Promoting effect of Nb2O5 addition to Ni-Cu catalysts on hydrogen production via methane decomposition

Article abstract of DOI:10.1246/cl.2004.652

The catalytic decomposition of methane to hydrogen and filamentous carbon was carried out over Ni-Cu-Nb₂O₅ catalysts. The results indicate that the addition of Nb₂O₅ can promote the carbon capacity of Ni-Cu cata

Full text of DOI:10.1246/cl.2004.652

652
Chemistry Letters Vol.33, No.6 (2004)
Promoting Effect of Nb2O5 Addition to Ni–Cu Catalysts on Hydrogen
Production via Methane Decomposition
Jianzhong Li, Gongxuan Lu,Ã and Ke Li
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics
Chinese Academy of Sciences, Lanzhou, 730000, P. R. China
(Received February 12, 2004; CL-040163)
The catalytic decomposition of methane to hydrogen and fil-
went on until the methane conversion decreased to about 3%. All
samples were reduced in situ with a 25% H2/N2 mixture
(40 mL/min, v/v) at 923 K for 30 min prior to the reaction.
The product mixtures were analyzed on a gas chromatography
using high purity Ar (99.99%) as the carrier gas. TEM images
of samples were obtained on a JEOL-1200EX transmission elec-
tron microscope operating at 80 kV.
Figure 1 depicts the range of methane conversion in the out-
let gas during methane decomposition as a function of time on
stream at 773–1023 K over 65Ni–25Cu–10Nb2O5, at a gas hour-
ly space velocity (GHSV) of 48000 mLgÀ1hÀ1. The results show
that the values of gC/gNi at 773, 873, 923, 973, and 1023 K are
339, 686, 666, 608, and 361, respectively. The catalyst presents a
steady carbon capacity for methane decomposition to hydrogen
between 873 and 973 K. The value of gC/gNi reaches its maxi-
mum of 686 at 873 K.
Figure 2 shows the promoting effect of Nb2O5 on methane
decomposition over Ni–Cu catalyst at 873 K. From the reactions,
we found that pure 65Ni–25Cu alloy has no catalytic activity for
methane decomposition at a GHSV of 48000 mLgÀ1hÀ1, which
shows that the Ni–Cu alloy agglomerates greatly after the reduc-
tion at 923 K. However, the results show that the maximal meth-
ane conversion over the 65Ni–25Cu alloy is about 42% at a
GHSV of 24000 mLgÀ1hÀ1 (hollow symbols), with a gC/gNi
value of about 516. The addition of Nb2O5 can increase the ac-
tivity of Ni–Cu catalysts at high GHSV (solid symbols). Further
addition of Nb2O5 will decrease the activity of the catalyst.
Comparing the results of Figures 1 and 2 with those of Ni–
Cu/Al2O3,8,9 we can see that the addition of Nb2O5 can increase
the gC/gNi value of Ni–Cu catalyst, and increase the lifetime
and gC/gNi value at high temperature greatly. 65Ni–25Cu–
5Nb2O5 has the best activity among the four Ni-Cu–Nb2O5 cat-
alysts (Figure 2) and the gC/gNi value of 743 is one of the high-
est values among those for catalysts5–9 reported so far. Effects of
other additives such as TiO2, SiO2 are still in progress in our lab-
oratory.
amentous carbon was carried out over Ni–Cu–Nb2O5 catalysts.
The results indicate that the addition of Nb2O5 can promote
the carbon capacity of Ni–Cu catalysts, and increase the lifetime
of the catalysts in methane decomposition at high temperature.
The highest yield of carbon is 743 gC/gNi for 65Ni–10Cu–
5Nb2O5, which is one of the highest values among those for cat-
alysts reported so far.
The catalytic decomposition of methane to hydrogen and
carbon is of current interest from the viewpoint of an alternative
route of hydrogen production from methane1 because no CO is
contained in the hydrogen formed by this process. Thus, hydro-
gen produced by this process can be directly introduced to the
fuel cells without any CO-removal process. However, it is in-
eluctable that the catalysts will lose their activity owing to car-
bon deposition and have to be regenerated with CO2,2 oxygen3
or steam,4 which makes the avoidance of ppm level COx in hy-
drogen product difficult. For these reasons, it is very important to
increase the capacity for methane decomposition to hydrogen.
In the published literatures, Ni–Cu catalysts for hydrocarbon
decomposition are more stable than Ni–Al2O3 catalysts5–9 under
conditions of carbon segregation. Niobium oxide, owing to its
considerable acidity, has been used as a support for bi-functional
catalysts, which have both acidic and hydrogenation activi-
ties.10,11 Niobia-supported catalysts have been investigated,
mainly in connection with the strong metal-support interaction
(SMSI) generated when these catalysts are heated in hydrogen
at high temperatures.12,13 However, few studies on the perform-
ance of Nb2O5 containing catalyst for methane decomposition to
hydrogen were reported. In the present work, we found that the
addition of Nb2O5 can promote the carbon capacity of Ni–Cu
catalysts for methane decomposition, and increase the lifetime
of the catalysts for methane decomposition at high temperature.
The highest yield of carbon is 743 gC/gNi (grams of carbon ac-
cumulated on per gram Ni) for 65Ni–10Cu–5Nb2O5 (molar ra-
tio), which is one of the highest values among those for catalysts
reported so far.
From Figure 2, the conversion of methane can be divided in-
60
773K
Ni–Cu–Nb2O5 catalysts were prepared by addition of
Nb2O5 into a solution of Ni(NO3)2 (0.1 g NiO/mL) and
Cu(NO3)2 (0.1 g CuO/mL). The mixtures were treated ultrasoni-
cally for 60 minutes, and then stirred violently for 6 h. The resid-
ual water in the mixtures was evaporated in a rotary evaporator
at 373 K. Finally, the solids were calcined at 873 K for 4 h, and
then ground and sieved to less than 150 meshes.
The reactions were carried out in a fixed-bed quartz reactor
(30-mm i.d.) under atmospheric pressure with pure methane.
50 mg of catalyst powder was added for each run. The reactions
873K
923K
973K
1023K
40
20
0
0
20
40
80 120
Time on stream (hours)
Figure 1. Effect of decomposition temperatures on methane con-
version over 65Ni–25Cu–10Nb2O5, GHSV = 48000 mLgÀ1hÀ1
.
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.6 (2004)
653
The commonly accepted model of methane decomposition
and carbon growth over nickel catalysts includes the stages of
activation and decomposition of methane on Ni(100) and
Ni(110) planes, carbon dissolution and diffusion through the par-
ticle, carbon segregation in the form of graphite-like phase on
Ni(111) planes due to crystallographic matching of Ni(111)
planes to (002) graphite planes.8,15 Thus, modifying the particle
shape, one increases the number of Ni(100) and Ni(110) planes
for methane decomposition and Ni(111) planes for carbon segre-
gation, which in turn, providing an increase in the carbon capaci-
ty of the catalysts and decrease of the deactivation rate.8 Recent-
ly, Nb2O5 has been used as a promoter in metal catalysts
supported on SiO2 or Al2O3.16 In our studies, we found that
the addition of Nb2O5 can increase the carbon capacity of Ni–
Cu catalyst for methane decomposition. XRD results (not
shown) reveal obviously the characteristic peaks of Nb2O5 of
the samples of Figures 3a and 3b, and no obvious peaks of
Nb2O5 of the samples of Figures 3c and 3d due to high amounts
of filamentous carbon formed. From the XRD results of the three
unreduced samples (i.e. 65Ni–25Cu, 65Ni–25Cu–5Nb2O5 and
65Ni–25Cu–10Nb2O5), we found that addition of Nb2O5 can
change the morphology of the Ni–Cu catalysts. The reduced
65Ni–25Cu–5Nb2O5 sample shows the largest relative intensity
of Ni(111) plane among the three samples, that is the intensity
ratio between Ni(111) and Ni(200) plane, which may be the rea-
son that the catalyst presents a high carbon capacity for methane
decomposition.
65-25-0
65-25-5
65-25-10
65-25-17
65-25-35
45
30
15
0
0
20
40
60
80
Time on stream (hours)
Figure 2. Methane conversion with time on stream over various
Ni–Cu–Nb2O5 catalysts at 873 K: ( ) GHSV = 24000 mLgÀ1hÀ1
( ), ( ), ( ), and ( ) GHSV = 48000 mLgÀ1hÀ1
;
.
to three stages: (1) the induction period, in which the methane
conversion increases, is from the beginning of the reaction to
about 8 h; (2) the steady state, in which the methane conversion
is constant, is from about 8 h to about 24 h; (3) the deactivation
stage, in which the methane conversion decreases to about 3%.
TEM images of carbon formed at different stages during meth-
ane decomposition over 65Ni–25Cu–10Nb2O5 at 873 K are
shown in Figure 3. The images show that the induction period
and the steady growth of filamentous carbon occur mostly on
Ni–Cu alloy particles lager than 60 nm, which are significantly
larger than those of the reduced catalyst before methane decom-
position (about 25 nm, not shown). Therefore, only coalesced
metal particles produce filamentous carbon intensively.14 It
can be seen that the quasi-octahedral particles sizes of Ni–Cu al-
loy are about 60–170 nm (Figures 3a–3c). In this case, several
filaments grow on one particle to form the so-called ‘‘octopus’’
structure. The diameter of the grown filaments is less than that
of the metal particle. Several fish-bone-like filaments about 30
to 65 nm in diameter are presented in Figure 3b, they are the
branches extended from the backbone of the filaments formed
during the reaction. Comparing Figures 3c and 3d with Figures
3a and 3b, we can see that the filaments are very fragile which
are easily crushed into filaments of very small sizes in length.
Nanocarbon particles are separated from the filaments because
the samples have been treated by ultrasonic before the TEM
images were taken.
The authors are grateful to the financial support of the 973
Project of China (G20000264).
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Figure 3. TEM images of carbon formed at different stages dur-
ing methane decomposition over 65Ni–25Cu–10Nb2O5 at 873 K:
Methane decomposition for 20 min (a), 120 min (b), and 18 h (c);
methane conversion decreased to about 3% (d).
Published on the web (Advance View) April 26, 2004; DOI 10.1246/cl.2004.652