In Situ Solid-State NMR Studies of Trichloroethylene Photocatalysis: Formation and Characterization of Surface-Bound Intermediates

Abstract

In situ solid-state NMR methodologies have been employed to investigate the photocatalytic oxidation of trichloroethylene (TCE) over two TiO2-based catalysts, Degussa P-25 powder and a monolayer TiO2 catalyst dispersed on porous Vycor glass. 13C magic angle spinning (MAS) experiments reveal that similar reaction intermediates form on the surfaces of both catalysts. Long-lived intermediates, including dichloroacetyl chloride (Cl2HCCOCl, DCAC), carbon monoxide, and pentachloroethane and final products CO2, phosgene (Cl2CO), and HCl were observed under dry conditions. The presence of molecular oxygen was found to be essential for TCE photooxidation to proceed. Adsorbed water was found to greatly reduce the formation of phosgene. The formation of surface-bound dichloroacetate and trichloroacetate species was observed and identified via 13C cross polarization MAS experiments. Dichloroacetate, which forms from mobile DCAC, appears to be bound to the nonirradiated surfaces of the powdered TiO2 catalyst and further degradation was not possible. Formation of di- and trichloroacetate also takes place on the TiO2/PVG catalyst in the absence of light; however, their concentrations are low. Degradation studies of these surface-bound species indicate that the photooxidation of dichloroacetate is slow and results in the formation of phosgene and CO2, while trichloroacetate remains resistive to degradation on the TiO2/PVG catalyst. Our results also indicate that the formation of DCAC and phosgene seems to be a general result of TCE degradation which is not limited to TiO2 photocatalysis but instead may be more characteristic of the types of initiating species which are formed by UV irradiation. However, the TiO2 surface is the most effective in terms of the observed initial rates of degradation.

Solid state NMR studies of molecular recognition at protein–mineral interfaces

“…Solid state NMR has long been used to identify and characterize the structures of small organic molecules adsorbed onto catalytic surfaces [10][11][12], argon matrices [13], silica and alumina surfaces [14][15][16]. There similarly exist a number of solid state NMR pulse techniques capable in principle of reporting the structures of surface-adsorbed proteins.…”

“…These models are derived from the full set of 1,000 models. Alignment of the models in the figure was based on root-mean-square deviation calculation of segments (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15) and (33)(34)(35)(36)(37)(38) between the 8 models. Table 1 31 P-31 P nearest neighbor distances lying within a sphere of radius 6 Å for the 004 crystal plane in in monoclinic HAP.…”

Degradation of trichloroethylene using highly adsorptive allophane–TiO2 nanocomposite

“…The amount of DCAC adsorbed on Sample T was three times greater than the initial amount of TCE. It is reported that TiO 2 can adsorb DCAC to some extent [17][18][19]24,29]. The adsorption capacities of Samples M and C for DCAC are about four times greater than that of Sample T. Figure 3 shows the changes in the average degradation rate of TCE during the repeated runs using Samples T, M, and C. The degradation rate increased with the repeated run for all the samples.…”

Characterization of Surface and Photooxidative Properties of Supported Metal Oxide Photocatalysts Using Solid-State NMR

“…We have introduced a new approach, the use of in situ solid-state NMR (SSNMR) spectroscopy to the study of the surface chemistry of photocatalytic oxidation reactions. [7][8][9] The use of in situ SSNMR to study the near-UV light driven oxidation reactions yields an abundance of information about the reaction chemistry of organic molecules on the catalyst surface because of the atomic specificity, high resolution, and quantitative capabilities of NMR as well as its ability to distinguish between strongly or weakly bound or mobile species. In this paper, we use ethanol to probe the different electronic environments of a number of single and multiple monolayer metal oxide catalysts supported on porous Vycor glass.…”