A Study of Pt Dissolution during Formic Acid Oxidation

Abstract

The studies on Pt or Pt-alloy based catalysts for formic acid oxidation (FAO) have been focused mostly on their activity with little analysis of durability and long-term performance. While durability of Pt-based FAO catalysts has been associated with CO poisoning, recent studies have brought evidence of correlation between catalyst performance and dissolution of Pt. Aimed at assessing the impact of that correlation, a detailed quantitative durability analysis of Pt ultrathin films grown on Au by surface limited redox replacement of a Pb underpotentially deposited layer is presented in this work. Long-term catalytic performance of those films featuring well-defined thickness and structure has been examined by potential cycling in 2 M HCOOH+0.1 M HClO4 solution at sweep rate of 0.050 V s–1 over an extended potential range limited positively by the onset of Pt oxidation. A clear proportionality between overall life and thickness of the catalyst observed in the tests emphasizes the dissolution of Pt as the key life-limiting factor. The steady loss of Pt during cycling has been also confirmed by XPS compositional analysis. Ex situ imaging, performed by SEM and AFM at characteristic stages of the activity decay (associated with distinctive voltammetric behavior), reveals dramatic morphological changes accompanying the Pt dissolution process. An average Pt dissolution rate of 272 ± 30 ng·cm–2·h–1 has been determined based on the FAO activity decay of Pt films of different thickness. For comparison, identical experiments performed in the absence of formic acid showed about 4–5 times lower Pt dissolution rate, in the range of 61 ± 8 ng·cm–2·h–1. The significant impact of the FAO on the dissolution rate suggests a possibility of reaction intermediate contribution to the dissolution of Pt