Applied Catalysis B, Environmental (v.45, #3)


A study of the regeneration of fresh and aged SO x adsorbers under reducing conditions by L Limousy; H Mahzoul; J.F Brilhac; F Garin; G Maire; P Gilot (169-179).
Sorption of SO2 over a commercial SO x adsorber was studied at a laboratory scale using a fixed bed reactor. The adsorbent material contains mainly alumina, barium oxide and precious metals. Sulphates formed by SO2 sorption are reduced by carbon monoxide or hydrogen in the presence of water. The main species emitted during the reduction was H2S. The species COS was emitted at a very low level when carbon monoxide was used as a reducing agent. The total removal of sulphur was not possible in the absence of water, the sulphur remaining bonded to platinum (or palladium) sites. The role of water is to hydrolyse these sulphides, leading to H2S or COS. The species SO2 was also emitted when an aged catalyst was used, without a loss of the catalytic reduction efficiency. After a first sequence adsorption–regeneration, the adsorption capacity was not affected and the second regeneration was more efficient. Such a SO x adsorber can be placed upstream a NO x adsorber to avoid poisoning of this latter by SO2.
Keywords: SO x adsorber; Lean burn; Reduction; SO2 adsorption;

State of Ru on CeO2 and its catalytic activity in the wet oxidation of acetic acid by Saburo Hosokawa; Hiroyoshi Kanai; Kazunori Utani; Yo-ichi Taniguchi; Yoshio Saito; Seiichiro Imamura (181-187).
The relationship between the state of Ru on CeO2 and catalytic activity in the wet oxidation of acetic acid was investigated for Ru/CeO2 catalysts prepared by different methods. The temperature programmed reduction (TPR) experiments of Ru/CeO2 showed that the oxygen species of RuO2 was reduced at different temperatures depending upon the methods of preparation. Ru species reduced at low temperatures could not be observed by TEM and XRD. It was concluded that RuOCe bonds in the well-dispersed Ru species are highly fragile and its mobile oxygen is the active species in the wet oxidation.
Keywords: Ru/CeO2; Wet oxidation; TPR; Acetic acid;

The reaction products and the stoichiometry of the urea catalytic decomposition in the system HNO3–HCOOH–Pt/SiO2 were studied with an application of the IR spectroscopy, the mass spectroscopy methods and chemical analysis. It was found that in the presence of 0.1 g ml−1 1% Pt/SiO2, the 0.1–0.4 M (NH2)2CO decomposes at 50–90 °C in the solutions of 2.5–6.4 M HNO3 containing 0.25–1.5 M HCOOH according to a stoichiometric equation: 3(NH 2)2 CO+3HCOOH+5HNO 3 →
2N 2+N 2 O+2NH 4 NO 3+NH 2 CHO+5CO 2+6H 2 O. Soluble reaction products—the ammonium nitrate and the formamide—undergo further a gradual degradation under the conditions of a catalytic denitration reaction that starts when the urea decomposition is completed.
Keywords: Urea decomposition; Catalytic denitration; Nitric acid;

The denitration by HCO2H of highly concentrated HNO3 media (3–5 M) was studied on Pt/SiO2 catalysts with Pt particle size varying from 1 to 10 nm. The reaction was carried out in batch mode at 343 K. The actual active intermediate is HNO2, which is formed very slowly in the absence of Pt/SiO2. The chief function of Pt/SiO2 is the initial fast generation of HNO2 in order to reach a threshold concentration of 5×10−3  mol l−1. Above this threshold, the homogeneous process between HNO3 and HCO2H, with HNO2 as an autocatalytic species, prevails. The initial generation of HNO3 on Pt/SiO2 is regulated by a catalytic cycle involving “Pt-H” and Pt0 species. This was demonstrated by redox cycle experiments in which Pt-H was reacted with HNO3 to yield Pt0, which was back reactivated to Pt-H by HCO2H. Large Pt clusters were the most active, possibly due to a faster reaction rate between Pt-H and HNO3.
Keywords: Formic acid; Nitric acid; Platinum; Chemical denitration; Nuclear fuel reprocessing;

Reductive regeneration of sulfated CuO/Al2O3 catalyst-sorbent in ammonia by Guoyong Xie; Zhenyu Liu; Zhenping Zhu; Qingya Liu; Jianrong Ma (213-221).
Reductive regeneration of a sulfated CuO/Al2O3 catalyst-sorbent suitable for simultaneous SO2 and NO x removal from flue gases was carried out in 5 vol.% NH3/Ar. Effect of regeneration temperature on SO2 removal activity of the regenerated catalyst-sorbent was investigated. Chemical morphology and physical structure of the catalyst-sorbent before and after the regeneration were characterized using elemental analysis, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and physical absorption. The results show that copper sulfate, the main copper species in the sulfated catalyst-sorbent, can be effectively regenerated at 350–450 °C. Aluminum sulfate, resulted from the reaction between Al2O3 and SO2 in flue gas, is unreductable under the conditions, which leads to reduced SO2 removal activity of the regenerated catalyst-sorbent compared to the fresh catalyst-sorbent. The main copper species after the regeneration at 400 °C is Cu3N. The nonexistance of copper sulfide suggests that over-reduction occurring in H2 is avoided in NH3. BET surface area and pore size distribution of the regenerated catalyst-sorbent are almost the same as that of the fresh one when the reduction temperature is 400 °C or higher. Besides as a reductant, ammonia also reacts with SO2 formed in the regeneration to form hydroxyamine sulfate at the outlet of the reactor.
Keywords: Flue gas desulfurization; CuO/Al2O3 catalyst-sorbent; Regeneration in NH3;

In our previous studies, three variations of a kinetic model for the transient gas–solid photocatalytic oxidation of aromatic contaminants on titanium dioxide (TiO2) were developed and compared with experimental data. Two of the models, based upon a single type of catalyst site, were not capable of replicating transient experimental data from the photocatalytic oxidation of benzene, toluene, or m-xylene in air at a single feed concentration (50 mg/m3). The remaining kinetic model, the Two-Site model, presumed the presence of a more hydrophilic type of site and was capable of replicating the time-varying behavior seen with all three aromatic contaminants considered. In the present study, this Two-Site kinetic model is extended to separately consider the photocatalytic oxidation of gas-phase toluene at various feed concentrations (20–100 mg/m3) and the regeneration of used catalysts via flowing, humidified air and UV illumination. Our Two-Site model provides reasonable fits for experimental data collected during the photocatalytic oxidation of toluene at several concentration levels with no significant changes to the prior model. It is also capable of representing catalyst regeneration, although some significant differences between the model predictions and experimental results are noted.
Keywords: Photocatalysis; Titanium dioxide; TiO2; Aromatics; Benzene; Toluene; Xylene; Deactivation; Two-site; Multi-site; Modeling;