J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 895-898 Effect of Potassium on the Surface Potential of Titania Dorninique Courcot, Leon Gengembre, Michel Guelton and Yolande Barbaux Laboratoire de Catalyse Heterogene et Homogene, URA C.N.R.S.402, Ba^t.C3, U.S.T.LiIIe. F-59655, Villeneuve d'Ascq, France Barbara Grzybowska" Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krako w 30239, Poland The surface potential, x, of several commercial titania powders has been measured by the vibrating condenser method in the temperature range 100-450"C in air. The K impurity detected by XPS on some of titanias lowers the x value of TiO, considerably. A series of K-doped TiO, anatase samples containing from 1.1 to 11 K atoms nm-* was prepared and examined by surface potential and XPS techniques.The surface potential of anatase decreases linearly with increasing potassium content up to ca. 2.4 K atoms nm-2, and then remains constant. The surface potential of K-doped samples varies to a smaller extent with temperature compared with undoped anatase. On the basis of the surface potential and XPS data obtained, it is suggested that the mode of K deposition on the surface of anatase changes at a coverage of about 2 K atoms nm-2. Using potassium as an additive to TiO, ,which is essential in the production of white titania pigments, can have an unfa- vourable effect on catalytic performance when commercial titanias, containing potassium as an impurity, are used as supports for vanadia catalysts for oxidation of hydrocar-bon~.'-~On the other hand, potassium is often added as pro- moter to industrial metal or oxide catalysts dispersed on support^.^*^ Characterization of surface properties of TiO, with added potassium is of great interest in catalytic studies.Recent work on this subject has shown considerable modifi- cation of the acidic properties of K+-doped anata~e~9~ and of the electronic properties when K atoms are adsorbed on a monocrystalline TiO,( 100)(r~tile).~ The effect of K addition on the reactivity of anatase in chlorination reactions has been also rep~rted.~ In the present work, surface potential and XPS measure-ments have been applied to characterize various commercial TiO, supports and to study the effect of K addition on the surface properties of anatase powders.Experimental Several commercial TiO, preparations were used in the pre- liminary experiments. Their characteristics are given in Table 1. TiO, Tioxide (Tiox-1), pure anatase (within the accuracy of the XRD and Raman methods) and without surface impu- rities, as verified by XPS, was selected for further studies on the effect of potassium. Before the introduction of potassium onto its surface, the sample was calcined for 4 h at 700°C. Table 1 Characteristics of some commercial titanias specific surface rutile content" producer area/m2 g-' (%) P-25 ET-1 (Eurotitania) Tiox.-1 Degussa Tioxide Tioxide 53.2 47 27.4 39.6 10.4 0 AN-PO AN-PI Chemical Works 8.4 6.9 4.7 trace RT-P Police, Poland 7.4 96 Ti-Pro1 Prolabo 8.7 4.7 Calculated from XRD data using the formula:26 rutile = [I1 -1/ (1 + 1.261,/IA)] x 100%, where I, and I, are the intensities of the diffraction peaks at d = 3.53 A and 3.26 A in diffractograms of anatase and rutile, respectively.The specific surface area decreased slightly to 23.8 m2 g-'; however, no rutile was observed after the calcination. Different amounts of K, varying from 1.1 to 11 K atoms nm-(which corresponds, respectively, to surface coverage of 1 K atom 100 A-2 to 1 K atom 10 A-2) were introduced by adding appropriate quantities of KHCO, (Fluka purissima) solution to a suspension of the support in water, followed by evaporation under vacuum at 50 "C,drying at 95 "C and cal- cination for 4 h at 500°C in a stream of air.The specific area of the K-doped samples did not vary within 10%from that of pure Tiox-1 after additional calcination. The K-promoted samples are denoted in the text by symbol K-X, where X indicates the calculated number of K atoms nrn-,. XP spectra of the samples were recorded with an AEI ES 200B spectrometer. The atomic ratio of the elements on the surface, n,/nB, was calculated from the intensity ratio IJIB with the formula: values of CT being taken after Scofield." The surface potential was measured by the vibrating con- denser method.' '-14 The method involves recording the Volta potential difference AV between two plates of a con- denser made of the sample and the reference electrode.AV is equal to the difference in the work functions of the solids which form the two plates of the condenser. As the work function of the reference electrode is constant under the experimental conditions adopted, the variations in A V reflect the changes in the work function of the sample. In the cases in which the structure of the bulk of the solids is not changed by chemisorption of gases or surface doping, the work func- tion is a measure of the surface potential, defined as the dif- ference between the electrostatic potential at the surface immediately outside a solid and the internal electric potential. The surface potential represents a potential barrier which has to be surmounted by an electron distribution in the surface layer and depends on the dipole structure of the solid surface.The apparatus used in the present work consisted of a cell made of stainless steel containing two electrodes mounted vertically; the cell was connected to a controlled gas-flow system. The sample electrode was constructed of a stainless- steel plate 20 mm x 30 mm and 1.5 mm thick, covered on one side with gold foil, on which a sample of the powder under study was deposited from a suspension in amyl alcohol. The heating Thermocoax element and thermocouple for recording the sample temperature were placed inside the steel plate. The vibrating reference electrode facing the sample electrode at a distance of 1 mm was made of a graph- ite plate, 3 mm thick. The choice of the reference electrode was a subject of a separate paper.The work function of the reference electrode was constant under measurement condi- tions. The measurements were performed under a flow of 20 vol% 0, in Ar in the temperature range 50-450°C. The values of surface potential, x, reported in the text are relative to the graphite electrode, an increase in x indicating that the surface becomes more negatively charged. The surface poten- tials were reproducible for a given sample within 5 mV. Results and Discussion Commercial Titanias Table 2 gives the surface potential data for commercial TiO, powders and compares them with the impurity contents on the surface as determined by XPS. The x values at 450°C, ~450,are much lower (by ca.1 V) for those titanias which contain impurities than for the preparations for which no surface contaminants were detected. At the same time, x varies only slightly with temperature for the contaminated samples, as indicated by the difference Ax450-200 (column 3 in Table 2). Washing one of the samples (AN P1) with water for 12 h, which removed most of the potassium from the surface, brought about an increase in x and Ax. This suggests that the differences in the x values observed between different titanias are due in the first place to the K impurity. Addition of small amounts of P fin the form of H(NH4),P04] to the sample of Tiox-1, free of contaminants, did not lead to any changes in the values of x450. Kdoped Anatase Tiox-1 XPS Measurements Table 3 lists binding energies (fib) of different elements in the series of K-doped TiO,-Tioxide samples, the C 1s signal with E, = 285 eV being taken as the reference.All the samples exhibited a small peak at Eb = 289 k0.5 eV, most probably due to carbonates; its intensity, however, did not vary signifi- cantly with increasing K content and was similar to that of adventitious carbon observed on pure Ti0,-Tiox-1. The binding energies of the Ti 2p and 0 1s levels are slightly lower for K-containing samples than for undoped TiO, , indicating some electron transfer from potassium to the support, which leads to an increase in the anionic charac- ter of oxygen and a decrease in the positive charge on tita- nium atoms. A similar effect was observed in ref.9. The binding energy of K 2p decreases when the amount of pot- Table 2 Surface potential values and surface impurities for com- mercial titanias impurity (atoms per 100 Ti atoms) 1450 Ax450-200 /mV /mV K P Ca P-25 1840 400 ET-1 (Eurotitania) 1740 440 ---Tiox.-1 1990 540 -AN-P1 310 130 9 3 -AN-P 1 washed 1100 250 2 3 AN-PO 210 70 9.2 6 1.1 RT-P 40 0 9.2 4.9 0.5 -Ti-Pro1 360 60 9 7 J. CHEM. SOC. FARADAY 'TRANS., 1994, VOL. 90 Table 3 Binding energies in Kdoped anatase Tiox-1 EdeV K content" /atoms nm-2 Ti 2p K 2p 0 1s ~ Tiox.-1 0 459.1 - 530.4 K-1 1.1 459.2 293.2 530.2 K-2 2.1 458.5 292.8 529.9 K-2 washedb 459.3 293.3 530.4 K-4 4.0 458.7 293.1 530 K-11 11.5 458.8 292.6 529.8 ~ ~ " Introduced onto the Tiox-1 surface. * Sample K-2 after washing with doubly distilled water at 25 "C.assium on the surface increases, suggesting some loss of its cationic character at higher potassium coverage. The variation in the surface atomic ratio K: Ti, derived from the XPS intensity data, with the total amount of pot- assium introduced is presented in Fig. 1. A break in the curve observed at a K content of ca. 2 K atom nm-2 suggests that the mode of potassium dispersion may change in this region. A similar course of changes in surface composition was observed when oxides (e.g. V,05, MOO,) were deposited on supports and has been ascribed to the change from mono- layer two-dimensional VO, or MOO, species (at low contents of the deposited oxide) to three-dimensional microcrystals of V205 or MOO^.'^-'^ It cannot be excluded that at higher K content the atomic dispersion of this additive changes to the dispersion in form of the K,O phase.Note that Busca and Ramis' ascribed the complete modification of the titania surface, observed in an FTIR study for the sample containing one K+ per 0.11 nmz to the formation of a 'surface K20 phase '. Surface Potential Studies Fig. 2 shows the change in the surface potential in air with temperature for the K-doped anatase Tioxide samples. The surface potentials are lower for K-containing samples than for pure TiO, over the whole temperature range studied. For pure TiO, the x values increase significantly with tem-perature, indicating an increase in the negative charge on the surface.Such an increase has previously been ascribed to transformation of the chemisorbed oxygen species from less to more negatively charged forms, e.g. 0, + e--+0; or ~0-+ e-02-,.19,20 it could be also due to an increase in the total amount of chemisorbed oxygen with temperature. 2 K atoms nniv2 Fig. 1 Variation of the surface atomic ratio nK : nTi with total pot- assium content for K-doped Tiox-1 anatase J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1750 *---*-*--*---+----+ /*' Fig. 2 Change of the surface potential with temperature for a series of K-doped Tiox-1 anatase samples: A, Tiox-1; 0,K-2 washed; a, K-1; +, K-2; 0,K-4; A, K-11 The changes of x with temperature are smaller for K-containing samples; for the sample K-11, which has the highest potassium content, a small decrease in x at higher temperatures is observed.It appears that the presence of potassium also affects the chemisorption of oxygen on the anatase surface: it could block the oxygen adsorption centres or hinder the transform- ation of the species shown above. Separate studies on the type of oxygen species and on oxygen chemisorptive proper- ties are in progress to decide between these possibilities. The lowering of the surface potential on addition of K can be explained by the change of sign of the dipole in the topmost layer of Ti02, potassium ions K+ covering the negatively charged surface oxygen anions.Injection of elec- trons from potassium to titania, changing the density of current carriers in the space-charge region of TiO, and hence the value of potential barrier (considered as the cause of work function changes when K atoms are adsorbed on semi-conductors8) seems less probable in our case since potassium was introduced in a cationic form. Electron transfer cannot be completely excluded in view of the changes in binding energies for Ti4+ and 02-ions described above. In Fig. 3, the values of x at 450°C are plotted as a function of the K content. At low doping levels of potassium, the potential decreases linearly with increasing K coverage up to ca. 2.5 K atoms nm-2 and changes only slightly with further increase in the K content.This corresponds to about 18% of the total surface occupied by terminal oxygen atoms on the anatase surface, ca. 14.2 atoms nm-2 for the 001 cleavage plane or 16.1 atoms nmV2 for the 110 plane. Note that a break in the nK :nTi us. K content plot (Fig. 1) is observed at similar value of potassium coverage. It appears that potassium ions are adsorbed on certain particular centres on the anatase surface which contribute considerably to the surface potential. Once these centres are filled, the mode of dispersion of K changes. Identification and localization of these centres remains a matter of speculation. They can be ascribed for instance to 897 1600 1200 >E 800 \::x" 400 0 -400 0246810' K atoms nm-2 Fig.3 Dependence of surface potential on total K-content for K-doped anatase Tiox-1 (T = 45OoC, stream of 20 vol0/0 0,-Ar, 10 dm3 h-') unsaturated 0 atoms on the 001 plane, coordinated to two Ti atoms, carrying a charge of -& and located 0.041 nm above this plane.21 Localization of potassium atoms on the OH groups of anatase, replacing an H atom, can be also envis- aged; some of the many hydroxy groups of Ti02 remain on the surface even after it is heated at temperatures >250 0C.22-24 It is interesting to note that a similar value was found for the number of sites on an anatase surface, which is capable of binding a vanadia phase strongly in V,O,/TiO, catalysts. Strongly bound VO, species (2.6 VO, nm-2) exhibits only dehydrogenating properties in isopropyl alcohol decomposi- tion and is the only VO, species that remains after washing the soluble V,O, with aqueous NH, .References 1 A. 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