Pt monolayer catalysts

FIGURE 13.4 Schematic showing the preparation of Pt monolayer and PdAu sublayer on a Pd core. Soluble forms of both Pd and Au are in the solution for the first galvanic step. Reprinted with permission from Ref. [50]. American Chemical Society. (See insertfor color representation of the figure.) 

Studying metal submonolayer (or monolayer islands) deposited on Au(lll), Kongkanand and Kuwabata showed that the ORR activities of these islands do not only depend on the their substrate but also depend strongly on the size of the islands [53, 54], Brankovic and coworkers later showed similar activity trend for HOR, and 

ELECTROCATALYST DESIGN IN PROTON EXCHANGE MEMBRANE FUEL CELLS 

FIGURE 13.5 (a) Model for the decrease of the OH coverage on Pt, caused by a high OH or O coverage on a second metal M. (b) Experimental kinetic current at 0.80V as a function of the calculated interaction energy between two OHs, or OH and O (vs. PtML/Pd(lll)). Reprinted with permission from Ref. [58]. American Chemical Society. 

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Similarly to the Pt monolayer catalysts, a series of Pd monolayers deposited on different metal single crystals were tested for ORR activity. The results are shown in Fig. 9.22. The ORR activity increases in the order PdML/Ru(0001) < PdML/fr(lll) < PdMi,/ Rh(lll)[Pg.299]

The Pt alloy monolayer nanoparticle catalysts (e.g., Pt-Re layer on Pd cores) showed a clearly improved specific (Pt surface normalized) ORR activity their Pt mass-based electrocatalytic activity, however, exceeded that of pure Pt catalysts by an impressive factor of 18 x— 20 x. Their noble metal (Pt, Re, and Pd) mass-based activity improvement was still about a factor 4x. The Tafel slope in the 800-950 mV/RHE range suggested that the surface accumulation of Pt-OH species is delayed on the Pt monolayer catalyst. The enormous increase in Pt mass-based activity is obviously due to the small amount of Pt metal inside the Pt monolayer. 

Pt alloy monolayer catalysts exhibited even more active ORR behavior compared to Pt monolayer catalysts. To understand this phenomenon computational DFT studies were carried out. The hypothesis to be tested was that, for instance, Ru metal atoms in the Pt—Ru monolayer are OH-covered and could inhibit the adsorption of additional OH on neighboring surface sites (adsorbate-adsorbate repulsion effect). A very similar hypothesis was put forward about three years earlier by Paulus et al. [105] who postulated that Co surface atoms might exhibit a so-called common-ion effect, that is, they could repel like species from neighboring sites. A combined computational-experimental study finally confirmed this hypothesis [123] If oxophilic atoms such as Ru or Os were incorporated into the Pt monolayer catalysts, the formation of adjacent surface OH was delayed, if not inhibited. Oxo-phobic atoms, such as Au, displayed the opposite effect, would not inhibit Pt—OH formation, and were found to be detrimental to the overall ORR activity. 

While the stability of the monolayer Pt alloy catalyst concept was initially unclear and therefore threatened to make the monolayer catalyst concept a questionable longer term solution, a very recent discovery seems to lend support to the claim that Pt monolayer catalyst could be made into stable catalyst structures Zhang et al. [94] reported the stabilizing effect of Au clusters when deposited on top of Pt catalysts. The presence of Au clusters resulted in a stable ORR and surface area profile of the catalysts over the course of about 30,000 potential cycles. X-ray absorption studies provided evidence that the presence of the Au clusters modified the Pt oxidation potentials in such a way as to shift the Pt surface oxidation towards higher electrode potentials. 

Catalyst Structure C in Figure 6.21 is commonly referred to as the Pt-skin catalysts [87,95,107,124,125]. The term Pt skin catalysts will here be used to refer to a monolayer of pure Pt sitting on a Pt-depleted Pt alloy core in contrast, a Pt monolayer catalyst was referred above to as a monolayer of pure Pt on top of... 

The concept of a Pt monolayer catalyst was first verified with a Pt submonolayer on Ru substrate. This approach radically changed the design of the Pt-Ru catalysts and it is likely to similarly affect a broad range of catalysts. It facilitates an ultimate reduction of Pt loadings in Pt-Ru catalysts by depositing Pt only at the surface of Ru nanoparticles, so that the most of the Pt atoms become available for the catalytic reaction. Ru (10%) nanoparticles on Vulcan XC-72 carbon were heated in an H2 atmosphere at 3()0°C for 2 h. This temperature is much lower than that required for bulk Ru... 

F ute 9.11 Model for the synthesis of Pt monolayer catalyst on non-noble metal-noble metal core-shell nanoparticles. ... 

DFT approach is consistently used to resolve electronic structure features of various advanced catalyst concepts including core-shell catalyst, nanostructured shape-selective catalysts, Pt monolayer catalyst, and selectively dealloyed catalysts... 

Pt monolayer catalyst introduced by the Adzic group, is a particularly promising concept that allows ultra low Pt content [49]. The correlation between the position of i/-band center relative to the Fermi energy and measured kinetic current density has been reported by Zhang et al. for the Pt monolayer catalyst supported on single... 

Pt monolayer catalysts show a promising pathway toward solving one of the major problems facing PEM fuel cells by enhancing the Pt-specific activity and the utilization of Pt atoms and therefore reducing the cost of the cathode catalyst, although more fuel cell tests of durability are needed before the monolayer catalysts can be put in fuel cell vehicles. There is still a need for a reduction in the total noble metals in these catalysts. 

The advantages of Pt monolayer catalysts include (1) full utilization of the Pt atoms that are all on the surface, and (2) that the Pt activity and stability can be tailored by the selection of the substrate metals. For example [102], when a Pt monolayer is deposited onto different substrate metals, as shown in Fig. 10.6, due to the lattice mismatch between the metals, it can experience compressive or tensile stress, which is known to affect the Pt activity by adjusting its d-band center energy [43, 47] and consequently its ORR activity. 

For further fine-tuning of the monolayer Pt/Pd ORR activity, they further introduced mixed metal + Pt monolayer catalysts [115], which contained 0.2 monolayer of a foreign metal from selection of (Au, Pd, Rh, Ir, Ru, Os, and Re) combined with 0.8 monolayer of Pt co-deposited on Pd(lll) or on Pd/C nanoparticles. The foreign metals have either a weaker M-OH btmd (for the case of Au-OH), or a stronger M-OH bond (for the rest of the cases) than the Pt-OH bond. DFT calculations [115] showed that, in addition to altering the Pt d-band center energies, the OH( m) OH( pq (or 0( m>—OH( pt)) repulsion plays an important role in augmenting the ORR activity, as shown below in Fig. 10.9. 

Pt-alloys PtMe (Me = Cr, Mn, Co, Ni, V, Ti) were investigated early on, since they show an increased specific activity with respect to ORR [1, 2]. Enrichments and depletions of alloying metals on the particle interface were found, which influenced the electrochemical activity significantly. Predominant scientific success was achieved in the field of Pt-monolayer catalysts [3], the concept of Pt-skin electrocatalysts [4], and the concept of unalloyed Pt bimetallic catalysts [5]. 

Zhang Y, Ma C, Zhu Y et al (2013) Hollow core supported Pt monolayer catalysts for oxygen reduction. Catal Today 202 50-54... 

FIGURE 4.6 Synthesis steps for Pt monolayer catalysts on nonnoble metal core-noble metal shell nanopartides. Reprinted with permission from Ref. [21]. Copyright 2005 American Chemical Society. 

FIGURE 4.14 Profiles of x-ray powder diffraction intensity for three Pt monolayer catalysts on Pd(solid) (blue, top), Pd9Au(solid) (green, middle), and PdgAu(hollow) (red, bottom) cores. The dotted lines are the fits, yielding the average particle diameters and lattice constants. The lattice strains listed are calculated with respect to the lattice constant of bulk Pt, 3.923 A. 

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