Panagiotis G. Smirniotis

Low-Temperature Selective Catalytic Reduction (SCR) of NO with NH3 by Using Mn, Cr, and Cu Oxides Supported on Hombikat TiO2

A Highly active, time stable, and water resistant, Hombikat TiO2 supported Mn catalyst has been developed for the selective reduction of NO by NH3 [Eq. (1)]. The analogous Cu and Cr supported catalysts also provide 100 % N2 selectivity at ≤120°C. Lewis acidity, redox properties, and a high surface metal oxide concentration are essential for good catalytic performance.

Sonophotocatalytic destruction of organic contaminants in aqueous systems on TiO2 powders

Abstract The effect of ultrasound on the photodegradation of salicylic acid on four commercial titania powders was studied. The system exhibiting the highest enhancement was isolated. The use of ultrasound during photocatalysis had a pronounced effect on the rate and efficiency of salicylic acid destruction as compared with UV-light photocatalysis alone. The possible reasons of the increased activity under ultrasonication were proposed: aggregate breakage and photocatalytic utilization of species produced by the ultrasound. The combination of the action of ultrasonic waves and UV-assisted photocatalysis yielded synergistic effects for the catalysts with smaller particle size (such as Hombikat), while no enhancement was observed for the largest particle size photocatalyst (Aldrich anatase). Degussa P25 exhibited the highest overall activity for the degradation of salicylic acid and moderate enhancement of activity by ultrasound. The presence of intermediates in the bulk solution was observed during the purely photocatalytic degradation of phenol. The presence of ultrasound, however, allows to eliminate the toxic intermediates by the sonolysis in the bulk solution.

Gas-phase photocatalytic oxidation of diethyl sulfide over TiO2: kinetic investigations and catalyst deactivation

Abstract The gas-phase heterogeneous photocatalytic oxidation of diethyl sulfide (DES), a simulant for chemical agent mustard gas, was investigated in a batch reactor using detection of gaseous products and FT-IR identification of surface species. Acetaldehyde and ethylene were detected as gaseous intermediates and diethylsulfone and carboxylates were detected as surface intermediate products. It was found that DES is oxidized completely and the final products of its oxidation are carbon dioxide, water and surface sulfate species. Deactivation of the TiO 2 photocatalyst was evidenced by increased time of DES complete mineralization in consecutive treatment of fixed amount of DES in the batch reactor. Carbonate and sulfate species were detected on the TiO 2 surface after complete DES oxidation by means of FT-IR diffuse reflectance spectroscopy. These structures turned out to be responsible for TiO 2 deactivation.

Pathways of photocatalytic gas phase destruction of HD simulant 2-chloroethyl ethyl sulfide

Abstract Photocatalytic oxidation of HD simulant, 2-chloroethyl ethyl sulfide (CEES), was studied in a specially designed coil and flow photocatalytic reactor by means of GC-MS and FTIR techniques. TiO 2 Hombikat UV 100 photocatalyst was deactivated after a few hours of operation, which was signaled by the appearance of incomplete oxidation products in the reactor effluent and accumulation of incomplete oxidation products on the TiO 2 surface. Complete reactivation of the photocatalyst was achieved by washing the photocatalyst with water. Compared to diethyl sulfide, CEES showed lower reactivity in photocatalytic oxidation and was accumulated on the TiO 2 surface after catalyst deactivation. Without UV irradiation, hydrolysis of CEES proceeded on the TiO 2 surface. Major gaseous products of CEES incomplete photocatalytic oxidation are acetaldehyde, chloroacetaldehyde, SO 2 , diethyl disulfide, and chloroethylene. Surface products extracted from the TiO 2 surface with acetonitrile and water include mainly 2-chloroethyl ethyl sulfoxide and ethanesulfinic and ethanesulfonic acids, as well as diethyl di-, tri-, and tetrasulfides mono and disubstituted in the β position with a chlorine or hydroxyl group. While surface monodentate sulfates can be removed upon washing, surface bidentate species stayed on the surface and possibly contribute to the permanent catalyst deactivation.

Characterization of the acidity of ultrastable Y, mordenite, and ZSM-12 via NH3-stepwise temperature programmed desorption and Fourier transform infrared spectroscopy

The acidity of ultrastable Y (USY), mordenite, and ZSM-12 of variable Si/Al ratios has been characterized by coupling NH3-stepwise temperature programmed desorption (STPD) and FT-IR. The former technique allows us to quantify accurately the ammonia chemisorbed on acid sites of different strengths. In contrast to other acidic oxide catalysts, it was found that protonated zeolites possess distinct limits of acid strength and that chemisorbed ammonia desorbs from each type of site only within particular temperature ranges. It was observed that for zeolites with Si/Al ratios smaller than about 20 the number of ammonia molecules chemisorbed on both Lewis and Bronsted sites is smaller than the total number of Al atoms present in the zeolite. However, an increase in the Si/Al ratio above this threshold results in a 1:1 relation between the total number of acid sites and the amount of ammonia adsorbed. It was found that for mordenite and ZSM-12 an increase of the dealumination severity results in a decrease of the acid site strength. For USY the acid strength decreased with steam treatment, but then increased with acid leaching. The FT-IR studies revealed that for the protonated USY, Lewis-bound ammonia is evolved within each temperature stage. For mordenite, the Lewis sites are associated with desorption up to 250°C. The non-dealuminated ZSM-12 (Si/Al=35) sample possesses a relatively small number of Lewis sites, whereas the ZSM-12 samples with higher Si/Al ratios had only Bronsted acid sites. A peak deconvolution procedure was performed in order to decompose accurately the complex hydroxyl region of USY and mordenite and was used to separate the Lewis peak from structural vibrations for ZSM-12. As a result, important information concerning the nature of the aluminum species present in these zeolites (extraframework species, framework species present within the side pockets of mordenite, framework species present within the β cages of USY, etc.) was obtained.

Vibrofluidized- and fixed-bed photocatalytic reactors: case of gaseous acetone photooxidation

Abstract Vibrofluidized- and multiple fixed-bed photoreactors were compared under identical operating conditions. The comparison was based on the quantum efficiency for the gas-phase photocatalytic oxidation of acetone using TiO 2 (Hombikat UV 100). Multiple fixed-bed reactor configurations were used, along with a vibrofluidized-bed configuration. Quantum efficiency decreased in the following order: vibrofluidized bed (8.7%)>fixed-bed granules (6.9%)>fixed film bed (5.9%) ∼fixed powdered bed (5.8%). The increased activity of the vibrofluidized-bed could not be attributed to enhanced external mass transport, as all reactor systems used in the present study demonstrated negligible external mass transfer resistances. Instead, the increased activity is most likely credited to the effect of periodic illumination phenomenon taking place because of the random motion of catalyst granules in the fluidized-bed and higher light absorption of scattered light. The enhanced activity observed for the granular fixed bed could be related to mechanical activation of TiO 2 during the preparation of granules, as well as to increased light absorption. A maximum of acetone oxidation rate with respect to humidity level was observed. Even at high humidity levels, ultrasound did not affect the rate of gas-phase acetone oxidation. A model has been developed to quantify the absorption of light in a fluidized-bed photoreactor. The model takes into account absorption and single-light scattering and accurately describes the dependence of acetone oxidation rate on the quantity of TiO 2 granules in the vibrofluidized bed.

COMPARATIVE STUDY ON PHOTOCATALYTIC OXIDATION OF FOUR ORGANOPHOSPHORUS SIMULANTS OF CHEMICAL WARFARE AGENTS IN AQUEOUS SUSPENSION OF TITANIUM DIOXIDE

Abstract Photocatalytic oxidation by oxygen of air was carried out for dimethyl methyl phosphonate (DMMP), trimethyl phosphate (TMP), triethyl phosphate (TEP), and diethyl phosphoramidate (DEPA) in different concentration. The initial rate of organophosphorus compounds consumption increases with the initial concentration at relatively low concentrations but decreases at higher initial concentrations. If the concentration is higher than the concentration in maximum, the rate decreases because of the lack of adsorbed oxygen. These summit-like dependences are well approximated by one site Langmuir–Hinshelwood equation with competitive adsorption of oxygen and organophosphorus compound. Parameters of the Langmuir–Hinshelwood equation are reported. Complete mineralization of the organophosphorus compounds at the end of reaction was evidenced by the total organic carbon concentration profiles. These profiles have sigmoidal shape. GC–MS technique was used to identify intermediates of TEP and TMP oxidation. The main intermediates are dimethyl phosphate and methyl phosphate in the case of TMP and diethyl phosphate and ethyl phosphate in the case of TEP. The set of intermediates shows that photocatalytic oxidation proceeds primarily at α carbon atoms of TEP. The distribution of intermediates corroborates that photocatalytic oxidation is initiated by reaction with hydroxyl radicals.

Synthesis of Al-rich ZSM-12 using TEAOH as template

Abstract Hydrothermal synthesis of ZSM-12 (MTW) with tetraethylammonium hydroxide (TEAOH) as template was systematically studied with the objective of obtaining an aluminum rich zeolite. In contrast to other zeolites, published information on the synthesis of ZSM-12, especially aluminum rich samples, is limited since its preparation is a difficult task due to its inherent high silica character. The effect of aluminum content of the gel, the OH − /SiO 2 ratio and the TEA/SiO 2 ratio were examined and the crystallization domain of pure ZSM-12 phase was identified as a function of these parameters. TEA/SiO 2 and OH − /SiO 2 ratios much lower than what is typically used in zeolite synthesis were found to be favorable for crystallization of ZSM-12, especially at Si/Al ratios ranging from 30 to 50. In this study, we were able to successfully synthesize ZSM-12 with Si/Al ratio of 31. This was achieved from gels having TEA/SiO 2 ratios of 0.125 as well as 0.16 and OH − /SiO 2 ratio of 0.07. The products obtained were characterized by XRD, SEM, BET and TGA. The results clearly showed that pure ZSM-12 with high crystallinity was obtained in all the batches. The amount of organic retained in the zeolites after synthesis was constant but the state of TEA was very different depending on the Si/Al ratio of the zeolite.

Routes of photocatalytic destruction of chemical warfare agent simulants

Selected imitants of chemical warfare agents such as dimethyl methylphosphonate (DMMP), diethyl phosphoramidate (DEPA), pinacolyl methylphosphonate (PMP), butylaminoethanethiol (BAET) were subjected to photocatalytic and sonophotocatalytic treatment in aqueous suspensions of TiO2. Complete conversion of the same mass of imitants to inorganic products was obtained within 600 min for DMMP, DEPA, PMP, but required a longer time for BAET. Sonolysis accelerated photodegradation of DMMP. No degradation was observed without ultraviolet illumination. Final products of degradation were PO43−, CO2 for DMMP and PMP, PO43−, NO3− (25%), NH4+ (75%), CO2 for DEPA, and SO42−, NH4+, CO2 for BAET. The number of main detected intermediate products increases in the order DMMP (7), DEPA (9), PMP (21), and exceeds 34 for BAET. Degradation of DMMP mainly proceeds through consecutive oxidation of methoxy groups and then the methyl group. Dimethyl hydroxymethylphosphonate and dimethylphosphate testify to the parallel oxidation of the methyl group. Destruction of DEPA mainly starts with cleavage of the P–NH2 bond to form diethyl phosphate, which transforms further into ethyl phosphate. Oxidation of α and β carbons of ethoxy groups to form ethylphosphonoamidate, hydroxyethyl ethylphosphonoamidate and other products also contributes to the destruction. Photocatalytic degradation of PMP mainly starts with oxidation of the pinacolyl fragment, methylphosphonic acid and acetone being the major products. Oxidation of BAET begins with dark dimerization to disulfide, which undergoes oxidation of sulfur forming sulfinic and sulfonic acids as well as oxidation of carbons to form butanal, aminobutane, etc., and cyclic products such as 2-propylthiazole. A scheme of degradation was proposed for DMMP and DEPA, and starting routes for PMP and BAET. Quantum efficiencies of complete mineralization calculated as reaction rate to photon flux ratio approximate 10−3%.

TiO2 reactivation in photocatalytic destruction of gaseous diethyl sulfide in a coil reactor

Abstract Diethyl sulfide (DES) photocatalytic oxidation was carried out in a flow research reactor with TiO2 Hombikat UV 100 deposited onto the internal surface of a Pyrex coil. The reactor allowed easy catalyst reactivation by washing with water. Mass transfer limitations were not detected in the reactor. Catalyst deactivated after several hours of complete DES mineralization, which was expressed by decrease of effluent CO2 concentration and appearance of gaseous intermediates. Catalyst reactivation was achieved by two procedures: (1) irradiating photocatalyst until complete mineralization of adsorbed organic products with subsequent water washing; and (2) immediate washing with water. The water used after the first procedure contained only H2SO4, while after the second procedure it contained organic intermediates. The second procedure was much faster. Lower water concentration and higher catalyst loading allowed longer catalyst stability in DES oxidation. Low feed DES concentration resulted in much longer deactivation. Twelve consecutive runs were done to test reactivation completeness. Some permanent catalyst deactivation was noted and explained by TiO2 etching with H2SO4. Gaseous and surface intermediate products were detected in the reactor effluent and catalyst wash water using solid phase microextraction (SPME) and trimethylsilyl derivatization. Main gaseous products were acetaldehyde, diethyl disulfide, ethylene, SO2, and main surface products were diethyl disulfide, diethyl trisulfide, 1,2-bis(ethylthio)ethane, ethanesulfinic, ethanesulfonic acids, diethyl sulfoxide, diethyl sulfone, and sulfuric acid.