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Coupling of Carbon Monoxide Molecules over Oxygen-Defected UO2(111) Single Crystal and Thin Film Surfaces S. D. Senanayake, G. I. N. Waterhouse, and H. Idriss* The Department of Chemistry, The University of Auckland, Private Bag 92019, Auckland, New Zealand
Theodore E. Madey Department of Physics and Astronomy, and Laboratory for Surface Modification, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903 Received July 14, 2005. In Final Form: August 28, 2005 While coupling reactions of carbon-containing compounds are numerous in organometallic chemistry, they are very rare on well-defined solid surfaces. In this work we show that the reductive coupling of two molecules of carbon monoxide to C2 compounds (acetylene and ethylene) could be achieved on oxygendefected UO2(111) single crystal and thin film surfaces. This result allows in situ electron spectroscopic investigation of a typical organometallic reaction such as carbon coupling and extends it to heterogeneous catalysis and solids. By using high-resolution photoelectron spectroscopy (HRXPS) it was possible to track the changes in surface states of the U and O atoms as well as identify the intermediate of the reaction. Upon CO adsorption U cations in low oxidation states are oxidized to U4+ ions; this was accompanied by an increase of the O-to-U surface ratios. The HRXPS C 1s lines show the presence of adsorbed species assigned to diolate species (-OCHdCHO-) that are most likely the reaction intermediate in the coupling of two CO molecules to acetylene and ethylene.
I. Introduction The surface chemistry of the uranium oxide system offers an ideal environment for fundamental investigation of carbon coupling reactions. The fact that uranium ions can accommodate high coordination numbers and several oxidation states in addition to the presence of the f-orbitals (with different symmetry compared to the d-orbitals) makes them active for oxidative and reductive coupling reactions.1-3 CO reactions have been studied in detail over early transition metals for many years. The most common chemical reaction studied for CO is its oxidation to CO2 over both metal and oxide surfaces. In that regard U3O8 is a very active catalytic oxide material, far superior to most early transition metal oxides for this reaction and for oxidation of hydrocarbons in general.4,5 Far less work has been done for the reduction (including coupling) of CO on metal oxide surfaces and none in surface science. On the contrary, examples of coupling of CO and carbonyl compounds (in general) in coordination chemistry are common.6,7 On U and Th complexes the unique coupling of two molecules of CO to cis-endiolates has been reported * Corresponding author. Fax:
[email protected].
74 9 373 7422. E-mail:
(1) Kahn, B. E.; Rieke, R. D. Chem. Rev. 1988, 88, 733 and references therein. (2) Maury, O.; Villiers, C.; Ephritikhine, M. Angew. Chem., Int. Ed. Engl. 1996, 35, 1129. (3) (a) Madhavaram, H.; Idriss, H. J. Catal. 2002, 206, 155. (b) Zhou, M.; Andrews, L.; Li, J.; Bursten, B. E. J. Am. Chem. Soc. 1999, 121, 9712. (4) Nozaki, F.; Ohki, K. Bull. Chem. Soc. Jpn. 1972, 45, 3473. (5) Hutchings, G.; Heneghan, C. S.; Hudson, I. D.; Taylor, S. H. Nature 1996, 384, 341. (6) Villiers, C.; Ephritikhine, M. Angew. Chem., Int. Ed. Engl. 1997, 36, 2380. (7) Villiers, C.; Ephritikhine, M. Chem.sEur. J. 2001, 7, 3043.
a while ago.1,8 Other metals, such as V, Mo, and W, are also active for CO coupling reactions.9,10 In this work we show that one can selectively make C2H2 from CO and H2 over the surface of UO2-x(111) single crystal and extend it to a similar surface of a thin film. The oxidation/reduction of UO2-x was followed by the U and O core levels (high-resolution photoelectron spectroscopy, HRXPS) while the reaction products were monitored by temperature programmed desorption (TPD), in addition. II. Experimental Section TPD experiments were conducted in an ultrahigh vacuum (pressure ≈ 1 × 10-10 Torr) stainless steel chamber equipped with several surface science techniques, as described elsewhere, at the University of Auckland.11 HRXPS is conducted at the U12a beamline at the National Synchrotron Light Source of Brookhaven National Laboratory (BNL). The UO2(111) single crystal surface was cleaned prior to all experimentation with several cycles of annealing to 800 K and Ar+ sputtering; surface stoichiometry (or substoichiometry) was analyzed by XPS while surface order was checked by low energy electron diffraction (LEED) with the latter showing a sharp hexagonal structure (Figure 1). The UO2 thin film (about 1000 Å thick), prepared by sputter deposition on a Mo substrate,12a was cleaned using a similar method but with annealing temperatures not exceeding 600 K to maintain the integrity of the thin film. The defected surfaces (UO2-x) were obtained with extended periods of Ar+ sputtering and analyzed (8) Katahira, D. A.; Moloy, K. G.; Marks, T. J. Organometallics 1982, 1, 1723. (9) Protasiewicz, J. D.; Lippard, S. J. J. Am. Chem. Soc. 1991, 113, 6564. (10) Carnahan, E. M.; Protasiewicz, J. D.; Lippard, S. J. Acc. Chem. Res. 1993, 26, 90. (11) Senanayake, S. D.; Idriss, H. Surf. Sci. 2004, 563, 135. (12) (a) Gouder, T.; Colmenars, C. A.; Naegele, J. R.; Spirlet, J. C.; Verbist, J. Surf. Sci. 1992, 264, 354. (b) Pearce, R. J.; Whittle, I.; Hilton, D. A. J. Nucl. Mater. 1969, 33, 1.
10.1021/la0519103 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/12/2005
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Figure 1. Ball model of the UO2(111) surface showing the hexagonal arrangement of U and O atoms and lattice parameters: red balls, U atoms; yellow balls, O atoms (A, top view; B, side view). (C) Low energy electron diffraction (LEED) of the UO2(111) surface showing the extended order (131 eV). Figure 3. HRXPS U 4f lines of Ar+-sputtered UO2 thin film before (a) and after (b-d) reaction with CO at the indicated exposures in langmuirs (1 L ) 10-6 Torr s). Table 1. Product Yield (%) during TPD Following CO and (CO + 3H2) Adsorption at 300 K over “Reduced” UO2-x(111) Surfacea product
CO + trace H2
CO + 3H2
C2 CO CO2 H2O
13 (5) 20 54 13
10 (0.1) 13 40 37b
a The numbers in parentheses are for the acetylene/ethylene ratios. b The increase in H2O yield is due to the preferential reaction of adsorbed H atoms with the weakly bonded O formed upon sputtering.
Figure 2. TPD following CO adsorption at 300 K on UO2-x (x ≈ 0.3) (111) single crystal. by HRXPS at BNL or by XPS at UA. Dosing of CO, formaldehyde, or ethylene glycol was conducted following a back-filling mode; thus reported exposures are chamber pressure × time during dosing without enhancement of the flux of the molecules on the surface. HRXPS pass energy was 10 eV for U 4f and O 1s and 20 eV for C 1s. The excitation energy was 625 eV for O 1s, 525 eV for U 4f, and 400 eV for C 1s and the valence band (containing U 5f, O 2p, U 6d, and U 6p lines). Spectra were normalized to the U 5f line at 1 eV below the Fermi level.
III. Results Figure 2 shows acetylene desorption during TPD following CO adsorption (1 and 9 L exposures; 1 langmuir (L) ) 10-6 Torr s) at 300 K over a UO2-x surface (x is estimated from XPS O 1s/U 4f lines to be equal to 0.3). The formation of acetylene is due to coupling of two molecules of CO and their association with the inevitable traces of H2 in the background of the chamber (as well as during sputtering). As the figure shows, desorption of acetylene is not coverage dependent, so one can neglect geometric reasons (modes of adsorption). A simple explanation would be related to the extent of reduction of the surface prior to adsorption. The very reduced states are easily oxidized (low temperature), while the less reduced states require additional energies (higher temp-
Table 2. Product Yield (%) during CO TPD for the Stoichiometric UO2(111) Surface, 15 min Ar+ Sputtering, 30 min Ar+ Sputtering, and 45 min Ar+ Sputtering product
oxidized
15 min redn
30 min redn
45 min redn
CO CO2 C2
0 0 0
42 31 28
20.5 54 24
8 70 23
erature). This point is subject to further work. Table 1 shows the computed yield for C2 compounds and other products during TPD. Co-dosing H2 with CO (with H2/ CO ratios ) 3) has resulted in the selective reduction of acetylene to ethylene (Table 1). Nonnegligible amounts of CO2 are also formed. Repeating the CO-TPD experiments after different sputtering times shows that the amount of CO2 increases with increasing reduction, while the yield for C2H2 remains constant (within a few percent); see Table 2. This result may indicate an additional route for CO reaction (besides the coupling to acetylene) on defected surfaces; the Boudouard reaction (2CO(a) f CO2(g) + C(a); (a) for adsorbed, (g) for gas) is a likely pathway.12b Figure 3 shows HRXP spectra of the U 4f region before and after exposure to CO at 300 K. Spectrum a is that of the freshly sputtered surface before adsorption. In addition to the U 4f7/2 and U 4f5/2 lines at 380.4 and 391.1 eV, respectively, and attributed to U4+, a clear broadening at 376-378 eV is seen. This complex region contains several oxidation states of U atoms including U metal and is at present the subject of further study. Upon adsorption of CO the intensity of these lines decreases and almost
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surface, since it also desorbs by 140 K. Figure 6 shows the
Figure 4. HRXPS O 1s lines of Ar+-sputtered UO2 thin film before (a, top) and after (b-d; d, bottom) reaction with CO at the indicated exposures in langmuirs (1 L ) 10-6 Torr s).
disappears at surface saturation. The decrease of this shoulder is thus direct evidence of surface oxidation following the adsorption of CO. Figure 4 shows the corresponding HRXPS of the O 1s lines. The main peak at 531.3 eV is that of surface O atoms of UO2-x. The shoulder at 533 eV is clear upon CO adsorption. This additional O 1s line is another indication for the reaction of CO with the surface. CO adsorption on stoichiometric UO2 has also been performed as a reference (Figure 5). CO was found to adsorb at temperatures below 140 K, and no evidence of other peaks besides that of the lattice oxygen is seen in the O 1s region. From the U 4f attenuation after CO adsorption at 113 K, the surface coverage is found equal to 0.18. The shoulder at the high binding energy side of the C 1s is probably due to surface CO2 species formed upon partial CO oxidation on the
Figure 6. HRXPS C 1s lines of Ar+-sputtered UO2 thin film before (a, top) and after (b-d; d, bottom) reaction with CO at the indicated exposures in langmuirs (L).
C 1s lines corresponding to Figures 3 and 4. The freshly sputtered surface contains some carbide left, close to 282 eV. One can remove it by annealing the surface, but while annealing the surface oxygen diffusion results in oxidation of most of the U atoms in reduced states. The main point from this figure is the presence of a C 1s peak at 286.5 eV far from the carbide or graphite region. This peak is considerably attenuated by heating the surface to 500 K (not shown). The presence of this peak and its decrease with increasing temperature indicate that it is a precursor of the hydrocarbon molecules (acetylene and ethylene). IV. Discussion There are two possible ways for making acetylene from CO on the oxygen-defected surface (UO2-x): either by totally dissociating the C-O bonds of CO and making
Figure 5. High-resolution spectra of U 4f, O 1s, and C 1s lines following adsorption of CO at 113 K over stoichiometric UO2 thin film: blue dots, before adsorption; red dots, after adsorption.
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Scheme 1: CO Insertion into U-H Bonds Making a Bis-formyl Intermediate (I)
Senanayake et al. Scheme 3: Formaldehyde Adsorption on a Model UO2(111) Surface in η1-Adsorbed Mode, Species (III)
Scheme 2: Formaldehyde Reaction with U Center Making a Bis-formaldehyde Intermediate (II)
uranium carbide or by initial partial C-O bond dissociation. While making UC from CO is an exothermic reaction,carbide hydrogenation to acetylene is very endothermic.
2CO + 2U f UO2 + UC2
∆G ) -844.4 kJ mol-1 (1)
2CO + U f UC + CO2
∆G ) -206 kJ mol-1 (2)
UC2 + H2 f U + C2H2
∆G ) +297.7 kJ mol-1 (3)
UC2 + 2H2 f U + C2H4
∆G ) +155.4 kJ mol-1 (4)
However, as seen in Figure 6, only traces of uranium carbide are formed upon CO adsorption on the defected surface. This and the fact that UC or UC2 hydrogenation is a very uphill reaction tend to suggest that the reaction pathway occurs somewhere else. The overall reaction for making acetylene from CO over reduced uranium oxide surfaces can be expressed as
UO2-x + xCO(a) + xH(a) f UO2 + (x/2)C2H2 (5) The driving force is the oxidation of UO2-x to UO2. However, this is not sufficient to make the coupling. The accommodation of CO molecules on either one U atom or two adjacent U atoms is required to allow for orbital interaction in the process of making the carbon-carbon bond. In a seminal work on the reactivity of a U compound (Cp2UR2) vis-a`-vis a stepwise CO insertion to making the unique η2-η2 bis-acyl ligand, it has been indicated that
this was possible due to the presence of fxy2 orbitals of the actinide.14 This allows us to write the first step of the reaction, as shown in Scheme 1. In a previous study we have obtained ethylene from formaldehyde over UO2(111) single crystal via the reductive coupling reaction.15 In the process of making ethylene from formaldehyde, one may obtain the species shown in Scheme 2. The C 1s peak positions of I and II are very close since in both cases the carbon atom is coordinated to one O atom and one U atom. This is in sharp contrast to an adsorbed formaldehyde on a stoichiometric surface (the stoichiometric UO2(111) surface, the most stable surface of the fluorite structure, is oxygen terminated, Figure 1), where the bonding occurs only in a η1-mode; see Scheme 3.16 III would give formate species (by oxidation) upon heating, while II would give pinacolates (by reductive coupling). We have performed HRXPS following formaldehyde adsorption over both stoichiometric and reduced UO2 thin films. Figure 7 shows the C 1s region and the effect of heating. Upon adsorption on both surfaces, at 225 K, monolayer plus multilayer formaldehyde is seen at 287.3-287.7 eV, with a small oxidation noticed at 289.2-289.6 eV (due to the presence of formate species HCOO(a)). Upon heating to 300 K most of the multilayer coverage has desorbed. On the stoichiometric surface only formate species are seen, while on the reduced surface a peak at 286.0-286.2 eV is also formed. This peak is due to a C 1s of C-O species that can be attributed to a combination of methoxides (CH3O(a) and pinacolates ((a)O-CH2-CH2-O(a)). Further heating resulted in the attenuation of both peaks with the formates more stable on the stoichiometric surface. To further assign the peak at 286.0-286.2 eV, experiments using ethylene glycol (EG) were conducted since the enolate species ((a)OCH2CH2O(a)) can be formed from dissociatively adsorbed ethylene glycol (EG), HOCH2CH2OH, on the stoichiometric UO2 thin film. Figure 8 shows the C 1s upon the EG adsorption; the peak position is very close to that observed upon CO
Figure 7. (a, top) HRXPS C 1s lines of Ar+-sputtered UO2 thin film after formaldehyde adsorption at 225 K (33 L) and subsequent heating to the indicated temperature. All spectra except that of the 225 K are recorded at 273 K. (b, bottom) HRXPS C 1s lines of stoichiometric UO2 thin film after formaldehyde adsorption at 225 K (2.2 L) and subsequent heating to the indicated temperature. All spectra except that of the 225 K are recorded at 273 K.
CO Molecule Coupling on Oxygen-Defected UO2(111)
Langmuir, Vol. 21, No. 24, 2005 11145 Scheme 5: Pinacolate Rearrangement To Give UO2 and Release Acetylene in the Gas Phase
Figure 8. HRXPS C 1s lines of stoichiometric UO2 thin film after ethylene glycol adsorption at 273 K (7.2 L) and subsequent heating to the indicated temperature. All spectra are recorded at 273 K. Scheme 4: Rearrangement of Bis-formyl (and Bis-formaldehyde) Species into Pinacolate
adsorption (Figure 6) as well as from HCHO adsorption (Figure 7a). The stability of this peak has been studied with respect to beam time and surface temperature. Negligible effect of the beam time on the quantity and the nature of the adsorbate is seen. The diolate species formed from ethylene glycol desorb upon heating and disappear by 553 K. This is consistent with desorption of ethylene from formaldehyde15 as well as with the first peak for acetylene desorption from CO (Figure 2) and reflects similar reaction pathways for both CO coupling and formaldehyde coupling. The next step of the reaction is the rearrangement of bis-formyl compound to a diolate species. This rearrangement is necessary to restore the surface oxygen and thus return U ions to their more stable +4 oxidation state (as seen by HRXPS U 4f in Figure 3). In other words, the conversion of I or II to diolates is the most reasonable explanation as shown in Scheme 4. Pinacolate-type species ((a)O-CHdCH-O(a)) can be either in a bridging or in a chelating mode. It has been shown by density functional theory computation of complexes containing one/two Ti atoms that, for the
McMurry reaction of two molecules of formaldehyde to pinacolate intermediate on low valent Ti ions, the intramolecular reaction is more favored thermodynamically.17 Another experimental work on UCl4 and Li/Hg has also shown that intramolecular (unimolecular) pinacolate formed from the reactant acetone is the only one converted to the symmetric alkene (Me2CdCMe2) product.2 Based on the above and our photoelectron spectra on CO, formaldehyde, and ethylene glycol, the endiolate route appears the most plausible one; see Scheme 5. V. Conclusions The rich chemistry of the uranium oxide system can serve as a model for studying chemical reactions in general and coupling reactions in particular. The work shows that, while stoichiometric UO2 surface is inactive for CO coupling, the oxygen-defected one is active for the coupling of CO molecules to acetylene and ethylene. The observation of this typical organometallic reaction as a solid-gas reaction has allowed photoelectron spectroscopic identification of the reaction intermediate of one of the most interesting and typical carbon-carbon bond formation reactions. Acknowledgment. The authors thank Dr. D. R. Mullins and Dr. Q-Y. Dong for assistance with experiments at U12a/NSLS/BNL. G.I.N.W. thanks the Foundation for Research Science and Technology (FORST) for provision of a postdoctoral fellowship (Contract No. UOAX0412) and funding to undertake this work. T.E.M. acknowledges partial support from US Department of Energy, Office of Basic Energy Sciences. The research was carried out in part at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under Contract No. DE-AC02-98CH10886. LA0519103 (13) Senanayake, S. D.; Soon, A.; Kohlmeyer, A.; So¨hnel, T.; Idriss, H. J. Vac. Sci. Technol., A 2005, 23, 1078. (14) Tatsumi, K.; Nakamura, A.; Hofmann, P.; Hoffmann, R.; Moloy, K. G.; Marks, T. J. J. Am. Chem. Soc. 1986, 108, 4467. (15) Senanayake, S. D.; Chong, S. V.; Idriss, H. Catal. Today 2003, 85, 311. (16) Idriss, H.; Kim, K. S.; Barteau, M. A. Surf. Sci. 1992, 262, 113. (17) Stahl, M.; Pidun, U.; Frenking, G. Angew. Chem., Int. Ed. Engl. 1997, 36, 2234.
