Titanium Dioxide as a Catalyst Support in Heterogeneous Catalysis

The lack of stability is a challenge for most heterogeneous catalysts. During operations, the agglomeration of particles may block the active sites of the catalyst, which is believed to contribute to its instability. Recently, titanium oxide (TiO2) was introduced as an alternative support material for heterogeneous catalyst due to the effect of its high surface area stabilizing the catalysts in its mesoporous structure. TiO2 supported metal catalysts have attracted interest due to TiO2 nanoparticles high activity for various reduction and oxidation reactions at low pressures and temperatures. Furthermore, TiO2 was found to be a good metal oxide catalyst support due to the strong metal support interaction, chemical stability, and acid-base property. The aforementioned properties make heterogeneous TiO2 supported catalysts show a high potential in photocatalyst-related applications, electrodes for wet solar cells, synthesis of fine chemicals, and others. This review focuses on TiO2 as a support material for heterogeneous catalysts and its potential applications.


Essential Principle of Catalyst.
The catalysis industry is a billion-dollar industry that accounts for the manufacture of 60% of all the chemicals that are utilized for most chemical processes [1][2][3][4][5][6]. Some of the products derived from catalytic processes include polymers [2], plastics [3], pharmaceuticals [4], and detergents [5]. After decades of research, systematic information on the catalytic properties of many catalysts has been established and accumulated. From the literature, it is concluded that one of the major limitations of the catalytic reaction is separation and distribution [7][8][9]. Many of real catalysts are made up of small (in the nanometer range) sizes. This consequently brought about the uncertainly and nonuniformity of the materials involved, the preparation methods, and surface conditions [8]. Heterogeneous catalyst was introduced to overcome the separation and distribution problems.

Importance of Heterogeneous Catalyst.
Heterogeneous catalysts have become a crucial part of many industrial activities, such as organic synthesis, oil refining, and pollution control [10][11][12][13][14][15]. Modern heterogeneous catalysts consist of several elements in precise proportions [12]. Currently, heterogeneous catalysts are optimized for the greatest reaction rate, which in turn results in optimal selectivity [11][12][13]. It is possible to improve the heterogeneous catalyst activity over modifying the support by approaches such as nanotechnology and nanoscience or controlling the pore structure [14][15][16]. For heterogeneous catalysis, the problem of catalyst separation and recovery from the reaction matrix are addressed by using various catalyst supports to immobilize the particle [15]. This in turn provides a large enough surface area for the heterogeneous catalyst for it not to dissolve into the solution matrix [16]. Therefore, the heterogeneous catalyst with broad supports such as Al 2 O 3 , TiO 2 , ZrO 2 , ZnO, and others is applied based on its broad availability and costeffective modes of synthesis.

Importance of Heterogeneous Catalyst Support.
Recently, the importance of an appropriate catalyst's support material has been of huge interest. The idea is that the main catalyst should be dispersed on a suitable support to make the catalytic nanoparticles stable and obtain optimal performance 2 The Scientific World Journal and decrease the amount of costly metal being utilized, which accordingly decrease the total catalyst expenses [11,15]. Furthermore, with porous characteristics, support materials offer a high dispersion of nanoparticle catalyst and simplify electron transfer, both of which contribute to better catalytic activities [17][18][19][20].
However, the heterogeneous catalyst support may sometime exert a structural effect, brought about by textural and active phase-linked effect [18]. Thus, the selection on support heterogeneous catalyst must retain its specific properties, such as porosity, surface area, dispersion, selectivity, and activity [19][20][21]. The morphology and pores size of the selected support materials play an important role in enhancing the heterogeneous catalyst's stability and performance [20].
According to the literature, the support of the heterogeneous catalyst can be alumina [22], zeolites [23], carbon nanofibers [19], active carbon [17], and metal oxides [13], such as TiO 2 , La 2 O 3 , CeO 2 , MnO 2 , and ZrO 2 . TiO 2 is a recognized heterogeneous catalyst support that is broadly utilized in fuel processing due to its tunable porous surface and distribution, high thermal stability, and mechanical strength [24,25]. Being used in this manner contributes to the ability of TiO 2 to develop Lewis acidity as well as redox properties [25].

TiO 2 : In General
TiO 2 has proven to be one of the promising n-type semiconductors due to its wide band gap (3.2 eV) under ultraviolet light [24]. Additionally, possessing high physical and chemical stability as well as the high refractive index makes this material widely researched [25]. Due to its electronic and optical properties, it can be utilized in several fields, such as solar cells, photocatalyst, sensors, and self-cleaning [26]. In electrochemistry, TiO 2 based materials play a key role due to their high conductivity and stability in alkaline and acid media. TiO 2 exists in three crystalline forms; anatase and rutile are the most common types, and the crystalline size of the rutile is always larger than the anatase phase. Brookite is the third structural form, an orthorhombic structure, which is rarely utilized, and is of no interest for most applications [27][28][29][30]. Rutile phase is the most thermally stable among the three phases. Brookite and anatase crystalline, above 600 ∘ C, experience a phase transition and convert into the rutile phase [28,29]. The anatase phase contains zigzag chains of octahedral molecules linked to each other, while the rutile consists of linear chains of opposite edge-shared octahedral structure [29][30][31][32]. Generally, the anatase-to-rutile phase transformation occurs between 600-700 ∘ C, but, for certain applications, it is required that TiO 2 anatase be stable at 900 ∘ C [31]. Generally, the anatase TiO 2 nanoparticles are stabilized by the addition of cations [32].
The synthesis techniques of TiO 2 usually require high temperatures to crystallize the amorphous material into one of the phases of TiO 2 , such as brookite, anatase, and rutile, consequently leading to larger particles and typically nonporous materials [33][34][35]. Recently, low temperature synthesis methods resulted in crystalline TiO 2 with a higher degree of control over the formed polymorph and its intra-or interparticle porosities [32]. There are reports on the formation of crystalline nanoscale TiO 2 particle via solution based approach without thermal treatment with special focus on the resulting polymorphs, surface area, particle dimensions, and crystal morphology [34]. There are exceptional emphases on the sol-gel method via glycosylated precursor and also the miniemulsion method [30][31][32].
TiO 2 , due to its nontoxicity, long-term photo stability, and high effectiveness, has been widely utilized in mineralizing toxic and nonbiodegradable environmental contaminants. TiO 2 possesses good mechanical resistance and stabilities in acidic and oxidative environments. These properties make TiO 2 a prime candidate for heterogeneous catalyst support.
There are also some obvious drawbacks in using TiO 2 as a heterogeneous catalysis. The limitations included small specific surface areas, low quantum efficiency, and low adsorption abilities [36,37]. Furthermore, both costs and difficulties in the separation of catalyst from the reaction media and inadequacy for continuous processing limited the applications of TiO 2 as a heterogeneous catalyst in largescale industries [38]. Despite these drawbacks, a number of studies have focused on catalytic reaction with TiO 2 as a support material. The mesoporous TiO 2 of pure anatase phase with sharp pore distribution and large surface area was synthesized to increase the degree of distribution and homogeneity of immobilized catalyst [39,40]. The influences of TiO 2 support on heterogeneous catalysts affect electronic effects and bifunctional mechanism [41]. TiO 2 , as a catalyst support, enforces an electronic effect where the hypo-delectronic Ti 3+ promotes electrocatalytic features of hyperd-electronic noble catalyst surface atoms [42]. This, in turn, decreases the adsorption energy of CO intermediates, while enhancing the mobility of CO groups. At the same time, the adsorption of OH species on TiO 2 tends to facilitate the conversion of the catalytically toxic CO intermediates in CO 2 , thus improving the durability of the heterogeneous catalyst [43,44]. Both factors indirectly assist the dispersion and anchor of the heterogeneous catalyst particle [44]. Further improvement in the catalytic stability and activity of the heterogeneous catalysts involves modifying the TiO 2 support material with semiconductor metal oxides.

TiO 2 : As Support in Heterogeneous Catalysis
Among different material candidates such as nitrides, perovskites, and carbides, TiO 2 based catalyst support materials are known to have excellent properties [44], due to TiO 2 nanoparticles high chemical and thermal stability. TiO 2 based catalyst supports have outstanding resistance towards corrosion in different electrolytic media. TiO 2 can be regarded The Scientific World Journal 3 as a support for heterogeneous catalysts which guarantees stability in electrochemical environment and commercial availability [45]. Meanwhile, strong interactions between the catalytic particles and mesoporous TiO 2 have been recorded, which, in the end, resulted in both improved catalytic stability and activity. TiO 2 as a catalyst support material also indicated a certain degree of proton conductivity, which may potentially enhance the regime of the triple phase boundary for catalytic reactions [44][45][46]. In general, the advantages and disadvantages of the other heterogeneous catalyst system are listed in Table 1.

TiO 2 : As Support in Metal Heterogeneous Catalysis.
The study of metal nanoparticle on TiO 2 support is important in heterogeneous catalysis due to the size and nature of the interaction of a metal nanoparticle with TiO 2 support [45]. This interaction strongly influences the determination of catalytic activity and selectivity of the metal heterogeneous catalyst [46]. Reduction and oxidation at elevated temperature are compulsory steps in the preparation of metal supported TiO 2 heterogeneous catalysts [47,48]. However, both treatments caused morphological changes to the dispersed metal nanoparticles from the sintering of TiO 2 . Therefore, it is important that the optimal conditions for catalyst supported TiO 2 preparation be optimized, both in terms of pretreatment and activation [48,49]. Besides, depending on the particular metal heterogeneous catalyst, different morphological changes will result from metal-TiO 2 support interaction [50][51][52], such as sintering [50], alloy formation [52] encapsulation, and interdiffusion [51].
Among the TiO 2 modifications, anatase is frequently utilized as a catalyst support for metal heterogeneous catalyst due to its high specific surface area and strong interaction with metal nanoparticles [37,40]. There are only a few studies reporting a rutile catalyst support which resulted in higher catalytic activity compared to anatase, such as the oxidation of toluene, xylene, and benzene over rutilesupported Cu catalyst. In comparison, rutile is preferred as a model support for particles of metals in surface science studies [53][54][55], due to its high crystal phase's thermodynamic stability. Furthermore, it is indicated that rutile and anatase differ noticeably in their ability of fixing particles of metals onto their respective surface [49,55]; whereas the strong metal support interaction is normally shown on anatase, this effect is not as significant on rutile. Inopportunely, the thermodynamic stability of TiO 2 is comparatively low, and calcination would usually lead to the collapse of the porous structures [54]. Additionally, it is reported that calcination above 465 ∘ C has always resulted in the phase transition from anatase to rutile [36]. The phase transition could be connected to the growth of crystal size, which results in a severe reduction in specific surface area [35]. Consequently, this should also influence the overall catalytic performance of metal heterogeneous catalysts.
3.1.1. Au/TiO 2 Heterogeneous Catalyst. Gold (Au) is an excellent catalyst for the oxidation of alcohol by molecular O 2 in the liquid phase with high activity, selectivity, and promising resistance to deactivation [56][57][58]. The catalytic performance of Au heterogeneous catalyst is mainly determined via the 4 The Scientific World Journal particle size and properties of the support [57]. Amongst all catalyst supports, TiO 2 was determined to be a good support for the Au heterogeneous catalyst system due to the strong interaction in metal support, chemical stability, and acid-base properties [44,46].
Au-supported TiO 2 nanoparticle has been prepared by changing the several different synthesis parameters, including precipitation pH, drying pretreatment, Au-cluster size/morphology, catalyst conditions, the nature of the support, acid-base treatment of TiO 2 support, loading of TiO 2 nanoparticle, incorporation of impurities, CO adsorption, catalytic reaction conditions, and chemical/electronic state in catalysts [53,58]. The valence band of Au/TiO 2 indicates the presence of the Au 5d band, with a significant contribution to the O 2p nonbonding states, which probably is derived from TiO 2 structure [46]. Therefore, the morphology images showed an interaction among the particles of Au with TiO 2 support affecting the Ti-O bonds at the surface, which leads to a lower Ti 2p binding energy in an intact Au/TiO 2 compared to native TiO 2 ( Figure 1) [59][60][61][62]. Generally, Au nanoparticles, as a catalyst, have a negative charge [57]. This is due to the electron transfer from oxygen vacancies of the TiO 2 acting as a catalyst support [58].
The transformation was clearly observed in the case of acetate to ketenylidene reaction [60] (Figure 2). Thus, the charge density and level binding energy of Au is 0.15-0.45 eV lower than that in pure Au [56]. It is concluded that the presence of TiO 2 as a support in Au is necessary for the crystallization of the support. This, in turn, reduces the number of Ti-OH functions to be proportional to the deposition of Au [60].
The aforementioned theory brought about the next advantage of Au/TiO 2 hybrid catalyst. The incorporation of Au is believed to prevent radiation-induced changes in the Au/TiO 2 heterogeneous catalyst composition, particularly for as-synthesized and dried samples [63,64]. For example, Xray irradiation limits a detectable further reduction in the Au/TiO 2 due to the presence of Au 3+ state and binding energy of the Au itself [57]. The binding energy and half-width of the Au 4f 7/2 spectra of Au particles deposited on the TiO 2 in a range of concentrations were sensitive to irradiation. Indeed, the local heating mechanism occurred at higher X-ray concentrations [65]. Exposure of Au/TiO 2 heterogeneous catalyst to X-ray irradiation induces the breaking of Ti-O bonds, Ti 4+ state reduction, and the O 2 desorption from the surface layer [64][65][66]. This is demonstrated by an increase in the fraction of Ti 3+ species, a nonmonotonic diminution of the O 2 − /Ti 4+ atomic ratio, improvement of the valence band, and the resultant variation of core binding energy [65]. Even though Ti 2p and O 1s binding energies in TiO 2 remain almost unaffected under extended X-ray irradiation, in Au/TiO 2 , a gradual increase in the Ti 2p binding energy is observed [67]. Thus, the effect of X-ray irradiation of Au/TiO 2 is relatively smaller than Au due to the direct evidence for charge transfer processes triggered by the production of O 2 vacancies in TiO 2 as a catalyst support [63,64].
Many scientists have documented that Au/TiO 2 , as a heterogeneous catalyst, showed higher reaction activities for the oxidation of primary alcohols to carboxylic acid compared to Au/zeolite [68]. The comparative study of CO oxidation reaction and deactivation behavior between the mesoporous Au/TiO 2 and Au demonstrated that Au/TiO 2 has greater catalytic selectivity and activity due to its higher surface area. Another study found that the deposition of Au into TiO 2 led to the highly active heterogeneous catalyst for the oxidation of methanol [69][70][71]. The catalytic performance was determined to increase with Au loading, provided that the deposited particle size remained unchanged [70][71][72]. In this case, the methoxy species bound to the oxide surface are reasonable reaction intermediates, with the formation of significant bound on TiO 2 [71]. The catalytic performance was correlated with the number of Au atoms. Thus, the methanol oxidation occurs at the interface with O 2 being activated at Au nanoparticle and the oxide acting as a reservoir of methoxy species (Figure 3) [73][74][75].
Furthermore, there are several reports on the catalytic application of Au/TiO 2 on the removal of CO and NO . Most of these reports investigated the catalytic and photocatalytic of Au/TiO 2 in the gas and liquid phase reactions, as well as their reaction mechanisms [74]. In other studies a higher stability was observed for Au/TiO 2 heterogeneous catalyst compared to Au/Co 3 O 4 , which indicates a comparatively fast deactivation [70,72]. This is supported by the aforementioned report; a significant enhancement in the stability of Au particles against calcination can be realized by utilizing TiO 2 as a catalyst support [71]. With these promising properties, Au/TiO 2 catalyst was also found to catalyze the epoxidation of propylene, hydrogenation reaction, water-gas shift reaction, and the oxidation of alkanes [68][69][70][71][72][73][74][75][76].
Meanwhile, the porosity and phase transformation are other factors that affect the catalytic performance of Au/TiO 2 as a heterogeneous catalyst [77,78]. It is estimated that various TiO 2 crystalline phases, as a catalyst support, could affect the interaction of support-metal, electronic density, oxidation state, Au-size, and Au dispersion of deposit material in heterogeneous catalyst system. In a comparative study of the activity of porous and nonporous Au/TiO 2 , heterogeneous catalyst prepared from different crystalline phase of TiO 2 included the anatase, rutile, and rutile+anatase [69,79]. For example, certain studies found that the activity of nonporous Au/TiO 2 heterogeneous catalyst decreased from brookite via anatase to rutile for catalyst activated by calcination at 300 ∘ C, while, after reduction at 150 ∘ C, the activities became rather comparable [80,81]. Similar observation was reported by other studies for mesoporous TiO 2 acting as support for Au heterogeneous catalyst [82]. The tendency in the increment for Au particle sintering during calcination at 300 ∘ C is brookite, anatase, and rutile, followed by mesoporous. In contrast, such effect is insignificant at the reduction of 150 ∘ C [82]. Some studies reported that the crystalline phase of TiO 2 influences also the deactivation of Au/TiO 2 heterogeneous catalyst with comparable surface area of 46-54 m 2 g −1 , increased in the order of Au/TiO 2 anatase and Au/TiO 2 nonporous rutile/anatase and followed by Au/TiO 2 rutile [83].   Contrarily, some studies concluded that the crystalline phase has no significant influence on the catalytic performance of the unconditioned Au/TiO 2 heterogeneous catalyst [84]. This was different, as the Au/TiO 2 heterogeneous catalyst was calcined prior to the reaction, where the Au/TiO 2 rutile+anatase heterogeneous catalyst indicated a significantly higher stability than Au/TiO 2 rutile and Au/TiO 2 anatase [85]. After calcination at 250 ∘ C, the activities are comparable to those of the unconditioned heterogeneous catalyst and are similar for all catalysts. However, calcination at 350 ∘ C leads to lower activities for the mesoporous rutile or anatase, while the rutile+anatase supported Au retains its respective activities [83]. The catalytic performance of Au-supported TiO 2 anatase being superior to other heterogeneous catalyst was examined for the decrease of NO using propane due to the smaller size of the Au particles on TiO 2 anatase [86,87].
Although Au/TiO 2 is active as a heterogeneous catalyst, it often complied with quick deactivate process which limits its commercial applications [70][71][72][73][74]. In general, in order for the Au particle to indicate higher performance at near subambient temperatures, its cluster size has to be less than 5 nm [65]. Therefore, some attempt has to be made to synthesize Au hydroxide [AuO (OH) 4−2 ] deposited on the TiO 2 support, followed by drying in the air. The idea is to prevent the in situ formation and the agglomeration of metallic Au nanoparticles, as seen with coprecipitation [64]. Another approach is by synthesizing nearly monodispersed thiol-protected Au nanoparticles and is deposited into TiO 2 as a support. In this case, the thiol ligands were utilized to control the size of the cluster [82,88]. The advantages offered by both methods included a better control size of Au nanoparticle due to ex situ synthesis and the formation of Au cluster in the solution prior to the deposition on the TiO 2 as a catalyst support [88].
In conclusion, the most common factors affecting the Au/TiO 2 heterogeneous catalyst activity are the size of the Au nanoparticles, preparation method, pretreatment method, Au loading, the oxidation state of Au, and the binding strength to TiO 2 as a catalyst support.

Co/TiO 2 Heterogeneous
Catalyst. The activity of cobalt supported TiO 2 (Co/TiO 2 ) heterogeneous catalyst is greatly related to the TiO 2 crystal phase and the loading of Co 3+ ions on the catalyst support [89][90][91]. The case of Co/TiO 2 , TiO 2 in the rutile phase is more appropriate as a catalyst support material than those with a 100% anatase phase [25,90]. Therefore, Co/TiO 2 heterogeneous catalyst synthesized with more than 15% rutile phase is shown to have 4 times higher CO conversion rate than those that only consist of the anatase phase [92,93]. Furthermore, the Co/TiO 2 , when modified with alkaline earth metals, resulted in a greater CO conversion rate, whereas modification with Mn and V resulted in high C 5+ and low methane selectivity, respectively [94][95][96]. Furthermore, with only 0.8% Ca modification, Co/TiO 2 obtained the highest CO conversion rate and site-time yield of C 5+ products. The conversion rate is 1.3-1.5 times higher than cosupported SiO 2 catalyst ( Figure 4) [97]. Therefore, the CO conversion reaction rate over Co/TiO 2 catalyst was proportionately amplified by increasing the surface area of Co [93].
Actually, the production of highly dispersed Co on TiO 2 as a catalyst support requires a strong interaction [91,92]. Nevertheless, too strong of an interaction generates the TiO 2 compound as a suboxide at an interface, which is highly resistant to reduction [89]. In this case, while it has known that the dominant surface sites of TiO 2 support consist of two main sites, which are Ti 3+ and Ti 4+ , the surface site's effect on the formation of Co to TiO 2 interface indicates that the reaction should be structured to be insensitive and based on the number of the exposed Co metal sites [93][94][95][96][97][98][99]. It is also proposed that this growth in the reaction rate might be due to the strong Co to TiO 2 interaction altering the catalytic properties [100]. of important Ni-based catalyst. This is due to the strong interaction between Ni as a metal and TiO 2 as a catalyst support [101][102][103]. For example, the catalytic properties of supported Ni/TiO 2 heterogeneous catalysts prepared by the incipient wetness impregnation method were evaluated for the vapor phase hydrogenation of maleic anhydride [104]. It was discovered that the hydrogenation process of maleic anhydride is strongly affected by the calcination temperature of Ni/TiO 2 . The Ni/TiO 2 heterogeneous samples recorded an optimum catalytic performance with 96% maleic anhydride conversion as the calcination temperature reached 1023 K [104,105]. This is attributed to the change of surface properties of TiO 2 support to the increase of calcination temperature. In addition, the strong metal support interaction between Ni and rutile surface over Ni/TiO 2 catalyst was the key reason for the better activity of the catalyst in acetophenone hydrogenation [105]. However, the deactivation of Ni/TiO 2 heterogeneous catalyst occurred as the carbonaceous species of maleic anhydride was deposited onto the Ni surface [106,107]. To regenerate the catalytic performance of Ni/TiO 2 , thermal treatment in the oxidant atmosphere approach is applied.

Pd/TiO 2 Heterogeneous
Catalyst. Anatase TiO 2 effectively engenders OH species. Thus, palladium-supported TiO 2 (Pd/TiO 2 ) anatase possesses excellent catalytic activity vis-à-vis methanol electrooxidation. There are some reports claiming that Pd-supported TiO 2 anatase heterogeneous catalyst demonstrated a greater activity than TiO 2 rutile in selective hydrogenation reactions due to the superior metal supporting the behavior of TiO 2 anatase [108][109][110][111][112]. Consequently, negligible CO intermediates are produced in direct formic acid fuel cells and formic acid electrooxidation, which consequently possess striking catalytic activity. In contradiction, the acetoxylation of toluene study indicates that Pd supported by rutile TiO 2 has greater selectivity of almost 100% without any deactivation of the catalyst compared to the ones supported by anatase [110][111][112][113], due to the high thermal stability of rutile. Meanwhile, the treatment of Pd/TiO 2 anatase heterogeneous catalyst by an H 2 reduction at 200 ∘ C resulted in greater reactivity in hydrogenation of alkadienes than that of the Pd/TiO 2 in rutile phase [112]. The strong metal support interaction was only observed in the anatase phase of supported Pd heterogeneous catalyst [109,110]. There is also an attempt to synthesize the genesis of the Pd cluster on TiO 2 -grafted SiO 2 . The results showed that TiO 2 anchors Pd particles during air calcination and maintains its small ensemble during H 2 reduction [114,115]. These observations serve to demonstrate how TiO 2 , as a catalyst 8 The Scientific World Journal support, influences the structure and catalytic performance, specifically for sulfur-resistant catalysts [114]. It was also concluded that TiO 2 -grafted SiO 2 offers better support due to the presence of high surface area and thermal stability for Pd heterogeneous catalyst compared to pure TiO 2 [116][117][118].
3.1.5. Pt/TiO 2 Heterogeneous Catalyst. The dispersion and loading of platinum (Pt) nanoparticles onto TiO 2 support is controlled by the structure and porosity of TiO 2 . Consequently, a suitable combination of passable electronic conductivity and nanostructured morphology with controlled porosity could result in very promising Pt/TiO 2 heterogeneous catalysts [119][120][121][122][123][124]. For example, a novel electrocatalyst based on mesoporous TiO 2 supported Pt nanoparticles indicates a high stability under accelerated stress test conditions and activity compared to commercial Pt-supported carbon catalyst [121,122]. Some studies have investigated the influence of the reductive treatment on structural properties of Pt/TiO 2 , with their catalytic activity for formaldehyde oxidation [123]. It is claimed that the enhanced catalytic performance is reflected by a uniform dispersion of Pt nanoparticles and the interaction between Pt and TiO 2 [124]. Thus, the application of Pt as a heterogeneous catalyst becomes more encouraging, especially in electrochemical and photoelectrochemical context [125,126]. For example, H 2 production could be significantly improved by photocatalytic water splitting over Pt/TiO 2 nanosheets compared to native Pt [126].
Several methods have applied in the preparation of Pt nanoparticle deposited on TiO 2 substrates included underpotential deposition [127][128][129], hydrothermal treatment [130,131], photoassisted reduction [132], and vacuum deposition [133,134]. Electrodeposition would be a straightforward approach for the synthesis of Pt-supported TiO 2 , which is highly required for the production of complex electrode architectures in fuel cells [132][133][134]. This in turn resulted in the H 2 evolution rate of 3 wt% Pt/TiO 2 , which was relatively greater than that of 3 wt% Pt/Al 2 O 3 . Other studies found that TiO 2 , as a support material, improves Pt O 2 reduction activity by assisting mechanisms such as reactant surface diffusion and O 2 spillover [135][136][137]. Furthermore, Pt/TiO 2 showed similar performance to Pt/C in H 2 -fuel cell operated at 60 ∘ C and 0.8 V ( Figure 5) [136].
Certain studies on the effect of particle size indicated that amorphous TiO 2 could powerfully suppress the O 2 reduction reaction activity of supported Pt at smaller sizes [138][139][140]. For example, a series of Pt/TiO 2 heterogeneous catalyst with various Pt particle sizes was prepared and tested for low temperature CO oxidation [139]. The result indicated that the Pt/TiO 2 heterogeneous catalyst that resulted in superior activity at the weight percent of Pt in Pt/TiO 2 heterogeneous catalyst was 5.0%, with the complete conversion temperature being 120 ∘ C [141][142][143].  constitute the main family of catalyst in the heterogeneous category. Furthermore, certain metal salts and organometallic compounds using a heterogeneous catalyst precursor have the tendency to be decomposed via exposure to light irradiation. Some reports show that, as the oxide-containing catalyst under light irradiation analysis needs great care, considerable damage is possible even at low irradiation dose. For catalytic applications, metal oxides have also been employed for various applications such as gas sensor, the photocatalyst thin film, and fuel cells. TiO 2 is often used to modify the supported metal oxide heterogeneous catalyst due to properties such as reducible surface and possible electron transfer via the spontaneous alignment of the Fermi levels.

CuO/TiO 2 Heterogeneous
Catalyst. It has been reported that copper oxide (CuO) heterogeneous catalyst is highly active for CO oxidation and lower in cost compared to other noble metal heterogeneous catalysts [144][145][146][147]. CuO has additional advantages, such as a high thermal stability and the fact that it is economical [145][146][147][148][149]. The highest activities of CuO heterogeneous catalyst are attributed to the synergy between the Cu species and the support, especially in the case of TiO 2 as support material. For example, the increment of activity for CO oxidation over the CuO/TiO 2 heterogeneous catalyst is attributed to the sites for CO chemisorption that is responsible for O 2 activation [150,151]. Furthermore, since CO oxidation over the supported metal heterogeneous catalyst takes place at the metal support interface, TiO 2 , as reducible oxide, could provide the intrinsic activity to the entire reaction [149]. Therefore, only a small amount of CuO is loaded onto TiO 2 and influence of TiO 2 crystalline phase, which are indicative of the fact that the support system has higher efficiency and selectivity than the CO oxidation being examined [152].
On the other hand, CuO/TiO 2 heterogeneous catalyst is also found suitable for NO reductions [150]. Rutile TiO 2 is the most stable phase compared to anatase and brookite, and Cu-based heterogeneous catalysts show promising activity towards NO reduction [151]. Therefore, it is necessary and important to further approach the change of heterogeneous catalyst surface with plasma-assisted processing. NO reduction by CO reaction was comparatively studied over microwave plasma pretreated CuO/TiO 2 heterogeneous catalysts employing transmission electron microscopy, H 2 temperature programmed reduction, and in situ Fourier transform infrared spectroscopy [152][153][154]. The CuO/TiO 2 heterogeneous catalytic performances indicated that a remarkable improvement in activity and selectivity was achieved after microwave plasma pretreatment, which depends on the microwave plasma pretreatment time [153]. The results also suggested that high active oxygen species are formed on the surface of plasma pretreated CuO/TiO 2 heterogeneous catalysts, which led to the easy oxidation of NO to NO 2 at low temperatures even without the introduction of any additional O 2 gas. Therefore, these high active oxygen species should play an important role in the exaltation of the catalytic performances of the CuO/TiO 2 heterogeneous catalysts [155]. It is found that CuO supported on the anatase phase of TiO 2 -support calcined at 400 ∘ C demonstrated better catalytic activity than those supported on TiO 2 calcined at 500 or 700 ∘ C. Among all of the investigated heterogeneous catalysts with CuO loading from 2% to 12%, the CuO/TiO 2 heterogeneous catalyst with 8 wt% CuO loading exhibited the highest catalytic activity [156]. The optimum calcination temperature of the CuO/TiO 2 heterogeneous catalysts was recorded at 300 ∘ C.
CuO/TiO 2 is also known as a promising heterogeneous catalyst in the photocatalytic applications. Whilst it is widely recognized that CuO facilitates charge separation and acts as a H 2 O reduction site, conjecture still exists as to the exact nature of the dispersed CuO, specifically on TiO 2 and the optimum CuO loading for efficient H 2 production [157,158]. The surface chemistry of the CuO/TiO 2 heterogeneous catalyst is the subject of a number of investigations to explain the excellent activity of the photocatalysts. It is generally recognized that CuO exists in several different forms on TiO 2 , with the specification depending on the CuO content and catalyst pretreatment conditions [159]. Experimental evidence suggested that, at low CuO loadings, Cu 2+ is highly dispersed on TiO 2 as a surface complex or CuO monolayer. This, in turn, can easily reduce the CO or H 2 at low temperatures [150]. For example, CuO/TiO 2 photocatalyst is very active in the reduction of water under sacrificial conditions (95 vol.% H 2 O, 5 vol.% CH 3 OH) and UV excitation, with the optimal CuO loading of 5 to 10 wt% of the heterogeneous catalyst system [153]. The high catalytic activity caused photo-citation of electrons in the conduction bands of both CuO and TiO 2 , followed by the migration of the conduction band electrons in TiO 2 into the conduction band of CuO heterogeneous catalyst [155,156]. The accumulation of excess electrons in the conduction band of CuO caused a negative shift in the Fermi level of CuO to give the required overvoltage necessary for H 2 O reduction [154]. However, once the CuO loading exceeds the dispersion capacity, nanocrystalline of the CuO begins to form. CuO is itself inactive for H 2 production from water or alcoholwater mixtures under UV or visible irradiation, since the conduction band of CuO is more positive than the H 2 O/H 2 redox potential [160][161][162]. Accordingly, the onset of the CuO nanoparticle formation should coincide with a decrease in the photocatalytic activity of CuO/TiO 2 photocatalysts for H 2 production, as the presence of inert CuO nanoparticles will reduce the number of surface sites on TiO 2 available for photoreactions [161,162]. Experimental evidence to support this hypothesis is currently lacking, which motivates the present investigation.
CuO/TiO 2 heterogeneous catalyst could be synthesized by a deposition-precipitation method. For example, CuO has been deposited on TiO 2 nanotubes with Cu/Ti atom ratio of 10 and then evaluated by H 2 generation activity in methanol H 2 O mixtures under UV excitation. It is reported that the optimal CuO loading is around 1.3 wt%, giving the H 2 production rate of 2061 mol g −1 h −1 [161]. Meanwhile, the CuO/TiO 2 heterogeneous catalyst prepared by a complex precipitation method indicates the highest H 2 production observed in 10 wt% of the CuO loading [162]. Another study evaluated the performance of 2.5 wt% CuO-TiO 2 photocatalyst for H 2 production from methanol and found that 27-29% of H 2 evolution at rate of 1350 mol g −1 h −1 is produced for ethanol-H 2 O mixture of 900-1000 g −1 h −1 [158,159]. It was recorded that 1 wt% CuO/TiO 2 photocatalyst is active for H 2 O splitting under visible light in the presence of triethanolamine.

V 2 O 5 /TiO 2 Heterogeneous
Catalyst. Initially, vanadium oxide (V 2 O 5 ) supported TiO 2 heterogeneous catalyst was applied for the o-xylene oxidation to phthalic anhydride reaction. Then, the potential of V 2 O 5 /TiO 2 was recorded in the pollution abatement of NO with NH 3 process [163,164]. Thus, V 2 O 5 /TiO 2 is one of the most efficient catalysts in the oxidative dehydrogenation of propane, with only a few byproducts [165]. A stable deposit of TiO 2 in V 2 O 5 /TiO 2 heterogeneous catalyst system was established with dip coating of V 2 O 5 in an aqueous suspension of titanium isopropoxide [166,167].
Furthermore, V 2 O 5 /TiO 2 heterogeneous catalyst found another promising application in the dehydrogenation of alkanes and selective oxidation of alcohols/alkanes [168]. In this respect, the activity and product distribution of the heterogeneous catalyst for methanol oxidation strongly depend on its surface acidity and redox ability, and this can be regulated through proper modifications [169]. The heterogeneous catalyst of V 2 O 5 supported on TiO 2 exhibited high conversion of methanol and selectivity of dimethylmethane for the oxidation of methanol under mild conditions [170]. The catalytic oxidation of V 2 O 5 /TiO 2 heterogeneous catalyst was investigated for 1,2-dichlorobenzene [171], and it was surmised that the catalytic oxidation of 1,2-dichlorobenzene on chemical vapor condensation which prepared V 2 O 5 /TiO 2 heterogeneous catalyst demonstrated excellent performance at lower temperatures [171]. The surface structure of TiO 2 allows the development of the Lewis acidity, as well as redox The Scientific World Journal properties. Consequently, redox properties TiO 2 modified by the presence of V 2 O 5 lead to an electronic interaction between this support and V 2 O 5 species [169]. V 2 O 5 loading is a key point for the activity, and it has been suggested that a single redox surface site participates in the kinetically significant steps, with the formation of crystalline V 2 O 5 being detrimental to oxidation activity [168,172].
The effect on surface acidity and redox ability of V 2 O 5 /TiO 2 is also found to be interesting for the selective catalytic reduction by NH 3 . The proposed mechanism suggested that NH 3 is activated and reacts from a strongly adsorbed state with gaseous or weakly adsorbed NO [173][174][175]. It is believed that reliable structure-activity relationship is based on the understanding of the V 2 O 5 /TiO 2 heterogeneous catalyst molecular structure under operating conditions [174]. Thus, a dual-site mechanism involving a surface V 2 O 5 redox site and an adjacent nonreducible site appears to be more favorable. Furthermore, the reported influence of specific oxide supports, along with the observed stability of terminal V= 18 O bonds during NH 3 reaction, suggests that the V-Osupport bond is involved in the rate-determining step [167][168][169]. Similarly, the NH 3 action of transition for V 2 O 5 /TiO 2 heterogeneous catalyst correlates well with the extent of interactions between the active phase and the support [172]. The Raman studies of V 2 O 5 /TiO 2 heterogeneous catalyst revealed that the presence of multiple structures of surface V 2 O 5 species on TiO 2 , including monomeric and polymeric V 2 O 5 species at submonolayer coverage [175,176]. Therefore, Raman studies of V 2 O 5 /TiO 2 heterogeneous catalysts under SCR or reducing (NH 3 ) conditions are scarce.
The influence on preparation procedure towards surface of V 2 O 5 state plays an important role in selective catalytic reduction. Some studies have investigated the surface of V 2 O 5 prepared by incipient wetness impregnation, with TiO 2 being a support, and found that both isolated and polymeric surface V 2 O 5 species existed with medium VO surface coverage [177][178][179]. This is supported by other studies mentioning that polymeric V 2 O 5 species demonstrated greater catalytic activity than monomeric V 2 O 5 [178]. This is due to the greater mobility of lattice O 2 atoms resultant experience faster reduction and reoxidation by gaseous O 2 [179,180]. Furthermore, the redox properties of V 2 O 5 are modified by the incorporation of TiO 2 , leading to an electronic interaction between TiO 2 and V 2 O 5 . There are some reports discussing the fact that the addition of molybdenum oxides (MoO 2 ) species to V 2 O 5 /TiO 2 heterogeneous catalyst causes dramatic changes in the 1,2-dichlorobenzene oxidation, most importantly, by improving the catalytic activity [181,182].

MnO/TiO 2 Heterogeneous Catalyst.
Manganese oxides (MnO), containing several types of labile oxygen, which are necessary to complete the catalytic cycle, have relatively high activity, but their optimal temperature (above 150 ∘ C) is still high [183][184][185]. For example, the activity and selectivity of pure MnO on NO conversion reached 90% selectivity [184]. To enhance the catalytic activity, MnO are usually supported on TiO 2 as a carrier. In this regard, the dispersion of the MnO towards TiO 2 support had an important influence on the reaction, since crystalline MnO contributed little to activity [186,187]. This, in turn, possesses profound surface acid-base properties and provides high surface area, strong mechanical strength, and high thermal stability [188].
Existing research on supported MnO to TiO 2 heterogeneous catalyst for NO oxidation demonstrated high catalytic activity [189][190][191][192]. For example, a series of MnO/TiO 2 heterogeneous catalyst prepared by the deposition-precipitation method and the sample with the Mn/Ti ratio of 0.3 showed a superior activity for NO catalytic oxidation to NO 2 [188]. Therefore, the maximum NO conversion over the MnO/TiO 2 heterogeneous catalyst could reach 89% in 250 ∘ C [189]. Indeed, MnO/TiO 2 heterogeneous catalyst showed an interesting development as a highly active heterogeneous catalyst with low pollution for the low temperature NO conversion process [190]. Meanwhile, some studies reported on MnO/TiO 2 heterogeneous catalyst being prepared by sol-gel, impregnation, and coprecipitation methods for lowtemperature selective catalytic reduction of NO with NH 3 [191,192]. Strong interaction, high concentration of hydroxyl groups, large surface area, and high concentration of amorphous Mn on the surface might be the main reasons for the excellent performance of the catalysts [191].
It was documented that the crystal phase of the TiO 2 influences catalysts' activity [193][194][195][196][197]. Therefore, some studies on the effect of the crystalline phase of TiO 2 towards the catalytic performance of MnO/TiO 2 heterogeneous catalyst were carried out in [194], and it was discovered that compared to anatase and rutile anatase+rutile resulted in better dispersion of MnO on the support surface, suppressed the agglomeration of catalyst particles, and produced more Mn 2 O 3 , which is more active for the oxidation of NO [195,196]. In addition, anatase+rutile enhanced the reduction of MnO, especially for Mn 2 O 3 , and the formation of easily desorbed O 2− generated from the Mn 3+ -O bond [197].

RuO 2 /TiO 2 Heterogeneous
Catalyst. Ruthenium oxide supported TiO 2 (RuO 2 /TiO 2 ) heterogeneous catalyst found an attraction in the oxidation process [198,199]. Generally, RuO 2 /TiO 2 heterogeneous catalysts are prepared by the impregnation of Ru salt, followed by calcination, with limited control on the properties of the RuO 2 species formation [199]. Furthermore, a recent development of green method to prepare calibrated RuO 2 nanoparticles has been developed and analyzed. It is expected that, for this kind of preparation method, the RuO 2 /TiO 2 heterogeneous catalyst exhibits outstanding oxidation activity [200].

TiO 2 : As Support in Bimetallic Heterogeneous Catalysis.
In this area, the availability, affordability, and lack of toxicity of TiO 2 as a robust solid with outstanding photochemical stability make it an attractive support for the bimetallic heterogeneous catalyst. TiO 2 is used for its well-known ability to interact with bimetallic through the formation of Ti 3+ ions. Generally, any electronic conductivity of TiO 2 is due to the presence of Ti 3+ ions. There are two ways to create Ti 3+ ions in the TiO 2 structure. The formation of Ti 3+ ions either through the O 2 vacancy creation or through shear planes by introducing appropriate donor dopants. It is well known that strong bimetallic support interaction occurred as the bimetallic catalyst was reduced by H 2 . The H 2 reduction over TiO 2 supported bimetallic catalyst generates O 2 vacancies in the form of coordinate unsaturated cations in the vicinity of active bimetallic, which in the end results in changes in the catalytic activity and stability.
3.3.1. PdNi/TiO 2 Heterogeneous Catalyst. Spherical TiO 2 nanoparticles were used to synthesize the PdNi-supported TiO 2 electrocatalyst for methanol oxidation [201]. It was found that the electrocatalytic activity of PdNi/TiO 2 catalyst is much more promising, better than the antipoisoning capability, and comparatively favorable as compared to commercial 31 PtRu-supported carbon [201]. The methanol oxidation mechanism of the PdNi/TiO 2 heterogeneous catalyst mainly results from the high catalytic activity of the hybrid system without UV light illumination. Therefore, PdNi/TiO 2 catalyst might become a promising candidate for a direct-methanol fuel cell.

AuCu/TiO 2 Heterogeneous
Catalyst. The AuCu/TiO 2 heterogeneous catalyst was studied for several other reactions, including water, gas shift, and total oxidation of methane, ethane, propane, and epoxidation of propane. The incorporation of Au into Cu complements each other in terms of electronic properties, O 2 mobility, and surface stability [202,203]. The interaction of Au and Cu in AuCu/TiO 2 bimetallic heterogeneous catalyst was analyzed for methanol oxidation, and it was found that the catalytic activity and selectivity of the bimetallic heterogeneous catalyst system are greater than the one with the monolithic catalyst [202].

CoMn/TiO 2 Heterogeneous Catalyst.
A series of CoMn/TiO 2 heterogeneous catalysts, with a composition range of 2-12 wt% containing 25% Co and 75% Mn, have been prepared by the coimpregnation method [204][205][206]. The produced CoMn/TiO 2 heterogeneous catalyst was tested in Fischer-Tropsch synthesis for the production of C2-C4 olefins [205], and it was discovered that the heterogeneous catalyst containing 8 wt% (CoMn)/TiO 2 is the optimal formulation for the production of C2-C4 olefins. It should also be pointed out that the operating conditions, such as the H 2 /CO molar feed ratio, temperature, Gas Hourly Space Velocity, and total reaction pressure affect the heterogeneous catalytic performance of an optimal catalyst [204].

Applications of TiO 2 Supported
Heterogeneous Catalysis 4.1. Environmental Security: Photocatalysis. Recently, TiO 2supported semiconductor is extensively used to mineralize toxic and nonbiodegradable environmental pollutants due to its high effectiveness, long-term photostability, and nontoxicity [66,76]. This is also attributed to the limitation of most semiconductors, such as low quantum efficiency, small specific surface area, and low adsorption ability. This in turn limits the efficiency of the photocatalyst. On the other hand, both costly and difficult separation of reaction media and the inadequacy for continuous processing are some of the restrictive factors [110]. As a result, a number of studies have focused on the immobilization of semiconductor materials onto porous TiO 2 nanoparticles [156]. This is believed to not only promote photocatalytic reactions by offering more active sites but also allow the recycling and reuse of semiconductor as a heterogeneous catalyst. Some data also reported that porous TiO 2 nanoparticle has shown some advantages in the preparation of highly supported catalysts due to its special physicochemical properties, including high adsorption capabilities [66]. Indeed, it is well known that TiO 2 , with its crystallographic forms, small particle size, and highly porous structure, greatly influence the photocatalytic performance of composite materials [76]. Among metal oxides suitable for photocatalytic processes, TiO 2 is the most widely used, due to both its high photocatalytic activity and its chemical/photocorrosion stability in the reaction conditions. TiO 2 has increased the photoactivities, due to the photoinduced electron-hole pairs on its surface that can be harvested to increase electron transfer and chemical reactivity. The semiconductor nature of TiO 2 has made it possible for the utilization of UV-Visible radiation to harvest the conduction band electrons that are subsequently used to reduce metallic ions onto TiO 2 's surface [66,156]. Therefore, the immobilization of semiconductor on a TiO 2 nanoparticle can exhibit a higher photodecomposition of organic and inorganic pollutant compared to nonsupported semiconductors [110].

Chemical Reaction/Conversion.
As a versatile metal/metal oxide supported TiO 2 heterogeneous catalyst, it is broadly studied in a variety of mild oxidation reactions, such as ethane to acetic acid, ethanol to acetaldehyde, and oxidative dehydrogenation of propane to propylene [8,9,16]. For example, various TiO 2 supported catalysts, including Au/TiO 2 , Pd/TiO 2 , Co/TiO 2 , and Pt/TiO 2 , have recently been developed for frequent industrial applications, including the hydrosulfurization of hydrocarbon oils, the epoxidation of propane, and the selective catalytic reduction of NO [36,43,[46][47][48]. However, since these aforementioned reactions are powerfully exothermic, it is important to avoid the hot spots that are responsible for structural damages and early deactivation of heterogeneous catalyst [53,[56][57][58][59]. Generally, the presence of hot spot leading from products productions included CO 2 , especially in oxidation reaction [60]. As an alternative, recent studies focused on the 3D structure of metal/metal oxide supported TiO 2 with an open structure [63]. The open structure of metal/metal oxide supported TiO 2 catalyst favor efficient heat and mass transfers between the gaseous reactants, the catalytic active phase, and the wall of the chemical reactors.
Furthermore, metal oxide supported TiO 2 heterogeneous catalysts are commonly used in several industrial important reactions, including selective reduction of NO by NH 3 [109,121]. It should be pointed out that metal oxide supported TiO 2 heterogeneous catalyst demonstrated excellent performance in slurry reactions [123,128]. Indeed, TiO 2 , utilized as support  for metal oxide, attracts considerable interest due to its favorable properties of low pressure drop, thermal shock resistance, chemical durability, low manufacturing cost, and high structural strengths [134][135][136]160].

Small Molecules Transformation.
Recently, heterogeneous catalyst supported TiO 2 for small molecules transformation has received considerable attention because of its simplicity and the advantages, over most other methods of preparing highly pure mixed oxides and a variety of other materials [13]. This included sulfides and phosphates, which are obtained under very similar experimental conditions. The preparation of metal particles with TiO 2 supported catalyst is commonly applied because of its mild reducing performance which has a chelating effect, which avoids agglomeration of particles during preparation [57]. The synthesis of monoor polymetal particles of Co, Ni, Cu, and noble metals in submicrometer and -nanometer size range has been reported and the materials obtained by catalyzed supported of TiO 2 show homogenous phase composition, narrow particle distribution, and high specific surface area [203]. For example, polyol-mediated preparation of nanoscale oxides can carry out by dissolving a suitable metal precursor (acetate, alcoholate, and halogenide) in diethylene glycol or other polyalcohol with assisted of heterogeneous catalyst supported TiO 2 . During this step, the surface of growing particles will be immediately complexed by TiO 2 as a catalyst support material, which limit grain growth [49].
Heterogeneous catalyst supported TiO 2 also applied for the nitrate to nitrite reduction with bimetallic catalyst. It is expected that H 2 molecules produced through the reaction and adsorbed on noble metal subsequently reduces nitrite to harmless N 2 gas [207]. A wide range of metal pairs included Au-Pd, Sn-Pd, Ni-Rh, Rh-Cu, and Pd-Cu supported on TiO 2 were extensively studied to maximize the efficiency of nitrate reduction to N 2 gas. In advances, some studies focus on the effect of pH and zwitterionic buffer on catalytic nitrate reduction by Cu-Pd supported TiO 2 and found that the nitrate reduction decreased from 100% to 72% as suspension of pH increased from 6 to 10 of which range was kept by zwitterionic buffers [207] (Figure 6).

Organic Synthesis.
It also revealed that Pd catalyst supported TiO 2 could be functionalized with various amounts of 3-aminopropyltriethoxysilane via a post synthesis grafting method combined with electroless deposition of Pd [108]. The supported catalyst system gave promising catalytic properties in the solvent-free selective oxidative of benzyl alcohol. It  Figure 7: Oxidative removal of dibenzothiophene in a biphasic system using sol-gel Fe/TiO 2 catalysts and H 2 O 2 promoted with acetic acid [208]. correlated well with the highest amount of Pd distribution (with particle size of 3.4 nm) with the 1% of Pd-supported TiO 2 . Increasing of surface basicity via the hydrolysis of NH 2 suggested enhancing the dehydrogenation of benzyl alcohol and, as a consequence, the selectivity towards benzaldehyde [109][110][111]. In addition, the presence of TiO 2 support gave high catalytic activity in benzyl alcohol oxidation, emphasizing that the reduction of PdOx species by the adsorbed benzyl alcohol is an essential step to form highly active metallic PdO sites [112].

Organic Reactions.
Another important application of heterogeneous catalyst supported TiO 2 derived from the Fisher-Tropsch synthesis, which can convert various carbon sources (coal, natural gas, and biomass) into long chain hydrocarbon via syngas [103]. It is a promising way to produce environmentally benign fuels with no sulfur and nitrogen compounds [116]. In this case, certain transition metals supported with TiO 2 have frequently applied as catalyst. Among them, Co/TiO 2 is considered as the preferred catalyst due to its high selectivity for long chain linear paraffin, high resistance toward deactivation by water, and low activity for the competitive water gas shift reaction [25]. Several studies indicated that activity of Co catalyst depended on the number of exposed metal sites [90,93,95,96]. Therefore, the Co/TiO 2 catalyst system may increase the dispersion of active Co metal species. Meanwhile, TiO 2 is suitable for the practical application due to the low cost, safety, and the chemical stability. Furthermore, it is reported that the strength of Co support interaction of Co/TiO 2 was in the middle of those on Co/SiO 2 and Co/Al 2 O 3 [99,100]. It found that the activity of the Co/TiO 2 catalyst for Fischer-Tropsch synthesis largely depended on the crystal phase of TiO 2 support, the reduction degree of Co, and the surface area of Co metal. Rutile TiO 2 gives more optimum reduction degree of Co with almost 60% as compared to the anatase [204,205].
Another interesting application of TiO 2 supported catalyst is the use of Fe/TiO 2 in petroleum refining industries. Several routes can apply the Fe/TiO 2 catalyst system including oxidation, adsorption, hydrodesulfurization, oxidative desulfurization, and biodesulfurization for organosulfur compounds removal from crude oil and refined petroleum products [2,59]. This is due to the surface of Fe/TiO 2 catalyst possess electron-hole pairs and free OH radicals [92]. Highly reactive OH radicals can also be formed by the reaction of the hole with OH-, which are able to generate oxidizing radicals in the presence of peroxides and may perform the oxidative desulfurization better due to recombination reactions occurring via free radical mechanisms [204]. Rather than that, the hydrophilic-hydrophobic character of Fe/TiO 2 facilitate dibenzothiophene oxidation as a phase transfer catalyst. The coordination of acetic acid and peroxyacetic acid with Fe 3+ and Ti 4+ present at the Fe/TiO 2 surface which in turn perform a superoxides its surface [208] (Figure 7).

Electrochemical Applications.
TiO 2 is an interesting material to evaluate as a heterogeneous catalyst support for electrochemical applications, not only because of its high stability cathode potentials in acidic and hydrous environment [19,34], but also because of the recent reports introducing Ti-containing material into the electrodes [118,142]. Substoichiometric Ti O 2 −2 has been studied and performs well as a catalyst support in cathodes [90,126]. It has been reported that once metals are deposited on TiO 2 , it indicates an increased electrochemically active area [139,142]. The incorporation of TiO 2 into the cathode indicated the improved methanol tolerance and its proton conductor properties [111].

Conclusion and Suggestions
TiO 2 is a reducible metal oxide and strongly reacts with noble metals compared to other metal oxides. For this reason, TiO 2 has attracted much attention for application as heterogeneous catalyst support in many reactions. It is inferred that catalyst support on TiO 2 with different structures might exhibit different physicochemical properties and catalytic activities. Generally, pure TiO 2 possesses abysmal electronic conductivity. It is proposed that substoichiometric TiO 2 is prepared in order to improve its conductivity. Although the electronic conductivity improved by utilizing TiO 2 as a heterogeneous catalyst support, the stability was compromised after extensive polarization at high oxygen electrode potentials. Therefore, another method that could improve electronic conductivity is to dope TiO 2 with n-type dopants, including niobium (Nb) or tantalum (Ta). It has been reported in literature that TiO 2 , with a/an rutile/anatase structure doped Nb, had a significantly greater electronic conductivity compared to the native TiO 2 . The presence of Ti 3+ species was observed, which was induced by the partial replacement of Ti 4+ by Nb 5+ . Therefore, in all cases, Nb TiO 2 is both electrochemically and thermally stable, which can further promote explorations for heterogeneous catalyst support.

Conflict of Interests
The authors declare that all the written materials related to the text, tables, and figures for paper of "Titanium Dioxide as a Catalyst Support in Heterogeneous Catalysis" do not have any conflict of interests concerning the validity of research and financial gain.