118
N. Dhachapally et al. / Applied Catalysis A: General 443–444 (2012) 111–118
V/M ratio shows the highest activity and lowest selectivity and vice
versa. Otherwise, one should have in mind that AlVO4 reveals the
highest BET surface area (ca. 36 m2/g) whereas the one of BiVO4
is very small (ca. 1 m2/g). The near-surface enrichment of vana-
dium observed in the present study (in particular for Nb-, La-
and Bi-containing MVs) might be due to the formation of some
indicate that the enrichment of vanadium in this region depends
on the type of the metal used. High near-surface-region V/M ratios
enhance the selectivity to CP but decrease the conversion of MP
(in particular over Bi- and La-containing catalysts). In contrast, the
AlVO4 sample reveals a high catalytic activity but poor selectivity
to CP. But it shows larger proportions of crystalline V2O5, how-
ever, they are not present in the near-surface-region according to
the XPS studies, obviously. Moreover, it displays the highest sur-
face area of all solids studied. Anyway, this example shows that the
formation of crystalline V2O5 and the generation of high surface
areas have to be avoided. Among all, NbVO5 exhibits the supe-
rior performance; small proportions of crystalline V2O5 seem to be
beneficial. However, further detailed characterization studies have
to be conducted to find out additional effective surface structures
(e.g. by UV–vis or Raman spectroscopy under operando conditions)
positively influencing the catalytic behaviour.
V
5+-containing amorphous phases, non-stoichiometric V5+/V4+ or
V4+-containing phases. If it would be well crystalline V2O5, higher
conversion might be reached at much lower reaction temperature
and hence, the selectivity to CP should be lower. This can be clearly
seen with AlVO4 showing some well crystalline, larger V2O5 parti-
cles probably extending to deeper layers of the surface.
LaVO4 and BiVO4 showed well crystalline metal vanadate
phases (as detected from XRD). Thus, it seems logical that there is
no excess of easily removable lattice oxygen in these catalysts due
to absence of crystalline proportion of V2O5 and thereby V–O–V
linkages as well. The opposite is true in the case of AlVO4 solid
where crystalline V2O5 is present in excess. As a result, the LaVO4
and BiVO4 samples gave rather low conversion but high selectivity.
made up of isolated VO4 or small domains of VO4 units that are
separated from each other by MOx units. In such materials, most of
the lattice oxygen ions are located in the solid bridge between Vm+
and Mn+ ions and hence less availability of readily removable lat-
tice oxygen [34], which makes low combustion or total oxidation. In
order to obtain higher activity, i.e. X-MP, the energy input has to be
adjusted usually by higher reaction temperature for those catalysts.
This is the case with BiVO4 catalyst being a very crystalline sample
too, where maximum conversion reached only 77% despite a sig-
nificant increase in reaction temperature (i.e. 440 ◦C). In contrast,
the AlVO4 catalyst, which is expected to have highly available (eas-
ily removable) lattice oxygen, displayed complete conversion even
at low reaction temperature (<360 ◦C). However, Y-CP obtained on
this sample is very low while the total oxidation and by-products
formation is very high. On the whole, NbVO5 catalyst has shown
the best performance of this series and seems to be a good bal-
ance between catalytic activity and desired product selectivity; i.e.
moveable lattice oxygen, which resulted in both high conversion of
MP and high yield of CP. Furthermore, some indirect hints can also
be obtained if one carefully examines the surface areas of the cata-
lysts and the temperature required to obtain maximum conversion
of MP (Fig. 8). In other words, the catalysts having high surface areas
require low reaction temperatures compared to the ones having
low surface areas and vice versa. The trend of required temperature
to get high conversion is well correlated with the characterisation
data. The above results obviously indicate that the nature of metal
controls the structure and thereby activity and selectivity proper-
ties of the catalysts. Thus, the surface enrichment of vanadium with
near-surface-region V/M ratios higher than stoichiometry reveals
increasing selectivity to CP and decreasing activity showing the
following trend: AlVO4 < FeVO4 < CrVO4 < NbVO5 < LaVO4 < BiVO4.
Acknowledgements
Thanks are due to Drs. M. Schneider, U. Bentrup, J. Radnik, and
Mr. U. Marx of LIKAT for XRD, FTIR, XPS and ESR measurements.
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