Abstract
Water formation is relevant in many technological processes and is also an important model reaction. Although water formation over Pd surfaces is widely studied, questions regarding the active site and the main reaction path (OH* + OH*) or (OH* + H*) are still open. Combining first-principles density functional theory calculations and kinetic Monte Carlo simulations, we find that the reaction rate is dominated by surface steps and point defects over a wide range of conditions. The main reaction path is found to be temperature dependent where the OH* + OH* path dominates at low temperatures, whereas the OH* + H* path is the main path at high temperatures. Steps facilitate the OH* formation, which is the rate limiting step under all conditions. OH* is formed via O* + H* association or OOH* splitting at low temperatures, whereas OH* is exclusively formed via O* + H* association at high temperatures. The results of the first-principles-based kinetic model are in excellent agreement with experimental observations at high and low temperatures as well as different gas-phase compositions.
Highlights
Water is a simple molecule and one of the most abundant compounds on earth
Over pristine Pd(111), we find that only the OH∗ + H∗ reaction path contributes to water formation
Using a first-principles-based kinetic Monte Carlo approach, we have investigated water formation from H2 and O2 over metallic Pd surfaces including reaction paths that are relevant for both H2O and H2O2 formation
Summary
Water is a simple molecule and one of the most abundant compounds on earth. Water is, relevant in a range of technological applications. Water splitting (H2O → H2 + 1/2O2) and the water–gas-shift reaction (H2O + CO → H2 + CO2) are two examples of possible routes to use H2O to obtain hydrogen, which is important in a fossil free society.1,2 Another example is the formation of water from H2 and O2 in proton-exchange membrane fuel cells, which is a promising technology in a hydrogen-based energy system.. The central role of water for sustainable energy systems is one reason to develop detailed knowledge on catalytic splitting and formation of water over solid surfaces. Another reason is to develop general concepts and understanding by studying a model reaction with a limited number of reaction steps. Scitation.org/journal/jcp activity and that the main path for water formation is temperature dependent
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