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

Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal–support interaction, and metal–reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results o...

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles.

Advanced oxidation processes (AOP) for water purification and recovery

Since the time of the industrial revolution, the atmospheric CO(2) concentration has risen by nearly 35 % to its current level of 383 ppm. The increased carbon dioxide concentration in the atmosphere has been suggested to be a leading contributor to global climate change. To slow the increase, reductions in anthropogenic CO(2) emissions are necessary. Large emission point sources, such as fossil-fuel-based power generation facilities, are the first targets for these reductions. A benchmark, mature technology for the separation of dilute CO(2) from gas streams is via absorption with aqueous amines. However, the use of solid adsorbents is now being widely considered as an alternative, potentially less-energy-intensive separation technology. This Review describes the CO(2) adsorption behavior of several different classes of solid carbon dioxide adsorbents, including zeolites, activated carbons, calcium oxides, hydrotalcites, organic-inorganic hybrids, and metal-organic frameworks. These adsorbents are evaluated in terms of their equilibrium CO(2) capacities as well as other important parameters such as adsorption-desorption kinetics, operating windows, stability, and regenerability. The scope of currently available CO(2) adsorbents and their critical properties that will ultimately affect their incorporation into large-scale separation processes is presented.

Adsorbent Materials for Carbon Dioxide Capture from Large Anthropogenic Point Sources

Carbon capture and storage (CCS) is broadly recognised as having the potential to play a key role in meeting climate change targets, delivering low carbon heat and power, decarbonising industry and, more recently, its ability to facilitate the net removal of CO2 from the atmosphere. However, despite this broad consensus and its technical maturity, CCS has not yet been deployed on a scale commensurate with the ambitions articulated a decade ago. Thus, in this paper we review the current state-of-the-art of CO2 capture, transport, utilisation and storage from a multi-scale perspective, moving from the global to molecular scales. In light of the COP21 commitments to limit warming to less than 2 °C, we extend the remit of this study to include the key negative emissions technologies (NETs) of bioenergy with CCS (BECCS), and direct air capture (DAC). Cognisant of the non-technical barriers to deploying CCS, we reflect on recent experience from the UK's CCS commercialisation programme and consider the commercial and political barriers to the large-scale deployment of CCS. In all areas, we focus on identifying and clearly articulating the key research challenges that could usefully be addressed in the coming decade.

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Carbon capture and storage (CCS): the way forward

Mass transfer coefficient ( k L a ) and Power Number ( N P ) have been measured in three flat-bottomed cylindrical acrylic tanks of 0.5, 1.0 and 1.5 m diameter. Three different impeller designs were employed [ viz . pitched blade upflow turbine (PBTU), pitched blade downflow turbine (PBTD) and disc turbine (DT)]. Measurements have been made at two submergence levels of 0.23 D and 0.3 D . The power consumption per unit volume ( P/V ) was varied in the range of 10–250 W m −3 which is normally employed in surface aeration applications. It is observed that at given submergence the power number is independent of the off bottom clearance and the power number changes with submergence. In case of PBTD and DT the dependence is higher as compared to PBTU. It has also been observed that the gas hold-up distribution significantly changes with the impeller design, submergence and the rotation speed. Gamma ray attenuation technique has been employed for the measurement of gas hold-up distribution for all the three impellers and its influence on the power number behaviour has also been discussed. In addition, the axial velocity measurements have been carried out for all the three types of impellers under surface aeration conditions for 0.5 m diameter tank with Ultrasound Doppler velocimeter. The validation of the measurement technique has also been done with the help of Laser Doppler Anemometer (LDA) measurements as well as with the CFD code. The velocity profile depends on the inherent gas hold-up in the impeller region, which in turn is governed by the impeller design, submergence and rotational speed. An attempt has also been made to shed some light on the interdependence of mass transfer coefficient, power number, gas hold-up and flow pattern. Finally the correlations have been proposed for the estimation of power number and mass transfer coefficient independently for each type of impeller, which are expected to be useful to practicing engineers.

Surface aerators: power number, mass transfer coefficient, gas hold up profiles and flow patterns

Regeneration of deoiled spent earth by wet oxidation has been investigated. Molecular oxygen was used as an oxidizing agent. Effects of operating parameters such as speed of agitation, temperature, oxygen partial pressure, slurry concentration, and cycle of regeneration have been studied. Kinetics of regeneration was found to be first order with substrate concentration and also with dissolved oxygen concentration. Decolorization capacity of regenerated earth was the same as that of its virgin bleaching earth. Adsorption isotherm data was fitted in Freundlich equation. The values of Freundlich constants of regenerated earth were comparable with the values of virgin bleaching earth.

Regeneration of spent earth by wet oxidation

Weeping through perforated plates occurs when the pressure drop of the gas/vapor passing through it is insufficient to support the liquid. The critical weep velocities (VO, crit) were measured for the plates having hole diameter range of 1 to 6mm. The thickness to hole diameter ratio (t/dO ) and the pitch to hole diameter ratio (P/dO) were varied in the range of 1 to 6 and 3.9 to 25.8, respectively. It was found that the VO, crit, crit is a strong function of plate parameters and the clear liquid height. A mathematical model for "No weep" condition was developed under the following two extreme conditions: (i) holes are partially open, (ii) holes are completely active. The results obtained from the experimentation and the mathematical correlation were put into a coherent form. The weeping criterion in terms of the Froude number was correlated with the plate parameters and the clear liquid height.

Design of Sieve Plate Spargers for Bubble Columns: Role of Weeping

Deactivation of lipase at gas–liquid interface in stirred vessel

Continuous counter-current column operation provides operating convenience for contacting two-phase aqueous partitioning systems for protein extraction. We discuss in detail the important parameters for designing spray, packed, plate and York-Scheibel columns for protein recovery using both polymer-polymer and polymer-salt two-phase aqueous systems. We compare the various contractors for their operating and extraction efficiency. The work also provides a step-by-step design procedure and specific recommendations for future data needs.

Jyeshtharaj B. Joshi

Papers

Surface aerators: power number, mass transfer coefficient, gas hold up profiles and flow patterns

Regeneration of spent earth by wet oxidation

Design of Sieve Plate Spargers for Bubble Columns: Role of Weeping

Deactivation of lipase at gas–liquid interface in stirred vessel

Continuous counter-current two-phase aqueous extraction.