(207e) Improving the Performance of Sn Promoted Ni/Al2O3 Catalysts for the Dry Reforming of Methane

Improving
the Performance of Sn Promoted Ni/Al₂O₃ Catalysts for the
Dry Reforming of Methane

T.
Stroud, T. Smith, E. Le Saché, H. Arellano-Garcia, T.R. Reina^*

Department
of Chemical and Process Engineering, Faculty of Engineering and Physical
Sciences, University of Surrey, Guildford, UK

Corresponding
author: t.ramirezreina@surrey.ac.uk

  Introduction

Prevention of CO₂ reaching the
upper atmosphere is a topic of great interest in the world today. Co-currently,
green energy production and chemical feedstock sources are being highly
researched, where combining these issues represents the most efficient solution.
Dry reforming of methane (DRM) represents one such solution:

CH₄
+ CO₂ ⇌ 2CO + 2H₂

DH_298K = +247 kJ/mol        DG_298K = +170 kJ/mol

The products of this reaction (known
colloquially as syngas) are a highly useful precursor for a number of chemical
processes, such as the production of methanol and synthesis of liquid fuels via
Fischer-Tropsch (FTS).

Given the endothermic nature of the
reaction, high reaction temperatures are required to achieve favourable
conversions of the reactants to syngas. Unfortunately, elevated temperatures
also promote the formation of solid carbon via a number of further side
reactions [1]. This carbon then deposits on the surface of the catalyst which
(alongside metal sintering) causes the catalyst to deactivate.

Hence an effective catalyst must resist
sintering and be resilient towards the formation of carbon upon the surface;
whilst also being relatively inexpensive, such that it can be realistically
employed within the wider industry. Nickel has previously been identified as a
viable active metal to use, whilst an alumina (Al₂O₃)
support helps disperse the Ni over a wider surface area, improving the activity
over that of just Ni alone [1].

However, this still suffers from severe
carbon deactivation and hence there remains potential for improvement. The aim
of this work is to develop advanced catalysts for chemical CO₂
recycling via DRM using tin and ceria as promoters of Ni/Al₂O₃ Experimental

Catalysts were prepared by the method of
sequential impregnation, where a base of 10 wt.% Ni was first prepared by
adding the appropriate amount of Ni(NO₃)•6H₂O (Sigma-Aldrich)
to an alumina (SASOL) base, using acetone (Sigma-Aldrich) as the solvent. This was
then thoroughly mixed for 30 minutes, before evaporating the acetone off with
the aid of a vacuum pump. The sample was then further air dried in an oven
overnight at 80 °C, after which calcination was performed at 700 °C for 4
hours. The resultant Ni/Al catalyst was employed as a reference material in
this paper.  Following the same preparation method, 3 promoted catalysts were
prepared: Sn 0.02 mol.% Ni/Al, Sn 0.04 mol.% Ni/Al, Sn 0.02 mol.% Ni/Ce-Al. The
tin was added using a SnCl₂ (Sigma-Aldrich) precursor and the ceria
by Ce(NO₃)₂•H₂O (Sigma-Aldrich) aiming for a
nominal CeO₂ loading of 20 wt.%.

Experiments were run in a quartz tube
reactor at atmospheric pressure, utilising quartz wool as a reactor bed. An
electric fired oven was used to ramp up (15 °C/min) and maintain the
temperatures used for reduction (800 °C) and reaction (700 °C). Reduction was
performed in a total flow of 100 ml/min for 1 hour in an atmosphere consisting
20 % H₂ (BOC) and 80 % N₂ (BOC).

Each reaction was run for over 20 hours, using
0.1 g of catalyst and a total inlet flow of 100 ml/min, corresponding to a
weight hourly space velocity (WHSV) of 60, 000 ml/(g-cat h). Of the inlet flow,
12.5 % was CO₂ (BOC), 12.5 % CH₄ (BOC) and the remainder being
that of N₂.

All fresh catalysts were characterised by
means of XRD, H₂-TPR and N₂ physisorption (BET) analysis.
Selected spent catalysts also underwent SEM and Raman. Results and Discussion

H₂-TPR analysis (not shown
here) confirms that a reduction temperature of 800 °C is sufficient to reduce
the metal oxides to their elemental forms, whilst the XRD data (Figure 1) shows
the Ni particles have been well dispersed. The increase of Sn loading as a promoter
appears to have little effect on the crystalline structure of the samples,
whereas cerium shows typical diffraction peaks of CeO₂ fluorite phases.

Figure 1 XRD profiles of the sample catalysts

N₂ physisorption results (Table 1) show
the addition of Sn does not affect the textural properties of the reference
Ni/Al material. However CeO₂ reduces the surface area and volume of
the pores, which is down to the CeO₂ nanoparticles covering the
surface of the primary support (Al₂O₃).

Table 1
Results of N₂ physisorption
analysis on fresh catalysts

In terms of catalytic performance, both methane and CO₂
conversions (Figure 2 and Figure 3
respectively) are improved by the addition of Sn, showing less deactivation
with time when compared with the un-promoted Ni/Al catalyst. Initially, both
the un-promoted and 0.04 Sn sample appear to perform better than the 0.02 Sn
catalyst, but this quickly changes around the 500 minute mark for CH₄
conversion and 450 minute mark for CO₂. Increasing the quantity of
Sn from 0.02 mol.% to 0.04 mol.% exhibits a decrease in performance with time.
This is theorised to be down to the extra Sn blocking the active sites [2], therefore reducing performance; hence a ratio of 0.02 mol.% of Sn was selected as the
optimum to be further improved upon.

Figure 2
Methane conversions for the catalysts over a
period of approximately 20 hours

Ceria is an excellent redox promoter due
to its high oxygen storage capacity [3]. Therefore by adding CeO₂,
the amount of surface oxygen available increases [1]. This helps with the
removal of carbon species from the catalyst surface, increasing the lifespan
and improving activity. This has been observed in the case of the
multicomponent catalyst (Ni-Sn/Ce-Al), with conversions being greater for longer
periods of time in the case of methane. An impressively high peak of 95 % CO₂
conversion can be seen, with a seemingly stable 66 % conversion after
approximately 20 hours of on-stream operation. Lower conversions of methane
(around      20 % after 20 hours) in comparison to CO₂ are to be
expected, due to decomposition of CH₄ being the rate determining
step and also the primary depositor of carbon [4].

Figure 3
Carbon dioxide conversions for the catalysts
over a period of approximately 20 hours

For the H₂/CO ratio (Figure 4), the
Ce doped sample appears the best candidate early on, peaking at a ratio of 0.97
after ca. one hour of operation. However this eventually decreases down
to the same level (a ratio of around 0.75) of the un-promoted and 0.04 Sn
samples after 20 hours. The Sn0.02 Ni/Al catalyst shows a slightly more
favourable ratio of 0.81 after 20 hours, showing the addition of tin is also
effective at improving the H₂/CO ratio; with the optimum ratio again
being 0.02 mol.%.

With the addition of ceria, a slight drop
in performance is observed when compared with the 0.02Sn promoted catalyst. At
the end of the experiment, a higher conversion of CO₂ is present for
Ce, with a similar methane conversion. This therefore results in a
comparatively greater abundance of CO being present in the product stream (as
the H₂ is only formed by methane decomposition), giving a lower
ratio. The observed trend could be due to the ability of ceria to catalyse the
reverse water shift reaction, RWGS (CO₂
+ H₂ ⇌ H₂O + CO) which explains greater CO₂
conversion and the smaller H₂/CO ratio for the ceria promoted
sample.

Figure 4 Syngas product ratio for each catalyst over a period
of approximately 20 hours Conclusions

The addition of tin as a promoter to a
nickel alumina catalyst shows a marked improvement in conversions of both
carbon dioxide and methane to syngas. However, adding too large amounts of tin
can have a detrimental effect on catalytic performance by restricting access of
reactants to the active nickel sites. It has been found that 0.02 mol.% is an optimum
tin loading.

Addition of ceria to the alumina support
increases resistance to carbon formation on the surface, exhibiting higher
conversions for a longer period of time. In parallel, ceria aids the RWGS
reaction boosting the CO₂ conversion. When included together, tin
and ceria work synergistically, resulting in a very promising catalyst for dry
reforming reactions.
References

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1.	Charisiou N.D., Baklavaridis A., Papadakis V.G., Goula M.A. Syn Gas Production via the Biogas Reforming Reaction Over Ni/MgO–Al2O3 and Ni/CaO-Al2O3 Catalysts. Waste and Biomass Valorization. 2016; 7: p. 725-736.
2.	Hou Z., Yokota O., Tanaka T., Yashima T. Surface Properties of a Coke-Free Sn Doped Nickel Catalyst for the CO2 Reforming of Methane. Applied Surface Science. 2004; 233:  p. 58-68.
3.	Reina T.R., Ivanova S., Centeno M.A., Odriolzola J.A. Boosting the Activity of a Au/CeO2/Al2O3 Catalyst for the WGS Reaction. Catalysis Today. 2015; 253: p. 149-154.
4.	Wei J., Iglesia E. Isotopic and Kinetic Assessment of the Mechanism of Reactions of CH4 With CO2 or H2O to Form Synthesis Gas and Carbon on Nickel Catalysts. Journal of Catalysis. 2004; 224(2): p. 370-383.

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