Segregation of Deuterium and Hydrogen on Surfaces of Palladium

Chemical and structural components of work function changes in the process of palladium hydride formation within thin Pd film. R. Duś , R. Nowakowski...
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Langmuir 2000, 16, 584-591

Segregation of Deuterium and Hydrogen on Surfaces of Palladium Deuteride and Hydride at Low Temperatures R. Dus´* and E. Nowicka Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224 Warszawa, Poland Received July 5, 1999. In Final Form: September 28, 1999 The distribution of deuterium (hydrogen) between the surface and the bulk of palladium deuteride (hydride) within the interval of temperatures 78-160 K was studied by simultaneously measuring the work function changes and D2 (H2) pressure in the process of thin PdDy (0.005 < y < 0.855) and PdHx (0.005 < x < 0.900) film formation. It has been found that at 78 K the concentration of the positively polarized, atomic deuterium (hydrogen) adspecies on the outer surface of palladium deuteride (hydride) approaches 1.9 (1.5), while the atomic ratio D/Pd (H/Pd) in the bulk does not exceed 1. The work function strongly decreases (by ∼2400 mV for PdD0.785). Surface segregation of deuterium predominates that of hydrogen. Above 120 K an increase of the work function is observed in the process of PdDy (PdHx) formation. This is interpreted as deuterium (hydrogen) positioning below the surface image plane within the first layer of Pd atoms. Under these conditions the surface concentration of deuterium (hydrogen) adatoms does not differ significantly from that in the bulk, approaching 1.

Introduction For many transition metals (Me) interaction with molecular hydrogen (deuterium) can lead to the formation of hydrides MeHx (deuterides MeDy) if critical conditions of temperature and pressure are reached.1,2 Transition metal hydrides (deuterides) are nonstoichiometric compounds with hydrogen (deuterium) distributed along interstitial sites of a metal lattice1,2 and held in place by a combination of covalent and ionic bonding.2-4 For palladium hydride and palladium deuteride the critical temperatures are 565 and 549 K, while the critical pressures reach 19.7 atm H2 and 35.0 atm D2, respectively.5-7 H2 (D2) equilibrium pressure Peq over hydrides (deuterides) strongly depends on hydrogen (deuterium) content, often expressed as the atomic ratio H/Me (D/Me) and temperature. For PdH0.5 (PdD0.5) at 298 K the equilibrium pressures reach ∼1.07 × 103 Pa and ∼5.20 × 103 Pa respectively.8,9 The Peq value for palladium hydride (deuteride) drops sharply with decrease of temperature, and as we have found it is of the order of 10-4 Pa at 78 K.10,11 This allows use of an ultrahigh vacuum (UHV) apparatus to obtain a clean surface of palladium and study “in situ” surface phenomena which occur in the process of Pd transformation into PdHx (PdDy). It has been found that at low temperatures (78-90 K), surface phenomena in the process of hydride (deuteride) formation in thin films of several transition metals (1) Hydrogen in Metals. Allefeld, G., Vo¨lkl, J., Eds.; Topics in Applied Physics; Springer: Berlin, Heidelberg, New York, 1978; Vol. 29. (2) Fukai, Y. The Metal-Hydrogen Systems; The Springer Series in Material Science; Springer: Berlin, 1993; Vol. 21. (3) Nagel, H.; Goretzki, H. J. Phys. Chem. Solids 1975, 36, 431. (4) Stalin˜ski, B.; Coogan, C. K.; Gutowsky, H. S. J. Chem. Phys. 1961, 34, 1191. (5) Gillespie, L. J.; Glastaun, L. S. J. Am. Chem. Soc. 1936, 58, 2565. (6) Gillespie, L. J.; Downs, W. R. J. Am. Chem. Soc. 1939, 61, 2496. (7) Friske, H.; Wicke, E. Ber. Bunsen-Ges. Phys. Chem. 1973, 77, 50. (8) Wicke, E.; Nernst, E. Ber. Bunsen-Ges. Phys. Chem. 1964, 68, 224. (9) Lewis, A. F. The Palladium-Hydrogen System; Academic Press: London, New York, 1967. (10) Dus´, R. Surface Sci. 1973, 42, 324. (11) Dus´, R.; Nowicka, E.; Wolfram, Z. Surf. Sci. 1991, 216, 1.

(for example, Pd, Ti, V, Nb) exhibit some common features:11-16 (I) At the beginning of adsorption an atomic, surface stable, negatively polarized adspecies (negative pole of the dipole pointing away from the surface) increasing work function appears, referred to as β-. (ΙΙ) With increasing coverage a positively polarized, atomic form of the adsorbate, referred to as β+ occurs. The β+ adspecies are not stable on the surface as the β- are, but rather quickly incorporate into the bulk. This leads to hydride (deuteride) formation. The incorporation is not a complete one, but with the progress of the reaction, the concentration of the β+ adspecies remaining on the outer surface also increases, leading to a strong decrease of the work function Φ. (III) At high concentration of hydrogen (deuterium) in the bulk of MeHx (MeDy), under H2 (D2) pressure above 10-2 Pa, on surfaces of the investigated hydrides (deuterides) there exists a weakly bound, positively polarized deposit desorbing during an isothermal evacuation. Redosing of hydrogen (deuterium) completely rebuilds the adsorbate. There is evidence that this is a unique form of weakly adsorbed atomic hydrogen (deuterium).11,12,16 The presence of the induced, positively charged hydrogen adspecies on the surface of a transition metal precovered with an amount of H adatoms was predicted theoretically in Grimley’s model.17 The induced positively polarized hydrogen adspecies were recently found experimentally also on a Pd (210) single-crystal surface by Muschiol, Schmidt, and Christmann.18 Measuring work function changes ∆Φ caused by exposition of palladium to H2 at several constant temperatures, these authors observed at 90 K an increase of Φ followed by its decrease. This corresponds well to the formation of the adspecies which we have named β- and β+. Isothermal lowering of (12) Nowicka, E.; Dus´, R. J. Alloys Compd 1997, 253/254, 506. (13) Dus´, R.; Nowicka, E.; Wolfram, Z. Surf. Sci. 1992, 269/270, 545. (14) Dus´, R.; Nowicka, E.; Wolfram, Z. Langmuir 1998, 14, 5487. (15) Dus´, R. Surf. Sci. 1975, 52, 440. (16) Nowicka, E.; Dus´, R. Langmuir 1996, 12, 1520. (17) Grimley, T. B. In Chemisorption; Butterworth: London, 1957. (18) Muschiol, U. I.; Schmidt, R. K.; Christmann, K. Surf. Sci. 1998, 395, 182.

10.1021/la990870y CCC: $19.00 © 2000 American Chemical Society Published on Web 11/13/1999

Segregation of Deuterium and Hydrogen

H2 pressure at 90 K resulted in an increase of Φ (by ∼160 mV) caused by desorption of the weakly bound, positively polarized adsorbate. Investigation of TD spectra of the adsorbate obtained in the course of H2 interaction with single Pd crystal planes (210),18 (110),19 and (111)20 within the temperature interval 115-140 K revealed formation of a subsurface hydrogen at pressure as low as 10-4 Pa. The question arises as to the relation between β+ concentration on the surface of MeHx (MeDy) and hydrogen (deuterium) content in the bulk of hydrides (deuterides). We have reported recently that in the course of thin vanadium hydride film formation at 78 K the surface concentration of β+ approaches 2, while the average concentration H/V in the bulk does not exceed 0.25.14 On the other hand it is well-known that VH2 can be obtained in bulk at elevated temperature under H2 pressure of several atmospheres.1 A low-temperature deviation in hydrogen distribution in vanadium can be caused by hindering from the phase transitions, which occur in the reaction of VHx formation and demand some activation energies. Various phases of VHx are characterized by different contents of hydrogen. The system Pd-H2 (D2) is of special interest since it is well established that the isolated ground state 4d10 Pd atoms react spontaneously at low temperature with molecular hydrogen (deuterium) to produce PdH2 (PdD2) complexes,21 or the complex ions PdH2+,22 while atomic concentration of hydrogen (deuterium) in the bulk of hydrides (deuterides) does not exceed H(D)/Pd ) 1, despite applied H2 (D2) pressure as high as 104 atm.1,2 Yet phase transition does not occur in the reaction of PdHx (PdDy) formation. Preliminary experiments have shown that a concentration of the β+ adspecies approaching 2 can indeed be obtained at 78 K on a PdH0.8 surface.23 The aim of this work was to study deuterium segregation on the PdDy surface and determine the isotope effect for this phenomenon as a function of temperature. A strong isotope effect was previously observed11 in work function changes during PdDy and PdHx formation at 78 K. It has been an open question however whether this phenomenon is associated with a difference in the dipole moments of deuterium and hydrogen adspecies or with the difference in their surface concentration. The isotope effect in segregation on the surface could be expected since the H2 (D2) equilibrium pressure over palladium hydride and deuteride of the same H/Pd and D/Pd ratios differs.1,2,9 Experimental Section To carry out the above-mentioned studies one has to use a palladium sample which can be converted into palladium hydride (deuteride) within the whole bulk under the applied experimental conditions, which means at low hydrogen (deuterium) pressure. Single Pd crystals are strongly resistive against deuteride (hydride) formation,18,20 and a large hysteresis in the pressurecomposition relationship is observed at PdDy (PdHx) formation and decomposition. On the other hand, previous experience has shown that thin Pd films deposited under UHV conditions can be successfully applied.10,11 The present investigation also required an experimental method capable of distinguishing between various hydrogen (deuterium) adspecies over a wide pressure interval, 10-7-101 Pa, at temperatures within the (19) Behm, R. J.; Penka, V.; Cattania, M.-G.; Christmann, K.; Ertl, G. J. Chem. Phys. 1983, 78, 7486. (20) Gdowski, G. E.; Felter, T. E.; Stuhlen, R. H. Surf. Sci. 1987, 181, L147. (21) Ozin, G. A.; Garcia-Prieto, J. J. Am. Chem. Soc. 1986, 108, 3099. (22) Knight, L. B., Jr.; Cobranchi, S. K.; Herlog, J.; Kirk, T.; Balasambramanian, K.; Das, K. K. J. Chem. Phys. 1990, 92, 2721. (23) Dus´, R.; Nowicka, E. Prog.Surf. Sci. 1998, 59, 289.

Langmuir, Vol. 16, No. 2, 2000 585 boundaries (78-160 K) thermodynamically determined by the possibility of PdHx (PdDy) formation under the applied conditions. These requirements can be fulfilled while simultaneously measuring the work function changes ∆Φ and pressure P.10-15 Precise ∆Φ measurements allow determination of the electrostatic features of deuterium (hydrogen) adspecies and elucidate the dynamics of elementary surface processes. Simultaneous pressure measurements allow correlation of these features with the uptake of the adsorbate on the basis of volumetric calculations if the area of the sample is large enough and the volume of the reactor is small. Application of a short response time detection circuit is important, since this allows differentiation between deuterium (hydrogen) uptake due to fast adsorption and the slower absorption that follows. 1. Apparatus and Samples. In the present studies an UHV apparatus made of Pyrex glass, capable of routinely reaching pressures (1-2) × 10-8 Pa, was employed. The static capacitor method was appied to measure work function changes.24,25 The scheme of the UHV apparatus and the static capacitor cell is shown in Figure 1. The samples were prepared by complete evaporation under the above-mentioned pressure of a fine Pd wire (Johnson-Matthey, Grade I) of known weight, wound around a tungsten heater, and deposition of the thin film on the wall of the cylindrical static capacitor maintained at 78 K. In this way a thin film of known weight (containing a known total amount of Pd atoms) was obtained. The films were next sintered at 320 K for 20 min under UHV conditions. Their geometrical area was ∼1.50 × 10-2 m2, while calculated average thickness was ∼200 nm. The average roughness factor (14-16) was determined on the basis of hydrogen-oxygen titration in the course of a separate experiment.26 The roughness factor determined in this way represents the ratio of the real area of the thin film to its geometrical area. The density of Pd atoms on the thin film surface: F ) 1.27 × 1019 Pd atoms/m2 was taken as the average density for the planes (111), (100), and (211).27 On this basis the amount of Pd atoms on the surface of the thin film can be determined. The amount of palladium atoms in the bulk of the thin film can be calculated by subtraction from the known, total amount of Pd atoms in the film. It should be mentioned that the majority of thin film (but not all) was deposited on the part of the static capacitor cell which was active in ∆Φ measurements. On the basis of the geometrical considerations, it was estimated that it reaches ∼90% of the total real area of the thin Pd film surface. This introduces a systematic error in the determination of deuterium (hydrogen) distribution between the surface and the bulk of the thin Pd film, since the whole film (active and inactive in ∆Φ measurements) reacts with D2 (H2) leading to changes of pressure P, while the concentration of the adsorbate on the surface is determined taking into account ∆Φ active film and P measurements. Because of this, the surface concentration of the adsorbate is overestimated by ∼10%. Precise ∆Φ measurements were performed by means of a continuously recording, sensitive (1 mV), high stability (1 mV/ h), short response time (1 ms), static capacitor circuit.24,25,28 The static capacitor cell (Figure 1) was made of Pyrex glass and consisted of two coaxial cylindrical electrodes. The inner one, which served as a reference electrode, was prepared by melting a conducting layer of (SnO + Sb2O5) into the glass. This electrode was completely inert against H2 (D2) adsorption under our experimental conditions as tested by means of the thermal desorption mass spectrometry (TDMS) method. The reference electrode was movable. It could be lifted up during thin film deposition and next repositioned for ∆Φ measurements. Every experiment was performed using a fresh thin Pd film. It was important to avoid the production of atomic deuterium (hydrogen) on the hot filament of the ionization gauge and the gauge pumping effect, since this would influence the equilibrium in the adsorbate-adsorbent system and the calculated hydrogen (24) Delchar, T.; Eberhagen, A.; Tompkins, F. C. J. Sci. Instrum. 1963, 40, 105. (25) Nowicka, E.; Dus´, R. Surf. Sci. 1984, 144, 665. (26) Dus´, R.; Lisowski, W. Surf. Sci. 1976, 59, 141. (27) Brennan, D.; Hayward, D. O.; Trapnell, B. M. W. Proc. R. Soc., London, Ser. A 1960, 56, 81. (28) Bachtin, A. Vacuum 1985, 12, 519.

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Figure 1. Diagram of the UHV system and the static capacitor cell. Symbols used in the UHV system scheme: PR, rotary pump; PD, mercury diffusion pump; M, manostate; CT, cold traps immersed in liquid nitrogen; D2, container with spectroscopically pure deuterium (hydrogen); FPd, palladium thimble for additional purification of deuterium (hydrogen) in the course of the experiment; MHg, mercury manometer; V1 and V2, calibrated volumes for D2 (H2) dosing; ML, McLeod manometer; BA, Bayard-Alpert ionization gauge; PG, ultrasensitive Pirani gauge; JG, ionization gauge with modulation; MS, mass spectrometer gauge (Topatron type, Leybold-Heraus); DK, greaseless Dekker valve; T, UHV tap. The dashed line indicates the bakeable part of the apparatus. Symbols for the static capacitor cell: (1) the adsorbent thin Pd film; (2) movable reference electrode; (3) electrical contact to the adsorbent; (4) electrical contact to the conducting coating of the reference electrode; (5) holes in the reference electrode for the symmetrical introduction of gas to the adsorbent surface; (6) tungsten heater with Pd wire wound around; (7) tube joining the capacitor with the diffusion pumps; (8) glass-coated iron slugs for moving the reference electrode by means of magnet; (9) gas inlet. The outer walls of the static capacitor were painted with liquid bright platinum. (deuterium) consumption. For this reason the ionization gauge was applied only during thin film deposition but was switched off in the course of PdDy (PdHx) formation. During this process D2 (H2) pressure was measured by means of an ultrasensitive, short response time Pirani-type gauge immersed in a liquid nitrogen bath, reading from 10-4 to 10 Pa.29 Spectroscopically pure deuterium (hydrogen), purified additionally in the course of the experiment by diffusion through a palladium thimble, was used. 2. Experimental Procedure. D2 (H2) was introduced in successive, calibrated doses of known capacity into the static capacitor isolated from pumping by means of a system of greaseless Dekker valves. The time-dependent work function changes ∆Φ(t) and pressure P(t) were continuously recorded. Knowing the volume of the static capacitor, the uptake of the adsorbate following every dose introduction could be volumetrically determined. Thanks to the high sensitivity of our Pirani gauge an uptake as small as (2-3) × 1014 D2 (H2 ) molecules (∼10-4 of a monolayer) could be detected. The examination of time-dependent courses of ∆Φi(t) and Pi(t) functions corresponding to the ith introduced D2 (H2) dose allows (29) Dushman, S. In Scientific Foundations of Vacuum Technique; Lafferty, J., Ed.; J. Wiley and Sons: New York, London, Sydney, 1965.

Dus´ and Nowicka

Figure 2. Work function isotherms obtained at 78 K for palladium deuteride and palladium hydride denoted (×) and (O), respectively. The work function of the clean surface of thin Pd film is chosen as an arbitrary zero. Every point in the graph corresponds to the equilibrium value of work function change registered in respect to this arbitrary zero as a result of successive deuterium (hydrogen) dose introduction into the static capacitor. The work function corresponding to the weakly bound β+ adspecies, reversibly adsorbed on the outer surface, is marked in the frames. studies of the elementary steps of surface processes at determined [D/Pd ]tot and [H/Pd]tot ratios. Here the expressions [D/Pd]tot and [H/Pd]tot indicate the ratio of the total uptake of deuterium (hydrogen) to the total amount of palladium atoms, on the surface and in the bulk of the thin Pd film. Equilibrium was achieved after the ith introduced dose when d(∆Φ)i/dt ) 0 and dPi/dt ) 0. Having the equilibrium ∆Φi eq value and equilibrium pressure Pi eq achieved as a result of every successive dose introduction, two important relations can be determined for deuterium (hydrogen): (I) the work function isotherm ∆Φeq ) ∑i ∆Φi eq ) f1 [D/Pd]tot; (II) the thermodynamic isotherm Peq ) ∑i Pi eq ) f2 [D/Pd]tot. On the basis of these two functions distinct forms of the adsorbate can usually be differentiated. Further in this paper we shall denote the concentration of the β+ adspecies on the thin Pd film surface [D(β+)/Pd]surf or [H(β+)/Pd]surf and the concentration of deuterium (hydrogen) in the bulk [D/Pd]bulk or [H/Pd]bulk, respectively.

Experimental Results and Discussion 1. Work Function Response to PdDy (PdHx) Formation at 78 K. Work function isotherms for the process of PdDy and PdHx formation at 78 K are shown in Figure 2, while the courses of ∆Φ(t)i and P(t)i functions for the characteristic stages of this reaction for PdDy are presented in Figure 3. The work function of the clean surface of thin Pd film was chosen as an arbitrary zero for measurements of work function changes in the process of palladium deuteride (hydride) formation. Every successive ith dose was introduced when equilibrium between the gas phase, the surface, and the bulk within the preceding dose was reached. Every point in Figure 2 corresponds to the equilibrium value of ∆Φeq. Figure 3 is divided into sections

Segregation of Deuterium and Hydrogen

Figure 3. Work function and D2 pressure changes during the distinguished steps of PdDy formation at 78 K. ∆Φ(t)i and P(t)i changes registered in the course of relaxation of the adsorbateadsorbent system after a perturbation caused by the introduction of the ith dose of D2 into the static capacitor are represented in the successive sections a, b, c, and d. The total content of deuterium in thin palladium deuteride film [D/Pd]tot corresponding to the successive sections reaches the following values: 0.005 for section a, 0.05 for section b, 0.319 for section c, and 0.673 for section d.

a, b, c, and d to better illustrate the successive steps of the reaction. In this figure the changes of work function and pressure within the ith dose are shown with respect to ∆Φeq and Peq values from the preceding dose (i - 1). Production of PdHx is characterized by similar features except for a higher rate of ∆Φ(t)i and P(t)i changes.11 In good agreement with previous experiments,11 it was found that at the beginning of adsorption two successive doses of D2 create the negatively polarized β- adspecies (section a in Figure 2). Constant values of ∆Φ(t)1 and ∆Φ(t)2 were quickly achieved, suggesting the stability of this species on the surface. The consumption of deuterium was so rapid that no P(t)1 and P(t)2 signals could be registered by means of our ultrasensitive Pirani gauge. Several successive doses of D2 resulted in a small decrease of the work function followed by its strong increase (section b in Figure 2). A sharp signal of P(t)3 on the level of 1 × 10-4 Pa was detected. The decrease of the work function is caused by the creation of the β+ adspecies on the outer surface of the film, while according to Smoluchowski’s model30 increase of Φ is interpreted as their displacement below the surface image plane (SIP). The exact position of the surface image plane for a polycrystalline thin Pd (30) Smoluchowski, R. Phys. Rev. 1941, 60, 661.

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film is not known. However there are suggestions that for transition metals the (SIP) is situated within the first atomic layer.31 Since the radius of the β+ adspecies is at least 4 times smaller than the covalent radius of palladium atoms, these adspecies can be well situated in the gaps within the first layer of Pd atoms on the thin film surface above (SIP) decreasing work function or below the (SIP) increasing Φ. It should be expected that the β+ species incorporated below the second layer of atoms in thin Pd film do not influence ∆Φ because of screening by electrons of the metal. This expectation is based on the dependence of ∆Φ on hydrogen content in PdHx at 120 K23 and in VHx at 298 K.14 Also, the experiments carried out recently in Christmann’s laboratory confirmed that hydrogen dissolved in a single Pd crystal (R phase of hydrogen in palladium) does not influence ∆Φ.18 The height of the negative ∆Φ(t) signals progressively increased within several doses, and next as a result of successive adsorption, it was almost constant. About 60 successive D2 doses resulted in such negative ∆Φ(t) transients accompanied by the transients of P(t) (section c in Figure 3). The ∆Φeq values were increasingly more negative, indicating an increase of the β+ adspecies concentration on the outer surface with the increase of [D/Pd]tot and [H/Pd]tot (see Figure 2). Despite a large uptake of deuterium (or hydrogen) the equilibrium pressure was lower than 1 × 10-4 Pa, while the maximum of the P(t)i signal successively increased, indicating a decrease of the sticking probability. The negative ∆Φ(t)i transients were observed until [D/Pd]tot (or [H/Pd]tot) reached 0.65. Successive doses of D2 resulted in a monotonic decrease of ∆Φ(t)i, accompanied by an increase of the equilibrium pressure (section d in Figure 3). At this comparatively high D2 pressure, every site on the outer surface of the film released due to incorporation of the β+ adspecies below the surface is immediately reoccupied by deuterium. Thus at this stage of the process the consumption of deuterium within the ith D2 dose occurs at a constant ∆Φ(t) reached within a short time after the dose introduction. Under equilibrium pressure of the order of 1 Pa, palladium deuteride with concentration [D/Pd]tot ) 0.785 and palladium hydride [H/Pd]tot ) 0.950 (see Figure 2) could be obtained with an excellent reproducibility of all the features described above. Lowering the pressure caused an increase of the work function by ∼110 mV associated with desorption of some weakly bound β+ adspecies from the PdDy and PdHx surfaces. 2. Deuterium and Hydrogen Segregation on Surfaces of PdDy and PdHx at 78 K. Analysis of ∆Φ(t)i and P(t)i functions allows to differentiate between a fast adsorption and much slower incorporation of the β+ adspecies below the surface of thin Pd film at 78 K. One can notice in Figure 3 (section c) that ∼90% of the maximal signal of ∆Φi corresponding to adsorption within the ith dose was achieved within 5-6 s, while the ∆Φi eq established as a result of an equilibrium between the gas phase, the surface and the bulk, due to the incorporation of some of the β+ adspecies below the surface, is reached within ∼180 s. Thus within the short period of time required to achieve the above-mentioned ∆Φ(t)i signal the majority of deuterium (hydrogen) deposit remains on the outer surface. Deuterium (hydrogen) uptake corresponding to this value of ∆Φ(t)i can be volumetrically determined on the basis of the P(t)i function. It can be accepted that within the uptake from a single, small ith dose ∆Φ(t)i is proportional to the concentration of the β+ adspecies within (31) Kiejna, A.; Wojciechowski, K. F. In Metal Surface Electron Physics; Pergamon: Elsevier Science Ltd.: U.K., U.S.A., Japan. 1996.

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Figure 4. Examination of ∆Φ(t)i transients with respect to the first-order relaxation eq 2. The numbers of doses shown in the graph are marked as follow: (×) dose 11, (4) dose 21, (O) dose 32 and (0) dose 40.

the first layer of Pd atoms on the thin film surface [D(β+)/ Pd]i surf. Hence the deposit on the outer surface corresponding to ∆Φi eq at the equilibrium can also be determined.Thisprovidesinformationconcerningthedistribution of the adsorbate between the outer surface and the rest of the thin Pd film, since the capacity of the ith dose is known and P(t) is measured. The accuracy of these calculations can be improved even further taking into account that the negative ∆Φ(t)i transient can be considered as a relaxation process. Indeed, introduction of the ith D2 (H2) dose suddenly perturbs the equilibrium between the gas phase, the surface, and the bulk of the thin Pd film. In response to this perturbation the system readjusts to the new equilibrium, and this is registered as ∆Φ(t)i and P(t)i transients. When the system is sufficiently far from the new equilibrium, the rate of β+ incorporation below the surface strongly predominates the rate of the reverse process, and the negative ∆Φ(t)i transients can be described by a first-order kinetic equation:

-d[D(β+)/Pd]surf/dt ) +

Figure 5. Distribution of deuterium (×) and hydrogen (O) concentration on the outer surface and in the bulk of thin PdDy and PdHx films at 78 K.

by the relaxation equation when the system is sufficiently far from the equilibrium. Thus P(t)0)i corresponding to ∆Φ(t)0)i can be determined. This eliminates the inaccuracy associated with the incorporation of the adsorbate into the bulk within the time required to reach the ∆Φ(t)i maximum. Application of the analysis described above to every dose allows determination of the equilibrium distribution of deuterium (hydrogen) between the outer surface and the bulk of the thin film in the process of PdDy (PdHx) formation at 78 K:

[D(β+)/Pd]surf ) f3[D/Pd]bulk and

[H(β+)/Pd]surf ) f4[H/Pd]bulk Simultaneously the work function isotherms for the β+ adspecies can be determined

∆Φ(β+) )

∑i∆Φi eq ) f5[D(β+)/Pd]surf

∆Φ(β+) )

∑i ∆Φi eq ) f6[H(β+)/Pd]surf

and +

k{[D(β )/Pd ]surf - [D(β )/Pd]surf eq} (1) where k is a rate constant and [D(β+)/Pd]surf eq corresponds to the new equilibrium concentration of the β+ adspecies on the outer surface. Remembering the proportionality between ∆Φ(t)i and the adsorbate concentration achieved within a single ith dose, after integration we have

ln[∆Φ(t)i - ∆Φi eq] ) ln[∆Φ(t ) 0)i - ∆Φi eq] - kt (2) It has been found that this equation fits the experimental results very well within 100-140 s, until ∆Φ(t)i differs from ∆Φi eq by more than 10 mV as it is shown in Figure 4. On this basis the ∆Φ(t)0)i signal can be calculated for every dose and correlated with the uptake of the β+ adspecies. P(t)i transients can also be described

The courses of functions f3 and f4 are presented in Figure 5, while those of functions f5 and f6 are presented in Figure 6. It can be clearly seen that starting from low deuterium and hydrogen content in palladium deuteride (hydride), surface concentration predominates that in the bulk. Surface segregation of deuterium is higher than the segregation of hydrogen. At the maximal concentration [D(β+)/Pd]surf ∼ 2.45, while [H(β+)/Pd]surf ∼ 2. However, in our calculations we have taken into account the roughness factor determined for the origin thin Pd films. Recent studies of thin Pd film topography in the process of PdHx formation at 298 K carried out in our laboratory by means of the scanning tunneling microscopy (STM) and atomic force micrscopy (AFM) methods showed that in the course

Segregation of Deuterium and Hydrogen

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Figure 7. Work function isotherms for PdDy at 120 K (×) and 160 K (4) and for PdHx at 160 K (O). ∆Φeq dependences on [D/Pd]surf and [H/Pd]surf are also shown in the insert. Figure 6. Work function isotherms for the β+ adspecies of deuterium (×) and hydrogen (O) present on the outer surfaces of thin PdDy and PdHx films at 78 K.

of this reaction the roughness factor increases by ∼15%.32 Unfortunately our STM (AFM) apparatus cannot work at 78 K, but these studies suggest that in addition to the above-mentioned systematic error caused by the appearance of a small part of the deposited film surface inactive in ∆Φ measurements, there arises another source of overestimation of the surface concentration of deuterium (hydrogen) on the surface of palladium deuteride (hydride). Total overestimation approaches 20-25%. Taking this into account we estimate that the real concentration of deuterium on a palladium deuteride surface at 78 K under D2 pressure of the order of 1 Pa reaches 1.85-1.9, while the concentration of hydrogen on palladium hydride under similar conditions approaches 1.5. The error mentioned above does not significantly influence concentration in the bulk, since the bulk capacity of 200 nm thick Pd film for storing deuterium (hydrogen) in the form of PdDy∼1 or PdHx∼1 is much higher than the capacity of the surface. One can see in Figure 6 that ∆Φ decreases almost linearly with the increase of concentration of the β+ adspecies on the outer surfaces of PdDy (PdH)x, reaching very high values: -2408 mV for [D(β+)/Pd]surf ) 2.45 and -1500 mV for [H(β+)/Pd]surf ) 1.95. These values approach ∆Φ registered for alkali metal adsorption on transition metal surfaces.33,34 The linear dependence of ∆Φ on [D(β+)/Pd]surf and [H(β+)/Pd]surf allows the application of the Helmholtz equation for calculation of the normal component µ of the dipole moment of the β+ adspecies on the outer surfaces of PdDy and PdHx:

µD(β+) ) ∆Φ(β+)/4πF[D(β+)/Pd ]surf and

µΗ(β+) ) ∆Φ(β+)/4πF[Η(β+)/Pd]surf (32) Kobiela, T.; Dus´, R. To be published in J. Catal. (33) Gerlach, R. L.; Rohdin., T. N. Surf. Sci. 1970, 19, 403. (34) Fehrs, D. L.; Stickney, R. E. Surf. Sci. 1971, 24, 309.

(3)

Here F is the density of palladium surface atoms. The calculated normal components of the dipole moments for deuterium and hydrogen are very similar within the β+ adspecies concentration below 1, reaching 7.20 × 10-31 C m (or 0.216 D). At higher concentration deuterium exhibits µD(β+) ) 8.04 × 10-31 C m (or 0.241 D). The absolute values of these normal components are at least 10 times higher than those known for the negatively polarized β- hydrogen adspecies commonly observed on transition metal surfaces when hydrides are not formed µD(β-) ) - 6.67 × 10-32 C m (or -0.02 D) for the β- deuterium adspecies on thin Pd film at 298 K.11 Such high normal components of the dipole moments indicate the significance of an ionic component in the binding of the β+ adspecies on the outer surfaces of PdDy and PdHx. This suggests a fundamental difference between the nature of hydrogen adspecies binding with the surface palladium and palladium deuteride (hydride). The character of binding of the β+ adspecies with the surface of palladium deuteride (hydride) is not well understood yet. In light of the results presented above we can answer the question mentioned in the Introduction. The strong isotope effect observed in ∆Φ dependence on [D/Pd]tot and [H/Pd]tot and seen in Figure 3 arises because of the difference in surface concentration of deuterium and hydrogen adspecies at the same bulk concentration, while the value of µD(β+) is very similar to that of µΗ(β+). This corresponds to the well-known higher equilibrium pressure over palladium deuteride than over palladium hydride of the same composition.9 3. PdDy and PdHx Formation at 120 and 160 K. It has been found that palladium deuteride and palladium hydride of the composition [D/Pd]tot ) 0.88 and [H/Pd]tot ) 0.90 can be obtained under our experimental conditions with very good reproducibility at 120 K (a bath composed of a mixture of neopentane and liquid nitrogen) and in somewhat smaller concentration at 160 K (a bath composed of a mixture of 2-methylpentane and liquid nitrogen). The work function isotherms for this process are shown in Figure 7, while the features of the ∆Φ(t)i and P(t)i functions for the distinguished stages of PdDy formation at 120 K are presented in Figure 8. At the beginning of the process several D2 doses caused a monotonic increase of ∆Φ(t)i, accompanied by a small,

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Figure 8. Work function and D2 pressure changes during the distinguished steps of PdDy formation at 120 K. ∆Φ(t)i and P(t)i changes registered in the course of the relaxation of the adsorbate-adsorbent system after a perturbation caused by the ith dose of D2 introduction into the static capacitor are represented in sections a, b, and c. The total content of deuterium in thin palladium deuteride film [D/Pd]tot corresponding to the successive sections is as follows: 0.003 for section a, 0.200 for section b, and 0.832 for section c.

sharp signal of P(t)i of the order of 10-4 Pa (see section a in Figure 8). ∑i)1-6∆Φi eq ) 280 mV was achieved. When [D/Pd]tot exceeded 0.045, every successive D2 dose resulted in a small (∼8 mV) negative ∆Φ(t)i transient, accompanied by a P(t)i transient with a maximum of the order of 10-3 Pa (section b in Figure 8). This phenomenon was observed until [D/Pd]tot reached 0.65. The decay of every negative ∆Φ signal was complete. Hence, as it can be seen in Figure 7, contrary to the process carried out at 78 K (Figure 2), the ∆Φeq value was on a constant level. At higher deuterium content a small decrease (∼25 mV) of ∆Φeq was noticed (see Figure 7, and section c in Figure 8). The ∆Φ features during the process of PdHx (0.05 < x < 0.90) formation were very similar. In the course of the reaction carried out at 160 K a monotonic increase of ∆Φeq was observed until [D/Pd]tot was greater than 0.040, and further a plateau of the work function was recorded despite the large amount of deuterium (hydrogen) consumed (Figure 7). No negative ∆Φ transients were observed at this temperature, neither during PdDy nor during PdHx formation. One can see in Figure 8 that the ∆Φeq value characteristic for the plateau of surface potential was almost the same for PdDy and PdHx at both temperatures. On the basis of the observations presented here, the models of Grimley17 and Smoluchowski,30 and previously published results,11 we interpret the phenomena described above as follows: The negatively polarized β- adspecies are formed on a thin palladium film surface in the first step of its

Dus´ and Nowicka

interaction with H2 or D2 at every one of the temperatures maintained in our experiments, and also at 298 K.11 At 298 K palladium deuterides (hydrides) cannot be formed under the pressure applied in our experiments for thermodynamic reasons, or in other words the concentration of the adsorbate is too low to create the β+ adspecies. Then the increase of the work function is caused solely by the formation of the negatively polarized, surface-stable β- adspecies. ∆Φeq reaches 190 and 200 mV for deuterium and hydrogen adsorption, respectively. To achieve the critical concentration of deuterium (hydrogen) adsorbate required for the creation of the β+ adspecies under D2 (H2) pressure applied in our experiments (below 10 Pa), the temperature has to be lowered enough, e.g., to 160 K. The β+ adspecies create a precursor surface state for PdDy and PdHx formation in the bulk of the palladium at 160 or 120 K, similarly as at 78 K. However, at higher temperatures the preferable position for them is that below the surface image plane. The β+ adspecies positioned below (SIP) induce, according to Smoluchowski’s model30 a negative charge on the outer surface. This is observed as an increase of the work function at the beginning of the process of PdDy and PdHx formation. ∆Φeq is now more positive than that in the case of deuterium (hydrogen) adsorption at 298 K, reaching 280 mV (Figure 7). The small negative ∆Φ(t)i transients observed at 120 K (Figure 8, section b) as a result of successive D2 dose introduction are good evidence for the presence of the β+ adspecies on the outer surface. However, at 120 K their residence in this position is very short, and they move quickly below the (SIP). It can be expected that at the beginning of the process of PdDy (PdHx) formation the first layer of palladium atoms should be filled with deuterium (hydrogen) and next incorporation into the bulk might occur. In this second stage of the process every released site in the first layer of palladium atoms should be immediately filled due to adsorption from the gas phase. As a consequence, a plateau of the work function should be expected, as seen in fact in Figure 7. Hence the highest level of ∆Φeq should be reached when [D(β+)/Pd]surf and [H(β+)/Pd]surf approach 1, corresponding to the surface compound PdD and PdH, respectively, or could significantly exceed 1, even approaching 2 if palladium dideuteride (dihydride) was formed on the surface. It can be calculated that in our experiments [D(β+)/Pd]surf and [H(β+)/Pd]surf approach 1 when [D/Pd]tot and [H/Pd]tot reach 0.045. This is almost exactly the point where ∆Φeq is at its highest value before a plateau of the work function is reached in the process of PdDy (PdHx) formation at 120 and 160 K (see Figure 7). The work function isotherms for [D/Pd]surf and [H/Pd]surf shown in the insert in Figure 7 clearly demonstrate these features of the adsorbateadsorbent systems. Thus the concentration of the β+ adspecies of deuterium (hydrogen) placed below the (SIP) on the surface of PdDy (PdHx) does not exceed 1, similarly as in the bulk of these compounds. It should be mentioned that isothermal evacuation at 120 or 160 K does not alter ∆Φeq of palladium deuteride (hydride). That means that the β+ adspecies positioned below the (SIP) are more strongly bound than those present on the outer surface of palladium deuteride (hydride) at 78 K. This again agrees with the results obtained in Christmann’s laboratory in the course of the investigation of the H2-Pd (211) system.18 The negatively polarized adsorbate deposited at 130 K did not desorb during the isothermal evacuation. However, at lower temperatures (90-110 K) evacuation caused at least partial desorption of the hydrogen deposit, increasing the work function. This suggests removal of the positively polarized adspecies from the outer surface.

Segregation of Deuterium and Hydrogen

Figure 9. Work function isotherm for palladium deuteride at 88 K. ∆Φ(β+)eq dependence on [D/Pd]surf is shown in the insert.

4. Deuterium and Hydrogen Segregation on PdDy and PdHx Surfaces at 88 K. If the above interpretation of surface phenomena in the process of palladium deuteride (hydride) formation is valid, then at a temperature somewhat higher than 78 K, but below 120 K, negative ∆Φ(t)i transients should be clearly observed, while segregation of the β+ adspecies on the outer surface of palladium deuteride (hydride) should be less pronounced. Consequently ∆Φeq should reach a much less negative value. To verify this expectation, the experiments were carried out at 88 K (liquid argon bath). In full agreement with the prediction described above, distinct, negative ∆Φ(t)i transients following the β- arising at the beginning of adsorption were clearly observed, similarly as at 78 K. However, further consumption of the adsorbate from every successive dose resulted in a smaller negative ∆Φ(t)0)i signal. ∆Φi eq values were also less negative, as can be seen in the work function isotherm for PdDy formation presented in Figure 9. This suggests that at 88 K simultaneous adsorption of the β+ adspecies on the outer surface above the (SIP) and below the (SIP) occurs or the residence time of these adspecies on some sites on the outer surface is very short, beyond the detection possibility of our system. Consequently the registered work function changes are a superposition of ∆Φ above and below the (SIP). It is reasonable to expect that the dipole moment of the β+ adspecies of deuterium on the outer surface of PdDy at 78 and 88 K is the same (a similar expectation concerns hydrogen, of course). With this assumption, the work function isotherm for the β+ adspecies on the outer surface of PdDy (PdHx) and the segregation of deuterium (hydrogen) at 88 K can be determined on the basis of eq 3. The work function isotherm for the β+ adspecies of deuterium on the outer surface of PdDy is shown in the insert in Figure 9, while the segregation of these adspecies on the outer surfaces of PdDy (PdHx) as a function of bulk concentration is presented in Figure 10. It is clearly seen

Langmuir, Vol. 16, No. 2, 2000 591

Figure 10. Distribution of deuterium (×) and hydrogen (O) on the outer surface and in the bulk of thin PdDy and PdHx films at 88 K.

that at 88 K the concentration of β+ adspecies on the outer surface of PdDy (PdHx) is much lower than that registered at 78 K and does not exceed 0.9. Similarly as at 78 K, isothermal evacuation resulted in desorption of a part of this deposit leading to an increase of ∆Φ. Conclusions 1. The equilibrium concentration of the positively polarized deuterium (hydrogen) β+ adspecies existing on the outer surface of PdD0.785 (PdH0.950) at 78 K under D2 (H2) pressure of the order of 10-2 Pa approaches 1.9 (1.5) while the concentration in the bulk does not exceed 0.645 (0.910). 2. The equilibrium concentration of deuterium on the outer surface of palladium deuteride predominates that of hydrogen on palladium hydride at the same bulk concentration, while the normal components of the dipole moments of these two adsorbates are similar reaching values as high as 7.20 × 10-31 C m (0.216 D). This suggests that the binding character of deuterium (hydrogen) adsorbate to PdDy (PdHx) surface strongly differs from that known for these adsorbates on palladium, characterized by the normal component of the dipole moment -6.67 × 10-32 C m (0.02 D). 3. In the process of palladium deuteride (hydride) formation at 78 K the work function strongly decreases (by 2314 mV for PdD0.785 and 1350 mV for PdH0.875), while above 120 K an increase of the work function (by 280 mV) is observed. This is interpreted as the positioning of the positively polarized β+ adspecies below the surface image plane. 4. Above 120 K the surface concentration of deuterium (hydrogen) on PdDy and PdHx approaches 1, simlarly as the bulk concentration. LA990870Y