The chemisorption energy of CO on a disordered binary alloy

m

Surface Science 269/270 (1992) 331-334 North-Holland

'surface

science

The chemisorption energy of CO on a disordered binary alloy Hui Zhang Department of Phystcs, Shenyang Normal College, Shenyang 110031, People's Repubhc of China Recewed 20 August 1991; accepted for pubhcatmn 11 September 1991

The one-dimensional tight-binding model and the one-electron chemisorpt~on theory are used to calculate the chem~sorption energy of CO on a Nl-Cu d~sordered binary alloy surface, in which the coherent-potentml approximation and the techmque of complex-energy-planemtegration are adopted The change of surface segregation due to chem~sorption ~s also investigated The chem~sorption of the CO/N~-Cu system can change the surface component concentratton drastically, thereby influencing the chemlsorption energy. The result shows that the chem~sorptionof the system strongly constrains surface segregauon, resJtmg m chemisorptmn properties more hke those of CO/N! rather than those of CO/Cu.

I. Introduction

Since alloys are widely used in catalysis in recent years [1], it is, therefore, important to study the chemisorption theory of alloy surfaces. Van Santen and Sachtler [2,3] employed a cluster model of adsorption to examine the effects on the chemisorption bond of alloying two substrate metals. Sulston et at [4] used the coherent-potential approximation (CPA) and the A n d e r s o n - Newns (AN) [5] approach to calculate the chemisorption energy and the adatom charge transfer of H on disordered binary alloys (DBA). Tracy [6] studied the adsorption of CO on Ni as well as Cu using LEED, A u g e r and work function measurements and obtained a lot of experimental results. In the present work, the chemisorption energy of C O on a N i - C u D B A is calculated by using the CPA 'nd the chem~sorption theory of Einstein and Schrieffer (ES) [7]. The phenomenon of surface segregation is also incorporated into the model of DBA, m the stmplest way, by confining it to the surface layer. In p a r - e ular, the c h a n g e of surface segregation due to chemisorption is investigated in detail. The surface c o m p o n e n t concentration Xs (Xc) is calculated employing the expressions [8] of surface concentration of an A , B l_~ alloy before (after)

chemisorption. Different from atomic hydrogen adsorption [4], the change of the surface component concentration due to the chemisorption fo~ the C O / N i - C u system cannot be neglected. which affects drastically the chemisorption energy. The chemisorption energy changes greatly with the increase of the adsorbate coverage.

2. Model and formalism

Within the one-dimensional tight-binding approximation, the model of an effective adsorbate is introduced to describe CO molecules with various adsorbate coverages, that is, the attached CO molecules are effectively considered as an adatom and an occupied site a. The chemisorption system consists of an a d a t o m interacting with a semi-infinite, monatomic chain of the D B A (fig. 1) The model of a substitutional DBA developed by Ueba a

Effective adsorbate

1

2

q

4

DBA

Fig 1 One-dimensional model deplctmg the chem~sorptlon

0039-6028/92/$05 00 © 1992 - Elsewer Science Pubhshers B V All rights reserved

process

H Zhang / Chermsorption energy of CO on Nl-Cu

332

et al. [9] is adopted. The virtual-crystal Hamiltonian for the substrate is then H~= Y'. {a, l t > < i l - T ( I,><, I)}. t=l

(1)

In (1), the effective bond strength - T between nearest-neighbor atoms may be approximated by

T= X~TA+Xb(l - Xb) (rA+ TB) +(1 --Xb)2TB,

i> 1 i = 1 before chemisorption i = 1 after chemisorption,

where A = eA - E B, e a (%) is the electronic energy of atom A (B) and X~ (Xc) is the surface concentration of the A component at the first site of the DBA before (aftra) chemisorption. Within the CPA, the coherent-potential (CP) [%10] assocmted with the ~th atomic site is mtroduced as i>1 i = 1 before chemisorption i = 1 after chemisorption,

(7)

The self-consistency condition [11] for the bulk and surface CP's is

~( E) = [a(I-X,) -O',( E)]

xG,(E)[aX,+O',(E)],

(3)

o'b,

y=XcTA + (1--Xc)ys.

i=b,s,c,

(2)

- T A ( - T B) being the bond energy between two A (B) atoms and Xb the bulk concentration of the A component. By confining surface segregation to the first (surface) layer, the virtual-crystal electronic energy [4] at atom i is XbA + eB, a, = ~Xszl + e B, (X¢A + % ,

In (6), Eett is the effecti've level of the effective adsorbate attached to the surface atom of the substrate by a bond of energy 3' which can be represented by

(8) where Gb(m, m) is a diagonal element of the effective Green's function (GF) for the infinite DBA, and G~(1, 1) [Gc(1, 1)] which can be determined from the bulk G F via the Dyson equation is that of the surface G F at site 1 before (after) chemisorption. In addition, o-b, o's and o-c can be determined from the self-consistent equation (8). The effective GF of the effective adsorbate is defined by G~ = ( E - Eefr + i0 + ) -I

According to ES's chemisorption theory [7], the change in the density of states (DOS) caused by chemisorption Is 1

0

Ap(E) = - --Im oE[lndet(1-G°V)], 77

where

0)

(4)

where o'b is the bulk CP and o"s (%) is the surface CP before (after) chemisorption. So that, before chemisorption the effective Hamiltonian for the semi-DBA is

(9)

V=

Gs '

(0

( x c - xs)a + (o'c- o's)

)

.

(10)

(11) (12)

The chemisorption energy can be written as

O~

Heff= Hv + ~cr,(E)lt>(il,

(5)

aE = 2fE~( E - EF)Ap(E) dE, 4

(13)

- -~.

t=l

and after chemisorption the effective Hamdtoman of the chemisorption system ~s

HcCn=

Eeff[

a>(a[ + H e f t + [( X c - g s ) A

+ (o'¢- o's)] I 1><11 + ~ , ( l a ) < l l + I I>
(6)

where f is the Fermi level. In the usual computation of the chem~sorption energy of (13) by numerical quadrature, the integrand can have sharp peaks due to the presence of resonances. For strong adatom-substrate interaction ,/, localized states exist outside the substrate band [12-14], whose energy levels must be

H Zhang / Chemtsorpuon energy of CO on Nt-Cu

determined separately. A direct calculation of this integral is rather cumbersome. The technique of complex-energy-plane lntegrahon [15] is, therefore, adopted to calculate the chem~sorption energy, i.e., 2 AE=Tf ° Ref(z)

dy,

(14)

where

f(z)

=

(Z--EF) a

X az XGs(z)

In (14) and Z =

lr,{1 - [(X~-X~)A

+

(o-c-C,)]

(15)

-~]2ea(z)es(z)}.

333

Table 2 Surface ( X , or X , ) versus bulk ( X b) Ni c o n c e n t r a n o n for segregated N I - C u alloy u n d e r the adsorbate c m c r a g e s 0 : 1~5 and 1 0

Xb

X,

Xc/(o = 0.5)

Xc/(0 = 1 o)

0.0 0 1 0.2 03 0.4 05 0 6 0.7 08 09 10

0.00000 0 00009 0.00020 0.00035 0.00054 0.00081 0.00122 0.00189 0.00324 0.00725 1.00000

0.00000 0.04314 0.09209 0.14813 0 21290 0 28863 0 37834 0 48632 0 61875 0 78502 1 00000

0.00000 0.96495 0.98411 0.99067 0.99398 0.99598 0.99732 0 99827 0 99899 0 99955 1 00000

(15),

E F + i y,

(16)

y being real. In actual computation, the integral of (14) is transformed into one with finite limits by the substitution y = x / ( l - x), and then evaluated with standard Gaussian quadrature.

-0.6

-0.7

3. Results and discussion

Calculations of the chemlsorption energy of the C O / N i - C u system have been performed over the full range of bulk concentrations (Xb), utilizing the pure-substrate parameters found m table 1. E~ff is chosen as - 7 . 7 eV. The surface component concentration X s (X c) is shown in table 2. The chemisorption energy A E versus Ni bulk concentration X b is shown in fig. 2. The results are as follows: (1) In the case of no surface segregation (curve a), A E decreases with the increase of X b, which displays a monotonic and almost hnear behavior.

-0.8

-0.9

-1 o0

g, o~

<1

-1.1

-1,2

Table 1 Parameters for pure metals Ni, Cu ( a d a p t e d from ref [16,]) Parameter

EA(B) (eV) E F (eV) TA(B) (eV) ~'A(m(eV)

-1,3

Element

Nt

Cu

- 6 260 - 4 500 0 950 2 763

- 7 370 - 4 460 0 675 3 320

- 1 . 4 0,0

'

' o,2

'

0.4

0:6

o:8

.o

xb Fig 2 Chemlsorptlonenergy AE for CO/Nt-Cu versus bulk Ni concentration for (a) X c = X~ = X b, (b) X c = X~ ¢ X~,, (c) for O = 0 5 X c ~s X, ¢ X b, and (d) for 0 = 1 0 A'~ ¢ X, ~ X h

334

H Zhang / Chem,.sorptton energy of CO o~, Nt-Cu

(2) Curve b is the result of the assumption that the surface segregation is not changed by the chemisorption, and it is well established [16] that N~-Cu alloys possess an enriched Ct: concentration in the surface layer for all compositions (table 2), resulting in chemisorption properties of CO/Cu-like behavior. (3) Curve c means that under the adsorbate coverage 0 =0.5 the chemisorption constrains surface segregation, so, its shape is similar to that of curve a. (4) When 0 = 1.0 (curve d), the chemisorption strongly constrains surface segregation, resulting in an enriched Ni concentration in the surface layer for all compositions and chemisorption properties more of CO/Ni-like rather than of CO/Cu-like behavior. Even a small amount of Ni, added to a pure Cu substrate, produces a significant change in AE, compared to cases (1) and (2). Important conclusions are that the chemisorption for the system of C O / N i - C u t,an change the surface concentration, constrains surface segregation and makes that the chemisorption energy decreases. Certainly, the conclusions do not suit all binary alloys Although the model adopted in this pape~ ix stmple, the results and conclusions obtained are new and encouraging. Thus, the model is utilized for restructure purposes rather than as an earnest attempt to reproduce reahty.

Acknowledgements It is a pleasure to thank Professors Shan-jun Yu of Liaonmg University and Zong-xian Yang of Henan Normal Unwersity, for their advice and useftd discussions throughout this work.

References Ill J.H. Smfelt, J. Catalysis 24 (1972) 283; 29 (1973) 308. 12] R.A. van Santen, Surf. Scl. 53 (1975) 35. 13] R.A. van Santen and W M.H. Sachtler, Surf. Scl. 63 (1977) 358 [4] K.W. Sulston, S.G. Davison and W.K. Lm, Phys. Rev. B 33 (1986) 2263. [5] D.M. Newns, Phys. Rev. 178 (1969) 1123 [61 J.C Traey, J. Chem. Phys. 56 (1972) 2736; 56 (1972) 2748. [7] T L Einstein and J.R Schneffer, Phys Rev B 7 (1979) 3629. [8] D. Tomanek, S. Mukherjee, V. Kumar and K.H. Bennemann, Surf Sel 114 (1982) 11 [9] H. Ueba and S. Ichimura, Phys. Status Solidi (b) 92 (1979) 307 [10] N.F. Berk, Surf. Sct. 48 (1975) 289. [11] W H Butler, Phys Rev B 8 (1973)4499 [12] K W Sulston, S G. Davlson and W K Lm, Surf. So 311 (1984) 148. [13] T Zhang and W K Lm, Surf Scl. 605 (1987) 180 [14] T Zhang and W K Llu, Surf Scl 343 (1987) 185. [15] H Dreysse and R Rledlnger, Phys Rev B 28 (1983) 5669 [lb] Z X Yang and T Zhang, Acta Plays Smlca 40 (1991) 269