The Platinum Catalysed Reduction of Nitric Oxide by Ammonia A SOLID ELECTROLYTE POTENTIOMETRY AIDED STUDY (original) (raw)

Abstract

The reduction of nitric oxide by ammonia on platinum catalysts has been investigated in the temperature range 300 to 400°C at reactant partial pressures between 0.05 and 5 mbar. During a combined potentiometric and kinetic study, a discontinuous change in both the reaction rates and the surface state has been observed at a partial pressure ratio for nitric 0xide:ammonia of 1.

Figures (7)

Results The reaction has been investigated in the tem- perature range 300 to 400°C. Measurements were made at a constant temperature, and with one reactant at a constant partial pressure of 0.5 mbar, while the partial pressure of the second reactant was increased sequentially from 0.05 to 5 mbar. All results were obtained under station- ary conditions. The stationary state was attained, by definition, when the variation in the gas phase concentrations was less than 1 per cent and the variation in the potential difference was less than 5 mV over a 30 minute period.

Results The reaction has been investigated in the tem- perature range 300 to 400°C. Measurements were made at a constant temperature, and with one reactant at a constant partial pressure of 0.5 mbar, while the partial pressure of the second reactant was increased sequentially from 0.05 to 5 mbar. All results were obtained under station- ary conditions. The stationary state was attained, by definition, when the variation in the gas phase concentrations was less than 1 per cent and the variation in the potential difference was less than 5 mV over a 30 minute period.

Typical results obtained during the nitric oxide reduction are shown in Figures 3 and 4. In the Figures the rates of nitrogen, r,', and nitrous oxide, r.’, formation, and the oxygen activity at the catalyst surface, aq’, are plotted against the nitric oxide partial pressure in Figure 3, and against

Typical results obtained during the nitric oxide reduction are shown in Figures 3 and 4. In the Figures the rates of nitrogen, r,', and nitrous oxide, r.’, formation, and the oxygen activity at the catalyst surface, aq’, are plotted against the nitric oxide partial pressure in Figure 3, and against

the ammonia partial pressure in Figure 4. The discontinuity of the reaction order in both the nitrogen and nitrous oxide formation rates has been confirmed by all results. It occurs at the same ratio of the nitric oxide to ammonia par- tial pressures, that is 1.5, and is independent of the fact that the measurements are made by vary- ing either the nitric oxide or the ammonia partial pressures. It can be seen from Figure 4 that the rates are only slightly affected by temperature in the domain examined; nitrous oxide formation is increased somewhat at lower temperatures. seen confirmed by all results. It occurs at the

the ammonia partial pressure in Figure 4. The discontinuity of the reaction order in both the nitrogen and nitrous oxide formation rates has been confirmed by all results. It occurs at the same ratio of the nitric oxide to ammonia par- tial pressures, that is 1.5, and is independent of the fact that the measurements are made by vary- ing either the nitric oxide or the ammonia partial pressures. It can be seen from Figure 4 that the rates are only slightly affected by temperature in the domain examined; nitrous oxide formation is increased somewhat at lower temperatures. seen confirmed by all results. It occurs at the

reducing atmosphere (Py,o:Pnu,<1.5), seen in Figure 5, are thus correlated to low values of the nitroie avide farmanon rate. ceen in Figure 4. We have verified that the measured reaction rates are not influenced by outer or inner mass transfer effects. The results are consistent, the same values being obtained when the partial pres- sures of nitric oxide and ammonia are each 0.5 mbar, independent of the experimental proce- dure, that is increasing the partial pressure of ammonia or of nitric oxide. The measurements are reproducible within the limits of experimen- tal precision. We have verified that the measured reaction

reducing atmosphere (Py,o:Pnu,<1.5), seen in Figure 5, are thus correlated to low values of the nitroie avide farmanon rate. ceen in Figure 4. We have verified that the measured reaction rates are not influenced by outer or inner mass transfer effects. The results are consistent, the same values being obtained when the partial pres- sures of nitric oxide and ammonia are each 0.5 mbar, independent of the experimental proce- dure, that is increasing the partial pressure of ammonia or of nitric oxide. The measurements are reproducible within the limits of experimen- tal precision. We have verified that the measured reaction

conditions of excess ammonia, is due to the fact that the nitrous oxide formation rate is low, or that the nitrous oxide reduction rate is very high? At the moment we cannot answer this question quantitatively, but we can get an indication by using the results obtained in the separate mea- surements of the nitrous oxide reduction. Let us assume, as a rough approximation, that the reduc- tion of nitrous oxide is not influenced by the presence of nitric oxide, that is, the reaction of the two nitrogen oxides can be superposed. Then we can calculate, at least at a constant ammonia partial pressure of 0.5 mbar, the rate of the nitrous oxide reduction (Reaction d) in the nitric oxide/ammonia system, and thus obtain the amount of nitrogen formed in step d, n'y,4. In Figure 8 the ratio of n'y,,4 and the total amount of nitrogen formed in the nitric oxide/ammonia system, f'n, totais 18 plotted as a function of the par- tial pressure of nitric oxide. It is clearly shown that under reducing conditions the nitrogen for- mation proceeds exclusively through the direct reduction of nitric oxide (path b in the Scheme), since ni'y,,4/N'n,, tow iS less than 0.1, whereas under oxidising conditions the nitrogen formation is merely due to paths b and d. Tt ic intereetine to correlate the qhcerved hehav- selectivity to nitrogen, while at higher values vig- orous nitrous oxide formation occurs, and between the two domains a discontinuous vari- ation of reaction order, selectivity and oxygen activity is observed. This may be explained by the postulation of two different states of the sur- face: if Pyo > 1.5 Pya the surface is covered by oxygen or an oxygen containing species, where- as it is in a reduced state if the reducing compound is predominant in the gas phase.

conditions of excess ammonia, is due to the fact that the nitrous oxide formation rate is low, or that the nitrous oxide reduction rate is very high? At the moment we cannot answer this question quantitatively, but we can get an indication by using the results obtained in the separate mea- surements of the nitrous oxide reduction. Let us assume, as a rough approximation, that the reduc- tion of nitrous oxide is not influenced by the presence of nitric oxide, that is, the reaction of the two nitrogen oxides can be superposed. Then we can calculate, at least at a constant ammonia partial pressure of 0.5 mbar, the rate of the nitrous oxide reduction (Reaction d) in the nitric oxide/ammonia system, and thus obtain the amount of nitrogen formed in step d, n'y,4. In Figure 8 the ratio of n'y,,4 and the total amount of nitrogen formed in the nitric oxide/ammonia system, f'n, totais 18 plotted as a function of the par- tial pressure of nitric oxide. It is clearly shown that under reducing conditions the nitrogen for- mation proceeds exclusively through the direct reduction of nitric oxide (path b in the Scheme), since ni'y,,4/N'n,, tow iS less than 0.1, whereas under oxidising conditions the nitrogen formation is merely due to paths b and d. Tt ic intereetine to correlate the qhcerved hehav- selectivity to nitrogen, while at higher values vig- orous nitrous oxide formation occurs, and between the two domains a discontinuous vari- ation of reaction order, selectivity and oxygen activity is observed. This may be explained by the postulation of two different states of the sur- face: if Pyo > 1.5 Pya the surface is covered by oxygen or an oxygen containing species, where- as it is in a reduced state if the reducing compound is predominant in the gas phase.

compound is predominant in the gas phase.

compound is predominant in the gas phase.

Loading...

Loading Preview

Sorry, preview is currently unavailable. You can download the paper by clicking the button above.

References (15)

  1. J. Ando, "Air Pollution by Nitrogen Oxides", ed. T. Schneider and L. Grant, Amsterdam, Elsevier, 1982, p. 699
  2. H. Bosch and F. J. J. G. Janssen, C d . T h , 1988, 2,369
  3. B. Harrison, A. F. Diwell and M. Wyatt, Platinum Metals Rev., 1985,29, (Z), 50
  4. H. C. Andersen, W. J. Green and D. R. Steele, I d Eng. Chem., 1961,53,199
  5. H.-G. Lintz and C. G. Vayenas, Angm. Chem. Int. Ed. Engl., 1989, 28, 708
  6. E. Hafele and H.-G. Lintz, Ber. Bunsenges. Phys. Chem., 1988,92, 188 324
  7. C. G. Takoudis and L. D. Schmidt, J. Phys. Chem.,
  8. T. Katona, L. Guczi and A. Somorjai, J. Curd.,
  9. R. J. Pusateri, J. R. Katzer and W. H. Manogue,
  10. M. Markvart and V. Pour,J. Cutul., 1967,7,297
  11. G. L. Bauerle, S. C. W u and K. Nobe, I d . Eng.
  12. J. Blanco and P. Avila, An. Quim., 1984,80,645
  13. J. L. Gland and V. N. Korchak,J. C u d . , 1978,55, 1983,87,958 1991,132,440
  14. AICM J., 1974, 20, 219
  15. Chem., 1975,14, 123