CaO Surface Research Articles

Effective biodiesel synthesis from waste cooking oil and biomass residue solid green catalyst

Biodiesel has recently received greater concern as one of the reliable and sustainable source of energy. Production of biodiesel is hampered by both feedstock availability and catalyst system. This work successfully utilized the waste egg shell to develop a bimetallic mixed oxide catalyst, and waste cooking oil characterised by high free fatty acid (FFA) as feedstock, towards methyl ester production under mild reaction condition in one-step transesterification process. The properties of the catalyst were assessed using XRD, BET, SEM, EDX and TPD-CO2. The prepared catalyst was successfully recorded a high biodiesel yield of 92.1% under the optimized reaction conditions of 15:1 methanol to oil molar ratio, 3 wt% catalyst loading, 80 °C reaction temperature and 3 h reaction time. The recovered catalyst was reused in five cycles without significant loss in activity. The leaching of catalytic Ca2+ active site was reduced when transition bimetallic mixed oxides were added to the CaO surface.

Experiment on NOx reduction by advanced reburning in cement precalciner

Experiment on NOx reduction by advanced reburning in cement precalciner was performed on a laboratory-scale circulating fluidized bed reactor. In reburning zone, the order of the adsorption ability on CaO surface is CO2 > NO > CO, CO2 occupies a large fraction of the active sites to inhibits NO reduction by CO, the decrement of NO reduction rate is about 35% when CO2 increases from 15% to 30%, and the increment of NO reduction rate is about 40% when temperature increases from 850 °C to 1050 °C. In SNCR zone, CO2 concentration has an optimal range (25–27%) due to the competitive adsorption of CO2 and NH3 and the formation of (NH2)2CO on CaO surface, the optimal temperature range becomes narrow (850–975 °C) in comparison with that in utility boiler, the optimal concentration of O2 is in the range of 2% to 3.3%, and NO reduction rate reaches above 75% at 900–970 °C.

Zerovalent Selenium Adsorption Mechanisms on CaO Surface: DFT Calculation and Experimental Study.

Zerovalent Se (Se atom and small Se2 molecule) adsorption mechanisms on a CaO surface were studied by both density functional theory (DFT) calculations and adsorption experiments. Nonvalent Se adsorption on the CaO(001) surface was simulated using a slab model. The adsorption energy, adsorption structure, electron density clouds, and electron properties were calculated. Different Se surface coverages were investigated to elucidate the adsorption process. In the experiments, the Se adsorption products were prepared in a U-shaped quartz reactor at 300 °C. The properties were investigated by X-ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), field emission scanning electron microscopy/energy dispersive X-ray spectroscopy (FE-SEM/EDS), and X-ray diffraction (XRD), respectively. The experimental results match up with the DFT results, which reveal fundamentalmonochemisorption mechanisms of zerovalent Se on the CaO surface.

Pressurized calcium looping in the presence of steam in a spout-fluidized-bed reactor with DFT analysis

Calcium looping is a high-temperature solid-looping process for CO2 capture, exploiting cyclical carbonation of CaO. Previous work investigating the effects of steam on the carbonation reaction has produced conflicting results, with the majority of work conducted using thermogravimetric analyzers (TGA). Here, pressurized carbonation kinetics in the presence of steam in a 3kWe pressurized spout-fluidized bed reactor, gives a rigorous insight into the effects of steam. Pseudo-intrinsic kinetics were determined using an effectiveness factor model along with activation energies and kinetic expressions. The mechanism in which steam promotes CO2 adsorption on the surface of CaO was investigated using density functional theory (DFT). The molecular-scale changes on the CaO surface owing to the presence of steam compared to the base case of CO2 adsorption on a ‘clean’ (without steam) surface were simulated with the Cambridge Serial Total Energy Package (CASTEP) software. The results suggest that steam promotes CO2 adsorption via the formation of surface OH groups on the CaO surface.

Investigation into acid­basic equilibrium on the surface of oxides with various chemical nature

The acid-base equilibrium on the surface of CaO, MgO, FeO, Al2O3, Fe2O3, SiO2, TiO2 oxides was studied. As a result of the study, the main provisions have been formulated which made it possible to propose models of surface active centers and schemes of their acid-base equilibrium. The acid-base and energy equilibrium parameters of the active centers in the deprotonation reactions were calculated. Regularities of changes in the acid-base and energy parameters of the active centers were determined depending on the nature of the central element of the crystal lattice, the number of OH-groups and the number of hydration. Acidity of all considered types of active centers depends to a large extent on the nature of the central element, namely its charge and coordination number. It was found that with increase in the coordination number of the central elements with the same charge, pKa of anhydrous centers and the value of their Gibbs free energy in deprotonation reactions increase. Acidity of isolated anhydrous centers increases in a direct proportion as the number of OH groups increases. The obtained regularity preserves for the surfaces of all oxides considered. It was shown for all oxides that when the amount of adsorbed water molecules increases, acidity of the active centers of all types increases and this dependence is identical to the growth of acidity with an increase in the number of OH groups. This indicates identity of the nature of acidity growth with an increase in thickness of hydroxyl and hydration surface layers. The obtained results will ensure prediction of oxide filler behavior in compositions and adjustment of operating properties of composite materials

First principles study on the adsorption of Au dimer on metal-oxide surfaces: The implications for Au growing

The adsorption of Au dimer on MgO(100), CaO(100), BaO(100), TiO2(110) and YSZ(100) surfaces is comparatively studied using ab initio density functional theory calculations. It is found that Au dimer prefers upright adsorption on MgO(100), CaO(100), BaO(100) surfaces and parallel adsorption on TiO2(110) and YSZ(111) surfaces. According to the analysis of the metal-metal cohesive energy (EAu-Au) and the metal-substrate adhesion energy (EAu-Support), we find that Au adatoms prefer 2D cluster on BaO(100) and TiO2(110) surfaces and 3D cluster on MgO(100), CaO(100) and YSZ(111) surfaces. We also find a linear relationship between EAu-Support/EAu-Au and Eb (binding energy of metal atom on the substrate). Furthermore, an interesting correlation between association energies and the amount of transferred charge is found. The findings may help to gain further insight into the structure-property correlation and provide valuable information on crucial parameters in catalyst design.

CO2 and SO2 sorption on the alkali metals doped CaO(100)surface: A DFT-D study

The electronic properties of the pristine and alkali metals doped CaO(100) surfaces, as well as their propensity toward CO2 and SO2sorption are studied using the dispersion-corrected density functional theory (DFT-D). Various interaction modes between the acidic gas and surfaces are considered. The results show that the basic oxygen surface sites are active adsorption sites for CO2 and SO2, with monodentate-like coordination formed on each surface. Further, charge analysis is conducted, showing a charge transfer fromeach surface to the adsorbate. The comparison of adsorption energies indicates that CO2 and SO2 show stronger adsorption on alkali metal-doped CaO than that on pristine CaO. The metal Csis the preferred dopant for high sorption capacity, whichhave agreat strengthening effect onCO2 and SO2 binding.

Surface and bulk carbonate formation in calcium oxide during CO2 capture

Calcium looping (CaL), the cyclic carbonation and calcination of limestone, is a prominent carbon capture option considering reduced parasitic energy consumption compared to amine scrubbing. The main issue preventing application is sorbent performance decay during cycling. Therefore hydration and extended carbonation reactivation strategies, as well as synthetic approaches to enhance or sustain reactivity, are of interest. Results of an investigation are presented on the impact of reactivation strategies on carbonate formation through detailed study using in-situ infrared spectroscopy. Surface and bulk carbonate formation regimes were readily distinguished allowing the influence of temporal, hydration and thermodynamic conditions within each regime to be studied. Surface chemistry of CaO during two initial cycles was shown to change. Significantly, it was shown that the CaO surface produced from the calcination of Ca(OH)2 possessed more highly reactive sites compared to the surface produced from CaCO3. The presence of water enhanced bulk carbonate formation at 300°C and influenced the complexation of surface carbonates during looping. The results are of interest in the CaL community, offering molecular scale explanations for macroscale observations, whilst also informing on the role of humidity in direct air capture and sorption enhanced reaction processes.

Diffusion of asphaltene, resin, aromatic and saturate components of asphalt on mineral aggregates surface: molecular dynamics simulation

In this research, the models of four asphalt components (asphaltene, resin, aromatics and saturate) and five minerals (SiO2, Al2O3, CaO, MgO and Fe2O3) were constructed individually, and then the interface models were constructed by adding the asphalt components and minerals together. The interfacial behaviour at molecular scale was simulated by setting boundary conditions, optimising the structure and canonical ensemble. The mean square displacement and diffusion coefficient of particles were selected to study the diffusion of asphalt components on the surface of different minerals. The results show that increasing the temperature can accelerate the diffusion of asphalt components. The diffusion speed of asphalt components on the surface of Al2O3 is faster than other mineral crystals. The temperature sensitivity of diffusion coefficient of asphalt components on the surface of CaO is the maximum. The diffusion speed of asphalt components ranked roughly as their molecular weight: saturate > aromatics > resin > asphaltene.

Theoretical Study of Mercury Species Adsorption on CaO(100) Surface

As an important air pollutant, mercury produced by power generation can be captured by CaO incoal-firedfly ash. In order to understand the interaction mechanism of CaO with mercury in HCl atmosphere, the density functional theory method is utilizedto investigate the adsorption of mercury species on the fly ash components. The CaO(100) surface is built to study the stable adsorption configurations of Hg0, HCl, HgCl and HgCl2 on all possible adsorption sites. The results show that CaO(100) surface cannot absorb Hg0 directly, but the intermediate HgCl can be adsorbed on the surface stably, and then continue to combine with a Cl radical to produce HgCl2, which is easy to be removed.

SeO2 adsorption on CaO surface: DFT and experimental study on the adsorption of multiple SeO2 molecules

SeO2 adsorption mechanisms on CaO surface were firstly investigated by both density functional theory (DFT) calculations and adsorption experiments. Adsorption of multiple SeO2 on the CaO (001) surface was investigated using slab model. Based on the results of adsorption energy and surface property, a double-layer adsorption mechanisms were proposed. In experiments, the SeO2 adsorption products were prepared in a U-shaped quartz reactor at 200°C. The surface morphology was investigated by field emission scanning electron microscopy (FE-SEM). The superficial and total SeO2 mass fractions were measured by X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES), respectively. The surface valence state and bulk structure are determined by XPS and X-Ray Diffraction (XRD). The experimental results are in good agreement with the DFT results. In conclusion, the fundamental SeO2 chemisorption mechanisms on CaO surface were suggested.

Large-scale high-temperature solar energy storage using natural minerals

Abstract––– The present work is focused on thermochemical energy storage (TCES) in Concentrated Solar Power (CSP) plants by means of the Calcium-Looping (CaL) process using cheap, abundant and non-toxic natural carbonate minerals. CaL conditions for CSP storage involve calcination of CaCO3 in the solar receiver at relatively low temperature whereas carbonation of CaO is carried out at high temperature and high CO2 concentration to use the heat of reaction for power production by means of a CO2 closed power cycle. Under these conditions, large CaO particles derived from limestone to be used in industrial processes are rapidly deactivated due to pore-plugging, which limits the extent of the reaction. This is favored by the relatively small pores of the CaO skeleton generated by low temperature calcination, the large thickness of the CaCO3 layer built upon the CaO surface and the very fast carbonation kinetics. On the other hand, at CaL conditions for CSP storage does not limit carbonation of CaO derived from dolomite (dolime). Dolime is shown to exhibit a high multicycle conversion regardless of particle size, which is explained by the presence of inert MgO grains that allow the reacting gas to percolate inside the porous particles.

In situ DRIFTS study of O3 adsorption on CaO, γ-Al2O3, CuO, α-Fe2O3 and ZnO at room temperature for the catalytic ozonation of cinnamaldehyde

In situ DRIFTS were conducted to identify adsorbed ozone and/or adsorbed oxygen species on CaO, ZnO, γ-Al2O3, CuO and α-Fe2O3 surfaces at room temperature. Samples were characterized by means of TG, XRD, N2 adsorption–desorption, pyridine-IR, nitrobenzene-IR, chloroform-IR, and CO2-TPD. Pyridine-DRIFTS measurements evidence two kinds of acid sites in all the samples. Nitrobenzene, chloroform-DRIFTS, and CO2-TPD reveal that there are large amounts of medium-strength base sites on all the metal oxides, and only CaO, ZnO, and γ-Al2O3 have strong base sites. And the benzaldehyde selectivity was increased in the same order of the alkalinity of the metal oxides. With weaker sites, ozone molecules form coordinative complexes bound via the terminal oxygen atom, observed by vibrational frequencies at 2095–2122 and 1026–1054cm−1. The formation of ozonide O3− at 790cm−1, atomic oxygen at 1317cm−1, and superoxide O2− at 1124cm−1 was detected; these species are believed to be intermediates of O3 decomposition on strong acid/base sites. The adsorption of ozone on metal oxides is a weak adsorption, and other gases, such as CO2, will compete with O3 adsorption. The mechanism of cinnamaldehyde ozonation at room temperature over CaO shows that cinnamaldehyde can not only be oxidized into cinnamic acid, but also be further oxidized into benzaldehyde, benzoic acid, maleic anhydride, and ultimately mineralized to CO2 in the presence of O3.

SeO2 adsorption on CaO surface: DFT study on the adsorption of a single SeO2 molecule

Selenium is a hazardous element in coal. During coal combustion, most of the selenium will convert to SeO2 in the flue gas. Ca-based adsorbents, especially CaO, have been considered as a potential sorbent to adsorb SeO2 due to its low cost. In this paper, the adsorption mechanisms of single SeO2 on CaO surface were investigated by density functional theory (DFT) calculation. Both the physisorption and chemisorption structures were determined. It has been identified that the adsorption of SeO2 on CaO surface is primarily chemisorption, while physisorption takes effects at the initial stage of the process. Under O2 atmosphere, selenate is hard to form. Most of the adsorption products are selenite. Additionally, the electron density maps were obtained to reveal the surface active sites. The partial density of states (PDOS) was calculated for analyzing the electronic structural change of SeO2 and CaO surface during adsorption. The results provide fundamental information of the adsorption process, which could be meaningful for the development of new absorbents.

First-principles study on the adsorption properties of transition-metal atoms on CaO(001) surfaces

By using first-principles electronic-structure calculations based on the density functional theory, we systematically investigated the adsorption properties of transition-metal (TM) adatoms on CaO(001) surfaces. Optimized adsorption structures and energetics of TM adatoms on CaO(001) are reported for various adsorption structures. The results are different from those of TM adatoms on MgO(001). Concomitantly, this suggests different dynamical properties of TM adatoms on CaO(001) surfaces as compared with TM adatoms on MgO(001) surfaces. Also performed was an analysis of the electronic structures of the TM adatoms on CaO(001) by using the energy positions of the adsorbate states with respect to the valence band maximum of CaO. The results are discussed in connection with the charge states of the TM adatoms on doped CaO(001).

Initial stages of CO2 adsorption on CaO: a combined experimental and computational study.

Room temperature adsorption of carbon dioxide (CO2) on monocrystalline CaO(001) thin films grown on a Mo(001) substrate was studied by infrared reflection-absorption spectroscopy (IRAS) and quantum chemical calculations. For comparison, CO2 adsorption was examined on poorly ordered, nanoparticulate CaO films prepared on Ru(0001). For both systems, CO2 readily adsorbs on the clean CaO surface. However, additional bands were observable on the CaO/Ru(0001) films compared with CaO/Mo(001), because the stricter IRAS surface selection rules do not apply to adsorption on the disordered thin films grown on Ru(0001). Spectral evolution with increasing exposure of the IRA bands suggested the presence of several adsorption sites which are consecutively populated by CO2. Density functional calculations showed that CO2 adsorption occurs as monodentate surface carbonate (CO32-) species at monatomic step sites and other low-coordinated sites, followed by formation of carbonates on terraces, which dominate at increasing CO2 exposure. To explain the coverage-dependent IRAS results, we propose CO2 surface islanding from the onset, most likely in the form of pairs and other chain-like species, which were calculated as thermodynamically favorable. The calculated adsorption energy for isolated CO2 on the terrace sites (184 ± 10 kJ mol-1) is larger than the adsorption energy obtained by temperature programmed desorption (∼120-140 kJ mol-1) and heat of adsorption taken from microcalorimetry measurements at low coverage (∼125 kJ mol-1). However, the calculated adsorption energies become less favorable when carbonate chains intersect on CaO terraces, forming kinks. Furthermore, our assignments of the initial stages of CO2 adsorption are consistent with the observed coverage effect on the CO2 adsorption energy measured by microcalorimetry and the IRAS results.

Theoretical study on influence of CaO and MgO on the reduction of FeO by CO

Coating of CaO or MgO on the particle surface can prevent the sticking among iron ore particles effectively during fluidization process. However, CaO and MgO promote the formation of iron whiskers at high temperature, leading to the catastrophic defluidization. The density functional theory (DFT) calculations were implemented to investigate the influence of CaO and MgO on reduction of FeO/Fe2O2 by CO. Our results show that the CO molecule tends to bind to FeO/Fe2O2 on CaO(100) and MgO(100) surfaces through newly formed C-Fe and CO bonds. The CaO(100) surface will accelerate the reduction reactions which occur on it, in particular, in the initial stage of reactions, however, will slow down the reactions in the posterior stage. For the MgO(100) surface, the reduction reactions which occur on it will be promoted. The positive roles displayed by CaO and MgO in promoting the reduction of FeO by CO accelerate the precipitation of fresh iron and therefore, leading to the formation of iron whiskers.

Hydration Structures of MgO, CaO, and SrO (001) Surfaces

Using density functional theory (PBE functional), we show that the degree of surface hydroxylation increases in the MgO, CaO, SrO series, accompanied by an increase in water adsorption energy. Already for water coverage of two monolayers, structures with dissolved M2+ ions are considerably more stable than the intact, nondissolved surface. The dissolved ions above the surface form different patterns including ordered ones (e.g., an infinite stripe) that are preferred for MgO(001) and CaO(001) and disordered ones that are favored for SrO(001). Contrary to previous assignments, an analysis of calculated X-ray photoelectron spectra shows that O(1s) signals arising from OH and H2O groups might coincide in the experimental spectrum.

On the Modification of Hydration Resistance of CaO with ZrO2 Additive

Various ZrO2/CaO samples were fabricated by cold isostatic pressing and sintered at 1750°C for 4 h. It was observed that the sample with 12% ZrO2 additive possessed the good hydration resistance and had the lowest apparent porosity of about 0.75%; its weight additive stored after 56 days was less than 0.6 wt%, and it contributed to the occurrence of CaZrO3 on the surface of CaO. The CaO crucible with 12 mol% ZrO2 additive did not react with titanium melt during melting TiNi alloy. This provides a support for searching a new refractory with the good hydration resistance for induction melting titanium alloys.

High efficient Aldol condensation reaction utilizing modified calcium oxide as stable solid base catalyst

Aldol condensation of acetophenone and benzaldehyde, as well as various benzaldehydes was carried out efficiently to produce chalcone with a good selectivity and high yields by using modified CaO as a solid base catalyst. Stability and catalytic activity of commercial CaO were significantly improved after modifying calcium oxide with bromobenzene in a simple way. An Aldol yield higher than 98.9% was obtained after the reaction was conducted for 3 h. This time interval is considerably shorter when compared to a period of 12 h needed for the commercial CaO to reach 92.1% yield under optimum activation. The high catalytic activity of modified CaO suggests that heterogeneous aldolisation was greatly improved by changing its hydrophilic properties. The influence of several reaction parameters, such as temperature and catalyst loading, was investigated. The humidity test over modified CaO reveals that the basic sites of modified CaO are resistant to CO2 and moisture. The type of aldehyde has great influence on the yield of Aldol condensation. Based on the results of characterization by Fourier transform-infrared spectrometry (FT-IR) and thermogravimetric measurements (TG), it was concluded that the modifier was chemically bonded to the surface of CaO and nearly no Ca(OH)2 was formed during the modification process.