Evaluation of CO₂ adsorption capacity with a nano-CaO synthesized by chemical combustion/ball milling

The analytical-grade reagents used for the chemical combustion synthesis of nano-CaO adsorbent were calcium nitrate tetrahydrate (Merck ACS, 98.0% purity) as the oxidizing agent and urea (Sigma Aldrich ACS, 99.9% purity) as the chemical fuel; these reagents were used without further purification.

The nano-CaO adsorbent was prepared via a typical chemical combustion followed by a ball milling process [21]. The redox mixture contained urea as the chemical fuel and calcium nitrate tetrahydrate as the oxidizing agent, according to the following reaction: 2[Ca(NO3)2⋅4H2O](s)+4CH4N2O(s)+O2(g)→2CaO(S)+16H2O(g)+4CO2(g)+6N2(g) \matrix{{2{{[{\rm{Ca}}{{({\rm{N}}{{\rm{O}}_3})}_2} \cdot 4{{\rm{H}}_2}{\rm{O}}]}_{({\rm{s}})}} + 4{\rm{C}}{{\rm{H}}_4}{{\rm{N}}_2}{{\rm{O}}_{({\rm{s}})}} + {{\rm{O}}_{2({\rm{g}})}}} \hfill \cr {\to 2{\rm{Ca}}{{\rm{O}}_{({\rm{S}})}} + 16{{\rm{H}}_2}{\rm{O}}_{({\rm{g}})} + 4{\rm{C}}{{\rm{O}}_{2({\rm{g}})}} + 6{{\rm{N}}_{2({\rm{g}})}}} \hfill \cr}

A 2:1 molar ratio of urea: calcium nitrate and a minimum volume of distilled water were thoroughly mixed in a porcelain crucible, so as to obtain a homogeneous mixture. This slurry was warmed to almost dryness and then 1 mL of distilled water was added. Finally, the resulting product was heated for 5 min at 800°C. The process was rich in fuel, which facilitated the formation of CaO because enough energy was available during combustion. The chemical combustion method has been traditionally used to prepare several metal oxides and other materials with fluffy structure, large surface areas, high porosity, and high yield [24]. The dried powders derived from the chemical combustion were subjected to mechanical ball milling for 2.5 h under argon atmosphere with a Spex-type mechanical ball mill, with stainless steel balls of 1 mm diameter, and a ball-to-powder weight ratio of 6:1. The dry ground CaO was the starting material for the study of CO₂ adsorption.

The CaO was characterized using the techniques described below. (A)

X-ray diffraction (XRD) analysis with a SIEMENS D-5000 powder diffractometer in a wide-angle region of 10°–100° in the 2θ scale, and the diffraction pattern was compared to the database in the International Centre for Diffraction Data, Pennsylvania, USA.

Kinetic adsorption experiments of CO₂ onto the nano-CaO adsorbent were conducted at different temperatures (25°C, 50°C, and 75°C) and at room pressure using a 50 mL capacity Parr 4592 stainless steel pressure reactor coupled to a temperature-controlled system. Samples of 10 mg of the nano-CaO adsorbent were exposed to a high-purity (99.98%) dry CO₂ flow at 1 atm pressure for 1 min, 3 min, 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 45 min, 60 min, 90 min, and 180 min. Prior to the CO₂ adsorption tests, the adsorbent samples were degassed for 30 min under vacuum at 325°C to remove some possible impurities. Measurements of the CO₂ adsorption capacity were performed by thermogravimetric analysis (TGA) with a TGA analyzer (SDT Q600; TA Instruments) coupled with a mass spectrum analyzer (TA Instruments, LLC). For these analyses, the CaO sample (~6 mg) was put in a ceramic cell and then heated from 20°C to 850°C at the rate of 10°C/min, in an inert atmosphere of helium (100 mL/min). The quantity of CO₂ (millimoles) captured per gram of adsorbent (millimoles per gram [mmol/g]) was calculated from the TGA calcination profile for different contact times and temperatures based on the weight loss of CO₂; the first derivatives of the TGA curves (differential thermal analysis [DTA]) were also computed.

The adsorption isotherms of CO₂ in the porous nano-CaO adsorbent were determined at 25°C, 50°C, and 75°C and under different CO₂ gas pressures (1 atm, 5 atm, 10 atm, and 15 atm). The adsorption capacity of CO₂ was measured by TGA as for the kinetic tests. Classic Langmuir and Freundlich isotherm models were used to provide a mathematical description of the CO₂ adsorption process on the nano-CaO adsorbent, and their respective constants were determined. These isotherm models generally provide information on the CO₂ adsorption mechanisms and affinity properties of the adsorbents.

The effect of temperature on the amount of CO₂ adsorbed by the nano-CaO adsorbent was studied at 298 K, 323 K, and 348 K. The nano-CaO adsorbent dosage, the flow, and the CO₂ pressure were constant. The most important thermodynamic parameters, such as standard enthalpy (ΔH°), standard entropy (ΔS°), standard free energy (ΔG°), and activation energy (E_a), of CO₂ adsorption were determined.

The CO₂ adsorption-desorption activity of the nano-CaO adsorbent was determined as follows. First, adsorption of CO₂ onto nano-CaO adsorbent was carried out at 75°C and 1 atm, and the adsorption capacity q_ad was measured in these conditions. Then, these CO₂/nano-CaO samples were exposed to a helium flow from 20°C to 850°C for eliminating CO₂. Thereafter, CO₂ was again adsorbed onto the same samples of the nano-CaO adsorbent. Five cycles of adsorption-desorption were achieved.

The chemical combustion/ball milling method allowed the preparation of a large amount of voluminous, spongy, and nanocrystalline CaO adsorbent with a large surface area and high porosity. Figure 1 shows the XRD pattern of the adsorbent, in which the CaO is the most important mineral component of the sample (International Centre for Diffraction Data, file: 37-1497). The crystallite size of the CaO particles using the Scherrer equation was 18.61 nm, showing a nanostructured material. In addition, a crystalline structure of CaCO₃ appears as a trace component (International Centre for Diffraction Data, file: 05-0586).

Fig. 1

The SEM micrograph of the CaO (Figure 2) adsorbent shows an agglomeration of nanoparticles with a well-developed and irregularly shaped porous structure, rough surface, and a small particle size (<10 μm) due to the fragmentation caused by the mechanical ball milling. In this regard, the agglomerates have substantial surface texture, porosity, and big surface area. A porous structure facilitates CO₂ access to the active sites and thereby increases the rates of CO₂ physisorption, chemisorption, and diffusion. The quantitative microanalysis of the nano-CaO adsorbent using SEM with X-ray energy dispersion only confirmed the presence of calcium and oxygen elements in this sample; a high CaO content in the adsorbent structure favors the conversion of CaO into CaCO₃.

Fig. 2

N₂ physisorption at 77 K showed that the CaO adsorbent had a large BET surface area of 50.73 m²/g (which is indicative of highly developed porosity), a high total pore volume of 0.548 cm³/g, and a narrow and uniform size distribution of the pore radius, which was determined by the BJH method to be 19.24 nm with an ~80% distribution (Figure 3). These improved structural and textural properties all played an important role in capturing CO₂ and showed a high CO₂ adsorption performance of the adsorbent when it was exposed to a flow of CO₂ at different temperatures.

Fig. 3

The adsorption kinetics describes the adsorption rate of the adsorbate, which governs the residence time of the adsorption reaction. The rate at which adsorption occurs is therefore of paramount importance in the design of an appropriate adsorption system, and thus, understanding the kinetics of adsorption is vital for capturing CO₂ because it provides essential information on the possible speed control steps (e.g., chemical reaction, diffusion control, and mass transfer) [25]. In the present study, kinetic models were used to investigate the CO₂ adsorption behavior on nano-CaO adsorbent under a range of temperatures and durations, ranging from 1 min to 180 min. The correlation coefficients (R²) determined the conformity between the experimental data and each kinetic model, which were the pseudo-first-order [26], pseudo-second-order [27], Elovich [28], and intraparticle diffusion [29] models, the most widely used for describing adsorption processes. The results obtained are shown in Table 1.

Data of Table 1 show that the correlation coefficient (R²) for the pseudo-second-order model is higher than for the other ones, which indicates that this model was the most appropriate for describing the kinetics of CO₂ adsorption onto the nano-CaO adsorbent. The corresponding equation is as follows (Figure 4): (1) 1qt=1k2qe2+(1qe)t, {1 \over {{q_t}}} = {1 \over {{k_2}{q_e}^2}} + \left({{1 \over {{q_e}}}} \right)t, where q_e and q_t are the amounts of CO₂ adsorbed (in mmol/g) at equilibrium and at time t (in minutes), respectively, and k₂ is the pseudo-second-order adsorption rate constant (/min). According to this model, it is assumed that the rate-limiting step can be chemisorption, which involves valence forces through the sharing or exchange of electrons between adsorbate (gas molecules) and adsorbent [29]. Moreover, the occupation of the velocity adsorption sites is proportional to the number of unoccupied sites, and the adsorption site occupancy rate is proportional to the square of the number of unoccupied sites.

Fig. 4

For the process CO₂/nano-CaO, the maximum adsorption capacity (q_e, mmol/g) increases with temperature (25°C, 50°C, and 75°C) at room pressure, whereas the rate constant diminishes with this parameter. This effect suggests a greater driving force of the gas molecules when temperature increases, leading to faster diffusion of CO₂ and an increase in CO₂ capture. These kinetic results are important regarding cost-effective adsorption technologies because the pollutant CO₂ is removed in a relatively short time. For all the studied conditions, CO₂ adsorption was observed in the first minute of contact, equilibrium was achieved in approximately 15 min, and no further changes in CO₂ adsorption were observed afterward. The increasing of CO₂ adsorption with the contact time prior to reaching equilibrium is due to the high availability of the active sites of the adsorbent, and the process slowed in the later stages of contact time due to a decrease in the number of active sites. The experiments for determining other adsorption parameters were carried out with 20 min contact time.

The values of R² for the intraparticle diffusion model are good enough as well. The equation of this model is the following: (2) qt=KiPt+I, {q_t} = {K_{iP}}\sqrt t + I, where K_iP corresponds to the rate constant of each step of the process and I is the intercept that provides an idea about the boundary layer thickness: i.e., the larger the intercept, the greater is the boundary layer effect (in mmol/g). According to this model, the gas molecules may diffuse into the particles when the solid adsorbent is porous enough. The adsorption of the gaseous phase on and in the adsorbent may occur via three main stages: (1) transport of the gas molecules from the volumetric phase to the adsorbent surface; (2) diffusion of the gas molecules from the surface to the adsorbent pores; and (3) diffusion in the interior of the pores.

Data on the adsorption of CO₂ by the CaO nanoparticle adsorbent treated according to Eq. (2) showed that it was a multilinear process (Figure 5). The first line corresponds to the instant adsorption from the volumetric phase to the surface of the adsorbent; the second section is attributed to the gradual intraparticle diffusion; and the third line corresponds to the final equilibrium stage [25]. The first stage was very fast and was completed in the first 1 min; the second stage of intraparticle diffusion was reached at 10 min; thereafter, equilibrium was achieved. The value of K_iP for the first stage was higher than that of the second one (Table 1). Therefore, the adsorption of CO₂ on the highly porous nano-CaO adsorbent was probably not only due to intraparticle diffusion but this mechanism may be a consequence of the adsorption itself.

Fig. 5

Data of q_e obtained under different supply pressures (1 atm, 5 atm, 10 atm, and 15 atm) and at different temperatures (25°C, 50°C, and 75°C) were treated using the linear equations of both Langmuir [30] and Freundlich [31] adsorption isotherm models. The results are presented in Table 2, where the higher value of the correlation coefficient (R²) corresponds to the Langmuir model. The linear form of this model is expressed as follows: (3) PCO2qe=(1qmax)PCO2+(1KL qmax) {{{P_{{CO}_2}}} \over {{q_e}}} = \left({{1 \over {{q_{max}}}}} \right){P_{{CO}_2}} + \left({{1 \over {{K_{L\,{q_{max}}}}}}} \right) where P_CO2 is the pressure (in atmospheres) of the adsorbed gas CO₂, q_e is the adsorption capacity of CO₂ at equilibrium (mmol/g), q_e is the maximum adsorption capacity of CO₂ at equilibrium (mmol/g), and K_L (/atm) is the Langmuir isotherm constant, which represents the affinity between the adsorbent and adsorbate and may be used to predict whether an adsorption system is favorable or unfavorable.

The Langmuir isotherm parameters at 25°C, 50°C, and 75°C were evaluated from the slopes and the intersections of the plots P_CO2/q_e versus P_CO2 (Figure 6). As expected, the CO₂ adsorption capacity increased with the CO₂ pressure, which is the driving force of the adsorption process. Based on the data of Table 2, both K_L and q_max increase with temperature. This indicates that the adsorbent surface was well coated with CO₂ molecules as a result of their higher surface affinity by implementing a chemisorption behavior. CO₂ must be chemisorbed on fixed surface sites.

Fig. 6

Moreover, according to the Langmuir model, the surface of the porous nano-CaO adsorbent is homogeneous, and CO₂ adsorption occurs in a monolayer of CaO; in this case, the formation of multilayers, as proposed by the Freundlich model, is restricted [32].

The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant called the separation factor (R_L), which examines the favorable acceptance of the adsorption process [33]. An adsorption process is irreversible if R_L =0, favorable if 0 < R_L < 1, linear if R_L = 1, and unfavorable if R_L > 1. The equation for determining the dimensionless R_L values is the following: (4) RL=11+KLPCO2 {R_L} = {1 \over {1 + {K_L}{P_{{CO}_2}}}} where K_L is the Langmuir constant or the affinity constant (/atm), and P_CO2 is the equilibrium pressure of the adsorbed gas (in atmospheres). From the experimental results, the R_L terms showed values from 0.67 to 0.12 (25°C), from 0.74 to 0.16 (50°C), and from 0.77 to 0.18 (75°C), for the range of pressure from 1 atm to 15 atm. These values of R_L, which are <1, indicate that the conditions of the adsorption process are favorable.

Important factors, such as energy changes in a reaction, should be considered when developing an adsorption system, and thermodynamic concepts are required to achieve this outcome. The thermodynamic parameters provide in-depth information on the inherent energy changes associated with adsorption mechanisms. To evaluate these parameters, the linearized form of the van’t Hoff modified equation is the following: (5) lnKL=−ΔH0R(1T)+ΔS0R, \ln {K_L} = - {{\Delta {H^0}} \over R}\left({{1 \over T}} \right) + {{\Delta {S^0}} \over R}, where K_L is the Langmuir equilibrium constant; ΔH° and ΔS° are the enthalpy and entropy changes, T is the absolute temperature (in Kelvin), and R is the universal gas constant (8.314 × 10⁻³ kJ/mol K). The slope and intersection of the straight-line diagram of lnK_L versus 1/T allows the determination of the values of ΔH° and ΔS° (Figure 7).

Fig. 7

The thermodynamic analysis revealed that the adsorption process is endothermic due to the positive value of ΔH° = 32.6 KJ/mol. The magnitude of ΔH° can also be used to distinguish between the mechanisms of physisorption (<20 kJ/mol) and chemisorption (~80 kJ/mol to 200 kJ/mol) [34]. As physisorption arises from relatively weak interactions, such as the van der Waals force, chemisorption involves stronger chemical interactions. According to Moroto-Valer et al. [35], the process of physisorption involves a high surface adsorption of energy and the diffusion of molecules at elevated temperatures. The ΔH° value obtained for CO₂ adsorption onto the CaO sample was 32.6 kJ/mol, which is indicative of both physisorption and chemisorption processes. The ΔS° value was 0.102 kJ/mol K, and the positive value reflects the affinity of CO₂ for the nano-CaO adsorbent and suggests a high disorder of the adsorbate molecules during the fixation of the CO₂ molecules onto the active sites of CaO. According to Pighini et al. [36], entropy change is related to the mobility of CO₂ and to the coverage on the adsorbent; a high entropy value must be related to chemisorption.

The Gibbs free energy change (ΔG°) is an important criterion for spontaneity, and both energetic and entropy factors must be considered for its evaluation; according to the following equation: (6) ΔG∘=ΔH∘−TΔS∘. \Delta {G^ \circ} = \Delta {H^ \circ} - T\Delta {S^ \circ}.

The results of ΔG° were of a positive sign, which implies a nonspontaneous adsorption reaction [37]; moreover, these values diminish with temperature (3.2 KJ/mol, 1.7 KJ/mol, and 0.7 KJ/mol at 298 K, 232 K, and 348 K, respectively), which suggests that the adsorption of CO₂ on the CaO-based nanoadsorbent is more favored at higher temperatures.

The adsorption activation energy (E_a) was calculated by using the Arrhenius linear equation: (7) lnKv=−EaR(1T)+lnA \ln {K_v} = - {{{E_a}} \over R}\left({{1 \over T}} \right) + lnA where k_v is the velocity constant, A is the independent temperature factor (millimoles of substrate per milligram per minute [mmol/mg min]), R is the universal gas constant (8.314 × 10⁻³ kJ/mol K), T is the temperature (in Kelvin), and E_a is the activation energy of adsorption (in kilojoules per mole [kJ/mol]). This is the minimum energy that must be overcome by the adsorbate molecules and it defines the velocity of the process [38].

The values of the velocity constants of the pseudo-second-order kinetic model at 298 K, 323 K, and 348 K (Table 1) were incorporated into Eq. (7) as a function of 1/T (Figure 8). The values of the slope and the ln k_V = 0 intersection of that plot allowed us to obtain E_a = −73.4 KJ/mol and ln A = ~26.9. Otherwise, considering the experimental data of k_V at 298 K and 348 K and the Arrhenius equation, in which the factor A is eliminated, we get the following expression: (8) ln(Kv1Kv2)=−EaR(1T1−1T2) ln\left({{{{K_{v1}}} \over {{K_{v2}}}}} \right) = - {{{E_a}} \over R}\left({{1 \over {{T_1}}} - {1 \over {{T_2}}}} \right)

Fig. 8

The calculated value of E_a = −74.3 KJ/mol was virtually the same as that obtained from Eq. (7). The value of E_a (~80 KJ/mol) confirms that the adsorption mechanism is chemisorption. Typically, physisorption requires low E_a values, whereas chemisorption is characterized by higher values [39]. In general and based on the signs and magnitudes of the thermodynamic parameters obtained in the present study, it can be established that the adsorption of CO₂ on the porous nano-CaO adsorbent is mainly produced by chemical adsorption through a thermodynamically nonspontaneous process, and the adsorption of CO₂ molecules is positively temperature dependent. These data provide valuable information for the design of improved adsorption schemes to capture CO₂ using a new nanoadsorbent based on CaO.

CO₂ capture technologies require the understanding of the adsorption mechanism to enable use of an adsorbent at the industrial level. According to the literature, a gas-solid carbonation reaction occurs when CaO is directly combined with CO₂ [40]. In the CaO-CO₂ reaction, the CO₂ molecules can interact with the CaO surface, and when this occurs, carbonation directly follows. The chemical reaction may progress until the complete formation of the carbonate thin layer on the surface of the CaO particles, but this layer may block the access of CO₂ to the internal active CaO sites of the particles. For chemisorption, the CO₂ molecule must diffuse into the pores of the adsorbent and interact with the internal surface of the adsorbent [41]; it has also been shown that reducing the crystallinity of the adsorbent favors diffusion and promotes recharging [42].

In this study, the crystalline structure of the adsorbent, based on CaO, and both the particle and crystal sizes were reduced by the ball milling treatment. This caused a beneficial effect of more conversion from CaO to CaCO₃, attributed to the CaO nanoadsorbent’s fluffy structure, significant surface area, and large pore volume, which allows CO₂ molecules to easily diffuse into the pore structure and reach deeper active sites, which leads to maximum CO₂ capture.

In this framework, the nanostructured nature of CaO caused reactivity and increased adsorption effects involving many active surface sites available for CO₂ adsorption. This also allowed the introduction of a large amount of CO₂ into its mesopores in a well-dispersed manner that favors the gas-solid diffusion reaction and results in increased CO₂ capture. As observed in our results, a mesoporous nano-CaO adsorbent, with a mean pore radius of 19.24 nm at approximately 80%, was obtained. In this way, we improved intraparticle diffusion in the CO₂ capture process in the porous CaO because the kinetic radius was sufficient for the diffusion of CO₂ molecules within the CaO porous structure when considering a CO₂ molecular size of 0.33 nm. The carbonation reaction in the micro-pores is hindered by a limited space that restricts the growth of CaCO₃. For CO₂ capture with CaO-based adsorbents, mesopores (2–50 nm in diameter) are expected to play an important role based on previous studies on carbonation. Furthermore, these pore types are the dominant contributors to the carbonation reaction and have a high surface area and high pore volume that accommodates the bulky CaCO₃ product [43]. The chemical adsorption for the whole process in this material has also been confirmed. Yet, to the best of our knowledge, the physical adsorption process is also controlled by the pore characteristics of the adsorbent, and the micropores (d_p <2 nm) create more selective CO₂ adsorption. According to Lee and Park [44], the gas adsorption potential reaches a maximum at the subnanometer scale, when the gas molecules begin to overlap with reducing pore size. This results in a much higher bonding energy emanating from the formation of deep potential fields from the neighboring walls, which leads to an increase in the surface area. In our case, narrow pores of the prepared nano-CaO adsorbent were approximately 15% of the total, which were also responsible for the physical capture of CO₂.

Finally, it is possible that the most appropriate reaction mechanism in this solid-gas system mainly occurred by chemisorption, with a carbonation reaction on the surface amid a favorable pore morphology of the CaO adsorbent obtained after treatment by ball milling. This was clearly verified by XRD, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) analysis. However, a very small fraction of physisorption, as corroborated by FTIR [21], was present along with the chemisorption process. In general, all the previously mentioned analysis results confirm the remarkable contribution of chemical adsorption throughout the CO₂ adsorption process in the nano-CaO adsorbent. One of the innovative aspects of this work was the favorable capacity of CO₂ capture in this adsorbent at moderate temperature and pressure.

In postcombustion processes from large stationary sources, the flow gas is released at temperatures in the range of 40–160°C and at a total pressure of approximately 1 atm [45]. The CO₂ capture behavior at moderate temperature and pressure using the prepared porous nano-CaO adsorbent is a feasible preprocess, which determined the intrinsic CO₂ capture capacity under typical postcombustion operating conditions. Furthermore, these results are of critical importance for carbon capture sequestration (CCS) technologies using adsorption processes.

Cycling processes may be the cause of CaO-based adsorbent decay. Therefore, knowing the stability of the adsorbent is necessary for CCS technology implementation on a large scale. A high stability would result in the reduction of capital cost and energy. The q_ad of five cycles was 11.0 ± 0.1 meq/g; therefore, the adsorbent could be regenerated five times without any significant loss of CO₂ adsorption performance. Moreover, the material remains stable because the loss of the original samples after the five cycles was <2 wt%. Based on future energy scenarios, it is obvious that this porous nano-CaO adsorbent is an efficient material for balancing the costs and engineering feasibilities for different CO₂ capture processes. Previous experimental reports over multiple cycles of adsorption-desorption activity at different adsorption temperatures of CaO-based adsorbent showed a loss of carbonation activity with a high number of cycles [46].