zirconia systems

365

Catalysis Today, 12 (1992) 365-373 Elsevier Science Publishers B.V., Amsterdam

QUANTITATIYE ALKALI

IONS

X-RAY PHOTOELECTRON / ZIRCONIA

SPECTROSCOPY

ANALYSIS

OF

SYSTEMS

D. GAZZOLI, A. CIMINO, G. MINBLLI and M. VALIGI Centro CNR SACSO, c/o DipChimica, WY)

Univ. “La Sapienza”, P.le A. Moro,S - 00185 Roma

SUMMARY Zirconia-supported alkali ions, containing Na, from 0.74 to 3.61 wt.%, and K, from 0.91 to 4.76 wt.%, have been studied by X-Ray Photoelectron Spectroscopy, X-ray diffraction and chemical analyses. The results show that the alkali ions remain dispersed on the zirconia surface. After treatment at 773 K in air, 5h, the Na containing samples maintain a good dispersion at low Na content, whereas for the more concentrated samples, agglomeration and incorporation in solid solution have been detected. The K based system shows neither agglomeration nor incorporation in solid solution of potassium ions. The XPS intensity ratios are compared with calculated values, according to different models, to account for the changes in surface morphology. INTRODUCTION X-Ray Photoelectron Spectroscopy (XPS) is applied extensively for the characterization of different classes of materials because of its ability to give quantitative information about the surface and near-surface composition. For supported catalysts the quantitative analysis can provide not only the average surface composition but also certain morphological information. In fact, the quantitative analysis is based on peak intensity ratios which depend on factors related to the features of the supported species (loading, dispersion) as well as to the characteristics of the carrier (specific surface area, pore distribution). The use of the intensity ratios as a tool for quantitative determinations requires the development of models that include the knowledge of terms related to the photoemission

process together with informations

regarding sample

chamcteristics. The present contribution defines the effect of parameters such as surface area and chemical composition on the XPS intensity ratios for systems obtained by sodium or potassium hydroxide dispersal on a ximonia support. One of the problems relevant to the surface chemistry and catalytic behaviour of the solid is the fate of the promoter after the thermal treatment of the “as prepared” catalysts. This problem is also addressed here with regard to the alkali ion promoter. EXPERIMENTAL The ximonia-supported alkali ions materials wem prepared by contacting hydrous zirconium oxide [l] with solutions of NaOH or KOH at pH from 6.0 to 13.5 for 72 h, followed by drying

1992 Elsevier Science Publishers B.V.

366

at 383 K in air for 24 h. Portions of the starting material, indicated “as prepax#

samples, were

submitted to heat treatments in air at 773 K for 5h. The samples are designated as ENax

or

ZKx(T) where: x is the. Na or K metal content (wt. %), T is the treatment temperature (in K) of the support before sodium or potassium uptake. The sample characterization was performed by chemical analyses, specific surface area (SA) and porn distribution measuremen ts, and by X-ray difi?a&on (XKD) analysis. The alkali ion content was determined by atomic absorption. For the thermally treated specimens, in addition to the total ion content, the amount of Na or K rinsable with water and those in the solid residue were also analyzed. XPS Meas-. The XPS spectra were collected by a Leybold-Heraeus LHS-10 spectrometer, interfaced to a 2113B HP computer, using Al Ka radiation (1486.6 eV, 12kV/2OmA). Constant pass energy mode (50 eV) was then operated. The pressure inside the analysis chamber was typically 5x10-8 Torr. The samples, reduced to a fine powder, were manually pressed onto a gokldecommd tantalum plate attached to the sample rod. The following regions were mcorded in a sequential manner: Na(ls), O(ls), Na(KLL), C(ls) and Zr(3d) for the ENax system; O(ls), K(2p), C(ls) and Zr(3d) for the ZKx(I’) samples. All core electron binding energy (BE) were referenced to the ZrQd5~) peak at 182.5 eV. Data analysis involved smoothing and non-linear

background subtraction. Peak areas were determined from the

integration of the appmpriate signal after smaxhing and background mmoval. Q.The

application of XPS to quantitative analysis of

solid surfaces has been the object of several reviews [2,3.4]. Intensity ratios of components in a given specimen rather than absolute intensities ate generally utilized to avoid the ambiguity in instNmentalterms. Among the different models proposed to represent supported systems [4, 51 the following ones am considered (1) Kerkhof-Mouliin (K-M) [5]: the porous support (s ) consists of stacked sheets with thickness evaluated by the SA value, and the density of the carrier (rs), t = U(rs. SA). The supportedspecies or “pmnorer” (p ) is described as uniformly distributed on the surface of the support, in&ding the internal surface of the sheets: $/I~ = p/s’. F. sp/ss - bin - [(l+ e-bzW(l- e-k01

(1)

where p/s = promoter / support atomic ratios; sp, ss = cross sections ; lps. lss = escape depth of electrons from promoter and support respectively through the support; bl= t/Iss ; b2 = t/lps ; F= instrumental factors. (2) w

over&r

on a m

[2]: no attenuation due to the supported species

is considered. The sample is described as a monammically d@ersed promomr phase on a semiinfinite support : Ipi& = F . sp/ss . s’/& Is

(2)

367

with s’, the promomsurface density (atom.%m2). (3) Attenuatinp [2]: the overlayer of supported species with thickness 11t’ ” is present over a semi-infinite support causing an exponential attenuation. when Q
(3)

131:the model describes the dispersed phase-carrier system as a homogeneous

solid solution: ‘p/Is = F . sp/ss . np/ns. II&

(4)

be m and support atomic concentration ratio The surfacesensitivity factor method was also considered, as proposed by Wagner [a],

with X@S,

and deteunined on the instrument used ln this study PI. The photoionixation cross section s were taken from the Scofield’s theoretical values of 181: 1 values were calculated as proposed by Seah for inorganic com&unds

[9] and the

instrumental factor correction was applied using a detectability efficiency dependence of the kinetic energy as l/KE [ 10, 111. RESULTS

AND DISCUSSION

The Na( 1s) line shape is symmetrical and its binding energy (BE) was found for all ZNax(383) samples at 1072.6 f 0.3 eV, both before and after thermal treatments. The kinetic energy (KE) value of the Na(KLL) Auger emission enables us to calculate the modified Auger parameter a’ = KE + BE whose experimental values 2061.5 f 0.2 eV agrees closely with Na2CO3 values [ 121. The sodium analytical content, SA and surface concentration values, s’, are reported in Table 1. Fig.1 shows the KPS intensity ratios, I(Na)/I(Zr), plotted as a function of the surface concentration. In the same figure, the calculated intensity ratios predicted by the K-M model [7] are also reported. This is done after transformation of the overall atomic concentration p and s into surface concentration, atoms/nm2. A linear correlation was found by increasing sodium loading and the agreement between experimental and calculated values is excellent. The agreement is also excellent if the intensity ratios are plotted as a function of the atomic ratio N(Na)/N(Zr),

as proposed by K-M. The surface-sensitivity

factors method [lo] gives

satisfactory results. All these results am indicative of a high dispersion of the sodium ions on the xhconia surface.

363

TABLE 1 Analytical sodium content, SA (mzg-1) and surface concentration, s’ (Na/ atoms nm-2). values for the ZNax(383) specimens. Samples W(383)

thermallyueatedat773K

“as prepared” S’

SA

-

360

lWt0t

inair5h

tWrinS.

[NGeS.

SA 60

ZNaO.74(383)

0.558

347

0.92

0.92

0.08

54

ZNaO.92(383)

0.757

318

1.15

1.18

0.09

55

ZNa1.09(383)

0.924

309

1.39

1.34

0.15

47

2.08

0.49

39

3.62

0.80

44

ZNa1.91(383)

1.793

279

2.29

ZNa3.07(383)

3.045

264

3.89

ZNa3.61(383)

4.075

232

4.42

42

0

s’ (Na / atoms nmm2) Fig.1. XPS intensity ratios I(Nals)/I(Zr3d) ys sodium surface concentration, s’, for o - experimentah x - calculated values ZNax(383) “as preparedkmples:

Table 1 illustrates the results of the sodium concentration

determinations

and SA

measurements after thermal treatment. When ZNa(383) samples are heated in air at 773 K for 5h. several phenomena occur, both to the supported species and to the carrier. Chemical analyses show that only part of the sodium ions are present on the support surface. In fact, not all the sodium is removed by tiater extraction, since the amount of sodium in the solid residue increases with the sodium content. In addition to the ZrO2 crystalline modifications (tetragonal and/ or cubic, monoclinic) traces of Na20 were found by XRD in the more concentrated sample,

369

ZNa3.61(383). The measuremen toftheunitcell pammeErofthissamplegaveavalueofg= 5.114 f 0.002 A. This value was higher than that of pure cubic ZTO~, 4 = 5.085 A [ 131, which in~~sthatpartofthc~~ionsarcincorporatedinsolidsdution. In Fig. 2 the expcrbmmtal intensity ratios, I(Na)/I(Zr). am plotted against the sodium surface concentration (atom&m&, calculated from the total Na content. The thermally treated samplescannot

beoonJidrredasathinwalledpoaousmaterial,asthetvalueisof

the6.Onm

order. One hypothesis is that if the Na ions axe still dispersed over the surface, the model of a non-muating

overlayer on a thick support (model 2) would be mace appmp&te. ‘Ihe results of

the calculation are then plotted in Fig.2. The agmement between experimental and calculated values appears to be gocd, except for the most concehtrated samples. This deviation is indicative of Na+ agglomexation and/or of solid solution formation.

1.60 g L

1.20

g

0.80 0.40 0.00 0.0

5.0

10.0

15.0

20.0

25.0

SO.0

s’ (Naw / atoms nme2)

Fig.2. XPS intensity ratios, I(Na)/I(Zr), u . Natot surface concentration (atoms nm-2) for ZNax(383) samples treated at 773 Kin air, 5h: o - exp$mena x - calculated values; + calculatedval~ (apladon 5)

The incorporation of sodium atoms in the ZrO2 structure is also supported by XPS measurements on the ZNa3.61(383) sample after rinsing with water. The experimental intensity ratio obtained is, in fact, in agreement with the value calculated by the solid solution model. On the basis of the above results a model can be set for the concentrated ZNa3.61(383) sample heated at 773 K. The Na+ ions are formed in 3 motphologically distinct regions: a- solid solution in ZXO~; b- as an overlayer anchored to the 2102 surface, g- as a separate Na20 phase. The intensity ratio I(Na)/I(ik) can be calcukdastbetiofthe3contributions: QJaKI(zr) = [I(NaKI(zrlla + l?(Na)/I(Wlb + [I(Na)/r(zr)lg

(5)

A quantitative assessment of the relative amounts can be given on the following basis. The (220) Na20 reflection intensity relative to the (220) ZrO2 reflection gives an order of magnitude of 1% by wt. for the Na20 phase The detectability of the Na20 gives a lower limit of 5 nm for the crystal sire, and the broadening enables us to estimate 10 nm as an upper limit. If a value of 7 nm is assumed, we can calculate the number of Na20 particles (as small cubes) present. Consequently, the relative contribution of each region can be calculated as (0.09)a + (1.002)b + (0.05)g = 1.14 to be compared with the experimental value of 1.25 f 0.1 (two determinations). Agreement is fairly good, and it is quite acceptable in view of the uncertainties.~ The g term is also seen to have little weight, so that a change from 7 to 10 nm for size changes the calculated value only slightly. One significant finding is that the morphological picture deduced from the different techniques allows us to justify the decrease of the KPS intensity ratio. TABLE 2 Analytical potassium content. SA (m2g1) and surface concentration, s’ (atoms nm-2), values for the ZKx(383) system. Samples

“as prepared” S’

ZrU2(383)

SA

thermally treated at 773 K in air 5h Kltot

Klrins.

[Klres.

SA

-

380

60

ZK2.16(383)

0.91

365

2.31

2.26

0.10

99

ZK2.61(383)

1.20

335

2.71

2.54

0.17

90

ZK4.00(383)

2.00

307

4.32

3.84

0.42

56

ZK4.76(383)

2.40

306

4.93

4.43

0.47

44

ZK x(383) Svstem One of the problems in the K(2p) peak ama determinations is the partial overlapping between the K(2p3/2) component and the C( 1s) contribution from carbonate lie species. This effect was into account, by subtracting the background from the whole K(2p) + C(ls) region. The intensity due to carbonate species was then subtracted from the total spectrum by a curve fitting procedure. II 11 Table 2 shows the analytical results, the SA determinations, and surface concentration values, s’. Fig.3 shows the experimental intensity ratios, I(K(2p)/I(%3d),

as a function of

potassium surface concentration, s’ (atomsAtm2). A linear correlation between intensity ratios and potassium surface concentration was then observed. The results obtained by applying the KM model [7] are also shown in the same figure. A good agreement between experimental and calculated values is seen for the mom diluted samples, whereas at high potassium loading a slight surface enrichment is observed.

371

s’

(K /

atoms

nm*)

Fig.3. KF% intensity ratios, I(K2p)/I(Z&l). ys potassium surface concentration s’ (atoms nm-2): 0 - experimental; x - calculated values The ssme behaviour was detected when the intensity ratios were calculated by the sensitivityfactors method, using values both hrn

[8] and [9]. This tinding can be explained by the ease of

carbonation of the KOH species, which are thought to be uniformly dispersed on the xirconia Surface.

The analytical and SA (m2g1) determinations are reported in Table 2. After thermal treatment in air at 773 (Sh). no phases other than those of Zroz (tetragonal and monoclinic), and

no evidence of potassium incorporation in solid solution was found Only a small amount of potassium species was retained in the solid residue (Table 2). Fig.4 represents the linear correlation between the experimental I(K2p)/I(Z&l),

intensity ratios,

and the potassium surface concentration s’. The values calculated by model (2)

are also shown. As in the case of sodium, the agreement between experimental and calculated values was not satisfactory for the mom concentrated samples. However, a difference exists between the two cases. For the Na case, segregation into Na20 (and ensuing carbonation of Na20)

particles occurs, together with the formation of solid solution. For K, neither

agglomeration into particles nor solid solution were dekcted It is surmised that in both systems a lowering of surface free energy drives the alkali onto the surface, while the K ions remain more widely dispersed.

372

0.24-

0.080.00

I I I I / I I1 0.0 5.0

I, 11 10.0

I I, I I I I 15.0 2 I.0

s'(K/atoms

Fig.4. XPS intensity ratios, I’Kzp,‘)/Iz)d~ . (atomsnm-2): 0 _expcrimntal;

nm*)

$as$un

surface concentration

CONCLUSIONS The main aspects of this work can be summa+&

as follows. In the ZNax(383) system, the

“as prepared” samples show sodium hydroxide species highly dispersed on the surface of the zirconia support, as shown by the match between experimental and calculated XPS results. Thermal treatmeuts at 773 K in air modify the sample structure, causing: i) segregation of some sodium ions, such as Na20 (and reactions to yield carbonates and hydroxide) and ii) incorpomdon in the Z102 lattice. The XPS quantitative analysis on these samples supports these points: a good agreement between calculated and experimental values is obtained when the contribution of the incorporated and segmgatal species other than that coming from the d@ersed ones is considered. As for the ZKx(383) system on the “as prepared” samples, potassium hydroxide species are dispersed on the carrier surface, although a small enrichment seems to be present with increasing potassium loading. After thermal treatments at 773 K in air, no phases besides 2~02 ones, and no potassium ions agglomeration as K20 are detected. The quantitative analysis indicates a dispersion of the potassium species on the zirconia surface. Some degree of enxichment at high potassium content is also observed, which may be explained by the ease of formation

ofcarbonatespecies on the hydroxide

surface.

REFERENCES 1 2

A. Cimino. D. Cord&hi, S. De Rossi, G. Ferraris, D. Gazzoli, V. Indovina. G. Minelli, M. Occhiuzzi and M. Valigi, J. Catal. 127 (1991) 744 -760. C.S. Fadley in: “Electron Spectroscopy Theory,Techniques and A plications”, C.R. Brundle and D.A. Baker Eds., Vol. 2, Academic Press. New York, 197TI,2-145.

373

A. Cimino, Mater. Chem. and Phys. 13 (1985) 221-241. JB. Fulghum and R.W. Linton, Surf. Inte&cc Anal. 13 (1988) 186-192. F.P.J.M. Kerkhof and J& Moulijn, J. Phys. Chem. 83 (1979) 1612-1619. ~.D~~JJiIc&x~Spcctrpsc.clat Phenom. 32 (1983) 99-102. CD. Wagner, J.Electron Spcctmsc. Relat. Phenom., 8(1976) 129-137. .M.P. Se&, Surf. Interface Anal. 9 (1986) 85-98. M.P. Se& Surf. Interface Anal. 2 (1980) 222-239. A.Van Eenbergen and ELBnmix, J.Electron.Specmxc. Relat. Phenom. 33 (1984) 51-60. CD. Wagner and A, Joshi, J.Eleqmn Spectrosc. Relat. Phenom. 47 (1988) 283-313. &&bison. R Kershaw, K Dw@t and A. Weld, J. Solld State Chem. 72 (1988) 131-