(308g) High Throughput Study of Catalysis on Pd-Based Alloys
AIChE Annual Meeting
2017
2017 Annual Meeting
Catalysis and Reaction Engineering Division
Reaction Chemistry and Engineering I
Tuesday, October 31, 2017 - 10:12am to 10:34am
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 CuxAuyPd1-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 â 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 CuxAuyPd1-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 CuxAuyPd1-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 CuxAuyPd1-x-y CSAF and high across Pd-rich region of the ternary CSAF. The kinetics of ethylene hydrogenation will be measured across CuxAuyPd1-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.