Titanium Dioxide

Titanium Dioxide as a Catalyst Support

Synopsis

Titanium dioxide is a remarkable ceramic material with unique properties that have proved useful in a range of applications. It has a very high refractive index, which gives it the ability to disperse light better than most other materials. This property has been exploited to give the white pigment used in paints, and in sunscreen to reflect ultraviolet radiation away from the skin.

Thin layer films of titanium dioxide are used to cover medical implants. These films allow integration between the implant and bone tissue, which greatly reduces the possibility of implant rejection by the body. This use illustrates the inertness and biocompatibility of titanium dioxide.

In the presence of ultra violet light, titanium dioxide behaves as a very efficient photocatalyst. This property, attributed to the materials strong oxidizing power, is utilized in chemical cleaning processes. Applications in self-cleaning glass, air purification and treatment of cancerous tumours have all benefited from this property.

While titanium dioxide is used as a catalyst itself, in recent years, studies have shown that it is just as effective as a support to a catalyst.

The review will begin by explaining the importance of catalysts in industrial-scale processes and the significance and reasons of interest for using titanium dioxide as a catalyst support.

The topic areas chosen for discussion are hydrodesulfurization, polymer electrolyte membrane fuel cells and the Fischer-Tropsch reaction. The principal aim of the review will be to identify and discuss the trends and conflicts across research papers.

Hydrodesulfurization concerns the removal of sulfur impurities from hydrocarbons. The process is aided by a catalyst, which is supported on another material, and studies where this material is titanium dioxide have shown, in general, positive results.

Polymer electrolyte membrane fuel cells (PEMFCs) are a type of fuel cell being developed for use in many applications, such as transportation and portable electronics. In PEMFCs, hydrogen and oxygen react to produce electrochemical energy. Studies have shown that using a titanium dioxide-supported platinum catalyst deposited upon a Nafion membrane enhances this reaction in PEMFCs.

In Fischer-Tropsch synthesis, carbon dioxide and hydrogen are reacted to form different chain-length hydrocarbons. Transition metals are used to catalyse the reaction and cobalt is the most effective in this use. The catalysts also contain a promoter, and a high surface area support. Research carried out where titanium dioxide has been used as the catalyst support has shown interesting results.

The review will close with a conclusion containing a summary of the results and ideas learnt throughout the research. Problems and limitations of using titanium dioxide as a catalyst support will be discussed, and as the research in this area is current, and still developing, future developments will be accounted.

Introduction

Titanium dioxide is a remarkable ceramic material with unique properties that have proved useful in a range of applications. It has a very high refractive index, which gives it the ability to disperse light better than most other materials. This property has been exploited to give the white pigment used in paints, and in sunscreen to reflect ultraviolet radiation away from the skin.

Thin layer films of titanium dioxide are used to cover medical implants. These films allow integration between the implant and bone tissue, which greatly reduces the possibility of implant rejection by the body. This use illustrates the inertness and biocompatibility of titanium dioxide.

In the presence of ultra violet light, titanium dioxide behaves as a very efficient photocatalyst. This property, attributed to the materials strong oxidizing power, is utilized in chemical cleaning processes. Applications in self-cleaning glass, air purification and treatment of cancerous tumours have all benefited from this property.

While titanium dioxide is used as a catalyst itself, in recent years, studies have shown that it is just as effective as a support to a catalyst.

1.1. Heterogeneous Catalysis

The term catalysis was first defined in 1835 by Baron J. J. Berzelius to “describe the property of substances that facilitate chemical reactions without being consumed in them” [1]. This definition allows for the possibility that a catalyst may be able to either increase or decrease the rate of a chemical reaction. However, in most occurrences, the addition of a catalyst affects the rate by increasing it. Catalysts are employed in over 90% of chemical processes, and so their importance cannot be underestimated [2].

Catalysts can be of two types: heterogeneous or homogeneous. The difference between these two forms is the composition of the phases present during reaction. Heterogeneous catalysts are present in a different phase to reactants and products, usually solid, whereas homogeneous catalysts are present in the same phase as reactants and products, usually liquid.

Research carried out in the topic area of this review has involved using titanium dioxide as the support for a catalyst, of which the catalyst has been the solid phase. This concise introduction will therefore focus on heterogeneous catalysis.

When a catalyst is present in a different phase to the reactants and products, such as in heterogeneous catalysis, the advantage is that it is relatively simple to separate the catalyst from the product e.g. separating a liquid and a solid. In addition, heterogeneous catalysts are able to withstand more extreme conditions (e.g. temperature, pressure) than an equivalent homogeneous catalyst [1].

Ethene and hydrogen are passed over a metal catalyst (usually nickel) and adsorb onto its surface, shown in the first step. Hydrogen dissociatively chemisorbs on to the surface of the metal catalyst, and then migrates to the adsorbed ethene molecule (second step). This initially yields a surface alkyl, as shown in the third step, and following migration of the second hydrogen atom, the saturated hydrocarbon ethane is formed (fourth step) [3].

So, in general, the process that occurs in a chemical reaction involving a heterogeneous catalyst follows:

i. Adsorption of reactants onto catalyst surface

ii. Chemical reaction of reactants

iii. Desorption of reaction products.

In a chemical reaction, the catalyst must provide an alternative pathway by which the reactants can follow to yield products. If the pathway provided by the catalyst has lower energy barriers than those of the non-catalysed form of the same reaction, the presence of the catalyst would give significant improvements in the rate of the reaction [1].

The following discusses the properties of a catalyst that may affect the energy barriers of a pathway provided by the catalyst.

Arguably, the most important physical characteristic of a heterogeneous catalyst and its support is its surface area and porosity. The catalyst should either be formed as “finely divided substrates or crystallites” with ample internal pores that are readily accessible. The specific surface area is the surface area of the catalyst divided by the mass of the sample. A high surface area results from many fine connected particles, and one gram of a catalyst support may have a surface area equivalent to that of a tennis court [3].

1.2. Catalyst Supports

Many heterogeneous catalysts show optimum functionality when supported upon another material, often single- or mixed-metal oxides.

Supported catalysts consist of three key components: the active phase, the promoter and the support. The active phase is the only phase that directly takes part in a chemical reaction, and the catalytic effect is dependent on the presence of this phase. The promoter is a phase that adds enhancement or stability to the catalytic activity. The support is the material upon which the active phase is deposited.

In a heterogeneous catalyst, the active phase functions through its surface and so its exposure to reactants is important if it is to have a high catalytic activity. This is most effectively achieved by dispersing the active phase upon another material, namely the support, which allows the active phase to be composed of particles as small as 1 nm. Without the support, this is not possible. The mass of the active phase can range from less than 1 wt. % (e.g. platinum metals in reforming) to 50 wt. % (e.g. metal sulfide catalysts) [2].

Utilising a support has many advantages in catalysis. Efficient dispersion of the active phase can maximise the surface area covered, and reduce the amount of the phase required for its application. This can contribute greatly to reducing costs, a major factor taken into consideration in industry. The support provides “mechanical properties needed for a long-lasting operation (hardness, resistance to crushing or erosion)” [2] and also provides better resistance to sintering.

Initially, the materials used as a support were naturally derived such as silicates, aluminium and magnesium hydroxysilicates and minerals. Since the 1940s, the catalyst manufacturing industry expanded and there was a need to develop catalyst supports that exhibited properties that could be adapted, because natural materials did not always display the ideal properties required (ibid.).

Synthetic catalyst supports are manufactured from metal oxides which have high surface areas, are relatively cheap and exhibit excellent chemical and thermal stability. For example, an unsupported copper oxide catalyst is thermally unstable at high temperatures, but copper oxide supported upon high surface area alumina is thermally stable [4]. Theoretically, it is possible to use any material that displays a high surface area as a catalyst support, but ideally it should conform to other criteria [2].

The support should have a porous structure that allows reactants and products to diffuse in and out of easily. The ability to form the support into different physical shapes must also be taken into account as this allows its use in a range of physical conditions. The ability of the support to form interactions with the active phase is important because it can provide stabilisation to the active phase if it has an unusual particle structure (ibid.).

Catalysts used in processes involving hydrogen (e.g. hydrogenation, hydrodesulfurization, hydrodenitrogenation) use supports that have high surface areas, whereas low surface area supports are used in selective oxidation processes (e.g. olefin epoxidation). In cases where the active phase is too reactive, a nonporous low surface area support can be used to reduce the contact times of the reactants on the catalyst. In cases where the active phase has a moderate reactivity, a porous low surface area support is often utilised (ibid.).

The nature and size of the pores of a catalyst support affect the catalytic activity greatly. An open pore structure ensures the best use of the available pore volume, but if molecular size and shape selectivity is required in the reaction, a bimodal pore distribution is recommended. This distribution consists of large channels connected by narrower regions, and only molecules of a certain size or shape may pass through. This type of distribution is observed in zeolite catalysts (ibid.).

Supported catalysts can be prepared by a range of methods. One method may be more efficient in preparing the desired catalyst over another method, and this would depend on the properties that are required of the prepared catalyst. The methods and their key steps are summarised below.

Impregnation: In this method the pores of the support are filled with a solution of the active phase, and the solvent is then evaporated. The pores can be filled either by spraying the support with the active species solution or by adding the support to the solution in known amounts. This is beneficial in that it prevents the excess use of the active species, which can often be costly. Drying ensures the solvent evaporates and this is carried out at high temperature. If the active phase consists of more than one metal catalyst it is important that selective adsorption does not occur; this may lead to the formation of an undesired concentration of the metals in the prepared catalyst. The impregnation method is commonly used when preparing small amounts of a catalyst [4].

Adsorption: Metal salts or metal ion species can be selectively adsorbed onto a support by either physisorption or by the formation of chemical bonds with the active phase on the support. This method is suitable when control is required over the amount of active phase present and dispersion over the surface of the support, and as such is widely used in industry when preparing supported catalysts (ibid.).

Coprecipitation: Salt solutions of the active phase and the catalyst support are prepared and a precipitating agent (e.g. NaOH or NaHCO3) is added. Hydroxy salts or hydroxides precipitate and these are filtered off. During the drying and calcination stages carbon dioxide and water are removed, and during the reduction stage oxygen is removed. The final product is a porous catalyst that exhibits a high surface area. This procedure is favoured when the primary aim is to obtain a material with the optimum catalytic activity per unit volume and materials are cheaply available. It is important to keep the initial solution homogeneous so that both the active phase and the support precipitate simultaneously [5].

Deposition: This is the process of laying down an active phase on the surface of a support. It is achieved by condensing a metal vapour over an agitated support, a process known as sputtering. A drawback of this method is the requirement of a high vacuum, which may limit its use to only preparing ‘model' catalysts [4].

Chemical Vapour Deposition (CVD): Similar to the Deposition method, a support is plated with a vapour, but in this technique the vapour is either a volatile inorganic or an organometallic compound (ibid.).

Hydrodesulfurization

Global energy consumption continues to increase every year, with the majority of energy provided by non-renewable sources such as coal, oil and natural gas. Over the past 50 years, oil has been the world's dominant energy resource. In 2008, 35% of the world's total energy consumption was sourced from oil[1], although this is a small decrease over the past decade the importance of this fuel cannot be emphasized enough.

Oil is composed of hydrocarbon chains of different lengths, and along with these very useful products there are other non-hydrocarbon compounds which must be removed prior to use. These are compounds of sulfur, nitrogen and oxygen, and metals and salts.

Sulfur, which is present in the highest proportion of all the non-hydrocarbon compounds, can cause serious problems if it is not removed from oil. During combustion, it reacts with oxygen to form sulfur dioxide, a major contributor to pollution. Sulfur species are also a poison in catalysis. In catalytic converters, sulfur chemisorbs onto active sites on catalysts and reacts, forming strong sulfur-metal bonds [6]. This reduces the life of the catalyst drastically.

Authorities worldwide have imposed environmental legislation to reduce the harmful emissions from fuels, and this has increased the need to use ‘cleaner' fuel technology. This demand has prompted studies into more efficient refining processes that remove these non-essential compounds from oil.

A key step in the refining of oil involves the removal of sulfur and its compounds (e.g. thiols and sulfides), a process known as hydrodesulfurization. It is a catalysed reaction and can occur via two different pathways.

In the direct route a displacement occurs, where a hydrogen atom replaces the sulfur atom in the hydrocarbon chain without hydrogenation of any carbon-carbon double bonds. In the hydrogenative route, at least one aromatic ring adjacent to the sulfur-containing ring is hydrogenated, followed by removal and replacement of the sulfur atom by hydrogen. It is also possible for the sulfur atom to be removed before hydrogenation of the aromatic ring [7].

These two pathways are illustrated in 2, where DDS represents the direct route, and HYD represents the hydrogenative route.

The HYD route shows the hydrogenation of carbon-carbon double bonds followed by removal of sulfur, whereas the DDS route shows the removal of sulfur without hydrogenation of any carbon-carbon double bonds.

In the hydrogenative route, hydrogenation of a carbon-carbon double bond destabilises the aromatic ring, which weakens the carbon-sulfur bond. This facilitates the insertion of the metal (catalyst) atom which results in the removal of the sulfur atom.

Insertion of the metal (catalyst) atom in the carbon-sulfur bond in the direct route occurs because the metal-sulfur bond that forms is energetically favourable. The sulfur is removed in the form of hydrogen sulfide [7].

The most commonly used catalysts in hydrodesulfurization are Co(Ni)Mo(W)/Al2O3 where molybdenum or tungsten (sulfide) catalysts have promoters of either cobalt or nickel, and are dispersed on an alumina support [7, 9].

The alumina support has a high surface area, typically of the order of 230 m2/g [10], and this results in a very efficient dispersion of the active molybdenum or tungsten species. In order to meet the stringent regulations on sulfur content in fuels, research is underway into higher activity catalysts.

However, an alternative to researching new catalysts is to vary the catalyst support, which plays an important role by modifying the active species and taking part in the hydrodesulfurization reaction by acting as a co-catalyst [7, 11]. Studies have shown that some supports promote excellent catalytic activities, but are limited by their low surface area, which reduces the effect of increased activity.

A common method used to produce catalysts is known as the incipient wetness impregnation. A known amount of the active species in solution is added to the powdered support to ensure complete coverage of the powder, followed by solvent evaporation. The result of this method is an even dispersal of catalyst over the active species material.

In recent years, titanium dioxide-supported catalysts have been investigated. They have shown higher activity than those supported on alumina [9]: initial studies have shown that molybdenum catalysts supported on titanium dioxide are 4.4 times more active than those supported on aluminium oxide [12].

Despite the increased activity of titania in comparison to conventional alumina supports, its surface area is much smaller: Dzwigaj et al. reports the surface area as 50 m2/g [10].

The study by Dzwigaj et al. focuses on conventional titania support (72 m2/g) and a high surface area support prepared by a novel method developed by the Chiyoda Corporation that has a higher surface area (120 m2/g). The molybdenum loading for the Chiyoda support was between 8 – 12 wt. %, which is comparable to that of alumina supported catalysts. Conventional titania limited the loading to 6 wt. %. Molybdenum was deposited on the titania supports and the catalytic properties were investigated in the hydrodesulfurization of dibenzothiophene [10].

The results of this study showed that up to molybdenum loading of 7 wt. % the catalytic activities of the conventional support and the Chiyoda support were similar, but for loading above this value the Chiyoda support achieved higher activities. The report also states that the catalytic activities of the conventional titania support surpass those of the alumina support (whose surface area is not given) [10].

A study by Gulková et al. further investigated the effect on hydrodesulfurization activity by varying the surface area of the titania support as a function of molybdenum loading. Three titania supports with surface areas of 140, 230 and 407 m2/g were used, and the loading was varied from 2 – 32 wt. % molybdenum. The catalytic properties were investigated in the hydrodesulfurization of thiophene and were compared against a commercial alumina catalyst (15 wt. % molybdenum loading; surface area: 210 m2/g). The authors expected catalytic activity to increase with higher molybdenum loadings, however this was not observed [13].

As shown in 3, the highest level of activity was achieved at a loading of approximately 20 wt. % for all three titania supports. The highest catalytic activity was observed for the titania with a surface area of 407 m2/g, followed by 140 m2/g titania and the support with a surface area of 230 m2/g had the lowest catalytic activity. These are surprising results; it is generally expected that catalytic activity should increase as surface area increases.

The authors' assume that these results are due to differences in impurities of the titania supports used in the investigation. The credibility of this article is debatable as there are many instances where the authors' simply make assumptions or speculations, and the research conducted has been very restricted to just investigating surface areas of titania supports with very little reasoning behind the results.

Key: Square: 407 m2/g; open circle: 140 m2/g; full circle: 230 m2/g; star: Alumina support

Other... have been conducted studies investigating the use of titanium dioxide as a catalyst support in the hydrodesulfurization process. For simplicity, the results of these studies, and those already discussed, are summarised in Table 1.

The methods used by different authors to prepare and investigate the catalytic activity and effect of titania have been very similar. In general, the molybdenum active species is added to the titania support by the incipient wetness impregnation method with a ammonium heptamolybdate solution as a precursor. The solution is left to diffuse into the pores of the support and then dried to remove any solvent. A solution containing a cobalt species is added to impregnate the cobalt promoter, which is followed by a calcination step [10, 14, 15, 16].

During the preparation of supporting the catalyst species on titania, results have shown that the surface area of the titania support has reduced from its initial surface area when in a dry powder form. Escobar et al. [14] initially prepared a titania support with a surface area of 343 m2/g, which, after the processes of impregnation, sulfidation and calcination, reduced to 174 m2/g. Similarly, Cortes-Jacome et al. [15] prepared a titania support with a surface area of 369 m2/g which reduced to between 204 – 229 m2/g, depended on the pH used during the preparation of the supported catalyst.

Discuss the use of titania-alumina mixed oxide

Polymer Electrolyte Membrane Fuel Cells

Increasing energy consumption and declining amounts of non-renewable energy sources have prompted large investments into researching alternative energy production. Research is directed at creating new technologies that allow for these energy sources to be renewable, seen as an investment in the future of generations to come, rather than just for the short-term progress. Many renewable sources already exist, such as wind and solar power. However, it is difficult to maintain these sources because humans have no effect on the ‘fuel' of these sources; it's not possible to create wind to make wind turbines turn, or to make the sun shine when we require solar power.

Therefore, although the naturally available renewable energy sources have proved to be a success, they make up only a small percentage of the global energy consumption.

In order to reduce the use of non-renewable sources, effective renewable technologies must be created that are reliable. One such technology is the polymer electrolyte (or proton exchange) membrane fuel cell (PEMFC).

References

1. M. E. Davis and R. J. Davis, "Heterogeneous catalysis," Fundamentals of chemical reaction engineering, McGraw-Hill Higher Education, New York, NY, 2003, pp. 133 - 183.

2. A. Contescu and C. Contescu, "Oxides and related surfaces as catalyst supports," Encyclopedia of surface and colloid science, P. Somasundaran (Editor), vol. 7, CRC Press, New York, 2006, pp. 4405 - 4414.

3. P. Atkins, D. Shriver, T. Overton, J. Rourke, M. Weller and F. Armstrong, "Heterogeneous catalysis," Inorganic chemistry, Oxford University Press, Oxford, 2006, pp. 694 - 707.

4. G. J. K. Acres, A. J. Bird, J. W. Jenkins and F. King, "The design and preparation of supported catalysts," Catalysis, C. Kemball and D. A. Dowden (Editors), vol. 4, Royal Society of Chemistry, London, 1981, pp. 1 - 30.

5. I. Chorkendorff and J. W. Niemantsverdriet, "Catalyst supports and preparation of supported catalysts," Concepts of modern catalysis and kinetics, Wiley-VCH, Weinheim, 2003, pp. 189 - 198.

6. J. K. Dunleavy, Sulfur as a catalyst poison, Platinum Metals Review 50 (2006), no. 2, 110.

7. I. Mochida and K. H. Choi, An overview of hydrodesulfurization and hydrodenitrogenation, Journal of the Japan Petroleum Institute 47 (2004), no. 3, 145-163.

8. D. Zuo, D. Li, H. Nie, Y. Shi, M. Lacroix and M. Vrinat, Acid-base properties of niw/al2o3 sulfided catalysts: Relationship with hydrogenation, isomerization and hydrodesulfurization reactions, Journal of Molecular Catalysis a-Chemical 211 (2004), no. 1-2, 179-189.

9. A. Spojakina, E. Kraleva, K. Jiratova and L. Petrov, Tio2 supported iron-molybdenum hydrodesulfurization catalysts, Applied Catalysis a-General 288 (2005), no. 1-2, 10-17.

10. S. Dzwigaj, C. Louis, A. Breysse, M. Cattenot, V. Belliere, C. Geantet, M. Vrinat, P. Blanchard, E. Payen, S. Inoue, H. Kudo and Y. Yoshimura, New generation of titanium dioxide support for hydrodesulfurization, 3rd International Conference on Environmental Catalysis, 2001, p.^pp. 181-191.

11. S. K. Maity, M. S. Rana, S. K. Bej, J. Ancheyta, G. M. Dhar and T. Rao, Studies on physico-chemical characterization and catalysis on high surface area titania supported molybdenum hydrotreating catalysts, Applied Catalysis a-General 205 (2001), no. 1-2, 215-225.

12. J. Ramirez, S. Fuentes, G. Diaz, M. Vrinat, M. Breysse and M. Lacroix, Hydrodesulfurization activity and characterization of sulfided molybdenum and cobalt molybdenum catalysts - comparison of alumina-supported, silica alumina-supported and titania-supported catalysts, Applied Catalysis 52 (1989), no. 3, 211-224.

13. D. Gulkova, L. Kaluza, Z. Vit, J. Horacek, E. Machackova and M. Zdrazil, High surface area moo3/tio2 hydrodesulfurization catalysts, Reaction Kinetics and Catalysis Letters 94 (2008), no. 2, 219-226.

14. J. Escobar, J. A. Toledo, M. A. Cortes, M. L. Mosqueira, V. Perez, G. Ferrat, E. Lopez-Salinas and E. Torres-Garcia, Highly active sulfided como catalyst on nano-structured tio2, International Conference on Gas-Fuel 05, 2005, p.^pp. 222-226.

15. M. A. Cortes-Jacome, J. Escobar, C. A. Chavez, E. Lopez-Salinas, E. Romero, G. Ferrat and J. A. Toledo-Antonio, Highly dispersed comos phase on titania nanotubes as efficient hds catalysts, 4th International Symposium on Molecular Aspects of Catalysis by Sulfides, 2007, p.^pp. 56-62.

16. K. Suriye, P. Praserthdam and B. Jongsomjit, Effect of surface sites of tio2 support on the formation of cobalt-support compound in co/tio2 catalysts, Catalysis Communications 8 (2007), 1772-1780.

[1] BP, Statistical Review of World Energy June 2009

 

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