The control of valence state: How V/TiO2 catalyst is hindering the deactivation using the mechanochemical method

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Abstract

Various experiments were conducted to improve durability against SO2 by impregnating the same amount of vanadium in TiO2 which had the various physical properties. According to those catalysts, the degree of deactivation by SO2 had various results, and it was found that the production of unreacted NH3 in selective catalytic reduction reaction should be low. Based on X-ray photoelectron spectroscopy analysis, O2 on–off test, O2 reoxidation test and H2-temperature programmed reduction experiment, the redox capacity of catalyst was improved due to increasing of non-stoichiometric compounds. Such a non-stoichiometric oxide and redox capacity of catalyst can be enhanced by the ball-milling process, and the production of ammonium sulfate salt can be more easily inhibited by the superior oxidation–reduction capacity of catalyst. We found that this result is caused by producing and increasing of Vx+ (x  4), Tiy+ (y  3) which are non-stoichiometric chemical species of catalyst.

Introduction

Among several technologies proposed for removing nitrogen oxide emitted from stationary sources, the selective catalytic reduction (SCR) process in flue gas treatment (FGT) is recognized to be most desirable in the technological and economic aspect. The keys of this technology are the composition of catalyst, the form of the reactor, operation conditions, etc., but most of researches have been focused on the development of catalyst. Until now, hundreds of catalysts have been proposed from noble metal catalysts to basic metal catalysts, and most of developed catalysts are V2O5–WO3/TiO2 and zeolite, iron oxide, copper oxide, manganese oxide, etc. [1], [2], [3], which are manufactured in the form of honeycomb and plate and operated at high temperature of 300–500 °C.

Particularly because of the durability of catalyst against SO2 in the SCR process, research has been conducted on various supports, and durability against SO2 is different according to supports as titania = silica > α-alumina > η-alumina > γ-alumina [4].

Active elements were mostly noble metals in early researches, but according to the results of studies on transition metals such as Co3O4, Cu2O, Fe2O3, MnO, NiO, CrO3 and V2O5, activity and selectivity were similar but resistance to SOx was highest in V2O5 [5].

In general, the SCR process in power plants is installed before or after the desulfurization facility. If it is installed before, it causes problems such as the thermal fatigue of catalyst, abrasion by dust and shortening of life by SO2 poisoning. In most cases, accordingly, a wet or semi-dry flue gas desulfurization process is used in connection to a denitrification process using catalyst for simultaneous treatment of SOx/NOx [6]. However, if the SCR process is installed after the desulfurization facility, because the desulfurization process is of wet type, the temperature of exhaust gas decreases rapidly after desulfurization, and in order to obtain the optimal efficiency, an additional source of heat is necessary, which in turn consumes a large amount of energy. In addition, salt such as NH4NO3 and NH4HSO4 is produced by unreacted NH3 from the low-temperature region and it corrodes the equipment, shortens the life of catalyst, and lowers the efficiency of denitrification.

In order to prevent this, several attempts have been made to develop low-temperature denitrification catalyst, but few researches have been made on the poisoning of catalyst by SO2 and catalysts durable against SO2.

Thus, for an efficient process in the thermodynamic and economic aspect, the present study purposed to explain the SCR reaction characteristic of high-activity denitrification catalyst resistant to SO2 poisoning at temperature of 250 °C, focused on power plants among stationary sources of emission.

For this, based on the results of researches [7] for determining differences in the redox characteristic caused by the existence of non-stoichiometric chemical species {Vx+ (x  4), Tiy+ (y  3)} as a method of improving durability against SO2 in V/TiO2 catalyst, we inhibited the production of ammonium sulfate salt by improving the oxidation/reduction characteristic of catalyst through non-stoichiometric improvement using ball milling, one of high-energy metal dissolution methods, and by doing so, enhanced the durability of the catalyst against SO2.

In addition, for examining how denitrification catalyst prepared by this method has increased durability against SO2 and the reaction characteristic of the catalyst, we conducted reactivity experiments and several physicochemical characteristic analysis using a fixed bed reactor fit for the composition of the actual process.

Section snippets

Wet impregnation method

The catalyst used in this study is one of V2O5/TiO2 line, which was prepared by impregnating vanadium with several commercial TiO2 supports with different specific surface area and crystal structure. The used TiO2 supports are largely divided into anatase supports, rutile supports, and supports of the mixture of the two phases. The BET area (m2/g) alphabetically 92(A), 94(B), 106(C) and 80(D). Each sample that contains W and SO3 with TiO2. TiO2(A) and TiO2(B) contain 1.2% and 0.4% of SO3

Characteristics of SCR reaction in the presence of SO2

Catalyst applicable to the SCR process in the presence of SO2 should be low in SO2 adsorption power, and should not produce unreacted ammonia by maintaining its reactivity even after SO2 has reached adsorption/desorption equilibrium in the catalyst.

Fig. 3(a) shows a SCR long-run test using commercial catalysts at reaction temperature of 250 °C by putting 500 ppm of SO2 when SCR reaction reached the normal state at space velocity 60,000 h−1, NOx 800 ppm, NH3/NOx mole ratio 1.0, O2 3% and H2O 6%.

In

Conclusions

The present study conducted reactivity experiments and several physicochemical property analysis using a fixed bed reactor fit for the actual process in order to improve durability against SO2 based on the properties of V/TiO2 catalyst by the high-energy metal dissolution method using ball milling and to explain the causes of the improvement, and drew conclusions as follows:

  • (1)

    The decrease of durability against SO2 resulting from the deactivation of catalyst by SO2 is different according to

Acknowledgement

This research was supported by a grant (code #10024184) from ‘Korea Institute of Industrial Technology Evaluation and Technology’ under ‘Ministry of Commerce, Industry and Energy, Korea’.

References (17)

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