386
W. Piskorz and F. Zasada
NO x Catalytic Reduction by NH 3
The molecular aspects of vanadia-based catalysts performance in selective reduction
of NO by NH 3 were also studied by quantum-chemical modelling. Using V 2 O 9 H
model cluster, Gilardoni et al. postulated that the reaction is initiated by NH 3 adsorption on a Brønsted site with formation of NH
+
4 surface cation [457]. Afterwards,
gaseous NO reacts with such activated ammonia yielding NH 2 NO intermediate and
then the reaction products N 2 and H 2 O. The computational results agree with isotopic
labelling studies giving support to a dual-site Eley-Rideal-type mechanism. Quite
similar conclusions were reported by Soyer et al. following their V 2 O 9 H 8 cluster
simulations performed at B3LYP/6–31G** level [458].
The influence of vanadia loading in V 2 O 5 /TiO 2 catalyst on NH 3 activation was
studied theoretically by Anstrom et al. [459] with conclusion that vanadia species
formed at low loadings (monomeric clusters) are less active than oligomeric clusters
expected for higher V 2 O 5 coverages. It was reasoned by the fact that Brønsted acid
sites may be formed by the transfer of hydrogen atoms from Ti–OH groups to vanadia
oxide oligomeric moieties, and that NH 3 adsorption is facilitated by presence of
neighbouring vanadyl V=O bonds. The adsorption of NO on pure and Mo defected
(001)-V 2 O 5 surface (in both pristine and reduced state) was also studied [460]. The
detailed analysis of electronic structure revealed that substituting a V atom with Mo
leads to a reduction of the adjacent metal atom, practically without changing the
electronic properties of the oxygen atoms. Furthermore, it was shown that the NO
adsorption on the reduced surfaces of V 2 O 5 and V 2−x Mo x O 5 is exothermic and that
the effect of the molybdenum presence for energetic stabilisation of adsorbed NO is
not distinct. The role of Brønsted acidic centres in NH 3 activation on other low-index
vanadium pentoxide surfaces was also studied [461]. It was shown that in all cases,
the hydrogen bonds play a major role in this process and that the active sites for
ammonia activation in the SCR mechanism are localised not only at saturated (010)
sites but also at the unsaturated (001) and (100) V 2 O 5 surfaces.
Based on the earlier work on adsorption, diffusion, and reaction of the different
surface species, Gruber [462] and Hermann proposed detailed theoretical description
of the elementary steps of the catalytic NO x reduction with NH 3 [462]. In this paper,
geometric and energetic details and reaction paths at Brønsted sites of the perfect
surface as well as at Lewis sites of the reduced surface were evaluated using extended
cluster models. The results suggest that Brønsted and Lewis reaction mechanisms
differ in many aspects such as overall barrier energy, number of elementary steps,
and reaction intermediates.
The DFT calculations (B3LYP) were also used to complement experimental findings for the dehydrogenation of CH 3 OH mediated by VO
+ [463]. The reaction stationary points for both triplet and singlet potential energy surface (PES) have been
considered, with conclusion that initial C–H bond activation is the rate determining step in both cases. Interestingly, the reported mechanism cannot be considered
as an VO-induced rearrangement of a CH 3 OH cation [464], because an electron
transfer in the adduct (CH 3 OH)VO
+ from the methanol moiety to VO
+ does not
occur to a notable extent. The mechanism of the CH 3 OH to CH 2 O conversion on
W. Piskorz and F. Zasada
NO x Catalytic Reduction by NH 3
The molecular aspects of vanadia-based catalysts performance in selective reduction
of NO by NH 3 were also studied by quantum-chemical modelling. Using V 2 O 9 H
model cluster, Gilardoni et al. postulated that the reaction is initiated by NH 3 adsorption on a Brønsted site with formation of NH
+
4 surface cation [457]. Afterwards,
gaseous NO reacts with such activated ammonia yielding NH 2 NO intermediate and
then the reaction products N 2 and H 2 O. The computational results agree with isotopic
labelling studies giving support to a dual-site Eley-Rideal-type mechanism. Quite
similar conclusions were reported by Soyer et al. following their V 2 O 9 H 8 cluster
simulations performed at B3LYP/6–31G** level [458].
The influence of vanadia loading in V 2 O 5 /TiO 2 catalyst on NH 3 activation was
studied theoretically by Anstrom et al. [459] with conclusion that vanadia species
formed at low loadings (monomeric clusters) are less active than oligomeric clusters
expected for higher V 2 O 5 coverages. It was reasoned by the fact that Brønsted acid
sites may be formed by the transfer of hydrogen atoms from Ti–OH groups to vanadia
oxide oligomeric moieties, and that NH 3 adsorption is facilitated by presence of
neighbouring vanadyl V=O bonds. The adsorption of NO on pure and Mo defected
(001)-V 2 O 5 surface (in both pristine and reduced state) was also studied [460]. The
detailed analysis of electronic structure revealed that substituting a V atom with Mo
leads to a reduction of the adjacent metal atom, practically without changing the
electronic properties of the oxygen atoms. Furthermore, it was shown that the NO
adsorption on the reduced surfaces of V 2 O 5 and V 2−x Mo x O 5 is exothermic and that
the effect of the molybdenum presence for energetic stabilisation of adsorbed NO is
not distinct. The role of Brønsted acidic centres in NH 3 activation on other low-index
vanadium pentoxide surfaces was also studied [461]. It was shown that in all cases,
the hydrogen bonds play a major role in this process and that the active sites for
ammonia activation in the SCR mechanism are localised not only at saturated (010)
sites but also at the unsaturated (001) and (100) V 2 O 5 surfaces.
Based on the earlier work on adsorption, diffusion, and reaction of the different
surface species, Gruber [462] and Hermann proposed detailed theoretical description
of the elementary steps of the catalytic NO x reduction with NH 3 [462]. In this paper,
geometric and energetic details and reaction paths at Brønsted sites of the perfect
surface as well as at Lewis sites of the reduced surface were evaluated using extended
cluster models. The results suggest that Brønsted and Lewis reaction mechanisms
differ in many aspects such as overall barrier energy, number of elementary steps,
and reaction intermediates.
The DFT calculations (B3LYP) were also used to complement experimental findings for the dehydrogenation of CH 3 OH mediated by VO
+ [463]. The reaction stationary points for both triplet and singlet potential energy surface (PES) have been
considered, with conclusion that initial C–H bond activation is the rate determining step in both cases. Interestingly, the reported mechanism cannot be considered
as an VO-induced rearrangement of a CH 3 OH cation [464], because an electron
transfer in the adduct (CH 3 OH)VO
+ from the methanol moiety to VO
+ does not
occur to a notable extent. The mechanism of the CH 3 OH to CH 2 O conversion on