Electronic and molecular dynamics of chemisorption on α-Fe2O3 with time-delayed injection of donor–acceptor gases

Abstract

We have developed a new method which can examine the time response of the electrical conductivity of semiconductive oxides and the total gas pressure upon successive admission of electron donor and acceptor gases with a time delay of injection of 1–999 ms. Synthetic microporous α-Fe₂O₃ of n-type character was used as a semiconductor. First, an electron donor gas (CO₂ or NH₃) was injected, then an electron acceptor gas (NO) was injected after a time delay at 303 K. Chemisorption of CO₂ or NH₃ increased the conductivity a little, while chemisorption of NO lowered the conductivity markedly. Effects of prechemisorption of the donor molecule on the conductivity decrease due to charge transfer from α-Fe₂ O₃ to NO were examined kinetically with the aid of the time-delay injection method. The decrease in electrical conductivity due to NO chemisorption in the presence of the preadsorbed CO₂ is much faster than that in the absence of CO₂ and is dependent on the time delay of the coverage of preadsorbed CO₂ for coverages of less than 0.005. This behaviour is assumed to be caused by a lowering of the electronic barrier height in the space charge layer due to the prechemisorbed CO₂. In the case of NH₃ prechemisorption, the rate and amount of the electrical conductivity drop on admission of NO is diminished as a consequence of preadsorption of NH₃. Preadsorption of NH₃ to a coverage of 0.07 at 1 s blocks the postadsorption of NO, inducing a lower NO chemisorption rate.