Article pubs.acs.org/IECR
Support Effect on the Water Gas Shift Activity of Chemical Vapor Deposition-Tailored-Pt/TiO2 Catalysts Matthias Faust,*,† Mirja Dinkel,‡ Michael Bruns,§ Stefan Bras̈ e,‡ and Martin Seipenbusch*,∥ †
Institute for Mechanical Process Engineering and Mechanics, Karlsruhe Institute of Technology (KIT), Straße am Forum 8, D-76131 Karlsruhe, Germany ‡ Institute of Organic Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 6, D-76131 Karlsruhe, Germany § Institute for Applied Materials and Karlsruhe Nano Micro Facility, Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen D-76344, Germany ∥ Institute of Chemical Process Engineering, University of Stuttgart, Boeblingerstr. 78, D-70199 Stuttgart, Germany ABSTRACT: In contrast to most wet chemical synthesis routes, chemical vapor deposition (CVD) offers many degrees of freedom for the structuring of Pt nanoparticle catalyst systems, which is a prerequisite for the examination of structure−function correlations in heterogeneous catalysis. Two different suitable metal−organic Pt precursors [MeCpPtMe3 and novel PtMe2(iBu-COD)] were employed to generate highly defined, very narrowly distributed Pt nanoparticles with high active surface areas on well-specified TiO2 supports of differing surface properties. The TiO2 supports and the Pt nanoparticles were thoroughly characterized and tested in the water gas shift reaction. The influence of the support particle size and its surface chemistry (hydroxyl groups) as well as the nature of the metal−organic precursors used on the water gas shift (WGS) activity were investigated by nanotechnological methods within this study. A further modification of the support material was done by Na deposition. This catalyst was found to be a highly efficient WGS catalyst with an increase of the catalytic activity by a factor of 3 compared to the nonmodified system. structure−function correlations.7 Also, bifunctional catalyst systems are known, where the catalytic cycle may be actively influenced by the precious metal and by the support, a phenomenon known as strong metal−support interaction (SMSI).8 An important area of catalyst development is in the generation of electrical power using alternative energy sources (e.g., fuel cells). Because of the very high energy density of 120 MJ/kg,9 hydrogen has become one of the most promising energy sources of the future. The environmentally friendly generation of electric energy by the conversion of hydrogen and oxygen within compact and modular polymer electrolyte fuel cells is of high public and scientific interest.10 Beside the usage of industrially generated hydrogen by steam reforming, by electrolysis, or by conversion of biomass, the concept of socalled fuel generators offers many advantages.11 In a first step, hydrogen is generated by a highly efficient, compact steam reforming process. A second water gas shift (WGS) reactor decreases the carbon monoxide content and increases the amount of hydrogen. The location-independent process
1. INTRODUCTION Industrial applications usually define performance requirements, which are typically met by screening heterogeneous catalysts of varied parameters such as composition, structure, and morphology. This approach, together with the optimization of reaction conditions, resulted in increased efficiency of many chemical processes within the last decades. Many of these industrial catalyst systems are based on dispersed precious metals on metal oxide supports, which were usually optimized by wet chemical (liquid-phase synthesis) or surface chemical (e.g., monocrystal analysis) procedures. The influence of structural parameters on catalytic performance of heterogeneous precious metal catalysts has been known in principle for nearly 50 years.1 These correlations are widely based on the particle size on a support and its statistical distribution, which determine dispersion and active surface area of the catalyst material.2 Depending on the particle size, particular types of atomic configurations (for example edges)3,4 and crystal planes5,6 appear at the surface, which can influence the catalytic activity of a particulate catalyst. The current research, however, goes beyond the focus on the single-particle level and takes a holistic approach encompassing the entire catalyst system. For instance, the combination of materials, surface structure, and size (or surface area) of the support particles were found to play an important role in © 2017 American Chemical Society
Received: Revised: Accepted: Published: 3194
November 20, 2016 March 7, 2017 March 9, 2017 March 20, 2017 DOI: 10.1021/acs.iecr.6b04512 Ind. Eng. Chem. Res. 2017, 56, 3194−3203
Article
Industrial & Engineering Chemistry Research
the possibility of generating not only similar amounts of Pt on TiO2 but also comparable median Pt particle size and active surface area, which is a prerequisite for similar Pt−support interfaces. Only a few metal−organic precursors were used by different groups for the generation of Pt nanoparticles on catalyst supports by CVD. Platinum acetylacetonate, Pt(acac)2, is a well-known CVD precursor which is easily synthesized but has the disadvantage of relatively high decomposition temperature and high carbon residues.36 An example for the use of Pt(acac)2 is the generation of Pt nanoparticles on zeolite by Jacobs et al.36 Another possible precursor for the coating of particulate substrates is platinum bis-hexafluoracetylacetonate, Pt(hfa)2. Pt(hfa)2 was used by Hierso et al. for the synthesis of Pt/SiO2 catalysts by a fluidized bed reactor, but the resulting Pt particles were characterized by an unwanted increased amount of carbon residues.37 Within this study we present the usage of commercially available MeCpPtMe3 (1) and novel PtMe2(iBu-COD) (2) as precursors for the preparation of Pt/TiO2 nanoparticles. MeCpPtMe3 (trimethyl(methylcyclopentadienyl)platinum) is an easy to handle, well-characterized precursor which was used for the generation of Pt films and nanoparticles within a number of studies.38−40 A few examples are the synthesis of Pt/ Al2O3 catalysts by CVD41 or the generation of Pt on different particulate supports by fluidized bed ALD.39,40 PtMe2(COD) is characterized by a relatively low volatility and a high melting point, which implies problems during the evaporation and transport of the precursor to the reactor. The synthesis and characterization of PtMe2(iBu-COD) (2) was described in our previous publications.42,43 Alkyl groups were added to increase the volatility and to decrease the melting point of the metal− organic substance. Homogeneously distributed defined Pt nanoparticles with very narrow size distribution were generated by a well-understood coating processes on different supports by our group.35,42,43 Within this study, highly controlled Pt nanoparticles with high Pt-ASA (active surface area) were structured by fixed-bed CVD, and the correlation between the CVD precursor and the WGS catalytic activity was investigated. To our knowledge the usage of PtMe2(iBu-COD) (2) structured Pt nanoparticle within a catalytic reaction is described for the first time in this paper. The Pt loading and dispersion of Pt/TiO2 WGS-catalysts was not found to influence the turnover frequency, which led to the statement that the active species are transferred to or from the support by the Pt-controlled CO adsorption very rapidly.20 An influence of the TiO2 particle size on the catalytic activity of Pt/ TiO2 in the WGS reaction was found by Panagiotopoulou and Kondarides;20 furthermore, the crystal modification of TiO2 did not influence the catalytic activity.44 The enhanced activity of small TiO2 supports was associated with the increased reducibility of very small TiO2 particles.45 Two different TiO2 nanoparticles (P25 and UV 100) of different size were coated by Pt species with a defined similar Pt-ASA, size distribution, and Pt loading within our studies, in order to minimize the influence of very different Pt structures on the catalytic reaction. This similar Pt−support interface was not investigated by the liquid-phase prepared catalysts before. During these catalyst support-based experiments, the influence of the hydroxyl groups on the surface of the support particle falls into the focus of our observations. The hydroxyl groups on the surface of the TiO2 support are important during the catalytic
requires only natural gas and water. Because of a decentralized and easily available natural gas supply, the difficult development of functional, secure, and space-saving hydrogen storage can be avoided. In the past decade many researchers focused on the development of WGS reactors and heterogeneous catalysts for those. Typical commercially used WGS catalysts are Fe3O4/ Cr2O3 (high-temperature shift, 350−450 °C) and Cu/Zn/Alcatalyst (low-temperature shift,
