Heterogeneous Single-Atom Catalysts for Electrochemical CO2 Reduction Reaction

The electrochemical CO₂ reduction reaction (CO₂RR) is of great importance to tackle the rising CO₂ concentration in the atmosphere. The CO₂RR can be driven by renewable energy sources, producing precious chemicals and fuels, with the implementation of this process largely relying on the development of low-cost and efficient electrocatalysts. Recently, a range of heterogeneous and potentially low-cost single-atom catalysts (SACs) containing non-precious metals coordinated to earth-abundant elements have emerged as promising candidates for the CO₂RR. Unfortunately, the real catalytically active centers and the key factors that govern the catalytic performance of these SACs remain ambiguous. Here, this ambiguity is addressed by developing a fundamental understanding of the CO₂RR-to-CO process on SACs, as CO accounts for the major product from CO₂RR on SACs. The reaction mechanism, the rate-determining steps, and the key factors that control the activity and selectivity are analyzed from both experimental and theoretical studies. Then, the synthesis, characterization, and the CO₂RR performance of SACs are discussed. Finally, the challenges and future pathways are highlighted in the hope of guiding the design of the SACs to promote and understand the CO₂RR on SACs.     

On the performance of different control charting rules

In the literature a number of control charting rules are proposed to decide whether a process is in control or out of control. Some issues with these rules will be highlighted in this article. By redefining and listing a set of rules we will evaluate their performance on the , R, S and S² charts. Also we will compare the performance of these rules using their power curves to figure out the superior ones. Application of a few of these rules with real data sets will show their detection ability and use for practitioners. Copyright © 2011 John Wiley & Sons, Ltd.     

A maturity approach to estimate compressive strength development of CO2-cured concrete blocks

An alternative CO₂ curing method for precast concrete products has been proposed in order to achieve rapid strength development at early age, as well as to capture and store greenhouse gas (CO₂). In this paper, an experimental study for the development of a maturity approach is presented to estimate the strength development of carbonated concrete blocks. In order to promote the use of industrial flue gas containing CO₂, a flow-through CO₂ curing regime at ambient pressure and temperature was employed using different atmospheric conditions, such as various CO₂ concentrations, RH values and gas flow rates. The experimental results showed that the compressive strength or maturity of the carbonated concrete blocks was affected by two factors: accelerated cement hydration and carbonation extent. A high CO₂ concentration, a fast gas flow rate and a moderate relative humidity were essential for enhancing the maturity and the strength development. The developed model based on the maturity approach may accurately predict the strength development of the carbonated concrete blocks.     

A comparison study on effectiveness and robustness of control charts for monitoring process mean and variance

This article compares the effectiveness and robustness of nine typical control charts for monitoring both process mean and variance, including the most effective optimal and adaptive sequential probability ratio test (SPRT) charts. The nine charts are categorized into three types (the type, CUSUM type and SPRT type) and three versions (the basic version, optimal version and adaptive version). While the charting parameters of the basic charts are determined by common wisdoms, the parameters of the optimal and adaptive charts are designed optimally in order to minimize an index average extra quadratic loss for the best overall performance. Moreover, the probability distributions of the mean shift δ_µ and standard deviation shift δ_σ are studied explicitly as the influential factors in a factorial experiment. The main findings obtained in this study include: (1) From an overall viewpoint, the SPRT‐type chart is more effective than the CUSUM‐type chart and type chart by 15 and 73%, respectively; (2) in general, the adaptive chart outperforms the optimal chart and basic chart by 16 and 97%, respectively; (3) the optimal CUSUM chart is the most effective fixed sample size and sampling interval chart and the optimal SPRT chart is the best choice among the adaptive charts; and (4) the optimal sample sizes of both the charts and the CUSUM charts are always equal to one. Furthermore, this article provides several design tables which contain the optimal parameter values and performance indices of 54 charts under different specifications. Copyright © 2011 John Wiley & Sons, Ltd.     

Numerical simulation of multiphase flows of CO2 storage in saline aquifers in Daqingzijing oilfield, China

In this article, according to in situ geological and geophysical data archived of Daqingzijing oilfield, a 3D multiphase flow model based on hydrodynamic trapping mechanism is set up, with the phase interface mechanism considered. A high-order CE/SE (space–time conservation element and solution element) method coupled with (HPLS) Hybrid level-set method is updated to simulate migration and build-up of CO₂ in saline aquifers for short-term time scales. Results revealed that both the lateral variation of stratigraphic thickness and the heterogeneity of permeability control migration and accumulation of CO₂ plume. After 20?years of injection, CO₂ front propagates 8–9?km away from injection wells. The saline aquifer formation with high permeability is the dominant channel for CO₂ migration. The present work provides a novel approach for simulation of hydrodynamic trapping mechanism for CO₂ geological storage in saline aquifers of Songliao Basin of China, which could be a suitable location for a CO₂ storage demonstration project.     

High-temperature reaction networks in graphite furnaces

The intensity of gas-phase species present in a graphite furnace/dilatometer from room temperature to 1400°C were recorded by a mass spectrometer in real time. Based on the relative intensities and fragmentation factors, the species could be assigned to carbon dioxide, carbon monoxide, oxygen, water, and cyanogen. At low temperature, carbon dioxide was the main species observed whereas at high temperature, carbon monoxide was the predominant compound. The appearance of these species at low and high temperature was consistent with the results of equilibrium calculations for the coupled reactions of C + O₂ = CO₂ and CO₂ + C = 2CO. The intensity data from the mass spectrometer indicate that trace oxygen reacts with the graphite to form the carbon dioxide and carbon monoxide.     

Fault Diagnosis in Multivariate Control Charts Using Artificial Neural Networks

Most multivariate quality control procedures evaluate the in‐control or out‐of‐control condition based upon an overall statistic, like Hotelling's T². Although T² is optimal for finding a general shift in mean vectors, it is not optimal for shifts that occur for some subset of variables. This introduces a persistent problem in multivariate control charts, namely the interpretation of a signal that often discourages practitioners in applying them. In this paper, we propose an artificial neural network based model to diagnose faults in out‐of‐control conditions and to help identify aberrant variables when Shewhart‐type multivariate control charts based on Hotelling's T² are used. The results of the model implementation on two numerical examples and one case of real world data are encouraging. Copyright © 2005 John Wiley & Sons, Ltd.     

Multishelled CaO Microspheres Stabilized by Atomic Layer Deposition of Al2O3 for Enhanced CO2 Capture Performance

CO₂ capture and storage is a promising concept to reduce anthropogenic CO₂ emissions. The most established technology for capturing CO₂ relies on amine scrubbing that is, however, associated with high costs. Technoeconomic studies show that using CaO as a high‐temperature CO₂ sorbent can significantly reduce the costs of CO₂ capture. A serious disadvantage of CaO derived from earth‐abundant precursors, e.g., limestone, is the rapid, sintering‐induced decay of its cyclic CO₂ uptake. Here, a template‐assisted hydrothermal approach to develop CaO‐based sorbents exhibiting a very high and cyclically stable CO₂ uptake is exploited. The morphological characteristics of these sorbents, i.e., a porous shell comprised of CaO nanoparticles coated by a thin layer of Al₂O₃ (<3 nm) containing a central void, ensure (i) minimal diffusion limitations, (ii) space to accompany the substantial volumetric changes during CO₂ capture and release, and (iii) a minimal quantity of Al₂O₃ for structural stabilization, thus maximizing the fraction of CO₂‐capture‐active CaO.     

Critical curves in mixtures of carbon dioxide with ethane and ethylene based on a generalized fluctuation form of the van der waals equation of state

Critical curves of CO₂-C₂H₄ and CO₂-C₂H₄ mixtures are investigated using a new fluctuation equation of state. Particular attention is paid to the behavior of thermodynamic sensitives in the vicinity of the critical azeotropic points, at which the derivative of the molar volume with respect to the composition, which has no special features in the vicinity of the critical points of the pure components, becomes zero. Reliable agreement between the results of describing the critical curves of the mixtures and the known (including volumetric measurements) experiment is obtained. Possible intensification of supercritical extraction in using the CO₂-C₂H₆ mixture in the vicinity of the azeotropic point is predicted. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 73, No. 2, pp. 407–413, March–April, 2000.     

Gas transmission properties of polyvinyl chloride (PVC) films studied under subambient and ambient conditions for modified atmosphere packaging applications

This study was undertaken to determine the gas transmission properties of three polyvinyl chloride (PVC) films mostly used as overwrap for fresh produce under subambient and ambient conditions (5–40°C). Three different kinds of curves or groups of data are presented: gas transmission rates, activation energy measurements and the permeability coefficient. Hydrophobic by nature, the PVCs are not affected in their gas transmission properties (O₂, CO₂) by the moisture level. Reliable gas transmission data were obtained with method variability (≤9%) over a wide range of temperature. Regression constants, activation energies for prediction equations and permeability ratios (CO₂/O₂) are presented for the three films. Greater uniformity in test procedures, and the presentation of permeability data normalized to 1 μm instead of 1 mil would simplify comparative studies within and between countries, laboratory services and from the scientific literature. More research is needed to study the wide variety of polymers available under subambient conditions.     

Cement and carbon emissions

Because of its low cost, its ease of use and relative robustness to misuse, its versatility, and its local availability, concrete is by far the most widely used building material in the world today. Intrinsically, concrete has a very low energy and carbon footprint compared to most other materials. However, the volume of Portland cement required for concrete construction makes the cement industry a large emitter of CO₂. The International Energy Agency recently proposed a global CO₂ reduction plan. This plan has three main elements: long term CO₂ targets, a sectorial approach based on the lowest cost to society, and technology roadmaps that demonstrate the means to achieve the CO₂ reductions. For the cement industry, this plan calls for a reduction in CO₂ emissions from 2 Gt in 2007 to 1.55 Gt in 2050, while over the same period cement production is projected to increase by about 50 %. The authors of the cement industry roadmap point out that the extrapolation of existing technologies (fuel efficiency, alternative fuels and biomass, and clinker substitution) will only take us half the way towards these goals. According to the roadmap, the industry will have to rely on costly and unproven carbon capture and storage technologies for the other half of the required reduction. This will result in significant additional costs for society. Most of the CO₂ footprint of cement is due to the decarbonation of limestone during the clinkering process. Designing new clinkers that require less limestone is one means to significantly reduce the CO₂ footprint of cement and concrete. A new class of clinkers described in this paper can reduce CO₂ emissions by 20 to 30 % when compared to the manufacture of traditional PC Clinker.     

Solid fraction modelling for CO2 and CO2–THF hydrate slurries used as secondary refrigerants

In the present paper, the suitability of hydrate slurries in secondary refrigeration was investigated by the means of a new hydrate solid-fraction model. Considering the high melting enthalpy of CO₂-containing hydrates, slurries presenting high hydrate solid fractions can carry sufficient latent heat to be useful for a two-phase secondary-refrigerant application. The model presented in this paper allowed to calculate the solid fraction of CO₂ and CO₂–THF hydrate from thermodynamic conditions of pressure and temperature. Contrary to a previous work on single CO₂ hydrates in a closed system, the present model can take into account hydrate mixture and is well adapted to additional CO₂ injections (opened system). By relying on the hydrate-conversion model results, the study of hydrates in suspension in a carrying liquid was also studied in an experimental loop and was based on a formation process by CO₂ injection in a cooled aqueous solution.     

Stress corrosion cracking of 2205 duplex stainless steel in H<Subscript>2</Subscript>S–CO<Subscript>2</Subscript> environment

Stress corrosion cracking (SCC) behavior of 2205 duplex stainless steel (DSS) in H₂S–CO₂ environment was investigated by electrochemical measurements, slow strain rate test (SSRT), and scanning electron microscopy (SEM) characterization. Results demonstrated that the passive current density of steel increases with the decrease of solution pH and the presence of CO₂. When solutions pH was 2.7, the steel SCC in the absence and presence of CO₂ is expected to be a hydrogen-based process, i.e., hydrogen-induced cracking (HIC) dominates the SCC of the steel. The presence of CO₂ in solution does not affect the fracture mechanism. However, the SCC susceptibility is enhanced when the solution is saturated simultaneously with H₂S and CO₂. With elevation of solution pH to 4.5, the hydrogen evolution is inhibited, and dissolution is involved in cracking process. Even in the presence of CO₂, the additional cathodic reduction of H₂CO₃ would enhance the anodic reaction rate. Therefore, in addition to the hydrogen effect, anodic dissolution plays an important role in SCC of duplex stainless steel at solution pH of 4.5.     

Prozessberechnungen für eine CO<Subscript>2</Subscript>-Anlage zur Kälte- und Wärmeerzeugung für Supermärkte. Teil 1: Weitestgehende CO<Subscript>2</Subscript>-Abkühlung durch Umgebungsluft

CO₂ processes are used in supermarkets for medium- and low-temperature refrigeration and by now even for room heating and hot tap-water preparation via heat recovery. Through systematic thermodynamic process calculations, the limits of the heat recovery from a CO₂ plant and its influence on the refrigeration process as well as the complete system are investigated for a supermarket with 100 kW medium-temperature refrigeration capacity. This investigation focuses on the energy efficiency of the heat supply by the extended CO₂ plant. By using the medium-temperature refrigeration capacity as a reference, the results for ambient temperatures between ??15?°C and?+?10?°C are applicable for other supermarket sizes as well.In part 1, a plant is investigated with a ratio between low- and medium-temperature refrigeration capacity of 0.2 and CO₂ cooling via the ambient air as far as possible. The contributions of refrigeration and heat supply are specified separately for a range of high-pressure values for sub-critical and for trans-critical operation of the CO₂ plant. Relevant data for design and operation of the CO₂ plant are made available: the supplied heating capacity (relative to the medium-temperature refrigeration capacity), the coefficients of performance of refrigeration and of combined refrigeration and heating, and the exergy efficiencies of refrigeration and of combined refrigeration and heating. With this knowledge, preferred operation parameters for the CO₂ process can be chosen depending on the ambient temperature. Moreover, the decision is prepared, whether an additional heating system is required or the heat recovery from the refrigeration plant is sufficient. In the following second part, results for limited CO₂ cooling via ambient air will be considered.     

Harnessing the Beneficial Attributes of Ceria and Titania in a Mixed-Oxide Support for Nickel-Catalyzed Photothermal CO2 Methanation

Solar-powered carbon dioxide (CO₂)-to-fuel conversion presents itself as an ideal solution for both CO₂ mitigation and the rapidly growing world energy demand. In this work, the heating effect of light irradiation onto a bed of supported nickel (Ni) catalyst was utilized to facilitate CO₂ conversion. Ceria (CeO₂)-titania (TiO₂) oxide supports of different compositions were employed and their effects on photothermal CO₂ conversion examined. Two factors are shown to be crucial for instigating photothermal CO₂ methanation activity: ① Fine nickel deposits are required for both higher active catalyst area and greater light absorption capacity for the initial heating of the catalyst bed; and ② the presence of defect sites on the support are necessary to promote adsorption of CO₂ for its subsequent activation. Titania inclusion within the support plays a crucial role in maintaining the oxygen vacancy defect sites on the (titanium-doped) cerium oxide. The combination of elevated light absorption and stabilized reduced states for CO₂ adsorption subsequently invokes effective photothermal CO₂ methanation when the ceria and titania are blended in the ideal ratio(s).     

Artificial versus Natural Reuse of CO2 for DME Production: Are We Any Closer?

This work uses a mathematical optimization approach to analyze and compare facilities that either capture carbon dioxide (CO₂) artificially or use naturally captured CO₂ in the form of lignocellulosic biomass toward the production of the same product, dimethyl ether (DME). In nature, plants capture CO₂ via photosynthesis in order to grow. The design of the first process discussed here is based on a superstructure optimization approach in order to select technologies that transform lignocellulosic biomass into DME. Biomass is gasified; next, the raw syngas must be purified using reforming, scrubbing, and carbon capture technologies before it can be used to directly produce DME. Alternatively, CO₂ can be captured and used to produce DME via hydrogenation. Hydrogen (H₂) is produced by splitting water using solar energy. Facilities based on both photovoltaic (PV) solar or concentrated solar power (CSP) technologies have been designed; their monthly operation, which is based on solar availability, is determined using a multi-period approach. The current level of technological development gives biomass an advantage as a carbon capture technology, since both water consumption and economic parameters are in its favor. However, due to the area required for growing biomass and the total amount of water consumed (if plant growing is also accounted for), the decision to use biomass is not a straightforward one.     

Electrocatalytic Reduction of CO2 to Ethylene by Molecular Cu‐Complex Immobilized on Graphitized Mesoporous Carbon

The electrochemical reduction of carbon dioxide (CO₂) to hydrocarbons is a challenging task because of the issues in controlling the efficiency and selectivity of the products. Among the various transition metals, copper has attracted attention as it yields more reduced and C2 products even while using mononuclear copper center as catalysts. In addition, it is found that reversible formation of copper nanoparticle acts as the real catalytically active site for the conversion of CO₂ to reduced products. Here, it is demonstrated that the dinuclear molecular copper complex immobilized over graphitized mesoporous carbon can act as catalysts for the conversion of CO₂ to hydrocarbons (methane and ethylene) up to 60%. Interestingly, high selectivity toward C2 product (40% faradaic efficiency) is achieved by a molecular complex based hybrid material from CO₂ in 0.1 m KCl. In addition, the role of local pH, porous structure, and carbon support in limiting the mass transport to achieve the highly reduced products is demonstrated. Although the spectroscopic analysis of the catalysts exhibits molecular nature of the complex after 2 h bulk electrolysis, morphological study reveals that the newly generated copper cluster is the real active site during the catalytic reactions.     

Liquid-in-Aerogel Porous Composite Allows Efficient CO2 Capture and CO2/N2 Separation

The pursuit of efficient CO₂ capture materials remains an unmet challenge. Especially, meeting both high sorption capacity and fast uptake kinetics is an ongoing effort in the development of CO₂ sorbents. Here, a strategy to exploit liquid-in-aerogel porous composites (LIAPCs) that allow for highly effective CO₂ capture and selective CO₂/N₂ separation, is reported. Interestingly, the functional liquid tetraethylenepentamine (TEPA) is partially filled into the air pockets of SiO₂ aerogel with left permanent porosity. Notably, the confined liquid thickness is 10.9–19.5 nm, which can be vividly probed by the atomic force microscope and rationalized by tailoring the liquid composition and amount. LIAPCs achieve high affinity between the functional liquid and solid porous counterpart, good structure integrity, and robust thermal stability. LIAPCs exhibit superb CO₂ uptake capacity (5.44 mmol g⁻¹, 75 °C, and 15 vol% CO₂), fast sorption kinetics, and high amine efficiency. Furthermore, LIAPCs ensure long-term adsorption–desorption cycle stability and offer exceptional CO₂/N₂ selectivity both in dry and humid conditions, with a separation factor up to 1182.68 at a humidity of 1%. This approach offers the prospect of efficient CO₂ capture and gas separation, shedding light on new possibilities to make the next-generation sorption materials for CO₂ utilization.     

Pinch point analysis and design considerations of CO2 gas cooler for heat pump water heaters

Due to strong nonlinear variation of supercritical CO₂ specific heat capacity with temperature, pinch point would occur in water-cooled CO₂ gas cooler, which has great impacts on the heat transfer characteristics of gas cooler and overall system performance. Pinch point analysis was conducted for CO₂ gas cooler in the present study. The effects of refrigerant pressure, mass flow ratio (m_w/m_c), inlet water temperature and heat transfer area on pinch point location, approach temperature difference and heat transfer rate were analyzed in detail. Based on the analysis of pinch point location in CO₂ gas cooler, the critical flow ratios were proposed to effectively control the approach temperature difference. Furthermore, the actual conductance of gas cooler was calculated and compared with that estimated by LMTD method. The results showed that CO₂ gas cooler may be undersized by as much as a factor of 30–60% for different pressures if LMTD method is used. However, the UA value evaluated by LMTD method also may be overestimated under high refrigerant pressures when the approach temperature difference tends to be zero. Results of the present study are helpful to practical designs of CO₂ gas cooler and heat pump water heaters.     

Regulating π-Conjugation in sp2-Carbon-Linked Covalent Organic Frameworks for Efficient Metal-Free CO2 Photoreduction with H2O

The development of sp²-carbon-linked covalent organic frameworks (sp²c-COFs) as artificial photocatalysts for solar-driven conversion of CO₂ into chemical feedstock has captured growing attention, but catalytic performance has been significantly limited by their intrinsic organic linkages. Here, a simple, yet efficient approach is reported to improve the CO₂ photoreduction on metal-free sp²c-COFs by rationally regulating their intrinsic π-conjugation. The incorporation of ethynyl groups into conjugated skeletons affords a significant improvement in π-conjugation and facilitates the photogenerated charge separation and transfer, thereby boosting the CO₂ photoreduction in a solid-gas mode with only water vapor and CO₂. The resultant CO production rate reaches as high as 382.0 µmol g⁻¹ h⁻¹, ranking at the top among all additive-free CO₂ photoreduction catalysts. The simple modulation approach not only enables to achieve enhanced CO₂ reduction performance but also simultaneously gives a rise to extend the understanding of structure-property relationship and offer new possibilities for the development of new π-conjugated COF-based artificial photocatalysts.