High impact of the reducing agent on palladium nanomaterials: new insights from X-ray photoelectron spectroscopy and oxygen reduction reaction

Abstract

Palladium has exceptional affinity with hydrogen and the evolution of the surface of its nanomaterials prepared from chemical methods over time is still unclear. Here, the reducing agent effect on Pd nanomaterials and their long-term chemical stability were scrutinized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The subsequent impact on the catalytic properties was examined using the electrochemical oxygen reduction reaction (ORR). We have discovered that the nature of the reducing agent has noteworthy effects on the final composition of Pd nanomaterials prepared from chemical methods. The surface state of the nanomaterials prepared by using sodium borohydride as reducing agent (Pd/C–NaBH₄) is radically different from those obtained from L-ascorbic acid (Pd/C–AA). In addition to pure metal, two oxides were identified: PdO and PdO_x (x > 1). XRD analysis has upheld the presence of PdO only in Pd/C–NaBH₄, thus underpinning the conclusion that NaBH₄ has drastically changed the Pd structure. Furthermore, the reducing agent substantially affects the electrocatalytic properties. The ORR starts with enhanced kinetics (E > 1 V vs. RHE) by a 4-electron process, producing p(H₂O₂) < 0.5% associated with excellent durability over 5000 cycles. Both catalysts outperform all reported data for Pd electrocatalysts. The novelty of this work is combining ex/in situ XPS and XRD analyses together with ORR as a catalytic model. Overall, this work represents a clear development in our understanding of Pd affinity towards hydrogen and paves new ways for the successful synthesis of Pd-based nanomaterials free from hydrides and oxides, and having impressive catalytic activities.