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Catalysts 2019, 9(9), 757; https://doi.org/10.3390/catal9090757

Article
Adsorption Characteristics of Gas Molecules (H2O, CO2, CO, CH4, and H2) on CaO-Based Catalysts during Biomass Thermal Conversion with in Situ CO2 Capture
1
Key Laboratory for Biomass Gasification Technology of Shandong Province, Energy Research Institute, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250014, China
2
School of Civil Engineering, University of Science and Technology LiaoNing, Anshan 114051, China
*
Author to whom correspondence should be addressed.
Received: 7 August 2019 / Accepted: 2 September 2019 / Published: 9 September 2019

Abstract

:
Biomass thermochemical conversion with in situ CO2 capture is a promising technology in the production of high-quality gas. The adsorption competition mechanism of gas molecules (H2O, CO2, CO, CH4, and H2) on CaO-based catalyst surfaces was studied using density functional theory (DFT) and experimental methods. The adsorption characteristics of CO2 on CaO and 10 wt % Ni/CaO (100) surfaces were investigated in a temperature range of 550–700 °C. The adsorption energies were increased and then weakened, reaching their maximum at 650 °C. The simulation results were verified by CO2 temperature-programmed desorption (CO2-TPD) experiments. By the density of states and Mulliken population analysis, CaO doped with Ni caused a change in the electronic structure of the Osurf atom and decreased the C–O bond stability. The molecular competition mechanism on the CaO-based catalyst surface was identified by DFT simulation. As a result, the adsorption energies decreased in the following order: H2O > CO2 > CO > CH4 > H2. The increase of CO2 adsorption energy on the 10 wt % Ni/CaO surface, compared with the CaO surface, was the largest among those of the studied molecules, and its value increased from 1.45 eV to 1.81 eV. Therefore, the 10 wt % Ni/CaO catalyst is conducive to in situ CO2 capture in biomass pyrolysis.
Keywords:
in situ CO2 capture; DFT; 10 wt % Ni/CaO; adsorption; CO2-TPD

1. Introduction

Biomass thermochemical conversion with in situ CO2 capture technology is one of the hotspots in biomass energy utilization research, which can produce high-quality fuel gas with a wide range of applications (alcohol/ether fuel synthesis, hydrogen fuel cells, etc.) [1,2,3,4,5]. The in situ CO2 capture is the core process for the research and development of this technology because it can effectively remove CO2, adjust the gas composition, and improve the quality of pyrolysis gas [6,7,8,9,10]. In order to enhance CO2 adsorption capacity, CaO-based catalysts have been studied in biomass thermochemical conversion with in situ CO2 capture [1,2,3,11,12,13,14,15,16]. Naeem et al. [11] utilized the Pechini method to synthesize Al2O3-, MgO-, Y2O3-stabilized, and CaO-based CO2 sorbents. The experimental results exhibited high CO2 uptake with improved cyclic stability and superior performance when compared to the performance of limestone-derived benchmark sorbent. Charisiou et al. [12] investigated the catalytic performance of H2 production through the glycerol steam reforming reaction (GSR) of nickel catalysts supported on CaO–MgO–Al2O3. It was proved that the presence of CaO–MgO modifiers had an important effect on gaseous product distribution, leading to increased conversion to gaseous products. In our previous studies [1,2,3], a Ni/CaO catalyst was developed in biomass pyrolysis for high-quality gas, which had a profound impact on the CO2 adsorption capacity and catalytic cracking of tar effect at about 700 °C. However, at present, there are few researches on the mechanism of adsorption of CaO-based catalysts during biomass thermal conversion with in situ CO2 capture.
The adsorption characteristics of CaO-based catalysts are impacted by multiple reactions and a large number of intermediates (CO2, H2O, CO, H2, CH4, toluene, etc.) in the process of biomass thermal conversion with in situ CO2 capture, which cannot be simply explained by experiments. Molecular simulation can be utilized to study the reaction mechanism of a whole system, since it can save large amounts of manpower, materials, and time. Density functional theory (DFT) [17] plays an important role in the study of molecular simulation and is very convenient in the investigation of the adsorption mechanism of gas molecules on catalysts’ surfaces. In recent years, studies on adsorption simulation using the DFT method have mainly focused on the optimization of different adsorption configurations and on the theoretical analysis of the adsorption behaviors of adsorbent surfaces and molecules. Hong et al. [18] studied the adsorption of CO2 on the surface of CaO (001) at different coverages, from 1/9 monolayer (ML) to 1 ML using DFT calculations. The most stable adsorption structures at different coverages were obtained by analyzing the results. Sun et al. [19] simulated the dissociative adsorption of methane (DAM) on the surface of CaO (001) doped with Li, Na, K, and Cu. The calculations showed that the dopant lowers the energy of oxygen vacancy formation, increases the energy of the DAM reaction, and lowers the activation energy of DAM. Although some studies on the adsorption of gas molecules on the CaO surface have been reported, there are few studies on the simulation of small gas molecule adsorption on CaO-based sorbents in a biomass catalytic pyrolysis system.
In this paper, the adsorption characteristics of CO2 on the surface of CaO and 10 wt % Ni/CaO catalysts at different temperatures in a biomass catalytic pyrolysis system were studied by DFT simulation and experimental methods. Hence the feasibility of DFT method was verified. Based on this premise, the adsorption behaviors of other molecules (CO, H2O, H2, and CH4) on CaO and 10 wt % Ni/CaO (100) surfaces were further simulated to provide theoretical support for an in-depth analysis of complex catalytic reactions in biomass pyrolysis with in situ CO2 capture and production of high-quality gas.

2. Results and Discussion

2.1. Comparison of Adsorption Characteristics of CO2 on Two Catalyst Surfaces at Different Temperatures

The process of CO2 adsorption on CaO and 10 wt % Ni/CaO (100) surfaces, at temperatures of 550, 600, 650, and 700 °C, was simulated by the DFT method. Table 1 shows the adsorption energies and geometrical parameters of CO2 adsorbed on catalysts’ surfaces. In the initial configuration of CO2, the bond angle of O1–C–O2 was 180°, which changed after adsorption on the surface. Table 1 shows that CO2 could be stably chemisorbed on the surface of the two catalysts. Moreover, the adsorption energy of CO2 on the 10 wt % Ni/CaO (100) surface was significantly higher than on the CaO (100) surface, which proves that CO2 is more likely to form a stable adsorption structure on the 10 wt % Ni/CaO (100) surface. Ni-doped CaO is conducive to enhancing the adsorption of CO2 molecules on its surface, which promotes the in situ CO2 capture in the reaction system. In the temperature range of 550–700 °C, the adsorption energies of CO2 on CaO and 10 wt % Ni/CaO (100) surfaces increased and then weakened, reaching the maximum at 650 °C with 1.45 eV and 1.81 eV. The adsorption configurations of a single CO2 molecule adsorbed on CaO and 10 wt % Ni/CaO (100) surfaces at 650 °C are presented in Figure 1, which shows that the C–Osurf bond lengths were 1.487 and 0.1343 nm, while the bond angles of O1–C–O2 were 133.461° and 137.173°, respectively.
The density of states (DOS) can be used to characterize the electronic structure of a material. Figure 2 shows the partial density of states (PDOS) spectra of the Osurf atom at 650 °C. It should be noted from Figure 2a,b, which illustrates the PDOS of the Osurf atom on CaO and 10 wt % Ni/CaO (100) surfaces, that there were Osurf 2s and 2p peaks in the regions between −12 and −17 eV and 0 and −5 eV, respectively. Compared with the intensity of the Osurf 2p peak on the CaO (100) surface, the 10 wt % Ni/CaO surface decreased significantly, which can be explained by the Ni-doping affecting the electronic band structure of the Osurf atom, resulting in a decrease of the gravitational force between the nucleus and the outer electrons. It is clear from Figure 2c,d, which illustrates the PDOS of the Osurf atom on CaO-based catalyst surfaces after CO2 adsorption, that the Osurf 2p peak split and moved to lower energy levels, indicating that the Osurf 2p orbital electrons participated in the bonding and transferred to the C atom. Similarly, the Osurf 2s peak also changed, which reveals that the Osurf 2s and the Osurf 2p orbitals both contributed to the C–Osurf chemical bond. The above results are taken as a proof that the CaO catalyst doped with Ni changes the electronic structure of the Osurf atom and is beneficial for strengthening the adsorption of CO2 on the surface.
Mulliken’s population analysis reveals the strength of the chemical bond between atoms. The stability of the bonds decreases in a smaller bond population, while the possibility of the bond breaking increases during chemical reactions. Table 2 shows the bond populations of a CO2 molecule before and after adsorption on the catalysts’ surfaces. After adsorption, the Mulliken orbital population of C–O1 and C–O2 covalent bonds decreased. The bond population of CO2 on the 10 wt % Ni/CaO (100) surface was smaller than on the CaO (100) surface, especially the population of the C–O2 bond near the Ni atom, which reduced from 1.15 to 0.84. This indicates that Ni-doping decreases C–O bond stability in favor of the chemisorption reaction. This result provides strong evidence that CaO-based catalysts are beneficial to in situ CO2 capture in biomass catalytic pyrolysis systems, and the decarbonization capability of the 10 wt % Ni/CaO catalyst is better than that of the CaO catalyst.

2.2. Analysis of CO2 Temperature-Programmed Desorption (CO2-TPD) Experimental Results at Different Temperatures

The CO2-TPD experimental results are shown in Figure 3. At the adsorption temperatures of 550 °C, 600 °C, 650 °C, and 700 °C, both CaO and 10 wt % Ni/CaO catalysts presented a temperature-programmed desorption (TPD) peak. Figure 3 shows that when the adsorption temperatures increased, the intensity of the peaks rose and then decreased, reaching the highest values at 650 °C. The experimental results showed that the CO2 adsorption capacities of the CaO-based catalysts were enhanced in the following order: 550 °C < 600 °C < 700 °C < 650 °C, in agreement with the trend of adsorption energies in the above DFT simulation. It can be concluded that the DFT simulation method is feasible to study the adsorption mechanism of other molecules on the catalysts’ surfaces, which is beneficial for better understanding the complex catalytic reactions in biomass in situ CO2 capture pyrolysis systems.

2.3. Adsorption of H2O, CO, H2, and CH4 on Catalysts’ Surfaces

The above studies show that 650 °C is the most suitable temperature in the range of 550 °C–700 °C for the adsorption of CO2 on the catalyst surface, which is conducive to the decarburizing reaction. Therefore, the adsorption energies of H2O, CO, H2, and CH4 on CaO and 10 wt % Ni/CaO (100) surfaces at 650 °C were also calculated using the DFT method. By comparing the calculated adsorption energies, the adsorption competition mechanism of gas molecules on the catalyst surface was preliminarily identified.
Figure 4 shows the adsorption configurations of H2O, CO, H2, and CH4 on CaO and 10 wt % Ni/CaO (100) surfaces at 650 °C. After adsorption on the CaO (100) surface, the adsorption energies were 2.05 eV, 0.351 eV, 0.136 eV, and 0.199 eV, respectively. Additionally, the adsorption energies of H2O and CO molecules on the 10 wt % Ni/CaO (100) surface increased to 2.23 eV and 0.461 eV, respectively. The interaction between H2O and the 10 wt % Ni/CaO (100) surface was relatively strong, indicating obvious chemical adsorption. However, the weak adsorption energies of H2 and CH4 on the surface were 0.162 eV and 0.183 eV, respectively, indicating that the adsorption should not be viewed as traditional chemisorption.
The molecular competition mechanism on CaO-based catalyst surfaces was identified by DFT simulation. Table 3 shows the adsorption energies, which decreased in the following order: H2O > CO2 > CO > CH4 > H2. Among the molecules, the adsorption energy of H2O was the largest, resulting in a preferential reaction which competed with the adsorption of CO2 on the catalyst surface. The adsorption energies of CO, CH4, and H2 were so small that their effect on CO2 adsorption was not worth considering. The increase of CO2 adsorption energy on the 10 wt % Ni/CaO surface, in contrast with the CaO surface, was the largest, with the value increasing from 1.45 eV to 1.81 eV. Ni-doping obviously improved CO2 adsorption capability of CaO; therefore, the 10 wt % Ni/CaO catalyst is conducive to in situ CO2 capture in biomass pyrolysis. The adsorption capacity of each gas molecule on the catalyst surface affected the progress of each catalytic reaction in the system, so the molecular competition mechanism on the Ca-based catalyst provides a basis for adjusting the gas composition and preparing gas products directionally.

3. Computational and Experimental Methods

3.1. Computational Details

The first-principles calculations based on DFT were performed using the Cambridge Sequential Total Energy Package (CASTEP) and the Perdew-Wang-91 (PW91) functional within the generalized gradient approximation (GGA) [20]. The energy cutoff was set to 400 eV, and a 2 × 2 × 1 k-point grid sampling was used for Brillouin-zone integration throughout the study [21,22,23,24]. The dynamics were calculated using the NTV ensemble, and the time step for the solution of dynamic KS equation was configured at 1.0 fs. The total simulate time was 1.5 ps, and the total number of steps was 1500 [25,26,27].
CaO has the same crystal structure as sodium chloride with a lattice parameter of 4.8105 nm, the space group of 225, Fm3m. In this study, the CaO (100) surface was selected for the investigation of the adsorption behavior because it is the most stable surface of the CaO crystal [28]. Supercells (3 × 3) of the slab configurations, with a thickness of three layers, were used to study the adsorption properties of a CO2 molecule, where only the adsorbate could relax. The thickness of the vacuum layer between the two adjacent layer planes was 1.5 nm, ensuring that the interaction force between the planes was small enough [29].The adsorption energy (Eads) is defined as:
E a d s = E a d s o r b a t e + E s u r f a c e E a d s o r p t i o n + s y s t e m
where Eadsorbate, Esurface, and Eadsorbate + surface represent the energy of the catalyst surface, a gas molecule, and the total energy after adsorbing a gas molecule, respectively. If the adsorption energy obtained by simulation is within the range of 40–800 kJ/mol the process is chemical adsorption with strong adsorption. Otherwise, it is physical adsorption with relatively weak adsorption.

3.2. Experimental Details

CO2 temperature-programmed desorption (CO2-TPD) experiments were conducted using the chemical adsorption instrument FINESORB-3010 (Fantai, Hangzhou, Zhejiang, China). The samples were CaO catalyst prepared by calcining at 900 °C for 4 h, and 10 wt % Ni/CaO composite catalyst prepared by the method described in our previous study [5]. The samples were treated in high-purity carbon dioxide CO2 (≥ 99.999%) flow (15 mL/min) at 550, 600, 650, and 700 °C for 30 min. After that, they were cooled down to 100 °C in pure argon (Ar) (≥ 99.999%) flow (20 mL/min) and then heated from 100 °C to 900 °C at a heating rate of 10 °C /min, while the TPD spectra was recorded continuously [30].

4. Conclusions

In this paper, the adsorption of CO2 on Ca-based catalyst surfaces at different temperatures (550, 600, 650, and 700 °C) in the catalytic biomass pyrolysis system was studied using the DFT method and CO2-TPD experiments. The results indicated that the adsorption capability of CO2 on catalysts’ surfaces was strongest at 650 °C. In addition, the adsorption competition mechanism of gas molecules was investigated using the DFT method, proving that, in contrast with a CaO surface, a 10 wt % Ni/CaO surface is conducive to in situ CO2 capture in biomass pyrolysis. The above results provide theoretical support for in-depth studies on complex catalytic reactions in biomass pyrolysis with in situ CO2 capture.

Author Contributions

Conceptualization, H.G.; Formal analysis, J.W.; Funding acquisition, B.Z. and D.Z.; Investigation, J.W., G.S., and H.Y.; Methodology, J.W., L.S., and S.Y.; Project administration, B.Z.; Resources, L. C. and X.X.; Supervision, B.Z.; Validation, B.Z.; Writing—original draft, J.W.; Writing—review & editing, B.Z.

Funding

This work was supported by National Key R&D Program of China (2018YFE0106400), Shandong Provincial Key Research and Development Plan (2018GGX104028 and 2018GGX104026), Natural Science Foundation of Shandong Province of China (ZR2019MEE069, ZR2019MB061, ZR2016YL010 and ZR2016YL012).

Acknowledgments

The authors would like to thank all the laboratory technicians.

Conflicts of Interest

There are no conflicts to declare.

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Figure 1. Adsorption configurations of CO2 on the surfaces of CaO-based catalysts at 650 °C: (a) CO2 adsorption on CaO (100) surface; (b) CO2 adsorption on 10 wt % Ni/CaO (100) surface. Green, red, gray, and purple balls represent Ca, O, C, and Ni atoms.
Figure 1. Adsorption configurations of CO2 on the surfaces of CaO-based catalysts at 650 °C: (a) CO2 adsorption on CaO (100) surface; (b) CO2 adsorption on 10 wt % Ni/CaO (100) surface. Green, red, gray, and purple balls represent Ca, O, C, and Ni atoms.
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Figure 2. Partial density of states (PDOS) spectra of Osurf atoms s: (a) the PDOS of the Osurf atom on CaO (100) surface; (b) the PDOS of the Osurf atom on 10 wt % Ni/CaO (100) surface; (c) the PDOS of the Osurf atom on CaO (100) surface after CO2 adsorption; (d) the PDOS of the Osurf atom on 10 wt % Ni/CaO (100) surface after CO2 adsorption.
Figure 2. Partial density of states (PDOS) spectra of Osurf atoms s: (a) the PDOS of the Osurf atom on CaO (100) surface; (b) the PDOS of the Osurf atom on 10 wt % Ni/CaO (100) surface; (c) the PDOS of the Osurf atom on CaO (100) surface after CO2 adsorption; (d) the PDOS of the Osurf atom on 10 wt % Ni/CaO (100) surface after CO2 adsorption.
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Figure 3. CO2-TPD results of the examined catalysts: (a) the CO2-TPD results of the CaO catalysts; (b) the CO2-TPD results of the CaO catalysts.
Figure 3. CO2-TPD results of the examined catalysts: (a) the CO2-TPD results of the CaO catalysts; (b) the CO2-TPD results of the CaO catalysts.
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Figure 4. The adsorption configurations of gas molecules on CaO-based surfaces at 650 °C, left and right represent CaO and Ni/CaO: (a) the adsorption configuration of H2O; (b) the adsorption configuration of CO; (c) the adsorption configuration of H2; (d) the adsorption configuration of CH4. Green, red, gray, white, and purple balls represent Ca, O, C, H, and Ni atoms.
Figure 4. The adsorption configurations of gas molecules on CaO-based surfaces at 650 °C, left and right represent CaO and Ni/CaO: (a) the adsorption configuration of H2O; (b) the adsorption configuration of CO; (c) the adsorption configuration of H2; (d) the adsorption configuration of CH4. Green, red, gray, white, and purple balls represent Ca, O, C, H, and Ni atoms.
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Table 1. Geometrical parameters and adsorption energies of CO2 on catalysts’ surfaces at different temperatures.
Table 1. Geometrical parameters and adsorption energies of CO2 on catalysts’ surfaces at different temperatures.
Temperature (°C)Bond Length (Å) C–OsurfBond Angle (°) O1–C–O2Eads(eV)
CaO5501.529121.9241.39
6001.517127.3501.41
6501.487133.4611.45
7001.493138.3411.43
10 wt % Ni/CaO5501.392132.6041.69
6001.373136.2491.74
6501.343137.1731.81
7001.357127.2281.77
Table 2. Bond populations of CO2.
Table 2. Bond populations of CO2.
Orbital PopulationCO2CO2 on CaO (100)CO2 on 10 wt % Ni/CaO (100)
C–O11.150.970.92
C–O21.150.970.84
Table 3. Adsorption energies of gas molecules on 10 wt % Ni/CaO (100) surfaces.
Table 3. Adsorption energies of gas molecules on 10 wt % Ni/CaO (100) surfaces.
Eads (eV)CO2H2OCOH2CH4
CaO1.452.050.3610.1360.199
10 wt % Ni/CaO1.812.230.4610.1620.183

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