Improved Catalytic Reactor for the Electrochemical Promotion of Highly Dispersed Ru Nanoparticles with CeO2 Support

Electrochemical promotion of catalysis (EPOC) is a promising method for enhancing catalytic activity through the application of a small electrical stimulus between the catalyst-working and counter electrode deposited on a solid electrolyte ¹. The electronic properties of the catalyst can be modified resulting in a change in catalytic activity. In the case of yttria-stabilized zirconia (YSZ) as a solid electrolyte, the addition or removal of O^2- species on the catalyst surface can be controlled in situ depending on the specified reaction conditions. Fully reversible and “permanent” or “persistent” EPOC has been reported for more than 70 various catalytic systems ¹. In reversible EPOC experiments, the reaction rate returns to its initial value after the electrical stimulus is interrupted. For permanent EPOC (P-EPOC), the reaction rate remains at a higher value than the initial open circuit value ^2,3.  Despite receiving much attention, this phenomenon has not yet reached commercial application. One of the main technical factors preventing such development is the use of thick film catalysts with low surface areas and high material costs ⁴.

Ceria, CeO₂, is a mixed ionic-electronic conducting (MIEC) material that conducts O^2- due to oxygen vacancies in the crystallographic structure in addition to conducting electrons at elevated temperatures. Furthermore, due to its non-stoichiometry, CeO₂ has the ability to undergo conversion between Ce⁴⁺ and Ce³⁺ quite easily ⁵. These properties make the use of ceria-containing catalysts of interest for many applications. In heterogeneous catalysis, Pt group metals deposited on CeO₂ show a metal-support interaction (MSI) effect associated with charge transfer between the two solids that are in contact. In EPOC studies, using a MIEC can also ensure electrical connectivity between highly dispersed nanoparticle catalysts ⁶.

In this study, electrochemical enhancement of catalytic activity of a low particle size (1.9 nm) ruthenium nanoparticles catalyst for ethylene oxidation was investigated. Ru nanoparticles, synthesized using a modified polyol reduction method, were supported on CeO₂ resulting in a 1 wt% Ru loading (RuNPs/CeO₂) (i.e., typical in heterogeneous catalysis studies).The highly dispersed RuNPs/CeO₂ catalyst powder was supported on a YSZ solid electrolyte in order to apply polarization. The discussion of this study includes the effect of the partial pressure of ethylene with constant partial pressure of oxygen, temperature, and applied positive and negative current on the catalytic activity of the RuNPs/CeO₂ catalyst as well as the role of the cerium redox state in the observed persistent effect. In addition, the catalytic properties of the RuNPs/CeO₂ catalyst is compared to that of larger Ru particles supported on CeO₂ (TD-Ru/CeO₂) (same metal loading) and blank CeO₂, both supported on a YSZ solid electrolyte.

Characterizations of the catalysts were carried out using TEM and SEM. In addition, XPS analysis was done for the RuNPs/CeO₂catalyst as prepared and “spent” (after the reaction).  

Overall, it was observed that only the RuNPs/CeO₂ catalyst could be catalytically enhanced, showing a pronounced enhancement (up to 2.5 times) of the catalytic rate for negative polarization. The opposite effect was observed for positive polarization. This effect of both positive and negative polarization is illustrated by Fig.1.

Fig. 1.Transient effect of current application for C₂H₄ oxidation over Ru/CeO₂ on YSZ electrolyte at 350°C for an applied current of -2 μA for 4 hours and +2 μA for 6 hours (0.012 kPa C₂H₄).

Apparent Faradaic efficiencies up to 100 were also determined, indicating a non-Faradaic effect. In addition, a persistent effect was observed, showing stability up to 16 hours after current interruption. The modification of the cerium oxidation state (i.e., reduction from Ce⁴⁺ to Ce³⁺) is proposed to enhancethe catalytic performance of the Ru nanoparticles. This is due to the presence of more oxygen vacancies in the ceria interlayer causing a stronger metal-support interaction.These results demonstrate the feasibility of in-situ modification of the metal support-interaction between Ru nanoparticles and CeO₂catalytic support.

References

1. C. G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda, and D. Tsiplakides, Electrochemical Activation of Catalysis: Promotion, Electrochemical Promotion, and Metal-Support Interactions, Kluwer Academic/Plenum Publishers, New York, (2001).

2. J. Nicole and C. Comninellis, J. Appl. Electrochem., 28, 223–226 (1998).

3. S. Wodiunig, V. Patsis, and C. Comninellis, Solid State Ionics, 137, 813–817 (2000).

4. D. Tsiplakides and S. Balomenou, Catal. Today, 146, 312–318 (2009).

5. A. Trovarelli, Ed., Catalysis by Ceria and Related Materials, Imperial College Press, London, (2002).

6. A. Kambolis et al., Electrochem. commun., 19, 5–8 (2012).