In situ concentration of mercury vapour in a palladium-coated graphite tube: determination of mercury by atomic absorption spectrometry

Analyttca Chunrca Acta, 272 (1993) 105-114

105

Elsevler Science Pubhshers B V , Amsterdam

In situ concentration of mercury vapour in a palladium-coated graphite tube: determination of mercury by atomic absorption spectrometry Xm-Pmg Yan and Zhe-Mmg Nl Research Cenrre for Eco-Envuonmenral

Scrences, Academua Suaca, P 0 Box 2871, Belpng loo085 (Chma)

Qm-Lm Guo In&We

of Physcal ChemWy,

Pekmg Umversrty, Beqmg (Chma)

(Received 11th May 1992, revised manuscript received 16th September 1992)

Abstract

A method was developed for the determmabon of mercury by cold vapour generation electrothermal atonuc absorption spectrometty mth m mtu concentration m a palladmm-coated graphite tube The mercury vapour generated by using NaBH, IS rapidly trapped m the graphite tube, coated with PdCl,, at 250°C and atormzed at 2200°C The expertmental results show that the trapping efficiency for mercury by PdCl, IS better than that by reduced palladium Redution of PdCl, to palladmm metal gives less favourable mercury trappmg Calibration IS aclueved wtth sunple aqueous solutrons An absolute detection hnut (3~) of 314 pg, correspondmg to a concentration detection hnut of 628 pg I-’ for SO-ml samples, 1s obtained The charactenstx mass IS 114 pg The prectsion for ten replicate determmabons IS 2 0% (relattve standard devlabon) at the I-ng level and 18% at the 5-ng level The method was successfully apphed to the determmation of mercury m certified water samples, sea water and waste water The most favourable aspects of the method are its simplicity, high-speed and good reproducxlxhty Keywords Atormc absorption

spectrometv,

Mercury, Preconcentratlon,

To establish sources of mercury contammatlon and to evaluate levels of mercury pollution, extensive research on mercury determmatlon has been carried out In particular, cold vapour atomic absorption spectrometry (CVAAS) has received great attention owing to its snnphclty, high sensltlvlty and relative freedom from interferences [1,2] In order to lower the detection hmlt and to improve the sensltlvlty further, the combination of CVAAS with noble metal amalgamation techniques has been investigated [2-71 Usually the Correspondence to Zhe-Mmg Ni, Research Centre for EcoEnvironmental Saences, Academia Smsa, P 0 Box 2871, Beipng 100085 (Chma)

Waters

copper, sliver, gold and platinum metals were used for this purpose [61 However, there are two disadvantages wrth this amalgamation technique [8] First, the efficiency of mercury collection may be unpaired by moisture or other gaseous reaction products which poison the surface of the amalgamation medium, necessitating occasional cleaning Second, during heatmg of the collector to release the mercury, a gas flow 1s used to transport the vapour to the absorption cell, which means that the sensltlvlty 1s flow-rate llrmted, Slight changes m the flow-rate between measurements wrll also impair the reproduclblllty 181 To circumvent these problems, methods for the determmatlon of mercury have been devel-

0003-2670/93/$06 00 0 1993 - Elsevler Science Publishers B V All nghts reserved

106

oped using a combmatlon of cold vapour generation, trapping m a porous gold-plated graphite mmltube [9], a gold-coated [lO,ll] or a platmumlmed graphite tube [8], followed by atomic absorption spectrometrlc detectlon In earlier work, Slemer and Hageman [9] used porous gold-plated graphite mmltubes (CRA-63) mounted m a speclally designed holder as filters to collect mercury from a gas stream drawn through the reaction cell by a vacuum pump Owmg to the variation m the blank levels obtained, the reported detectlon lrmlt was 14 ng 1-l [9] Methods developed by Lee et al [lo] and Hladky et al [ll] using a gold-coated graphite furnace as both mercury trapping medmm and atomlzatlon cell permit the determmatlon to be carried out at ng 1-l levels Unfortunately, m the method proposed by Lee et al [lo], the commercially available electrothermal atomizer (CRA-90) needed modlflcatlon so that it could be coupled directly to a mercury vapour generator [lo] This poses a problem m applying this method to routme analysis Further, m these methods [lO,ll], an additional pretreatment step was required to reduce the gold(II1) to the metal before mercury trappmg Baxter and Frech [81 reported the posslblhty of using a platmum-hned graphite furnace for mercury determmatlon and obtamed a detection hmlt (2~) of 2 ng 1-l for SO-ml samples However, the adsorption of mercury m a platmum-lined furnace was slow and a collectlon time of more than 5 mm was needed to achieve the maxnnum absorbance signal In the method reported by Hladky et al [ll], a much longer collection time, over 20 mm, was used to trap mercury m a gold-coated tube Chemical modlflcatlon techniques are widely used m electrothermal atomic absorption spectrometry (ETAAS) Palladmm 1s a very effective chemical modtiler and can be used to stab&e many elements to several hundred degrees higher than the temperatures possible with current methods [12-221 The use of palladium-coated tubes as both the hydride-trappmg medium and atomlzatlon cell greatly improved the sensltlvlty and detection hmlts for hydnde-formmg elements [23-Z] In this work, a method was developed for the

X-P Yan et al /Anal Chun Acta 272 (1993) 105-114

determmatlon of mercury by m situ concentration of mercury m a PdCl,-coated graphite tube with subsequent ETAAS detection There 1s no need to modify the graphite furnace m this method Moreover, no additional pretreatment step for the reduction of palladium salt 1s required before mercury trapping The mercury vapour can be effectively adsorbed by the PdCl,-coated tube wlthm 20 s The most favourable aspects of this method are its amphaty, high speed and good reproduclblhty The valence state of palladium on the graphite surface after heat treatment was studied using x-ray photoelectron spectrometry (XPS)

EXPERIMENTAL

Instrumentation The measurement

of analyte absorbance was carried out m the peak height mode under “gas stop” condltlons using a Perkm-Elmer Model 4000 atomic absorption spectrometer with deutermm background corrector, equipped with an HGA-400 graphite furnace and a Model 056 chart recorder A mercury hollow-cathode lamp was used as the hne source at 253 7 nm The sample mtroductlon port of the graphite tube was enlarged to a diameter of 2 5 mm wrth a drdl bit Pyrolytic graphite-coated graphite tubes were used Mercury vapour was generated m a laboratorybuilt hydride generator, HG-100 [261, mto which sodium tetrahydroborate solution and sample solution were introduced by two channels of a penstaltlc pump The mercury vapour evolved was stripped from the solution and swept into the preheated palladmm-coated graphite tube using an argon purge gas The generator assembly and sequence of operations used to generate and trap mercury vapour were slmllar to the earlier detailed descnptlon [23,261 The x-ray photoelectron spectra were measured at room temperature usmg an ESCA LAB-5 (VG Saentlflc) surface spectrometer which had a base pressure of 4 X lo-l3 bar Mg Ku x-radlatlon (hv = 1253 6 eV) was used The XPS data were recorded and processed by a PDP LS 11/2

X-P Yan et al /Anal

107

Chtm Acta 272 (1993) 105-114

100,200, 300 and 400 pg ml-’ were obtained by diluting the above solution with delomzed water

TABLE 1 Recommended Wavelength

experimental condltlons 253 7 nm

Band width

07nm

Lamp current

4 mA

Carrier gas flow-rate

570 ml mu--l

Uptake rate of NaBH, solution Uptake rate of HCI solution NaBH, concentration HCI concentration

6mlmu-’

6mlmu-’ 2% (w/v)

05 mol dm-3

computer The bmdmg energy scale was cahbrated by asslgmng a value of 284 5 eV to the C 1s signal of graphite Reagents All chemicals were of analytical-reagent grade A stock standard solution of mercury (1000 pg ml-‘) was prepared by dlssolvmg mercury(II) chloride m delomzed water Workmg standard solutions were obtained through appropriate dllutlon of the stock standard solution Just before use Solutions of sodmm tetrahydroborate of l%, 15%, 2%, 4% and 6% (w/v) were prepared dally or as required by dlssolvmg NaBH, m delomzed water and used without further filtration or stablhzation Palladium solution (1000 pg ml-‘) was prepared by dlssolvmg palladmm(I1) chloride m dllute nitric acid and subsequently dllutmg with delomzed water Palladium solutions of 25, 50,

Procedure Mercury vapour generation, adsorptlon and ETAAS measurement were done as follows A 50-~1 ahquot of 200 pg ml - ’ Pd as PdCl 2 solution was injected mto the graphite tube with an Eppendorf mlcrohtre pipette fitted with dlsposable polypropylene tip, and dried at 100°C The tip of the quartz tube connected with the outlet of the hydride generator was mserted mto the sample mtroductlon port at the centre of the graphite tube and was held near the opposite Interior wall When the furnace reached the adsorption temperature, the NaBH, solution and acid sample solution containing mercury were delivered to the hydride generator The mercury vapour evolved was swept mto the furnace with argon and adsorbed on the graphite tube coated with PdCl, When the adsorption was complete, the quartz hydride delivery tube was automatltally withdrawn from the furnace, and the analyte was atomized at 2200°C The recommended experimental condltlons and furnace programme are summanzed m Tables 1 and 2 Graphite platforms were used as base matenals on which three samples were prepared for subsequent mvestlgatlons by XPS Samples 1 and 2 were prepared by repeatmg the followmg operation five times 20 ~1 of 200 pg ml-’ Pd as PdCl, solutions were inJected on to the graphite platform, dried at 100°C for 30 s, and then heated at 250°C (sample 1) or 900°C (sample 2) for 40 s Sample 3 contammg PdCl, and Hg was prepared

TABLE 2 Furnace. programme Step 1 2

3 4

Temperature (“C) 100 250

2200 2650

Ramp time (s)

Hold time (s.1

Internal gas flow

Procedure

5 5

50 5 40

Normal

Dry Pd solution Insert quartz tube Generate and trap mercury vapour Remove quartz tube Atonuzatlon Clean the furnace

1 1

5 5 2

Normal

Stopped Normal

108

as follows the operation of mJectlon and drying of the PdCI, solution was the same as for sample 1, the mercury vapour was adsorbed by PdCl, at 250°C for 40 s and this process was repeated until a required amount (pg level) of sample was accumulated After the graphite tube had been cooled to room temperature, the platform was removed and transferred mto the vacuum system of the x-ray photoelectron spectrometer for XPS measurement

X-P Yan et al /Anal Chm Acta 272 (1993) 105-114

03.

u

021 O-\

100

200

RESULTS AND DISCUSSION

Effect of ascorbic acid and heat pretreatment In direct aqueous sample mtroductlon m ETAAS, it has been reported that steps taken to ensure that the palladium modlfler was reduced to the metal as early as possible greatly unproved Its performance The palladmm modlfler could be reduced, for example, by hydrogen-argon (5 + 95), ascorbic acid or preheating palladium modlflers to 1000°C before mjectmg analytes 127-291 In this work, the effects of ascorbic acid and heat pretreatment on the mercury trapping were mvestlgated m order to establish whether the reduction of PdCl, was a prerequlslte for mercury trapping The results m Fig 1 show that the adsorption efficiency of mixed PdCl,-ascorbic acid for mercury (curve b) 1s much lower than that of PdCl, alone (curve a) Figure 2a indicates that the trappmg efficiency for Hg by PdCl, decreases as the pretreatment temperature Increases However, the results m Fig 2b show that the pretreatment temperature has no influence on the trapping efficiency of mixed PdCl,-ascorbic acid, as the PdCl, has been reduced to the metal by ascorbic acid It 1s well documented that palladmm salts would be reduced to metalhc palladmm m a graphite tube at temperatures around 500°C or on mung with a reducing reagent such as ascorbic acid [27,28,30,31] From the above results, it 1s evident that the reduction of PdCl, to the metal 1s less favourable for mercury trappmg This is contrary to the results obtained from the direct mjectlon method, In which higher sensltlvltles are usually achieved

,

“o?/T& 300

500

400

600

Temperoture.‘C

Fig 1 Effect of adsorption temperature on the peak absorbance of 5 ng of mercury generated m 0 5 mol 1-l HCl and 2% (w/v) NaBH, (a) 50 ~1 of 200 pg ml-’ Pd as PdCl,, (b) 50 pl of 200 pg ml-’ Pd as PdCI,+20 ~1 of 1% (w/v) ascorbic acid

when the reduced palladium 1s used as a matrur moddier [27,28,30,31] Therefore, PdCI, was used to trap mercury vapour with no addltlon of ascorbic acid or heat pretreatment In order to mvestlgate further the mechanism of mercury trappmg, the oxldatlon state of the palladium on the graphite surface was studied by XPS Optimization of expenmental parameters Aa’sorptron temperature and collectwn time As can be seen from Fig 1, the maximum ab-

01 200

400

600

600

Temperature

1000

1200

‘C

Fig 2 Dependence of the peak absorbance of 5 ng of mercury generated m 0 5 mol I-’ HCl and 2% (w/v) NaBH,, adsorbed at 25O“C, on pretreatment temperature (a) 50 ~1 of 200 pg ml-’ Pd as PdCl,, (b) 50 ~1 of 200 pg ml-’ Pd as PdCI, + 20 ~1 of 1% (w/v) ascorbic acid

X-P Yan et al /Anal

109

Chun Acta 272 (1993) 105-114

0

u

P

0

ct

x

5 9

%

0

A

0

20

60

40 Time,

80

100

100

200

Concentration

S

of

Pd

500

400

300

pg/ml

I9g 3 Influence of collection tune on the peak absorbance of 5 ng of mercury generated m 05 mol dmm3 HCl and 2% (w/v) NaBH+ adsorbed at 250°C m the PdCl,-coated graphite tube

Fig 4 Effect of Pd concentration on the peak absorbance of 5 ng of mercury generated m 0 5 mol I-’ HCl and 2% (w/v) NaBH,, adsorbed at 250°C m the graplute tube coated with PdCI,

sorbance of mercury 1s obtamed with adsorption temperatures between 150 and 300°C usmg either (a) PdCI, or (b) mixed PdCl,-ascorbic acid as an adsorber Above 300°C the trappmg efficiency rapidly decreases This 1s probably due to the reduction of PdCl, to the metal (curve a) and mcomplete adsorptlon of mercury (curves a and b) at higher temperatures In subsequent expenments, a temperature of 250°C was selected for mercury trapping Figure 3 shows the dependence of the peak absorbance of mercury on collection tune Even a collection trme of 20 s is sufficient for trappmg mercury m a PdCl,-coated tube, no slgmficant change 111the absorbance 1s observed as the collectlon tune mcreases further These results nnply that mercury vapour can be rapldly adsorbed by the graphite tube coated wth PdCl, To ensure that all of the mercury vapour was completely adsorbed, a collectlon tune of 40 s was used Amount of palladaum added The mfluence of the concentration of Pd solution on the trappmg efficiency was exammed when the volume of Pd solution mjected was kept at 50 ~1 The results m Fig 4 show that the trappmg efficiency mcreases slgmficantly as the concentration of Pd solution mcreases from 25 to 100 pg ml-‘, then levels out above 100 pg ml-’ The effect of the volume of Pd solution mJetted on the trappmg efficiency was also mvestl-

gated for a constant 10 pg of Pd No significant change m the trapping efflclency was found when the volume of Pd solution mjected varied from 10 to 100 ~1 These results suggest that the adsorption of mercury m the PdCl,-coated tube depends only on the amount of Pd added All subsequent expernnents utlhzed a 50-~1 inSectton volume of 200 pg ml-’ Pd solution flow-rate The variation of the mercury absorbance with carrier gas flow-rate was examined under the recommended condltlons, and 1s illustrated 111Fig 5 It 1s obvious that a carrter gas flow-rate of 480 ml nun-l is necessary to strip the generated mercury vapour completely

03

t 6

f

;:_

/--

0

200

400 Corr!cr

600 *s

fIQW

600 rote

1000

1200

ml/-m”

Fig 5 Influence of tamer gas flow-rate on the peak absorbance of 5 ng of mercury generated at 0 5 mol 1-l HCl and 2% (w/v) NaBH,, trapped m the PdCl,-coated tube at 250°C

110

X-P Yan et aL /Anal Chun Acta 272 (1993) 105-114

0

02

4

; B b 2 u

o2/-

b 0; 6 f 5: : 01

t

1

01

0

1 Concentration

of

HCI

C

4

3

2

0

mol/dm3

1

2 Ramp

time.

3

4

S

Fig 6 Dependence of the peak absorbance of 5 ng of mercury generated m 2% (w/v) NaBH,, trapped m the PdCl,-coated tube at 25OT, on HCl concentration

Fig 8 Influence of heatmg rate on the peak absorbance of 5 ng of mercury generated m 0 5 mol I-’ HCI and 2% (w/v) NaBH,, trapped m the PdCI,-coated tube at 250°C

from the solution A carrier gas flow-rate of 570 ml mu- ’ was used HCl and NaBH4 concentratwns Figure 6 shows the mfluence of HCl concentration on the peak absorbance of mercury Below 0 25 mol 1-l the mercury absorbance mcreases with mcreasmg concentration of HCl, but above 0 25 mol dmm3 it remams almost unchanged A concentration of 0 5 mol 1-l HCI was needed for the generation of mercury vapour The vanatlon of the peak absorbance of mercury with the concentration of NaBH, 1s depicted m Fig 7 Obviously, below 15% the peak absorbance increases rapidly Hrlth mcreasmg con-

centratlon of NaBH, However, above 15% (w/v) the mercury absorbance gradually decreases as the concentration of NaBH, increases It seems unreasonable to conclude that excessive NaBH, ~111 hinder the generation of mercury vapour However, as interaction between Pd species and H, at low furnace temperatures has been suggested [27], the hydrogen generated by excessive NaBH, may decrease the trappmg efficiency through reduction of palladmm chlonde to metalhc palladium, which 1s less favourable for the adsorptlon of mercury, as described above For subsequent expenments 2% (w/v) NaBH, solution was used Heatmg rate and atomuatwn temperature For the determmatlon of refractory elements, the “maximum power” mode IS often reqmred to achieve maxlmum sensltlvlty, as fast heating mcreases the rate of analyte release However, for the analysis of highly volatile mercury, the “mmmum power” mode IS unnecessary Figure 8 depicts the influence of ramp tnne m the atomlzatlon step on the peak absorbance of mercury The highest signal 1s obtamed usmg a ramp time of 1 s rather than “maxnnum power” Although rapid heatmg facilitates the release of analyte from the graphite surface, it also speeds up the dissipation of gaseous analyte atoms within the atomizer through expulsion Therefore the “maximum power” mode IS not favourable for mercury determmatlon

03.

01

0

4

1

2

4

3

concantratton

of

NaBH,,

5 %

6

7

m/v

Fig 7 Vanatlon of the peak absorbance of 5 ng of mercury generated m 05 mol 1-l HCI, trapped m the PdCl,-coated tube at 25OT, with NaBH, concentration

X-P Yan et al /Anal

111

Chrm Acta 272 (1993) 105-114 TABLE 4 Determmatlon

of mercury m standard water samples a Concentration (ng ml-‘)

Sample

Certified GSBZ 50016-90 Hg 1400203 (China) GSBZ 50016-90 Hg 1410104 (China) 1100

1500

1900 Temperature,

2300

2700

‘C

Fig 9 Effect of atomlzatlon temperature on the peak absorbance of 5 ng of mercury generated m 0 5 mol I- * HCI and 2% (w/v) NaBH,, adsorbed m the PdCl,-coated tube at 250°C

Figure 9 shows the dependence of the peak absorbance of mercury on atomlzatlon temperature The optimum atomlzatlon temperature ranges from 1900 to 2400°C Compared wth the direct aqueous solutlon mJectlon method for mercury determmatlon m which a palladmm modifier was used and optnnum atomlzatlon temperatures of 900-2400°C were obtained [12,13], the mercury adsorbed by PdCl, seems to be more dlfflcult to atomize This probably results from the different stablhzatlon and atomlzatlon mechamsms 4

Determined b

49*10

48+04

147*14

145+02

a Standard water samples were prepared and certdied by the Chma Environmental Momtormg Statlon, Beijmg b Mean f standard devlatlon based on tnphcate determmatlons

temperature tion

of 2200°C was chosen for atomlza-

Analytical results The results for the recovery of mercury added

to water samples are summarized m Table 3 The recoveries are m the range 96-103% The slope of the cahbratlon graph prepared with standard aqueous solutions was found to be almost ldentlcal with that of the standard addltlons plot prepared for water samples, so cahbratlon was achieved Hrlth a simple cahbratlon graph The results for certified water samples given m Table 4 show that the concentrations of mercury determined by the present method are m good

TABLE 3 Recovery of mercury from water samples Sample

Hg m water (ng)

Hg added (ng)

Total Hg found

Recovery

(n.$

(%)

GSBZ 50016-90 Hg 1400203 (China) GSBZ 50016-90 Hg 1410104 (China) Sea &ter 1

490

400 600

890 1120

100 103

705

250 500

958 12 11

101 101

009

Sea water 2

006

Waste water 1

005

Waste water 2

006

500 1000 500 1000 500 1000 500 1000

5 20 10 10 5 18 980 490 10 10 485 1000

102 100 102 97 97 101 96 99

112

X-P Yan et al /Anal Chm Acta 272 (1993) 105-114

TABLE 5 Determmatlon of mercury III water samples Sample

Concentration (ng ml-‘) a

Sea water 1 Sea water 2 Waste water 1 Waste water 2

0 0 0 0

a Mean *standard tlons

18fOOl 12*002 10*001 11*002

devlatlon based on tnphcate

determma-

agreement with the certified values Results for sea water and waste water are given m Table 5 Fgures of ments

A character&c mass of 114 pg was obtained, where characterlstlc mass 1s defined as the amount of analyte that provides a defined peak absorbance of 0 0044 The absolute detection hmlt, based on the varrabdlty of the blank (3a), 1s 314 pg This corresponds to a concentration detection hmlt of 628 pg 1 -I for SO-ml samples The precision of the method was evaluated by replicate determmatlons for 1 ng and 5 ng of mercury The relative standard devlatlons for ten replicate determmatlons are 2 0% for 1 ng and 18% for 5 ng of mercury The regression equatlon for the cahbratlon graph 1s y = 0 0386x + 0 0005 with a correlation coefficient of 0 9998 (n = 81, where x = the analyte mass (ng) and y = peak absorbance The calibration graph is linear up to 30 ng, corresponding to a linear range of three orders of magnitude The blank 1s found to be 5 pg Adsorptwnmechamsm Palladium 1s a very useful chemical modifier for ETAAS Recently, a great deal of mterest has be generated m the mechanism of palladium as a chemical modifier [25,27-29,32-361 The formation of mtermetalhcs between palladmm and the analytes has been ascribed as the primary mechanism by which palladnnn stab&es elements m ETAAS [33,34,36] The high efficiency and umversa1 effect of palladium modifiers are due to the fact that the palladmm metal can be easily formed from its compounds [291 Differences m the physical form of the palladrum obtained by

various reduction methods may influence the performance of the palladium modifier [28] The effectwe hydride trapping m a palladnun-coated graphite tube has been explained by the catalytic act&y of the palladnnn metal [24,25] In order to elucidate the mechanism of mercury trapping m the graphite-tube coated with PdCl,, the effects of the physlcal and the chemlcal forms of the palladium on mercury adsorption were Investigated Scanmng electron mlcrographs showed that the size and dlstrlbutlon of palladium particles on the graphite surface were dlfferent depending on the reduction method used [281 When ascorbic aad was used as a reducing agent, relatively small particles of palladium were obtained, but they were not evenly distributed, with the case of using hydrogen as a reducing agent, most of the palladnnn particles appeared to be clustered together, however, preheating a palladium solution to 1000°C resulted m a conslderable variation m palladmm particle size Although the physical form of palladmm produced by ascorbic acid reduction 1s dtierent from that obtamed by preheatmg the palladmm modifier to lWC, as mentioned above, no slgmflcant changes m the peak absorbance of mercury were found (see Figs lb and 2) This suggests that the physical form of palladium has no significant mfluence on mercury trapping To evaluate the role of the chemical form of palladium m mercury adsorption, XPS was used to identify the chemical state of elements Figure 1Oa shows the 3d XP spectrum of palladmm for pure PdCl, powder The 3d,,, and 3d,,, bmdmg energies are 343 50 and 338 12 eV, respectively, m good agreement wth the tabulated values [37] The 3d XP spectrum of palladium for the PdCl, deposit prepared by heating at 250°C on the graphite surface 1s depicted m Fig lob In comparrson with the 3d XP spectrum of pure PdCl,, no slgmflcant shift m the 3d bmdmg energies 1s observed, implying that PdCl, has not been converted mto the metal on the graphite surface at 250°C However, the 3d spectrum m Fig lOc, obtained from the PdCl, deposit heated at WC, slgmflcantly shifts to lower bmdmg energies of 341 12 eV for the 3d,,, and 335 78 eV for the level, correspondmg to the values for 3d,,,

X-P Yan et al /Anal

3d 5/z

320

113

Chm Acta 272 (1993) 105-114

338 12 343 50 B~ndlng

energy,

360

ev

Fig 10 Palladmm 3d x-ray photoelectron spectra for (a) pure PdCI, powder, (b) PdCI, deposit, heated at 250°C on the graphite platform, (cl PdCl, deposit, heated at 900°C on the graphite platform, (d) PdCI, deposit on which Hg was adsorbed at 250°C

metallic palladium [37] This indicates that PdCl, has been reduced to palladmm metal on the graphite surface at 900°C Therefore, the reduction of PdCl, to the metal by preheating or ascorbic acid treatment 1s responsible for the considerable decrease m the mercury trapping efficiency The much higher trapping efficiency for Hg by PdCl, than by the reduced palladium can be ascribed to the chemical reaction that occurred between PdCl, and Hg It has been reported that PdCl, could be used to remove mercury vapour m the ongmal vapour stream due to the reaction PdCl, + Hg + Pd + HgCl, [38,39] According to this reaction, palladium metal and mercury(I1)

chloride seem to be formed during mercury trappmg Figure 10d shows the 3d XP spectrum of palladium for a PdCI, deposit on which mercury vapour was trapped at 250°C In contrast with Fig lob, it can be seen that mtroducmg mercury vapour to the PdCl, surface (Fig 10d) makes the bmdmg energies of Pd 3d,,, and 3d,,, shift to 335 30 and 340 66 eV, respectively, close to those of metallic palladium [37] A bmdmg energy of 99 42 eV for the Hg 4f,,2 level was also observed by XPS (not shown), nearly corresponding to that of metallic mercury [37] It 1s possible that mercury@) chloride produced from the oxldatlon of mercury vapour by PdCl, was reduced to metallic mercury by the reducing atmosphere m the graphite tube during mercury trapping The posslblhty of mtermetalhc compound formation between mercury and palladium during mercury trapping still exists, although no ngmflcant evidence of this formation can be provided by XPS experments as the bmdmg energy shifts of 3d,,, and 3d,,, shown m Fig 1Oc and d are less than 0 6 eV Hedman et al [40] studied the electronic structure of some palladium alloys (Cu-Pd, Rh-Pd, Ag-Pd and Au-Pd) by XPS and found no significant shifts (not more than 0 3 eV) of Pd 3d bmdmg energies for these alloys Vesely and Langer [41] investigated the bmdmg energies of some simple compounds of Zn, Cu, Cd and Hg and pointed out that m general the chemical shifts of these elements were fairly small As no XPS results for the Hg-Pd system are available, it 1s assumed that the chemuxl shifts of Pd 3d and Hg 4f m this system are very small Hence it 1s difficult to confirm the presence of mtermetalhc formation between mercury and palladium during mercury trapping by XPS However, the fact that the reduced palladium did trap mercury vapour at temperatures of 150-300°C indicates that an mtermetalhc compound or alloy is probably formed between Hg and Pd The rapid oxldatlon of mercury vapour by PdCl, makes the PdCl, more favourable for mercury trapping than the reduced metal This work was supported tional Science Foundation 29170240

by the China Naunder Grant No

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