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عنوان فارسی مقاله:
افزایش کاتالیزور فلزی درجه حرارت بالا: تیتانات آلومینیوم در سیستم های نیکل زیرکونیا
عنوان انگلیسی مقاله:
Enhancement of high temperature metallic catalysts: Aluminum titanate in the nickel-zirconia system
سال انتشار : 2016
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مقدمه انگلیسی مقاله:
1. Introduction
Stability of catalyst microstructure, particularly for hightemperature metallic catalysts, is an important issue in a multitude of applications, particularly where nano-scale, high surface area materials are utilized. The attrition of precious metals from automotive catalytic converters has been sufficient to motivate studies that investigate the environmental impacts or harvesting of platinum group metals from roadside soils [1–4]. In another application, microstructure instability is a defining issue in implementing infiltration based anodes in Solid Oxide Fuel Cells (SOFC). The general consequences ofmetal catalyst coarsening are reflected by the need for addition of excess catalyst during device fabrication and in decreased device efficacy as fine catalyst microstructure coarsens. In this work, improvements to the microstructural stability of high-temperature metallic catalysts are explored using SOFC anodes as a model system in which catalyst percolation adds a primary constraint on system function. Specifically, this work uses the nickel metal/yttria-stabilized zirconia (YSZ) composition. Traditionally, SOFC anodes are made of a ceramic/metal (cermet) composite material consisting of a nickel metal catalyst and yttria-stabilized zirconia (YSZ). Nickel provides an inexpensive catalyst when compared to less abundant materials in the platinum metals group. Ni retains high catalytic activity at the elevated operating temperatures of SOFCs, and over years of development has been established as the standard material for benchmarking performance [5–7]. Fabricating Ni-based cermet electrodes usually involves mixing nickel oxide, YSZ, and a pyrolizable thermal fugitive to achieve an equal volume mixture. This distribution of materials satisfies the percolation limit (3-D connectivity) allowing for simultaneous gas flow and electron flow through the anode, with ion transport extended farther into the anode than the planar anode-electrolyte interface. While this method is convenient for commercial production, it has several shortcomings. The high nickel concentration of over 33 vol% limits the mechanical strength of the YSZ and shifts the coefficient of thermal expansion for the anode to approximately 13.4 ppm/◦C, much higher than that of the YSZ electrolyte (∼10.8 ppm/◦C). Mechanical stresses induced by thermal cycling can be detrimental to a cell/stack stability and cause catastrophic failures [8]. From a weight perspective, the density of nickel, 8.9 g/cc, is substantially higher than that of YSZ at 5.9 g/cc. The high solids loading of nickel therefore can also result in increased system mass which can be critically important in mobile applications. This consideration is compounded by a percentage of the nickel existing within the bulk of the cermet, not electroni cally connected to the anode structure and not contributing to the electrochemically active three phase boundary. A different approach to anode fabrication is to introduce the nickel electrocatalyst by infiltration methods. Instead of being a simple mixture of sintered Ni and YSZ particles, an infiltrated electrode starts with a YSZ scaffold rendered porous by one of several available methods. These methods include pyrolyzable thermal fugitives, freeze tape casting which leaves porosity in a green state, or a chemical leaching method [9–11]. The sintered, porous YSZ is then infiltrated with a solution-based nickel precursor such as nickel nitrate. Similar methods have also been established for infiltrationof commonlanthanum-based perovskite cathodes [8,12–15]. Use of infiltration for both the fuel and air electrodes lends itself to a symmetrical cell architecture where porous scaffolds can be applied to both sides of the electrolyte resulting in a design that is scalable both in size and in quantity [16,17]. Depositing catalysts on a pre-existing scaffold via infiltration significantly reduces the amount of catalyst material required. Given an appropriate scaffold geometry, as little as 15 vol% of expensive [18], high mass density catalysts could be used to produce fuel cell electrodes with excellent thermal expansion coefficient matching. Cells produced in this manner have already shown excellent redox stability while retaining high catalytic activity [12,19–21]. Because the scaffold and catalyst are processed in discreetly different steps means that catalysts previously precluded due to high temperature reactivity with electrolyte materials can be considered as viable options for development. For example, lanthanum strontium manganite (LSM) reacts with YSZ at temperatures above 1200◦ C to form La2Zr2O7, an ionic insulator [22]. This undesired outcome constrains how SOFC membrane electrode assemblies are fabricated, given that LSM is often used as an SOFC cathode and YSZ is one of the most commonly used SOFC electrolytes. Moreover, material combinations for low temperature-SOFCs that were previously considered impractical due to reactivity between phases at processing temperatures also become feasible with infiltration [23]. Infiltrated cells localize catalysts where they are most needed for activity and catalyst particles formed from infiltration tend to be much smaller than those used in mechanically mixed electrodes. Furthermore, smaller catalysts havepowerdensities thatfar surpass theperformance oftraditional SOFC cermet electrodes [17,19,24–26]. In high temperature applications, the advantages of electrode fabrication by infiltration have not been fully realized. This is due largely to the fact that very fine nickel metal introduced by infiltration at particle diameters < 100 nanometers migrates and coarsens substantially, degrading cell performance at rates much greater than traditional anodes. At SOFC operational temperatures (700 ◦C to 1000 ◦C) small nickel catalysts have been shown to coarsen and agglomerate into larger particles that break up the original nickel network. This behavior leads to two important problems; first, the larger particles are less catalytically active than the smaller, original network of particles due to the substantial decrease in surface area (reduced triple phase boundary length). Second, the coarsening of nickel causes voids and separation in the nickel network, breaking down the electron path for current collection and hindering overall performance of the anode [25–29]. Recent reports have shown that adding a minor amount of secondary phase additive, such as aluminum titanate (Al2TiO5 or ALT), improves the performance of Ni-infiltrated SOFC cermet anodes [30]. The proposed mechanism of improved anode stability cited chemical interactions between the catalyst and substrate in which portions of the nickel network that had previously necked down to a discontinuous path were instead stabilized at 800 ◦C [30,31]. This work suggested that high temperature reaction of the Ni-YSZ-ALT system led to the formation of additional oxide phases, NiAl2O4 and ZrTiO4, within the anode; however, the spatial distribution and phase evolution of the secondary phases remained uncharacterized. Furthermore, NiAl2O4 and ZrTiO4 are not expected to exhibit electro-catalytic behavior to facilitate electrochemicalfunction despite the loss of primary phases [32,33]. If these secondary phases stabilize small particles at high temperatures, these materials can be strategically added to stabilize nanoscale supported catalysts. This approach may be applied to several industries where migration of a catalyst or metal phases on a ceramic oxide surface has been shown to lead to failures such as automotive catalytic converters [34], methane reforming systems [35], and multilayer capacitors [36], as well as SOFCs [7,28,29,37]. In order to explore the electro-catalyst enhancement of ALT additions in traditional SOFC anodes, experiments described below identify the reaction pathways that form multiple secondary phases in the nickel/zirconia system. Phase formation as a function oftemperature during standard anode processing temperature regimes was evaluated using complementary XRD and Raman spectroscopy data. The spatial distribution and morphology of the secondary phases with regard to the nickel/zirconia interfacial region were assessed with FIB/TEM analysis and the mitigation of nickel coarsening/spallation identified by SEM analysis of posttested nickel electro-catalysts. Voltammetry and Electrochemical Impedance Spectroscopy (EIS) measurements were performed to examine how the dopant influences degradation in the doped and undoped systems. The merits of using SOFC anodes as a model catalyst/support system to study these enhancement mechanism(s) are bolstered by the attributes of a truly high temperature system (700–1000 ◦C) combined with the added rigor of requiring catalyst percolation for electronic conductivity.
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کلمات کلیدی:
A Sacrificial Coating Strategy Toward Enhancement of Metal-Support ... pubs.acs.org/doi/abs/10.1021/jacs.6b10472 by W Zhan - 2016 - Cited by 3 - Related articles Nov 22, 2016 - Upon annealing at high temperature under an inert atmosphere, the ... be extended to the stabilization of other supported catalysts on a broad ... X-ray Absorption Fine Structure for Catalysts and Surfaces https://books.google.com/books?isbn=9810223234 Yasuhiro Iwasawa - 1996 - Science Debye temperature was due to the larger contribution of surface atoms which have ... TiOx species migrates onto the Rh particles after high temperature reduction. ... enhancement phenomenon of SiO2 -supported noble metal catalysts (Rh, Pt, ... Understanding Carbon Nanotubes: From Basics to Applications https://books.google.com/books?isbn=3540269223 Annick Loiseau, Pascale Launois-Bernede, Pierre Petit - 2006 - Science Such configurations are not found for high temperature routes, since actual ... of the enhancement of the chemical activity due to the presence of metal catalyst ... A Sacrificial Coating Strategy Toward Enhancement of Metal-Support ... pubs.acs.org/doi/pdf/10.1021/jacs.6b10472 by W Zhan - 2016 - Cited by 3 - Related articles discrete metal nanoparticles are made resistant to sintering, preserving their catalytic activities in high-temperature oxi- dation processes. Taking advantage of ... [PDF]Enhancement of the catalytic performance of supported-metal ... citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.514.3222&rep=rep1... by Z Qu - 2005 - Cited by 61 - Related articles Supported-metal catalysts are widely used in the chemi- cal industry and environmental .... higher temperatures dramatically decreases the surface area. Fig. 2. Transition Metal Catalyzed Polymerizations: Ziegler-Natta and ... https://books.google.com/books?isbn=0521332893 Roderic P. Quirk, Raymond E. Hoff, University of Akron. Institute of Polymer Science - 1988 - Science ... Natta catalyst capabilities functioning at high temperature and high pressure ... various treatments were identified which resulted in enhancement of catalyst ...
