(581a) Atomic-Scale Insights Into the Reactivity of Catalytically Important Pd/Cu Alloys

Matthew B. Boucher¹, Timothy J. Lawton², April D. Jewell²,  George Kyriakou², Maria Flytzani-Stephanopoulos¹, E. Charles H. Sykes²

1 – Department of Chemical and Biological Engineering, Tufts University, 4 Colby Street, Medford, MA 02155, USA

2 – Department of Chemistry, Tufts University, 62 Talbot Avenue, Medford, MA 02155, USA

Introduction

Understanding the atomic-scale surface structure of alloys is a crucial step on the path towards designing optimal heterogeneous catalysts. Here we have investigated the formation and the catalytic performance of Pd/Cu(111) alloys towards the decomposition of methanol. A range of alloy compositions has been tested, extending from single atom alloys (SAAs) to high coverage disordered alloy systems. The catalytic dehydrogenation of methanol was used as a probe reaction due to its great importance to the chemical industry and for energy transport. It comprises an elementary step in the steam reforming process and offers a potential source of hydrogen for fuel cells. It is also a promising method for producing water-free formaldehyde.

Results

By means of high-resolution scanning tunneling microscopy (STM) we find that depending on the deposition temperature, alloying of Pd on Cu(111) can take place at the topmost layer (for relatively low sample temperatures ~ 350 K) or subsurface at higher sample temperature (~ 500 K). The effect is attributed to the hetero Pd-Cu bonds being stronger than homo Cu-Cu and Pd-Pd bonds and also to the steric repulsion between the larger Pd atoms. The alloy phase appears in the STM images as brims running across the step edges of the Cu(111) sample Figure 1(a) shows an atomic resolution STM image acquired after deposition of 0.01ML of Pd on the Cu(111) surface at 380 K. The bright protrusions indicate Pd atoms present as isolated monoatomic entities on the surface.^1-3 In addition to brims increasing the surface coverage of Pd leads to the nucleation of islands. 

Conclusion

We have demonstrated that low- surface concentration Pd/Cu(111) alloys can act as efficient catalysts in the dehydrogenation of methanol. We suggest that Pd/Cu alloy dispersed catalyst can function in a similar way. Such measurements are currently in progress in our laboratories.

Acknowledgments

The financial support by the National Science Foundation CBET grant no. 0828666 is gratefully acknowledged. MF-S also acknowledges support from the U.S. Department of Energy, Basic Energy Sciences, under grant no. FG02-05ER15730.

References

1. D D. O. Bellisario, J. W. Han, H. L. Tierney, A. E. Baber, D. S. Sholl, E. C. H. Sykes, importance of kinetics in surface alloying: A comparison of the diffusion pathways of Pd and Ag atoms on Cu(111), J. Phys. Chem. C 113, 12863-12869 (2009)

2. H. L. Tierney, A. E. Baber and E. C. H. Sykes, Atomic-scale imaging and electronic structure determination of catalytic sites on Pd/Cu near surface alloys, J. Phys. Chem. C 113, 7246-7250 (2009)

3. H. L. Tierney, A. E. Baber, J. R. Kitchin, E. C. H. Sykes, Hydrogen dissociation and spillover on individual isolated palladium atoms, Phys. Rev. Lett. 103, 2461021-2461024 (2009)

4. S. M. Johnston, A. Mulligan, V. Dhanak, M. Kadodwala, The structure of methanol and methoxy on Cu(111).  Surface Science 530, 111-119 (2003).