39 research outputs found
Activation of CO2 at chromia-nanocluster-modified rutile and anatase TiO2
Converting CO<sub>2</sub>
to fuels is required to enable the production of sustainable fuels and to
contribute to alleviating CO<sub>2</sub> emissions. In considering conversion
of CO<sub>2</sub>, the initial step of adsorption and activation by the catalyst
is crucial. In addressing this difficult problem, we have examined how
nanoclusters of reducible metal oxides supported on TiO<sub>2</sub> can promote
CO<sub>2</sub> activation. In this paper we present density functional theory
(DFT) simulations of CO<sub>2</sub> activation on heterostructures composed of extended
rutile and anatase TiO<sub>2</sub> surfaces modified with chromia nanoclusters.
The heterostructures show non-bulk Cr and O sites in the nanoclusters and an
upshifted valence band edge that is dominated by Cr 3d- O 2p interactions. We
show that the supported chromia nanoclusters can adsorb and activate CO<sub>2 </sub>and
that activation of CO<sub>2</sub> is promoted whether the TiO<sub>2</sub>
support is oxidised or hydroxylated. Reduced heterostructures, formed by
removal of oxygen from the chromia nanocluster, also promote CO<sub>2</sub>
activation. In the strong CO<sub>2</sub> adsorption modes, the molecule bends
giving O-C-O angles of 127 - 132<sup>o</sup> and elongation of C-O distances up
to 1.30 Ã…; no carbonates are formed. The electronic properties show a strong CO<sub>2</sub>-Cr-O
interaction that drives the interaction of CO<sub>2</sub> with the nanocluster
and induces the structural distortions. These results highlight that a metal
oxide support modified with reducible metal oxide nanoclusters can activate CO<sub>2</sub>,
thus helping to overcome difficulties associated with the difficult first step
in CO<sub>2</sub> conversion
Reactivity of metal oxide nanocluster modified rutile and anatase TiO2: Oxygen vacancy formation and CO2 interaction
The reduction of CO2 to fuels is an active research topic with much interest in using solar radiation and photocatalysts to transform CO2 into higher value chemicals. However, to date there are no photocatalysts known that can use solar radiation to efficiently reduce CO2. One particularly difficult problem is activating CO2 due to its high stability. In this paper we use density functional theory simulations to study novel surface modified TiO2 composites, based on modifying rutile and anatase TiO2 with molecular-sized metal oxide nanoclusters of SnO, ZrO2 and CeO2 and the interaction between CO2 and nanocluster-modified TiO2. We show that reduction of the supported nanocluster is favourable which then provides reduced cations and sites for CO2 adsorption. The atomic structures and energies of different adsorption configurations of CO2 on the reduced modified TiO2 composites are studied. Generally on reduced SnO and CeO2 nanoclusters, the interaction of CO2 is weak producing adsorbed carbonates. On reduced ZrO2, we find a stronger interaction with CO2 and carbonate formation. The role of the energies of oxygen vacancy formation in CO2 adsorption is important because if reduction is too favourable, the interaction with CO2 is not so favourable. We do find an adsorption configuration of CO2 at reduced CeO2 where a CO bond breaks, releasing CO and filling the oxygen vacancy site in the supported ceria nanocluster. These initial results for the interaction of CO2 at surface modified TiO2 provide important insights for future work on CO2 reduction using novel materials
Metal oxide nanocluster-modified TiO2 as solar activated photocatalyst materials
In this review we describe our work on new TiO2 based photocatalysts. The key concept in our work is to form new composite structures by the modification of rutile and anatase TiO2 with nanoclusters of metal oxides and our density functional theory (DFT) level simulations are validated by experimental work synthesizing and characterizing surface-modified TiO2. We use DFT to show that nanoclusters of different metal oxides, TiO2, SnO/SnO2, PbO/PbO2, NiO and CuO can be adsorbed at rutile and anatase surfaces and can induce red shifts in the absorption edge to enable visible light absorption which is the first key requirement for a practical photocatalyst. We furthermore determine the origin of the red shift and discuss the factors influencing this shift and the fate of excited electrons and holes. For p-block metal oxides we show how the oxidation state of Sn and Pb can be used to tune both the magnitude of the red shift and also its mechanism. Finally, aiming to make our models more realistic, we present some new results on the stability of water at rutile and anatase surfaces and the effect of water on oxygen vacancy formation and on nanocluster modification. These nanocluster-modified TiO2 structures form the basis of a new class of photocatalysts which will be useful in oxidation reactions and with the suitable choice of nanocluster modifier can be applied to CO2 reduction
Theoretical insights into the hydrophobicity of low index CeO2 surfaces
The hydrophobicity of CeO2 surfaces is examined here. Since wettability
measurements are extremely sensitive to experimental conditions, we propose a
general approach to obtain contact angles between water and ceria surfaces of
specified orientations based on density functional calculations. In particular,
we analysed the low index surfaces of this oxide to establish their
interactions with water. According to our calculations, the CeO2 (111) surface
was the most hydrophobic with a contact angle of {\Theta} = 112.53{\deg}
followed by (100) with {\Theta} = 93.91{\deg}. The CeO2 (110) surface was, on
the other hand, mildly hydrophilic with {\Theta} = 64.09{\deg}. By combining
our calculations with an atomistic thermodynamic approach, we found that the O
terminated (100) surface was unstable unless fully covered by molecularly
adsorbed water. We also identified a strong attractive interaction between the
hydrogen atoms in water molecules and surface oxygen, which gives rise to the
hydrophilic behaviour of (110) surfaces. Interestingly, the adsorption of water
molecules on the lower-energy (111) surface stabilises oxygen vacancies, which
are expected to enhance the catalytic activity of this plane. The findings here
shed light on the origin of the intrinsic wettability of rare earth oxides in
general and CeO2 surfaces in particular and also explain why CeO2 (100) surface
properties are so critically dependant on applied synthesis methods
Design of novel visible light active photocatalyst materials: Surface modified TiO2
Work on the design of new TiO2 based photocatalysts is described. The key concept is the formation of composite structures through the modification of anatase and rutile TiO2 with molecular-sized nanoclusters of metal oxides. Density functional theory (DFT) level simulations are compared with experimental work synthesizing and characterizing surface modified TiO2. DFT calculations are used to show that nanoclusters of metal oxides such as TiO2, SnO/SnO2, PbO/PbO2, ZnO and CuO are stable when adsorbed at rutile and anatase surfaces, and can lead to a significant red shift in the absorption edge which will induce visible light absorption; this is the first requirement for a useful photocatalyst. The origin of the red shift and the fate of excited electrons and holes are determined. For p-block metal oxides the oxidation state of Sn and Pb can be used to modify the magnitude of the red shift and its mechanism. Comparisons of recent experimental studies of surface modified TiO2 that validate our DFT simulations are described. These nanocluster-modified TiO2 structures form the basis of a new class of photocatalysts which will be useful in oxidation reactions and with a correct choice of nanocluster modified can be applied to other reactions
Length-dependent resistance model for a single-wall Carbon nanotube
The non-linear length-dependent resistance, observed in
single-wall Carbon nanotubes (SNTs) is explained through the recently proposed
ionization energy () based Fermi-Dirac statistics (FDS). The length
here corresponds to the Carbon atoms number () along the SNT. It
is also shown that is associated
with , which can be attributed to different semiconducting
properties in their respective and directions.Comment: Publishe