(308g) High Throughput Study of Catalysis on Pd-Based Alloys

Alloys of transition metals often makes better catalysts than their pure components. It is challenging to understand the reasons behind this fact and to design new multicomponent catalysts with desirable and predictable properties due to the need to perform many catalyst preparation, characterization and reactivity measurements across composition space. This search can be accelerated using Composition Spread Alloy Films (CSAFs) which are thin multicomponent films that have composition gradients parallel to their surfaces, A_xB_yC_1-x-y with x = 0 → 1 and y = 0 → 1-x. Study of alloy catalysis across composition space using a CSAF requires a multichannel reactor system that can be used to run steady state catalytic reactions at many different positions or alloy compositions on the CSAF. For this purpose we have developed a 100 channel microreactor array that can sample product distributions from 100 different alloy catalysts of about 10 minutes. Cu_xAu_yPd_1-x-y CSAFs spanning all of binary and ternary composition space have been prepared using a rotating shadow mask CSAF deposition tool which is designed and developed in our group. CSAF surface composition and valence electron energy distributions are measured using X-ray Photoemission Spectroscopy (XPS).

High-throughput techniques have been used to characterize the activity of ethylene hydrogenation reaction over ternary alloy catalysts. Ethylene hydrogenation reaction has been chosen for this work since it is a mechanistically simple reaction and it has been investigated on binary alloy catalysts before. It is a simple reaction producing only ethane, but it has also been referred as deceptively simple reaction, which in fact possesses an extremely rich and complicated chemistry. Horiuti-Polanyi mechanism is widely accepted for this reaction. In this mechanism, ethylene adsorbs onto catalyst surface molecularly and H₂ adsorbs dissociatively. Adsorbed H atoms combine with adsorbed ethylene to form ethyl groups on the surface. As the last step, another H atom is added to the ethyl group, ethane is formed and immediately desorbed from the surface.

In order to study this reaction, as a first step, H₂ dissociation properties were investigated over ternary CSAFs via characterizing H₂-D₂ exchange activity versus composition at various temperatures, partial pressures and flow rates. The relationship between alloy catalyst activity and electronic structure has been investigated experimentally across a broad, continuous span of Cu_xAu_yPd_1-x-y compositionspace. The CSAF was used as a catalyst library with a multichannel microreactor to measure H₂-D₂ exchange kinetics at 100 discrete compositions on the CSAFs over a temperature range of 333 – 597 K at atmospheric pressure. H₂ conversion was chosen to be the indicator of activity. It was found that H₂-D₂ exchange activity varies across the CSAF and it tends to increase with increasing Pd content. A microkinetic model that has been validated using a number of single component Cu-Pd catalysts in a fixed bed reactor was used to estimate the energy barriers to dissociative adsorption ( ) and associative desorption ( ) of H₂ as functions of alloy composition, x and y. On the Cu_xAu_yPd_1-x-y CSAF, increasing Pd content from 0 to 1 was found to decrease adsorption barrier from 0.44 to 0.12 eV. Increasing Pd content from 0.25 to 1 was found to increase desorption barrier from 0.4 to 0.74 eV which suggests H₂-D₂ exchange reaction is limited by H₂ desorption step within this Pd content. Spatially resolved X-ray photoelectron spectra were obtained from the CSAFs and used to estimate the energy of the valence-band center as a function of alloy composition. The v-band center shifted monotonically from -3.4 to 5.6 eV across the Cu_xAu_yPd_1-x-y CSAF. The barrier to dissociative adsorption of H₂ was found to decrease as the v-band energy increases. This data proves that we have successfully developed a high-throughput experimental strategy for understanding and designing ternary alloy catalysts. And it provides the first experimental correlation of elementary reaction barriers with valence band energy across a continuous span of alloy composition space.

The same experimental strategy was then used to study ethylene, ethyl and ethane adsorption and desorption characteristics. The preliminary results showed that ethylene hydrogenation activity can be monitored using our high-throughput methodology and it was found to be low at Cu- and Au-rich region of Cu_xAu_yPd_1-x-y CSAF and high across Pd-rich region of the ternary CSAF. The kinetics of ethylene hydrogenation will be measured across Cu_xAu_yPd_1-x-y CSAF so that the kinetic analysis can be performed across a wide composition range. The obtained rate law will provide insight into the reaction mechanism across alloy compositions.