(157c) Geometric and Electronic Effects in Pd and Pt Alloy Alkane Dehydrogenation Catalysts

The recent shale gas boom and corresponding decrease in light alkane prices have made on-purpose olefin production via catalytic alkane dehydrogenation economically competitive. High dehydrogenation rates while decreasing the C-C bond hydrogenolysis is critical for selectivity control to olefins. Recently, it has been found that olefin selectivity can be controlled by catalysts with specific intermetallic alloy (or intermetallic compound, IMC) structures.^1,2

Silica supported Pd and Pd-In catalysts with different In:Pd atomic ratios and similar particle size (~2 nm) were tested for ethane dehydrogenation at 600 °C. For a monometallic Pd catalyst, the dehydrogenation selectivity was 53 %. Addition of In to Pd increased the dehydrogenation selectivity to near 100 % and also greatly increased the TOR. Combined carbon monoxide IR, in situ synchrotron XRD and XAS analysis were able to resolve the detailed nanoparticle geometric structure and showed that for Pd-In catalysts with increasing In loading, an increasing fraction of the Pd nanoparticle surface was transformed into PdIn IMC with a cubic CsCl structure.

A series of Pt-In catalysts with various In:Pt ratios were also prepared and compared to a monometallic Pt catalyst for ethane dehydrogenation. As with Pd, the addition of In to Pt catalysts promoted ethane dehydrogenation selectivity. However, where Pd and In formed a single intermetallic compound over a wide range of bulk In:Pd ratios, changes to the bulk In:Pt ratio led to the formation of multiple intermetallic phases. In-situ x-ray characterizations revealed that the Pt₃In phase formed when the bulk catalyst composition had a low In:Pt ratio whereas the Pt₂In₃ phase was formed at a high In:Pt ratio. Both phases exhibited superior catalytic performance compared to the monometallic Pt catalyst.

Formation of these PdIn and PtIn IMC structures on the catalyst surface geometrically isolates the Pd and Pt catalytic sites by non-catalytic, metallic In neighbors, which is suggested to be responsible for the high olefin selectivity. The changes in initial TOR and E_app seen in these IMC catalysts are also indicative of electronic changes to the catalytic atoms by the inactive In neighbors. It appears that a combination of geometric changes and electronic changes to Pt due to the formation of the IMC phases are responsible for the promotional effects of In. A fundamental understanding of these geometric and electronic changes can lead to improved catalyst design and new approaches to catalyst optimization through the formation of intermetallic compounds.

 References:

1) Childers D. J., Schweitzer N. M., Shahari S. M. K., Rioux R. M., Miller J. T. and Meyer R. J., Journal of Catalysis, 2014, 318, 75-84.

2) Gallagher J. R., Childers D. J., Zhao H., Winans R. E., Meyer R. J. and Miller J. T., Physical Chemistry Chemical Physics, 2015, 17, 28144-28153