(771d) Development of Continuous Co-Electroless Deposition and Improvements Made to Catalyst Activity and Stability

Despite the importance of bimetallic catalysts, the most common methods of preparation (co-impregnation and co-deposition precipitation) offer little control over surface or bulk composition. Electroless deposition (ED) was developed to remedy these problems and is an effective way to create bimetallic catalysts by depositing a secondary metal shell onto a primary metal core.[1] However, some problems remain with this method arising from the inherently unstable nature of the ED bath. To address these issues, continuous ED was developed. The addition of secondary metal salt and reducing agent on the order of magnitude matching the deposition reaction prevents buildup of reagents and subsequent thermal reduction, an undesirable side reaction forming monometallic secondary metal nanoparticles.[2] The addition of reagents along the course of synthesis allows for greater control over surface composition; from this continuous co-ED was developed, wherein two or more metals are added along with the reducing agent to create bimetallic or trimetallic catalysts. The advantage of this method lies in the ability to create mixed-metal shells with tight control over composition. In this manner, one does not have to rely on thermodynamics of alloy formation to drive bimetallic catalyst formation, and instead allows for surface composition to be directed by simply varying the ratio of reagent addition.

This method was used to synthesize two classes of catalysts to demonstrate improved activity and increased catalyst stability. Methanol electrooxidation catalysts for DMFCs were synthesized by depositing Cu²⁺/Pt⁴⁺, Ni²⁺/Pt⁴⁺, and Co²⁺/Pt⁴⁺ onto Pd/C cores. By controlling ratio of M:Pt (where M is a non-Pt transition metal), maxima in activities were shown. All elemental compositions had Pt mass activity maxima over that of supported Pt/C alone. Additionally, shell compositions showing the highest activities matched those predicted by computational modeling of this reaction reported previously in the literature.[3][4] The second class of catalysts were Ir@Pt-Ir /h-BN for a high temperature (800°C) decomposition reaction. The sintering behavior of Pt is widely reported and is made worse by weak interaction between Pt and support (as in the case with h-BN, a support lacking oxide species).[5] Catalysts prepared by IWI tested under these conditions show rapid deactivation by means of Pt particle growth. However, catalysts made by the co-deposition of Pt and Ir together resisted Pt particle growth, evidenced by in situ XRD analysis under relevant high temperature, oxidizing conditions. This demonstrates a large improvement in catalyst stability (lifetime) over monometallic Pt and bimetallic Pt-Ir catalysts made by other methods.

1. Beard, K., Schaal, M., Zee, J. V., Monnier, J., “Preparation of highly dispersed PEM fuel cell catalysts using electroless deposition methods,” Applied Catalysis B: Environmental, 72(3-4) pp. 262-271. (2007)
2. Tate, G., Kenvin, A., Diao, W., Monnier, J. R., “Preparation of Pt-containing bimetallic and trimetallic catalysts using continuous electroless deposition methods,” Catalysis Today, (2018)
3. Rossmeisl, J., Ferrin, P., Tritsaris, G. A., Nilekar, A. U., Koh, S., Bae, S. E., Brankovic, S. R., Strasser, P., Mavrikakis, M., “Bifunctional anode catalysts for direct methanol fuel cells,” Energy & Environmental Science, 5(8) pp. 8335-8342. (2012)
4. Li, Z., Wang, S., Chin, W. S., Achenie, L. E., Xin, H., “High-throughput screening of bimetallic catalysts enabled by machine learning,” Journal of Materials Chemistry A, 5(46) pp. 24131-24138. (2017)
5. Hansen, T. W., Delariva, A. T., Challa, S. R., Datye, A. K., “Sintering of Catalytic Nanoparticles: Particle Migration or Ostwald Ripening?” Accounts of Chemical Research, 46(8) pp. 1720-1730. (2013)