J Mol Model (2015) 21: 187 DOI 10.1007/s00894-015-2740-z
ORIGINAL PAPER
Using of TiN-nanotubes and Cu-nanoparticles for conversion of CO2 to hydrocarbon fuels Leila Mahdavian 1
Received: 17 November 2014 / Accepted: 15 June 2015 / Published online: 5 July 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract In this study, conversion of CO2 and H2O to CH3OH or CH4 on Cu-nanoparticles in TiN- nanotubes was optimized by density functional theory (DFT) methods. DFT optimized the intermediates and transient states using the GAMESS program package, but the structures, total energy, and thermodynamic properties were calculated by semiempirical methods using ZINDO/1 at room temperature. The results show a sensitivity enhancement in resistance and capacitance when CO2 and H2O are converted to CH3OH and other products. TiN-nanotubes use photo-catalytic reactivity for the reduction of CO2 and H2O to form CH3OH or CH4 at 298 K. The endohedral location of interaction of these reactants in TiN-nanotubes with Cu-nanoparticles was investigated. Calculations show that these processes were endothermic, thus the reactions need solar or other energies in the presence of visible light to progress favorably. Keywords Carbon dioxide . Hydrocarbon fuels . TiN-nanotube . Environment . ZINDO/1
Introduction Climate change is an extraordinary policy challenge. The atmosphere–ocean system integrates greenhouse gas emissions from every place and every sector of human society, and mediates the impacts of climate change globally. The elimination
* Leila Mahdavian [email protected]; [email protected] 1
Department of Chemistry, Doroud Branch, Islamic Azad University, PO Box: 133, Doroud, Iran
of chemical pollutants from contaminated environments is one of the most important steps towards achieving the goal of environmental remediation. For this reason it is important to consider the possibility of capturing carbon dioxide directly from the air [1, 2], therefore reuse or conversion of greenhouse gases from sources could be an effective approach. Conversion of CO2 to useful chemicals is most important because, in addition to the removal of pollutants from the air, new fuel sources could result. Conversion of CO2 leads not only a reduction in greenhouse gases, if also prevents additional emissions [3–7]. Because implementation of this reaction normally requires large amounts of energy, it will be acceptable economically only in rare cases. If the procedure could be done simply, liquid fuels could be produced commercially from the exhaust gases of fossil-fuel power plants [8]. Regarding this point, there are some methods and processes currently in existence for CO2 removal from the air, including afforestation and chemical approaches such as direct air capture of CO2 from the atmosphere or reactions of CO2 with minerals to form carbonates. Such technologies include: bioenergy with carbon capture and storage, direct air capture, ocean fertilization and enhanced weathering [9–13, 15]. Methane and methanol are one of the major products of the chemical industries and also a feedstock for many manufacturers. However, CO2 conversion to methanol and other products is an increasing challenge. For the reactions mentioned above, the use of catalysts is essential to decrease energy usage. The current commercial catalyst is the co-catalyst of titanium nitride (TiN) nanotubes with a Cu-nanoparticle that helps to convert CO2 and water into methanol or methane using sunlight as the power source (Fig. 1). As shown in Fig. 2, there are four locations within the TiN-NT matrix that place the copper nanoparticles correctly to catalyze the formation of methanol from CO2. In this work, the first of these (endohedral) is investigated.
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memory available in the GAMESS program package during optimization of all interacting structures: Fig. 1 Conversion of CO2 into methanol and other products using titanium nitride (TiN) nanotubes containing Cu-nanoparticles
%mem ¼ 3GB %nprocshared ¼ 4 #B3LY P=6−31G opt maxdisk ¼ 30GB test
Figure 3 displays the conversion of CO2 and H2O to CH3OH or CH4 and O2. In this figure TiN-nanotubes with Cu-nanoparticles are simulated by ball and stick models. Interaction of CO2 and H2O passing through a TiN-nanotube doped with Cu-nanoparticle was investigated with ZINDO/1 by semi empirical methods. In this study, the structural, total energy, thermodynamic properties of the interactions were calculated at room temperature. Density functional theory (DFT) was utilized to optimize intermediates and transient states using the GAMESS program package. Previous studies have shown that the heat of formation (ΔH) for this reaction is positive; thus this reaction requires an external energy source such as sunlight or other energy in the presence of visible light in order to occur. These techniques have attracted considerable attentions from researchers and industries because of their low cost, high adsorption efficiency, selective operation, as well as their easy and hazardless application.
Methods The geometrical structures of all molecules were optimized by DFT in B3LYP [14, 16, 17] using the 6-31G basis set at a constant pressure of 1 atm. The input files were very large and the following methods were used to increase computing
Fig. 2a,b Potential locations for interaction between pollutants and copper nanoparticles in TiNnanotubes. a TiN-nanotube array. b (1)–(4) Potential locations for catalysis
Following optimization of their geometrical structures, CO2 and H2O were attached to the nano-surface and converted to other safer products as follows: CO2 þ 2H2 O þ Cu−TiN nanotube→ CO2 −2H2 O−Cu−TiN nanotube CO2 −2H2 O−Cu−TiN nanotube→ CH3 OH þ 3=2O2 þ Cu−TiN nanotube CO2 − 2 H2 O − C u − TiN nanotube → CH4 þ 2 O2 þ C u − TiN nanotube
These interactions were calculated using semi-empirical methods. Note that the accuracy of semi-empirical quantum mechanical methods depends on the database used for parameterization. Configuration interaction (or electron correlation) improves energy calculations using CNDO, INDO, MINDO/ 3, MNDO, AM1, PM3, ZINDO/1, and ZINDO/S for these electron configurations. We can use the information obtained from semi-empirical calculations to investigate many thermodynamic and kinetic aspects of chemical processes, leading to well-defined relationship between energies and geometries of molecules and chemical phenomena. Heats of formation were calculated by subtracting atomic heats of formation from the binding energy. ZINDO/1 has been applied widely to calculate heats of formation, molecular geometries, dipole moments, ionization energies, electron affinities, and other properties [18, 19]. ZINDO/1 has been parameterized to generate geometries of molecules containing transition metals. The optimized geometry was obtained by the sequential conjugate gradient method known as the Polak-Ribiere and Eigenvector
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Fig. 3a,b Ball-and-stick models of configuration. Top view of the first location at which CO2 and H2O pass through the TiN-nanotube with Cunanoparticle with conversion to: a methanol and b methane
Table 1
The thermodynamic properties of the CO2 and H2O interaction on Cu-TiN nanotube are calculated by ZINDO/1 method at 298 K
CO2 and H2O converted to CH3OH Steps Etotal mJ mol−1 TiN-Cu 1845.45 1 2401.63 2 1959.95 3 2141.16 4 1843.16 5 2368.93 6 3101.20 7 1733.80 CO2 and H2O converted to CH4 1 2401.63 2 1959.95 3 2141.16 4 1843.16 5 2571.47 6 2606.10 7 1969.73
Enuc mJ mol−1 8280.83 9700.69 9900.11 10149.70 10467.32 11489.20 11168.56 10464.03
Dipol moment (D) 1.63×104 1.53×104 1.56×104 1.58×104 1.61×104 1.50×104 1.53×104 1.57×104
RMS kcal mol−1 Å−1 3012 3027 3383 3227 3191 2980 3066 3949
Ebin mJ mol−1 2984.40 3689.87 3248.19 3429.41 3131.40 3747.65 4479.92 3112.52
H mJ mol−1 3007.80 3715.16 3273.48 3454.70 3156.69 3773.88 4506.15 3138.75
Eele (v) −15.88 −18.01 −19.59 −19.76 −21.28 −22.49 −19.91 −21.54
9700.69 9900.11 10,149.70 10,467.32 10,876.39 10,311.59 9891.81
1.53×104 1.56×104 1.58×104 1.61×104 1.47×104 1.52×104 1.53×104
3027 3383 3227 3191 3069 3138 3241
3689.87 3248.19 3429.41 3131.40 3906.21 3940.85 3304.48
3715.16 3273.48 3454.70 3156.69 3932.20 3966.83 3330.46
−18.01 −19.60 −19.76 −21.28 −20.49 −19.01 −19.55
187 Page 4 of 6 Table 2 Thermodynamic properties of CO2 and H2O passing over Cu-TiN and being converted to CH3OH and CH4
J Mol Model (2015) 21: 187
CO2 and H2O pass over Cu-TiN
ΔGele mJ mol−1
ΔHele mJ mol−1
ΔSele mJ K−1 mol−1
K
Methanol Methane
−491.90 −1325.10
617.18 −558.47
3.72 2.57
198,542.77 534,839.80
Following methods [14, 16]. Their optimization algorithm has a convergence limit of 0.01 kcal mol−1 and an RMS gradient of 0.05 kcal mol−1 Å−1. After geometry optimization, the parameters involved in converting CO2 and H2O molecules on Cu-TiN nanotube structures were obtained from a study of the nano-surface; the interactions between them were also correct for this work in DFT calculations. Ball-and-stick models of these configurations are depicted in Fig. 3. The electronic structure and the conductance properties were computed using DFT.
Results and discussion The aim of this study was to develop an inexpensive method for CO2 conversion into useful chemicals that would be able to reduce local CO2 storage and lead to a reduction in anthropogenic CO2 emissions. This novel catalytic method for the continuous chemical conversion of CO2 was simulated and investigated thoroughly mechanistically. Geometry optimization of TiN nanotubes with Cu nanoparticles was performed at the B3LYP/6-31G level (Fig. 3). The effects of CO2 and H2O passing through the Cu-TiN nanotube and its conversion to CH3OH and CH4 are shown in Table 1.
Fig. 4 Resistance (Ω) recorded for conversion of CO2 and H2O to a methanol via inter-TiNnanotubes with Cu- nanoparticles, and b methane between TiNnanotubes with Cu-nanoparticles
In these simulations, CO2 and H2O pass from inside to outside of the nanotube in several stages. The study included conformational searches (and further refinement by DFT) and ab initio semi empirical calculations of ZINDO/1 methods and the dipole moments for all steps using the GAMESS program package. The most important property is ZINDO/1, which revealed a significant correlation between substrate and the substitution pattern in this conversation. ZINDO/1 has been used widely to calculate thermodynamics properties. The studied reactions are: 3 CO2 þ 2H2 O→CH3 OH þ O2 2 CO2 þ 2 H2 O → CH4 þ 2 O2 The thermodynamic parameters of these simulations for methanol and methane from steps 1 to 4 are the same, only from step 5 onwards do they differ because the product obtained is methanol or methane. In step 4, results show the greatest value for the dipole moment (D) in a nanotube. These places in the nanotubes and on the Cu surface snare CO2 and convert it to other products. The dipole moment (D) for methanol is more than that for methane because methanol is a polar molecule while methane is non-polar. The total energy (Etotal),
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Fig. 5 Total energy (MJ mol−1) needed to convert CO2 and H2O to a CH3OH and b CH4
nuclear energy (Enuc), binding energy (Ebin) and heat of formation (enthalpy) ΔH are displayed in Table 1 for methanol formation; all are greater than for methane (Table 2). The RMS gradient (kcal mol−1 Å−1) is increased for the formation of CH 3 OH and CH 4 in these interactions at 298 K. The enthalpy of all steps is positive, suggesting that these interactions are endothermic, and thus require ultraviolet solar radiation. In Fig. 4, the resistance (Ω) recorded for conversion of CO2 and H2O to methanol and methane shows a sudden decrease. The minimum amount of total energy was located in the middle of the tube because the field of the TiNnanotube is greater inthe middle than at other locations on the tubes. The total energy for the steps involved is shown in Fig. 5. The total energy decreases in the middle of the nanotubes, which creates the potential for CO2 conversion to CH3OH and CH4. To correlate sensor signals with changes in electrical resistance, the calculated data (Table 1) need to be converted to electrical resistance as pictured in Fig. 4. TiN-NTs are so-called Bup hole-doped semiconductors^, as can be gleaned from the current versus gate voltage curve shown in Fig. 4 (middle curve), where their resistance is observed to decrease. Band bending induced by charging molecules causes the increase or decrease in surface conductivity that is responsible for the gas response signal. Their ΔGele values are negative; the interactions are exothermic and spontaneous during CO2 separation from the air. Table 2 presents the calculated thermodynamic parameters (ΔGele, ΔHele, and ΔSele). The results suggest that the nature of adsorption is exothermic, spontaneous
and favorable. This method is best for CO2 conversion to CH3OH and CH4. TiN-nanotubes are used in photocatalysis processes, because they are semiconductors. The absorption of a photon (hν) with ultra-band energy from a UV irradiation source causes TiN activation (Fig. 6). The transmission of an electron (e−) from the valence band to the conduction band leads to high reactive positive holes (h+) in the valence band. TiN−NT þ hν→ TiN−NT ðecb − þ holevb þ Þ H2 O → OH− þ Hþ Oxidative reaction: holevb þ OHads − →• O H • OH þ CO2 þ HO2 → CH3 OH þ O2 • O H þ CO2 þ HO2 → CH4 þ O2
Fig. 6 Electron transition from valance band (VB) to conduction band (CB) in semiconductor Cu-TiN-nanotubes
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Conclusions The combination of CO2 capture and conversion is an attractive strategy with which to reduce CO2 emissions. It is essential that the materials used in this strategy are able to work at atmospheric pressure and room temperature with any external thermal source to avoid the generation of new CO2. No material has yet been identified that can fulfil these requirements. The unique catalytic efficiency of TiN-nanotubes doped with Cu nanoparticles has been developed for applications in contexts such as gas sorption molecular separation, electronics and catalysis. Our results showed that the geometry and electrostatic properties of TiN-nanotubes with Cu can be useful for CO2 conversion. In this work, we chose armchair TiNnanotube (4, 4) to investigate CO2 conversion. The interaction between them was simulated by ab initio calculations. A change in the potential of all atoms of the inner surface of the TiN-nanotubes was observed in passing and converting CO2. These surface phases have different properties, resulting in altered molecule–surface interactions. For TiN-nanotubes, one site of high potential was located in the middle of the tube. This point in the nanotube acts to ensnare CO2. Acknowledgments We would like to thank the Amirkabir University of Technology and Center for High Performance Computing Research [http://hpcrc.aut.ac.ir/] for computing time on the High Performance Computing Cluster and for access to other supercomputing facilities.
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ORIGINAL PAPER
Using of TiN-nanotubes and Cu-nanoparticles for conversion of CO2 to hydrocarbon fuels Leila Mahdavian 1
Received: 17 November 2014 / Accepted: 15 June 2015 / Published online: 5 July 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract In this study, conversion of CO2 and H2O to CH3OH or CH4 on Cu-nanoparticles in TiN- nanotubes was optimized by density functional theory (DFT) methods. DFT optimized the intermediates and transient states using the GAMESS program package, but the structures, total energy, and thermodynamic properties were calculated by semiempirical methods using ZINDO/1 at room temperature. The results show a sensitivity enhancement in resistance and capacitance when CO2 and H2O are converted to CH3OH and other products. TiN-nanotubes use photo-catalytic reactivity for the reduction of CO2 and H2O to form CH3OH or CH4 at 298 K. The endohedral location of interaction of these reactants in TiN-nanotubes with Cu-nanoparticles was investigated. Calculations show that these processes were endothermic, thus the reactions need solar or other energies in the presence of visible light to progress favorably. Keywords Carbon dioxide . Hydrocarbon fuels . TiN-nanotube . Environment . ZINDO/1
Introduction Climate change is an extraordinary policy challenge. The atmosphere–ocean system integrates greenhouse gas emissions from every place and every sector of human society, and mediates the impacts of climate change globally. The elimination
* Leila Mahdavian [email protected]; [email protected] 1
Department of Chemistry, Doroud Branch, Islamic Azad University, PO Box: 133, Doroud, Iran
of chemical pollutants from contaminated environments is one of the most important steps towards achieving the goal of environmental remediation. For this reason it is important to consider the possibility of capturing carbon dioxide directly from the air [1, 2], therefore reuse or conversion of greenhouse gases from sources could be an effective approach. Conversion of CO2 to useful chemicals is most important because, in addition to the removal of pollutants from the air, new fuel sources could result. Conversion of CO2 leads not only a reduction in greenhouse gases, if also prevents additional emissions [3–7]. Because implementation of this reaction normally requires large amounts of energy, it will be acceptable economically only in rare cases. If the procedure could be done simply, liquid fuels could be produced commercially from the exhaust gases of fossil-fuel power plants [8]. Regarding this point, there are some methods and processes currently in existence for CO2 removal from the air, including afforestation and chemical approaches such as direct air capture of CO2 from the atmosphere or reactions of CO2 with minerals to form carbonates. Such technologies include: bioenergy with carbon capture and storage, direct air capture, ocean fertilization and enhanced weathering [9–13, 15]. Methane and methanol are one of the major products of the chemical industries and also a feedstock for many manufacturers. However, CO2 conversion to methanol and other products is an increasing challenge. For the reactions mentioned above, the use of catalysts is essential to decrease energy usage. The current commercial catalyst is the co-catalyst of titanium nitride (TiN) nanotubes with a Cu-nanoparticle that helps to convert CO2 and water into methanol or methane using sunlight as the power source (Fig. 1). As shown in Fig. 2, there are four locations within the TiN-NT matrix that place the copper nanoparticles correctly to catalyze the formation of methanol from CO2. In this work, the first of these (endohedral) is investigated.
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memory available in the GAMESS program package during optimization of all interacting structures: Fig. 1 Conversion of CO2 into methanol and other products using titanium nitride (TiN) nanotubes containing Cu-nanoparticles
%mem ¼ 3GB %nprocshared ¼ 4 #B3LY P=6−31G opt maxdisk ¼ 30GB test
Figure 3 displays the conversion of CO2 and H2O to CH3OH or CH4 and O2. In this figure TiN-nanotubes with Cu-nanoparticles are simulated by ball and stick models. Interaction of CO2 and H2O passing through a TiN-nanotube doped with Cu-nanoparticle was investigated with ZINDO/1 by semi empirical methods. In this study, the structural, total energy, thermodynamic properties of the interactions were calculated at room temperature. Density functional theory (DFT) was utilized to optimize intermediates and transient states using the GAMESS program package. Previous studies have shown that the heat of formation (ΔH) for this reaction is positive; thus this reaction requires an external energy source such as sunlight or other energy in the presence of visible light in order to occur. These techniques have attracted considerable attentions from researchers and industries because of their low cost, high adsorption efficiency, selective operation, as well as their easy and hazardless application.
Methods The geometrical structures of all molecules were optimized by DFT in B3LYP [14, 16, 17] using the 6-31G basis set at a constant pressure of 1 atm. The input files were very large and the following methods were used to increase computing
Fig. 2a,b Potential locations for interaction between pollutants and copper nanoparticles in TiNnanotubes. a TiN-nanotube array. b (1)–(4) Potential locations for catalysis
Following optimization of their geometrical structures, CO2 and H2O were attached to the nano-surface and converted to other safer products as follows: CO2 þ 2H2 O þ Cu−TiN nanotube→ CO2 −2H2 O−Cu−TiN nanotube CO2 −2H2 O−Cu−TiN nanotube→ CH3 OH þ 3=2O2 þ Cu−TiN nanotube CO2 − 2 H2 O − C u − TiN nanotube → CH4 þ 2 O2 þ C u − TiN nanotube
These interactions were calculated using semi-empirical methods. Note that the accuracy of semi-empirical quantum mechanical methods depends on the database used for parameterization. Configuration interaction (or electron correlation) improves energy calculations using CNDO, INDO, MINDO/ 3, MNDO, AM1, PM3, ZINDO/1, and ZINDO/S for these electron configurations. We can use the information obtained from semi-empirical calculations to investigate many thermodynamic and kinetic aspects of chemical processes, leading to well-defined relationship between energies and geometries of molecules and chemical phenomena. Heats of formation were calculated by subtracting atomic heats of formation from the binding energy. ZINDO/1 has been applied widely to calculate heats of formation, molecular geometries, dipole moments, ionization energies, electron affinities, and other properties [18, 19]. ZINDO/1 has been parameterized to generate geometries of molecules containing transition metals. The optimized geometry was obtained by the sequential conjugate gradient method known as the Polak-Ribiere and Eigenvector
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Fig. 3a,b Ball-and-stick models of configuration. Top view of the first location at which CO2 and H2O pass through the TiN-nanotube with Cunanoparticle with conversion to: a methanol and b methane
Table 1
The thermodynamic properties of the CO2 and H2O interaction on Cu-TiN nanotube are calculated by ZINDO/1 method at 298 K
CO2 and H2O converted to CH3OH Steps Etotal mJ mol−1 TiN-Cu 1845.45 1 2401.63 2 1959.95 3 2141.16 4 1843.16 5 2368.93 6 3101.20 7 1733.80 CO2 and H2O converted to CH4 1 2401.63 2 1959.95 3 2141.16 4 1843.16 5 2571.47 6 2606.10 7 1969.73
Enuc mJ mol−1 8280.83 9700.69 9900.11 10149.70 10467.32 11489.20 11168.56 10464.03
Dipol moment (D) 1.63×104 1.53×104 1.56×104 1.58×104 1.61×104 1.50×104 1.53×104 1.57×104
RMS kcal mol−1 Å−1 3012 3027 3383 3227 3191 2980 3066 3949
Ebin mJ mol−1 2984.40 3689.87 3248.19 3429.41 3131.40 3747.65 4479.92 3112.52
H mJ mol−1 3007.80 3715.16 3273.48 3454.70 3156.69 3773.88 4506.15 3138.75
Eele (v) −15.88 −18.01 −19.59 −19.76 −21.28 −22.49 −19.91 −21.54
9700.69 9900.11 10,149.70 10,467.32 10,876.39 10,311.59 9891.81
1.53×104 1.56×104 1.58×104 1.61×104 1.47×104 1.52×104 1.53×104
3027 3383 3227 3191 3069 3138 3241
3689.87 3248.19 3429.41 3131.40 3906.21 3940.85 3304.48
3715.16 3273.48 3454.70 3156.69 3932.20 3966.83 3330.46
−18.01 −19.60 −19.76 −21.28 −20.49 −19.01 −19.55
187 Page 4 of 6 Table 2 Thermodynamic properties of CO2 and H2O passing over Cu-TiN and being converted to CH3OH and CH4
J Mol Model (2015) 21: 187
CO2 and H2O pass over Cu-TiN
ΔGele mJ mol−1
ΔHele mJ mol−1
ΔSele mJ K−1 mol−1
K
Methanol Methane
−491.90 −1325.10
617.18 −558.47
3.72 2.57
198,542.77 534,839.80
Following methods [14, 16]. Their optimization algorithm has a convergence limit of 0.01 kcal mol−1 and an RMS gradient of 0.05 kcal mol−1 Å−1. After geometry optimization, the parameters involved in converting CO2 and H2O molecules on Cu-TiN nanotube structures were obtained from a study of the nano-surface; the interactions between them were also correct for this work in DFT calculations. Ball-and-stick models of these configurations are depicted in Fig. 3. The electronic structure and the conductance properties were computed using DFT.
Results and discussion The aim of this study was to develop an inexpensive method for CO2 conversion into useful chemicals that would be able to reduce local CO2 storage and lead to a reduction in anthropogenic CO2 emissions. This novel catalytic method for the continuous chemical conversion of CO2 was simulated and investigated thoroughly mechanistically. Geometry optimization of TiN nanotubes with Cu nanoparticles was performed at the B3LYP/6-31G level (Fig. 3). The effects of CO2 and H2O passing through the Cu-TiN nanotube and its conversion to CH3OH and CH4 are shown in Table 1.
Fig. 4 Resistance (Ω) recorded for conversion of CO2 and H2O to a methanol via inter-TiNnanotubes with Cu- nanoparticles, and b methane between TiNnanotubes with Cu-nanoparticles
In these simulations, CO2 and H2O pass from inside to outside of the nanotube in several stages. The study included conformational searches (and further refinement by DFT) and ab initio semi empirical calculations of ZINDO/1 methods and the dipole moments for all steps using the GAMESS program package. The most important property is ZINDO/1, which revealed a significant correlation between substrate and the substitution pattern in this conversation. ZINDO/1 has been used widely to calculate thermodynamics properties. The studied reactions are: 3 CO2 þ 2H2 O→CH3 OH þ O2 2 CO2 þ 2 H2 O → CH4 þ 2 O2 The thermodynamic parameters of these simulations for methanol and methane from steps 1 to 4 are the same, only from step 5 onwards do they differ because the product obtained is methanol or methane. In step 4, results show the greatest value for the dipole moment (D) in a nanotube. These places in the nanotubes and on the Cu surface snare CO2 and convert it to other products. The dipole moment (D) for methanol is more than that for methane because methanol is a polar molecule while methane is non-polar. The total energy (Etotal),
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Fig. 5 Total energy (MJ mol−1) needed to convert CO2 and H2O to a CH3OH and b CH4
nuclear energy (Enuc), binding energy (Ebin) and heat of formation (enthalpy) ΔH are displayed in Table 1 for methanol formation; all are greater than for methane (Table 2). The RMS gradient (kcal mol−1 Å−1) is increased for the formation of CH 3 OH and CH 4 in these interactions at 298 K. The enthalpy of all steps is positive, suggesting that these interactions are endothermic, and thus require ultraviolet solar radiation. In Fig. 4, the resistance (Ω) recorded for conversion of CO2 and H2O to methanol and methane shows a sudden decrease. The minimum amount of total energy was located in the middle of the tube because the field of the TiNnanotube is greater inthe middle than at other locations on the tubes. The total energy for the steps involved is shown in Fig. 5. The total energy decreases in the middle of the nanotubes, which creates the potential for CO2 conversion to CH3OH and CH4. To correlate sensor signals with changes in electrical resistance, the calculated data (Table 1) need to be converted to electrical resistance as pictured in Fig. 4. TiN-NTs are so-called Bup hole-doped semiconductors^, as can be gleaned from the current versus gate voltage curve shown in Fig. 4 (middle curve), where their resistance is observed to decrease. Band bending induced by charging molecules causes the increase or decrease in surface conductivity that is responsible for the gas response signal. Their ΔGele values are negative; the interactions are exothermic and spontaneous during CO2 separation from the air. Table 2 presents the calculated thermodynamic parameters (ΔGele, ΔHele, and ΔSele). The results suggest that the nature of adsorption is exothermic, spontaneous
and favorable. This method is best for CO2 conversion to CH3OH and CH4. TiN-nanotubes are used in photocatalysis processes, because they are semiconductors. The absorption of a photon (hν) with ultra-band energy from a UV irradiation source causes TiN activation (Fig. 6). The transmission of an electron (e−) from the valence band to the conduction band leads to high reactive positive holes (h+) in the valence band. TiN−NT þ hν→ TiN−NT ðecb − þ holevb þ Þ H2 O → OH− þ Hþ Oxidative reaction: holevb þ OHads − →• O H • OH þ CO2 þ HO2 → CH3 OH þ O2 • O H þ CO2 þ HO2 → CH4 þ O2
Fig. 6 Electron transition from valance band (VB) to conduction band (CB) in semiconductor Cu-TiN-nanotubes
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Conclusions The combination of CO2 capture and conversion is an attractive strategy with which to reduce CO2 emissions. It is essential that the materials used in this strategy are able to work at atmospheric pressure and room temperature with any external thermal source to avoid the generation of new CO2. No material has yet been identified that can fulfil these requirements. The unique catalytic efficiency of TiN-nanotubes doped with Cu nanoparticles has been developed for applications in contexts such as gas sorption molecular separation, electronics and catalysis. Our results showed that the geometry and electrostatic properties of TiN-nanotubes with Cu can be useful for CO2 conversion. In this work, we chose armchair TiNnanotube (4, 4) to investigate CO2 conversion. The interaction between them was simulated by ab initio calculations. A change in the potential of all atoms of the inner surface of the TiN-nanotubes was observed in passing and converting CO2. These surface phases have different properties, resulting in altered molecule–surface interactions. For TiN-nanotubes, one site of high potential was located in the middle of the tube. This point in the nanotube acts to ensnare CO2. Acknowledgments We would like to thank the Amirkabir University of Technology and Center for High Performance Computing Research [http://hpcrc.aut.ac.ir/] for computing time on the High Performance Computing Cluster and for access to other supercomputing facilities.
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