ELECTRONIC AND MOLECULAR-DYNAMICS OF CHEMISORPTION ON ALPHA-FE2O3 WITH TIME-DELAYED INJECTION OF DONOR-ACCEPTOR GASES

We have developed a new method which can examine the time response of the electrical conductivity of semi-conductive 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 alpha-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 alpha-Fe2O3 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.