Isothermal Reduction Kinetics of Titanium Dioxide-Based Materials

Abstract

As metal oxide reduction may be a limiting or otherwise important step in a reaction cycle, a complete description of the kinetics of the reduction can be critical to the successful choice of catalytic material. Unfortunately, such information is often lacking. Such is the case in our attempts to develop a catalytic cycle from the stoichiometric reductive carbonyl coupling reaction on reduced TiO2 surfaces. To provide the necessary reduction kinetics, reaction of the anatase and rutile forms of TiO2 with H2 has been studied from 573 to 773 K. A novel flow-through microreactor which provides time-resolved catalyst mass measurements to ±1 μg while maintaining a conventional, tubular reactor, gas−solid contacting pattern has been employed. A shift in the kinetic order with respect to H2 with increasing temperature occurs, from one-half order at 573 K to zero order at 673 K and above. A discontinuity was also observed within this same temperature range in Arrhenius plots of the reduction rates of both anatase and rutile TiO2; apparent activation energies determined were approximately 12 kcal mol-1 above and 29 kcal mol-1 below 623 K. Modification of the surface of anatase TiO2 with a sufficient loading of group VIII metals removes the Arrhenius plot discontinuity, increasing the rate of reduction and decreasing the apparent activation energy at low temperatures. A change in rate-determining step is indicated by these observations, and a mechanistic scheme which combines the current and previous observations within a single framework is proposed.

The enhancement of TiO2 photocatalytic activity by hydrogen thermal treatment

“…The falling of photocatalytic activity might result from the phase transition from anatase to rutile at around 700°C, which is generally considered to be not favorable to TiO 2 photocatalytic activity (Rekoske and Barteau, 1997). However, it seems that there was an optimal temperature for the H 2 treatment between 500 and 600°C.…”

“…In the meantime, the photoactivity of the TiO 2 pigment can be enhanced by this reduction reaction in a H 2 atmosphere at a thermal condition (Heller et al, 1987). The H 2 treatment was a usual method to improve the surface and photoelectrochemical properties of TiO 2 (Chen et al, 1983;Howe and Gr€ a atzel, 1987;Qin et al, 1993;Rekoske and Barteau, 1997). More recently, some researchers have investigated the reduction mechanism in the temperature range of 300-500°C (Khader et al, 1993) and the surface stoichiometry (Haerudin et al, 1998) of the TiO 2 with H 2 treatment by means of Chemosphere 50 (2003) [39][40][41][42][43][44][45][46] www.elsevier.com/locate/chemosphere electrical conductivity (Khader et al, 1993) or Fourier transform infrared (FTIR) spectroscopy (Haerudin et al, 1998).…”

Preparation of TiO2/SiO2 nanocomposite with non-ionic surfactants via sol-gel process and their photocatalytic study

“…For more than a decade, studies have mainly concentrated on the suspension of TiO 2 fine powder because of its higher photocatalytic activity photodegrade pollutant molecules when radiated with UV radiation compared to TiO 2 thin films 15 . TiO 2 powders possess interesting optical, dielectrical, and catalytical properties, which results in industrial applications 16,18 . In recent years great efforts have been devoted to the study of TiO 2 materials for photocatalytic degradation of organic and inorganic contaminants of water and air [19][20] .…”

Modifying oxide nanomaterials’ properties by hydrogenation

“…and/or oxygen vacancies, [22][23][24][27][28][29] Ti-OH groups, [22,25,26,28] Ti-H groups, [22,25,26,30] and the modification of the valence band, [4,18,23,28] heavily depend on the characteristics of the starting TiO 2 nanomaterials and the hydrogenation conditions. The former includes the fabrication history (which may affect the refined surface chemical properties possibly out of the detection limits of many analytical techniques), and the physical properties (size, shape, morphology, phase, crystallinity, etc) of the starting TiO 2 nanomaterials.…”

“…[25,49,61] Meanwhile, there are a few reports mentioning possible decrease in the photocatalytic activities, if the hydrogenated TiO 2 nanomaterials are not properly prepared. [37,38] So far, hydrogenation has been conducted on TiO 2 nanoparticles, [4,21,31] nanorods, [48] nanotubes, [45] nanowires, [46] nanosheets, [35,36] with anatase or rutile phase, [4,29,33] under high-pressure, [4,21] ambient pressure, [29,30,39] or low-pressure [43] pure hydrogen environment, or hydrogen-argon, [44][45][46][47][48] hydrogen-nitrogen [52][53][54] gas flow, in the temperature range from room temperature [21] to 700°C, [27] with a hydrogenation time from a few minutes [27] to 20 days. [21] With no doubt, we can image that the so-formed TiO 2 nanomaterials will display variations in their characteristics and performances.…”