Structural dynamics of an iron molybdate catalyst under redox cycling conditions studied with <i>in situ</i> multi edge XAS and XRD

Gaur, Abhijeet; Stehle, M.; Raun, Kristian Viegaard; Thrane, Joachim; Jensen, Anker Degn; Grunwaldt, Jan‐Dierk; Høj, Martin

doi:10.1039/d0cp01506g

Cited by 26 publications

(24 citation statements)

References 32 publications

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“…Mo K-edge XANES (Figure D, see bulk GI-XAS in Figure S6) of the as-deposited film shows an increase in the pre-edge feature at the surface relative to the bulk. This peak increase has been correlated with an increase in the forbidden 1s → 4d transition that becomes more allowed with increased distortion around the Mo atom and consequent p-d hybridization. − However, while this feature does not provide a direct correlation to oxidation because there are Mo oxides (e.g., MoO 2 ) that do not exhibit this distortion, the similarity in position to the pre-edge peak measured for the MoO 3 reference powder supports the interpretation of this feature as an O incorporation event. This is consistent with the surface oxidation and O incorporation as determined by XPS and ToF-SIMS, respectively.…”

Section: Resultsmentioning

confidence: 97%

Identifying and Tuning the In Situ Oxygen-Rich Surface of Molybdenum Nitride Electrocatalysts for Oxygen Reduction

Stevens

Kreider

Patel

et al. 2020

ACS Appl. Energy Mater.

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Rigorous in situ studies of electrocatalysts are required to enable the design of higher performing catalysts. Non-platinum group metals for oxygen reduction (ORR) catalysis containing light elements such as oxygen, nitrogen, and carbon are known to be susceptible to both ex situ and in situ oxidation leading to challenges associated with ex situ characterization methods. We have previously shown that bulk O content plays an important role in the activity and selectivity of Mo-N catalysts, but further understanding the role of composition and morphological changes at the surface is needed. Here, we report the measurement of in situ surface changes to a molybdenum nitride (MoN) thin film under ORR conditions using grazing incidence x-ray absorption and 2 reflectivity. We show that the halfwave potential of MoN can be improved by ~ 90 mV by potential conditioning up to 0.8 V vs RHE. Utilizing electrochemical analysis, dissolution monitoring, and surface sensitive x-ray techniques, we show that under moderate polarization (0.3 -0.7 V vs RHE) there is local ligand distortion, O incorporation, and amorphization of the MoN surface, without changes in roughness. Furthermore, with a controlled potential hold procedure, we show that the surface changes concurrent with potential conditioning are stable under ORR relevant potentials.Conversely, at higher potentials (≥ 0.8 V vs RHE) the film incorporates O, dissolves, and roughens, suggesting that in this higher potential regime the performance enhancements are due to increased access to active sites. Density functional theory calculations and Pourbaix analysis provide insight into film stability and oxygen incorporation as a function of potential. These findings coupled with in situ electrochemical-surface sensitive x-ray techniques demonstrate an approach to studying non-traditional surfaces in which we can leverage our understanding of surface dynamics to improve performance with the rational, in situ tuning of active sites.

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Section: Resultsmentioning

confidence: 97%

Identifying and Tuning the In Situ Oxygen-Rich Surface of Molybdenum Nitride Electrocatalysts for Oxygen Reduction

Stevens

Kreider

Patel

et al. 2020

ACS Appl. Energy Mater.

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“…Molybdenum-oxy-methoxy or molybdenum-oxy-hydroxy species are formed from surface MoO 3 under reaction conditions and are believed to be the volatile compounds that lead to a loss of molybdenum. [37][38][39][40] Interestingly, Mössbauer spectroscopy also showed an enrichment of Fe 2 O 3 during testing, which is a second indication for a slight molybdenum loss during the stability testing. It seems, that in ethanol ODH a similar deactivation mechanism applies as for the methanol ODH.…”

Section: Discussionmentioning

confidence: 92%

“…Under reaction conditions, volatile molybdenum-oxy-methoxy or molybdenum-oxyhydroxy species are formed and sublimated from the surface leaving behind a molybdenum depleted catalyst. [37][38][39][40] These volatile species decompose in colder zones of the reactor and deposit between the catalyst pellets, blocking the reactor and causing increased pressure drops. [41] Nevertheless, iron molybdate catalysts show great performance in methanol ODH and are available on large scale.…”

Section: Introductionmentioning

confidence: 99%

Activity, Selectivity and Initial Degradation of Iron Molybdate in the Oxidative Dehydrogenation of Ethanol

et al. 2022

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Iron molybdate catalysts are applied in the industrial FormOx process to produce formaldehyde by oxidative dehydrogenation (ODH) of methanol. Only few studies are available about the applicability of iron molybdate catalysts for the ODH of ethanol to produce acetaldehyde. Herein, an iron molybdate synthesized by co-precipitation (p) and an iron molybdate prepared by a ball-milling solid-state synthesis (bm) are applied as ethanol ODH catalysts. Both materials show attractive acetaldehyde selectivites of > 90 % (280 °C: p-Fe 2 (MoO 4 ) 3 : Y AcH = 90.3 %; bm-Fe 2 (MoO 4 ) 3 : Y AcH = 60.4 %) and a stable performance.The bulk composition and crystal structure could be confirmed by various characterization techniques and is maintained during ethanol ODH. XPS reveals an enrichment of Mo on the catalyst surface which is slightly decreasing after the catalytic tests. This observation could be a first sign of long-term deactivation like known from methanol ODH. Comparing the performance of both materials, p-Fe 2 (MoO 4 ) 3 shows higher activity and aldehyde selectivity. We propose the higher Mo/Fe surface ratio and the lower acidity to be the reasons for these differences.

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“…The volatilization rate was also observed to be highly affected by the particle size [89]. Furthermore, in situ measurements during redox cycling on a FeMo catalyst showed phase reduction and segregation into MoO 2 and FeMoO 4 , which needed to be oxidized at 500 • C in order to fully regenerate (not including the volatilized Mo) [90]. Other studies [70,[91][92][93][94][95] show similar trends and results for deactivation and phase reduction.…”

Section: Deactivationmentioning

confidence: 52%

A Review and Experimental Revisit of Alternative Catalysts for Selective Oxidation of Methanol to Formaldehyde

et al. 2021

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The selective oxidation of methanol to formaldehyde is a growing million-dollar industry, and has been commercial for close to a century. The Formox process, which is the largest production process today, utilizes an iron molybdate catalyst, which is highly selective, but has a short lifetime of 6 months due to volatilization of the active molybdenum oxide. Improvements of the process’s lifetime is, thus, desirable. This paper provides an overview of the efforts reported in the scientific literature to find alternative catalysts for the Formox process and critically assess these alternatives for their industrial potential. The catalysts can be grouped into three main categories: Mo containing, V containing, and those not containing Mo or V. Furthermore, selected interesting catalysts were synthesized, tested for their performance in the title reaction, and the results critically compared with previously published results. Lastly, an outlook on the progress for finding new catalytic materials is provided as well as suggestions for the future focus of Formox catalyst research.

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Structural dynamics of an iron molybdate catalyst under redox cycling conditions studied with in situ multi edge XAS and XRD

Abstract: Combination of in situ multi-edge X-ray absorption spectroscopy at the Mo K- and Fe K-edges in combination with X-ray diffraction successfully uncovered structural dynamics and phase transformations of an iron molybdate catalyst during redox cycling.

Cited by 26 publications

References 32 publications

Identifying and Tuning the In Situ Oxygen-Rich Surface of Molybdenum Nitride Electrocatalysts for Oxygen Reduction

Identifying and Tuning the In Situ Oxygen-Rich Surface of Molybdenum Nitride Electrocatalysts for Oxygen Reduction

Activity, Selectivity and Initial Degradation of Iron Molybdate in the Oxidative Dehydrogenation of Ethanol

A Review and Experimental Revisit of Alternative Catalysts for Selective Oxidation of Methanol to Formaldehyde

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