Untersuchungen an oxidischen Katalysatoren. XXXIV. Redoxverhalten des Nickels in NiNaY‐Zeolithen 1. Reduzierbarkeit und Reoxydierbarkeit von Nickel in NiNaY‐Zeolithen

Abstract

Die Eigenschaften von metallischem Nickel in reduzierten (470–870 K) und reoxydierten (470, 670 K) NiNaY‐Proben wurden durch chemische Analyse (Reaktion mit K2Cr2O7) und spektroskopische Untersuchungen (FMR, IR nach CO‐Chemisorption und UV/VIS) charakterisiert. Die Reduktion der Ni2+‐Kationen oxidischer Cluster erfolgt einstufig, während isoliert lokalisierte Ni2+‐Kationen über Ni+‐Kationen zum Metall reduziert werden. Entsprechend der Lokalisierung und dem Austauschgrad ändert sich der Reduktionsgrad des Nickels in charakteristischer Weise mit der Reduktionstemperatur. Metallisches Nickel, das bei Temperaturen < 620 K erhalten wurde, kann bei 470 K vollständig reoxydiert werden. Höhere Reduktionstemperaturen führen zu Nickelaggregationen, die selbst bei 670 K nicht vollständig reoxydierbar sind.

FTIR Study of CO and NO Adsorption and Coadsorption on Ni-ZSM-5 and Ni/SiO2

“…In agreement with data from the literature, ,,− the bands at 2220 and 2212 cm -1 are assigned to Ni 2+ −CO species. Here, the carbonyls are much more stable than the Ni 2+ −CO carbonyls detected with the Ni/SiO 2 sample.…”

“…Adsorption of CO on a Ni/SiO 2 sample activated at 773 K results in formation of a band at 2192 cm -1 , which decreases in intensity with the equilibrium pressure and disappears after evacuation. In agreement with data from the literature, ,,− the band at 2192 cm -1 is assigned to Ni 2+ −CO species.…”

“…The electrostatic bond (which is weaker than the σ-bond) also leads to an increase of the C−O stretching frequency because of the vibrational Stark effect. That is why the CO stretching frequency in the Ni 2+ −CO species is higher than the frequency of gaseous CO (2143 cm -1 ) and is usually observed in the 2220−2180 cm -1 region. ,,− Only with NiO and bulk mixed oxides containing nickel the carbonyls of Ni 2+ are detected at lower frequencies. , Since both bonds (σ and electrostatic) are relatively weak, the Ni 2+ −CO species are usually detected under a certain CO equilibrium pressure. Stable Ni 2+ −CO species have been found with nickel-exchanged zeolites only. ,, …”

“…Evacuation leads to the disappearance of the bands characterizing gem-species and appearance, on their place, of a band at 2109 cm -1 , assigned to linear Ni + monocarbonyls. 16,20,24,[26][27][28][29][30][31][32][33]42,43 These results indicate that a ZSM-5 matrix stabilizes the Ni + cations. As was described above, the attempts to produce Ni + ions on the Ni/SiO 2 sample failed; in the latter case the Ni 2+ ions were directly reduced to metallic nickel.…”

High-dispersion d.c. sputtered platinum-titania powder catalyst active in ethane hydrogenolysis

“…It is shown that under identical experimental conditions the degree of reduction follows the order zeolite X > zeolite Y and mordenite > zeolite Y.2,4 While the coexchanged cations determine the overall chemical properties of the zeolites through the average electronegativity,5 they also compete with Ni2+ for exchange sites. Thus, under the same experimental conditions the degree of reduction of Ni2+ in alkaline-earth-exchanged Y exceeds that of NiNa-Y and follows the order Mg < Ba < Sr < Ca.4 However, the degree of reduction in NiNaX exceeds that of NiCaX.6 In X-type zeolites Ce3+ is known to promote the reduction of Ni2+, while La3+ retards it.7-10 In the presence of acidic OH groups the reduction of Ni2+ is difficult, because the equilibrium Ni2+ + H2 ^Ni(0) + 2H+ (1) is driven to the left.4…”