Quantum chemical studies of formate on copper surfaces

Vacuum/volume 38/numbers 4 / 5 / p a g e s 389 to 3 9 2 / 1 9 8 8

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Q u a n t u m c h e m i c a l s t u d i e s of f o r m a t e on c o p p e r surfaces D W B u l l e t t and W G Dawson, School of Physics, University of Bath, Bath BA2 7AY, UK

The chemisorption of formate on Cu(1 10) and Cu( l O0) is investigated via electronic structure calculations of formate overlayers on the two surfaces. Peaks at binding energies 5-6, 8-10, and ~ 12.3 eV below the Fermi energy are associated with the adsorbed formate species, and correlate well with features identified in angleresolved photoelectron spectroscopy. The main adsorbate-surface interaction is shown to be via the 4b r molecular orbital of the formate species.

"]7he surface science of adsorbed formate on the (100) and (110) surfaces of copper has recently been much studied experimentallyTM. Formate has been identified as a surface intermediate species in the catalytic decomposition of formic acid 14. Below about 270 K formic acid adsorbs in the molecular form, but near and above room temperature H 2 desorbs leaving adsorbed formate on the surface, until above about 400 K the formate dissociates into hydrogen and CO23. Experimental studies which have identified and characterized the adsorbed formate have utilized ultraviolet photoelectron spectroscopy 3-s (including an angle-resolved synchrotron radiation study on Cu(ll0)S), high-resolution electron energy loss spectroscopy ~, temperature-programmed desorption and infrared reflectionabsorption spectroscopy 6. Combined with very recent NEXAFS and SEXAFS studies 7-~ 1, these techniques provide us with one of the very few molecular adsorption systems for which the full surface geometry (both molecular orientation and adsorption site) is now fairly reliably established. In this paper we report the results of quantum mechanical calculations to investigate the changes in electronic structure of the molecule and copper surface associated with the adsorption process, and make comparisons with the photoemission spectra for formate on Cu(100) and (110). Few such quantum chemical studies of molecular chemisorption processes have been carried out; for this particular system we know only of the very recent semiempirical INDO cluster calculations of Rodriguez and Campbell ~2. Our own calculations use a non-empirical atomicorbital-based method ~3 in the two-centred approximation for a tperiodic) chemisorbed molecular overlayer on the surface of a slab of copper. We begin with the the molecular orbitals of the free formate radical and H C O O - ion, and for the formic acid molecule. Since our calculations assume neutral-atom potentials around each atomic site and are not spin-polarized, neutral HCOO and H C O O - differ only by virtue of the different molecular geome-

HCOOH.

-4 -

.... "

HC00 ........

2bJrt}

3b(~}

~

~O ~ C~~ O ~

-

E>o <9 V 0

-10

~ 1 0 a _ _

-12

<9

6

-8

2b(rC) -

-

_ _ 9 a

O~C--O

6 __lb{~)

lbJrt)

-14

8a

666

\

-15

4 o , 48

O--=C~0

__so,~

6

60

Figure 1. Calculated molecular orbital levels in the isolated formate radical and in formic acid. tries in the two cases. We adopt the optimized geometries suggested by previous molecular studies14-17: bond distances C - H = 0 . 1 0 9 nm, C - O = 0 . 1 2 6 nm, and bond angles O - C - O of 114° in the neutral radical 16 and 125.6° in the negative ion 1~. Both molecules have C2v symmetry, and we define a coordinate 389

D W Bullett and W G Dawson," Q u a n t u m chemical studies o f formate on copper surfaces

Table l. Calculated molecular orbital energies (eV) in HCOO and HCOOH (a) HCOO 0 = 114° -29.0 3a 1

-26.3 2b 1

-17.3 4a 1

-14.9 5a I

-13.7 lb2(n)

-13.7 3b 1

-10.8 la2(z)

-10.3 6al

-9.9 4b I

(-4.2) (2bzOz)

0 = 126° -28.7 3al

-26.6 2b I

-17.3 4a 1

-14.9 5al

-13.7 3b I

-13.7 lb2(n)

-•0.9 la2(rc)

-10.3 6al

-9.9 4b I

(-4.2) (2bz(Tt))

--27.2 5a

-17.8 6a

-15.3 7a

-14.6 8a

-13.6 lb(rt)

-12.0 9a

-11.0 2b(n)

-10.2 lOa

(-4.7) (3b(rc))

(b) HCOOH -29.5 4a

©@

()

0

-

0

) 1~'1oi ..--~.

Ocu

HCO0

Cu (100)

Cu (110}

l

C)

Cu( 1 }0)

()

0 0

or]

0 } -

~

()

Qo

© c ° H Figure 2. Model geometry for adsorbed formate system: (a) 0.5 monolayers on Cu(110), (b) 0.25 monolayers on Cu(100). In each case we show a single unit cell in the surface, with the plane of the adsorbate lying perpendicular to the surface.

system with the z-axis along the C - H b o n d direction a n d the molecule in the x z plane, consistent with the discussion in ref 5. (Rodriguez a n d C a m p b e l l ~2 use the alternative coordinate system with the same z-axis but the molecule in the yz plane, thereby i n t e r c h a n g i n g the b~ a n d b 2 irreducible representations). Figure 1 indicates the calculated energies (Table 1), together with a schematic indication of the c o r r e s p o n d i n g molecular orbitals: orbitals of a 2 a n d b 2 s y m m e t r y form by hybridization of the py (re) orbitals on c a r b o n a n d oxygen, while the aa a n d b 1 representations label the a - m o l e c u l a r orbitals, formed by mixing of s, Px a n d p= orbitals on C, O a n d H. Individual molecular orbital energies shift by no m o r e t h a n 0.3 eV between the two different b o n d angles. In each case the highest occupied orbital, singly occupied in the case of the neutral radical, has b~ symmetry. Also s h o w n are the molecular orbitals for the formic acid molecule (C, symmetry) in its equilibrium geometry. The local a d s o r p t i o n geometry for formate on C u ( l l 0 ) is s h o w n in Figure 2. T h e molecule is believed to a d s o r b immediately a b o v e a surface a t o m a n d to lie perpendicular to the surface a n d oriented in the r l l O ] a z i m u t h . W e assume a b o n d angle of 124 °, u n c h a n g e d b o n d lengths within the a d s o r b e d molecule, a n d nearest C u - O b o n d distances of 0.188 nm; oxygen a t o m s sit in asymmetric bridge sites a b o v e the ridges on the C u ( l l 0 ) surface so that each oxygen has a second Cu near n e i g h b o u r at a b o u t 0.208 nm. F o r Cu(100) the formate again sits perpendicular to the surface, b u t deductions of o t h e r aspects of surface geometry from S E X A F S d a t a have been more controversial. It n o w seems likely t° t h a t the a d s o r p t i o n site is again a t o p a 390

% 5

19 -17 -15 -13 -11 -9 Ene~g~

-7

-g

-3

I

(eV)

Figure 3. Calculated distribution of electron states for formate overlayer

on three layers Cu(110): (A) total density of states, and its projection on (B) formate radical, (C)-(D) surface Cu atoms, (E) central layer Cu atoms, (F) back Cu surface. surface Cu a t o m , with the plane of the molecule at 45 ° relative to the C u - C u nearest n e i g h b o u r directions in the surface. This is the geometry we use in o u r model here, with the same nearest C u - O distances as o n Cu(110). Figures 3 a n d 4 illustrate the resulting distributions of electron states for formate on a three-layer slab of copper in the (110) a n d (100) orientations respectively. In the first case we use a 1/2 m o n o l a y e r coverage (which allows intermolecular distances close to v a n der Waals radii), in the second we model a 1/4 monolayer, with a considerably larger molecular separation. We show the total density of states a n d its projections on the orbitals of the a d s o r b a t e a n d on those of the copper a t o m at the a d s o r p t i o n site, as well as the weights o n other surface copper atoms, on the central layer a n d on the back surface. C h e m i s o r p t i o n - i n d u c e d shifts in the calculated molecularorbital energies of occupied states derived from the formate are in

D W Bullett and W G Dawson." Quantum chemical studies of formate on copper surfaces

HCO0 om C u ( 1 0 0 )

H

G

F t~ 0

~

E o c

c3

ct. 0 2~

~b £3

A 19 -17 - 1 S - 1 3 -1 1 - 9 Emergy

-7

-5

-3

1

(eV)

Figure 4. Calculated electronic structure for formate on Cu(100) surface: (A) total density ofstates, and projections on (B) formate orbitals, (C)-(F) surface Cu atoms, (G) central Cu layer, (H) back Cu surface.

IsotaLed

HCO@ m o n o t ~ y e ~ s

~q d w ~L 0

W

-~-17

_

I

'I

5-1~-~1

/91

17 I

I~

-s

-1

Ene~'gy ( e V )

Figure 5. Calculated electronic structure for formate monolayers after removal of metal: (A) (110) surface, (B) (100) surface.

fact relatively small, indicating that the covalent bonding component of the formate to the surface is rather weak. In the energy range shown, the adsorbate bands form three main groups, centred approximately 12, 9 and 6 eV below the Fermi energy Ep. (The highest occupied orbital lies at - 5 . 3 eV in our Cu(100) calculation, and at - 5 . 5 eV in our Cu(ll0) calculation.) The lowest of these is the 4a I molecular level, ~ 12.3 eV below E v and essentially unshifted from its original position, but broadened slightly, mainly by the interaction between neighbouring formate units. The middle peak is a composite from the 5al, Ib 2 and 3b~ orbitals of the formate; the 5a 1 contribution is dominant at about 9.8 eV below E e, while both the lb 2 and 3b I orbitals contribute in the energy region 7.8-9.3 eV below E r. The upper peak derives from the la z, 6a I and 4b t formate levels. These three orbitals produce only two peaks in the chemisorbed system. The 4b~ orbital is the state which interacts most strongly with the Cu d-orbitals at the adsorption site, so that during adsorption the energy of the 4b t state is pushed down about 1 eV, to be coincident with the l a : energy. The calculations for chemisorption on either surface thus show a weak (6a 1) peak 4.9 eV below E r, and a stronger (la2+4b~) peak about 5.6 eV below Ev. It is apparent from the projection of the local density of states on the copper atom of the adsorption site (curves D and F in Figures 3 and 4 respectively) that the only appreciable interaction of the metal surface is the feature at 5.6eV below E r , the ~r-bonding interaction of copper with the formate 4b~ state, together with a small peak pushed above the top of the d-band from the corresponding antibonding interaction. An electron in the 4bl state of the molecule spends most of its time on the oxygen atoms; this state can be approximately represented by antibonding combinations of p-orbitals on the two oxygens, with some participation from the carbon Px orbital. It thus lies fairly high in energy (and is in fact the singly occupied orbital in our calculation for neutral HCOO) and has a strong overlap with copper d and s, p orbitals at the surface site when formate adsorbs on a metal. Relative to the negatively charged formate ion, the d-orbital contribution to the chemisorption bond energy appears to be rather modest, since both bonding and antibonding Cu-O peaks are filled, nor do we see any substantial hybridization of occupied states in the metal with previously unoccupied ~r, or ~zstates of the formate. Another way of seeing the weakness of the covalent contribution to the adsorbate-substrate interaction is to compare Figures 3 and 4 with Figure 5, where we plot the density of states for isolated monolayers of formate in the two geometrical arrangements. Removal of the metal surface causes the 4bl-derived band to revert towards its original energy. Our calculated electronic energy levels for the surface formate species correlate fairly well with features reported by Lindner et al 5 in the angle-resolved photoemission spectra for formate on Cu(ll0). These workers identified peaks in the photoemission spectra at the following binding energies (eV) relative to Er: 13.0 (4al); 9.7, 9.6 (5a 1, 3bl); 7.8 (lb2); 5.1, 4.7, 4.8 (6a 1, la2, 4bx). Detailed comparisons of the calculated electronic structure with the ARUPS data will be reported more fully elsewhere.

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392

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