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Chemisorption of C2H2 on Pd(111) and Pt(111): formation of a thermally activated olefinic surface complex
CHEMICALPHYSICS LETI-ERS
Volume 45, number 1
1
January 1977
CHEMISOP~ION OF C,H, ON Pd(ll1) AND Pt(ll1): FORMATION OF A THERMALLY ACTIVATED OLEFINIC SURFACE COMPLEX* 3.E. DEMUTH ZBBZThomas J. Watson Yorktown
Heights, New
Received 9 August Revised manuscript
Research Center, York 10598, USA
19 76 received 5 October
1976
Ultraviolet photoemission spectroscopy s:udics indicate that acetylene chcmisorbs on Pd( 111) and Pt(I 11) as x-bonded acetylene for T 5 180 K but upon warming (or room tcmpcraturc exposure) forms a new surface species. We identify the electronic structure of this new phase as being characteristic of an olefinic CzI12 surface species - the result of rehybridiTatIon of the mltially chemisorbed molecule. T!lese xsults are discussed relative to previous work on Ni(ll1).
1_ Introduction The identification of the chemical nature and bonding interactions of hydrocarbon molecules adsorbed on transition metal (TM) surfaces is of fundamental importance to understand surface reactions, and possibly catalytic processes on these surfaces. We have previously reported UV photoemission spectroscopy (UPS) results for kv = 2 1.2 eV for low exposures of acetylene to Nl(11 I) (80 K 2 Ts400 K) and interpreted these results as indicative of a n-d chemisorption interaction between aceiylene and the surface [I ] where little rehybridization of the chemisorbed species occurs relative to that of the free molecule [2]. In this wo*l: we report similar UPS studies for low exposures of acetylene to Pd( 111) and Pt (111) at photon energies of 2 1.2 and 40.8 eV for T 5 180 K where we not only observe a chemisorbed phase similar to that on Ni(17 l), but upon warming we directly observe the thermally activated formation of a new chemisorbed species from initial!y chemisorbed acetylene. The ionization levels of this new phase are different from those of chemisorbed or gaseous acetylene and are characteristic of an olefin. Room temperature exposures of either Pd(ll1) or Pt(ll1) to C,H2 produce this new olefinic species directly. We show that this species is likely the result * Partiauy
supported
by ONR
contract
N00014-75-C-0346.
of rehybridization of initially chemisorbed acetylene to form an olefinic surface complex with the Pd or Pt surface.
2. Experimental procedures The present studies were performed in a turbomolecular pumped TJHV system (base pressure < 1 X lo-lo torr) equipped with a He resonance lamp, electron energy analyzer, LEED optics, quadrupole mass spectrometer and auxiliary electron gun (for Auger and energy loss spectroscopy). Single crystal samples of Ni(l1 l), Pd(l11) and Pt(l11) were prepared by conventional techniques and mounted on a multiple sample holder which permitted the samples to be liquid nitrogen cooled to T * 78 K or independently resistivity heated to T = 1600 K as measured with a chromelalumel thermocouple. The single crystals were cleaned by mild oxidation treatments, argon ion sputter-etching and subsequent annealing. Surface characterization was performed by LEED, Auger and photoemission analy ses. Energy analysis of the photoemitted electrons was performed with a double pass (angle-integrating) cylindrical mirror anaIyzer (CMA) operated in a fvted pass mode so as to have a resolution of eO.1 S(O.25) eV or better
for He I(II)
UPS work. The samples were
pOSi-
Volume 45, number 1
CHEMICAL PHYSICS LETTERS
tioned with their normal direction =220” from the axis of the CMA into the photon beam, which is 73” off the axis of the CMA. We note that we always observe the same adsorbate ionization energies for adsorbed species independent of sample orientation - the sample orientation may change relative intensities, e.g. via polarization effects. Also, we do not observe any photon- or photoelectron-induced changes in chemisorbed
I kmuuy
1977
0) 3L C,H,, A9 = -14eV
CLEAN Pd (111) W=56eV) /
species or in the thermally activated reaction we discuss here. Work function changes were measured using the low-energy cutoffs of the photoemission energy distributions
[3]. hv= 408eV
3. Experimental
results
for acetylene
on Pd( 111)
In fig. la we show the photoemission energy distribution curve for clean Pd( 111) (solid line) and after exposure to 3 X lop6 torr s acetylene (dashed iine) for hv = 40.8 eV with a sample temperature of 180 K. We observe strong overall d-band changes and new levels at 8.8 and 11 eV below the Fermi level E, . To enhance the chemisorption induced features, we show the difference spectrum AN@‘) between these two spectra in fig. lb. The dashed line in the d-band region represents the emission decrease if only uniform attenuation of the d-states had occurred. We observe preferential attenuation near the top of the d-band and additional emission near the bottom of the d-bands a behavior commonly observed for chemisorbed species on Ni [4,5] or Pd 161. In fig. lc we also show for comparison both the AN(E) spectra for an identical exposure of Ni(l1 I) to acetylene at T = 300 K as well as the gas phase spectrum obtained in a gas phase spectrometer of similar geometry and resolution [7]. The results for Ni(ll1) are similar to those reported previously for tzv = 21.2 eV [ 1,2] except that for hv = 40.8
eV we can also observe
a lower
lying
level at 15.8
eV below E, which corresponds to the 20~ molecular orbital of acetylene. Clearly the same chemisorbed phase of acetylene forms on Pd(1 I I) at 180 K as on Ni(l I I), with details of the interaction being different. Namely, the saturation coverage work function change on Pd is -1.4 eV (-1.6 eV on Pt) versus -i -2 eV on Ni** while the re-
GASEOUS IP --
-----p
IleV _-_ CEHE/Pd(III).T- 3mK --
-0
:--. I6
I4 I2 IO 8 6 4 2 ELECTRON BINDINGENERGY (eV)
Fig. 1. Photoemission energy distribution spectra N(m reIative to EF for (a) clean Pd( 111) at T = 180 K and after exposure to 3L (3 X IO+ torr s) acetylene (dashed line) and (b) their corresponding difference spectra ti(f?), (c) the ti(fZ) spectra for 3L acetylene on Nd 111) at T = 300 K and the gas spectra [ 71, and (d) the A&k) spectra For 3.5 L acetylene exposure to Pd(l I I) at F = 300 K. In the d-band region of the &V(E) curves the short dashed lines indicate a uniform attenuation of the d-band emission of the clean surface while the longer dashed tines estimate the background intensity. AU results are for hv = 40.8 eV. (The gas phase spectra ionization features are assigned to the In& 30 ,2ou and 20 moIecuIar orbitals of acetylene respectively w a ere the In; &bital lies at s 11.5 eV_)
13
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laxation shift as defined previously [I-3] is 3.7 eV on Pd (3.5 eV on Pt) versus 3.5 eV on Ni. Also, the 7r-orbit& of acetylene on Pd would appear to have a larger “bonding shift” (a I .9 eV versus 1.2 eV for Ni *) although on Pd the d-band overlaps the n-level and prohibits its clear delineation from d-band changes. The greater work function change. n-orbital bonding shift, and relaxation shifts for chemisorbed acetylene on Pd (or Pt> than on Ni are indicative of a stronger interaction of chemisorbed acetylene wide Pd (or Pt) than with Ni. The stronger sr-d ir,teraction on Pd or Pt than on Ni is likely related to the greater spatial delocalizaticn of d-wavefunctions of Pd 01 Pt relative to Ni - the latter being a direct result of nodes in the dwavefunctions of Pd and Pt. The exposure of Pd(l11) at T = 300 K to acetylene does not produce the same chemisorbed acetylene phase as found for T 5 180 K, but inster;d produces a new phase characterized by AN(E) of fig. Id. We find that this new species also forms upon momentarily heating initially chemisorbed acetylene at T,< 180 K. to 200 K or above. That is, the 8.9 eV and 11 eVionization features of chemisorbed acetylene decrease partiaily and completely in intensity, respectively, upon warming while new emission features at ~7.2 and 12 eV form which are characteristic of the room tempcrature species. We also find that this new C2H2-derived phase on Pd(ll1) appears to be shghtly more stable than chemisorbed acetylene on Ni( 111). Namely, the former species starts to deccmpose at temperatures of 460 K versus 400 K for Ni. The results for Pt( 111) are qualitatively the same as those for Pd(ll1) and are not discussed here. Both the conversion of chemisorbed acetylene to this new species on Pd and Pt as well as further decomposition are discussed in more detail elsewhere [3].
4. The olefinic surface complex Room
temperature
exposure
of Pd( 11 i) or Pt( 111)
suggest a Luger satuiation work function change and a smaller n-orbital bonding shift than observed previously in ref. [ 11, Also note that the breadth and lack of structure in the x-orbital of i&ally chcmisorbcd acetylene on Ni aF-d Pd suggests that the n orbital degeneracy is not removed upon adsorption and that both n-orbitals intcrdct equally
** Our present results on Ni(l11)
14
1 January
1977
to acetylene produces a new species whose ionization features are very similar to those of gaseous ethylene and to ethylene chemisorbed on Ni, Pd or Pt at low temperatures. This simiIarity is seen by compriison of the various spectra shown in fig. 2. In addition, we find that the frequency dependence between /IV = 21.2 and 40.8 eV for the I2 eV ionization level of &is new species corresponds to that observed for molecular orbjtals whose character is largely derived from the antibonding combination of atomic carbon Zs-orbitals, in particular. the 2a, orbital of chemisorbed or free ethyIene. Since the relative location of such a low lying antibondmg valence orbital relative to the higher lying I
,a) 3L I
r
I
,
,
C,H, /Pt (III),T- 300K
g’ a
II i
;b) ZL
CzH,/PdW,T-300K
z z
c) 2L
czH4/
Pd(lll)
,T+
80K
k
-. 4’ a
I6 GASEOUS I P
16
I 20
,
,
,
, J2&,
14
:2
IO
8
6
ELECTRON
BINDING ENESGY
4 (eV)
I’@. 2. Comparison of the AA’(E) spectra (corrected so as to give a constant background) relative to ITI: for (a) a 3L (3 X 10” torr s) exposure of3cetyIene to Pt(I11) at room temperature, (b) a 2L exposure of acetylene to Pd(l11) at room temperature and (c) a 2L exposure of ethylene to Pd(l11) at T== 80 K and the gas phase ethylene spectra 171. (In (c) the energy scales have been shifted to align with the spectra in (a) and (b), while the ionization features for gaseous ethylene are assigned to the molecular orbitals of ethylene.) For chemisorption results we show M(E) spectra for both hv = 21.2 and 40.8 eV. Note that the dotted lines for the 40.8 eV spec-
tra in (a) - (c) indicate a region of uncertainty in assigning features in the LW(&Jspectra to adsorbate derived molecular orbitals due to overlap with Pt or Pd d-band features.
Volume 45, number
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valence orbitals in hydrocarbon molecules strongly reflects the nature of the C-C bond [2,8], the close sirnilarities between the ionization spectra of this new species and ethylene strongly argues that the new species is of olefinic character*. We have considered several other explanations to interpret the close similarity between the UPS spectra of this new phase and chemisorbed ethylene but these have been excluded based upon other experimenral evidence. First, we exclude the possibility that dehydrogenation of one chemisorbed acetylene molecule with hydrogen transfer to another chemisorbed acetylene molecule produces chernisorbed C2H4 + 2 Cads . This is unlikely as chemisorbed ethylene on Pd (T 4 80 K) also decomposes for T> 200 K to this new species pius chemisorbed hydrogen. Also, we might expect that a dehydrogenation process which involves half the chemisorbed acetylene species would noticeably change the surface dipole, i.e. the work function. We see negligible if any change in work function when this conversion occurs. Further, the presence of preadsorbcd hydrogen does not appear to alter this conversion or the room temperature formation of this new species - suggestive that adsorbed hydrogen is not important for the formation of this new species. The possibility that dimerization or polymerization of chemisorbed acetylene as well as fragmentation of chemisorbed acetylene to form this new species is aIso excluded. We exclude the former possibility as this new phase is observed to form at the lowest observable coverages and for a range of low coverage exposures. Also, the simplest dimer, diacetylene [ 111, has a gas phase orbital ionization spectrum markedly different from that observed here. Fragmentation of C,H, to form CC, CH, CH2 or C,H radicals on Pd is also exchrded as such radicals would have different ionization levels than observed here. (We have observed and identified such radicals on Ni(l1 l), Ni(ll0) and Si(lll) surfaces as is discussed in more detail elsewhere [3] .) Also, from temperature programmed mass spectroscopic measurements we find no significant desorption products nor * Note that IR work on silica supported
metal particles of Pd [9] and also for Pt [lo] have suggested that olefinic species of several stoichiometries may form from room temperature exposure to acetylene. However, the experimental conditions of such measurements - i.e., higher ambient pressure, poorly defied crystal surfaces and sample purity preclude a direct comparison with our results.
1 January
L977
any evidence for the formation of chernisorbed hydrogen during this conversion. Thus, we conclude that this olefinic species has the same chemical composition as acetylene. Such a composition would imply that irrrtially chemisorbed acetylene rehybridizes from an sp to an sp2 configuration so as to form an olefinic species with o-like bonds to substrate orbitals. We note that departures in the ionization features from those of ethylene (fig. 2) are expected as substrate orbitals are likely admixed differently into the higher lying orbitals to give different orbital energies and photoemission cross sections than in the case of ethylene. Also another difference exists between this new phase and chemisorbed ethylene or acetylene - i.e. assuming that the 12 eV ionization level corresponds to the 2a, molecular orbital of ethyfene, the relaxation shift of this new phase on Pd appears to be -0.8 eV more than for chemisorbed ethylene (x2.1 eV) and x 1 eV less than for chemisorbed acetylene. To summarize our proposed modes of bonding, we schematically show in fig. 3 the cases of (a) the free C2H, molecule, (b) n-bonded C2HZ and (c) the C,H, olefinic surface complex. We neglect substrate crystallography and only indicate charge density contours of one of the n-orbitals of acetylene. For initially chemisorbed acetylene on Ni, Pd or Pt, little rehybridization
b,
H-c---_-c-H a /I///
I//‘/
/
Fig. 3. Schematic of the geometry and a-orbital charge density for (a) free acetylene (b) initially chemisorbed acetyhme and (c) the oiefinic CzH2 -Pd( 11 l)/Pt( 11 I) phase discussed in the text. The “double” bond of the molecuIe indicated h each diagram corresponds to the o-bond and other n-orbital (dashed line) not represented as a charge density. For the olefinic &Hz -Pd/Pt phase it is not clear whether only one or both n-orbitals arc involved in bonding.
15
Volume 45, number
CHEMICAL
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6f the free mofecule occurs as is illustrated in fig. 3b and was discussed previously for Ni [2]. We believe that both Ir-orbitals interact equally with the surface atoms for this phase. However, we do expect a greater n-orbital charge density distortion for chemisorbed acetylene on Pd or Pt than on Ni - the result ofa stronger or-ci interaction which we observe and have discussed earlier. In the conversion
of n-bonded
acetylene
to the sta-
ble olefinic C,H,-Pd( 111) phase, we observe some interesting features which we note but do not discuss here. First, the dependence of the conversion rate (or conversion temperature) on-the initial coverage on Pd suggests that adsorbate-adsorbate interactions and a site change may occur. Second, the smaller relaxation/ screening shift for this new species suggests that the new species lies from the surface, possibly even in a !ower symmetry bond site than initially chemisorbed acetylene. Thus, we speculate that a geometry and/or site change occurs during this conversion. In fig. 3c we schematicalljr show the C,H,-PdjPt olefinic complex. (We note that recent LEED structure work for acetylene exposed to Pt( 111) at room temperature indicates that what is likely our olefinic species lies parallel to the plane of the surface [ 12]-) The rehybridization of chemisorbed acetylene on Pd or Pt to form the surface olefin involves strong distortion and polarization of the Ir-orbit& * so as to interact more strongly with Pd or Pt d-orbitals than in the initially chemisorbed phase. As a result of this interaction the hydrogen atoms are strongly bent away from the C-C plane while the C-C bond length is increased to a value characteristic of sp2 hybridization (* 1.35 A). Such a rehybridized species is not only analogous to valence bond descriptions of divalent alkyne or alkene organometallic complexes of Pd and Pt [ 131 but also to the classical di-o adsorbed species discussed in the 1i:erature [14]. However, here on the surface the number of neighboring Pd or Pt atoms which are involved in forming the new u-like bonding charges with the surface may not necessarily be restricted to a single surface atom but may arise from interactions with surface d-orbitals of several neighboring Pd or Pt atoms. * For the new oletkic
phase on Pd(l i 1) and particularly on the adsorbate n-orbitals from the d-band changes, and cannot say whether one Rorbltal of C2H2 remains after forming this new olefinic surfact species.
Pt(l11) we cannot clearly delineate
15
1 January
1977
The formation of a “surface olefin” on Pd or Pt and not on Ni is consistent with the fact that Pd and Pt form either zerovalent ot divalent organometallic transition metal complexes whereas Ni forms almost exclusively zerovalent complexes [ 13]_ We can further correlate the formation of this olefinic surface complex on Pd and Pt not only with the spatial extent of Pd and Pt d-wavefunctions but also with their greater polarizability relative to Ni [ 151.
5. Summary We conclude that chemisorbed acetylene n-bonds to Pd(l11) and Pt(l11) for T,< IS0 K in a similar manner but more strongly than on Ni(l1 l), and that on Pd(l11) and Pt(ll1) a thermally activated conversion occurs, for Ts 200 K to form a CzH2-Pd(1 1 I)/ Pt (111) olefinic surface complex not found on Ni (111). The formation of this new species on Pd (111) and Pt(ll1) involves significant rehybridization of initially chemisorbed acetylene via distortion of acetylenic n-orbitals and the formation of new di-a like valence bonds with substrate d-electrons_
Acknowledgement The author wishes to acknowledge useful discussions with D.E. Eastman and G.W. Rubloff.
References [l] J.E. Demuth and D.E. Eastman, (1974) 1123. [2] J.E. Dcmuth and D.E. Eastman,
Phys. Rev. Letters
32
Phys. Rev. B 13 (19761
1523. 131 J.E. Demuth, to be published. r41 D.E. Eastman and J.E. Demuth, Japan. J. Appl. Phys. Suppl. 2, Part 2 (1974) 827. [51 P.J. Page and P.M. Williams, Faraday Discussions 58 (1974). [61 H. Conrad, G. Ertl, J. Kdppers and E.E. Latta, Faraday Discussions 58 (1974) 80. 171 W.D. Grobman, private communication. 181 J.N. Murrell and W. Schmidt, J. Chem. Sot. Faraday Trans. II 68 (1972) 1709; D.G. Streets and A.W. Potts, J. Chem. Sot. Faraday Trans. II 70 (1974) 1505.
Volume 45, numoer
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[9] L.H. Little, Infrared spectra of adsorbed species (Academic Press, New York, 1966) p. 100. [lo] N. Sheppard and J.W. Ward, J. Catalysis 15 (1969) 50. [ll] D-W. Turner, C. Baker, A.D. Baker and C.R. Brundle, Molecular photoelectron spectroscopy (Interscience, New York, 1970) p. 194. 1121 L.L. Kesmodal, PC. Stair, R.C. Bactzold and G.A. Somorjai, Phys. Rev. Letters 36 (1976) 1316.
1 Ianuary
1977
[ 131 P-W. Jolly and G. Wrlke, The organic chemistry of nicke1, Vol. 1 (Academic Press, New York, 1974) pp. 245-250. [ 141 G-C. Bond, m: Advances in catalysis, Vol. i.5 (Academic Press, New York, 1964) p. 91. [IS] R.P. hlessncr, in: The physical basis for hetcrogcncous catalysis, eds. E. Drauglis and R.I. Jaffec (Plenum Press, New York, 1975) p_ 270.
17
Volume 45, number 1
1
January 1977
CHEMISOP~ION OF C,H, ON Pd(ll1) AND Pt(ll1): FORMATION OF A THERMALLY ACTIVATED OLEFINIC SURFACE COMPLEX* 3.E. DEMUTH ZBBZThomas J. Watson Yorktown
Heights, New
Received 9 August Revised manuscript
Research Center, York 10598, USA
19 76 received 5 October
1976
Ultraviolet photoemission spectroscopy s:udics indicate that acetylene chcmisorbs on Pd( 111) and Pt(I 11) as x-bonded acetylene for T 5 180 K but upon warming (or room tcmpcraturc exposure) forms a new surface species. We identify the electronic structure of this new phase as being characteristic of an olefinic CzI12 surface species - the result of rehybridiTatIon of the mltially chemisorbed molecule. T!lese xsults are discussed relative to previous work on Ni(ll1).
1_ Introduction The identification of the chemical nature and bonding interactions of hydrocarbon molecules adsorbed on transition metal (TM) surfaces is of fundamental importance to understand surface reactions, and possibly catalytic processes on these surfaces. We have previously reported UV photoemission spectroscopy (UPS) results for kv = 2 1.2 eV for low exposures of acetylene to Nl(11 I) (80 K 2 Ts400 K) and interpreted these results as indicative of a n-d chemisorption interaction between aceiylene and the surface [I ] where little rehybridization of the chemisorbed species occurs relative to that of the free molecule [2]. In this wo*l: we report similar UPS studies for low exposures of acetylene to Pd( 111) and Pt (111) at photon energies of 2 1.2 and 40.8 eV for T 5 180 K where we not only observe a chemisorbed phase similar to that on Ni(17 l), but upon warming we directly observe the thermally activated formation of a new chemisorbed species from initial!y chemisorbed acetylene. The ionization levels of this new phase are different from those of chemisorbed or gaseous acetylene and are characteristic of an olefin. Room temperature exposures of either Pd(ll1) or Pt(ll1) to C,H2 produce this new olefinic species directly. We show that this species is likely the result * Partiauy
supported
by ONR
contract
N00014-75-C-0346.
of rehybridization of initially chemisorbed acetylene to form an olefinic surface complex with the Pd or Pt surface.
2. Experimental procedures The present studies were performed in a turbomolecular pumped TJHV system (base pressure < 1 X lo-lo torr) equipped with a He resonance lamp, electron energy analyzer, LEED optics, quadrupole mass spectrometer and auxiliary electron gun (for Auger and energy loss spectroscopy). Single crystal samples of Ni(l1 l), Pd(l11) and Pt(l11) were prepared by conventional techniques and mounted on a multiple sample holder which permitted the samples to be liquid nitrogen cooled to T * 78 K or independently resistivity heated to T = 1600 K as measured with a chromelalumel thermocouple. The single crystals were cleaned by mild oxidation treatments, argon ion sputter-etching and subsequent annealing. Surface characterization was performed by LEED, Auger and photoemission analy ses. Energy analysis of the photoemitted electrons was performed with a double pass (angle-integrating) cylindrical mirror anaIyzer (CMA) operated in a fvted pass mode so as to have a resolution of eO.1 S(O.25) eV or better
for He I(II)
UPS work. The samples were
pOSi-
Volume 45, number 1
CHEMICAL PHYSICS LETTERS
tioned with their normal direction =220” from the axis of the CMA into the photon beam, which is 73” off the axis of the CMA. We note that we always observe the same adsorbate ionization energies for adsorbed species independent of sample orientation - the sample orientation may change relative intensities, e.g. via polarization effects. Also, we do not observe any photon- or photoelectron-induced changes in chemisorbed
I kmuuy
1977
0) 3L C,H,, A9 = -14eV
CLEAN Pd (111) W=56eV) /
species or in the thermally activated reaction we discuss here. Work function changes were measured using the low-energy cutoffs of the photoemission energy distributions
[3]. hv= 408eV
3. Experimental
results
for acetylene
on Pd( 111)
In fig. la we show the photoemission energy distribution curve for clean Pd( 111) (solid line) and after exposure to 3 X lop6 torr s acetylene (dashed iine) for hv = 40.8 eV with a sample temperature of 180 K. We observe strong overall d-band changes and new levels at 8.8 and 11 eV below the Fermi level E, . To enhance the chemisorption induced features, we show the difference spectrum AN@‘) between these two spectra in fig. lb. The dashed line in the d-band region represents the emission decrease if only uniform attenuation of the d-states had occurred. We observe preferential attenuation near the top of the d-band and additional emission near the bottom of the d-bands a behavior commonly observed for chemisorbed species on Ni [4,5] or Pd 161. In fig. lc we also show for comparison both the AN(E) spectra for an identical exposure of Ni(l1 I) to acetylene at T = 300 K as well as the gas phase spectrum obtained in a gas phase spectrometer of similar geometry and resolution [7]. The results for Ni(ll1) are similar to those reported previously for tzv = 21.2 eV [ 1,2] except that for hv = 40.8
eV we can also observe
a lower
lying
level at 15.8
eV below E, which corresponds to the 20~ molecular orbital of acetylene. Clearly the same chemisorbed phase of acetylene forms on Pd(1 I I) at 180 K as on Ni(l I I), with details of the interaction being different. Namely, the saturation coverage work function change on Pd is -1.4 eV (-1.6 eV on Pt) versus -i -2 eV on Ni** while the re-
GASEOUS IP --
-----p
IleV _-_ CEHE/Pd(III).T- 3mK --
-0
:--. I6
I4 I2 IO 8 6 4 2 ELECTRON BINDINGENERGY (eV)
Fig. 1. Photoemission energy distribution spectra N(m reIative to EF for (a) clean Pd( 111) at T = 180 K and after exposure to 3L (3 X IO+ torr s) acetylene (dashed line) and (b) their corresponding difference spectra ti(f?), (c) the ti(fZ) spectra for 3L acetylene on Nd 111) at T = 300 K and the gas spectra [ 71, and (d) the A&k) spectra For 3.5 L acetylene exposure to Pd(l I I) at F = 300 K. In the d-band region of the &V(E) curves the short dashed lines indicate a uniform attenuation of the d-band emission of the clean surface while the longer dashed tines estimate the background intensity. AU results are for hv = 40.8 eV. (The gas phase spectra ionization features are assigned to the In& 30 ,2ou and 20 moIecuIar orbitals of acetylene respectively w a ere the In; &bital lies at s 11.5 eV_)
13
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laxation shift as defined previously [I-3] is 3.7 eV on Pd (3.5 eV on Pt) versus 3.5 eV on Ni. Also, the 7r-orbit& of acetylene on Pd would appear to have a larger “bonding shift” (a I .9 eV versus 1.2 eV for Ni *) although on Pd the d-band overlaps the n-level and prohibits its clear delineation from d-band changes. The greater work function change. n-orbital bonding shift, and relaxation shifts for chemisorbed acetylene on Pd (or Pt> than on Ni are indicative of a stronger interaction of chemisorbed acetylene wide Pd (or Pt) than with Ni. The stronger sr-d ir,teraction on Pd or Pt than on Ni is likely related to the greater spatial delocalizaticn of d-wavefunctions of Pd 01 Pt relative to Ni - the latter being a direct result of nodes in the dwavefunctions of Pd and Pt. The exposure of Pd(l11) at T = 300 K to acetylene does not produce the same chemisorbed acetylene phase as found for T 5 180 K, but inster;d produces a new phase characterized by AN(E) of fig. Id. We find that this new species also forms upon momentarily heating initially chemisorbed acetylene at T,< 180 K. to 200 K or above. That is, the 8.9 eV and 11 eVionization features of chemisorbed acetylene decrease partiaily and completely in intensity, respectively, upon warming while new emission features at ~7.2 and 12 eV form which are characteristic of the room tempcrature species. We also find that this new C2H2-derived phase on Pd(ll1) appears to be shghtly more stable than chemisorbed acetylene on Ni( 111). Namely, the former species starts to deccmpose at temperatures of 460 K versus 400 K for Ni. The results for Pt( 111) are qualitatively the same as those for Pd(ll1) and are not discussed here. Both the conversion of chemisorbed acetylene to this new species on Pd and Pt as well as further decomposition are discussed in more detail elsewhere [3].
4. The olefinic surface complex Room
temperature
exposure
of Pd( 11 i) or Pt( 111)
suggest a Luger satuiation work function change and a smaller n-orbital bonding shift than observed previously in ref. [ 11, Also note that the breadth and lack of structure in the x-orbital of i&ally chcmisorbcd acetylene on Ni aF-d Pd suggests that the n orbital degeneracy is not removed upon adsorption and that both n-orbitals intcrdct equally
** Our present results on Ni(l11)
14
1 January
1977
to acetylene produces a new species whose ionization features are very similar to those of gaseous ethylene and to ethylene chemisorbed on Ni, Pd or Pt at low temperatures. This simiIarity is seen by compriison of the various spectra shown in fig. 2. In addition, we find that the frequency dependence between /IV = 21.2 and 40.8 eV for the I2 eV ionization level of &is new species corresponds to that observed for molecular orbjtals whose character is largely derived from the antibonding combination of atomic carbon Zs-orbitals, in particular. the 2a, orbital of chemisorbed or free ethyIene. Since the relative location of such a low lying antibondmg valence orbital relative to the higher lying I
,a) 3L I
r
I
,
,
C,H, /Pt (III),T- 300K
g’ a
II i
;b) ZL
CzH,/PdW,T-300K
z z
c) 2L
czH4/
Pd(lll)
,T+
80K
k
-. 4’ a
I6 GASEOUS I P
16
I 20
,
,
,
, J2&,
14
:2
IO
8
6
ELECTRON
BINDING ENESGY
4 (eV)
I’@. 2. Comparison of the AA’(E) spectra (corrected so as to give a constant background) relative to ITI: for (a) a 3L (3 X 10” torr s) exposure of3cetyIene to Pt(I11) at room temperature, (b) a 2L exposure of acetylene to Pd(l11) at room temperature and (c) a 2L exposure of ethylene to Pd(l11) at T== 80 K and the gas phase ethylene spectra 171. (In (c) the energy scales have been shifted to align with the spectra in (a) and (b), while the ionization features for gaseous ethylene are assigned to the molecular orbitals of ethylene.) For chemisorption results we show M(E) spectra for both hv = 21.2 and 40.8 eV. Note that the dotted lines for the 40.8 eV spec-
tra in (a) - (c) indicate a region of uncertainty in assigning features in the LW(&Jspectra to adsorbate derived molecular orbitals due to overlap with Pt or Pd d-band features.
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valence orbitals in hydrocarbon molecules strongly reflects the nature of the C-C bond [2,8], the close sirnilarities between the ionization spectra of this new species and ethylene strongly argues that the new species is of olefinic character*. We have considered several other explanations to interpret the close similarity between the UPS spectra of this new phase and chemisorbed ethylene but these have been excluded based upon other experimenral evidence. First, we exclude the possibility that dehydrogenation of one chemisorbed acetylene molecule with hydrogen transfer to another chemisorbed acetylene molecule produces chernisorbed C2H4 + 2 Cads . This is unlikely as chemisorbed ethylene on Pd (T 4 80 K) also decomposes for T> 200 K to this new species pius chemisorbed hydrogen. Also, we might expect that a dehydrogenation process which involves half the chemisorbed acetylene species would noticeably change the surface dipole, i.e. the work function. We see negligible if any change in work function when this conversion occurs. Further, the presence of preadsorbcd hydrogen does not appear to alter this conversion or the room temperature formation of this new species - suggestive that adsorbed hydrogen is not important for the formation of this new species. The possibility that dimerization or polymerization of chemisorbed acetylene as well as fragmentation of chemisorbed acetylene to form this new species is aIso excluded. We exclude the former possibility as this new phase is observed to form at the lowest observable coverages and for a range of low coverage exposures. Also, the simplest dimer, diacetylene [ 111, has a gas phase orbital ionization spectrum markedly different from that observed here. Fragmentation of C,H, to form CC, CH, CH2 or C,H radicals on Pd is also exchrded as such radicals would have different ionization levels than observed here. (We have observed and identified such radicals on Ni(l1 l), Ni(ll0) and Si(lll) surfaces as is discussed in more detail elsewhere [3] .) Also, from temperature programmed mass spectroscopic measurements we find no significant desorption products nor * Note that IR work on silica supported
metal particles of Pd [9] and also for Pt [lo] have suggested that olefinic species of several stoichiometries may form from room temperature exposure to acetylene. However, the experimental conditions of such measurements - i.e., higher ambient pressure, poorly defied crystal surfaces and sample purity preclude a direct comparison with our results.
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any evidence for the formation of chernisorbed hydrogen during this conversion. Thus, we conclude that this olefinic species has the same chemical composition as acetylene. Such a composition would imply that irrrtially chemisorbed acetylene rehybridizes from an sp to an sp2 configuration so as to form an olefinic species with o-like bonds to substrate orbitals. We note that departures in the ionization features from those of ethylene (fig. 2) are expected as substrate orbitals are likely admixed differently into the higher lying orbitals to give different orbital energies and photoemission cross sections than in the case of ethylene. Also another difference exists between this new phase and chemisorbed ethylene or acetylene - i.e. assuming that the 12 eV ionization level corresponds to the 2a, molecular orbital of ethyfene, the relaxation shift of this new phase on Pd appears to be -0.8 eV more than for chemisorbed ethylene (x2.1 eV) and x 1 eV less than for chemisorbed acetylene. To summarize our proposed modes of bonding, we schematically show in fig. 3 the cases of (a) the free C2H, molecule, (b) n-bonded C2HZ and (c) the C,H, olefinic surface complex. We neglect substrate crystallography and only indicate charge density contours of one of the n-orbitals of acetylene. For initially chemisorbed acetylene on Ni, Pd or Pt, little rehybridization
b,
H-c---_-c-H a /I///
I//‘/
/
Fig. 3. Schematic of the geometry and a-orbital charge density for (a) free acetylene (b) initially chemisorbed acetyhme and (c) the oiefinic CzH2 -Pd( 11 l)/Pt( 11 I) phase discussed in the text. The “double” bond of the molecuIe indicated h each diagram corresponds to the o-bond and other n-orbital (dashed line) not represented as a charge density. For the olefinic &Hz -Pd/Pt phase it is not clear whether only one or both n-orbitals arc involved in bonding.
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6f the free mofecule occurs as is illustrated in fig. 3b and was discussed previously for Ni [2]. We believe that both Ir-orbitals interact equally with the surface atoms for this phase. However, we do expect a greater n-orbital charge density distortion for chemisorbed acetylene on Pd or Pt than on Ni - the result ofa stronger or-ci interaction which we observe and have discussed earlier. In the conversion
of n-bonded
acetylene
to the sta-
ble olefinic C,H,-Pd( 111) phase, we observe some interesting features which we note but do not discuss here. First, the dependence of the conversion rate (or conversion temperature) on-the initial coverage on Pd suggests that adsorbate-adsorbate interactions and a site change may occur. Second, the smaller relaxation/ screening shift for this new species suggests that the new species lies from the surface, possibly even in a !ower symmetry bond site than initially chemisorbed acetylene. Thus, we speculate that a geometry and/or site change occurs during this conversion. In fig. 3c we schematicalljr show the C,H,-PdjPt olefinic complex. (We note that recent LEED structure work for acetylene exposed to Pt( 111) at room temperature indicates that what is likely our olefinic species lies parallel to the plane of the surface [ 12]-) The rehybridization of chemisorbed acetylene on Pd or Pt to form the surface olefin involves strong distortion and polarization of the Ir-orbit& * so as to interact more strongly with Pd or Pt d-orbitals than in the initially chemisorbed phase. As a result of this interaction the hydrogen atoms are strongly bent away from the C-C plane while the C-C bond length is increased to a value characteristic of sp2 hybridization (* 1.35 A). Such a rehybridized species is not only analogous to valence bond descriptions of divalent alkyne or alkene organometallic complexes of Pd and Pt [ 131 but also to the classical di-o adsorbed species discussed in the 1i:erature [14]. However, here on the surface the number of neighboring Pd or Pt atoms which are involved in forming the new u-like bonding charges with the surface may not necessarily be restricted to a single surface atom but may arise from interactions with surface d-orbitals of several neighboring Pd or Pt atoms. * For the new oletkic
phase on Pd(l i 1) and particularly on the adsorbate n-orbitals from the d-band changes, and cannot say whether one Rorbltal of C2H2 remains after forming this new olefinic surfact species.
Pt(l11) we cannot clearly delineate
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The formation of a “surface olefin” on Pd or Pt and not on Ni is consistent with the fact that Pd and Pt form either zerovalent ot divalent organometallic transition metal complexes whereas Ni forms almost exclusively zerovalent complexes [ 13]_ We can further correlate the formation of this olefinic surface complex on Pd and Pt not only with the spatial extent of Pd and Pt d-wavefunctions but also with their greater polarizability relative to Ni [ 151.
5. Summary We conclude that chemisorbed acetylene n-bonds to Pd(l11) and Pt(l11) for T,< IS0 K in a similar manner but more strongly than on Ni(l1 l), and that on Pd(l11) and Pt(ll1) a thermally activated conversion occurs, for Ts 200 K to form a CzH2-Pd(1 1 I)/ Pt (111) olefinic surface complex not found on Ni (111). The formation of this new species on Pd (111) and Pt(ll1) involves significant rehybridization of initially chemisorbed acetylene via distortion of acetylenic n-orbitals and the formation of new di-a like valence bonds with substrate d-electrons_
Acknowledgement The author wishes to acknowledge useful discussions with D.E. Eastman and G.W. Rubloff.
References [l] J.E. Demuth and D.E. Eastman, (1974) 1123. [2] J.E. Dcmuth and D.E. Eastman,
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Phys. Rev. B 13 (19761
1523. 131 J.E. Demuth, to be published. r41 D.E. Eastman and J.E. Demuth, Japan. J. Appl. Phys. Suppl. 2, Part 2 (1974) 827. [51 P.J. Page and P.M. Williams, Faraday Discussions 58 (1974). [61 H. Conrad, G. Ertl, J. Kdppers and E.E. Latta, Faraday Discussions 58 (1974) 80. 171 W.D. Grobman, private communication. 181 J.N. Murrell and W. Schmidt, J. Chem. Sot. Faraday Trans. II 68 (1972) 1709; D.G. Streets and A.W. Potts, J. Chem. Sot. Faraday Trans. II 70 (1974) 1505.
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[9] L.H. Little, Infrared spectra of adsorbed species (Academic Press, New York, 1966) p. 100. [lo] N. Sheppard and J.W. Ward, J. Catalysis 15 (1969) 50. [ll] D-W. Turner, C. Baker, A.D. Baker and C.R. Brundle, Molecular photoelectron spectroscopy (Interscience, New York, 1970) p. 194. 1121 L.L. Kesmodal, PC. Stair, R.C. Bactzold and G.A. Somorjai, Phys. Rev. Letters 36 (1976) 1316.
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[ 131 P-W. Jolly and G. Wrlke, The organic chemistry of nicke1, Vol. 1 (Academic Press, New York, 1974) pp. 245-250. [ 141 G-C. Bond, m: Advances in catalysis, Vol. i.5 (Academic Press, New York, 1964) p. 91. [IS] R.P. hlessncr, in: The physical basis for hetcrogcncous catalysis, eds. E. Drauglis and R.I. Jaffec (Plenum Press, New York, 1975) p_ 270.
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