in B2O3form (Fig. 63). When the doped structues were heated to higher temperatures, significant changes were detected in the XP spectra of both nanotubes and nanowires. The intensities radically decreased and in addition the B 1s shifted to lower binding energy (192.4 eV). The intensity change can be explained by the dimerization of B which was observed on different substrates: B–B interaction was detected on Rh [388,389], Fe [390] and Mo [391] surfaces. The binding energy shift to 192.4 eV may be due to the formation of certain kinds of suboxide-like species [388], which contain groups and may form linear chains containing B–B bonds. The presence of Ti–O–B structures in the linear chain cannot be exluded. The fact that after ion etching the B 1s intensity does not change indicates that the suboxide-like species do not penetrate to the subsurface significantly.
Multi-walled (B, N)-doped titanate nanotubes (TNTs) were prepared by a simple hydrothermal method. The effect of the doping amounts of B and N on the photocatalytic activity of TNTs was studied [379]. The chemical states of B and N in codoped TNTs were investigated using high-resolution XPS. The peak at around 191.8 eV appears in the B 1s spectrum, which can be ascribed to Ti–O–B bond [392]. The formed Ti–O–B structures suggest that B could be localized at the intersticial position or act as substitute for the H in the lattice of H2Ti3O7.
5. Titanate nanowires and nanotubes as supports in
Titanium oxide nanotubes with diameter of 8–10 nm and lengths of several tens to several hundred of nanometers have been used as support for preparation of nano-sized gold based catalyst [396].The catalyst was active in the water–gas shift reaction. It is quite interesting to observe that nano-sized gold can well fit into nanotubes. The application potential of the prepared Au/TNTs was investigated in water–gas shift reac-tion. The observed catalytic behavior of gold-supported TNTs correlated with the structural particularity and the nature of the TNTs. The catalytic performance of Au/TNTs may be impro-vable by increasing the diameter of the nanotubes or modifying the structure of the nanotubes to anatase phase.
Fe-doped trititanate nanotubes which were prepared via wet-chemistry exhibited noticeable catalytic activity in the water– gas shift reaction [266]. The Fe-doped trititanate nanotubes were clearly active around 650 K, and their catalytic activity increased as the temperature rose to 700 and 800 K. It is well-known that although pure TiO2is thermodynamically able to act as a catalyst, it is inactive for kinetic reasons. The control experiments show the inactivity of pure H-form titanate nanotubes or pure TiO2nanorods.
Very recently a comparative study on the reaction of COþH2O reaction catalyzed by gold supported on titanate nanowires and nanotubes and by rhodium supported titanate nanowires and nanotubes was carried out, respectively [397].
In the interpretation of the catalytic activities of the metals supported on titanate nanowires and nanotubes the role of the size and the oxidation states of active components were considered. The primary products in the COþH2O reaction are hydrogen and CO2besides a trace amount of CH3OH on both Au and Rh containing titanate nanowires and nanotubes catalysts. The conversions as a function of reaction time are shown inFig. 65.
200 198 196 194 192 190 188 186 200 198 196 194 192 190 188 186 193.2
B2O3 192.4
BOx
10%B after Ar+ etching
10% B, heated at 200 °C, 30 min
1% B 10% B
nanotube
193.2
B2O3 1000 c
ps
Binding energy [eV]
1000 cps
192.4 BOx
10% B after Ar+ etching
10 B%, heated at 200 °C, 30 min
nanowire
1% B 10% B
Fig. 63. B 1s Xp spectra on titanate nanotubes and nanowires during heat treatment and after ion eching.
Fig. 64. Catalytic activity of Au/TiO2 nanotube for CO and H2 oxidation.
Reproduced from Ref.[323].
The Au/TiONW obtained with NaBH4reduction has higher activity than that obtained with H2 reduction at 473–573 K.
Reduction with molecular hydrogen causes bigger particle sizes (5–9 nm) while NaBH4reduction resulted in smaller sizes (2–8 nm). At the same time a higher portion of gold adatoms exists in partially positive states (Auþ) after reduction with molecular H2 [52]. Finally it was concluded that the smaller Au particles play significantly more important role in the WGS reaction than the partially positive gold sizes.
A catalyst prepared from gold and deposited on titanate nanotubes has demonstrated a high activity for CO2reduction by hydrogen [398]. Recently the investigation of this reaction was expanded to Au- and Rh-doped titania and titante catalysts. Supported gold, rhodium and bimetallic rhodium-core-gold-shell catalysts were prepared. The supports were TiO2as well as titanate nanotube and nanowire formed in the hydrothermal conversion of titania. The catalytic properties were tested in the CO2hydrogenation at 493 K [304]. It was found that Rh containing catalysts exhibited the highest activity among different supported catalysts. The main product was CH4 in all cases and CO was formed only on catalysts supported on nanotube. The methane formation rates obtained on all catalysts are displayed inFig. 66for comparison.
DRIFT spectra revealed the existence of CO and formate groups on the Rh/NW, Rh/NT and on all bimetallic catalysts.
Additionally, a new band around 1770 cm1 was identified which was attributed to tilted CO that is bonded to Rh and interacts with a nearby oxygen vacancy of the support. This intermediate could dissociate easier and forms methane.
Although the activity of Rh showed higher activity than Au supported catalysts it is remarkable that Au supported on titanate nanowires exhibited higher activity than the Au/TiO2
where the support was P25 titania.
Protonated titanate nanotube (PTNT) was proved to be an effective solid acid catalyst for the hydroxyalkylation/alkyla-tion (HAA) of 2-methyfuran (2-MF) and n-butanal from lignocellulose [399]. Compared with some often used inor-ganic solid acid catalysts, the PTNT has higher activity for the HAA of 2-MF with n-butanol. The summarized results are presented inFig. 67.
According to the characterization results, the transformation of TiO2P25 to PTNT by hydrothermal treatment with NaOH solution and ion-exchange with acid solution leads to (1) the evident increase in the specific BET surface area and the acidity (including the amount acid sites and acid strength) of the catalyst, (2) the generation of Brönsted acid sites. All of these changes are responsible for the excellent performance of titanate nanotubes.
Nanotubular TiO2 based catalyst were synthesized for the enantioselective hydrogenation of 1-phenyl-1,2-propanedione (PPD) [400]. Cinchonidine was tethered directly with prior silanisation modification over activated TNTs. The character-ization data provide evidence of the covalent immobilcharacter-ization of cinchonidine. The HRTEM images and XPS demonstrated that the cinchonidine content affects the active phase dispersion and particle size distribution. It can be deduced from the XPS results that the Pt nanoparticles exhibited a positively charged
surface, which was attributed to metal deposition method and the strong interaction with the TNTs surface. The most selective catalyst showed poor reusability under optimum conditions and was deactivated by hydrogenation of 1-trimethoxysilyl-cinchonidine (TMS-CD) and Pt leaching after thefifth cycle.
A simple two-step procedure was developed for preparing highly efficient AuNPs/TNWs composite catalyst for 4-nitrophenol reduction [401]. The results showed that AuNP were crystalline with active planes such as (111), (200), (220), (311) and (222). Gold nanoparticles with an average diameter of 3 nm were also homogenously growing on the nanowire (TNWs) scaffold surface. Moreover, the catalytic activity of as-prepared AuNPs/TNWs nanocomposite exhibited a good catalytic activity for reduction of 4-nitrophenol. Meanwhile, it was noted that both gold particles size and gold content played an important role for influencing the catalytic efficiency of AuNPs/TNWs catalyst. This new route provided a useful platform for the fabrication of nanocatalysts based on noble metal NPs/TNWs nanocomposites.
Sodium titanate nanotubes doped with potassium were synthesized by the Kasuga method and tested as catalysts for biodiesel production [402]. Potassium was added to the nanotubes in order to increase their basicity and, consequently, improve their performance in the transesterification of soybean oil with methanol. The synthesis temperature and NaOH:KOH molar ratio used in the preparation were changed in order to define the best experimental conditions leading to solids with nanotubular morphology and improved potassium loading.
Sodium titanate nanotubes doped with potassium showed higher amounts of medium and strong basic sites than the pure sodium counterpart used as a reference. Their catalytic activity in the transesterication was also higher than that of the reference NaTNT samples. The best results were obtained with the sample containing 3.2–3.3 wt% of potassium.
The selected examples revealed that sodium titanate nano-tubes and acid treated titanates could exhibit significant catalytic activity in certain reactions by the generation of Brönsted acid sites. Metal (especially Au) loaded titanates are sometimes more active catalytically than Au on anatase or rutile TiO2. Titanates
Fig. 65. Conversion data in the COþH2O reaction on Au and Rh containing titanate nanowire and nanotubes. *The samples were obtained with NaBH4
reduction at 273 K. For comparison the activity of Rh supported on commercial TiO2P25 support is also shown[397].
can stabilize the gold in small sizes. Moreover, the contact structure between Au and nanotubes and nanowires is different from that of Au on anatase or rutile TiO2.