Theoretical Characterization of Ammonia Oxidation Species on Platinum Clusters

Home , Deprotonation

A dissertation presented to

the faculty of

the Russ College of Engineering and Technology of Ohio University

In partial fulfillment

of the requirements for the degree

Doctor of Philosophy

Oludamilola A. Daramola

November 2011

This dissertation titled

Theoretical Characterization of Ammonia Oxidation Species on Platinum Clusters

OLUDAMILOLA A. DARAMOLA

has been approved for

the Department of Chemical and Biomolecular Engineering

and the Russ College of Engineering and Technology by

Gerardine G. Botte

Professor of Chemical and Biomolecular Engineering

Dennis Irwin

Dean, Russ College of Engineering and Technology 3

ABSTRACT

DARAMOLA, OLUDAMILOLA A., Ph.D., November 2011, Chemical Engineering

Theoretical Characterization of Ammonia Oxidation Species on Platinum Clusters (166 pp.)

Director of Dissertation: Gerardine G. Botte

Ammonia oxidation is being considered as a viable technology for hydrogen production for use in fuel cells. This study was undertaken to gain insight into current issues related to catalytic inactivity with time. Density Functional Theory was used in modeling the chemical species present during ammonia oxidation: NHx (x = 0 – 3), OHy

(y = 1 & 2) and N2Hz (z = 0 - 4) and the adsorption of these molecules on the surface of platinum clusters. Using comparison with experimental measurements where possible, it was found that the strength of adsorption for these molecules followed this trend: N2 <

H2O < NH3 < N2H2 < N2H4 < N2H < N2H3 < OH < NH2 < NH < N. This suggests that the species present towards the right of this spectrum were especially relevant to surface blockage and could play a role in catalytic inactivity.

In addition, the formation and oxidation of the N2Hz molecules could possibly be tracked by spectrochemical analysis of the position of the N – N bond, which went from single (N2H4) to double (N2H2) to triple (N2) as the oxidation of ammonia progressed.

The presence or absence of this peak is an indicator of the orientation of the molecule formed and an indicator of the progress of the reaction.

Finally, an exploratory investigation of a mechanism of ammonia oxidation, where ammonia is deprotonated in successive steps, predicted that the conversion of the 4 imide radical to nitrogen, although thermodynamically favorable, exhibits slow kinetics in comparison to deprotonation of ammonia or amidogen.

Approved: ______

Gerardine G. Botte

Professor of Chemical and Biomolecular Engineering

DEDICATION

To Olalekan and Funmilayo Daramola

For all that I am and will become, I have you to thank.

ACKNOWLEDGMENTS

First and foremost, thanks go to my parents Lekan and Funmi Daramola, who had a vision of sending their son to Ohio University so he could create his future, just like they had created theirs. This dissertation is a testament of that investment and I am forever indebted and forever grateful. Dad, thank you for the countless times you reminded me: “Don’t forget who you are and where you are coming from.” Mum, thank you for always keeping me in your prayers and ensuring that I kept a balance between the intellectual with the spiritual.

Secondly, to my wife Brandi, our life together is just beginning, but our relationship has always involved me staying up late nights to run tests and perform calculations. Your relocation to Ohio helped in keeping me balanced and sane. Thank you for the countless times you wielded a big stick, just so I would get the ample sleep and the right diet. Your patience through this process has now been rewarded.

The research contained within would never have been possible without the guidance, support and enthusiasm of an amazing advisor. Dr. Botte, you have provided me a template of what a research professor is supposed to do: allow students to take ownership of their work by creating an avenue for them to test their ideas, inspire students by continually receiving accolades based on your own research progress and create an environment that fosters creativity and interaction by ensuring students have mentors. Thank you for giving me the opportunity to work with you and allowing me to develop professionally as an author, a researcher and a teacher.

I am fortunate enough to have taken a class with most of the professors present on my committee and I am thankful that I am able to share my work with you. Dr. Young, your statistics course was the most useful class I ever took in my undergraduate career; I am still 7 applying the analyses you spent so much time harping on 8 years ago. Your husband started me on the path to research by letting me work with him in my sophomore year, so one can say I have now come full circle. Dr. Sandler, I want to especially thank you for accepting to be on my dissertation committee on such short notice and providing the relevant feedback needed. Dr. Gulino, as my undergraduate advisor, you ensured that I was taking the appropriate courses and was on track to finish in four years. Your jovial approach to life as always helped me feel at ease in your presence. Dr. Dewald, your approach to teaching by tying textbook material with fundamental journal articles ensured the ideas taught in class were crystallized by further study.

I would be remiss if I didn’t mention administrative staff at the Department of

Chemical and Biomolecular Engineering and the Center for Electrochemical Engineering

Research: from making copies (Carrie Linscott) to installing software (Jim Caesar) to stocking up on coffee and processing orders (Leisa Ostermann and Shannon Bruce). These are the non-academic things that ensure a student’s academic life goes smoothly.

I would also like to thank my uncle, Dr. Simbo Odunaiya who was my guardian when all this began in 2000 as a freshman at Ohio University. Your support provided the valuable foundation for my academic career.

Finally, to my colleagues at the Center for Electrochemical Engineering, I am thankful for all the discussions we have during our monthly research meetings. It reminds me that “Iron sharpens iron.” 8

TABLE OF CONTENTS

Page

Abstract ...... 3 Dedication ...... 5 Acknowledgments...... 6 List of Tables ...... 11 List of Figures ...... 14 Chapter 1: Introduction ...... 17 1.1 Project Overview ...... 17 1.2 Statement of Objectives ...... 18 1.3 Significance of Research...... 20 1.4 References ...... 21 Chapter 2: Literature Review...... 22 2.1 Background ...... 22 2.2 Experimental Observations ...... 22 2.3 Molecular Modeling of Ammonia Oxidation ...... 25 2.4 Summary ...... 28 2.5 References ...... 29

Chapter 3: Characterization of NHx (x = 0 - 3) and OHy (y = 1 & 2) molecules on platinum clusters ...... 30 The contents of this chapter have been submitted to the Chemistry of Materials for publication consideration...... 30 3.1 Abstract ...... 30 3.2 Introduction ...... 30 3.3 Computational Details ...... 35 3.4 Results and Discussion ...... 38 3.4.1 Bare Clusters ...... 38 3.4.2 Gas Phase Adsorbate Molecules ...... 42 3.4.3 Adsorption Energies and Geometry ...... 43 3.4.4 Atomic Spin Density ...... 54 3.4.5 Frequency Calculations ...... 57 9

3.5 Conclusions ...... 69 3.6 References ...... 70

Chapter 4: Characterization of N2Hz (z = 0 - 4) molecules on a Platinum cluster of 20 Atoms 74 The contents of this chapter have been submitted to the Chemistry of Materials for publication consideration...... 74 4.1 Abstract ...... 74 4.2 Introduction ...... 75 4.3 Computational Details ...... 78 4.4 Results and Discussion ...... 79 4.4.1 Geometry of free molecules ...... 79 4.4.2 Energies, Geometries and Frequencies of adsorbed molecules ...... 84 4.5 Conclusions ...... 114 4.6 References ...... 115 Chapter 5: Preliminary Investigation of the Oswin and Salomon Mechanism for Ammonia Oxidation...... 117 5.1 Abstract ...... 117 5.2 Introduction ...... 117 5.3 Computational Details ...... 118 5.4 Results and Discussion ...... 120 5.4.1 Adsorption of ammonia ...... 120 5.4.2 Deprotonation of ammonia ...... 121 5.4.3 Deprotonation of amidogen ...... 124 5.4.4 Deprotonation of imide ...... 128 5.5 Conclusion ...... 130 5.6 References ...... 131 Chapter 6: Conclusions and Recommendations ...... 132 6.1 Conclusions ...... 132 6.2 Recommendations ...... 133 Glossary of terms ...... 135 Appendix A – X, Y and Z Coordinates, Energies and Spin multiplicities for calculated systems ...... 136 10

Appendix B – Table of Values from digitized graphs ...... 160

LIST OF TABLES

Page

-1 Table 2.1. Frequency values (in cm ) for deformation of the N-H bond in different NHx (x = 0 - 3) molecules ...... 27 Table 2.2. Calculated activation energies (in kJ/mol) for the Oswin and Salomon mechanism (Equations 4 – 9) depicting different rate-limiting steps on two different surface ...... 27 Table 3.1. Energy calculations on platinum clusters at various spin densities. Energy differences (kJ/mol) are calculated with respect to the ground states predicted by the Interstitial Electron Model. Cohesive energies (kJ/mol) of the clusters approach the cohesive energy of bulk platinum...... 39 Table 3.2. Calculated electronic energies (in hartree) of the adsorbed molecules indicating the ground state when multiple spins are possible. The difference between neighboring spins and the ground state is about 0.1 Ha...... 43

Table 3.3. Pt – N and Pt – O bond lengths (in Å) after full geometry optimization of NHx (x = 0 – 4) and OHy (y = 1 & 2) molecules on four platinum clusters of varying size. There are one, two and three bond lengths shown for the top, bridge and face-centered cubic hollow positions, respectively). The platinum clusters are unrelaxed as an initial approximation. Atomic coordinates used in this calculation are present in Appendix A. 45

Table 3.4. Ground state calculated adsorption energies (in kJ/mol) for NHx (x = 0 – 4) and OHy (y = 1 & 2) molecules on platinum clusters of various sizes. The platinum clusters are unrelaxed as an initial approximation...... 46

Table 3.5. Adsorption energies (in kJ/mol) calculated in this study for NHx (x = 0 - 3) and OHy (y = 1 & 2) molecules on the Pt(111) surface in comparison with experimental and calculated values found in literature...... 50 -1 Table 3.6. Vibration modes and vibration frequencies (in cm ) for free and adsorbed NH3 on Pt(111) from experimental observation and DFT calculations. Values in this study are shown to predict the shift observed in experiment upon adsorption of NH3...... 58 -1 Table 3.7. Vibration modes and vibration frequencies (in cm ) for free and adsorbed NH2 on Pt(111) from experimental observation and DFT calculations. Values in this study are shown to predict the shift observed in experiment upon adsorption of NH2...... 60 Table 3.8. Vibration modes and vibration frequencies (in cm-1) for free and adsorbed NH on Pt(111) from experimental observation and DFT calculations. Values in this study are shown to predict the shift observed in experiment upon adsorption of NH...... 63 Table 3.9. Vibration modes and vibration frequencies (in cm-1) for adsorbed N on Pt(111) from experimental observation and DFT calculations...... 64 12

Table 3.10. Vibration modes and vibration frequencies (in cm-1) for free and adsorbed H2O on Pt(111) from experimental observation and DFT calculations. Values in this study are shown to predict the shift observed in experiment upon adsorption of H2O. ... 66 Table 3.11. Vibration modes and vibration frequencies (in cm-1) for free and adsorbed OH on Pt(111) from experimental observation and DFT calculations. Values in this study are shown to predict the shift observed in experiment upon adsorption of OH...... 68

Table 4.1. Initial configuration for adsorbed N2Hz molecules prior to performing geometry optimization. N1 and N2 denote the different nitrogen atoms within the molecule. In the case of N1 and N2 in bridge positions, the N – N bond is parallel to the surface of the cluster ...... 79

Table 4.2. Bond lengths (in Å) and bond angles (in º) of N2Hz (z = 0 – 4) molecules calculated in this study compared with experimental measurements where available. .... 80

Table 4.3. Energy and geometry of hydrazine adsorbed on the Pt20 cluster. The end-on configuration is favored in both the relaxed and frozen platinum cluster calculations. ... 86 Table 4.4. Vibration modes and vibration frequencies (in cm-1) for free, adsorbed and bridge-ligated N2H4 from experimental observations and calculations in this study. The frequencies from this study are based on the favored end-on configuration adsorbed on relaxed platinum. Calculations show the expected shift in N – N and N – H stretches also observed in experiment upon adsorption of N2H4...... 94

Table 4.5. Energy and geometry of hydrazyl adsorbed on the Pt20 cluster. The side-on configuration is favored in both the relaxed and frozen platinum cluster calculations. ... 99 Table 4.6. Effect of surface dihedral angle rotation on geometry of the adsorbed hydrazyl radical in the side-on configuration on the Pt20 cluster. The angle difference of 0º is the configuration from Figure 4.11 ...... 101 Table 4.7. Vibration modes and vibration frequencies (in cm-1) for free and adsorbed hydrazyl. The frequencies are based on the favored side-on configuration adsorbed on relaxed platinum. The adsorbed radical yields a vibration frequency similar to adsorbed hydrazine for the N – N bond stretch...... 102

Table 4.8. Energy and geometry of diazene adsorbed on the Pt20 cluster. The side-on configuration is favored in both the relaxed and frozen platinum cluster calculations. . 103 Table 4.9. Vibration modes and vibration frequencies (in cm-1) for free and adsorbed diazene from experimental observations and calculations in this study. The frequencies are based on molecules adsorbed on relaxed platinum. The calculated N – N stretch for trans diazene occurs at a much higher frequency than hydrazine and hydrazyl...... 107

Table 4.10. Energy and geometry of N2H adsorbed on the Pt20 cluster...... 109 Table 4.11. Vibration modes and vibration frequencies (in cm-1) for free and adsorbed N2H from calculations in this study. The frequencies are based on molecules adsorbed on relaxed platinum. The calculated N – N stretch occurs at around the same value calculated for trans diazene...... 110 13

Table 5.1. Thermodynamics of ammonia adsorption on Pt20 predict an exothermic and spontaneous reaction...... 120

Table 5.2. Thermodynamics of ammonia and hydroxyl co-adsorption on Pt20 predict an exothermic and spontaneous reaction...... 121

Table 5.3. Thermodynamics and kinetics of ammonia deprotonation on Pt20 predicts an endothermic and non-spontaneous reaction (Equation 1). This predicts that the calculated product state will required an increase in entropy to favor a spontaneous reaction...... 123 Table 5.4. Thermodynamics and kinetics for deprotonation of amidogen by the hydroxyl radical (Equation 2). ΔHtop and ΔGtop use Figure 5.6 as the product, while ΔHfcc and ΔGfcc use figure 5.8 as the product. This shows the effect of diffusion on favoring a spontaneous reaction ...... 126

Table 5.5. Thermodynamics and kinetics of imide deprotonation on Pt20 predicts an exothermic and spontaneous reaction (Equation 3)...... 130

LIST OF FIGURES

Page

Figure 2.1 Illustration of special positions for adsorption of molecules on a platinum surface (T = top, B = bridge, H = hexagonal close-packed (HCP) hollow and F = face- centered cubic (FCC) hollow) ...... 26

Figure 3.1. Clusters of Pt10, Pt15, Pt20 and Pt25 used in modeling the Pt(111) surface orientation. The atom numbers are used as a later reference within the text when adsorption is considered...... 36

Figure 3.2. Special positions in Pt(111) illustrated with Pt10 (left) and Pt20 (right) with T, B, H and F representing top, bridge, hexagonal close-packed (HCP) and face-centered cubic (FCC) positions, respectively...... 37 Figure 3.3. Spin densities of platinum atoms within the four clusters. The model shows a good representation for Pt15 and Pt20 with spins close to 1 for all atoms, while Pt10 and Pt25 have some atoms which are poorly represented with little to no spin density...... 41

Figure 3.4. Optimized geometries of adsorbed NH3, NH2, NH, N, OH and H2O (white H atoms, blue N atoms and red O atoms) on unrelaxed Pt20 cluster (grey Pt atoms) showing the occupation of special positions as predicted by previous DFT calculations. Atomic coordinates for these molecules on Pt10, Pt15 and Pt25 can be found in Appendix A...... 44

Figure 3.5. Comparison between OH adsorbed on unrelaxed (top) and relaxed Pt20 (bottom) cluster to show the final position of the relaxed platinum atoms relative to initial positions. Upon relaxation, Pt5 and Pt6 atoms relax upwards while Pt2 and Pt9 atoms relax slightly inwards. Atomic coordinates for all relaxed Pt20 cluster calculations with NHx and OHy molecules are in Appendix A...... 52 Figure 3.6. Comparison between spin density of bare cluster (broken line) and cluster with adsorbate (solid line). The non-metallic atoms (N or O where applicable) are represented as atom 0 in each graph. Radicals spin pair with the unpaired electrons of the platinum cluster thus causing the spin on the N or O atom and the bonding Pt atom (Figures 3.1 and 3.4) to be reduced to zero...... 55

Figure 4.1. Conformation of minimum energy for free N2Hz (z = 1 - 4) molecules. Clockwise from the top are N2H radical, diazene, hydrazine and hydrazyl radical with nitrogen atoms in blue and hydrogen atoms in white...... 80 Figure 4.2. Energy barrier for the dihedral angle rotation of the hydrazine molecule. The global minimum is the gauche conformation shown in Figure 4.1...... 82 Figure 4.3. Energy barrier for the dihedral angle rotation of hydrazyl. The global minimum is the conformation shown in Figure 4.1 ...... 83 Figure 4.4. Energy barrier for the dihedral angle rotation of diazene. The global minimum is the trans conformation shown in Figure 4.1...... 84 15

Figure 4.5. Orientation of end-on (top) and side-on (bottom) configurations of adsorbed hydrazine on frozen Pt20 cluster with all frozen coordinates...... 85 Figure 4.6. Orientation of end-on (top) and side-on (bottom) configurations of adsorbed hydrazine on Pt20 cluster with Pt2, Pt5, Pt6 and Pt9 relaxed. The atomic coordinates for the relaxed calculations for both configurations can be found in Appendix A...... 87

Figure 4.7. Energy barrier related to surface dihedral rotation (Pt4 – Pt5 – N21 – N22 from Figure 4.5) for the end-on configuration of N2H4 on Pt20 with each point representing the degree of rotation with respect to the optimized geometry...... 88

Figure 4.8. Energy barrier related to internal molecular dihedral rotation (H24 – N21 – N22 – H26 from Figure 4.5) for the end-on configuration of N2H4 on Pt20 with each point representing the degree of rotation from the optimized geometry...... 89 Figure 4.9. Digitized version of Figure 1 from Binder and Sellmann14. This was used to challenge the FWHM of 3.8 eV reported by Grunze12...... 91 Figure 4.10. Digitized version of the N1s XPS Spectra of Figure 8 from Alberas et al. study for 0.5 ML coverage8. This was closer to the value of the digitized verion of the Binder and Sellman figure14 (Figure 4.9) and challenges the notion of side-on adsorption...... 92 Figure 4.11. Orientation of end-on (top) and side-on (bottom) configurations of adsorbed hydrazyl on frozen Pt20 cluster. The atomic coordinates for the relaxed calculations for both configurations can be found in Appendix A...... 98

Figure 4.12. Energy barrier related to surface dihedral rotation (Pt4 – Pt5 – N21 – N22 from Figure 4.11) for the side-on configuration of N2H3 on Pt20 with each point representing the degree of rotation with respect to the optimized geometry...... 100 Figure 4.13. Orientation of end-on (top) and side-on (bottom) configurations of adsorbed diazene on the frozen Pt20 cluster. The atomic coordinates for the relaxed calculations for both configurations can be found in Appendix A...... 103

Figure 4.14. Energy barrier related to surface dihedral rotation (Pt4 – Pt5 – N21 – N22 from Figure 4.13) for the end-on configuration of N2H2 on Pt20 with each point representing the degree of rotation with respect to the optimized geometry...... 105

Figure 4.15. Energy barrier related to internal molecular dihedral rotation (H23 – N21 – N22 – H24 from Figure 4.13) for the end-on configuration of N2H4 on Pt20 with each point representing the degree of rotation with respect to the optimized geometry...... 106

Figure 4.16. Orientation of the side-on configuration of adsorbed N2H on frozen Pt20 cluster. The atomic coordinates for the relaxed calculation can be found in Appendix A...... 109 Figure 4.17. Orientation of end-on (top) and side-on (bottom) configurations of adsorbed nitrogen on frozen Pt20 cluster. The atomic coordinates for the initial step for both configurations can be found in Appendix A...... 112 16

Figure 4.18. An illustration of the effect of Cluster – Nitrogen distance (Pt5 – N21) on overall electronic energy of the Pt20 + N2 system. This shows that the minimum energy is achieved at ≥ 3.9 Å suggesting no adsorption of N2 on the surface of platinum...... 113

Figure 5.1. Orientation of adsorbed ammonia on Pt20 Cluster ...... 120

Figure 5.2. Orientation of co-adsorbed hydroxyl and ammonia on Pt20 cluster. The atomic coordinates for this system can be found in Appendix A...... 121

Figure 5.3. Orientation of co-adsorbed water and amidogen radical on Pt20. The atomic coordinates for this system can be found in Appendix A...... 122 Figure 5.4. Orientation of calculated transition state for oxidation of ammonia to amidogen on Pt20 (Equation 1) ...... 123

Figure 5.5. Orientation of co-adsorbed hydroxyl and amidogen radicals on Pt20. The atomic coordinates for this system can be found in Appendix A...... 124

Figure 5.6. Orientation of co-adsorbed hydroxyl and imide radicals on Pt20. The atomic coordinates for this system can be found in Appendix A...... 125 Figure 5.7. Orientation of calculated transition state for oxidation of amidogen to imide on Pt20 (Equation 2) ...... 126 Figure 5.8. Orientation of products for equation after diffusion of imide radical. The atomic coordinates for this system can be found in Appendix A...... 127

Figure 5.9. Orientation of co-adsorbed hydroxyl and imide radicals on Pt20. The atomic coordinates for this system can be found in Appendix A...... 128

Figure 5.10. Orientation of co-adsorbed water and nitrogen atom on Pt20. The atomic coordinates for this system can be found in Appendix A...... 129 Figure 5.11. Orientation of calculated transition state for oxidation of imide to nitrogen on Pt20 (Equation 3) ...... 129

CHAPTER 1: INTRODUCTION

1.1 Project Overview

Fossil fuels are a major source of power generation in the United States as coal is currently the largest source of electricity generation in the United States and gasoline powers most vehicles. However, coal and gasoline combustion produce various pollutants. As a result, alternatives to these fossil fuels have been a focal point of current research. The use of hydrogen as an alternative fuel is desirable as it can be reacted with oxygen to generate energy without pollution. Unfortunately, most of the hydrogen produced worldwide is also fossil-fuel based. Therefore, raw materials for hydrogen production also need to mitigate pollution. Although electrolysis of water has been considered, a new technology to produce hydrogen by the electrochemical oxidation of ammonia is also plausible.

In order for any hydrogen generation process to be feasible and competitive with current methods, the Department of Energy has set a cost-goal of $3/kg of hydrogen.

Preliminary research within the Center has shown that while the aforementioned oxidation processes are viable hydrogen sources for energy generation, improvements need to be made before they can be considered for large-scale use. Specifically, there needs to be:

 A characterization of the substances undergoing the reactions involved in

hydrogen production.

 A determination of the rates at which these reactions occur. 18

The accomplishment of these objectives will provide an understanding of the atomic level interactions of the materials involved in the conversion of ammonia to hydrogen. As these molecules and their reaction rates are studied, the areas for improvement can be noted, thereby increasing the feasibility of the processes. In addition to the importance of these electrochemical conversions for hydrogen production, ammonia is also readily available in animal and human waste as well as plant fertilizers

(which can leach into the soil). As such, the processing of these materials can also help to avoid pollution.

In order to accomplish the objectives mentioned, the catalytic activity will be investigated using commercially available chemistry software with additional insight from experimental studies.

1.2 Statement of Objectives

With current energy sources based mostly on fossil fuels1, whose by-products are major environmental pollutants, the focus of power generation is being shifted to alternative energy sources. These alternatives have included wind and solar energy but also include the conversion of hydrogen to water. The consideration of hydrogen as an energy source has become more popular in recent years; however, issues with storage and costs have not made the process completely feasible. As an economic guide, the

Department of Energy has set a goal for the cost of hydrogen to be $3/kg (equivalent to producing an equal untaxed quantity of gasoline)2. 19

Research efforts within the Center for Electrochemical Engineering Research have shown that the oxidation of ammonia3 can be a considerable source of hydrogen.

Due to the availability of this compound in animal waste and plant fertilizer (which can subsequently run-off into water streams), the oxidation of this will also serve as a waste remediation process.

However, it was noted that the catalyst (platinum) for ammonia oxidation deactivated over time4. Consequently, it has become imperative to:

 Characterize the intermediate molecules formed while ammonia is converted

to hydrogen and

 Determine the kinetics of the reactions involving these intermediates

Specifically, the tasks to be accomplished include

1. Characterization of the catalytic surface (Chapter 3)

2. Characterization of the adsorption of intermediates on the catalytic surface

(Chapter 3 and Chapter 4)

3. Determination of the kinetics of intermediate reaction (Chapter 5)

Pursuing these objectives will help to understand the process of ammonia oxidation to hydrogen through mechanistic studies. In addition, these studies can further aid in the improvement of the catalysts that convert these substances to hydrogen. As catalytic activity improves, the production of hydrogen from ammonia can become more feasible.

To accomplish these objectives, computational chemistry will be employed in describing the reaction mechanisms through the calculation of the structural and 20 vibrational characteristics of the molecules and catalyst; thus providing the thermodynamics and kinetics for the reactions that will occur.

Overall, the premise for this project is to improve a novel approach to hydrogen production for power generation. Simultaneously, this project will provide possible insight into phenomena observed in experimental conditions.

1.3 Significance of Research

This project is devoted to understanding the chemical nature of the interaction of ammonia with catalysts for oxidation to hydrogen. Preliminary studies (vide ante) have shown the importance of these interactions because the catalytic activity deteriorates over time. It is relevant to note that one of the desirable characteristics of a catalyst is its ability to maintain activity for prolonged periods of time5. This activity is governed by the rate at which molecular conversion is achieved as well as the molecules present on the catalytic surface during reactions. Depending on the molecules formed during intermediate reactions, the blockage of active sites can occur which could lead to the reduction of catalytic activity. Consequently, an investigation considering formation of the intermediates and the relative rate of molecular collisions will provide an understanding of the chemistry occurring at the catalytic surface.

The results obtained can provide information useful for improving current catalysts used in the conversion of ammonia for hydrogen production. As an improved catalytic activity is imperative in making these processes feasible for hydrogen production, this project is useful in clean energy generation. 21

From an environmental perspective the reduction of these waste materials is also of added interest. Ammonia is used as a fertilizer or as a raw material for fertilizer production6 and can eventually leach into the soil. Prior to expulsion of this waste to the environment, due to erosion, this substance can be remediated to generate hydrogen. As an example, on a farm where ammonia is present in a comparatively large amount, waste remediation can be used to produce power. This can further help in lowering the cost of food production as farmers can either use the power generated within their farms or sell to other power sinks.

From an economic perspective, a comparison with water electrolysis (an alternative method of hydrogen generation) has shown a 95% in the cost of energy used in the conversion of ammonia to hydrogen3 (based on theoretical free energy required).

Therefore, optimization of the catalytic activity involved in this conversion should approach these savings, thereby making this hydrogen source competitive with other hydrogen generation sources.

1.4 References

1. Wachsman, E.; Williams, M. The Electrochemical Society INTERFACE; The Society: Pennington, NJ :, 2004. 2. NETL, United States Department of Defense, http://www.netl.doe.gov/technologies/hydrogen_clean_fuels/systems_studies.html, 2005. 3. Vitse, F.; Cooper, M.; Botte, G. G. Journal of Power Sources 2005, 142(1-2), 18- 26. 4. Gerischer, H.; Mauerer, A. Journal of Electroanalytical Chemistry 1970, 25(3), 421-433. 5. van Santen, R. A.; Neurock, M. Molecular Heterogeneous Catalysis: a conceptual and computational approach; Wiley-VCH: Weinheim :, 2006. 6. Slack, A. V.; James, G. R. Ammonia; M. Dekker: New York, 1973.

CHAPTER 2: LITERATURE REVIEW

2.1 Background

In the electrochemical conversion of ammonia to hydrogen, the oxidation of ammonia occurs at the anode,

- - 2NH3 (aq) + 6OH (aq) → N2 (g) + 6H2O (l) + 6e (1)

while water is reduced at the cathode:

- - 6H2O (l) + 6e → 3H2 (g) + 6OH (aq) (2)

Overall, these coupled reactions result in

2NH3 (aq) → N2 (g) + 3H2 (g) (3)

The overall cell will consume 0.06V under standard conditions, which is 95% less the theoretical voltage for water electrolysis (1.23V)1. However, the platinum catalyst has been noted to deactivate over time.

2.2 Experimental Observations

Early work by Oswin and Salomon examined the mechanism of ammonia oxidation using platinum black2 and proposed a mechanism similar to decomposition of ammonia in the gas phase. Initially, ammonia is adsorbed on the surface of the catalyst

NH3 (aq) ⇄ NH3 ad (4)

This adsorbed ammonia is then oxidized in three steps by hydroxide ions.

- - NH3 ad + OH → NH2 ad + H2O + e (5)

- - NH2 ad + OH → NH ad + H2O + e (6)

- - NH ad + OH → N ad + H2O + e (7) 23

These reactions result in the formation of adsorbed nitrogen atoms which recombine to form adsorbed nitrogen gas which subsequently desorbs as nitrogen gas

Nad + Nad → N2 ad (8)

N2 ad → N2 (g) (9)

Using Tafel slope calculations, the oxidation of NH2 to NH (equation 6) was suggested to be the rate-limiting step at low currents, while nitrogen recombination

(equation 8) would be rate-limiting at higher currents. In addition, at positive potentials versus a standard hydrogen electode (SHE), Oswin and Salomon pointed out that the formation of nitrogen oxides would occur due to oxidation of the adsorbed nitrogen atoms.

In a subsequent study, Mauerer and Gerischer proposed an alternative mechanism which included the steps proposed by Oswin and Salomon, and also recombination of the

3 NH3, NH2 and NH species .

NHx ad + NHy ad → N2Hz ad (10)

where x and y = 1,2 or 3 and

z = x + y.

These dimers are subsequently oxidized by hydroxide ions to form adsorbed nitrogen gas

- - N2Hz ad + zOH ad → N2 ad + zH2O + ze (11)

This adsorbed nitrogen gas formed is subsequently desorbed as in equation 9.

Based on an ex situ analysis, this study also found that N atoms remain adsorbed on the 24 surface of the anode. Therefore, these atoms were proposed as the reason for deactivation of the platinum catalyst.

Subsequently, de Vooys et al. were able to corroborate some of the findings by the aforementioned authors, while extending the study of ammonia oxidation to other noble metals4. With a combination of reduction of the formed adsorbate at various potentials and an integrated charge analysis, the authors suggested adsorption of NH3 between -0.48 V and -0.4 V, NH2 and NH between -0.4 V and -0.2 V and N above -0.2

V. These potentials are measured with respect to a SHE and used a platinum electrode.

For other metals considered, Ir yielded N2, while Ru, Rh and Pd did not. The authors attributed this to the affinity of the 4d metals mentioned towards Nad, leading to the instability of NH2 and NH intermediates on these surfaces. Binding energies calculated using Density Functional Theory (DFT) were also supplied which agreed with the experimental observations. However, the recombination reactions proposed by Gerischer and Maurer were not mentioned in the deVooys et. al study even though these reactions are considered relevant to the electrochemical oxidation of ammonia.

In single crystal electrode studies, ammonia electro-oxidation on Pt(111), Pt(110) and Pt(100) is shown to be surface sensitive5-7. While Pt(100) exhibited a peak oxidation potential at -0.20 V vs SHE, Pt(110) and Pt(111) showed negligible oxidation at this potential5. Interestingly, this peak oxidation potential shown by Pt(100) coincided with the shoulder of cyclic voltammetry on polycrystalline Pt which peaked around -0.07 V vs SHE. Vidal-Iglesias et al. suggested that ammonia oxidation began on (100) sites and 25 possible rearrangement of other intermediates on the (111) and (100) surfaces caused the polycrystalline peak potential to exceed that of Pt (100)5.

Rosca and Koper extended this study by comparing ammonia oxidation on

Pt(111) and Pt(100)7 using cyclic voltammetry, chronoamperometry and in situ infrared spectroscopy. Charge analysis showed NHad and NH2,ad as the dominant products in ammonia oxidation on Pt(111) and Pt(100). This was used to explain the slow kinetics on

Pt(111), as the NH precursor for N2 formation would be strongly coordinated to this electrode. Therefore, the formation of N2H4 would be the limiting step on Pt(100). These

8 conclusions were further bolstered by theoretical calculations showing that NH2 was more strongly adsorbed on Pt(100) than on Pt(111) while NH and N showed similar adsorption energies on both surfaces.

2.3 Molecular Modeling of Ammonia Oxidation

In one of the initial molecular models by Garcia-Hernandez et al., NH3 preferentially adsorbed in the top position (Figure 2.1) with its hydrogen atoms positioned above the Pt atoms in the lower plane9.

Figure 2.1 Illustration of special positions for adsorption of molecules on a platinum surface (T = top, B = bridge, H = hexagonal close-packed (HCP) hollow and F = face- centered cubic (FCC) hollow)

Novell-Leruth et al. further investigated the energy, geometry and vibrational

8 spectra of NHx (x = 0 - 3) molecules adsorbed on both Pt(100) and Pt(111) . Based on the adsorption energy (Equation 12), NH3 preferred to adsorb in the top site, NH2 preferred to adsorb in the bridge site while NH and N preferred to adsorb in fcc hollow site.

Eadsorbed = Esurface+molecule – Esurface – Emolecule (12)

In addition, the magnitude of the adsorption energies increased with a decreasing number of hydrogens, due to the increasing stabilization by surface binding8. The study also pointed out that the deformation of the N-H bond could be used to determine which adsorbate was present (Table 2.1). This could be used at certain stages of the ammonia electro-oxidation reaction as in situ vibrational spectrum can be measured. 27

-1 Table 2.1. Frequency values (in cm ) for deformation of the N-H bond in different NHx (x = 0 - 3) molecules

Molecule Calculated8 Experimental10 NH3 1063 1088 NH2 1454 1555 NH 819

A study by Offermans, Jansen and van Santen focused solely on Pt(111)11, further extended by Novell-Leruth et al. on Pt(100)12, examined the kinetics of ammonia dehydrogenation by proton abstraction, O oxidation and OH oxidation. The study calculated activation energies and reaction energies for the Oswin and Salomon mechanism (Table 2.2).

Table 2.2. Calculated activation energies (in kJ/mol) for the Oswin and Salomon mechanism (Equations 4 – 9) depicting different rate-limiting steps on two different surface

Equation Pt(111)11 Pt(100)12 5 73 59 6 22 76 7 35 70

The lower activation energy for NH3 on Pt(100), compared with the other NHx

5 fragments, agrees with the hypothesis of the initiation of NH3 oxidation on (100) sites .

Although dimerization of NH2 could occur, this reaction was disqualified on the basis of repulsion between hydrogen atoms of neighboring NH2 molecules in bridge sites. In 28 addition, migration of NH2 molecules to top sites would require 107 kJ/mol and this was unfavorable compared to a competing oxidation reaction.

Novell-Leruth et al. further investigated the dehydrogenation of ammonia on Pd and Rh13. Calculated adsorption energies showed stronger adsorption on the (100) orientation than the (111) for NH3 and NH2. In addition, Rh(100) was the most favorable surface overall. This final observation shows a disagreement with the de Vooys et al.

4 study, which concluded that Ru and Rh provided the strongest affinity for Nads as this increased affinity should reduce the possibility of N2 formation.

2.4 Summary

In characterizing the mechanism of ammonia oxidation on platinum, empirical evidence points to the Gerischer-Mauerer mechanism as the more probable pathway for formation of adsorbed nitrogen gas, with the structure of the electrode playing a role in the kinetics. However, there is still a need to characterize the energy barriers associated with formation of the dimer adsorbates (N2H4, N2H3 and N2H2). These reactions have to be considered due to the N – N bond formed prior to desorption of the nitrogen gas.

While the dimers as free molecules have been well modeled14, fewer models exist for their adsorption on catalysts as shown by recent calculations15,16. Of these three

17 18 dimers, N2H4 has been the most experimentally characterized on surfaces of Ru , Pt , Si

19,20 and Ni21 and the frequency and energy analyses presented could prove useful for comparison to molecular models of N2Hy.

2.5 References

1. Vitse, F.; Cooper, M.; Botte, G. G. Journal of Power Sources 2005, 142(1-2), 18- 26. 2. Oswin, H. G.; Salomon, M. Canadian Journal of Chemistry 1963, 41(7), 1686- 1694. 3. Gerischer, H.; Mauerer, A. Journal of Electroanalytical Chemistry 1970, 25(3), 421-433. 4. de Vooys, A. C. A.; Koper, M. T. M.; van Santen, R. A.; van Veen, J. A. R. Journal of Electroanalytical Chemistry 2001, 506(2), 127-137. 5. Vidal-Iglesias, F. J.; Garcia-Araez, N.; Montiel, V.; Feliu, J. M.; Aldaz, A. Electrochemistry Communications 2003, 5(1), 22-26. 6. Vidal-Iglesias, F. J.; Solla-Gullon, J.; Montiel, V.; Feliu, J. M.; Aldaz, A. Journal of Physical Chemistry B 2005, 109(26), 12914-12919. 7. Rosca, V.; Koper, M. T. M. Physical Chemistry Chemical Physics 2006, 8(21), 2513-2524. 8. Novell-Leruth, G.; Valcarcel, A.; Clotet, A.; Ricart, J. M.; Perez-Ramirez, J. Journal of Physical Chemistry B 2005, 109(38), 18061-18069. 9. Garcia-Hernandez, M.; Lopez, N.; Moreira, I. D.; Paniagua, J. C.; Illas, F. Surface Science 1999, 430(1-3), 18-28. 10. Sun, Y. M.; Sloan, D.; Ihm, H.; White, J. M. Journal of Vacuum Science & Technology A - Vacuum Surfaces and Films 1996, 14(3), 1516-1521. 11. Offermans, W. K.; Jansen, A. P. J.; van Santen, R. A. Surface Science 2006, 600(9), 1714-1734. 12. Novell-Leruth, G.; Ricart, J. M.; Perez-Ramirez, J. Journal of Physical Chemistry C 2008, 112(35), 13554-13562. 13. Novell-Leruth, G.; Valcarcel, A.; Perez-Ramirez, J.; Ricart, J. M. Journal of Physical Chemistry C 2007, 111(2), 860-868. 14. Matus, M. H.; Arduengo, A. J.; Dixon, D. A. Journal of Physical Chemistry A 2006, 110(33), 10116-10121. 15. Agusta, M. K.; David, M.; Nakanishi, H.; Kasai, H. Surface Science 2010, 604(3 - 4), 245 - 251. 16. Daff, T. D.; Costa, D.; Lisiecki, I.; de Leeuw, N. H. Journal of Physical Chemistry C 2009, 113(35), 15714-15722. 17. Rauscher, H.; Kostov, K. L.; Menzel, D. Chemical Physics 1993, 177(2), 473- 496. 18. Alberas, D. J.; Kiss, J.; Liu, Z. M.; White, J. M. Surface Science 1992, 278(1-2), 51-61. 19. Bu, Y.; Lin, M. C. Surface Science 1994, 311(3), 385-394. 20. Bu, Y.; Shinn, D. W.; Lin, M. C. Surface Science 1992, 276(1-3), 184-199. 21. Gland, J. L.; Fisher, G. B.; Mitchell, G. E. Chemical Physics Letters 1985, 119(1), 89-92.

CHAPTER 3: CHARACTERIZATION OF NHX (X = 0 - 3) AND OHY (Y = 1 & 2)

MOLECULES ON PLATINUM CLUSTERS

The contents of this chapter have been submitted to the Chemistry of Materials for publication consideration.

3.1 Abstract

This study examines the adsorption of reactants (NH3 and OH) and intermediates

(NH2, NH, N and H2O) formed during ammonia oxidation by hydroxyl on platinum.

Specifically, four clusters were used to model the catalytic surface in the (111) orientation and structural, electronic and vibrational properties have been calculated using Density Functional Theory (DFT). The molecules reside in the favored positions predicted by prior experimental observation and DFT calculations, while the adsorption energies follow the trend: H2O < NH3 < OH < NH2 < NH < N, with the weakest bonds formed by charge transfer and the strongest bonds by orbital overlap of unpaired electron with the unpaired electron in the d orbital of adjacent Pt atoms. Calculated frequency vibrations in this work show sufficient agreement with experimental observation, challenge previously assigned frequency modes for NH2, NH and H2O and predict the correct shift in frequency vibrations upon adsorption on platinum when compared with prior DFT calculations.

3.2 Introduction

Ammonia is used as a fertilizer or as a raw material for fertilizer production; making it an integral substance in the chemical industry1. When used in this form, 31 ammonia acts as a nitrogen source for plants2. Alternatively, ammonia can be electrochemically oxidized to hydrogen, in the presence of platinum and platinum alloys, for renewable energy generation3. Its presence in wastewater runoff from farm lands

(from fertilizers), livestock facilities, and chemical plants (as a raw material) suggest that this is a cheap hydrogen source and ammonia oxidation could serve as a water remediation process.

Ammonia oxidation occurs in the presence of transition metal catalysts and has been studied in conditions where ammonia is either in the gas phase4-10 or dissolved in solution3,11-17, with theoretical calculations18-23 providing additional valuable insight.

Initial studies performed by Sexton and Mitchell investigated the vibrational modes of the ammonia molecule in monolayer and multilayer coverage on Pt(111)4 and found that the vibrations exhibited by adsorbed ammonia resided between the vibrations of the free molecule and in ammine-coordinated platinum complexes. This suggested a bond of intermediate strength was formed due to adsorption of ammonia on platinum. In extending this work, Fisher concluded that ammonia adsorbed on platinum through the nitrogen atom as a consequence of charge transfer from nitrogen to the catalyst5. This was based on the change in work function observed for the metal and the increased dipole moment of the ammonia molecule from 1.47 D to 2.0 D. Subsequent work by Mieher and

Ho, as well as, Sun et al. found that exposure of an ammonia-covered platinum surface to

7,9 an electron beam converted the ammonia to molecules like NH2, NH and N . This was used to elucidate the vibrations of the intermediates formed in the oxidation of ammonia to hydrogen and nitrogen. Their analyses also proved that ammonia did not decompose at 32 high temperatures, but was converted in the presence of electrons and adsorbed oxygen.

These aforementioned studies were also carried out on Pt(111) which is a close-packed surface of platinum. Although ammonia does not favor a specific crystal orientation for adsorption24, it was found that Pt(100)10 and Pt(554)6 showed higher catalytic conversion of ammonia. This suggests that the oxidation of ammonia is surface sensitive.

With regards to studies of electrochemical conversion of ammonia, Oswin and

Salomon proposed a mechanism for ammonia oxidation (where M is platinum)11:

- - M + NH3 + OH → MNH2 + H2O + e (1)

- - MNH2 + OH → MNH + H2O + e (2)

- - MNH + OH → MN + H2O + e (3)

MN + MN → 2M + N2 (4)

This work was further extended by Gerischer and Maurer to include the

13 recombination of NHx molecules (x = 1 – 3) , which were subsequently oxidized to nitrogen disputing the prior claim by Oswin and Salomon that adsorbed nitrogen atoms were responsible for the formation of the nitrogen molecule. To further bolster this claim, ex-situ analysis performed showed an increased concentration of nitrogen on the deactivated platinum surface. This suggested that the adsorbed nitrogen atoms acted as a hindrance for further ammonia oxidation. DeVooys et al. extended this electrochemical analyses to other transition metals, concluding that nitrogen was formed only in the presence of iridium and platinum, while other metals showed minimal selectivity for nitrogen production14 . Additional surface analyses on platinum performed by Vidal-

Iglesias et al. showed that the current density generated varied with crystal 33 orientation15,16. This work suggested that the observed improved catalytic activity of

6 Pt(554) on gas-phase NH3 was also possible in electrochemical oxidation. This surface sensitivity for ammonia oxidation was confirmed by Rosca and Koper’s work where larger current densities were generated with Pt(100) than with Pt(111) or with Pt(110) during ammonia oxidation17.

In more recent studies, theoretical analyses were performed using Density

Functional Theory (DFT) and Hartree-Fock (HF) calculations to understand the possible phenomena that occurred during ammonia adsorption and oxidation. Initial studies by

Illas et al.19 and Garcia-Hernandez et al.18 on Pt clusters with 10 atoms confirmed the N- down adsorption geometry of NH3 with negligible preference to the orientation of the hydrogens: rotational energy (around normal to platinum surface) was ≈ 0.8 kJ/mol

(B3LYP§) (§ is used for terms found in the glossary) and ≈ 4 kcal/mol (HF). In addition, it was shown that the N atom of NH3 preferred to be bonded at the top of a platinum atom. However, these cluster calculations were not extended to the other intermediates formed in the mechanisms previously proposed. Furthermore, these calculations did not include spin effects which have been shown to play a role in calculating adsorption geometries and energies on Pt clusters25-28. Further theoretical analyses were performed on 4-layer20,23 and 5-layer21 Pt slabs§ modeling Pt(111) and Pt(100) surfaces to examine the structural and vibrational properties of the intermediates suggested by the Oswin and

Salomon mechanism (vide infra). Based on calculated adsorption energies, these studies showed that successive removal of hydrogen atoms from NH3 resulted in stronger adsorption to the catalyst surface. The studies also showed (independently) that on the 34

Pt(111) surface, NH2 prefers to adsorb in the bridge position between 2 platinum atoms, while NH and N preferred to adsorb in the hollow position between 3 (Pt(111)) or 4

(Pt(100)) platinum atoms. Offermans et al. also provided additional calculations on the

21 kinetics of NH3 by decomposition and reaction with O and OH . This study further expanded on the vibrational analyses previously performed in vacuum and pointed out

7 that one of the vibrations of the NH2 molecule predicted by Mieher and Ho was incorrect

(also previously noted by Novell-Leruth et al.20). Although these slab calculations provided extensive insight into ammonia oxidation, it has been noted that cluster studies are often better suited for analyses of the local bonding environment29. In addition, these calculations were performed within the Generalized Gradient Approximation (GGA) and a comparison to a hybrid level of theory such as B3LYP could prove advantageous.

Consequently, this study seeks to characterize the intermediates formed during ammonia oxidation on the platinum surface using various cluster sizes and a popular hybrid functional – B3LYP. Specifically, the electronic, structural and vibrational properties of the NHx molecules on several platinum clusters will be calculated and used to elucidate the localized adsorbate-platinum environment. In addition, the adsorption of hydroxyl (OH) and water (H2O) molecules will be included in this study as the mechanisms previously proposed are related to oxidation in a basic medium. Overall, the values calculated using platinum clusters will be compared to available experimental observations and the aforementioned theoretical calculations and will provide a background for the use of clusters to calculate the proposed recombination reactions suggested by Gerischer and Mauerer. 35

3.3 Computational Details

Unrestricted spin calculations were performed based on Density Functional

Theory as applied in Gaussian 0930. The hybrid B3LYP31 functional was the level of theory with the Vosko, Wilk and Nusair functional III32 used for local correlation.

Different basis sets were used in representing the heavy and light atoms – the pseudopotential-based LANL2DZ33-35 for platinum and the 6-311++g**36,37 basis with diffuse38 and polarization39 functions added for hydrogen, nitrogen and oxygen.

The clusters were arranged similar to the Pt(111) surface with four clusters built to investigate size effects (Figure 3.1). Pt(111) was used as it exhibits the least relaxation of all the platinum surfaces40; therefore the clusters could be held at the fixed Pt – Pt distance of 2.775 Å as a plausible approximation.

Figure 3.1. Clusters of Pt10, Pt15, Pt20 and Pt25 used in modeling the Pt(111) surface orientation. The atom numbers are used as a later reference within the text when adsorption is considered.

Platinum, in the ground state, has an electronic configuration of d9s1, and therefore the clusters are expected to have some unpaired electrons. According to the

Interstitial Electron Model (IEM) proposed by Kua and Goddard and explained in their paper25, the number of tetrahedrons in the cluster provides a basis for approximating the unpaired electrons in each cluster. All bare clusters were represented with the Cs symmetry during energy and spin density calculations. The Pt(111) surface has special 37 positions of top (T), bridge (B), hexagonal close-packed hollow (HCP) and face-centered cubic hollow (FCC) (Figure 3.2) and adsorbed molecules were initially placed at these sites according to their preferred positions20,21. In addition, to differentiate between the different hollow positions (HCP and FCC) a minimum of two layers was needed.

Geometry optimizations were performed within a self-consistent field approximation with a convergence values set at the default (for Gaussian 09) of 10-8 Ha, with only the adsorbed molecules allowed to relax. Subsequent frequency calculations were performed for these optimized structures, also with the default options in Gaussian

09 i.e. analytical first and second derivatives of the energy at the stationary point. As expected, some negative frequencies were obtained as platinum atoms held at fixed 38 coordinates (no relaxation); however, the pertinent frequency values with regards to displacement of only atoms in the adsorbed molecule were reported and are much higher

(NHx and OHy). Images of the molecules and animations of frequencies were visualized using both J-ICE41 and GaussView 542.

3.4 Results and Discussion

3.4.1 Bare Clusters

25 Based on the IEM , there are four interstitial bonding orbitals (IBO) for Pt10: three within each tetrahedron (occupying the HCP sites as shown in Figure 3.1) and one in the face of the bottom tetrahedron. For a cluster of 10 platinum atoms, 100 valence electrons are present (10 × 10), of which there are two 6s electrons per bonding orbital.

Therefore, there are 92 (100 – 2 × 4) electrons left to distribute among the 5d orbitals (as the antibonding 6s orbitals are left unoccupied). This will result in 8 total unpaired 5d electrons for Pt10. Similar analyses were performed for Pt15, Pt20 and Pt25 resulting in predicted spin states of 12, 14 and 12 respectively. Note that for the three layer clusters, the number of IBOs present is halved as orbitals can occupy alternate tetrahedrons as noted by Jacob, Muller and Goddard27. To verify the accuracy of this model, energy calculations were performed at the predicted spin multiplicities and neighboring spin multiplicities as well (Table 3.1).

Table 3.1. Energy calculations on platinum clusters at various spin densities. Energy differences (kJ/mol) are calculated with respect to the ground states predicted by the Interstitial Electron Model. Cohesive energies (kJ/mol) of the clusters approach the cohesive energy of bulk platinum.

Unpaired E – E E Cluster cluster ground cohesive electrons (kJ/mol) (kJ/mol) 6 2.94 -235 Pt10 8 0.00 -235 10 54.21 -229 10 11.88 -254 Pt15 12 0.00 -255 14 42.90 -252 12 16.72 -272 Pt20 14 0.00 -273 16 5.38 -273 12 0.00 -277 a Pt25 14 5.46 -277 16 18.42 -276 a For Pt25, energy calculations for unpaired electrons below 12 did not converge

The calculated energy differences show that the IEM correctly predicts the ground state of each cluster as the aforementioned predicted spins have the minimum energy.

Since the energy values for more than 12 unpaired electrons began to increase, the ground state for Pt25 was chosen as the cluster with 12 unpaired electrons. These values also show that an incorrect spin assignment would yield higher energies, which could lead to errors in subsequent binding energy calculations. Based on prior experience, exclusion of spin also results in even higher predicted energies for the clusters calculated.

The cluster’s cohesive energy (Ecohesive) is based on the energy required to hold the platinum atoms together and is calculated using the following formula:

E E  Cluster  E (5) cohesive n Pt atom

Here, n represents the number of atoms in the cluster and EPt atom is the energy of the platinum atom calculated in the s1d9 state (-119.078 Ha), which is the ground state compared with s0d10 (-119.057 Ha). As can be seen (Table 3.1), the calculated cohesive energies approach the value for the atomization energy for bulk platinum which is 565.7

± 4.2 kJ/mol43.

As previously mentioned, the Cs symmetry is applied in the energy calculations and this is also illustrated by the calculated spin density for the platinum atoms within the clusters (Figure 3.3).

Figure 3.3. Spin densities of platinum atoms within the four clusters. The model shows a good representation for Pt15 and Pt20 with spins close to 1 for all atoms, while Pt10 and Pt25 have some atoms which are poorly represented with little to no spin density.

Overall, each platinum atom is shown to have some electron spin due to the presence of an unpaired electron, however Pt10 and Pt25 deviate from this overall trend.

The atom representing the top position in Pt10 (Pt4) exhibits minimal spin density. This is expected to yield overestimated binding energies when molecules are adsorbed on this atom. In addition, while Pt10, Pt15 and Pt20 yield average spin densities of 0.8, 0.8 and 0.7, respectively, Pt25 yields spin densities of ≈ 0.5. Upon further examination of Pt25, the atoms showing minimal spin density are in the middle of the cluster and thus are surrounded by neighboring platinum atoms as would be found in bulk platinum (i.e. 12 atoms). A possible reason for the low spin calculated is the possible delocalization of electrons to the surface platinum atoms, which are closer to the overall average of 0.5.

This is not the case for Pt20 which, although having three layers, has none of the atoms in the middle layer completely surrounded by the neighboring platinum atoms.

3.4.2 Gas Phase Adsorbate Molecules

In the mechanisms proposed by Oswin and Salomon (1 to 4), the molecules involved in ammonia oxidation are NH3, NH2, NH, N, OH, and H2O. In calculating the energies of the molecules, the use of unrestricted spin DFT plays a role in calculating an accurate energy for these small molecules (Table 3.2).

Table 3.2. Calculated electronic energies (in hartree) of the adsorbed molecules indicating the ground state when multiple spins are possible. The difference between neighboring spins and the ground state is about 0.1 Ha.

Molecule Unpaired electrons Energy

NH3 0 -56.58 NH2 1 -55.90 0 -55.16 NH 2 -55.24 1 -54.50 N 3 -54.60 H2O 0 -76.46 OH 1 -75.76

There is a difference of ≈ 0.1 Ha between the possible states for NH and N, also showing the importance of representing the correct spin state (as previously observed for the platinum clusters). In addition, these spin states play an important role in the bond formation occurring during the adsorption of the molecules on the platinum clusters.

3.4.3 Adsorption Energies and Geometry

For each aforementioned molecule, prior calculations have shown that NH3, H2O, and OH prefer the top position, NH2 prefers the bridge position while NH and N prefer the FCC position18-21. As such, each molecule was initially placed in this position and then full geometry optimizations were performed. In each case, the adsorbed molecule ended in the aforementioned favorable position (Figure 3.4). 44

To obtain an idea of the bond type and bond strength between the adsorbate molecule and the platinum clusters, the Pt – N/Pt – O bond distance were reported (Table 45

3.3), while the adsorption energy (Table 3.4) was calculated according to the following formula:

Eadsorption  Esystem  Ecluster  Emolecule (6)

Cluster NH3 NH2 NH N OH H2O 2.02 1.94 2.15 Pt 2.30 1.94 1.90 2.02 2.54 10 2.06 1.94 1.90 1.95 1.93 2.12 Pt 2.23 1.93 1.91 2.04 2.52 15 2.13 2.05 1.95 1.95 1.93 2.07 Pt 2.26 1.93 1.91 2.02 2.47 20 2.26 2.02 1.96 1.98 1.94 2.10 Pt 2.22 1.98 1.94 25 2.20 2.00 1.97

As observed in the bond distances reported, an increase in the number of unpaired electrons in the adsorbate molecule yields a decrease in the distance between the molecule and the cluster surface. This leads to a stronger bond between the molecule and the platinum cluster. Molecules with no unpaired electrons (NH3 and H2O), are predicted to bond through charge transfer from the lone pairs to the platinum cluster i.e. donor- 46 acceptor bond, while molecules with unpaired electrons will bond through spin pairing between the molecule and platinum atom(s) i.e. covalent bond. Therefore, the bond distances for NH3 and H2O are generally larger than the bond distances for NH2, NH and

N. This argument is further bolstered by the difference in bond distances when both a covalent and donor-acceptor bond is possible for the same molecule as is the case for NH

(1 lone pair and 2 unpaired electrons) and NH2 (1 lone pair and 1 unpaired electron). This difference in bond distance has been previously calculated by Goddard et al.27,44 to be about ≈ 0.2 Å in the case of oxygen bonded to platinum and was also measured in comparing the chemisorption of OH (2.02 ± 0.05 Å) and H2O (2.21± 0.02 Å) on

45 TiO2(110) where adsorption occurs in the top position .

Cluster NH3 NH2 NH N H2O OH Pt10 -56 -150 -274 -354 -17 -178 Pt15 -65 -167 -259 -322 -19 -157 Pt20 -71 -152 -248 -325 -24 -156 Pt -75 -179 -314 -350 25

An examination of the calculated adsorption energies (Table 3.4) confirms this observation. The binding energies increase in the trend NH3 < NH2 < NH < N and H2O <

OH and this trend is independent of the cluster size involved in the adsorption. It has been shown that the calculated cohesive energies increase as the number of atoms within 47 a cluster increases (Table 3.1). Therefore, the binding energies for adsorbed molecules will also increase as the cluster size increases. This is observed to be the case for NH3 and

H2O, where the bonds are weaker and are formed by charge transfer to the platinum surface through lone pair interactions. However, there does not appear to be a trend in the energies for the other molecules bound to different clusters. This difference in the expected trend can be explained based on the bond distances calculated and the calculated spin density of the bare clusters (Table 3 and Figure 3.3).

For Pt10, the binding energies calculated are relatively higher compared to the other cluster sizes. In calculating the spin density of the Pt10 (Figure 3.3), it was observed that the central atom (Pt4) exhibited a very low spin compared to the other atoms. As such, calculations involving molecules with unpaired electrons (NH2, NH, N and OH) resulted in a quenching of the spin on the molecule through charge donation to the Pt4 atom. This resulted in an overestimation in the binding energies calculated. This observation has also been made in prior calculations where the spin density was not included in the cluster. This observation also led to the conclusion that the Pt10 cluster did not accurately represent a platinum cluster for the adsorption of the molecules presented in this study.

For Pt15 the binding energies follow the same trend for all the clusters i.e. NH3 <

NH2 < NH < N and H2O < OH; however the binding energy of NH2 is higher than all other clusters except Pt25. This NH2 molecule possesses a lone pair of electrons and one unpaired electron and has been shown to preferentially adsorb in the bridge position between Pt5 and Pt6 (Figure 3.1). Based on this position, it is expected that the orbital 48 with the lone pair would overlap with one of the platinum atoms, while the orbital with the unpaired electron will overlap with the orbital of the other platinum atom, in keeping with a tetrahedral molecular shape. This difference in the type of orbital overlap expected should yield two different bond distances based on the difference in bond strength (due to orbital overlap). However, the bond distances are 2.12 Å and 2.13 Å to Pt5 and Pt6 respectively (Table 3.3), suggesting a similar bond strength. In addition, both bond distances are shorter than the surface bond distance for NH3 suggesting a stronger bond as well. Further evidence for this difference in bond length is observed in the difference between the bond lengths for the NH molecule on Pt15. The NH molecule has a lone pair and 2 unpaired electrons (Table 3.2) and prefers the FCC hollow position. Keeping with the analogy of the tetrahedral molecule, the lone pair and unpaired electron orbitals will interact with the three closest Pt atoms resulting in two similar bond distances and a third slightly longer bond. This is observed with bond distances of 1.95 Å, 1.93 Å and 2.05 Å to Pt2, Pt3 and Pt6 (Table 3.3). This suggests a difference of ≥ 0.1 Å (i.e. 1 decimal place) between bonds represented by the lone pair charge transfer and bonds represented by spin pairing. On the other hand, the N atom in the FCC hollow position should have three equal bond distances due to three bonds formed by spin pairing of the 3 unpaired electrons of N with the unpaired electron of each adjacent Pt atom. This is reflected in the bond distances – 1.93 Å, 1.91 Å and 1.95 Å – which are essentially equal. In summary, the binding energies and bond distances of adsorbed molecules on Pt15 are well represented except for NH2. 49

A similar analysis for Pt20 shows that the bond distances accurately reflect the bonds formed by adsorption of NH3, NH2, NH and N; especially in the case of adsorbed

NH2 which was not well represented in Pt15 as previously explained. There are two bond distances of 2.07 Å and 2.26 Å exhibiting the aforementioned ≥ 0.1 Å difference between the lone pair orbital overlap and the single electron overlap. The bond distances for NH and N are also represented appropriately with 2 and 3 single electron overlap bonds respectively with the lone pair orbital overlap for NH larger than the other two bonds by

≈ 0.1Å. This suggests that Pt20 is an adequately sized cluster as it is a better representation than Pt10 and Pt15. This was tested by calculating the adsorption energies and adsorption geometry for NHx molecules on Pt25. Bond distances comparable to Pt20 were obtained for the molecules except for NH, where all bond distances calculated were essentially equal – 1.98 Å, 1.98 Å and 2.00 Å. These similar distances result in a binding energy for NH (-314 kJ/mol) close to that calculated for adsorbed N on Pt25 (-350 kJ/mol). In comparison to the other clusters, this is a smaller difference in adsorption energies indicating that the bond strength of Pt25 – NH is close to the Pt25 – N bond strength. This also appears to confirm that the Pt20 cluster performs the best out of the four clusters examined for the system of molecules considered.

A comparison to prior DFT calculations and experimental measurements lends some agreement to the energy values calculated with the Pt20 cluster (Table 3.5).

Method NH NH NH N H O OH 3 2 2 Experimental -106 ± 108 -42.346 -260 ± 2047

Pt20 unrelaxed UB3LYP -71 -152 -248 -325 -24 -156

Pt20 relaxed UB3LYP -119 -215 -360 -414 -66 -203 Pt Cluster18 B3LYP -111 10 Pt Cluster48 LDAa -44 -221 10 Pt Cluster49 B3LYP -30 10 Pt Cluster28 UB3LYP -58 -199 35 Pt Cluster50 LDA -121 91 4 layer slab20 GGAb -72 -238 -409 -463

5 layer slab21 GGA -68 -298 -387 -449 -23 -213 4 - 6 layer slab51 GGA -29

7 layer slab50 LDA -119 aLocal Density Approximation bGeneralized Gradient Approximation

In general, the values calculated for Pt20 underestimated all the experimental and calculated adsorption energies. The largest difference is observed for the adsorption of

OH, where literature values are circa -200 kJ/mol while the value calculated in this work is -156 kJ/mol. Experimentally, the enthalpy of adsorption of OH on Pt(111) has been estimated as ≈ -260 kJ/mol in the gas phase (using deuterium)47, from -235 to -26552 in acidic media and -26853 kJ/mol in an acidic medium. A more recent work by Lew et al. found a range from -263 to -274 kJ/mol under UHV conditions54. Michaelides and Hu explained that DFT calculations of OH adsorption on bare Pt(111) should be comparable to the higher experimental values of -250 kJ/mol calculated under high vacuum conditions. This also explains the higher enthalpy values calculated in the acidic media as the anions present were not expected to adsorb on Pt(111)52,53. The reason for this appears to be the lack of surface relaxation which was used as an approximation. This conclusion is bolstered by a comparison with OH adsorbed on Pt35 spin-unrestricted cluster, where surface relaxation for some Pt atoms was included, and yielded an

28 adsorption energy of -199 kJ/mol . To investigate this, adsorption on Pt20 was recalculated with Pt2, Pt5, Pt6 and Pt9 allowed to relax (Figure 3.1). These atoms were chosen as they are present in the bonding in all the molecules and also have at least 2 neighbors so as to reduce edge effects. Upon reoptimization, Pt5 and Pt6 are raised above the cluster surface while Pt2 and Pt9 shift towards the center of the cluster (Figure 3.5).

Recalculated adsorption energies show better agreement with experimental and other

DFT calculations (Table 3.5), proving that relaxation is indeed considerable for bonding to these small molecules. 52

In the adsorption of NH3 on Pt(111), the value of -106 ± 10 kJ/mol measured by

Szulczewski and Levis8 was described as the upper bound for the adsorption energy. The 53 relaxation of platinum atoms in the Pt20 cluster resulted in closer values to the experiment and other cluster calculations showing the importance of relaxation in the Pt(111) cluster upon adsorption of the molecules considered in this work. On the other hand, the slab calculations resulted in an underestimation of the adsorption energies, while the slab calculation using a Local Density Approximation (LDA) functional50 resulted in a value closer to the clusters. This is actually an overestimation since LDA calculations generally

55 overestimate binding energies . This suggests that the chemisorption of NH3 is not as well represented by the slab calculations in comparison to the cluster calculations, a conclusion that will be further bolstered by vibration comparisons (Table 3.6).

The adsorption energy for H2O on Pt(111) is much lower than the other molecules with earlier measurements by Sexton and Hughes predicting -42.3 kJ/mol46 and a more

56 recent measurement by Lew et al. predicting -51.3 ± 1.6 kJ/mol . The relaxed Pt20 calculation resulted in an adsorption energy of -66 kJ/mol, while prior DFT calculations reported also predict low energy values ranging from -23 to -58 kJ/mol (Table 3.5). This energy value is also reflected in the calculated Pt – O distance as the H2O molecule is the farthest of all the molecules from the cluster surface: 2.5Å for all clusters in this work

(Table 3.3), indicative of a weak bond. As observed in the adsorption of NH3, adsorption energies calculated by platinum clusters are higher than the slab calculations values.

Therefore, it can be concluded that the slab calculations are not as accurate as clusters for the weak bonding of NH3 and H2O on Pt(111).

Although there are no experimental values for the adsorption energies of the smaller nitrogen fragments (NH2, NH and N), frequency peaks observed at temperatures 54 above 285 K after electron bombardment9 and oxidation7,57 suggest that these smaller molecules are more strongly bound to the Pt(111) surface than NH3. As such, there is qualitative agreement between the Pt20 relaxed calculations and experimental observation as the adsorption energies of the aforementioned smaller fragments are greater than those for NH3.

3.4.4 Atomic Spin Density

In further examining the adsorption of these molecules to the Pt20 cluster, the spin densities of the cluster with and without an adsorbate will be compared (Figure 3.6). As previously illustrated (Figure 3.3), the atoms in Pt20 have spin densities averaging 0.7 approximating the presence of an unpaired electron per platinum atom. 55

Figure 3.6. Comparison between spin density of bare cluster (broken line) and cluster with adsorbate (solid line). The non-metallic atoms (N or O where applicable) are represented as atom 0 in each graph. Radicals spin pair with the unpaired electrons of the platinum cluster thus causing the spin on the N or O atom and the bonding Pt atom (Figures 3.1 and 3.4) to be reduced to zero.

The adsorption of NH3 shows no appreciable change in the spin density of the cluster as the spin densities values of the platinum atoms are still > 0.5. It is important to point out that the spin density on Pt5 (which is the position where NH3 is adsorbed) is decreased due to the charge transfer from the NH3. Similar results are observed for H2O with decreased spin on Pt6, which is where H2O adsorbed at the top position (Figure 3.4).

Upon adsorption of NH2, the spin on Pt5 and N is eliminated due to spin pairing.

On the other hand, the spin density on Pt6 is only slightly reduced due to the charge transfer from the lone pair on N. This further lends credence to the aforementioned differences in the bond lengths of Pt5 – N and Pt6 – N, based on the interactions between the different N orbitals. A similar phenomenon is observed in the adsorption of OH with the spin density on Pt6 reduced to almost zero due to spin pairing with O which is also reduced to zero.

Spin densities for adsorbed NH on Pt20 show the spin densities of Pt5 and Pt6 are reduced to almost zero, while spin density on Pt9 is reduced to 0.1. Although this value is lower than expected since the bond to Pt9 should be due to charge transfer not spin pairing, it can be observed that the spin on the atoms in the bottom layer have been increased due to redistribution of the spin on Pt9. For the Pt20 + N system, Pt5, Pt6 and Pt9 are all reduced to zero due to the spin pairing of N with all three atoms with no redistribution to the bottom layer. These spin density observations are therefore concurrent with the bond geometries previously reported (Table 3.3).

3.4.5 Frequency Calculations

Frequency analyses were performed on the geometrically optimized structures to further characterize the interaction of the molecules with the Pt clusters. These calculations provided the best comparison to prior experimental work as frequency measurements are more facile than energy measurements and are readily available for all the molecules in this work. Using common terminology for vibration modes, a comparison of the values calculated for Pt20, previous DFT calculations and experimental measurements show further evidence of the usefulness of cluster calculations to explain and elucidate adsorption on platinum (Tables 3.6 – 3.11)

3.4.5.1 NH3

Comparison between the vibration modes of the free NH3 molecule calculated in this work and the the coupled cluster method58 show no significant difference (average difference of 22 cm-1), providing further credence to the method and basis set used in modeling the molecules.

-1 Table 3.6. Vibration modes and vibration frequencies (in cm ) for free and adsorbed NH3 on Pt(111) from experimental observation and DFT calculations. Values in this study are shown to predict the shift observed in experiment upon adsorption of NH3. Gas Phase Adsorbed on Pt(111) Vibration Experimental Calculated Experimentala Calculated modes aVTZ/ IR59 IR60 This work EELS4 c EELS7 EELS9 Pt GGA20 GGA21 CCSD(T)58 b 20 3600 3470 3529 ν (N – H) 3414 3444 3605 3593 3340 3360 3320 a 3588 3470 3528 νs(N – H) 3337 3337 3479 3464 3240 3150 3320 3470 3331 3376 1651 1556 1555 δ (HNH) 1628 1627 1669 1671 1600 1610 1640 a 1646 1553 1552 δs(HNH) 950 950 1007 1064 1140 1170 1190 1161 1063 1040 632 630 646 ρ(NH ) 690 720 3 626 622 638 ν (Pt – N) 360 323 348 353 aValues depicted are for low-coverage amounts of ammonia bCalculation was based on coupled cluster theory (CCSD(T)) and Dunning’s correlation basis set of triple zeta quality (aVTZ) cElectron energy loss spectroscopy 59

In comparison with the measured values, the calculated values in this investigation overestimate the values for the N – H stretching frequencies for both adsorbed and free ammonia. This is because anharmonic§ frequencies were not calculated and are relevant for stretches involving hydrogen atoms. It should be noted however that the calculated bending modes agree well with the measured modes in both the adsorbed and gas phase measurements, while the slab calculations underestimate these values by about 100cm-1. In addition, the cluster calculations predict the measured shift in frequencies for adsorbed ammonia i.e. red shift for the asymmetric bending mode

(δa(HNH)) and blue shift for the symmetric bending modes (δs(HNH)). This blue shifted frequency for δs(HNH) is also the most intense peak in the calculated spectrum as this mode experiences the highest dipole moment change for adsorbed ammonia. Although the measurement by Sun et al. yields a higher frequency value for δs(HNH), this is a measurement for higher ammonia coverage where the ammonia molecules farther from the surface experience less of a dipole moment change. This increased ammonia coverage also causes a reduction in intensity as observed by Mieher and Ho7. The rocking mode (ρ

(NH3)) is underestimated by all calculations performed while the surface stretch mode

(ν(Pt – N)) is underestimated by the current cluster calculations.

3.4.5.2 NH2

In analyzing the measured and calculated vibrations for the free and adsorbed

NH2 radical (Table 3.7), similar comparisons to phenomena observed for NH3 can be made. 60

-1 Table 3.7. Vibration modes and vibration frequencies (in cm ) for free and adsorbed NH2 on Pt(111) from experimental observation and DFT calculations. Values in this study are shown to predict the shift observed in experiment upon adsorption of NH2.

Gas Phase Adsorbed on Pt(111) Vibration Experimental Calculated Experimental Calculated modes IR62 This 6-311G**/ IR 61 EELS7 EELS9 Pt GGA20 GGA21 FTIR63 Work UB3LYP64 20 ν (N – H) 3301 3440 3413 3605 3470 3534 d νs(N – H) 3220 3219 3349 3325 3310 3250 3499 3360 3414 1610, δ(HNH) 1499 1497 1507 1535 1555 1522 1454 1445 1770 Wagging 1392 799 780 764

Twisting 830a 830 771 757

ρ(NH ) 657 665 652 2 ν(Pt – N) 488 505 468 475

320 346 T (Pt – N) || 181 203 aReassigned from original paper based on comparison with with amido complexes and the calculations in this study 61

Calculated N – H stretching frequencies are much higher than the measured values for the radical for reasons previously mentioned. The measured frequency values also predict a blue shift for the bending mode of NH2 (δ(HNH)) with Mieher and Ho observing two modes at 1613 and 1774 cm-1. To further increase the complexity of comparison, the values measured by Sun et al. predict the presence of a wagging mode at

-1 -1 1392 cm and a rocking mode (ρ(NH2)) at 830 cm . To clarify these modes a

65 comparison to previously measured values for amido complexes: [Hg(NH2)]Cl and

66 [Cr2(NH2)2(H2O)2](SO4)2•2H2O can be made. The assignments for the mercury complex were 1543 cm-1(bending), 1022 cm-1(wagging) and 668 cm-1(rocking) while those for the chromium complex were 1490 cm-1 (bending), 985 cm-1(wagging) and 923 cm-1(twisting). Due to the difference in bonding type for complex ligands and surface adsorbed molecules, the frequencies will not be the same but will reflect the proximity for the modes observed for adsorbed NH2. The complexes predict the following trends for

-1 the modes: δ(HNH) > wagging > twisting > ρ(NH2). Therefore the 1392 cm mode has to be erroneous since it cannot be assigned to the bending mode due to the presence of 1555 cm-1 in the spectra and is much higher (by ≈ 400 cm-1) than the wagging modes observed for these complexes. Offermans et al. also point out that 1392 cm-1 cannot be a wagging

§ mode as libration modes (wagging, twisting and ρ(NH2)) are expected to be much lower

(400 - 1050 cm-1)21. In addition, although Offermans et al. compare their rocking mode

-1 -1 (652 cm ) to the experimental value of 830 cm , the frequency value for Pt20 calculations (830 cm-1) and comparison with amido complexes (1022 cm-1 and 985 cm-1) 62 suggest that this is more appropriate as a twisting mode as also postulated by Novell-

Leruth et al.20.

The values assigned by Mieher and Ho to the bending mode for NH2 (1610 or

1770 cm-1) could be erroneous due to the large difference from the bending mode observed for the amido complexes. However, Nakata and Matsushita have also assigned

-1 67 δ(HNH) to 1610 cm for adsorbed NH2 formed during ammonia synthesis on Fe , suggesting that other interactions (coverage effects or hydrogen bonding) not investigated in this work (and prior DFT calculations) could be possible for causing this large shift (≈

100 cm-1) from the free radical. Albeit, the measurements considered predict a blue shift for δ(HNH) with respect to the free radical as was also observed in the calculations in this study. On the other hand, slab calculations incorrectly predict a redshift. The surface stretch (ν(Pt - N)) is well represented by the models, while lower wavenumbers assigned to modes for NH2 translated parallel to the surface (T||(Pt – NH2)) were calculated but not observed in the measurements.

3.4.5.3 NH

Frequency values for the vibrations of the NH molecule (Table 3.8) were also calculated at the previously optimized geometry in the FCC site (Figure 3.4). Two bending modes (δ(NH)) are calculated at 976 and 952 cm-1 with similar intensities while the stretching frequency (ν(N – H)) is the most intense peak calculated at 3441 cm-1.

These relative intensities are expected since the largest dipole change experienced for adsorbed NH is due to the N – H stretch. 63

Table 3.8. Vibration modes and vibration frequencies (in cm-1) for free and adsorbed NH on Pt(111) from experimental observation and DFT calculations. Values in this study are shown to predict the shift observed in experiment upon adsorption of NH.

Gas Phase Adsorbed on Pt(111) vibration Experimental Calculated Experimental Calculated modes 68 7 9 20 21 IR This work EELS EELS Pt20 GGA GGA ν(N – H) 3133 3254 3307 3281 3441 3396 3441 976 819 802 δ(NH) 1430 952 819 800 ν(Pt – N) 503 536 538

601 490 T (Pt – N) || 579 485

Mieher and Ho assign the 1428 cm-1 to the bending mode of NH suggesting that the DFT calculations severely underestimate this mode (Table 3.8). In comparison with previously observed NH bending modes: 1270 cm-1 (NH adsorbed on Ni(111))69, 1200 –

-1 69 -1 70 1420 cm (imides) and 1414 cm ([Os2(NH)bipy4Cl2](ClO4)2) , this assignment by

Mieher and Ho is plausible but questionable as this mode is observed with other species

(NH3 and NH2) present. This led Novell-Leruth et al. to suggest that the mode at 1428

-1 20 cm is more applicable to the bending mode for NH2 (δ(HNH) in Table 3.7). Based on the cluster calculations and the measurement by Sun et al. the bending mode for NH2 would exhibit a blueshift§ upon adsorption and therefore 1428 cm-1 would be a severe underestimation in comparison. It is more plausible that this mode is from NH radicals in the bridge position, similar to the orientation of the NH radical in imides and the bridged osmium complex vide supra. In addition, considering the low intensity of this mode in comparison with the ν(N – H) mode for adsorbed NH, enhanced intensity measurements 64 using Surface Enhanced Raman Spectroscopy, could be more useful for precise detection for the frequency value of δ(NH) mode.

In comparison with previous GGA calculations, the bending mode is expected at

≈ 800 cm-1 and therefore there is a disagreement in these values. It should be initially noted that prior bending modes for NH3 (δa(HNH)) predicted by GGA, which also involved translation of H parallel to the surface, underestimated measured frequencies by

≈ 80 cm-1(Table 3.6). Finally, the predicted surface stretch for adsorbed NH – 503 cm-1 is slightly lower than the other calculated values.

3.4.5.4 N

In the case of adsorbed N, calculated values are in excellent agreement with experiment for the surface stretch (Table 3.9).

Table 3.9. Vibration modes and vibration frequencies (in cm-1) for adsorbed N on Pt(111) from experimental observation and DFT calculations.

Adsorbed on Pt(111) vibration Experimental Calculated modes 7 9 21 EELS EELS Pt20 GGA ν(Pt – N) 480 488 487 492 623 520 T (Pt – N) || 584 510

3.4.5.5 H2O

Gas phase vibration frequencies for H2O agree well with the calculated values shown and provide an ability to predict the trends for adsorbed water molecules (Table

3.10).

-1 Table 3.10. Vibration modes and vibration frequencies (in cm ) for free and adsorbed H2O on Pt(111) from experimental observation and DFT calculations. Values in this study are shown to predict the shift observed in experiment upon adsorption of H2O. Gas Phase Adsorbed vibration Experimental Calculated Experimental Calculated

modes 6-311G**/ IR71 IR72 This work EELS74 EELS75 Pt GGA21 B3LYP73 20 ν (O – H) 3756 3924 3907 3852 3716 d 3400 3670 ν (O – H) 3657 3819 3657 3737 3613 s δ(HOH) 1595 1602 1595 1625 1613 1609 1536

ρ(OH ) 700a 742 534 574 2 wagging 550a 589 490 450

ν (Pt – O) 250a 121 202 137

T (Pt – O) 112 58 || aReassigned from original paper based on calculations and comparison with EELS from Reference 75 67

The Pt20 calculations predict a negligible change in the bending mode of H2O

(also similar to NH2) and therefore underestimate the change in this mode as measurements show a blue shift for this vibration mode. Conversely, the slab calculations

§ predict a redshift in this bending mode, as was observed for NH3 and NH2. This is not accurate as the bending mode for ice also shows a blue shift (1620 cm-1)74 and since adsorbed water experiences a similar hindrance in motion, a similar shift in vibration is expected. Other libration modes (wagging and ρ(OH2)) calculated on Pt20 are also underestimated in comparison to the experimental values. However it should be pointed out that while the models represented low coverage conditions for water, the values by

Sexton74 are based on high coverage conditions. For comparison to the models, the low coverage conditions presented by Jacobi et al. are more representative of the model calculations; however there are two observed peaks (230 and 290 cm-1)75 not observed in the Pt20 and the previous DFT calculation (Table 3.10). This shows the difficulty in properly assigning and quantifying the vibration modes for water in comparison to the other molecules studied in this work. To attempt to clarify these low frequency modes

(400 – 900 cm-1), comparisons to hydrate salts and aquo complexes have proven useful.

Lattice water experiences weak bonds and as such is in a similar chemical environment to water adsorbed on Pt(111). As summarized by Nakamoto, libration modes for lattice water have been observed between 300 cm-1 and 600 cm-1 for first group, second group and some transition metal halide hydrates59. Conversely, in aqua complexes where water

-1 76 experiences a bond similar to a covalent bond, ρ(OH2) is shifted as high as 887 cm .

This suggests that the modes measured around 700 cm-1 must be caused by stronger 68 bonding due to hydrogen bonding effects from multilayers and/or a change in the adsorption geometry of water. This is pointed out by Jacobi et al., where H2O monomers at low coverage are expected to be concentrated at “Pt(111) step edges”75. In conclusion based on the proposed geometry, the calculated modes 450 – 574 cm-1 are more representative of weakly bonded surface water molecules on Pt. Finally, the surface stretches calculated are on the order measured by Jacobi et al. for monolayer water.

3.4.5.6 OH

The vibration modes exhibited by adsorbed OH radical are the O – H stretch, O –

H bend, Pt – O stretch and Pt – O bend that is parallel to the surface (Table 3.11).

Table 3.11. Vibration modes and vibration frequencies (in cm-1) for free and adsorbed OH on Pt(111) from experimental observation and DFT calculations. Values in this study are shown to predict the shift observed in experiment upon adsorption of OH.

Gas Phase Adsorbed on Pt(111) Vibration Experimental Calculated Experimental Calculated modes This 6-311G**/ IR77 EELS78 EELS79 Pt GGA21 work UB3LYP73 20 ν(O – H) 3452, 3428 3709 3707 3480 3774 3689

δ (OH) 1015 968 994 929

ν(Pt – O) 430 546 537

115 T (Pt – O) 158 || 62

Although the O – H stretching mode (ν(O – H)) is overestimated by calculations on the Pt20 cluster, the blueshift with respect to experimental values is still observed. In 69 addition, the calculated O – H bending mode (δ (OH)) agrees very well with the experimental measurements. The low frequency mode calculated for the Pt – O stretch is overestimated as compared to the experimental value.

3.5 Conclusions

The calculations performed on clusters simulating Pt(111) show the Pt20 cluster to perform with the most plausible geometry and adsorption energies, while showing viable agreement with experimental measurements and previous DFT calculations where available. The difference in adsorption energies calculation with and without relaxation of the cluster showed that the adsorption of the molecules considered will cause a change in the geometry of Pt(111) which experiences minimal relaxation when bare. Calculated spin densities explained the favored positions for the adsorbed molecules as the number of bonds to the cluster increased with the number of unpaired electrons present. Vibration modes calculated for adsorbed molecules on Pt20 showed agreement with experimental values in most cases. Although anharmonic corrections were not made for the N – H and

O – H stretching modes, modes between 400 – 1600 cm-1 (bending and libration modes) showed valid agreement with experimental values and were used to explain deviations that occurred with some prior DFT calculated values for adsorbed NH2, NH and H2O modes.

Although clusters larger than 25 platinum atoms were not included in the scope of this work, the comparison to experimental measurements showed the Pt20 cluster can be used to model the intermediates which are formed during ammonia oxidation on Pt. In 70 the future, this study will be extended to molecules of the N2Hz (where y = 0 – 4) which will be formed during ammonia oxidation to nitrogen as predicted by the Gerischer and

Mauerer mechanism.

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CHAPTER 4: CHARACTERIZATION OF N2HZ (Z = 0 - 4) MOLECULES ON A

PLATINUM CLUSTER OF 20 ATOMS

The contents of this chapter have been submitted to the Chemistry of Materials for publication consideration.

4.1 Abstract

Density Functional Theory Calculations with the hybrid B3LYP functional,

LANL2DZ and 6-311++g** basis sets has been used to describe the adsorption of N2Hz

(z = 0 – 4) molecules on a cluster of 20 platinum atoms. Using binding energy, the trans conformations of N2H4 and N2H2 were predicted to adsorb with one nitrogen in contact with the cluster surface. On the other hand, N2H3 and N2H radicals adsorb with both nitrogen atoms in contact with the catalytic surface, while N2 was not found to adsorb to any appreciable degree. The calculated adsorption configuration for N2H4 disagreed with a separate experimental measurement which predicts that both nitrogen atoms in hydrazine are in contact with the surface. A closer examination of the analyses in this work suggests that this conclusion is not entirely accurate based on the X-ray photoelectron spectroscopy and High resolution electron energy loss spectroscopy obtained on other surfaces. Overall, the N – N bond stretching frequency occurs at 913

-1 -1 § -1 cm and 953 cm for N2H4 and N2H3 respectively and is blueshifted to 1603 cm and

-1 1631 cm for N2H and N2H2. Although the calculated values overestimate the measured values where available, comparison with calculated free molecules show the same redshift§ observed in experiment upon molecular adsorption. This suggests that this bond 75 formation could be indicative of these species during ammonia oxidation as a sudden shift from 900 to 1600 cm-1 is expected as the single bond becomes a double bond.

4.2 Introduction

Electrochemical oxidation of ammonia, in the presence of platinum, has been considered as a plausible source of hydrogen for fuel cells1. While two mechanisms have been proposed for the oxidation of ammonia to nitrogen in basic conditions2,3, the more extensive mechanism suggests the recombination of oxidized ammonia fragments (NH2 and NH) prior to complete removal of hydrogens3 (where M is platinum):

- - MNH3 + OH → MNH2 + H2O + e (1)

- - MNH2 + OH → MNH + H2O + e (2)

MNH2 + MNH2 → M2N2H4 (3)

MNH2 + MNH → M2N2H3 (4)

MNH + MNH → M2N2H2 (5)

- - M2N2H4 + 4OH → M2N2 + 4H2O + 4e (6)

- - M2N2H3 + 3OH → M2N2 + 3H2O + 3e (7)

- - M2N2H2 + 2OH → M2N2 + 2H2O + 2e (8)

M2N2 → 2M + N2 (9)

As such, the characterization of adsorbed N2Hz (z = 0 - 4) molecules on the platinum surface similar to the investigation performed on NHx and OHy molecules would prove beneficial to the study of ammonia oxidation on platinum. 76

4 While the hydrazyl radical (N2H3) has not been observed in isolation , hydrazine

(N2H4) and diazene (N2H2) are found to occur naturally. Prior experimental studies on free hydrazine found it existed in the gauche conformation5,6, while diazene would mostly exist in the trans conformation although evidence for the iso and cis isomers also exist7. Furthermore, characterization of hydrazine adsorbed on surfaces8-12 expected both nitrogen atoms to be in contact with the surface i.e. side-on adsorption with N – N bond parallel to the surface as opposed to end-on adsorption with one nitrogen in contact with the surface and the N – N bond at an angle to the surface normal. In the first study found,

Grunze compared the N(1s) (X-ray photoelectron spectroscopy) (XPS) of hydrazine on

12 13,14 Fe(111) with XPS studies performed on C5H5Mn(CO)2N2 and C5H5Mn(CO)2N2H4 where the nitrogen and hydrazine molecules in these complexes exhibit end-on coordination. The dinitrogen complex yielded a doublet indicating a difference in the binding energies for the two nitrogen atoms further indicating a difference in electronic properties of both atoms. On the other hand, the hydrazine complex yielded a single peak of about the same width as the doublet for dinitrogen. Although the authors did not provide a full width at half maximum§ (FWHM) value for this spectrum14, Grunze subsequently provides a value for the half-width of ≈3.8eV. Based on this value, Grunze concluded that a peak of this width has to be indicative of end-on bonding for hydrazine.

Subsequent studies for hydrazine on Pt(111)8, Si(111)9 and Si(100)10, quoted the value mentioned by Grunze and as such concluded that the XPS studies for hydrazine on these three surfaces suggest a side-on bonding. However, a digitization of this graph (using

GetData Graph Digitizer15) yielded a FWHM value of ≈2.5 eV. This raised some 77 questions with regards to the previous conclusions since the N(1s) XPS spectrum for hydrazine adsorbed on Si(111) at a low coverage provides FWHM of 2.3 eV9 and this value decreased with increasing coverage. Similarly, a coverage of 0.2 L N2H4 on Si(100) yielded a FWHM of 2.4 eV. In addition, while the XPS study of N2H4 on Pt(111) was predicted to yield FWHM of 1.8 eV, the spectra at lower coverages (spectra for ~0.5 ML and ~1 L found in Figure 8 of the article8) are wider than the spectrum at the higher coverage. In other words, adsorbed hydrazine on these three surfaces exhibits N(1s) spectra closer to the value for end-on N2H4 based on the FWHM values found for the reproduced graph (≈2.3 eV) than the value of ≈3.8 eV mentioned by Grunze.

Recently, multiple DFT studies of hydrazine adsorption on metallic surfaces have yielded varying favored conformations of N2H4 on the surface. A study performed on Ni

(100)16 suggested that the end-on adsorption with a trans conformation was the most favored configuration with a binding energy (B.E.) of -80 kJ/mol, although the B.E. for the end-on gauche conformation was only higher by 2 kJ/mol. This Ni(100) essentially suggests that within the end-on configuration, there is also a plausible rearrangement in the molecular geometry. Another study performed on copper surfaces17 by Daff et al. calculated the most favorable configurations on Cu(111), Cu(110) and Cu(100) to be the end-on gauche (B.E. = 41 kJ/mol), side-on gauche (B.E. = -79 kJ/mol) and end-on gauche

(B.E. = -55 kJ/mol) conformations respectively. Finally, an investigation of adsorbed hydrazine and other N2Hz molecules in the Gerischer and Mauerer mechanism was

18 performed by Zhang et al. on Ir(111) predicting side-on adsorbates for N2H4, N2H3, 78

N2H2, N2H while N2 exhibited an end-on adsorption with the N – N axis perpendicular to the Ir surface.

These prior experimental and theoretical studies indicate the complexity in investigating the adsorption of N2H4 on metallic surfaces as the crystal surface, adsorbate position and molecular conformation play relevant roles in the analyses. Considering these molecules are also relevant in understanding the overall kinetics of ammonia oxidation in the presence of hydroxides, the investigation of the adsorption of these molecules is relevant and paramount. Consequently, the purpose of this study is to investigate the adsorption of N2H4, N2H3 and N2H on Pt(111) based on binding energies, dihedral angle rotations and vibration analysis to highlight the expected configurations of these transient intermediates on the catalytic surface upon formation.

4.3 Computational Details

Prior characterization of NHx and OHy molecules on various cluster sizes representative of Pt(111) showed that Pt20 performed best and this was used for all the

N2Hz molecules. In order to characterize the adsorption of N2Hz molecules, conformation of the molecule as well as binding position needs to be investigated. Initial characterization of the energy of the free molecules was performed through angular rotations to predict the most favorable conformation of the individual molecules.

Subsequently, various configurations in relation to the aforementioned special positions on Pt(111) were considered to characterize the most favorable binding position (Table

4.1) 79

Position for N1 Position for N2 Top FCC hollow Bridge Top FCC hollow HCP hollow HCP hollow FCC hollow Bridge

No further changes were made to the methodology previously used for the adsorption of the NHx and OHy molecules studied.

4.4 Results and Discussion

4.4.1 Geometry of free molecules

The lowest energy configurations for N2H4, N2H3, N2H2 and N2H were calculated

(Figure 4.1) with comparison to experimental bond angles and lengths where available

(Table 4.2). 80

Table 4.2. Bond lengths (in Å) and bond angles (in º) of N2Hz (z = 0 – 4) molecules calculated in this study compared with experimental measurements where available.

Bond lengths Bond angles Molecule Source N – H N – N N – N – H H – N – N – H 1.0129 108.23 90.87 This work 1.4309 1.0168 112.86 -29.36 N2H4 106(2) Experimental19 1.015(2) 1.447(2) 91(2) 112(2) 1.0240 105.97 -169.08 N H This work 1.0098 1.3502 114.15 2 3 -24.44 1.0146 121.39 This work 1.0350 1.2383 107.00 180.00 N H 2 2 Experimental7 1.247(1) 1.029(1) 106.3(2)

N H This work 1.0595 1.1714 117.99 2 This work 1.0956 N 2 Experimental20 1.0977 81

Due to the possible orientations that could occur within these molecules, further analyses with respect to angular rotations were performed on each molecule. Beginning with the gauche conformation of N2H4, an angular rotation was performed by changing the dihedral angle (5H – 4N – 1N – 3H) by 10° and calculating the energy of the molecule at each new angle. This resulted in a potential energy surface for the rotation

(Figure 4.2) showing stable points at the cis, trans and gauche conformations which existed as global maximum (Energy difference = 40 kJ/mol), local maximum (Energy difference = 13 kJ/mol) and global minimum points respectively. This confirmed the experimental observation of the gauche conformation as the favored position.

Figure 4.2. Energy barrier for the dihedral angle rotation of the hydrazine molecule. The global minimum is the gauche conformation shown in Figure 4.1.

Similarly, an angular rotation (2H – 1N – 3N – 4H) for N2H3 starting with the optimized structure (Figure 4.1) resulted in a potential energy diagram where the maximum energy difference occurred when 2H bisected the angle between 4H and 5H

(Figure 4.3), while a local maximum occurs when the hydrogen is adjacent with the lone pair on the NH2 portion of the N2H3 molecule.

Figure 4.3. Energy barrier for the dihedral angle rotation of hydrazyl. The global minimum is the conformation shown in Figure 4.1

Angular rotation performed on N2H2 resulted in both trans and cis conformations being minima along the angular scan (Figure 4.4) with the trans conformation favored by

28 kJ/mol.

Figure 4.4. Energy barrier for the dihedral angle rotation of diazene. The global minimum is the trans conformation shown in Figure 4.1.

4.4.2 Energies, Geometries and Frequencies of adsorbed molecules

4.4.2.1 Adsorbed N2H4

Using the guidelines from Table 4.1, the final configurations for adsorbed N2H4 yielded stable side-on and end-on configurations (Figure 4.5), with other initial configurations optimizing to or approaching these configurations. 85

Figure 4.5. Orientation of end-on (top) and side-on (bottom) configurations of adsorbed hydrazine on frozen Pt20 cluster with all frozen coordinates.

The adsorbed end-on configuration exhibits geometry similar to trans-N2H4 while the adsorbed side-on configuration exhibits a geometry between cis-N2H4 and gauche-

N2H4: -75º (adsorbed dihedral), -175.94º (free cis dihedral) and -28.9º (free gauche dihedral). The bond distance between hydrazine and the closest Pt atom is 2.24 Å for end-on bonded N2H4 compared to 2.37 Å and 2.24 Å for the side-on bonded configuration (Table 4.3), showing a stronger adsorption bond for the end-on configuration. 86

Table 4.3. Energy and geometry of hydrazine adsorbed on the Pt20 cluster. The end-on configuration is favored in both the relaxed and frozen platinum cluster calculations.

Frozen Relaxed Frozen Relaxed Platinum Cluster Cluster Cluster Cluster Cluster Configuration side-on end-on Adsorption Energy (kJ/mol) -56 -101 -76 -127 2.37 2.33 Pt - N (Å) 2.24 2.19 2.39 2.33 N - N (Å) 1.44 1.45 1.45 1.45

H25 - N21 - N22 - H26 (º) -74.61 -72.01 68.41 67.68

A comparison of the N – N bond distance between the adsorbed (Table 4.3) and free trans-N2H4 (1.48 Å) indicates that the end-on adsorption strengthens the N – N bond.

This can be explained considering the charge transfer from the molecule to the platinum cluster and lone pair orbital positions. For end-on adsorption, the Milliken charges calculated for the nitrogen atoms are -0.29e and -0.02e indicative of a depletion of electrons from the nitrogen in contact with the surface. Delocalization of the lone pair present on the nitrogen atom farther from the surface will result in a lower energy for the molecule. On the other hand, side-on adsorption yields nitrogen Milliken charges of -

0.18e and -0.13e indicative of similar charge transfer from both nitrogens to the platinum cluster without a delocalization of electrons. As such, the trans-conformation is stabilized upon adsorption relative to the preferred gauche conformation for the free N2H4 molecule

(Figure 4.1). Consequently, the adsorption energy predicted the side-on configuration to be favored by 20 kJ/mol (Table 4.3). 87

The effect of hydrazine adsorption on surface relaxation was examined by unfreezing the coordinates of Pt2, Pt5, Pt6 and Pt9 atoms of Pt20 cluster (Figure 4.6). The binding energies were recalculated as -127 kJ/mol (end-on) and -101 kJ/mol (side-on).

These values are postulated to be more reasonable for experimental comparison as was the case for the smaller molecules. Inasmuch as the adsorption energies increased, the end-on configuration was still favored in comparison with the side-on configuration i.e. the same qualitative trend is observed.

Figure 4.6. Orientation of end-on (top) and side-on (bottom) configurations of adsorbed hydrazine on Pt20 cluster with Pt2, Pt5, Pt6 and Pt9 relaxed. The atomic coordinates for the relaxed calculations for both configurations can be found in Appendix A. 88

To further investigate the most favored configurations for adsorption, azimuthal rotation along the surface normal (Pt4 – Pt5 – N21 – N22) and dihedral angle rotation within the molecule (H24 – N21 – N22 – H26) were performed on the previously optimized geometries for the unrelaxed cluster (Figures 4.7).

Rotation along the surfaces yielded a maximum energy barrier of 2.2 kJ/mol

(Figure 4.7). This suggests that for the adsorption of N2H4, the second nitrogen is not restricted by the special positions on Pt(111). On the other hand, rotation within the molecule (Figure 4.6) yielded a maximum energy barrier of 42 kJ/mol corresponding to the cis conformation. There are also inflection points at ≈7 kJ/mol corresponding to the gauche conformation. Based on this energy difference, one can conclude that rotation to the gauche conformation is facile while rotation to the cis conformation would not be 90 expected to occur. These energy scans showed the trans conformation to be the overall favored configuration as predicted by the initial geometry optimization.

Alberas et al.8 conducted N1s XPS measurements for low coverage hydrazine on

Pt(111) generating spectra with FWHM widths of 1.8eV for low coverage and multilayer

12 coverage N2H4 (~0.5 ML – ~2 ML). Using the paper by Grunze as a reference, this meant the nitrogens were in the same chemical environment and as such, represented a side-on adsorption. However, a redigitized version of the N1s XPS spectrum from the original study by Binder and Sellmann14 (Figure 4.9) yielded an FWHM of 2.5 eV as opposed to an FWHM of 3.8 eV that Grunze provided.

Figure 4.9. Digitized version of Figure 1 from Binder and Sellmann14. This was used to challenge the FWHM of 3.8 eV reported by Grunze12.

In addition, a visual inspection of the spectra provided by Alberas et al. reveals the peak narrows as the coverage of N2H4 increase. As such, a redigitizing of this graph for the lower coverages was also performed (Figure 4.10). A recalculation of the FWHM yielded 2.2 eV for 0.5 ML N2H4 and 2.0 eV for 1 ML N2H4. These recalculated values are also in agreement with the N1s XPS spectra for N2H4 on Si(111) and Si(100) with

FWHMs of 2.3 eV and 2.4 eV respectively.

Figure 4.10. Digitized version of the N1s XPS Spectra of Figure 8 from Alberas et al. study for 0.5 ML coverage8. This was closer to the value of the digitized verion of the Binder and Sellman figure14 (Figure 4.9) and challenges the notion of side-on adsorption.

This suggests that it is more plausible for the end-on configuration of N2H4 on

Pt(111) to exist, especially at very low coverages (~ 0.5 ML). Similarly, this orientation could also occur in the other studies for other surfaces where widths close to 2.4 eV were measured. At this point it should be mentioned that the presence of Si – H stretches in the

HREELS on Si(111) and Si(100) show that some dissociation of N2H4 occurs in the initial adsorption of this molecule on the silicon surfaces. Inspection of the HREEL 93 spectra for low coverage N2H4 on Pt(111) did not reveal Pt – H vibrational modes, which are expected at 548 and 1234 cm-1 21.

Subsequent frequency vibration spectra using HREELS was performed on adsorbed N2H4 on Pt(111) and comparison to the free solid vibrations was performed to further clarify the structure of N2H4 present (Table 4.4).

-1 Table 4.4. Vibration modes and vibration frequencies (in cm ) for free, adsorbed and bridge-ligated N2H4 from experimental observations and calculations in this study. The frequencies from this study are based on the favored end-on configuration adsorbed on relaxed platinum. Calculations show the expected shift in N – N and N – H stretches also observed in experiment upon adsorption of N2H4. Free Adsorbed Metal Complex Vibration Mode This Measured22 This work Pt(111)8 Ni(111)23 Ni(N H ) Cl 24 work 2 4 2 2 3573 3554 3288 ν (N - H) 3310 Unresolved a 3568 3511 3230

3469 3484 ν (N - H) 3200 3156 3146 s 3461 3437

δ (HNH) 1655 1696 1690 1608 s 1558 1580 δa (HNH) 1603 1678 1631 1568 symmetric NH twisting 1350 1322 1487 1342 2 1392 1340 asymmetric NH2 twisting 1304 1296 1147 1313 ν (N - N) 1126 1112 1040 953 1070 978 asymmetric NH wagging 1066 956 1230 1201 2 836 900 symmetric NH2 wagging 884 798 1110 1172 Torsion 627 460

symmetric NH rocking 586 570 649 2 symmetric NH rocking 271 340 613 2 ν (Pt - N) 383 409 ν (Pt - N) 206

Based on the basis set and method chosen, frequencies < 1300 cm-1 underestimate the measured values for the infrared spectrum of solid hydrazine22, while frequencies >

1600 cm-1 overestimate the measured values. The infrared spectrum for solid hydrazine was chosen as a basis for comparison because the studies on surface-adsorbed N2H4 on metals also used vibrations of solid hydrazine as reference values.

Upon adsorption, the largest change in the frequency is experienced by the wagging modes which are increased by ≈ 300 cm-1, with an additional decrease in the N –

N stretching mode. This is due in part to the change in the electron density of N2H4 upon adsorption and the change in the dipole moment of the molecule which now has a polar bond. These effects contribute to both the infrared and raman spectra of hydrazine and the N – N stretching mode is considered a relevant indicator of N2H4 binding mode i.e. monodentate (end-on) or bridging (side-on) modes24-26. A class of transition metal complexes of the form M(N2H4)2X2, where N2H4 is the bridging ligand predict the N – N

-1 stretch and NH2 wagging modes are redshifted (960 – 985 cm ) and blueshifted (1179 –

-1 24 1352 cm ) respectively . On the other hand, N2H4 as a unidentate ligand showed a larger degree of decrease with the N – N stretch around 930 cm-1 26. This suggests that the side- on mode experiences a large shift than an end-on mode. As an example, a study on a cobalt salt containing both the unidentate and bridging hydrazine ligand showed that as the temperature increased, favoring the formation of the bridging ligand, N – N at 928 cm-1 disappeared as the 970 cm-1 became more intense25. While all studies of surface

-1 N2H4 predicted the N – N peak at 1070 – 1113 cm , the Pt(111) spectra predicted this peak to be 1031 cm-1 at low coverage and 1070 cm-1 at 300K. When compared to the 96 trend observed for N2H4 complexes, this suggests that binding on Pt(111) is end-on due to the higher degree of shift experienced, just as monodentate N2H4 ligands experience a higher magnitude of shift in comparison with bridging N2H4. Although Si(111) and

Si(100) surfaces, which are expected to be exhibit a similar adsorption geometry to

Pt(111), also predict frequencies in the higher range (1081 cm-1 and 1113 cm-1 respectively), it is believed that this could be due to the aforementioned N2H2 or N2H3 molecules formed from N2H4 dissociation at low coverage.

11 23 In considering HREEL spectra for N2H4 adsorbed on Ru(001) and Ni(111) , further evidence points to the geometry of the adsorbate geometry on Pt(111) being different from the proposed side-on configuration. Low coverage spectra on Ru(001) predict side-on geometry for N2H4 due to the presence of 4 intense peaks based on the dipole selection rule: dipole moments perpendicular to the surface are enhanced by a corresponding image dipole in the metal27. These peaks represent the ν (Pt - N), ν (N -

-1 N), δs (HNH) and ν (N - H) modes at 395, 1110, 1630 and 3310 cm respectively.

Similarly, spectra on Ni(111), shows 3 intense peaks with the twisting and wagging modes barely visible above background noise. Furthermore, the N – N peak is about ten times as intense as the adjacent wagging peaks on both Ni(111) and Ru(001). On the other hand, low coverage peaks for Pt(111) include the wagging and twisting modes as intense peaks with the wagging mode at ≈ 836 cm-1 showing a similar intensity to the N –

N at 1031 cm-1.

In conclusion, while other authors have predicted a side-on adsorption for

Pt(111), this work argues that the end-on adsorption is a more apt description based on 97 the binding energy, dihedral rotation and comparison with available experimental values.

One point of contention which is yet to be resolved is the shift in frequency upon adsorption. While bridging complexes predict the wagging mode will increase in frequency, the Pt(111) surface (assumed to now be monodentate) predicts a decrease in this frequency. This disagrees with the shift observed based on the values calculated in this present study which predict a large increase relative to the free structure for this mode. The reason for this is not currently understood, but due to the complexity in conformation as well as adsorbate geometry changes, it is plausible that the expected shifts observed in complexes may not follow similar trends. As an example for NH3 and platinum, frequencies shifted with increasing bond strengths of ammonia and platinum.

This may not be the case for hydrazine. To confirm the proposed orientation of N2H4 on

Pt(111), a Raman study would be useful as the nitrogen atoms have a similar charge in the side-on configuration while the charges would be very different for the end-on configuration, thus exhibiting a difference in polarizability.

4.4.2.2 Adsorbed N2H3

For the N2H3 radical, adsorption was considered for both the NH and NH2 side for end on adsorption, however only the configuration with NH in contact with the surface converged to an optimized structure. The converged configuration of N2H3 on the Pt(111) surface yielded two plausible configurations similar to N2H4 (Figure 4.11). 98

Figure 4.11. Orientation of end-on (top) and side-on (bottom) configurations of adsorbed hydrazyl on frozen Pt20 cluster. The atomic coordinates for the relaxed calculations for both configurations can be found in Appendix A.

The favored configuration for this calculation is the side-on configuration, with the end-on configuration possessing a lower adsorption energy by 30 kJ/mol and 43 kJ/mol for the relaxed and unrelaxed platinum clusters respectively (Table 4.5).

Table 4.5. Energy and geometry of hydrazyl adsorbed on the Pt20 cluster. The side-on configuration is favored in both the relaxed and frozen platinum cluster calculations.

Frozen Relaxed Frozen Relaxed Cluster Cluster Cluster Cluster Configuration side-on end-on Adsorption Energy (kJ/mol) -106 -162 -76 -119 2.12 2.08 Pt - N (Å) 2.02 1.98 2.22 2.18 N - N (Å) 1.42 1.42 1.33 1.33 H24 - N21 - N22 - H23 (º) -19.22 -21.40 -28.59 -27.96

This suggests that relaxation is relevant in the adsorption of this molecule, however the inclusion of relaxation does not change the favorability of the adsorbate. In further investigating the binding mode, a surface rotation (Pt4 – Pt5 – N21 – N22) of the side-on configuration was performed around the normal to the surface (Figure 4.12).

100

Based on energy differences, it is apparent that the position of N2H3 is more sensitive to surface rotation than the N2H4 molecule. Significant changes were observed in N2H3 geometry as the dihedral angle was increased as the molecule rotated along the surface normal (Table 4.6). This suggested that this molecule must have both nitrogen atoms in contact with the surface for stability.

101

Table 4.6. Effect of surface dihedral angle rotation on geometry of the adsorbed hydrazyl radical in the side-on configuration on the Pt20 cluster. The angle difference of 0º is the configuration from Figure 4.11

Angle difference (º) 0 30 90 135 Energy difference (kJ/mol) 0 20 29 32 H24 - N21 - N22 - H23 (º) -19.22 41.64 8.88 15.15

Due to the large energy differences calculated from the surface rotation, molecular dihedral rotations were not performed.

Vibrational calculations were performed on the minimum side-on configuration obtained (Figure 4.11) and compared with vibrations of the adsorbed N2H3 radical (Table

4.7)

The N – N stretch registers a redshift of 300 cm-1 upon adsorption representing the most affected mode in comparison with the free radical. This is due to the weakening of the bond upon adsorption. The shift in frequency is more significant than that observed for hydrazine due to the formation of two bonds with the surface: one due to the spin pairing of the unpaired electron on NH and the other through charge donation from the lone pair on NH2. In addition, as in the case of adsorbed N2H4, the NH2 wagging mode is blueshifted for adsorbed N2H3. The torsion mode in the free radical has been split into two rocking modes due to adsorption.

102

Table 4.7. Vibration modes and vibration frequencies (in cm-1) for free and adsorbed hydrazyl. The frequencies are based on the favored side-on configuration adsorbed on relaxed platinum. The adsorbed radical yields a vibration frequency similar to adsorbed hydrazine for the N – N bond stretch.

Free Adsorbed Vibration Mode CCSD(T)/ This work This work aVDZ 3622 3635 3555

νa (N - H) 3472 3485 3460 3377 3417 3446

δs (HNH) 1647 1668 1628

symm. NH2 twisting 1478 1479 1474

asymm. NH2 twisting 1136 1136 1196

NH2 wagging 1101 ν (N - N) 1213 1236 913

ν (N - N) + NH2 rocking 829 Torsion 709 695

NH2 wag 606 513

symm. NH2 rocking 638 ν (Pt - N) 508 ν (Pt - N) 373

T|| (Pt - N) 205

4.4.2.3 Adsorbed N2H2

Two structures were also obtained with a similar end-on and side-on configuration as in the case of hydrazine and hydrazyl (Figure 4.13 and Table 4.8) 103

Figure 4.13. Orientation of end-on (top) and side-on (bottom) configurations of adsorbed diazene on the frozen Pt20 cluster. The atomic coordinates for the relaxed calculations for both configurations can be found in Appendix A.

Table 4.8. Energy and geometry of diazene adsorbed on the Pt20 cluster. The side-on configuration is favored in both the relaxed and frozen platinum cluster calculations.

Frozen Relaxed Frozen Relaxed Platinum Cluster Cluster Cluster Cluster Cluster Configuration side-on end-on Adsorption Energy (kJ/mol) -23 -83 -68 -121 2.16 2.10 Pt – N (Å) 2.16 2.10 2.12 2.07 N – N (Å) 1.29 1.29 1.23 1.23 H – N – N – H (º) -0.43 -0.77 -179.98 -179.98

104

The end-on configuration is favored by 44.6 kJ/mol and 38.4 kJ/mol for the unrelaxed and relaxed Pt cluster respectively and is indicative of the trans conformation of N2H2. The cis- N2H2 has the N – N bond perpendicular to the surface, however the plane of the molecule is tilted with respect to the cluster surface at ≈37º in the unrelaxed calculation. As in the previous calculations, the relaxation of the cluster did not change the favorability of the configuration, but the increase in adsorption energies was significant. This also showed the importance of relaxation especially for a quantitative analysis. In comparison with the smaller single nitrogen molecules, trans- N2H2 has an adsorption energy on the order of ammonia adsorption due to the similarity in charge transfer.

Subsequent dihedral angle rotations were performed with respect to the surface normal (Figure 4.14) and within the molecule (Figure 4.15) to examine the effect of position and geometry on the adsorption energy of the N2H2 molecule.

105

106

Based on the energy differences, the adsorption of N2H2 on the surface relative to the special positions on Pt becomes less facile as the NH group above the surface rotates toward the edge of the cluster. The maximum energy difference (4 kJ/mol) is observed when the second nitrogen atom is above an FCC hollow position and the hydrogen atom is in between two platinum atoms. This suggests there are some edge effects occurring as the hydrogen atoms are possibly stabilized by the platinum atoms. In rotating the molecular dihedral angle, it is evident that the trans conformation is the most favorable 107 for end-on adsorption while the cis conformation requires 41 kJ/mol for stabilization in this position. The maximum energy difference (148 kJ/mol) occurs when the dihedral angle between i.e. when the molecule is between the cis and trans conformation. This is also the conformation with maximum energy when the dihedral rotation of the free molecule was performed (Figure 4.3).

Vibrational modes were subsequently calculated at the favored end-on configuration with the relaxed platinum cluster and compared with the free molecules for shifts (Table 4.9).

Table 4.9. Vibration modes and vibration frequencies (in cm-1) for free and adsorbed diazene from experimental observations and calculations in this study. The frequencies are based on molecules adsorbed on relaxed platinum. The calculated N – N stretch for trans diazene occurs at a much higher frequency than hydrazine and hydrazyl.

Free Adsorbed Vibration Modes 7 Measured trans-N2H2 cis-N2H2 trans-N2H2 cis-N2H2

νa (N - H) 3120 3266 3179 3384 3373

νa (N - H) 3128 3234 3082 3409 3164

δs (NNH) 1583 1592 1342 1260 1551 ν (N - N) 1529 1655 1655 1337 1631

δs (NNH) 1316 1351 1543 1441 1343 Torsion 1289 1345 1270 symmetric 1256 529 NH rocking asymmetric 458 866 NH rocking νa (Pt - N) 405 477 νs (Pt - N) 257 358 260 T (Pt - N) || 230

108

Excluding the ν(N – H) modes, the average difference between the calculated and measured frequencies for cis and trans diazene are -57 and -23 cm-1 respectively, indicative of a general overestimation by the method used. The magnitude of this average difference is mostly due to a difference of 126 cm-1 for the calculated ν(N – N) mode.

Due to its orientation, the N – N peak is absent in the infrared spectrum and can thus be a plausible indicator for the formation of cis-N2H2. In addition, metal complexes with diazene have both the nitrogen atoms in contact with the metals similar to the orientation for the adsorbed cis in this calculation. The N – N stretches for these complexes vary from 1358 cm-1 for Cu28, 1382 cm-1 for Fe(II)29, 1385 cm-1 for W30 and 1415 cm-1 for

30 -1 Cr . On the other hand, based on the calculated value of 1631 cm for trans-N2H2, higher values redshifted with respect to the free trans-N2H2 are expected as a consequence of the weakening of the N – N bond. As such, the value of 1512 cm-1 suggested by Alberas et al. would indicate the formation of N2H2 with an end-on

8 configuration from hydrazine oxidation to N2 as temperature increases . In comparison to the Si(111) surface where peaks between 800 and 1600 cm-1 disappear as temperature increases, it is evident that Pt(111) stabilizes the N – N bond while serving to break N –

H bonds.

4.4.2.4 Adsorbed N2H

There was only one minimum configuration obtained for this adsorbed radical

(Figure 4.16): the side-on configuration with the N – N bond parallel to the surface.

109

Figure 4.16. Orientation of the side-on configuration of adsorbed N2H on frozen Pt20 cluster. The atomic coordinates for the relaxed calculation can be found in Appendix A.

This N – N bond elongates to 1.21 Å upon adsorption (Table 4.10), while the N –

H bond is shortened to 1.03 Å to reflect the electronic rearrangement due to charge transfer to the surface.

Table 4.10. Energy and geometry of N2H adsorbed on the Pt20 cluster. Frozen Relaxed Cluster Cluster Adsorption Energy (kJ/mol) -80 -145 2.08 2.03 Pt - N (Å) 2.13 2.07 N - N (Å) 1.21 1.22 Pt – N – N – H (º) -179.19 -179.97

110

Vibrational analyses showed ν(N – H) is blueshifted while ν(N – N) is redshifted with respected to the modes for the free radical, as a reflection of the aforementioned bond length changes (Table 4.11).

Table 4.11. Vibration modes and vibration frequencies (in cm-1) for free and adsorbed N2H from calculations in this study. The frequencies are based on molecules adsorbed on relaxed platinum. The calculated N – N stretch occurs at around the same value calculated for trans diazene.

Free Adsorbed vibration modes aVDZ/ This work This work CCSD(T)20

νa (N - H) 2867 2787 3376 ν (N - N) 1786 1881 1603 δ (NH) 1099 1108 1249 δ (NH) 715

νa (Pt - N) 538

νs (Pt - N) 395 343 T|| (Pt - N) 247

The lower δ(NH) is based on motion of the hydrogen atom parallel to the surface as was the case for the NH molecule (Chapter 3).

4.4.2.5 Adsorbed N2

Several attempts to optimize nitrogen molecule on the surface failed. Although the energy of the system reduced as the molecule moved away from the surface, 111 convergence at the default for Gaussian 09 was not achieved. This suggests that nitrogen diffuses from the surface of platinum as it is formed. In the Alberas et al. study on

Pt(111), the authors mention that Pt – N modes were absent upon oxidation of N2Hz species thus indicating N2 does not adsorb on the surface at room temperature. A separate study on Pd (same group as platinum) arrived at the same conclusion31.

In an attempt to characterize N2’s interaction with the surface, single point energy calculations were performed (all coordinates frozen) for various distances between the platinum surface and the nitrogen molecule for the bridge and top positions (Figure 4.17).

The N – N bond was kept at the previously optimized value for the free molecule (1.0956

Å).

112

Figure 4.17. Orientation of end-on (top) and side-on (bottom) configurations of adsorbed nitrogen on frozen Pt20 cluster. The atomic coordinates for the initial step for both configurations can be found in Appendix A.

113

The side-on configuration with the N – N bond parallel to the surface appears to be more favorable. In addition, the system appears to have minimum energy when the nitrogen molecule is at 4.61 Å and 4.20 Å for the side-on and end-on configuration respectively. However, however as there is essentially no interaction between these two systems due to the distance (> 4Å), this type of interaction is more of a weak dispersion force. As such, the problems encountered are not unexpected as DFT does not describe these types of interactions effectively without some empirical correction32. Therefore, it 114 is satisfactory to note that the nitrogen would not be expected to stay on the surface of the catalyst upon formation and if there is interaction, the forces are not relevant enough to affect the system.

4.5 Conclusions

Using Density Functional Theory and prior experimental measurements for comparison, the adsorption of N2Hz molecules in Gerischer and Mauerer’s mechanism have been characterized on Pt20. Based on adsorption energies, N2H4 and N2H2 adsorb in the end-on configuration and in the trans conformation of the molecule, while N2H3 and

N2H adsorb in the side-on configuration with the N – N bond parallel to the surface. A comparison of the adsorption energies calculated suggests that none of these molecules would cause to surface blockage as the maximum adsorption energy calculated is -162 kJ/mol for N2H3 which is much lower than the values for NH2, NH, N and OH. In addition, N2 does not adsorb on the Pt(111) surface. Although the work of Alberas et al. suggested hydrazine existed in the side-on configuration, a digitization of their XPS studies predicts that at low coverages, the preferred configuration is end-on. Based on dihedral rotation, N2H3 appears to be the molecule most sensitive to surface rotation, while hydrazine and diazene are less hindered with respect to the position of the second nitrogen atom not in contact with the surface.

Overall, as these molecules are being considered as intermediates in the oxidation of ammonia to nitrogen, an indicator of their formation and presence is the N – N bond.

-1 -1 Adsorbed N2H4 and N2H3 exhibit calculated frequencies of 913 cm and 953 cm while 115

-1 -1 adsorbed N2H2 and N2H exhibit frequencies at 1603 cm and 1631 cm . This showed agreement with the HREELS of Alberas et al.’s, which exhibited peaks at 1512 cm-1 as temperature increased above 200 K. Therefore as these dimers are oxidized by hydroxide, a spectroscopic analysis using in situ techniques could provide information on the potential at which the N – N bond is formed, thus showing providing an idea of the kinetics of the formation of the nitrogen molecule.

Finally, the characterization of these dimers could help in future calculations related to the formation of the N – N bond as these molecules are the resulting products of NHx combinations.

4.6 References

13. Binder, H.; Sellmann, D. Angewandte Chemie 1973, 85(24), 1120-1121. 14. Binder, H.; Sellmann, D. Angewandte Chemie-International Edition in English 1973, 12(12), 1017-1019. 15. Fedorov, S.; http://getdata-graph-digitizer.com/: Moscow, Russia, 2008 16. Agusta, M. K.; David, M.; Nakanishi, H.; Kasai, H. Surface Science 2010, 604(3- 4), 245-251. 17. Daff, T. D.; Costa, D.; Lisiecki, I.; de Leeuw, N. H. Journal of Physical Chemistry C 2009, 113(35), 15714-15722. 18. Zhang, P.-X.; Wang, Y.-G.; Huang, Y.-Q.; Zhang, T.; Wu, G.-S.; Li, J. Catalysis Today 2011, 165(1), 80-88. 19. Kohata, K.; Fukuyama, T.; Kuchitsu, K. Journal of Physical Chemistry 1982, 86(5), 602-606. 20. Matus, M. H.; Arduengo, A. J.; Dixon, D. A. Journal of Physical Chemistry A 2006, 110(33), 10116-10121. 21. Davenport, J. W.; Estrup, P. J. in The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A.; Woodruff, D. P., Eds.; Elsevier Scientific Pub. Co. ; Distributors for the U.S. and Canada, Elsevier North-Holland: Amsterdam ; New York : New York :, 1981, p 1 - 33. 22. Durig, J. R.; Fush, S. F.; Mercer, E. E. Journal of Chemical Physics 1966, 44(11), 4238-4247. 23. Gland, J. L.; Fisher, G. B.; Mitchell, G. E. Chemical Physics Letters 1985, 119(1), 89-92. 24. Sacconi, L.; Sabatini, A. Journal of Inorganic and Nuclear Chemistry 1963, 25, 1389-1393. 25. Nicholls, D.; Rowley, M.; Swindell, R. Journal of the Chemical Society A - Inorganic Physical Theoretical 1966(7), 950-952. 26. Bottomley, F. Quarterly Reviews, Chemical Society 1970, 24(4), 617-638. 27. Willis, R. F.; Lucas, A. A.; Mahan, G. D. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A.; Woodruff, D. P., Eds.; Elsevier Scientific Pub. Co. ; Distributors for the U.S. and Canada, Elsevier North-Holland: Amsterdam ; New York : New York :, 1981. 28. Fujisawa, K.; Lehnert, N.; Ishikawa, Y.; Okamoto, K. Angewandte Chemie International Edition 2004, 43(37), 4944-4947. 29. Lehnert, N.; Wiesler, B. E.; Tuczek, F.; Hennige, A.; Sellmann, D. Journal of the American Chemical Society 1997, 119(38), 8869-8878. 30. Sellmann, D.; Brandl, A.; Endell, R. Angewandte Chemie International Edition in English 1973, 12(12), 1019-1020. 31. Raval, R.; Harrison, M. A.; King, D. A. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A.; Woodruff, D. P., Eds.; Elsevier Scientific Pub. Co. ; Distributors for the U.S. and Canada, Elsevier North-Holland: Amsterdam ; New York : New York :, 1981, p 39 - 124. 32. Grimme, S. Journal of Computational Chemistry 2006, 27(15), 1787-1799. 117

CHAPTER 5: PRELIMINARY INVESTIGATION OF THE OSWIN AND SALOMON

MECHANISM FOR AMMONIA OXIDATION

5.1 Abstract

In an attempt to investigate the rate at which intermediate reactions occur, the transition state theory has been used to calculate the kinetics of ammonia deprotonation.

The kinetics predicts that as NH3 is oxidized to the nitrogen atom, the speed of the reaction begins to decrease. Specifically, the rate constants for deprotonation of ammonia, amidogen and imide were calculated as 1.77 × 1011 s-1, 4.61 × 1010 s-1 and 1.16

× 104 s-1 respectively. Thus the removal of hydrogen from the strongly surface-bound imide radical is predicted to be the rate-limiting reaction. In addition, thermodynamics predict that the ammonia and amidogen deprotonation were possibly entropy-driven as the optimized products predicted these reactions to be endothermic.

5.2 Introduction

As highlighted in the initial chapter, the catalytic activity of the catalyst is observed to decrease over time due to some surface adsorption. To evaluate the process by which this could occur, it is relevant to examine the kinetics of the intermediate formation on the cluster surface. With the previous studies on the relative position of all molecules involved in the mechanisms for ammonia oxidation, rate constants and thermodynamic properties of the reactants and products of the simpler Oswin and

Salomon mechanism can be investigated1 (Equations 1 - 3).

- - M + NH3 + OH → MNH2 + H2O + e (1) 118

- - MNH2 + OH → MNH + H2O + e (2)

- - MNH + OH → MN + H2O + e (3)

In their studies, Oswin and Salomon predicted the oxidation of NH2 to NH was the rate-limiting step at low currents, while nitrogen recombination would be rate- limiting at higher currents. However, considering the changes in the intermediates upon adsorption e.g. N – H decreasing due to charge transfer to the surface, it is beneficial to explore these reactions with theoretical electronic structure methods. It appears plausible that the subsequent removal of protons to reduce adsorbed NH3 to adsorbed N would be more and more difficult and thus predict some other rate-limiting step.

5.3 Computational Details

The reaction coordinate chosen is the removal of hydrogen by the hydroxyl radical. Thus the initial state is the adsorption of the reactants on the platinum cluster.

Subsequently, the products are formed by transferring the hydrogen to the hydroxyl radical to form water and this is optimized to yield the final state.

Subsequently, a transition state search is performed. In Gaussian 092, this is done using the Synchronous Transit-Guided Quasi-Newton (STQN) method3. In this method, optimized structures for the initial and final states of a reaction are used as the input and the saddle point between these two states is calculated. Geometry optimization and vibrational frequencies of the transition state is subsequently calculated and thermochemistry data is yielded. To further characterize this state, an animation of the 119 imaginary frequency calculated for this transition state (using GaussView4) should show the reaction coordinate being studied.

The rate constant can then be calculated using the following equation from transition state theory5:

Gact k BT k  e RT (4) h

where,

k = rate constant

kB = Boltzmann’s constant

h = Planck’s constant

ΔGact = Gtransition state - Greactant (kJ/mol)

R = Universal gas constant (kJ/mol/K)

T = Temperature (K)

No further changes were made to the methodology previously used for the adsorption of the NHx and OHy molecules studied. It should be noted that no relaxation of the cluster was performed in these calculations as was done for the NHx, OHy and

N2Hz molecules previously studied.

120

5.4 Results and Discussion

5.4.1 Adsorption of ammonia

Prior to oxidation, the ammonia is expected to adsorb in the top position (Figure

5.1) and the adsorption is predicted to be exothermic and spontaneous (Table 5.1).

Table 5.1. Thermodynamics of ammonia adsorption on Pt20 predict an exothermic and spontaneous reaction.

Property Value ΔH (kJ/mol) -62.72 ΔG (kJ/mol) -25.42

Figure 5.1. Orientation of adsorbed ammonia on Pt20 Cluster

121

5.4.2 Deprotonation of ammonia

Co-adsorption of the hydroxyl radical (Figure 5.2) is predicted to be highly exothermic and even more spontaneous than the adsorption of NH3 (Table 5.2). This is due to the high binding energy for adsorption of OH which is only less favored to adsorb than N and NH (Chapter 3).

Figure 5.2. Orientation of co-adsorbed hydroxyl and ammonia on Pt20 cluster. The atomic coordinates for this system can be found in Appendix A.

Table 5.2. Thermodynamics of ammonia and hydroxyl co-adsorption on Pt20 predict an exothermic and spontaneous reaction.

Property Value ΔH (kJ/mol) -247.94 ΔG (kJ/mol) -152.55

122

The Pt – O and Pt – N distances are 2.05 Å and 2.16 Å respectively close to the values calculated for single molecule adsorption (2.02 Å and 2.26 Å respectively), but also showing that the bond strength between NH3 and the surface has increased.

To generate the product of this reaction (Equation 1), 23H is moved to OH to form H2O, while ammonia becomes the amidogen radical. The resulting product predicts both molecules adsorbed in the top position with some hydrogen bonding stabilizing the amidogen radical in this position (Figure 5.3).

Figure 5.3. Orientation of co-adsorbed water and amidogen radical on Pt20. The atomic coordinates for this system can be found in Appendix A.

Subsequently, using the reactant (Figure 5.2) and product (Figure 5.3) as the initial and final states for equation 1, the transition state is calculated as the hydrogen translated between the amidogen and hydroxide radicals (Figure 5.4). 123

Figure 5.4. Orientation of calculated transition state for oxidation of ammonia to amidogen on Pt20 (Equation 1)

The activation energy for this reaction is kJ/mol and the reaction is predicted to occur at the typical rate of a monomolecular reaction i.e.1012 – 1013s-1 5. However, the reaction is also predicted to be non-spontaneous and endothermic (Table 5.3).

Property Value ΔH (kJ/mol) 12.61 ΔG (kJ/mol) 5.48 k (s-1) 1.77 × 1011

124

This suggests the final state is not currently optimized and some diffusion of the molecules would need to occur prior to reaching the true final state. Considering the reaction will now become entropy driven i.e. TΔS > ΔH, this appears to be a valid reasoning.

5.4.3 Deprotonation of amidogen

Subsequently, NH2 adsorbed at the top position is optimized with an adjacent hydroxyl radical as the reactants for this step (Figure 5.5).

Figure 5.5. Orientation of co-adsorbed hydroxyl and amidogen radicals on Pt20. The atomic coordinates for this system can be found in Appendix A.

The product of this reaction has water in the top position and the imide radical in the top position as well (Figure 5.6). 125

Figure 5.6. Orientation of co-adsorbed hydroxyl and imide radicals on Pt20. The atomic coordinates for this system can be found in Appendix A.

Using this reactant (Figure 5.5) and product (Figure 5.6) as the initial and final states respectively, the transition state shows the proton (22H) translated between the imide and hydroxide radicals (Figure 5.7).

126

Figure 5.7. Orientation of calculated transition state for oxidation of amidogen to imide on Pt20 (Equation 2)

The reaction is predicted to occur at a slightly slower rate than the deprotonation of ammonia by an order of magnitude; however it is also predicted to be non-spontaneous and endothermic in this initial state (Table 5.4).

Table 5.4. Thermodynamics and kinetics for deprotonation of amidogen by the hydroxyl radical (Equation 2). ΔHtop and ΔGtop use Figure 5.6 as the product, while ΔHfcc and ΔGfcc use figure 5.8 as the product. This shows the effect of diffusion on favoring a spontaneous reaction

Property Value

ΔHtop (kJ/mol) 13.21

ΔGtop (kJ/mol) 10.54

ΔHfcc (kJ/mol) -47.78

ΔGfcc (kJ/mol) -53.68 k (s-1) 4.60 × 1010

127

The calculated also suggests that final state is not thermodynamically favorable for the products. To investigate the possibility of diffusion, the imide radical was then optimized in the face-centered cubic (FCC) position (where imide adsorption is favored) with water in the top position (Figure 5.8).

Figure 5.8. Orientation of products for equation after diffusion of imide radical. The atomic coordinates for this system can be found in Appendix A.

With the products thus modeled, the reaction is predicted to be spontaneous and exothermic (Table 5.4). This suggests that upon oxidation of the amidogen radical, the imide radical diffuses to the FCC position. The kinetics of this diffusion was not investigated in the scope of this study, but provides a possible explanation for the positive

Gibbs free energy change observed in the deprotonation of ammonia.

128

5.4.4 Deprotonation of imide

The reactants for this step are imide adsorbed at the FCC position with an adjacent hydroxyl radical at the top position (Figure 5.9).

Figure 5.9. Orientation of co-adsorbed hydroxyl and imide radicals on Pt20. The atomic coordinates for this system can be found in Appendix A.

The product of this reaction calculates water in the top position and the nitrogen atom in the top position as well (Figure 5.10). 129

Figure 5.10. Orientation of co-adsorbed water and nitrogen atom on Pt20. The atomic coordinates for this system can be found in Appendix A.

With these initial and final states, the transition state shows the hydrogen translated between the nitrogen and hydroxide radical (Figure 5.11).

Figure 5.11. Orientation of calculated transition state for oxidation of imide to nitrogen on Pt20 (Equation 3) 130

The reaction is predicted to occur at a much slower rate than the deprotonation of ammonia and amidogen, however, it is also the most thermodynamically favorable (Table

5.5).

Table 5.5. Thermodynamics and kinetics of imide deprotonation on Pt20 predicts an exothermic and spontaneous reaction (Equation 3).

ΔH (kJ/mol) -1574.53 ΔG (kJ/mol) -1576.34 k (s-1) 1.16 × 104

The calculated thermodynamic values suggest that there is no need for molecular diffusion as the reaction is spontaneous. This is different from the models for the deprotonation of ammonia and amidogen, which require diffusion of the molecules prior to being thermodynamically favorable.

5.5 Conclusion

This exploratory study into the kinetics of oxidation of ammonia to the nitrogen atom by co-adsorbed hydroxyl suggests entropy plays a critical role in the progress of the first two steps of the reaction. This is due to the molecules being displaced from the favorable adsorbed positions i.e. amidogen resided in the top position as opposed to bridge, while imide resided in the top position as opposed to FCC hollow. 131

While the thermodynamics predict the deprotonation of imide as the most product-driven reaction, the kinetics predict this step to be the slowest. It is suggested that the effect of atom relaxation be included in these calculations as their effect has been known to affect the energy of the system.

5.6 References

1. Oswin, H. G.; Salomon, M. Canadian Journal of Chemistry 1963, 41(7), 1686- 1694. 2. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.; Gaussian, Inc.: Wallingford CT, 2009. 3. Frisch, A.; Frisch, Æ.; Frisch, M. J.; Clemente, F. R.; Trucks, G. W. Gaussian 09 User's Reference; Gaussian, Incorporated, 2009. 4. Dennington, R.; Keith, T.; Millam, J. In GaussView 5; Semichem Inc.: Shawnee Mission KS, 2009. 5. Santen, R. A. v. Chemical kinetics and catalysis / R.A. van Santen and J.W. Niemantsverdriet; Plenum Press: New York :, 1995. 132

CHAPTER 6: CONCLUSIONS AND RECOMMENDATIONS

6.1 Conclusions

The use of platinum clusters has helped to characterize the interaction of the intermediates formed during ammonia oxidation with the catalyst. In modeling the catalytic surface, the inclusion of electron spin, multiple layers and relaxation of the top layer have shown better agreement between calculated energies and frequencies and experimental measurements. As ammonia is oxidized to nitrogen atoms by hydroxyl radicals, the strength of the bond between the intermediate and the catalytic layer is increased.

Other intermediates formed in the oxidation of ammonia to nitrogen contain nitrogen – nitrogen bonds. These cluster models are the first to completely calculate the strength of the interactions of these molecules with platinum theoretically. It was observed that the radicals would adsorb on the catalytic surface with the N – N bond parallel to the surface, while the molecules (without unpaired electrons) would adsorb with only one of the nitrogen atoms in contact with the surface. In addition, the interaction of the nitrogen molecule is an extremely weak and

As a result of the complete characterization of the intermediates and products of ammonia oxidation, the strength of adsorption of these molecules were expected to follow the trend: N2 < H2O < NH3 < N2H2 < N2H4 < N2H < N2H3 < OH < NH2 < NH < N.

This suggests it is plausible for continuous deprotonation of ammonia, prior to recombination of the intermediates, to lead to surface blockage. This is due to the ability of molecules with unpaired electrons to bind strongly to the platinum surface. 133

Finally, the preliminary study of the Oswin and Salomon mechanism suggests that while the removal of hydrogen from the adsorbed amide radical is the most thermodynamically favorable, it is also the slowest of the deprotonation reactions and is proposed as the rate-limiting step in the mechanism. However, it should be noted that the initial deprotonation of ammonia was predicted to have Gibbs free energy of reaction greater than zero. This suggests this reaction will not proceed under the conditions modeled. It is also plausible that the product calculated from this reaction is actually an intermediate step where the water or amidogen radical have to diffuse to another position on the platinum surface to model the positions of the final products.

6.2 Recommendations

The electrochemical oxidation of ammonia occurs in a basic media, therefore the oxidizing agent should be the hydroxide ion. Although the hydroxyl radical was used in this work as an initial step, the use of the charged hydroxide ion will provide a more realistic environment as the excess charge lost during ammonia deprotonation will be donated to the surface. It is plausible that the excess charge donated to the surface weakens the interactions of intermediates like NH2 and NH on the surface as back- donation of charge may become dominant.

In addition, although the interactions studied are based on the electronic interactions of the molecules with the platinum surface, the interactions between surface molecules may also be important. This is especially true with the molecules in this study as hydrogen bonding is plausible. Since DFT does not characterize these interactions 134 properly, it could be beneficial to attempt a different theoretical method, especially when increased surface coverage is considered.

Finally, as the conditions to be modeled are electrochemical, it is relevant to include solvent effects.

135

GLOSSARY OF TERMS

B3LYP: A hybrid functional developed by Axel Becke as detailed in “Becke, A. D.

Journal of Chemical Physics 1993, 98(7), 5648-5652”.

Slab Calculations: These are systems where the solid state material is represented as a periodic slab with a finite amount of layers as opposed to an aperiodic cluster as was done in this study.

Anharmonic: Frequencies associated with stretching of the hydrogen atom and a much heavier atom. These frequencies deviate from the harmonic approximation that is used and require correction for accuracy.

Libration modes: These are frequency vibrations restricted by adsorption on the surface usually wagging, rocking, twisting and rocking modes on the molecule.

Blueshift: A shift in frequency values to higher values i.e. towards the blue end of the visible spectrum.

Redshift: A shift in frequency values to lower values i.e. towards the red end of the visible spectrum.

Full width at half maximum: This is the width of a peak at a position which is half of the maximum intensity.

136

APPENDIX A – X, Y AND Z COORDINATES, ENERGIES AND SPIN

MULTIPLICITIES FOR CALCULATED SYSTEMS

All energy values provided are in units of Hartree. Multiplicity – 1 yields the total number of unpaired electrons in the system. For instance Pt10 + NH3 has 8 unpaired electrons.

System: Pt10 + NH3 Energy: -1248.27364990 Multiplicity: 9

Pt 2.29222000 1.56404000 -0.64076000 Pt -0.20841600 2.76711900 -0.64127600 Pt 2.50064500 -1.20309100 -0.64022100 Pt -0.00002200 -0.00003400 -0.64077100 Pt -2.50063300 1.20303700 -0.64125700 Pt 0.20843900 -2.76717500 -0.64020000 Pt -2.29220900 -1.56410100 -0.64071900 Pt 0.90279100 -1.32298600 1.62538100 Pt 0.69436400 1.44411400 1.62484300 Pt -1.59786700 -0.11993500 1.62486000 H 0.41735000 0.85836200 -3.27495000 N 0.00479700 -0.00728500 -2.94152500 H 0.55245600 -0.79806200 -3.26627800 H -0.94960400 -0.08631500 -3.27881000

System: Pt15 + NH3 Energy: -1844.22760441 Multiplicity: 13

Pt 2.77940500 2.49351900 -0.61788400 Pt 0.00447800 2.49114900 -0.59789100 Pt -2.77045000 2.48877800 -0.57789700 Pt 4.16800100 0.09486900 -0.75533400 Pt 1.39307300 0.09249800 -0.73534100 Pt -1.38185400 0.09012700 -0.71534700 Pt -4.15677900 0.08775400 -0.69535500 Pt 2.78166900 -2.30615700 -0.87279100 Pt 0.00674200 -2.30852800 -0.85279700 Pt -2.76818500 -2.31089800 -0.83280500 Pt -2.75329100 0.76873300 1.59966400 Pt 2.79656400 0.77347500 1.55967700 137

Pt 0.02163700 0.77110300 1.57967100 Pt 1.41023100 -1.62754900 1.44222100 Pt -1.36469500 -1.62992000 1.46221500 H -1.93613000 0.96487000 -3.25563500 N -1.30351500 0.23530400 -2.94182700 H -0.35458900 0.46411900 -3.22320500 H -1.57526200 -0.65452000 -3.34884000

System: Relaxed Pt20 + NH3 Energy: -2440.25810616 Multiplicity: 15

Pt -2.73836000 -1.54558500 2.51632600 Pt 0.08017400 -1.43120800 2.42664300 Pt 2.81002600 -1.40806700 2.49952700 Pt -4.12852100 -1.74378600 0.12284100 Pt -1.36302000 -1.93768500 0.19047400 Pt 1.44971300 -1.94142300 0.10334400 Pt 4.19381900 -1.53749800 0.09773700 Pt -2.74456800 -1.87313900 -2.27885300 Pt -0.02766600 -1.79128800 -2.21166400 Pt 2.80351600 -1.73567000 -2.29566200 Pt -2.79682600 0.60496800 0.76391800 Pt -0.02239200 0.67362900 0.75384300 Pt 2.75105100 0.74248100 0.74738800 Pt -1.41335800 0.47559100 -1.63766600 Pt 1.36156600 0.54436300 -1.64594000 Pt -1.46523400 2.95385300 1.40462900 Pt 1.30890100 3.02255000 1.39638500 Pt -0.08137100 2.82440800 -0.99776200 Pt 2.69274600 2.89316700 -1.00573600 Pt -2.85544500 2.75562800 -0.98901400 H 0.48653900 -4.46272800 0.41399900 N 1.44695400 -4.15228500 0.29603800 H 1.84178300 -4.58244200 -0.53438600 H 1.99249100 -4.42149200 1.10880300

System: Pt25 + NH3 Energy: -3036.18977471 Multiplicity: 13

Pt 2.11823100 3.67173500 -0.11331000 Pt 3.02744200 1.04988900 -0.11348000 Pt 3.93667000 -1.57190600 -0.11351600 138

Pt -0.60699400 4.19524700 -0.11331900 Pt 0.30225300 1.57343100 -0.11350200 Pt 1.21148600 -1.04842900 -0.11360600 Pt 2.12068300 -3.67026500 -0.11362700 Pt -2.42293400 2.09691800 -0.11349700 Pt -1.51373000 -0.52491100 -0.11361400 Pt -0.60448800 -3.14674500 -0.11370300 Pt -4.23890200 -0.00138800 -0.11354100 Pt -3.32966400 -2.62325800 -0.11364400 Pt 0.60453700 3.14697400 -2.37916000 Pt 1.51381300 0.52501800 -2.37933100 Pt 2.42303300 -2.09679800 -2.37937300 Pt -1.21147600 1.04860300 -2.37934000 Pt -0.30222500 -1.57330800 -2.37944400 Pt -3.02746200 -1.04981800 -2.37938200 Pt 2.72512700 -0.52363200 2.15217900 Pt 1.81587700 2.09827900 2.15228200 Pt 0.90925300 -2.62185800 2.15206700 Pt -0.00005200 -0.00002300 2.15215200 Pt -0.90917900 2.62175900 2.15227300 Pt -1.81601500 -2.09833800 2.15205800 Pt -2.72519400 0.52337000 2.15216200 N -0.00061400 -0.00415200 4.36967400 H 0.24145600 0.92240000 4.70916100 H 0.68088600 -0.67774100 4.70771300 H -0.92505900 -0.25825600 4.70621100

System: Pt10 + NH2 Energy: -1247.62703049 Multiplicity: 7

Pt 1.44258600 -2.37038900 -0.68922800 Pt 2.77357300 0.06433900 -0.72284600 Pt -1.33076700 -2.43472500 -0.61909700 Pt 0.00029000 0.00002100 -0.65286500 Pt 1.33118800 2.43474200 -0.68633900 Pt -2.77322200 -0.06434600 -0.58248900 Pt -1.44217000 2.37039200 -0.61616100 Pt -1.31059200 -0.83310700 1.64690200 Pt 1.46265000 -0.76868800 1.57683700 Pt 0.02035800 1.60165800 1.61329400 N -1.49881300 0.00083300 -2.19628300 H -1.53394600 -0.83737200 -2.76597900 H -1.53801700 0.83965700 -2.76475500 139

System: Pt15 + NH2 Energy: -1843.58359404 Multiplicity: 12

Pt 2.77497700 2.49072800 -0.61027600 Pt -0.00002400 2.49073900 -0.61028200 Pt -2.77502400 2.49074600 -0.61028600 Pt 4.16246900 0.09061800 -0.73255500 Pt 1.38747200 0.09062700 -0.73256500 Pt -1.38753500 0.09063700 -0.73257100 Pt -4.16253000 0.09064400 -0.73257400 Pt 2.77496100 -2.30948500 -0.85484100 Pt -0.00003900 -2.30947800 -0.85484700 Pt -2.77503900 -2.30946600 -0.85485400 Pt -2.77503100 0.77538400 1.57103300 Pt 2.77496400 0.77536700 1.57104400 Pt -0.00003300 0.77537700 1.57104000 Pt 1.38745800 -1.62473500 1.44876000 Pt -1.38754100 -1.62472700 1.44875500 H 0.00995200 1.00836400 -2.86413900 N 0.00361700 0.13949700 -2.34223700 H 0.00330500 -0.65704400 -2.96899200

System: Relaxed Pt20 + NH2 Energy: -2439.61407368 Multiplicity: 14

Pt -2.77738700 -1.47809500 2.49890000 Pt 0.02040200 -1.30329700 2.51928600 Pt 2.77198600 -1.50709600 2.49619000 Pt -4.16548400 -1.63309700 0.10184000 Pt -1.46072900 -1.95559600 0.17414500 Pt 1.35288600 -1.99807500 0.18765400 Pt 4.15565100 -1.67443600 0.09923300 Pt -2.78022200 -1.80315500 -2.29438500 Pt 0.00747400 -1.75434600 -2.24183100 Pt 2.76827300 -1.83046800 -2.29466700 Pt -2.76677300 0.67310700 0.74859900 Pt 0.00862900 0.65882200 0.74317200 Pt 2.78109000 0.64575300 0.74402200 Pt -1.38128900 0.50475200 -1.64936400 Pt 1.39448300 0.49075000 -1.64906200 Pt -1.36803200 2.98164700 1.39467100 140

Pt 1.40683700 2.96766200 1.39404900 Pt 0.01747700 2.81226900 -1.00427600 Pt 2.79271000 2.79857200 -1.00444400 Pt -2.75765500 2.82626400 -1.00278700 N -0.15940400 -3.53109700 0.32536200 H -0.23809600 -4.18339700 -0.44732700 H -0.23155900 -4.00995900 1.21603300

System: Pt25 + NH2 Energy: -3035.54660454 Multiplicity: 11

Pt -2.87304300 -3.12408000 0.06830400 Pt -0.09941700 -3.20300800 0.10737700 Pt 2.67416600 -3.28195500 0.14631000 Pt -4.19151100 -0.68231700 0.04974700 Pt -1.41785500 -0.76125100 0.08886300 Pt 1.35573100 -0.84023500 0.12783200 Pt 4.12931300 -0.91909100 0.16678300 Pt -2.73634700 1.68044500 0.07025500 Pt 0.03725700 1.60152200 0.10927100 Pt 2.81086500 1.52258100 0.14827200 Pt -1.28122000 4.04323000 0.09062000 Pt 1.49243900 3.96430200 0.12962900 Pt -2.85941000 -1.52257900 2.33451800 Pt -0.08566300 -1.60153600 2.37359300 Pt 2.68793200 -1.68047000 2.41253300 Pt -1.40422000 0.84027100 2.35503400 Pt 1.36946900 0.76133200 2.39403600 Pt 0.05100800 3.20311600 2.37539800 Pt 1.34206200 -2.44168400 -2.13832200 Pt -1.43163800 -2.36275700 -2.17733100 Pt 2.79710500 -0.07898700 -2.11785900 Pt 0.02348900 -0.00006800 -2.15695700 Pt -2.75003300 0.07885200 -2.19589200 Pt 1.47864800 2.36278200 -2.13643000 Pt -1.29483200 2.44171200 -2.17545200 N 1.51188500 0.00031600 -3.77103900 H 1.54576200 -0.83434300 -4.34910200 H 1.57607400 0.82224600 -4.36388100

System: Pt10 + NH Energy: -1247.01646189 Multiplicity: 7 141

Pt -2.41398800 1.37383700 -0.70959100 Pt -2.39815300 -1.40087400 -0.71006700 Pt -0.01916500 2.77517700 -0.66000000 Pt -0.00310000 0.00106700 -0.66021100 Pt 0.01239600 -2.77462500 -0.66068100 Pt 2.39109400 1.40024900 -0.61043000 Pt 2.40714600 -1.37356900 -0.61034300 Pt 0.74264800 1.39164800 1.62190000 Pt -1.65191200 -0.01035800 1.57204500 Pt 0.75855900 -1.38348700 1.62150300 N 1.69111400 0.00915900 -1.76585400 H 1.77130100 0.00894700 -2.78067100

System: Pt15 + NH Energy: -1842.96086966 Multiplicity: 11

Pt 2.76781100 2.50321000 -0.59394000 Pt -0.00712200 2.48713800 -0.58326200 Pt -2.78205500 2.47106600 -0.57258400 Pt 4.16857600 0.11300500 -0.75330500 Pt 1.39364200 0.09693200 -0.74262700 Pt -1.38129100 0.08086000 -0.73194900 Pt -4.15622300 0.06478700 -0.72127300 Pt 2.79440600 -2.29327600 -0.90199300 Pt 0.01947400 -2.30934700 -0.89131500 Pt -2.75545900 -2.32541900 -0.88063700 Pt -2.76364800 0.72707200 1.58583400 Pt 2.78621700 0.75921600 1.56447900 Pt 0.01128500 0.74314300 1.57515700 Pt 1.41204800 -1.64706300 1.41579100 Pt -1.36288400 -1.66313500 1.42647000 N -1.40794100 1.84729300 -1.77458500 H -1.43714600 2.03022800 -2.77588800

System: Relaxed Pt20 + NH Energy: -2439.00927560 Multiplicity: 13

Pt -2.77534200 -1.33647100 2.61193000 Pt -0.00109800 -1.38074700 2.37019900 Pt 2.77470100 -1.33180500 2.61362600 Pt -4.16131200 -1.61517200 0.22612600 142

Pt -1.54099200 -2.20140900 0.22791100 Pt 1.55049800 -2.19828400 0.22883900 Pt 4.16246800 -1.60850500 0.22835700 Pt -2.77288100 -1.89203500 -2.16026700 Pt 0.00747400 -1.87924900 -2.44565600 Pt 2.77579200 -1.88745700 -2.15873200 Pt -2.77561800 0.72941900 0.76038700 Pt -0.00169200 0.73234000 0.75840600 Pt 2.77351600 0.73368100 0.76087300 Pt -1.38786100 0.45261100 -1.62618900 Pt 1.38657500 0.45493800 -1.62514100 Pt -1.39090900 3.07354300 1.29373200 Pt 1.38394200 3.07579800 1.29461900 Pt -0.00258500 2.79664700 -1.09332600 Pt 2.77264300 2.79877900 -1.09224100 Pt -2.77797400 2.79429000 -1.09381200 N 0.00634000 -2.90579400 -0.75317100 H 0.00686700 -3.91066400 -0.93993200

System: Pt25 + NH Energy: -3034.94041461 Multiplicity: 11

Pt 3.19657300 -2.77463200 0.14282000 Pt 3.19628700 0.00041300 0.14285500 Pt 3.19588500 2.77541800 0.14277200 Pt 0.79368700 -4.16243200 0.11425800 Pt 0.79339100 -1.38741400 0.11432500 Pt 0.79305000 1.38760700 0.11430100 Pt 0.79266600 4.16262500 0.11418300 Pt -1.60951600 -2.77521500 0.08572800 Pt -1.60986800 -0.00020200 0.08571500 Pt -1.61019700 2.77481900 0.08567800 Pt -4.01275300 -1.38800700 0.05704100 Pt -4.01309100 1.38702100 0.05701600 Pt 1.56764600 -2.77486600 2.38942900 Pt 1.56733000 0.00028000 2.38948400 Pt 1.56697500 2.77527300 2.38938600 Pt -0.83564600 -1.38763200 2.36089500 Pt -0.83598700 1.38746600 2.36086900 Pt -3.23893300 -0.00033800 2.33221200 Pt 2.42193300 1.38764600 -2.13225800 Pt 2.42229200 -1.38710600 -2.13225000 Pt 0.01887900 2.77493500 -2.16084200 143

Pt 0.01925800 -0.00007400 -2.16071600 Pt 0.01956100 -2.77497400 -2.16079400 Pt -2.38407500 1.38713300 -2.18941400 Pt -2.38373500 -1.38775900 -2.18938800 N 1.64482300 0.00013200 -3.31759400 H 1.62067600 0.00011600 -4.33459500

System: Pt10 + N Energy: -1246.40642463 Multiplicity: 6

Pt -2.40566200 1.38999500 -0.70198000 Pt -2.40853600 -1.38456900 -0.70201500 Pt -0.00119800 2.77524800 -0.66416300 Pt -0.00298400 0.00112600 -0.66377500 Pt -0.00718200 -2.77467100 -0.66526000 Pt 2.39969000 1.38372300 -0.62732800 Pt 2.39648900 -1.38915000 -0.62726200 Pt 0.76301400 1.38653800 1.61391200 Pt -1.64125800 0.00068400 1.57615200 Pt 0.76005400 -1.38879000 1.61328900 N 1.64440100 -0.00148300 -1.68891100

System: Pt15 + N Energy: -1842.34456909 Multiplicity: 10

Pt 2.76957300 2.50020100 -0.60451800 Pt -0.00538300 2.48689800 -0.59614000 Pt -2.78033800 2.47359400 -0.58776200 Pt 4.16811700 0.10799400 -0.75301100 Pt 1.39316100 0.09469100 -0.74463300 Pt -1.38179500 0.08138700 -0.73625500 Pt -4.15675100 0.06808600 -0.72787700 Pt 2.79170200 -2.29751700 -0.89312500 Pt 0.01674600 -2.31082000 -0.88474600 Pt -2.75821000 -2.32412400 -0.87636900 Pt -2.76548100 0.73833800 1.57771500 Pt 2.78443100 0.76494400 1.56095800 Pt 0.00947500 0.75164200 1.56933700 Pt 1.40801600 -1.64056500 1.42084400 Pt -1.36693900 -1.65386900 1.42922200 N -1.40761500 1.77304300 -1.71207000

144

System: Relaxed Pt20 + N Energy: -2438.38980052 Multiplicity: 12

Pt -2.77548600 -1.42165200 2.53962000 Pt -0.00326700 -1.39202100 2.39630300 Pt 2.77388100 -1.42119100 2.54149100 Pt -4.16144100 -1.62009700 0.14585200 Pt -1.49688100 -2.25606800 0.22532300 Pt 1.50070900 -2.25038700 0.22354700 Pt 4.16130600 -1.61904100 0.14813700 Pt -2.77395100 -1.81868600 -2.24753700 Pt 0.00386300 -1.78878800 -2.31908000 Pt 2.77503100 -1.81820100 -2.24613500 Pt -2.77596000 0.70285300 0.75557400 Pt -0.00098900 0.70342000 0.75424300 Pt 2.77539900 0.70338000 0.75677800 Pt -1.38828800 0.50559700 -1.63473900 Pt 1.38848000 0.50627600 -1.63371000 Pt -1.38828200 3.02839300 1.36972600 Pt 1.38647400 3.02882300 1.37014700 Pt -0.00036600 2.82979000 -1.02675400 Pt 2.77461000 2.83012200 -1.02477900 Pt -2.77509800 2.82956500 -1.02638100 N 0.00284400 -2.92041500 -0.75352800

System: Pt25 + N Energy: -3034.31359726 Multiplicity: 10

Pt 3.19779600 -2.77443700 0.13400200 Pt 3.19732100 0.00059600 0.13403900 Pt 3.19679000 2.77559000 0.13395200 Pt 0.79492000 -4.16238600 0.11021300 Pt 0.79446100 -1.38737000 0.11027900 Pt 0.79396000 1.38765500 0.11025400 Pt 0.79342200 4.16267100 0.11013600 Pt -1.60841900 -2.77530500 0.08645900 Pt -1.60892800 -0.00029400 0.08644700 Pt -1.60941800 2.77472600 0.08640800 Pt -4.01178600 -1.38823700 0.06255100 Pt -4.01228400 1.38679200 0.06252600 Pt 1.57332000 -2.77477500 2.38384000 Pt 1.57283800 0.00037200 2.38389300 145

Pt 1.57233000 2.77536500 2.38379600 Pt -0.83010700 -1.38767900 2.36008200 Pt -0.83060700 1.38742100 2.36005600 Pt -3.23352400 -0.00052300 2.33617900 Pt 2.41835600 1.38780900 -2.13952900 Pt 2.41887500 -1.38699400 -2.13952100 Pt 0.01518700 2.77494000 -2.16334600 Pt 0.01572600 -0.00007400 -2.16322900 Pt 0.01618700 -2.77497900 -2.16329600 Pt -2.38773900 1.38699500 -2.18714100 Pt -2.38723900 -1.38789700 -2.18711500 N 1.65539200 0.00017500 -3.25301200

System: Pt10 + H2O Energy: -1268.13604494 Multiplicity: 9

Pt 0.79280300 2.66031800 -0.62375000 Pt -1.90640800 2.01643500 -0.63946500 Pt 2.70006100 0.64467800 -0.62307800 Pt 0.00084100 0.00076000 -0.63886000 Pt -2.69833800 -0.64312100 -0.65451000 Pt 1.90813500 -2.01485900 -0.63812900 Pt -0.79107600 -2.65876200 -0.65384400 Pt 1.52543200 -0.46600200 1.63239500 Pt -0.38177500 1.54959700 1.63172700 Pt -1.17373600 -1.10991800 1.61667700 H -0.16092400 0.98875200 -3.26963900 O 0.11916900 0.06863800 -3.17691600 H 1.08429900 0.09012200 -3.22976500

System: Pt15 + H2O Energy: -1864.08679427 Multiplicity: 13

Pt 2.77798100 2.49361100 -0.61066600 Pt 0.00303100 2.49132100 -0.59390700 Pt -2.77192700 2.48904100 -0.57712700 Pt 4.16664400 0.09504200 -0.74874600 Pt 1.39169400 0.09272900 -0.73196800 Pt -1.38331400 0.09049800 -0.71520300 Pt -4.15820500 0.08819700 -0.69843100 Pt 2.78036500 -2.30582700 -0.87002600 Pt 0.00543000 -2.30809200 -0.85329000 146

Pt -2.76952600 -2.31038500 -0.83649600 Pt -2.75735500 0.76697400 1.59885200 Pt 2.79255400 0.77154600 1.56529600 Pt 0.01760500 0.76918000 1.58210600 Pt 1.40627900 -1.62934000 1.44399600 Pt -1.36861300 -1.63158200 1.46080000 H -0.59796200 1.07709200 -3.23760100 O -1.15041200 0.28342700 -3.22037700 H -0.54484700 -0.45174400 -3.38424300

System: Relaxed Pt20 + H2O Energy: -2460.11478880 Multiplicity: 15

Pt -2.80157600 -1.42934100 2.49131800 Pt -0.07560100 -1.43801800 2.42258500 Pt 2.74428800 -1.53845600 2.50881200 Pt -4.18486900 -1.56099900 0.09218100 Pt -1.44128600 -1.88898700 0.09469500 Pt 1.36774800 -1.94669800 0.18681400 Pt 4.13422900 -1.72273000 0.11788700 Pt -2.79357400 -1.74627200 -2.29610100 Pt 0.03599800 -1.79358200 -2.21314300 Pt 2.75241100 -1.85357400 -2.27769400 Pt -2.75492800 0.72503900 0.74950600 Pt 0.01768500 0.66735500 0.74731300 Pt 2.79227000 0.61579600 0.76575000 Pt -1.36440300 0.53682700 -1.64249400 Pt 1.41205300 0.48400200 -1.63162100 Pt -1.32552800 3.01170300 1.40149500 Pt 1.44821100 2.95755700 1.41048900 Pt 0.06607600 2.82481600 -0.99571000 Pt 2.84135400 2.77077600 -0.98321500 Pt -2.71001000 2.87930300 -1.00038300 H -0.35519900 -4.39191700 0.32880200 H -1.61540000 -4.46196100 1.21380500 O -1.31902400 -4.29981400 0.30944600

System: Pt10 + OH Energy: -1267.50096633 Multiplicity: 8

Pt -2.61609100 0.92585400 -0.64836200 Pt -2.10996900 -1.80259200 -0.64695800 147

Pt -0.50626800 2.72838100 -0.65000100 Pt -0.00011100 -0.00007800 -0.64862900 Pt 0.50600200 -2.72850000 -0.64719400 Pt 2.10969900 1.80248400 -0.65023600 Pt 2.61583100 -0.92596900 -0.64883300 Pt 0.53505600 1.51156400 1.61615600 Pt -1.57474000 -0.29094600 1.61779500 Pt 1.04121300 -1.21688400 1.61755600 O 0.09670200 -0.02674200 -2.66492300 H -0.82227600 -0.04458600 -2.96163600

System: Pt15 + OH Energy: -1863.44328992 Multiplicity: 12

Pt 2.77931200 2.49213200 -0.62034300 Pt 0.00434500 2.49090500 -0.60639000 Pt -2.77062900 2.48968800 -0.59241600 Pt 4.16722400 0.09272000 -0.75113100 Pt 1.39225800 0.09147000 -0.73715900 Pt -1.38276600 0.09030200 -0.72320000 Pt -4.15767300 0.08906500 -0.70923400 Pt 2.78017800 -2.30792900 -0.86792400 Pt 0.00522700 -2.30913100 -0.85399400 Pt -2.76974500 -2.31036000 -0.84000600 Pt -2.75889800 0.77296100 1.58779500 Pt 2.79104300 0.77540600 1.55985100 Pt 0.01607800 0.77410400 1.57385500 Pt 1.40400200 -1.62525900 1.44303800 Pt -1.37090600 -1.62643800 1.45703600 H -0.90664600 0.97975300 -2.98865500 O -1.14492200 0.07607800 -2.74424000

System: Relaxed Pt20 + OH Energy: -2459.47101393 Multiplicity: 14

Pt -2.74540600 -1.52424200 2.52321200 Pt 0.07501000 -1.42928000 2.41954000 Pt 2.80118500 -1.41926100 2.50732600 Pt -4.13620900 -1.72394200 0.13120800 Pt -1.37380400 -1.93320900 0.19595700 Pt 1.43169700 -1.92756200 0.09778800 Pt 4.18485700 -1.56675700 0.10777700 148

Pt -2.75270700 -1.87163000 -2.26755300 Pt -0.03789200 -1.81472700 -2.20636900 Pt 2.79464600 -1.76716800 -2.28300000 Pt -2.79141900 0.61925500 0.76257200 Pt -0.01729700 0.67115300 0.75355200 Pt 2.75595600 0.72414500 0.74711600 Pt -1.40841500 0.47119900 -1.63722700 Pt 1.36629900 0.52357900 -1.64491900 Pt -1.44668500 2.96301800 1.39329900 Pt 1.32751700 3.01545300 1.38556700 Pt -0.06308500 2.81497000 -1.00772000 Pt 2.71150100 2.86757600 -1.01490000 Pt -2.83776800 2.76263200 -0.99931700 H 0.58755500 -4.21371700 0.29941500 O 1.50625600 -3.91151000 0.31446800

System: Relaxed Pt20 + N2H4 (side-on configuration) Energy: -2495.58047294 Multiplicity: 15

Pt -2.77597700 -1.47813400 2.49397300 Pt -0.00972100 -1.39381300 2.44650000 Pt 2.77437400 -1.47817100 2.49581200 Pt -4.16267200 -1.62690700 0.09516200 Pt -1.42060100 -1.90240300 0.12339100 Pt 1.41961100 -1.89589800 0.16466800 Pt 4.16259500 -1.62698300 0.09786000 Pt -2.77449200 -1.77590100 -2.30258400 Pt 0.00770300 -1.72793000 -2.23940400 Pt 2.77583500 -1.77588900 -2.30072400 Pt -2.77546500 0.68376000 0.75501600 Pt -0.00013500 0.68384400 0.75454500 Pt 2.77540100 0.68367700 0.75641000 Pt -1.38721700 0.53516300 -1.64256600 Pt 1.38818500 0.53523700 -1.64153700 Pt -1.38784500 2.99502400 1.41470600 Pt 1.38711800 2.99499600 1.41547000 Pt 0.00029600 2.84607400 -0.98392500 Pt 2.77542800 2.84610100 -0.98265600 Pt -2.77471500 2.84615300 -0.98436800 N -0.68763600 -4.09060900 0.46526700 N 0.70603300 -4.11109300 0.07780800 H 0.75647100 -4.26828800 -0.92547700 H -0.73289800 -4.09859300 1.48114800 149

H -1.20316100 -4.88662500 0.09079000 H 1.22981400 -4.83860100 0.56354900

System: Relaxed Pt20 + N2H4 (end-on configuration) Energy: -2495.59012377 Multiplicity: 15

Pt -2.82857100 -1.40517200 2.45238300 Pt -0.09628500 -1.45854400 2.38566300 Pt 2.71612900 -1.61678900 2.47589000 Pt -4.20875100 -1.45854700 0.04660900 Pt -1.46929900 -1.90569100 0.04507900 Pt 1.34217900 -1.94417500 0.14005700 Pt 4.10866800 -1.77599000 0.08147000 Pt -2.81680200 -1.61807700 -2.34662800 Pt 0.01449400 -1.71469900 -2.26023600 Pt 2.72818900 -1.82958100 -2.32384200 Pt -2.73788800 0.78562400 0.75273800 Pt 0.03434400 0.67948800 0.76325400 Pt 2.80693100 0.57402000 0.77633000 Pt -1.34623400 0.62601900 -1.64120500 Pt 1.42714900 0.52018000 -1.62982000 Pt -1.26711600 3.03026300 1.45854300 Pt 1.50566300 2.92443200 1.47025500 Pt 0.12531800 2.87066500 -0.93675400 Pt 2.89834200 2.76491800 -0.92458100 Pt -2.64783900 2.97654900 -0.94782100 N -1.52234300 -4.09803600 0.13183000 N -0.97868900 -4.79258300 1.27972200 H -1.46740600 -4.41211100 2.08928000 H -2.49065100 -4.39296900 0.03092800 H -1.02771900 -4.44657500 -0.68552100 H -0.01932100 -4.45565600 1.36853800

System: Relaxed Pt20 + N2H3 (side-on configuration) Energy: -2494.96497366 Multiplicity: 14 Pt -2.76766800 -1.52729700 2.46873200 Pt 0.00681400 -1.33475000 2.52798900 Pt 2.78003300 -1.48973600 2.46439500 Pt -4.15632300 -1.66333200 0.07113500 Pt -1.32524700 -1.94782000 0.18742000 Pt 1.44897700 -1.88998900 0.18393500 Pt 4.16694100 -1.60729700 0.06405400 150

Pt -2.77076300 -1.78163200 -2.32759000 Pt 0.00364100 -1.72078400 -2.23452900 Pt 2.77829300 -1.74438400 -2.33303100 Pt -2.78464800 0.65031300 0.74966800 Pt -0.01017500 0.66933200 0.74610100 Pt 2.76476200 0.68787500 0.74516200 Pt -1.39925400 0.53228000 -1.64966600 Pt 1.37635100 0.55108300 -1.65239800 Pt -1.41229700 2.96509400 1.42826400 Pt 1.36243100 2.98384300 1.42582800 Pt -0.02620900 2.84677300 -0.97307400 Pt 2.74889000 2.86559700 -0.97476000 Pt -2.80124400 2.82802100 -0.96998600 N -0.70351100 -3.93635200 0.20887300 N 0.70978000 -3.93954400 0.34539000 H 1.18484200 -4.52711500 -0.33702300 H -0.88133800 -4.23326000 -0.75174400 H 0.95482300 -4.21723800 1.29226200

System: Relaxed Pt20 + N2H3 (end-on configuration) Energy: -2494.94855032 Multiplicity: 14

Pt -2.80257700 -1.43533100 2.47793800 Pt 0.03167600 -1.41279400 2.43650100 Pt 2.74394300 -1.56150600 2.48055100 Pt -4.19166200 -1.53235700 0.07861400 Pt -1.45424300 -1.88897700 0.11605200 Pt 1.35042500 -1.92587300 0.11789100 Pt 4.12940300 -1.72167900 0.08239700 Pt -2.80676500 -1.69317700 -2.31875000 Pt 0.00207100 -1.72974700 -2.26033700 Pt 2.74077600 -1.81914900 -2.31608400 Pt -2.75242300 0.74072600 0.75783600 Pt 0.02124800 0.67739600 0.75769800 Pt 2.79482900 0.61443800 0.76048000 Pt -1.36787500 0.57999400 -1.64030200 Pt 1.40690600 0.51682600 -1.63895100 Pt -1.31373600 3.01422700 1.43632600 Pt 1.46032000 2.95103700 1.43775000 Pt 0.07156000 2.85345000 -0.96299400 Pt 2.84591000 2.79031300 -0.96104300 Pt -2.70295500 2.91669600 -0.96362700 151

N -1.50602800 -3.85294100 0.35484200 N -0.45238900 -4.62302800 0.62611600 H -0.49412600 -5.18433500 1.47289000 H -2.36975600 -4.22883900 0.73885400 H 0.43991000 -4.14693200 0.44172800

System: Relaxed Pt20 + N2H2 (side-on configuration) Energy: -2494.34245267 Multiplicity: 15

Pt -2.76617000 -1.51852200 2.48199600 Pt -0.00233500 -1.32459900 2.50803800 Pt 2.78505500 -1.46738900 2.47929800 Pt -4.15355800 -1.67315800 0.08309900 Pt -1.34943400 -1.95184800 0.18857000 Pt 1.49410100 -1.95474000 0.19582600 Pt 4.17253400 -1.59626500 0.07907100 Pt -2.76583400 -1.80199800 -2.31690700 Pt 0.00122100 -1.72123400 -2.21747700 Pt 2.78498500 -1.75084400 -2.31959300 Pt -2.78724600 0.64839100 0.74824000 Pt -0.01168200 0.67410500 0.74790400 Pt 2.76444000 0.69967900 0.74555200 Pt -1.39899200 0.52003100 -1.65077200 Pt 1.37636700 0.54550500 -1.65232800 Pt -1.42010800 2.97009900 1.41488700 Pt 1.35477900 2.99583600 1.41358200 Pt -0.03218500 2.84159100 -0.98504800 Pt 2.74240900 2.86715300 -0.98669800 Pt -2.80727400 2.81600800 -0.98391000 N -0.55310900 -3.89897400 0.19517300 N 0.73483700 -3.88336500 0.19547700 H -0.97861800 -4.66635000 -0.33317800 H 1.18285000 -4.64562600 -0.32127100

System: Relaxed Pt20 + N2H2 (end-on configuration) Energy: -2494.35707726 Multiplicity: 15

Pt -2.81286900 -1.39261300 2.48920300 Pt -0.07707100 -1.42200700 2.40681100 Pt 2.73268900 -1.54763800 2.50500300 Pt -4.19693900 -1.50372800 0.08851500 Pt -1.45777200 -1.95434500 0.08971300 152

Pt 1.36615300 -1.93841400 0.17413300 Pt 4.12223300 -1.73624200 0.11149500 Pt -2.80846700 -1.69302600 -2.30318700 Pt 0.02414500 -1.75329100 -2.22139200 Pt 2.73775400 -1.84776000 -2.28852000 Pt -2.74783900 0.76756500 0.75054100 Pt 0.02478700 0.68955200 0.75621800 Pt 2.79816700 0.61258700 0.76626300 Pt -1.35986900 0.57820300 -1.64232300 Pt 1.41473200 0.50068500 -1.63499700 Pt -1.29893900 3.03960800 1.41194800 Pt 1.47466200 2.96208800 1.41968500 Pt 0.09034200 2.85023100 -0.98320100 Pt 2.86442400 2.77282200 -0.97458800 Pt -2.68387100 2.92784500 -0.98988900 N -1.46413300 -4.04668500 0.25969700 N -0.54511100 -4.85660800 0.41004200 H -2.38222300 -4.50653300 0.22910900 H 0.34362100 -4.31599400 0.43107700

System: Relaxed Pt20 + N2H Energy: -2493.75384501 Multiplicity: 14

Pt -2.78317800 -1.47959100 2.47145600 Pt -0.00633300 -1.32518900 2.51070200 Pt 2.76831900 -1.52313900 2.47506200 Pt -4.16877200 -1.60478000 0.07165600 Pt -1.46688000 -1.96394100 0.16571100 Pt 1.32553700 -1.98807400 0.18912800 Pt 4.15439800 -1.66670200 0.07725900 Pt -2.78117400 -1.74659300 -2.32552700 Pt 0.01541300 -1.72064400 -2.22750500 Pt 2.76834100 -1.78931200 -2.32107200 Pt -2.76402500 0.68841000 0.74346300 Pt 0.00915400 0.66890600 0.74751500 Pt 2.78425500 0.64587400 0.74812700 Pt -1.37680600 0.54441800 -1.64933000 Pt 1.39904600 0.52247200 -1.64635400 Pt -1.36120500 2.98827100 1.42181900 Pt 1.41457200 2.96770000 1.42357100 Pt 0.02839700 2.84584400 -0.97736500 Pt 2.80301100 2.82357800 -0.97524600 Pt -2.74754100 2.86587900 -0.97881100 153

N -0.60860800 -3.84063500 0.27789000 N 0.60667200 -3.87847300 0.29656000 H -1.11958000 -4.73047500 0.32665400

System: Pt20 + N2 (N – N parallel to surface) Energy: -2493.136906 Multiplicity: 15

Pt -2.78135100 -1.51976300 2.47877300 Pt -0.00623600 -1.53232400 2.48017400 Pt 2.76902000 -1.54414000 2.48159600 Pt -4.16842000 -1.63757000 0.07789500 Pt -1.38945100 -1.65120500 0.08191700 Pt 1.37748200 -1.65972500 0.07960700 Pt 4.15724700 -1.67401700 0.08214700 Pt -2.78027400 -1.76741700 -2.32120800 Pt -0.00493900 -1.77972100 -2.31969300 Pt 2.77021600 -1.79179500 -2.31841800 Pt -2.77142400 0.66022100 0.76132300 Pt 0.00391900 0.64793500 0.76458700 Pt 2.77985700 0.63595300 0.76399700 Pt -1.38272600 0.53045700 -1.63740500 Pt 1.39233300 0.51818100 -1.63616100 Pt -1.37391100 2.95808200 1.44621000 Pt 1.40118100 2.94608000 1.44763900 Pt 0.01457600 2.82856500 -0.95291500 Pt 2.78934500 2.81626800 -0.95205500 Pt -2.76065600 2.84058000 -0.95473900 N -0.67072951 -3.50726162 0.24520200 N 0.42352325 -3.50391067 0.29900926

System: Pt20 + N2 (N – N perpendicular to surface) Energy: -2493.180741 Multiplicity: 15 Pt -2.78135100 -1.51976300 2.47877300 Pt -0.00623600 -1.53232400 2.48017400 Pt 2.76902000 -1.54414000 2.48159600 Pt -4.16842000 -1.63757000 0.07789500 Pt -1.38945100 -1.65120500 0.08191700 Pt 1.37748200 -1.65972500 0.07960700 Pt 4.15724700 -1.67401700 0.08214700 Pt -2.78027400 -1.76741700 -2.32120800 Pt -0.00493900 -1.77972100 -2.31969300 Pt 2.77021600 -1.79179500 -2.31841800 154

Pt -2.77142400 0.66022100 0.76132300 Pt 0.00391900 0.64793500 0.76458700 Pt 2.77985700 0.63595300 0.76399700 Pt -1.38272600 0.53045700 -1.63740500 Pt 1.39233300 0.51818100 -1.63616100 Pt -1.37391100 2.95808200 1.44621000 Pt 1.40118100 2.94608000 1.44763900 Pt 0.01457600 2.82856500 -0.95291500 Pt 2.78934500 2.81626800 -0.95205500 Pt -2.76065600 2.84058000 -0.95473900 N -1.39946414 -3.64060573 0.25613874 N -1.40495736 -4.73199482 0.35171712

System: Pt20 + NH3 + OH Energy: -2516.07538825 Multiplicity: 14

Pt -2.77432400 -1.48379700 2.51194200 Pt 0.00054400 -1.48076600 2.51271900 Pt 2.77567500 -1.47772500 2.51342500 Pt -4.16091000 -1.65320400 0.11420100 Pt -1.38592300 -1.65020600 0.11516100 Pt 1.38886400 -1.64724100 0.11598700 Pt 4.16414400 -1.64414500 0.11647900 Pt -2.77253700 -1.81967900 -2.28271200 Pt 0.00256300 -1.81648100 -2.28193500 Pt 2.77763400 -1.81365900 -2.28098000 Pt -2.77618200 0.66451100 0.75521000 Pt -0.00106300 0.66740100 0.75626400 Pt 2.77412300 0.67057300 0.75617800 Pt -1.38771900 0.49802200 -1.64165500 Pt 1.38745500 0.50099500 -1.64086300 Pt -1.39125400 2.98225100 1.39626800 Pt 1.38386300 2.98529400 1.39679300 Pt -0.00276700 2.81571800 -1.00066300 Pt 2.77239200 2.81862000 -1.00014300 Pt -2.77785800 2.81275500 -1.00154900 H 1.83631600 -4.10177300 1.08530200 N 1.33151300 -3.80528900 0.25662300 H 0.31918800 -4.03394500 0.32634400 H 1.74682700 -4.23220500 -0.56489700 O -1.36523200 -3.69452500 0.24012400 H -2.04521400 -3.91950900 0.88598200

155

System: Pt20 + NH2 + H2O Energy: -2516.06872493 Multiplicity: 14

Pt -2.77591400 -1.48192600 2.51109400 Pt -0.00104400 -1.48166300 2.51164400 Pt 2.77408800 -1.48139000 2.51212400 Pt -4.16286400 -1.64842700 0.11336000 Pt -1.38787600 -1.64819800 0.11409300 Pt 1.38691300 -1.64800000 0.11469300 Pt 4.16219500 -1.64767200 0.11495800 Pt -2.77485400 -1.81476500 -2.28377300 Pt 0.00024800 -1.81433500 -2.28322200 Pt 2.77532000 -1.81428100 -2.28249400 Pt -2.77577300 0.66749800 0.75572700 Pt -0.00065300 0.66762000 0.75655400 Pt 2.77453500 0.66802400 0.75624100 Pt -1.38767300 0.50114700 -1.64135800 Pt 1.38750300 0.50135100 -1.64079300 Pt -1.38848200 2.98344800 1.39814200 Pt 1.38663700 2.98372300 1.39844100 Pt -0.00035700 2.81705300 -0.99900800 Pt 2.77480300 2.81718700 -0.99871500 Pt -2.77545000 2.81685800 -0.99966800 H 1.86329200 -3.94560400 1.08328200 N 1.29739300 -3.70208000 0.27282400 H -0.23638000 -3.99710400 0.30021600 H 1.79593500 -4.06911100 -0.53629900 O -1.27164100 -3.93256700 0.22306900 H -1.65289600 -4.18672800 1.07146300

System: Pt20 + NH2 + OH Energy: -2515.40915381 Multiplicity: 13

Pt -2.77811000 -1.47767000 2.51305400 Pt -0.00272100 -1.48202200 2.51412400 Pt 2.77209900 -1.48640900 2.51534400 Pt -4.16460200 -1.64546300 0.11501800 Pt -1.39059900 -1.64929600 0.11887600 Pt 1.38677800 -1.65448400 0.11535500 Pt 4.16071800 -1.65868000 0.11890100 Pt -2.77587400 -1.81760600 -2.28129600 Pt -0.00068000 -1.82195000 -2.28018800 156

Pt 2.77423700 -1.82656400 -2.27925800 Pt -2.77452600 0.66889800 0.75393000 Pt 0.00218500 0.66447900 0.75530100 Pt 2.77618600 0.66035200 0.75735500 Pt -1.38481500 0.49698200 -1.64187300 Pt 1.39023200 0.49235000 -1.64101800 Pt -1.38260100 2.98380500 1.39295200 Pt 1.39248800 2.97924600 1.39506700 Pt 0.00626400 2.81129000 -1.00309300 Pt 2.78069600 2.80694600 -1.00127400 Pt -2.76932900 2.81578700 -1.00446600 N 1.40950200 -3.68530800 0.37883100 H 0.42460000 -3.97214400 0.32895700 H 1.86019200 -4.07971400 -0.44811500 O -1.43655600 -3.66906700 0.22281100 H -2.06486800 -3.87778800 0.92558300

System: Pt20 + NH + H2O (NH in top position) Energy: -2515.40284234 Multiplicity: 13

Pt -2.77868500 -1.46818900 2.51661100 Pt -0.00371600 -1.47761700 2.51631500 Pt 2.77114300 -1.48688200 2.51594700 Pt -4.16708100 -1.63651200 0.11983700 Pt -1.39288600 -1.64561200 0.12133100 Pt 1.38347100 -1.65546500 0.11753200 Pt 4.15768700 -1.66462400 0.11876500 Pt -2.78064100 -1.81422300 -2.27739200 Pt -0.00561500 -1.82359600 -2.27773700 Pt 2.76916100 -1.83294900 -2.27809200 Pt -2.77231200 0.67611500 0.75528900 Pt 0.00360400 0.66701400 0.75462000 Pt 2.77781600 0.65765900 0.75535800 Pt -1.38499200 0.49881000 -1.64143100 Pt 1.38971900 0.48925600 -1.64197000 Pt -1.37638700 2.98928100 1.39121700 Pt 1.39857100 2.97987200 1.39089000 Pt 0.01031100 2.81149900 -1.00595700 Pt 2.78503400 2.80214800 -1.00617700 Pt -2.76476700 2.82092700 -1.00539800 N 1.26432500 -3.59620700 0.30956300 H -0.32120800 -4.05016000 0.32624300 H 2.20960200 -3.98921100 0.24343600 157

O -1.32740300 -3.95700300 0.31341200 H -1.63531700 -4.31029000 -0.52943500

System: Pt20 + NH + H2O (NH in fcc position) Energy: -2515.42632097 Multiplicity: 13

Pt -2.77742100 -1.50430300 2.48932600 Pt -0.00246500 -1.52873500 2.47870200 Pt 2.77194500 -1.55318900 2.46805600 Pt -4.17519600 -1.61870100 0.09481000 Pt -1.40107100 -1.64293100 0.08604500 Pt 1.37515400 -1.66780700 0.07204100 Pt 4.14923900 -1.69217300 0.06295800 Pt -2.79822400 -1.75759600 -2.31042200 Pt -0.02329300 -1.78213900 -2.32104400 Pt 2.75132500 -1.80655000 -2.33164800 Pt -2.76547000 0.67358900 0.76969600 Pt 0.01034200 0.64940400 0.75877800 Pt 2.78443900 0.62489800 0.74923200 Pt -1.38761900 0.53507700 -1.63502100 Pt 1.38691300 0.51041100 -1.64582000 Pt -1.35457800 2.96640200 1.44507300 Pt 1.42028400 2.94189600 1.43448700 Pt 0.02264900 2.82742600 -0.96010500 Pt 2.79726300 2.80294700 -0.97060700 Pt -2.75231300 2.85193800 -0.94929300 N 1.38837900 -2.72109000 1.81924000 H -1.52344100 -4.34859400 1.07571200 H 1.43391600 -3.72292400 1.99322000 O -1.44567400 -4.15093200 0.13456300 H -0.55211700 -4.40284300 -0.12929100

System: Pt20 + NH + OH Energy: -2514.79249390 Multiplicity: 12

Pt -2.77223100 -1.52250100 2.48880400 Pt 0.00268200 -1.53375500 2.47906000 Pt 2.77767500 -1.54505500 2.46926700 Pt -4.16868300 -1.64423400 0.09384900 Pt -1.39368100 -1.65550600 0.08401000 Pt 1.38121800 -1.66681400 0.07434900 158

Pt 4.15616800 -1.67803700 0.06453300 Pt -2.79021100 -1.77723400 -2.31087700 Pt -0.01526600 -1.78849500 -2.32064700 Pt 2.75970200 -1.79978300 -2.33039500 Pt -2.76940900 0.65515000 0.76886800 Pt 0.00548300 0.64385400 0.75911600 Pt 2.78043600 0.63262100 0.74928000 Pt -1.39107000 0.52210700 -1.63596100 Pt 1.38396800 0.51090800 -1.64564000 Pt -1.37025700 2.95449800 1.44390800 Pt 1.40470200 2.94325200 1.43406300 Pt 0.00818300 2.82156900 -0.96086100 Pt 2.78318100 2.81031600 -0.97067700 Pt -2.76673000 2.83280800 -0.95110300 N 1.39341500 -2.69591200 1.77608400 H 1.36982800 -3.71553400 1.78904200 H -2.02564600 -3.95350500 0.65203200 O -1.19440300 -3.66022900 0.25705800

System: Pt20 + N + H2O Energy: -2514.82412983 Multiplicity: 12

Pt -2.77919200 -1.50998900 2.48628600 Pt -0.00433400 -1.53178300 2.47885400 Pt 2.77060300 -1.55362300 2.47137400 Pt -4.17409500 -1.62583500 0.09013600 Pt -1.39914900 -1.64764700 0.08261000 Pt 1.37569500 -1.66949500 0.07526200 Pt 4.15058900 -1.69125800 0.06775900 Pt -2.79413000 -1.76348800 -2.31347100 Pt -0.01924000 -1.78528900 -2.32092800 Pt 2.75567200 -1.80711700 -2.32836400 Pt -2.76666000 0.66805300 0.76688800 Pt 0.00817600 0.64621700 0.75944800 Pt 2.78307400 0.62444400 0.75192500 Pt -1.38682800 0.53035700 -1.63682300 Pt 1.38815400 0.50861800 -1.64418900 Pt -1.35934800 2.96190500 1.44366000 Pt 1.41555600 2.94011900 1.43612700 Pt 0.02058600 2.82432300 -0.95999100 Pt 2.79552800 2.80253000 -0.96749400 Pt -2.75427200 2.84610200 -0.95254600 N 1.38020500 -2.66367900 1.76975700 159

H -0.31752100 -4.19788400 0.70592000 H -1.82267600 -4.26865800 1.02979700 O -1.19740000 -4.09062900 0.31589800 160

APPENDIX B – TABLE OF VALUES FROM DIGITIZED GRAPHS

1 Table B.1 Values for Figure 1 (C5H5Mn(CO)2N2H4) from Binder and Sellman

Binding energy Intensity 403.05 29.94 (eV) (Counts/sec) 403.08 30.74 401.60 6.74 403.11 31.59 401.70 6.92 403.12 34.15 401.79 7.36 403.12 33.29 401.86 7.96 403.12 32.43 401.93 8.63 403.13 34.98 401.99 9.30 403.16 35.82 402.07 9.86 403.18 36.67 402.16 10.31 403.19 37.54 402.25 10.69 403.21 38.38 402.35 11.09 403.22 39.22 402.43 11.58 403.24 40.09 402.48 12.20 403.25 40.96 402.49 12.98 403.26 41.82 402.50 13.84 403.27 42.69 402.53 14.68 403.28 43.55 402.56 15.50 403.31 44.40 402.59 16.33 403.33 45.23 402.62 17.15 403.35 46.08 402.66 17.92 403.37 46.93 402.70 18.70 403.39 47.79 402.74 19.49 403.41 48.64 402.78 20.29 403.43 49.47 402.81 21.08 403.45 50.31 402.83 21.91 403.46 51.16 402.85 22.77 403.48 52.00 402.86 23.62 403.51 52.84 402.87 24.46 403.52 53.68 402.90 25.27 403.54 54.53 402.94 26.05 403.56 55.37 402.98 26.85 403.58 56.22 402.99 27.65 403.60 57.09 402.99 28.43 403.62 57.94 403.02 29.18 403.64 58.77 161

403.68 59.57 405.25 52.72 403.72 60.34 405.27 51.87 403.78 61.10 405.28 51.02 403.83 61.85 405.30 50.17 403.88 62.59 405.32 49.31 403.93 63.35 405.35 48.49 403.98 64.13 405.37 47.66 404.02 64.90 405.39 46.80 404.08 65.60 405.41 45.95 404.15 66.21 405.42 45.10 404.20 67.55 405.44 44.25 404.23 66.73 405.48 43.44 404.32 66.30 405.51 42.62 404.34 66.62 405.53 41.78 404.36 67.48 405.54 40.91 404.36 68.34 405.55 40.06 404.44 67.76 405.57 39.21 404.51 67.14 405.58 38.35 404.52 66.28 405.60 37.49 404.59 68.12 405.61 36.63 404.69 67.98 405.62 35.78 404.77 67.49 405.63 34.91 404.82 66.75 405.64 34.05 404.84 65.90 405.66 33.21 404.86 65.05 405.70 32.43 404.89 64.22 405.76 31.69 404.92 63.39 405.79 31.31 404.95 62.57 405.80 30.44 405.00 61.81 405.80 30.91 405.05 61.08 405.83 29.63 405.09 60.31 405.86 28.81 405.10 58.62 405.89 27.97 405.12 59.47 405.90 27.11 405.12 57.78 405.90 26.25 405.14 56.92 405.92 25.40 405.16 56.06 405.95 24.59 405.18 55.22 405.99 23.81 405.20 54.39 406.03 23.00 405.23 53.56 406.05 22.16 162

406.06 21.31 406.80 8.40 406.09 20.46 406.87 7.89 406.12 19.66 406.93 6.82 406.19 19.02 406.96 7.36 406.20 18.15 406.97 7.62 406.22 17.28 407.05 7.98 406.24 16.44 407.16 7.98 406.28 15.64 407.26 7.73 406.31 14.86 407.35 7.34 406.35 14.03 407.44 6.83 406.37 11.56 407.52 6.29 406.37 13.20 407.61 5.83 406.37 12.37 407.68 6.44 406.42 11.00 407.74 6.86 406.51 10.65 407.78 6.49 406.61 10.28 407.81 6.23 406.68 9.70 407.89 5.66 406.73 9.00 408.00 5.66 1Binder, H.; Sellmann, D. Angewandte Chemie-International Edition in English 1973, 12(12), 1017-1019

Table B.2. Table of Values for Figure 8 from Alberas et al. (~ 0.5 ML)2

Binding energy Intensity 401.38 333.05 (eV) (Counts/sec) 401.33 368.29 401.99 218.24 401.26 400.68 401.89 211.42 401.22 438.76 401.81 227.34 401.19 477.41 401.76 265.98 401.12 475.70 401.71 301.22 401.04 450.13 401.68 292.13 401.00 450.13 401.66 286.45 400.98 465.47 401.61 249.50 400.92 506.39 401.55 245.52 400.87 526.85 401.50 265.98 400.82 511.51 401.49 277.35 400.77 521.74 401.45 301.22 400.74 552.43 163

400.71 578.01 398.89 306.91 400.69 613.81 398.85 286.45 400.65 651.89 398.81 244.96 400.63 664.96 398.76 210.29 400.59 703.04 398.70 174.48 400.58 713.84 398.64 143.22 400.53 721.23 398.48 138.11 400.48 705.88 398.43 132.99 400.46 665.53 398.33 143.22 400.45 663.82 398.25 122.76 400.38 695.08 398.21 107.42 400.37 691.11 398.15 132.99 400.35 704.75 398.13 147.20 400.27 731.46 398.09 143.22 400.17 720.66 398.04 132.99 400.11 700.77 397.98 97.19 400.07 708.72 397.92 65.36 400.00 721.80 397.87 75.59 399.96 716.11 397.82 111.40 399.90 705.88 397.77 122.76 399.81 686.56 397.71 112.53 399.73 659.85 397.61 101.73 399.67 625.75 397.56 143.22 399.66 623.47 397.52 163.68 399.63 584.83 397.48 162.55 399.56 554.70 397.46 122.76 399.50 532.54 397.42 92.07 399.49 522.31 397.39 61.38 399.46 495.03 397.31 51.72 399.41 458.65 397.25 82.98 399.37 434.78 397.18 92.07 399.32 439.90 397.14 82.41 399.22 455.24 397.11 61.95 399.18 429.67 397.05 61.38 399.14 393.86 397.01 77.86 399.09 348.96 396.98 93.21 399.03 319.41 396.98 102.30 398.93 309.75 396.91 132.99 164

396.87 143.22 396.43 110.26 396.79 118.22 396.39 122.76 396.75 89.23 396.35 117.65 396.73 88.09 396.33 99.46 396.70 53.99 396.29 85.25 396.65 66.50 396.24 76.73 396.64 73.32 396.21 76.73 396.62 80.14 396.19 71.61 396.58 94.91 396.17 66.50 396.54 76.16 396.14 61.38 396.52 61.95 396.12 46.04 396.47 72.18 396.07 40.92 396.44 92.07 396.03 30.69 2Alberas, D. J.; Kiss, J.; Liu, Z. M.; White, J. M. Surface Science 1992, 278(1-2), 51-61.

Table B.3. Table of Values for Figure 8 from Alberas et al. (~ 1 ML)2

Binding energy Intensity 396.84 593.35 (eV) (Counts/sec) 396.89 610.40 395.60 511.51 396.95 593.35 395.67 539.93 396.97 583.12 395.72 575.73 397.04 572.89 395.81 595.06 397.14 572.89 395.89 568.34 397.33 552.43 395.95 533.11 397.42 578.01 395.96 531.97 397.44 617.22 396.05 553.00 397.51 649.05 396.11 587.67 397.67 613.81 396.32 583.69 397.77 613.81 396.42 595.62 397.83 598.47 396.51 575.16 397.85 587.67 396.60 550.16 397.91 572.32 396.69 531.97 397.99 588.80 396.79 542.20 398.08 618.36 396.83 578.57 398.14 623.47 165

398.18 629.16 400.11 1720.94 398.23 643.93 400.24 1765.27 398.31 670.08 400.29 1788.01 398.49 680.31 400.32 1793.12 398.59 694.52 400.40 1785.17 398.72 743.96 400.49 1765.84 398.78 777.49 400.59 1749.36 398.84 813.30 400.61 1744.25 398.88 851.38 400.68 1715.83 398.97 869.57 400.71 1676.61 399.00 874.11 400.74 1636.83 399.05 909.92 400.82 1545.33 399.11 942.31 400.90 1519.18 399.14 969.03 400.92 1479.40 399.15 980.96 400.98 1443.59 399.17 1007.67 401.17 1407.22 399.22 1044.05 401.21 1369.14 399.28 1077.01 401.24 1329.92 399.32 1113.95 401.28 1291.28 399.36 1130.43 401.32 1253.20 399.39 1145.78 401.37 1175.90 399.40 1156.01 401.42 1140.66 399.41 1166.24 401.47 1109.97 399.43 1194.66 401.56 1084.40 399.46 1233.87 401.64 1058.82 399.50 1273.09 401.79 997.44 399.54 1310.03 402.33 769.54 399.60 1345.27 402.39 827.51 399.65 1381.07 402.42 788.29 399.69 1418.58 402.46 749.64 399.71 1458.37 402.49 736.57 399.73 1498.72 402.59 745.10 399.74 1539.07 402.66 762.15 399.81 1620.92 403.02 646.77 399.90 1643.08 403.08 679.74 399.94 1681.16 403.18 675.19 400.01 1711.85 403.31 716.11 403.35 632.57 166

403.41 597.90 403.63 618.93 403.47 630.29 403.72 631.43 403.54 649.62 403.76 670.08 2Alberas, D. J.; Kiss, J.; Liu, Z. M.; White, J. M. Surface Science 1992, 278(1-2), 51-61.

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