Towards Physical Descriptors of Active and Selective Catalysts for the Oxidation of n‐Butane to Maleic Anhydride

Abstract

Based on our newly developed microwave cavity perturbation technique, the microwave conductivity of diverse vanadium(III), (IV), and (V) phosphate catalysts was measured under reaction conditions for the selective oxidation of n‐butane. The conductivity response on the gas phase was identified as a very sensitive measure for the redox kinetics, reversibility, and stability of the samples, which are important prerequisites for highly selective and active catalysts. The sensitivity achieved by our method was comparable to surface‐sensitive methods such as X‐ray photoelectron spectroscopy, whereas more conventional analytic techniques such as X‐ray diffractometry or Raman spectroscopy only indicated the stability of the bulk crystal phase under the same reaction conditions.

Heterogeneous Catalysis

“…In the form of semiconductor properties from many active phases, these have an important and not yet fully understood effect80c, 204b in catalysis because they directly influence the degree of coverage of the reacting surface. This effect cannot be considered to be independent because it is influenced in many ways by the other factors of catalyst function; it acts rather in a combined51i, 204a fashion with the other elements of dynamics. In metals the situation is somewhat easier as the bath of free electrons at the Fermi level can be assumed to be a reservoir for redox equivalents required for surface chemical reactions.…”

The Electronic Factor in Alkane Oxidation Catalysis

“…[22] Due to its increasing conductivity in air and decreasing conductivity in n-butane-containing gas mixtures, VPP was identified as a ptype semiconductor with electron holes as the majority charge carriers. [10][11][12][13][14][23][24][25] However, the gas-phase-dependent conductivity response alone is not a sufficient descriptor for selectivity, since vanadium(V) oxide exhibits a reversible conductivity response under reaction conditions [26] but catalyzes only the total oxidation of n-butane to CO and CO 2 .…”

Heterogene Katalysatoren – fundamental betrachtet

“…Die Eigenschaften der Ladungsträgerdynamik werden in vielfacher Weise von den übrigen Steuergrößen der Katalysatorfunktion beeinflusst. Die “Elektronenstruktur” eines reagierenden Katalysators kann deshalb kaum als eine vom Reaktionsgeschehen unabhängige konstante Materialgröße angesehen werden; sie wirkt stark verbindend51i, 204a zwischen den unterschiedlichen Einflussfaktoren. Dies zeigt sich weiter in der Entwicklung, welche die Theorie im Grenzbereich zwischen heterogener und homogener Katalyse nimmt: Dort spricht man von der Dynamik von “Ultrananokatalysatoren”232 und meint damit Systeme, bei denen alle Atome von Reaktant und Katalysator233 gleichermaßen eine Einheit in der Betrachtung der Reaktion bilden; dies wurde auch für größere Partikel in der Hydrierkatalyse mit dem Begriff “Fernwirkung”234 eingeführt.…”