WO_x‐CeO₂ and WO_x‐Nb₂O₅ catalysts deactivation during hexane isomerization

Abstract

in Wiley InterScience (www.interscience.wiley.com).Two series of WO x -CeO 2 catalysts containing 10, 12, and 15 wt % W were prepared by co-precipitation and incipient wetness impregnation using a reported procedure. WO x -Nb 2 O 5 catalysts with the same amount of W were prepared by co-precipitation starting from an aqueous solution of ammonium niobium oxalate and ammonium metatungstate with ammonia, until reaching pH 7. After calcining for 3 h in static air at 973 K, the deactivation of the catalysts was investigated using isomerization of hexane as probe reaction at temperatures in the range 393-513 K, both in the absence and in the presence of hydrogen. In-situ Raman spectra during deactivation were recorded as well. The deactivation rate was fitted by an empirical Voorhies equation: R 5 m t 2n , where the values of n correlated very well with the mass loss determined from TG analysis.

Comparison of the Structures and Mechanism of Arsenic Deactivation of CeO2–MoO3 and CeO2–WO3 SCR Catalysts

“…To confirm this conjecture, Raman spectroscopy was used to explore the changes in chemical bonds and functional groups before and after deactivation. As shown in Figure S3, for the CM system, the main peak of the cubic-phase CeO 2 at 460 cm –1 , due to the F 2g mode, is reduced sharply after deactivation, but this peak remains nearly the same after As poisoning of the CW catalyst . Moreover, compared with the fresh catalyst, CWP exhibits an obvious blue shift from 958 to 968 cm –1 , corresponding to the WO stretching modes of the isopolytungstate species.…”

“…As shown in Figure S3, for the CM system, the main peak of the cubic-phase CeO 2 at 460 cm −1 , due to the F 2g mode, is reduced sharply after deactivation, but this peak remains nearly the same after As poisoning of the CW catalyst. 26 Moreover, compared with the fresh catalyst, CWP exhibits an obvious blue shift from 958 to 968 cm −1 , corresponding to the WO stretching modes of the isopolytungstate species. Additionally, some new peaks at 804 [ν as (W−O)] and 636 cm −1 [ν s (W−O−W)] attributed to the octahedral unit can also be shown.…”

“…For the CW catalyst (Figure a), when the catalyst was purged with NO + O 2 after pretreatment with NH 3 , the υ s mode of the WO vibrations due to WO x (998 cm –1 ) reemerged, while the characteristic crystalline CeO 2 vibrations (457 and 248 cm –1 ) were obviously enhanced, indicating that NH 3 can be chemisorbed onto the surface of both WO x and CeO x at low temperatures. Moreover, when the gaseous order was reversed, the band at 930 cm –1 , attributed to the WO vibrations of Ce 2 (WO 4 ) 3 compounds, and the band at 817 cm –1 due to the oxotungstate species were strengthened to some extent after the NH 3 was introduced (Figure b). , Therefore, Ce 2 (WO 4 ) 3 compounds and W–O bonds from [WO 4 ] or [WO 6 ] units may also be active sites during the SCR reaction. Nevertheless, the changes described above become inconspicuous after deactivation by As oxide because the peak for the WO vibration of WO x weakens.…”

“…Moreover, when the gaseous order was reversed, the band at 930 cm −1 , attributed to the WO vibrations of Ce 2 (WO 4 ) 3 compounds, and the band at 817 cm −1 due to the oxotungstate species were strengthened to some extent after the NH 3 was introduced (Figure 9b). 25,26 Therefore, Ce 2 (WO 4 ) 3 compounds and W−O bonds from [WO 4 ] or [WO 6 ] units may also be active sites during the SCR reaction. Nevertheless, the changes described above become inconspicuous after deactivation by As oxide because the peak for the WO vibration of WO x weakens.…”

In situ growth of epitaxial cerium tungstate (100) thin films

“…Further, the combination of cerium oxide with tungsten oxide, which is another material used as a gas sensor 3 and a catalyst, 4 results in interesting optical 5,6 and catalytic properties, for example in isomerisation of hexane, [7][8][9] dehydrogenation of alkanes used in rechargeable solid oxygen reservoirs for clean hydrogen oxidation, 10 or in inorganic ion exchangers. 11,12 Eight phases of cerium-tungsten mixed oxides were found by Yoshimura et al: 13 The properties of the most studied compound, Ce 2 O 3 Á3WO 3 , also known as cerium tungstate, Ce 2 (WO 4 ) 3 , were briefly summarized by Nelson and McKee already in 1946, 5 and its crystal structure was extensively studied by Gressling and Mu¨ller-Buschbaum.…”