long time a constant rate of reaction is obtained. By fitting the curves presented in Fig. 3 it appears that they can be described by the formula
R(t) = R, + (Ro - R,) exp(-&t). (1)
In this formula R. and R, stand for the initial rate of reaction and the rate of reac- tion at infinite time, respectively, and Kn is the so-called deactivation constant which determines the rate of deactivation. Kn can be obtained from the slope of the plot of In[R(t) - R,] versus time.
In Fig. 4 the results are given in this way for three experiments carried out at differ- ent pH values. Dirkx et al. (IO) showed that the pH is an important parameter for the rate of reaction, and this is confirmed by the results in Fig. 4. However, Fig. 4 proves that the deactivation constant is hardly influenced by the pH. In Part 2 (24) it will be shown that the deactivation of
310 n T T W n P A A P T7T A T
4 i reactor or stopping the stirrer for a few minutes (the rate of deactivation of the catalyst after resuming the experiment, I1UWCVCI, IS l l l g l lC l i i u s i c a v u v c t i i u i i
is accomplished by reducing compounds in the reaction mixture which reactivate the deactivated platinum sites at the catalyst surface. Using a similar procedure, regen-eration or a platinum catalyst was acnievea in the case of the oxidation of other com-nounds such as ethvlene elvcol (3. 4). am-
FIG. 5. Deactivation constant as a function of partial monia (2), and sugar acids (8-10). This regeneration process at the platinum sur-
pressure 01 UAygeu anu suuiuiii-D-giui-uuiiic luuic i i -tration. Sodium-D-gluconate concentrations: (O) 0.25 mole/liter, (x) 0.5 mole/liter, (•) = 1.0 mole/liter, ( + )
face will also occur during normal oxidation experiments. However, deactivation then Hr\mir»Qt(=»c ctnr\ ihf> npt rpcn l t ic a ornHiml
decrease in the catalyst activity. Equation (1) can also be derived starting
platinum catalysts during the oxidation of D-gluconate can be ascribed entirely to the
from the three elementary processes that take place on the catalyst surface, namely,
have been performed using a constant concentration of D-gluconate and different oxy-
ation. Because of these general starting points the proposed model does not have to
gen pressures. The deactivation constants refer only to the oxidation of D-gluconate. belonging to these experiments are given in It may possibly also hold for other oxida-tig. J as a runction or tne oxygen pressure and the D-gluconate concentration. It is striking that the deactivation constant de-
iion processes using precious meiai caia-lysts under comparable reaction conditions, during which (weak) reducing
pends on both parameters and decreases using a lower oxygen pressure or a higher
compounds are converted to their accessory products. The three elementary
D-gluconate concentration, rrom this result steps will now be discussed in some detail, the assumption arises that the coverage of The oxidation reaction. As reported by thf* nlntinum <nrfarp. with rvxvaen i* a nre.- Hftvns et nl (75. 7ft\ the reaction is initiated
dominant factor for the deactivation process. Lowering the oxygen pressure and
by an abstraction of a proton of a hydroxyl group of the sixth carbon atom in the chain
increasing the D-gluconate concentration actually both decrease the part of the plati-
by an OH ion, yielding water. Alter this step a hydride ion is transferred to the r\ la t in 1 i m c i i r f u r p a i v i n o thp r p u r t i n n intf*r-
This is also supported by the results in Fig. mediate, L-guluronate. An O H " ion is ob-4. Although the pH of this solution is of tained by reaction of the hydride ion with great importance for the rate of reaction an adsorbed oxygen atom. The reaction (10) there is no relation between the pH path of the consecutive reaction of L-gulu-
confirms our assumption because the pH is only of minor influence on the part of the
way. It appears (27) that the rate of oxidation is proportional to the fractions of the
platinum surface covered by oxygen. platinum surface which are covered by oxy-In the case of a deactivated catalyst the gen,/0> and the organic reactant,/A- If the
interruption of the oxygen supply to the the catalyst surface which is still not deac-
332 DIJKGRAAF ET AL.
4
-1 c: 0 0
-0 r 2
2
0
0 u I 0 + + 0.5
‘12 1.0
P 02
(bad/‘)
FIG. 5. Deactivation constant as a function of partial pressure of oxygen and sodium-D-gluconate concen- tration. Sodium-D-ghconate concentrations: (0) 0.25 mole/liter, (X) 0.5 mole/liter, (0) = I .O mole/liter, (+) 1.67 mole/liter.
platinum catalysts during the oxidation of D-gluconate can be ascribed entirely to the presence of oxygen. Series of experiments have been performed using a constant con- centration of D-gluconate and different oxy- gen pressures. The deactivation constants belonging to these experiments are given in Fig. 5 as a function of the oxygen pressure and the D-gluconate concentration. It is striking that the deactivation constant de- pends on both parameters and decreases using a lower oxygen pressure or a higher D-gluconate concentration. From this result the assumption arises that the coverage of the platinum surface with oxygen is a pre- dominant factor for the deactivation pro- cess. Lowering the oxygen pressure and increasing the D-gluconate concentration actually both decrease the part of the plati- num surface which is covered by oxygen. This is also supported by the results in Fig. 4. Although the pH of this solution is of great importance for the rate of reaction (20) there is no relation between the pH applied and the deactivation constant. This confirms our assumption because the pH is only of minor influence on the part of the platinum surface covered by oxygen.
In the case of a deactivated catalyst the original rate of reaction may be restored by interruption of the oxygen supply to the
reactor or stopping the stirrer for a few minutes (the rate of deactivation of the catalyst after resuming the experiment, however, is higher (24)!). This reactivation is accomplished by reducing compounds in the reaction mixture which reactivate the deactivated platinum sites at the catalyst surface. Using a similar procedure, regen- eration of a platinum catalyst was achieved in the case of the oxidation of other com- pounds such as ethylene glycol (3, 4), am- monia (2), and sugar acids (8-10). This regeneration process at the platinum sur- face will also occur during normal oxidation experiments. However, deactivation then dominates and the net result is a gradual decrease in the catalyst activity.
Equation (1) can also be derived starting from the three elementary processes that take place on the catalyst surface, namely, an oxidation, a deactivation, and a regener- ation. Because of these general starting points the proposed model does not have to refer only to the oxidation of D-gluconate. It may possibly also hold for other oxida- tion processes using precious metal cata- lysts under comparable reaction condi- tions, during which (weak) reducing compounds are converted to their ac- cessory products. The three elementary steps will now be discussed in some detail.
The oxidation reaction. As reported by Heyns et al. (25,26) the reaction is initiated by an abstraction of a proton of a hydroxyl group of the sixth carbon atom in the chain by an OH- ion, yielding water. After this step a hydride ion is transferred to the platinum surface giving the reaction inter- mediate, L-guluronate. An OH- ion is ob- tained by reaction of the hydride ion with an adsorbed oxygen atom. The reaction path of the consecutive reaction of L-gulu- ronate to D-glucarate proceeds in a similar way. It appears (27) that the rate of oxida- tion is proportional to the fractions of the platinum surface which are covered by oxy- gen, fO, and the organic reactant, f~. If the oxidation reaction occurs only at the part of the catalyst surface which is still not deac-
nC A P T n / A T T r i M H P DT AXTXTTTA4 P A T A T V Q T C 1 S *J
tivated, 1 - the rate of oxidation is the platinum surface at infinite time and described by equals, k d f 0 / ( k d f 0 + k r f A ) , */,o is the deac
tivated fraction of the platinum surface at / r„M) = A O X / A / O H - Xi(t)). (2) r\ _ 1 rs- * i i J ±: _ ^ „ A ^
The deactivation reaction. The deac tivation of the catalvst is caused by disso
u, anu A D is ine ueaciivauun cuiisictiu "') and equals ( k d f 0 + k r f A ) / S . Wolf and Petersen (28) derived a similar
ciative chemisorption of oxygen followed type of relation for a reaction with a self by penetration of oxygen atoms into the poisoning parallel reaction due to an irre-platinum lattice, the nature ot trie deac-tivation is described in more detail in Part 2 (?4\ Tt is verv likelv that the rate of the
versible interaction ot a reactant adsorbed on an active site. The introduction of a regeneration reaction as in our case.
deactivation reaction depends on the fraction of platinum sites at the surface which is
however, does not result in a greatly different expression.
not yet deactivated, 1 - Xi(t), and the The total rate of reaction is obtained by fraction of sites which is covered with the summation of Eqs. (2) and (4). Equation
reaction also contributes to the conversion of the organic reactant into products:
The regeneration reaction. The interaction between a reducing compound A and a deactivated site may result in a regeneration of this site. The rate of regeneration
} ib aisu ULUCUIICU uy LUC 1I1U1L1(J1IL;<111U11 U l
the initial rate of reaction, Rq, and Eq. (6). In this way a relation similar to Eq. (1) is
win proDauiy ucpcnu un uic 11 aiuuii ui platinum sites which are deactivated, Xj(t), and the fraction of the surface covered by
obtained. Thus the theoretical result fits the experimental results very well.
the reducin reactant f ' o u r a e a c t l v a t I o n e x P e n _
e re u ing r , A - merits the catalyst deactivates faster than nredicted bv the theoretical model. Khan et
As a first approach it is assumed that no a/, (3) obtained curves similar to those in Fig. 4 when oxidizing ethylene glycol in an
dilierence exists between tne adsorption equilibria of a reactant on active or deactivated nlatimim sites. As the reaction is
aqueous slurry or a Pt/C catalyst. Sarkany and Gonzalez (29) observed this phenomen o n w/hf^n r\YiHi*7ino f ^ O cit Inw t n m n p n .
performed under steady-state conditions (i.e., a constant composition of the reaction
tures and explained it by a rapid adsorption of unreactive oxygen on Pt sites of low
mixture) f A a n d / 0 will remain constant in surface coordination. Until now, however, time. no satisfactory evidence was available for
The* nhnnof* n f thf* a m n n n t rvf sirtivf* nlat i - ±ki f\ c i i nrt *^ • * r\
num sites per unit of time is given by the difference of the rates of the deactivation
The rates of reaction attained are rather low. When D-glucose was oxidized, higher
r e a c t i o n a n d the r e g e n e r a t i o n r e a c t i o n : r a t e s o f r e a c t i o n w e r e o b t a i n e d w i t h r e s p e c t
r - J V t o t he o x i d a t i o n r a t e w h e n s o d i u m - D - g l u -^ a x i _ / . . . x sc\ . . . . . .
Substitution of Eqs. (3) and (4) in (5) results
conaie was oxiaizea unaer me same reaction conditions. With regard to the molecular structure both compounds are rather
after integration in
x;(t) = x / x 4- U / n - x i x ) exp(-AW) (6)
similar. Accordingly, limitation of the rate of reaction by mass transfer of either reac-tan t , D - g i u c o n a t e o r o x y g e n , i r o m tne
i n w h i c h JC,> i s t he d e a c t i v a t e d f r a c t i o n o f a q u e o u s p h a s e t o the c a t a l y s t s u r f a c e i s n o t
DEACTIVATION OF PLATINUM CATALYSTS, 1 333
tivated, 1 - xi(t), the rate of oxidation is described by
The deactivation reaction. The deac- tivation of the catalyst is caused by disso- ciative chemisorption of oxygen followed by penetration of oxygen atoms into the platinum lattice. The nature of the deac- tivation is described in more detail in Part 2 (24). It is very likely that the rate of the deactivation reaction depends on the frac- tion of platinum sites at the surface which is not yet deactivated, 1 - xi(t), and the fraction of sites which is covered with oxygen, h :
rd(t) = kd.h(l - .x,(f)). (3)
The regeneration reaction. The interac- tion between a reducing compound A and a deactivated site may result in a regenera- tion of this site. The rate of regeneration will probably depend on the fraction of platinum sites which are deactivated, x;(t), and the fraction of the surface covered by the reducing reactant, f~ :
r&l = kfAXi(t)* (4)
As a first approach it is assumed that no difference exists between the adsorption equilibria of a reactant on active or deac- tivated platinum sites. As the reaction is performed under steady-state conditions (i.e., a constant composition of the reaction mixture) fA and f. will remain constant in time.
The change of the amount of active plati- num sites per unit of time is given by the difference of the rates of the deactivation reaction and the regeneration reaction:
(5)
Substitution of Eqs. (3) and (4) in (5) results after integration in
x;(t) = x,,, + (x,,,, - xiJ exp(-Ki,t) (6)
in which x;,~ is the deactivated fraction of
the platinum surface at infinite time and equals, k&/(k& + krfA), Xi,0 is the deac- tivated fraction of the platinum surface at t = 0, and Kn is the deactivation constant (s-l) and equals (k&o + krfA)/S.
Wolf and Petersen (28) derived a similar type of relation for a reaction with a self- poisoning parallel reaction due to an irre- versible interaction of a reactant adsorbed on an active site. The introduction of a regeneration reaction as in our case, however, does not result in a greatly differ- ent expression.
The total rate of reaction is obtained by the summation of Eqs. (2) and (4). Equation (4) is included because the regeneration reaction also contributes to the conversion of the organic reactant into products:
R(t) = r,,(t) + r,(t). (7)
R(t) is also obtained by the multiplication of the initial rate of reaction, Ro, and Eq. (6). In this way a relation similar to Eq. (1) is obtained. Thus the theoretical result fits the experimental results very well.
At the start of our deactivation experi- ments the catalyst deactivates faster than predicted by the theoretical model. Khan et al. (3) obtained curves similar to those in Fig. 4 when oxidizing ethylene glycol in an aqueous slurry of a PtiC catalyst. Sarkany and Gonzalez (29) observed this phenome- non when oxidizing CO at low tempera- tures and explained it by a rapid adsorption of unreactive oxygen on Pt sites of low surface coordination. Until now, however, no satisfactory evidence was available for this assumption.
The rates of reaction attained are rather low. When o-glucose was oxidized, higher rates of reaction were obtained with respect to the oxidation rate when sodium-D-glu- conate was oxidized under the same reac- tion conditions. With regard to the molecu- lar structure both compounds are rather similar. Accordingly, limitation of the rate of reaction by mass transfer of either reac- tant, D-gluconate or oxygen, from the aqueous phase to the catalyst surface is not
I'XA mnicii> A A TH F T A T
very likely. As regards the equilibrium con-dition for adsorption, it is very likely that the Langmuir theory is applicable. CA
i n e ucacuvciiiuii uuiisuuu can nuvv uc written as O 4
S(l + (KnCn.)05 + KACA + ZKXCX) n
(8) 40 50 60
T ( ° C >
The oxygen concentration in the slurry is proportional to the partial pressure of oxy-
FIG. 7. Deactivation constant as a iunction or temperature (P0l = 1 bar, C G 0 Z = 0.5 mole/liter).
gen m trie reaction mixture ^Co2 may oe obtained by multiplication of Po2 with the Henrv coefficient which is determined as a
£d(^o,C 0 ,) 0 , 5 . From Eqs. (3) and (4) it then follows that the regeneration reaction is
function of the concentration of several compounds). Hence the deactivation con-
much less signmcant than the deactivation reaction. All together Kb may be simplified to
stant is proportional to (Po2) as long as it holds that
0.5
c /1 _i_ ir r< (10)
(K0lCof -5 < 1 + KACA + ZKXCX. (9) From Eq. (9) it follows that the fraction of
According 10 tne resuus in rig. d 11 was found that a linear relationship exists between and CPn.)05. Aooarentlv the con-
uie piaimiun suiiciL-c uuvcicu uy uAygcn atoms must be rather low. This means that in view of the high rates of deactivation of
dition as given by Eq. (9) is fulfilled. In Fig. 5 the interception of all lines with the KD
the catalyst observed during our experiments, the reaction rate constant k& in Eq.
axis is located m the origin, lhis means that (3) is quite mgn. the term ^ / A C A in the numerator of Eq. (8) In Fig. 6 the deactivation constant is ic vprv Qmall rnmnarprl tn the other term nlotted as a function of the sodium-D-elu-
conate concentration (all these experiments were performed using a reaction mixture saturated with oxygen at 1 bar). I he path 01 the curve corresponds with the general formula oivf»n hv P n (\(W Thp Hftar-
tivation constant at a zero concentration of D-gluconate in Fig. 6 was obtained by batch experiments as described under Experimental.
1,0 2.0 r fmolR r 1 l
constant appears to decrease linearly with increasing temperature (all experiments
wGOZ
FIG. 6. Deactivation constant as a function of so-
were performed using a reaction mixture saturated with oxygen at 1 bar). Ostermaier s>t n / ctnH\/ino tin* 1rv\x/_t**fTmf»ratiirf*
IV • T V V 111 i-* %r A. U t U l ^
334 DIJKGRAAF ET AL.
very likely. As regards the equilibrium con- dition for adsorption, it is very likely that the Langmuir theory is applicable.
The deactivation constant can now be written as
K D
= kti + krfi S
kd(Ko,Co2)0~5 + k&CA
= S(l + (Ko,Co,)O.’ + KACA + XK,G)
(8)
The oxygen concentration in the slurry is proportional to the partial pressure of oxy- gen in the reaction mixture (Co, may be obtained by multiplication of Pq with the Henry coefficient which is determined as a function of the concentration of several compounds). Hence the deactivation con- stant is proportional to (PO,)‘.’ as long as it holds that
(Ko,CO,)~.~ + 1 + KACA + XK,C,. (9)
According to the results in Fig. 5 it was found that a linear relationship exists be- tween Kn and (PO,)‘.‘. Apparently the con- dition as given by Eq. (9) is fulfilled. In Fig. 5 the interception of all lines with the Kn axis is located in the origin. This means that the term krfACA in the numerator of Eq. (8) is very small compared to the other term
0 1.0 2.0
cGoz (mole I-‘)
FIG. 6. Deactivation constant as a function of so- dium-D-ghconate concentration (Ps = 1 bar).
6.
‘; ”
% .-
2 4
2
oL----- 40 50 60
T(“C)
FIG. 7. Deactivation constant as a function of tem- perature (PO2 = 1 bar, Co,, = 0.5 mole/liter).
kd(KO,CO,)o~S. From Eqs. (3) and (4) it then follows that the regeneration reaction is much less significant than the deactivation reaction. All together KD may be simplified to
h(Ko co >O” KD =
S(1 + KACy +’ XK,C,)’ (10)
From Eq. (9) it follows that the fraction of the platinum surface covered by oxygen atoms must be rather low. This means that in view of the high rates of deactivation of the catalyst observed during our experi- ments, the reaction rate constant kd in Eq. (3) is quite high.
In Fig. 6 the deactivation constant is plotted as a function of the sodium-D-glu- conate concentration (all these experiments were performed using a reaction mixture saturated with oxygen at 1 bar). The path of the curve corresponds with the general formula as given by Eq. (10). The deac- tivation constant at a zero concentration of D-gluconate in Fig. 6 was obtained by batch experiments as described under Experi- mental.
As illustrated in Fig. 7, the deactivation constant appears to decrease linearly with increasing temperature (all experiments were performed using a reaction mixture saturated with oxygen at 1 bar). Ostermaier et al. (2), studying the low-temperature
n F A P T T V A T T O N OF PT ATTNTTTM P AT AT V S T S 1 11*
oxidation of N H 3 , also noted a decreasing A C K N O W L E D G M E N T S
extent of deactivation with increasing tem- T he authors gratefully acknowledge financial sup-perature. They ascribed this effect to an port from the Dutch Foundation for Applied Techno-
of N H 3 at elevated temperatures. It is difficult, however, to predict in the light of Eq.
I V g l V U l 1WOVU1 V U . M. * T T • / X \ J X U l l O L^l V J V V L ^ *. X i -V «/ X / -
The authors also thank Degussa for supplying the catalysts.
(10) what kind of relation exists between the deactivation constant and the tempera-mre. A H rates or reaction ana adsorption equilibria presumably depend on the temperature to a different extent, while it is
/. " Kirk-Othmer Encyclopedia of Chemical Technology," 3rd ed. Wiley, New York, 1978.
impossible to determine the influence of the temperature on all rate and adsorption con-
i. ustermaier, j . j . , Katzer, j . K., ana Manogue, w. H . , 7 . Catai 41, 277 (1976).
3. Khan, M. I. A . , Miwa, Y . , Morita, S., and Okada, stants. J., Chem. Fharm. Bull. 31, 1141 (1983).
Khan, M. I. A . , Miwa, Y . , Morita, S., and Okada, J., Chem. Pharm. Bull 31, 1827 (1983).
APPENDIX: N O M E N C L A T U R E
rate of reaction (mol g
5. Morozov, L. G. , and Druz, V. A . , Kinet. Katal. 21, 1071 (1980).
6. Nagal, M . , and Gonzalez, R. D. , Ind. Eng. Chem. initial rate of reaction (mol g
r o t a n f ror»r*tiAti o f 11 rv* c%
Prod. Res. Dev. 24, 525 (1985). 7. Franklin, T. C , and Miyakoshi, Y . , Swrf. Tech-
nol. 5, 119 (1977).
(mol g"1 s"1) KD deactivation constant (s_1)
8. Dirkx, J. M. H . , and van der Baan, H . S., J Catal. 67, 1 (1981).
9. Dirkx, J. M. H . , and van der Baan, H. S., J time (s) rate of oxidation reaction
Catal. 67, 14 (1981). 10. Dirkx, J. M. H . , van der Baan, H. S., and van den
Broek, J. M. A. J. J., Carbohydr. Res. 59, 63
rate constant of oxidation reaction (mol e"1 s - 1)
(1977). Alper, E . , Wichtendahl, B . , and Deckwer, W. D., Chem. Ens. Commun. 10. 369 (1981).
1 . . . . . . /.?. Amirna7mi. A and Rnnrlart M 1 C.ntnl V)_ /cd rate constant or tne deac-
tiyation reaction (mol g 1
/ 4 Neth. Appl. Patent, N L 7,106,590 (1970) to Hoff-
rate of regeneration reaction 15. U.S. Patent, US 4,190, 605 (1980) to Muench, w (mol g" 1 S~ !) C., Strand, G. O., and Hormel, T. S.
rate constant of the regeneration reaction (mol g 1 s 1 )
tntal amnnnt nf nlatirmm citpc
Inorg. Nucl. Chem. 38, 889 (1976). 17. Velasco, J. G. , Allyon, S., and Sancho, J
J AT 7 *t 1 A T C / 1 (\1C\\
per gram of catalyst (mol -•)
inufg. lyutt. Ksiivm. f i , I U / J \ i y / y j . 18. Wilham, C. A . , and Mehltretter, C. L . , J. Amer.
Oil Chem. Soc. 48, 682 (1971). j n M « * L A 1 r * „ 4- A. X T i -f ^ t c i o n /1 r \ i a \ *. _
the inactive fraction of the platinum sites
o t i n i i m o i l * *
Heesen, J. G. 20. Dijkgraaf, P. J. M . , Verkuylen, M. E. C. G. , and
face covered by compound * i
van aer wieie, K. , Carbohydr. Kes. lbJ, 12/ (1987).
27. U.S. Patent, US 3,407,220 (1968) to Shell Oil Co.,
Ko2, KA, Kx adsorption constants Po2 partial oxygen pressure in the
rsew I OFK. 22. German Patent, DE 2836327 (1980) to Fiege, H . ,
and Wademeyer, K. LUjKgraar, v. j . M . , vernaar, L. A . , in . , Lrroen-
DEACTIVATION OF PLATINUM CATALYSTS, 1 335
oxidation of NH3, also noted a decreasing ACKNOWLEDGMENTS
extent of deactivation with increasing tem- The authors gratefully acknowledge financial sup- perature. They ascribed this effect to an port from the Dutch Foundation for Applied Techno- increasing reduction (regeneration) ability logical Research (S.T.W.) for-this project (1 l-20-318).
of NH3 at elevated temperatures. It is diffi- The authors also thank Degussa for supplying the
cult, however, to predict in the light of Eq. catalysts.
(10) what kind of relation exists between the deactivation constant and the tempera- ture. All rates of reaction and adsorption equilibria presumably depend on the tern- ‘. perature to a different extent, while it is impossible to determine the influence of the
2,
temperature on all rate and adsorption con- 3. stants.
R Ro
R,
KD
t
rox
k ox
rd
kd
APPENDIX: NOMENCLATURE
rate of reaction (mol g-’ s-9 initial rate of reaction (mol g-l
SF’) rate of reaction at infinite time
(mol g-’ SC’) deactivation constant (s-l) time (s) rate of oxidation reaction
(mol g-’ s-l) rate constant of oxidation re-
action (mol g-’ SC’) rate of deactivation reaction
(mol g-’ ss’) rate constant of the deac-
tivation reaction (mol g-’ s-l)
rate of regeneration reaction (mol g-’ SC’)
rate constant of the regenera- tion reaction (mol g-’ ss’)
total amount of platinum sites per gram of catalyst (mol s-9
the inactive fraction of the platinum sites
fraction of the platinum sur- face covered by compound
4.
5.
6.
7.
8.
9.
10.
II.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
Ko,, KA, K, adsorption constants PO, partial oxygen pressure in the
22.
slurry 23.
ERENCES
“Kirk-0thmer Encyclopedia of Chemical Tech- nology,” 3rd ed. Wiley, New York, 1978. Ostermaier, J. J., Katzer, J. R., and Manogue, W. H., J. Catal. 41, 277 (1976). Khan, M. I. A., Miwa, Y., Morita, S., and Okada, J., Chem. Pharm. Bulk 31, 1141 (1983). Khan, M. I. A., Miwa, Y ., Morita, S., and Okada, J., Chem. Pharm. Bull. 31, 1827 (1983). Morozov, L. G., and Druz, V. A., Kinet. Katal. 21, 1071 (1980). Nagal, M., and Gonzalez, R. D., Ind. Eng. Chem. Prod. Res. Dev. 24, 525 (1985). Franklin, T. C., and Miyakoshi, Y., Surf. Tech- nol. 5, 119 (1977). Dirkx, J. M. H., and van der Baan, H. S., J. Catnl. 67, 1 (1981). Dirkx, J. M. H., and van der Baan, H. S., J. Catnl. 67, 14 (1981). Dirkx, J. M. H., van der Baan, H. S., and van den Broek, J. M. A. J. J., Carbohydr. Res. 59, 63 (1977). Alper, E., Wichtendahl, B., and Deckwer, W. D., Chem. Eng. Commun. 10, 369 (1981). Tsukamoto, T., Morita, S., and Okada, J., Chem. Pharm. Bull. 28, 2188 (1980). Amirnazmi, A., and Boudart, M., J. Cutnl. 39,383
(5 eth. Patent, NL 7,106,590 (1970) to Hoff- man-La
.Patent,
US 4,190, 605 (1980) to Muench, W. C., Strand, G. O., and Hormel, T. S. Velasco, J. G., Ortega, J., and Sancho, J., J. Inorg. Nucl. Chem. 38, 889 (1976). Velasco, J. G., Allyon, S., and Sancho, J., J. Inorg. Nucl. Chem. 41, 1075 (1979). Wilham, C. A., and Mehltretter, C. L., J. Amer. Oil Chem. Sot. 48, 682 (1971). Neth. Appl. Patent, NL 7,215,180 (1974) to Heesen, J. G. Dijkgraaf, P. J. M., Verkuylen, M. E. C. G., and van der Wiele, K., Carbohydr. Res. 163, 127 (1987). U.S. Patent, US 3,407,220 (1968) to Shell Oil Co., New York. German Patent, DE 2836327 (1980) to Fiege, H., and Wademeyer, K. Dijkgraaf, P. J. M., Verhaar, L. A., Th., Groen-
nUVni? A AT7 r?T AT
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24. Dijkgraaf, P. J. M . , Duisters, H. A. M . , Kuster, B. F. M . , and van der Wiele ; K . , J. Catal. 112, 337
J., Fortschr. Chem. Forsch. 11, 285 (1969). 27. Dijkgraaf, P. J. M . , submitted for publication. 28. Wolf, E. E . , and Petersen, E. E . , J. Catal. 47, 28
(1988). 25. Heyns, K . , and Paulsen, H . , Adv. Carbohydr.
Chem. 17, 169 (1962).
(1977). 29. Sarkany, J., and Gonzalez, R. D., Appl. Catal. 5,
85 (1983).
336 DIJKGRAAF ET AL.
land, W. P. T., and van der Wiele, K., J. Chroma- 26. Heyns, K., Paulsen, H., Rudiger, G., and Weyer, togr. 329, 371 (1985). J., Fortschr. Chem. Forsch. 11, 285 (1969).
24. Dijkgraaf, P. J. M., Duisters, H. A. M., Kuster, B. 27. Dijkgraaf, P. J. M., submitted for publication. F. M., and van der Wiele. K., J. Caral. 112, 337 28. Wolf, E. E., and Petersen, E. E., J. Catal. 47,28 (1988). (1977).
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