Molecular Dynamic Simulation of Aluminum/Water Reaction Using Reactive Force Field

Open Access
Author:
Li, Rong
Graduate Program:
Mechanical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
December 09, 2010
Committee Members:
  • Clinton Matthew Mench, Thesis Advisor
  • Matthew M Mench, Thesis Advisor
  • Adrianus C Van Duin, Thesis Advisor
Keywords:
  • aluminum water reaction
  • reactive force field
Abstract:
Over the past few years, it is becoming more likely that the emphasis on cleaner fuel will lead to use of hydrogen in a significant way. The worldwide increasing demand of hydrogen, such as in hydrogen fuel cells, has made it crucial to find hydrogen generation methods from inexpensive simple processes. One of the most promising approaches is hydrogen generation from the aluminum-water reaction. The hydrogen produced via such aluminum-water reactions can be employed to power fuel cell devices for portable applications such as emergency generators and laptop computers. Also, aluminum-water reactors can be used for emergency hydrogen storage on fuel cell-powered vehicles. In this work, molecular dynamic simulations were conducted to study the aluminum-water reaction using the reactive force field (ReaxFF), which is optimized especially for Al and aluminum oxide. The initial reaction between water molecules and an Al cluster was considered to be the chemisorption of water molecules, and then the dissociation of adsorbed water molecules. Both water self-assisting effect and self poisoning effect on H2O reaction are observed. The dissociation of adsorbed water molecule is assisted by other water molecules due to the interactions between them, which is referred to as water self-assisting effect on H2O reaction. On the other hand, adsorbed water molecule can also prevent further reaction with the Al cluster, which is the water self poisoning effect. When too many water molecules are present in the system, considerable surface reaction sites are lost because of the chemisorption of water molecules. As a result, these adsorbed water molecules can’t further dissociate, since the hydrogen atoms needs a free site to move into. The adsorbed water molecules tend to connect and form layers of water molecules starting from the adsorbed ones and stretching to the ones in the gas phase around the cluster. These layers of water molecules will also block the pathways for the reactants to arrive at the Al cluster surface. Water dissociation (conversion) rate depends on the number of water molecules present in the system. The dissociation rate firstly increases with the number of water molecules because of the water self-assisting effect and then decreases with the number of water molecules due to water self poisoning effect. A maximum water dissociation rate of 100% is obtained for a 100 molecule Al cluster when 30 water molecules were originally added. Among the intermediate species produced during the reaction, the hydronium ion is believed to be crucial to assist water molecule dissociation and facilitate hydrogen generation. Acting as an inert specie, noble gas neon (Ne) was added to the aluminum-water mixture. Direct collisions between Ne atoms and water molecules are not observed, but Ne atoms can temporarily adsorb on the aluminum cluster. Due to water self-assistance and self poisoning effect, an optimum Ne concentration (corresponding to an optimum water concentration) is found with respect to higher water dissociation rate and hydrogen generation rate. Different aluminum cluster sizes were also studied in this work. When the same average surface coverage of aluminum cluster by water molecules is maintained, higher water molecule dissociation rate and better hydrogen generation performance are obtained from larger aluminum cluster, due to the additional surface reaction sites provided by the larger aluminum cluster. However, for aluminum clusters with different sizes, the same optimum water concentration with respect to better water dissociation and hydrogen generation performance was found. The influences of aluminum oxide layer on aluminum-water reaction are also investigated in this work. The presence of aluminum oxide layer causes the loss of available surface reaction sites, which is demonstrated by the reduction of chemisorption and dissociation of water molecules during the reaction. As the aluminum oxide layer becomes thicker, the spontaneous reaction of aluminum with water is severely prohibited or even stopped. To activate aluminum-water reaction, continuous removal of aluminum oxide either by mechanical or chemical methods is required. For future studies, modifying aluminum cluster structure or cluster surface configuration, or adding other materials to the aluminum cluster, may have the potential of promoting aluminum-water reaction.