ZnO Catalysts for Water-Gas Shift and Methanol Synthesis Based on Single Crystals

·J .w. \Yard (Editor). Cotolv»!» 1.91'.;

1988 Elsevier Science Publishers H. V.. Arnst erdam -

Printed in The Ner herlands

783

KODEL Cu/ZnO CATALYSTS fOR WATER-GAS SHIFT AND METHANOL SYNTHESIS BASED ON SINGLE CRYSTALS C.T. CAMPBELLChealstry Departaent. Indlana Unlverslty. BlooMlngton. IN

47405
ABSTRACT We have prepared aodel catalysts based on Cu
INTRODUCTION Mlxtures of Cu and Zn oXldes are well-accepted catalysts for the water-gas shift
~

H2 • C02) and Methanol synthesls <2H2 •

CO-~

CH30H)

reactlons. and have therefore been the subJect of Much study [1-13 and ref. therelnJ.

However. there reaalns a conslderable controversy over the nature of

the actlve sltes.

SOMe authors belleve that Metallic Cu serves as the actlve

surface specles 16-8J. whereas other authors argue equally convinclngly that ·Cu l•• specles assoclated wlth the ZnO lattlce (as substitut10nal or lnterstltlal lMpurlties) play the dOMlnant role [1-23.

The actlvity of Cu-Cr

oXlde catalysts also appears to correlate wlth the concentration of surface eu1> specles [14J.

With the question In Mlnd, we began an experl.ental study

of these reactlons and thelr klnetlcs over

Mode~

catalysts based on eu and

ZnO slngle crystals. whereby surfaces were prepared WhlCh were extreaely well-

• Alfred P. Sloan Fellow

784 ,nd hoaogeneous In 3 slngle specles of lnterest (e.g •• pure Cu

ch~r,c~erl=eo

pure

Sl~es, ~he

~nu

sltes, or (u

bondeo

s~o.s

dlrec~ly

klnetlc results we have obtalnco so tar In

are presented In detall el.ewhere suggesteo nuaerous theoretlcal

ThcGe

(15-1~~.

ZnO>.

~o

ihlS paper reVlewa

experlaental study, WhlCh

~hlS

results have

experl.en~sl

concernlng these catalysts, WhlCh we

ques~lons

have addressed wlth seal-eaplrlcal quantua chealcal calculatlons U.lng Cu, ZnO and Cu-substltuted ZnO cluaters.

These re.ult. wlll al.o be brlefly

dlscus.ed here, although the aa)Orlty of the results are to be found elsewhere (19-211. THE WATER-GAS SHIFT REACTION A cle.n Cu surface .hows speclflc


'C~lvlty

the low-converslon WGS r ••ctlon .t IS torr H20 .nd 200 torr CO, WhlCh lS very slallar to that reported for hlgh-surface area CullnO .nd Cu/ZnO/Alz03 catalysts (15).

No ad.orbed specles reaalned on the surface for

reactlon surface

analY~ls.

If

~he

pos~

surface was coapletely oXldlzed to CuO

prlor to reactlon, lt rapldly reduced ln a aatter of seconds to coapletely Cu under reactlon condltlons.

.e~,!llC

The .pparent

energy

aC~lvation

<17 kcal/aole> and reactlon orders with respect to HZO <0.75> and CO <0> pressures are also the saae a. expected at these condltlon., based on results for Cu/ZnO catalysts.

Ooplng the Cu ll11> surface with ZnOx cause.

no slgnlflcant lncrease In cat.lyst

aC~lvlty.

These results suggest

strongly that the actlve slte for the WGS resctlon lS .etaillc Cu. at lea.t at our pressure 3nd dls~oclatlve

condltlons.

~eaper.ture

The klnetics .uggest that

adsorptlon of H20 18 the rate-deter.lnlng step.

The cleavage

of 3n O-H bond durlng the dlssoclatlon of ad.orbed H20 requlre. an actlvatlon energy of ,27 kcal/.ole, and therefore proceeds very slowly. The coverages of the varlOUS sdsorbed specles under reactlon condltlons were estlaated based on kinetlc para••ter. derlved froa ultrah1gh vacuu.
desorptlon spectroscopy <105> .tudle. (IS).

All coverage. were

predlcted to be very low under our re.ctlon conditlons. fr ••ework of the fora.te aechanlsa (4,15), thlS 1apl1es

Wlthln the the co.bin.tlon

th.~

of .dsorbed CO and OH to produce .uri.ce for ••te 1. very rapld, once an ad.orbed hydrvxy1 spec1e.

1&

prOduced.

Thl' lS conslstent

w1~n

the

recen~

obaervetloll by vlbretlonel spectroscopy (22) that OH e and

CO~

read11y below rooa teaperature to prOduce toraate

on Rh.

Sulfur Pvlson1ng

~pec1.s

coab1ne

(l~)

The lllteraetlon of HZS wlth Cu<111> h•• been stud1es w1th and LEEO: and the effect. of sulfur

ad&~o.s

X~S,

upon tne Klnetlcs oi the

AES, ISS

aedlum-pressure

At 12v h.

H?~

·'''0

li·~::'~'rb"

t.I)o;

I f•.

10

('(7' x-r'T'f<;l'"

t~~~tl0tl

p~rt1311y

~v~J·

aol~cul~r.

~IJ{i11

J

have been .easured.

By 200 K.

50ae

of the kZS

o;t d15aOC1;)tea '.0 pro<:luce sultur adato_s bonded on top of

t.he CUI 111' suriace. packed

Shitt

w~ter'q~s

~CDcrpt10~

The surfac(' saturates .H rooa teaperature 11'1 a

stl ..ct.ure ( s ' 0.4j, cr.ar ..cterl.tlc of a nearly close,)verlayer. very alal)ar to layers 1n (111,-orlentea CuZS.

~ultur

Sulfur adatoa. are qUlte ataole tc

he~t1ng

11'1 vacuua or HZ. Wlth a reactl0n

probaOl1lty for reaoval per HZ collls1cn wlth the surtace of

at &3& K.

~i(,-9

The rate oi the forward water-gas sn1ft react10n (H?u • CO -+ HZ • COZ' decreases dneluly wltn auHul covelage as \l-2.b,·s>'

Thl& pOlson1ng 1.

attr1Duted to sterlc blocklng by the aulfur adatoa. of the altes requlred tor dlssoclat1ve wat.r adsorptlon. 1a saall <1-2 Cu atoas>.

Th~

The slte ens._ble r.qu1red for thla step

well-known aensltivlty of .hlft catalysts to

.ulfur pOlson1nq 15 dlscuS5ed 11'1 llght of theae results 1161. C.flua Proaotion (17) Th. klnel1cs of the forward water-gas Shlft reactlon have b.en aaasured on a Cs-dop.d Cu<111> single-cry.tal .urfac. at pressures near 40 torr. in air.

Th. Cs la doaed froa aqueous CsOH solutlon and drlad

The surface atructure. have been charact.rlzed by XPS. AES. LEED. and

ISS both befole and after r.achon.

The actlvlty of the Cu(lll> surface

increase. with increaslng Cs coverage until it r.ach.s a aaXlaua at eCs 0.13. where the rate 15 flfte.n tla•• that for C.-fr•• Cutll1>. coveragea. the rate decrea.e.. depth

~

At hlgh.r

The Ca-do.ed surface ia oXldlzed to CU20 to a

Aft.r brle! treataent t40a.) under re.cll0n condlt10n•• the

~201~.

surface 1& coapletely reduced lO aetallic Cutlll>. wlth a Ca overlayer showlng an O:C. atoalC ratlon of about one. pattern when

~C.

Thi. over layer d.aonatrat••• p<2x2> LEED

exceed. 0.13 twhere the rate a.xlalzes). WhlCh 1.

interpret.d in tera. of l.land. of .1'1 oXldlC or hydroxidlC Ca ov.rl.yer of local cover.ge eCs : 1/4 (17). for the optiaally-proaot.d surface. the reactlon ord.r wlth r ••pect to H20 pres.ure atill i. l.rg_

<~O.7S).

which lndic.t•• that the r.te-d.teralnlng

st.p reaalna the di.soci.tlve ad.orptl0n of water even 11'1 the pre.ence of C•• The .pparent activation en.rgy for the over.ll re.ctlon . Accordlng to the aod.l d••cribed above for the clean .urface. E.p p i. re.lly the diff.r.nc. between the activotlon energl_. for dl.SOClation
Eapp

= Edi ••

- Edes'

Sine. Edea for wat.r i • •xp.cted to lncr•••• ln the

pre.enc. of alkall 1171. the barrler EdiS. auat al.o incr.ase at l.a.t alightly.

The lS-fold lncr_a.e 11'1 the WGS rate i. th.r.fore (withln this

model> not due to

reduction In the energy barrler for

~

ttl18

reactlon. but

lnstead must be due to an lncrease ln the pre-exponentlal factor.

Th,S result

argues agalnst a promotlon mechanlsm Whereby the maln role of Cs lS to modlfy the

propertles of the Cu.

electr~

the Cs promoter

Instead, lt favors a mechanIsm whereoy

partlclpates ln H20 dlssoclat,on.

dlrec_~

There are several

posslbilltles that can be lmaglned for such a mechanlsm. especlally glven the fact that alkall metals are often observed to tacll1tate H20 dlssoclatlon on transtlon metal surfaces [23-251.

o -

°

H, "H ..

C"

I

I

I

,?' ,./. '}

H I

o I

), I.' / 1 , / / / /

One example pathway lS shown schematlcally

H,

0 - Cs I

I

/7///////////////

"'0 - Ca , ,

H I

+

0"""

COa

I

H ,

-(C02

+

1/2

C

'0 I

H,

oI -

Cs I

oI -

0I - Cs ,

-112 H2

Cs ,

IItl'l'l
Another posslbllity could involve dlrect CO attack of the OH group whlle lt lS bonded to Cs to produce a Cs-stabilized formate intermediate such as proposed by the Klier group [261.

The fact that the Klier group also sees promotlon of

the WGS rate upon Cs additlon to high-surface-area Cu/ZnO catalysts [261 further supports the viability of Cu(lll> as a very good model for practlca1 Cu/ZnO WGS catalysts. and it suggests that the promotion mechanism is unrelated to the ZnO phase. METHANOL SYNTHESIS [181 The growth of vapor-depoaited Cu over layers on the oxygen-terminated ZnO(OOOl> polar single-crystal surface was atudied with X-ray photoelectron spectroscopy (XPS>, Auger electron spectroscopy (AES>, He+ lon scattering spectroscopy and low energy electron diffraction (LEED> (181.

The

results are consistent wlth a model in which, at room temperature, the first monolayer spreads unlformly across the surface ln a p(lX1> atructure. With Cu atoms bonding to oxygen atoms.

The Cu(2p> XPS spectrum of this Cu monolayera

is consistent with known spectra for metaillc Cu or Cu 1+ compounds, but

787 lnconsistent wlth Cu 2+ compounds.

For Cu coverages ln excess of one

Monoleyers. the Cu segregetes into thick islends of (lll)-oriented Meteillc Cu. rotationally allgned wlth the substrate.

Increas1ng teMperature favors thicker

lslands. The ebove structures appeared to be steble under Methenol synthesls conditlons (SOO-600K. 2 atM. CO + H2 + C02), except fOl- the expected therMal agglomeratlon above one monolayer.

Interestlngly, a SSOK 02 treatment of the

above structures produced e Mlxed Cu 2+/Zn 2+ oXlde as eV1denced by XPS [181, similer to the calclned state of practlcel Cu/ZnO cetalysts [271.

Th,S oxide

15 reedily reduced 1n less then f,ve M1nutes under reection cond1t10ns to an over layer structure of the type produced siMply by annealling the freshly deposlted Cu.

Thus far, we can only set an upper limlt on the actlv1ty of

these Model catelysts for methanol productlon et 2 atM. total pressure (CO + H2 + C02) and 600K.

The turn-over frequency limit of <2X10 l 2 Molecules CM-2

s-l for these Model catalysts 1S not unexpected for these cond1t,ons, based on results frOM h1gh-surfece-aree Cu/ZnO end Cu/ZnO/A1203 catalysts [18J. The Methanol synthesis activ1ty of a clean and ZnOx-doped Cullll) surface were also measured et these conditIons, end ege1n we can only set en upper lIMit on the actlv1ty of these model catalysts at <2Xl012 Molecules cM-2 s-1 [181.

IWe are currently modifyIng our equ1pMent 1n order to allow

Meesurements of lower reection retes on these smell-surface-aree I~l

c. 2)

sa.ples at pressures up to ten atmospheres.) It 1S Interest1ng to note that no surface oxygen was observable on Cullll) after reectlon treet.ent at SOO-600K even when C02/CO ratIos 1n excess of 200 were used 1n the reactant feed.

Th,s highlIghts the low probabl11ty for

oxygen depos1tlon on Cu(lll) uS1ng C02 es the oxygen source.

FrOM separate

.easurements with pure C02 on clean Cul1ll), we can set an upper liMit of <10- 6 for the dlssocietion probabIlity for C02 Molecules per coll,s,on WIth Cullll) at SOaK [181.

In comparIson, the reduct10n of adsorbed oxygen on Cul111> by CO

and H2 is More probable [18,281.

Since the Cu(lll) surface May be considered

as representetlve of the metallIC Cu surfaces in Cu/ZnO catelysts, these results suggest that It is very unlIkely that SIgnificant concentrations of adsorbed oxygen exist on the Cu surfaces of Cu/ZnO catalysts under Methanol syntheSIS condltions. QUANTUM CHEMICAL (INDO) STUDIES [19-2lJ We have e.ployed seMl-emplrlcal quantUM cheMIcal celculatlons (INDO) to study the electronIc propertIes of Cu. ZnO.

CuO and CU20 clusters

1~26

atOMS)

and of Cu adsorbed on or substituted ,n these ZnO clusters, and of SMall

7R8

aolecul.s ed.orbed on Cu and ZnO clu.ter. 119-213.

The paraaeterlzation and

approxiaetion. of INDOIS and INoo/2 w.re ••••nti.lly adopted froa work by the Zern.r group 129-30).

(See 119-213 for detail. of the aethod•• )

The

geoa.trl•• of the clu.ter. w.re a.suaed to be that of lhe bulk: and .xperlaental adaorbate geoaetrle. were used when avallabl•• Table I .uaaarlze. r.sults 1201 of calculated atoalC charges on Cu and Zn lone wlthln cluater aodels of reference coapounds of accepted foraal oXldation statea 1+ and 2+.

TABLE 1.

Atoalc Charg•• Calculated by INDOIS ln Cluster "odels of Cu and Zn OXldes and Cu/ZnO Calculate Charge (e)

Specles Reference Specl's Cu Cu Zn Zn

ln (Cu20 37 - bulk in (Cu0314 - bulk ln IZnO) 13 - bulk ln (2n0313 - withln (0001) .urfac. plan.

0.15 0.37 0.44 0.53

I II II II

Unknown Specl" Cu substituted in 12nO)13 - bulk Cu .ub.tituted ln IZnO)13 • wlthln (0001) surface plane Cu ad.orbed on ZnO<00011 -three-fold hollow on IZnOl9 -a-top alt. on IZnO)13

0.42 0.48

().45 0.49

The 2+ coapounds show an atoalc charge of ··0.4e for both Cu and Zn, and the 1- coapound shows a charge ot roughly half thls &lze.

The calcu13tad

charge for Cu substltuted lnto Slolchloaetrlc ZnO clu&ters
Th1S lndicates that Cu substituted lllto ZnO

1&

1n the

2- oXldatlon at.at.e, rn agreeaent wlth nuaerous experiMent"l results (,321. The saae can be sald of Cu adsorbed on the oxygen-teraineted 2nO(0001) surface.

ThlS final concluslon

de5cr1bed above.

clear, but aay be related t.o 1201.

s~e.s

1n contrad1cllon to our XPS resu:tn

The reason for this apparent d1acrepency ~u<2pl

15

atlll not

XPS llneshape changes wlth aite

5y.~,·t,t

ThlS hyp?thesis 15 currently belnq axs.ined. The lnteractlona of CO, foraate, C02, H20, aethoxy and for.aldehyde wlth

Cu snd ZnO clusters have been studied With INDO

i

n an alteapt t.o under"t",nd

..etChanls," of chealsorpt1ve bcondlng for these ... ystetas. sod ln ord"r to h"ll'

t.h.·'

789 a881gn and interpret electronlc spectra for these surface speCles 119-21).

As

an exasple of the types of inforsatlon avallable in these calculatlons, we orlefly dl&CUSS here r ••ult. (19] for for sate cheslsorption on Cu and ZnO clusters WhlCh sodel the Cu, Cu
On all three surfaces, HCOO actr. as a net

electron acceptor and "-electron donor. roughly oalance one another.

~

On Cu. these two lnteractions

The Zn ato.s of ZnO
aee. to be better at --eleclron donell0n. and the for.ate specles on thlS .urface plcka up a sub.tantlal negatlve charge

<~-O.4e"

It appears that the

favorable coulo.blC lnteractlon between thlS negatlve charge and the posltlve end of the .urface dipole of thl. tranafer.

~

ZnO surface helps drive this charge

The co.plex angle-re.olved ultravlolet photoelectron .pectru.

recently .ea.ured by Llndner et al. (31] of for.ate on Cu<110> .atche. very well the orbltal energle. and ay••etrle. we have predicted wlth I"DO/S (191. and help. confir. our understandlng of the che.lsorptlve bond. ACKNOWLEDGEKENTS The author would like to acknowledge the Depart••nt of Energy. Office of 8eslc Energy SClences. Che.lcel SClence. Dlvlslon, end the Deport.ent of Energy through Korgantown Energy Technology Center for support of varlOUS aspects of thl. work.

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