Titania surface calculations

Figure 2 Surface atomic ratios of n(Si)/n(Ti) of titania/silicas calculated by XPS Spectra...

The secondary ring carbon atoms are preferentially oxidized with respect to that of the methyl group. A simple statistical calculation demonstrates that they are 6.5 times more Teactive. This behaviour is opposite to the gas phase photocatalytic oxidation of toluene (ref. 4), which produces only traces of benzaldehyde, whereas the aromatic ring withstands oxidation, at least in pure gas or liquid organic phase and in the absence of water. The above selectivities seem to be correlated to steric factors governing the mode of adsorption of methylcyclohexane on the surface of titania. 

Parameters of the porous structure of titania samples (pores volume Vs, specific surface area Ssp) were calculated using BET theory [34] from the adsorption isotherms of methanol. The average pore diameter (Dp) values were estimated from the differential curves of pore size distribution. 

The influence of the support is undoubted and spillover was further confirmed by the excess of hydrogen chemisorbed by a mechanical mixture of unsupported alloy and TJ-A1203 above that calculated from the known values for the separate components. It was also observed that the chemisorption was slower on the supported than on the unsupported metal and that the greater part of the adsorbate was held reversibly no comment could be made on the possible mediation by traces of water. On the other hand, spillover from platinum-rhenium onto alumina appears to be inhibited for ratios Re/(Pt Re) > 0.6. In an infrared investigation of isocyanate complexes formed between nitric oxide and carbon monoxide, on the surface of rhodium-titania and rhodium-silica catalysts, it seems that the number of complexes exceeded the number of rhodium surface atoms.The supports have a pronounced effect on the location of the isocyanate bond and on the stability of the complexes, with some suggestion of spillover. 

Figure 9. Comparison of the turnover frequency (TOF) for methane synthesis over a clean Ni and a titania-containing Ni surface based on model calculations at a total pressure of 120 Torr and an H /CO ratio of 4.

Reference [2749] reports results from [1472], and [2750] reports results from [951]. The IEP of alumina at pH 7.3 is reported in [2751] without any specific information about the source of material or about experimental conditions (probably from the literature). The IEP of titania at pH 5.6 is reported in [2752] without experimental conditions (probably from the literature). The IEP of mica at pH 3-3.5 is reported in [2753] without experimental conditions (probably from the literature). Reference [2754] reports charging curves of titania, obtained under unspecified experimental conditions, probably from previous paper. Reference [2755] reports lEPs from the literature and estimated from X-ray photoelectron spectroscopy. The PZCs reported in [2756] are probably from the literature (no experimental details are provided). lEPs from the literature ar e reported in [2757-2765,2767-2792,2794-2804,2842,2852,2894,2900,2905]. lEPs and PZCs from the literature are reported in [169,2805-2807]. The lEPs/PZCs reported in [2808] are also probably from the literature. Charging and electrokinetic curves from the literature are reported in [2809]. Electrokinetic curves from the literature are reported in [2810-2812]. PZCs from previous papers by the same authors are reported in [1165,2813-2821]. lEPs from previous papers by the same authors are reported in [111,2009,2822-2825]. Electrokinetic curves from previous papers by the same authors are reported in [2826,2827]. Reference [2828] reports PZC for an ill-defined material, and PZC from the literature. Reference [2829] reports calculated charging curves based on results from the literature. References [222, 1784,2830,2831] report surface charging data from the literature. Reference [2832] reports surface charging curves and PZCs from the literature. The PZCs in Table 1 of [947] are probably taken from the literature. References [2835,2836] probably report PZCs from the literature and [2838,2839] probably report lEPs from the literature. Reference [2840] reports PZCs from the literature that were confirmed by a nonstandard method. PZCs from the literature are also reported in [84,87,92, 114,118,188,723,780,945,968,1115,1162,1505,1533,1699,1766,1773,1975,1976,1996, 2035,2708,2766,2793,2841,2843-2845,2847-2851,2853-2893,2895-2899,2901-2904,2906-2917]. Reference [2846] reports a result of coagulation study from the literature. 

The textural nature of the titania and TiCex were characterised by nitrogen ad/desorption isotherms. The specific surface areas are presented in Table la. None of the materials were found to be microporous from t-plot analyses of the adsorption isotherms. From the desorption curves, the mesopore size distribution was calculated using the BJH method. All of the samples had bimodal mesopore size distributions. The volumes of the narrow and wide mesopores, presented in Table lb), were calculated from the minima between the two distributions. These results indicated, that as the amount of ceria incorporation rose, the bimodal mesopore size distribution became narrower. For the titania sample the mesopores were centred in diameters of approximately 14 and 17 nm. With ceria incorporation both of these diameters were reduced until at the highest ceria content they were approximately 9 and 15 nm. 

The results of surface Si/Ti compositions of titania/silicas characterized by XPS are shown in figure 3. In order to calculate the surface atomic ratios, n(Si)/n(Ti), the following equation was used [5] ... 

The properties of the raw materials used to prepare the supports are shown in Table 1. With the exception of type A, which is a hydrated non-heat treated titania, all other types were calcined in the industrial production process and therefore no significant changes in the BET area with heat treatment were observed. The titania particle sizes were calculated from the BET surface areas, assuming that the heat treated titania was constituted by non-porous spherical particles. Their XRD patterns (not shown) indicate that they all had the anatase structure. 

The yields of ammonia generally ranged from 0 to 606 x 10 8 g NH, m 2 of powder surface. Absolute yields (which we calculated from the surface area and amount of putative catalyst used in each experiment) ranged from 0 to 52.3/rmol NH3. The highest yield was with a sample of coprecipitated Ti02 (0.2 mol % Fe) which was irradiated for 28 h. The most effective catalysts appeared to be Ti02 coprecipitated with 0.2 and 0.5 mol % Fe. For some samples there appeared to be a correspondence between ammonia yield and reaction temperature, but in others no consistent trend was reported. No correlation was observed between the surface area of titania samples and their apparent catalytic activity, but the surface area was never varied without also varying the composition and manner of preparation of the powder. A net decline in apparent rate of ammonia production was observed after a few hours of irradiation the decline did not depend on the reactor temperature, the composition of the putative catalyst, or the apparent yield of ammonia. The maximum yield was reported to correspond to a turnover of 6 electrons per iron atom before the powder was deactivated. 

Table 1. Properties of the titania networks obtained by using polymer gel templates calculated porosity, surface area obtained from nitrogen adsorption, pore, and titania nanoparticle diameters. Adapted with permission from [8]. Copyright 2001 American Chemical Society ...

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