Self-assembly of noble metal monolayers on transition metal carbide nanoparticle catalysts

Abstract

Abstract:We demonstrate the self-assembly of transition metal carbide nanoparticles coated with atomically-thin noble metal monolayers by carburizing mixtures of noble metal salts and transition metal oxides encapsulated in removable silica templates. This approach allows for control of the final core-shell architecture, including particle size, monolayer coverage, and heterometallic composition. Carbon-supported Ti0.1W0.9C nanoparticles coated with Pt or bimetallic PtRu monolayers exhibited enhanced resistance to sintering and CO poisoning, achieving an order of magnitude increase in specific activity over commercial catalysts for methanol electrooxidation after 10,000 cycles. These core-shell materials provide a new direction to reduce the loading, enhance the activity, and increase the stability of noble metal catalysts. One Sentence Summary:The self-assembly of transition metal carbide nanoparticles coated with atomically-thin noble metal monolayers results in a highly active, stable, and tunable catalytic platform.Noble metal (NM) catalysts critically enable many existing and emerging technologies, such as catalytic converters (1), reforming (2), and fuel cells (3). However, their scarcity and high cost necessitate the development of catalytic systems with reduced NM loadings, increased activity, and improved durability. In this respect, various nanostructured architectures have been investigated, including atomically-dispersed NM catalysts (4), hollow nanocages (5, 6), alloyed nanoparticles (NPs) (7), and core-shell structures (8, 9). In particular, core-shell NPs composed of an earth-abundant core coated with an atomically-thin NM shell are a promising platform that offers both design flexibility and reduced precious metal loadings. However, achieving independent control over the particle size, core composition, shell composition, and shell thickness poses a substantial challenge (8, 9). State-of-the-art synthetic methods are predominantly limited to a few earth-abundant metallic cores (e.g., Fe, Co, Ni, and Cu) that allow for more precise synthetic control; however, these metal cores form intrinsically metastable core-shell particles that restructure during heating (10-12) or electrochemical cycling (13, 14).Early transition metal carbides (TMCs) are earth-abundant ceramics with ideal topochemical properties for supporting precious metal shells (15-17). First, TMCs exhibit metallic electrical conductivity, corrosion resistance, and high melting points (18). Second, precious metals tend to bind strongly to metal-terminated early TMC surfaces (Fig. S1), but cannot readily form stable carbides (19). Thus, NMs should coat TMC surfaces, but should not alloy with the underlying core. In particular, tungsten carbide (WC) is inexpensive (Fig. S2), exhibits a "platinum-like" density of electronic states (20, 21), and its metal-terminated surface forms interfacial Pt-WC bonds that are ca. 90 kJ mol -1 stronger than interfacial Pt-Pt bonds (Fig. S1). Although experimental studies on model thin film systems have co...

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