Time-, space- and energy-resolved in situ characterization of catalysts by X-ray absorption spectroscopy

Abstract

A setup for dispersive X-ray absorption spectroscopy (XAS) with spatial, temporal and energy resolution is presented. Through investigation of Mo/HZSM-5 catalyst during the dehydro-aromatization of methane we observed a reduction...

Stabilization of intermediate Mo oxidation states by Nb doping enhancing methane aromatization on Mo/HZSM-5 catalysts

“…Meanwhile, a higher concentration of carbidic C together with a lower amount of oxygen was found. We have observed a strong increase in the Mo total surface concentration for the undoped 6Mo/HZSM-5 catalyst under NAP reaction conditions before, 39 which was also apparent for the Nb-doped sample in this study. Despite its higher initial surface Mo concentration before the reaction compared to 6Mo/HZSM-5 (see Table 2), the 6Mo1Nb/HZSM-5 sample showed a ≈22% lower total concentration of Mo after comparable reaction times (see Table S4† and ref.…”

“…First, we investigated the dynamic change in Mo and Nb oxidation states via in situ NAP-XPS analogous to our previous investigations performed on 6Mo/HZSM-5. 37,39 During the initial measurement in N 2 at 127 °C the surface composition of 6Mo1Nb/HZSM-5 was slightly different from that observed in the ex situ results, with the major fraction of Mo present as Mo 6+ together with only a minor amount of Mo 5+ (see Fig. 8d).…”

“…8c and Table S4†) compared to 8% for the 6Mo/HZSM-5 catalyst under similar reaction conditions. 39 Additionally, binding energies for reduced Mo (Mo 5+ to Mo 2+ ) were shifted slightly lower in comparison, further supporting an interaction between Mo and Nb. The formation of carbidic Mo species was accompanied by a significant relative concentration increase in the Nb species at higher binding energy (up to 34% of detected Nb, see Fig.…”

“…Despite its higher initial surface Mo concentration before the reaction compared to 6Mo/HZSM-5 (see Table 2), the 6Mo1Nb/HZSM-5 sample showed a ≈22% lower total concentration of Mo after comparable reaction times (see Table S4† and ref. 39). This suggests a higher retention of Mo inside the pores under the employed reaction conditions.…”

“…38 A more detailed view of the geometrical arrangement of this setup has been recently described. 39 The experiments were performed in a specially designed reaction cell. The sample cell is constructed from Inconel steel and equipped with a SiN window, gas connections and two heating cartridges controlled by a thermocouple.…”