Indian Journal of ChemistryVol. 31A, January 1992, pp, 39-42

Solvent extraction--electrothermal atomic absorption analysis of platinumin heterogeneous supported catalysts]

G Kalpana & V J Koshy*

Research Centre, Indian Petrochemicals Corporation Limited, PO: Petrochemicals, Vadodara 391 346

Received 22 April 1991; revised 9 July 1991; accepted 18 November 1991

A method based on extraction of the analyte from the interfering matrix and atomisation byelectrothermal technique (ETAAS) for the determination of platinum in heterogeneous catalysts has beendeveloped. Interference levelsof the probable catalyst coelements are checked. Beer's law is obeyed in theconcentration range of 0.5-5.0Ilg/mlplatinum. The detection limit is0.15ug/ml with 3.3% RSD. The resultsare discussed in terms of the application of this method to the analysis of catalysts.

The increasing industrial application of platinum,especially in the use of hydrocarbon processingcatalysts, demands its accurate analysis forperformance as well as economic considerations'.Atomic absorption spectrophotometry is a methodof choice for the estimation oflow levels of platinumin a variety of samples. However, matrix constituentslimit its determination by FAAS as well as ETAASdue to interferences and loss of sensitivity". Becauseof these reasons AAS has not been widely used forplatinum analysis in catalyst materials. AlthoughPotter and Waldo" have attempted its analysis inautomotive catalysts, their method is limited only toy-alumina supports. They have observed 25%enhancement in platinum signal due to the presenceof alumina. During our investigations on the analysisof platinum in heterogeneous catalysts, we have alsocome across varying levels of interference fromsupport materials like y-alumina, alumina-silica andzeolites. Analysis under such conditions requiresrigorous matrix matching for each type of sampleattempted. This procedure as such is very tedious andmight fail in the case of samples with unknowncomposition. We report here a method for thedetermination of platinum in which these difficultiesare avoided by first extracting it from the interferingmatrix and then analysing it by ET AAS.

Materials and MethodsAll absorbance measurements were made using a

Varian Techtron 1200 atomic absorptionspectrophotometer coupled with a Varian CRA-63carbon rod atomiser. The graphite cup furnace was

+IPCL communication No. 175

kept cooled with water flowing at the rate of 2 l.min - 1 .

A platinum hollow cathode lamp (Varian,S6-100041-00) served as the line source. Absorptionsignals at 26S.9 nm were measured as peak heights ona Perkin- Elmer 10m V recorder operated at low chartspeed (S.O mm. min -I). Aliquots of samples andstandards (Sill) were delivered into graphite cupsusing a Hamilton microlitre syringe. Argon (99 .S%)was used as sheath and carrier gas. Hydrogen (99.9%)was used as supplementary fuel to sustaincombustion.

All the reagents used were of AR/GR grade anddoubly distilled water was used for dilution purposes.Methyl isobutyl ketone (MIBK, Fluka) was usedwithout further puri fication.The dithizone solutionwas prepared by dissolving dithizone (10 mg) inMIBK (100 ml) with stirring, stannous chloridesolution was prepared by dissolving SnCI2.2H20 (25g) in cone. HCI (2S ml), boiling till the solutionbecame clear, transferring it to a 100 ml volumetricflask and making up the volume with water.

A stock solution of platinum (1000 Ilg/ml) wasprepared by dissolving pure platinum metal (O.Sg) inaqua regia (20 ml). The oxides of nitrogen wereremoved by three successive evaporations (almost todryness) with cone. HCl. The residue was dissolved incone. HCI (I Oml),and the solution was transferred toa SOOml volumetric flask and volume made up withwater. The solution was standardised against atomicabsorption calibration standards (Aldrich, No.20736-S). The stock solution (SOO ug/rnl) of theelements underlined were prepared from thefollowing salts: AgN03, AICI).6H20. As20J•AuCl3.2H20, CoCI2.6H20, K2Cr207, CuS04.SH20.(NH4hFe(S04h.12H20, NiCb.6H~O PdCl~- , -,

40 INDIAN J CHEM, SEe. A, JANUARY 1992

NH4Re04, RhCI3·nH20,KSbC4H407, Se02, Na2Si03,YOS04.2H20, ZnS04.7H20.

RuCh.nH20,Te metal,

Sample preparationThe catalyst samples and y-alumina,

silica-alimina (53% Si02/45% A1203) and ZSM-5supports (200 mesh size) were dried at 200T for2h. HCl (8N, 30 ml) was added to the flask containing0.2 g of catalyst/support material and the contentswere refluxed for 1 hr with constant stirring. In thecase of alumina based catalysts, where thedissolution was complete, the contents weretransferred to a 50 ml volumetric flask and volumemade up with water. Since dissolution of silica basedcatalysts was partial, the digests were filtered and thesilica particles thoroughly washed with HCl (8N). Allthe washings were collected and combined with theearlier fraction and volume made up.

Determination of platinumAn aliquot (4 ml) of catalyst digest, alongwith

cone. HCI (3 mI) and stannous chloride solution (2 mI)was taken in a separating funnel. Dithizone solution(3 ml) was added and the platinum dithizonateextracted into the organic layer. The extraction wasrepeated, organic layers were combined and volumemade upto 10 ml by adding MIBK. Platinum wasdetermined by ET AAS under the followingdry-ash-atomize operating conditions i.e. TempCC)/Time (s): for aqueous solution85/20-1500/20-2200/5 and for nonaqueous solution85/30-1500/30-2200/5 respectively. Thebackground absorbance was checked using extract ofblank alumina digest. The absence of absorbancesignal under the instrument settings employed,enabled us to perform the experiments withoutbackground correction. The calibration graph wasprepared by treating 0.5-5.0 ml of aqueous standardsolution containing 10J.1g/mlof platinum by a similarextraction procedure.

Results and DiscussionSeveral methods are reported in literature for the

dissolution of catalyst samples. Potter? has usedH2S04-HCl mixture, Maziekien et al," have useddilute HCI (1:1) and ASTM5 suggests use of6N HCI inpresence of H202. By following the procedure asdescribed by us, complete dissolution of y-aluminasupported catalysts was achieved. However, in thecase of alumina-silica based catalysts, a filtration stepwas found essential.

In y-alumina supported catalysts, aluminium isgenerally present in more than 100 fold excess of

Table I-Interference levels of foreign ions on the ETAASdetermination of 3.0 Ilg/ml of platinum

Element Interferent Apparent Recoverycone. (Ilg/ml) cone. (ug/rnl) (%)

AI 300.0 3.00 100.0Sb 300.0 3.10 103.3As 300.0 3.00 100.0Cr 300.0 2.95 98.3Co 300.0 3.05 101.6Cu 300.0 2.85 95.0Au 300.0 3.05 101.6Fe 300.0 3.00 100.0Mo 300.0 3.00 100.0Ni 300.0 3.00 100.0Pd 300.0 3.85 128.3

30 3.60 120.06.0 3.40 113.3

Re 300.0 1.80 60.030.0 2.15 71.76.0 2.95 98.3

Rh 300.0 2.05 68.330.0 2.15 71.76.0 2.65 88.3

Ru 300.0 3.05 101.6Se 300.0 0.75 25.0

30.0 2.25 78.36.0 2.90 96.6

Si 300.0 3.05 101.6Ag 300.0 2.95 98.3Te 300.0 2.95 98.3V 300.0 3.15 105.0Zn 300.0 2.95 98.3

platinum. In alumina-silica or zeolite supportedcatalysts, most of the silica is filtered off, but solublesilica may remain along with varying amounts ofaluminium. To check the possibility of direct analysisof platinum, interference from alunimium and siliconwas investigated. Aluminium was found to causepositive interference whereas silicon showed anegative trend. Aluminium increased the platinumsignal from 15 to 35% (when Al:Pt ratio changedfrom 100:I to 1000:I) and silicon depressed it by 15%even at 10:1ratio (Si:Pt). A higher amount of siliconcaused high loss of platinum (~40% decrease). Toovercome these interferences, a method of separationwas adopted. Dithizone is known to separateplatinum selectively from geological samples". Thismethod was extended to catalyst samples andplatinum could be successfully separated from thesupport materials.

The behaviour of platinum was studied in aqueoussolution aswell as in organic extract. This was done

KALPANA et al.: SOLVENT EXTRACTION-ELECTROTHERMAL ATOMIC ABSORPTION ANALYSIS OF Pt 41

Table 2-Platinum determination in catalyst samplesSample PlatinumContent (%)

(.J

Expected Obtained"

A 0.410 0.415±0.004B 0.375 0.376 ± 0.002C 0.340 o 337±0.010D 0.375 O.377±0.003E 0.600 0.592 ±0.008F 0.300 0.3 \0 ±0.009G 0.300 0.296±0.004H 0.300 0.308±0.008I <0.100 0.078±0.01J 0.400 0.397 ± 0.009

• Average of 6 determinationsA-D: Multirnetallic/y-alumina, EiMonometallic/v-alumina,F-H:Bimetallicfy-alumina, I: Monometallic/zeolite,J: Monometallic/alumina-silica

Table 3-Recovery of platinum from catalyst sample solutions

Catalyst Pt Concentration (I!g/ml) RecoverySamples (%)

Present Added Foundy-A1203 0.50 0.50 100.0

B 3.10 0.50 3.65 101.4E 2.55 0.50 3.00 98.3G 2.45 0.50 2.90 98.3H 2.45 0.50 3.00 \0\7J 1.65 0.50 2.10 97.6

with a view to examining the possibility of usingaqueous standard solutions of chloroplatinic acid forcalibration while the analyte is present as thedithizonate in the organic extract. It was observedthat the calibration graphs in both cases were linear inthe range of 0.5 to 5.0 J.lg/ml platinum with a slightvariation in their slopes. This indicated the necessityof plotting a calibration graph with extractedstandards.

The analytical performance of the system is asfollows: The detection limit of platinum expressed astwice the standard deviation of the signal caused bythe blank was 0.15 ug/ml, The amount of analyteyielding 0.0044 absorbance was 0.15 J.lg/ml. Thecalibration graphs conformed to the followingequations:y= 12.8857 X -0.3810 (aq.) and Y= 13.4857X - 0.0476 (non aq.) (in terms of peak height andconcentration ).

In addition to support materials like alumina,silica, zeolites. activated carbon, etc., most catalysts

also contain certain promoter elements. From patentliterature 7, it is seen that several transition, alkali,alkaline earth as well as noble metals are alsoincorporated into these catalysts along withplatinum. In order to study their effect, syntheticsolutions of the analyte and probable co-elements inthe ratio I: 100 were prepared and the platinumatomisation signal was monitored after extracting itinto MIBK. The-results listed in Table 1clearly showthat many metals such as AI, As, Sb, Te, Cu, Si, Cr, V,Mo, Ag, Au and Ru do not interfere significantly withthe estimation of platinum. This may be because theydo not get extracted along with platinum. On theother hand, serious interferences are observed in thecase of Se, Rh, Pd and Re; Re and Se could betolerated up to 2 and 5 fold excess respectively, Pdboosted the Pt signal even when present in the ratio1:2 (Pt:Pd). This interference can be removed byselective extraction ofPd by the method suggested byMarczenko"; Rh also interfered even at 1:2 ratio(Pt:Rh).

Eventhough most of the elements tested do notshow interference, the possibility of interference dueto matrix variations in catalyst samples cannot beruled out. Hence, synthetic solutions ofrepresentative catalyst matrices (Table 2) weresubjected to platinum analysis by this method. Nodeviation was observed from the expected platinumcontent indicating that the catalyst matrices do notcause any additional interference.

The platinum contents of various laboratory-prepared catalysts estimated by this method arepresented in Table 2. The results obtained agree wellwith the expected values. Since certified catalystsamples were not available in our laboratory,recovery experiments were carried out by spikingcatalyst sample solutions with platinum stocksolutions (Table 3). Recoveries of the order of99.7 ± 2.1% indicate that the method can be appliedto catalysts with different support materials. Thismethod can be utilised for analysing very lowcontents of platinum in a variety of samples becauseseparation by complexation-extraction also servesas a preconcentration step.

AcknowledgementThe authors wish to thank Dr V N Garg and Dr R N

Nigam for their valuable suggestions and Dr I SBharadwa], Director (R&D), for his permission topublish this work. Thanks are also due to Mr A RShah and Ms R H Patel for their technicalassistance.

42 INDIAN J CHEM, SEe. A, JANUARY 1992

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