Abstract
Background: Ceria (CeO2) is an important catalyst component and support/ carrier and high surface area ceria is extremely useful for increasing catalytic activity in several low temperature applications such as emissions control, water gas shift (WGS), CO oxidation, and VOC combustion/destruction. The highest surface areas reported in the literature using aqueous synthetic routes, in the absence of organic solvents, are in the range of 200–260 m2/g. We report here the preparation of high surface area ceria using three different synthesis methods: 1) dry decomposition of common cerium salts as precursors, 2) precipitation, and 3) soft combustion synthesis.
Methods: The three routes were studied using common and readily available cerium precursors, without using expensive templates, surfactants, alcoholic solvents, supercritical drying, or high pressure equipment. We obtained unprecedented high surface areas of >300m2/g by precipitation, after mapping out vast parameter spaces including Ce salt precursor selection, choice of base, pH, method (precipitation at constant pH or titration (pH ramp)), temperature, and aging conditions. We screened more than half of the periodic table and all the rare earth metals from Pr to Lu.
Results: For dry decomposition, BET surface areas of ~170 m2/g were obtained from Ce (III) acetate and ~135 m2/g from Ce (III) oxalate and Ce (III) carbonate precursors. Using wet combustion synthesis with aqueous glyoxylic acid and ketoglutaric acid as dispersants we measured ~160 m2/g when starting from Ce (III) acetate but lower surface areas of ~120 m2/g from Ce (III) nitrate. An unprecedented surface area of ~300 m2/g was obtained by precipitation of Ce (IV) nitrate with NMe4OH after a multiparameter optimization using a 64-vessel co-precipitation robotic synthesis station. Supported ceria catalysts were prepared by impregnation with active metals and found to be highly active for the low temperature water gas shift reaction, CO oxidation, and VOC combustion.
Conclusion: This work demonstrates the potential to significantly improve conventional inexpensive synthetic routes to high surface area materials with, in many cases, unprecedented high surface areas, by using high throughput mapping of multi-dimensional parameter spaces. It provides practical alternatives to both sol-gel and hydrothermal synthesis methods.
Keywords: Ceria, cerium oxide, rare earth, high surface area, BET, precipitation, combustion synthesis, pechini, water gas shift, VOC removal, CO oxidation, carriers, supports, high throughput, combinatorial.
Graphical Abstract
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