Surface reactivity of the borided Pd(111) with respect to hydrogen, ethyne and ethene

Vacuum/volume

46humbers

Pergamon 0042-207x(95)00128-x

Surface reactivity of the borided to hydrogen, ethyne and ethene

8-IO/pages

1151 to 1153/1995 Elsevier Science Ltd Printed in Great Britain 0042-207x/95 $9.50+.00

Pd(l ‘I I) with respect

M Krawczyk* and W Palczewska, Department of Applied Surface Science, institute of Physical Chemistry of the Polish Academy of Sciences, Kasprzaka 44/52,01-224 Warszawa, Poland

In the present AES-LEED-TDS study, the surface reactivity of the borided Pd(l1 I! sample exposed to f-f*, C,H, or C,H, at 300 K was investigated. Quantitative AES surface analysis allowed determination of the concentration ofboron. Interaction ofhydrogen and the hydrocarbons with clean and borided Pd(l1 I) surfaces has been compared, The results were interpreted in terms of the catalytic role of surface boron on palladium.

1. Introduction Boron as surface or bulk component of metals or elemental semiconductors has found an extensive application in modern material science. In recent years, numerous studies have been devoted to the role of boron as a surface modifier of transition metal catalysts. Boron and its compounds have manifested their catalytic qualities not only as main components, but also as subtle admixtures modifying selectivity of catalyst action’. In the present paper, it has been attempted to present a surface-science model system representing some primary stages of the catalytic semihydrogenation : ethyne =+ ethene on palladium modified with boron. The aim was to suggest a reasoned interpretation of the role of this modifier in the catalytic system investigated. 2. Experimental The experimental equipment used in the present investigation has been described in earlier papers’, *. Therefore, only the general features of this routine set-up are mentioned here. The experiments were carried out in an UHV chamber (base pressure < 1 x 10e7 Pa) supplied with LEED, AES and TDS techniques. The preparation of the Pd( 111) single-crystal sample, its mounting, cleaning and methods of monitoring of the surface purity have been already reported in detail elsewhere3. Sample temperatures in the range 30&1500 K were monitored by a NiCr-Ni thermocouple spot-welded to the edge of the crystal. Hydrogen (VEB Technische Gase Werke, Germany, specpurity), ethyne and ethene (Matheson, USA, 99.6% purity) were introduced separately into the chamber and then adsorbed on the sample surface at 300 K by backfilling the UHV chamber to P = 1 x 10m6 Pa. Gas exposures are given in Langmuirs (1 L = 1.33 x 10e4 Pas) uncorrected for the ionization gauge factor. The purity of the reactant gases was verified in situ by quadrupole mass spectrometry (QMS). The evolution of desorption products

*Author to whom correspondence should be addressed.

during the TDS experiments was also checked mass-spectrometrically. The thermal desorption spectra of M/e = 2, M/e = 26 and M/e = 28 QMS intensities were acquired in consecutive separate TDS measurements, which followed the preadsorption process on each investigated sample. However, during a programmed increase of sample temperature, hydrogen (M/e = 2 amu) desorbed into a well evacuated UHV chamber (‘p < 2 x 10d7 Pa) ; molecules of ethyne (M/e = 26 amu) or ethene (M/e = 28 amu) into a low pressure flow of respective gases admitted gently to the chamber (P = 1 x 10m6 Pa). Boron-covered surfaces of the Pd( 111) sample were prepared in situ by exposing the clean crystal at 300 K to B,H, (1% diborane in argor?, diborane pressure 5 x 1O-7 Pa; Chemipan, Poland). According to the earlier AES-TDS results’, ‘, this treatment leads to the complete dissociation of the B2H6 molecules with the formation of boron and hydrogen adatoms ; a part of hydrogen desorbs into the gas phase but a certain amount becomes incorporated into the Pd(ll1) bulk phase and forms a Pd-H solid solution. The B concentration in the Pd(ll1) surface layer, XB, expressed as atomic ratio n,/~+.~, was determined from AES’. ‘, 6. During the AES measurements, the intensity of Pd MNN (330 eV) Auger signal was acquired for the clean, i.e. as H&(330), or precovered with boron Pd(ll1) surface, denoted as @$(330). The detailed calibration procedure of the computed AES intensity ratio H&(330)/&,(330) to an X, value can be found elsewhere’s *. 6.

3. Results and discussion 3.1. Interaction of H2 with clean and boron-covered Pd(ll1) surfaces. TDS and AES revealed effects of interactions between hydrogen and clean Pd(ll1) or Pd(lll)-B surfaces at 300 K’. Figure 1 represents the decreasing amount of hydrogen desorbing from the Pd( 111)-B sample surface with the increase of X,. The area determined from the shape of respective TD curves’ revealed the corresponding quantity of hydrogen that desorbs during the controlled temperature increase. As shown in Figure 1, the 1151

M Krawczykand

WPalczewska:

H2/Pd

I

0.00

I

0.05

Surface

(Ill)

reactivity

of borided

Pd(l11)

- B

I

I

!

0.10

0.15

0.20

l

300

400

XB

500

600

TEMPERATURE

Figure 1. Dependence of the TDS peak area (m/e = 2 amu), representing the amount of hydrogen (PH2 = 1 x 10d6 Pa, T= 300 K, H, exposure = 2.25 L) sorbed on clean and B-covered Pd(l1 l), as a function of boron surface concentration X,.

700

c

(K)

lb) C2H2/Pd(111)-B iz k

amount of desorbing H, rapidly diminishes with increasing boron surface concentration X,, e.g. by more than a factor of 8 within changes of X, between 0.00 and 0.16. A more detailed consideration of the results (such as a lack of changes of TDS line shapes and their peak positions’) does not manifest any other effect of the B adatoms than this simple blockage of Pd surface sites. This is not surprising if one takes into account that the electronegativity of boron (2.0 on the Pauling scale) has a value near to that of palladium (2.2) ; thus, a charge transfer between B and Pd should be minimal, not affecting their interactions’. 3.2. Interaction of C,H, or C,H, with clean and boron-covered Pd(ll1) surfaces. The effect of preadsorbed boron on the chemisorption of ethyne (C2H2) or ethene (C,H,) on Pd(ll1) at 300 K was investigated in earlier work’. Thermal desorption spectra (seen in Figure 2 (a)) acquired after the exposure of Pd(l 11)-B to C2H, (2.25 L) exhibit only the presence of desorbing H2 (M/e = 2 amu). The value of X, varies from 0 (clean surface) to 0.3 (that represents the maximal value of X,). The TD spectrum of hydrogen from C2H2/Pd(l 11) consists of two desorption peaks (curve 1 in Figure 2(a)). The first sharp peak at T, E 480 K is assigned to hydrogen desorbing from irreversibly adsorbed ethyne, identified as vinylidene adspecies (=C=CH3,d8. 9. The second broad feature, detected between 550 and 750 K, can be ascribed to the further stage of hydrogen desorption from the hydrogen-deficient acetylide irreversible adspecies (-C-CH),, or products of their surface oligomerisation’, lo. This H2 desorption signal can be also partly attributed to on the hydrogen evolved from the Pd(ll1) subsurface region”. “. After both CzHz or C,H4 adsorption on platinum group metals within the temperature range 300-700 K, the various hydrogen-deficient carbonaceous adspecies (C&H,),, have been successfully recognized and the surface phenomena, mentioned above, have been described in the literature on unsaturated hydrocarbons’ adsorption on transition metals*, 9. I33r4. Exposure of boron-covered Pd(ll1) to ethyne at 300 K results in H, desorption curves (curves 24 in Figure 2(a)) similar to curve 1 in Figure 2(a) recorded during the thermal decomposition 1152

300

400 TEMPERATURE

(KI

Figure 2. Thermal

desorption spectra acquired after the adsorption of ethyne (PcaHI = 1 x 10e6 Pa, CZHz exposure = 2.25 L) at 300 K. Heating rate 31.5 K/s. Top : (a) hydrogen (m/e = 2 amu) TD spectra from clean and B-covered Pd( 111) with various boron surface concentrations X, : 1. X, = 0.00, clean Pd(ll1); 2. X, = 0.05; 3. X, = 0.12; 4. X, = 0.30; bottom : (b) ethyne (m/e = 26 amu) TD spectra from B-covered Pd( 111) with various boron surface concentrations X, : 1. X, = 0.04; 2. x, = 0.11; 3. x, = 0.15; 4. x, = 0.30.

of C2H, chemisorbed on clean Pd(ll1). An unique, but very distinct, difference consists in a much lower amount of desorbing hydrogen, still diminishing with an increase of X, up to its saturation value of 0.3. The results presented for H, (M/e = 2 amu)TDS clearly demonstrate that preadsorbed boron causes effectively the suppression of the amount of chemisorbed ethyne. Any change of the form of carbonaceous moieties on the Pd( 111)-B surface could be detected’. Therefore, one can conclude that B exerts only a site blocking local geometric effect on the C,H, chemisorption, similarly to the case of HZ chemisorption. Figure 2(b) displays a series of C,H2 (M/e = 26 amu) TD spectra recorded after the exposure of Pd(l11) or Pd(l 11)-B at 300 K to C,H, (2.25 L). The clean Pd(ll1) surface (as confirmed

M Krswczyk

and W Palczewska:

Surface reactivity of borided Pd(l11)

by other authors’ results, e.g. for Ni, Ru, Rh, etc13) does not reveal any molecular ethyne (M/e = 26 amu) desorbing at T > 300 K into vacuum’. The same concerns Pd(lll)-B or Pd(lll)-B,O, surfaces*. However, when the process of C2H, adsorption is performed with a constant residual ethyne background pressure as high as 1 x lop6 Pa, the palladium-boron samples (but only those) are able to adsorb reversibly a part of molecular C,H, within T = 300-400 K and to desorb it back in the process of the TD. The TD spectra of molecular C,H, exhibit only one peak with T,,, E 365 K (see Figure 2(b)) invariant with increasing X,. However, the area under these C,H, (M/e = 26 amu) desorption peaks varies with the X, values : at first it increases with the rise of the X, value, then reaches a distinct maximum at X, g 0.11 (i.e. somewhere between 0.04 and 0.15) and finally it decreases, while the X, value is still increasing. It should be noted that even for the highest X, value of 0.3 the amount of desorbing molecular ethyne does not fall to zero. The TDS give evidence for the presence of molecular C2H, desorbate from both Pd(1 11)-B and Pd(l1 l)-B,O, ; the phenomenon is absent in the case of a pristine Pd(l11) surface under the same conditions. The H,-TDS results of ethene adsorption on clean Pd(ll1) or B-covered Pd(ll1) at 300 K are similar to those of ethyne. The two points of significance are: (i) the desorption of hydrogen reveals dissociative adsorption of C,H, on Pd(l11)“~ 15.l6 or Pd(lll)-B to form probably 2Had and (CCH, * CCH,),, in both cases, and (ii) preadsorbed boron suppresses ethene adsorption without any distinct change of the binding energy of resulting adspecies. Figure 3 demonstrates a series of TD spectra recorded after the exposure of both Pd(ll1) or Pd(1 11)-B surfaces at 300 K to 1.12 L of C,H,. The thermal desorption spectrum of preadsorbed C2H, from clean Pd( 111) exhibits one intense peak of desorbing molecular C,H, (M/e = 28 amu) with T,,, z 330 K (curve 1 in Figure 3), in agreement with the previous results15. The peak

intensity drastically attenuates with the X, increase, but the position of its maximum remains unaltered. Already at X, E 0.16 the molecular C2H4 desorbate disappears in the respective TD spectra (curve 4 in Figure 3). 4. Conclusions Present studies on the surface reactivity of the boron-precovered Pd( 111) surface (with controlled boron concentration X,) with regard to adsorption-desorption processes of hydrogen, ethyne or ethene at 300-700 K suggest the following : 1. The Pd(lll)-B surface reveals a decrease of its surface coverage with adsorbed H,, C,H1 or C2H4 (when X, increases) ; however, respective adspecies do not reveal in their first adsorptive 2D layer any change of character of their interaction in comparison with the pristine palladium surface. 2. The presence of surface boron results in a partial blocking of respective Pd adsorptive sites (proper to H2, C2H, or C,H,). It may be postulated, that the boron adatoms determine a particular distribution of the ‘free’ palladium adsorptive sites on the Pd(ll1) surface, similarly to that earlier observed directly in the case of Pb adsorbed on that same surface”. The free Pd adsorptive sites may form surface ensembles of suitable size and geometry for the accommodation of relevant gas molecules. 3. The molecules C,H2, monitored as adsorbate weakly bound with the Pd(lll)-B and easily desorbing in molecular form (within T = 300-450 K; T,,, g 365 K), present a new observation. It is tempting to interpret it in terms of Thomson and Webb ideas concerning the mechanism of unsaturated hydrocarbons’ adsorption on transition metals17. “. The first stage of their adsorption results in the formation of a strongly adsorbed layer of hydrogen-deficient moieties (C&H,),. It is only on the top of these that an overlayer of ‘complete’ molecules of the hydrocarbon forms17. “. Those admolecules would be able to adapt themselves properly to the 2-D steric arrangement of the (C,H,),,/Pd-B interface and could become the relevant reactant in the catalytic selective semihydrogenation system that has inspired the present study. References

C2H4/Pd(lll)-B

‘M Krawczyk, J Sobczak and W Palczewska, Catal Lett, 17,21 (1993). ‘M Krawczyk and W Palczewska, Surf&e Sci, 287/288,212 (1993). 31 Ratajczykowa, J Vuc Sci Technol, A 1, 1512 (1983). 4G Zweifel and H Brown, In Organic Reactions, Vol 13, p 1. Wiley, New York (1963). ‘T B Fryberger, J L Grant and P C Stair, Langmuir, 3, 10 15 (1987). 6A Jablonski, Surf Interface Anal, 4, 135 (1982) ; A Jablonski, M Krawczyk and B Lesiak, J Electron Specfrosc Rel Phenom, 46, 131 (1988). 7D J Joyner and RF Willis, Philos A4a.q,A 43, 815 (1981). ‘R M drmerod, R M Lambert, H Hoffmann, F Za&a, L P Wang, D W Bennett and W T Tvsoe, JPhvs Chem, 98.2134 (1994). ‘G A Somorjai, Caialyst Design Progiess’and Pbspec’fives (Edited by L L Hegedus), p 11. J Wiley, New York (1987). “-8 Leslak, A Jablonskl, W Palczewska, I Kulszewicz-Baier and M Zaaorska, Surf Znferface Anal, 18,430 (1992). “G E Gdowski, T E Felter and R H Stulen. Surface Sci. 181., L147 I

t

(1987). ‘*I Ratajczykowa, Surface Sci, 172, 691 (1986).

300

13PA P Nascente, M A Van Hove and G A Somorjal, Surface Sci, 253,

400 TEMPERATURE

(K)

Figure 3. Thermal desorption spectra of ethene (m/e = 28 amu) acquired after the adsorption of ethene (PcZH,= 1 x 10e6 Pa, T = 300 K, C,H, exposure = 1.12 L) on clean and B-covered Pd(ll1) with various boron surface concentrations X, : 1. X, = 0.00, clean Pd(ll1) ; 2. X, = 0.04; 3. X, = 0.12; 4. X, = 0.16. Heating rate 30 K/s.

167 (1991). 14C Xu, J W Peck and B E Koel, J Am Chem Sot, 115,751 (1993). “W Palczewska I Ratajczykowa, I Szymerska and M Krawczyk, In Proc 8th Znf Congr &al, Vo14, p 173, Verlag-Chemie, Weinheim (1984). 16D R Lloyd and F P Netzer, Surface Sci, 129, L249 (1983). I’S J Thomson and G J Webb, J Chem Sot, Chem Commun, 526 (1976). 18G Webb, Catal Todalj, 7, 139 (1990). 1153