ft-ir emission spectra of chemisorbed species, with

CROATICA CHEMICA ACTA CCACAA 61 (4) 731-744 (1988)

CCA-1821YU ISSN 0011-1643

UDC 543.42Original Scientific Paper

FT-IR Emission Spectra of Chemisorbed Species,with Application to Species Adsorbed on Alumina

J. Mink

Institute of Isotopes of the Hungarian Academy of Sciences, H-1525 Budapest,P. O. B. 77, Hungary

and

G. KereszturyCentral Research Institute for Chemistry, H-1525 Budapest, P. O. B. 17, Hungary

Received February 22, 1988

The principles of infrared emission spectroscopy are brieflyreviewed with emphasis on the aspects of its application to thestudy of chemisorbed species. The ma in problems of sample pre-paration, selection of measurement conditions and the most suit-able methods of data treatment are discussed.

IR emittance spectra of two typical support materials forsupported metal catalysts, aJumina and silica, are presented. Onthe examp1e of a rhenium complex, tetrakisttricarbonyl-us-hy-droxo-rhenium), formed on alumina support in catalytic amounts(1.5 to 5% Re) it is shown that the four-measurement techniquecan .Iead to observation of the adsorbate bands also in the regionsof high substrate emission (between 1300 and 400 cm-I).

I. lNTRODUCTlON

Spectroscopic methods providing information on the structure of adsorbedmolecules, and thus on the nature of inter action between adsorbent andadsorbate, are of utmost importance for heterogeno us catalysis. As regardsvibrational spectroscopy, the relevant are the attenuated total reflection(ATR), IR reflection-absorption spectroscopy (IRRAS), electron energy lossspectroscopy (EELS) and surface enhanced Raman scattering (SERS), but theusual transmission IR technique can be applied as well if the substrate doesnot absorb very strongly in the spectral region of interest.

The birth of Fourier-transform infrared (FT-IR) spectroscopy with itsfast singal accumulation possibilities has opened up a new way towardsincreased sensitivity in IR spectrometry enabling spectroscopists to deal withspectra recorded under poor energy conditions, e. g. very weak emissionspectra or spectra in regi ons of low transmission.

In this paper we intend to review the principles of infrared emissionspectroscopy (IRES), give ashort overview of the experimental methods thathave been suggested for measuring IR emission spectra of molecular species

732 J. MINK AND G. KERESZTURY

chemisorbed on different types of surfaces, and also add same of our ownrecent experience.

II. BASIC PRINCIPLES OF IRES

The theoretical background of infrared emission spectroscopy has beendescribed in detail in the reviews by Bates' and Huong-.

Considering the inter action of thermal radiation with condensed phases(solids and liquids), one can start with the basic relationship according towhich the portions of radiation absorbed, reflected and transmitted by thesample add up to unity, i. e.

a+r+t=l. (1)

where a, l' and t are the absorptance, reflectance and transmittance, respe-ctivley. According to Kirchoff's law, the emittance, e, of an object at a giventemperature and wavenumber is equal to its absorptance:

e (JI, T) = a (JI, T) (2)

which means that a good absorber is a good emitter as well, and vice versa.Consequently, Eq. (1) can be rewritten in the form:

e+r+t=1. (3)

This relationship has to be taken into account when designing an emissionexperiment (choosing the substrate and cell materials) and also when inter-preting the spectra obtained, since spectral features due to emission, absorp-tion and reflection can appear in the spectra simultaneously. Thus, in emis-sion experiments, a specular mirror substrate is preferred due to its highreflectivity which means a very low absorptance and, hence, negligiblebackground emittance.

The blackbody, on the other hand, is an ideal thermal emitter. No objectcan emit more energy at any frequency than a blackbody at the sametemperature. The emittance of an object is defined as the radiant power,J, emitted by that object and is expressed as a fraction of the power emittedby a blackbody at the same temperature:

e (1', T) = J (1', T)/JBB (1', T). (4)

Thus, the emittance, of a blackbody is unity, and that of any real object(greybody) is less than unity.

The spectral distribution and temperature dependence of the radiantpower from a blackbody are described by Planck's law.' According to this,the emitted energy at a given frequency increases with increasing temper-ature, while the maximum in the spectral distribution shifts towards higherfrequencies. Stemming from this, high frequency vibrations (>2000 cm'")can be more favourably studied at temperatures exceeding 300 K.

When using commercial FT-IR instruments to measure IR emission spec-tra, the sample is usually mounted at the place of the radiation source orat an optically equivalent spot nearby (although another arrangement hasalso been proposed)". The detector responds to any deviation from the radi-ation balance between the sample and the detector. This means that aspectrum is detected if the temperature of the sample is either higher or

FT-IR EMISSION SPECTRA 733

lower than that of the detector.' The common practice is to heat the samplefrom the rear using an attached heater block (substrate).

Emission spectra can be presented as radiant power vs. wavenumber,i. e. single beam spectra on an arbitrary energy scale. However, for quanti-tative work or more precise intensity cornparisons double beam emittancespectra (intensities between O and 100010) can be generated by ratioing theemission spectrum of the sample against that of the blackbody at the sametemperature.š-"

nr. APPLICATION OF FT-IR TO EMISSION SPECTROSCOPY: PRACTICAL ASPECTS

The first but not quite successful attempts to apply IR emission spectro-scopy to the study of absorbed species using a grating spectrometer werereported by Eischens and Pliskin? in 1955-56. In the 1960's and early 1970'sIR emission measurements were published on molten salts8-11, fatty acids-",minerals-š-!-, greases smeared over metal surfaces-š-!", and polycrystalline andpowdered organic and inorganic compourids-?"!".

The Fourier-iransform technique was first applied to the measurementof infrared emission spectra by Gebbie et aL20 in 1961. It soon became clearthat the increased throughput of interferometric spectrometers and the easeand speed of spectrum accumulation made it possible to study very weakradiations like emission of stars and atmospheres of planets.21-23 Regardingthe study of condensed phase materials, FT-IR emission spectroscopy seemedto be very prornising, especially in case of thin surface layers such as che-misorbed monomolecular layers on strongly absorbing or opaque substrates.It was believed that sample preparation would be less critical than in trans-mission or ATR measurements, since even rough surfaces of any shape couldbe used to measure emission spectra. Systematic studies that followed reve-aled that, regarding sample preparation, the situation is far from being sofavourable, since several conditions have to be fulfilled to get good qualityemission spectra.

One of the most important factors determining the quality of emissionspectra is sample thickness. Griffithsš! has shown that characteristic emissionspectra can only be obtained fromoptically thin layers of a sample on aheated specular mirror substrate. lf the sample is thick, spectral contrastgets lost and the emissicn spectrum becomes more like that of a blackbody.When the sample is heated from the rear, the ccoler outer layers partiallyabsorb the radiation emitted from the hotter inner layers, especially in theregions of high absorbances. This can lead to apparent splitting of strongemission ban.ds or to inversion at band centers.

Concerning this reduced emission phenomenon, Kember and Sheppard"have put forward an alternative explanation involving the selective reflec-tivity of the sample in the vicinity of strong absorption bands. As a resultof a thorough analysis of the problem, Rytter and coworkers="" have eon-cluded that dispersion of the surface reflectivity dominates in these spectraldistortions. In order to avoid the problem, they suggest ratioing the samplespectrum against the emission spectrum of a sufficiently thick (opaque)sample as reference instead of that of a blackbody.š"

Since emission intensity is strongly dependent on the temperature, thesample temperature should be stable during the measurement. Even small

r

734 J. MITNK AND G. KERESZTURY

temperature variations {Jr a temperature mismatch between the sample andreference can lead to a sloping and curved background."

Because of their higher speed and sensitivity, liquid nitrogen cooled MCTdetectors are preferred in the majority of FT-IR applications. In case ofemission work, one must realize that room temperature parts of the spec-trometer or any object in the surroundi:ngs can contribute to the emissionbackground if the detector is cooled. An efficient way of background elimi-nation is to cool down the whole spectrometer to liquid nitrogen or liquidhelium temperature, as it has been done by Durana and coworkers=-" andTobin and coworkers.30-32 A much simpler and cheaper alternative is tomake a four-measurement experiment as suggested by Kember et aL27: boththe sample and the blackbody reference are measured at two temperatures(without any movement of the samples between the two measurements) andthe emittance of the sample is calculated as:

e = [Es (Tz) - Es (Tl)]/[EBB (Tz) - EBB (Tl)], (5)

where Es and EBB denote emission of the sample and the blackbody, respec-tively.

The use of room temperature TGS detectors, though they are 10 to 17times less sensitive than a cooled MCT detector, has the advantage that thereis no need for background eliminatiorr' unless the interferometer is equippedwith a thermostated (heated) Ge/KBr beamsplitter (which may give rise tespurious emission bands at 850 and 1125 cm" due to a thin Ge02 layer onthe beamsplitter surface)..6,27Correction for beamsplitter emission can be madeby the four-measurement method, and there is indication that subtractionof interferograms may lead to better results than subtraction of the spectrathemselves.

A number of papers published in the last decade on IR emission spec-troscopy deal with surface species on metal substrates.w"" It has generallybeen accepted that polished metal surfaces make the best substrates, thanksto their very low emittance.P However, if the sample is very thin (in theorder of the thickness of a monomolecular layer), no spectrum can be detectedin the direction perpendicular to the sample surface, but increasing emissionis observed when the sample is tilted, until grazing angles of incidence areapproached.š-" An explanation for this behaviour was given by Greenler'"on the basis of model calculations for the angular distribution of light emit-ted by dipoles located very close to the reflecting metal surface. Taking intoaccount the dipole radiation pattern and the combination of the direct andreflected rays, his calculati:ons have proven that(i) the maximum of emitted mtensity is expected at viewing angles in therange of 70-80 degrees (measured from the normal to the surface), and(ii) only the transition dipole component perpendicular to the metal surfacecan contribute to the emitted intensity (a condition similar to the so calledmetal surface selection rule proposed by Pearce and Sheppard+' for absorptionspectra of molecules absorbed on metal surfaces).

For thicker samples (> 1-2 micrometers), emission spectra can be re-corded at a normal viewing angle as well, but then another problem mayoccur. As pointed out by Chase", the radiation returning from the interfero-meter to the sample may be reflected back into the interferometer by thereflecting substrate. Thus, multiple passing ·of the interferometer and, hence,


multiple modulation of the beam can occur, which may lead to the appearanceof spurious bands in the overtone regions. Therefore, tilting of the sampleis recommended, but care must be taken to avoid reflection of outside thermalradiation into the interferometer,

IV. IRES STUDIES OF CHEMISORBED SPECIES

Few publications deal with IRES studies of chemisorbed species. As tothe type of samples, these works can be divided into two categories: adsor-bates on pure metal sheets or on metal single crystal surfaces28-32 on the onehand, and adsorbates on metal oxides or on oxide supported metal cata-lysts33-39,42-49 on the other. The two categories involve different experimentaldifficulties. In the case of metal supports, the weak emission bands of thesmall amount of adsorbate may be masked by the background radiation,thus reduction of the latter (e. g. by cooling the instrument) can lead to thesolution.28-32

In the case of metal oxides or oxide supported metal catalysts, the over-all emission intensity can be orders of magnitude greater, and the mainproblem is caused by the emission of the adsorbent or support material whichcannot be reduced by cooling since the support is part of the sample andmust have the same temperature as the adsorbate. Griff'iths=' has calledattention to the problem that in this case the emission of the adsorbate (thespecies of interest) may be compensated by the absorption of the substrateemission at the same frequencies.

Detection of the emission bands of adsorbates in the spectral regionsdear from support emission presents no difficulty, but these regions can bestudied in transmission as well. Primet et al.39,42 claim that the IR emissionmethod is capable of yielding information also in the regi on of .lattice vibra-tions of the support, provided that the sample is thin enough (about 0.1mg/crn"). This spectral region (the region below about 1200 cm-i) is of greatimportance from the catalytic point of view, since bands in this region couldgive key information on the nature of bonding of adsorbates to the surface.But the situation is not simple even in the case of thin samples: Johnsonet al.,47 studying toluene adsorption on metal oxides, observed that the bandsof adsorbate above 1000 cm'" appeared as emission and below 1000 cm-i asabsorption bands 'on the emission background of the support.

In an earlier work carried out in this Iaboratory.v" we could observetwo weak bands in the 600 to 400 cm-i region of the emission spectrum ofchemisorbed CO on Pt/ Ah03 which could possibly belong to the metal--adsorbate modes.

Other workers43-45 suggested the use of much thicker samples (0.5 mmor 15 mg/crn"). Kember and Sheppard+' have shown that in this case theadsorbate bands can appear either in emission or in absorption, dependingon the actual temperature distribution within the sample. Temperature inho-mogeneities depending on the gas pressure in the cell make the equilibriumof emission and absorption processes hard to control.

The sample cells used in this type of studies should operate like cata-lytic microreactors, so that the samples can be studied under real catalyticconditions. Important aspects of microreactor design and most of the problemsinherent in emission spectroscopic measurements of supported catalysts are


discussed by van Woerkom et aL45 According to the authors, the four-mea-surement technique is not applicable in the case of adsorbed species becauseemissivity is not independent of temperature.

v. EXPERIMENTAL

IR emission spectra were recorded on a Digilab FTS-20C spectrometer withthe source unit modified (by inserting an additional mirror) to accept a home-madeexternal emission accessory. The accessory allows temperature control and samplepositioning in the vertical plane using a 3-D translation stage. The sample mountis a heatable copper block with exchangeable disk shaped sample holders, 13 mmin diameter. Sample holders can be covered with aluminum foil to elirninatecleaning. The samples were obtained from ethanol suspension by evaporation ofthe solvent, or by pressing layers (cea. 20 mg/crn") on the AI-foU support. Thesample holder covered with soot was used for reference blackbody.

All emission measurements were made at 8 cm'" resolution accumulating 2000or more scans using an ambient temperature high sensitivity DTGS detector.

The diffuse reflectance spectra recorded for comparison purposes were measuredon a Nicolet 170sx FT-IR spectrometer at 4 cm" resolution using a custom madeDR accessory (incorpcrating two off-axis parabolic mirrors and transmitting about17% of the radiation when KBr powder is used as sample).

VI. RESULTS AND DISCUSSION

The IR emission spectra of two typical support materials for supportedmetal catalysts, alumina (Alz03) and silica (Si02), are shown in Figure 1 inthe region between 2200 and 400 cm-lo The emittance of both oxides is quitehigh at low wavenumbers (80-90010 of the emittance of our blackbody), andthat of silica is more structured in the whole region. The upward slope tolow wavenumbers in both spectra might be due to a small temperature mis-match between the samples and the reference blackbody. (Common featuresin the two spectra not belonging to the oxides include a small bump at 1260cm? and a broad feature centered at 1620 cm-lo The former may belong tobidentate carbonate impurity, whereas the latter is most probably due tomoisture. Uncompensated atmospheric water vapor and carbon dioxide alsoappear in the spectra.)

To check the overall effects to be expected when asurface layer ofanother substance is added to the sample, a fair amount of silicone greasewas spread over the surface of the pressed silica sample. Silicone grease waschosen because of its negligible vapor pressure, ease of handling, and itsvery strong emission bands just in the regi on of substrate emissions (seeFigure 2A for the emission spectrum of a thin film of silicone grease on apolished copper plate). The emission spectrum of this sample record ed at343 K was ratioed against that of the blackbody, then the double beamemittance spectrum of the silica disc was subtracted from ito The resultingspectrum is shown in Figure 2B. (Ratioing the two spectra ins tead ofsubtracting leads to a very similar result, with a slightly different base-line profile.) It is surprising at first sight that there is strong emission inthe 2000 to 1300 cm" region, and that only weak emission remained in theregi:on below 1300 cm? showing strange details. A closer inspection revealsthat the strong emission bands appearing above 1300 cm" are not artefactsat all, but greatly enhanced overtone and combination bands, the traces ofwhich can be seen also in the emission spectrum of pure sili cone grease

FT-IR EMISSION SPECTRA

88

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Az<ct-'I-

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lA.

B

oB Đ 4 .0

Figure 1. Emission spectra of thin pellets on Al-foil support at 341 K ratioedagainst blackbody:

A - Ah03 (Alon-C, Degussa)B - Si02 (Aerosil, Degussa)

1 .0.0 1'2iJ.0liAVENUMBERS

(Figure 2A). In the region below 1300 cm-I, however, where the substrateemission is high, the absorpticn and emission of silicone grease are at near--equilibrium. In the regions 1300 to 950 and 870 to 750 cm" absorption seemsto dominate, but some of the peaks still point in the direction of emission(e. g. 1265 and 1020 cm"). From this we have drawn the conclusion that, inthe region of substrate emission, one has to look for absorption as well asemission bands in order to loeate the adsorbate vibrations.

The above experience was utilized when studying a rhenium complex,tetrakis(tricarbonyl-IJ.rhydroxo-rhenium), formed on the surface of powderedalumina in catalytic amounts (1.5 to 50/0 of Re). The tetrameric molecule ofthis rh enium compound (which is very hard to obtain in pure form) is knownto have a cube-like structure, shown in .Figure 3. Its IR absorption frequen-cies are known from Ref. [50] and listed in Table 1. The reaction leading tothe f'orrnation of this compound

2 Re2 (CO)lO+ 4 Hp -+ [Re (CO)3 OH14 + 8 CO + 2 Hp


TABLE I

Emission FT-IR Spectrum of [Re(CO)3 ORh Supported on Al203

[Re(CO)sOH]41Ah03 [Re(CO)30H]4(1.50/0Re) KBr pellet* Assignment

cm= cm?

2065sh2055sh2045sh

2038s 2038s v, (CO) stretching1960sh

1925vs,b 1940vs,b va(CO)1912vs,b

1883s,sh 1895vs,b890vw(?)870w 873m 8 (OH) deformation

812m,s801m 804w,sh653m,s 660m (5(ReCO) linear bending

650m,sh545sh 555sh v (ReC) stretching

540m518m 517m,s500m,w 512m,sh470m,s 457m,b

* Ref. 50.

can be performed at 473 K on the surface of alumina impregnated with thesolution of the starting material. The tetramer, once formed, is stable up to670 K, but it cannot be separated from the alumina support without beingdestroyed. The product was studied by IR transmission spectroscopy, and thecarbonyl stretching bands near 2000 cm" could be easily detected, but noinformation could be obtained in the region of the substrate absorptionbelow 1200 cm-l;5l thus, IR emission spectroscopy and, following the sug-gestion of a referee, diffuse reflection spectroscopy were evoked.

The diffuse reflection (DR) spectra of alumina and of the Re complexon alumina were measured as mixtures with KBr (pure alumina has so highabsorbance that no diffusely reflected signal is transmitted to the detector).Both single beam spectra were ratioed against the DR spectrum of pureKBr, and the % reflectance intensity scales obtained in this way were trans-formed to Kubelka-Munk units. The spectrum of the sample was then sub-tracted from that of the support material (using appropriate scaling). Theresulting difference spectrum is shown in Figure 4: the CO stretching bandsof the Re complex were obtained with high SNR at 2030 and 1910/1885 cm-i,but the spectrum is completely useless in the region of high absorption ofthe support below 1000 cm". The noise level proportional to the subtractedsignals in the low wavenumber part of the spectrum exceeds the intensity ofthe otherwise very strong carbonyl bands, thus leaving no hope of observingthe weaker, low frequency bands of the complex investigated.

r


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wuZq:l-I-

I:W

VI.

o

90

wuZ<!:l-I-

I:wVI.

o4 .I:l

Figure 2. Emission spectra of silicone grease at 343 K ratioed against blackbody.A - Thin Iayer on polished copper plate.B - A fair amount of grease spread over the surface of the pressed Si02 pellet.

Figure 3. Structure of tetrakis(tricarbonyl-~l3-hydroxo-rhenium).;o

":;t<

7~ 1.m•o-r--~...:z::JN

~ o

Z::J:E

~m-'Ow-IDI::J~

lO

NI 2+:-00c:---::-:r:-::--:-r-::-c.


1~OO liloo d~oo 1600WAVENUMBERS

800 600

Figure 4. Difference of diff~se reflection spectra of [Re(CO)S130HJ4on alumina(1.50/0Re) and of alumina (both samples were 0.50/0mixtures with KBr).

The emission spectra of the alumina support and of the rhenium complexon alumina (5010 Re) are shown in Figure 5 (curves A and B, respectively).The direct ratio of the two spectra gave the trace in Figure 5C. Beside thecarbonyl stretching bands, no other spectral feature of the Re complex canbe discerned. Some small sharp features pointing downwards in the ratioedspectrum (Figure 5C: 1590, 1380 and 1260 cm-I) are due to »impurity« bandsin the reference alumina spectrum; the broad bands, pointing upwards around960 and 570 cm-I probably indicate same structural changes in alumina duringimpregnation with n-hexane solution of Re2(CO)!Oand during further treatmentto form [Re(COhOHk The spectral changes in the region below 1200 cm "cannot be ascribed unambiguosly to the presence of the rhenium complex.

In fact, the direct ratioing of the above two single beam spectra (C andB in Figure 5) is not reasonable, since it enhances noise in the regions oflow emittance (above 1200 cm-I) and reduces the sensitivity to small intensitychanges In the regi ons of high emittance of the support.

The standard procedure, i. e. ratioing against the blackbody, leads to ahigher SIN ratio in the carbonyl stretching region (see Figure 6), allowingone to determine the three main CO stretching frequencies with greaterconfidence.

A better solution to the problem is the four-measurement experiment asdefined by Eq. (5). The two temperatures used were 345 and 373 K, and theemittances were calculated according to Eq. (5) for both the sample (eon-taining 1.5010 Re) and the pure alumina support. The emittance spectrum ofthe support was then subtracted from that of the sample yielding the diffe-rence spectrum shown in Figure 7 in the region below 1400 cm-I. Severalnew bands pointing downwards (i. e. absorption bands) appeared at wave-numbers very close to those reported for the Re cluster by Herberhold etal.50 (see Table I). Slight frequency shifts, broadening and intensity redis-tributions of the bands as compared to those of the isolated [Re(C03)OH14moleculeš? suggest that the tetrameric molecule is weakly bonded to the


60

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98

wuZq:I--I--

:EW...

o1

HA V[NUMlJERS4 l'l

Figure 5. Directly observed emission spectrum of deposited thin layers on polishedcopper plate at 340 K.

A - Ah03 (Alon-C, Degussa)B - [Re(C0l30Hl4 on Ah03 (5010 Re)C - ratio spectrum of ElA

surface of the alumina support. Additional spectral features that were ob-served can be attributed, likewise as before, to impurities ar changes of emis-sion band shapes of the support. It is difficult to speculate on the origin ofthe strong band pointing downwards ne ar 670 cm", because of the coinci-dence with the absorption of atmospheric CO2•

Thus, on the basis of the new emission IR data it seems most likely th 1tthe rhenium complex formed on alumina as suppcrt has a cuba-like tetra-meric structure similar to that ascribed to the pure compounc1.

In a broader sense, the near-success of this study makes us believe thatFT-IR emission spectroscopy is worthy of efforts, and might live up to our

742

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JR

J. MINK AND G. KERESZTURY

1.0

z iaa

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1 XlXl

Figure 6. The emittance spectrum of [Re(CO)aOH]4 on alumina with blackbodyreference in the CO stretching region.

izaa 1 B\lAVENUHBER S

Figure 7. Emission spectrum of [Re(CO)sOH]4 supported on A1203 using the four--measurement experiment at two different temperatures (347 and 337 K) followed

by subtraction of the corresponding alumina spectrum.

2 .0

wuZ<I:t-t-

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o


expectations in the field of structural studies of chemisorbed molecules onsupported metal catalysts in a wide frequency range inc1uding the spectralregion of adsorbate-substrate vibrations.

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SAŽETAK

FT-IR emisljski spektrt kemisorbiranih vrsta, s primjenom na vrste adsorbiranena aluminij-oksidu

J. Mink i G. Keresztury

Ukratko su prikazana načela infracrvene emisijske spektroskopije s naglaskomna aspekte njezine primjene na studij kemisorbiranih vrsta. Razmatraju se glavniproblemi priprave uzoraka, izbora mjernih uvjeta i najprikladnije metode obradbepodataka,

Prikazani su infracrveni emisijski spektri dvaju tipičnih materijala-nosačaza metalne katalizatore, aluminij i silicij oksida. Na primjeru renijeva kompleksa,tetrakis(trikarbonil-fl3-hidrokso-renija), stvorenog na aluminij-oksidu kao nosačuu katalitičkim uvjetima (1,5 do 5% Re) pokazalo se da tehnika četiriju mjerenjamože rezultirati vrpcama sorbata u područjima visoke emisije supstrata (između1300 i 400 cm").

ft-ir emission spectra of chemisorbed species, with

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