Co₂ Adsorption Study on Nio and Pr₂o₃ -Nio Catalyst Synthesis Using Simple Sol-Gel Method

Introduction

Over the past few years, the amount of carbon dioxide (CO₂) in the atmosphere are exponentially increasing and it is predicted to follow a similar trend in the future.Atmospheric carbon dioxide (CO₂) levels have grown exponentially in the last two centuries as a consequence of the larger anthropogenic CO₂ emissions due to the high demand of fossil fuels by an increment of the world population and the industrial development. This fact has caused that levels of atmospheric CO2 increase to above 400 ppm, which has entailed global warming and ocean acidification [1]. This increment is significant and it was expected to increase further due toenergy demand for economic and population growth. As known, at this moment the fossil fuels are the dominant energy resources which provide 86% share in the global energy utilization [2]. This large utilization of fossil fuels accounts for 75% carbon dioxide (CO₂) emissions to the atmosphere from various industries such as fossil fuelled power plants, cement industry, refinery and synthetic ammonia production units [3]. Burning of fossil fuels for transportation, electricity and heat are responsible for almost all of the increase of CO₂ in the atmosphere over the last 100 years.

As CO₂ is the main greenhouse gas (GHG) and the root cause of global warming which influence the climate change [4], extensive studies into creating effective CO₂ capture solution to mitigate CO₂ emission is explored.Among the known methods, adsorption shows the best results in terms of efficiency, energy costs and versatility to different compounds [5]. Thus, the development of novel adsorbent materials for CO₂ adsorption is a greatly concerned step for practical CO₂ capture and storage (CCS)applications. In general, ideal adsorbents should have a high CO₂ adsorption capacity, excellent adsorption selectivity over other gases, and a good chemical and mechanical stability. Recently, wide variety of porous materials including mesoporous silica [6], amine functionalized mesoporous silica [7], mesoporous alumina [8], metal organic frameworks (MOFs) [9-11], activated carbon [12] as well as mesoporous carbons [13]have been tried as solid adsorbents for CO₂. However the performance of these materials for CO₂ adsorption is still low. Latest study by Li et al., (2017), shows that the modification of mesoporous carbon by nickel oxide had increased the adsorption capacityof CO₂ and their selectivity due to better interaction between CO₂with the introduced metal oxides [14]. This finding suggested that metal oxide has a great potential to be used as CO₂adsorbent. Thus in this study nickel oxide (NiO) and praseodymium oxide mixed nickel oxide (Pr₂O₃-NiO) has been synthesized and applied for CO₂ adsorption. The interaction of NiO and Pr₂O₃ with CO₂ were also investigated.

Experimental

Catalyst Preparation

Pr₂O₃-NiO was prepared using simple sol-gel method. 2.00 gof Ni(NO₃)₂•6H₂Owas dissolved with minimum amount of distilled water and stirred for 15 minutes. Then, 2.00 g of Pr(NO₃)₃•6H₂O was added and continuously stirred. The mixture was then transferred into an evaporating disc and aged in an oven at 80^oC for 24 hours. A greenish solid gel was formed and calcined at 400^°C for 2 hours.NiO catalyst was prepared using similar procedure without the addition of Pr(NO₃)₃•6H₂O.

Catalyst Characterization

The crystal structure was studied using X-Ray diffraction (XRD), Bruker D8 Diffractometer with Cu-Kα (λ = 1.54021 Å) and scans were performed in step of 0.2^o/second over the range of 2θ from 10 up to 70^o.The morphology of samples was observed via SEMusing Philip XL 40.N₂gas adsorption isotherm was taken after pre-treatment in vacum at 473 K and surface area was analyzed from the isotherm using Brunauer-Emett-Teller equation.

Catalytic Performance

Temperature Programmed Desorption of CO₂ (TPD-CO₂) was recorder on a Thermo-Finnigan TPD/R/O 1100 fitted with a thermo-conductivity detector (TCD) and controlled by a computer. In order to remove surface contaminants, the sample (0.1 g) loaded in a quartz reactor was pre-treated at 100^oC in a nitrogen stream for 1 hour. After cooling to room temperature, a flow of 5% ml/min (30ml/min) of CO₂ gas was passed through the sample and the temperature was raise at the rate of 10^oC/min form room temperature up to 600^oC, while the TCD signal was recorded.

Results and Discussion

Figure 1 shows the XRD pattern of NiO and Pr₂O₃-NiO catalyst. For NiO catalyst, three sharp and narrow peaks were observed at 2θ of 37.24, 43.27 and 62.87^o with d values of 2.41, 2.09 and 1.48 Å (Figure 1a). These peaks can be indexed as (1 1 1), (2 0 0), and (2 2 0)crystal planes of the crystalline cubic NiO[15]. Sharp and narrow diffractionpeaks indicating that NiO catalyst sample is highly crystalline because the sample was calcined at high temperature (400°C). During the calcination, the crystallite size NiOincreased, thus enhanced the crystallite growth in order to minimize the interfacial surface energy[16].On the other hand, broad and big XRD peaks was observed for Pr₂O₃-NiO catalyst (Figure 1b). Besides the peaks matched to cubic NiO phase at 2θ of 37.46, 43.78, 63.50 and one additional peak was appeared at 2θ of 28.54° with d values of 3.14 Å which is corresponded to cubic Pr₂O₃ (PDF 1995 d values (Å): 3.13 Å) [17]. Broad and big peaks suggesting that Pr₂O₃-NiO sample is less crystalline as compared to pure NiO. This is probably due to the present of Pr₂O₃, which can prevent the agglomeration of particles by occupying the position of nickel oxide [18]. The XRD pattern also revealed the absence of binary or ternary compounds as NiO and Pr₂O₃existed as individual structure proved that solid state reaction did not occur.

Figure 1: The XRD pattern of (a) NiO (b) Pr2O3-NiO Click here to View figure

 

Figure 2 shows the SEM micrograph of NiOand Pr₂O₃-NiO catalysts. As shown figure 2(a), NiO catalyst particlestend to accumulate, which resulted in the smooth catalyst surface and pack particles. Upon the addition of Pr₂O₃, an irregularity of the particle shape was observed in which smaller particles were dispersed within the matrix of larger particles (Figure 2b). The near spherical shape particles were uniformly distributed on the catalyst surface. The surface of resultant sample is relatively rough without obvious agglomeration phenomenon. This indicated that the SEM results of Pr₂O₃-NiO catalyst is in good agreement with the XRD analysis which exhibited very broad peaks denoting a less crystalline character as compared to NiO catalyst.

Figure 2: FESEM micrographs of (a) NiO (b) Pr2O3-NiO Click here to View figure

 

Figure 3(a) illustrated the EDX spectra of NiO catalyst, which revealed that the sample is only consist of Ni and O elements existed as NiO compound which is consistent with XRD analysis. Nickel and oxygen emitted the X-ray signal at 7.386 kV and 0.886 kV respectively. Meanwhile, the EDX spectra of Pr₂O₃-NiO demonstrates the presence of Pr, Ni and O elements without other impurity elements (Figure3(a)). Praseodymium emitted the X-ray signal at 5.166 kV.

Figure 3: EDX of (a) NiO (b) Pr2O3-NiO   Click here to View figure

 

The N₂ adsorption-desorption isotherms of catalysts are illustrated in Figure4. In this research, all studied catalysts exhibited typical type IV isotherms with hysteresis loops attributing to capillary condensation in mesopores, which further demonstrates that the catalysts have mesoporous structure. It is noteworthy that the types of hysteresis loop between NiO and Pr₂O₃-NiO catalysts are slightly different. The NiOshowed H1-type hysteresis loop, while Pr₂O₃-NiO catalysts exhibited H2-type hysteresis loop. Type IV isotherm with H2-type hysteresis loop is closely associated with the “ink-bottle” pore structure, which leads to a not well-defined pore shape. Meanwhile, type IV isotherm with H1-type hysteresis loop appears in mesoporous materials comprising nearly spherical-shaped particles [19]. The BET surface area of NiO and Pr₂O₃-NiO catalysts was found to be 5.60 m²/g and 75.58 m²/g, respectively. Larger BET surface area of Pr₂O₃-NiO attributed to their smaller particle size, as Pr₂O₃suppressed the grain growth of NiO.

Figure 4: Isotherm plot of (a) NiO (b) Pr2O3-NiO Click here to View figure

 

Figure 5 shows the TPD profile of CO₂ desorption on NiO catalyst. Three desorption peaks at 90^oC, 140^oC and 240^oC was observed. These three peaks were attributed to the existence of three desorption sites, corresponding to different interaction of CO₂ with NiO. First peak at 90^°C was assigned to two dative covalent bonds between oxygen from CO₂ with Ni²⁺. Each oxygen atom donated lone pair electrons and occupied hybrid orbital of Ni²⁺ to form pure oxygen coordination as shown in figure 6[20]. As a results constrained coordination sphere was formed due to weak dative covalent bonds, therefore CO₂gas can be easily desorbed and released at low temperature.

Figure 5: TPD profile of CO2 desorption onNiO catalyst. Click here to View figure

 

Figure 6: Coordination of CO2 to Ni2+ via pure oxygen coordination [19]. Click here to View figure

 

The second peak at 170^oC was attributed to the pure covalent bond between carbon from CO₂with Ni²⁺ in NiO (figure 7) [19]. This bonding was formed when single electron of CO₂combined with the unpaired d electron on the Ni atom in its d configuration. This bonding is strong because the existence of pure covalent bond thus CO₂desorb at higher temperature.

Figure 7:Coordination of CO2 to Ni2+ via pure carbon coordination [19]. Click here to View figure

 

The third peak was appeared at the highest temperature of 240^oC owing to the mixed carbon-oxygen coordination (figure 8) [19]. The highest desorption temperature of this coordination maybe due to the existence of both pure covalent bond and dative covalent bond. The pure covalent bond was formed from the sharing of electrons between the Ni and electron from the carbon. In addition, the lone pairs from the oxygen were donated into the diffused sp hybrid orbitals of Ni to form dative covalent bond.

Figure 8: Coordination of CO2 to Ni2+ via mixed carbon-oxygen coordination [19]. Click here to View figure

 

The TPD profile of CO₂ desorption obtained for Pr₂O₃-NiO catalyst asshown in figure 9. It is slightly different with NiO catalyst, as four peaks was observed at temperature of 100°C, 190°C, 240°C and 440°C.

Figure 9: TPD profile of CO2 desorption by NiO-Pr2O3 catalyst.  Click here to View figure

 

The first three peaks at lowertemperature corresponded to the three peaks as observed in NiO catalyst. That peaks related to the chemical interaction between NiO and CO₂. An additional peak at 440°C is belong to Pr₂O₃ desorption sites, suggesting that incorporation of Pr₂O₃ species in NiO increased the adsorption site of CO₂.In fact, CO₂ adsorption site on Pr₂O₃ is more than NiO, as the signal of forth peak is the highest. The presence of Pr₂O₃ created a tight interface interaction between NiO-Pr₂O₃catalyst and CO₂. This is due to the weak basicity of Pr₂O₃ which resultedin the formation of carbonates. In fact, this is a common reaction in the acid base interaction to give an unstable surface carbonate species as an intermediate (Equation 1).

Pr₂O₃ + CO₂ → Pr₂O₂CO₃   (Equation 1)

As a results, the amount of CO₂adsorbed on Pr₂O₃-NiO catalyst was found to be 10 times greater than NiO catalyst as shown Table 1.

Table 1:  The amount of adsorbed gas by the catalysts.

Catalyst	Amount CO2 gas adsorbed (μmol/g)
NiO	32.53
Pr2O3-NiO	331.40

 

Conclusion

CO₂ adsorption studies over NiO and Pr₂O₃-NiO catalysts were carried out. Both catalysts was synthesized using simple sol-gel method. Pr₂O₃-NiO catalyst possessed less crystallinity as compared to NiO catalyst. Somehow, the CO₂ adsorption performance was found better for Pr₂O₃-NiO than NiO. The introduction of Pr₂O₃ increased the adsorption capacity of the samples because more CO₂ adsorption sites presence on the catalyst. Moreover, large surface area of Pr₂O₃-NiO support their good performance for CO₂ adsorption.

Acknowledgement

The authors are grateful to Universiti Malaysia Terengganu (UMT) for providing the facilities to carry out this project and Ministry of Higher Education of Malaysia for the financial support vote FRGS 59358.

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