Enhancement of acrylic acid yields in propane and propylene oxidation by selective P Doping of MoV(Nb)TeO-based M1 and M2 catalysts

Article abstract of DOI:10.1016/j.cattod.2010.01.044

The selective doping of M1 and M2 phases for the selective oxidation of propane and propylene to acrylic acid (AA) was investigated. A series of catalytic materials for the oxidation of propane were prepared with the general precursor composition Mo₁V_0.31-Te_0.37Nb _xP_yO_n. A first solution was prepared by dissolving ammonium heptamolybdate, ammonium vanadate, and telluric acid in water at 60°C. This solution was allowed to cool to room temperature. A second solution was prepared by dissolving niobic acid in oxalic acid at 60°C. The appropriate amount of the first solution, second solution, and phosphoric acid were mixed and then dried by rotary evaporation. Each catalyst was tested at four different space velocities, starting with the highest, at several reaction temperatures starting at the lowest temperature. Doping of crystalline M1 and M2 phases with P in selective oxidation of propane or propylene enhances significantly the desired AA yields at commercially relevant high hydrocarbon conversions.

Full text of DOI:10.1016/j.cattod.2010.01.044

Catalysis Today 157 (2010) 33–38
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Enhancement of acrylic acid yields in propane and propylene oxidation by
selective P Doping of MoV(Nb)TeO-based M1 and M2 catalysts
R.K. Grasselli a, ,b, C.G. Lugmair c, A.F. Volpe Jr.c, A. Andersson d, J.D. Burrington e
*
a Center for Catalytic Science and Technology, University of Delaware, Newark, DE 19716, USA
b Department of Chemistry, Technische Universita¨t Mu¨nchen, D-85748 Garching, Germany
c Symyx Technologies Inc., 3100 Central Expressway, Santa Clara, CA 95051, USA
d Department of Chemical Engineering, Lund University, Chemical Center, P.O. Box 124, SE-22100 Lund, Sweden
e The Lubrizol Corporation, 29400 Lakeland Boulevard, Wickliffe, OH 44092, USA
Our current study concentrates on the selective doping of
M1and M2 phases for the selective oxidation of propane and
propylene to acrylic acid. It centers on the doping of these
structures with low levels of phosphorous (P); well known to
possess substantial acidic properties [31,32].
A R T I C L E I N F O
Article history:
Available online 1 March 2010
Keywords:
M1
M2
2. Experimental
Acrylic acid
Propane
Propylene
A series of catalytic materials for the oxidation of propane were
prepared with the general precursor composition Mo1V0.31-
Te0.37NbxPyOn using published methods [33–35]. A first solution
was prepared by dissolving ammonium heptamolybdate, ammo-
nium vanadate, and telluric acid in water at 60 8C. This solution
was allowed to cool to room temperature. A second solution was
prepared by dissolving niobic acid in oxalic acid at 60 8C and
allowing the solution to cool to room temperature. The Nb
concentration of the final solution was 0.4 M and the oxalate/Nb
ratio was 3. The appropriate amount of the first solution, second
solution, and phosphoric acid (0.1 M) were mixed and then dried
by rotary evaporation. The solid product was dried at 120 8C for
12 h and calcined at 600 8C for 2 h under N2. The nominal
compositions of the catalysts were Mo1V0.31Te0.37NbxPyOn where
x = 0, 0.02 and 0.09 and y = 0, 0.001 and 0.005. The BET surface
areas of the samples with Nb0 and Nb0.02 (M2’s) both with and
without P doping were about 4 m2/g. The BET surface areas of the
samples with Nb0.09 (primarily M1) with and without P doping
were about 7 m2/g.
The catalysts for propylene oxidation were prepared in a similar
manner as described above and as described earlier [20,31] and
evaluated by high throughput experimental techniques [36]. The
nominal catalyst compositions were: Mo1V0.31Nb0.06Te0.37 (M2-1),
Mo1V0.31Nb0.06Te0.37P0.0005 (M2-2), Mo1V0.31Nb0.06Te0.37P0.001 (M2-
3), and Mo1V0.31Nb0.06Te0.37P0.005 (M2-4). A specific weight of
catalyst (200 mg) was diluted with SiC (about 3 ꢀ volume of
catalyst) to establish a consistent bed height and tested for catalytic
activity in a tertiary screening six-channel reactor module. The
reaction conditions were: atmospheric pressure, 290 to 370 8C,
WHSV = 0.07 to0.2 hꢁ1, and feed ratioC3H8/O2/He/H2O = 1/2/10/1.4
for propane oxidation and C3H6/O2/He/H2O = 1/2/8.4/2.9 for pro-
pylene oxidation. Each catalyst was tested at four different space
1. Introduction
MoV(Nb,Ta)(Te,Sb)O is one of the best known catalytic systems
for the selective oxidation of propane to acrylic acid (AA) and
ammoxidation to acrylonitrile (AN) [1,2]. Since its discovery [3–7]
it has been studied extensively [8–24]. It is comprised of two
phases: M1 (orthorhombic) Mo7.8V1.2NbTe0.94O28.9 and M2 (pseu-
do-hexagonal) Mo4.67V1.33Te1.82O19.82 [9,10,13]. The M1 phase
performs all of the catalytic functions needed to convert propane to
acrylic acid or acrylonitrile, since all key catalytic functions are
located at its active center within bonding distance of each other to
perform these complex transformations [9]. At high throughputs
(high wwh) the M2 phase, if commingled with the M1 phase,
becomes a co-catalyst in symbiosis with M1, performing a mop-up
operation converting free intermediately formed propylene to the
respective desired end products (AA or AN) [9,16].
It is postulated that if the catalytic efficiency of either or both
phases could be improved, then the yields of the desired products
of the combined M1/M2 system should be improved [20]. Various
attempts to influence the catalytic properties of these phases
through substitution of select elements are reported in the
literature, with varying degrees of success [25–30].
* Corresponding author.
E-mail address: rkgrasselli@yahoo.com (R.K. Grasselli).
0920-5861/$ – see front matter ß 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.01.044

34
R.K. Grasselli et al. / Catalysis Today 157 (2010) 33–38
velocities,startingwiththehighest, atseveralreactiontemperatures
starting at the lowest temperature.
3. Results and discussion
The main aim of the current study was to enhance the
performance of M1 and M2 phases for the selective oxidation of
propane and propylene to acrylic acid. Our thesis was that addition
of the acidic element, P, to these phases would enhance their
ability to convert intermediately formed, chemisorbed or des-
orbed, acrolein more efficiently to the desired acrylic acid end
product. This postulate was essentially confirmed, since measured
additions of P to M1 and M2 lead, as will be seen below, to
improved AA yields. Conversely, the addition of basic elements
such as Cs has the opposite effect, decreasing the AA yield, because
acidic sites needed for AA catalysis are neutralised [31]. Somewhat
unexpected was, however the significant activity enhancement of
M1 with P doping, which might find an explanation in how P
effects the V5+ = O ! ꢂV4+ ꢁ Oꢂ activating site of the catalyst’s
active center.
3.1. Oxidation of propane over M1 and M2 catalysts
We prepared three sets of catalysts (Table 1) with varying
amounts of Nb (0, 0.02, 0.09), while the Mo, V, and Te contents were
kept constant (Mo 1, V 0.31, Te 0.37). These base catalysts were then
also doped with 0.001 and 0.005 P. The powder XRD’s (Fig. 1) reveal
that the Nb0 and Nb0.02 containing systems crystallize as the M2
phase; while the Nb0.09 sample gives predominantly M1 phase. The
P doped systems donot showany obvious structural deviationsfrom
the respective base systems (XRD’s in Fig. 1).
These catalysts were tested for the oxidation of propane
(Fig. 2) to assess their respective abilities to give acrylic acid (AA).
It is well known from the literature [9,16,19–21] that MoV(Nb)-
TeO compositions containing only the M2 structure are
essentially inactive for the conversion of propane to AA.
Nonetheless, we wanted to see if doping with P might prod
the M2 structure at least partly into the desired reaction path.
Not so; the two P doped M2 structures are still very inactive
towards propane activation; they still lack the V5+ = O paraffin
activating moieties. Yet, interestingly (Fig. 2) they convert more
effectively whatever little propylene is formed to acrolein and AA
than the unpromoted base composition. Thus in M2 composi-
tions the P doping appears to intercede in the later stages of the
consecutive reaction path; i.e., at the adsorbed propylene stage
leading finally to acrolein and AA. This will be discussed more
fully in the Mechanistic section below.
Fig. 1. Powder X-ray diffraction patterns of the P doped M2 and M1 + (M2) catalysts
and their respective undoped base systems.
appears to be most pronounced in the earliest, the activating stage
of the catalytic process. This too will be discussed more thoroughly
in the Mechanistic section below.
It is an entirely different matter with P doped M1 catalysts
(Fig. 2). Again, it is well known (same as above) that MoV(Nb)TeO
compositions having the M1 structure are effective for the
conversion of propane to AA. We confirm this here also. But what
is new is that the P doped M1 compositions are much more active
for propane oxidation than the base. Here, the doping effect
The selectivities to AA at maximum conversion of propane are
shown in Fig. 3 for the three sets of catalysts. The selectivity to AA
of the M2 Nb0 base composition is practically nil and is markedly
increased to 15% with P 0.001 and to 17% with P 0.005 doping.
The AA selectivity of the M2 Nb0.02 base is also practically nil
and rises steeply to 27% at P 0.001 and 34% at P 0.005 doping.
Conversely, as expected, the AA selectivity of the M1 Nb0.09 base
lies already at a respectable 43%, but is further improved by P
doping to 60% at P 0.001, falling off slightly to 58% at P 0.005. One
has to use the P doping with caution; the useful range is low and
very narrow!
Table 1
M2 and M1+ (M2) Catalysts with P added to the synthesis mixture.
Sample #
Mo
V
Te
Nb
P
Nb-0
1
1
1
1
1
1
1
1
1
0.31
0.31
0.31
0.31
0.31
0.31
0.31
0.31
0.31
0.37
0.37
0.37
0.37
0.37
0.37
0.37
0.37
0.37
0
0
Nb-0-P 0.001
Nb-0-P 0.005
Nb-0.02
0
0.001
0.005
0
The remarkable effect of the P doping can be illustrated even
more dramatically by comparing the maximum AA yields of these
three sets of catalysts (Fig. 4). The yields of AA are minute using M2
0
0.02
0.02
0.02
0.09
0.09
0.09
Nb-0.02-P 0.001
Nb-0.02-P 0.005
Nb-0.09
0.001
0.005
0
phase catalysts, unpromoted or
P promoted. Although the
selectivity to AA is greatly increased with P doping over the base
(Fig. 3), minimal yields of AA are obtained with all M2’s, since an
almost zero activity multiplied by a reasonably large selectivity
Nb-0.09-P 0.001
Nb-0.09-P 0.005
0.001
0.005

R.K. Grasselli et al. / Catalysis Today 157 (2010) 33–38
35
Fig. 2. Propane oxidation over P doped M2 and M1 + (M2) catalysts. Reaction temperature 390 8C; feed composition C3H8/O2/He/H2O = 1/2/8.4/2.9; WHSV = 0.08 to 0.15 hꢁ1
still amounts to very little. Thus the maximum AA yields of M2 Nb0
P 0.001 and P 0.005 are in the 0.1% range and the M2 Nb0.02 P 0.001
and P 0.005 are in the less than 1% range.
In contrast to M2, the M1 catalysts give respectable AA yields
from propane (Fig. 4). The unpromoted M1 base gives a maximum
AA yield of about 3%. Although this is a low value for an M1
catalyst, the value is low because the M1 composition was not
optimized in these experiments (i.e., we worked with an Nb0.09
rather than the optimum Nb0.12 composition) and, because of the
high propane throughput (WWH) used in these experiments. The
idea was to simulate real commercial conditions. However, the P
promoted catalysts show exceptional potential. The M1 Nb0.09
P 0.001 catalyst gives 27% AA yield and the M1 Nb0.09 P 0.005
catalyst a 26% AA yield. Thus the improvement over the M1 base is
about a factor of 9, which is remarkable. There might be an effect of
Fig. 3. Propane oxidation over P doped M2 and M1 + (M2) catalysts. Acrylic acid
selectivities at maximum observed yields vs catalyst composition (Mo1V0.31Te
0.37NbxPy).
Fig. 4. Propane oxidation over P doped M2 and M1 + (M2) catalysts. Maximum
observed acrylic acid yields vs catalyst composition Mo1V0.31Te0.37NbxPy.

36
R.K. Grasselli et al. / Catalysis Today 157 (2010) 33–38
doping (and Nb level). Nevertheless, our experimental results are
catalytically very encouraging.
3.2. Oxidation of propylene over M2 catalysts
Our thesis is that symbiosis or phase cooperation exists
between M1 and M2 phases under commercially demanding
conditions of high WWH and that if either of the two phases were
improved, the overall AA yields of an M1/M2 mixture should also
be improved. We have shown above that P doping of the M1 phase
significantly enhances the AA yields in the oxidation of propane.
Drawing on our earlier observations [31,37], we can state that
substantial improvements in the oxidation of propylene to AA can
also be obtained by doping the M2 structure with P. As illustrative
examples it is shown in Fig. 5 that under comparable conditions
(47% propylene conversion) the net AA yield of the unpromoted
M2 base is improved from 24% to 26% at P 0.0005, 27% at P 0.001
and 32% at P 0.005. The latter is a respectable 33% AA yield
enhancement over the base.
Fig. 5. Propylene oxidation over P doped M2 catalysts. Maximum observed acrylic
acid yields vs catalyst composition Mo1V0.31Te0.37NbxPy.
P on the favorable formation of M1 during catalyst preparation
even at levels below the optimum Nb0.12 range. But this is not
obvious from the XRD’s (Fig. 1).
3.3. Symbiosis between M1 and M2 phases in propane oxidation to AA
Of course if a more active M1 base were to be made (e.g. an
Nb0.12-based M1), the P doping improvement cannot be expected
to be as large as the one observed here. Additional experiments are
necessary to more exactly define the exact range and limits of P
While we have now shown significantly improved AA yields in
propane oxidation over P doped M1 phase and substantially
improved AA yields in propylene oxidation over P doped M2
phase, the P doped improvement of M1/M2 mixtures in propane
Fig. 6. Proposed propane/propylene oxidation mechanism over MoV(Nb)TeO catalysts (M1 domains: #1–#4 and #4–#12; M2 domain: #4–#12). Adapted from reference [35].

R.K. Grasselli et al. / Catalysis Today 157 (2010) 33–38
37
Fig. 7. Symyx draw illustration of the catalytically active center of Mo7.8V1.2NbTe0.94O28.9 (M1). Adapted from reference [9].
oxidation, while on the agenda, still needs to be demonstrated.
Since both phases benefit by P doping, it should follow that
optimal mixtures of M1/M2 phases, if synergy exists between the
phases in oxidation of propane as it exists in ammoxidation
of propane [9,15,16,20], should show the desired doping effect,
to waste products CO and CO2. The beneficial effect of P doping in
the later stages of the reaction mechanism (steps #4 through
#12) can occur on either M1 or M2.
A word about the very low level of P doping which is greatly
effective in enhancing AA yields: it is remarkable that such low
levels of P have such a large effect on both the activity and
selectivity of AA production. With an M1 composition containing
Nb0.09P 0.005 only 20% of the ‘‘active sites’’ contain a P; meaning
that only every fifth site has a P. This assumes that the P is
distributed throughout the whole bulk and surface uniformly. If
we assume that only the active and selective sites attract P, then
the P population increases to about every second useful site
possessing a P (see reference [39] for the distribution of active/
selective, waste, and inactive sites at the surface of M1 basal plane
[ab direction]). Actually, these P site concentrations are not all
that out of line to expect a doping influence on the catalytic
behavior. Since we see a beneficial doping effect even at P 0.001
levels, one might, but not necessarily has, to reason that there
could be a surface enhancement of P, which is possible; perhaps
akin to VPO catalysts. Conversely, too much P is undesirable,
actually also logical, since at the higher concentrations the dopant
changes drastically and deleteriously the nature of the catalyst.
From our study we have learned that the window of P doping is
very narrow and still needs to be further optimized for best
catalytic results.
and thus
a higher AA overall yield. Verification is still
outstanding.
3.4. Mechanistic consequences of P doping
The observed improvement in AA selectivity and yield with
P doping of M1 and M2 phases, respectively can be explained
on the basis of our proposed reaction mechanism shown in
Fig. 6.
We are of the opinion that there are two effects, which P doping
brings about in these compositions. In the M1 phase, the P doping
exhibits itself in both an improved selectivity to AA but primarily
in a much higher activity of propane conversion. This leads us to
believe that P must be located on the M1 surface in the vicinity of
V5+ = O ! ꢂV4+ ꢁ Oꢂ activating sites of the active site ensemble
as shown in Fig. 7.
Under the premise of such P/V adjacency, the P will tend to
electronically push the V5+ = O moiety to its complimentary,
paraffin activating ꢂV4+ ꢁ Oꢂ resonance structure, thereby in-
creasing the time averaged number of the activating sites. This of
course would automatically lead to enhanced activity. In the
mechanism (Fig. 6) steps #1 to #2, #2 to #3, and #3 to #4 are
accelerated.
The second type of enhancement towards AA formation due to
P doping comes in the later stages of the mechanism, with
phosphorous now placed next to the spectator Mo6+ [38] of the
Mo6+-O-Mo4+ site as depicted in complex #4 and #8 of the
mechanistic illustration. The acidic nature of the P (acting as a
pseudo surface heteropoly acid) in complex #4 enhances the
chemisorption of the propylene moiety, preventing its desorp-
tion and favouring its path towards the allylic intermediate
(complex #6) and on to the chemisorbed acrolein intermediate
(complex #8). At this juncture, the P induces and facilitates
the charge transfer from the spectator Mo6+ to the adjacent,
reacting Mo4+ moiety and stabilises it as Mo6+ in complex #9.
This charge transfer facilitates the transition from complex #8 to
#9 and activates complex #9 towards hydrolysis again by
stabilising Mo6+, thereby enhancing the transition from complex
#9 to #10. The higher acidity of the P doped Mo centers enhances
not only the chemisorption of propylene (complex #4) but also
the surface acrolein intermediate (complex #8), inhibiting it’s
desorption and facilitates further surface transformations
toward AA as shown in Fig. 6. Hence, path a prevails over
As to the life of the P doped catalysts, no extensive life tests
were conducted, but we did not observe any decline in
performance over limited time on stream. Actually, there is ample
precedent [1,2] that P containing Bi-molybdates exhibit excellent
life properties over years of commercial operation. By analogy, we
do not expect P containing M1 and M2 compositions tending
towards life problems.
4. Conclusions
Doping of crystalline M1 and M2 phases with P in selective
oxidation of propane or propylene to AA enhances significantly the
desired AA yields at commercially relevant high hydrocarbon
conversions. It comprises a promising approach to improve AA
yields and is worth further study with the possibility of
commercial implementation (Table 2).
Table 2
M2 catalysts with P added to synthesis mixture.
Sample #
Mo
V
Te
Nb
P
M2–1
M2–2
M2–3
M2–4
1
1
1
1
0.31
0.31
0.31
0.31
0.37
0.37
0.37
0.37
0.06
0.06
0.06
0.06
0
0.0005
0.001
0.005
path
b in the mechanism. Path b leading to acrolein is
undesirable, since under the reaction conditions it would lead

38
R.K. Grasselli et al. / Catalysis Today 157 (2010) 33–38
[20] R.K. Grasselli, D.J. Buttrey, J.D. Burrington, A. Andersson, J. Holmberg, W. Ueda, J.
References
Kubo, C.G. Lugmair, A.F. Volpe Jr., Top. Catal 38 (2006) 7.
[21] A.C. Sanfiz, T.W. Hansen, F. Girgsdies, O. Timple, E. Ro¨del, T. Ressler, A. Trunschke,
R. Schlo¨gl, Top. Catal 50 (2008) 19.
[22] B. Deniau, G. Bergeret, B. Jouguet, J.L. Dubois, J.M.M. Millet, Top. Catal 50 (2008)
33.
[23] P. Korovchenko, N.R. Shiju, A.K. Dozier, U.M. Graham, M.O. Guerro-Perez, V.V.
Guliants, Top. Catal 50 (2008) 43.
[24] R. Ha¨ggblad, J.B. Wagner, B. Deniau, J.-M.M. Millet, J. Holmberg, R.K. Grasselli, S.
Hansen, A. Andersson, Top. Catal 50 (2008) 52.
[1] R.K. Grasselli, J.D. Burrington,, in: G. Ertl, H. Kno¨zinger, F. Schu¨th, J. Weitkamp
(Eds.), 2nd edn, Handbook of Heterogeneous Catalysis, 8, Wiley-VCH Verlag GmbH
& Co, 2008, p. 3479 Ch. 14.11.8.
[2] R.K. Grasselli, M.A. Tenhover,, in: G. Ertl, H. Kno¨zinger, F. Schu¨ th, J. Weitkamp
(Eds.), 2nd edn, Handbook of Heterogeneous Catalysis, 8, Wiley-VCH Verlag GmbH
& Co, 2008, p. 3489 Ch. 14.11.9.
[3] M. Hatano and A. Kayo, European Patent 318 295 (1988).
[4] T. Ushikubo, K. Oshima, A. Kayo, T. Umezawa, K. Kiyono and I. Sawaki, European
Patent 529 853 (1992).
[5] T. Ushikubo, H. Nakamura, Y. Koyasuand S. Wajiki, European Patent608 838 (1994).
[6] S. Komada, H. Hinago, M. Kaneta and M. Watanabe, EP Patent 895 809 (1998).
[7] M. Vaarkamp, T. Ushikubo, Appl. Catal. A 174 (1998) 99.
[8] T. Ushikubo, Catal. Today 57 (2000) 331.
[9] R.K. Grasselli, J.D. Burrington, D.J. Buttrey, P. DeSanto Jr., C.G. Lugmair, A.F. Volpe
Jr., T. Weingand, Top. Catal. 23 (2003) 5.
[10] P. DeSanto Jr., D.J. Buttrey, R.K. Grasselli, C.G. Lugmair, A.F. Volpe Jr., B.H. Toby, T.
Vogt, Top. Catal 23 (2003) 23.
[11] M. Baca, A. Pigamo, J.L. Dubois, J.M.M. Millet, Top. Catal 23 (2003) 39.
[12] W.V. Guiliants, R. Bhandari, J.N. Al-Saeedi, V.K. Vasudevan, R.S. Soman, O. Guer-
rero-Pe´rez, M.A. Ban˜ares, Appl. Catal. A 274 (2004) 123.
[13] P. DeSanto Jr., D.J. Buttrey, R.K. Grasselli, C.G. Lugmair, A.F. Volpe Jr., B.H. Toby and
T. Vogt, Z. Kristallogr. 219 (2004) 152.
[25] S. Chaturvedi, A.M. Gaffney, S. Han and E.M. Vickery, US Patent 6 403 525 B1
(2002).
[26] K. Karim, Y.S. Bhat, S. I. Zaheer, A.B. Nafisah and A.A. Khan, US 6 646 158 B1 (2003).
[27] C.G. Lugmair, J. Zysk and R.K. Grasselli, US Patent Appl. 2005/0054869 A1.
[28] J. Holmberg, S. Hansen, R.K. Grasselli, A. Andersson, Top. Catal 38 (2006) 17.
[29] N.R. Shiju, V.V. Guliants, Catal. Comm 9 (2008) 2253.
[30] B. Deniau, J.M.M. Millet, S. Loridant, N. Christin, J.L. Dubiois, J. Catal 260 (2008).
[31] R.K. Grasselli, C.G. Lugmair, A.F. Volpe Jr., Top. Catal 50 (2008) 66.
[32] E. Bordes-Richard, Top. Catal 50 (2008) 82.
[33] T. Ushikubo, K. Oshima, A. Kayo, T. Umezawa, K. Kiyono and I. Sawaki, EP 529,853
(1992).
[34] S. Komoda, H. Hinago, M. Kaneta and M. Watanabe, EP 895,809 (1998).
[35] T. Ushikubo, H. Nakamura, Y. Koyasu and S. Wajiki, US Pat. 5,380,933 (1995).
[36] A. Hagemeyer, B. Jandeleit, Y. Lui, D.M. Poojary, H.W. Turner, A.F. Volpe Jr., H.
Weinberg, Appl. Catal. A Gen. 211 (2001) 23.
[14] J.M. Oliver, J.M. Lo´pez Nieto, P. Botella, Catal. Today 96 (2004) 241.
[15] J. Holmberg, R.K. Grasselli, A. Andersson, Appl. Catal. A 270 (2004) 121.
[16] R.K. Grasselli, Catal. Today 99 (2005) 23.
[17] W. Ueda, D. Vitry, T. Katou, Catal. Today 99 (2005) 43.
[18] M. Baca, M. Aouine, J.L. Dubois, J.M.M. Millet, J. Catal 233 (2005) 234.
[19] J. Holmberg, R. Ha¨ggblad, A. Andersson, J. Catal 243 (2006) 350.
[37] R.K. Grasselli, C.G. Lugmair, A.F. Volpe Jr., A. Andersson, J.D. Burrington, Catal. Lett
126 (2008) 231.
[38] Y.H. Jang, W.A. Goddard III, Top. Catal 15 (2001) 273.
[39] R.K. Grasselli, D.J. Buttrey, P. DeSanto Jr., J.D. Burrington, C.G. Lugmair, A.F. Volpe
Jr., T. Weingand, Catal. Today 91–92 (2004) 251.