Incorporation
of carbonaceous templates (CT) into TiO2 composites is
a promising alternative to increase the photocatalytic
activity of TiO2. In this work, the effects of carbon sphere
(CS) and carbon nanotube (CNT) incorporation (as CT) in TiO2 composites were thoroughly investigated and the roles of these CTs
as template, cocatalyst, and adsorbent were studied. To this end,
three different methods were utilized to form a layer of TiO2 on the CT: alcoholic phase sol–gel, aqueous phase sol–gel,
and hydrothermal. The role of CT as template was examined through
morphology analysis of the prepared composites. The cocatalyst and
adsorbent roles of CT were investigated based on photocatalytic hydrogen
production from glycerol.
Interestingly, it was found that the incorporation of CNT into TiO2 composite can approximately double the rate of hydrogen production
(i) in the absence of Pt or (ii) at low glycerol concentration. Accordingly,
it was concluded that in addition to being a template, the CNT can
play two important roles as cocatalyst and adsorbent.
The
present work investigates the sustainable production of high-purity
hydrogen through sorption enhanced steam reforming of glycerol (SESRG)
over Ca9Al6O18–CaO/xNiO (x = 15, 20, and 25 wt %) and Ca9Al6O18–CaO/20NiO–yCeO2 (y = 5, 10, and 15 wt
%) bifunctional catalyst-sorbent materials. A wet mixing method involving
limestone acidification coupled with two-step calcination was employed
to prepare the bifunctional materials. Cyclic carbonation/calcination
tests revealed that the bifunctional materials promoted with 10 and
15 wt % of CeO2 possessed an excellent CaO conversion (97%
in both cases) and a remarkable cyclic stability (up to 15 cycles).
This was mainly attributed to the thin shell-connected structure formed
by the addition of CeO2 and the oxygen mobility characteristic
of CeO2. The use of Ca9Al6O18–CaO/xNiO materials in five consecutive SESRG/regeneration
cycles revealed that they suffered from fast deactivation mainly
due to CaO sintering and coke deposition. Despite the high H2 purity obtained (∼98%), the prebreakthrough time and hydrogen
yield decreased significantly over five cycles. Interestingly, the
addition of CeO2 to the most efficient catalyst (Ca9Al6O18–CaO/20NiO) resulted in
a significant improvement in material stability during cyclic operation.
The performance of CeO2-promoted materials was shown to
depend strongly on the CeO2 content which controlled the
number of adjacent Ni active sites, the amount of coke deposition,
and the degree of CaO sintering. The bifunctional material promoted
with 10 wt % of CeO2 showed the best performance over five
consecutive SESRG/regeneration cycles, with a stable H2 purity of ∼98%, H2 yield of ∼91%, and prebreakthrough
time of 48 min. The long-term cyclic stability test of Ca9Al6O18–CaO/20NiO–10CeO2 over 20 cycles exhibited a very stable performance with a H2 yield of 91% and H2 purity of 98% within 20 cycles,
confirming the high potential of this material for SESRG process.
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