On the underlying mechanisms of the low observed nitrate selectivity in photocatalytic NO_xabatement and the importance of the oxygen reduction reaction

Abstract

Semiconductor photocatalysis could be an effective means to combat air pollution, especially nitrogen oxides, which can be mineralized to nitrate. However, the reaction typically shows poor selectivity, releasing a number of unwanted and possibly toxic intermediates such as nitrogen dioxide. Up to now, the underlying principles that lead to this poor selectivity were not understood so a knowledge-based catalyst design for more selective materials was impossible. Herein, we present strong evidence for the slow oxygen reduction being one the causes, as the competing back-reduction of nitrate leads to the release of nitrogen dioxide. Consequently, engineering the photocatalyst for a better oxygen reduction efficiency should also increase the nitrate selectivity.

Grafted iron(iii) ions significantly enhance NO2 oxidation rate and selectivity of TiO2 for photocatalytic NOx abatement

“…Also displayed are the best fits to the previously published model obtained using eqn (24) and non-linear regression as dotted lines. 22 The onset of this effect can even be seen for the freshly prepared catalyst which accumulate traces of nitrate during the test, which is already sufficient to slightly deviate from perfect linear behavior (Fig. 8, blue triangles).…”

“…† This case was analyzed using Langmuir-Hinshelwood (L-H) kinetics and all other materials were analyzed according to the simplied rstorder kinetics (which in the cases yields the same rst-order rate constants as when using L-H kinetics) to calculate the respective rate constants using eqn (13). 22,31,32 As summarized in Table 1, the rst-order rate constants for NO oxidation are virtually unaffected by both the washing procedure and the iron graing and range from 1.6 to 1.8 s À1 . Only at the highest studied iron loadings of $0.1 at%, the rate constant notably decreases.…”

“…This illustrates the robustness of the catalyst towards increasing nitrate poisoning and is a measure for the long-term performance of the materials. 22 The nitrate surface coverage can be calculated according to eqn (16) with the volume ux ( _ V ), pressure (p), Avogadro's constant (N A ), gas constant (R), absolute temperature (T), mass of catalyst (m), its specic surface area (S A ) and the maximum nitrate surface coverage (q max ¼ 2 nm À2 ). 22,42 qðtÞ ¼…”

“…13,[17][18][19][20][21] Both the poor kinetics of NO 2 oxidation as well as the backreaction of already formed nitrate have been identied as causes for the low selectivity. 22,23 As has already been outlined in one of our earlier publications, 22 the introduction of oxygen reduction enhancing co-catalysts onto the surface of TiO 2 should prevent re-reduction of already formed nitrate and thereby increase the observed selectivity. 22 This effect should be especially signicant at high nitrate coverages, i.e., aer longer times of operation.…”

“…22,23 As has already been outlined in one of our earlier publications, 22 the introduction of oxygen reduction enhancing co-catalysts onto the surface of TiO 2 should prevent re-reduction of already formed nitrate and thereby increase the observed selectivity. 22 This effect should be especially signicant at high nitrate coverages, i.e., aer longer times of operation. One method to achieve this is by graing small concentrations of metal ions such as Fe or Cu onto the titanium dioxide.…”