Bimetallic Pt-Metal catalysts for the decomposition of methanol: Effect of secondary metal on the oxidation state, activity, and selectivity of Pt

Abstract

We present here a study of methanol (MeOH) decomposition over a series of bimetallic Pt-M catalysts, with M = Au, Pd, Ru, Fe. All samples have the same initial size distribution (∼3 nm nanoparticle height), support (ZrO₂), and preparation conditions. Therefore, differences in the electronic and catalytic properties of the samples tested are related directly to the addition of the secondary metals (M). We find that the oxidation state as well as the activity of Pt is heavily influenced by the addition of the secondary metal. PtO is found to be highly stable in these systems and increasing concentrations of metallic Pt are associated with the surface segregation of metal M due to its affinity for the oxygen present during air annealing.

Introduction

Pt-Metal (Pt-M) bimetallic catalysts are important in a variety of applications ranging from fuel cells [1] to thermal coatings [2]. In addition, recent years have witnessed a surge in the interest of methanol (MeOH), especially as a potential storage fuel for hydrogen, ultimately used for the production of electricity in on-board applications such as the direct methanol fuel cell (DMFC), portable electronics, or stationary power generation. These applications potentially involve electro-oxidation, steam reforming, and the direct decomposition of MeOH, each encompassing unique reaction conditions [3].

In order to take advantage of Pt-M systems in the design of new catalysts for any of these applications, the structural, chemical, and electronic modifications, brought about by the addition of secondary metals [4], [5], [6], [7], [8], [9], [10], [11], need to be fully understood. In particular, the surface compositions of such catalysts are influenced by a number of factors. Besides such familiar properties as surface energy, atomic volume, and heats of sublimation, nanoscale systems require additional considerations. For instance, it has been shown theoretically that Pt atoms may preferentially segregate to sites of low (edges, vertices, etc.) or high (facets) coordination depending on the structure of the particles as well as the metal M [12]. In addition, the presence of oxygen has been shown to heavily influence atomic segregation in nanoparticles (NPs), and the presence of metal M on the NP surface can affect the stability of oxide species on active nanocatalysts [13], [14]. In reference to fuel cells, alloying Pt with metals such as Fe, Ru, Ni, Co, as well as others, has been reported to enhance the oxygen reduction reaction (ORR) [5], [6], increase activity [15], and enhance resistance to CO poisoning [16]. In connection with the latter, Pt-Ru catalysts are known to be efficient and some detailed theoretical studies already exist concerning this catalyst’s role in CO oxidation [17]. However, as stated earlier, these systems hold importance in a broader sense than power generation alone [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32] and a large number of works have been dedicated to understanding their synthesis, characterization, and catalytic properties [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44]

Furthermore, with the ever increasing industrial use of nanomaterials, their impact on the environment has become an issue of great importance. Specifically, thermally or mechanically induced emissions of particulate Pt from automobile catalytic converters is a source of toxicological concern [45], [46], [47], [48]. Therefore, not only are the activity, selectivity, and stability of the working catalysts important, but so also is the state in which they might be emitted into the environment upon reaction (oxidated, chlorinated, etc.). Although catalysts are prepared in a certain state (i.e. Pt⁰), the oxidation state of the working catalyst might be different, and how this state evolves and reacts under environmental conditions is of interest.

In general, it is the surface properties of alloys which are credited with observed catalytic improvements, and as we will show in this article, the nature of the secondary metal, as well as the preparation and pretreatment conditions, have a large influence over the final composition and oxidation state of the surface and its active components. We present here an investigation of the influence that the addition of M = Au, Pd, Ru, and Fe has on the oxidation state, activity, and selectivity of supported Pt_0.8M_0.2 nanoparticles. We use as a probe reaction the decomposition of MeOH. Previous studies by our group [49], [50] have shown that the size of Pt nanoparticle catalysts, as well as the particle support, can influence the type (PtO, PtO₂) and stability of the metal oxide shell present in “real-world” catalysts under realistic reaction conditions. Indeed, there has been recent interest in the role of oxidized versus metallic species in the activity of several systems where oxidized metals may contain some catalytic advantage [51], [52]. To remove any ambiguities in the present study, we have used particles of the same average size for all samples and a common support, zirconia. Under these conditions the observed effects can be directly related to the addition of the secondary metal.