Catalytic Stability Studies Employing Dedicated Model Catalysts

Accounts of Chemical Research, vol. 53, pp. 380–389

Abstract

Conspectus Long-term stability of heterogeneous catalysts is an omnipresent and pressing concern in industrial processes. Catalysts with high activity and selectivity can be searched for by high-throughput screening methods based maybe on educated guesses provided by ab initio thermodynamics or scaling relations. However, high-throughput screening is not feasible and is hardly able to identify long-term stable catalyst so that a rational and knowledge-driven approach is called for to identify potentially stable and active catalysts. Unfortunately, our current microscopic understanding on stability issues is quite poor. We propose that this gap in knowledge can be at least partly closed by investigating dedicated model catalyst materials with well-defined morphology that allow for a tight link to theory and the application of standard characterization methods. This topic is highly interdisciplinary, combining sophisticated inorganic synthesis with catalysis research, surface chemistry, and powerful theoretical modeling. In this Account, we focus on the stability issues of Deacon catalysts (RuO 2 and CeO 2 -based materials) for recovering Cl 2 from HCl by aerobic oxidation and how to deepen our microscopic insight into the underlying processes. The main stability problems under harsh Deacon reaction conditions concomitant with a substantial loss in activity arise from deep chlorination of the catalyst, leaching of volatile chlorides and oxychlorides, and decrease in active surface area by particle sintering. In general, powder materials with undefined particle shape are not well suited for examining catalyst stability, because changes in the morphology are difficult to recognize, for instance, by electron microscopy. Rather, we focus here on model materials with well-defined starting morphologies, including electrospun nanofibers, shape-controlled nanoparticles, and well-defined ultrathin crystalline layers. CeO 2 is able to stabilize shape-controlled particles, exposing a single facet orientation so that comparing activity and stability studies can reveal structure sensitive properties. We develop a quasi-steady-state kinetic approach that allows us to model the catalyst chlorination as a function of temperature and gas feed composition. For the case of pure CeO 2 nanocubes, this simple approach predicts chlorination to be efficiently suppressed by addition of little amounts of water in the reaction feed or by keeping the catalyst at higher temperature. Both process parameters have great impact on the actual reactor design. Thermal stabilization of CeO 2 by intermixing Zr has been known in automotive exhaust catalysis for decades, but this does not necessarily imply also chemical stabilization of CeO 2 against bulk-chlorination since Zr can readily form volatile ZrCl 4 and may quickly lose its stabilizing effect. Nevertheless, with model experiments the stabilizing effect of Zr in the Deacon process over mixed Ce x Zr 1– x O 2 nanorods is clearly evidenced. Even higher stability can be accomplished with ultrathin CeO 2 coatings on preformed ZrO 2 particles, demonstrating the great promise of atomic layer deposition (ALD) in catalysis synthesis.

Authors 3

1. Franziska Heß Aachen Institute of Physical Chemistry

   RWTH Aachen University · Massachusetts Institute of Technology · Justus-Liebig-Universität Gießen

   Affiliation as printed

   Institute of Physical Chemistry, RWTH Aachen, Landoltweg 2, 52074 Aachen, Germany

   Laboratory of Electrochemical Interfaces, Department of Nuclear Science & Engineering, MIT, 77 Massachuetts Avenue, 13-3034, Cambridge, Massachusetts 02139, United States

   Physikalisch-Chemisches Institut, Justus Liebig University, Heinrich-Buff-Ring 17, 35392 Giessen, Germany

2. Bernd M. Smarsly corresponding

   Justus-Liebig-Universität Gießen

   Affiliation as printed

   Physikalisch-Chemisches Institut, Justus Liebig University, Heinrich-Buff-Ring 17, 35392 Giessen, Germany

3. Herbert Over corresponding

   Justus-Liebig-Universität Gießen

   Affiliation as printed

   Physikalisch-Chemisches Institut, Justus Liebig University, Heinrich-Buff-Ring 17, 35392 Giessen, Germany