Catalytic effects on ZrO2 oxygen sensors exposed to non-equilibrium gas mixtures

Eiigineeri& and Research-StufJ, Ford koror

Conip&y.

i~81;‘ipxvised f&i&h ..... :.. -_

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Michigun 481Fl.. (U.S.A.) 1981)

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We. deport some unusual steady-state. charatiteristics.of. ZrOz oxygen sensors : exposed’to ~ga&m&urks that are not in che&cal e&ilib&m. z we..ZrO; oxy&tiLsensor [-l].:iS ,a.-concentration cell where a solid oxygen-ion conduc.ting.el&rdiyte, -YiOj-doped $!rO,; separates (a)’ -a referende ‘gas of known oxygen~‘+tial-~pr&&re PO,- .froti -co) ’ a sampie &s df ~~unk;lotin-oxygen partiil p+sure .Pnz_ -The : potential .-difference, AI?; ~acr&S’:the .electroljrte is related to (PO;-/P&$ by.the Net=& equatidn ’

For, the’@resent’purposesit should be emphasized that PO2.and PO; refer to oxygen ~aitkl pre&ures at the solid.electrolyte/gas.interfacesj i.e.; oxygen partial pressures within surface boundary layers. These may or may not be identical to oxygen partial ‘- : .: ‘.pressures m .the.bulk’gases.. ‘In many ap$icatiens it is-cdmmon practice to equilibrate sample gases chemically before sen&r. detection,:This. is,c&veniently .dOne-by. passing the gases.over a high ‘activity catalyst. Gas pre-equilibratidn is sometimes.impractical when high flow rates are involved or. when rapid-response times are. required. One example .of this occurs in .en&ne feedback Confrol on’autemobiles; ‘here ZrO; sensors tie. used to. monitor kygk conce&iations in engine exhaust g&s& that are -not in’ chkitiai equilibrium -: .;. -:_ : ...~ ..- .. .,. -, [2;31_,. -~ ,-..::_-;,.. -_. ] -:, ; ..: _-.- ” A’ schem&ec drawink of our flow’ system,is shown in Fig. 1. Capillaries ;A and C -par&al 1insure’ ..thSt.* gases: introduce;i - tii. the.%: points-. are -maint&ed at ~c6nstzrnt

p&Sure kid&$&dent &he_ addition bf k&h&& thrqugh a..Math&tin Se&s &&IO at B. :Fl& raks t&o&&_ ~h~:&ntr~llkr w&e e&&ted tti. g& ~~@&&;kssuies’:I tiiia.-~brati6n{~~.flthl :a.:.Validy& .‘absoh&:-pressure gauge::. The .~t~e$&ink .~+a& m’$$fied. so’,-tik..exte&i: &$t~&l~&,nals Icould. be used to -s@zcify- th&“p&r&l, &ssure;of : the:-gas -‘z&l&~‘at: B; ,IIJ. @actike, -we ‘used 5 voltage :-&ikss Flow’ &&oll+<.

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Pa/s). No significant differences in sensor output were found between experiments performed at this sweep rate and at slower sweep rates. A constant N, partial pressure of 0.033 atm (3.3 kPa) and an O2 partial pressure of 0.0012 atm (0.12 kPa), applied via A and C, formed the baseline for our experiments. The total volume flow past the sensor was 13 L/min. The ZrO? sensor consisted of inner and outer platinum electrodes attached to opposite sides of a closed 8% Y,03-doped ZrOz tube. The outer electrode, in contact with the sample gas, had a geometric surface area of approx. 0.5 cm’. The inner reference electrode was exposed to air at atmospheric pressure. The sensor was housed in a tube furnace at 75OOC. Temperature measurements were made with a chromel-alumel thermocouple that was set in place of the sensor before and after runs. The potential difference across the sensor was measured with an high-impedence circuit and converted to oxygen partial pressure via eqn. (1) using a analog anti-log circuit. In some experiments, gas mixtures were equilibrated chemicahy before sensor detection by passing them over a platinum foil catalyst at 800°C. This catalyst had a geometric surface area of 30 cm’. Figure2 compares sensor characteristics obtained with addition of each of fourgases to a 3.6% 0, -96.4% Nz mixture. Curve (1) demonstrates linearity between the PO2 derived from the sensor voltage and the partial pressure of oxygen added via the mass flow controller. Curve (2) affirms that the ZrO, cell is sensitive to oxygen partial pressure rather than total pressure; (i.e. addition of a quasi-inert gas, nitrogen, has no effect on detected PO; levels). Curve (2) also represents sensor response as a function of added COz_ Thus demonstrates that the sensor is inseilsitive to CO, partiai pressure. Curve (3) shows data when n-butane is added{and the OJC,H,, mixture is equilibrated over the platinum foil catalyst before detection. This curve represents expected behavior when the initial 0, partial p&sure is.. reduced by an irreversible reaction going to completion. In- our experimental apparatus, an irreversible reaction of the form C,,H,,

+ { (4~2 + m)/4}02

- n CO, + ( m/2)H20

exhibits a (d PO2/d PCeHm,)slope of - ((4n + nz)/4}

(2) and an intercept

on the-abscissa

PARTIAL

PRESSURE

OF AOOED

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Fig. 2. Oxygen partial pressure, derived from the ZrO, sensor voltage, plotted against the gas partial pressure added via the mass fIow controller. Curve (1): oxygen; curve (2): nitrogen or carbon dioxide: curve (3): butane with &ui!ibration over the platinum catalyst; curve (4): butane, without equilibration.

where PC H is {4/(4~ + nz)} times the original oxygen partial pressure. The experimeitar (d PO, ./dPc,& s!ope is -6.5 reflecting the stoichiometry qf the C,H,, + 6.5 0, reaction. Curve (4) shows_dattifor butane addition wiihout pre-equilibration ovei~ die platinum foil .ctitalyst. It illustrates expected behavior when the sensor/electrode is an inefficient oxidation catalyst; When the surface reaction does not go to completion, detecied PO; levels lie between bounds corresponding to (a) no reaction (curve 2) and (b) compleie reaction (cuive 3). Figure 3 dompares -sensor output when hydrogen and carbon- monoxide. are individ-ually added..to ‘a 3.6% O,-96.4% .N, -mixture: Data were taken with and without tire-equilibra@i over. the platinum catalyst. As in the butane study, distinction b&een experiment%done .with Bnd.tiithout pre-equilibration lies in the chemical composition of the-:bulk. -g&s flowing past the sensor. In the present ex@erime&u&z of the caialyst -pr&iuces a bul,k gas .con&ining nitrogen together with equilib&m amo&n+ of -water atid residual ‘oxygen, 0; r&r&n together with equilibrium :&n&ii% ~f;.tiarb&i :dio$de ancl &dual oxjrgen;’ Without the cat&lys& the ‘bulk-_&1con&ed_ i&&i&,- hydrcig& ,&d- &cy&n,~&.nitrogen, :carbo.n.mono.. .-.. i.. .;- ; .” xide.an-d’6~~g~~~_~.~___; :_:.: -. ‘_.! ~. _::::- _:.‘__;.:.: ;.I. ’ When ~~&&ilibi~~i~n :w& -u&d, @rv& :i i) --and,(2)
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Fig. 3. Oxygen partial prt~~ure derived from ZrO, sensor. Curves (I) and (2) correspond to added H, and CO with gas equilibration; CUNCS (3) and (4) were obtained with added Ha and CO without equilibration.

they indicate that more oxygen is being consumed than the amount calculated for stoichiometry; curves (3) and (4) lie below the bounds for (a) no reaction and (b) complete reaction. The experimental slope of curve (3) (for 0z/H2) is -2.0; that of curve (4) (for 02/CO) is - 1.2. Both slopes are substantially greater in magnitude than the -0.5 slope predicted by stoichiometry for each system. Curve (3) of Fig. 3 (for 0,/H,) can be explained in terms of a diffusion limited surface reaction_ We write two coupled kinetic equations linking boundary layer partial pressures of oxygen (P,J and hydrogen (Pn,) to corresponding partial pressures in the bulk gas, P,$bulk) and P,z(bulk): d Po_/dr

= -ko,(

PO, - Po2(bulk)}

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- P,JbuW}

- R

(3)

- 2R

(4)

where R is the rate of the irreversible reaction, and k,_ and kH, are diffusion exchange coefficients of oxygen and hydrogen between- the bulk gas and. the boundary layer adjacent to the sensor/electrode surface. Under steady-state conditions R can be eliminated from eqns. (3) and (4) to give PO> = Po+lk)

- O-5( ktrL/kol)

(P&bulk]

- Pu,)

(5)

When R is much larger than the bulk gas/boundary layer diffusiorrexchange rates; P,_ is effectively zero in the presence of excess 0,. This condition is satisfied for the O$‘H, reaction on a platinum sensor/electrode surface. Kinetic .gas, theory [4] predicts that diffusion coefficients vary as M -‘/2, where M,is the gas molai &a&_ This implies (kHl/ko,)=4 and leads to a slope [dPoZ/dP~~(bulk)]=.-_2 in. .,- . . agreement with experiment.: _ ..

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&qjq&& ~~&~~o,&.$u@&,& 1 b$H; +$ ;&/rj2’.&~ mix&r& [5;6]; p’l+& .~ :’ _caily~~M&&ff&t$ %s&rom thd’high .diffusivit~~oflHi (and. &).relative- to O,;This 1 .-:..perriiics:~Llk._~z:~o;:~~pi~ni~~lIngsurface boundary layer faster thanbulk 0, as ttie_ ~e~ct&&&@eedsi .The riet cffe&is ;dispJac&&nt of oxyg.en from the boundary layer -an+: lo&e&g the oxygen. @&i&l $essu?e d&e&id ‘by: ihe 21-0~ sensor below -levels correspondjng.tii._s~~,Ichiometj;.’. 1’ ; j:_i .. : __. ’ .. ;. .y piffusion .effects’ -cannot account for either. the O,‘@& the..Oz &Hi0 results. obtained without gas equihbrationover the p]atinum catalyst. Theoretical ( kco/&oL) :~d~(k,,,b/kpz),~~iios_~re ~lJ-.an~ pi74 respectively. A,rguments _analogous‘to-those. used inobtaimng eqns; (3)~(5) predict:(a) a d P,,‘/d~,,(hulk) slope of ,.-_4_8. These-predictions deviate. substantially, from. the experiment+ .va_luesof. roughly - 1.2 and LO.23 respectively. Note that these c&ulated’.sloIjes- a&me surface reaction’ rates,. R, that are much faster than diffusion exchange :mtes. This &sump&on is invalid for butane oxidation over platinum [7]. As:noted ear&&,- catalytic inefficiency provides a reasonable exptanationof. our O,/CiH,, results.. The 02/C0 system probably requires more systematic study. It is possible that the present d&a,. obtained without pre-equilibration, reflect a situation where the rates of -the surface .O, /CO reaction is slower than the. rate of diffusion exchange between the-bulk gas and the boundary layer. This would follow from the secondorder ;depende_nceofthe overall reaction rate on CO partial pressure. Another paperfrom this laboratory has shown that concentration cell vohage may become sensitive to the relative rates. of surface adsorption, desorption and reaction under -these circumstances [6],.rather than to-the boundary layer partial pressure of oxygen-alone. Results similar to those seen experimentally. with the O,/CO system have been obtained in modeling studies with judicious choice of surface kinetic parameters. Any analogy bletween the modeling results and experimental values is speculative at this time. REF-ERENCES I T.k Etseii and .S.N;.Flengas, Chem. Rev., 70 (1970) 339. 2 I&S; Eddy, !~_Trans’Vehicle Tech., 4 (1974) 125. 3 W.J; _FIemitig,~SAE T&s. fro; 77&%JO( 1978). SAE T&a& X& 800020 ( 1980). .4 -R .Jacksan, Tr&s~o~ in P?@us Catalysts. Ekevier. Amsterda& 1977. 5 .T. Takk&i;-K. !&ji tid I. Igarashi,-Ele+&hem; Sot. Ext. .Abstr.; 78. No. 74 (1978). .. 6 J.E Anaersqn ;i;ld Y.B; Grap& J:Electkchein. Sot., I28 (1981) 294. 7 C.F. C&, DE. Keeked I%&. Tri+m. i. C$alysis !9 ( 1970) 378.

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