Quantitative Determination Of The Number Of Surface Active .
Catalysis Today 78 (2003) 257–268Quantitative determination of the number of surface activesites and the turnover frequency for methanol oxidationover bulk metal vanadatesLaura E. Briand a , Jih-Mirn Jehng b , Laura Cornaglia c ,Andrew M. Hirt d , Israel E. Wachs e, aeCentro de Investigación y Desarrollo en Ciencias Aplicadas, CONICET, Univ. Nacional de La Plata,Calle 47 No. 257, B1900AJK La Plata, Buenos Aires, Argentinab Department of Chemical Engineering, National Chung Hsing University, 250 Kuokuang Road, Taichung 402, Taiwan, ROCc Instituto Nacional de Catálisis y Petroquimica (FIQ, UNL-CONICET), Santiago del Estero 2829, 3000 Santa Fe, Argentinad Materials Research Laboratories, Inc., 290 North Bridge Street, Struthers, OH 44471, USADepartment of Chemical Engineering, Zettlemoyer Center for Surface Studies, Lehigh University, 7 Asa Drive, Bethlehem, PA 18015, USAAbstractThe present work investigates the number and nature of the surface active sites, selectivity and turnover frequency towardsmethanol selective oxidation of a series of bulk metal vanadates. The catalysts were synthesized through an organic route andcharacterized by laser Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and specificsurface area analysis (BET). The number of surface active sites (Ns ) was determined by measuring the concentration ofsurface methoxy species adsorbed on the catalysts exposed to an atmosphere of 2000 ppm of methanol in helium at 100 C.The specific activity values (TOFs) were calculated by normalizing the methanol oxidation reaction rate by the number ofsurface active sites probed by methanol chemisorption. The comparison of the methanol oxidation products distribution frombulk metal vanadates, pure V2 O5 and corresponding metal oxides (NiO, MnO, etc.) strongly suggests that the metal vanadatecatalysts consist of only surface vanadium oxide sites. The comparison of the TOF values demonstrated that bulk metalvanadates possess similar activity to monolayer vanadium oxide supported catalysts and are more active than bulk metalmolybdates for methanol selective oxidation. Moreover, bulk metal vanadates are as active and selective as the commercialMoO3 /Fe2 (MoO4 )3 catalysts at high methanol conversion. 2002 Elsevier Science B.V. All rights reserved.Keywords: Catalysts; Bulk vanadates; Bulk molybdates; Supported vanadium oxides; Supported molybdenum oxides; Methanolchemisorption; Surface active sites; Methanol oxidation; Turnover frequency1. IntroductionThe chemisorption of chemical probe moleculesis a well-known method to characterize the surface Corresponding author. Tel.: 1-610-758-4274;fax: 1-610-758-6555.E-mail address: iew0@lehigh.edu (I.E. Wachs).properties of catalytic materials. Recently, methanolchemisorption was shown to be a reliable method todetermine the number of surface active sites of metaloxide catalysts [1–4]. Wachs et al. demonstrated thatmethanol chemisorbs as a stable monolayer of surface methoxy species on an oxide surface exposedto an atmosphere of 2000 ppm of methanol in Heat 100 C. Surface methoxy species, CH3 O(ads) , are0920-5861/02/ – see front matter 2002 Elsevier Science B.V. All rights reserved.PII: S 0 9 2 0 - 5 8 6 1 ( 0 2 ) 0 0 3 5 0 - 4
258L.E. Briand et al. / Catalysis Today 78 (2003) 257–268also the reaction intermediates in the selective oxidation/dehydration of methanol and, therefore, the quantification of the amount of surface methoxy speciesallows the quantitative determination of the densityof surface active sites for methanol selective oxidation. The number of chemisorbed surface methoxyspecies can be determined both gravimetrically andthrough in situ infrared spectroscopy. Both methodswere successfully applied to a series of monolayer supported catalysts (oxides of molybdenum, vanadium,chromium, tungsten), bulk metal molybdates and bulkmetal oxides [1–6].Knowledge of the surface active site density alsoallows for the calculation of the catalytic activityper active site (turnover frequency (TOF) or specificactivity) and a reliable comparison of the catalyticactivity of different materials. The determination ofthe active surface site density and specific activityof monolayer molybdenum oxide supported catalystsand bulk metal molybdates demonstrated for the firsttime in the literature that both systems possess similarsurface compositions and activity towards methanolselective oxidation [5]. The present investigation extends the novel methanol chemisorption technique tobulk metal vanadates and compares the TOF valuesof these systems with monolayer vanadium oxidesupported and bulk metal molybdates catalysts formethanol selective oxidation.Bulk metal vanadates are active catalysts in theselective oxidation of methanol to formaldehyde athigh conversions. Recent investigations establishedthat metal vanadates possess a higher activity andstability than the MoO3 /Fe2 (MoO4 )3 mixture commercially employed in the industrial production offormaldehyde via methanol oxidation [7].The most studied topic concerning bulk metalvanadates in recent years has been the oxidative dehydrogenation of propane to propylene over bulk magnesium vanadate phases (pyro-vanadate -Mg2 V2 O7 ,meta-vanadate Mg3 V2 O8 and ortho-vanadate -MgV2O6 ). The higher catalytic activity and selectivityobserved on magnesium pyro-vanadate phase was attributed to the liability of its surface O2 ions [8,9].Au et al. [10] reported that rare earth ortho-vanadatessuch as SmVO4 , LaVO4 and YVO4 are as active asmagnesium vanadate in ODH of propane.Owen and Kung [11] demonstrated that magnesium, zinc, chromium, nickel, copper and ironortho-vanadates are active in the ODH of butane. Theselectivity towards butenes and butadiene correlatedwith the reducibility of the cation in aqueous media, but ODH is a vapor–solid process rather than anaqueous process.Mixtures of bulk metal vanadates with chlorideshave also been applied as catalysts in soot combustion.Saracco et al. [12,13] found that copper and potassiumvanadates along with cesium, rubidium or potassiumchloride catalyze the combustion of amorphous carbonat 382 C (the non-catalyzed reaction takes place at617 C).Although, several scientific studies established thatbulk metal vanadates are active catalysts in selectiveoxidation reactions, there are no reported investigations that have determined the number of surface activesites and the specific catalytic activity (TOF) values.2. Experimental2.1. SynthesisMg2 (VO4 )2 , NbVO5 , CrVO4 , FeVO4 , Ni3 (VO4 )2 ,Co3 (VO4 )2 , Mn3 (VO4 )2 , AlVO4 , AgVO3 , Cu3 (VO4 )2and Zn3 (VO4 )2 were synthesized through an organicroute from NH4 VO3 (Alpha Aesar Products, 99.9%),the corresponding metal nitrates (Mg3 (NO3 )2 ·6H2 O,Cr(NO3 )3 ·9H2 O, Fe(NO3 )3 ·9H2 O, Ni(NO3 )2 ·6H2 O,Co(NO3 )2 ·6H2 O, AgNO3 , Cu(NO3 )2 ·6H2 O andZn(NO3 )2 ·6H2 O; Alfa Aesar or J.T. Baker, 99.9%)or chlorides (MnCl2 ·3H2 O and NbCl5 ; Alfa Aesar,99.9%) and citric acid (HOC(COOH)(CH2 COOH)2 ·H2 O; Alfa Aesar, 99.9%). The details of the synthesisare under patent revision [7].2.2. Characterization2.2.1. Laser Raman spectroscopyThe phase purity of the bulk metal vanadate phaseswas determined by Raman spectroscopy because of itsultra-sensitivity to metal oxide microcrystalline phases(especially below 40 Å that are XRD amorphous). Thespectra were obtained under ambient conditions withan Ar ion laser (Spectra Physics model 2020-50, excitation line 514.5 nm) delivering 15–40 mW of incident radiation. The powdered solid (approximately100–200 mg) was pressed into a thin wafer about 1 mm
L.E. Briand et al. / Catalysis Today 78 (2003) 257–268thick that was mounted onto a spinning sample holderand rotated at 2000 rpm to avoid local heating effects.The scattered radiation from the sample was directedinto a Spex Triplemate spectrometer (model 1877)coupled to a Princeton Applied Research OMA III optical multichanneled analyzer (model 1463) equippedwith an intensified photodiode array detector cooledto 243 K. The spectral resolution and reproducibilityare 2 cm 1 .2.2.2. Specific surface areaThe BET surface areas of the samples were determined by N2 adsorption at 77 K on a MicromeriticsAccusorb surface area analyzer.2.2.3. X-ray photoelectron spectroscopyThe near-surface composition of the bulk metalvanadates was investigated using XPS. The analysiswas performed in a model DS800XPS surface analysis system (Kratos Analytical Plc, Manchester, UK)that operates with an X-ray beam of either Mg K orAl K X-rays and a base pressure of 5 10 19 Torr.The details of the technique have been publishedpreviously [5].2.2.4. X-ray diffractionX-ray diffraction (XRD) spectra were measuredwith a Philips PW 1390 that operates with Cu K radiation and a Ni filter. The details of the techniquehave been published previously [14].2.3. Determination of the number of surface activesites (Ns ) through methanol chemisorptionThe number of surface active sites was quantified bymethanol chemisorption by exposure to a mixture of2000 ppm of methanol vapor in He at 100 C that generated a stable monolayer of surface methoxy species.The amount of adsorbed surface methoxy species wasdetermined gravimetrically in the present investigation. A detailed flow diagram of the equipment andthe chemisorption technique has been previously published [1].2.4. Methanol oxidationMethanol oxidation steady-state kinetics were obtained in a fix-bed catalytic reactor under differential259reaction conditions (methanol conversion 10%) aswell as high methanol conversion. The followingoperating parameters were used in order to maintainmethanol conversion below 10% for methanol reaction over metal vanadates: sample weight, 10 mg; reaction temperature, 300 C; flow rate, 100 cm3 (NTP)min 1 and feed gas composition methanol/oxygen/helium, 6/13/81 mol%.The experiments at high methanol conversion wereperformed under the following operating conditions:sample weight, 30–200 mg; reaction temperature,300 C; flow rate, 100 cm3 (NTP) min 1 and feed gascomposition methanol/oxygen/helium, 6/13/81 mol%.The catalysts were tested for 24 h. at high methanolconversion in order to determine their stability underreaction conditions.Methanol conversion and the amount of productswere quantified with an on-line gas chromatograph(HP 5840) equipped with TCD and FID detectorsand two columns: capillary column (CP-sil 5CB)for methylal, dimethyl ether, methyl formate andmethanol analysis and a packed column (Carboxene1000) for CO, CO2 , O2 , formaldehyde and methanolanalysis.3. Results and discussion3.1. Bulk phase analysis of metal vanadatesBulk metal vanadates were synthesized through anorganic route that was applied successfully to obtaina series of bulk metal molybdates in previous publication [5]. The literature also shows that bulk vanadatesof rare earth elements and magnesium vanadates aresuitable to be obtained with that method [8,10]. Thepresent work extends the application of the organicroute synthesis to the preparation of manganese,cobalt, niobium and aluminum ortho-vanadates, andsilver meta-vanadate (the ortho-vanadate phase is unstable and decomposes at 400 C). The Raman analyses of the samples are presented in Fig. 1. The spectraof Mg3 (VO4 )2 , Ni3 (VO4 )2 , Cu3 (VO4 )2 , Zn3 (VO4 )2 ,CrVO4 , Mn3 (VO4 )2 and FeVO4 are in agreementwith those reported in the literature for pure bulkmetal ortho-vanadate compounds [11]. The structures of NbVO5 , Co3 (VO4 )2 , AlVO4 and -AgVO3were further analyzed with XRD diffraction since
260L.E. Briand et al. / Catalysis Today 78 (2003) 257–268Fig. 1. Raman spectra of bulk metal vanadates.
L.E. Briand et al. / Catalysis Today 78 (2003) 257–268Fig. 1. (Continued ).261
262L.E. Briand et al. / Catalysis Today 78 (2003) 257–268Fig. 1. (Continued ).
L.E. Briand et al. / Catalysis Today 78 (2003) 257–268263Fig. 2. XRD spectra of NbVO5 , Co3 (VO4 )2 , AlVO4 and -AgVO3 .no Raman spectra are available in the literature forthese vanadates.1 The XRD analysis showed that silver meta-vanadate also contains the Ag2 V4 O11 phase(2θ 23.6 , 29.0 , 30.5 , 32.2 ) and NbVO5 containsNb18 V4 O55 (shoulder at 2θ 22 ) (see Fig. 2). Nosignals belonging to crystalline V2 O5 (Raman bandsat 994, 702, 527, 404, 284 and 146 cm 1 and XRDsignals at 2θ 14.9 , 18.0 , 21.3 , 23.5 , 28.1 )or other metal oxide microcrystalline phases weredetected which ensures the purity of the catalysts [5].3.2. XPS surface analysis before and aftermethanol oxidationFig. 3 shows the surface atomic vanadium/metal ratio of bulk metal vanadates before and after the reaction with methanol. The figure compares the surfaceatomic ratio obtained through XPS analysis and thetheoretical (bulk) ratio. Nickel, copper, zinc, silver,niobium and iron vanadate catalysts possess a higher1 X-ray diffraction spectra coincide with the following JCPDSInternational Centre for Diffraction Data Files, 1997–1999:46-0046 for NbVO5 , 37-352 for Co3 (VO4 )2 , 39-276 for AlVO4and 29-1154, 20-1385 for -AgVO3 and Ag2 V4 O11 , respectively.surface atomic vanadium/metal ratio than the value expected according to the bulk phase stoichiometry. Thisobservation indicates a surface enrichment of vanadium species on bulk metal vanadates that cannot beattributed to the presence of an excess of bulk V2 O5according to the bulk phase analyses of the catalysts.The XPS analyses of magnesium, cobalt, manganese,aluminum and chromium vanadates show the oppositeresult. The surface atomic vanadium/metal ratio arelower than the theoretical ratio suggesting a surfaceenrichment of the metal cation (Mg, Co, Mn, Al, Cr).The differences in the nature of the surface activesites of bulk metal vanadates would also lead to adifferent product distribution in the methanol reaction.The next section compares the selectivity of bulk metalvanadate catalysts, V2 O5 and bulk metal oxides inorder to obtain more information on the true surfacecomposition of bulk metal vanadates.3.3. Nature of the surface active sites of bulkmetal vanadates from CH3 OH oxidationselectivity at low conversionsComparison of the product distribution formethanol oxidation over bulk metal vanadates and
264L.E. Briand et al. / Catalysis Today 78 (2003) 257–268Fig. 3. Surface vanadium/metal atomic ratio of bulk metal vanadates: (䊏) before and (the corresponding pure metal oxide provides morefundamental information about the nature of the surface active sites. Methanol is a probe molecule todetermine the nature of surface active sites since theproduct distribution of methanol reaction is sensitiveto the redox and/or acid–base properties of the surfaceactive species. Redox sites catalyze the selective oxidation of methanol to formaldehyde, acid sites yielddimethyl ether and basic species produce CO2 [15].Tables 1 and 2 show the selectivity of bulk metalvanadates and pure metal oxides at low methanol conversions, respectively. Bulk metal vanadates possess ahigh selectivity to formaldehyde with some selectivity to dimethoxy methane (nickel vanadate), dimethylether (niobium, chromium and aluminum vanadates),methyl formate (magnesium, chromium and coppervanadates) and CO2 (niobium and silver vanadates).TOFs and selectivity results of methanol oxidation over pure metal oxide catalysts were determinedin previous studies [5,6]. The data were obtainedat different temperatures (typically 300 C) in orderto maintain low methanol conversions. The surface) after reaction with methanol.redox/acid sites of bulk V2 O5 lead to a high selectivity to formaldehyde, along with dimethoxy methaneand dimethyl ether. The redox/basic character of thesurface active sites of MgO, NiO, MnO, Cr2 O3 , CoOTable 1Specific surface area and selectivity of bulk metal vanadates towardmethanol oxidation at low conversionsCatalystMg3 (VO4 )2NbVO5CrVO4Mn3 (VO4 )2AlVO4AgVO3Ni3 (VO4 )2Co3 (VO4 )2Cu3 (VO4 )2FeVO4Zn3 (VO4 )2SBET(m2 y ��–5.0–––8.5–––7.4–––––a FA: formaldehyde; DMM: dimethoxy methane; DME:dimethyl ether; MF: methyl formate.
L.E. Briand et al. / Catalysis Today 78 (2003) 257–268Table 2TOF and selectivity of pure metal oxide catalysts toward methanoloxidation at low conversionsCatalyst (reactiontemperature, C)TOFa(s 1 )Selectivityb (%)FACO2DMEOtherscMgO (300)Nb2 O5 (300)Cr2 O3 (290)MnO (300)Al2 O3 (300)Ag2 O (300)NiO (300)CoO (270)CuO (330)Fe2 O3 (300)ZnO (380)V2 O5 �4.7–5.735.710.4a Based on selective oxidation products (formaldehyde, methylformate and dimethoxy methane).b TOF and selectivity data from Refs. [5,6].c Other products are methyl formate and dimethoxy methane.and ZnO yield formaldehyde and CO2 . The surfaceacidic sites of Al2 O3 , Nb2 O5 and Fe2 O3 catalyzemethanol dehydration to dimethyl ether. Formaldehyde is the only product of the methanol oxidationreaction over CuO and Ag2 O.The XPS analyses discussed in the previous sectionsuggested a surface enrichment of the metal cationon Mg3 (VO4 )2 , Co3 (VO4 )2 , Mn3 (VO4 )2 , AlVO4 andCrVO4 . The presence of surface magnesium, cobalt,manganese and chromium sites would catalyze thetotal oxidation of methanol to CO2 and an aluminumsite would yield dimethyl ether according to the results obtained on the methanol oxidation over puremetal oxides. However, no CO2 was observed onMg3 (VO4 )2 , Co3 (VO4 )2 , Mn3 (VO4 )2 and CrVO4 ,and only 2% selectivity towards dimethyl ether is produced by AlVO4 (see Table 1). The observation thatbulk metal vanadates posses a high selectivity towardsformaldehyde strongly suggests that the surface ofbulk metal vanadates is composed of vanadium oxidesites with redox properties that cover the metal oxidesites and, thus, inhibit methanol total oxidation.It is well known that V2 O5 is unstable under themethanol oxidation environment due to the formationof a V-alkoxy complex at temperatures as low as150 C. The complex removes V2 O5 from the catalytic bed and produces a deposit at the exit, the cooler265side, of the reactor. The current observation that thebulk metal vanadates being investigated for methanoloxidation were stable for 24 h at high methanol conversion further supports the Raman analyses that thebulk metal vanadates do not possess bulk microcrystalline V2 O5 on their surfaces. Moreover, the selectivity results further suggest that surface vanadium oxidespecies are anchored to metal oxide atoms (MgO,NiO, MnO, Cr2 O3 , CoO, ZnO, Al2 O3 , Nb2 O5 andFe2 O3 ) in a similar way to vanadium oxide supportedcatalysts.3.4. Specific activity, TOF, for methanol selectiveoxidation over bulk metal vanadates, bulkmetal molybdates and supported vanadiumoxide catalystsThe number of surface active sites, reaction ratesand turnover frequencies of the bulk metal vanadatesduring methanol oxidation are presented in Table 3.No correlation was observed between the density ofactive surface sites of the bulk metal vanadates andtheir reaction rates during methanol oxidation. Thespecific activity (TOF) of the bulk metal vanadateswas calculated as the reaction rate towards redoxproducts (formaldehyde, dimethoxy methane andmethyl formate) per surface active site per second.The TOF values of the bulk metal vanadates are similar indicating that there is no significant influence ofTable 3Number of surface active sites, reaction rate and turnover frequencies of bulk metal vanadates at low methanol conversionBulk metalvanadatesNs ( mol/m2 )Reaction ratea at300 C ( mol/m2 s)TOFb(s 1 )Mg3 (VO4 )2NbVO5CrVO4Mn3 (VO4 )2AlVO4AgVO3Ni3 (VO4 )2Co3 (VO4 )2Cu3 (VO4 )2FeVO4Zn3 (VO4 .00.2Activity based on overall methanol conversion at 3
are 2cm 1. 2.2.2. Specific surface area The BET surface areas of the samples were deter-mined by N2 adsorption at 77K on a Micromeritics Accusorb surface area analyzer. 2.2.3. X-ray photoelectron spectroscopy The near-surface composition of the bulk metal vanadates was investigated using XPS. The analysis was performed in a model DS800XPS .
May 02, 2018 · D. Program Evaluation ͟The organization has provided a description of the framework for how each program will be evaluated. The framework should include all the elements below: ͟The evaluation methods are cost-effective for the organization ͟Quantitative and qualitative data is being collected (at Basics tier, data collection must have begun)
Silat is a combative art of self-defense and survival rooted from Matay archipelago. It was traced at thé early of Langkasuka Kingdom (2nd century CE) till thé reign of Melaka (Malaysia) Sultanate era (13th century). Silat has now evolved to become part of social culture and tradition with thé appearance of a fine physical and spiritual .
On an exceptional basis, Member States may request UNESCO to provide thé candidates with access to thé platform so they can complète thé form by themselves. Thèse requests must be addressed to esd rize unesco. or by 15 A ril 2021 UNESCO will provide thé nomineewith accessto thé platform via their émail address.
̶The leading indicator of employee engagement is based on the quality of the relationship between employee and supervisor Empower your managers! ̶Help them understand the impact on the organization ̶Share important changes, plan options, tasks, and deadlines ̶Provide key messages and talking points ̶Prepare them to answer employee questions
Dr. Sunita Bharatwal** Dr. Pawan Garga*** Abstract Customer satisfaction is derived from thè functionalities and values, a product or Service can provide. The current study aims to segregate thè dimensions of ordine Service quality and gather insights on its impact on web shopping. The trends of purchases have
Chính Văn.- Còn đức Thế tôn thì tuệ giác cực kỳ trong sạch 8: hiện hành bất nhị 9, đạt đến vô tướng 10, đứng vào chỗ đứng của các đức Thế tôn 11, thể hiện tính bình đẳng của các Ngài, đến chỗ không còn chướng ngại 12, giáo pháp không thể khuynh đảo, tâm thức không bị cản trở, cái được
Le genou de Lucy. Odile Jacob. 1999. Coppens Y. Pré-textes. L’homme préhistorique en morceaux. Eds Odile Jacob. 2011. Costentin J., Delaveau P. Café, thé, chocolat, les bons effets sur le cerveau et pour le corps. Editions Odile Jacob. 2010. Crawford M., Marsh D. The driving force : food in human evolution and the future.
Le genou de Lucy. Odile Jacob. 1999. Coppens Y. Pré-textes. L’homme préhistorique en morceaux. Eds Odile Jacob. 2011. Costentin J., Delaveau P. Café, thé, chocolat, les bons effets sur le cerveau et pour le corps. Editions Odile Jacob. 2010. 3 Crawford M., Marsh D. The driving force : food in human evolution and the future.
