5 research outputs found
Photoinduced Reactions of Surface-Bound Species on Titania Nanotubes and Platinized Titania Nanotubes: An in Situ FTIR Study
Photoinduced
conversion of surface-bound species on titania nanotubes
that were first oxidized and then reduced (Ti–NT–O<sub>2</sub>–H<sub>2</sub>) and on platinized titania nanotubes
subjected to oxidation and reduction (Pt–Ti–NT–O<sub>2</sub>–H<sub>2</sub>) has been investigated by means of in
situ FTIR spectroscopy. Bidentate and monodentate carbonates as well
as bicarbonates and carboxylates are formed subsequent to exposure
of both Ti–NT–O<sub>2</sub>–H<sub>2</sub> and
Pt–Ti–NT–O<sub>2</sub>–H<sub>2</sub> to
CO<sub>2</sub>. Formic acid was only observed on Pt–Ti–NT–O<sub>2</sub>–H<sub>2</sub>. UV illumination of the nanotubes led
to an increase in the number of surface-bound species as a result
of the further reaction with gas-phase CO<sub>2</sub> with a greater
increase in surface species on Ti–NT–O<sub>2</sub>–H<sub>2</sub> than on Pt–Ti–NT–O<sub>2</sub>–H<sub>2</sub>. The underlying basis of the photoinduced increase in adsorbed
species is discussed for both types of nanotubes. Photoinduced reactions
of surface species also take place and are remarkably different on
the two types of nanotubes. UV illumination of Ti–NT–O<sub>2</sub>–H<sub>2</sub> converts bidentate carbonates and bicarbonates
to monodentate carbonates and carboxylates. There are less, and different,
photoinduced reactions of surface species on Pt–Ti–NT–O<sub>2</sub>–H<sub>2</sub>: bicarbonates and monodentate carbonates
convert to bidentate carbonates on the platinized titania nanotubes,
and there is no obvious reaction involving carboxylates and formic
acid upon irradiation of the platinized nanotubes. These differences
in reactive behavior are discussed in the context of platinum acting
as an efficient trap for photoelectrons which mitigates against reduction
of Ti<sup>4+</sup> to Ti<sup>3+</sup>, stabilizes holes, and alters
the surface photochemistry taking place on the two different types
of nanotubes. Photoinduced holes play an important role in photochemistry
via oxidation of “structural water” and concomitant
production of undercoordinated titania sites
Methanol Oxidation to Formate on ALD-Prepared VO<sub><i>x</i></sub>/θ-Al<sub>2</sub>O<sub>3</sub> Catalysts: A Mechanistic Study
Well-defined
supported VO<sub><i>x</i></sub>/θ-Al<sub>2</sub>O<sub>3</sub> catalysts were prepared by atomic layer deposition
(ALD) with vanadium coverages of 0.48, 1.20, and 3.40 wt %. In-situ
Raman and UV–vis diffuse reflectance spectroscopy confirm that
the monovanadate, VO<sub>4</sub>, is the predominant vanadium species
at low loadings (0.48 and 1.20 wt %), while polyvanadate VO<sub>4</sub> is the predominant vanadium species for the 3.40 wt % VO<sub><i>x</i></sub>/θ-Al<sub>2</sub>O<sub>3</sub> catalyst. In-situ
FTIR spectroscopy of methanol oxidation to formate, in the absence
of gas-phase oxygen, on the 0.48 wt % VO<sub><i>x</i></sub>/θ-Al<sub>2</sub>O<sub>3</sub>, identified two different formates.
A comparison of the frequencies for the formates adsorbed on just
V<sub>2</sub>O<sub>5</sub> and on just θ-Al<sub>2</sub>O<sub>3</sub> demonstrates that one of these formates is located on aluminum
sites of VO<sub><i>x</i></sub>/θ-Al<sub>2</sub>O<sub>3</sub> while the other is located on vanadium sites. The oxidation
state of vanadium for the VO<sub><i>x</i></sub>/θ-Al<sub>2</sub>O<sub>3</sub> catalyst was determined by XPS after different
reaction times. On the basis of the time dependence of the formate
absorptions and the change in the oxidation state of vanadium in VO<sub><i>x</i></sub>/θ-Al<sub>2</sub>O<sub>3</sub>, a mechanism
is proposed for methanol oxidation and we discuss the role of the
alumina support in the mechanism
Role of the Surface Lewis Acid and Base Sites in the Adsorption of CO<sub>2</sub> on Titania Nanotubes and Platinized Titania Nanotubes: An in Situ FT-IR Study
An
understanding of the adsorption of CO<sub>2</sub>, the first
step in its photoreduction, is necessary for a full understanding
of the photoreduction process. As such, the reactive adsorption of
CO<sub>2</sub> on oxidized, reduced, and platinized TiO<sub>2</sub> nanotubes (Ti-NTs) was studied using infrared spectroscopy. The
Ti-NTs were characterized with TEM and XRD, and XPS was used to determine
the oxidation state as a function of oxidation, reduction, and platinization.
The XPS data demonstrate that upon oxidation, surface O atoms become
more electronegative, producing sites that can be characterized as
strong Lewis bases, and the corresponding Ti becomes more electropositive
producing sites that can be characterized as strong Lewis acids. Reduction
of the Ti-NTs produces Ti<sup>3+</sup> species, a very weak Lewis
acid, along with a splitting of the Ti<sup>4+</sup> peak, representing
two sites, which correlate with O sites with a corresponding change
in oxidation state. Ti<sup>3+</sup> is not observed on reduction of
the platinized Ti-NTs, presumably because Pt acts as an electron sink.
Exposure of the treated Ti-NTs to CO<sub>2</sub> leads to the formation
of differing amounts of bidentate and monodentate carbonates, as well
as bicarbonates, where the preference for formation of a given species
is rationalized in terms of surface Lewis acidity and or Lewis basicity
and the availability of hydrogen. Our data suggest that one source
of hydrogen is water that remains adsorbed to the Ti-NTs even after
heating to 350 °C and that reduced platinized NTs can activate
H<sub>2</sub>. Carboxylates, which involve CO<sub>2</sub><sup>–</sup> moieties and are similar to what would be expected for adsorbed
CO<sub>2</sub><sup>–</sup>, a postulated intermediate in CO<sub>2</sub> photoreduction, are also observed but only on the reduced
Ti-NTs, which is the only surface on which Ti<sup>3+</sup>/O vacancy
formation is observed
A novel CO2 utilization technology for the synergistic co-production of multi-walled carbon nanotubes and syngas
Dry reforming of methane (DRM) is a well-known process in which CH4 and CO2 catalytically react to produce syngas. Solid carbon is a well-known byproduct of the DRM but is undesirable as it leads to catalyst deactivation. However, converting CO2 and CH4 into solid carbon serves as a promising carbon capture and sequestration technique that has been demonstrated in this study by two patented processes. In the first process, known as CARGEN technology (CARbon GENerator), a novel concept of two reactors in series is developed that separately convert the greenhouse gases (GHGs) into multi-walled carbon nanotubes (MWCNTs) and syngas. CARGEN enables at least a 50% reduction in energy requirement with at least 65% CO2 conversion compared to the DRM process. The second process presents an alternative pathway for the regeneration/reactivation of the spent DRM/CARGEN catalyst using CO2. Provided herein is the first report on an experimental demonstration of a 'switching' technology in which CO2 is utilized in both the operation and the regeneration cycles and thus, finally contributing to the overall goal of CO2 fixation. The following studies support all the results in this work: physisorption, chemisorption, XRD, XPS, SEM, TEM, TGA, ICP, and Raman analysis.Other Information Published in: Scientific Reports License: https://creativecommons.org/licenses/by/4.0See article on publisher's website: http://dx.doi.org/10.1038/s41598-021-80986-2</p
Acceptorless Dehydrogenative Coupling of Neat Alcohols Using Group VI Sulfide Catalysts
Group VI sulfides
were synthesized via coprecipitation of elemental
sulfur and metal hexacarbonyl and characterized with XRD, XPS, and
TEM. These materials were then demonstrated as active catalysts for
the acceptorless dehydrogenative coupling of neat ethanol to ethyl
acetate, rapidly reaching equilibrium conversion and up to 90% selectivity.
Other primary alcohols form the corresponding esters, while diols
formed the corresponding cyclic ethers and oligomers