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Effects of potassium on the chemisorption of CO2 and CO on the Pd(100) surface
L498
Surface Science 171 (1986) L498-L502 North-Holland, Amsterdam
S U R F A C E S C I E N C E LETTERS E F F E C T S OF P O T A S S I U M O N T H E C H E M I S O R P T I O N OF C O s AND C O O N T H E Pd(100) SURFACE Andrfis B E R K 0 and Frigyes SOLYMOSI * Reaction Kinetics Research Group of the Hungarian Academy of Sciences and Institute of Solid State and Radiochemistry, Unirersity of Szeged. PO Box 105. H-6701 Szeged, ltungar}, Received 27 November 1985; accepted for publication 21 February 1986
It has been found that preadsorbed potassium dramatically affects the adsorption behaviour of CO 2 and CO on Pd(lO0) surface. In increases the rate of adsorption, the binding energy of CO 2 and CO and it induces the dissociation of CO 2.
There has been an increasing interest in the catalytic transformation of CO 2 into more valuable compounds. Recently, it has been observed that palladium is an effective catalyst in the hydrogenation of CO 2 into methanol [1-4]. and the performance of the catalyst can be improved by means of alkali metal additives [5]. Whereas the effects of alkali metals on the chemisorption of CO over transition metals have been extensively studied in the past years, no reports were found in the literature on their influence on the interaction of CO 2 with metal single crystal surfaces. Furthermore, we found no information as regards the adsorption of potassium on Pd surfaces and the effect of preadsorbed potassium on the chemisorption of CO on Pd. The primary aim of the present study is to investigate the main characteristics of effects of potassium on the adsorption behaviour of CO 2 and CO on the Pd(100) surface. A standard U H V system containing facilities for LEED, AES, EELS, thermal desorption and work function (A~) measurements was used for the experiments. This was described in detail in our previous paper dealing with the adsorption of CO 2 on the R h ( l l l ) surface [6]. Sample preparation and cleaning followed previous procedures [7]. Potassium was deposited onto the Pal(100) surface by heating a commercial SAES Getter source situated 3 cm from the sample. Adsorption of potassium. The deposition of potassium on a clean Pd(100) surface at 300 K lowered the work function of Pd by 3.65 eV at monolayer coverage. A monolayer of potassium was found to correspond to a surface * To whom all correspondence should be addressed.
0039-6028/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
.4. Berkb, F. Solymosi / Effects of K on chemisorption of C02 and CO on Pd(lO0)
L499
density of 6.2 × 10 ~4 a t o m s / c m 2, or OK -- 0.47 potassium atoms per surface palladium atom. The adsorption of potassium was accompanied by the appearance of new loss features in the electron energy~loss spectrum (EELS) in the electronic range at 19.4, 18.3 and 3.5 eV in accord with the results obtained for the K-Ni(100) system [8]. The peak temperature for the desorption of potassium is 460 K at monolayer and 345 K for the multilayer. The adsorbed CO 2 was found to stabilize the adsorption of potassium on the surface, and in the saturated adlayer desorption of potassium occurred simultaneously with that of CO 2. Adsorption of CO2. As concerns the adsorption of CO 2 the behaviour of Pd resembles that of Rh [6]; the adsorption is also weak and non-dissociative. Adsorption was observed only at 100 K. CO 2 desorbed mainly in one peak (denoted by a), with Tp = 135 K. With the increase of the exposure, the peak broadened, but no shift occurred in Tp. However, shoulders or breaks could be clearly seen on both sides of the peak. The weak adsorption of CO2 suggests that CO 2 bonds at the surface via a lone pair of an O, with the molecular bond vertically as proposed for Pt and Cu surfaces [9-11]. However, the preadsorbed potassium on the Pd surface dramatically affects the adsorption behaviour of CO 2 on Pd(100). It increases (i) the amount of weakly adsorbed CO2, (ii) the initial sticking coefficient of CO 2 adsorption in the range of 0 K = 0.0-0.15 from = 0.01 to near 1, (iii) the binding energy of CO 2 and (iv) it induces the formation of new adsorption states and lowers the activation barriers for dissociation. At 0 K = 0.05, a new adsorption state (fl) appeared with Tp = 185 K. With the increase of potassium coverage the peak temperature markedly shifted to higher values and from OK -- 0.42 another adsorption state (3') developed (fig. 1). The formation of these adsorption states also depended on the CO 2 exposure. At higher CO 2 coverages, new discrete, but rather broad desorption states with lower activation energies (between 89 and 62 k J / m o l ) also developed. The dissociation of CO 2 also occurred on potassium-dosed Pd. The lowest potassium coverage where the production of CO was clearly detected was OK = 0.21. CO desorbed from this surface in one peak (fl). The amount of CO formed increased considerably with the coverage of potassium up to 0 K = 0.47. At and above 0 K = 0.36 a new CO desorption peak (3') emerged. The peak temperatures for CO formation were 624 and 693 K, which are significantly higher than measured for the clean Pd(100) surface [7]. From the study of the effect of CO z exposure it appeared that the amount of CO produced attains saturation well before the saturation coverage for CO 2 is attained. The maximum amount of CO 2 adsorbed at monolayer K coverage is 2.8 x 1014 CO 2 molecules/cm 2. From this amount 1.6 × 1014 CO molecules/cm 2 are formed in the dissociation of CO 2. The adsorption of CO 2 on potassium-dosed Pd led to a work function
A. Berkb, E So6'most / Effects of K on chemtsorptmn of C02 und CO on Pd( lO0)
L500
zr-
8K--0.05 eK:O.lO eK=O2T
~=o26
e~ o.z,2
"4--
0
-
IE
100
300
500
I (K)
~
700
Fig. I. The effect of p o t a s s i u m coverage on the thermal d e s o r p t i o n of C O 2 ( OCO2 : 0.03) from Pd(100) surface. The a d s o r p t i o n t e m p e r a t u r e of C O 2 (0.1 L) was 100 K. The l o w - t e m p e r a t u r e state ( a ) is not shown.
increase of 2.25 eV at nearly a monolayer of potassium. This indicates a substantial charge transfer from the potassium-dosed metal to an empty CO 2 ~r* orbital and a corresponding large dipole moment of the chemisorbed CO,. This leads to a strengthening of the P d - C O bond and to a weakening of the C - O bond of the adsorbed CO 2, which results in its dissociation. It is very likely that the extended electronic interaction involves changes in the bonding and structure of the adsorbed CO 2, i.e. the formation of a metal-carbon bond in the form of a monodentate or bidentate structure. As the peak temperature of the high-temperature adsorption state of CO2(Y), coincides well with that of the ~, state of CO and also with the ), state of K desorption (fig. 2), we may conclude that this coincidence is a result of the formation and decomposition of a surface compound, very probably a carbonate-like species. Adsorption of CO. Thermal desorption spectra for CO from potassium-dosed Pd(100) have ben studied as functions of OK and CO exposure. Independently of the potassium coverage, CO desorbed only in one state, which can be attributed to molecular desorption. The peak temperature shifted from 470 K (clean surface) [7] to 619 K at monolayer K coverage, which means a 38 kJ / t o o l increase in the activation energy of the CO desorption. In this case the adsorption of CO at OK = 0.40 caused a work function increase of 1.53 eV. These features can be ascribed to the enhanced back donation of metal electrons into the lowest unoccupied molecular orbital (2~r*) of CO. There was no indication for the dissociation of CO and for the formation of a high-temperature desorption state for CO observed following CO 2 adsorption on the
A. Berkb, F. Solymosi / Effects of K on chemisorption of CO 2 and CO on Pd(lO0)
L501
cleonPd
OK=Ot*O
,
COIEO
Ix
OK=O.~2 -6 t[:D
~ /3 /';',,
COIC02
2
T x~
~ 'IF
8E02=0.19 (8K=0.6,2)
OJ
E o t_
W
KIK
~E
lOx
cleon Pd lO× KIK (OK= 0 . ~
300
K
560
T(K)
"
7()0
Fig. 2. Thermal desorption of K and CO from a clean Pd(100) surface and from the coadsorbed layers.
same surface. Accordingly preadsorbed potassium alters the bonding of CO to the Pd surface but its effect is not so dramatic as observed for example for Ni(100) [12], R h ( l l l ) [131, Ru(001) [141 surfaces. From this respect the behaviour of K + P d resembles that of K + Pt [15,16] and K - C u ( l l 0 ) systems [171.
References [1] E. Ramaroson, R. Kieffer and A. Kiennemann, J. Chem. Soc. Chem. Commun. 645 (1981). [2] E. Ramaroson, R. Kieffer and A. Kiennemann, J. Chim. Phys. 79 (1982) 749. [3] A. ErdiShelyi, M. Lancz and F. Solymosi, Proc. 5th Intern. Syrup. on Heterogeneous Catalysis, Varna, 1983, Part !I, p. 115.
L502 [4] [5] [6] [7] [8] [9] [10] [11] [12] 1131 [14] [15] [16] [17]
A. Berkb, F. Solymosi / Effects of K on chemisorption o / C O 2 and CO on Pd(lO0)
A. Erd~Shelyi, M. Pb.sztor and F. Solymosi, J. Catalysis, in press. F. Solymosi and A. ErdiShelyi, to be published. F. Solymosi and J. Kiss, Surface Sci. 149 (1985) 17. A. Ortega, F.M. Hoffmann and A.M. Bradshaw, Surface Sci. 119 (1982) 79. S. Anderson and U. Jostell, Surface Sci. 46 (1974) 625. N.K. Ray and B. Anderson, Surface Sci. 119 (1982) 35. P.R. Norton and P.J. Richards, Surface Sci. 49 (1975) 567. P.R. Norton and R.L. Trapping, Chem. Phys. Letters 38 0976) 207. H.S. Luftman, Y.M. Sun and J.M. White, Appl. Surface Sci. 19 (1984) 59. J.E. Crowell and G.A. Somorjai. Appl. Surface Sci. 19 (1984) 73. J.J. Weimer, E. Umbach and D. Menzel, Surface Sci. 155 (1985) 132. E.L. Garfunkel, J.E. Crowell and G.A. Somorjai, J. Phys. Chem. 86 (1982) 310. M. Kiskinova, G. Pirug and H.P. Bonzel, Surface Sci. 133 (1983) 321. D. Lackey. M. Surman, S. Jacobs, D. Grider and D.A. King, Surface Sci. 152/153 (1985) 513.
Surface Science 171 (1986) L498-L502 North-Holland, Amsterdam
S U R F A C E S C I E N C E LETTERS E F F E C T S OF P O T A S S I U M O N T H E C H E M I S O R P T I O N OF C O s AND C O O N T H E Pd(100) SURFACE Andrfis B E R K 0 and Frigyes SOLYMOSI * Reaction Kinetics Research Group of the Hungarian Academy of Sciences and Institute of Solid State and Radiochemistry, Unirersity of Szeged. PO Box 105. H-6701 Szeged, ltungar}, Received 27 November 1985; accepted for publication 21 February 1986
It has been found that preadsorbed potassium dramatically affects the adsorption behaviour of CO 2 and CO on Pd(lO0) surface. In increases the rate of adsorption, the binding energy of CO 2 and CO and it induces the dissociation of CO 2.
There has been an increasing interest in the catalytic transformation of CO 2 into more valuable compounds. Recently, it has been observed that palladium is an effective catalyst in the hydrogenation of CO 2 into methanol [1-4]. and the performance of the catalyst can be improved by means of alkali metal additives [5]. Whereas the effects of alkali metals on the chemisorption of CO over transition metals have been extensively studied in the past years, no reports were found in the literature on their influence on the interaction of CO 2 with metal single crystal surfaces. Furthermore, we found no information as regards the adsorption of potassium on Pd surfaces and the effect of preadsorbed potassium on the chemisorption of CO on Pd. The primary aim of the present study is to investigate the main characteristics of effects of potassium on the adsorption behaviour of CO 2 and CO on the Pd(100) surface. A standard U H V system containing facilities for LEED, AES, EELS, thermal desorption and work function (A~) measurements was used for the experiments. This was described in detail in our previous paper dealing with the adsorption of CO 2 on the R h ( l l l ) surface [6]. Sample preparation and cleaning followed previous procedures [7]. Potassium was deposited onto the Pal(100) surface by heating a commercial SAES Getter source situated 3 cm from the sample. Adsorption of potassium. The deposition of potassium on a clean Pd(100) surface at 300 K lowered the work function of Pd by 3.65 eV at monolayer coverage. A monolayer of potassium was found to correspond to a surface * To whom all correspondence should be addressed.
0039-6028/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
.4. Berkb, F. Solymosi / Effects of K on chemisorption of C02 and CO on Pd(lO0)
L499
density of 6.2 × 10 ~4 a t o m s / c m 2, or OK -- 0.47 potassium atoms per surface palladium atom. The adsorption of potassium was accompanied by the appearance of new loss features in the electron energy~loss spectrum (EELS) in the electronic range at 19.4, 18.3 and 3.5 eV in accord with the results obtained for the K-Ni(100) system [8]. The peak temperature for the desorption of potassium is 460 K at monolayer and 345 K for the multilayer. The adsorbed CO 2 was found to stabilize the adsorption of potassium on the surface, and in the saturated adlayer desorption of potassium occurred simultaneously with that of CO 2. Adsorption of CO2. As concerns the adsorption of CO 2 the behaviour of Pd resembles that of Rh [6]; the adsorption is also weak and non-dissociative. Adsorption was observed only at 100 K. CO 2 desorbed mainly in one peak (denoted by a), with Tp = 135 K. With the increase of the exposure, the peak broadened, but no shift occurred in Tp. However, shoulders or breaks could be clearly seen on both sides of the peak. The weak adsorption of CO2 suggests that CO 2 bonds at the surface via a lone pair of an O, with the molecular bond vertically as proposed for Pt and Cu surfaces [9-11]. However, the preadsorbed potassium on the Pd surface dramatically affects the adsorption behaviour of CO 2 on Pd(100). It increases (i) the amount of weakly adsorbed CO2, (ii) the initial sticking coefficient of CO 2 adsorption in the range of 0 K = 0.0-0.15 from = 0.01 to near 1, (iii) the binding energy of CO 2 and (iv) it induces the formation of new adsorption states and lowers the activation barriers for dissociation. At 0 K = 0.05, a new adsorption state (fl) appeared with Tp = 185 K. With the increase of potassium coverage the peak temperature markedly shifted to higher values and from OK -- 0.42 another adsorption state (3') developed (fig. 1). The formation of these adsorption states also depended on the CO 2 exposure. At higher CO 2 coverages, new discrete, but rather broad desorption states with lower activation energies (between 89 and 62 k J / m o l ) also developed. The dissociation of CO 2 also occurred on potassium-dosed Pd. The lowest potassium coverage where the production of CO was clearly detected was OK = 0.21. CO desorbed from this surface in one peak (fl). The amount of CO formed increased considerably with the coverage of potassium up to 0 K = 0.47. At and above 0 K = 0.36 a new CO desorption peak (3') emerged. The peak temperatures for CO formation were 624 and 693 K, which are significantly higher than measured for the clean Pd(100) surface [7]. From the study of the effect of CO z exposure it appeared that the amount of CO produced attains saturation well before the saturation coverage for CO 2 is attained. The maximum amount of CO 2 adsorbed at monolayer K coverage is 2.8 x 1014 CO 2 molecules/cm 2. From this amount 1.6 × 1014 CO molecules/cm 2 are formed in the dissociation of CO 2. The adsorption of CO 2 on potassium-dosed Pd led to a work function
A. Berkb, E So6'most / Effects of K on chemtsorptmn of C02 und CO on Pd( lO0)
L500
zr-
8K--0.05 eK:O.lO eK=O2T
~=o26
e~ o.z,2
"4--
0
-
IE
100
300
500
I (K)
~
700
Fig. I. The effect of p o t a s s i u m coverage on the thermal d e s o r p t i o n of C O 2 ( OCO2 : 0.03) from Pd(100) surface. The a d s o r p t i o n t e m p e r a t u r e of C O 2 (0.1 L) was 100 K. The l o w - t e m p e r a t u r e state ( a ) is not shown.
increase of 2.25 eV at nearly a monolayer of potassium. This indicates a substantial charge transfer from the potassium-dosed metal to an empty CO 2 ~r* orbital and a corresponding large dipole moment of the chemisorbed CO,. This leads to a strengthening of the P d - C O bond and to a weakening of the C - O bond of the adsorbed CO 2, which results in its dissociation. It is very likely that the extended electronic interaction involves changes in the bonding and structure of the adsorbed CO 2, i.e. the formation of a metal-carbon bond in the form of a monodentate or bidentate structure. As the peak temperature of the high-temperature adsorption state of CO2(Y), coincides well with that of the ~, state of CO and also with the ), state of K desorption (fig. 2), we may conclude that this coincidence is a result of the formation and decomposition of a surface compound, very probably a carbonate-like species. Adsorption of CO. Thermal desorption spectra for CO from potassium-dosed Pd(100) have ben studied as functions of OK and CO exposure. Independently of the potassium coverage, CO desorbed only in one state, which can be attributed to molecular desorption. The peak temperature shifted from 470 K (clean surface) [7] to 619 K at monolayer K coverage, which means a 38 kJ / t o o l increase in the activation energy of the CO desorption. In this case the adsorption of CO at OK = 0.40 caused a work function increase of 1.53 eV. These features can be ascribed to the enhanced back donation of metal electrons into the lowest unoccupied molecular orbital (2~r*) of CO. There was no indication for the dissociation of CO and for the formation of a high-temperature desorption state for CO observed following CO 2 adsorption on the
A. Berkb, F. Solymosi / Effects of K on chemisorption of CO 2 and CO on Pd(lO0)
L501
cleonPd
OK=Ot*O
,
COIEO
Ix
OK=O.~2 -6 t[:D
~ /3 /';',,
COIC02
2
T x~
~ 'IF
8E02=0.19 (8K=0.6,2)
OJ
E o t_
W
KIK
~E
lOx
cleon Pd lO× KIK (OK= 0 . ~
300
K
560
T(K)
"
7()0
Fig. 2. Thermal desorption of K and CO from a clean Pd(100) surface and from the coadsorbed layers.
same surface. Accordingly preadsorbed potassium alters the bonding of CO to the Pd surface but its effect is not so dramatic as observed for example for Ni(100) [12], R h ( l l l ) [131, Ru(001) [141 surfaces. From this respect the behaviour of K + P d resembles that of K + Pt [15,16] and K - C u ( l l 0 ) systems [171.
References [1] E. Ramaroson, R. Kieffer and A. Kiennemann, J. Chem. Soc. Chem. Commun. 645 (1981). [2] E. Ramaroson, R. Kieffer and A. Kiennemann, J. Chim. Phys. 79 (1982) 749. [3] A. ErdiShelyi, M. Lancz and F. Solymosi, Proc. 5th Intern. Syrup. on Heterogeneous Catalysis, Varna, 1983, Part !I, p. 115.
L502 [4] [5] [6] [7] [8] [9] [10] [11] [12] 1131 [14] [15] [16] [17]
A. Berkb, F. Solymosi / Effects of K on chemisorption o / C O 2 and CO on Pd(lO0)
A. Erd~Shelyi, M. Pb.sztor and F. Solymosi, J. Catalysis, in press. F. Solymosi and A. ErdiShelyi, to be published. F. Solymosi and J. Kiss, Surface Sci. 149 (1985) 17. A. Ortega, F.M. Hoffmann and A.M. Bradshaw, Surface Sci. 119 (1982) 79. S. Anderson and U. Jostell, Surface Sci. 46 (1974) 625. N.K. Ray and B. Anderson, Surface Sci. 119 (1982) 35. P.R. Norton and P.J. Richards, Surface Sci. 49 (1975) 567. P.R. Norton and R.L. Trapping, Chem. Phys. Letters 38 0976) 207. H.S. Luftman, Y.M. Sun and J.M. White, Appl. Surface Sci. 19 (1984) 59. J.E. Crowell and G.A. Somorjai. Appl. Surface Sci. 19 (1984) 73. J.J. Weimer, E. Umbach and D. Menzel, Surface Sci. 155 (1985) 132. E.L. Garfunkel, J.E. Crowell and G.A. Somorjai, J. Phys. Chem. 86 (1982) 310. M. Kiskinova, G. Pirug and H.P. Bonzel, Surface Sci. 133 (1983) 321. D. Lackey. M. Surman, S. Jacobs, D. Grider and D.A. King, Surface Sci. 152/153 (1985) 513.