7841
2007, 111, 7841-7844 Published on Web 05/11/2007
Mechanistic Implications of Aun/Ti-Lattice Proximity for Propylene Epoxidation Ajay M. Joshi, W. Nicholas Delgass, and Kendall T. Thomson* School of Chemical Engineering, Purdue UniVersity, West Lafayette, Indiana 47907 ReceiVed: April 12, 2007
Previous work on propylene epoxidation using H2/O2 over Au/TS-1 catalysts indirectly supports a “sequential” mechanism: (1) H2O2 formation from H2/O2 on Au and (2) propylene epoxidation using H2O2 on Ti-defect sites. On the contrary, recent kinetic studies suggest a “simultaneous” mechanism which involves attack of adsorbed propylene on H-Au-OOH species. We have employed QM/MM calculations to examine whether the “sequential” mechanism is viable if Au/Ti sites are in proximity inside the TS-1 pores. On the bare Ti-defect site, the calculated ∆Eact for Ti-OOH formation (16.8 kcal/mol) and for subsequent propylene epoxidation (20.3 kcal/mol) suggest that epoxidation is viable. However, the Ti-defect site is also the most favorable binding site for small Au clusters. Interestingly, the ∆Eact for Ti-OOH formation on Au3/Ti-defect site is 32.1 kcal/mol, suggesting that the “sequential” mechanism is kinetically inhibited due to the proximity between Au clusters and Ti-defect sites. In the “simultaneous” mechanism, propylene is likely to be adsorbed on Au-Ti interface sites (∆Eads ∼ -20.0 kcal/mol) rather than on Ti sites (∆Eads ∼ -10.0 kcal/mol). The predicted adsorption energies are consistent with the reaction order of propylene (0.18 ( 0.04) in the powerlaw model. We propose that the “simultaneous” mechanism dominates if Ti-defect sites are covered by Au.
Introduction Although bulk Au is noble,1 oxide-supported nanoscale ( +8 kcal/mol and ∆G°rxn > +10 kcal/mol) suggesting that Ti-OOH (or Si-OOH) formation is unlikely. Formation of Si-OOH species on Sidefect sites is ruled out due to endothermicity and very high activation barrier (Table 1). However, the Ti-defect site is a viable candidate for Ti-OOH formation. First, H2O2 adsorbs on the Ti-defect site (∆Eads ) -14.9 kcal/mol) and then rearranges itself to form η-1 Ti-OOH species and water. Due to differences in the level of theory employed, our ∆Eact (16.8 kcal/mol) is somewhat higher than the previously reported value (13.6 kcal/mol).42 Propylene adsorption (∆Eads ) -8.8 kcal/ mol) around the η-1 Ti-OOH species is followed by the propylene epoxidation step (∆Eact ) 20.3 kcal/mol). The transition state now exhibits η-2 character and facilitates the
Figure 1. Adsorbed configuration of H2O2 on Au3/T6-Ti-defect site inside the TS-1 pores. All of the MM atoms and some atoms in the QM region were removed for clarity. The atomic distances in Å are indicated.
formation of Ti-OH species and PO. The results discussed till now suggest that bare (no Au) Ti-defect sites inside the TS-1 pores form Ti-OOH species which can epoxidize propylene. Now we address the most important unanswered question: Is the aforementioned mechanism viable if Au clusters are adsorbed on Ti sites? In fact, the Ti-defect is likely to be the most favorable site for adsorption of Au clusters inside the TS-1 pores,41 and the experimentally observed correlation between Ti and Au loadings on Au/TS-1 catalysts16 indirectly supports this notion of proximity of Au and Ti sites. Previous gas-phase DFT calculations showed that Au3 is the smallest neutral cluster which can form H2O2.22,24 Therefore, we selected Au3 adsorbed on a Ti-defect site (∆Eads ) -28.2 kcal/mol, ∆G°ads ) -15.0 kcal/mol) to model the proximal Au-Ti sites in TS-1 pores. One of the Au atoms of Au3 is coordinated with O of the TiOH group (Au-O distance ) 2.17 Å). We find stronger interaction of H2O2 with the Au3/Ti-defect site (∆Eads ) -20.3 kcal/mol) than that with bare Ti-defect site. On the bare Ti-defect site, one H in adsorbed H2O2 forms a hydrogen bond with the Ti-OH group (which later gives H2O). However, on the Au3/Ti-defect site, the access to the Ti-OH group is sterically hindered due to Au3 which strongly interacts with H in adsorbed H2O2 (Figure 1). In the transition state (Figure 2) for the formation of η-1 Ti-OOH species on the Au3/Ti-defect site, this H (of H2O2) moves away from Au3 and forms a partial bond with the O of the Ti-OH group which is coordinated to Au. The OH group later separates as H2O. Interestingly, the ∆Eact for this Ti-OOH formation step on the
Letters
J. Phys. Chem. C, Vol. 111, No. 22, 2007 7843 discussed earlier, propylene adsorption is much stronger at the Au-Ti interface than on the bare Ti sites, and the “simultaneous” mechanism is likely to dominate. Nevertheless, the possibility of the “sequential” mechanism also playing an important role on other Au/TS-1 and Au/Ti catalysts differing in preparation method, pretreatment, metal loadings, and Au particle size, etc., cannot be ruled out. Conclusions
Figure 2. Transition state geometry for attack of H2O2 on Au3/T6Ti-defect site to form Ti-OOH species inside the TS-1 pores. All of the MM atoms and some atoms in the QM region were removed for clarity. The atomic distances in Å are indicated.
Au3/Ti-defect site is 32.1 kcal/mol, almost 15 kcal/mol higher than the corresponding step on bare Ti-defect site and ∼12 kcal/ mol higher than that of the RDS (Table 1). This dramatic increase is outside the domain of errors (3-4 kcal/mol) in our calculations. About one-third of the 15 kcal/mol increase in the intrinsic ∆Eact can be attributed to stronger adsorption of H2O2 on the Au3/Ti-defect site than that on the bare Ti-defect site. The rest is due to steric hindrance, i.e., interaction of Au3 with the O of the Ti-OH group and with adjacent H in adsorbed H2O2. Perhaps even higher activation energies for Ti-OOH formation are likely on Au4-5/Ti-defect sites where steric effects will be more severe. Since formation of the Ti-OOH species is a prerequisite of the “sequential” propylene epoxidation mechanism, we have shown here for the first time that the “sequential” mechanism is kinetically inhibited due to the proximity between Au clusters and Ti-defect sites in Au/TS-1 catalysts. On the contrary, the “simultaneous” mechanism explaining the kinetic data involves attack of propylene (adsorbed on Ti or at the Au-Ti interface) on the H-Au-OOH species and does not require Ti-OOH species or H2O2 species.33 In fact, Ti plays an indirect role, and the “simultaneous” mechanism can, in principle, work on both Au/Ti-nondefect and Au/Ti-defect sites. Our calculations also suggest that propylene adsorption in TS-1 pores without any Au (∆Eads ∼ -10.0 kcal/mol) is significantly weaker than propylene adsorption on Au3/Ti sites (Au-Ti interface: ∆Eads ∼ -20.0 kcal/mol), consistent with the experimentally observed stronger propylene adsorption at the Au-Ti interface on Au/ TiO2.43,44 Moreover, such a strong adsorption of propylene at the Au-Ti interface in TS-1 is consistent with the reaction order of propylene (0.18 ( 0.04) in the power-law rate model.33 Therefore, we provide another significant insight that the “site 2” in the “simultaneous” mechanism is likely to be the Au-Ti interface site rather than the bare Ti site. Although Au clusters are likely to adsorb near Ti sites, some bare Ti-defect sites may still be available in Au/TS-1 catalysts. The H2O2 formed on Au must diffuse along the catalyst surface to reach these distal Ti-defect sites. A majority of such H2O2 and OOH species formed in situ are likely to decompose to form water,23 if they are not consumed immediately to form PO, consistent with the known poor H2 efficiency (
