An understanding of the adsorption of CO 2 , the first step in its photoreduction, is necessary for a full understanding of the photoreduction process. As such, the reactive adsorption of CO 2 on oxidized, reduced, and platinized TiO 2 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 3+ species, a very weak Lewis acid, along with a splitting of the Ti 4+ peak, representing two sites, which correlate with O sites with a corresponding change in oxidation state. Ti 3+ 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 2 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 2 . Carboxylates, which involve CO 2 − moieties and are similar to what would be expected for adsorbed CO 2 − , a postulated intermediate in CO 2 photoreduction, are also observed but only on the reduced Ti-NTs, which is the only surface on which Ti 3+ /O vacancy formation is observed.
Single phasic anatase titania with up to 10 mol % vanadium doping, having a particle size of about 12 nm, was synthesized by a sol-gel route and its photocatalytic behavior evaluated for the photo-oxidation of ethene using sunlight-type excitation. Incorporation of vanadia in the titania lattice was established using techniques like XRD, XPS, surface area analysis, TEM, DR-UV-visible spectroscopy, Raman spectroscopy, and FTIR. Vanadium doping led to a red shift in the UV-visible absorbance spectra compared to pristine titania, thus enhancing the absorption in the visible region. Synthesized nanotitania and vanadia-doped titania exhibit enhanced photocatalytic activity compared to the commercial bulk anatase TiO 2 , with maximum activity obtained for the 5 mol % vanadium-doped titania sample, which was attributed to an optimum concentration of V 4+ and V 5+ species. The role of these species in enhancement of catalytic activity and reaction mechanism has been discussed.
The development of noble metal-free catalysts for hydrogen evolution is required for energy applications. In this regard, ternary heterojunction nanocomposites consisting of ZnO nanoparticles anchored on MoS -RGO (RGO=reduced graphene oxide) nanosheets as heterogeneous catalysts show highly efficient photocatalytic H evolution. In the photocatalytic process, the catalyst dispersed in an electrolytic solution (S and SO ions) exhibits an enhanced rate of H evolution, and optimization experiments reveal that ZnO with 4.0 wt % of MoS -RGO nanosheets gives the highest photocatalytic H production of 28.616 mmol h g under sunlight irradiation; approximately 56 times higher than that on bare ZnO and several times higher than those of other ternary photocatalysts. The superior catalytic activity can be attributed to the in situ generation of ZnS, which leads to improved interfacial charge transfer to the MoS cocatalyst and RGO, which has plenty of active sites available for photocatalytic reactions. Recycling experiments also proved the stability of the optimized photocatalyst. In addition, the ternary nanocomposite displayed multifunctional properties for hydrogen evolution activity under electrocatalytic and photoelectrocatalytic conditions owing to the high electrode-electrolyte contact area. Thus, the present work provides very useful insights for the development of inexpensive, multifunctional catalysts without noble metal loading to achieve a high rate of H generation.
Developing
highly active oxygen evolution and reduction reaction
(OER/ORR) bifunctional electrocatalysts is key to multiple technologies,
including regenerative fuel cells and metal-air batteries. To this
end, we have investigated structure–activity relationships
in Pb2Ru2O7–x
having pyrochlore structure by tuning the structural oxygen vacancy
(Ovac) and metal oxidation states. Increase in Ovac with temperature boosts the ORR activity by facilitating molecular
oxygen dissociation via decrease in work function. Ovac formation is accompanied by lowering of the Ru(V)/Ru(IV) ratio due
to charge-compensation which leads to decreased OER activity. Air-annealing
of Pb2Ru2O7–x
accelerates the formation of Ovac in comparison to Ar-annealing
since atmospheric oxygen facilitates the reduction and phase-segregation
of Ru from Pb2Ru2O7–x
as RuO2. A maximum bifunctionality index and specific
bifunctionality index of 0.69 V and 274.0 μA/cm2
BET, respectively, are observed for pristine Pb2Ru2O7–x
. However, the
activity is skewed toward OER for pristine Pb2Ru2O7–x
, creating an asymmetric bifunctional
property which is not desirable for practical applications. To reduce
the asymmetric behavior, pristine and air-annealed Pb2Ru2O7–x
samples at 700 °C
are physically mixed which yields a higher symmetric OER/ORR activity
(|Δi
OER(η = 0.25 V)‑ORR(η = −0.45 V)
specific|: pristine = 0.25 mA/cm2
BET, Air-700 °C = 0.20 mA/cm2
BET, physical mixture = 0.037 mA/cm2
BET). The
inverse OER/ORR relationship in Pb2Ru2O7–x
is attributed to the presence of
an optimal ratio of 0.75 for Ru(V)/Ru(IV) and Ovac/Olattice, which provides symmetric bifunctional activity essential
in electrochemical devices. An increase in Ru(V)/Ru(IV) ratio in pristine
Pb2Ru2O7–x
with no detectable Ru dissolution in the electrolyte observed subsequent
to a 5-h OER hold-test, confirming high stability.
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