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–O₂–H₂) and on platinized titania nanotubes subjected to oxidation and reduction (Pt–Ti–NT–O₂–H₂) 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–O₂–H₂ and Pt–Ti–NT–O₂–H₂ to CO₂. Formic acid was only observed on Pt–Ti–NT–O₂–H₂. 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 CO₂ with a greater increase in surface species on Ti–NT–O₂–H₂ than on Pt–Ti–NT–O₂–H₂. 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–O₂–H₂ converts bidentate carbonates and bicarbonates to monodentate carbonates and carboxylates. There are less, and different, photoinduced reactions of surface species on Pt–Ti–NT–O₂–H₂: 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 Ti⁴⁺ to Ti³⁺, 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_<i>x</i>/θ-Al₂O₃ Catalysts: A Mechanistic Study

Well-defined supported VO_<i>x</i>/θ-Al₂O₃ 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, VO₄, is the predominant vanadium species at low loadings (0.48 and 1.20 wt %), while polyvanadate VO₄ is the predominant vanadium species for the 3.40 wt % VO_<i>x</i>/θ-Al₂O₃ catalyst. In-situ FTIR spectroscopy of methanol oxidation to formate, in the absence of gas-phase oxygen, on the 0.48 wt % VO_<i>x</i>/θ-Al₂O₃, identified two different formates. A comparison of the frequencies for the formates adsorbed on just V₂O₅ and on just θ-Al₂O₃ demonstrates that one of these formates is located on aluminum sites of VO_<i>x</i>/θ-Al₂O₃ while the other is located on vanadium sites. The oxidation state of vanadium for the VO_<i>x</i>/θ-Al₂O₃ 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_<i>x</i>/θ-Al₂O₃, 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 CO₂ on Titania Nanotubes and Platinized Titania Nanotubes: An in Situ FT-IR Study

An understanding of the adsorption of CO₂, the first step in its photoreduction, is necessary for a full understanding of the photoreduction process. As such, the reactive adsorption of CO₂ on oxidized, reduced, and platinized TiO₂ 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 Ti³⁺ species, a very weak Lewis acid, along with a splitting of the Ti⁴⁺ peak, representing two sites, which correlate with O sites with a corresponding change in oxidation state. Ti³⁺ 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 CO₂ 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 H₂. Carboxylates, which involve CO₂^– moieties and are similar to what would be expected for adsorbed CO₂^–, a postulated intermediate in CO₂ photoreduction, are also observed but only on the reduced Ti-NTs, which is the only surface on which Ti³⁺/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