Adsorption of pyridine on RU(001): a study by high resolution electron energy loss spectroscopy

Journal of Electron Spectroscopy and Related Phenomena, 44 (1987) 131-139

131

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

ADSORPTION

OF PYRIDINE ON RU(O01):

A STUDY BY HIGH RESOLUIION ELECTRON ENERGY

LOSS SPECTROSCOPY

P. JAKOB, D.R. LLOYD* and D. MENZEL Physik-Department E 20, TU MOnchen, D-8046 Garching b. MOnchen, Germany

ABSTRACT The adsorption states of pyridine on Ru(O01) have been studied as a function of exposure and temperature using HREELS. An unusually stable parallel-bonded state I is formed at low coverage below 250 K; at large exposure below 150 K a tilted form II can be additionally formed. A dehydrogenated a-pyridyl state III can be generated by heating multilayers or dense monolayers of type,IT to 190 K, or b)' adsorbing at this temperature. Neither IIl nor IT can be formed b)" heating I. INTRODUCTION Pyridine, almost

C5HsN ,

invariably

guration

(ref,

initially

is isoelectronic with benzene,

C6H6,

but whereas

coordinates to metal surfaces with an q6 ,

I),

pyridine adsorption is more complex.

benzene

parallel,

The models

confiproposed

were the similar parallel q6and a N-coordinated perpendicular ql form

(ref. 2); in the subsequent discussion these will be referred to as models I and IV respectively. Man) subsequent studies have suggested an intermediate form II, in which the ring is at an angle to the surface (refs.

3-9),

and there is

now

increasing evidence for a reaction product III in which the ring is vertical but the a -,

or

2-,

hydrogen atom has probably been lost so that q2 coordination

through N and C2 occurs (refs. 4,5,8,10). The species lit is usually referred to ass -pyridyl. The

prototype

pyridine simple and

system

stud) by vibrational spectroscopy (HREELS)

crowding of the surface:

adsorbate is weak (ref.

5).

in both states interaction However,

have been reported using NEXAFS (ref. adsorption

on

temperatures, around

of

the

Ag(lll)-

was interpreted as evidence for a transition from I to between

II

by

substrata

conflicting results for this system

9) and ARUPS (ref. Ii). HREELS studies of

Ni(lO0) show a similar crowding transition from I to IT though with stronger interaction,

and a transition to

room temperature. On Pd(lll) there is evidence

at

low

state III

for I at low temperature

and II at room temperature, i.e. a transition induced b)' temperature rather than crowding,

and

no evidence of reaction to form III (ref.

6).

In

contrast

* Permanent address: Chemistry Department, T r i n i t y College, Dublin 2, Ireland

0368-2048/87/$03.50

© 1987 Elsevier Science Publishers B.V.

on

132

Pt(III)

there is no evidence for a parallel state I:

coordination

is the inclined form II,

the only low

temperature

which reacts well below room temperature

to form III (refs. 5,8,10,12). These observations show some patterns, coordination

but the factors which determine

to the surface are by no means clear.

the

Accordingly we have studied

the interaction of pyridine with the reactive Ru(O01) surface,

with

particular

interest in crowding and temperature effects, using HREELS.

EXPERIMENTAL The spectrometer,

and the cleaning procedure for the Ru(O01)

been described previously (ref. migration crystal could

of

an

face. not

sputtering

l~).

be made to disappear by oxygen cycling, ( 10 -5 A cm -2,

(nD:n H ~

dosing

400 V,

needle,

99%).

i0 min).

and was

pure material (GC 99.97%):

band

in

specarea.

and one or

two

cycles

Despite these precautions slight

Although not identified,

possibly

from

these materials gave rise

the spectrum in the region of 500-600 cm -I.

HREELS

is

to the presence of CO on the surface and most of the spectra

to very

show

a

in the region of 2000 cm -1 which can be assigned as due to the presence of

0.5 to 2% of a monolayer of CO. of

argon freshly

pyridine -d 5 was

of the crystal surface was occasionally observed,

decomposition products.

by

the

carried out before each day's operation.

sensitive

the

using an inlet system of minimum metal surface

were repetitively degassed by freeze-thaw cycles,

losses

removed

The samples were attached close to

They

broad

to

Pyridine ~as taken from a

were

contamination

on

This could readily be recognised by a complex LEED pattern which

quality

trometer

have

Occasionally trouble was experienced with

oxide of tantalum from the crystal support wires

opened bottle of spectroscopically n.m.r,

surface,

a

similar intensity to the

observed

in all spectra,

CO also has a band at 480 cm -1 which is usually 2000 cm -I band.

A band in this

position

but with considerably higher intensity,

present when the CO contamination was marginal,

was

and was even

so it must be pyridine induced.

This band disappears in the off-specular spectra, i.e. is almost entirely dipole excited. Spectrometer resolution was typically 80-100 cm -1

for the C5H5N

work,

with

specular beam count rates ~ 2

x 105 sec -1 for the clean surface, but the studies -1 with C5D5N were carried out after the resolution had been improved to 50-70 cm with similar peak count rates. carried needle and

All spectra were measured at ~ llO K. Dosing was

out from the system ambient, of 2 mm i.d.

or,

for higher exposures,

from a dosing

terminating approximately lO mm from the crystal

with the crystal cooled to 120 K unless otherwise

adsorbed

reversibly

pressure

falls slowly,

on

the chamber wails,

specified.

so that after a dose

which causes problems in assessing absolute

surface,

Pyridine its

is

partial

exposures.

133

RESULTS AND DISCUSSION By varying exposure and temperature, three different states can be picked out which show similarities to I,

II and Ill discussed above.

In

addition,

large

exposures at low temperatures give a characteristic multilayer spectrum, similar to that reported on Ag(lll) (ref. 3). The multilayer is readily recognised by an -I intense peak at 720 cm At

llO K and low exposures the spectrum in the specular direction

nated

is

domi-

by an intense band at 770 cm -I with a shoulder at 880 cm -1 (Fig.l,

upper

panel).

Except for the feature at 480 cm -I (see above) the rest of the spectrum

is very weak. In the off-specular spectrum the 880 cm -1 shoulder has become more intense,

and

creasing

exposure these bands increase in intensity up to about

the bands at 1400 cm -1 and 3020 cm -I become prominent.

With . Ex ;

0.7

inthe

1410 cm -1 band is detectable in the specular spectrum at these higher coverages, but

there

is

very little sign of the 3020 cm -1 band

stantially greater exposures (ca. change

in

the

multilayer (Fig.l,

spectrum,

spectrum.

The

principal

changes induced by

this

880 cm -1,

large

of

a general poorly-resolved increase in intensity between 850

better defined,

the

exposure 3060

a decrease in intensity of the bands at 770

and the appearance of a band at around

is

Sub-

noticeable

central panel) are a substantial increase in the intensity of the

1200 cm -1 , spectrum

stretching).

and it is difficult to avoid the production

cm -1 band in the specular spectrum, and

(C-H

5 x) are required to produce much

650 cm -1

and

The off-specular

and there is an increase in intensity of the

C-H

stretch. Equivalent

observations can be made when analyzing the

spectra which are shown in fig.

2.

corresponding

C5DsN

The mode assignments, which are closely re-

lated to the gas phase, are given in Table I. These

observations have many resemblances to those for law

temperature

ad-

sorption on Ni(lO0); the band positions, which are reported in Table l, agree in most

cases

spectra

to

to within the experimental precision.

We assign the

low

a parallel species I and the high coverage spectra with

coverage the

addi-

tional bands to a system which contains both state I and the inclined state II. The appearance of the CH stretch in the specular spectrum as II builds up can be associated with the dipole character of this excitation: figuration the C-H stretch dipole is parallel to

the

in

a parallel con-

metal surfaceand therfore

* The exposure unit i Ex i x iO 14 collisions cm -2 (ref. 14); C~H5N at room temperature, 2.3 Ex I L. A gauge sensitivity factor of 5.8 has ~een used, but see the note above on absolute values.

134

C5H5 N / Ru(001} ATE I

t L

,. J "

V) I,.-

vk,,.

~ STATE II

I--,,I

Z

::3

rd

I:E

~__._-_

__. }@S0

>I-..(.t)

Z

Iii I,-Z

~ , , ~~ , , ~,s~ t . TM

0

1000

"

2000

STATE m"

3000 ELOSS [cm.l]

Fig. I HREEL spectra of the three states of CsHsN adsorbed on Ru(O01). In each panel the lower spectrum has been obtained in specular reflection (De = 0°), and the upper spectrum with De = 6 °. Formation conditions: State I, 120 K, 0.7 Ex; State II, 120 K, 4 Ex (a considerable amount of I is still present); state III, multilayer heated to 270 K and cooled.

135

CsD5N I Ru(O01) $65

13~

2250

TATE I

560

830

U3 I'--

'~

~

~ STATE II

Z =)

rd rt,. '

>l--

CO

2270

t/) Z kI.J I-Z I,,,-I

~,~31~~~~~W~I ,~', II I I STATEN,,~oo 0

1000

21~00 ELOSS [cm-1]

Fig. 2 HREEL spectra of CsDsN on Ru(O01), corresponding to Fig. I.

136

not

dipole-coupled.

The increase in intensity of the C-H stretch in

the

off-

specular spectrum ms)' represent the effect of increasing the number of molecules on the surface. The well resolved band positions in C5D5N in

excellent

Pt(lll)

and

agreement at

spectra (Table l) are

with those reported for C5D5N at low

room temperature on Pd(lll) (ref.

5,6),

both

temperatures of

which

on are

believed to be in state II.

Table I: Vibrational assiqnments of state I and state II of p)'ridine on Ru(O01) C5H5N S)'mmetr)' A1

82

B1

Mode a 1 2 3 4 5 6 7 8 9 10 23 24 25 26 27 ii 12 13 14 15 16 17 18 19

C5D5N

Gas phase ~ 3094 2302 3072 2276 3030 2268 1583 1554 1483 1340 1218 882 1072 823 1032 1014 991 963 601 579 1007 828 936 765 744 631 700 526 403 367 3087 2289 3042 2256 1581 1546 1442 1303 1362 1046, 1227 1226 1143 856 1079 835 652 625

State I

State II

h5 3020 3020 3020 1530 1400

d5 2250 2250 2250

h5 3060 3060 3060 1570 1410

d5 2270 2270 2270 1495 1340

1080

840 840

1120 960 860

830 950 830

880

880 770

725 565

(860 (770

730} c 560) c

3020 3020 1530 1400

2250 2250

3060 3060 1570 1410

2270 2270 1495 1275

1120 1120 650 d

1180 830 830 630 d

1080

1340

1170 840

a There are man)' labelling systems used for the vibrational modes of pyridine. We used that of Long (ref. 20), as do Grassian and Muetterties (refs. 5 and 6), in which B. and B 2 are reversed from the standard use. b Referencei21 c Bands probably due to residual state I d Mode not resolved in the shown spectrum.

On Ni(lO0) the strong bands of state I at 770 and 880 cm -I were both assigned to~26,

the in-phase out of plane C-H bend;

proposed as an interpretation

site geometry is quite different, on Ni(lO0),

two different adsorption sites were

(ref. 4). On the close-packed Ru(O01) surface, the but the band positions are identical to those

and there is no comparable splitting for C5D5N. Therefore we prefer

the assignment shown in TabIe I, in which the bands are assigned to the two out-

137

of-plane bending vibrations ~b andre. We have investigated the temperature stability of state I;

spectrum

stays

effectively

starts. Around 450 K strong

one

constant up to about

K

where

the

decomposition

the only hands which can be observed for the product are a

at 800 cm -1 and

correlates

350

on warming,

weaker ones at 500-600 cm -1 and

3060

cm -I

well with the spectra reported for the inclined CH species

This

detected

on Ru(OOl) in studies of the decomposition of ethene and ethyne (refs. 15,16). Attempts the

were made to generate state II by raising the temperature at

exposure

was

carried out to 190 K (conversion of I to II on

observed at 170 K (ref.

4).

was

An exposure of 1.8 Ex gave only the spectrum of I,

suggesting that the state I is much more stable than on Ni(lO0). 7

which

Ni(lO0)

An exposure of

Ex showed a substantial change in the spectrum to give a new species Ill

only partial conversion was observed; cannot with

be excluded, species

III

but

formation of small amounts of species

II

since strong overlap of the spectral features of this type occurs.

The spectrum was

virtually

unchanged

by

further

can be produced w i t h a minimum o f o t h e r species

present

exposure, The new species I I I by

h e a t i n g a condensed m u l t i l a y e r t o above 190 K;

lower

the spectrum i s shown i n the

panels o f Figures 1 and 2. E s s e n t i a l l y the same s p e c t r a are

heating

obtained

by

a high coverage monolayer containing a maximum of type I[ to T > 190 K.

The band positions are given in Table 2; the)' are very similar to those reported for state III on

Ni(lO0) and Pt(lll) (ref.

4,5).

On Pt(lll), this species has

been well characterised by other techniques (NEXAFS, In

addition,

agreements

ref 8, and ARUPS, ref I0).

comparison to~ -pyridyl cluster compounds (Table 2)

of vibrational losses.

interpretation

of

species

Taken together,

Ill as~-pyridyl

shows

close

these comparisons make

safe even though

there

the

are

some

intensity variations. Like state I, substantially

state III is also thermally stable at room temperature;

unchanged

on leaving overnight in the

heating to ]40 K species III starts to decompose, a

spectrometer

it is

vacuum.

On

as indicated by the growth of

peak at 780 cm -1 which shifts to 800 cm -I at 440 K.

due to formation of increasing amounts of angled CH,

We believe that this

is

as in the decomposition of

I, but other bands are present and the decomposition is clearly complex. ~-pyridyl pyridine

has been proposed as an intermediate in H-D exchange reactions

(ref. 18),

so

attempts were made to induce H-D exchange for III

of on

Ru(O01). However, no change in the intensity of the C-H stretch, and no intensity increase between 2000 and 2500 cm -1,

could be detected when the crystal

exposed t o a p p r o x i m a t e l y 600 Ex o f D2 a t 110 K and at 250 K. the ~ - p y r i d y l , surface H or D.

once formed, i s t i g h t l y

bound and w i l l

was

This suggests t h a t

not r e v e r s i b l y r e a c t w i t h

138

Table 2: Vibrational assignments of state III of pyridine on Ru(O01) HOs3(CO)Io(NCsH4!a Mode C-H stretch 4 5 6 7 8 9 I0 23 24 25 26 27 13 14 15 16 17 18 19

S)'mmetr T A1

B2

B1

h5

d5

1592 1459 1222 1082 1057 1029 680 1029 758 745, 740

1554 1339 869 869 1055 1000

1549 1421 1269 1162, 1117 1082 770

State

III

h5 3050 1570 1420 1150 1020 1020 1020

d5 2250 1505 1285 780b 840 1020b 960

1519 1298

1570 1420

1505 1285

1216 825 869 738

1260 b 1215 1150 840, 780b 1020 840 685 645

869 738 ? ?

a We use the values of Grassian and Muetterties (ref. 5), but mode 19 has been reassigned by Anson and Sheppard (ref. 17) who have obtained spectra at higher resolution, and we quote their value here. These authors have observed the C-H stretching modes; the frequency reduction from the gas phase is well within our resolution. b Mode is not resolved in the spectrum shown. Our

results show that on RuiOO1) the parallel orientation of adsorbed

pyri-

dine is energetically preferred at low coverages. When the coverage is increased (T <150 K) a tilted configuration is formed by crowding. configuration

Formation of a tilted

of intact pyridine iN-bonded) and of an~-pyridyl species seem to

be competing processes at higher temperatures (T = 150 - 250 K), the first being preferred at low, and the latter at higher adsorption temperatures. A I:

suprising aspect of the present observation is the lack of it

cannot

be converted to II or III by heating.

strongly bound to the surface, on

Ni,

Pd

or

interaction thermally which

for

that the

Recent theoretical work on benzene suggests a

Ru than for Pd (ref.

ring

19).

The failure to convert

the

breaks up before enough thermal activation

ring.

The

N-bonded

tilted pyridine which

temperatures iT <150 K) by crowding, -pyridyl

This suggests that

I

formation.

No

of is

z bonding is stronger on Ru than stronger I

to

can be associated with a decomposition channel becoming available

the

tilting

Pt.

i.e.

reactivity

is is

available formed

at

Ill in for low

appears to be a necessary intermediate in

evidence for the existence of a type IV

species

(N-

139

coordinated, perpendicular) has been found.

ACKNOWLEDGEMENTS We

thank Mr.

C.E.

Anson and Professor N.

Sheppard for discussions on

the

infrared spectra of a-pyridyl complexes and for communicating data in advance of publication and Dr. C. Minot for an advance copy of a manuscript. DRL thanks the DAAD

for

a visiting fellowship.

Forschungsgemeinschaft

This work has been supported by the

Deutsche

through SFB 128.

REFERENCES 1 See for example, M. Neumann, J.U. Mack, E. Bertel and F.P. Netzer, Surface Sci. 155 (1985) 629, snd references therein. 2 B.J. Band)', D.R. Lloyd and N.V. Richardson, Surface Sci. 89 (1979) 344. 3 J.E. Demuth, K. Christmsnn and P.N. Ssnda, Chem. Phys. Lett. 76 (1980) 201. 4 N.J. DiNardo, Ph. Avouris and J.E. Demuth, J. Chem. Phys. 81 (1984) 2169. 5 V.H. Grassian and E.L. Muetterties, J. Phys. Chem. 90 (1986) 5900. 6 V.H. Grassian and E.L. Muetterties, J. Phys. Chem. 91 (1987) 389. 7 F.P. Netzer and J.U. Mack, J. Chem. Phys. 79 (1983) 1017. 8 A.L. Johnson, E.L. Muetterties, J. Stohr and F. Sette, J. Phys. Chem. 89 (1985) 4071. 9 M. Bader, J. Haase, K.-H. Franck, C. Ocal and A. Puschmann, J. Physique Co11. C8 47 (1986) 491. 10 M. Connolly, J. Somers, M.E. Bridge and D.R. Lloyd, Surf. Sci. 185 (1987) 559. 11R. Dudde, E.E. Koch, N. Veno and R. Engelhardt, Surf. Sci. 17B (1986) 646. 12 E. Taylor, M.E. Bridge and D.R. Lloyd, unpublished. 13 M.A. Barteau, P. Feulner, R. Stengl, J.Q. Broughton and D. Menzel, J. Catal y s i s 96 (1985) 51. 14 D. Menzel and J.C. Fuggle, Surf. Sci. 74 (1978) 321. 15 M.A. Barteau, J.Q. Broughton and D. Menzel, App. Surface Sci. 19 (1984) 92. 16 P. Oakob, A.Cassuto and D. Menzel, Surface Sci. to be published. 17 C.E. Anson and N. Sheppard, personal communication. 18 R.B. Moyes and P.B. Wells, J. Catalysis 21 (1971) 86. 19 C. Minot, personal communication. 20 D.A. Long and E.L. Thomas, Trans. Faraday Soc. 59 (1963} 783. 21K.B. Wiberg, V . A . Walters, K . N . Wongand D.S. Colson, J. Phys. Chem. 88 (1984) 6067.