Promotion of Platinum‐Ruthenium Catalyst for Electrooxidation of Methanol by Crystalline Ruthenium Dioxide

Abstract

The fuel cell is currently being established as a future 'green' energy source. The direct methanol fuel cell (DMFC) uses aqueous methanol solution as a fuel and has potential application in mobile electronics.[1] However, the high electrocatalyst cost and poor anode performance are two key handicaps which restrain the commercialization of DMFC. [2,3] The fabrication of low-loaded but active Pt catalysts for the anode is essential for the DMFC development. Practical approaches include 1) a decrease in the particle size (or high density of active sites) of Pt electrocatalysts, [4][5][6] 2) an increase in the accessibility of active sites for methanol, [7][8][9] and 3) a high resistance against CO poison.[10]CO is an intermediate in the oxidation of methanol [Eq. (1)] and poisons active Pt sites at the anode of DMFC:Adsorbed CO can be oxidized to poisonless CO 2 by water [Eq. (2)] at a cost of overpotential (h):The h required depends on the electrocatalysts used. [11,12] For monometallic Pt/C catalysts, a high h % 0.6 V is generally needed. Advanced electrocatalyst design relies on "bifunctional catalysts" [13][14][15] in which other metals are added to promote Equation (2). Ruthenium has been extensively explored as the best promoter. [1][2][3] A decrease of h to % 0.2 V was generally found for Pt-Ru/C bimetallic catalysts.Recently, Steigerwalt and co-workers [16] prepared a Pt-Ru/ GCNF nanocomposite (oxidized at 623 K) as the anode catalyst in a DMFC, and found a 50 % increase in the cell performance from an unsupported Pt-Ru catalyst. The enhanced performance was attributed to the existence of oxidized Ru species in the Pt-Ru/GCNF catalyst. In a previous report, [17] electroactivity of methanol oxidation with PtRu/C catalyst was also found promoted by oxidation treatments. The extent of promotion, however, depends on the temperature (T o ) of oxidation treatment. The promotion increased initially with T o value and optimized at T o = 520 K when the crystalline RuO 2 (c-RuO 2 ) phase began (seen through XRD) on the studied Pt-Ru/C catalyst. Herein, we report our experience on the reversibility between RuO 2 and Ru, during repetitive redox treatments of reduction (by hydrogen at T r = 620 K) and oxidation (by air at T o = 520 K) on a Pt-Ru/C bimetallic catalyst, and on the stability of c-RuO 2 during electrooxidation of methanol. A supported bimetallic catalyst of 11.1 wt % Pt-Ru/C (with a Pt/Ru atomic ratio of 1.0) was prepared by the method of incipient wetness impregnation.[10] The freshly impregnated catalyst was subsequently treated by the repetitive redox treatment. The treated catalysts were coded as Red m or Ox m , in which symbols Red and Ox denote the type (reduction or oxidation) of final treatment while the subscript m represents the number of repetitive treatments. For example, Red 1 presents an electrocatalyst from reduction of the freshly impregnated catalyst while Ox 2 catalyst is obtained from oxidation of catalyst Red 2 .

Effects of Pt Shell Thicknesses on the Atomic Structure of Ru–Pt Core–Shell Nanoparticles for Methanol Electrooxidation Applications

“…[6,29,30] The Pt/Ru bimetallic system plays an important role in energy production and storage technologies, such as direct methanol fuel cells (DMFC). [7,8,10,33,34] Although catalysts in such architectures have been widely studied, [12,28,29] the detailed atomic structure and the surface activity on methanol oxidation are still not well understood. Hence, the purpose of the present paper is to identify the evolution of atomic structure and the corresponding surface activity of core-shell type electrocatalysts with just a few ALs of Pt atoms coated on the Ru nanoparticles.…”

“…The bifunctional effect is often cited to explain the promotion of specific current density (I SC ) and CO tolerance on electrocatalyst system of Pt-M NPs (M denotes the hydrophilic transition metals). Typical examples include the alloyed NPs of PtRu, [7,8] multi-element NPs of Fe 3 O 4 /Au/Pt, [9] and Pt-based catalysts supported by different matrices. [10,11] The promotion of catalytic activity on such materials originates from the formation of surface MÀ (OH) ads /MÀhydrate that provides low-energy reaction pathways.…”

“…The second element can form MÀ (OH) ads sites and consequently promote the surface activity and selectivity of bimetal systems. [8,[22][23][24][25][26] Recent experimental [27] and DFT studies [6] on Pt monolayers coated on Ru (0001) thin-films show substantially enhanced low temperature (~30 8C) hydrogen light-off and carbon monoxide oxidation activity. It is attributed to the compressive strain at the surface Pt layer.…”

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

“…On the other hand, air treatment of the PtRu/Ce 10 C samples resulted in a surface composed of RuO 2 and AO x . Huang et al [22,23] and Wang et al [15] have reported that the electroactivity can be improved by the presence of Ru c O 2 . However, the air treatment has a negative effect on the electroactivity in this study.…”

“…Steigerwalt et al [21] found that the presence of oxidized Ru species in PtRu catalysts resulted in an enhanced DMFC performance. Huang et al [22,23] demonstrated that Ru atoms segregate to the surface of bimetallic crystallites and are oxidized to crystalline RuO 2 at oxidation temperatures above 520 K. Guo et al [24] indicated that the oxygen atoms released by CeO 2 particles can easily react with unalloyed surface Ru atoms. The released oxygen species can then form RuO x compounds, which improve the electrocatalytic activity, thus leading to methanol decomposition.…”