Novel Method for Synthesis of Nano-Materials: Combustion of Active Impregnated Layers**

Transcription

1 DOI: 1.12/dem.2776 Novel Method for Synthesis of Nno-Mterils: Comustion of Active Impregnted Lyers** By Alexnder S. Muksyn*ndPeter Dink Nnomterils with chrcteristic structurl dimensions on the order of few nnometers (1 1 nm) ttrct wide ttention owing to their unique properties. They found vriety of pplictions in different rnches of science nd industry including ctlysis, micro nd iotechnologies, energy storge nd conversion devices, such s fuel cells. Different techniques hve een developed for nnomteril preprtions, solid-stte, wet-type nd CVD methods, mechno-, plsm- nd comustion synthesis. A comintion of comustion nd rective solution pproches leds to soclled solution (queous) comustion synthesis (SCS) method. [1 3] Typiclly SCS involves self-sustined rection in solutions of metl nitrtes nd different fuels, which cn e clssified sed on their chemicl structure, i.e. type of rective groups (e.g. mino, hydroxyl, croxyl) onded to the hydrocron chin. [4] The rection etween fuel nd oxygen contining species provides high temperture rpid interction. In selfsustined propgtion mode fter locl initition the comustion front stedily moves long the sol-gel medi forming extremely fine solid products with tilored composition. [3,5] Severl distinctive fetures of SCS contriute to the unique properties of the synthesized products. First, the initil rection medi eing liquid solution llows mixing the rectnts on the moleculr level thus permitting precise nd uniform formultion of the desired composition on nno scle. Second, the high comustion tempertures ensure the formtion of the desired phse composition directly in SCS wve. This feture llows one to skip dditionl step, i.e. high-temperture product clcintion, which typiclly follows the conventionl sol-gel pproch. Third, short process durtion (seconds) nd formtion lrge mount of gses during SCS, inhiit solid prticle size growth nd fvor synthesis of nno-size powders with high specific surfce re. Forth, lmost no externl energy is required to produce mterils, since SCS occurs owing solely to the self-sustined highly exothermic rections. Finlly, simple equipments re typiclly used for this process. A wide vriety of nno-mterils ws synthesized y the SCS pproch, including unique pigments nd ctlysts, mterils for SOFC nd lithium tteries, oxygen sensors nd storges, novel dielectric nd piezoelectric compositions. [1,2] Recently we reported method for synthesis of high surfce re supported ctlysts y rective solution impregntion to the inert porous solid support, followed y initition of the comustion rection in such complex heterogeneous medi. [6] An unusul effect ws found. By using this, so-clled Impregnted Support Comustion (ISC) method, mterils with very high specific surfce re (> 2 m 2 /g) were synthesized. However, not every mteril cn e otined y ove techniques. Mny low exothermic systems, which llow synthesize of vlule compositions, do not provide self-sustined rection mode. To overcome this ostcle novel, so-clled, Impregnted Active Lyer Comustion (IALC) method ws developed nd descried elow. IALC llows not only one step high yield production of powders with high surfce re ut lso suggests n pproch for continuous synthesis of such mterils. Impregnted Active Lyer Comustion [*] Plese check title nd ddresses of uthors! Dr. A. S. Muksyn, Dr. P. Dink Deprtment of Chemicl nd Biomoleculr Engineering Center for Moleculrly Engineered Mterils University of Notre Dme Notre Dme, IN 46556, USA E-mil: [**] This work ws supported y the VU.S. Army CECOM RDEC through Agreement AAB7-3-3-K414. Such support does not constitute endorsement y the U.S. Army of the views expressed in this puliction. This work ws lso prtilly supported y the Indin 21st Century Reserch nd Technology Fund. The concept of IALC hs some similrity with ISC pproch, [6] however in the former cse the rection solution is impregnted to the porous medi, which is lso rective nd thus cn ssist in propgtion of comustion rection. In order to control composition of the synthesized fine prticles it is convenient tht the rection products of ctive lyer do not contriute to the finl structure of nno-mteril. This effect cn e chieved if ll lyer relted products re in gs phse. Also, for synthesis of nno-prticles the comustion temperture should e reltively low nd process durtion s short s possile. All ove requirements suggest using thin porous lyer of cron-sed comustile support, e.g. cellulose pper. Indeed such medi hs excellent infiltrtion chrcteristics nd cn e esily impregnted with liquid rective solution. Also the experiments show tht residul sh fter urning of the pper in ir is less thn.2 wt.%. Finlly, urning of thin lyer, owing to lrge het loss, leds to so-clled ADVANCED ENGINEERING MATERIALS,, No. 27 WILEY-VCH Verlg GmH & Co. KGA, Weinheim 1

2 Muksyn, Dink/Novel Method for Synthesis of Nno-Mterils: Comustion of Active Impregnted Pper Product quenching effect, i.e. rpid temperture drop fter the rection front, which fvors the formtion of fine powders. The scheme for Impregnted Active Lyer Comustion is presented in Figure 1(). On the first stge the desired queous solution of fuels nd oxidizers is impregnted into the thin lyer of porous medi (e.g. cellulose pper, cron nnotues, etc). Next step ssumes drying of thus formed complex rection system to decrese the mount of wter. This component, which plys constructive role on the stge of solution preprtion, while hinders the process on the stge of rection propgtion. Ignition is n importnt prt of the method, ecuse two different comustion modes cn e ccomplished (Fig. 1() nd (c)), i.e. slow smoldering () nd rpid flme (c) comustion wves. Smoldering mode cn e initited y electriclly heted metl wire with reltively low temperture ( 5 K), while pplying higher tempertures results in flme comustion mode. Slowly propgting (.1 1 mm/s) smoldering mode leds to higher product yield nd lrger surfce re of the synthesized powders. During IALC the finl product forms s gglomertes of nno-prticles. It is importnt tht chemicl nlysis shows smll (.1.15 wt. %) free cron content in vriety of thus synthesized products. IALC of SrRuO 3 Perovskite Rection Front Pper Flme Product Fig. 1. Stges of Impregnted Active Lyer Comustion () nd different IALC modes: smoldering (); flme (c). c Let us consider IALC pproch in more detils on the exmple of synthesis of SrRuO 3 perovskite, which is vlule ctlyst for direct lcohol fuel cells. [7] This system, ecuse of high melting point of strontium nitrite, Sr(NO 3 ) 2 nd low exothermicity of ruthenium nitrite reduction, does not llow self-propgting modes descried ove. In ddition, the conventionl, so-clled volume solution comustion mode, ecuse of its therml explosion nture, provides only 3 % of product yield. In this work, chromium nitrte, Sr(NO 3 ) 3, ruthenium nitrosyl nitrte, Ru(NO)(NO 3 ) 3, nd glycine (C 2 H 5 NO 2 ) (ll from Alf Aesr) were used to synthesize different perovskitesed ctlysts. In generl, under equilirium conditions, the comustion rection in such systems cn e represented s follows: Me m (NO 3 ) m +( 5 9 m f)ch 2NH 2 CO 2 H+m 5 4 (f 1) O 2 MeO (s) m/2 +( 1 9 m f)co 2 (g) f H 2O (g) + m ( 5 9 f + 1)/2 N 2 (g) where Me m is metl with vlence m, f is fuel to oxidizer rtio. f = 1 mens tht the initil mixture does not require tmospheric oxygen for complete oxidtion of the fuel, while f > 1 (< 1) implies fuel-rich (len) conditions. Specificlly for synthesis of the SrRuO 3 perovskite the following stoichiometric (f=1) rection is considered: Sr(NO 3 ) 3 +Ru(NO)(NO 3 ) 3 +32/5 C 2 H 5 NO 2 SrRuO 3 +32/5 CO 2 " g +16H 2 O" g +67/1 N 2 " g (2) Decomposition rtes nd tempertures for ll precursors re shown in Figure 2(). It cn e seen tht glycine (CH 2 NH 2 CO 2 H), rpidly decomposes in the rnge of K. Clcultion shows tht the weight decrese of 46 % (T = 566 K) corresponds to the removl of mmoni nd wter. Note tht this rection is endothermic. Initil (T = 468 K) decrese of weight ( 9 %) during decomposition of RuNO(NO 3 ) 3 occurs owing to relese of NO. Next reltively shrp drop occurs in the temperture rnge K leding to formtion of RuO 2. Endothermic decomposition of Sr(NO 3 ) 2 tkes plce t high temperture T > 87 K nd results in formtion of SrO t T= 12 K. Cellulose (C 6 H 1 O 5 ) n is long-chin polymeric polyscchride of et-glucose. It ws shown tht the cellulose pper (SCOTT Hrd Roll Towel) used in this study s n ctive support medi lmost completely urn in ir, i.e. weight of residul sh (T = 647 K) is less thn.2 wt % (see Fig. 2()). Note tht rpid highly exothermic oxidtion rection strts t reltively low temperture 556 K. Results of DT/TG nlysis for Sr(NO 3 ) 3 +Ru(NO) (NO 3 ) 3 +32/5 C 2 H 5 NO 2 rection solution (Fig. 3()) indictes two exothermic peks. First, reltively wek pek coincides with decomposition temperture of the glycine ( 525 K) nd is result of its interction with ruthenium (III) nitrosylnitrte. The second lrge exothermic pek strts t T 65 K is relted to the reduction of solid Sr(NO 3 ) 2 y cron-contining phse, which formed during the glycine decomposition. Thus, ecuse of the high decomposition temperture for strontium nitrite, to initite rection one hs to prehet system to reltively high temperture ( 65 K). But even under such conditions rection occurs not in the homogeneous (1) WILEY-VCH Verlg GmH & Co. KGA, Weinheim ADVANCED ENGINEERING MATERIALS,, No.

3 Muksyn, Dink/Novel Method for Synthesis of Nno-Mterils: Comustion of Active Impregnted Weight (%) Weight (%) K; 91% 566K; 54% 523K 87K 65K; 54.98% 12K; 48.8% 2 Ru(NO)(NO 3 ) 3 Sr(NO 3 ) 2 915K; 4.8% Glycine: C 2 H 5 NO % 376K Temperture, K 556K.18wt.% 647K Temperture [K] TG DTA Fig. 2. DTA for: () rective precursors; () cellulose (2 C/min, oxygen tmosphere. 5 ml/min) T Weight % Weight % % K T ig 523K 566K 65K Temperture [ C] complete urning TG DTA (TG) (DTA) 9.9 % Temperture, K Fig. 3. DTA-TG of: (): Sr(NO 3 ) 3 +Ru(NO)(NO 3 ) 3 +32/5 C 2 H 5 NO 2 solution () cellulose impregnted y Sr(NO 3 ) 3 +Ru(NO)(NO 3 ) 3 +32/5 C 2 H 5 NO 2 solution (2 C/min, oxygen tmosphere. 5 ml/min) T T solution, ut in the heterogeneous mixture of ruthenium oxide- cron-sed phse nd Sr(NO 3 ) 2. The ltter mke it difficult direct synthesis of desired perovskite structure nd n dditionl clcintion step is required. Also fter preheting to 65 K the system explodes, giving only 3 % of theoreticl product yield. The TGA-DTA dte of the cellulose impregnted y glycine- RuNO(NO 3 ) 3 Sr(NO 3 ) 2, solution is shown in Figure 3(). It cn e seen tht in this cse only one exothermic pek presents nd rections strts t T 525 K, which correspond to ignition temperture of such complex medi in ir. Note tht this temperture is elow decompositions tempertures of ll precursors (see Fig. 2()) nd s result the rection leds to one step formtion of the desired perovskite. It is more importnt tht rection in such medi cn e esily initited t room temperture without ny preheting. For this, rective solution ws prepred y mixing ove mentioned nitrites nd glycine in stoichiometric rtio (f = 1), nd ws impregnted into the ulk of thin (.3 mm) lyer of cellulose pper with pper/solution rtio 1:1. By using low temperture (T ig = 5 K) ignition device, smoldering mode ws ccomplished nd rection propgted stedily with velocity 5 mm/s long the impregnted medi. A typicl temperture time profile for such comustion front propgtion is shown in Figure 4() (Profile 1). Two distinctive temperture peks, corresponding to two rection fronts, cn e resolved. XRD mesurements of the quenched products revel tht the first stge is primrily cron oxidtion y tmospheric oxygen, while on the second stge the desired perovskite formed (see Fig. 4()) owing to metl nitrites glycine interction. It is interesting tht pure cellulose cnnot urn in smoldering mode: initited y low temperture source this rection quickly extinguishes owing to lrge het loss. Also the comustion temperture for pure pper (Profile 2; Fig. 4) is much higher thn the first pek for smoldering mode of complex solution-cellulose system. This effect cn e explined tking into ccount the suppression of cron oxidtion rection y evportion of wter constrined in metl nitrites. Finlly, note tht for highly exothermic solutions (e.g. iron nitrite/ glycine) the sequence of rections in the smoldering wve cn e reverse, i.e. first the solid oxide is formed, followed y cron urning (Profile 3; Fig. 4). The initil microstructure of this conventionl pper towel is shown in Figure 5(, ). It cn e seen (Fig. 6()) tht it consists of long yrns with dimeters of 5 lm nd lrge pores ADVANCED ENGINEERING MATERIALS,, No. 27 WILEY-VCH Verlg GmH & Co. KGA, Weinheim 3

4 12 c d e f 1: Sr-Ru-O system on pper 2: pure pper 3: Fe-O system on pper cron oxidtion Temperture, K 1 8 first front: cron oxidtion 6 second front: cron oxidtion 2 first front: oxide formtion second front: oxide formtion Time, s 1 SrRuO3 prepred y IALC 8 6 CPS Muksyn, Dink/Novel Method for Synthesis of Nno-Mterils: Comustion of Active Impregnted g nm Θ Fig. 5. Typicl microstructures of the rective medi nd comustion products: (), () cellulose; (c) (d) impregnted lyer; (e), (f) s synthesized product (g) product fter milling (1 minute). Fig. 4. Typicl temperture profiles for IALC in different systems () nd XRD-ptterns for s-synthesized product in Sr-Ru-O3 system (). (1 lm). More close inspection shows tht yrns surfces (Fig. 6()) re well developed with chrcteristic scle of heterogeneity on the order of 1 lm. Microstructure of the cellulose pper impregnted y rection solution fter drying is shown in Figure 5(c,d). It is interesting tht formed sol-gel type medi does not occupy the lrge pores of the cellulose structure, ut uniformly distriuted long the surfce of the cellulose yrns, mking thin ( 1 lm) rective lyers (compre Fig. 6() nd 6(d)). Microstructure of s-synthesized product is shown in Figure 5(e f). It cn e seen tht on the scle of 1 lm the product structure exctly mimic those for cellulose (compre Fig. 6() nd 6(e)). However, the microgrphs with higher mgnifiction (Fig. 5(f)) indicte tht yrns re hollow, with thin wlls, which in turn re very porous. This porosity is result of comustion/ evportion of the cellulose, leding to the formtion of medi with high surfce re ( 16 m2/g) for this type of perovskites Also ecuse the produced gglomertes re very porous just one minute of conventionl solid milling (powder: ll 4 rtio = 3:1, rottion speed 15 min 1) leds to development of nno-prticles with the verge size on the order of 2 nm (see Fig. 5g). Tle 1. Some properties for vrious complex-oxides synthesized y IALC method. Composition Surfce re*, BET (m2/g) C content s synthesized [wt.%] Fe2O CRuO SrRuO LRuO L.6Ce.4Fe.8Ni.2O3-d L.6Ce.4Fe.68Ni.2K.12O3-d CuO/ZnO/ZrO ll presented vlues were mesured for s-synthesized products, i.e. without milling 27 WILEY-VCH Verlg GmH & Co. KGA, Weinheim ADVANCED ENGINEERING MATERIALS,, No.

5 Muksyn, Dink/Novel Method for Synthesis of Nno-Mterils: Comustion of Active Impregnted The IALC method ws used to produce vriety of mterils with high specific surfce re (see T. 1). For exmple, y this pproch severl compositions were synthesized nd tested s ctlysts in direct ethnol nd methnol fuel cells, [8] s well s for reforming of kerosene-type fuels to produce hydrogen. [9] It ws demonstrted tht these ctlysts possess high ctlytic ctivities nd currently they re under longterm testing in different leding industril compnies. Finlly, it is importnt to note tht IALC method llows continuous method for synthesis of nno-powders. In this scheme the ctive lyer lso serves s creer of the rective medi. [9] Conclusion A novel effective method, i.e. Impregnted Active Lyer Comustion, for synthesis of oxide-sed nno-mterils is descried. The unique future of considered pproch includes high surfce re nd crystllinity of the synthesized powders, exceptionlly high product yield nd opportunity for continuous technology. It is lso importnt tht method llows synthesis of complex metl-oxide nno-mterils for low exothermic systems, which cn not e otherwise produced y comustion pproch. Received: Mrch 9, 27 Finl version: My 7, 27 [1] K. Ptil, S. Arun, S. Ekmrm, Curr. Opin. Solid Stte Mter. Sci. 1997, 2, 158. [2] K. Ptil, S. Arun, T. Mimni, Curr. Opin. Sol. Stte Mter. Sci. 22, 6, 57. [3] A. S. Muksyn, P. Epstein, P. Dink, Proc. Comus. Inst. 27, 31, [4] A. Vrm, A. S. Muksyn, K. Deshpnde, P. Prnd, P. P. Erii, Mter. Res. Soc. Symp. Proc. 23, 8, 113. [5] K. Deshpnde, A. S. Muksyn, A. Vrm, Chem. Mter. 24, 16, [6] P. Dink, A. S. Muksyn, J. Phys. Chem. 25, 19, [7] A. Ln, A. S. Muksyn, ECS Trns. 27, 2,1. [8] P. Dink, A. S. Muksyn, J. Power Source 27, 167, 472. [9] A. S. Muksyn, P. Dink, PCT Appl. No. PCT/US26/ 3486, Filed: August 4, 26. Novel Method for Synthesis of Nno-Mterils: Comustion of Active Impregnted Lyers A. S. Muksyn*, P. Dink Solution Comustion of Active Impregnted Lyer is novel method for synthesis of nno-mterils. It comines fetures of impregntion nd comustion pproches nd llows production of nno-powders using low exothermic rection systems, which cnnot e ccomplished y conventionl solution comustion method. ADV. ENG. MATER. 27, ADVANCED ENGINEERING MATERIALS,, No. 27 WILEY-VCH Verlg GmH & Co. KGA, Weinheim 5