Effect of chemical composition of isomorphous metavanadates on their catalytic activity toward carbon combustion

Article abstract of DOI:10.1016/S0025-5408(99)00077-X

Metal vanadates of K, Rb, and Cs and their solid solutions were prepared by reaction between carbonates and vanadium(V) oxide, characterized by X-ray diffraction and tested as catalysts for carbon combustion. These vanadates are all orthorhombic but show different lattice parameters depending on the ionic radius of alkali metals. A complete solubility in the solid state was found to exist for the systems KVO₃-RbVO₃ and RbVO₃-CsVO₃, while only terminal solid solutions were found in the KVO₃-CsVO₃ system. The lattice parameters (mainly b₀) of the orthorhombic cell increased with the increase of the ionic radius of the alkali metal. This increase was found to be closely linked to the substitution of a larger metal for a smaller one when a complete solubility occurs. The catalytic activity, investigated by temperature programmed oxidation (TPO) experiments, improves progressively along with the introduction of a more electropositive and larger alkaline metal in the vanadate crystal structure. The catalytic performance, however, does not seem to be dependent on the crystal structure because, due to polymorphic transformations, not all these vanadates keep the orthorhombic structure in the temperature range suitable for the catalytic carbon combustion (350-600 °C). The catalytic activity of these vanadates towards carbon combustion thus seems to be strictly related to their chemical composition only.

Full text of DOI:10.1016/S0025-5408(99)00077-X

Materials Research Bulletin, Vol. 34, No. 6, pp. 851–862, 1999
Copyright © 1999 Elsevier Science Ltd
Printed in the USA. All rights reserved
0025-5408/99/$–see front matter
PII S0025-5408(99)00077-X
EFFECT OF CHEMICAL COMPOSITION OF ISOMORPHOUS
METAVANADATES ON THEIR CATALYTIC ACTIVITY
TOWARD CARBON COMBUSTION
C. Badini, D. Mazza, S. Ronchetti*, and G. Saracco
Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino,
Corso Duca degli Abruzzi 24, 10129 Torino, Italy
(Refereed)
(Received March 20, 1998; Accepted August 21, 1998)
ABSTRACT
Metal vanadates of K, Rb, and Cs and their solid solutions were prepared by
reaction between carbonates and vanadium(V) oxide, characterized by X-ray
diffraction and tested as catalysts for carbon combustion. These vanadates are
all orthorhombic but show different lattice parameters depending on the ionic
radius of alkali metals. A complete solubility in the solid state was found to
exist for the systems KVO3–RbVO3 and RbVO3–CsVO3, while only terminal
solid solutions were found in the KVO3–CsVO3 system. The lattice parame-
ters (mainly b0) of the orthorhombic cell increased with the increase of the
ionic radius of the alkali metal. This increase was found to be closely linked
to the substitution of a larger metal for a smaller one when a complete
solubility occurs. The catalytic activity, investigated by temperature pro-
grammed oxidation (TPO) experiments, improves progressively along with
the introduction of a more electropositive and larger alkaline metal in the
vanadate crystal structure. The catalytic performance, however, does not seem
to be dependent on the crystal structure because, due to polymorphic trans-
formations, not all these vanadates keep the orthorhombic structure in the
temperature range suitable for the catalytic carbon combustion (350–600°C).
The catalytic activity of these vanadates towards carbon combustion thus
seems to be strictly related to their chemical composition only. © 1999 Elsevier
Science Ltd
*To whom correspondence should be addressed.
851

852
C. BADINI et al.
Vol. 34, No. 6
KEYWORDS: A. inorganic compounds, B. chemical synthesis, C. X-ray
diffraction, D. crystal structure, D. catalytic properties
INTRODUCTION
Metal vanadates (mostly metavanadates) are often used in the formulation of more or less
complex catalysts that in recent years have been proposed for the low-temperature combus-
tion of carbonaceous materials. The crystal structure of several metavanadates has been
investigated extensively [1–3], but only poor information can be found about the formation
of solid solutions among these compounds [4] and the relationship between the catalytic
activity and their crystal structures. The above-mentioned catalysts have been developed
mainly with the aim of reducing the particulate content in diesel emission by burning it at
temperatures typical of diesel exhausts (250–400°C). In fact, the progressive tightening of
standards adopted by the major industrial countries for this pollutant has greatly enhanced the
practical interest in this research field. For instance, the European limits for diesel soot are
0.08 and 0.04 g/km, starting in 1996 and 2000, respectively. Because it appears it will be very
difficult to accomplish the year 2000 standards by improving only the engine design and fuel
formulation, a catalytic trap suitable for the continuous filtration and combustion of soot
should be developed. To date, one of the more effective catalysts proposed for this purpose
is a mixture of compounds containing K, Cu, V, Cl, and O that is able to lower the particulate
ignition temperature to 380°C [5–8]. Its catalytic activity is probably due to the synergetic
effect of several chemical compounds (mostly metal vanadates and chlorides).
In a recent paper [9], the high activity of catalysts based on metavanadates of K, Rb, and
Cs was pointed out. In the present work, the crystalline structure of some metavanadates of
alkali metals and of the solid solutions among these pure compounds has been reviewed.
Their catalytic activity towards carbon combustion has been studied.
EXPERIMENTAL
Pure metavanadates of alkali metals were prepared by melting together M2CO3 (M ϭ K, Rb,
Cs) and V2O5, according to the reaction
M2CO3 ϩ V2O5 3 2 MVO3 ϩ CO2
These samples were annealed for 24 h at 400°C to obtain well-crystallized specimens suitable
for X-ray diffraction (XRD) analysis. In order to prepare solid solutions among these
vanadates, the pure compounds were mixed in suitable proportions, melted, and annealed at
400°C after solidification. Solid solutions of composition M1ϪxMЈxVO3 (with x ϭ 0.25, 0.5,
0.75) were prepared, where M and MЈ are different K, Rb, Cs metals.
Powders of all the specimens were submitted to X-ray diffraction analysis using a PW1710
Philips diffractometer equipped with a monochromator on the diffracted beam (Cu K␣
radiation). The X-ray diffraction patterns were processed to calculate the lattice parameters
of the phases present in each sample with standard least-squares procedures. The program
used [10] allows contemporary refinement of the orthorhombic unit cell and of zero-theta
correction. Silicon was used as external standard. The metavanadates were also characterized
by differential thermal analysis (DTA) under air, at a scanning rate of 10°C/min, in order to
assess their melting points and the possibility of polymorphic transformations.

Vol. 34, No. 6
VANADATE CATALYSTS
853
TABLE 1
Lattice Parameters (Å) of Orthorhombic Alkali Metal Vanadates
Lattice parameters
Vanadate
a0
b0
c0
KVO3
RbVO3
CsVO3
5.178
5.271
5.397
10.801
11.446
12.279
5.688
5.723
5.801
The catalytic activity of these samples towards the combustion of amorphous carbon was
studied by temperature programmed oxidation tests (TPO) using the experimental method
detailed in a previous paper [11]. The samples for TPO experiments were prepared by mixing
accurately in a mortar the vanadate with amorphous carbon (average particle size: 45 nm;
0.34 wt% ashes after calcination at 800°C; 12.2 wt% of moisture lost after drying at 110°C)
and 0.3–0.7 mm inert silica pellets (weight ratio 9:1:15). Each sample was inserted into a
quartz reactor and submitted to a linear rise of temperature (10°C/min) up to 700°C under air
flow (100 ml/min). The flowing gas was analyzed at the exit of the reactor by a CO2 IR
analyzer (Hartmann–Braun URAS 10E). A computer-controlled data acquisition system was
employed to collect the carbon dioxide concentration data during the temperature rise. The
temperature corresponding to the maximum carbon dioxide concentration in the outlet gas
flow was used as the main parameter to evaluate the catalyst activity.
RESULTS AND DISCUSSION
Crystal Structures of Metavanadates and Their Solid Solutions. The lattice parameters,
as calculated for the orthorhombic KVO3, RbVO3, and CsVO3 compounds, are collected in
Table 1. The XRD diffraction patterns of a pure MVO3 vanadate progressively changes when
the alkali metal M is substituted by a different one (MЈ) (Figs. 1 and 2). The results show that
a complete solubility exists between potassium and rubidium vanadates as well as between
rubidium and cesium vanadates.
The variations of the lattice parameters with the composition of the solid solutions are
shown in Figure 3 for the solid solutions prepared from KVO3 and RbVO3 and in Figure 4
for those prepared from RbVO3 and CsVO3. With the substitution of one alkali metal by a
larger one, the b0 parameter appreciably increased, while only slight increases were observed
for a0 and c0. In contrast, it was observed that only a partial solubility between the vanadates
KVO3 and CsVO3 was possible, likely due to the higher difference between the ionic radii
of potassium and cesium. XRD patterns (Fig. 5) showed that there was a partial solubility of
potassium in cesium vanadate up to the composition Cs0.75K0.25VO3, while the substitution
of cesium for potassium in KVO3 caused the formation of a new phase starting from the
composition Cs0.15K0.85VO3.
The K–Rb and Rb–Cs substitutions markedly influence the b0 unit-cell edge when a
complete solubility between two vanadates occurs. This structural evolution can be discussed
on the basis of literature data [1]. The crystal structure of these vanadates (orthorhombic,
space group Pbcm) is characterized by parallel infinite chains, based on tetrahedral VO4 units
sharing corners, which extend along the c-direction. Between these chains, the alkali metal

854
C. BADINI et al.
Vol. 34, No. 6
FIG. 1
XRD patterns of K1ϪxRbxVO3 vanadates (x ϭ 0, 0.25, 0.5, 0.75, 1).
FIG. 2
XRD patterns of Rb1ϪxCsxVO3 vanadates (x ϭ 0, 0.25, 0.5, 0.75, 1).

Vol. 34, No. 6
VANADATE CATALYSTS
855
FIG. 3
Variation of lattice parameters for the solid solutions K1ϪxRbxVO3 (x ϭ 0, 0.25, 0.50, 0.75,
1.00).
cations lie with eightfold coordination with oxygen; these are linked with eightfold coordi-
nation polyhedra by sharing edges, to form octahedral unit chains.
The spacing between faced chains of the tetrahedral units increases along with the mean
ionic radius of the alkali metal. On the contrary, the different size of the cation scarcely
affects the a0 and c0 parameter values. According to Hawthorne and Calvo [1], the c-axis
expansion is not due to expansion of the V–O bond length, but results from the angular
distortion of the bond angles in the tetrahedral VO4 units.
Catalytic Activity. The catalytic activity of the different vanadates was evaluated by
comparing the temperature (Tp) at which the maximum concentration of carbon dioxide was
reached in the outlet gases of the TPO equipment. This temperature corresponds to the
temperature at which the catalyst under investigation promoted the maximum carbon com-
bustion rate. It was also possible to calculate, on the basis of the measured CO2 percent in

856
C. BADINI et al.
Vol. 34, No. 6
FIG. 4
Variation of lattice parameters for the solid solutions Rb1ϪxCsxVO3 (x ϭ 0, 0.25, 0.50, 0.75,
1.00).
the gaseous atmosphere and the gas flow rate, the amount of carbon dioxide produced during
the overall carbon combustion and compare it with the theoretical one corresponding to the
complete carbon burning, the balance being CO. This allowed for evaluation of the CO2 yield
during each TPO run.
The Tp values and the CO2 yields obtained are plotted in Figures 6 and 7, from which the
effect of the composition variations on the activity of the catalysts with formulae
K1ϪxRbxVO3 and Rb1ϪxCsxVO3 (with x ϭ 0, 0.25, 0.5, 0.75, 1) can be appreciated. A TPO
run carried out in the absence of catalyst gave the following reference results: Tp ϭ 635°C;
CO2 yield ϭ 38.5%.
It is quite clear that the activity of these metavanadates progressively improved with the
increase in content of the more electropositive alkali metal. For a solid solution
M1ϪxMЈxVO3 (where MЈ was less electronegative than M), the peak combustion temperature
decreased and the CO2 yield increased with the content of MЈ. Among the pure vanadates

Vol. 34, No. 6
VANADATE CATALYSTS
857
FIG. 5
XRD patterns of samples obtained by reaction between KVO3 and CsVO3 in different ratios.
FIG. 6
Peak temperature (°C) (ᮀ) and CO2 yield (■) in TPO runs for reaction catalyzed by
K1ϪxRbxVO3 vanadates (x ϭ 0, 0.25, 0.50, 0.75, 1.00).

858
C. BADINI et al.
Vol. 34, No. 6
FIG. 7
Peak temperature (°C) (ᮀ) and CO2 yield (■) in TPO runs for reaction catalyzed by
Rb1ϪxCsxVO3 vanadates (x ϭ 0, 0.25, 0.50, 0.75, 1.00).
(containing only a single alkali metal), the following order for the activity was assessed:
CsVO3 Ͼ RbVO3 Ͼ KVO3.
The samples obtained by reaction between KVO3 and CsVO3, which, according to X-ray
evidence, were heterogeneous, showed a different behavior with respect to the above-
mentioned solid solutions. For compositions ranging between K0.25Cs0.75VO3 and
K0.75Cs0.25VO3, the catalyst samples were polyphasic and their activity seemed better (at
least on the basis of the Tp value) than those of pure potassium or cesium vanadate (Fig. 8).
For this reason, the phases present in these samples should be identified, and further
investigations are in progress on this topic.
Effect of Composition and Structure on the Catalytic Activity of Vanadates. The
catalytic activity of alkali metals and their compounds in the carbonaceous material com-
bustion has been well assessed and generally attributed to the chemisorption of oxygen
(because of the low electronegativity of these elements) and/or to the oxidation-reduction
cycles involving the formation of oxides and peroxides [12,13]. When alkali-metal com-
pounds are coupled with oxides of transition metals, in the formulation of catalysts, their
electron-donating effect has been proved to increase the reactivity of the oxygen in the M–O
bonds. Particularly in the partial oxidation of hydrocarbons, the doping of molybdena
catalysts with small quantities of alkali leads to an increase in the rate of reaction in the order
K Ͻ Rb Ͻ Cs [14], which is exactly the same activity order found for the catalysts

Vol. 34, No. 6
VANADATE CATALYSTS
859
FIG. 8
Peak temperature (°C) (ᮀ) and CO2 yield (■) in TPO runs for carbon combustion catalyzed
by catalysts obtained through the reaction between KVO3 and CsVO3 in the molar ratio
(1Ϫx):x (x ϭ 0, 0.25, 0.50, 0.75, 1.00).
investigated in this work. This weakening effect of alkali metals on the V–O bonds might be
checked and, perhaps, rationalized by performing redox cycles on the studied materials.
The catalytic activity of alkali metal compounds, however, is not only dependent on the
nature of alkali metal, but also on the anion associated to the metal cation [15]. For instance,
both KCl and KVO3 are suitable to lower the ignition temperature of carbon, but the vanadate
shows a higher activity (ignition temperature of 585°C for KCl [9] and of 500°C for KVO3).
Furthermore, the activity of vanadates is not merely attributable to the presence of alkali
metals in their structure. The fact is that vanadates of copper (hexagonal) and silver
(monoclinic), as well as vanadium(V) oxide itself, are well known to display an appreciable
catalytic activity in carbon combustion [11,16,17]. This feature can be ascribed to the
possibility of vanadium to change its valence, thus setting oxygen free to take part in an
oxidation reaction (17). The same holds for other transition-metal salts employed as catalysts
for diesel soot combustion (e.g., molybdates [18]). Actually, in the case of the compound
CuVO3, both copper and vanadium show a valence fluctuating between I and II (for Cu) and
IV and V (for V) [2]. The fluctuation of valences can also result in a change of oxygen
content in the vanadate. Raveau [4] assessed that copper vanadate is nonstoichiometric,
showing limit compositions CuVO3 and CuVO2.8
.
The hypothesis that vanadates can act as oxygen pumps combining and delivering oxygen
through a change of vanadium valence is further strengthened by experiments carried out by
Ciambelli et al. [7] on the Cu–K–V–Cl catalyst. This catalyst (which mainly contains

860
C. BADINI et al.
Vol. 34, No. 6
TABLE 2
Powder X-ray Diffraction Pattern of
CsVO3 at 450°C
dobs
I/I0
4.7881
4.3236
4.0865
3.8692
3.7014
3.3918
3.2624
3.0404
2.7407
2.5446
2.4374
2.3794
16
100
48
31
13
34
16
18
16
19
12
11
metavanadates) was found to deliver the oxygen needed for the oxidation of carbonaceous
materials, even though the reaction was performed under inert atmosphere; furthermore, the
oxygen content was restored after a subsequent exposure to air. In the case of the alkaline
metavanadates studied in this work, only a valence fluctuation of vanadium could change
FIG. 9
DTA thermogram of CsVO3.

Vol. 34, No. 6
VANADATE CATALYSTS
861
TABLE 3
Powder X-ray Diffraction Pattern of CsVO3 at 25°C
dcalc
dobs
h,k,l
I/I0
4.060
3.770
3.330
3.070
2.905
2.710
2.670
2.406
2.110
1.916
4.051
3.757
3.320
3.065
2.900
2.713
2.668
2.399
2.334
1.912
120
111
121
040
002
041, 200
140
211
042
240, 160
11
17
100
26
21
30
17
16
14
12
oxygen stoichiometry, due to the fixed valence of alkaline metals. This change of valence
took place only during the reaction, because no modification of the XRD patterns of the
catalysts was observed after repeated employment in several combustion processes. This
further supports the fact that vanadates act as redox oxygen pumps.
The catalytic behavior of these phases is closely related to their chemical composition. For
the solid solutions KVO3–RbVO3 and RbVO3–CsVO3, a continuous increase in both cata-
lytic activity and lattice parameters with the content of the more electropositive alkali metal
was observed.
An influence of the lattice parameter variation (mainly consisting of an increase of the
distance among the chains of VOϪ4 units with the increase of the alkali atom size) on the
catalytic performance might be hypothesized; however, some experimental results show
otherwise. For instance, DTA runs showed that RbVO3 and CsVO3 undergo an endothermal
solid-state transformation at 530 and 416°C (Fig. 9), respectively, that is, below their melting
points (567°C for RbVO3 and 641°C for CsVO3). In the case of RbVO3, this transformation
occurred well above the TPO combustion peak, while CsVO3 experienced its transformation
just at the temperature corresponding to the combustion peak maximum. The X-ray diffrac-
tion pattern for CsVO3 obtained at 450°C showed that this compound changes its crystalline
structure with respect to the orthorhombic one, stable at room temperature (Tables 2 and 3).
Both polymorphic solid structures of CsVO3, therefore, exhibit appreciable catalytic activity.
Further, at the TPO peak temperature (Tp), when the catalysts reach their highest activity,
the vanadates are all solid. KVO3, however, melts just above Tp (Tp ϭ 500°C; Tm ϭ 520°C).
After melting, the KVO3 catalyst promotes the combustion of the residual part of carbon as
well. It can therefore be concluded that this vanadate in a liquid state also shows a catalytic
activity.
CONCLUSIONS
Orthorhombic metavanadates of alkali metals (K, Rb, Cs) were found to be isostructural at
room temperature. The monophasic solid solutions KxRb1ϪxVO3 and RbxCs1ϪxVO3 are
orthorhombic and stable in the whole composition range. Their lattice parameters change

862
C. BADINI et al.
Vol. 34, No. 6
linearly along the chemical composition. In contrast, a wide polyphasic field and only a
narrow range of solubility were observed in the KVO3–CsVO3 system.
All of these vanadates are good catalysts for the carbon combustion reaction, since they
appreciably lower the ignition temperature of carbon. Their catalytic activity improves with
the electropositivity of the alkali metal contained in the metavanadate. This catalytic activity
is strictly linked to the vanadate chemical composition and depends on the presence of alkali
metals and V–O bonds. In contrast, a correlation between catalytic activity and crystal
structure is less significant; in fact, some of these vanadates also demonstrate catalytic
activity after melting (KVO3) or after a polymorphic transformation that gives rise to a
crystalline structure different from the room-temperature one (CsVO3).
REFERENCES
1. F.C. Hawthorne and C. Calvo, J. Solid State Chem. 22, 157 (1977).
2. M. Nakamura, H. Namatame, A. Fujimori, A. Misu, S. Okatabe, M. Onoda, and H. Nagasawa, J.
Solid State Chem. 112, 100 (1994).
3. J.R. Rea and E. Kostiner, J. Solid State Chem. 7, 163 (1973).
4. B. Raveau, Rev. Chim. Miner. 4, 729 (1967).
5. Y. Watabe, K. Yrako, T. Miyajima, T. Yoshimoto, and Y. Murakami, SAE Paper 830082 (1983).
6. P. Ciambelli, P. Corbo, M. Scialo`, and S. Vaccaro, Italian Patent No. 1,221,416, 1988.
7. P. Ciambelli, V. Palma, and S. Vaccaro, Catal. Today 17, 71 (1993).
8. C. Badini, V. Serra, G. Saracco, and M. Montorsi, Catal. Lett. 37, 247 (1996).
9. G. Saracco, V. Serra, C. Badini, and V. Specchia, Ind. Eng. Chem. Res. 36, 2051 (1997).
10. D. Mazza, La diffrazione dei raggi X dai materiali policristallini, C.L.U.T, Torino (1996).
11. V. Serra, G. Saracco, C. Badini, and V. Specchia, Appl. Catal., B: Environ. 11, 329 (1997).
12. B. Krutzsch and G. Wenninger, SAE Paper 922188 (1992).
13. D.W. McKee, Fuel 62, 170 (1983).
14. W.D. Mross, Catal. Rev.-Sci. Eng. 25, 591 (1983).
15. D.W. McKee and D. Chatterji, Carbon 13, 381 (1975).
16. A.F. Ahlstrom, G.U.I. Odenbrand, Appl. Catal. 60, 143 (1990).
17. D.W. McKee, Carbon 8, 623 (1970).
18. G. Mul, J.P.A. Neeft, F. Kapteijn, M. Makkee, and J.A. Moulijn, Appl. Catal., B: Environ. 6, 339
(1995).