Alloy Catalysis Across Composition Space

Alloys of transition metals often possess superior catalytic properties than their pure components. Finding new alloy catalysts with predictable and desirable catalytic properties is the major challenge in multicomponent catalyst design due to the need to perform many catalyst preparations, characterizations and reactivity measurements across composition space. To accelerate this search Composition Spread Alloy Films (CSAFs), thin multicomponent films that have composition gradients parallel to their surfaces, AxByC1-x-y with x = 0 → 1 and y = 0 → 1-x, are prepared. Many otherwise intractable fundamental scientific problems in alloy science and catalysis can be effectively addressed through the use of CSAFs as high throughput materials libraries. High throughput characterization of composition and electronic structure of these CSAFs can be done using spatially resolved X-ray photoelectron spectroscopy (XPS). Coupling these techniques with a multichannel microreactor, we can sample product distributions from 100 different alloy catalysts across a CSAF in about 10 minutes. We focused on AgxPd1-x CSAFs and we investigated H2-D2 exchange reaction across these libraries using our multichannel microreactor. We investigated the energetics of this reaction using a microkinetic model and successfully came up with a correlation between the catalytic activity, the composition of the alloy and the electronic structure of the alloy. For the next step, the same H2-D2 exchange reaction was investigated experimentally across a CuxAuyPd1-x-y composition space. Increasing Pd content was found to decrease adsorption barriers and to increase desorption barriers. The v-band center moves toward the Fermi level as Pd content increases and the barrier to dissociative adsorption of H2 was found to decrease as the v-band energy increases. This data provides the first experimental correlation of elementary reaction barriers with valence band energy across a continuous span of alloy composition space. We also tested our methodology for a more complex reaction, ethylene hydrogenation. Catalytic activity - composition of the alloy - electronic structure of the alloy correlation has been obtained across a continuous composition space of CuxAuyPd1-x-y. This study is the first work to show this correlation experimentally. All in all, in this study we have come up with a new methodology that can be applied to alloy catalysis studies. Using this methodology 100 different alloy compositions can be tested in one experiment which accelerates the search for a proper catalyst for a given application as well as provides a fundamental understanding of multicomponent materials.