TY - JOUR
T1 - Chemisorption of oxygen, chlorine, hydrogen, hydroxide, and ethylene on silver clusters
T2 - A model for the olefin epoxidation reaction
AU - Carter, Emily A.
AU - Goddard, William A.
N1 - Funding Information:
This work was initiated and supported by the Shell Development Corporation (198221985) and the Shell Companies Foundation (1985-1987). We would particularly like to thank Drs. Charles Adams, Rutger van Santen, and John Cole for initiating and continuing their industrial support for our research. Further support was provided by the National Science Foundation [Grant No. DMR82-15650 (1983-1986) and Grant No. CHE83-18041 (1986-1987)]. EAC acknowledges a National Science Foundation Predoctoral Fellowship (1982-1985) a research grant award from the International Precious Metals Institute and Gemini Industries (1985-1986) and a SOHIO fellowship in Catalysis (1986-1987). We would also like to thank Professors C.T. Campbell and B.E. Koel for lively and helpful discussions.
PY - 1989/2/2
Y1 - 1989/2/2
N2 - We have examined various postulated pathways for the catalytic epoxidation of olefins by silver using ab initio quantum mechanical methods to study likely intermediates in this reaction. In particular, we predict preferred binding sites, geometries, vibrational frequencies, and binding energies for O, O2, Cl, H, OH, and C2H4 on a cluster model for Ag aggregates present on supported catalysts. These calculations suggest the presence of two nearly degenerate states of chemisorbed atomic oxygen. The calculated binding energy for these surface oxides are 78-79 kcal/mol (experimental values are 77-78 kcal/mol). One surface oxide has the form of an oxyradical anion and is predicted to be selective for olefin epoxidation. The other surface oxide is a closed shell state expected to be less active and nonselective for olefin epoxidation. These results lead to a detailed mechanistic model that explains: (i) why C2H2 exhibits high selectivity for epoxide while higher olefins do not; (ii) the difference in activity per surface site between the (110) and (111) surfaces of Ag; and (iii) the role of both electropositive (e.g. Cs) and electronegative (e.g. Cl) promoters in increasing selectivity. A number of experiments are proposed which would test key points of this new mechanism.
AB - We have examined various postulated pathways for the catalytic epoxidation of olefins by silver using ab initio quantum mechanical methods to study likely intermediates in this reaction. In particular, we predict preferred binding sites, geometries, vibrational frequencies, and binding energies for O, O2, Cl, H, OH, and C2H4 on a cluster model for Ag aggregates present on supported catalysts. These calculations suggest the presence of two nearly degenerate states of chemisorbed atomic oxygen. The calculated binding energy for these surface oxides are 78-79 kcal/mol (experimental values are 77-78 kcal/mol). One surface oxide has the form of an oxyradical anion and is predicted to be selective for olefin epoxidation. The other surface oxide is a closed shell state expected to be less active and nonselective for olefin epoxidation. These results lead to a detailed mechanistic model that explains: (i) why C2H2 exhibits high selectivity for epoxide while higher olefins do not; (ii) the difference in activity per surface site between the (110) and (111) surfaces of Ag; and (iii) the role of both electropositive (e.g. Cs) and electronegative (e.g. Cl) promoters in increasing selectivity. A number of experiments are proposed which would test key points of this new mechanism.
UR - http://www.scopus.com/inward/record.url?scp=0001372202&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=0001372202&partnerID=8YFLogxK
U2 - 10.1016/0039-6028(89)90071-X
DO - 10.1016/0039-6028(89)90071-X
M3 - Article
AN - SCOPUS:0001372202
SN - 0039-6028
VL - 209
SP - 243
EP - 289
JO - Surface Science
JF - Surface Science
IS - 1-2
ER -