Methane steam reforming for hydrogen production using low water-ratios without carbon formation over ceria coated Ni catalysts Jiahui Xu a , Connie M.Y. Yeung c , Jun Ni b , Frederic Meunier b,1 , Nadia Acerbi a , Martin Fowles c , Shik Chi Tsang a, * a Wolfson Catalysis Centre, Inorganic Chemistry Laboratory, University of Oxford, Oxford OX1 3QR, UK b CenTACat, School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast, Northern Ireland BT41 1PB, UK c Johnson Matthey Catalysts, P.O. Box 1, Belasis Avenue, Billingham, Cleveland TS23 1LB, UK Received 12 December 2007; received in revised form 28 January 2008; accepted 11 February 2008 Available online 18 March 2008 Abstract There are many advantages for hydrogen production from the catalytic methane steam reforming reaction (MSR) using low water to methane ratios. However, conventional Ni based catalysts suffer from severe carbon deposition under these conditions. A typical supported Ni catalyst at water to methane ratios <1 at 800 8C shows rapid deactivation (within few hours). Incorporation of CeO 2 is known to offer a kinetic resistance to carbon deposition for many hydrocarbon oxidation reactions because of its facilitated redox activity. As a result, a study of blending ceria into a supported Ni catalyst by a number of deposition methods has been carried out. Catalyst characterization including DRIFTS, XRD, TPR, TGA, TEM and EDX suggests the prime importance of an intimate contact between Ni and CeO 2 in reducing the extent of carbon deposition during the reforming reaction, which critically depends on the preparation method in catalyst synthesis. As a result, a stable and high level of hydrogen production with no significant carbon deposition for over 110 h is demonstrated over the same Ni commercial catalyst pre-coated with ceria using a sol–gel method, which offers the best interface for the reaction. # 2008 Published by Elsevier B.V. Keywords: Sol–gel method; Ceria; Ni catalyst; Methane steam reforming (MSR); Carbon deposition 1. Introduction Methane steam reforming (MSR) is an established process for the large scale production of hydrogen in industry [1–4]. Recently, there has been tremendous renewed interest in the process as hydrogen is considered a clean carrier (for fuel cells or internal combustion engines [4]) for future energy provision [5]. Thus, current intense research effort is being placed on hydrogen generation by means of methane steam reforming (MSR-Eq. (1) below) and water-gas shift (WGS-Eq. (2) below) followed by carbon capture or sequestration. Also, hydrogen purification from hydrogen reforming rich mixtures using membrane technology or pressure swing adsorption [6,7] receives much attention. Particularly, targets are small hydrogen generators giving high levels of hydrogen using MSR have recently been demonstrated. CH 4 þ H 2 O ¼ CO þ 3H 2 ðDH 298 ¼ 206 kJ=molÞ (1) CO þ H 2 O ¼ CO 2 þ H 2 ðDH 298 ¼41 kJ=molÞ (2) CH 4 ¼ C þ 2H 2 ðDH 298 ¼ 76 kJ=molÞ (3) 2CO ¼ C þ CO 2 ðDH 298 ¼173 kJ=molÞ (4) The typical reformate produced from MSR consists of hydrogen, carbon monoxide and carbon dioxide. Depending on the actual pressure, temperature and steam-to-carbon ratio, different equilibrium conditions are achieved that determine the exact composition of the gas. A higher water/methane ratio in the feedstock favors higher conversions, but unnecessary generation of more steam than the reaction stoichiometry is energetically unfavorable and will also dilute the hydrogen content from the reformate. In addition, the excess H 2 /CO ratios www.elsevier.com/locate/apcata Available online at www.sciencedirect.com Applied Catalysis A: General 345 (2008) 119–127 * Corresponding author. Tel.: +44 1865 282610; fax: +44 1865 282600. E-mail address: [email protected](S.C. Tsang). 1 Present address: Laboratoire Catalyse et Spectrochimie, CNRS - University of Caen – ENSICAEN, 6 Boulevard du Marechal Juin, 14050 Caen Cedex, France. 0926-860X/$ – see front matter # 2008 Published by Elsevier B.V. doi:10.1016/j.apcata.2008.02.044
Methane steam reforming for hydrogen production using low water-ratios without carbon formation over ceria coated Ni catalysts
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Methane steam reforming for hydrogen production using low
water-ratios without carbon formation over ceria coated Ni catalysts
Jiahui Xu a, Connie M.Y. Yeung c, Jun Ni b, Frederic Meunier b,1, Nadia Acerbi a,Martin Fowles c, Shik Chi Tsang a,*
a Wolfson Catalysis Centre, Inorganic Chemistry Laboratory, University of Oxford, Oxford OX1 3QR, UKb CenTACat, School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast, Northern Ireland BT41 1PB, UK
c Johnson Matthey Catalysts, P.O. Box 1, Belasis Avenue, Billingham, Cleveland TS23 1LB, UK
Received 12 December 2007; received in revised form 28 January 2008; accepted 11 February 2008
Available online 18 March 2008
www.elsevier.com/locate/apcata
Available online at www.sciencedirect.com
Applied Catalysis A: General 345 (2008) 119–127
Abstract
There are many advantages for hydrogen production from the catalytic methane steam reforming reaction (MSR) using low water to methane
ratios. However, conventional Ni based catalysts suffer from severe carbon deposition under these conditions. A typical supported Ni catalyst at
water to methane ratios <1 at 800 8C shows rapid deactivation (within few hours). Incorporation of CeO2 is known to offer a kinetic resistance to
carbon deposition for many hydrocarbon oxidation reactions because of its facilitated redox activity. As a result, a study of blending ceria into a
supported Ni catalyst by a number of deposition methods has been carried out. Catalyst characterization including DRIFTS, XRD, TPR, TGA,
TEM and EDX suggests the prime importance of an intimate contact between Ni and CeO2 in reducing the extent of carbon deposition during the
reforming reaction, which critically depends on the preparation method in catalyst synthesis. As a result, a stable and high level of hydrogen
production with no significant carbon deposition for over 110 h is demonstrated over the same Ni commercial catalyst pre-coated with ceria using a
sol–gel method, which offers the best interface for the reaction.
a Carbon deposition was calculated based on the difference in the total carbon count of reactant and product gas before and after the MSR reaction:
[(CH4in � (CO + CO2 + CH4)out)/CH4
in] � 100%. The result was averaged by the first three data collected for each reaction temperature. The error was estimated
to be �0.1% using the pure silica as the catalyst. The negative value for NASC-M2 was assigned to the possible disturbance of gas flow or/and volume change(s)
during the MSR over the sample.b Methane decomposition to carbon was evaluated by TGA method: 10 mg samples was pre-treated in a flowing stream of 2% methane/N2 with the flow rate of
60 mL/min before the temperature was ramped at 30 8C/min from room temperature to 800 8C and kept there for 900 min. The %weight gain relative to the sample
weight at 800 8C after 900 min was measured by the gravimetric method.
Fig. 3. A stability test over NSAC-M2 using 6.3% H2O and 25.1% CH4 in N2 at
GHSV of 11,600 h�1 at 800 8C.
J. Xu et al. / Applied Catalysis A: General 345 (2008) 119–127 123
reaction from 300 to 800 8C as previously described. At low
temperature (<600 8C), there was effectively very little carbon
deposition over all of these samples. At above 700 8C severe
carbon deposition was encountered over the samples NSA and
NSAC while NSAC-CNP gave the intermediate degree of
carbon deposition amongst all the samples. However, it is
interesting to find that there was an extremely small amount of
carbon loss, if any (carbon deposition) over samples NSAC and
NSAC-M2 during the short testing period within experimental
error (the measured carbon balances of the two samples were
within 3%, which indicated no significant carbon deposition).
Also, an independent evaluation for carbon deposition from
methane decomposition over the samples under a flowing
stream of 2% methane/nitrogen using gravimetric thermal
analysis (TGA) at 800 8C was carried out. The weight increase
relative to the sample at 800 8C ranked the extent of carbon
deposition from methane as the following order: NSA > N-
NSAC-CN > NSAC-CNP > NSAC� NSAC-M2. This order
is the similar to those calculated carbon deposition under the
MSR conditions (in this case NSA apparently showed a lower
carbon loss than the NSAC-CN, however, we noted that the
deactivation over the NSA was so rapid that the actual values of
carbon losses were expected to be much higher than those
measured values at above 600 8C). It should be noted that
the small extent of carbon gain in the case of NSAC-M2 from
the TGA is comparable to the result using pure silica as a
sample indicating that its catalytic surface is as inert as the
container material for carbon deposition by thermal methane
decomposition. The results clearly suggest that the extent of
carbon deposition over Ni catalysts under MSR conditions can
be eliminated or substantially reduced while maintaining the
high activity for MSR reaction by blending the commercial Ni
catalyst with ceria. Also, an important point to note is that this
kinetic inhibition of carbon deposition critically depends on the
preparation method as how the ceria is blended.
A stability test for NSAC-M2 under the MSR conditions was
evaluated at 800 8C under a flowing stream of 6.3% H2O and
25.1% CH4 in N2 at the GHSVof 11,600 h�1. It is noted that the
water to methane was deliberately set at 0.25. Under such
highly reducing conditions conventional catalysts would be
more susceptible to carbon deposition leading to rapid
deactivation. As seen from Fig. 3 it is interesting to find that
the catalyst gave about 25% methane conversion presumably
achieving the total consumption of water to CO and H2
(unsteady conversions along the testing period were primarily
due to a slight fluctuation in the steam generation). The activity
was maintained for more than 5 days (about 112 h) with a
constant high rate of hydrogen production giving the H2/COx
value at about 3.1. On the other hand, the parent catalyst, NSA
almost immediately lost all its activity for hydrogen production
within a short time (<5 h) with the reactor entirely blocked up
by the carbon deposited over this sample under the same
conditions (hence the stability study could not be continued
beyond this time).
3.3. Sample characterization
Fig. 4 shows the XRD patterns of NSA, NSAC-CN, NSAC-
CNP, NSAC and NSA-M2 samples before (after calcination)
and after the MSR reaction. Before the reaction the original
NSA sample shows diffraction peaks which match well with Ni
(44.5, 51.9 and 76.48) and NiO (36.7 and 62.88) phases. This
suggests the co-existence of metallic Ni and NiO phases in the
original sample. After its treatment with CeO2 by the deposition
precipitation, the peak intensities of Ni and NiO in NSAC-CN
Fig. 4. XRD patterns of NSA and CeO2 modified NSA samples ((a) before MSR reaction; (b) after MSR reaction).
J. Xu et al. / Applied Catalysis A: General 345 (2008) 119–127124
and NSAC-CNP samples are much attenuated, presumably due
to the dilution of these phases by the additive. It should be noted
that the dense NSA (0.5 g) was greatly diluted by the ceria
precursor (1.2 g), and thus the representative sample for XRD
contained relatively lesser quantity of NSA. However, the
metallic Ni peaks were still visible in those samples with the
ceria inclusion despite the size attenuation. Also, new but
characteristic diffraction peaks of CeO2 (2u of 28.6, 33.1, 47.5
and 56.3) indeed appear in the two samples. Similarly, the
diffraction peaks of NiO and Ni almost disappear from the
background in the NSAC and NSAC-M2 samples (synthesized
using sol–gel method) but this time no significant peak intensity
of CeO2 is detected (the size could be too small to be
distinguished from the background). On the other hand, after
the MSR reaction all the samples show sharp peaks of Ni and
CeO2 indicative some degree of sintering under the reaction
conditions. It is interesting to find a new peak at 26.2 (2u) which
can be attributed to graphite material with an interlayer distance
of 3.45 A in the NSA, NSAC-CN and NSAC-CNP samples.
This fact supports the observation of carbon deposition at least
partially in form of graphite over the samples. However, no
graphite formation is evident in the NSAC and NSAC-M2
samples. This evidence reinforces that the earlier observation
carbon was formed over the NSA, NSAC-CN and NSAC-CNP
samples but not on the sol–gel prepared samples, NSAC and
NSAC-M2 during the reaction. According to TEM images in
Fig. 5. (a) TEM image of NSA – 65%Ni/SiO2/Al2O3 (b) TEM image of NSAC-M2
the circles) both ranged from 5 to 10 nm. (The scale bar is 50 nm in both pictures
Fig. 5 the average particle size of Ni of all the samples is about
5–10 nm indicating that there was no major alteration in
particle size encountered despite various deposition methods.
Attempts were made at obtaining elemental analysis at the near
surface region of these catalyst materials. According to Monte
Carlo simulations using SANYL [24] 6 keV electrons have the
penetrating depth of about 0.5 mm, hence the electron beam
energy for EDAX analysis was attenuated to about 6 keV.
Table 2 shows the analysis for each fresh catalyst before testing.
It is found that % Ni on the surface of parent sample; the NSA is
39.3%, while the modified NSAC-CN, NSAC-CNP, NSAC and
NSAC-M2 show 14.2% and 12.3%, 5.9% and 3.8%,
respectively, with increasing cerium contents indicative of
external ceria coatings. It is envisaged that the thin ceria coating
prepared by the sol–gel method particularly in the NSAC-M2
sample must have covered the Ni catalyst very evenly with no
sharp diffraction ceria peaks detected (refer to Fig. 4) despite
the fact that a high content of ceria was used. It is noteworthy
that there seems to be a slight systematic variation in the Al/Si
atomic ratio as the table is descended. This could modify the
acid characteristics of the silica–alumina component of the
catalysts possibly also affecting the catalyst propensity for
carbon deposition.
Fig. 6 shows the H2-TPR profiles of CeO2, parent NSA and
CeO2 modified samples. For the pure CeO2, primarily two
peaks are detected at approximately 500 and 780 8C [25]. These
– NSA modified with CeO2 by sol–gel method—the Ni metal size (indicated by
.)
Table 2
Near surface elemental analysis by EDX at 6 keV
Catalyst Atomic (%) Ce/Ni (calculated
from EDX)
Ce/Ni (recipe
ratio)O Al Si Ni Ce
NSA 50.6 5.6 4.5 39.3 / / /
NSAC-CN 55.3 3.2 2.7 14.2 9.4 0.66 0.5
NSAC-CNP 55.3 2.4 2.3 12.3 9.8 0.80 0.5
NSAC 46.8 1.3 1.2 5.9 3.8 0.64 0.5
NSAC-M2 48.2 0.8 1.0 3.8 8.6 2.26 2.0
J. Xu et al. / Applied Catalysis A: General 345 (2008) 119–127 125
peaks could be assigned to the reductions of surface and lattice
oxygen from ceria. For the NSA sample two broad reduction
peaks can be clearly seen: the first reduction peak of NSA starts
at 230 8C and ends at about 480 8C, the second peak starts at
500 8C and ends at 820 8C. The low temperature peak can be
attributed to the reduction of lattice [O] from NiO phase while
the high temperature peak to the reduction of lattice [O] from
NiO with a strong support interaction, possibly relating to the
formation of aluminate or silicate [25–29]. On the other hand,
the TPR profile of NSAC-CN in Fig. 6 (NSAC-CNP showed
almost identical features as the NSAC-CN which was not
shown) basically represents the combination of TPR profiles of
NSA and CeO2 giving huge reduction humps. This suggests
that there was no strong interaction between the parent NSA
catalyst with the ‘ceria coating’ with their solid phases
physically separated from each other. This sample may
represent the generally poor interface between Ni and ceria
created using precipitation–deposition or related impregnation
methods. Interestingly, for the NSAC prepared by the sol–gel
method, there was no trace of the characteristic high
temperature reduction peak of the bulk ceria. The reduction
of oxygen species from the sample also appeared to take place
readily with all the peaks shifted towards lower temperatures.
The strong metal-support interaction between the NSA catalyst
and the ceria undoubtedly facilitates reducibility of the sample.
This is thought to arise from the effective ceria coating onto Ni
containing phases by the sol–gel method. To confirm this