(487k) Support Effect On the Low-Temperature Hydrogenation of Benzene Over PtCo Bimetallic Catalysts

Bimetallic catalysts are of great interest because they often display properties that differ from either of their parent metals. Previous studies on the Ni/Pt(111) bimetallic system have shown that the location of Ni atoms in the Pt(111) surface has a strong influence on the electronic and chemical properites of the surface [1]. The bimetallic surface consisting of a monolayer of Ni on top of bulk Pt(111), designated Ni?Pt?Pt(111), binds hydrogen and alkenes much more strongly than either parent metal, resulting in decreased hydrogenation activity. In contrast the surface consisting of a monolayer of Ni atoms in the subsurface region designated as Pt?Ni?Pt(111), has been shown to weaken metal-hydrogen bonds in comparison to Ni?Pt?Pt(111) or either parent metal surface. The resulting abundance of weakly bound hydrogen and alkenes on the Pt?Ni?Pt(111) surface increases its activity for novel low temperature hydrogenation pathways [2, 3]. In a previous attempt to extend these single crystal surface science studies to supported catalysts, PtCo bimetallic catalysts were found to be more active to hydrogenation reactions than PtNi bimetallic catalysts [4]. Therefore this study will focus on PtCo catalysts and the effect of different support materials on catalyst structure and activity.

Co and Pt monometallic and PtCo bimetallic catalysts were synthesized and supported on γ-Al₂O₃, SiO₂, TiO₂, and activated carbon (AC). Temperature programmed reduction (TPR) studies were performed to characterize the reduction behavior of the catalysts and to identify suitable reduction conditions. CO chemisorption was used to determine the number of active sites on each catalyst. Flow reactor studies of benzene hydrogenation the series of catalysts were used to compare the activity of PtCo on the different supports. Normalizing the hydrogenation activity with the number of active sites yielded the following trend in activity: SiO₂ > Al₂O₃ > TiO₂ ~ AC. Activation barriers were also calculated and followed a trend consistent with the normalized activity trend, that is: SiO₂2O₃2 ~ AC. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to characterize the extent of bimetallic formation. EXAFS analysis found that the SiO2 and Al2O3 supported catalysts showed a greater extent of bimetallic formation (larger Pt-Co coordination number) than either the TiO₂ or AC supported catalysts. Both the TiO₂ and AC supported catalysts possessed large Pt-Pt coordination numbers, suggesting the presence of larger particles. Transmission electron microscopy (TEM) was utilized to characterize the particle sizes. Particles on the SiO₂ supported catalyst appeared to be the smallest and most dispersed, while larger particles were observed in both the TiO₂ and AC supported catalysts.

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[2] Hwu, H., Eng, J., and Chen, J., J. Am. Chem. Soc. 124, 702 (2002).

[3] Chen, J., Menning, C., and Zellner, M., Surf. Sci. Rep. (2008).

[4] Lu, S., Lonergan, W.W., Bosco, J.P., Wang, S., Zhu, Y., Xie, Y., and Chen, J.G., J. Catal. 259, (260) (2008).