Activity–Structure Correlation of Pt/Ru Catalysts for the Electrodecomposition of Methanol: The Importance of RuO₂and PtRu Alloying

Abstract

Bimetallic catalysts: The effect of PtRu alloying and the influence of RuO(2) species on the methanol oxidation activity of PtRu/C catalysts is studied. Different heat treatments-utilizing either N(2) or air-are applied to the bimetallic materials to enhance the degree of alloying or produce RuO(2) [picture: see text]. The catalysts with the best performance are characterized by a small particle size, a high degree of PtRu alloying, and the presence of a Pt-related species on their surface.Herein, we study the effect of both PtRu alloying and the presence of RuO(2) species on the promotion of the methanol oxidation activity of PtRu/C catalysts. Bimetallic catalysts composed of 15 wt % PtRu/Ce(x)C (x=0 or 10) are prepared by using the precipitation-deposition method and activated through hydrogen reduction at 470 K. Different heat treatments, utilizing either N(2) or air, are applied to the as-prepared catalysts to enhance the degree of alloying or produce RuO(2), respectively. The electrocatalytic properties, the structure, and the surface composition of the alloys are investigated systematically by means of electrochemical measurements coupled with X-ray diffraction (XRD) and temperature-programmed reduction (TPR) experiments. We find that the N(2) heat treatment improves the catalytic activity of the alloys more significantly than the air heat treatment. Also, the current density and long-term durability toward methanol oxidation can be significantly enhanced by combining a loading of 10 % CeO(2) and N(2) with a heat treatment at 570 K. Physical characterization performed by means of TPR reveals that the surface of the N(2)-treated sample is covered with Pt, thereby presenting a higher methanol oxidation current than the air-treated sample whose surface is composed of RuO(2) and some alloy species. Moreover, a model for describing the physical structures of the deposited bimetallic crystallites obtained after the N(2) and air treatments is proposed. This model suggests that the catalysts with the best performance should have a small particle size and exhibit a structure characterized by a high degree of PtRu alloying and a Pt-related surface species. Therefore, we can conclude that the effect of PtRu alloying on the electro-oxidation activity of the catalysts is superior to that of the presence of RuO(2) species under practical conditions.

Promotion of Pt–Ru/C catalysts driven by heat treated induced surface segregation for methanol oxidation reaction

“…Previously, a comparison of surface species from N 2 -treated and O 2 -treated Pt-Ru alloy catalysts measured by temperature programmed reduction (TPR) has been reported [12]. The results suggest that an alloy with a Pt surface species resulted from N 2 treatment, rather than RuO 2 coming from O 2 treatment, is essential for obtaining a high MOR activity [12].…”

“…Up to the present, the Pt-Ru/C anode catalysts of DMFCs still have some shortcomings and need to be improved, such as slow kinetics of MOR and poor CO de-poisoning ability. Many methods, such as addition of modifiers or the third elements, adjustment of preparation parameters, use of alterative supports, and heat treatments on the Pt-Ru catalysts, have been proposed comprehensively in the literature for promotion of home-made catalysts [11][12][13][14][15][16][17][18]. However, the structure-activity relationship (SAR) and promotional effect depends on the preparation methods, alloy structures, and surface compositions of the catalysts significantly, and only some conclusive results can be obtained.…”

“…The performance of alloy catalysts can be improved by virtue of the post heat treatments at various temperatures and atmospheres [11,12,[19][20][21][22]. Moreover, the thermal annealing processes are also extensively applied in the Pt-based alloy catalysts to further enhance their catalytic activity [23,24].…”

“…The heat treatment in N 2 on the catalysts has many advantages, including removal of residual surfactants [20,34] and promotion of the alloying degrees [12,20]. The degrees of alloying of Pt-Ru catalysts depending on the proximity between the Pt and Ru atoms are generally considered as an important structural parameter for MOR efficiency.…”

“…Moreover, the thermal annealing processes are also extensively applied in the Pt-based alloy catalysts to further enhance their catalytic activity [23,24]. The electro-catalytic activity of Pt-Ru alloy catalysts can be greatly improved due to the alterations in physical structures [25][26][27], surface compositions [11,12,28], or degrees of Pt-Ru alloying [20,29] after the post heat treatments. Specifically, the post-treatment may induce the change of surface areas and phase separation of Pt and Ru that modify the surface structures along with the intrinsic and massspecific activities of methanol oxidation [22].…”