Tuning Catalytic Activity of Metal Catalysts through Alloy, Ensemble Effects and Forced Dynamic Operations




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Catalytic performance of metal catalyst can be improved via not only traditional surface design but also forced dynamic operation on reaction conditions. The former approach relies on detailed knowledge of electronic structures of metal surface and interactions between surface and adsorbates, which are controlled by alloy and ensemble effects. Nevertheless, it is usually difficult to distinguish these two effects and study them separately, because varying the surface composition will most likely affect both the electronic structure of individual atoms and the distribution of ensembles.

In our attempt to study separately alloy and ensemble effects of Formic Acid decomposition over PdCu catalyst, we have showed that the alloy effect, which causes electron transfer between Pd and Cu atoms, is responsible for the up-shift of Cu d-band center, which activates the Cu site and improve it’s reactivity by nearly 100-fold. The ensemble effect, which is more pronounced when Pd atoms are grouped together, causes a locally Pd-like surface, which contributes to the even higher reaction activity at the cost of selectivity. By successfully showing the distinguished functionality of the two effects, we have demonstrated a approach to control the reaction activity and selectivity by carefully designing the catalyst surface at atomic level.

Another approach to improve the catalytic performance is by imposing forced dynamics over reaction conditions, which does not include redesign of the catalytic surface. Dauenhauer et al. has proved that by oscillating binding energies, rate of reactions such as A→B can be improved drastically.1 Oscillating feed pressures, theoretically should result in same improvement. To test this hypothesis, we choose CO oxidation over Pd surface as a probe reaction, and studied the transient kinetics with kinetic Monte Carlo simulation. By fixing O2 and temperature, we showed that the CO pressure amplitude cannot be within a certain range, in order to exhibit a TOF spike higher than the static optimum. Although a broader pressure amplitude provides a larger driving force, the oscillation time scale is also shorter, which can be very challenging in real operation when below the scale of 0.1 s. The impact of frequency is demonstrated by its effect on the cycle average CO pressure. While the static operation has low tolerance for CO, forced dynamic operation can increase the CO tolerance by up to two-fold depending on oscillating frequencies, which offers the advantage of increased stability and durability under harsh reaction conditions. The forced dynamic operation, however, is not able to speed the cycle average reaction rate over static optimum. This is probably because the reaction mechanism does not involve a Eley-Radial like reaction, and the maximum reaction rate is limited by the surface coverage.



Alloy, Forced Dynamic Operation, Catalytic Performance