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Transition metal bonding functions and their applications in catalytic adsorptions and reactions
Journal of Molecular Catalysis, 64 (1991) 85-101
85
Transition metal bonding functions and their applications in catalytic adsorptions and reactions Part II. Micromechanism of CO chemisorption and a new concept of CT-T coordination K. H. Huang P. 0 Box 276, Department of Chemistry and Institute of Physical Chemistry, Xiamen University, Xiarnen 361005 (China) (Received January 22, 1990; accepted July 2, 1990)
Abstract A micromechanism of CO adsorption and a new concept of u-r coordination on transition metal are proposed in this article. Based on experimental facts, we assume CO 5u and/ or CO 1~ interacts with the representative M.0.s of the metal valence band, WMi, Vs) and ly(Mi, Vd), to form the bonding M.O. group and antibonding M.O. group. The bonding group is located below the Fermi level (EF), in which some M.0.s are much more characteristic of metal orbitals (denoted as M-CO u-bondings) while some M.0.s exhibit slight metal orbital characteristics, which belong to the excited valence M.0.s of adsorbed CO, conventionally assigned as adsorbed CO 50, CO lw and CO 4~. The calculated data indicate that the peak positions of adsorbed CO 50, CO 1~ and CO 4u are significantly higher than their corresponding M.0.s in the gaseous CO molecule, i.e. adsorbed CO is in an excited (or activated) state. The total energy generated (AE) from adsorbed CO 5u, CO 1~ and CO 4u can be used as a qualitative parameter for characterizing the ability for CO dissociation. On the other hand, the antibonding empty M.O. group of M-CO is located above the EF, which exhibits some characteristics of metal d orbitals. The hybridization of CO 27r with dT orbit& in the Vs, Vd bands and dr orbit& of the antibonding M.O. group of M-CO bondings results in the formation of unoccupied M.0.s with CO 2~r-M dr character. These M.0.s plus those unoccupied M.0.s without CO 27r-M dr character contribute the adsorbate-derived resonances, located 3-5 eV above E, and observed by Inverse Photo-Emission (IPE) difference spectra. We have used orbital overlap integrals of S(C0 50, du, Vd) and S(C0 2 r, dr, Vd) to characterize the relative competitive abilities for hybridization of CO 50 and CO 27r with d orbitals. The calculated results show that CO 5a possesses a stronger ability to hybridize d orbitals in the Vd band than does CO 2~r, thus the peaks of adsorbate-induced empty levels are shifted farther from the d band when the competitive hybridizing factor [CHF=S(CO 5u, do, Vd)/S(CO 2rr, dr, Vd)] is increased. The calculated data demonstrate that the peak positions of CO adsorbate-derived resonances of Cu, Ni, Pd and Pt metals, observed by IPE difference spectra, are in good parallel with their CHF values. Moreover, the values of CHE also demonstrate that CO u-bonding stimulates d electrons to transfer upward from the d band to the Vs band, where much more CO 2~-M drr character exists. We propose here a new concept of d back-donation, i.e. d electrons transfer from the occupied d band to the unoccupied M.0.s exhibiting CO 2-M drr character in the Vs and Vd bands, which weakens the rr bond of C-O and simultaneously strengthens the M-C bond; these phenomena have been confirmed by IR spectroscopy and EELS. The d back-donation is represented by the B bonding function. The calculations of A, B and Al3 bonding
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86 functions indicate that the AB bonding function of CO adsorption on Cu is signitlcantly smaller than that on Ni, Pd and Pt, so that CO adsorbtion is weak on Cu and is strong on Ni, Pd and Pt. Our micromechanism and our new concept of CT-~ coordination provide a unified interpretation of various CO adsorption electronic spectra from below to above the EF, i.e. from occupied orbitals to empty orbit&; and a unified interpretation of the adsorbate vibration spectra measured by EELS and IR spectroscopy. The advantages of our new concept have been discussed and compared with the conventional concepts of Blyholder and CO 2r-derived resonances.
Introduction
The theoretical model of transition metal bonding functions has been given in our previous paper [ 11. In subsequent papers, we will apply this model to characterize adsorptions, dissociations, catalytic reactions, support and promoter effects occurring on transition metals. A wealth of new experimental methods have been used for studying these processes on the surface of metals in atomic detail; if an effective bonding function theory can be further applied, it might be very useful to explain the experimental facts and disclose the nature of the processes. It is also possible to develop some new concepts for designing novel catalysts, which is final goal of our efforts. Research on CO catalytic hydrogenation has received much attention since 1974 and many data such as CO adsorption spectra, structures, heats and dissociations on various transition metal surfaces, have been collected, investigated and reviewed [ 2-41, although there are still many unanswered questions. In this article, we discuss the concept of CO u-rr coordination, which deals with the microscopic mechanism of CO adsorption and the relationships between CO adsorption bonds and the electronic structure of the metal. Furthermore, in catalytic adsorptions and reactions on transition metals, we employ the u-r coordination concept to discuss bond forming and breaking in a molecule, including how to activate the saturated u bond of H-H, C-H, C-C and the unsaturated rr bond of C=C, C=C, N=O and N=N. Obviously, this is most important to establish a correct concept of u-r coordination. Recently, Ishi et al. [5] addressed a question of the inadequacy of the d back-donation concept of Blyholder [ 61. Based on recent measurements by ARUPS [7], I-UPS [8], HREELS [9] and NEXAFS [lo], it seems more likely that the adsorbed CO 27r orbital is unoccupied in the neutral ground state in both strong and weak adsorption systems. Gumhalter et al. [ 1 l] proposed the hybridization of the CO 27? orbital with the substrate s, p and d bands producing the CO 27ir-derived resonances. These authors believed that even if the center of the CO 2#-derived resonances appears above the Fermi level (note: CO 2n’c is equal to CO 2~ and is noted in our paper similarly hereinafter), its wing may still extend below the Fermi level, allowing fractional charge to transfer into the resonance (i.e. backdonation into the resonance); moreover, the occupied and unoccupied parts of the resonances (which may be regarded as a fractionally occupied localized band of states)
87
may become a source of intra-resonance dynamic relaxation and polarization processes, which are largely localized on the adsorbate. But the authors did not address the following questions: What are the effects of other adsorbate M.0.s such as CO 5~ and CO 1~ on the formation of the adsorbate-derived resonances? What are the detailed wave functions of metal s, p, d bands which hybridize with the CO 27r orbital? What is the driving force for the d electron to transfer to the empty levels of adsorbate-derived resonances? Can we calculate rapidly the relative amounts of d back-donation on various transition metals? How can we reach a unified interpretation of electronic and adsorbate vibration spectra for CO adsorption? These five questions are important to enable us to use this concept in resolving the practical problems of catalytic adsorptions and reactions. In this article, we shall revise the conventional concept of U-W coordination, extend the concept of CO 27rderived resonances, and propose our new concept of CO (T-T coordination.
The method
of calculating
metal bonding functions
Calculation formulas
The molecular orbitals (M.0.s) of the CO molecule are shown in Fig. 1, where 5a is the Highest Occupied Molecular Orbital (HOMO), 2rZ, 27rr, are the doubly degenerate Lowest Unoccupied Molecular Orbitals (LUMO), and lr,, 171; are the doubly degenerate occupied M.O.s, whose levels are lower than that of the 5u M.O. According to eqns. (L4) and (L5) of our previous paper [ 11, the following metal bonding functions can be calculated if we insert their appropriate R.A.O. (reference atomic orbital) and R.A.E. (reference atomic exponent) into the corresponding equations.
+ ~P(d,Vs)S(~(d,z,~),C2p,)
(1)
+ ~dP(s,Vd)S{~(SMi,CLd),C2p+} +
WOW
cL,P(d,Vd>s(~(d,,,E~),C2p,)l
= Icbm /wpt)(l ICLd)s{~(~~,CLd),Czpy)P(d,d.x,)
(2)
where C 2p, and C 2p, are the R.A.0.s of the A(C05a) and B(CO2r) M.O.s, respectively, their corresponding R.A.E. values being 1.725 and 1.425, which will be discussed in the next paragraph. &(COlr) and B,(C029~) have similar R.A.O.s, C 2p, and C 2p,, but with different R.A.E. values of 1.825 and 1.525, the reason for which will be discussed in next paragraph. Thus eqns. (1) and (2) can be also used to calculate A, and B,, but with different RAE. values. For the calculations of A and 4, it should be noted that only the metal orbitals which are electron-unoccupied can be used for accepting the two electrons from CO 5a or CO 17r.
88 A.0
Hybrid
A.0
.s
CZS-2Pz( \
Fig. 1. The synthesis Orbitals (LCAO).
.S
\
&
,)
Hybrid
of CO molecular
2 OZS-2Pz
orbitals
(M.0.s)
by Linear
Combination
of Atomic
When CO is dissociated into its C atom and 0 atom, the metal-donating bonding function as described in eqn. (L3) of our previous paper [ 1 ] can be shown as follows: De = I(dGJlJ)]U
l~,)p(s,Vs)s(~(~i,~),C2~,)
+ (1 lcL,)p(d,Vs)S{~(~,,~),C2p,)
(3)
+ (1 ICLd)P(s,Vd)S{~(sM(,Il~),C2pt) + (1 IE.Ld)p(d,Vd)SGf(d,2,~~),C2p,)J where Dc is the bonding function for characterizing the metal donating electrons to the dissociated carbon atom, using C 2p, as its R.A.O. and 1.625 as its R.A.E. By similar reasoning, Do is the bonding function for characterizing the metal donating electrons to the dissociated oxygen atom. We use 0 2p, as its R.A.O., and let the corresponding R.A.E. be 2.275.
89
How to select the R.A.0.s and R.A.E.s of CO adsorbate for calculating metal bonding functions As indicated in the previous paper [ 11, in order to prevent any error arising from the calculation of M.0.s from influencing our calculated conclusion, we select an appropriate Reference Atomic Orbital (R.A.O.) to represent the M.O. of CO bonding, and we select an appropriate atomic exponent for representing the energy level of this orbital (R.A.O.), denoted as the Reference Atomic Exponent (R.A.E.). The conditions of these selections are as follows: 1. The symmetry of R.A.O. must match that of the M.O. available for CO bonding; 2. The sequence of R.A.E.s must coincide with the energy sequence of the corresponding M.0.s for CO bonding. Based on the above selection conditions, the R.A.0.s and R.A.E.s for A(C05a), B(C027r), &(COlr) and B,(C023~) are shown in the second column of Fig. 2(a), (b), (c) and (d), respectively. From these figures, it is obvious that all selected R.A.0.s satisfy the first condition described above, i.e. the symmetries of the selected R.A.0.s and the M.0.s of adsorbate are matching. The second condition is also satisfied, for example, (1) the R.A.E. of the CO 50 M.O. is selected as 1.725, which is smaller than that of the CO 1~ M.O. (1.825). This sequence indicates that the energy level of the CO 5u M.O. is higher than that of the CO 1~ M.O. (see Fig. 1); (2) the R.A.E. of CO 27~ M.O. for end-on d back-donation is selected as 1.425, which is less than the R.A.E. of CO 2~ M.O. for side-one d back-donation, 1.525; this sequence corresponds with their energy level sequence, because CO 27~M.O. of B(C027r) consists of a C 2p, constituent, while the CO 2rrM.0. of B,(C027r) consists of both C 2p, and 0 2p, constituents, so the energy level of the former M.O. is higher than that of the latter. Parameters and structural configurations The width of the d band (IV”) and the work function (U) are listed in our previous paper [ 11, which is adapted from [ 121. The CO end-on adsorption is assumed to occur atop the atom, while the CO bridge adsorption is assumed be bridging two surface atoms at the shortest atomic distance of the normal crystal lattice. The distance C-M is assumed to be the sum of the carbon covalent radius (0.728 A) and the metal radius (half of the shortest atomic distance in the normal crystal lattice). Calculation schedule -
The micromechanism coordination
see [I]
of CO adsorption
and a new concept of -
Micromechanism of CO adsorption on transition metals According to our metal bonding functions model [ 11, adsorption proceeds on the metal valence band, i.e. adsorbate interacts with the three represen-
Metal d
(b)
R.A.E. = 1.425
COZIT
R.A.E. =1.725
CO56
B (COZlT)bonding
A(CO567 bonding
(d)
Metal d Y=
Metal S-d=2
P
=1.525
R.A.E.
QD
C
+ + 0
+
C02K
R.A.E. =1.825
COIX
-
-
Q-,0
Bs(C02K) bonding
A~(CO~TI) bonding
Fig. 2. The diagram of W-T bonding functions for CO adsorption on transition metals: (a), A(CO5w) end-on bonding; (b), B(C02r) end-on bonding (located near terminal carbon); (c), &(COl?r) side-on bonding; (d), B,(COBlr) side-on bonding (located on the side of CO).
;
Metal s-d=2
(a)
Cc)
g
91
tative M.0.s of the metal valence band. The three M.0.s are shown as follows: !P(Mi,Vs) =P(s,VS)+(%~,& !P(Mi,Vd) =P(s,Vd)&ki,& %Mi&,)
=P(~&&NsM~,E)
+P(d,Vs)4(d,i,h) +P(d,Vd)+(d,i,& +P(d7d&Hd~i,~)
on the Vs band on the Vd band on the L,
band
CO adsorption can be regarded as a process in which the CO 5a M.O. and/or CO 1~ M.O. first bonds (end-on or side-on, then changes to bridging) with the vacant M.0.s of ?P(Mi, Vs) and !P(Mi, Vd), in which the vacant metal orbitals can ‘accept’ electrons from CO 5a or CO 1~ to form a ubond. We use the ‘A’ bonding function to characterize the relative ability of a-bonding. If we wish to use the language of valence bond, it can be said that two electrons of CO 5u or CO 1~ are accepted by the vacant orbital. ‘A’ bonding makes the various M.0.s of CO approach the valence bands of metal within the chemical interaction range. Because CO 5u, CO lr, CO 29r and CO 4a M.0.s make bonding or hybridizing or perturbating relationships with the valence bands of the metal, the MOGs of IF’(Mi,Vs) + !P(Mi, Vd) + ?P(Mi, d,,,) will be changed to new MOG states of P*(Mi, Vs) + ??*(Mi, Vd) + ??*(Mi, d,,,). Simultaneously, the d and s electrons of metal and electron charges of various occupied M.0.s of adsorbate will be reconstructed in the new MOG states. The new interpretation of CO ao!sorptti electronic spectra and further disclosure of the micromechanism of CO adsorption In order to further understand the mechanism of CO 5a and CO lrr bonding, we have analyzed the electronic adsorption spectra of CO 5u, CO 1~ and CO 4u M.0.s over Group VIII metals [5], as shown in Table 1, where the negative value of the exit work function (U, eV) is selected as the value of the Fermi level (Er). The peak position (eV) of the M.O. with respect to vacuum is equal to the peak position (eV) with respect to EF plus to U, and listed in parentheses. The AE is equal to the peak position (eV) of the gaseous molecule state minus the peak position (eV) of the adsorbed state, i.e. the separation of peak positions between the gaseous molecule state and the adsorbed state termed the generated energy position (eV) of the M.O. after adsorption, compared with the corresponding energy position (eV) of the gaseous CO molecule. Table 1 indicates that the values of A(la-5~) on Group VIII metals are negative or zero. If negative, it means that the energy positions of adsorbed CO 5u and CO lrr are opposite their original positions of free CO molecule; if the A(llr-5~) is zero, it means the CO 5u and CO 1r levels are mixed. The data of A(lr- 5~) revealed that the CO molecule structure is reconstructed in adsorption. On the other hand, the calculated values of AE(C05u), AE(COl$ and AE(C04u) and AE (total) [the sum of AE(C05u), AE(COlr) and AE(C04u)l are all positive, except for AE(C05u) on Ir(ll1) and Pt(lll), which means that after adsorption almost all peak positions of CO 5u, CO 1~ and CO 4u occur at higher energy
92
TABLE 1 Peak positions (eV) of adsorbed CO M.0.s with respect to Er or to vacuum (in parentheses) and the higher energy position compared with the correspondingM.O. of gaseous CO molecule (AE, eV Substrates
u
Peak positions
(eV) 4u(ads)
AE (total) 11.1
0
1131
4.80
10.8 (15.30) 4.40
-0.2
1141
1.10
10.7 (15.7) 4.00
9.3
(1;:;) 4.20
8.1 (13.25) 0.75
11.2 (16.35) 3.35
8.75
-1.0
1151
(lZ5) 4.65
0
1161
1.69
10.7 (15.41) 4.29
10.57
(Z) 4.59
-0.50
(1::;8) 0.72
(1;:878) 4.12
11.2 (16.18) 3.52
8.36
4.98
1171
-0.70
(lK2) 0.68
(lZ2) 4.28
11.2 (16.32) 3.38
8.34
5.12
1181
- 0.60
(lZ7) - 0.47
1191
(lki7) 3.03
11.7 (16.97) 2.73
5.29
5.27
- 0.80
PO1
(lZ5) -0.85
(lZ5) 2.85
11.90 (17.55) 2.15
4.15
5.65
14.0
16.9
19.7
Sa(ah) Fe(ll1)
4.50 AE
$0) 1.90
1 n(ads)
(127:Yo)
co(ooo1)
5.00
(12)
AE Ni(ll1)
5.15
AE Ru(0001) 4.71 AE
&)
Rh(ll1) AE Pd(ll1) AE Ir(ll1) AE Pt(ll1) AE CC (gas)
Ref.
0
A(lr-
2.90
5~)
WI
positions compared with their corresponding M.0.s of the gaseous CO molecule. The positive values of AE (total) are reliable, although it is possible for some error from the selection of the Fermi level to occur, but it would not influence the positive value of AE (total). As is well known, the chemical adsorption would evolve heat, i.e. the total energy of the system after adsorption must decrease. Thus on account of the energy balance, it is more likely that there would be some ways to supply sufficient energy for reconstruction of the CO molecule, i.e to balance this positive value of AE (total). We propose that CO 5u or CO 1~ bonds directly with the M.O. group of !P(Mi, Vs) + ?P(Mi, Vd), and simuhaneously supplies the binding energy for the structure reconstruction AE (total). It is suggested that adsorption bonds are formed directly by coupling CO 5a and/or CO 1~ orbit& with the M.0.s
93
of ?J’(Mi, Vs) + flMi, Vd), where two new M.O. groups are formed. The first M.O. group is the bonding M.O. group, in which some M.O’s exhibit much more character of metal orbitals, located below the EF, and present a broad peak in the range O-5 eV below EF, denoted as a-bonds; other bonding M.0.s exhibit little character of metal orbitals, and belong to the excited valence M.0.s of adsorbed CO, conventionally assigned as adsorbed peaks of CO 5u, CO 1~ and CO 4a, located 5-12 eV below the Er, as shown in Table 1. The second M.O. group is the antibonding empty M.O. group, which hybridizes with CO 27r and metal s, p, d vacant orbitals to form new bonding empty M.0.s and new antibonding empty M.0.s; the bonding empty M.0.s are located 3-5 eV above the EF, and exhibit some characteristics of metal orbitals. From Table 1, the following regular pattern can be seen: AE(COlrr) > AE(C04u) > AE(C05a). This means that the r bond of CO 1~ is most significantly weakened (the mechanism will be shown below), and more energy is lost in adsorbed CO 1~ M.O. compared with other adsorbed valence M.0.s. The CO 50 and CO 4a M.0.s of the gaseous CO molecule, being formed from C 2s-C 2p, hybridization as shown in Fig. 1, can be described as belonging to a pair of bonding and antibonding M.0.s. When gaseous CO 5~7and CO 1~ bond with metal, simultaneously reconstructing this pair of M.O.s, it is equivalent to recombining C2s, C2p, with the metal M.O. group, reforming the CO molecule in an excited state, so that CO 4a is shifted to a higher energy position compared with its original position. The AE (total) values can be used to characterize the activated state (or excited state) of adsorbed CO. The facility for CO dissociation qualitatively parallels the AE (total) values. For example, CO is most easily dissociated on Fe and Ru, whose AE (total) values are the largest among the Group VIII metals, and CO is difficult to dissociate on Pt, whose AE (total) is the smallest. The new concept of adsorbate-induced empty M.O.s, relative hybridizing abilities of CO 27&H drr and CO 5~M da on Vs and Vd bands, and the new concept of d back-donut&m When studying CO bonding, another effect should be considered, i.e. the fact that the CO 2~ M.O. will hybridize with the M.0.s of ly(Mi, Vs) + WMi, Vd) and with the antibonding M.0.s of M-CO bonds to form empty M.O.s, which can be measured by IPE spectroscopy and are denoted as CO adsorbateinduced resonances. When CO 2~, CO 5u and CO 1r simultaneously hybridize with the orbit& of ?#(Mi, Vs) + flMi, Vd) (noted as Vs and Vd bands, similarly hereinafter), the question arises as to their relative abilities of hybridization. In order to analyze the relative hybridizing abilities, we consider the orbital overlap integrals of CO 2~, CO 5u and CO 1~ with the dr, du orbitals in Vs and Vd bands, as shown in Table 2, in which S(CO277, dr, Vs) denotes the orbital overlap between CO 2~ and dv orbitals in the Vs band, while S(CO5u, du, Vd) denotes the orbital overlap integral between CO 5u and da orbitals in the Vd band, etc. We use the R.A.0.s of CO 27
94 TABLE 2 Orbital overlap
integrals
between
metal d orbital and CO 2n; CO 5a and CO 1~ M.0.s
co
Metal S(CO2q S(CO2q S(CO5q S(CO5u, S(COlq S(COlq
drr, dq du, du, do, du,
Vs) Vd) Vs) Vd) Vs) Vd)
0.3300 0.2997 0.1042 0.1377 0.1033 0.1681
Ti 0.3619 0.2886 0.0947 0.2078 0.0964 0.2028
V 0.3913 0.2535 0.0828 0.2245 0.0871 0.2209
Cr 0.4022 0.2641 0.0224 0.2277 0.0322 0.2251
Mn 0.3771 0.1620 0.1234 0.2035 0.1253 0.2547
Fe 0.3965 0.1463 0.1180 0.1924 0.1218 0.1882
0.3862 0.1120 0.1368 0.1644 0.1395 0.1600
Ni 0.3803 0.0890 0.1498 0.1403 0.1520 0.1360
Metal S(CO2q S(CO2q S(CO5u, S(CO50, S(COlq S(COl?r,
dw, dr, du, du, da, da,
Vs) Vd) Vs) Vd) Vs) Vd)
Y 0.2988 0.2997 0.0334 0.0602 0.0353 0.0603
Zr 0.3238 0.3333 0.0190 0.1057 0.0232 0.1049
Nb 0.3158 0.3636 0.0431 0.1074 0.0345 0.1086
MO 0.3358 0.3566 0.0436 0.1578 0.0345 0.1575
TC 0.3771 0.3075 0.0166 0.2177 0.0231 0.2143
Ru 0.3637 0.2887 0.0255 0.2235 0.0168 0.2220
Rh 0.3699 0.2402 0.0060 0.0473 0.0015 0.3677
Pd 0.3715 0.2126 0.0171 0.2279 0.0233 0.2044
Metal S(CO2q S(CO27r, S(CO5u, S(CO5u, S(COlw, S(COlp,
dq dq do, do, da, du,
Vs) Vd) Vs) Vd) Vs) Vd)
La 0.2711 0.2795 0.0041 0.0196 0.0074 0.0219
Hf 0.3101 0.3458 0.0313 0.0388 0.0242 0.0421
Ta 0.3342 0.3765 0.0442 0.0828 0.0355 0.0852
W 0.3519 0.3764 0.0441 0.1368 0.0350 0.1375
Re 0.3626 0.3248 0.0304 0.1905 0.0220 0.1887
OS 0.3757 0.3198 0.0241 0.2218 0.0156 0.2187
Ir 0.3805 0.2736 0.0084 0.2537 0.0035 0.2441
Pt 0.3582 0.2505 0.0346 0.2340 0.0200 0.2297
SC
CO 5a and CO 1~ to calculate the overlap integrals, as described in the first section of this text, which is useful for the purpose of relative comparisons. Table 2 indicates that the overlap integrals between CO 27r and metal dr decrease significantly from the Vs band to the Vd band on all transition metals except those whose d levels are fairly high, such as 4d’-4d4 and 5d’-5d4 metals, in which the d orbitals on the Vd band match better with CO 27r than those on the Vs band. In contrast, the overlap integrals between CO 5a and metal du orbital increase greatly from the Vs band to the Vd band on all transition metals except Ni, whose A.0.E.s of s and d orbitals are the largest of all transition metals (d’-d’), in this case CO 5a matches less well with the da orbital in the Vd band than in the Vs band. Thus for all transition metals except 4d’-4d4 and 5d’-5d4 metals, the abilities for CO 277-M dr couplings are larger on the Vs band than those on the Vd band. On the contrary, on all transition metals except Ni, the values of S(CO5q du, Vd) are greater on the Vd band than on the Vs band. Even on Ni, the value of S(CO5q da, Vd) is still larger than S(COBr, drr, Vd), and the character of CO 5~ would be greater than that of CO 2~ in the Vd band. In summary, the CO 5u bond is more likely to involve the lower d energy levels in the Vd band. This result is very interesting, since it means that the formation of CO u-bonds favours movement of d electrons from the occupied levels upward to the higher empty M.0.s. The empty M.0.s consist of those M.0.s with CO27r-M2r character and those M.0.s without C02+Mdr character. In catalysis, special attention is paid to those M.0.s with CO 27~M
96
dT character; if d electrons transfer to these M.O.s, it is called d backdonation in the CO 27r M.O. If the dr orbital is placed in the Vs band, on basis of the above conclusion, CO 2+M dr will be stronger in the Vs band than in the Vd band. The probability of d electron direct back-donation to the CO 27r vacant M.O. is very small, because the d electron must first transfer to the empty M.0.s of the Vs, Vd bands and the unoccupied antibonding M.0.s of the M-CO bonds, which agrees with the facts indicated by Ishi et al. [ 51. According to quantum chemistry, if the antibonding M.O. (CO 274 is filled by d backdonated electrons, the 7 bond (CO lrr-bonding M.O.) between C-O will be weakened; and simultaneously, the new bond between M-C (i.e. C02r-Mdw coupling) will strengthen the M-C bond; so the vibration frequencies of C-O and M-C are decreased and increased, respectively, as observed by IR and EELS. This will be further discussed in two subsequent papers. Representation of d backdonation As described above, if we can place the da orbital in the Vs band, it is equivalent to raising the d orbital energy position to approach the CO 27r level, and the overlap integral between CO 27r and da will be increased. The formation of a metal energy band is an effective way to increase the d percentage function in the Vs and Vd bands. We use the ‘B’ bonding function to characterized d back-donation. This function depends on four parameters (see eqn. 2). First, the overlap integral between CO 27r and M dr orbital, noted as S(CO2rr, dr) or S[@(%=, &, CBp,], where C2py is the R.A.O. of CO27r, and dJZ is the R.A.O. of Mdrr, which characterizes the hybridizing ability between CO 2a and the reference metal da orbital. The larger S(CO27r, dn) is, the greater the percentage characteristic of CO 27~M dr in the Vs and Vd bands would be. Second, the mean energy potential of the d occupied band, represented by the effective eXpOnent of d orbital, j,&&The Smaller ,_&d is, the smaller the separation between CO 27~and the d occupied band would be, which is equivalent to an increase in the d orbital position to approach CO 27~and results in enhancing the percentage CO 2rr-dr character in the unoccupied M.0.s located between CO 27r and the d occupied band. The third parameter is the width of the d band (wd); a larger wd results in increasing the overlap of s-d bands and decreasing the energy level separation between CO 27r and the d occupied level, both effects which enhance the COB+d7r character in the unoccupied M.0.s located between the C02~ and the d occupied band. The last factor is the percentage function of d orbital in the d occupied band, P(d, d,,,). The larger P(d, d,,,,) is, the greater the probability of d electron back-donation. According to the above analysis, the B bonding function can easily be derived, as shown in eqn. (2). The r bond of C-O would be weakened with increasing B bonding function, and at the same time the r bond of M-C would be strengthened, with increasing AB bonding function. Our calculated results are in good agreement with the relative amounts of vibration frequencies of C-O and M-C on various Group VIII metals as measured by IR and EELS.
96
In brief, d back-donation is defined as that d electron charge transfered from the d occupied orbital to the unoccupied M.O. consisting of CO 2rr-M dr character, to form a CO 2?r-M drr bond. The d back-donation weakens the bond of C-O and strengthens the M-C bond simultaneously. On the other hand, CO cT-bonding promotes d back-donation. This new concept provides a unified interpretation of the broad peak in the range of O-3 eV below the Fermi level, measured by FEED, UPS and SPIES with CO adsorption and the adsorption vibration spectra measured by EELS and IR.
Comparison of our new concept of a--a coordination with the conventional concept proposed originally by Blyholder and the concept of CO 2pderived resonances
The conventional concept for u-r end-on adsorption over transition metals was proposed originally by Blyholder [6 1. Under this conventional concept, a (+ bond infers that a vacant metal atomic (T orbital (hybrid of s and ds) accepts electrons from CO 5u, as shown in the first column of Fig. 2(a); a r bond infers that some electrons are back-donated from a metal atomic dr orbital, such as dyz, to the LUMO CO 27r M.O. (belonging to the terminal carbon), as shown in the first column of Fig. 2(b). If CO adsorption is side-on instead of end-on, the (T bond means that an unoccupied metal atomic u orbital (hybrid of s and d,z) accepts electrons from CO lq as shown in the first column of Fig. 2(c). The r bond of side-on CO adsorption means that electrons are back-donated from metal atomic drr orbital such as duz to the side part of LUMO CO 27~M.O., as shown in the first column of Fig. 2(d). According to our model of metal bonding functions, the CO adsorption g bond occurs when the fractional vacant cr orbitals (hybrid of s and d,z) of the WMi, Vs) and !&‘(Mi, Vd) M.0.s accept electrons from CO 5a, and the CO adsorption r bond occurs when d electron charges are back-donated from Iy(Mi, d,,,) into the unoccupied M.0.s exhibiting CO 27r-M dr character in the Vs and Vd bands. In brief, if we use the fractional vacant u orbitals of Vs and Vd bands instead of the conventional vacant atomic s orbital, use the CO 277-M drr unoccupied orbitals of Vs and Vd bands instead of the conventional vacant CO 297 M.O., and use the IY(Mi, d,,,) instead of conventional atomic d occupied orbital, then we can employ the language of the valence bond, and use the conventional language of U-T coordination. In other words, all the pictures in Fig. 2 are still correct if the new model of metal bonding functions is employed, but note that all conventional metal atomic orbitals would be changed to their corresponding fractional orbitals of the metal valence band. The metal u-r bonding functions for CO endon bonding and side-on bonding are denoted as A(CO5u)-B(CO2r) and &(COl79-B,(CO27r), as shown in Fig. 2(a), (b) and Fig. 2(c), (d), respectively; where A(C05a) or B(C027r) indicates that the metal valence band accepts electrons from CO 5uor back-donates into CO 2rduring CO end-on adsorption,
97
and &(COln) or B,(C027r) means that the metal valence band accepts electrons from CO 1r or back-donates to CO 27rduring CO side-on adsorption, respectively. The advantage of using the language of the valence bond is obvious; for example, in B(C027+) bonding, it makes a rr bond between M-C, resulting in an increase in the bond strength of M-C, which is clearly depicted in Fig. 2(b). This theoretical image has been confirmed by CO adsorption vibration spectra and will be further discussed in two subsequent papers. The difference between the concept of CO 2#-derived resonances and ours lies mainly in the following five points. First, our model emphasizes that the empty levels above the Fermi level were derived not only from CO 27?, but also from other M.0.s of adsorbed CO, especially CO 5a and CO 171: Second, the model of CO 2+-derived resonances did not account for the wave composition of the valence band. Our model can rapidly calculate the representative wave functions in the valence band. Third, we propose that orbital overlap integrals to be used to estimate the competitive hybridizing abilities of CO 2~ and CO 5a or CO 1~ with respect to d orbit& in the Vs and Vd bands, interpretating successfully the peak position of CO adsorbateinduced resonances, which will be further discussed below. Fourth, in fact CO adsorbed on Group VIII metals is quite stable, if its stability is only due to the relaxation effect as proposed by the concept of CO 27ir-derived resonances, which is difficult for chemists to accept; however, our model emphasizes that the chemical bonds M-CO o-bondings and d back-donation originate in the metal valence band such as, resulting in the formation of a r bond between M-C, which further stabilizes CO adsorption and at the same time activates the r bond of C-O; thus the vibration frequencies of M - C and C - 0 are increased and decreased, respectively. Our model provides a unified interpretation of adsorption electronic spectra and adsorbate vibration spectra. The model of CO 2+-derived resonances never interprets the adsorbate vibration spectra, its advantage being to provide a unified interpretation of various electronic spectra including the adsorbate core-to-valence transitions. Last, our new concept of d back-donation suggests that d electrons transfer only to the empty M.0.s having CO 27r-M drr character, not to all empty adsorbate-derived M.0.s.
Using metal bonding functions to interpret adsorption electronic spectra and CO adsorption strength on Cu, Ni, Pd and Pt metals
When CO was adsorbed weakly on Cu(1 lo), inverse photoemission revealed the existence of a single asymmetric peak located 0.9 eV below the vacuum level (i.e. 3.4 eV above the Fermi level) with a half-width varying from 1.9 to 2.6 eV as a function of increasing coverage [6, 221. Similar peaks with larger widths and located further below the vacuum level have been observed for CO strongly adsorbed on Ni [23], Pd [24] and Pt (251. For instance, in the system CO-Ni( 111) it has been found [ 231 by analyzing
98
the IPES difference spectra that the lowest empty virtual state of the adsorbate is centered between 3.5 and 3 eV above the Fermi level and exhibits FWHM of about 4 eV at a coverage of l/3 monolayer (see Fig. 9 in [6]). Using our metal bonding functions, we can rapidly answer the following questions: (1) Why is CO adsorption weak on Cu, and strong on Ni, Pd and Pt? (2) Why is the width of the peak of adsorbate-derived resonances larger on Ni, Pd and Pt than on Cu? (3) Why does the separation of peak positions between the peak of adsorbate-induced resonances and the peak of the d band on clean metal decrease in the following order: Cu > Ni > Pd > Pt? [note: if there are two peaks in the d band on clean metal, such as on Pd and Pt, one is located just above the top of the d,,, band and other located at the Vd band, in this case, the separation of peaks is assigned as the separation between the peak of resonances and the peak of the Vd band.] As shown in Table 3, the u-r bonding function for CO end-on adsorption (AB) and the u-r bonding functions for CO side-on adsorption (LB,) are significantly smaller on Cu than on Ni, Pd and Pt, so CO adsorption on Cu is rather weaker than those on Ni, Pd and Pt metals. The CO dissociation function is also listed in Table 3 only for comparison with AE (total) values shown in Table 1, and both calculated results are qualitatively in agreement with most experimental facts. For example, CO is easily hydrogenated to methane on Ni but not on Pt, which indicates that the ease of CO dissociation would be greater on Ni than on Pt. CO dissociation will further be discussed in Part VI of our series of papers. Table 3 also indicates that the peak width of CO adsorbate-induced resonances increases in the order: Cu Cu > Pt> Pd. From the data in Table 3, the parallel relation between the peak width and the Al3 function is seen to be better than that between peak width and the B function. This is in agreement with our concept of d back-donation, because the B function represents only part of the adsorbate-derived resonances which contain CO 2+M dn- character. The AI3 bonding function is not directly correlated with the peak width of adsorbate-derived resonances, thus we obtain only qualitative results. We use the relative competitive hybridizing factor (CHF), S(CO5u, da, Vd)/S(C0271; dr, Vd), to characterize the relative hybridizing abilities of CO
3
0.1244 0.1403 0.2279 0.2340
cu
da, Vd)
4.65 5.15 5.12 5.65
dw)
0.0753 0.0890 0.2127 0.2505
S(CO2q
0.9035 0.7667 0.7203 0.7048
P(d,
dq
Vd)
10.91 15.99 4.153 5.001
Ax 10”
1.65 1.58 1.07 0.93
CHP
1.085 1.539 5.935 9.807
BxlO’
3.4 3.0-3.5 1.8 O-2
(eVY
R
11.89 24.62 24.65 49.04
ABx104
I
W Pd
10.37 15.51 4.360 4.137
AJO”
I221 1231 I241 1251
Ref.
1.002 1.430 5.516 9.128
BJO”
0.57 0.60 0.16 0.15
AE3DcDo x 10’
10.39 22.18 24.05 37.76
&B,lO’
“CHF= competitive hybridizing factor = S(C0517, da, Vd)/S(C02T, drr, Vd). bR(eV)= the separation of peak positions between the peak of adsorbate-derived resonances and the peak of the d band on clean metal measured by IPES.
Ni Pd Pt
S(CO5q
3.41 4.64 5.86 8.17
cu Ni Pd Pt
Metal
w,
Metal
Metal bonding functions and CO 5u/CO 2rr competitive hybridizing factors in the Vd band on Cu, Ni, Pd and Pt metals, and their relations with the adsorption strength and the spectra of adsorbate-induced resonances
TABLE
100
5~ and CO27r with the d orbitals in the Vd band. As shown in Table 3, the values of CHF decrease with the sequence Cu, Ni, Pd and Pt, which parallels the sequence of the decrease in R (eV), where R (ev) represents the separation of peak positions between the peak of adsorbate-derived resonances and the peak of the metal d band. As described above, the increase of S(CO5a, du, Vd) will strengthen the CO5g bond; according to the principle of quantum chemistry, the stronger the bonding, the greater the separation between the antibonding M.O. and the d band. On the other hand, the decrease in S(CO297 drr, Vd) causes the CO 2r-derived empty levels to be farther removed from the Vd band. Our calculated results are good in agreement with experiment [22-251.
Acknowledgement
The author would like to acknowledge the support of International Research Cooperation Fund of the European Economic Community (EEC) for extending his stay at Universit6 Catholique de Louvain to accomplish this work. Many thanks are due to Professor B. Delmon for his hospitality, helpful discussions and comments, and to Professor K. Tamaru and the reviewers for their helpful suggestions and comments. He should also acknowledge Dr. E. Andreta (EEC) for his encouragement during the completion of this work.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
K. H. Huang, J. Mol. Catal., 64 (1990) 53. A. T. Bell, Catal. Rev. Sci. Eng., 23 (1981) 203. W. M. H. SachtIer, Proc. 8th Int. Congr. Catal., Berlin, 1984, Vol. 1, p. 151. A. T. Bell, Proc. 9th Znt. Con@-. Catal., Calgary, 1988, Vol. V, p. 134. S. I. Ishi, Y. Ohno and B. Viowanathan, Sur$ Sci., I61 (1985) 349. G. Blyholder, J. Phys. Chem., 68 (1964) 2772. D. G. Castner, B. A. Sexton and G. A. Somorjai, Surf: Sci., 71 (1978) 519. L. H. Dubois and G. A. Somorjai, Su?$ Sci., 91 (1980) 514. Y. Kim, H. C. Peebles and J. M. White, Surf: Sci., I14 (1980) 363. R. A. Marbrow and R. M. Lambert, Su?rf: Sci., 67 (1977) 489. (a) B. Gumhalter, Phys. Rev., B33 (1986) 5245; (b) B. Gumhalter, K. Wandelt and P. Avouris, Phys. Rev., B37 (1988) 8048. V. M. Varma and A. J. Wilson, Phys. Rev., 22 (1980) 3795. M. Texter, I. D. Gay and R. Mason, Proc. R. Sot. CLandon), A356 (1977) 37. F. Greuter, D. Heskett, E. W. Phunmer and F. J. Freund, Phys. Rev., B27 (1983) 7117. P. M. Williums, P. Butcher, .I. Wood and K. Jacobi, Phys. Rev., B14 (1976) 3215. J. C. Fuggle, T. E. Madey, M. Steinkiberg and D. Menzel, Phys. Letters 51A (1975) 163; J. C. Fuggie, M. Steinkiberg and D. Menzel, Chem. Phys., 11 (1975) 307. W. Braum, M. Neumann, M. Iwan and E. E. Koch, Phys. Status Solidi B, 90 (1978) 525. D. R. Lloyd, C. M. Quinn and N. V. Richardson, Solid State Commun., 20 (1976) 409. C. W. Seaburg, T. N. Rhodin, M. M. Traum, B. Benbow and Z. Hurgch, SUT$ Sci., 97 (1980) 363. P. R. Norton, J. W. Goodole and E. B. SeIkirk, Surf: Sci., 83 (1979) 189.
101 2 1 D. W. Turner, G. Baker,A. D. Baker and C. R. Brundle,MolecularPhotoelectronSpectroscopy, Wiley, New York, 1970.
22 J. Rogozik, V. Dose, K. C. Prince, A. M. Bradshaw,P. S. Bagus, K. Hermann and P. Avouris, Phys. Rev., B32 (1985) 4296. 23 (a) F. J. Himpsel and Th. Fauster, Phys. Rev. L.&t., 49 (1982) 1583; (b) T. Fauster and F. J. Himpsel, Phys. Rev., B (1983) 1390. 24 J. Rogozik, J. Kuppers and V. Dose, Suyf Sci., 148 (1984) L653. 25 S. Ferrer, K. H. Frank and B. Reihl, Surf: Sci., 162 (1985) 264.
85
Transition metal bonding functions and their applications in catalytic adsorptions and reactions Part II. Micromechanism of CO chemisorption and a new concept of CT-T coordination K. H. Huang P. 0 Box 276, Department of Chemistry and Institute of Physical Chemistry, Xiamen University, Xiarnen 361005 (China) (Received January 22, 1990; accepted July 2, 1990)
Abstract A micromechanism of CO adsorption and a new concept of u-r coordination on transition metal are proposed in this article. Based on experimental facts, we assume CO 5u and/ or CO 1~ interacts with the representative M.0.s of the metal valence band, WMi, Vs) and ly(Mi, Vd), to form the bonding M.O. group and antibonding M.O. group. The bonding group is located below the Fermi level (EF), in which some M.0.s are much more characteristic of metal orbitals (denoted as M-CO u-bondings) while some M.0.s exhibit slight metal orbital characteristics, which belong to the excited valence M.0.s of adsorbed CO, conventionally assigned as adsorbed CO 50, CO lw and CO 4~. The calculated data indicate that the peak positions of adsorbed CO 50, CO 1~ and CO 4u are significantly higher than their corresponding M.0.s in the gaseous CO molecule, i.e. adsorbed CO is in an excited (or activated) state. The total energy generated (AE) from adsorbed CO 5u, CO 1~ and CO 4u can be used as a qualitative parameter for characterizing the ability for CO dissociation. On the other hand, the antibonding empty M.O. group of M-CO is located above the EF, which exhibits some characteristics of metal d orbitals. The hybridization of CO 27r with dT orbit& in the Vs, Vd bands and dr orbit& of the antibonding M.O. group of M-CO bondings results in the formation of unoccupied M.0.s with CO 2~r-M dr character. These M.0.s plus those unoccupied M.0.s without CO 27r-M dr character contribute the adsorbate-derived resonances, located 3-5 eV above E, and observed by Inverse Photo-Emission (IPE) difference spectra. We have used orbital overlap integrals of S(C0 50, du, Vd) and S(C0 2 r, dr, Vd) to characterize the relative competitive abilities for hybridization of CO 50 and CO 27r with d orbitals. The calculated results show that CO 5a possesses a stronger ability to hybridize d orbitals in the Vd band than does CO 2~r, thus the peaks of adsorbate-induced empty levels are shifted farther from the d band when the competitive hybridizing factor [CHF=S(CO 5u, do, Vd)/S(CO 2rr, dr, Vd)] is increased. The calculated data demonstrate that the peak positions of CO adsorbate-derived resonances of Cu, Ni, Pd and Pt metals, observed by IPE difference spectra, are in good parallel with their CHF values. Moreover, the values of CHE also demonstrate that CO u-bonding stimulates d electrons to transfer upward from the d band to the Vs band, where much more CO 2~-M drr character exists. We propose here a new concept of d back-donation, i.e. d electrons transfer from the occupied d band to the unoccupied M.0.s exhibiting CO 2-M drr character in the Vs and Vd bands, which weakens the rr bond of C-O and simultaneously strengthens the M-C bond; these phenomena have been confirmed by IR spectroscopy and EELS. The d back-donation is represented by the B bonding function. The calculations of A, B and Al3 bonding
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86 functions indicate that the AB bonding function of CO adsorption on Cu is signitlcantly smaller than that on Ni, Pd and Pt, so that CO adsorbtion is weak on Cu and is strong on Ni, Pd and Pt. Our micromechanism and our new concept of CT-~ coordination provide a unified interpretation of various CO adsorption electronic spectra from below to above the EF, i.e. from occupied orbitals to empty orbit&; and a unified interpretation of the adsorbate vibration spectra measured by EELS and IR spectroscopy. The advantages of our new concept have been discussed and compared with the conventional concepts of Blyholder and CO 2r-derived resonances.
Introduction
The theoretical model of transition metal bonding functions has been given in our previous paper [ 11. In subsequent papers, we will apply this model to characterize adsorptions, dissociations, catalytic reactions, support and promoter effects occurring on transition metals. A wealth of new experimental methods have been used for studying these processes on the surface of metals in atomic detail; if an effective bonding function theory can be further applied, it might be very useful to explain the experimental facts and disclose the nature of the processes. It is also possible to develop some new concepts for designing novel catalysts, which is final goal of our efforts. Research on CO catalytic hydrogenation has received much attention since 1974 and many data such as CO adsorption spectra, structures, heats and dissociations on various transition metal surfaces, have been collected, investigated and reviewed [ 2-41, although there are still many unanswered questions. In this article, we discuss the concept of CO u-rr coordination, which deals with the microscopic mechanism of CO adsorption and the relationships between CO adsorption bonds and the electronic structure of the metal. Furthermore, in catalytic adsorptions and reactions on transition metals, we employ the u-r coordination concept to discuss bond forming and breaking in a molecule, including how to activate the saturated u bond of H-H, C-H, C-C and the unsaturated rr bond of C=C, C=C, N=O and N=N. Obviously, this is most important to establish a correct concept of u-r coordination. Recently, Ishi et al. [5] addressed a question of the inadequacy of the d back-donation concept of Blyholder [ 61. Based on recent measurements by ARUPS [7], I-UPS [8], HREELS [9] and NEXAFS [lo], it seems more likely that the adsorbed CO 27r orbital is unoccupied in the neutral ground state in both strong and weak adsorption systems. Gumhalter et al. [ 1 l] proposed the hybridization of the CO 27? orbital with the substrate s, p and d bands producing the CO 27ir-derived resonances. These authors believed that even if the center of the CO 2#-derived resonances appears above the Fermi level (note: CO 2n’c is equal to CO 2~ and is noted in our paper similarly hereinafter), its wing may still extend below the Fermi level, allowing fractional charge to transfer into the resonance (i.e. backdonation into the resonance); moreover, the occupied and unoccupied parts of the resonances (which may be regarded as a fractionally occupied localized band of states)
87
may become a source of intra-resonance dynamic relaxation and polarization processes, which are largely localized on the adsorbate. But the authors did not address the following questions: What are the effects of other adsorbate M.0.s such as CO 5~ and CO 1~ on the formation of the adsorbate-derived resonances? What are the detailed wave functions of metal s, p, d bands which hybridize with the CO 27r orbital? What is the driving force for the d electron to transfer to the empty levels of adsorbate-derived resonances? Can we calculate rapidly the relative amounts of d back-donation on various transition metals? How can we reach a unified interpretation of electronic and adsorbate vibration spectra for CO adsorption? These five questions are important to enable us to use this concept in resolving the practical problems of catalytic adsorptions and reactions. In this article, we shall revise the conventional concept of U-W coordination, extend the concept of CO 27rderived resonances, and propose our new concept of CO (T-T coordination.
The method
of calculating
metal bonding functions
Calculation formulas
The molecular orbitals (M.0.s) of the CO molecule are shown in Fig. 1, where 5a is the Highest Occupied Molecular Orbital (HOMO), 2rZ, 27rr, are the doubly degenerate Lowest Unoccupied Molecular Orbitals (LUMO), and lr,, 171; are the doubly degenerate occupied M.O.s, whose levels are lower than that of the 5u M.O. According to eqns. (L4) and (L5) of our previous paper [ 11, the following metal bonding functions can be calculated if we insert their appropriate R.A.O. (reference atomic orbital) and R.A.E. (reference atomic exponent) into the corresponding equations.
+ ~P(d,Vs)S(~(d,z,~),C2p,)
(1)
+ ~dP(s,Vd)S{~(SMi,CLd),C2p+} +
WOW
cL,P(d,Vd>s(~(d,,,E~),C2p,)l
= Icbm /wpt)(l ICLd)s{~(~~,CLd),Czpy)P(d,d.x,)
(2)
where C 2p, and C 2p, are the R.A.0.s of the A(C05a) and B(CO2r) M.O.s, respectively, their corresponding R.A.E. values being 1.725 and 1.425, which will be discussed in the next paragraph. &(COlr) and B,(C029~) have similar R.A.O.s, C 2p, and C 2p,, but with different R.A.E. values of 1.825 and 1.525, the reason for which will be discussed in next paragraph. Thus eqns. (1) and (2) can be also used to calculate A, and B,, but with different RAE. values. For the calculations of A and 4, it should be noted that only the metal orbitals which are electron-unoccupied can be used for accepting the two electrons from CO 5a or CO 17r.
88 A.0
Hybrid
A.0
.s
CZS-2Pz( \
Fig. 1. The synthesis Orbitals (LCAO).
.S
\
&
,)
Hybrid
of CO molecular
2 OZS-2Pz
orbitals
(M.0.s)
by Linear
Combination
of Atomic
When CO is dissociated into its C atom and 0 atom, the metal-donating bonding function as described in eqn. (L3) of our previous paper [ 1 ] can be shown as follows: De = I(dGJlJ)]U
l~,)p(s,Vs)s(~(~i,~),C2~,)
+ (1 lcL,)p(d,Vs)S{~(~,,~),C2p,)
(3)
+ (1 ICLd)P(s,Vd)S{~(sM(,Il~),C2pt) + (1 IE.Ld)p(d,Vd)SGf(d,2,~~),C2p,)J where Dc is the bonding function for characterizing the metal donating electrons to the dissociated carbon atom, using C 2p, as its R.A.O. and 1.625 as its R.A.E. By similar reasoning, Do is the bonding function for characterizing the metal donating electrons to the dissociated oxygen atom. We use 0 2p, as its R.A.O., and let the corresponding R.A.E. be 2.275.
89
How to select the R.A.0.s and R.A.E.s of CO adsorbate for calculating metal bonding functions As indicated in the previous paper [ 11, in order to prevent any error arising from the calculation of M.0.s from influencing our calculated conclusion, we select an appropriate Reference Atomic Orbital (R.A.O.) to represent the M.O. of CO bonding, and we select an appropriate atomic exponent for representing the energy level of this orbital (R.A.O.), denoted as the Reference Atomic Exponent (R.A.E.). The conditions of these selections are as follows: 1. The symmetry of R.A.O. must match that of the M.O. available for CO bonding; 2. The sequence of R.A.E.s must coincide with the energy sequence of the corresponding M.0.s for CO bonding. Based on the above selection conditions, the R.A.0.s and R.A.E.s for A(C05a), B(C027r), &(COlr) and B,(C023~) are shown in the second column of Fig. 2(a), (b), (c) and (d), respectively. From these figures, it is obvious that all selected R.A.0.s satisfy the first condition described above, i.e. the symmetries of the selected R.A.0.s and the M.0.s of adsorbate are matching. The second condition is also satisfied, for example, (1) the R.A.E. of the CO 50 M.O. is selected as 1.725, which is smaller than that of the CO 1~ M.O. (1.825). This sequence indicates that the energy level of the CO 5u M.O. is higher than that of the CO 1~ M.O. (see Fig. 1); (2) the R.A.E. of CO 27~ M.O. for end-on d back-donation is selected as 1.425, which is less than the R.A.E. of CO 2~ M.O. for side-one d back-donation, 1.525; this sequence corresponds with their energy level sequence, because CO 27~M.O. of B(C027r) consists of a C 2p, constituent, while the CO 2rrM.0. of B,(C027r) consists of both C 2p, and 0 2p, constituents, so the energy level of the former M.O. is higher than that of the latter. Parameters and structural configurations The width of the d band (IV”) and the work function (U) are listed in our previous paper [ 11, which is adapted from [ 121. The CO end-on adsorption is assumed to occur atop the atom, while the CO bridge adsorption is assumed be bridging two surface atoms at the shortest atomic distance of the normal crystal lattice. The distance C-M is assumed to be the sum of the carbon covalent radius (0.728 A) and the metal radius (half of the shortest atomic distance in the normal crystal lattice). Calculation schedule -
The micromechanism coordination
see [I]
of CO adsorption
and a new concept of -
Micromechanism of CO adsorption on transition metals According to our metal bonding functions model [ 11, adsorption proceeds on the metal valence band, i.e. adsorbate interacts with the three represen-
Metal d
(b)
R.A.E. = 1.425
COZIT
R.A.E. =1.725
CO56
B (COZlT)bonding
A(CO567 bonding
(d)
Metal d Y=
Metal S-d=2
P
=1.525
R.A.E.
QD
C
+ + 0
+
C02K
R.A.E. =1.825
COIX
-
-
Q-,0
Bs(C02K) bonding
A~(CO~TI) bonding
Fig. 2. The diagram of W-T bonding functions for CO adsorption on transition metals: (a), A(CO5w) end-on bonding; (b), B(C02r) end-on bonding (located near terminal carbon); (c), &(COl?r) side-on bonding; (d), B,(COBlr) side-on bonding (located on the side of CO).
;
Metal s-d=2
(a)
Cc)
g
91
tative M.0.s of the metal valence band. The three M.0.s are shown as follows: !P(Mi,Vs) =P(s,VS)+(%~,& !P(Mi,Vd) =P(s,Vd)&ki,& %Mi&,)
=P(~&&NsM~,E)
+P(d,Vs)4(d,i,h) +P(d,Vd)+(d,i,& +P(d7d&Hd~i,~)
on the Vs band on the Vd band on the L,
band
CO adsorption can be regarded as a process in which the CO 5a M.O. and/or CO 1~ M.O. first bonds (end-on or side-on, then changes to bridging) with the vacant M.0.s of ?P(Mi, Vs) and !P(Mi, Vd), in which the vacant metal orbitals can ‘accept’ electrons from CO 5a or CO 1~ to form a ubond. We use the ‘A’ bonding function to characterize the relative ability of a-bonding. If we wish to use the language of valence bond, it can be said that two electrons of CO 5u or CO 1~ are accepted by the vacant orbital. ‘A’ bonding makes the various M.0.s of CO approach the valence bands of metal within the chemical interaction range. Because CO 5u, CO lr, CO 29r and CO 4a M.0.s make bonding or hybridizing or perturbating relationships with the valence bands of the metal, the MOGs of IF’(Mi,Vs) + !P(Mi, Vd) + ?P(Mi, d,,,) will be changed to new MOG states of P*(Mi, Vs) + ??*(Mi, Vd) + ??*(Mi, d,,,). Simultaneously, the d and s electrons of metal and electron charges of various occupied M.0.s of adsorbate will be reconstructed in the new MOG states. The new interpretation of CO ao!sorptti electronic spectra and further disclosure of the micromechanism of CO adsorption In order to further understand the mechanism of CO 5a and CO lrr bonding, we have analyzed the electronic adsorption spectra of CO 5u, CO 1~ and CO 4u M.0.s over Group VIII metals [5], as shown in Table 1, where the negative value of the exit work function (U, eV) is selected as the value of the Fermi level (Er). The peak position (eV) of the M.O. with respect to vacuum is equal to the peak position (eV) with respect to EF plus to U, and listed in parentheses. The AE is equal to the peak position (eV) of the gaseous molecule state minus the peak position (eV) of the adsorbed state, i.e. the separation of peak positions between the gaseous molecule state and the adsorbed state termed the generated energy position (eV) of the M.O. after adsorption, compared with the corresponding energy position (eV) of the gaseous CO molecule. Table 1 indicates that the values of A(la-5~) on Group VIII metals are negative or zero. If negative, it means that the energy positions of adsorbed CO 5u and CO lrr are opposite their original positions of free CO molecule; if the A(llr-5~) is zero, it means the CO 5u and CO 1r levels are mixed. The data of A(lr- 5~) revealed that the CO molecule structure is reconstructed in adsorption. On the other hand, the calculated values of AE(C05u), AE(COl$ and AE(C04u) and AE (total) [the sum of AE(C05u), AE(COlr) and AE(C04u)l are all positive, except for AE(C05u) on Ir(ll1) and Pt(lll), which means that after adsorption almost all peak positions of CO 5u, CO 1~ and CO 4u occur at higher energy
92
TABLE 1 Peak positions (eV) of adsorbed CO M.0.s with respect to Er or to vacuum (in parentheses) and the higher energy position compared with the correspondingM.O. of gaseous CO molecule (AE, eV Substrates
u
Peak positions
(eV) 4u(ads)
AE (total) 11.1
0
1131
4.80
10.8 (15.30) 4.40
-0.2
1141
1.10
10.7 (15.7) 4.00
9.3
(1;:;) 4.20
8.1 (13.25) 0.75
11.2 (16.35) 3.35
8.75
-1.0
1151
(lZ5) 4.65
0
1161
1.69
10.7 (15.41) 4.29
10.57
(Z) 4.59
-0.50
(1::;8) 0.72
(1;:878) 4.12
11.2 (16.18) 3.52
8.36
4.98
1171
-0.70
(lK2) 0.68
(lZ2) 4.28
11.2 (16.32) 3.38
8.34
5.12
1181
- 0.60
(lZ7) - 0.47
1191
(lki7) 3.03
11.7 (16.97) 2.73
5.29
5.27
- 0.80
PO1
(lZ5) -0.85
(lZ5) 2.85
11.90 (17.55) 2.15
4.15
5.65
14.0
16.9
19.7
Sa(ah) Fe(ll1)
4.50 AE
$0) 1.90
1 n(ads)
(127:Yo)
co(ooo1)
5.00
(12)
AE Ni(ll1)
5.15
AE Ru(0001) 4.71 AE
&)
Rh(ll1) AE Pd(ll1) AE Ir(ll1) AE Pt(ll1) AE CC (gas)
Ref.
0
A(lr-
2.90
5~)
WI
positions compared with their corresponding M.0.s of the gaseous CO molecule. The positive values of AE (total) are reliable, although it is possible for some error from the selection of the Fermi level to occur, but it would not influence the positive value of AE (total). As is well known, the chemical adsorption would evolve heat, i.e. the total energy of the system after adsorption must decrease. Thus on account of the energy balance, it is more likely that there would be some ways to supply sufficient energy for reconstruction of the CO molecule, i.e to balance this positive value of AE (total). We propose that CO 5u or CO 1~ bonds directly with the M.O. group of !P(Mi, Vs) + ?P(Mi, Vd), and simuhaneously supplies the binding energy for the structure reconstruction AE (total). It is suggested that adsorption bonds are formed directly by coupling CO 5a and/or CO 1~ orbit& with the M.0.s
93
of ?J’(Mi, Vs) + flMi, Vd), where two new M.O. groups are formed. The first M.O. group is the bonding M.O. group, in which some M.O’s exhibit much more character of metal orbitals, located below the EF, and present a broad peak in the range O-5 eV below EF, denoted as a-bonds; other bonding M.0.s exhibit little character of metal orbitals, and belong to the excited valence M.0.s of adsorbed CO, conventionally assigned as adsorbed peaks of CO 5u, CO 1~ and CO 4a, located 5-12 eV below the Er, as shown in Table 1. The second M.O. group is the antibonding empty M.O. group, which hybridizes with CO 27r and metal s, p, d vacant orbitals to form new bonding empty M.0.s and new antibonding empty M.0.s; the bonding empty M.0.s are located 3-5 eV above the EF, and exhibit some characteristics of metal orbitals. From Table 1, the following regular pattern can be seen: AE(COlrr) > AE(C04u) > AE(C05a). This means that the r bond of CO 1~ is most significantly weakened (the mechanism will be shown below), and more energy is lost in adsorbed CO 1~ M.O. compared with other adsorbed valence M.0.s. The CO 50 and CO 4a M.0.s of the gaseous CO molecule, being formed from C 2s-C 2p, hybridization as shown in Fig. 1, can be described as belonging to a pair of bonding and antibonding M.0.s. When gaseous CO 5~7and CO 1~ bond with metal, simultaneously reconstructing this pair of M.O.s, it is equivalent to recombining C2s, C2p, with the metal M.O. group, reforming the CO molecule in an excited state, so that CO 4a is shifted to a higher energy position compared with its original position. The AE (total) values can be used to characterize the activated state (or excited state) of adsorbed CO. The facility for CO dissociation qualitatively parallels the AE (total) values. For example, CO is most easily dissociated on Fe and Ru, whose AE (total) values are the largest among the Group VIII metals, and CO is difficult to dissociate on Pt, whose AE (total) is the smallest. The new concept of adsorbate-induced empty M.O.s, relative hybridizing abilities of CO 27&H drr and CO 5~M da on Vs and Vd bands, and the new concept of d back-donut&m When studying CO bonding, another effect should be considered, i.e. the fact that the CO 2~ M.O. will hybridize with the M.0.s of ly(Mi, Vs) + WMi, Vd) and with the antibonding M.0.s of M-CO bonds to form empty M.O.s, which can be measured by IPE spectroscopy and are denoted as CO adsorbateinduced resonances. When CO 2~, CO 5u and CO 1r simultaneously hybridize with the orbit& of ?#(Mi, Vs) + flMi, Vd) (noted as Vs and Vd bands, similarly hereinafter), the question arises as to their relative abilities of hybridization. In order to analyze the relative hybridizing abilities, we consider the orbital overlap integrals of CO 2~, CO 5u and CO 1~ with the dr, du orbitals in Vs and Vd bands, as shown in Table 2, in which S(CO277, dr, Vs) denotes the orbital overlap between CO 2~ and dv orbitals in the Vs band, while S(CO5u, du, Vd) denotes the orbital overlap integral between CO 5u and da orbitals in the Vd band, etc. We use the R.A.0.s of CO 27
94 TABLE 2 Orbital overlap
integrals
between
metal d orbital and CO 2n; CO 5a and CO 1~ M.0.s
co
Metal S(CO2q S(CO2q S(CO5q S(CO5u, S(COlq S(COlq
drr, dq du, du, do, du,
Vs) Vd) Vs) Vd) Vs) Vd)
0.3300 0.2997 0.1042 0.1377 0.1033 0.1681
Ti 0.3619 0.2886 0.0947 0.2078 0.0964 0.2028
V 0.3913 0.2535 0.0828 0.2245 0.0871 0.2209
Cr 0.4022 0.2641 0.0224 0.2277 0.0322 0.2251
Mn 0.3771 0.1620 0.1234 0.2035 0.1253 0.2547
Fe 0.3965 0.1463 0.1180 0.1924 0.1218 0.1882
0.3862 0.1120 0.1368 0.1644 0.1395 0.1600
Ni 0.3803 0.0890 0.1498 0.1403 0.1520 0.1360
Metal S(CO2q S(CO2q S(CO5u, S(CO50, S(COlq S(COl?r,
dw, dr, du, du, da, da,
Vs) Vd) Vs) Vd) Vs) Vd)
Y 0.2988 0.2997 0.0334 0.0602 0.0353 0.0603
Zr 0.3238 0.3333 0.0190 0.1057 0.0232 0.1049
Nb 0.3158 0.3636 0.0431 0.1074 0.0345 0.1086
MO 0.3358 0.3566 0.0436 0.1578 0.0345 0.1575
TC 0.3771 0.3075 0.0166 0.2177 0.0231 0.2143
Ru 0.3637 0.2887 0.0255 0.2235 0.0168 0.2220
Rh 0.3699 0.2402 0.0060 0.0473 0.0015 0.3677
Pd 0.3715 0.2126 0.0171 0.2279 0.0233 0.2044
Metal S(CO2q S(CO27r, S(CO5u, S(CO5u, S(COlw, S(COlp,
dq dq do, do, da, du,
Vs) Vd) Vs) Vd) Vs) Vd)
La 0.2711 0.2795 0.0041 0.0196 0.0074 0.0219
Hf 0.3101 0.3458 0.0313 0.0388 0.0242 0.0421
Ta 0.3342 0.3765 0.0442 0.0828 0.0355 0.0852
W 0.3519 0.3764 0.0441 0.1368 0.0350 0.1375
Re 0.3626 0.3248 0.0304 0.1905 0.0220 0.1887
OS 0.3757 0.3198 0.0241 0.2218 0.0156 0.2187
Ir 0.3805 0.2736 0.0084 0.2537 0.0035 0.2441
Pt 0.3582 0.2505 0.0346 0.2340 0.0200 0.2297
SC
CO 5a and CO 1~ to calculate the overlap integrals, as described in the first section of this text, which is useful for the purpose of relative comparisons. Table 2 indicates that the overlap integrals between CO 27r and metal dr decrease significantly from the Vs band to the Vd band on all transition metals except those whose d levels are fairly high, such as 4d’-4d4 and 5d’-5d4 metals, in which the d orbitals on the Vd band match better with CO 27r than those on the Vs band. In contrast, the overlap integrals between CO 5a and metal du orbital increase greatly from the Vs band to the Vd band on all transition metals except Ni, whose A.0.E.s of s and d orbitals are the largest of all transition metals (d’-d’), in this case CO 5a matches less well with the da orbital in the Vd band than in the Vs band. Thus for all transition metals except 4d’-4d4 and 5d’-5d4 metals, the abilities for CO 277-M dr couplings are larger on the Vs band than those on the Vd band. On the contrary, on all transition metals except Ni, the values of S(CO5q du, Vd) are greater on the Vd band than on the Vs band. Even on Ni, the value of S(CO5q da, Vd) is still larger than S(COBr, drr, Vd), and the character of CO 5~ would be greater than that of CO 2~ in the Vd band. In summary, the CO 5u bond is more likely to involve the lower d energy levels in the Vd band. This result is very interesting, since it means that the formation of CO u-bonds favours movement of d electrons from the occupied levels upward to the higher empty M.0.s. The empty M.0.s consist of those M.0.s with CO27r-M2r character and those M.0.s without C02+Mdr character. In catalysis, special attention is paid to those M.0.s with CO 27~M
96
dT character; if d electrons transfer to these M.O.s, it is called d backdonation in the CO 27r M.O. If the dr orbital is placed in the Vs band, on basis of the above conclusion, CO 2+M dr will be stronger in the Vs band than in the Vd band. The probability of d electron direct back-donation to the CO 27r vacant M.O. is very small, because the d electron must first transfer to the empty M.0.s of the Vs, Vd bands and the unoccupied antibonding M.0.s of the M-CO bonds, which agrees with the facts indicated by Ishi et al. [ 51. According to quantum chemistry, if the antibonding M.O. (CO 274 is filled by d backdonated electrons, the 7 bond (CO lrr-bonding M.O.) between C-O will be weakened; and simultaneously, the new bond between M-C (i.e. C02r-Mdw coupling) will strengthen the M-C bond; so the vibration frequencies of C-O and M-C are decreased and increased, respectively, as observed by IR and EELS. This will be further discussed in two subsequent papers. Representation of d backdonation As described above, if we can place the da orbital in the Vs band, it is equivalent to raising the d orbital energy position to approach the CO 27r level, and the overlap integral between CO 27r and da will be increased. The formation of a metal energy band is an effective way to increase the d percentage function in the Vs and Vd bands. We use the ‘B’ bonding function to characterized d back-donation. This function depends on four parameters (see eqn. 2). First, the overlap integral between CO 27r and M dr orbital, noted as S(CO2rr, dr) or S[@(%=, &, CBp,], where C2py is the R.A.O. of CO27r, and dJZ is the R.A.O. of Mdrr, which characterizes the hybridizing ability between CO 2a and the reference metal da orbital. The larger S(CO27r, dn) is, the greater the percentage characteristic of CO 27~M dr in the Vs and Vd bands would be. Second, the mean energy potential of the d occupied band, represented by the effective eXpOnent of d orbital, j,&&The Smaller ,_&d is, the smaller the separation between CO 27~and the d occupied band would be, which is equivalent to an increase in the d orbital position to approach CO 27~and results in enhancing the percentage CO 2rr-dr character in the unoccupied M.0.s located between CO 27r and the d occupied band. The third parameter is the width of the d band (wd); a larger wd results in increasing the overlap of s-d bands and decreasing the energy level separation between CO 27r and the d occupied level, both effects which enhance the COB+d7r character in the unoccupied M.0.s located between the C02~ and the d occupied band. The last factor is the percentage function of d orbital in the d occupied band, P(d, d,,,). The larger P(d, d,,,,) is, the greater the probability of d electron back-donation. According to the above analysis, the B bonding function can easily be derived, as shown in eqn. (2). The r bond of C-O would be weakened with increasing B bonding function, and at the same time the r bond of M-C would be strengthened, with increasing AB bonding function. Our calculated results are in good agreement with the relative amounts of vibration frequencies of C-O and M-C on various Group VIII metals as measured by IR and EELS.
96
In brief, d back-donation is defined as that d electron charge transfered from the d occupied orbital to the unoccupied M.O. consisting of CO 2rr-M dr character, to form a CO 2?r-M drr bond. The d back-donation weakens the bond of C-O and strengthens the M-C bond simultaneously. On the other hand, CO cT-bonding promotes d back-donation. This new concept provides a unified interpretation of the broad peak in the range of O-3 eV below the Fermi level, measured by FEED, UPS and SPIES with CO adsorption and the adsorption vibration spectra measured by EELS and IR.
Comparison of our new concept of a--a coordination with the conventional concept proposed originally by Blyholder and the concept of CO 2pderived resonances
The conventional concept for u-r end-on adsorption over transition metals was proposed originally by Blyholder [6 1. Under this conventional concept, a (+ bond infers that a vacant metal atomic (T orbital (hybrid of s and ds) accepts electrons from CO 5u, as shown in the first column of Fig. 2(a); a r bond infers that some electrons are back-donated from a metal atomic dr orbital, such as dyz, to the LUMO CO 27r M.O. (belonging to the terminal carbon), as shown in the first column of Fig. 2(b). If CO adsorption is side-on instead of end-on, the (T bond means that an unoccupied metal atomic u orbital (hybrid of s and d,z) accepts electrons from CO lq as shown in the first column of Fig. 2(c). The r bond of side-on CO adsorption means that electrons are back-donated from metal atomic drr orbital such as duz to the side part of LUMO CO 27~M.O., as shown in the first column of Fig. 2(d). According to our model of metal bonding functions, the CO adsorption g bond occurs when the fractional vacant cr orbitals (hybrid of s and d,z) of the WMi, Vs) and !&‘(Mi, Vd) M.0.s accept electrons from CO 5a, and the CO adsorption r bond occurs when d electron charges are back-donated from Iy(Mi, d,,,) into the unoccupied M.0.s exhibiting CO 27r-M dr character in the Vs and Vd bands. In brief, if we use the fractional vacant u orbitals of Vs and Vd bands instead of the conventional vacant atomic s orbital, use the CO 277-M drr unoccupied orbitals of Vs and Vd bands instead of the conventional vacant CO 297 M.O., and use the IY(Mi, d,,,) instead of conventional atomic d occupied orbital, then we can employ the language of the valence bond, and use the conventional language of U-T coordination. In other words, all the pictures in Fig. 2 are still correct if the new model of metal bonding functions is employed, but note that all conventional metal atomic orbitals would be changed to their corresponding fractional orbitals of the metal valence band. The metal u-r bonding functions for CO endon bonding and side-on bonding are denoted as A(CO5u)-B(CO2r) and &(COl79-B,(CO27r), as shown in Fig. 2(a), (b) and Fig. 2(c), (d), respectively; where A(C05a) or B(C027r) indicates that the metal valence band accepts electrons from CO 5uor back-donates into CO 2rduring CO end-on adsorption,
97
and &(COln) or B,(C027r) means that the metal valence band accepts electrons from CO 1r or back-donates to CO 27rduring CO side-on adsorption, respectively. The advantage of using the language of the valence bond is obvious; for example, in B(C027+) bonding, it makes a rr bond between M-C, resulting in an increase in the bond strength of M-C, which is clearly depicted in Fig. 2(b). This theoretical image has been confirmed by CO adsorption vibration spectra and will be further discussed in two subsequent papers. The difference between the concept of CO 2#-derived resonances and ours lies mainly in the following five points. First, our model emphasizes that the empty levels above the Fermi level were derived not only from CO 27?, but also from other M.0.s of adsorbed CO, especially CO 5a and CO 171: Second, the model of CO 2+-derived resonances did not account for the wave composition of the valence band. Our model can rapidly calculate the representative wave functions in the valence band. Third, we propose that orbital overlap integrals to be used to estimate the competitive hybridizing abilities of CO 2~ and CO 5a or CO 1~ with respect to d orbit& in the Vs and Vd bands, interpretating successfully the peak position of CO adsorbateinduced resonances, which will be further discussed below. Fourth, in fact CO adsorbed on Group VIII metals is quite stable, if its stability is only due to the relaxation effect as proposed by the concept of CO 27ir-derived resonances, which is difficult for chemists to accept; however, our model emphasizes that the chemical bonds M-CO o-bondings and d back-donation originate in the metal valence band such as, resulting in the formation of a r bond between M-C, which further stabilizes CO adsorption and at the same time activates the r bond of C-O; thus the vibration frequencies of M - C and C - 0 are increased and decreased, respectively. Our model provides a unified interpretation of adsorption electronic spectra and adsorbate vibration spectra. The model of CO 2+-derived resonances never interprets the adsorbate vibration spectra, its advantage being to provide a unified interpretation of various electronic spectra including the adsorbate core-to-valence transitions. Last, our new concept of d back-donation suggests that d electrons transfer only to the empty M.0.s having CO 27r-M drr character, not to all empty adsorbate-derived M.0.s.
Using metal bonding functions to interpret adsorption electronic spectra and CO adsorption strength on Cu, Ni, Pd and Pt metals
When CO was adsorbed weakly on Cu(1 lo), inverse photoemission revealed the existence of a single asymmetric peak located 0.9 eV below the vacuum level (i.e. 3.4 eV above the Fermi level) with a half-width varying from 1.9 to 2.6 eV as a function of increasing coverage [6, 221. Similar peaks with larger widths and located further below the vacuum level have been observed for CO strongly adsorbed on Ni [23], Pd [24] and Pt (251. For instance, in the system CO-Ni( 111) it has been found [ 231 by analyzing
98
the IPES difference spectra that the lowest empty virtual state of the adsorbate is centered between 3.5 and 3 eV above the Fermi level and exhibits FWHM of about 4 eV at a coverage of l/3 monolayer (see Fig. 9 in [6]). Using our metal bonding functions, we can rapidly answer the following questions: (1) Why is CO adsorption weak on Cu, and strong on Ni, Pd and Pt? (2) Why is the width of the peak of adsorbate-derived resonances larger on Ni, Pd and Pt than on Cu? (3) Why does the separation of peak positions between the peak of adsorbate-induced resonances and the peak of the d band on clean metal decrease in the following order: Cu > Ni > Pd > Pt? [note: if there are two peaks in the d band on clean metal, such as on Pd and Pt, one is located just above the top of the d,,, band and other located at the Vd band, in this case, the separation of peaks is assigned as the separation between the peak of resonances and the peak of the Vd band.] As shown in Table 3, the u-r bonding function for CO end-on adsorption (AB) and the u-r bonding functions for CO side-on adsorption (LB,) are significantly smaller on Cu than on Ni, Pd and Pt, so CO adsorption on Cu is rather weaker than those on Ni, Pd and Pt metals. The CO dissociation function is also listed in Table 3 only for comparison with AE (total) values shown in Table 1, and both calculated results are qualitatively in agreement with most experimental facts. For example, CO is easily hydrogenated to methane on Ni but not on Pt, which indicates that the ease of CO dissociation would be greater on Ni than on Pt. CO dissociation will further be discussed in Part VI of our series of papers. Table 3 also indicates that the peak width of CO adsorbate-induced resonances increases in the order: Cu
3
0.1244 0.1403 0.2279 0.2340
cu
da, Vd)
4.65 5.15 5.12 5.65
dw)
0.0753 0.0890 0.2127 0.2505
S(CO2q
0.9035 0.7667 0.7203 0.7048
P(d,
dq
Vd)
10.91 15.99 4.153 5.001
Ax 10”
1.65 1.58 1.07 0.93
CHP
1.085 1.539 5.935 9.807
BxlO’
3.4 3.0-3.5 1.8 O-2
(eVY
R
11.89 24.62 24.65 49.04
ABx104
I
W Pd
10.37 15.51 4.360 4.137
AJO”
I221 1231 I241 1251
Ref.
1.002 1.430 5.516 9.128
BJO”
0.57 0.60 0.16 0.15
AE3DcDo x 10’
10.39 22.18 24.05 37.76
&B,lO’
“CHF= competitive hybridizing factor = S(C0517, da, Vd)/S(C02T, drr, Vd). bR(eV)= the separation of peak positions between the peak of adsorbate-derived resonances and the peak of the d band on clean metal measured by IPES.
Ni Pd Pt
S(CO5q
3.41 4.64 5.86 8.17
cu Ni Pd Pt
Metal
w,
Metal
Metal bonding functions and CO 5u/CO 2rr competitive hybridizing factors in the Vd band on Cu, Ni, Pd and Pt metals, and their relations with the adsorption strength and the spectra of adsorbate-induced resonances
TABLE
100
5~ and CO27r with the d orbitals in the Vd band. As shown in Table 3, the values of CHF decrease with the sequence Cu, Ni, Pd and Pt, which parallels the sequence of the decrease in R (eV), where R (ev) represents the separation of peak positions between the peak of adsorbate-derived resonances and the peak of the metal d band. As described above, the increase of S(CO5a, du, Vd) will strengthen the CO5g bond; according to the principle of quantum chemistry, the stronger the bonding, the greater the separation between the antibonding M.O. and the d band. On the other hand, the decrease in S(CO297 drr, Vd) causes the CO 2r-derived empty levels to be farther removed from the Vd band. Our calculated results are good in agreement with experiment [22-251.
Acknowledgement
The author would like to acknowledge the support of International Research Cooperation Fund of the European Economic Community (EEC) for extending his stay at Universit6 Catholique de Louvain to accomplish this work. Many thanks are due to Professor B. Delmon for his hospitality, helpful discussions and comments, and to Professor K. Tamaru and the reviewers for their helpful suggestions and comments. He should also acknowledge Dr. E. Andreta (EEC) for his encouragement during the completion of this work.
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