Nickel Nanoparticles for Enhancing Carbon Capture

Hydration reaction of CO 2 is one of the rate limiting steps for CO 2 absorption (in aqueous solutions) and aqueous CO 2 mineralization.The catalytic activity of nickel nanoparticles (NiNPs) for CO 2 hydration is studied at different temperatures, pH, and lowCO 2 partial pressures tomimic the true flue gas conditions. Results show that NiNPs can work as active catalyst for hydration of CO 2 in applications such asCO 2 separation andCO 2 mineralization.TheNiNPs display optimumactivitywithin 20–30Cand at pH value <8. NiNPs show catalytic activity even at low CO 2 partial pressures (12 vol%). In 50wt% K 2 CO 3 solution, an enhancement of 77% is observed in the rate of CO 2 absorptionwithNiNPs. Commercially, CO 2 saturatedK 2 CO 3 solutions are usually regenerated at 150C; at these conditions, NiNPs show no considerable surface oxidation.They still exhibit catalytic activity for hydration reaction of CO 2 . CO 2 absorption and mineralization (as CaCO 3 ) in DI water are three times higher in presence of NiNPs. Calcite (CaCO 3 ) particles precipitated in presence of NiNPs are spherical in morphology.


Introduction
Recently an increase has been observed in global atmospheric temperatures due to the greenhouse gas emissions resulting from human anthropogenic activities [1]. A major contributor to this global warming is CO 2 emitted from combustion of fossil fuels used for power generation and transportation. Consequently, there is a need for reduction of emitted CO 2 from the atmosphere and its safe storage. The process of separation, transportation, and storage of CO 2 is called carbon capture and storage (CCS) [2,3].
Hydration of CO 2 is an important reaction in CO 2 absorption or wet chemical conversion of CO 2 to mineral carbonates. Amines and carbonates are widely used for CO 2 absorption separation [4][5][6]. Amine solutions have higher reaction rates with carbonic acid rather than CO 2(aq) [7]. Therefore, enhancing the rate of CO 2 hydration to carbonic acid can increase the rate of CO 2 separation and mineralization. The hydration of CO 2 is a pH dependent reaction with two different reaction mechanisms depending on the pH [8]. At pH values above 10, the reaction follows OH − reaction mechanism as shown by (1). At pH values below 8, it follows a H 2 O based reaction mechanism as shown in (2a) and (2b) [8]: The reaction in (1) is faster than in (2a) having the kinetic rate constant of OH 8500 M −1 s −1 and CO 2 0.03 s −1 , respectively [8]. Bhaduri andŠiller [9,10] observed that the hydration reaction of CO 2 with water is autocatalytic, due to the H + ions generated by the dissociation of carbonic acid (2b) [11]. Carbonic anhydrase (CA) is one of the most studied and effective catalysts for hydration of CO 2 with rate constant of 10 6 molecules per second [12]. Although CA is very efficient, its limitations include difficulty in recovery of catalyst, as it is a homogeneous catalyst [12], cost of immobilization [13][14][15], catalysing the reverse reaction at pH values below neutral (i.e., pH < 7) [16], and high extraction and processing costs [17,18]. Therefore, other homogeneous organic [19,20]

Experimental
The NiNPs are bought from NanoTechnology, Korea. Sodium hydroxide, calcium chloride, potassium carbonate, sodium carbonate, Bis-Tris buffer, and concentrated HCl (analytical grade) are purchased from Sigma Aldrich, UK. 99% CO 2 and 12% CO 2 -air synthetic gas were bought from BOC. 18.2 MΩ/cm resistivity water was used for all experiments obtained from Nanopure Barnstead UK.
The Bis-Tris-HCl buffer was prepared by mixing the Bis-Tris buffer in DI water (0.1 M). The pH of the buffer was adjusted by adding drop by drop of concentrated HCl. 0.1 M Na 2 CO 3 solution was also prepared by mixing Na 2 CO 3 in DI water. Saturated solution was made by bubbling CO 2 in DI water for 4 hours until it saturates. The procedure for qualitative kinetic analysis was carried out by the procedure described by Mirjafari et al. [30]. 20 mL of Bis-Tris-HCl buffer (or Na 2 CO 3 solution 0.1 M) was taken in a 50 mL beaker. The pH of the solution was monitored using HI2500 pH meter (Hanna Instruments) in Log mode with data logging after every 5 sec. After 5 stable readings logged in the pH log, 20 mL of saturated CO 2 solution was added to the Bis-Tris-HCl buffer (or Na 2 CO 3 solution 0.1 M). The pH was recorded until a stable reading was observed. For the NiNPs catalyst run, 30 ppm suspension of Bis-Tris-HCl buffer (or Na 2 CO 3 solution) was prepared and the above process was repeated for this solution. The experiment was performed three times and the average of the results is presented.
The CO 2 hydration kinetics experiment was performed similar to the method used by Bhaduri andŠiller [9]. The CO 2 hydration rate was carried out in a 250 mL glass jar within a fixed volume (200 mL) of DI water (or 30 ppm NiNPs suspension). The CO 2 gas was sparged into the solution at a flow rate of 50 mL/min at 0.1 MPa pressure using a glass sinter (Pyrex 1, Sigma Aldrich, UK). The pH changes of the solution are monitored during the bubbling experiments using a HI2550 pH meter (Hanna Instruments). The readings were at an interval of 20 seconds. The temperature of the sample was maintained by immersing the glass jar in a water bath (BS5, Fisher Scientific) at different temperatures (i.e., 10 ∘ C, 20 ∘ C, 30 ∘ C, 40 ∘ C, 50 ∘ C, and 60 ∘ C, resp.). The same experiment with 12% CO 2 -air synthetic gas 20 ∘ C is repeated five times and its average results are presented.
The CaCO 3 precipitation experiments were carried out using the following methodology. 100 mL of DI water (or 30 ppm NiNPs suspension) was taken in a 150 mL glass jar and was bubbled with CO 2 with a flow rate of 1.96 mM/min, at 0.1 MPa pressure different time intervals (i.e., 0.5 hr, 1 hr, 2 hrs, and 4 hrs, resp.). To the respective CO 2 saturated samples were added 10 mL of 1 M NaOH solution and 10 mL of 1 M CaCl 2 solution. The samples were stirred and the calcium carbonate was allowed to precipitate in the glass jar. The precipitates are filtered using a vacuum filter using a Watman 54, air dried at 60 ∘ C for 1 hour, and then weighted. The CaCO 3 precipitated in the presence of NiNPs were characterized using scanning electron microscopy (SEM) and Xray diffraction (XRD) for shape and structural determination. All experiments have been repeated three times and average data is presented.
To study the influence of NiNPs on the absorption rate of CO 2 in K 2 CO 3 solutions, the 50% (by weight) K 2 CO 3 solution was prepared in 50 mL DI water. For the NiNPs suspension-K 2 CO 3 solution, 1.5 mg of NiNPs was added into 50% K 2 CO 3 solution and sonicated in the ultrasonic bath at constant temperature to make a 30 ppm solution. The concentration of 30 ppm was selected based on our previous studies [9]. The CO 2 absorption experiments are performed in a 180 mL glass jar (Wheaton Industries, UK). The solution with the glass bubbler (Pyrex 1) was assembled and weighed before the introduction of CO 2 . The CO 2 gas was bubbled at a flow rate of 1.69 mM/min at a pressure of 0.1 MPa. The final weight was obtained when there was no further possibility of gas flow though the bubbler. The gas bubbler was blocked by the formation of potassium bicarbonate crystals on the glass bubbler. The entire absorption experiment was carried out at 20 ∘ C using a water bath (BS5, Fisher Scientific). All the above experiments were carried out in triplets and average data is presented.
For the heat treatment, the NiNPs were placed in a silica crucible and heated in an oven at 150 ∘ C for 8 hours. The NiNPs was kept in an air tight container with ample desiccator beads to absorb any moisture present until further use. X-ray photoelectron spectroscopy (XPS) of the NiNPs before and after heat treatment was carried out using (Kratos Axis Ultra) equipped with a monochromatic Al K X-ray source, with a pass energy of 20 eV. The spectra were fitted with Shirley background [31] and mixed singlet peaks [32]. The spectra were calibrated by assigning the C 1s peak at 284.4 eV to adventitious carbon [33,34]. All binding energy values reported are within an error of ±0.1 eV. The CO 2 hydration catalysis using the heat treated NiNPs was carried out using the same methodology used for temperature dependent kinetics mentioned above. CO 2 catalysis was carried out for the heat treated NiNPs using same methodology as Bhaduri andŠiller [9] at room temperature and atmospheric pressure. The kinetic analysis of the heat treated NiNPs and DI water was carried out in triplet and average data is presented. Figure 1(a) shows the high resolution transmission electron microscope images of the commercially purchased NiNPs to determine their size distribution. The particle size distribution of the nanoparticles based on statistical analysis is presented in Figure 1(b). The average particle size of the NiNPs was determined by fitting the distribution curve with a Gaussian function and was ∼43 nm. The majority of the particles smaller than the prescribed manufacture size of 100 nm.

Kinetics of CO 2 Hydration Reaction with NiNPs at Low
Partial Pressure of CO 2 . It is known that the rate of CO 2 hydration is dependent on the concentration of CO 2(aq) [4,9,35]. CO 2(aq) concentration depends on the partial pressure of CO 2 in the gas phase. As the partial pressure decreases in the gas phase, the concentration of CO 2 molecules decreases at the interface and thus the reaction kinetics should decrease [35]. Figure 2(a) shows the change in pH of DI water and NiNPs suspension when 100% CO 2 gas is bubble in them, respectively. Figure 2(b) shows that the pH change is still more rapid in the presence of NiNPs in suspension as compared to DI water when 12% CO 2 -air gas mixture is bubbled through them, respectively. This result is similar to the results observed previously [9]. The results in Figure 2(c) show that the catalytic hydration rate of CO 2 (in presence of NiNPs) slightly decreases when the partial pressure of the CO 2 in the gas phase is decreased. However, it can be DI water DI water + NiNPs  Figure 2: (a) pH changes when 100% CO 2 -air mixture is bubbled in DI water and 30 ppm NiNPs suspension, (b) pH changes when 12% CO 2 -air mixture is bubbled in DI water and 30 ppm NiNPs suspension, (c) pH change when 12% and 100% CO 2 is bubbled in 30 ppm NiNPs suspension, and (d) pH change when 12% and 100% CO 2 is bubbled in DI water. observed from Figure 2(d) that the autocatalytic activity of CO 2 hydration in DI water is highly reduced when the partial pressure of CO 2 is reduced. This proves the assumption made previously about partial of CO 2 affecting the rate of CO 2 hydration.
All the above data suggest that in the presence of the NiNPs the rate limiting step is the transfer of CO 2g to CO 2(aq) rather than the hydration of reaction of CO 2 . The reduction in rate of CO 2 hydration (observed as the rate of pH change in Figure 2(c)) is due to the slow gas-liquid mass transfer due to the low partial pressure of CO 2 . 2 Hydration Reaction. A qualitative method was used to test the pH dependence of the NiNPs' activity for CO 2 hydration. The methodology was adapted from kinetic study of CA hydration as described by Mirjafari et al. [30] on recommendation of Britt [36]. Bis-Tris-HCl buffer has terminal OH groups for regulation of pH. As the NiNPs also have OH groups on their surface when dispersed in water [9], the adsorption of buffer would thus be limited. Figure 3 shows the pH changes in Bis-Tris-HCl buffer (Figure 3(a)) and sodium carbonate solutions (Figure 3 saturated solution was added. Figure 3(a) shows a faster pH drop in Bis-Tris-HCl buffer solution with addition of NiNPs. This indicates an increase in catalytic activity due to the presence of NiNPs at pH values below 8 with CO 2 hydration following the reaction mechanism described by (2a) (see Section 1) [8]. Figure 3(b) shows the pH changes at alkaline pH values (i.e., pH > 10) in Na 2 CO 3 solution. The NiNPs catalysis is minimal at alkaline pH (i.e., pH > 10) but can be observed when the pH drops below 10 ( Figure 3(b)). Above pH 10, CO 2 hydration can be described by (1) (see Section 1) whilst it operates via a mixed mechanism ((1) and (2a)) at pH values between 10 and 8 [8]. Therefore, NiNPs catalysis cannot be observed at alkaline pH values above 10 but can be observed at pH values below 10.

Temperature Dependent Kinetics of Catalytic Hydration
Reaction of CO 2 with NiNPs. NiNPs catalysis is analysed by observing the rate of change of pH when CO 2 is bubbled in DI water or NiNPs suspension. The change in pH is due to the formation of carbonic acid in solution. The results show (Figures 4(a)-4(f)) that in the presence of NiNPs the pH drop is more rapid than in its absence, but in the DI water system, there is an initial slow decrease in pH followed by a rapid decrease in pH. This is an autocatalytic behaviour of CO 2 hydration first observed by Bhaduri andŠiller [9] and is explained to be caused by the presence of H + ions (see Supplementary Information of [11] in Supplemetary Material available online at http://dx.doi.org/10.1155/2015/581785). Similar behaviour is observed in the current study. Due to the autocatalytic activity of hydration of CO 2 in DI water, the interpretation of the catalytic activity of NiNPs is done by observing the proximity of the catalysed and uncatalysed curves to one another. If the catalysed and uncatalysed curves are close to one another, the catalytic activity of NiNPs is low but if they are further apart from one another the catalytic activity of the NiNPs is high. Figures 4(a)-4(f) show the catalytic activity of NiNPs at all the tested temperatures. As the temperature changes, the proximity between the two curves (catalysed which is indicated with filled circles and uncatalysed which is indicated with filled squares) also changes mainly due to a change in the reaction kinetics of the uncatalysed hydration of CO 2 . A higher catalytic activity was observed for the NiNPs to catalyse the hydration of CO 2 at ambient temperatures (20-30 ∘ C) than that at lower (10-20 ∘ C) or higher (40-60 ∘ C) temperatures, respectively. For all the profiles for the pH change for noncatalytic reaction (the filled squares in Figures 4(a)-4(f)) of CO 2 hydration in DI water there is an initial slow change in pH followed by a rapid change in pH; these results are similar to the observations made by Bhaduri andŠiller [9]. It is evident form Figure 4(c) that the reaction rate of CO 2 hydration in DI water is slower at 30 ∘ C than at other temperatures. The NiNPs suspension shows similar pH change profile at all tested temperatures. The observed changes in kinetic rates are likely due to the adsorption/desorption of water or CO 2 on the NiNPs surface which is affected with temperature [37,38] along with the solubility of CO 2 .

Enhancing CO 2 Uptake in Potassium Carbonate Solution with
NiNPs. In the current study, gravimetric method was used to study the enhancement of CO 2 in carbonate solutions. Figure 5 shows the results of gravimetric uptake of CO 2 in 50 wt% K 2 CO 3 solution in the absence and presence of NiNPs. The experiment was run until no additional change in the weight of the reaction vessel was observed. The reaction 6 Journal of Nanomaterials  Time (min) 50 wt% K 2 CO 3 50 wt% K 2 CO 3 + NiNPs in the reactor can be represented by the following reaction equations: Carbonate solutions (K 2 CO 3 and Na 2 CO 3 ) have a high pH (i.e., >11) and as CO 2 is bubbled into the carbonate solution its pH drops [11]. The reaction 3a takes place when the K 2 CO 3 is instantaneously dissolved in DI water and CO 2 hydration reaction takes place via reaction (3b) till the pH reaches 10 [8]. Reaction mechanisms (3b) and (3c) simultaneously occur between pH 10 and 8 with reaction (3c) being the slowest step [8]. Reactions (3d) and (3e) like (3a) are instantaneous [39]. Thus, CO 2 hydration is the most important step in the CO 2 capture when using potassium carbonate solutions. Therefore, in summary, as the CO 2 and carbonic acid start reacting with the carbonate solution, the pH of the solution changes and the reaction mechanisms of CO 2 hydration changes [11] from (3b) to (3c) depending on the pH of the solution [8]. From Figure 5, it can be observed that initial rate of CO 2 absorption is similar for 50 wt% K 2 CO 3 solution both with and without NiNPs catalyst but after 60 minutes the rate of reaction is faster in presence of the NiNPs. This kind of enhancement in CO 2 absorption is a result of the carbonatebicarbonate buffering ability of carbonate solutions. Pinsent and Roughton [40] report that the rate of CO 2 hydration increases as the bicarbonate/carbonate ([HCO 3 − ]/[CO 3 2− ]) ratio increases. Therefore, as CO 2 reacts with the carbonate solution, the bicarbonate ions are formed (following reaction (3b)) (the value of the numerator increases while that of the denominator decreases), leading to the observed autocatalytic enhancement absorption of CO 2 . Figure 5 shows that in the presence of NiNPs there is the increase in weight of the carbonate solution (after saturation) due to enhanced kinetic rate of CO 2 hydration (but lower than the full capacity of the carbonate solution, which is 15.92 g, if there is 100% conversion of K 2 CO 3 to KHCO 3 ). There is an increase in CO 2 loading (mol of CO 2 per mol of K 2 CO 3 ) by ∼27%.
In Figure 5, the rapid increase in CO 2 absorption after 60 min until saturation is used to estimate the enhancement in absorption rate of CO 2 in carbonate solutions in presence of NiNPs. The data points between 60 min and saturation are linearly fitted (see Supplementary Figure S1) and the data is shown in Table 1. It can be observed that the data showed a good linear fit ( 2 > 0.96) and the slope can be used to compare the enhancement in rate. There is ∼77% increase in the absorption rate of CO 2 in carbonate solution in the presence of NiNPs under the same operating conditions.

Heat Treatment of NiNPs.
Carbonate solutions are generally regenerated at a temperature of 150 ∘ C [4,26,29]; therefore, in order to use NiNPs as enhancers in carbonate separation technology a set of experiments were performed to test the effect of heating on the catalytic activity of NiNPs. The NiNPs were heated at 150 ∘ C for 8 hours to study the changes in surface chemistry and its catalytic activity for CO 2 hydration. XPS analysis was done before and after heat treatment in order to check any surface chemistry changes due to heating. Figure 6 shows the surface species present on the NiNPs surface before and after heat treatment. Three different species were observed in Ni 2p 3/2 spectra for NiNPs before and after heat treatment, respectively. In Figure 6(a), the peaks at 852.5 eV, 854.1 eV, and 855.7 eV can be assigned to Ni 0 , NiO, and Ni(OH) x , respectively [41][42][43][44], and in Figure 6(b) the peaks at 852.4 eV, 854.0 eV, and 855.7 eV can be assigned to Ni 0 , NiO, and Ni(OH) x , respectively [42][43][44]. The peak at 861.0 eV in Figure 6(a) (or 860.9 eV in Figure 6(b)) is assigned the value for a collective plasmon peak of Ni [9,33,34]. No change in the peak positions of the Ni and NiO is observed (peaks are within the experimental error limit of ±0.1 eV) after heat treatment, although after heat treatment there is a 27% increase of oxide on the surface. The presence of the two peaks in the O 1s spectra shows the presence of oxygen in two oxidation states. In Figure 6(c), the peak at 529.3 eV corresponds to NiO [41] whereas the peak at 531.0 eV corresponds to NiOH x [45]. The existence of the NiOH x is due to the dissociative adsorption of water from the atmosphere. It is known that the OH bond on the surface has a binding energy difference of 1.8 eV higher than that of   NiO [45]. Thus, the peak at 531.0 eV is assigned to NiOH x . The peaks in O 1s at 529.5 eV and 531.2 eV after heat treatment observed in Figure 6(d) also correspond to NiO [41] and NiOH x [45,46]. The C 1s peak in Figures 6(e) and 6(f) at 284.4 eV can be attributed to adventitious carbon [33,34]. The survey spectrum shows presence of Ni 3p, Ni 2p, C 1s, and O 1s peaks for NiNPs before (Figure 6(g)) and after (Figure 6(b)) heat treatment. Figure 7 shows the X-ray diffraction pattern of the NiNPs before and after heat treatment at 150 ∘ C for 8 hours. The XRD pattern detects the bulk changes taking place in the NiNPs due to heat treatment.  [47,48]. The absence of NiO peaks (2 = 43.20 ∘ (most intensive), 62.87 ∘ , 75.20 ∘ , and 79.38 ∘ ) from the XRD diffraction patterns shows that no any significant bulk oxidation of the NiNPs occurred during the heat treatment.
Based on the results of the XPS analysis, the catalytic activity of the heat treated NiNPs was studied. Figure 8 shows the pH change profile when CO 2 is bubbled in NiNPs suspension compared to DI water. The pH change profiles of heat treated (Figure 8(a)) and untreated (Figure 8(b)) NiNPs compared to DI water are similar. The catalytic activity of NiNPs was observed even after heat treatment (150 ∘ C for 8 hours in air) thus making the NiNPs a suitable candidate to be used as enhancers in carbonate absorber systems.    Figure 9 shows the amount of CaCO 3 precipitated in presence and absence of NiNPs in DI water. The CaCO 3 precipitate yield in the presence of the NiNPs was higher than that of DI water alone; therefore, the NiNPs increase the CO 2 uptake and hydration constant rate even under the presaturation conditions. The CaCO 3 precipitate yield in the presence of the NiNPs was three times higher than that of DI water, similar to the results observed by Bhaduri andŠiller [9]. Figure 10 shows the SEM images of the CaCO 3 precipitate in presence of NiNPs. Micron sized (2-10 m) spherical precipitates are observed. XRD of the precipitate is shown in Figure 11, which confirms the presence of the calcite phase of CaCO 3 [49]. The diffraction plane of Ni [111] is also present [50] which indicates also the presence of NiNPs in the precipitates. Therefore, the NiNPs could act as nucleation sites for the crystals to grow, since the NiNPs are spherical in shape (reported previously [9]), and the carbonate precipitates are also spherical (instead of normal cubic shape [51,52]). Bodor et al. [22] studied the application of NiNPs for mineralization of CO 2 using alkaline wastes slurries containing MgO or CaO. They suggested that the activity of NiNPs is dependent on the solid to liquid ratio of the slurry based on the observation of low enhancement at high solid to liquid ratio. They did not provide a reason for the decrease in the activity of NiNPs. We suggest, based on our mineralization results, that at higher solid to liquid ratio the in situ mineral precipitation of carbonates (MgCO 3 or CaCO 3 ) would have blocked the NiNPs active sites. The carbonate (MgCO 3 or CaCO 3 ) would have precipitated on the NiNPs coving the particles. Thus, this would have lowered the activity of NiNPs.

Summary
In the current study, the temperature, pH dependence, and CO 2 partial pressure dependence of NiNPs catalysis for hydration of CO 2 are studied. As compared to other homogenous catalysts used for CO 2 hydration, NiNPs are cost-effective and have ease of separation for repeatable use. The NiNPs catalysis was optimum within 20-30 ∘ C for hydration of CO 2 and at pH below 8. NiNPs catalysis for hydration of CO 2 is observed at lower partial pressure of CO 2 . NiNPs show an enhancement in CO 2 uptake of 50 wt% K 2 CO 3 solution. An enhancement of CO 2 absorption rate of  Figure 11: XRD of the precipitate of CaCO 3 obtained in presence of NiNPs catalysed hydration of CO 2 using NaOH and CaCl 2 . Insert is the magnification of the later end of the XRD pattern to show the Ni peak.
∼77% was observed in the presence of NiNPs more than in its absence. This suggests that NiNPs can be used as enhancers for CO 2 separation using carbonate solutions and could work for other alkaline absorbing solutions as well. It was observed the heat treatment does not have major changes on the surface of NiNPs and the catalytic activity of NiNPs is retained after heat treatment. As compared to other homogeneous catalysts, the NiNPs have an advantage to be used at high temperature CO 2 separation with low loss in activity. The NiNPs also increase the precipitation of CO 2 as CaCO 3 in DI water by up to three times as compared to in its absence. The NiNPs act as nucleates for CaCO 3 growth and thus the calcite precipitates are spherical in shape. This suggests that the NiNPs need to be separated from the solution before carbonate formation during aqueous mineralization carbon capture and storage. Alternatively, the NiNPs can be immobilised on supports for easy separation and reuse during aqueous mineralization of CO 2 as it has been recently demonstrated by Han et al. [23].