Normal unenhanced Raman spectra of CO adsorbed on Ni(100) at liquid-nitrogen temperature and at room temperature

462

Surface Science 161 (1985) 462-478 North-Holland, Amsterdam

NORMAL UNENHANCED RAMAN SPECTRA OF CO ADSORBED ON Ni(100) A T L I Q U I D - N I T R O G E N T E M P E R A T U R E A N D A T R O O M TEMPERATURE H a s s a n A. M A R Z O U K

a n d Eugene B. B R A D L E Y

Department of Electrical Engineering, University of Kentucky, Lexington, Kentucky 40506-0046, USA

and K.A. A R U N K U M A R Spectron Development Labs., Inc., 3303 Harbor Blvd., Costa Mesa, California 92626, USA

Received 8 January 1985; accepted for publication 5 June 1985

Normal unenhanced Raman spectra (NURS) of CO adsorbed on Ni(100) after an exposure of L CO have been recorded at both liquid-nitrogen temperature and room temperature at pressures below 10 8 Torr. At both temperatures three models are presented to explain the observed spectra in the C-O stretch region and the Ni-C stretch region. These models are based upon the electronic sharing of the adsorbed CO in different configurations (linear, two-fold bridge and four-fold bridge) with both the Ni surface atoms and co-adsorbed species (like H and oxygen) present in the residual gas in the chamber. 10 6

I. Introduction Laser R a m a n s p e c t r o s c o p y is emerging as a powerful tool for s t u d y i n g at e x p o s u r e pressure b e l o w 10 -8 T o r r the a d s o r p t i o n of small molecules of low p o l a r i z a b i l i t y on p o o r R a m a n enhancers. In their R a m a n work, Stencel a n d Bradley [1] r e c o r d e d C O a n d N i C stretches on N i single crystals; they used C O pressure of 500 m i c r o n s at t e m p e r a t u r e s b e l o w - 120°C. A d d i t i o n a l i m p r o v e m e n t in the scattered light collecting technique a n d d a t a acquisition techniques [2] allowed the use of R a m a n s p e c t r o s c o p y for new studies in this field of research. W e recently p r e s e n t e d tentative m o d e l s for the a d s o r p t i o n of CO, H a n d oxygen on N i ( l l l ) [3]. V i b r a t i o n s of these a d s o r b e d species were o b s e r v e d b y the R a m a n technique at b o t h residual gas pressure a n d at exposure of 106 L CO, at b o t h l i q u i d - n i t r o g e n t e m p e r a t u r e a n d r o o m temperature. A n o t h e r w o r k b y us on a c o b a l t (poly)crystal [4] was also p e r f o r m e d at residual gas pressure 0 0 3 9 - 6 0 2 8 / 8 5 / $ 0 3 . 3 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

H.A. Marzouk et al. / NURS of CO on Ni(lO0)

463

and room temperature where C o - C , C - O and C H 4 stretch frequencies were observed. In this paper we propose three models to explain our observed R a m a n spectra of CO adsorbed on Ni(100) at both liquid-nitrogen temperature and room temperature for a CO exposure of 106 L. We observe that C - O stretches remain, in general, unchanged on going from liquid-nitrogen temperature to room temperature while the N i - C stretches change. These observations will be discussed in this paper. The high resolution of R a m a n spectroscopy (1 cm 1) makes it possible to observe features that are inaccessible by electron energy loss spectroscopy (EELS) and hence it is possible to propose a more detailed picture of the species present on the surfaces.

2. Experiment The Ni crystal was purchased from Materials Research Corporation, then oriented and cut to a 6 m m cube with a diamond saw. The sample was mechanically polished in a water-alumina slurry which had successive particle sizes of 1.0, 0.3 and 0.05 /~m. After etching, the X-ray diffraction patterns showed well-defined Laue spots indicating the (100) plane within + 1.5 ° of the sample surface. The sample was examined by a scanning electron microscope (energy dispersion technique for element detection) and only strong nickel peaks showed at several spots on the surface. The crystal was then put in a U H V chamber described elsewhere [2] and it was heated to get rid of any water present on the surface. This was done by passing 100 mA of current through the heater, while maintaining U H V conditions. Then the sample was bombarded with Ar + at 2 kV for 20 min while the sample temperature was 200°C, this was repeated a number of times. Finally, the sample temperature was raised to 300°C and a number of reduction cycles using research grade hydrogen were performed for 15 min each, the H 2 pressure was kept at 4 x 10 _5 Torr during the reduction cycles. U H V conditions were regained afterward and base pressure in the 10 -9 Torr regime was achieved. The sample was then cooled to - 170°C, and exposed to 106L of research grade CO. Figs. 5 and 6 represent the spectra after subtracting the background. Mass spectrometer analysis of the residual gas in the U H V cell revealed the presence of H 2, H20, CO, 02, and CO 2.

3. Results 3.1. R a m a n spectra o f CO on-top species

After exposing the sample to 106 L CO at - 170°C, spectra were recorded a t - 170°C and at 23°C. At both temperatures the occurrence of a band at

H.A. Marzouk et al. / N U R S of CO on Ni(lO0)

464

1758 A

o9 1836

z D >-

1856 ~

1797

z

w _z

1706

1758

1770

1802

1834

1866

RAMAN SHIFT IN CM-I

1919 1968

1950

I

1865

I

I

1892

J

I

I

1919

1946

1973

2000

RAMAN SHIFT IN CM-I

2066 O3 Z D

2o4

2009

~:

A

\

2095

z t.u

I 2000

[ 2035

I

I 2070

I

I ~,~I 2105

I 2140

I

I 2175

RAMAN SHIFT IN CM-I Fig. 1. T h e R a m a n s p e c t r u m in the region 1 7 0 6 - 2 1 7 5 c m 1 o f C O a d s o r b e d o n N i ( 1 0 0 ) a f t e r a n e x p o s u r e of 106 L C O . T = - 1 7 0 ° C . T h e laser p o w e r is 2 W, 5154 ,~. T h e slit w i d t h is 300 ~ m a n d the c o u n t time is 20 s / s t e p .

H.A. Marzouk et al. / N U R S of CO on Ni(lO0)

465

1830

p-

z Ld I.-z

I

1706

I

I

1738

I

I

1770 RAMAN

I

I

I

1802

I

1834

I

1866

SHIFT IN CM-I

1909

I-

(]3

1957

1988

Z I

1865

I

1892

I

I

I

1919 RAMAN

I

I

1946

I~./

1973

2098

<

'/l

2135

>

v

C

I

2000

I

2035

~

I

I

21105

2070

RAMAN

I

2000

SHIFT IN CM-I

2071

z w

I

I

i 214(

2175

SHIFT IN CM-I

Fig. 2. The Raman spectrum in the region 1706-2175 cm t of CO adsorbed on Ni(100) after an exposure of 106 L CO. T = 23°C. The laser power is 2 W, 5145 /k. The slit width is 300 /lm and the count time is 20 s / s t e p .

H.A. Marzouk et al. / N U R S of CO on Ni(lO0)

466 2010

I

1980

I

2020

" I

I

2060 RAMAN

I

I

I

2100

"~JI

2140

I

I

2180

S H I F T IN CM-I

Fig. 3. The R a m a n s p e c t r u m in the region 1980-2180 c m I of C O a d s o r b e d on Ni(100) after an exposure of 106 L CO. T = - 170°C. The laser power is 2 W, 4880 A. The slit w i d t h is 300/Lm and the c o u n t time is 20 s / s t e p .

2143 cm ] (_+ 5 c m - 1 ) confirms the presence of physisorbed CO. Displayed in fig. 1 is the R a m a n spectrum between 1706 and 2175 cm i a t - 170°C. The region 1706-2175 cm -1 at 23°C is shown in fig. 2. Both spectra were recorded using the 5145 A line of an Ar + laser. The bands that we assign and discuss are those which we were able to reproduce 100% of the time (_+ 5 c m - ]). Shown in fig. 3 is a R a m a n spectrum between 1980 and 2180 cm 1 at - 170°C recorded using the 4880 A line of an Ar + laser. This portion is shown to confirm that R a m a n bands recorded using the 5145 A laser line are also seen using the 4880 A (where they were also reproducible within_+ 5 cm 1). In fig. 1 the three bands at 2048, 2066 and 2095 c m - 1 are assigned to the C - O stretch of CO molecules linearly b o n d e d to the surface. U p o n warming to r o o m temperature we observe four bands at 2036, 2071, 2098 and 2111 cm -]. These bands do not move much (we shall discuss this p h e n o m e n o n later). The 2111 c m - 1 b a n d is a new b a n d that appears at room temperature. We put forward the following tentative model to explain the different R a m a n bands of the C - O stretch of the on-top species seen in figs. 1, 2 and 3. Refer to figs. 4a and 4b. The N i - C O b o n d for on-top species b o n d e d to Ni atom 1 is a o bond. To neutralize the excess charge on this atom, the atom back donates to the 2~r * antibonding orbitals of CO. This filling of the antibonding orbitals weakens the CO b o n d and so the C O stretch falls from its gaseous state value of 2143 cm - ] to a value of 2036 cm ~ (fig. 4b). In the model shown in fig. 4a for liquid-nitrogen temperature (where H atoms have a greater chance to be adsorbed on the surface) the N i - H bond, though weak, reduces the Ni C b o n d strength c o m p a r e d to the room-temperature case (fig. 4b). This weakening of the N i - C O b o n d reduces the back donation to the

467

H.A. Marzouk et al. / N U R S of CO on Ni(lO0)

C- 0 --~ 2 0 4 8

2066

C

II

c

Nil

Ni2

Ni3

Ni4

Ni5

Ni6

N i-C - , " 6 8 2

a

C - O --~

NilO

cm-I

2071 2 0 9 8

C

C

U

IJ

Nil 670

b

Ni9

648

L i n e a r o n - T o p Species ( L i q u i d Nitrogen Temperature)

2036

N i -C --~

7\9\

NiT Ni8 658

cm-I

2095

Ni2

Ni3

Ni4 Ni5

2111

C

cm -I

C

U Ni6

NiT Ni8

63?' 6 2 7

Ni9

NilO 582_ cm -I

L i n e a r o n - T o p Species (Room Temperature)

Fig. 4. (a) A schematic showing a tentative model for the li'near on-top CO species co-adsorbed on the Ni(100) surface with non-linear CO species and hydrogen atoms at liquid-nitrogen temperature, (b) A schematic showing a tentative model for the linear on-top CO species co-adsorbed on the Ni(100) surface with non-linear CO species and oxygen atoms at room temperature.

a n t i b o n d i n g o r b i t a l t h e r e b y m a k i n g the C - O b o n d stronger, a n d hence its frequency increases to 2048 c m - t. T h e b a n d at 2066 c m - t (and that at 2071 c m - ] seen in fig. 4b) is assigned to the C - O stretch of the o n - t o p species b o n d e d to N i a t o m 7 (Ni a t o m 5 in the r o o m - t e m p e r a t u r e m o d e l seen in fig. 4b) which is also shared b y a four-fold C O molecule. This N i - C b o n d is slightly stronger than that of the N i - H b o n d ; therefore, less electrons fill the 2~* a n t i b o n d i n g o r b i t a l of the linear C O a t t a c h e d to N i a t o m 7 (Ni a t o m 5 in fig. 4b). Hence, the C - O b o n d b e c o m e s s t r o n g e r a n d the C - O stretch shifts up to 2066 c m t (2071 c m - ~ in R T case). T h e linear C O species b o n d e d to N i a t o m 8 in fig. 4a could either share a

468

H.A. Marzouk et al. / N U R S of CO on Ni(lO0)

bridged CO species or a molecular oxygen species and in both cases will give a consistent picture to our model. Ni atom 8 (fig. 4a) is shared by a bridged CO species, the corresponding N i - C bond is stronger than that of the four-fold CO species; therefore, the N i - C bond of the linear species attached to Ni atom 8 (fig. 4a) is weakened due to fewer electrons filling its 2~r* antibonding orbital. Hence its frequency shifts up to 2095 cm ~. This argument holds for the 2098 cm-~ band represented in the model of fig. 4b for the room-temperature case. In the liquid-nitrogen case (fig. 4a) the bridged CO species sharing Ni atom 8 could have been replaced with molecular oxygen. Oxygen is a highly electronegative species and it tends to attract the surface charges toward it, producing a stronger N i - O bond than the N i - C bond. Therefore the metal charges available for the 29* antibonding orbitals of the linear CO species sharing a Ni atom with oxygen become fewer, causing the CO bond to become stronger and the frequency shifts upward. The band at 2111 cm ~ which appears at room temperature and disappears at liquid-nitrogen temperature is modeled in fig. 4b as a linear CO species attached to Ni atom 10. This atom is shared by a four-fold oxygen atom. This oxygen species is probably of the radical type Upton and Goddard [5] postulated, based on generalized valence-bond calculation (Andersson [6] has also studied these species using EELS). This radical type, at higher coverage, converts to the surface oxide type which, in turn, penetrates the Ni surface and ends in the bulk. Hence, at liquid-nitrogen temperature the oxygen coverage from the residual gas increases and the conversion to surface oxide intensifies and these oxygen species penetrate the surface (as the temperature drops) which leads to the disappearance of the 2111 cm -~ linear CO species in the process. In an independent work, Severson, Tornquist and Overend [7] have proposed similar interpretation for their results. Their I R R A S study was confined to the region 2030-2130 cm 1 where they studied the adsorption of CO on

Pt(lll). 3.2. Raman spectra of N i - C stretch of CO on-top species Shown in fig. 5 is the R a m a n spectrum between 450 and 695 cm -~ a t - 1 7 0 ° C . The R a m a n spectrum between 430 and 700 cm -1 at 23°C is shown in fig. 6. Both these spectra were recorded using the 4880 ,~ laser line. We now assign the N i - C bond stretching frequencies shown in figs. 5 and 6 to each of the linear species shown in figs. 4a and 4b. It is established in the literature (see Sheppard and Nguyen [8], for example) that the weakest N i - C bond is associated with the CO bond having the highest frequency, so as the CO bond frequency increases the corresponding N i - C bond decreases. Also, it is established that the N i - C stretch corresponding to

H.A. Marzouk et al. / N U R S of CO on Ni(lO0)

a

469

472

)r~

,42

z b3

z_ I 450

/

d;A 470

, 490

RAMAN

, 510

1 550

550

S H I F T IN CM-I

562

b

~

556 586

z

550

560

570

580

590

600

RAMAN SHIFT IN CM-I

Fig. 5.

linearly bonded species has a higher frequency than that corresponding to the two-fold bridge bond which, in turn, has a higher frequency than that corresponding to the four-fold bridge. Therefore, for the N i - C bond stretch at Ni atom 1 we assign the observed 682 cm-1 band (at LN temperature). As expected, this frequency shifts down to 670 cm-1 as the temperature increases to room temperature. We assign the observed 658 cm -1 band to the N i - C bond at Ni atom 7 (fig. 4a). This frequency shifts down to 637 cm-1 at room temperature (fig. 4b, Ni atom 5). The N i - C bond at Ni atom 8 (fig. 4a) we assign to the observed 648 cm-1 band. This band shifts down to 627 cm-1 at room temperature (fig. 4b, Ni atom 6). The 582 cm -1 band is assigned to the N i - C bond at Ni atom 10 (fig. 4b).

H.A. Marzouk et al. / NURS of CO on Ni(lO0)

470

658

C z 627

615

z w I_z

605

617

648

l,

635

I

641

629

RAMAN

SHIFT

{

653

665

686

695

IN CM-q

z D

682

v

>658

w

,,"7, 650

659

668 RAMAN

677 SHIFT

IN CM I

Fig. 5. The Raman spectrum in the region 450 695 cm 1 of CO adsorbed on Ni(100) after an exposure of 1 0 6 I, CO. T = - - 170°C. The laser power is 2 W, 4880 ,~. The slit width is 300 ~m and the count time 20 s/step. 3.3. Raman spectra of two-fold C O bridged species

In fig. 1 we observe five R a m a n bands at 1908, 1919, 1950, 1968 and 2009 c m - ' . We assign these bands to the C - O stretch of CO molecules b o n d e d to the N i surface in two-fold bridge configurations (a m o d e l is presented in fig. 7a). U p o n warming the sample to r o o m temperature (fig. 2) we observe four bands at 1909, 1957, 1971 and 1988 c m - 1 . W e observe that there is no change in the frequency of the 1908, 1950 and 1968 c m ' bands on going from liquid-nitrogen temperature to r o o m temperature, a point we shall discuss later. The band at 1919 c m - ' ( L N ) is neighbored by an H a t o m (sharing N i a t o m 3 as seen in the m o d e l sketched in fig. 7a). W h e n the sample warms to r o o m temperature, hydrogen coverage is substantially reduced and the 1919 c m - ' band shifts d o w n to 1909 c m ' (based on the same argument of

H.A. Marzouk et aL / NURS of CO on Ni(lO0)

471

472

a

512

554

442

A

430

I

456

482 RAMAN

508

534

560

S H I F T IN C M - I

582

b

I-.z

627

(z:

IJJ t-z

A

A

560

588

616

,.. 644

672

,,J, 700

R A M A N S H I F T IN C M - I

Fig. 6. The Raman spectrum in the region 430-700 cm-1 of CO adsorbed on Ni(100) after an exposure of 106 L CO. T = 23°C. The laser power is 1 W, 4880 ,~. The slit width is 300/~m and the count time is 60 s/step.

electrons sharing presented above for the linearly b o n d e d species). The 2009 c m - ] band we assign to the C - O stretch of CO m o l e c u l e which shares N i a t o m 12 with a linear species and another two-fold bridge species (similar arrangem e n t was proposed by Gelin and Yates [9] in their I R work on CO adsorbed on P d / S i O 2 at 300 K); it drops to 1988 c m - ] at r o o m temperature. This could be explained using an argument based upon C o u l o m b repulsion. Referring to block A in fig. 7a, there will be a net ( + ) charge on all three carbon atoms and on the c o m m o n N i a t o m 12 [10]. Here, the C - C distance of adjacent carbons is smaller than the C - C distance of adjacent carbons in situations corresponding either to linear only or bridge only. Therefore, the center C a t o m is subjected to repulsive forces from both sides and also by the N i a t o m 12. Adding to these repulsive forces, a weakening of the N i - C bond of the linear species

472

H.A. Marzouk et al. / N U R S of CO on Ni(lO0) 1909

1957 (971 1988

c-o 1908

t919

1950 1968 2009

l l Nil

Ni2

635

627

(T) Ni3

Ni4

Ni5

Ni6 NiT

554 2- Fold Bridged (Liquid

C-O

Nitrogen

8~ R o o m

NiB N i 9

NilO Nill

Nil2Nil3

586

562

542

534

512 cm-IlRoom Temperoture)

Ternperotures.)

1830

1797

1836

Ni8

.

NilO

Nil2

cm -I (Liquid Nitrogen TemperQture)

Species

1775

Ni6

Nil4

?

Nil6

Nil8

--

~

Ni20

Ni22

542

525

472

L499

472

4 42

--

4 - Fold Bridged (Liquid

(Room Temperature) Nitrogen re,,p°,ot,re~

cm-I

1856 B

1556

b

}

615

1758

Ni4

cm-I(Liquid Nitrogen

l l I ! I T..........

j" 1745

Ni2

Ni-C

cm - I ( R o o m Temperature)

~

cm-I (Liquid

Nitrogen Temperature)

Ni24 cm-I(Liquid

cm-I ( Room Temperature )

Species

Nitrogen ~ Room Temperatures}

Fig. 7. (a) A schematic showing a tentative model for the two-fold bridged CO species co-adsorbed on the Ni(100) surface with other CO species and hydrogen atoms at both liquid-nitrogen temperature and at room temperature. (b) A schematic showing a tentative model for the four-fold bridged CO species co-adsorbed on the Ni(100) surface with other CO species and hydrogen atoms at both liquid-nitrogen temperature and at room temperature.

Table 1 N i - C frequencies (in c m - ] ) at both liquid-nitrogen temperature and room temperature N i - C (LN)

N i - C (RT)

CO two-fold bridge LN

635 554 627 615 586. 562

RT

1908

542 534 512

1909 1919 1950 1968 2009

1957 1971 1988

H.A. Marzouk et al. / NURS of CO on Ni(lO0)

473

upon going to room temperature, this bond breaks and the 2009 cm-~ band of the bridge species in question drops to 1988 cm -~ in accordance with the argument presented above.

3.4. Raman spectra of N i - C stretch of CO two-fold bridged species Referring to figs. 5 and 6, we list the different N i - C frequencies at both liquid-nitrogen temperature and room temperature in table !.

3.5. Raman spectra of four-fold CO bridged species In fig. 1 we observe four bands at 1758, 1797, 1836 and 1856 cm -1. These we assign to the C - O stretch of CO species bonded to Ni surface in four-fold bridge configurations (a tentative model is presented in fig. 7b). Upon warming to room temperature (fig. 2) we observe three bands at 1745, 1775 and 1830 cm -1. The two bands at 1758 and 1797 cm -1 are neighboring an H atom at liquid-nitrogen temperature. Upon warming, the hydrogen is reduced substantially and these two bands drop down to 1745 and 1775 cm -1, respectively (using the same charge transfer argument presented above). The band at 1856 cm -1 at liquid-nitrogen temperature shares Ni atom 23 with a linear CO species and another two-fold bridge species. The very same argument we presented to explain the disappearance of the 2009 cm -~ two-fold bridged species above holds true here (block B, fig. 7b) for explaining the disappearance of the 1856 cm-1 on going to room temperature.

3.6. Raman spectra of Ni-C stretch of CO four-fold bridged species Referring to figs. 5 and 6, we list the different N i - C frequencies at both liquid-nitrogen temperature and at room temperature in table 2.

3. 7. N i - H and N i - O stretches Raman bands Shown in fig. 8 is the Raman spectrum between 1240 and 1450 cm 1 at - 170°C. It is established in the literature [11,12] that the v ( N i - H ) depends

Table 2 N i - C frequencies (in c m - 1 ) at both liquid-nitrogen temperature and room temperature N i - C (LN)

556 542 525 472

N i - C (RT)

499 472 442 -

CO four-fold bridge LN

RT

1758 1797 1836 1856

1745 1775 1830 -

474

H.A. Marzouk et al. / N U R S of CO on Ni(lO0)

1~ 4 7

124'

1260

[

,

I

1280

1300

1520 RAMAN

I 1340

1360

SHIFT

1380

i

I

I

I

1400

1420

1440

1460

IN C M - I

Fig. 8. The R a m a n in the region 1240-1460 cm a for residual H and 02 adsorbed on Ni(100).

T = -170°C. The laser power is 2 W, 5145 A. The slit width is 300 ~m and the count time is 20 s/step.

on the distance between the Ni and H atoms for a given configuration. This N i - H distance for the Ni(100) surface is generally thought to be between 1.8 and 2.0 ,~. In their neutron inelastic spectroscopy work, Renouprez et al. [11] presented also theoretical calculations to determine the u ( N i - H ) value as a function of the N i - H distance. In the region of 1.8 2.0 A N i - H distance, the u ( N i 4 - H ) varies between 1350 and 1200 cm -1 for the Ni(100) surface. N o r s k o v et al. [12], in an elegent theoretical work, calculated the N i - H distance for the N i 4 H configuration on the Ni(100) surface to be 1.91 A. Using the theoretically calculated curve of Renouprez et al., and taking the region between 1.8 and 2.0 A, as a straight line, the anticipated N i 4 - H frequency is 1275 cm 1. In fig. 8 we observe a b a n d at 1280 cm -1. This b a n d we assign to the u ( N i 4 - H ). The bands at 1320, 1347 and 1376 cm -a could be assigned to the same H species coadsorbed with CO or oxygen. Black [13] presented the estimated oxygen a d a t o m vibrational frequencies when adsorbed on Ni(100). These frequencies were obtained with Green function calculations. These frequencies are dependent on the distance between the oxygen adatoms and the Ni substrate. W h e n this distance is equal to 0.26 ,~ the parallel vibration of the c(2 × 2) oxygen adatoms is calculated to be equal to 1444 cm -1. In fig. 8 we observe a b a n d at 1447 cm -1. This b a n d we assign to the abovementioned oxygen vibration. The bands observed at 1402 and 1437 cm -~ could be assigned to similar oxygen vibrations with slightly different O - N i distance or weaker O - N i bond.

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475

4. Discussion We observed two phenomena in our results: (a) The effect of the temperature difference between L N T and RT on the N i - C stretches. (b) The lack, relatively, of the above effect on the C - O stretches especially the bridge ones. These phenomena could be explained on the basis on the following experimental reports and observations from other workers. In his pioneering work, Andersson [14] examined the adsorption of CO on Ni(100) resulting from exposure of 0.3 L in one dose; electron energy loss spectra were recorded at both liquid-nitrogen temperature and room temperature. At liquid-nitrogen temperature ( - 100°C in this case), Andersson observed a band at 1930 c m and one at 358 cm-~; also, two shoulders (weak bands) were recorded at 2065 and 657 cm -~. U p o n going to room temperature (20°C in this case), the shoulder at 2065 cm-1 grew to a strong band at the same frequency, the band at 1930 cm -~ remained as a band at the same frequency, the weak band at 657 cm ~ disappeared and a new one appeared at 480 cm ~ and, finally, the band at 358 cm -~ disappeared. Let us examine these results more closely. The resolution of Andersson's device was 9.5 meV ( - 77 cm-~). The two features at 2065 and 1930 cm -~ remained in the same position with a temperature change of 120°C (these are assigned to linear and bridge frequencies, respectively). However, it is expected that as the temperature decreases, the bond between the Ni and the C becomes stronger and the corresponding frequency shifts upward (and vice versa). So if one assumes the band at 480 cm -1 is the N i - C stretch corresponding to the linear stretch (2065 cm -~) at room temperature, the question arises as to where is the N i - C stretch (358 cm -1 a t - 1 0 0 ° C ) corresponding to the 1930 cm -~ bridge stretch at room temperature? Since the resolution is 77 cm-1 and this band is expected to drop below 358 cm -1 this band should have been observed. We propose that the 358 cm -~ band has dropped and is concealed in the descending energy tail which extends to about 300 cm-1 (where no band in this particular work can be detected below 300 cm-1). If we assume that the 358 cm -1 has dropped to just the edge of this limit, then it has dropped about 60 cm-~ going to room temperature ( f r o m - 100°C) while its corresponding C - O stretch remained constant. Richardson and Bradshaw [15] expressed doubt in their theoretical work about Andersson assigning the 657 cm -~ to P3, and they suggested that this band was influenced by co-adsorbed hydrogen. It is argued by Sheppard and Nguyen [8]; among other workers, that the adsorbate stretch frequency can be expected to be in the 2025-2055 cm -~ region, depending upon surface coverage and metal-adsorbate bond strength which, in turn, depends on adsorption site. These stretch frequencies move up to the 2060-2080 cm -1 region in the presence of hydrogen co-adsorbed on the surface.

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Considering these two points raised in the preceding two paragraphs, we could re-examine Andersson's results. We believe that at LN temperature the two weak bands at 2065 and 657 cm-1 may represent (a) C - O stretch of CO species neighboring H atoms, and (b) its corresponding N i - C stretch, affected by the neighboring hydrogen, as suggested by Richardson and Bradshaw. This picture is consistent with our results presented earlier. Fedyk and Dignam [16] investigated by infrared C - O stretches of both 12CO and 13CO adsorbed on Ni(100). They dosed their Ni(100) surface with 12CO at 180 K in successive steps up to saturation (when the crystal was equilibrated with 10 - 7 Torr of CO). Three bands in the linear CO region were observed (and more clearly resolved when repeating the experiment with 13CO) at 2154, 2115 and 2080 cm -1. Also observed was a band in the 1900 cm -1 region. After achieving saturation, warm up from 180 to 285 K produced downward shifts of about 25 cm-~ in the linear bands (as expected), and the bridge species shifted down about 60 cm 1. After saturation was achieved by one dose of CO at low temperature, then warm up in steps shifted the linear bands downward about the same amount as in the first case ( - 25 cm-1), but the bridge band shifted down 15 cm -1 (instead of 60 cm -~ in the first case), an obvious dependence on the dosing procedure as the authors noted. This would also explain Andersson's results, where his observed C - O stretch frequencies remained in the same positions upon warming up the sample (as mentioned above the sample was exposed to one 0.3 L CO dose). In their conclusions, the authors "guessed" that the surface structure that gives rise to the three different bands for linearly-bonded CO might arise from linearly bonded CO moieties that share an Ni atom with one or more bridge-bonded CO molecules. They considered the matter unresolved. No N i - C modes were observed; only the region 1700-2300 cm -~ was displayed, even though they mentioned that their apparatus is capable of operating from 60 cm -1 upward. In recent work by Trenary, Uram, Bozso and Yates [17], the temperature dependence of both the vibrational lineshape and frequency shift for CO on N i ( l l l ) was studied with IRAS. The CO exposure was done in one dose. They observed that no frequency shifts result for CO at both the two-fold bridge and terminal sites in the temperature range 80-300 K. They also observed that the bridge-bonded CO undergoes pronounced broadening at higher temperatures ( F W H M of the order of 50 cm -~) while the linearly-bonded CO is slightly broadened. They put forward a plausible theoretical explanation based upon the vibrational dephasing model developed by Shelby, Harris and Cornelius [18]. In this model the CO stretch vl is anharmonically coupled to one or more of the other CO modes (v 2, v3, etc.) known as the exchanging mode(s). The exchanging mode undergoes rapid energy exchange with the nickel phonons known as the reservoir modes. Because of the anharmonic coupling, Trenary et al. argue, energy exchange involving the low-frequency modes will cause

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dephasing of the u a transition and hence a broadening of the line without any shifting. This model predicts also, that some high-frequency modes are broadened with increasing temperature while others are unaffected. This theoretical explanation put forward by Trenary et al. has been also proposed independently recently by Gadzuk and Luntz [19]. The effect of temperature rise on the N i - C stretch frequencies was not presented. More theoretical studies are needed to establish a rigorous explanation of the two phenomena we have been discussing so far in this section. Mitchell, Gland and White [20] conducted EELS studies of CO adsorbed on clean Ni(100). They list four bands at 375, 565, 1720 and 2110 cm i which appear at low temperature for CO(sat)/H(sat) co-adsorbed on this surface. In another work, when Gland, Madix, McCabe and DeMaggio [21] achieved sulfur saturation first on this Ni(100) crystal before exposing to CO, they observed these same bands 385, 565, 1740 and 2115 cm -1 (considering the resolution of 60 cm -1 of their spectrometer). The first two bands were interpreted differently than in the case of adsorption on a clean surface; even though these two sets of bands are practically the same, they were assigned to two different sulfur structures. The other two bands were assigned to C - O stretches affected by the sulfur, before they (the 1720 and 2110 cm -~ bands) were assigned to C - O bands when the CO(sat) was co-adsorbed with H(sat) at 90 K (co-adsorption at 170 K does not produce these bands nor less than 0.6 L CO exposure). Thus we conclude that the 1700 and 2100 cm -~ bands are due to co-adsorbed species at temperatures well below 170 K a t / o r near saturation coverage. These "surface modifiers" as called by Gland et al. have the effect of physically blocking the Ni sites or changing the distribution of electrons available for bonding, as we have presented in our tentative models in this paper and in the previous ones. In a recent work [22] we used 13CO to confirm independently the previously reported [3] R a m a n bands due to 12CO + N i ( l l l ) system. All these bands were confirmed by this isotopic substitution method.

5. Summary R a m a n spectra of CO adsorbed on Ni(100) after an exposure of 106 L CO have been presented. Three models tentatively advancing the picture of the adsorbed species on the surface were able to account for the observed bands of C - O and N i - C stretches at both liquid-nitrogen temperature and room temperature. The independence of the CO stretch frequencies as a function of temperature for a single exposure of CO has been observed in this work. Theoretical work is much needed to examine the models we have advanced. Comparison with EELS results is insufficient due to the poor resolution of the work presented so far in the literature.

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Acknowledgements This work was sponsored in part by the US Department of Energy, Division of Chemical Sciences, Office of Basic Energy Sciences, under contract No. DE -AS05-79ER10447. The computer was purchased with funds from NSF Grant No. CPE-8015071 and from the University of Kentucky Graduate School. We also acknowledge the grant by Coastal Science Associates for the MARS (Multiple Analysis and Reference System) computer and software development used in this research. Also the authors wish to extend their thanks to Mr. Robert L. Doll for his contribution in the signal detection and smoothing routines.

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