1. General Introduction
1.3. Catalytic Properties of Ceria
1.3.4. Catalytic Applications
Ceria is used alone or as a support and is widely used as a catalyst for various reactions, of which we would like to introduce some typical reactions and suggest the possibility of using ceria as a catalyst.
The most widely known application of ceria is the automotive catalytic converter (TWCs). Catalytic converters promote the reduction of CO, hydrocarbons, and nitrogen oxides (NOx) in automobile exhaust gases. In general, the three-way catalyst consists of an Al2O3 support, a CeO2 based promoter, and a noble metal (Pt or Rh) as an active site.98 The air to fuel ratio (A/F) has a great influence on the conversion efficiency of the exhaust gas and can effectively remove all pollutants at a stoichiometric value of 14.6.99 This is accomplished by controlling the flow of air and fuel, and the conversion of pollutants changes rapidly depending on the A/F ratio. The main role of ceria in TWCs is an oxygen buffer and can provide the oxygen needed for CO and hydrocarbon oxidation. Nowadays, OSC- enhanced CeO2-ZrO2 catalysts are in use.
Steam methane reforming (SMR) is the industry's most widely known method for producing hydrogen. Compared to other reforming processes, the theoretical H2/CO ratio is higher, but it is an endothermic reaction, so it must be carried out at high temperatures.100
CH H O ↔CO 3H ∆H 206 kJ mol SRM
The ceria-zirconia mixed oxide is the most studied system due to its excellent redox properties.
CeO2-ZrO2 oxide acts as active support for Group 8, 9 and 10 metal NPs. Kusakabe et al. reported that the SMR performance for Pt, Rh, Ru, Ni catalysts of a specific ceria-zirconia composition (CexZr1-xO2, 0.5 < x < 1) is high at relatively low temperatures (500 ~ 600 ℃).101 The effect of the Ce/Zr ratio was tested on a 10% Ni-supported catalyst. As a result, the highest SMR performance was shown in the Ni/Ce0.8Zr0.2O2 catalyst, which is related to thermal stability, redox capacity, and particle size of Ni.102 Unlike other noble metals, nickel is a metal that has been studied a lot for SMR, where high performance can be expected at a relatively low cost. When ceria is added to the Ni/SiO2/Al2O3 catalyst, carbon deposition is lowered and SMR performance is improved. In addition, when the ceria addition synthesis method was changed, the degree of carbon growth inhibition was different.103 In the case of Ni-based catalysts, the biggest problem is catalyst deactivation due to carbon deposition, such as coke during SMR.104 The Gorte group reported that coking was inhibited in steam reforming of n-butane when ceria was used instead of silica as the support of the Ni catalyst.105 The excellent redox property of ceria accelerates the surface reaction to prevent coke formation.
Dry reforming of methane (DRM) is the reaction of two major greenhouse gases (CH4 and CO2) to CO and H2. Obviously, it is the decomposition of stable chemicals that require a lot of energy.
CH CO ↔2CO 2H ∆H 247 kJ mol for DRM
Nevertheless, it is a catalytic reaction that gains attention due to its excellent potential to solve both environmental and energy resource problems at the same time. Here, Ni catalyst is mainly used in consideration of the price competitiveness of the catalytic reaction, but the role of CeO2 is still important to solving the coking problem mentioned above. The Rodriguez group reported that ceria in Ni/CeO2
catalyst plays a role of strong-metal support interaction on C−H bond breaking in the DRM reaction.106 Ambient pressure XPS results suggested that methane separates from Ni/CeO2 at temperatures as low as 300K, creating CHx and COx species on the catalyst surface. The SMSI played a key role in enhancing the catalytic activity of Ni/CeO2 by methane dissociation and prevent the deposition of carbon and deactivation during the reaction. According to DFT calculations, the methane activation barrier is 0.9 eV for Ni(111) and 0.15 eV for Ni/CeO2-x (Figure 1.15).
Processes such as Fischer-Tropsch synthesis, methanol synthesis or hydroformylation require a well-defined H2/CO/CO2 ratio of the feed gas to function efficiently. Therefore, for the subsequent reaction, the H2/CO ratio of the produced syngas must be adjusted to the desired value. The most promising process for purifying H2 streams from CO is water-gas shift reaction (WGSR).107 WGSR is an exothermic reaction (ΔH298 = −41 kJ mol−1) and the equilibrium constant (maximum CO conversion)
decreases with increasing temperature. Therefore, the CO concentration is determined by thermodynamics, which depends on the temperature at the outlet of the reactor, and a lower temperature is advantageous for high CO conversion. WGSR is divided into low-temperature reaction (LT-WGSR, operating at 200 to 300 °C) and high-temperature reaction (HT-WGSR, operating at 300 to 450 °C) depending on the reaction temperature. Ceria-based catalysts are effective for LT-WGSR due to the excellent inherent properties of ceria. Two mechanisms have been proposed for Pt/CeO2 catalysts under LT-WGSR (Figure 1.16).108 The first is the redox route, where oxygen vacancies are formed by the transfer of reactive oxygen atoms from the support to the metal particles. The active oxygen reacts with the adsorbed CO on the surface of metal particles which producing CO2. Meanwhile, H2 is generated by restoring surface oxide anions from O2 or H2O. Second, the reaction in the formate route involves the formation of an OH group on the Ce ion, which reacts with CO to form formate. The products of H2
and CO2 are generated by the decomposition of the intermediate species of carbonates or formats on the surface of oxides. The role of the metal is to promote the adsorption of CO and the cleavage of the C−H bond of formate. In the two proposed mechanisms, the role of ceria through interactions at the Pt-CeO2
interface is clear.
Figure 1.15. Atomic structures and reaction energy profile for methane decomposition. Reaction energy profile for the CH4 → CH3 + H reaction on Ni(111), Ni2+/CeO2(111), and Ni0/Ce2O3(0001). Reprinted with permission from ref. 106. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Ceria itself and ceria-based materials are efficient catalysts for the catalytic oxidation of volatile organic compounds (VOCs) thanks to their excellent reducibility and OSC.109 In general, VOC involves the organic compounds, such as saturated/unsaturated alkanes or alkenes, aromatic hydrocarbons, and
oxygen- or chlorine-containing compounds with boiling points below 250 °C under ambient pressure.110 The reaction is considered to follow a Mars-van Krevelen type mechanism in which ceria supply oxygen to the reaction and re-oxidize by gaseous oxygen.111 As an example, the superior performance of CeO2
in toluene removal and its role in oxidation reactions have been recently reported.112 Toluene is a solvent commonly used in the chemical industry and makes a serious contribution to the formation of photochemical smog, thus traces of substances must be removed. In the decomposition of aromatic rings, catalytic oxygen vacancies are the step that determines the rate at which toluene is oxidized to CO2. To account for the effects of oxygen vacancies, a different ratio of surface to bulk oxygen vacancies was prepared in the ceria system. In addition to the main role of oxygen vacancies at the surface, some of the vacancies at the bulk promote the redox properties of CeO2 in toluene catalytic combustion. Bulk vacancy improves the mobility and activity of lattice oxygen species through the transfer effect, which is pronounced at high temperature.
Figure 1.16. Redox (up) and formate (down) mechanisms for the water-gas shift reaction (WGSR) on Pt/CeO2 catalysts shown in atomic scale. Reprinted with permission from ref. 108. Copyright © 2015 Elsevier B.V.