Oxide/support interaction and surface reconstruction in the sodium tungstate(Na2WO4)/silica system

Jiang, Zhi Cheng; Yu, Chuangqi; Fang, Xue; Li, Shu Ben; Wang, Hong Li

doi:10.1021/j100151a038

Cited by 139 publications

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“…[10] Thep eak intensity of Na 1s was located at 1072.0 eV,w hich corresponds to Na 1s of Na 2 WO 4 and is consistent with the reported value. [11] To explore the surface state of Na 2 WO 4 in the OH radical generation condition at 800 8 8C, am ixture of O 2 and H 2 Ow as introduced into the flowing reaction cell for AP-XPS.…”

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confidence: 90%

“…This shift does not result from surface charging because there is no shift in W4f 7/2 .This upshift suggests adefinite change in the chemical environment of the Na cations after being heated from 20 to 800 8 8Ci namixture of O 2 and H 2 O. Theh igh-binding energy peak at 1073.2 eV for Na 1s at 800 8 8Cwas assigned to peroxide species (that is,Na 2 O 2 ), which is consistent with the observed binding energy of Na 1s of Na 2 O 2 reported previously. [13] To check the authentic chemical state of the Na 2 WO 4 catalyst during OCM catalysis at 800 8 8C, O 2 and CH 4 were mixed and then introduced to the reaction cell containing the Na 2 WO 4 catalyst for AP-XPS.A tr oom temperature in am ixture of O 2 and CH 4 ,t he photoemission features of Na 1s,O1s,and W4f(blue lines in Figure 3a-c) are the same as those in UHV at 20 8 8C(black lines in Figures 3d-f), except for the observation of an O1speak of gaseous O 2 .Byheating the sample to 800 8 8Cb ya ni nfrared laser beam and maintaining the catalyst at 800 8 8Ci namixture of O 2 and CH 4 during AP-XPS data acquisition, [10] Na 1s with abinding energy at 1073.1 eV was observed during catalysis.N otably, the photoemission feature of Na 1s of the surface of the catalyst during catalysis (pink line in Figure 3a)isvery similar to that observed at 800 8 8Cinamixture of O 2 and H 2 O(red line in Figure 3d). Clearly,the AP-XPS studies of nominal catalyst Na 2 WO 4 supported on TiO 2 during OCM at 800 8 8Cuncovered that the authentic surface phase of the Na species during catalysis was in fact Na 2 O 2 .T herefore,t his result was due to the quasi-equilibrated generation of Na 2 O 2 in the presence of O 2 under the studied conditions.…”

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Integrated In Situ Characterization of a Molten Salt Catalyst Surface: Evidence of Sodium Peroxide and Hydroxyl Radical Formation

et al. 2017

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Sodium-based catalysts (sucha sN a 2 WO 4 )w ere proposed to selectively catalyze OH radical formation from H 2 Oa nd O 2 at high temperatures.T his reaction mayp roceed on molten salt state surfaces owing to the lower melting point of the used Na salts compared to the reaction temperature.T his study provides direct evidence of the molten salt state of Na 2 WO 4 ,which can form OH radicals,using in situ techniques including X-ray diffraction (XRD), scanning transmission electron microscopy( STEM), laser induced fluorescence (LIF) spectrometry,a nd ambient-pressure X-rayp hotoelectron spectroscopy(AP-XPS). As aresult, Na 2 O 2 species,which were hypothesized to be responsible for the formation of OH radicals,h ave been identified on the outer surfaces at temperatures of ! 800 8 8C, and these species are useful for various gasphase hydrocarbon reactions,i ncluding the selective transformation of methane to ethane.Gas-phase radical chemistry involving OH radicals plays acrucial role in the oxidative coupling of methane (OCM), [1] the dehydrogenation of ethane, [2] atmospheric chemistry, [3] and combustion reactions.[4] Thec atalytic generation of OH radicals from O 2 and H 2 Oc an occur on Pt metal and alkali earth oxides at high temperatures (> 700 8 8C).[5] Alkaline metal containing catalysts (that is,N as upported on oxide) enhanced the rate of H 2 Oactivation in the presence of O 2 and enhanced both the CH 4 conversion rate and C 2 selectivity during OCM.[1] Otsuka et al. demonstrated that Na 2 O 2 reacts with CH 4 to form methyl radicals at relatively low temperatures.[6] Recently,k inetic evidence suggests that H 2 Oi s involved in the activation of CH 4 via the quasi-equilibrated formation of OH radicals when aN a-containing catalyst is used.[1] In as imilar context, an in situ Raman spectroscopic study identified Ba species on MgO that formed peroxide ions and was proposed as the active sites.[7] However,d irect evidence of the formation of this sodium peroxide species and an OH radical product are lacking. Here,i nsitu studies performed at high temperatures using different in situ characterization techniques were performed to identify the authentic active species during catalysis responsible for the OCM reaction. We report evidence for the formation of Na 2 O 2 and OH radicals during the catalysis,and these species play asignificant role in high-temperature gas-phase chemistry.Ad etailed kinetic investigation that was focused on the effects of H 2 Oo nt he CH 4 /O 2 reaction was reported in our previous studies.[1] Briefly,the kinetic expression for the CH 4 / O 2 reaction on Na-based catalysts at low conversion levels is given in (1):Thef irst term corresponds to CH 4 activation via surface O (O*), which is quasi-equilibrated with gas-phase O 2 (dry condition). Thes econd term corresponds to CH 4 activation via an OH radical that is formed from O 2 and H 2 Oi nq uasiequilibrated steps (wet condition). Among the studied catalysts,t he Na 2 WO 4 /SiO 2 catalyst exhibited the highest contributi...

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Integrated In Situ Characterization of a Molten Salt Catalyst Surface: Evidence of Sodium Peroxide and Hydroxyl Radical Formation

et al. 2017

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“…In the Na 2 WO 4 /SiO 2 system, the reconstruction of surface WO 4 tetrahedral units into a structure containing a W=O and three W-O-Si surface bonds was responsible for the OCM reaction via a redox mechanism [22,23], and molecular oxygen is activated by an F-center to produce lattice oxygen O 2- [24]. Cobalt catalysts supported on alumina can catalyze methane partial oxidation [25], as well as CO 2 reforming of methane [26].…”

Section: The Dual-catalytic Performances Of Na 2 Wo 4 /Co-mn/siomentioning

confidence: 99%

Na2WO4/Co–Mn/SiO2 Catalyst for the Simultaneous Production of Ethylene and Syngas from CH4

Qin

2007

Catal Lett

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“…Here, basic oxides have been shown to be good catalysts for the process typically carried out at temperatures between 600 and 800°C [3,4]. Two of the most intensively studied systems in this respect are lithium doped magnesia which was pioneered by Lunsford and coworkers [5], and Mn-Na 2 WO 4 /SiO 2 which was first discovered by Li [6]. While Mn-Na 2 WO 4 /SiO 2 is considered to be the state-of-the-art OCM catalyst, it is a very complex system, and the nature of the active sites is still under debate [7].…”

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confidence: 99%

Sol–Gel Preparation of Samaria Catalysts for the Oxidative Coupling of Methane

et al. 2015

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A new sol-gel synthesis route for aluminasamaria mixed aero-and xerogel catalysts based on the socalled epoxide addition method and the use of these systems as catalysts for the oxidative coupling of methane (OCM) is reported. As precursors simple chloride or nitrate salts can be used. The mesoporous materials are X-ray amorphous even after calcination to 800°C and show an intimate mixing of Al and Sm on the nanoscale. In the case of the xerogels derived from chlorides, C 2 yields comparable to pure samaria can be achieved under OCM reaction conditions with 100 % O 2 conversion. Even at lower O 2 conversions the activity of the xerogel is competitive with a pure samaria reference catalyst taking the lower samaria content of 20 % into account. Accordingly, the approach is suitable to reduce the costs associated with the rare earth oxide. In addition to the preparation of aerogel and xerogel particles, the presented synthesis also allows the fabrication of xerogel films which can be coated on a suitable (monolithic) support. First results of such films are presented. Graphical AbstractKeywords Oxidative methane coupling Á Sol-gel chemistry Á Rare earth oxide catalyst Electronic supplementary material The online version of this article (

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Oxide/support interaction and surface reconstruction in the sodium tungstate(Na2WO4)/silica system

Cited by 139 publications

References 1 publication

Integrated In Situ Characterization of a Molten Salt Catalyst Surface: Evidence of Sodium Peroxide and Hydroxyl Radical Formation

Integrated In Situ Characterization of a Molten Salt Catalyst Surface: Evidence of Sodium Peroxide and Hydroxyl Radical Formation

Na2WO4/Co–Mn/SiO2 Catalyst for the Simultaneous Production of Ethylene and Syngas from CH4

Sol–Gel Preparation of Samaria Catalysts for the Oxidative Coupling of Methane

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