Zeolite encapsulated vanadium oxo species for the selective catalytic reduction of nitric oxide by ammonia

Robert Chandler Adams, Purdue University

Abstract

Vanadium oxo species have been introduced into the supercages of Y-zeolite by adsorption and decomposition of VOCl$\sb3$ and VO(O-i-C$\rm\sb3H\sb7)\sb3$ to produce catalysts for the study of the effects of oxo vanadium site isolation on selective catalytic reduction (SCR) of NO with NH$\sb3.$ $\sp{51}$V solid-state magic angle spinning NMR of calcined samples with up to 4 vanadium atoms per supercage show that the host Y-zeolite typically stabilizes the vanadium in tetrahedral coordination. Octahedral and V$\sb2$O$\sb5$-like square pyramidal coordinations have been observed, and are more prevalent at higher vanadia loadings. Two distinct isolated tetrahedral environments were identified with NMR resonances around $-$530 and $-$830 ppm. Differentiation between these sites was based on the strength of interaction between the vanadium oxo species and the zeolite framework, such that the upfield species is more strongly bound to the zeolite. There was no NMR evidence for oxygen bridge-bonded vanadia structures in the tetrahedral species in any of these catalysts. DRIFTS studies show that ammonia adsorbs primarily on Bronsted acid sites associated with both the zeolite and the vanadium oxo species. DRIFTS experiments during NO reaction with preadsorbed NH$\sb3$ show the production of a new hydroxyl species at 3690 cm$\sp{-1},$ assigned to V$\sp{+4}$-OH, and a nitrosyl anion at 1380 cm$\sp{-1}.$ In situ DRIFTS results also show that the NH$\sb4\sp{+}$ ions associated with the zeolite Bronsted acid sites are preferentially consumed during reaction with NO. For the SCR reaction in the 350-450$\sp\circ$C range, the turnover frequencies (TOF), based on NO conversion and total V loading, range from $1\times10\sp{-5}$ to $3\times10\sp{-4}$ s$\sp{-1}$ with selectivity to N$\sb2$ of nearly 100% at 350 and 400$\sp\circ$C and 85-95% at 450$\sp\circ$C. Steady-state reaction experiments on catalysts with vanadium loadings less than 0.75 vanadium atoms per supercage indicate that isolated vanadia species are viable sites for the SCR reaction. Results also suggest that there are interactions between the vanadia species and the zeolite acid sites which increase the specific activity of the isolated vanadia sites.

Degree

Ph.D.

Advisors

Delgass, Purdue University.

Subject Area

Chemical engineering

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