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Deactivation of Platinum Catalysts by Oxygen Deactivation ... · PDF fileJOURNAL OF CATALYSIS...

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  • JOURNAL OF CATALYSIS 112, 329-336.(1988)

    Deactivation of Platinum Catalysts by Oxygen

    1. Kinetics of the Catalyst Deactivation Consultan Inc.ts, Ali TranTech Amirnazmi,

    E

    Received September 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-gluconate to D-glucarate at 50C. 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 results very well. 1988 Academic Press, inc.

    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 oc-curs during oxidation processes in the liq-

    ties in the feed, and deposition of carbona-ceous 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 impor-tance in the case of the application of noble m j=tQl r"sitc1\/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 pub-lished (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 oc-curs due to the presence of oxygen. Oster-

    iormea oy c-

  • 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 con-trolled 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 acti-vated 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 prefer-able 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 appara-tus 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 iu 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.

  • FF 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 50C. a D H of 9. and catalyst concentration of 10 kg/m 3 unless mentioned otherwise