Catalysis Electrocatalysis and Photocatalysis 521 Reaction Catalysis

Elongated nanostructured titanates are characterised by a relatively high specific surface area, which is typically in the range of 200 to 300m2g_1 for nanotubes, and 20 to 50m2g_1 for nanofibres or nanorods. These values contrast with <20m2g_1 for TiO2 nanotubular arrays produced by anodisa-tion. The range of pore sizes (from 2 to 10 nm) ranks these materials as mesoporous; such structures are widely used in as a support in heterogeneous catalytic processes. The high surface area of the support facilitates high dispersion of the catalyst, while the open mesopores provide efficient transport of reagents and products.

Protonated titanate nanotubes can provide a moderate acid-base catalyst for esterification61 and the hydrolysis of 2-chlorethyl ethylsulfide.62 The Bronsted acidity of the nanotubular surface can also be increased by treatment with sulfuric acid, to give a value of approximately -12.7 on the Hammett scale.61 Most of the catalytic studies of titanate nanotubes are, however, focussed on the utilization of its surface as a support for highly dispersed catalysts.

Several methods are used to deposit active catalysts into the pores of titanate nanotubes and nanofibres. Incipient wetness impregnation of the catalyst precursor, followed by thermal or chemical treatment is a common approach. This method allows the deposition of relatively large quantities of the catalyst; however, the dispersion and the distribution of catalyst are usually inferior to that obtained using other methods of deposition. A second method involves the precipitation of active materials on the surface of the nanotubes, which can be initiated by chemical, photo- or electro-chemical treatments. Such methods enable a better distribution of the catalyst, but the loading of metal is limited by the amount of precursor adsorbed onto the surface of the nanotubes. In addition, there is also the possibility of precursor precipitation in the bulk solution occurring as a side reaction.

Another in situ method for doping catalysts into titanate nanotubes is the addition of a catalyst precursor to the TiO2-aqueous NaOH mixture prior to hydrothermal synthesis. The method allows catalyst atoms to be embedded into the crystal structure of the titanate nanotubes. A limitation of the method is that catalyst loading and dispersion are not easily controlled.

Since titanate nanotubes are characterised by good ion-exchange proper-ties,63 one method of catalyst deposition involves preliminary ion-exchange of the catalyst precursor in its cationic form with protons within nanotubular titanates (see Chapter 4, Figure 4.5). This allows an atomic-scale distribution of metal cations in the titanate lattice, achieving a higher metal loading compared to the adsorption of the precursor on the surface. Washing the sample with clean solvent, followed by reduction or chemical treatment, avoids precipitation of the catalyst in the bulk solution and leads to the formation of clusters or nanoparticles of catalysts evenly distributed on the nanotube surface only. A suitable choice of the ionic form of the metal precursor can significantly help to increase catalyst loading and to maintain high catalyst dispersion. For example, the use of a di-ethylendiamine complex of gold, [Au(en)2]3 + , instead of tetra-chloroaurate, [AuCl4]~, increases the sorption of the gold precursor onto the titanate nanotubes by more than ten-fold.64,65

Examples of various metal catalysts deposited on the surface of titanate nanotubes are shown in Figure 5.5. The deposition method was ion-exchange followed by chemical treatment. Such an approach can achieve a relatively high catalyst loading, whilst maintaining a relatively small average particle size (see Table 5.3). Not all catalysts can be deposited using this method, for example, in cases where a suitable cationic form of the metal is unavailable in aqueous solution.

The most widely studied nanotubular supported catalyst is gold (Au/TiNT). The study of such materials parallels the work done on Au/TiO2 catalysts, which are promising for low-temperature CO oxidation.66,67 The early Au/ TiNT catalysts demonstrate an activity comparable to that of the standard Au/

Figure 5.5 Examples of titanate nanotubes decorated with metal nanoparticles which are used in catalysis: a) Au/TiNT, b) Pd/TiNT, c) Pt/TiNT, d) RuOOH/ TiNT, e) Ni/TiNT and f) CdS/TiNT. (Images a), c) and d) are reproduced with kind permission from ref. 65 and image f) from ref. 119).

TiO2 materials. However, recent improvements in performance have been attempted by the acid-assisted transformation of nanotubes (with deposited gold) to TiO2 nanoparticles,68 or by the high-temperature transformation of gold particles on nanotubes to those on nanorods.69 Some successful attempts to reduce the use of precious metals such as gold have also been made using CuO (ref. 70) or Cu-Au66 composites deposited on the surface of titanate nanotubes.

Catalysts prepared from gold and deposited on titanate nanotubes have also demonstrated a high activity for CO2 reduction by hydrogen71 and water shift reactions.72 Most of these catalysts were prepared by precipitation from HAuCl4 solution where the dispersity and loading of gold nanoparticles can be difficult to control. Further improvements in catalyst preparation utilising ionexchange methods should lead to an improved catalyst activity.

The activity of palladium nanoparticle catalysts in a metal or metal hydroxide form deposited on the surface of titanate nanotubes or nanofibres, has been studied for the hydrogenation of phenol to cyclohexanone;73 the hydrogenation of (o)-chloronitrobenzene to (o)-chloroaniline;74 and the isomerisation of allylbenzene (double bond migration),75 see Table 5.3. Copper(ii) catalyst embedded into titanate nanotubes in situ, followed by conversion to TiO2 nanotubes using calcination, showed good activity and high selectivity in the catalytic reduction of NO.76

Ruthenium(III) hydrated oxide (RuOOH) appears to be a promising catalyst for the selective oxidation of alcohols to aldehydes. The catalyst precursor is deposited onto the surface of the titanate nanotubes using ion-exchange with Ru31 in aqueous solution, followed by hydroxylation with NaOH.77 The resultant catalyst nanoparticles are evenly distributed on the surface of the titanate nanotubes, and the increase in RuOOH loading results in an increase in the catalyst particle density on the surface, rather than in the size of the particles (see Figure 5.6). Such conservation of the average particle size, and consequently the specific surface area, of RuOOH nanoparticles during changes in catalyst loading, is consistent with the independence of specific catalytic activity on loading. Figure 5.7 shows the specific catalytic activity (TOF) of the RuOOH/TiNT catalyst in the selective oxidation of benzyl alcohol to benzaldehyde as a function of catalyst loading.77 Within the margins of error, the value of TOF remains unchanged over a range of loadings from 0.5 to 9 wt%.

Potential catalysts for the selective oxidation of dibenzothiophene by hydrogen peroxide at 60 °C are WOx/TiO2 nanostructures, obtained by the calcination of titanate nanotubes impregnated with (NH4)2WO4 (ref. 78). This reaction models the process of the desulfurization of oil and the catalyst demonstrates a high activity. During preparation of the catalyst, an interesting morphological transformation has been reported.79 The calcination of titanate nanotubes impregnated with (NH4)2WO4 at 500 °C results in a collapse of the tubular structure and a release of the residue Na1 ions to the surface of fibrous anatase. This leads to the formation of highly dispersed Nax(WO4) nano-particles in which tungsten is in a tetrahedral coordination, providing a high activity for selective oxidation.

Table 5.3 Reported catalytic processes using nanostructured titanates and Ti02.

Catalyst

Method of preparation

Particle

Loading

Formula

size1 nm

Au/TiNT

Deposition -

4-17

0.1-2 wt%

Au-Cu/TiNT

precipitation

Au/TiNT

Deposition -

10

1 wt%

Au/Ti02 NR

precipitation

Au/TiNT

Adsorption -reduction

2-5

2-6 wt%

Au/Ti02 NP

Au/TiNT

Deposition -

3-5

1.5 wt%

precipitation

Cu(ii)/Ti02 NT

Impregnation or in situ

n/a

2 wt%

deposition

CuO/TiNT

Impregnation -calc.

>5

6 wt%

200 'C

Pd/TiNT

Impregnation -

n/a

1 wt%

Pd/TiNF

reduction

Pd/TiNT

Impregnation -

2-3

1-7 wt%

Pd/TiNF

reduction

Pd(ii)/TiNT

Ion-exchange

2-5

10wt%

Pt/Ti02 NT

In situ re-crystallisation

0.3- 3

30 wt%

Pt/TiNT

Adsorption-

2

2 wt%

Au/TiNT

photoreduction

10

Ru(iii)/TiNT

Ion-exchange-alkali

1-2

1- 9 wt%

treatment

TiNT

hydrothermal

n/a

n/a

WOJTiNT

Impregnation-calcined

<1

5-20

WOJTiOj NF

at 500'C

Activity orperformance

Benefits

Ref.

TWC) = 47

Competitive with com

66,67

mercial Au/Ti02

T5„.,(,ÎC)= 123

Conversion Au/TiNT to

69

T5Ir,rC) = 77

Au/Ti02 NR

T5Ir,rC) = 70

Conversion Au/TiNT to

68

T5Ir,(/C) = 25

Au/Ti02 NP

n/a

Early data show feasi

72

bility of catalyst

TWC) = 150

High dispersity of

76

catalyst

TWC) = 90

High activity

70

TOF = 94h_1

High activity, selectivity

73

Select = 99%

and deactivation

resistance

TOF= 186 h_1

Selectivity can be con

74

Select = 84 %

trolled by Pd size

TOF = 2.3 IT1

High dispersity of the

75

Sel. = 93 %

catalyst at high

loading

Sel(CH4) = 77%

Higher performance

81

TOF = 20h_1

than Pt/Ti02

n/a

Early data shows feasi

71

bility of catalyst

TOF = 450 h_1

High dispersity of cata

77

Sel. = 99

lyst at high loading

n/a

Feasibility of titanate

62

nanotubes in

hydrolysis

TOF = 54h~1

Route to obtain highly

78

dispersed WO^ on Ti02

CO + 0.5 02 -> C02 CO+ 0.5 02 -> C02 CO+ 0.5 02 -> C02

6NO + 4NH, -> 5 N2 +6 H20 CO+ 0.5 02 -> C02 OH O

C,H,SC,H4C1 + H,0 -> C,H,SC,H4OH + HCl o o dispersed WO^ on Ti02

TiNT: titanate nanotubes. TiNF: titanate nanofibres. NR: nanorods. NP: nanoparticles, n/a: not available.

Figure 5.6 TEM images of ruthenium(iii) hydrated oxide nanoparticles deposited onto TiO2 nanotubes and histograms of particle-size distribution for metal loadings of: a) 1.1 wt.%, b) 3.4 wt.% and c) 8.7 wt.%. (Images are reproduced with kind permission from ref. 64).

Particle size I nm

Figure 5.6 TEM images of ruthenium(iii) hydrated oxide nanoparticles deposited onto TiO2 nanotubes and histograms of particle-size distribution for metal loadings of: a) 1.1 wt.%, b) 3.4 wt.% and c) 8.7 wt.%. (Images are reproduced with kind permission from ref. 64).

Figure 5.7 The specific catalytic activity of a RuOOH/TiNT catalyst as a function of RuOOH nanoparticle loading. The turnover frequency (TOF) is the rate of benzyl alcohol oxidation with oxygen in a continuous flow reactor at 117 °C. (Data are reproduced with kind permission from ref. 77).

Platinum deposited on the surface of wide-pore nanotubular catalyst supports can be synthesised using [Pt(NH3)4](HCO3)2 complex as a template.80 In this method, the template is also a catalyst precursor. Such Pt/TiO2 nano-tubular catalysts can be characterized by their wide diameter (100-200 nm), and the Pt nanoparticles are well dispersed on the nanotubular surfaces with a high loading. Catalysts show a good performance in the CO reduction with H2 selectively forming CH4, as well as in the water shift reaction.81

All reported applications of titanate nanotubes in catalysis emphasise the benefits of a high surface area, together with the versatility of the surface chemistry and electronic interactions between the catalyst and the support, which allow an improved catalytic activity. The low cost of titanate nanotubes opens a novel route for nanostructured TiO2-supported catalysts using either acid or thermal transformations. The control of the localisation of catalyst (either inside the hollow of the tubes or on the external surface) is still chal-lenging64 and represents an important topic for further studies.

5.2.2 Supercapacitors and General Electrochemistry

Composite electrodes consisting of titanate nanotubes supporting nano-particles of precious metal have been also studied for electrocatalytic processes. These include the electrochemical reduction of CO2 to methanol using a RuO2/ TiNT composite electrode82 and the oxidation of hydrazine (N2H4) using palladium-decorated titanate nanotubes.83 In both cases, the composite electrodes demonstrated a better performance than TiO2-based ones.

The oxides of transition metals, which can have several valence states, are possible materials for electrical energy storage in electrochemical capacitors. Effective dispersion of these oxides on the electrode surface is critical to yield a high capacitance, which can be achieved using titanate nanotubes as a support for the metal oxide. Early reports of vanadium(v) oxide deposited on the surface of titanate nanotubes showed the feasibility of such composites as capacitors.84 Further improvements in capacitance stimulated the use of ruthenium (RuO2)85,86 or mixed ruthenium chromium oxides (Ru1 yCryO2; ref. 87) deposited onto the surface of titanate nanotube composites, allowing values of specific (per mass of RuCr oxide) capacitance of ca. 1272 F g 1 to be obtained for a 4 wt% of Ru1 yCryO2 loading (see Table 5.4). This value is almost twice that for the electrochemical capacitance of the bulk Ru1 yCryO2 positive electrode. The high cost of ruthenium has encouraged the search for lower-cost substitute elements, resulting in the synthesis of cobalt hydroxide88 and cobalt nickel double hydroxide deposited on TiNT, which have a specific (per mass of metal oxide) capacitance of approximately 1000 Fg-1.89

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