A combined multi-technique in situ approach used to probe the stability of iron molybdate catalysts during redox cycling

A setup combining a number of techniques (WAXS, XANES and UV–Vis) has been used to probe the stability of an iron molybdate catalyst during redox cycling. The catalyst was first reduced under anaerobic methanol/helium conditions, producing formaldehyde and then regenerated using air. Although in this test-case the catalyst and conditions differ from that of a commercial catalyst bed we demonstrate how such a setup can reveal new information on catalyst materials. In particular we observe the formation of two phases during reduction; one which we propose to be an oxygen deficient ‘pseudo-molybdate phase’, the other a molybdenum carbide-like phase, both produced as oxygen is removed from the catalyst. Standard in situ techniques could detect such transient phases, however, the information from multiple techniques, allows us to more accurately identify the nature of these materials and to carry out appropriate complementary ex situ measurements to aid in the analysis. This and similar setups therefore offer a way to more quickly and accurately observe reaction pathways within a catalyst, which may for example, result in the deactivation of the material by different routes to those observed previously. Additionally, the specific combination of these techniques with on-line mass spectrometry, allows us to monitor the activity of the catalyst surface and here observe that different catalytic mechanisms may occur during different stages of the redox process. Therefore this setup should allow for the observation of many novel variations in a catalyst’s reactivity, leading to the improvement of current and development of new materials

[Zn10(µ4-S)(µ3-S)6(Py)9(SO4)3] as a molecular model of ZnS surfaces: an experimental and theoretical study

Experimental and theoretical results pertaining to [Zn10(μ4-S)(μ3-S)6(Py)9(SO4)3], a possible molecular model of ZnS S-terminated polar surfaces, as well as a potential source of strictlymonodispersed ZnS quantum dots, are presented and discussed. The results of density functional theory (DFT) calculations provided a rationale for the peculiar arrangement of [Zn10(l4-S)(l3-S)6(Py)9(SO4)3] clusters in the solid state, contemporarily indicating the unsuitability of the isolated species to mimic whatever (polar or non-polar) ZnS surface. Despite the fact that such a failure is further confirmed by time-dependent DFT and UV–Vis diffuse reflectance spectroscopy, the combined use of theoretical outcomes, DRIFT measurements, and literature data pertaining to the surface chemical properties of ZnS (Hertl in Langmuir 4:594, 1988) ultimately testifies that [Zn10(μ4-S)(μ3-S)6(Py)9(SO4)3] is perfectly suited to model the interaction of pyridine molecules with ZnS surface Lewis acid sites. The herein reported theoretical results are expected to be a useful reference for the interpretation of chemisorption experiments of Py-based Lewis bases on single crystal ZnS surfaces