The influence of electron-withdrawing substituents on the adsorption of CO on copper

Volume 120, number 6

THE

CHEMICAL PHYSICS LEl-l-ERS

INFLUENCE

OF ELECI-RON-WITHDRAWING

ON THE ADSORPTION L-H. DUBOIS zi T&T

and B.R

SUBSTITUENTS

OF CO ON COPPER ZEGARSKI

Bcll Luborarorres_ Murrq~ HI/I

RealLed

25 October 1985

NJ 07971 USA

10 May 1985 in linnl rorm 19 July 1985

The chcmlsorpuon of carbon monoxldr on Cu(100) m thL presence or clther mclhyl chlorlds or lormlc ncld results m nn incrsae m the C-O srrruzhmg rrequrncy LOahwe the gas-phase t AIM orld rl umullmeous mcreasr m Ihs hat or adsorptron or CO We explam thrs bchavmr quahmwrl~ III term, of the ample LIZHIS ncld/bsse charxter or holh thr surface And the adsorbale

1 Introduction The adsorptron of carbon monoxide on metals is perhaps the most extensrvely studied chemisorption system in surface science, yet important questions still remain. CO generally bonds wrth the carbon end down and with Its molecular axis oriented perpendicular to the surface In the sunplest picture, bonding 1s thought to take place by electron donatron from the filled 50 orbital of CO (localized pnmarily on carbon) into the unfilled portion of the metal orbitals (marnly sp bands for the case of copper) and by backdonation of metal d electrons into the empty 2n* orbital of the adsorbate The frequency of the carbon-oxygen stretchmg vibration can be used to monitor the extent of this electron transfer_ Increased population in the 2n* orbrtal caused by increased coordination results in a decrease in the C-O stretching vibration. By analogy with metal cluster carbonyl chemistry, the followmg general classrficatron scheme has been developed +o > 2000 cm-1 : atop bonded; 2000 cm-l > vco 2 1880 cm-l 2-fold bridge bonded, and vco > 1870 cm-l - multiply coordinated [l] Previous work has shown that the coadsorption of electron donatmg or withdrawing species can srgnificantly shift the observed C-O stretching frequency wrthout changing adsorbate geometry. This shift IS caused by the added or decreased electron density in the 2a* antrbondmg orbital. Examples of tlus include 0 009-2614/85/S (North-Holland

03.30 0 Elsevier Science Publishers B V. Physics Pubhshing Division)

the interaction of CO with carbon and oxygen on Rh(lll) [2] and with numerous hydrocarbons on Ni(l11) [3] Large negative shifts in the C-O stretchtng vlbration on Pt(ll1) (-75 cm-l) have also been observed rn the presence of potassium [4]. In many instances the increase or decrease in electron backdonation can be correlated with an increase or decrease m the heat of adsorptron (&J,,& of CO. For example, the heat of adsorption of CO on Pt( 111) can increase from =27 kcal/mole to 39 kcal/mole ti the presence of coadsorbed potassmm [4]. Although one expects a decrease m the surface-CO bond strength m the presence of electron withdrawmg substrtuents, this is not always the case Below we demonstrate that the coadsorptron of erther methyl chloride or fonnrc acid with CO on Cu( 100) results in both an increase in the carbon-oxygen stretclung frequency (to above the gas phase value) und a sunultaneous increase in the heat of adsorptron of CO. We explain this behavrour quahtatrvely rn terms of the srmple Lewrs acrd/base character of the materials [5].

2. Experimental The high-resolutron electron energy loss (EELS) spectrometer, vacuum system and sample preparation techniques are described elsewhere [6]. For these studres, experiments were carried out on three Cu(lO0) 537

Volume

120, number 6

CHEMICAL

smgle crystals In addrtion to characterrzmg samples by low-energy electron diffractron Auger

electron

spectroscopy

(AES),

PHYSICS

25 October

LEITERS

these

7

cu II001

(LEED),

*

5OL

CHgzl

X50 J 3 Results

co

2160 I

and h&t-resolution

EELS, work functron changes (A@) were measured by the retarding potentral method.

1985

k-

33T

and drxussron

The chemrsorptron of carbon monoxrde on clean Cu( 100) has been extensively studied At low temperatures, adsorption is molecular wrth smgle metal-carbon (340 cm- r) [7,8] and carbon-oxygen (2090 cm-l) [7-IO] (fig 1) stretching vibrations whrch shift only slightly with coverage_ The molecule is orrented normal to be the surface and bonded to a smgle substrate atom [ 1 I] Slowly warmmg a CO saturated surface results m the desorption of all adsorbed species by z-80°C. No frequency shaft 1s observed within our resolution (k-5 cm -I)_ The coadsorptron of carbon monoxide with electron-wrthdrawing substituents wrll shift the clean surface C-O stretching vrbration to higher frequency of CO Thrs IS clearly shown m fig 1 for the coadsorption wrth 02, HCOOH, and CH,Cl on Cu(lO0) Attempts to measure shafts m the substrate-carbon stretching vrbration were hampered by the proxrmity of relatively intense modes at -300 cm-l_ The experimental condttions as well as the measured frequencies, intensrties, work function changes and approxrmate desorptron temperatures are summarized m table 1. Representative vrbratronal spectra for the chemisorption of HCl(l0 L at 30°C) and CH3C1 (20 L at -110°C warmed to room temperature) [ 161 on a clean Cu(lO0) surface are shown m fig 2 Representative spectra for the adsorptron of CO [7,8], 02 [17], and HCOOH [ 141 have been pubhshed prevrously. Two important observatrons recorded m fig 1 and table 1 should be pornted out First, both the C-O stretching frequency and the CO desorption temperature can increase simultaneously u-r the presence of electron-withdrawmg species- The details of the surFace sate m which the carbon monoxide IS bound are unclear, but we assume that the adsorbate is still linearly bonded to a smgle copper atom Second, only a small fraction of the surface 1s covered by the perturbed carbon monoxrde, implymg a very localrzed type of interaction (see below) Due-to-the low CO surface 538

I

l800

I

I

2000 ENERGY

I

I

2200

1

I

J

2400

LOSS ktn-‘1

Kg 1 HI&h-resolution EELS spectra of CO chemisorptron on clean Cu(100) (lowest tract) and coadsorbed with oxygen ((a x 2fi)R45” surface structure), surface formate from the decomposition of Iormrc and. and methyl chloride (top tray;) The oripm of the broad shoulder centered near 2000 m the latter spectrum is most likely due to an overtone Fz double loss) from the intense mode at 1010 cm-l m rig. 2. Expansion scales are relative to the clean surface value For reference, gas-phaseCO has a stretching vrbrauon at 2143 cm-’

coverage, complementary thermal desorptron mass spectrometry experiments to directly measure the increase in the CO heat of adsorption are complrcated by background desorptron from the sample holder, the crystal heater, etc However, it appears that carbon monoxide is stable on Cu(100) in the presence of methyl chloride even after flashmg (==4”C/s) the sample to 110°C in vacuum (the desorption/decomposition temperature of CH3Cl) Thrs places a minimum value of 23 kcal/mole on the heat of adsorption of

Volume 120, number 6 Table 1 The coadsorption Adsorbate

CHEMICAL

of carbon monoxrdc with electron-wlthdrawmg Evposure conchhons

02 HCOOH HCI CH3CI

subshtuents on Cu(100)

Adsorbatc LEED Pattern -

-

clean

(a (a h)

50 L at 2oo”c 1500 L at 200°C 5L at - 150°C. warm to 30-C 6) 0 5-l L at 30°C 10 L at 30°C

(a

0 S-10

h)

L at -12O”CJ)

X a, X 2Js,

R45’ R45”

x Jr,

R45”

-0.2 d) 0.39 e)

0.41 02 0.22

I/I0 (clean)

2090 2100 2120 r) 2090 2150

1 01
2090 2105 0

=0.2-O to 02

zogo'0 zogok) 2160

1985

a)

“CO =) (cm-l)

-0

h) h)

50 L at -12O’CJ) warm to 3O’C

25 October

PHYSICS LEITERS

= =des eo -00 -80 -50 -80 >35

6

02-05

0 l-0.2.

>o 011)

-80 -50 -so -80 >lOO

b) Malimum change m \\ork function (this work ua!esz indlcared a) >lO L CO exposure at a substrate temperature c - 120°C. otherwise) b) Observation of YCo-C 1s blocked by mtcnse low-frequency substrate-adsorbate modes exept on the clean and low-covcr~ge HCI and CH,CI exposed surfaces (uCu_c - 340 cm-‘). d) Ref. [ 121 ‘=) Ref [ 131. 0 No CO adsorption observed Flash to ==3OO’C to create defects in the ovcrlayer. 6) A surface formatc species is formed under these reaction conditions [ 141 h) No extra chffrnction features obscrvcd by LEED, but the ovcrlayer appears to be scnsltlvc to rhc electron bnm I) SignficantJy more mtense C-O stretching vlbratlons have been observed on occasion [15] J) Exposing a clean Cu(100) surfacc to CHsCl at temperatures below -90°C results m a physisorbed species [ 161 k) SlmiJar results have been obtained for CO adsorption on Cu(100) m the prcscnce of either methyl bromide or methyl iodide For the case of condsorbcd methyl fluoride, ~~0 shifts to =2100 cm-‘. ‘) Sample allowed to remam overnight in lO+ Torr of CO at =35’C

co*

cu l1001

ZOL

CH3CI

8

-IIODC

WARM TO 30-C

Ns measurable

sluft m the C-O

stretchmg

fre-

quency IS observed throughout this heating cycle The increase m the desorption temperature of carbon monoxide on copper with mcreasmg C-O stretchmg frequency U-Ithe presence of oxygen, surface formate, chlorine, or methyl chloride can be understood quahtatively m terms of increased u donation_ The chemlsorption of CO on clean Cu( 100) results m a decrease in the work function (by as much as -0.20 eV, dependmg on surface coverage) [ 121 mdlcatmg a net electron transfer toward the metal surface although +A ~urnmg t-u&order desorplmn kn-.ctlcs nnd n pre-c\ps nential factor of 10” [la]

0

1600

800 ENERGY

LOSS ltm-‘1

2400

fl Fg 2 High-resolution EELS spectra of HCI (lower trace) and CHsCI (upper trace) chermsorbed on clean Cu(100) The adsorption conditions are mdicated in the figure Representatwe EELS spectra for the adsorption of CO [7,8], 02 [ 171 and HCOOH [ 141 on Cu(100) have been pubhshhed previously. 539

Volume 120, number 6

CHEMICAL

recent calculations indicate that much of the COsurface bond strength may be denved from H backdonation [ 193, this backdonation 1s somewhat lmuted (compared to other 3d metals) smce the d bands of copper are located to far m energy (~2 eV) below the Fermi level to strongly interact with the CO 2~” orbital. The bond to the surface is therefore relatively weak (=I 6 kcal/mole) [ 12 3 Stdl, backbondrng must be significant [19] since vco is below that of gas-phase carbon monoxldc (see below) The presence of electron withdrawing substituents partially empties the sp band and more extensive charge transfer from CO to the substrate can occur A stronger bond to copper is then formed *_ Electron donation from the 50 orbital (the highest occupied molecular orbital of gas-phase CO) should be manifested as an increase in vco since the stretchmg frequency of the gas-phase molecular ion (CO+) is 2 184 cm-t Our observations are in sharp contrast to those of Klskanova and Goodman [21] where the preadsorptlon of electronegative atoms (Cl, S, and P) resulted m a decrease m the heat of adsorption of CO on Ni(l00). This difference, however, can be easily reconciled if one considers the LCWIS acid/base character of these two materials [5] For the case of CO chemlsorptlon on nickel, the surface acts hke a Lewis base since there IS a net electron transfer from the substrate to the molecule upon adsorptlon (A+ > 0 indicative of more n backbondmg than u donation) The presence of oxygen or chlonne withdraws electron density from the metal, the surface becomes less “basic” and, consequently, the COchemlsorptlon bond IS weakened In our experiments, the copper surface is “ncldic” towards CO (A@ < 0). The preadsorption of electron-withdrawing substituents makes tbs material even more “acidic” This results In l~zore charge transfer from the adsorbate and therefore a stronger chemlsorption bond This rather sunple analysis neglects the details of the adsorbate-copper and copper-CO interactions, but provides a useful qualitative understanding of the observed trends It IS important to pomt out that although the measured CO stretching frequency m the presence of surface formate or methyl chloride 1s 5-15 cm-’ above that of gas-phase carbon monoxide, the C-O stretching ’ Similar mcrcascs in surface bond strength have been observed recently for the chemisorptlon of molecular oxygen on a polycrystaUme copper foil predosed with chlonne [20]. 540

25 October

PHYSICS LEITERS

1985

force constant (kco) 1s still less than that of the free molecule (i-e significant backbondmg 1s still present) Thus becomes clear when one considers the effects of kinematic coupling between the metal-carbon and carbon-oxygen stretchmg vibrations [22]. For ex(a value typical of ample, if we fii vco at 2050 cm-l carbon monoxide linearly bonded to a metal surface [I]) and plot both the metal-carbon (khlc) and carbonoxygen stretching force constants as a function of the 20"5 m 5

I

,

YClS - -Cc----Ia-

L

I

L

'

I

'

1

s

-_-_ --__ -_

-_ -.

-E

-.

-co-

-_-_-_

UCO-2160Cm-'

IO

UC,-zosocm-

Fig 3 Plot of metal-carbon (khr) and carbon-oxygen (km) stretching force constants as a function of the metalcarbon stretching frequency (vm) in the harmonic approximanon A harmonic force field IS assumed and the mass of the substrate is infiitc The stretch-stretch Interaction force constant (kMm, typically on the order of 0 l-0.3 X-m) has been neglected Twn cases are considered. “~0 = 2050 cm-l (solid curve) ano VW = 2160 cm-’ (dashed curve). Values of km derived from w(C0),5 and IG(C0)4 as WI1 km for CO+ (one electron missing from the So orbital). CO. and CO- (one electron added to the 2n* orbital) are shown for comparison

CHEMICAL

Volume 120. number 6

PHYSlCS

observed metal-carbon stretching frequency (vMC); we find that, in all cases, kc0 IS far less than that of the free molecule (fig. 3, sohd curve) Even for V& = 2160 cm-’ (fig 3, dashed curve), reasonable values of vMc (300 < vMc < 500 cm-l) require &, to be well below the gas-phase value of 18.54 mdyne/A Clearly significant metal-to-molecule backdonation (to weaken the C-O bond) must still be takmg place. Subsequent coohng of 2 sample predoned wrth either formic acid or methyl chloride and further exposure to CO results in the growth of an intense peak at ~2090 cm-l _Wanmng once agam reveals 2 single, slufted peak The presence of two drstmct stretching vibrations in these spectra clearly indicate that the adsorption of CO m these cases must have a highly localued component_ This is sirmlar to the conclusrons of Wallden [23],

Somerton

et al. 1241,

and Dubois

et al. [25]

on their experiments with the adsorption of CO on alkali promoted Cu( 100). This highly localized type of interaction on copper is m sharp contrast to the numerous coadsorption studies on group VIII metals in which the presence of even small amounts of electron-donating or -withdrawing substituents severely affects adsorption over the entire surface 14, 211.

based

4 Conclusion From the above, it IS clear that the presence of coadsorbed electron-withdrawing substituents can readily increase the heat of adsorption of carbon monoxide on copper This increased bond strenght is most likely due to more extensive electron transfer from the 50 orbital and this IS manifest by an increase in the C-O stretching frequency. This 1s not to say that u donation IS the only important consrderatron when discussing the strength of the surface-CO bond, but its effects must not be underestunated

25 October

L?ZITER!3

1985

Acknowledgement We thank R G Nuzzo, H.S. Luftnran, and T H Elhs for their msiteful comments and for a critical reading of the manuscript prior to publication.

References 111 N. Sheppard and T-T. Nguyen, m_ Advances m infrared

and Raman spectroscopy, Vol 5, eds RJ.H. Clark and R-E. Hester (Heyden. London. 1978) p_ 67. PI L H Dubois and G-A Somorjai, Surface SCI 91 (1980) 514. 131 H Ibach and G A Somorjai, Appl Surface SCI 3 (1979) 293 141 E L. GarTunkel. J E Crolvell and G.A Somoqai, J_ Phys

151 [61 [7] [S] [9] [lOI Ill ] [ 121 [13] [ 141 [ 151 [ 161 1171 LlS] [ 191 [20] 1211 [2X] 1231 [24]

1251

Chem

86 (1982)

310

PC Starr, J Am Chcm Sot 104 (1982) 4044. L H Dubots and R G Nuuo. Lan_grmu. to be published B A Sexton, Chem. Phys Letters 63 (1979) 451 S Andersson, Surface Scr 89 (1979) 477. K Horn and J Prttchard, Surface Scr 55 (i976) 701. R. Ryberg, Surface Sci 113 (1982) 627. S. Andcrsson and J B Pendry, Phys Rev. Letters 43 (1977) 363. J.C Tracy, J Chem. Phys 56 (1972) 2748. T. Dclchar. Surface Sci 27 (1971) 11 B-A Sexton, Surface Sci 88 (1979) 319; J_ Vnc SCI Tcchnol 17 (1980) 141 T H. Ellis. pnvntc commumication L H Dubois. B R Zegrsks and R G Nuzzo, to be published. B A. Sexton, Surface SCI 66 (1979) 299 P A Redhead, Vacuum 12 (1962) 203 P S Bagus, K Hermann and C.W Bauschbcher Jr , J. Chem Phys 81 (1984) 1966. P.V. Kamath, K Prabhakaran and C N R Rae, Surface Sci. 146 (1984) L551 hf. Ksskinova and D-W. Goodman. Surface Sci. 108 (1981) 64 J IV_ Gadzuk, Phys Rev. B19 (1979) 5355 L. Wallddn, SurfxeScr 134 (1983) L513. C. Somerton, C F McConwlle, D.P. Woodruff, D E Grider and N-V. Richardson, Surface SCI 138 (1984) 31 L H Dubois. B R Zegarski and H S Luftmon, to be published

541