Computational design of zno based catalysts for chemical systems

Thus, in this study we investigate the pure ZnO and modification of ZnO 1010 surface by doping and depositing different types of metals in attempt to design better catalysts for several

National Taiwan University of Science and Technology Department of Chemical Engineering

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Acknowledgments

I have benefited from numerous people and many facilities during my graduate study at the National Taiwan University of Science and Technology First, I would like to acknowledge my advisor, Professor Jyh-Chiang Jiang, whose enthusiasm and expertise were greatly appreciated

I am very grateful for many current and former members of my lab (T2-513) for their support Of special note are Dr Wang, Dr Ni, and Mr Hung I spent a wonderful student life in the University of Science and Technology-Taiwan with numerous friends They supported lots information for me to understand the good living at school At last, I would like to thank my loving parents, beautiful wife, for their endless support This work has been supported by the National Science Council, Taiwan (NSC 99-2113-M-011-001-MY3) and the National Center of High- Performance Computing (NCHC) for computer time and facilities

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Abstract

As a semiconductor with a hexagonal wurtzite crystal structure and wide direct band gap, zinc oxide (ZnO) has a wide range of technological uses It is widely known that, in combination with other metal particles (e.g., Na, Cu, Pd and Ti) at the surface, ZnO, in general, has better selectivity and catalytic performance Thus, in this study we investigate the pure ZnO and modification of ZnO (1010) surface by doping and depositing different types of metals in attempt to design better catalysts for several important chemical reactions including; (1) methanol decomposition on ZnO surface; (2) CO oxidation on undoped and Ti doped ZnO surface; and (3) the water gas-shift reaction (WGS) on 2Cu deposited ZnO surface To detail, the adsorption and reaction mechanisms of methanol decomposition, CO oxidation, and water gas shift on pure and modified ZnO(1010) surfaces have been investigated using density functional theory (DFT) slab calculations, respectively In addition, the effect of the adsorption bonding between adsorbates and surfaces are studied also using density of states (DOS) and electron density difference (EDD) contour plots The understanding on these systems will help to shed more light on how to design better catalytic materials for different chemical systems

Keywords: Density Functional Theory (DFT); ZnO 1010 ; CO Oxidation, Methanol decomposition, Water Gas-Shift Reaction

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Table of Contents

Abstract 2  

Chapter 1: Introduction 15  

1.1   Fuel cell 15  

1.1.1   Background 15  

1.1.2   Fuel cell principle and fuel cell types 16  

1.1.3   The hydrogen generation via methanol 19  

1.1.4   CO oxidation 21  

1.1.5   Water gas shift reaction (WGSR) 22  

1.2   Catalytic performances 23  

1.2.1   Catalyst development 23  

1.2.2   Bravais-miller index - case of ZnO structures 25  

1.3   This research 26  

Chapter 2: Computational Details 28  

2.1   Theoretical background 28  

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2.2   Method and parameters in this work 34  

Chapter 3: Surface Models 37  

3.1   ZnO (1010) surface 37  

3.2   Ti doped ZnO(1010) surface 41  

3.3   Two Cu atoms deposited on ZnO(1010) surface 44  

Chapter 4: Methanol Decomposition on ZnO (1010) Surface 49  

4.1   Methanol adsorption 49  

4.2   Electron density difference analysis 51  

4.3   CH 3 OH decomposition on ZnO (1010) surface 53 

4.4   Conclusions 59  

Chapter 5: CO Oxidation on Undoped and Ti Doped on ZnO (1010) Surface 61  

5.1   CO adsorption on undoped ZnO surface 61  

5.2   CO adsorption on Ti doped ZnO surface 68  

5.3   Reaction mechanisms 70  

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5.3.1   Undoped ZnO(1010)surface 71  

5.3.2   Ti doped ZnO(1010)surface 74  

5.3.3   Restored surface 75  

5.4   Conclusions 77  

Chapter 6: Water Gas Shift Reaction on 2Cu Deposited ZnO (1010) Surface 78  

6.1   Adsorption of reactants, intermediates, and products on the 2Cu/ZnO(1010) surface 79  

6.2   Conversion of CO and H 2 O on the 2Cu/ZnO(1010)surface 85  

6.3   Desorption products 95  

6.4   Conclusions 98  

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Index of Figures

Figure 1 Chart to summarize the applications and main advantages of fuel cells of different types, and in different applications - 17  

Figure 2 Wurtzite structure of ZnO - 26  

Figure 3 The ZnO(1010)surface model: two-dimensional view with 15Å vacuum slab and six layers representation where the first two upper and the four bottom layers

in the (1010) direction The blue and red balls are Zn and O atoms, respectively - 35  

Figure 4 A side view of relaxation and reconstruction on ZnO(1010)surface In which Figure 4(a) shows six layer structure; Figure 4(b) is an extraction (bordering by the dashed circle) from Figure 4(a) to describe the relaxed surface Compared to its bulk structure, the surface Zn-O bond tilts, marked with parameter θ and the bond linked between the surface zinc and oxygen contracts shown by d - 38  

Figure 5 The side-view of the (2x2)-ZnOsupercell of Ti-doped systems Only first two outermost layers in the direction (1 010 ) are shown More detailed description about the tilt angle (α) can be found in the work of Wander - 43  

Figure 6 Local density of state (LDOS) analysis for the surface oxygen bonded to the undoped ZnO surface (grey line) and Ti doped ZnO surface (black line) - 44  

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Figure 7 The most stable site (site E from Table S2, see in appendix) was chosen for calculations in WGSR In which, (a), (b) noted a top view and site view of structural site E The black circle of in each (a) and (b) noted that different positions of Cu-site and IF-site on 2Cu/ZnO(1 010 ) surface - 46  

Figure 8 The electronic properties of 2Cu atoms adsorbed on ZnO(1 010 ) surface Figure 8(a) shows DOS analysis of 2Cu adsorption on the surface in which the top box is states of 2Cu atoms before (grey line) and after adsorption (dash line), and the bottom box notes the states of ZnO before (grey line) and after (dash line) adsorption; Figure 8(b) presents EDD contour plot of 2Cu adsorption on the surface, where the dash-line is represented Cu electrons loss, and solid line as electrons accumulation of adsorbate-surface - 48  

Figure 9 Configuration of CH 3 OH adsorbed on ZnO (1010) surface at three different adsorption sites All bond distances (black-arrow line) are in Å Shaded white spheres represent the hydrogen, the grey ones are the carbon atoms - 49  

Figure 10 Electron density difference contour plot of CH 3 OH adsorbed on ZnO

dash-lines represents the loss of electron and the solid-dash-lines show electron accumulation 52  

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Figure 11 In each box, the top and bottom shown transition and final states structures

of CH 3 OH decomposition on ZnO(1010) surface; (a), (b), and (c) present O-H, C-H, and C-O bond breaking of site A, respectively, (d), (e) as O-H-C, and C-O bond breaking of site C - 55  

Figure 12 Calculated activation barrier and reaction energies (in eV) characterizing C-O, C-H, and O-H bond breaking of MeOH on ZnO (1010) substrates with respect to the energy calculated for the corresponding MeOH adsorption - 56  

Figure 13 Reaction pathway of CH 3 OH decomposition of site A on clean ZnO (1010)surface - 57  

Figure 14.The most stable structures of CO adsorption on the undopedZnO (1010)

surface (top) and the Ti-doped (1010) surface (bottom) - 62 

Figure 15 The other less stable structures of CO adsorbed on the undoped ZnO (1010)

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Figure 17.The analysis of partial density of state (PDOS) of s and p orbitals of CO before adsorption (clean) and after adsorbed on undoped and Ti doped ZnO surfaces (bonded), respectively - 65  

Figure 18 The analysis of partial density of state (PDOS) of dz 2 , dyz, and dxz orbitals

of CO before adsorption (clean) and after adsorbed on undoped and Ti doped ZnO surfaces (bonded), respectively - 66  

Figure 19 Electron density difference (EDD) contour plots analysis of CO molecule binding to undoped and Ti-doped ZnO (1010) surface are shown in (a) and (b), respectively.The solid and dashed lines represent increasing and decreasing electron densities, respectively - 67  

Figure 20 The other less stable structures of CO adsorbed on the Ti doped ZnO

(1010) surface - 68  

Figure 21 Reaction pathway of CO oxidation on undoped (solid black lines) and

Ti-doped ZnO 1010 surface (solid grey line) In each box, the black box presents structures of CO oxidation at every state on undoped surface, whereas the grey box is structures of CO oxidation on Ti doped ZnO surface - 72  

Figure 22 The structures and reaction energies to restored surfaces via O 2 gas phase and activated CO then to CO 2 formation on undoped and Ti doped surfaces; the box

as side view of adsorption species at IF-site - 81  

Figure 24 From Figure 24(a) to Figure 24(j) are shown as DOS analysis of adsorbates, intermediates, and products adsorbed at IF-site and Cu-site on 2Cu/ZnO surface, in which corresponding to configurations adsorption in Figure 23(a-j), respectively The black dash lines and solid lines shown positions of molecule bonded

to surface at Cu-site and IF-site, respectively; the grey solid line represent positions of molecule in the gas phase The black circles point out the interaction section between molecules and surface - 84  

Figure 25 Reaction pathway profile of H atom abstracted from H 2 O on 2Cu/ZnO(1 010 ) surface at Cu-site (red-line) and IF-site (black-line) In each box, the box with solid colored-red and dash colored-black show structural side view of initial, transition, and final state at Cu-site and IF-site, respectively - 87  

Figure 29 Reaction pathway profile of COOH dissociated (Figure 29(a)) and COOH dissociation (Figure 29(b)) by OH group on 2Cu/ZnO(1 010 ) at Cu-site (red-line) and IF-site (black-line), respectively - 93  

Figure 30 Reaction pathway profile of H 2 recombined on 2Cu/ZnO(1 010 ) at site (red-line) and IF-site (black-line), respectively - 94  

Cu-Figure 31 Reaction pathway of CO 2 desorption at Cu-site on 2Cu/ZnO (1010) surface - 95  

Figure 32 Reaction pathway of H 2 desorption at Cu-site on 2Cu/ZnO (1010) surface - 96  

Figure 33 The graph of reaction network for WGSR, including both redox mechanism and carboxyl mechanism at Cu-site and IF-site are listed All of barrier

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and reaction energy values are given in electronvolts (eV) The dash-arrow lines point out reaction pathway is impossible The solid-arrows and block-arrows are presented different pathways of redox mechanism and carboxyl mechanism, respectively 97

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Index of Tables

Table 1 Data for different types of fuel cell 18  

Table 2 Relaxation of the ZnO (1010)surface 39  

Table 3 Binding energies, E b (in eV) of CH 3 OH on ZnO(1010)surface with the changes of cutoff energy (in eV), and k-points settings 41  

Table 4 The average adsorption energy per adsorbate ( ), and surface energy ( ) of 2Cu atoms adsorbed on ZnO(1010)surface 45  

Table 5 The bond length (R, in Å), bond distance (d, in Å), bond angle ( , ے in degree), and adsorption energies (E ads , in eV) for CH 3 OH adsorption on ZnO

Table 6.The calculated bond distances (d), bond angle and binding/adsorption energies (E ads ) for CO adsorption on undoped and Ti-doped ZnO(1010)surfaces (see Figure 14 for the specific sites) 62  

Table 7 The important parameters and structures of initial (SP), transition (TS), intermediate (IM), and final (FP) states for CO + O s →CO 2 In which, O s is noted as Oxygen neighboring atom of ZnO surface; the subscripts A and B represent path of

CO oxidation on undoped and Ti doped ZnO surfaces, respectively 73  

Table 8 Carboxyl mechanism vs redox mechanism for WGSR on 2Cu/ZnO a 78  

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Table 9 Adsorption sites, selected adsorbate (A)-surface (S), bond distance (d A-S , in Å), and adsorption energy E ad (in eV) for possible species involved in the WGSR at Cu-site and IF-site on 2Cu/ZnO surface 80  

Table 10 Result calculations of E a , ∆H, and IMF for the elementary steps at Cu-site and IF-site in WGSR 86    

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A fuel cell generates electrical power by continuously converting the chemical energy of a fuel into electrical energy by way of an electrochemical reaction A fuel cell consists of an electrolyte and two electrodes A fuel such as hydrogen is continuously oxidized at the negative anode while an oxidant such as oxygen is continuously reduced at the positive cathode The electrochemical reactions take place

at the electrodes to produce a direct electric current FCs use hydrogen as a fuel which results in the formation of water vapor only and thus they provide clean energy FCs offer high conversion efficiency and hence is promising The current status of fuel cell technology for mobile and stationary applications has recently been discussed 6

1.1.2 Fuel cell principle and fuel cell types

Leaving aside practical issues such as manufacturing and materials costs, the two fundamental technical problems with fuel cells are

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though there are always other important differences as well The situation now is that six classes of fuel cell have emerged as viable systems for the present and near future Basic information about these systems is given in Table 1 and Figure 1

Figure 1 Chart to summarize the applications and main advantages of fuel cells of

different types, and in different applications 5

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Table 1 Data for different types of fuel cell 5

Fuel cell type Mobile ion Operating

temperature Applications and notes Alkaline (AFC) OH 50–200°C Used in space vehicles, e.g Apollo, Shuttle

Proton exchange

membrane

(PEMFC)

H + 30–100°C Vehicles and mobile applications, and for lower power in

Combined Heat and Power (CHP) systems Direct methanol

+ 20–90°C Suitable for portable electronic systems of low power, running for

long times Phosphoric acid

2− 500–1000°C Suitable for all sizes of CHP systems, 2kW to multi-MW

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The use of hydrocarbon or alcohol fuels requires an external fuel processor to be incorporated into the system This item not only increases the complexity and cost of the system, but also decreases the overall efficiency In contrast, molten-carbonate fuel cells (MCFCs) and solid-oxide fuel cells (SOFCs) operating at higher temperatures have the advantage that both CO and H2 can be electrochemically oxidized at the anode Moreover, the fuel-processing reaction can be accomplished within the stack, which enables innovative thermal integration/management design features to provide excellent system efficiencies (~50%).7

Figure 1 shows the main advantages and applications of fuel cells of different types However, the most important disadvantage of fuel cells at the present time is the same for all types – the cost There are varied advantages, which feature more or less strongly for different types and lead to different applications, which include efficiency, simplicity, low emissions, and silence The advantages of fuel cells impact particularly strongly on combined heat and power systems (for both large- and small-scale applications), and on mobile power systems, especially for vehicles and electronic equipment such as portable computers, mobile telephones, and military communications equipment These areas are the major fields in which fuel cells are being used Several example applications are given in the chapter in which the specific fuel cell types are described

1.1.3 The hydrogen generation via methanol

Fuel cells run on hydrogen, the simplest element and most plentiful gas in the universe Hydrogen is never found alone on earth, it is always combined with other elements such as oxygen and carbon Hydrogen can be extracted from virtually any hydrogen compound and is the