Structure Sensitivity in Selective Oxidation of Propene over Cu2O

Chapter 10

Structure Sensitivity in Selective Oxidation of Propene over Cu O Surfaces 2

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 24, 2015 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch010

Kirk H. Schulz and David F. Cox Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

Studies of the interaction of propene (CH =CHCH ) with Cu O(111) and (100) single crystal surfaces have demonstrated that each step in the allylic oxidation to acrolein ( C H = C H C H O ) exhibits structure-sensitivity. Oxygen vacancies (i.e., surface defects) on the nonpolar, Cu O(111) surface have been found to be energetically-favorable sites for the dissociation of propene to allyl when compared to (100) and non-defective (111) surfaces. The oxygen insertion reaction requires the presence of coordinately-unsaturated surface lattice oxygen. The final reaction step is hydrogen elimination from the carbon α to oxygen in the σ-bonded allylic intermediate (identified as a surface allyloxy, C H = C H C H O - ) . The activation energy for this final reaction varies by over 7 kcal/mol depending on which Cu O surface is investigated. 2

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The partial oxidation of propene (CH = CHCH ) to acrolein ( C H = CHCHO) is a useful model for the class of allylic oxidation reactions of olefins. Several mixed oxides catalyze this reaction, but Cu O is the only reported single-component oxide catalyst to exhibit significant activity and selectivity (1-3). Because of its single-component nature, single crystal Cu O was chosen for investigating the structure sensitivity and site requirements for the propene selective oxidation reaction. The basic steps in the reaction pathway of propene oxidation to acrolein have been widely studied. In the rate determining step over Cu O and bismuth-molybdate catalysts, a methyl hydrogen is abstracted from propene to produce a symmetric, π-allyl intermediate (4,5). The order in which the second and third steps occur is not well understood, but involves a second hydrogen abstraction and lattice oxygen insertion into the symmetric π-allyl to form an oxygen-containing σ-allyl species. Grasselli and 2

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Current address: Department of Chemical Engineering, University of North Dakota, Box 8101, University Station, Grand Forks, ND 58202

0097-6156/93/0523-0122$06.00/0 © 1993 American Chemical Society In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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SCHULZ & COX

Structure Sensitivity in Selective Oxidation of Propene

coworkers (3) advocate the formation of a σ-allyl on bismuth-molybdate catalysts where oxygen insertion occurs prior to the final hydrogen abstraction (i.e., an allyloxy intermediate, C H 2 = C H C H 9 O - ) . However, several groups advocate the formation of a σ-allyl species on bismuth-molybdate (1) and Q12O (6) catalysts where lattice oxygen insertion occurs after the final hydrogen abstraction.


Surfaces Studied The use of Q12O single crystal surfaces as model catalysts allows for the testing of site requirements for propene adsorption and oxidation. The two low-index surfaces investigated differ both in the availability of surface lattice oxygen and the Cu+ coordination numbers. The ideal, stoichiometric, nonpolar, C u 2 0 ( l l l ) surface exposes singly- and doubly-coordinate Cu+ cations (bulk Cu+ coordination = 2) in the second atomic layer in the ratio of 1 to 4, respectively. However, the top atomic layer is composed exclusively of threefold-coordinate oxygen anions (bulk lattice coordination = 4). The ideal, stoichiometric C u 2 0 ( l l l ) surface is illustrated in Figure la. An oxygen-deficient C u 2 0 ( l l l ) surface may also be prepared by exposure to reducing gases. Such treatments lead to a (J 3xJ 3)R30° L E E D periodicity due to an ordered one-third of a layer of oxygen vacancies (7). Each oxygen vacancy gives rise to a threefold site of singly-coordinate Cu+ cations. The Qi20(lll)-(J 3xJ 3)R30° surface is illustrated in Figure lb. The Cu9O(100) surface used in this study is a polar, Cu -terminated, reconstructed surface which displayed a (3J2xJ2)R45° L E E D periodicity with many missing spots (7). Although there is no definitive model of the structure of the reconstructed surface, the periodicity of the reconstruction suggests a relaxation of top atomic layer Cu cations, possibly associated with a weak Cu - C u bonding interaction (7). In contrast to the (111) surface, the ideal, Cu -terminated (100) surface exposes no lattice oxygen in the top atomic layer. A l l top layer Cu cations are singly coordinated. The ideal (i.e., unreconstructed) Cu + terminated surface is illustrated in Figure 2a. An oxygen terminated surface can also be prepared by 10^ L (1 L - 10~° Torr - sec) exposures to Oo at room temperature. This preparation lifts the reconstruction on the (TOO) surface to form a (lxl), oxygen-terminated surface which contains doubly-coordinate oxygen in the outer atomic layer. This O-terminated (100) surface is shown in Figure 2b. A more complete description of the characterization of the Cu2O(100) and (111) surfaces has been reported previously (7). +

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+

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Propene Adsorption Adsorption at Low Pressure (P < 10"*> Torr). The adsorption of propene has been studied with thermal desorption spectroscopy (TDS) on all of the different forms of the (100) and (111) surfaces and under several different conditions of exposure. For exposures at low pressure (P < 10'" Torr), no selective oxidation is observed. For small exposures (< 5 L) at low-temperature (100K-120K), four propene desorption states are observed from the 0ι?0(111) surface compared to two desorption states from the Cu9O(100)-Cu surface. These TDS results are shown in Figure 3, and give a clear indication of a structure-sensitive interaction of propene with Q12O.

In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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CATALYTIC SELECTIVE OXIDATION

Figure 1. Ball model illustrations of (a) the ideal, stoichiometric Cu20(lll) surface and (b) the oxygen-deficient Oi2p(lll)-(J3xJ3)R30° surface. The small filled circles represent cations, and the larger open circles represent O^" anions. For clarity, only the top four atomic layers are shown.



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SCHULZ & COX


c ci i

II

n

n

i i

II

u

Figure 2. Ball model illustrations of (a) the ideal (i.e., unreconstructed) Cu + -terminated (100) surface and (b) the O-terminated (100) surface.

CO

c

Çj) CD /

01^0(111)

c ο 0)

Û ω c

Q) α ο

100

CL^O(100)-CU

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Temperature (Κ)

Figure 3. Propene thermal desorption traces from the (111) and Cu + -terminated (100) surface following 0.3 L doses in U H V at 110 K.



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For the low adsorption temperatures, propene dissociation is observed only on the C u 2 0 ( l l l ) surface. Dissociation and non-selective oxidation to C O occurs at low temperatures, and gives rise to desorption-limited CO as a product. Dissociation to a surface allyl (ΟτΗζ) has been confirmed also by adsorption of propene on a deuterium-predosed surface to yield singly-labeled propene (C3H5D). A comparison of the desorption behavior between Cii20(l 1 l)-(lxl) and Cu2O(lll)-(J3xJ3)R30° surfaces demonstrates that the dissociation of propene to allyl and/or the recombination of allyl with surface hydrogen occurs at oxygen vacancies (i.e., defects) on the (111) surface associated with three-fold cation sites. The highest temperature desorption feature at 325 Κ shown for the (111) surface in Figure 3 is characteristic of this process. Propene adsorption at room temperature for large exposures (3xl(P L) at low pressure shows that dissociation to allyl occurs on all surfaces investigated at higher temperature, but the probability for dissociative adsorption at room temperature is estimated to be less than 10"^. Even though dissociation to allyl is observed, no selective oxidation products are formed during thermal desorption. The low pressure results allow for bounds to be put on the energetics of propene dissociation to allyl assuming that the dissociative adsorption is exothermic. The projected limits are given in Table I, and indicate that the activation energy for propene dissociation to allyl is less than 22 kcal/mol on the surfaces studied (8).

Table I. Energetics for Propene Dissociation to Allyl on Q12O Cu2°( °) 10

Cu 0(lll) 2

16.9 kcal/mol < E E

a

a

< 21.1 kcal/mol

< 21.8 kcal/mol

The lack of selective oxidation products even when propene is dissociated to allyl suggests that the "activation" of propene for selective oxidation involves not just dissociation, but also the separate process involving oxygen insertion. Using the different single crystal surfaces, different preparations (7,8) and different adsorption temperatures allow one to test the interaction of propene with different forms of surface oxygen (i.e., molecular, doubly-coordinate and triply coordinate). Since no selective oxidation products are observed, lower limits on the energies for "activation" by surface oxygen can be estimated from the low pressure thermal desorption results. In all cases, no selective oxi dation is observed, and the maximum activation energies which can be accessed experimentally are limited by the desorption of reactants. The results are summarized in Table II. The results suggest that the activation energy for oxygen insertion into an allyl is greater than that for dissociation to allyl. All the results discussed above for low-pressure propene adsorption have been reported in detail elsewhere (8).


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SCHULZ & COX

Table II.

Energetics of Propene Activation for Oxygen Insertion ( -6 ) P < 1 0

T o r r

form of oxygen

Surface Cu O(100)-Cu 2

reaction

molecular o

no reaction

2

lower limit QnE

propene desorbs

E >15 kcal/mol

a

a

Cu O(100)-O

atomic, no reaction doubly coordinate

E >21 allylhydrogen kcal/mol recombination

Cu 0(lll)

atomic, CO formation, triply no selective coordinate oxidation

allylE >24 hydrogen kcal/mol recombination

2


limiting process

2

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a

Adsorption at Atmospheric Pressure. The lack of selective oxidation products following adsorption at low pressure indicates the existence of a 'pressure gap" for the selective oxidation process. To overcome the low pressure limitations, propene adsorption at 1 atm. was investigated (9). Different single crystal C u 0 surfaces were exposed to flowing propene at room temperature and 1 atm. for short periods (about 2 seconds), then returned to ultrahigh vacuum for thermal desorption studies. These conditions of exposure are reducing because hydrogen is released to the surface as a result of propene dissociation. Propane is the primary C3 product evolved. Non-selective oxidation products (CO, CO? and H 0 ) are also observed. However, following adsorption at atmospheric pressure, selective oxidation products including acrolein and allyl alcohol are observed also in TDS. Figure 4 shows the TDS traces for propene and the selective oxidation products from the O-terminated Cu?O(100) surface following propene adsorption at atmospheric pressure. The selectivity to acrolein is near 2% for the O-terminated (100) surface and the (111) surface, but less than 0.1% (and in many cases undetectable) for the Cu + -terminated (100) surface. The variation in acrolein yields from the different surfaces indicate that the primary site requirement for oxygen insertion and selective oxidation is the presence of coordinately-unsaturated surface oxygen anions like those present on the (111) and O-terminated (100) surfaces. The coordination number of these oxygen anions appears to make little difference in the selective oxidation process, with two-coordinate and three-coordinate anions both capable of producing acrolein. The lack of coordinately-unsaturated anions at the Cu +-terminated (100) surface accounts for the minimal selective oxidation products formed. 2

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Identification of the σ-Allyl Intermediate X-ray photoelectron spectroscopy (XPS) of the surface following propene adsorption at 1 atm. indicates that the oxygen insertion reaction occurs at room temperature (9). A high binding energy contribution to the C Is XPS signal is observed at 286.5 eV on the (111) and O-terminated (100) surfaces, but not on the Cu + -terminated (100). The binding energy is similar to that observed for alkoxides on C u 0 surfaces (10), and is attributed to the oxygenated carbon of the oxygen-containing σ-allyl postulated to be the 2


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final surface intermediate in the selective oxidation to acrolein (1,3,6). The similarity of the σ-allyl binding energy in XPS to that of surface alkoxides suggests that the final surface intermediate is an allyloxy ( C H = CHCH90-). To verity this pathway, the product distribution and kinetic arameters observed in TDS for acrolein formation from the σ-allyl formed y 1-atm. propene exposures were compared to those for the reaction of acrolein and allyl alcohol under ultrahigh vacuum conditions. The dissociative adsorption of allyl alcohol ( C H = C H C H O H ) to allyloxy ( C H = C H C H 0 - ) allows one to investigate the anticipated final intermediate if oxygen insertion precedes the final hydrogen abstraction. Adsorbed acrolein has the same composition as the anticipated σ-allyl formed if the final hydrogen abstraction precedes oxygen insertion. The thermal desorption data tor adsorbed acrolein and for acrolein produced from allyl alcohol and propene oxidation on the Cu O(100) surface is shown in Figure 5. For this data, the acrolein signal from adsorbed acrolein and from allyl alcohol are from U H V experiments, while the signal due to propene oxidation is the result of a 1 atm. exposure. For the two oxygenate exposures in UHV, the Cu terminated surface is used since the adsorbate itself provides the oxygen required for the associated oxygen-containing surface species. For propene oxidation, the O-terminated (100) surface is used since oxygen must be supplied by the lattice. Note that the acrolein product from propene and allyl alcohol is clearly reaction-limited by comparison to the desorption trace for adsorbed acrolein. Using the Redhead equation (11) assuming a normal preexponential of 10^ sec"\ the first-order activation energy for acrolein production in TDS from the propene-derived σ-allyl (470 K) and from the allyl alcohol-derived allyloxy (525 K) using a 2 K/sec temperature ramp is essentially the same: 31.5 ± 1.8 kcal/mol. In comparing the product distributions, it is noted that allyl alcohol is observed as a reaction product from propene TDS, as shown in Figure 4. The surface chemistry of acrolein has been checked to ensure that allyloxy (and therefore allyl alcohol and reaction-limited acrolein) is not due to simply to the subsequent hydrogénation of acrolein formed by an alternate pathway in the propene TDS run. The reactivity of acrolein and hydrogen with C u 0 does not produce allyl alcohol (12). Hence, the propene oxidation pathway clearly proceeds through an allyloxy surface intermediate. The final step in the reaction sequence is a ummolecular hydride elimination from allyloxy ( C H = CH-CH2O-) at the carbon a to the oxygen heteroatom. Hence, oxygen insertion precedes the final hydrogen abstraction in the allylic oxidation of propene over Cu?0. Details of the results for atmospheric pressure exposures are given elsewhere (9). 2

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Allyloxy Decomposition The similarity between the measured activation energies for the reaction-limited production of acrolein over Cu O(100) from allyl alcohol in U H V or propene following a 1 atm. exposure gives a clear indication that these reactions involve the same surface intermediate, an allyloxy. This similarity also suggests that the surface intermediates formed by these two routes behave in a chemically similar fashion. For the (100) surface, the Cu -alkoxide surface complex is similar regardless of whether oxygen from 2

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I

ι

300

400

ι

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ι

ι

500 600 700 Temperature (K)

800

Figure 4. Desorption of C3 compounds from the O-terminated Qi2O(100) surface following propene exposure at 1 atmosphere.

100

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400 500 600 Temperature

700 800

Figure 5. Comparison of acrolein desorption traces from the Cu2O(100) surface following (a) exposure to propene at 1 atm., (b) allyl alcohol adsorption in U H V and (c) acrolein adsorption in UHV.


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the O-terminated (100) surface is incorporated into propene or allyl alcohol dissociates to the alkoxide on the Cu + -terminated (100) surface. On the (111) surface, however, the activation energy for allyloxy decomposition is significantly lower than that for the (100) surface. This structure-sensitive variation in the activation energy for allyoxy decomposition is illustrated in Figure 6 which shows the reaction-limited acrolein product traces for U H V exposures of allyl alcohol over the stoichiometric (111) and Cu + -terminated (100) surfaces. The highest temperature features from each trace (400 Κ on the (111) surface and 525 Κ on the (100) surface) are known to be due to the decomposition of allyloxy to acrolein (10). For both surfaces, these high temperature desorption features are observed in conjunction with allyl alcohol desorption via recombination of allyloxy and surface hydrogen, behavior characteristic of an alkoxide decomposition (10). On the (111) surface, the activation energy is more than 7 kcal/mol lower than observed on the (100) surface. A similar difference in the activation energy for methoxy decomposition in the oxidation of methanol has also been observed over these two surfaces (13). Hence, as with the structure sensitivity displayed in both the adsorption of propene and the oxygen insertion step, the final elementary step in the reaction pathway to acrolein also displays structure sensitivity. Catalytic Consequences of the Structure Sensitivity The dissociation of propene to allyl at low temperatures on defect sites on the (111) surface suggests that point defects such as oxygen vacancies may provide lower energy sites for the initial dissociation reaction. However, since both surfaces investigated are capable of dissociating propene to allyl at room temperature and 1 atmosphere (relatively mild conditions), it is unlikely that such energetically favorable point defect sites would significantly effect the overall rate under catalytic reaction conditions. However, the effect of such sites on the selectivity of the reaction could be considerable. Note that even for adsorption at low temperatures on the stoichiometric C u 2 0 ( l l l ) - ( l x l ) surface, propene dissociation results in non-selective oxidation to CO. Surface allyl is verified only in the presence of point defects. While these low pressure and low temperature results for the (111) surface clearly cannot be assumed to be characteristic of real powder catalysts under reaction conditions, there are interesting parallels to another study in the literature. From studies of propene oxidation on polycrystalline Cu?0, Wood et al. found that a copper rich (i.e., oxygen deficient) surface favors selective oxidation to acrolein, while an oxygen rich surface favors nonselective oxidation (14). This compositional observation suggests that the potential importance of point defects like oxygen vacancies in controlling the selectivity of Ο ι 0 catalysts cannot not be overlooked. The site requirements for oxygen insertion, coordinately-unsaturated surface lattice oxygen, would not be expected to significantly limit the activity of C u 0 catalysts under reaction conditions. Under conditions for selective oxidation, the feed of oxidant with propene would be expected to sustain a steady state concentration of such species even on polar surfaces like the (100). Hence while it is possible to sustain C u * terminated surfaces like Cu2O(100)-Cu in our TDS experiments, the possibility of sustaining such surfaces under reaction conditions is small. Also, since exchange of oxygen between the bulk and the surface occurs at elevated temperatures (9,10,12), the availability of surface oxygen for selective 2

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300

400 500 600 Temperature

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Figure 6. Comparison of acrolein desorption traces from allyl alcohol decomposition over (a) the Cu2O(100)-Cu surface and (b) the stoichiometric (111) surface.




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oxidation is not expected to be compromised under reaction conditions. Since the coordination number of the surface oxygen does not determine its ability to produce acrolein, the different reactivity observed in TDS for the Cu+ and Ο terminated (100) surfaces is probably best thought of as compositional sensitivity rather than structure sensitivity, to the extent that they can be distinguished. As with the point defects discussed above, however, different selectivities associated with Cu rich or oxygen rich surfaces may effect the catalyst performance. The differences observed in the activation energy for allyoxy decomposition to acrolein over different crystallographic surfaces is also expected to have a negligible effect on the activity of the catalyst. It has been well established that under catalytic reaction conditions the rate-limiting step in propene oxidation over Q12O involves the abstraction of a methyl hydrogen in the initial dissociation step (4,5). Since the decomposition of allyloxy is not rate-limiting, the 7-8 kcal/mol difference in the activation energies for this reaction over different crystallographic planes should have no significant effect on the overall kinetics under catalytic reaction conditions. As for possible effects on the selectivity, differences in the selectivity for the allyloxy decomposition reaction over different surfaces has not yet been fully investigated. Acknowledgments We gratefully acknowledge the National Science Foundation for support of this work through CBT-870876. We also thank Professor L. Tapiero for providing the single crystal used in this study. Literature Cited 1. Keulks, G.W.; Krenzke, L.D. and Noterman, T.M. Advan. Catal. 1978, 27, 183. 2. Hucknall, D.J., Selective Oxidation of Hydrocarbons; Academic Press: New York, 1974. 3. Grasselli, R.K. and Burrington, J.D., Advan. Catal. 1981, 30, 133. 4. Adams, C.R. and Jennings, T.J., J. Catal. 1964,3,549. 5. Voge, H.H.;Wagner,C.D. and Stevenson, D.P., J. Catal. 1963, 2, 58. 6. Adams, C.R. and Jennings, T.J., J. Catal. 1963, 2, 63. 7. Schulz, K.H. and Cox, D.F., Phys. Rev. B. 1991, 43, 3061. 8. Schulz, K.H. and Cox, D.F., Surf. Sci. 1992, 262, 318. 9. Schulz, K.H. and Cox, D.F., J. Catal., Submitted. 10. Schulz, K.H. and Cox, D.F., J. Phys. Chem., Submitted. 11. Redhead, P.A., Vacuum 1962, 12, 203. 12. Schulz, K.H. and Cox, D.F., J. Phys. Chem., Submitted. 13. Cox, D.F. and Schulz, K.H., J. Vac. Sci. Technol. A 1990, 8, 2599. 14. Wood, B.J.; Wise H. and Yolles, R.S., J. Catal. 1969,15,355. RECEIVED October 30, 1992


Structure Sensitivity in Selective Oxidation of Propene over Cu2O

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