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MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF EDUCATION VAN THI MINH HUE THEORETICAL STUDY OF CO2 HYDROGENATION OVER Ni5, Ni5/MgO, Ni5/AC CATALYSTS USING DENSITY FUNCTIONAL THEORY Specialization: Theoretical and Physical Chemistry Code: 9.44.01.19 SUMMARY OF CHEMICAL PhD THESIS HA NOI - 2020 The thesis was completed at: Department of Chemistry - Hanoi University of Education Scientific Instructors: Assoc Prof NGUYEN NGOC HA Review 1: Prof Dr Lâm Ngọc Thiềm-Hanoi University of Science, VNU Review 2: Prof Dr Trần Đại Lâm-Institute for Tropical Technology Review 3: Assoc Prof Dr Nguyễn Thị Minh Huệ-Hanoi National University of Education The thesis will be presented before the Board of thesis review at Hanoi University of Education on .h day month year The thesis can be found at: National Library, Hanoi or the library of Hanoi National University of Education LIST OF WORKS PUBLISHED BY AUTHOR Văn Thị Minh Huệ, Nguyễn Ngọc Hà ( 2015), “Nghiên cứu hấp phụ CO2 H2 cluster Ni5 phương pháp phiếm hàm mật độ”, Tạp chí Xúc tác Hấp phụ, T4(1), Tr.23-28 Van Thi Minh Hue, Nguyen Ngoc Ha (2016), “–HCOO and –HOCO formations from CO2 on Ni5 cluster and Ni5/MgO(200): a theoretical study”, The 6th International Conference “Chemical Thermodynamics and Kinetics”, Tver, Russia, 31st May – 3rd June Văn Thị Minh Huệ, Bùi Cơng Trình, Nguyễn Thị Thu Hà, Nguyễn Ngọc Hà (2018), “Nghiên cứu phản ứng methane hóa CO2 xúc tác cluster Ni5 phương pháp phiếm hàm mật độ”, Tạp chí hóa học, 56, 6e2, Tr.194-198 Nguyen Thi Thu Ha, Van Thi Minh Hue, Bui Cong Trinh, Nguyen Ngoc Ha, and Le Minh Cam (2019), “Study on the Adsorption and Activation Behaviours of Carbon Dioxide over Copper Cluster (Cu4) and Alumina-Supported Copper Catalist (Cu4/Al2O3) by means of Density Functional Theory”, Journal of Chemistry, vol 2019, Article ID 4341056, 10 pages Văn Thị Minh Huệ, Nguyễn Thị Thu Hà, Phùng Thị Lan, Lê Minh Cầm, Nguyễn Ngọc Hà (2020), “Nghiên cứu lí thuyết phản ứng methane hóa CO2 xúc tác Ni5/AC phương pháp phiếm hàm mật độ Phần I : Giai đoạn hấp phụ hoạt hóa”, Tạp chí Xúc tác Hấp phụ, 9(1), Tr.33-38 Văn Thị Minh Huệ, Nguyễn Thị Thu Hà, Phùng Thị Lan, Lê Minh Cầm, Nguyễn Ngọc Hà (2020), “ Nghiên cứu lí thuyết phản ứng methane hóa CO2 xúc tác Ni5/AC phương pháp phiếm hàm mật độ Phần II : Các đường phản ứng”, Tạp chí Xúc tác Hấp phụ, 9(1), Tr.74-81 1 INTRODUCTION The reason for choosing topic Nowadays, the concentration of carbon dioxide in the atmosphere has consequently risen The increase in CO emissions arguably contributes to the increase in global temperatures and climate changes due to the „„greenhouse effect‟‟ However, carbon dioxide is also a cheap, safe source of carbon This gas can be converted into many organic compounds with important applications such as CH4, CH3OH, HCHO, HCOOH when suitable catalysts are available In most of the studies, nickel-based catalysts were chosen because of their higher CO2 conversion in terms of activity and selectivity, and cheaper compared to noble metal-based catalysts The study of CO2 conversion catalyst using nano-sized nickel metal on the support is expected to be a good catalyst, making CO2 conversion easier but there have not been many studies on catalysis Continuing research on carbon dioxide conversion is essential For the reasons mentioned above, we select the research problem: “Theoretical study of CO2 hydrogenation over Ni5, Ni5/MgO, Ni5/AC catalysts using density functional theory” Research purpose and tasks a) Purpose: Using the density functional method (DFT) to find out the mechanism of CO2 hydrogenation reaction, the main product, by-products, the optimal reaction path, compare and evaluate the catalytic ability of the Ni5 cluster, Ni5/MgO, and Ni5/AC b) Tasks: Overall document review for the theoretical basis of related quantum chemical problems Research situation to convert CO by theoretical and empirical methods in the country and in the world - Calculate to build reaction paths that occur in each system, evaluation and comparison of the operability, and selectivity of the catalysts Scope and object of the study - Molecules: CO2, H2, CH4, HCHO, HCOOH, CH3OH - Catalysts: Ni5, Ni5/MgO, Ni5/AC 2 Scientific and practical significance of the thesis The conversion of CO2 with H2 is not only a reduction of CO2 but also economical when converting CO2 into useful products Using theoretical calculations by modern and good approximation methods to find out the mechanism of hydrogenation CO2 over Ni5 based catalyst This will help guide empirical research in the selection of suitable catalysts and supports New points of the thesis - The adsorption of H2 on Ni5 is dissociated CO2 adsorption is chemically adsorbed and CO2 is strongly activated When placed on the supports, the capacity of Ni5 decreases due to the decrease in charge density; - Calculated the mechanism of CO2 hydrogenation to form CH4, CH3OH, HCHO, and HCOOH over Ni5, Ni5/MgO, and Ni5/AC including 33 reaction paths, building potential energy surfaces, and consider reaction mechanisms - On the three catalyst Ni5, Ni5/MgO and Ni5/AC, CH4 is the most preferred CO2 hydrogenation product The reaction path that forms the optimal CH4 is the reaction path that passes through intermediate products such as HOCO, CO, HCO, H2COH, etc (CO route); identified the step that determines the reaction rate in the hydrogenation of CO2 on the above catalysts; - Explained that MgO and AC not only act as supports but also change the electron structure of the metal cluster, leading to changes in activation and reaction mechanism The layout of the thesis Introduction: Introducing the reasons for choosing the topic, the purpose, and scope of the research, the new points of the thesis, the scientific and practical significance of the thesis Chapter 1: Introduce the theoretical basis including the problems of quantum chemical theory Chapter 2: Overview of the research system and method of calculation Chapter 3: Research results and discussion Conclusion: Briefly summarize the results of the thesis 3 Chapter THEORY BASIS Introduce the density functional theory: Schrödinger equation, Hohenberg-Kohn theorems, exchange-correlation approximations, the CINEB method determines the transition state Chapter LITERATURE REVIEW 2.1 Overview of hydrogenation CO2 reaction The thermodynamic stability of CO2 requires high energy substances for its transformation into valuable chemicals in which the carbon atom has a lower oxidation state than Hence, it is highly crucial to employ suitable catalysts which can decrease the energy barriers, thus resulting in lower energy states of the system 2.2 Catalytic CO2 hydrogenation CO2: Overview Due to the structure of CO2 molecule, to weaken the C=O bond, it is often used as a transition metal, in which Ni-based catalysts are commonly used in industrial purposes due to its good activity, high selectivity and high availability so the price is cheap There have been some theoretical and empirical studies on the CO2 conversion reaction over nickel-based catalysts but mainly on crystal nickel catalysts, very few studies on nano-sized catalysts 2.3 Research in the country and in the world Although the methanation of CO2 is a comparatively simple reaction, its reaction mechanism appears to be difficult to establish There are different opinions on the nature of the intermediate formation process Reaction mechanisms proposed for CO2 methanation fall into two main categories The first one involves the conversion of CO2 to CO prior to methanation, and the subsequent reaction follows the same mechanism as CO methanation The other one involves the direct hydrogenation of CO2 to methane without forming CO as intermediate Even for CO methanation, there is still no consensus on the kinetics and mechanism The theoretical studies of cluster catalysts, especially the cluster placed on the supports are still limited Therefore, more theoretical studies on this issue are needed to clearly understand the reaction mechanism, in order to guide empirical studies to synthesize suitable catalysts 2.4 Method of calculation All the geometry and energy calculations were performed using the density functional theory (DFT) approach The generalized gradient approximation (GGA) with the Perdew, Burke, and Ernzerhof (PBE) nonlocal gradient-corrected functional was employed to estimate the exchange-correlation energy The double zeta basis plus polarization orbitals (DZP) was used for valence electrons, while the core electrons were treated using the norm-conserving pseudopotentials (NCP) in its fully nonlocal (Kleinman-Bylander) form The Coulomb potential was expanded in a plane-wave basis with an energy cut-off of 2040,75eV The sizes of the simulation box were 25×25×25 (Å3) The DFT calculations were performed using the SIESTA code due to its accuracy The transition states were determined by using the climbing image nudged elastic band (CI-NEB) method Chapter 3: RESULTS AND DISCUSSION 3.1 Computational model In this thesis, we have built computational models including molecular molecules (CO2, H2 and Ni5 cluster), MgO (200), AC; Ni5 on MgO (200), and AC; Intermediate products and transition states of phases in CO2 hydrogenation reaction paths on the catalytic surfaces of Ni5, Ni5/MgO, Ni5/AC 3.2 CO2 hydrogenation reaction on Ni5 catalyst 3.2.1 Ni5 structure In this study, we chose Ni5 because this is the smallest cluster that can comprehensively study isomers, Ni5 can be considered to be stable and existent The calculation results show that the two most stable structures of the Ni5 cluster are square pyramid and trigonal bipyramid The trigonal bipyramid isomers are more stable than the square pyramid This result agrees with some previous theoretical and empirical studies on the structure of Ni clusters 3.2.2 Adsorption of CO2 and H2 on Ni5 - Adsorption of H2 on Ni5: In the most stable adsorption configurations, H2 is dissociated into H, Eads=243,9kJ.mol1 The result of CI-NEB calculation shows Figure Ea = configuration of H2 over Ni5 3.1 Adsorption - Adsorption of CO2 on Ni5: The most stable adsorption configuration is multi-center adsorption on Ni atoms with Eads=249,7 kJ.mol1, Ea=0 This is a chemical adsorption Figure 3.3 Adsorption configuration of CO2 over Ni5 3.2.3 Reaction paths on Ni5 catalyst The calculated mechanisms are summarized in Figure 3.6 Figure 3.6 The calculated reaction pathways of CO2 hydrogenation over Ni5 Based on the proposed reaction diagram, we can provide the following reaction pathways: - Carbon monoxide formation on Ni5 catalyst: P1-CO/Ni5: R1→R4→R10; P2-CO/Ni5: R3→R7→R11 -Methane formation on Ni5 catalyst: P1-CH4/Ni5: R1→R4→R10→R15→R18→R22→R28→R29 P2-CH4/Ni5: R1→R4→R10→R15→R19→R24→R32→R37→R38 P3-CH4/Ni5: R1→R4→R10→R15→R18→R23→R23‟→R32→R37→R38 P4-CH4/Ni5: R1→R4→R10→R15→R19→R25→R33→R37→R38 P5-CH4/Ni5: R1→R4→R10→R16→R21→R26→R34→R37→R38 P6-CH4/Ni5: R1→R4→R10→R16→R21→R27→R35→R37→R38 - Methanol formation on Ni5 catalyst: P1-CH3OH/Ni5: R1→R4→R10→R15→R18→R22→R30 P2-CH3OH/Ni5: R1→R4→R10→R15→R19→R24→R31 P3-CH3OH/Ni5: R1→R4→R10→R15→R18→R23→R23‟→R31 P4-CH3OH/Ni5: R1→R4→R10→R16→R21→R27→R36 - Formaldehyde formation on Ni5 catalyst: P-HCHO/Ni5: R1→R4→R10→R15→R18 -Formic acid formation on Ni5 catalyst: P-HCOOH/Ni5: R2→R6 - Coke formation on Ni5 catalyst: P-C/Ni5: R1→R4→R10→R16→R20 The corresponding energies and activation energies values of the steps are shown in Table 3.9 Table 3.9 Corresponding energy (∆E), activated energy (Ea) of the reaction steps on Ni5 Step R1 R2 R2‟ R3 R4 R5 Reaction Oa=C=Ob +Ha → HaOaCOb Oa=C=Ob +Ha → HaCOaOb R2 transposition Oa=C=Ob → COb + Oa HaOaCOb→HaOa +COb HaCOaOb+Hb→HbHaCOaOb Ea, kJ.mol 71.7 88.1 133.0 56.1 183.8 1 ΔE, kJ.mol 26.6 63.2 120.2 12.3 75.4 113.6 1 R6 R7 R8 R9 R9‟ R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 R20 R21 R22 R23 R23‟ R24 R25 R26 R27 R27‟ R28 R29 R30 R31 R32 R33 R34 R35 R36 R37 HaCOaOb+Hb→HaCOaObHb COb + Oa +Ha →HaOa +COb COb + Oa +Ha →Oa +CObHa COb + Oa +Ha →Oa +HaCOb R9 transposition HaOa+COb+Hb→COb+HaOaHb HaOa+COb+Hb→COb+HaOaHb Oa +HaCOb+Hb →OaHb +HaCOb Oa +HaCOb +Hb→Oa +HbHaCOb Oa +HaCOb+Hb →Oa +HaCObHb COb+Hc→HcCOb COb+Hc→CObHc COb→C+Ob HcCOb+Hd→HcHdCOb HcCOb+Hd→HcCObHd CObHc +Hd→HdObHc + C CObHc +Hd→HdCObHc HcHdCOb+He→HeHcHdCOb HcHdCOb+He→HcHdCObHe R23 transposition HcCObHd+He→HeHcCObHd HcCObHd+He→CHc+HeObHd HdCObHc+He→CHd+HeObHc HdCObHc+He→HeHdCObHc R27 transposition HeHcHdCOb→HeHcHdC+Ob HeHcHdC+Ob+Hf→HfHeHcHdC+Ob HeHcHdCOb+Hf→HeHcHdCObHf HeHcCObHd+Hf→HfHeHcCObHd HeHcCObHd+Hf→CHeHc+HfObHd CHc+Hf→CHcHf CHd+Hf →CHfHd HeHdCObHc+Hf→HeHdC+ HfObHc HeHdCObHc+Hf→HfHeHdCObHc+Hf CHcHf +Hg→ CHcHfHg 124.8 53.1 138.6 37.3 89.7 167.8 131.0 145.0 106.5 126.5 288.1 74.4 86.0 295.7 69.3 8.1 111.5 43.3 12.0 103.3 83.5 12.1 45.1 57.3 112.3 206.0 145.5 74.0 63.6 97.0 56.9 85.4 52.9 83.2 140.2 101.2 12.5 25.0 80.2 139.2 25.6 69.5 92.4 59.6 85.9 146.3 44.7 59.2 161.9 10.6 50.0 20.5 26.8 47.1 0.0 134.1 47.1 23.8 0.8 102.1 117.1 50.4 15.0 59.5 18.3 61.7 33.9 30.8 R38 CHcHfHg +Hh→CHcHfHgHh 80.5 69.8 In the following sections, we detailed analysis of calculation results for each reaction path 3.2.3.1 Carbon monoxide formation on Ni5 catalyst The most optimal reaction of the two paths forming CO is P1-CO/Ni5 The highest Ea value of this path is 71.7 kJ.mol1 for the R1 step 8 3.2.3.2 Methane formation on Ni5 catalyst The most optimal reaction of the six paths forming CH4 is the P2CH4/Ni5 reaction path The highest Ea value of this path is 106.5 kJ.mol1 for the formation of HCO in the R15 step 3.2.3.3 Methanol formation on Ni5 catalyst The most optimal reaction of the four paths forming CH3OH is P3CH3OH/Ni5, with the highest Ea of 126.5 kJ.mol1, while the other paths have Ea value of 145.5 and 206.0 kJ.mol1 respectively 3.2.3.4 Formaldehyde formation on Ni5 catalyst This reaction path is part of the P1-CH4/Ni5 HCHO formation is feasible The highest Ea value of this path is 106.5 kJ.mol1 3.2.3.5 Formic acid formation on Ni5 catalyst This reaction path has only two reaction steps, in which the R6 step has the highest Ea value, Ea = 124.8 kJ.mol1 3.2.3.6 Coke formation on Ni5 catalyst The formation of carbon (coke) is unlikely because the highest Ea in this reaction path is 295.7 kJ.mol1 The potential energy surface for the proposed reaction mechanism on the Ni5 catalyst shown in Figure 3.8 and Figure 3.9 Figure 3.8 The potential energy surface of the reaction steps between CO2 and Ha, Hb on Ni5 Figure 3.9 The potential energy surface from R15 to R38 step of conversion reaction on Ni5 According to our calculation results, on Ni5 catalysts, the main product is methane via the CO route This result is in agreement with experimentation, experimental studies have shown that nickel-based catalysts for productoriented catalysis are CH4 The highest activation energy is 106.5 kJ.mol1 from the hydrogenation CO into HCO so the elementary reaction H+CO → HCO is the rate-determining step When the reaction occurs at high temperatures, the by-products are CH3OH, CO, HCHO, HCOOH 3.3 CO2 hydrogenation reaction on Ni5/MgO catalyst 3.3.1 Ni5/MgO structure MgO is good support for nickel catalysts; MgO is a Lewis base, it will increase CO2 absorption and reduce or inhibit carbon formation by adsorbing high concentrations of CO2 on the surface of the catalyst In this thesis, the Ni5 cluster is placed on the surface of MgO(200), corresponding to the highest pick on the XRD diagram according to calculations and experiments A two-layer with 41 molecules of MgO was chosen to represent the MgO(200) surface The number of atoms and the number of layers is chosen to ensure the best possible simulation of the possible interactions between MgO and gas molecules but not too large as this will increase the computational cost Then Ni5 cluster of trigonal bipyramid and square 10 pyramid is placed on the surface of MgO(200) and optimized The results show that in the most stable geometric structure, the Ni5 cluster has a structure close to the square pyramid The ∆E of this process is very negative (–512.8 kJ.mol1) proving that this structure is stable In the next steps, we investigate the hydrogenation of CO2 on Ni5 in the square-pyramid shaped 3.3.2 Adsorption of CO2 and H2 on Ni5MgO - Adsorption of H2 on Ni5MgO: Adsorption configuration of 2.1a1 is the most stable H2 molecule is dissociated Figure 3.16 Adsorption into H, Eads=250,6kJ.mol1, Ea = configuration of H2 over Ni5/MgO - Adsorption of CO2 on Ni5/MgO: The most stable structure has Eads=224,3 kJ.mol1, Ea=0, corresponding to multicenter adsorption on Ni atoms After Figure 3.17 Adsorption adsorption, the CO2 molecule is activated configuration of CO2 over Ni5/MgO Another possibility to consider is the adsorption of CO2 and H2 onto the MgO support The calculation results show that the most stable absorption of CO2 and H2 were 105,3 kJ.mol1 and –44.9 kJ.mol1 Compared CO2 adsorption energy on MgO and nickel, it was observed that it is more favorable for CO2 activation on the nickel surface The adsorption on the cluster is more preferred on MgO, MgO only acts as a support The adsorbed CO2 molecule on the Ni5 cluster is stronger than the Ni5 cluster adopted on MgO The results of Cu4 adsorption on the Al2O3 support also showed similar results, the adsorption on Cu4 cluster (Eads = 150,49 kJ.mol1) was stronger than on Cu4/Al2O3 (Eads= 87,15 kJ.mol1) 3.3.3 Reaction paths on Ni5/MgO catalyst The calculated mechanisms are summarized in Figure 3.22 11 Figure 3.22 The calculated reaction pathways of CO2 hydrogenation over Ni5/MgO Based on the proposed reaction diagram, we can provide the following reaction pathways: - Carbon monoxide formation on Ni5/MgO catalyst: P1-CO/Ni5MgO: R1→R4→R10, P2-CO/Ni5MgO: R3→R7→R7‟→R10 - Methane formation on Ni5/MgO catalyst: P1-CH4/Ni5MgO: R1→R4→R10→R11→R14→R17→R17‟→R22→R23→R24 P2-CH4/Ni5MgO: R1→R4→R10→R11→R14→R18→R21→R25 - Methanol formation on Ni5/MgO catalyst: P1-CH3OH/Ni5MgO: R1→R4→R10→R11→R14→R17→R19 P2-CH3OH/Ni5MgO: R1→R4→R10→R11→R14→R18→R20 - Formaldehyde formation on Ni5/MgO catalyst: P-HCHO/Ni5MgO: R1→R4→R10→R11→R14 -Formic acid formation on Ni5/MgO catalyst: P-HCOOH/Ni5MgO: R2→R6 - Coke formation on Ni5/MgO catalyst: P-C/Ni5MgO: R1→R4→R10→R12→R16 12 The corresponding energy and activation energy values of the steps are shown in Table 3.18 Table 3.18 Corresponding energy (∆E), activated energy (E a) of the reaction steps on Ni5/MgO Step Reaction Ea, kJ.mol-1 ΔE, kJ.mol-1 117.8 R1 Oa=C=Ob +Ha → HaOaCOb 128.7 R2 Oa=C=Ob +Ha → HaCOaOb 199.7 97.1 R3 Oa=C=Ob → Oa +COb 172.0 133.9 R4 HaOaCOb→HaOa +COb - 114.5 R5 HaCOaOb+Hb→HbHaCOaOb 334.1 139.1 R6 HaCOaOb+Hb→HaCOaObHb 263.2 250.4 R7 Oa +COb + Ha→HaOa +COb 46.8 152.2 R7‟ R7 transposition - 60.0 R8 Oa +COb + Ha→Oa +HaCOb 120.4 76.6 R9 Oa +COb + Ha→Oa +CObHa 243.6 180.8 R10 HaOa+COb+Hb→COb+HaOaHb 142.0 88.5 R11 COb+Hc→HcCOb 97.0 49.0 R12 COb+Hc→CObHc 256.8 215.4 R13 COb→C+Ob 243.7 1.5 R14 HcCOb+Hd→HcHdCOb 75.2 21.3 R15 HcCOb + Hd → HcCObHd 223.0 148.2 R16 CObHc+Hd→C+HdObHc 27.8 47.4 R17 HcHdCOb+He→HcHdCObHe 34.6 67.6 R18 HcHdCOb+He→HeHcHdCOb 12.3 74.9 R17‟ R17 transposition 52.5 51.2 R19 HcHdCObHe+Hf→HfHcHdCObHe 97.1 35.0 R20 HeHcHdCOb+Hf→HeHcHdCObHf 192.0 151.8 R21 HeHcHdCOb→HeHcHdC+Ob 152.2 30.0 R22 HcHdCObHe+Hf→HcHdC+HfObHf R23 HcHdC+Hg→HcHdC+Hg R24 R25 - 34.2 42.6 28.3 HcHdCHg+Hf→HcHdCHgHf 87.8 68.6 HeHcHdC+Ob+Hf→HfHeHcHdC(CH4)+Ob 139.1 87.9 13 In the following sections, we detailed analysis of calculation results for each reaction path 3.3.3.1 Carbon monoxide formation on Ni5/MgO catalyst The most optimal reaction of the two paths forming CO is P1CO/Ni5MgO The highest Ea value of this path is 142.0 kJ.mol1 for the R10 step: CO+OH+ H→ CO+H2O 3.3.3.2 Methane formation on Ni5/MgO catalyst The most optimal reaction of the two paths forming CH4 is the P2CH4/Ni5MgO reaction path The highest Ea value of this path is 152.2 kJ.mol1 for the R10 step 3.3.3.3 Methanol formation on Ni5/MgO catalyst The most optimal reaction of the two paths forming CH3OH is P1CH3OH/Ni5MgO reaction path The highest Ea value of this path is 142.0 kJ.mol1 for the R10 step 3.3.3.4 Formaldehyde formation on Ni5/MgO catalyst The reaction steps in this pathway are part of the P1-CH4/Ni5MgO HCHO formation is feasible because of the step with the highest Ea in this reaction path is 142.0 kJ.mol1 3.3.3.4 Formic acid formation on Ni5/MgO catalyst The formation of HCOOH is not favorable on kinetics and thermodynamics because of Ea= 263.2 kJ.mol1 and ∆E = 250.4 kJ.mol1 3.3.3.5 Coke formation on Ni5/MgO catalyst Coke formation is unfavorable because of the R12 step has very high Ea and ∆E According to our opinion, it is not possible to create coke on Ni5/MgO After a detailed analysis of the reaction paths, we constructed the potential energy surface for the proposed reaction mechanism on the Ni5/MgO catalyst shown in Figure 3.23 and Figure 3.24 14 Figure 3.23 The potential energy surface of the reaction steps between CO2 and Ha, Hb on Ni5/MgO Figure 3.24 The potential energy surface from R11 to R25 of conversion reaction on Ni5/MgO Mechanism analysis shows that the CO2 hydrogenation reaction on Ni5/MgO catalyst, conversion of CO2 to CO prior to methanation (involving a CO intermediate) On the potential energy surface, CH4 is more stable 15 than CH3OH The reaction path that forms CH4 is also more favorable, so on the Ni5/MgO catalyst, the hydrogenated CO2 reaction creates the main product CH4 through the P2-CH4/ Ni5MgO reaction path On this path, the reaction step has the highest activation energy of 152.2 kJ.mol–1 for step CH3O*→CH3*+O*, so this step is expected to determine the reaction rate The conversion of CO2 into methane by the proposed mechanism is considered feasible The possible by-products are CO, HCHO, and CH3OH 3.4 CO2 hydrogenation reaction on Ni5/AC Activated carbon (AC) is the support of interest due to its many properties: large specific surface area, low cost, capillary structure, easy surface modification, etc Loading Ni5 on the AC surface is expected to be a good catalyst for the hydrogenation of CO2 3.4.1 Ni5/AC structure Simulation of activated carbon (AC) with a hexagonal arrangement of carbon ring, referenced from empirical data The selected AC model is C72, which is large enough for the reactions to occur on the surface of the AC but not too large because it will increase the computational cost Calculation results show that in the most stable geometric structure, the Ni5 cluster has a structure close to the square pyramid The results of charge analysis, bond order, spin polarization, HOMO images show that the interaction between Ni5 and AC can be considered as chemical adsorption due to the formation of NiC bonds 3.4.2 Adsorption of CO2 and H2 on Ni5/AC - Adsorption of H2 on Ni5/AC: The most stable adsorption configuration is 3.1a with absorption energy 240,8 kJ.mol1, Ea=0 Figure 3.29 Adsorption configuration of H2 over Ni5/AC 16 - Adsorption of CO2 on Ni5/AC: The most configuration Eads=219,0 stable is adsorption 3.1b1 1 kJ.mol , with Ea=0 - corresponding to the multi-center Figure 3.30 Adsorption configuration of CO2 over Ni5/AC chemical adsorption on two different Ni atoms on the bottom of the square pyramid The adsorption of CO2 and H2 on AC showed that corresponding to the most durable adsorption structures, the adsorption energy was 27.7 kJ.mol1 and 15.0 kJ.mol1, respectively After adsorption on AC, CO2 and H2 molecules not change significantly compared to free molecules The adsorption of CO2 and H2 on AC is much less favorable than adsorption on Ni5/AC 3.4.3 Reaction paths on Ni5/AC catalyst The calculated mechanisms are summarized in Figure 3.33 Figure 3.33 The calculated reaction pathways of CO2 hydrogenation over Ni5/AC Based on the proposed reaction diagram, we can provide the following reaction pathways: 17 - Carbon monoxide formation on Ni5AC catalyst: P1-CO/Ni5AC: R1→R4→R11; P2-CO/Ni5AC: R3→R8→R11 - Methane formation on Ni5AC catalyst: P1-CH4/Ni5AC: R3→R8→R11→R15→R18→R22→R24→R26→R27 P2-CH4/Ni5AC: R3→R8→R11→R15→R18→R22→R25→R30 P3-CH4/Ni5AC: R3→R8→R11→R15→R18→R22→R25→R28→R29 - Methanol formation on Ni5AC catalyst: P-CH3OH/Ni5AC:R3→R8→R11→R15→R18→R21→R23Formaldehyde formation on Ni5AC catalyst: P-HCHO/Ni5AC: R3→R8→R11→R15→R18 -Formic acid formation on Ni5AC catalyst: P-HCOOH/Ni5AC: R2→R7 - Coke formation on Ni5AC catalyst: P-C/Ni5AC: R3→R8→R11→R16→R20 The corresponding energy and activation energy values of the steps are shown in Table 3.26 Table 3.26 Corresponding energy (∆E), activated energy (E a) of the reaction steps on Ni5/AC Step Reaction Ea, kJ.mol-1 ΔE, kJ.mol-1 R1 Oa=C=Ob +Ha → HaOaCOb 177.4 105.0 R2 Oa=C=Ob +Ha → HaCOaOb 304.7 3.5 R3 Oa=C=Ob → Oa +COb 25.9 5.6 R4 HaOaCOb→HaOa +COb 28.4 126.4 R5 HaOaCOb+Hb→HaOaCObHb 254.0 162.0 R6 HaCOaOb+Hb→HbHaCOaOb 246.7 181.5 R7 HaCOaOb+Hb→HaCOaObHb 300.3 278.3 R8 Oa +COb+Ha→HaOa +COb - 166.9 R9 Oa +COb+Ha→ Oa +CObHa 132.6 59.6 R10 Oa +COb+Ha→ Oa +HaCOb - 6.5 R11 HaOa +COb+Hb→HaOaHb +COb 168.9 142.0 R12 Oa +HaCOb+Hb →OaHb HaCOb 192.3 3.3 18 R13 Oa +HaCOb +Hb→Oa +HbHaCOb 74.9 35.5 R14 Oa +HaCOb+Hb →Oa +HaCObHb 210.7 83.4 R15 COb+Hc→HcCOb 23.0 0.2 R16 COb+Hc→CObHc 140.3 29.4 R17 COb→C+Ob 276.8 78.0 R18 HcCOb+Hd→HcHdCOb 108.5 80.3 R19 HcCOb+Hd→HcCObHd 212.1 178.2 R20 CObHc+Hd→HdObHc + C 303.3 161.9 R21 HcHdCOb+He→HeHcHdCOb 93.6 48.7 R22 HcHdCOb+He→HcHdCObHe 113.0 72.4 R23 HeHcHdCOb+Hf→HeHcHdCObHf 191.6 160.7 R24 HcHdCObHe+Hf→HcHdC+ HfObHe 33.3 20.1 R25 HcHdCObHe+Hf→HfHcHdC+ObHe 79.9 68.9 R26 HcHdC+Hg→HgHcHdC R27 HgHcHdC+ Hh→HhHgHcHdC R28 HfHcHdC+ObHe+Hg→HfHcHdC+HgObHe R29 HfHcHdC+Hh →HhHfHcHdC 180.6 165.7 R30 HfHcHdC+ObHe+Hg→HgHfHcHdC+ObHe 59.6 116.0 6.8 100.9 180.9 148.5 2.7 99.4 In the following sections, we detailed analysis of calculation results for each reaction path 3.4.3.1 Carbon monoxide formation on Ni5AC catalyst The most optimal reaction of the two paths forming CO is P2-CO/Ni5AC The highest Ea value of this path is 168.9kJ.mol1 for the R11 step 3.4.3.2 Methane formation on Ni5AC catalyst On the three reaction paths forming CH4, the first steps are quite similar, the other steps are not significantly different, all three paths are likely to occur However, the P2-CH4/Ni5AC reaction path, the obtained structures are more stable, so in our opinion, the P2-CH4/Ni5AC is the most favorable in the reaction paths to form CH4 3.4.3.3 Methanol formation on Ni5AC catalyst 19 On the P-CH3OH /Ni5AC reaction path, the step with the highest Ea is R23, Ea = 191.6 kJ.mol1 so this path is not preferred, can only occur at very high temperatures 3.4.3.4 Formaldehyde formation on Ni5AC catalyst The reaction steps in this path are part of the P1-CH4/Ni5AC HCHO formation is feasible 3.4.3.5 Formic acid formation on Ni5AC catalyst The formation of HCOOH is not feasible because both phases have Ea > 300 kJ.mol1 3.2.3.6 Coke formation on Ni5 catalyst Step R20 has a very high Ea (Ea = 303.3 kJ.mol1) In our opinion, coke formation on Ni5/AC catalysts is unlikely The potential energy surface for the proposed reaction mechanism on the Ni5AC catalyst shown in Figure 3.34 and Figure 3.35 Figure 3.34 The potential energy surface of the reaction steps between CO2 and Ha, Hb on Ni5AC 20 Figure 3.35 The potential energy surface from R15 to R30 step of conversion reaction on Ni5AC Analysis of reaction paths shows that on Ni5/AC catalyst, CH4 is the main product of CO2 hydrogenation reaction according to the P2-CH4/Ni5AC reaction path The first step of this path starts is CO2 dissociation into CO and O The step R11 has the highest Ea of 168,9 kJ.mol–1, so this step is expected to determine the reaction rate 3.5 Compare and analyze reactivity on three catalyts 3.5.1 Adsorption of H2, CO2 The adsorption of CO2 and H2 on the catalysts Ni5, Ni5/MgO, and Ni5/AC systems can be considered as chemisorption due to the relative negative adsorption energies After adsorption the obtained structure is quite similar, the H2 molecule dissociates, the activated CO2 molecule increasing CO bonding length reduced bond order, favorable for the next steps The adsorption of H2 between the three catalysts was not significant, indicating that it was not affected by the electron density When the Ni5 is placed on the support, the adsorption of CO2 decreases due to the decrease in charge density on the cluster 21 The adsorption energy on Ni5, Ni5/MgO, and Ni5/AC systems are 249,7; 224,3 and 219,0 kJ.mol1 corresponding to QupQdown are 8, 7, Another study on the adsorption of CO2 on Cu4 and Cu4 /Al2O3 catalysts also showed the same results 3.5.2 CO2 hydrogenation According to our results, the mechanism of CO2 hydrogenation reaction on the catalysts considered all show the formation of CO However, on different catalysts, the reaction path is different H H - On Ni5 and Ni5/MgO: CO*2 CO* OH* CO* H2 O* * * H H - On Ni5/AC: CO*2 CO* O* CO* OH* CO* H2 O* * * This is due to the effect of the catalyst on the adsorption of CO 2, in the first steps of the reaction, step R3 on the Ni5/AC catalyst is more preferred than on the Ni5 and Ni5/MgO catalysts Because of the Ni5/AC catalyst, after adsorption of the CO2 molecule activated, the COa bond is longer, d(COa) = 1.39Ǻ compared to the values of 1.305Ǻ and 1.26Ǻ on Ni5 and Ni5/MgO catalysts, respectively, the CO bonds in CO2* adsorbed on Ni5/AC is more easily broken Coke formation on three catalysts is very unlikely because of the high Ea Other studies also suggested that small nickel clusters will help reduce coke formation on the catalytic surface Calculation results show that the supports increases the selectivity of the product On Ni5, there are reaction paths that can form CH3OH, on Ni5/MgO, there are reaction paths forming CH3OH while on Ni5/AC, there is only one path to form CH3OH On the Ni5 catalyst, there is a by-product of HCOOH, and when the Ni5 is placed on the MgO and AC, the hydrogenation of CO2 on these two catalysts makes it almost impossible to create this by-product Selectivity is an important factor, even more, important than reaction productivity In our opinion, Ni5/AC is the best catalyst among the three catalysts studied 22 GENERAL CONCLUSIONS In this thesis, the mechanism of CO2 hydrogenation reaction on Ni5, Ni5/MgO, and Ni5/AC was investigated systematically using the DFT method together with Climbing Image Nudged Elastic Band (CI-NEB) method We put out some conclusion as follows: - Propose and calculate the mechanism of CO hydrogenation to form CH4, CH3OH, HCHO, and HCOOH including 33 reaction paths, building potential surfaces; Calculation results show that on Ni5 based catalysts, CH4 is the most preferred product CO2 hydrogenation reaction over Ni5 based catalysts does not form carbon (coke) - On Ni5 catalyst, the optimal reaction of CO2 hydrogenation through intermediate products: HOCO*, CO*, HCO*, HCOH*, H2COH*, CH2*, CH3*, CH4* The rate-determining step for CH4 formation in CO*+H* → HCO*, Ea= 106.5 kJ.mol1 Some other by-products are CO, HCHO, HCOOH, CH3OH - On the Ni5/MgO catalyst, the optimal reaction of CO2 hydrogenation through intermediate products: HOCO*, CO*, HCO*, HCHO*, CH3O*, CH3*, CH4* The rate-determining step for CH4 formation is CH3O*→ CH3*+O*, Ea= 152.2 kJ.mol1 Some other by-products are CO, HCHO, CH3OH - On the Ni5/AC catalyst, the optimal reaction of CO2 hydrogenation through intermediate products: CO*+O*, CO*, HCO*, HCHO*, H2COH*, CH3*, CH4* The rate-determining step for CH4 formation is CO*+OH*+H* → CO*+H2O*, Ea=168.9 kJ.mol1 Some other by-products are CO, HCHO - When Ni5 is placed on the MgO and AC support, the selectivity of the reaction increases The Ni5/AC catalyst is the best catalyst of the three catalysts studied ... Cầm, Nguyễn Ngọc Hà (2020), “ Nghiên cứu lí thuyết phản ứng methane hóa CO2 xúc tác Ni5/ AC phương pháp phiếm hàm mật độ Phần II : Các đường phản ứng? ??, Tạp chí Xúc tác Hấp phụ, 9(1), Tr.74-81 1... Cầm, Nguyễn Ngọc Hà (2020), ? ?Nghiên cứu lí thuyết phản ứng methane hóa CO2 xúc tác Ni5/ AC phương pháp phiếm hàm mật độ Phần I : Giai đoạn hấp phụ hoạt hóa? ??, Tạp chí Xúc tác Hấp phụ, 9(1), Tr.33-38... Trình, Nguyễn Thị Thu Hà, Nguyễn Ngọc Hà (2018), ? ?Nghiên cứu phản ứng methane hóa CO2 xúc tác cluster Ni5 phương pháp phiếm hàm mật độ? ??, Tạp chí hóa học, 56, 6e2, Tr.194-198 Nguyen Thi Thu Ha,
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