Pt-loaded anatase
TiO
2
(Pt/TiO
2
-A) was found
to be a highly active and stable catalyst for SO
3
decomposition
at moderate temperatures (∼600 °C), which will prove to
be the key for solar thermochemical water-splitting processes used
to produce H
2
. The catalytic activity of Pt/TiO
2
-A was found to be markedly superior to that of a Pt catalyst supported
on rutile TiO
2
(Pt/TiO
2
-R), which has been extensively
studied at a higher reaction temperature range (≥800 °C);
this superior activity was found despite the two being tested with
similar surface areas and metal dispersions after the catalytic reactions.
The higher activity of Pt on anatase is in accordance with the abundance
of metallic Pt (Pt
0
) found for this catalyst, which favors
the dissociative adsorption of SO
3
and the fast removal
of the products (SO
2
and O
2
) from the surface.
Conversely, Pt was easily oxidized to the much less active PtO
2
(Pt
4+
), with the strong interactions between the
oxide and rutile TiO
2
forming a fully coherent interface
that limited the active sites. A long-term stability test of Pt/TiO
2
-A conducted for 1000 h at 600 °C demonstrated that there
was no indication of noticeable deactivation (activity loss ≤
4%) over the time period; this was because the phase transformation
from anatase to rutile was completely prevented. The small amount
of deactivation that occurred was due to the sintering of Pt and TiO
2
and the loss of Pt under the harsh reaction atmosphere.
This
study investigated the thermal decomposition behaviors of
platinum oxide (PtO2) nanoparticles deposited on polycrystalline
TiO2 in different crystal phases. The dissociation of PtO2 to metallic platinum in air occurred at 400 °C on anatase
TiO2 (Pt/TiO2-A), but required 650 °C or
higher on rutile TiO2 (Pt/TiO2-R). The higher
thermal stability of PtO2 on rutile TiO2 is
caused by thermodynamic effect and rather than kinetic effect. In
contrast to the thermodynamic prediction, metallic Pt (Pt0) on TiO2-R was reversibly oxidized to PtO2 (Pt4+) at 650 °C. This behavior was attributed to
the coherent interface structure formed by strong interactions between
PtO2 and rutile TiO2, as revealed by combined
extended X-ray adsorption spectroscopy (EXAFS) and density functional
theory (DFT) studies. At the optimized interface structure, between
the (100) planes of α-PtO2 and rutile TiO2, the interface formation energy was −17.04 kJ mol–1 Å–2 versus −9.84 kJ mol–1 Å–2 in the anatase TiO2 model.
The larger interface formation energy provides a stabilizing effect
against PtO2 dissociation. Therefore, the widely used Pt-loaded
rutile TiO2 typifies the interfacial interactions under
an oxidizing atmosphere, which differ from the strong metal–support
interactions prevailing under a reducing atmosphere.
Platinum
supported on Ta2O5 was found to be a very active
and stable catalyst for SO3 decomposition, which is a key
reaction in solar thermochemical water splitting processes. During
continuous reaction testing at 600 °C for 1,800 h, the Pt/Ta2O5 catalyst showed no noticeable deactivation (activity
loss ≤ 1.5% per 1,000 h). This observed stability is superior
to that of the Pt catalyst supported on anatase TiO2 developed
in our previous study and to those of Pt catalysts supported on other
SO3-resistant metal oxides Nb2O5 and
WO3. The higher stability of Pt/Ta2O5 is due to the abundance of metallic Pt (Pt0), which favors
the dissociative adsorption of SO3 and the smooth desorption
of the products (SO2 and O2). This feature is
in accordance with a lower activation energy and a less negative partial
order with respect to O2. Pt sintering under the harsh
reaction environment was also suppressed to a significant extent compared
to that observed with the use of other support materials. Although
a small fraction of the Pt particles were observed to have grown to
more than several tens of nanometers in size, nanoparticles smaller
than 5 nm were largely preserved and were found to play a key role
in stable SO3 decomposition.
Supported molten
cesium vanadate catalysts (Cs–V–O/SiO2) showed
activities comparable to that of a reference Pt catalyst (1 wt % Pt/TiO2) for SO3 decomposition at moderate temperatures
(∼600 °C), which is essential as an O2 evolution
reaction in solar thermochemical water splitting cycles. Stability
testing of the catalyst over a 1000 h continuous reaction at 600 °C
resulted in deactivation by ∼20% of the initial activity. Kinetic
analysis of the activity versus time-on-stream indicated that the
observed deactivation behavior can be divided into an induction period
(≤100 h) and an acceleration period (>100 h). The deactivation
is mainly caused by the vaporization loss of active components (Cs
and V) from the molten phase. At the earliest stage, most vapor is
generated in the upstream section of the catalyst bed and then redeposits
therebelow. Upon repeating these vaporization and deposition cycles,
Cs and V move gradually downstream. During this induction period,
the deactivation is not obvious because the total Cs and V content
of the catalyst bed remains almost unchanged. After this period, however,
detachment of Cs and V from the downstream end of the catalyst bed
induces accelerated deactivation. The vaporization loss was found
to be significantly suppressed by inverting the catalyst bed every
100 h during the stability test. Consequently, this operation reduced
the extent of catalyst deactivation from 20% to less than 10% of the
initial activity.
Alkaline
earth vanadates (Ae–V: Ae = Ca, Sr, and Ba) were
supported on mesoporous SiO2 by a wet impregnation method.
The catalytic activity of the prepared materials for the decomposition
of SO3 into SO2 and O2, which is
a key step in solar thermochemical water splitting cycles, was investigated.
In the temperature range 700–800 °C, the Ae–V/SiO2 catalysts exhibited remarkably high activities, which were
superior to those of supported Pt catalysts in a wide range of weight
hourly space velocities (55–220 g-H2SO4 g–1 h–1). Despite the melting
points of the materials exceeding 1000 °C, the high activity
was determined to be closely related to the unusual melting behavior
of Ae–V. Under the reaction atmosphere, the Ae–V phase
was converted to AeSO4 and molten V2O5 (melting point = 690 °C) via facile solid–gas reactions
between SO3 and alkaline earth elements displaying high
basicity. Notably, upon contact with the molten V2O5 phase, the as-deposited AeSO4 was immediately
decomposed into SO2 and O2 to regenerate the
Ae–V phase. The catalyst, which solidified at lower temperatures
(<690 °C), could not decompose the sulfate and was therefore
unable to drive the catalytic cycles. Consequently, the SO3 decomposition rate at <690 °C was lower than that of an
alkaline vanadate (Cs–V) with a melting point as low as 500
°C but higher than that of a rare earth vanadate (La–V)
with the highest melting point (>1800 °C).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.