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

The literature treating mechanisms of catalyst deactivation is reviewed. Intrinsic mechanisms of catalyst deactivation are many; nevertheless, they can be classified into six distinct types: (i) poisoning, (ii) fouling, (iii) thermal degradation, (iv) vapor compound formation accompanied by transport, (v) vapor-solid and/or solid-solid reactions, and (vi) attrition/crushing. As (i), (iv), and (v) are chemical in nature and (ii) and (v) are mechanical, the causes of deactivation are basically three-fold: chemical, mechanical and thermal. Each of these six mechanisms is defined and its features are illustrated by data and examples from the literature. The status of knowledge and needs for further work are also summarized for each type of deactivation mechanism. The development during the past two decades of more sophisticated surface spectroscopies and powerful computer technologies provides opportunities for obtaining substantially better understanding of deactivation mechanisms and building this understanding into comprehensive mathematical models that will enable more effective design and optimization of processes involving deactivating catalysts. © 2001 Elsevier Science B.V. All rights reserved.

/pdf/mechanisms-of-catalyst-deactivation-4zz8v6wnta.pdf

Mechanisms of catalyst deactivation

Dry (CO2) reforming of methane (DRM) is a well-studied reaction that is of both scientific and industrial importance. This reaction produces syngas that can be used to produce a wide range of products, such as higher alkanes and oxygenates by means of Fischer–Tropsch synthesis. DRM is inevitably accompanied by deactivation due to carbon deposition. DRM is also a highly endothermic reaction and requires operating temperatures of 800–1000 °C to attain high equilibrium conversion of CH4 and CO2 to H2 and CO and to minimize the thermodynamic driving force for carbon deposition. The most widely used catalysts for DRM are based on Ni. However, many of these catalysts undergo severe deactivation due to carbon deposition. Noble metals have also been studied and are typically found to be much more resistant to carbon deposition than Ni catalysts, but are generally uneconomical. Noble metals can also be used to promote the Ni catalysts in order to increase their resistance to deactivation. In order to design catalysts that minimize deactivation, it is necessary to understand the elementary steps involved in the activation and conversion of CH4 and CO2. This review will cover DRM literature for catalysts based on Rh, Ru, Pt, and Pd metals. This includes the effect of these noble metals on the kinetics, mechanism and deactivation of these catalysts.

A review of dry (CO2) reforming of methane over noble metal catalysts

CO2 conversion covers a wide range of possible application areas from fuels to bulk and commodity chemicals and even to specialty products with biological activity such as pharmaceuticals. In the present review, we discuss selected examples in these areas in a combined analysis of the state-of-the-art of synthetic methodologies and processes with their life cycle assessment. Thereby, we attempted to assess the potential to reduce the environmental footprint in these application fields relative to the current petrochemical value chain. This analysis and discussion differs significantly from a viewpoint on CO2 utilization as a measure for global CO2 mitigation. Whereas the latter focuses on reducing the end-of-pipe problem “CO2 emissions” from todays’ industries, the approach taken here tries to identify opportunities by exploiting a novel feedstock that avoids the utilization of fossil resource in transition toward more sustainable future production. Thus, the motivation to develop CO2-based chemistry does...

Sustainable Conversion of Carbon Dioxide: An Integrated Review of Catalysis and Life Cycle Assessment

Although technological practice should minimize environmental impact, this is not always economically feasible. During the past decade, for example, there has been increasing global concern over th...

CO2 Reforming of CH4

A comprehensive 10-step kinetic model of the oxidative coupling of methane to C2+ hydrocarbons over a La2O3/CaO catalyst was developed on the basis of kinetic measurements in a microcatalytic fixed-bed reactor covering a wide range of reaction conditions (1 < pO2 < 20 kPa, 10 < pCH4 < 95 kPa, 700 < T < 955 °C, 0.76 mCat/VSTP ≤ 250 kg·s/m3). The reaction scheme contains three primary and seven consecutive steps. The conversion of hydrocarbons and of carbon monoxide with oxygen were described by applying Hougen−Watson type rate equations. For the other reactions power-law rate equations were used. From the experimental data kinetic parameters, i.e. frequency factors, apparent activation energies, and adsorption enthalpies, were estimated. With the kinetic model the experimentally determined conversions of methane and oxygen, as well as yields to C2 hydrocarbons and carbon oxides, could be predicted with an average accuracy of ±20%.

Comprehensive Kinetics of Oxidative Coupling of Methane over the La2O3/CaO Catalyst

The reaction of methane with surface oxygen as well as the interaction of methane/oxygen mixtures with a Rh(1 wt%)/γ-Al2O3 catalyst was studied by applying the temporal-analysisof-product (TAP) reactor. The product distribution was strongly affected by the degree of surface reduction. CO2 is formed as a primary product via a redox mechanism with the participation of surface oxygen. The dehydrogenation of methane yielding carbon deposits on the surface occurs on reduced surface sites. The formation of CO proceeds with high selectivity (up to 96%)at 1013 K via fast reaction of surface carbon species with CO2.

Rhodium-catalyzed partial oxidation of methane to CO and H2. Transient studies on its mechanism

Oxidative coupling of methane in the gas phase. Kinetic simulation and experimental verification

The oxidative dehydrogenation of ethane in ethene has been investigated in the 753–863 K temperature range over V 2 O 5 /SiO 2 catalysts prepared by grafting or wet impregnation methods. The acidic features of fresh and used catalysts were studied by ammonia and pyridine adsorption using microcalorimetry and DRIFT spectroscopy. V 2 O 5 /SiO 2 catalysts can be regarded as medium acidic materials. At the same conversion level of ethane, selectivity to ethene was favored by low V 2 O 5 loadings. Ethane conversion increased with the vanadium content and the ammonia uptake of the V 2 O 5 /SiO 2 catalysts. Some Bronsted-type sites disappeared in the course of the catalytic reaction, leading to minor changes in the acidic character of the catalysts.

Manfred Baerns

Papers

Comprehensive Kinetics of Oxidative Coupling of Methane over the La2O3/CaO Catalyst

Rhodium-catalyzed partial oxidation of methane to CO and H2. Transient studies on its mechanism

Oxidative coupling of methane in the gas phase. Kinetic simulation and experimental verification

Role of surface acidity on vanadia/silica catalysts used in the oxidative dehydrogenation of ethane

Infrared Spectroscopic Studies of CO Adsorption on Rhodium Supported by SiO2, Al2O3, and TiO2