Theoretical Studies On Interactions Between Small Oxides And Aluminum Cluster And Aluminum Surface

H2O or CO2in the environment can be used as propellant fuel with metal (such as Al, Mg etc.). Because H2O or CO2do not need to be specially carried, the energy density of propellant can be greatly improved. So, propellant development of metal-small oxygen containing compounds has become the frontier and hot issue in the field of aeronautics and astronautics. On the other hand, in the solid propellant booster of some space shuttle, the combustion products of propellant and ammonium perchlorate contain approximately40%～50%H2O and～10%CO2. Nitramine propellants like HMX contain approximately20.9%NO. The study on the interaction of metal-small oxygen containing compounds is very important to explore the redox reactions between aluminum and these small oxides, and to elucidate the combustion mechanism of propellant.The adsorption and decomposition of small oxygen containing compounds (water, CO2and NO) on the aluminum surface, aluminum clusters and doped aluminum clusters were investigated using density functional theory (DFT) method. The main investigation contents are as follows:1. Density functional theory studies on the adsorption of NH2NO2on Al13clusterBoth dissociative and nondissociative adsorption structures were predicted with different NH2NO2molecule orientations on AI13cluster surfaces. In dissociative chemisorption, the main decomposition products of NH2NO2are0atom(s) and NH2NO or NH2N species. The decomposited products strongly adsorbed to the surface. The largest adsorption energy is-737.66kJ/mol when the NH2NO2molecule decomposes into two O atoms and a NH2N fragment. For nondissociative adsorption, the seriously deformed nitroamine forms various N-O-Al bonding configurations with Al. The significant charge transfer occurs for all adsorption configurations. The change of the electronic structures is obvious due to the adsorption or dissociation of NH2NO2molecule. Nitroamine readily oxidizes the aluminum surface.2. Adsorption and dissociation of H2O on Al12X clustersThe adsorption and reaction of H2O molecule on neutral X-centered icosahedronal Al12X clusters (X=Al, Mg, Zn, Ga, Ni, Fe, B, C, Si, P) were investigated by PW91, PBE and PWC methods. The spin states and the doped atoms have important influences on the Al12X geometries, density, electronic properties, and energy density of reaction between Al12X with a single H2O molecule. The energies of the neutral X-centered Al12X are lower than that of surface X-replaced Al12X with exception of Al12Mg. The H2O dissociation on the Al12X (X=Mg, Zn, Ga, Ni, Fe) clusters have relatively low activation barriers, but large activation barriers for Al12X (X=B, C, Si, P). The activation barrier of water dissociation on the singlet Al12Fe cluster is the lowest, whereas the highest barrier is with the Al12C. The most exothermic reaction is of H2O with Al12Fe occurs. The center-Fe atom can move out to the surface after the adsorption or dissociation of H2O with an energy barrier of172kJ/mol. The results showed that the water dissociation on the Al12X cluster can be tuned by controllable X doping.3. Geometric and electronic structures to adsorption and dissociation energies towards H2O on Al,,Bi (n=2-15) ClustersThe results reveal that the most stable AlnBi structures have similar configurations as the most stable host Aln+1cluster except Al7Bi and Al8Bi. Bi-doping introduces a significant modulation of the electronic structures, such as the HOMO-LUMO gaps and the s and p hybridization states. The adsorptions of H2O molecule on AlnBi clusters are all exothermic. The configurations hardly distorted for relatively stable AlnBi clusters while somewhat distorted for the less stable ones after adsorption of H2O. H2O dissociation on the AlnBi is also exothermic. The adsorption energyEads can fully compensate for the dissociation barrier except for Al14Bi and Al15Bi. Of all the considered AlnBi clusters, the adsorption and dissociation of H2O on Al5Bi can release the largest energies and the dissociation is almost spontaneous.4. Adsorption and dissociation of H2O on Al13cluster:origins of reactivity and mechanism for H2releaseCalculations reveal that H2O molecule adsorbs easily on the Al13surfaces and forms adlayers. The dissociation of the first H2O molecule from the bimolecular H2O structure via Grotthuss mechanism is the most kinetically favorable among the five pathways of breaking O-H bond. H2eliminating from the reaction of H2O molecule with the hydrogen atom on the Al cluster via Eley-Rideal mechanism has a lower activation barrier than that from the reaction of two adsorbing H atoms or reaction of OH and H. After adsorption and dissociation of H2O, the structure of Al13distorts in varying degrees.5. Investigation of surface reaction mechanism on Al surfaces:H20adsorption and dissociation on Al(111),(110) and (100) surfacesIt was found that H2O adsorbed preferentially at top sites. There are two steps for H2O decomposition, The reactivity of the Al planes for the first and the second steps are all Al(111)>Al(100)>A1(110). All reactions were found to have low energy barriers and to be highly exothermic. These reveal that the decomposition of H2O on Al surfaces is a readily process, especially on Al(111). By the energy decomposition scheme and Mulliken population analysis, both electronic and geometrical effects were found to affect the H2O dissociation reactions. Furthermore, the geometrical effect was obviously more important than the electronic effect for the dissociation of H2O(ad)→OH(ad)+H(ad) on the Al surfaces.6. Theoretical study of the gometries and dissociation energies of CO2on Al12XAdsorption energies, binding energies and barriers between the physic-and chemi-sorption states for CO2were determined. It was found that the kinds of doped atoms and spin states have important influences energies of adsorption and reaction between Al12X and a CO2molecule. CO2chemisorption on the AI12C cluster is energetically and kinetically unfavorable. CO2dissociation on the metallic doping Al12X (X=Fe, Ni, Cu) clusters have relatively low activation barriers, but large activation barriers for Al12X (X=B, C, Si, P). The barriers of CO2chemisorption and dissociation on the Al12Fe cluster are5.23kJ/mol and38.53kJ/mol, which are the lowest barriers among all the clusters discussed. The results showed that the carbon dioxide adsorption and dissociation on the Al12X cluster can be tuned by controllable X doping.7. Study of Adsorption and dissociation mechanism of NO and (NO)2dimer on Al(111),(110),(100) surfacesCalculations reveal that NO molecule adsorbed and dissociated easily on Al(111). cis-(NO)2adsorbed and dissociated easily on Al(111) and formed compounds with nitrogen, oxygen and aluminum elements. On the Al(110) and Al(100) surfaces, cis-(NO)2adsorbed easily and removed nitrogen and formed oxygen and aluminum compounds.