Alloy model catalyst

Using in situ STM also makes it possible to monitor the growth of supported alloy model catalysts. The simplest way to synthesize supported alloy model catalysts is to... 

Jerdev D, Olivas A, Koel BE (2002) Hydrogenation of crotonaldehyde over Sn/Pt(l 11) alloy model catalysts. J Catal 205 278... 

Table 8.4 Effectiveness of surface treatments to prolong the lifetimes of Aluchrom FeCrAIRE alloy model catalyst supports during air oxidation at 1100°C...

The examples introduced above refer to the characterization of the most common types of catalysts, usually supported metals or single, mixed, or supported metal oxides. Many other materials such as alloys [199,200], carbides [201-203], nitrides [204,205], and sulfides [206] are also frequently used in catalysis. Moreover, although modem surface science studies with model catalysts were only mentioned briefly toward the end of the review, this in no way suggests that these are of less significance. In fact, as the ultimate goal of catalyst characterization is to understand catalytic processes at a molecular level, surface studies on well-defined model catalysts is poised to be central in the future of the field [155,174], The reader is referred to the Chapter 10 in this book for more details on this topic. 

There are several ways to prepare thin films for use as model catalyst supports.30-31 For the purposes of this review, we will point the reader toward other sources that discuss two of these methods direct oxidation of a parent metal and selective oxidation of one component of a binary alloy. 32 34 The remaining method consists of the deposition and oxidation of a metal on a refractory metal substrate. This method has been used extensively in our group323131 11 and by others33-52-68 and will be the focus of the discussion here. The choice of the metal substrate is important, as lattice mismatch between the film and the substrate will determine the level of crystallinity achieved during film growth. 

Direct metal deposition from metallic sources has been extensively used for model catalyst deposition for high-throughput and combinatorial studies. However, these methods are also increasingly used to deposit practical electrocatalyst materials. The best known approach is the one developed by 3M researchers have used physical vapor deposition to deposit Pt and Ft alloys onto nanostructured (NS) films composed of perylene red whiskers. The approach has been recently been reviewed by Debe. ... 

In recent years, there has been a strong effort to use model catalyst surfaces combined with atomistic modeling approaches to understand why Ft alloys show activity enhancements and to predict new surfaces fhaf show even greater activity for oxygen reduction. 

Although model catalyst studies show the maximum possible activity obtainable, practical catalyst systems use nanoparticles of Pt or Pt alloys usually... 

In comparison to most other methods in surface science, STM offers two important advantages (1) it provides local information on the atomic scale and (2) it does so in situ [50]. As STM operates best on flat surfaces, applications of the technique in catalysis relate to models for catalysts, with the emphasis on metal single crystals. Several reviews have provided excellent overviews of the possibilities [51-54], and many studies of particles on model supports have been reported, such as graphite-supported Pt [55] and Pd [56] model catalysts. In the latter case, Humbert et al. [56] were able to recognize surface facets with (111) structure on palladium particles of 1.5 nm diameter, on an STM image taken in air. The use of ultra-thin oxide films, such as AI2O3 on a NiAl alloy, has enabled STM studies of oxide-supported metal particles to be performed, as reviewed by Freund [57]. 

In this chapter, we will illustrate with a few selected examples how well-defined, ordered Pt-Sn surface alloys have been used to elucidate the overall chemical reactivity of Pt-Sn alloys, clarify the role of Sn in altering this chemistry and catalysis, and develop general principles for understanding the reactivity and selectivity of bimetallic alloy catalysts. Most studies have involved chemisorption under UUV conditions, but the use of these alloys as model catalysts for investigating catalysis at pressures up to one atmosphere will also be discussed. 

Early higher pressure reaction smdies over Pt-Sn model catalysts by Paffett [62,63] and Somorjai [64, 65] and their coworkers revealed new insights into hydrocarbon catalysis in such systems. Szanyi et al. [62] showed that n-butane hydrogenolysis under moderate pressures (1-200 Torr H3/butane=20) and temperatures (up to 650 K) could be carried out without disruption of the ordered Sn/Pt(lll) surface alloys. This established that such catalytic reactions could be studied while maintaining the composition and geometric structure of these alloys under reducing reaction conditions (but not catalytic oxidation due to the aggressive interaction of O3 with Sn). These ordered Sn/Pt surfaces are qualitatively different from those in many studies of promoters and poisons, or disordered alloys, e.g., Au-Pt, in which the quantitative information on ensemble sizes available for reactions is difficult to determine. 

The studies reviewed here focus on Sn/Pt because of the opportunity afforded by the ordered alloys formed in this system for improving our basic understanding, as well as the commercial importance of Pt-Sn catalysts in naphtha reforming and their potential for other selective hydrogenation and dehydrogenation reactions. These studies combined detailed structural characterization of the alloy surfaces with UHV studies of adsorption and reaction of hydrocarbons and other small molecules, and measurements of the rate and selectivity of catalytic reactions at atmospheric pressure over these model catalysts. 

The sequence of elementary steps shown in Fig. 13.2 suggests that one can formulate the problem of carbon poisoning in terms of the selectivity associated with the formation of C-0 vs. C-C bonds on Ni. In order to prevent carbon-induced deactivation, a catalyst should be able to selectively oxidize C atoms (and CH fragments) rather than form C-C bonds. This elementary step mechanism was the basis for the DFT calculations that focused on the identification of catalysts (mainly Ni-containing alloys), which preferentially oxidize C atoms rather than form C-C bonds [15, 16]. In these DFT calculations, the potential energy surfaces for the formation of C-C and C-0 bonds were calculated for different Ni alloys. The alloy model system used in these calculations contained mainly Ni, with some Ni atoms displaced by another atom in the surface layer. While we have examined a number of different alloys, we will focus our discussion on the alloy material (Sn/Ni). We note that this alloy material has also been studied by others previously [35, 38, 41, 49, 50]. 

Beyond single component metal catalysts, the nanofabricated model catalysts can be used to study alloy catalysts, with compositions controlled by co-evaporation from two or more PVD sources. Alternatively, arrays of alternating particles or areas of two different materials can be made to study lateral communication between two types of catalysts at the nanoscale. For example, sequential reactions consisting of a first step on one type of catalyst and a second step on another catalyst particle could be studied systematically. The role of reactants and reaction intermediates as surfactants, affecting particle shape and morphology [163], will be possible to study in detail by in situ TEM studies in reactive environments. 

In both the case of the model catalysts treated in air at high temperature, and of the commercial aged catalysts, the formation of alloyed phases are shown. 

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