Peter C. Stair

Coking- and Sintering-Resistant Palladium Catalysts Achieved Through Atomic Layer Deposition

A Useful Cover-Up Many industrial catalysts that consist of metal nanoparticles adsorbed on metal oxide supports undergo deactivation after prolonged use. Organic reactants can decompose and cover the metal with carbon (“coking”), and other processes can push the size distribution to fewer but larger particles that have less overall surface area available for reaction (“sintering”). Lu et al. (p. 1205) used atomic-layer deposition to apply a uniform overlayer of alumina onto supported palladium nanoparticles. This coating greatly increased the resistance of the nanoparticles to coking and sintering during the oxidative dehydration of ethane to ethylene. Uniform oxide coating on palladium nanoparticles prevents carbon accumulation and particle growth during chemical reactions. We showed that alumina (Al2O3) overcoating of supported metal nanoparticles (NPs) effectively reduced deactivation by coking and sintering in high-temperature applications of heterogeneous catalysts. We overcoated palladium NPs with 45 layers of alumina through an atomic layer deposition (ALD) process that alternated exposures of the catalysts to trimethylaluminum and water at 200°C. When these catalysts were used for 1 hour in oxidative dehydrogenation of ethane to ethylene at 650°C, they were found by thermogravimetric analysis to contain less than 6% of the coke formed on the uncoated catalysts. Scanning transmission electron microscopy showed no visible morphology changes after reaction at 675°C for 28 hours. The yield of ethylene was improved on all ALD Al2O3 overcoated Pd catalysts.

Vapor-phase metalation by atomic layer deposition in a metal-organic framework

Metal-organic frameworks (MOFs) have received attention for a myriad of potential applications including catalysis, gas storage, and gas separation. Coordinatively unsaturated metal ions often enable key functional behavior of these materials. Most commonly, MOFs have been metalated from the condensed phase (i.e., from solution). Here we introduce a new synthetic strategy capable of metallating MOFs from the gas phase: atomic layer deposition (ALD). Key to enabling metalation by ALD In MOFs (AIM) was the synthesis of NU-1000, a new, thermally stable, Zr-based MOF with spatially oriented -OH groups and large 1D mesopores and apertures.

The surface reconstructions of the (100) crystal faces of iridium, platinum and gold : II. Structural determination by LEED intensity analysis

Abstract The investigation, in a companion paper, of the reconstructions of the Ir(100), Pt(100), and Au(100) crystal surfaces is completed here with an extensive analysis of low energy electron diffraction (LEED) intensities, using dynamical (multiple scattering) calculations. It is found that a hexagonal rearrangement of the top monolayer is a likely explanation of the surface reconstruction. At least for Ir and Pt (no calculations were made for Au), this hexagonal layer would have a registry involving bridge sites on the next square unit cell metal layer and it is contracted and buckled. Bond length contractions parallel and perpendicular to the surface occur; the Pt top layer is rotated by a small angle (0.7°) with respect to the substrate. A second model that cannot be ruled out by the LEED analysis, but disagrees with ion-scattering data, involves shifted close-packed rows of top-layer atoms and requires domain structures in the case of Pt and Au. Charge-density-wave and missing-row models are ruled out by our structure analysis. A correlation is found between the occurrence of surface reconstructions on metals and a small ratio of their Debye temperature to their melting point. This correlation singles out mainly the 5d metals as having a propensity to surface reconstruction. The effects of adsorbates on the reconstructions are also discussed.

Identification of active sites in CO oxidation and water-gas shift over supported Pt catalysts

Comparing active site reactivity Noble metal nanoparticles often exhibit behaviors distinct from atomic and bulk versions of the same material. Gold and platinum dispersed on metal oxide supports, for example, show remarkable low-temperature reactivity for carbon monoxide (CO) oxidation by oxygen or water. Ding et al. used infrared spectroscopy to identify CO adsorbed on isolated platinum atoms or nanoparticles dispersed on zeolite and oxide supports. Temperature-programmed desorption studies showed that CO reacted at much lower temperatures when adsorbed on nanoparticles versus on isolated metal atoms. Science, this issue p. 189 Infrared spectroscopy reveals that carbon monoxide oxidizes more readily on supported noble metal nanoparticles than on isolated atoms. [Also see Perspective by Stephens et al.] Identification and characterization of catalytic active sites are the prerequisites for an atomic-level understanding of the catalytic mechanism and rational design of high-performance heterogeneous catalysts. Indirect evidence in recent reports suggests that platinum (Pt) single atoms are exceptionally active catalytic sites. We demonstrate that infrared spectroscopy can be a fast and convenient characterization method with which to directly distinguish and quantify Pt single atoms from nanoparticles. In addition, we directly observe that only Pt nanoparticles show activity for carbon monoxide (CO) oxidation and water-gas shift at low temperatures, whereas Pt single atoms behave as spectators. The lack of catalytic activity of Pt single atoms can be partly attributed to the strong binding of CO molecules.

Supported ru-pt bimetallic nanoparticle catalysts prepared by atomic layer deposition.

Atomic layer deposition (ALD) is used to deposit ruthenium-platinum nanostructured catalysts using 2,4-(dimethylpentadienyl)(ethylcyclopentadienyl) ruthenium, trimethyl(methylcyclopentadienyl) platinum, and oxygen as precursors. Transmission electron microscopy shows discrete 1.2 nm nanoparticles decorating the surface of the spherical alumina support. The Ru-Pt particles are crystalline and have a crystal structure similar to pure platinum. X-ray fluorescence measurements show that the nanoparticle composition is controlled by the ratio of metal precursor ALD cycles. X-ray absorption spectroscopy at the Ru K-edge indicates a nearest neighbor Ru-Pt interaction consistent with a bimetallic composition. Methanol decomposition reactions further confirm a Ru-Pt interaction and show enhanced methanol conversion for the bimetallic nanoparticles when compared to catalysts comprised of a mixture of pure Pt and Ru nanoparticles of similar loading. These results demonstrate that ALD is a viable technique for synthesizing mixed-metal nanostructures suitable for catalysis and other applications.

Design Strategies for the Molecular Level Synthesis of Supported Catalysts

Supported catalysts, metal or oxide catalytic centers constructed on an underlying solid phase, are making an increasingly important contribution to heterogeneous catalysis. For example, in industry, supported catalysts are employed in selective oxidation, selective reduction, and polymerization reactions. Supported structures increase the thermal stability, dispersion, and surface area of the catalyst relative to the neat catalytic material. However, structural and mechanistic characterization of these catalysts presents a formidable challenge because traditional preparations typically afford complex mixtures of structures whose individual components cannot be isolated. As a result, the characterization of supported catalysts requires a combination of advanced spectroscopies for their characterization, unlike homogeneous catalysts, which have relatively uniform structures and can often be characterized using standard methods. Moreover, these advanced spectroscopic techniques only provide ensemble averages and therefore do not isolate the catalytic function of individual components within the mixture. New synthetic approaches are required to more controllably tailor supported catalyst structures. In this Account, we review advances in supported catalyst synthesis and characterization developed in our laboratories at Northwestern University. We first present an overview of traditional synthetic methods with a focus on supported vanadium oxide catalysts. We next describe approaches for the design and synthesis of supported polymerization and hydrogenation catalysts, using anchoring techniques which provide molecular catalyst structures with exceptional activity and high percentages of catalytically significant sites. We then highlight similar approaches for preparing supported metal oxide catalysts using atomic layer deposition and organometallic grafting. Throughout this Account, we describe the use of incisive spectroscopic techniques, including high-resolution solid state NMR, UV-visible diffuse reflectance (DRS), UV-Raman, and X-ray absorption spectroscopies to characterize supported catalysts. We demonstrate that it is possible to tailor and isolate defined surface species using a molecularly oriented approach. We anticipate that advances in catalyst design and synthesis will lead to a better understanding of catalyst structure and function and, thus, to advances in existing catalytic processes and the development of new technologies.

Characterization of surface species on iron synthesis catalysts by X-ray photoelectron spectroscopy

Abstract X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) have been used to characterize the surfaces of clean and oxidized iron foils, bulk iron oxide powders, and reduced and carbided iron foils and powders. Metallic, divalent, trivalent, and carbidic iron species were identified by their characteristic Fe(2p) XPS spectra, and spectra for mixed-valence surfaces were approximated by linear combinations of the spectra of the individual iron species. Results for iron carbides prepared from metallic iron indicated that surface carbides can also be identified by their characteristic iron Auger line shape. The results of this investigation show that the combination of XPS and AES can be used effectively for identification of the chemical states of iron on synthesis catalyst surfaces.

Synthesis and Stabilization of Supported Metal Catalysts by Atomic Layer Deposition

Supported metal nanoparticles are among the most important catalysts for many practical reactions, including petroleum refining, automobile exhaust treatment, and Fischer-Tropsch synthesis. The catalytic performance strongly depends on the size, composition, and structure of the metal nanoparticles, as well as the underlying support. Scientists have used conventional synthesis methods including impregnation, ion exchange, and deposition-precipitation to control and tune these factors, to establish structure-performance relationships, and to develop better catalysts. Meanwhile, chemists have improved the stability of metal nanoparticles against sintering by the application of protective layers, such as polymers and oxides that encapsulate the metal particle. This often leads to decreased catalytic activity due to a lack of precise control over the thickness of the protective layer. A promising method of catalyst synthesis is atomic layer deposition (ALD). ALD is a variation on chemical vapor deposition in which metals, oxides, and other materials are deposited on surfaces by a sequence of self-limiting reactions. The self-limiting character of these reactions makes it possible to achieve uniform deposits on high-surface-area porous solids. Therefore, design and synthesis of advanced catalysts on the nanoscale becomes possible through precise control over the structure and composition of the underlying support, the catalytic active sites, and the protective layer. In this Account, we describe our advances in the synthesis and stabilization of supported metal catalysts by ALD. After a short introduction to the technique of ALD, we show several strategies for metal catalyst synthesis by ALD that take advantage of its self-limiting feature. Monometallic and bimetallic catalysts with precise control over the metal particle size, composition, and structure were achieved by combining ALD sequences, surface treatments, and deposition temperature control. Next, we describe ALD oxide overcoats applied with atomically precise thickness control that stabilize metal catalysts while preserving their catalytic function. We also discuss strategies for generation and control over the porosity of the overcoats that allow the embedded metal particles to remain accessible by reactants, and the details for ALD alumina overcoats on metal catalysts. Moreover, using methanol decomposition and oxidative dehydrogenation of ethane as probe reactions, we demonstrate that selectively blocking low coordination metal sites by oxide overcoats can provide another strategy to enhance both the durability and selectivity of metal catalysts.

Controlled Growth of Platinum Nanoparticles on Strontium Titanate Nanocubes by Atomic Layer Deposition

With an eye toward using surface morphology to enhance heterogeneous catalysis, Pt nanoparticles are grown by atomic layer deposition (ALD) on the surfaces of SrTiO(3) nanocubes. The size, dispersion, and chemical state of the Pt nanoparticles are controlled by the number of ALD growth cycles. The SrTiO(3) nanocubes average 60 nm on a side with {001} faces. The Pt loading increases linearly with Pt ALD cycles to a value of 1.1 x 10(-6) g cm(-2) after five cycles. Scanning electron microscopy images reveal discrete, well-dispersed Pt nanoparticles. Small- and wide-angle X-ray scattering show that the Pt nanoparticle spacing and size increase as the number of ALD cycles increases. X-ray absorption spectroscopy shows a progression from platinum(II) oxide to metallic platinum and a decrease in Pt--O bonding with an increase in Pt--Pt bonding as the number of ALD cycles increases.

Ultraviolet Raman spectroscopy characterization of coke formation in zeolites

Abstract Ultraviolet (UV) Raman spectroscopy has been used to characterize coke formation in ZSM-5 and USY zeolites under propene at temperatures from 300K to 773K. The strong fluorescence background always present with normal Raman spectra is completely avoided in UV Raman spectra. Three groups of UV Raman bands near ≈ 1390, ≈ 1600 and ≈ 3000 cm−1 regions were detected for the two zeolites, and these bands varied significantly at different stages of coke formation. At room temperature, adsorbed propene was formed in the two zeolites and showed similar spectra. At elevated temperatures, the coke formation behavior in the two zeolites is quite different. For example, at 773K the coke species in ZSM-5 are mainly polyolefinic and aromatic species, but polyaromatic and pregraphite species are predominant in USY. The major portion of coke species formed in ZSM-5 can be removed even by He purging at 773K while the coke species in USY are very stable and can only be removed in O2 flow at temperatures above 773K. The difference in coke formation in ZSM-5 and USY is likely due to the different pore structure and acidity of the two zeolites.