Physicochemical and Electrical Properties of Praseodymium Oxides

SAGE-Hindawi Access to Research International Journal of Electrochemistry Volume 2011, Article ID 561204, 7 pages doi:10.4061/2011/561204 Review Article Physicochemical and Electrical Properties of Praseodymium Oxides Sergio Ferro Department of Biology and Evolution, University of Ferrara, Via L. Borsari 46, 44121 Ferrara, Italy Correspondence should be addressed to Sergio Ferro, Received 8 November 2010; Revised 28 February 2011; Accepted 21 March 2011 Academic Editor: Benjamı́n R. Scharifker Copyright © 2011 Sergio Ferro. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The industrial research is continuously looking for novelties that could improve the applied processes, increasing the yields, lowering the costs, or improving the performances. In industrial electrochemistry, one more aspect is the stability of electrode materials, which is generally balanced by the catalytic activity: the higher the latter, the lower the former. A compromise has to be found, and an optimization is often the result of new ideas that completely change the way of thinking. Praseodymium-oxidebased cathodes have been proved to be quite interesting devices: the hydrogen evolution reaction is guaranteed by the presence of a noble metal (platinum and/or rhodium), while the stability and poisoning resistance seem to be strongly improved by the presence of lanthanide oxides. 1. Introduction The reduction of costs, in terms of energy consumption and loading of precious metals for catalysts, is the main reason for the continuous research of more performing electrode materials. While the insoluble anodes adopted in the chlor-alkali industry have been investigated for some decades (starting from the pioneering works by Henry Beer in the seventies), the optimization of cathode performances is a recent request, and the research is still ongoing on this subject. At the moment, the membrane technology makes use of low-carbon steel cathodes, but both their relatively low exchange current density (high overpotential, i.e., from 300 to 400 mV) toward the hydrogen evolution reaction (HER) and their susceptibility to corrosion under the increasingly hard operating conditions (request for more concentrated sodium hydroxide solution production) have prompted for new materials. A significant reduction (from 100 to 200 mV) of HER overpotential has been obtained substituting the steel cathodes with nickel-based devices, possibly provided with catalytic coatings. However, the durability of such electrodes is limited, as the coating tends to dissolve, while impurities (mainly iron) are deposited on the electrode surface, reducing the catalytic activity. In the last years, the industrial research has started investigating the behavior of devices provided with coatings in which a noble-metal catalyst is mixed with one or more oxides of lanthanide elements. A mixed platinum-cerium oxide coating has been claimed by Permelec Electrode Ltd. [1]: it is assumed that the cerium component has a crucial role in maintaining the catalytic activity of the noble metal, impeding the electrochemical deposition of iron; at the same time, the cerium oxide shows a good stability in concentrated alkali solutions. Further stability improvements have been obtained with a catalytic layer containing at least three components, that is platinum, cerium, and lanthanum, in the form of metal, metal oxide or hydroxide [2]. From the examples discussed in the patent application, the ternary mixture shows performances that are not obtainable if one of the components is lacking (either a deterioration due to iron deposition or a high overvoltage is found). A somewhat similar coating has been recently claimed by Industrie De Nora S.p.A.: the nickel substrate is provided with a first protective layer comprising palladium (as such or in mixture with silver), and a second, external layer containing a mixture of platinum or ruthenium with chromium or praseodymium oxide [3]. The first layer would 2 International Journal of Electrochemistry Table 1: Stoichiometry of the known members of the homologous series Prn O2n−2 . Reproduced from J. Solid State Electrochem., 2001, 5, 531. Copyright 2001, Springer-Verlag [4]. 20 40 α 800 ι 600 ζ 400 The oxides of praseodymium represent a system of phases whose composition is variable, although being restricted to a well defined range, and whose structure may be extensively defective. In addition to Pr2 O3 and PrO2 , at least other five suboxide phases have to be taken into consideration, and they represent a series of homologues having the Prn O2n−2 general composition (see Table 1) [4, 5]. Among the different phases, Pr6 O11 is the stable one at room temperature is in air; also, it worth mentioning that the Pr6 O11 and PrO1.833 terminologies are completely equivalent (the former has not any special meaning). Phases can be quite easily interconverted by means of a simple heating (or cooling) and suitably adjusting the oxygen content in the atmosphere in contact with the solid, as it can be inferred from Figures 1 and 2. In addition, some scientific papers deal with the existence of intermediate phases (PrOx ), in which x would assume the following values: 1.585, 1.658, and 1.703 [7, 8]; since the latter papers are quite aged, it is reasonable that some mistakes could have been done in the evaluation of data, and the examined phases could have been mixtures in which one phase has not completely converted into another. In any case, the different phases differ for the dimension of the lattice parameters (see Figure 3) and, in some cases, also for the crystalline lattice and the color of the powder [5]: the β and ε phases have a face-centered cubic lattice (fcc), with lattice constants of 5.468 and 5.481 Å, respectively; the former phase is black-colored, while the latter is brown. β ε δ β 1.714 ι 1.5 1.6 1.818 1.7781.8 δ 1.833 ζ ε β 1.7 1.8 Composition (x in PrOx ) 1.9 2 Figure 1: Projection of the phase diagram of the praseodymiumoxygen system on the temperature-composition plane. Reproduced from Ionics, 1999, 5, 426. Copyright 1999, Springer-Verlag [6]. 1200 T (◦ C) 2. The Praseodymium Oxides 100 80 σ 1000 200 act as a sponge, absorbing hydrogen during the normal operation conditions and releasing it during the inversion events (accidental malfunctioning of the electrolyzer), thus preventing the cathode potential to be shifted to values high enough to give rise to significant dissolution phenomena. On the other hand, the presence of Cr or Pr oxides in the catalytic layer would preserve the catalyst activity, destroying the possible iron-based impurities while contributing to the film stability. On the basis of the above information, the present paper aims at reviewing the role of praseodymium, while representing a possible (...truncated)