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It excites me discovering the "secrets of the Creator", which may provide a speck of contribution to humankind.

Role of catalysts

 

A catalyst is a material that speeds up a reaction without being consumed in the overall process. This is accomplished by providing alternative routes for the reactions to follow that require less free energy to reach the transition state. Thus, catalyzed reactions have lower activation energy than the corresponding uncatalyzed reaction, resulting in higher reaction rate. Consider a simple heterogeneous reaction shown on the left. First, the reactants are adsorbed on the surface of the catalyst. This initial step entails the interaction between the electronic states of the reactants and surface atoms. Sabatier principle states that the interactions between the catalysts and reactants should neither be too strong nor too weak. If the interaction is too weak, the reactants will fail to bind to the catalyst and no reaction will take place. On the other hand, if the interaction is too strong, the catalysts will be poisoned by the reactants and further reactions will not take place. In the next step, the adsorbed reactants can then migrate over the surface of the catalyst and react to form a new adsorbed molecule. Diverse mechanisms for reactions on surfaces can take place depending on how the adsorption takes place. The product of this surface reaction can then desorb from the surface in the final step allowing them to move away from the surface for further reactions. Without a catalyst, the reaction will proceed in a path that requires large activation barrier as shown by the brown curve.

 

 

Rational Catalyst Design using First Principles Calculations

 

Despite the impressive advancements in experimental methods, most traditional experimental techniques in discovering catalysts for particular reactions involve combinatorial and trial-and-error methods (intuition) that are time-consuming, costly, and do not guarantee success. On the other hand, modern theoretical techniques use state-of-the-art computational methods at the molecular and catalyst levels to screen the best catalysts. Such theoretical techniques employ density functional theory (DFT) calculations at the molecular level, coupled with statistical mechanics-based approaches such as kinetic Monte Carlo (KMC) method at the catalyst level, to gain mechanistic insights into the reaction mechanisms (i.e., intermediate elementary steps that comprise the overall reaction) and understand the activity and selectivity of catalysts. Thus, it is now possible to theoretically screen for potentially active catalytic materials and rationally design catalysts that promote the desired selectivity and catalytic activity. 

 

A few groups have studied the trends in catalyst activity that sought to develop catalyst design principles based on reactivity descriptors. One such model for transition metal reactivity uses the concept of the d-band center and activity volcano plots to explain the trends in adsorption energies and activation barriers of elementary processes on transition metal catalysts. (e.g., Why Gold is the Noblest of All Metals) The Brønsted-Evans- Polanyi (BEP) relations were also quantified to find predictive models that relate the catalytic reactivity to adsorption energies of key relevant species. These methods paved the way to the rational design of catalysts for various heterogeneous reactions which were conrmed by experiments. 

 

In my works, an approach to rational catalyst design (RCD) is demonstrated which aims to theoretically engineer on the atomic-scale the desired catalysts for various heterogeneous reactions (see the figure below). To achieve this goal, the following specic objectives were set

 

1. Determine the reaction mechanism of a catalytic reaction on surfaces

2. Identify the design principles that will guide the design of new catalysts

3. Develop relationships between the properties of the catalyst and its reactivity

 

In the RCD approach, the reaction mechanism of a catalytic reaction on a conventional catalyst is first built. A conventional catalyst is defined to be the material that is commonly used in experiments and is known to be catalytically active for a particular reaction of interest. The reaction mechanism identifies the relevant features of the elementary steps that comprise the overall reaction such as the ratedetermining steps and exothermicity of intermediate reactions. From here, design principles are generated to guide the modication of the conventional catalyst into a new catalyst that promotes the desired reaction. Thus, as shown in the figure below, the RCD approach has two interconnected parts: design and mechanism. The design component of the RCD approach includes the determination of the material composition (choice of material elements), structure (crystal/surface structure and atomic ratio), and stability of the catalyst. The mechanism component includes the investigation of the intermediate elementary reactions of the overall process and electronic properties of the catalyst.

 

The results in my studies were obtained through first principles calculations based on DFT. In a nutshell, DFT is a computational quantum mechanical modelling method that is used to investigate the ground state electronic properties of many-body systems. In this theory, the ground state property of a many-electron system is a functional (i.e., function of a function) of electron density; hence the name density functional theory. The electron density thus uniquely determines the ground state properties of quantum mechanical systems. A more comprehensive discussion about DFT, as well as the software package used and calculation details, can be found in the literature. 

 

Starting from a surface reaction of interest (e.g., conversion of NO to NO2 on Pt), DFT calculations were used to identify the reaction mechanism which serves as input in the design of a new surface reaction (e.g., conversion of NO to NO2 on alloys) in a cyclic input-process output design. The major results from DFT calculations can be categorized into (1) fundamental properties of the catalysts and reactants, (2) thermodynamics and (3) kinetics of reactions. The fundamental properties include the structural, electronic and magnetic properties which were investigated through structural optimization, calculation of charges and charge densities, analysis the density of states, and calculation of the magnetic moment. The thermodynamics of reactions (i.e., exothermicity of reactions based on the energies of products and reactants) required the calculation of the adsorption energies and Gibbs free energies. The kinetics of reactions (i.e., rates of chemical processes) entailed the calculation of activation barriers and reaction rates of elementary processes. The reaction mechanisms are verified and compared with experimental results reported in the literature. 

 

 

Topics of Interest

 

Methane Activation

 

The initial breaking of the C–H bonds of methane molecule is the key reaction in the production of a wide range of higher hydrocarbons, alcohols, and other important chemicals for industrial and alternative energy applications. For most existing heterogeneous catalysts, the initial C–H bond cleavage is rate-limiting, which makes the subsequent elementary reaction steps occur rapidly and difficult to control. Achieving C–H bond activation at low temperature could remedy this problem and allow for the control of its selectivity towards the desired products. Also, one problem in the efficient catalysis of methane activation is the poisoning of the catalysts that result in its deactivation. To this end, first principles calculations were used to determine the mechanism of methane activation on benchmark surfaces such as stepped Ni and Ru catalysts to gain insights into the design of catalysts that are both resistant to poisoning and promote facile C–H bond cleavage.  

 

Water Splitting

 

The dissociation of water is one of the cornerstone problems in catalysis and is of profound importance in many physical, chemical, and biological phenomena. It is a key reaction in various industrial processes such as the water gas shift reaction, steam reforming of natural gas, thermochemical and photocatalytic hydrogen production, and electrode reactions in fuel cells or other electrocatalytic devices.

 

Carbon Dioxide Conversion

 

CO2 accounts for most of the greenhouse gas emissions from human activities such as in the combustion of fossil fuels and industrial processes. Despite the thermodynamic stability of CO2, some biological systems (such as the leaves of plants in photosynthesis) are capable of both activating the molecule and converting it into a range of hydrocarbons such as methane, formic acid, and methanol which can be used as fuels for alternative energy systems. Thus, if we can discover the “secret of Nature” in catalyzing this reaction without the need for extreme reaction conditions, the impact would be enormous: One of the major greenhouse gases responsible for accelerating global warming will become an important sustainable feedstock for energy! Indeed, the catalytic conversion of CO2 into hydrocarbons is one of the major challenges in contemporary science.

 

Electrochemical Reactions for Energy Conversion

 

The aqueous-phase electrochemical oxidation/reduction of ionic compounds is the key reaction in energy storage and utilization systems such as in batteries and fuel cells. It opens the possibility of transforming the free energy of spontaneous reaction into electricity which can be exploited for a worldwide implementation of clean, affordable, and renewable energy sources. Though there have been enormous advances in the experimental characterization of electrochemical interfaces, much of the molecular level insights into the atomic-scale processes on these surfaces have emerged dominantly from the impressive advances in DFT calculations. Such “first-principles computational electrochemistry” may well be the first to be fully embraced by experimental electrochemists to supplement and guide their experiments.

 

 

Computational Heterogeneous Catalysis

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Our modern-day society is highly dependent on heterogeneous catalysis (a form of catalysis where the catalysts and reactants have different phases). Its impact ranges from the field of pharmaceuticals to alternative energy and environmental concerns. For example, the reduction of greenhouse gases through catalytic oxidation and reduction is a key factor in slowing down the effects of global warming and climate change. Another notable example is the catalyzed production of ammonia (Haber process) which is used to generate fertilizers to sustain one-third of the Earth's population. Furthermore, heterogeneous catalysis is used to produce most of the industrially important key chemicals such as benzene, sulphuric acid, hydrogen and oxygen, polymers, among others. 

Mentors/Collaborators

Hideaki Kasai

National Institute of Technology, Akashi College

 Osaka University

 

Hiroshi Nakanishi

National Institute of Technology, Akashi College

Osaka University

 

Hiroshi Kitagawa

Kyoto University

 

Mary Clare Escano

University of Fukui

 

Michail Stamatakis

University College London

 

Susan Menez Aspera

National Institute of Technology, Akashi College

 

Elod Gyenge

University of British Columbia

 

Allan Abraham Padama

University of the Philippines

 

From: R.L. Arevalo, Ph.D. Dissertation (2015), Osaka University

Available: https://ir.library.osaka-u.ac.jp/repo/ouka/all/52142/27704_要旨.pdf

From: R.L. Arevalo, Ph.D. Dissertation (2015), Osaka University

Available: https://ir.library.osaka-u.ac.jp/repo/ouka/all/52142/27704_要旨.pdf

From: R.L. Arevalo, Ph.D. Dissertation (2015), Osaka University

Available: https://ir.library.osaka-u.ac.jp/repo/ouka/all/52142/27704_要旨.pdf