Incorporation of manganese and iron into the zirconia lattice in promoted sulfated zirconia catalysts

Abstract

Two series of Mn-or Fe-promoted zirconia samples were prepared, (i) a series of sulfated-free reference compounds via co-precipitation of aqueous solutions containing zirconium and the promoter cation, and (ii) a series of catalysts via incipient-wetness impregnation of a sulfated zirconium hydroxide. The promoter content was varied between 0 and 5 wt% metal. All promoter-containing materials were calcined at 923 K. The reference materials contained mainly isolated Mn or Fe species incorporated into the zirconia lattice as evidenced by stabilization of the tetragonal zirconia phase, EPR (isolated ions in highly symmetric environment), and a shrinking unit cell volume (XRD) of the tetragonal zirconia phase with increasing promoter content. Only the Mn-promoted catalysts showed such shrinkage in unit cell volume with increasing promoter content. At 2 wt% promoter content, Fe could, and Mn could not be detected by ion scattering spectroscopy on the surface of the catalysts. The Fepromoted catalysts contained Fe2O3-like surface species (EPR, XANES), which could at least in part be removed by washing with oxalic acid. Catalysts were tested for isomerization at 338 K using 1 kPa n-butane in balance of N2. At 0.5 wt% promoter content the maximum rates produced by the 0.5 wt% Mn-promoted and Fe-promoted sulfated zirconia were about 80 and 20 µmol g -1 h -1 , respectively. Mn was thus more effective as a promoter for n-butane isomerization than Fe, despite the more extensive incorporation into the zirconia lattice.

n-Butane Isomerization Catalyzed by Sulfated Zirconia Nanocrystals Supported on Silica or γ-Alumina

“…The activity of SZ can be improved by 1 to 2 orders of magnitude through the addition of small amounts of a second metal component as promoter, e.g., Mn or Fe [12][13][14]. It has been found that one of the effects of these promoters is the stabilization of the tetragonal (cubic) phase by incorporation of the Mn or Fe cations into the zirconia lattice [15,16]. Furthermore, mechanical stress in the form grinding, milling, or pressing-typical laboratory practices for homogenization or wafer fabrication-can convert tetragonal into monoclinic zirconia, and the phase change is accompanied by a decrease of catalytic activity for n-butane isomerization [17].…”

The Balance Between Reactivity and Stability of Modified Oxide Surfaces Illustrated by the Behavior of Sulfated Zirconia Catalysts

“…The various phases of zirconia are stable in various ranges of oxygen vacancy concentration. Promoter ions such as Fe 3+ or Mn x+ (x<4) have also been shown to stabilize the high temperature phases of zirconia [47][48][49][50][51]. They are incorporated into the lattice similarly to Y 3+ , and for charge balance, oxygen vacancies are formed.…”

“…Promoters with a valence lower than + IV enhance the stability of the tetragonal phase because oxygen vacancies are generated. The incorporation of the iron and manganese cations leads to a smaller unit cell size of the tetragonal phase, mostly through reduction of the unit cell constant c [47]. The doping affects the stability in a second way because the reduced unit cell size will affect the surface energy.…”

“…This behavior can be explained with the formation of a solid solution and formation of oxygen vacancies as a consequence of a valence lower than +IV of the incorporated promoter cations. Manganese cations are more easily incorporated into the zirconia lattice than iron cations, and consequently iron is more likely to form a separate oxide phase with increasing content [47]. Hence, at 0.5 wt% content and most likely complete incorporation, iron and manganese are equally effective in the stabilization, whereas at 2.0 wt% content, manganese is more effective than iron ( Table 4).…”

Physicochemical properties, cytotoxicity, and&nbsp;antimicrobial activity of sulphated zirconia&nbsp;nanoparticles

“…[1][2][3] It has been found that it is not just the strength of the acid but also the type of acidity (Brønsted or Lewis) that matters for improved activity and selectivity. The inclusion of superacidity in solids has attracted great attention.…”