Wavenumber (cm-1)81624324048566472808896%Transmittance 4576691636 28532926 3435 3500 3000 2500 2000 1500 1000 500 Wavenumber… [600215]

4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)81624324048566472808896%Transmittance
4576691636
28532926
3435
3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)81624324048566472808896%Transmittance 409438138514581636
3435
3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)081624324048566472808896104%Transmittance
488648754827920
1360
1385163919171952 2426
29392989
3398
RESULTS AND DISCUSSION
Inorder toconfirm thecomposition and thestructural formula ofthestudied coordination compound, aswell astoestablish thesteps ofits
thermal conversion andthedecomposition products formed during heating, thethermoanalytical methods have been used .TheTG,DTG andDTA curves,
corresponding totheaerobic thermal decomposition ofnickel(II) polyoxalate hydrate, areshown inFig.1,while the3DFTIR spectrum ofthegases evolved
during thesame process isshown inFig.2.
The 25
th
Symposium on Thermal Analysis and Calorimetry
–
Eugen Segal, Bucharest, Romania, April 15
th
, 2016
Thermal and spectroscopic analysis of nickel(II) polyoxalate
obtained through the reaction of ethylene glycol with Ni(NO
3
)
2
·6H
2
O
Mircea NICULESCU
1,2
*, Mihai
–
Cosmin PASCARIU
1,3
–
5
1“Chemeia Semper” Association, 6 Giuseppe Verdi, RO -300493, Timișoara
2University Politehnica Timișoara, Faculty of Industrial Chemistry and Environmental Engineering, 6 Vasile Pârvan Blvd., RO -3002 23, Timișoara
3“Vasile Goldiș” Western University of Arad, Faculty of Pharmacy, 86 Liviu Rebreanu, RO -310045, Arad
4“Victor Babeș” University of Medicine and Pharmacy of Timișoara, Faculty of Medicine, 2 Eftimie Murgu Sq., RO -300041, Timișoara
5National Institute of Research & Development for Electrochemistry and Condensed Matter –INCEMC Timișoara, 144 Dr. Aurel Păunes cu Podeanu, RO -300569, Timișoara
* Corresponding author: [anonimizat]
This paper analyzes thethermal decomposition ofnickel(II) polyoxalate hydrate, ahomopolynuclear coordination compound having theformula [NiL(H2O)2]n·0.1nH2O,where Listheoxalate dianion (C2O42-).
Thethermolysis was conducted indynamic oxidative atmosphere bysimultaneously applying theTG,DTG andDTA analytical techniques .Theproposed decomposition mechanism was confirmed bytheFourier
transform infrared spectroscopy (FTIR) analysis oftheevolved gases .Solid -state decomposition products formed during heating were investigated bychemical analysis, FTIR, Raman spectroscopy andX-ray
diffraction (XRD) .The structure, morphology and properties ofthefinal decomposition product were characterized byXRD, FTIR, energy dispersive X-rayspectroscopy (EDX) and transmission electron
microscopy (TEM) .Theanalysis ofX-raydiffraction data shows thatthefinal decomposition product isrelatively wellcrystallized .
Keywords : ethylene glycol; nickel(II) nitrate; nickel(II) polyoxalate; homopolynuclear coordination compound; thermal analysis; FTIR
CONCLUSIONS
Theaerobic thermal conversion ofnickel(II) polyoxalate hydrate gave nickel(II )oxide asthesolid-state product ,asshown byFTIR andRAMAN spectra .The
XRD results confirmed that thesynthesized NiO possesses rhombohedral phase crystallinity .The TEM images show that theNiO nanoparticles were
formed through aggregation .The particles exhibit irregular shapes and their size iswidely distributed between 5nmand 2μm.The results obtained
regarding theelectrocatalytic properties ofthenickel oxide synthetized through themethod described inthepresent paper willbedescribed inafuture work .
INTRODUCTION
Oxidic systems, generated byvarious methods, arenowadays required inincreasing quantities because ofthedevelopment ofmodern
technologies inmultiple fields :catalysis andelectrocatalysis, ceramic pigments, electronics, physical supports and carriers indevices intended fordata
processing, andalso pharmaceutical industry .One ofthesynthetic pathways used fortheir preparation, which hasrecently known arapid development, is
represented bythethermal conversion ofhomo -andheteropolynuclear metal complexes, which contain anions ofcarboxylic acids asligands .
Inourprevious papers wehave presented theproducts oftheredox reactions between several diols, such asethylene glycol, 1,2-propanediol
and1,3-propanediol, andcertain metal nitrates .Allthecoordination compounds obtained through thismethod contain carboxylate orhydroxycarboxilate
anions asligands, i.e.glyoxylate, oxalate, lactate or3-hydroxypropionate .These complexes, which contain relatively simple organic ligands, have onemain
advantage over other coordination compounds :they undergo thermal induced degradation tometals, alloys and oxidic structures atconsiderably low
temperatures, withtherelease ofgaseous species such ascarbon oxides (CO, CO2),hydrocarbons (e.g.,CH4)andwater .Thecomposition oftheobtained
powders depends onboth thestructure ofthecoordination compound andthethermal treatment thatwasapplied .
Theaimofthispaper istoclarify themechanism involved inthethermal decomposition ofnickel(II) polyoxalate obtained through thereaction of
ethylene glycol withNi(NO3)2∙6H2O.This study also shows that, following thethermal decomposition atrelatively lowtemperatures ofthiscompound, nickel
oxide isobtained inoxidative atmosphere .The properties and theapplicability ofNiO invarious domains were already presented inseveral studies
published bydifferent authors .
This paper ispart ofaseries ofstudies concerned with thedevelopment ofnew methods forobtaining coordination compounds through the
oxidation ofdiols with metal nitrates .Alarge variety ofcoordination compounds was prepared byusing thisoriginal synthesis .Themain objective ofsuch
research istohighlight theimportance oftheprecursor’s nature inthesynthesis ofsimple andmixed metal oxides withvarious properties andapplications .
MATERIALS AND METHODS
Thenickel(II) polyoxalate hydrate was prepared starting from nickel(II) nitrate andethylene glycol byusing anoriginal method, asdescribed ina
previous paper .Before being used, thecompound waspurified byrefluxing inanacetone/water (5:1v/v)mixture inanultrasonic bath.
Forthethermal analysis, aNetzsch STA 409PCcoupled with aBruker 27FTIR instrument were used, with thefollowing measuring conditions :
10°Cmin-1heating speed, 100mLmin-1synthetic airflowand20.58mgsample weight .
Thephase composition ofthepowders obtained through thecomplex’s thermolysis at400and1000 °Cwas investigated byX-raydiffractometry
(XRD) using aRigaku Ultima IVdiffractometer with CuKαradiation (λ=1.5406 Å).Thelattice parameters (a,b,c)andtheaverage crystallite size (d)were
calculated byusing thewhole pattern profile fitting method (WPPF) .The instrument influence over thelines’ broadening was subtracted byusing the
diffraction pattern ofaSistandard recorded inthesame conditions .
FortheTEM analyses, thematerial wasdeposited from ethanol ona200mesh TEM copper gridcovered withlacey carbon film.Weused aTitan
G280-200TEM/STEM (FEI Company, Netherlands) instrument withimage correction .Theimages were registered at200kVaccelerating voltage .ADigital
Micrograph v.2.12.1579 .0software was used forimages recorded inTEM mode, while aTEM Imaging &Analysis v.4.7software was used forrecording
theEDX spectrum .TheSTEM -EDX elemental distribution maps were recorded withtheEsprit v.1.9software .
The FTIR spectra were acquired onaVertex 70(Bruker, Germany) FT-IRspectrometer inthe400-4000 cm-1domain, using KBrpellets .The
Raman spectra were measured atroom temperature using aMultiView -1000 system (Nanonics Imaging, Israel) which incorporated theShamrock 500i
Spectrograph (Andor, UK);alaser wavelength of514.5nmwasused astheexcitation source, witha20sexposure time anda300l/mm grating .
LERF
–
Laboratorul de Energii
Regenerabile
–
Fotovoltaic
Timișoara
ACKNOWLEDGEMENT
Part ofthisresearch was done attheCenter ofGenomic Medicine from “Victor Babeș” University ofMedicine and Pharmacy ofTimișoara, POSCCE
185/48749 ,contract 677/09.04.2015 .
The thermal decomposition reactions were monitored bychemical analysis, FTIR, XRD, energy dispersive X-rayspectroscopy (EDX) and
transmission electron microscopy (TEM) .
Fig.3comparatively shows theFTIR andRaman spectra ofthecoordination compound anditsdecomposition products .
Fig.3.FTIR andRaman spectra of(from lefttoright) thecoordination compound anditsdecomposition products at400°Cand1000 °C;theFTIR band at
1636 cm-1isowned totraces ofabsorbed waterThecorresponding assignments oftheFTIR andRAMAN spectra ofnickel(II) polyoxalate hydrate aregiven inTable 1.
Table 1.Assignment oftheFTIR andRaman spectra ofnickel(II) polyoxalate
hydrate (band positions incm-1)Theintense band inFTIR at1639 cm-1isattributed to
asymmetrical vibration ofthecarboxylate ionand thevalue
shows that the resonance inthe carboxylate groups is
maintained during the complex’s formation, the metal –
carboxylate bond being mostly ionic.Theabsence ofthebands
inthe 1720 -1660 cm-1range, attributed tothe νasy(C=O)
vibration inthecase ofcoordination compounds inwhich C2O42-
isabidentate ligand, confirms theresonance inthecarboxylate
groups and shows that thefour oxygen atoms areequivalent,
theoxalate dianion being abridging ligand .
The FTIR spectra ofthe thermal decomposition
products show twoabsorption bands :at438 and 660 cm-1for
the400°Cthermal decomposition product, andat457and669
cm-1forthe1000 °Cthermal decomposition product, slightly
shifted withrespect tothecharacteristic bands ofnickel(II) oxide
mentioned intheliterature .Thebroadness ofthe457cm-1band
indicates thattheNiOpowder consists ofnanocrystals .
The Raman spectra ofthethermal decomposition
products, clearly distinct from that ofthe starting complex
compound, areinaccordance withliterature data.
The XRD patterns ofthepowders obtained through thecoordination compound’s annealing at400and1000 °Carepresented inFig.4.Both
patterns record thediffraction lines oftheNiOsingle phase (rhombohedral, ICDD file01-078-4374 ).Table 2presents thecharacteristics derived from the
XRD analysis forthese powders .Bycomparison, thepowder annealed at400°Ciscomposed ofmuch smaller crystallites (4.4nm)then theoneannealed
at1000 °C(37.4nm).Forboth ofthese powders, thevalues ofthelattice parameters areclose tothevalues found intheICDD file(a=b=2.9633 Å,c=
7.2553 Å).
Fig.4.XRD patterns ofthepowders obtained bycoordination
compound’s annealing at400and1000 °C
Table 2.Characteristics derived from theXRD analysis forthe
powders annealed at400and1000 °CThese results allow some conclusions tobedrawn regarding the
thermoanalytical curves presented inFig.1.
The TGprofile shows, inthefirststep, theremoval ofthelattice water until
around 160°C,followed, inthesecond step, byaninclined slope upto286 °C,
indicating theremoval ofcoordinated water molecules .Acompletely dehydrated
compound isproduced atabout 286°C.Anendothermic DTA peak located between
160-286°C(maximum at260°C)andachange ontheDTG curve inthesame range
correspond totheremoval ofthecoordinated molecules ofwater .These results arein
good agreement with those from theliterature forNiC2O4∙2H2Oobtained through
classical methods .Thebreakdown oftheanhydrous compound, inthethird step, takes
place within avery short temperature range (286-345°C)asshown byasteep slope on
theTGcurve withtheinflection point at313°C.Thetotal mass lossof58.80%suggests
thattheproduct oftheconversion isNiO.Theformation ofthisproduct isaccompanied
byavery sharp exothermic DTA peak at322°C.
The analysis oftheobtained data suggests that theprogressive aerobic
heating ofthecoordination compound leads tothefollowing processes :
After analyzing theFTIR spectrum from Fig.2wecanconfirm thewater release, which presents twopeaks :aweak onearound 140°Candthe
main onearound 260°C.Thecarbon dioxide shows apeak around 260°Candavery intense peak around 320°C.Thus, therelease ofCO2,although of
weak intensity, begins somewhere instep II.
Thegood agreement between thecalculated andexperimental mass losses, asseen inTable 3,confirms theproposed conversion mechanism .
AnEDX quantitative elemental analysis ofvery small areas revealed that, onthesurface, thethermal decomposition product at1000 °Cisanon-
stoichiometric oxide .Thecomposition ofthenickel oxide analyzed byEDX shows aNi:O=1.2atomic ratio.This indicates that, onthesurface, theproduct of
thermal decomposition ofthecoordination compound isanoxygen -deficient non-stoichiometric nickel oxide, impurified with Ni.This product hasavaried
crystalline morphology, asshown byTEM .Theparticles exhibit irregular forms andtheir sizeiswidely distributed between 5nmand2μm.
Table 3.Sample mass loss during thethermal conversion inairofthe
polynuclear coordination compound [NiL(H2O)2]n·0.1nH2O(M=184.55)
Fig.5.TEM images forthe1000 °Cdecomposition product
Fig.6.EDX analysis anddistribution ofNi/O inthescanned surface forthe1000 °Cdecomposition product (CuandCarefrom thesample support grid)
Fig. 1. Thermoanalytical curves for the aerobic heat degradation of nickel(II) polyoxalate hydrate 
Fig. 2. 3D FTIR spectrum of gaseous products generated during nickel(II) polyoxalate hydrate’s aerobic thermolysis 