Critical Thinking

1.3. compared to Co and shows a low

1.3. Co-based catalysts
for Fischer-Tropsch synthesis

Although several transition metals can be used as
catalysts for FTS process, of which Fe, Co, Ni and Ru are the commonly accepted to be the best
catalytic materials 32. Among all these metals, Ru
is the most active catalyst for FTS producing long chain hydrocarbons without the need for any promoters. However,
its limited availability and relatively high price make industrial applications
unfeasible 6. Ni is a very
active catalyst in FTS, but it shows a high selectivity towards the undesired
product CH4 mainly. Therefore, only Fe and Co are deemed the best catalyst materials for application
in industrial scale FTS processes 33. The advantage of
Fe is its availability and significantly lower price, but it is very less active compared to Co and shows
a low
selectivity to paraffins, favouring the production of olefins. The advantage of Co catalyst is its low
unwanted WGS activity, leading to less formation of CO2. Therefore,
Co is the preferred catalyst to produce long chain paraffins at industrial scale 4,32. For example, Shell’s Pearl GTL plant in Qatar and
another GTL plant in Bintulu, Malaysia are operating using Co-based FTS
catalysts 15,17.

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1.4. Deactivation of Co-based FTS catalysts

Co-based catalysts have been used in large scale
industries,
however, they have major drawbacks like their high cost and
deactivation in time 34–40. Therefore, it is very
important to increase the lifetime of the catalysts to make the process
economically more efficient. This requires a fundamental understanding of the
various catalyst deactivation mechanisms. Already, the deactivation mechanisms of Co-based FTS
catalysts have been studied by many research groups, but there is no consistent
picture. The general proposed mechanisms of
deactivation are poisoning of active phase, re-oxidation of Co, sintering of
active phase and deposition of carbonaceous and oxygenated compounds on the catalyst
surface 35,36. Here, the main catalyst
deactivation mechanisms in FTS are discussed briefly.

1.4.1. Poisoning

Fischer-Tropsch
catalysts are sensitive to poisoning by the sulphur, nitrogen compounds and
alkali or alkali earth metals in the feed gas. Poisoning is a strong
chemisorption of reactants or impurities on catalytic sites, thereby blocking
the sites for catalytic reaction. Sulphur poisons the metal since it adsorbs
strongly on active sites. The sulphur atom adsorbed on cobalt catalyst poisons
more than two cobalt atoms 35. This poisoning can be prevented by
purifying the synthesis gas, particularly in the process using coal and biomass
as feedstock. For example, zinc oxide or lead oxide beds can be used to remove
effectively trace amount of sulphur and other impurities 41–43.

1.4.2. Oxidation

Water is the
major by-product of the FT reaction and it may react with cobalt to form inactive
cobalt oxides. Thermodynamic calculations have shown that nanosized cobalt
particles can be oxidized in steam hydrogen environments at FTS conditions. Bezemer
et al. 44 found that oxidation is not a
deactivation mechanism during commercial operation, even for
the smallest cobalt crystallites, when they studied the cobalt supported
on carbon nanofiber. Also, van de Loosdrecht et al. 45 studied the deactivation of an
industrial Co/Al2O3 catalyst by pseudo in-situ X-ray diffraction, magnetic
measurements and X-ray absorption near-edge spectroscopy. From these studies,
it was concluded that oxidation of Co can be ruled out as a major deactivation
mechanism.

1.4.3. Sintering

Another
catalyst deactivation mechanism is sintering of small metallic particles, which
leads to a reduction of the active surface area either through Ostwald ripening
or/and coalescence. The high temperature operation and presence of water vapour
during the FTS reaction accelerate the sintering 46. A study by Storsæter et al. 47 suggested that the presence of water
during FTS promotes deactivation of Co-based FTS catalysts by sintering. Kistamurty
et al. 48 from transmission electron microscopy
(TEM) experiments on Co/SiO2 catalyst proposed that Ostwald ripening
is the dominant sintering mechanism and could contribute to catalyst
deactivation. Duff et al. 49 explained sintering of Co particles
via Ostwald ripening using density functional theory of the adsorption and
migration energies of different cobalt moieties. Sintering mechanism of Co-based
catalysts has been studied extensively to improve the activity by preventing
sintering or regeneration of sintered phase. Saib et al. 36,50–52 studied the sintering mechanism Co
particles supported on a flat SiO2 support and they reported an
oxidative regeneration method to restore the activity of the spent sintered
catalyst.

1.4.4. Carbon deposition

Carbon is a
major element in FTS process. During FTS reaction, the catalyst surface
consists of a wide range of carbon containing molecules. These
carbon-containing molecules might interact with the catalyst in different ways
via side reactions like Boudouard reaction and enhance carbon deposition on the
catalyst. Deposition of carbon species on the surface of the catalyst is a
major deactivation mechanism because of accumulation of these carbon species.
The potential carbon compounds that could be present on the catalyst are wax,
graphite, atomic carbon, carbide etc. and the activity of these species towards
oxygen, hydrogen and carbon monoxide would differ. The formation of different
kinds of carbon compounds formation during FTS is illustrated in Figure 1.4.

The formation of different types of carbon
compounds on the catalyst surface and their role on deactivation of Co-based FT
catalysts have also been studied earlier 36–38. Pena et al. 53,54 identified different types of carbon
species on the spent Co/Al2O3 FT catalyst using ex-situ characterisation techniques. They
found that strongly adsorbed hydrocarbons and polymeric carbon contribute to
catalyst deactivation. Moodley et al. 7 identified three different carbon species, when a spent Co/Pt/Al2O3
FT catalyst was studied with temperature-programmed hydrogenation mass
spectrometry (TPH-MS) experiments. They were atomic carbon, residual wax in the
pores of the catalyst and polymeric carbon. From their studies, it was
concluded that only polymeric carbon contributes to the long-term deactivation.

1.4.5. Oxygenated compounds deposition

Along with
olefins and paraffins, small amounts of oxygenated compounds (carboxylic
acids,  alcohols, ketones etc.) are
formed in FTS 11. These oxygenated compounds interact
strongly with the support and the catalyst because of their high polarity 55. In addition, the boiling points of
oxygenated compounds are higher compared to olefins and paraffins. These
oxygenated compounds deposited on the catalyst surface may block the active
sites because it may be difficult for these oxygenated compounds to deadsorb
from the catalyst surface. Scalbert et al. 56 found that adsorption and an increase in the amount of
oxygenated and unsaturated compounds with time on a Co/Alumina FTS catalyst
using X-ray diffraction–Diffusive reflective infrared Fourier-transform (XRD-DRIFT)
spectroscopy. They proposed that these strongly adsorbed species are
responsible for catalyst deactivation by covering the active sites. Pinard et al. 57 analysed the carbon species present on a Co/Ru/Al2O3 FTS
spent catalyst using Temperature
programmed hydrogenation–infrared (TPH-IR) technique. Atomic carbon, alcohols, carboxylic acids and polymeric carbon were
found on the spent catalyst surface. TPH-IR of spent catalyst indicated that complete
removal of carboxylate species required temperatures above 600 oC as
shown in Figure 1.5. Only carboxylic acids and polymeric carbon were
resistant to rejuvenation treatment under hydrogen. From these studies, Pinard et al. 57 proposed that carboxylates on the catalyst surface deactivate the catalyst,
but the exact role of the carboxylates on the catalyst surface is not yet fully
understood.

1.5. Effect of carboxylic
acids and alcohols addition

Kistamurthy
et al. 58 studied the effect of carboxylic acids addition
on a Co/Al2O3 FTS catalyst continuous in a stirred tank
reactor. From their studies, it is reported that the carboxylic acids can cause
of formation of atomic carbon on the support that leads to strongly adsorbed
carboxylates on Al2O3 support and do not
influence catalyst deactivation significantly. Jalama et al. 59 studied the effect of ethanol (2% and 6%) addition
during FTS over a 10% Co/TiO2 catalyst in a stirred basket reactor
at T = 220 oC, P = 0.8 MPa and H2/CO = 2. They observed
that ethanol addition increased the selectivity to light products, increased
the olefin to paraffin ratio and significantly decreased the catalyst activity.
They proposed that the decrease in catalyst activity by ethanol addition is due
to the oxidation of active Co to inactive CoO by ethanol.

1.6. Aim of the research

It has been
proposed that deposition of high molecular weight carboxylates on the surface
of the catalyst in FTS reaction deactivates the catalysts. The possible role of
carboxylates and their effect of catalyst deactivation are not yet fully
understood. Therefore, we are aimed to investigate this hypothetical
deactivation mechanism at industrial conditions by employing the in-situ operando spectroscopy techniques. In-situ operando characterisation techniques open new ways to
understand the phenomena occurring during the reaction that helps to develop
better industrial catalysts. In-situ/operando diffusive
reflective infrared
Fourier transform (DRIFT) spectroscopy to monitor the surface species and reaction intermediates
on the catalyst and 57Co Mössbauer emission
spectroscopy (MES) to study the state of the catalyst are
adopted for this study.  

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