Over the past several years, cerium oxide and CeO2-containing materials have come under intense scrutiny as catalysts and as structural and electronic promoters of heterogeneous catalytic reactions. Recent developments regarding the characterization of ceria and CeO2-containing catalysts are critically reviewed with a special focus towards catalyst interaction with small molecules such as hydrogen, carbon monoxide, oxygen, and nitric oxide. Relevant catalytic and technological applications such as the use of ceria in automotive exhaust emission control and in the formulation of SO x reduction catalysts is described. A survey of the use of CeO2-containing materials as oxidation and reduction catalysts is also presented.

Catalytic Properties of Ceria and CeO2-Containing Materials

Cerium dioxide (CeO2, ceria) is becoming an ubiquitous constituent in catalytic systems for a variety of applications. 2016 sees the 40th anniversary since ceria was first employed by Ford Motor Company as an oxygen storage component in car converters, to become in the years since its inception an irreplaceable component in three-way catalysts (TWCs). Apart from this well-established use, ceria is looming as a catalyst component for a wide range of catalytic applications. For some of these, such as fuel cells, CeO2-based materials have almost reached the market stage, while for some other catalytic reactions, such as reforming processes, photocatalysis, water-gas shift reaction, thermochemical water splitting, and organic reactions, ceria is emerging as a unique material, holding great promise for future market breakthroughs. While much knowledge about the fundamental characteristics of CeO2-based materials has already been acquired, new characterization techniques and powerful theoretical methods are dee...

Fundamentals and Catalytic Applications of CeO2-Based Materials

Use of CeO2-based oxides in the three-way catalysis

Single-crystalline and uniform nanopolyhedra, nanorods, and nanocubes of cubic CeO2 were selectively prepared by a hydrothermal method at temperatures in the range of 100-180 degrees C under different NaOH concentrations, using Ce(NO3)3 as the cerium source. According to high-resolution transmission electron microscopy, they have different exposed crystal planes: {111} and {100} for polyhedra, {110} and {100} for rods, and {100} for cubes. During the synthesis, the formation of hexagonal Ce(OH)3 intermediate species and their transformation into CeO2 at elevated temperature, together with the base concentration, have been demonstrated as the key factors responsible for the shape evolution. Oxygen storage capacity (OSC) measurements at 400 degrees C revealed that the oxygen storage takes place both at the surface and in the bulk for the as-obtained CeO2 nanorods and nanocubes, but is restricted at the surface for the nanopolyhedra just like the bulk one, because the {100}/{110}-dominated surface structures are more reactive for CO oxidation than the {111}-dominated one. This result suggests that high OSC materials might be designed and obtained by shape-selective synthetic strategy.

Shape-selective synthesis and oxygen storage behavior of ceria nanopolyhedra, nanorods, and nanocubes.

Automotive catalytic converters: current status and some perspectives

Nanostructured CeO2–ZrO2 mixed oxides have attracted great interest in the past 10 years, in particular as redox or oxygen storage promoters in the three-way catalysts. However, it has now became clear that both the chemistry of the simple oxides and the complex metal–CeO2-based oxide interactions can play a critical role in developing novel and highly active materials for a number of processes, ranging from catalysts for H2 production from fuels up to ceramic materials and solid state conductors for fuel cells. Despite the apparent simplicity of these systems, extreme variability of their chemical and textural properties has been observed. An attempt is made here to rationalise the various factors and the inter-connections to the structure, the texture, and the effects of the nanometre scale, which all contribute to the properties of these materials.

Nanostructured CeO2–ZrO2 mixed oxides

Heterogeneous environmental catalysis – a gentle art: CeO2–ZrO2 mixed oxides as a case history

Redox Property–Local Structure Relationships in the Rh-Loaded CeO2–ZrO2Mixed Oxides

The role of the so-called oxygen storage and release capacity (OSC) in promoting the activity of noble metals in the three-way catalysts (TWCs) is critically discussed. It is shown that the promoting effects of CeO2 in the TWCs cannot be attributed to a simple redox type of effect according to the reaction CeO2 ↔ CeO2 − x + x/2O2, since multiple and sometimes intriguing effects of CeO2 promoter have been observed, particularly in the latest generation of TWCs containing modified CeO2–ZrO2 mixed oxides.

On the Role of Oxygen Storage in Three-Way Catalysis

The effects of ZrO2 content on the CO oxidation activity in a series of CuOx/CexZr1−xO2 (x = 0, 0.15, 0.5, 0.7 and 1) catalysts were investigated, both in the absence and in the presence of H2, i.e. preferential CO oxidation—PROX. The investigation was performed under light-off conditions to focus the effects of transients and shut-down/start-up cycles on the performance; such phenomena are expected to affect the activity of PROX catalysts in small/delocalised fuel reformers. Evidence has been obtained for a transition from an “oxidized” towards a “reduced” state of the catalyst under the simulated PROX reaction conditions as a function of the reaction temperature, leading to different active species under the reaction conditions. Both CO oxidation activity and PROX selectivity appear to be affected by this process. IR characterisation of the surface copper species suggests an important role of reduced cerium sites in close contact with copper clusters on the CO oxidation activity at low temperatures.

Roberta Di Monte

Papers

Nanostructured CeO2–ZrO2 mixed oxides

Heterogeneous environmental catalysis – a gentle art: CeO2–ZrO2 mixed oxides as a case history

Redox Property–Local Structure Relationships in the Rh-Loaded CeO2–ZrO2Mixed Oxides

On the Role of Oxygen Storage in Three-Way Catalysis

CO oxidation over CuOx-CeO2-ZrO2 catalysts: Transient behaviour and role of copper clusters in contact with ceria