Ind. Eng. Chem. Res. 1991,30, 1814-1818
1814
Effects of As203 on the Phase Composition of V205-Mo03-Ti02(Anatase) DeNO, Catalysts Erich Hums* Siemens AG, Power Generation Group (KWU),P.O. Box 3220,D-8520Erlangen, Germany
Herbert E. Gobel Siemens AG, ZFE FKE 42, 0-8000Munchen 83, Germany
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The effecta of As203 on the phase composition of V205-MOO, and V205-Mo03-Ti02 (anatase) are investigated by X-ray diffraction and discussed in terms of phase transformation V9M060a
Introduction It is a well-established fact that nitrogen oxide removing (DeNO,) catalysts such as titania-supported vanadium/ molybdenum oxides or vanadium/tungsten oxides on the high-dust side of wet-bottom furnaces are deactivated far more rapidly than in dry-bottom furnaces. This is caused by the increased concentration of catalyst poisons in the flue gas due to process-specific ash recirculation into the combustion chamber. Various primary catalyst poisons, predominantly oxides of arsenic in several oxidation states and modifications,as well as accumulations of other heavy metals that represent the main components of catalyst poisons in the flue gas are discussed; arsenic oxide is carried both in fly ash and in the gas phase depending on the conditions that prevail in the flue gas. Gutberlet (1988) has pointed out that high arsenic concentrations are found primarily in fly ash fines and catalysts. A connection between the time behavior of arsenic oxide adsorption and the loss of activity during testing of DeNO, catalysts has been described by Hofmann et al. (1988). Experience gathered in two years of trial operation of DeNO, catalytic converter plants is given by Schallert (1988). Schmelz et al. (1988) state that arsenic oxide adsorption depends on the porosity of the catalyst. Ti02with a high specific surface area and small pore diameters is the primary cause of deactivation, since As203 condensation is supported preferentially. Reduction of the BET TiOa surface area does, as one would expect, reduce arsenic oxide adsorption. Empirical evidence shows, however, that certain catalysts can adsorb several times more As203 and not suffer a corresponding loss of activity (Figure 1). This suggests that the same linear relationship of As203adsorption to deactivation does not necessarily apply to all catalysts, but rather that deactivation factors and their causes can be categorized as either chemical or physical. The central issue here is therefore what effect As203 has on the catalyst and its phase composition, independent of the problems of physical deactivation. Catalysts typically used in wet-bottom furnaces consist of more than 90% by weight Ti02 (anatase), combined with 1-270 V206and 8-9% Moog. The first step of these investigations intentionally did not focus on Ti02content; investigations were instead restricted to the phase composition of mixtures of Moo3 and V206, since consistent descriptions of their relative proportions are not given in the literature. A further objective was to determine to what extent phase composition characteristics (bulk), revealed by X-ray diffraction, are reduced when successive increases in Ti02 concentration are made, ultimately to
* Author to whom correspondence should be addressed.
levels approaching those of catalysts used in industrial applications. Mixing proportions equivalent to industrial DeNO, catalysts lie on the molybdenum-rich side of the phase diagram (Figure 2). The phase composition consists of the @-phaseand the major phase Moo3. There is some disagreement in the literature regarding the stoichiometry of the @-phase. Magneli and Blomberg (1951) calculate the composition as vo.6~MO0.3302.4~, whereas Munch and ~ O nomenclature ~ of this Pierron (1964) state V ~ M O (The formula is not consistent in the literature; neither is v6Mo40%,therefore nonuniform in the present article.) and Eick and Kihlborg (1966) V2Mo08; Volkov et al. (1972) calculate V2-xM01+&+y(0Ix I0.3) for the composite oxide phase.
Experimental Section Moo3 and V206powder mixtures were prepared and tempered in quartz ampules at 700 "C (Table I, samples 1-3). The tempered samples were then exposed to A s 2 0 3 at 430 "C and allowed to react in quartz ampules (samples 5-7). Vanadium-free samples were also investigated for reference purposes (samples 4 and 8). Sample 6a was obtained by leaching sample 6 with H20. A portion of the material of samples 1-3 was mixed with Ti02(anatase) and reacted in quartz ampules at 500 "C. Finally, the powder (samples 9-13) was exposed to As203 at 430 "C in quartz ampules (samples 14-18). In a further series of samples for the catalyst system V20,-Mo03-Ti02 (anatase), intermediate calcination was foregone, and mixtures of Moo3, V206 and Ti02 were calcined in a single process step. Powder mixtures were tempered in quartz ampules at 600 "C (samples 19-23) and then allowed to react with b o 3at 430 OC (samples 24-28). All powders were measured by using a transmission powder X-ray diffractometer (Siemens Model F) with Guinier focusing. Identification was performed with the (Siemens) DIFRAC AT analysis system using the JCPDS file (up to 1988 version). Results In samples 1-3 the composite oxide phase MosVgOa (JCPDS Card No. 34-525) increases with increasing V206 content. Several weak lines (
