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Efficient Removal of Organic Ligands From Supported Nanocrystals by Fast Thermal Annealing Enables Catalytic Studies on Well Defined Active Phases Matteo Cargnello, Chen Chen, Benjamin T. Diroll, Vicky V. T. Doan-Nguyen, Raymond J. Gorte, and Christopher B. Murray J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 11 May 2015 Downloaded from http://pubs.acs.org on May 12, 2015
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Journal of the American Chemical Society
Efficient Removal of Organic Ligands From Supported Nanocrystals by Fast Thermal Annealing Enables Catalytic Studies on Well-Defined Active Phases Matteo Cargnello,1,† Chen Chen,2 Benjamin T. Diroll,1 Vicky V. T. Doan-Nguyen,3 Raymond J. Gorte2 and Christopher B. Murray1,3,* 1
Department of Chemistry, 2Department of Chemical and Biomolecular Engineering, and 3Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104 (USA)
ABSTRACT: A simple yet efficient method to remove organic ligands from supported nanocrystals is reported for activating uniform catalysts prepared by colloidal synthesis procedures. The method relies on a fast thermal treatment in which ligands are quickly removed in air, before sintering can cause changes in the size and shape of the supported nanocrystals. A short treatment at high temperatures is found to be sufficient for activating the systems for catalytic reactions. We show that this method is widely applicable to nanostructures of different sizes, shapes and compositions. Being rapid and effective, this procedure allows the production of monodisperse heterogeneous catalysts for studying a variety of structure-activity relationships. We show here results on methane steam reforming, where the particle size controls the CO/CO2 ratio on alumina-supported Pd, demonstrating the potential applications of the method in catalysis.
1. INTRODUCTION A variety of uniform nanocrystals (NCs) and heterostructured materials with different sizes, shapes and compositions can be prepared with nanometer precision using surfactant-assisted organic- or aqueous-based techniques.1-3 This nanoscale control over a wide variety of different crystallographic surfaces and compositions is very appealing for catalytic studies because it allows accurate structureactivity relationships and mechanisms to be made.4 Studying finely-controlled NC systems is important for answering fundamental questions that could allow the preparation of more active catalysts. The major drawback of colloidal synthetic methods is that they often require surface-bound surfactants to achieve colloidal stability and to control size and shape. Unfortunately, catalysis relies (in most although not all cases5) on having clean surfaces where reactants can bind to surface atoms and undergo chemical transformations into the desired products. Obtaining clean surfaces free of extraneous species in these uniform systems is therefore essential to prepare active, stable and selective catalytic materials for numerous applications. It has been clearly shown that the removal of the ligands is essential for observing full activity. Even small amounts of ligands left on the surface might be extremely detrimental for catalysis.6 Several methods have been reported in the literature to remove organic surfactants and stabilize the surfaces of nanoparticles, small clusters or even single atoms for catalysis.7 The most popular ways include thermal treatments at low temperatures,8,9 solvent extraction,10 chemical
treatments to strip the ligands off the particles,11-13 methods involving plasma,14 or UV-ozone cleaning procedures.15,16 All these methods can remove organic byproducts and activate catalysts to some extent. The problem lies in the fact that more gentle approaches may leave residues that block the active sites, and harsher treatments can cause unwanted changes in the morphology of the systems and sintering of the clusters/particles that compromise their uniformity. Furthermore, some of these methods require long treatments to be effective.17 In previous work, we found that the minimum temperature required for removal of organic ligands and activation of group VIII metal catalysts by thermal treatments was 300 °C.18 Despite having narrow particle-size distributions, the slow heating ramps and long annealing times led to a change in the initial particle shapes and a restructuring of the nanocrystals onto the support.18 The reconstruction was particularly noticeable with metal nanoparticles on CeO2 (ceria) support and can be explained by the propensity of metals to wet the ceria surface.19 Reconstruction must be avoided to study size and shape effects on catalytic activity. UV-ozone is one of the best methods because it is relatively mild and it works at room temperature. However, there are reports indicating that catalysts are not completely activated following this process, likely due to incomplete removal of surface-bound surfactants.17 Here we report on a simple yet effective thermal treatment to remove organic ligands from catalysts prepared by colloidal methods. We show that the uniformity of the particles is preserved while the systems are activated for catalytic
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reactions. We expect this method to be valuable to other materials and applications. As a demonstration of the usefulness of this approach, we present studies on methane activation through steam reforming on size-selected Pd NCs. 2. EXPERIMENTAL SECTION 2.1 Synthesis of Pd nanocrystals. The synthesis was performed using standard air-free Schlenk techniques. Reaction conditions for appropriate sizes are reported in Table 1. 76 mg of palladium acetylacetonate (0.25 mmol, 35% Pd, Acros Organics) were dissolved in 10 mL of either trioctylamine (TOA, 97%, Acros Organics) or benzyl ether (98%, Sigma-Aldrich) together with oleylamine (OLAM, 8090%, Acros Organics) and oleic acid (OLAC, 90%, SigmaAldrich), if needed. The mixture was evacuated at RT (pressure
