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Energetics and CO-induced lifting of a (1×2) surface reconstruction observed on Pt{311}
5 November 1999
Chemical Physics Letters 313 Ž1999. 1–6 www.elsevier.nlrlocatercplett
Energetics and CO-induced lifting of a ž1 = 2 / surface reconstruction observed on Pt ½311 5 Rickmer Kose 1, David A. King
)
Department of Chemistry, UniÕersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK Received 4 June 1999
Abstract Despite expectations from the literature, clean Pt3114 is shown to exhibit a Ž1 = 2. reconstruction, and CO adsorption leads to a lifting of this reconstruction to a Ž1 = 1. Pt structure. The reconstruction and its lifting are similar to those observed for CO on Pt1104. A Ž1 = 1. island nucleation process occurs above a critical coverage of around 0.16 ML. Sticking probabilities and calorimetric adsorption heats are reported, and from the data the energy difference between Pt3114-Ž1 = 1. and Ž1 = 2. is estimated to be in the order of ; 10 kJ moly1. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The restructuring of clean metal surfaces, and the alteration of the substrate structure by adsorption, has received considerable attention w1x. Amongst the fcc metals, it is well established that the clean 1004 and 1104 surfaces of Pt, Au and Ir are reconstructed w2x. In particular, the 1104 surfaces of these metals show a missing row Ž1 = 2. reconstruction, but, surprisingly, Blakely and Somorjai w3x reported some years ago that the structurally similar 3114 surface of Pt forms a stable Ž1 = 1. surface. Recently, a missing row Pt 3114 -Ž1 = 2. reconstruction was observed by Gaussmann and Kruse using field ion microscopy ŽFIM. w4x. In this study it was reported
) Corresponding author. Fax: Žq44. 1223 336 362; e-mail: [email protected] 1 Present address: Sandia National Laboratories, Livermore, California, 94551-0969, USA
that CO adsorption does not lift the reconstruction, in contrast to the behaviour of Pt 1104 -Ž1 = 2. w5x. Extensive studies of the oxidation of CO on Pt 3114 have been reported by Matsushima and coworkers w6–16x, performed using electron stimulated desorption ŽESD., time-of-flight mass spectrometry ŽTOF. and photodesorption Ž193 nm.. Low energy electron diffraction ŽLEED. studies of the clean or CO-covered Pt 3114 surface are not explicitly reported in these references, and indeed, despite the contrary suggestion, there are no other structural studies relevant to this system in the literature. The study cited, by Xu and Yates w17x, in fact refers to work on Pt 2114 , not Pt 3114 . However, Yamanaka et al. w10x describe the LEED pattern observed during their initial cleaning bulk procedure as ‘characteristic of stepped surfaces with two-atom wide terraces’. Here we provide clear evidence for the first time that the clean 3114 bulk crystal plane of platinum exhibits a stable Ž1 = 2. reconstruction, and that this reconstruction is lifted by CO adsorption above a
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 1 0 1 9 - 2
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R. Kose, D.A. King r Chemical Physics Letters 313 (1999) 1–6
critical coverage of 0.16 monolayers ŽML.. The system has been further studied by molecular beam single crystal adsorption calorimetry ŽSCAC., yielding sticking probabilities and adsorption heats. Further analysis allows an estimate to be made of the energy difference between the Pt 3114 -Ž1 = 1. and Ž1 = 2. planes.
and subsequent annealing to ; 700 K. Crystal cleanliness and structure were checked using Auger electron spectroscopy ŽAES. and LEED. The CO coverage is defined as 1 ML when every Pt atom in the top Ž1 = 1. layer is occupied by a CO molecule.
3. Results 2. Experimental 3.1. Low energy electron diffraction The experiments were carried out using the single crystal adsorption calorimeter developed in these laboratories, in an ultra-high vacuum chamber with a base pressure of F 7 = 10y1 1 mbar. CO was adsorbed at room temperature using a pulsed molecular beam w18x Ž50 ms pulse width, 2.5 s repetition period, ; 2 = 10 12 molecules per pulse.. To achieve a measurable change in crystal temperature upon adsorption, the crystal heat capacity is kept low by employing single crystals that are approximately 200 nm thick. The temperature is remotely monitored using a commercially available mercury cadmium telluride ŽMCT. infrared detector. Typically, the initial adsorption of a single pulse of gas leads to a temperature rise of ; 0.1 K at the crystal surface with a srn ratio of the amplified infrared signal of ; 100:1. Parallel to the heat measurements, sticking probabilities were determined using the King and Wells method w19x. We use the term ‘apparent coverage’ for the presentation; it refers to the coverage determination via integration of successive pulse dependent sticking probabilities. Any species desorbing between pulses cannot be detected and will thus indefinitely extend the coverage scale when close to or at adsorption-desorption equilibrium. We stress, however, that the coverages are absolutely determined, and for coverages where the adsorption heat is G 100 kJ moly1 , and the desorption rate is negligible, the coverage scale is accurate. The sticking probability and the adsorption heat measured at steady state are referred to as the ‘steady state sticking coefficient’ and the ‘steady state heat’, respectively. A detailed description of the experiment and principles can be found elsewhere w18,20x. Cleaning of the Pt 3114 crystal was achieved via Ar-ion sputtering at discharge currents below 8 mA
The Pt 3114 surface was investigated by LEED Žall experiments were performed at 300 K., and the diffraction patterns are reproduced in Fig. 1. Both images were taken at an incident electron energy of 69 eV. A representative image of the clean surface is given in Fig. 1a, clearly showing the formation of a Ž1 = 2. structure. Fig. 1b shows the corresponding LEED pattern after ; 1 L CO exposure. The fractional order spots have disappeared, leaving integral order beams only. The intensity of the fractional order Ž0.5,0. spot as a function of CO exposure is reproduced in Fig. 2. The fractional spot intensity changes only very slightly for coverages below ; 0.16 ML, but then falls linearly with coverage until the diffraction spots vanish at a coverage of ; 0.46 ML. It is noted that this behaviour is very similar to that reported for CO on Pt 1104 -Ž1 = 2. : at low
Fig. 1. LEED patterns for Ža. clean and Žb. CO-covered Pt3114 at 300 K. Both images were taken at an incident electron energy of 69 eV. The Ž0,0. spot is obscured by the electron gun, which is indicated by the dotted line.
R. Kose, D.A. King r Chemical Physics Letters 313 (1999) 1–6
Fig. 2. CO exposure dependent Ž0.5,0. spot intensity for Pt3114Ž1=2. ™ Ž1=1. . The line through the data points is based on an interpolating spline and only serves as a guide to the eye. The incident electron energy was 69 eV.
coverages CO adsorption occurs onto the Ž1 = 2. surface, but above 0.2 ML the reconstruction is lifted continuously with increasing CO coverage w21x. The local coverage in the Ž1 = 1. domains where CO has lifted the reconstruction is, from this experiment, ; 0.46 ML. A cŽ2 = 2. -CO overlayer is formed on the Ž1 = 1. Pt surface in these growing islands. 3.2. Adsorption heats and sticking probabilities Each of the data points shown in Figs. 3 and 4 is the result of averaging heat and sticking probability data from 5 experimental runs for the adsorption of
Fig. 3. Coverage-dependent differential heat of adsorption for CO on Pt3114 adsorbed at 300 K. Open circles are averaged experimental data points, and the line represents a guide to the eye.
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Fig. 4. Coverage-dependent sticking probability for CO adsorbed on Pt3114 at 300 K. Open circles are averaged experimental data points, and the solid line represents a guide to the eye through these points.
CO on Pt 3114 . One data point on the heat or sticking plot corresponds to ; 40 to 50 original data points in the experiment. The error bars shown indicate the standard deviation corresponding to this averaging procedure. Four adsorption regimes can be identified. The initial heat of adsorption ŽFig. 3. is 210 " 7 kJ moly1 , but drops quickly at a coverage of ; 0.16 ML to reach a plateau for the coverage range of 0.16 F Q F 0.5 ML, where the heat of adsorption is ; 200 " 7 kJ moly1 . The heat does not significantly change over this range. For coverages very close to 0.5 ML, the heat of adsorption suddenly drops sharply to reach a second plateau. Over the range between 0.5 and ; 1 ML, where again the heat of adsorption remains almost constant with a value of 190 " 6 kJ moly1 . At the end of this second plateau, the heat of adsorption undergoes another sudden change to drop off to its steady state value of 110 " 15 kJ moly1 , marking the beginning of the third plateau Žfor an apparent coverage above 1 ML.. The sticking probability in the zero coverage limit is 0.82 " 0.02. It falls to a value of 0.56 at a coverage of 0.5 ML, and suddenly changes slope at a coverage of ; 0.55 ML to fall at a higher rate until the saturation coverage of ; 1 ML is established. The sticking probability at the end of the second regime, at the point where steady state is reached, is 0.03.
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4. Discussion 4.1. Surface reconstruction The observed Ž1 = 2. LEED pattern demonstrates a significant surface reconstruction of the clean Pt 3114 substrate, which could be very similar to the missing row reconstruction observed on Pt 1104 . In Fig. 5, we compare these two surfaces in their Ž1 = 1. and Ž1 = 2. phases, clearly showing that there are only a few steric differences between Pt 1104 and Pt 3114 . These can be best expressed in terms of atomic coordination numbers. For both surfaces in their Ž1 = 1. and Ž1 = 2. phases, the coordination number for the top layer atoms is 7. For Pt 3114 , the rows have a different alignment than for
Pt 1104 , resulting in second layer coordination numbers of 8 and 9 and a third layer coordination number of 10 for Pt 3114 -Ž1 = 2. , whereas for Pt 1104 -Ž1 = 2. these are 9 Žsecond layer. and 11 Žthird layer.. While Pt 1104 exhibits 1114 facets only, Pt 3114 shows a mixture of 1004 and 1114 facets. The model for a Ž1 = 2. reconstruction of the clean Pt 3114 surface is not only accompanied by the observation of a similar reconstruction on the sterically similar Pt 1104 surface w2x, but strongly supported by observations made by Gaussmann and Kruse using FIM w4x. They have recently observed a Ž1 = 2. missing row reconstruction of the clean Pt 3114 facets on an FIM tip which is the only previous study further substantiating the reconstruc-
Fig. 5. CO induced lifting of the Pt1104-Ž1 = 2. and Pt3114-Ž1 = 2. structures. The numbers indicate corresponding atomic coordination numbers.
R. Kose, D.A. King r Chemical Physics Letters 313 (1999) 1–6
tion model for clean Pt 3114 . These results suggest that the Ž1 = 1. structure for the Pt 3114 substrate, observed by Blakely and Somorjai w3x, could be due to the presence of residual carbon andror poor UHV conditions. After adsorbing oxygen a Ž1 = 2. structure formed, which could be due to a clean-off of surface impurities, establishing the original Žclean. reconstructed Ž1 = 2. phase. This interpretation would be in agreement with our heat of adsorption measurements for O 2 on a Pt 3114 crystal covered with a small amount of carbon: an initial higher adsorption heat was observed, which is attributed to the formation of gaseous CO 2 from the surface reaction between oxygen and carbon w22x. It is noted, however, that there are no studies concerning the structure of adsorbed oxygen on Pt 3114 . 4.2. Adsorption kinetics, heats of adsorption and the process of reconstruction lifting The Pt 3114 -Ž1 = 2. reconstruction is presumably lifted with the formation of randomly distributed Ž1 = 1. islands with adsorbed CO once the adsorbate coverage exceeds 0.16 ML. As can be seen from the measurement of fractional LEED spot intensity ŽFig. 2., the lifting of the reconstruction occurs over the CO coverage range 0.2 to 0.5 ML. At 0.5 ML, the reconstruction is completely lifted and the substrate exhibits a Pt 3114 -Ž1 = 1. structure with an ordered CO layer, whereby every other top-layer Pt atom is occupied by a CO molecule. At coverages above 0.5 ML, the heat of CO adsorption is lowered due to nn CO–CO interactions, which is reflected by a second plateau in the heat of adsorption. The adsorption of CO molecules on sites other than on Pt atoms on the ridges would be highly unfavourable, as was clearly shown by density functional theory ŽDFT. calculations for CO adsorbed on Pd 1104 w23x and on Pt 1104 w24x. Assuming that the observed drop in adsorption heat Ž16 kJ moly1 . is entirely due to nn interactions, the CO–CO nn pairwise interaction energy can be estimated as ; 8 kJ moly1 which is in very close agreement with the CO nn interaction energy determined by SCAC for COrRh 1004 w25x. This reconstruction process is comparable to that observed on Pt 1104 , where there is a clear steric similarity of the structures in their respective Ž1 = 1. and Ž1 = 2. phases ŽFig. 5.. However, surface Pt
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atom coordination numbers Žexcept the top layer. are generally lower on Pt 3114 , compared with Pt 1104 w21,26x, which agrees with the overall higher heat of adsorption measured for CO adsorption on Pt 3114 . Although Gaussmann and Kruse observed a Ž1 = 2. reconstruction of the clean Pt 3114 surface, they reported no evidence for its CO-induced lifting. This does not necessarily imply a disagreement between the FIM observations and our experiments, as there are fundamental differences between FIM studies and studies on bulk single crystals. In FIM, the crystal is essentially three-dimensional, extending over a range of different crystal faces, each small in extension. Between these, material exchange can easily occur, especially if this lowers the total energy of the tip. Additionally, the extremely strong fields required for cleaning and imaging can potentially inhibit or favour specific processes that would otherwise not occur on a two-dimensional crystal w27x. Discrepancies between FIM and conventional surface science studies have also been found for CO adsorbed on Pt 1104 , where Gaussmann and Kruse found only a partial lifting of the Pt 1104 -Ž1 = 2. structure, whereas for Pt 1104 bulk crystals, the reconstruction is completely lifted w28–30x. As the drop in the adsorption heat for COrPt 3114 at a critical coverage of ; 0.16 ML is entirely due to the lifting of the surface reconstruction, the energy difference between the two phases can be estimated. Extrapolating the heats of adsorption at G 0.16 ML back to the zero coverage limit yields an energy difference between the Pt 3114 -Ž1 = 2. and Ž1 = 1. phases of ; 10 kJ moly1 . This energy compares well with the previously calorimetrically determined energy difference of 12 kJ moly1 between Pt 1004 hex and Ž1 = 1. w26,31x, and the theoretically estimated energy difference between Ir 1004 -Ž1 = 5. and Ž1 = 1. of 6 kJ moly1 w32x.
5. Summary and conclusions The clean Pt 3114 substrate is Ž1 = 2. reconstructed and CO adsorption leads to a lifting of the reconstruction. This reconstruction and the lifting process are similar to those observed for Pt 1104 and we note the steric similarities of the two bulk terminations, Pt 1104 and Pt 3114 . The energy difference
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R. Kose, D.A. King r Chemical Physics Letters 313 (1999) 1–6
between Pt 3114 -Ž1 = 2. and Pt 3114 -Ž1 = 1. is estimated as 10 kJ moly1 .
Acknowledgements Qingfeng Ge and Simon Titmuss are acknowledged for stimulating discussions, Andrew Karmazyn for technical assistance, and Jacques Chevallier for supplying and mounting the Pt crystals. The Oppenheimer Trust is acknowledged for a Fellowship ŽR.K.., and the EPSRC for an equipment grant.
References w1x S. Titmuss, A. Wander, D.A. King, Chem. Rev. 96 Ž1996. 1291. w2x D.A. King, D.P. Woodruff, The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis. Volume 7: Phase Transitions and Adsorbate Restructuring at Metal Surfaces, Elsevier, Amsterdam, 1994. w3x D.W. Blakely, G.A. Somorjai, Surf. Sci. 65 Ž1977. 419. w4x A. Gaussmann, N. Kruse, Surf. Sci. 266 Ž1992. 46. w5x A. Gaussmann, N. Kruse, Surf. Sci. 279 Ž1992. 319. w6x A. Lesar, T. Yamanaka, Y. Ohno, T. Matsushima, Chem. Phys. Lett. 234 Ž1995. 330. w7x T. Yamanaka, T. Matsushima, S. Tanaka, M. Kamada, Surf. Sci. 349 Ž1996. 119. w8x T. Yamanaka, T. Matsushima, S. Tanaka, M. Kamada, J. Elec. Spec. Rel. Phen. 80 Ž1996. 105. w9x T. Yamanaka, Y. Ohno, A. Lesar, T. Matsushima, C. Moise, Surf. Sci. 358 Ž1996. 759. w10x T. Yamanaka, Y. Inoue, T. Matsushima, Chem. Phys. Lett. 264 Ž1997. 180.
w11x H. Akiyama, C. Moise, T. Yamanaka, K. Jacobi, T. Matsushima, Chem. Phys. Lett. 272 Ž1997. 219. w12x T. Yamanaka, C. Moise, T. Matsushima, J. Chem. Phys. 107 Ž1997. 8138. w13x T. Yamanaka, Y. Inoue, M. Sano, C. Moise, T. Matsushima, Appl. Surf. Sci. 121r122 Ž1997. 601. w14x Y. Ohno, T. Yamanaka, H. Akiyama, Y. Seimiya, T. Matsushima, Appl. Surf. Sci. 121r122 Ž1997. 567. w15x M. Sano, Y. Ohno, T. Yamanaka, T. Matsushima, E.B. Quinay, K. Jacobi, J. Chem. Phys. 108 Ž1998. 10231. w16x G.Y. Cao, Y. Seimiya, Y. Ohno, T. Matsushima, Chem. Phys. Lett. 294 Ž1998. 419. w17x J.Z. Xu, J.T. Yates, Surf. Sci. 327 Ž1995. 193. w18x C.E. Borroni-Bird, D.A. King, Rev. Sci. Instrum. 62 Ž1991. 2177. w19x D.A. King, M.G. Wells, Surf. Sci. 29 Ž1972. 454. w20x A. Stuck, C.E. Wartnaby, Y.Y. Yeo, J.T. Stuckless, N. Al-Sarraf, D.A. King, Surf. Sci. 349 Ž1996. 229. w21x C.E. Wartnaby, A. Stuck, Y.Y. Yeo, D.A. King, J. Phys. Chem. 100 Ž1996. 12483. w22x R. Kose, W.A. Brown, D.A. King, in preparation. w23x P. Hu, D.A. King, S. Crampin, M.-H. Lee, M.C. Payne, J. Chem. Phys. 107 Ž1997. 8103. w24x Q. Ge, D.A. King, J. Chem. Phys., in press, 1999. w25x R. Kose, W.A. Brown, D.A. King, J. Chem. Phys., in press, 1999. w26x W.A. Brown, R. Kose, D.A. King, Chem. Rev. 98 Ž1998. 797. w27x N. Kruse, personal communication, 1998. w28x P. Hofmann, S.R. Bare, D.A. King, Surf. Sci. 117 Ž1982. 245. w29x S.R. Bare, P. Hofmann, D.A. King, Surf. Sci. 144 Ž1984. 347. w30x T. Gritsch, D. Coulman, R.J. Behm, G. Ertl, Phys. Rev. Lett. 63 Ž1989. 1086. w31x Y.Y. Yeo, C.E. Wartnaby, D.A. King, Science 268 Ž1995. 1731. w32x Q. Ge, D.A. King, N. Marzari, M.C. Payne, Surf. Sci. 418 Ž1998. 529.
Chemical Physics Letters 313 Ž1999. 1–6 www.elsevier.nlrlocatercplett
Energetics and CO-induced lifting of a ž1 = 2 / surface reconstruction observed on Pt ½311 5 Rickmer Kose 1, David A. King
)
Department of Chemistry, UniÕersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK Received 4 June 1999
Abstract Despite expectations from the literature, clean Pt3114 is shown to exhibit a Ž1 = 2. reconstruction, and CO adsorption leads to a lifting of this reconstruction to a Ž1 = 1. Pt structure. The reconstruction and its lifting are similar to those observed for CO on Pt1104. A Ž1 = 1. island nucleation process occurs above a critical coverage of around 0.16 ML. Sticking probabilities and calorimetric adsorption heats are reported, and from the data the energy difference between Pt3114-Ž1 = 1. and Ž1 = 2. is estimated to be in the order of ; 10 kJ moly1. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The restructuring of clean metal surfaces, and the alteration of the substrate structure by adsorption, has received considerable attention w1x. Amongst the fcc metals, it is well established that the clean 1004 and 1104 surfaces of Pt, Au and Ir are reconstructed w2x. In particular, the 1104 surfaces of these metals show a missing row Ž1 = 2. reconstruction, but, surprisingly, Blakely and Somorjai w3x reported some years ago that the structurally similar 3114 surface of Pt forms a stable Ž1 = 1. surface. Recently, a missing row Pt 3114 -Ž1 = 2. reconstruction was observed by Gaussmann and Kruse using field ion microscopy ŽFIM. w4x. In this study it was reported
) Corresponding author. Fax: Žq44. 1223 336 362; e-mail: [email protected] 1 Present address: Sandia National Laboratories, Livermore, California, 94551-0969, USA
that CO adsorption does not lift the reconstruction, in contrast to the behaviour of Pt 1104 -Ž1 = 2. w5x. Extensive studies of the oxidation of CO on Pt 3114 have been reported by Matsushima and coworkers w6–16x, performed using electron stimulated desorption ŽESD., time-of-flight mass spectrometry ŽTOF. and photodesorption Ž193 nm.. Low energy electron diffraction ŽLEED. studies of the clean or CO-covered Pt 3114 surface are not explicitly reported in these references, and indeed, despite the contrary suggestion, there are no other structural studies relevant to this system in the literature. The study cited, by Xu and Yates w17x, in fact refers to work on Pt 2114 , not Pt 3114 . However, Yamanaka et al. w10x describe the LEED pattern observed during their initial cleaning bulk procedure as ‘characteristic of stepped surfaces with two-atom wide terraces’. Here we provide clear evidence for the first time that the clean 3114 bulk crystal plane of platinum exhibits a stable Ž1 = 2. reconstruction, and that this reconstruction is lifted by CO adsorption above a
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 1 0 1 9 - 2
2
R. Kose, D.A. King r Chemical Physics Letters 313 (1999) 1–6
critical coverage of 0.16 monolayers ŽML.. The system has been further studied by molecular beam single crystal adsorption calorimetry ŽSCAC., yielding sticking probabilities and adsorption heats. Further analysis allows an estimate to be made of the energy difference between the Pt 3114 -Ž1 = 1. and Ž1 = 2. planes.
and subsequent annealing to ; 700 K. Crystal cleanliness and structure were checked using Auger electron spectroscopy ŽAES. and LEED. The CO coverage is defined as 1 ML when every Pt atom in the top Ž1 = 1. layer is occupied by a CO molecule.
3. Results 2. Experimental 3.1. Low energy electron diffraction The experiments were carried out using the single crystal adsorption calorimeter developed in these laboratories, in an ultra-high vacuum chamber with a base pressure of F 7 = 10y1 1 mbar. CO was adsorbed at room temperature using a pulsed molecular beam w18x Ž50 ms pulse width, 2.5 s repetition period, ; 2 = 10 12 molecules per pulse.. To achieve a measurable change in crystal temperature upon adsorption, the crystal heat capacity is kept low by employing single crystals that are approximately 200 nm thick. The temperature is remotely monitored using a commercially available mercury cadmium telluride ŽMCT. infrared detector. Typically, the initial adsorption of a single pulse of gas leads to a temperature rise of ; 0.1 K at the crystal surface with a srn ratio of the amplified infrared signal of ; 100:1. Parallel to the heat measurements, sticking probabilities were determined using the King and Wells method w19x. We use the term ‘apparent coverage’ for the presentation; it refers to the coverage determination via integration of successive pulse dependent sticking probabilities. Any species desorbing between pulses cannot be detected and will thus indefinitely extend the coverage scale when close to or at adsorption-desorption equilibrium. We stress, however, that the coverages are absolutely determined, and for coverages where the adsorption heat is G 100 kJ moly1 , and the desorption rate is negligible, the coverage scale is accurate. The sticking probability and the adsorption heat measured at steady state are referred to as the ‘steady state sticking coefficient’ and the ‘steady state heat’, respectively. A detailed description of the experiment and principles can be found elsewhere w18,20x. Cleaning of the Pt 3114 crystal was achieved via Ar-ion sputtering at discharge currents below 8 mA
The Pt 3114 surface was investigated by LEED Žall experiments were performed at 300 K., and the diffraction patterns are reproduced in Fig. 1. Both images were taken at an incident electron energy of 69 eV. A representative image of the clean surface is given in Fig. 1a, clearly showing the formation of a Ž1 = 2. structure. Fig. 1b shows the corresponding LEED pattern after ; 1 L CO exposure. The fractional order spots have disappeared, leaving integral order beams only. The intensity of the fractional order Ž0.5,0. spot as a function of CO exposure is reproduced in Fig. 2. The fractional spot intensity changes only very slightly for coverages below ; 0.16 ML, but then falls linearly with coverage until the diffraction spots vanish at a coverage of ; 0.46 ML. It is noted that this behaviour is very similar to that reported for CO on Pt 1104 -Ž1 = 2. : at low
Fig. 1. LEED patterns for Ža. clean and Žb. CO-covered Pt3114 at 300 K. Both images were taken at an incident electron energy of 69 eV. The Ž0,0. spot is obscured by the electron gun, which is indicated by the dotted line.
R. Kose, D.A. King r Chemical Physics Letters 313 (1999) 1–6
Fig. 2. CO exposure dependent Ž0.5,0. spot intensity for Pt3114Ž1=2. ™ Ž1=1. . The line through the data points is based on an interpolating spline and only serves as a guide to the eye. The incident electron energy was 69 eV.
coverages CO adsorption occurs onto the Ž1 = 2. surface, but above 0.2 ML the reconstruction is lifted continuously with increasing CO coverage w21x. The local coverage in the Ž1 = 1. domains where CO has lifted the reconstruction is, from this experiment, ; 0.46 ML. A cŽ2 = 2. -CO overlayer is formed on the Ž1 = 1. Pt surface in these growing islands. 3.2. Adsorption heats and sticking probabilities Each of the data points shown in Figs. 3 and 4 is the result of averaging heat and sticking probability data from 5 experimental runs for the adsorption of
Fig. 3. Coverage-dependent differential heat of adsorption for CO on Pt3114 adsorbed at 300 K. Open circles are averaged experimental data points, and the line represents a guide to the eye.
3
Fig. 4. Coverage-dependent sticking probability for CO adsorbed on Pt3114 at 300 K. Open circles are averaged experimental data points, and the solid line represents a guide to the eye through these points.
CO on Pt 3114 . One data point on the heat or sticking plot corresponds to ; 40 to 50 original data points in the experiment. The error bars shown indicate the standard deviation corresponding to this averaging procedure. Four adsorption regimes can be identified. The initial heat of adsorption ŽFig. 3. is 210 " 7 kJ moly1 , but drops quickly at a coverage of ; 0.16 ML to reach a plateau for the coverage range of 0.16 F Q F 0.5 ML, where the heat of adsorption is ; 200 " 7 kJ moly1 . The heat does not significantly change over this range. For coverages very close to 0.5 ML, the heat of adsorption suddenly drops sharply to reach a second plateau. Over the range between 0.5 and ; 1 ML, where again the heat of adsorption remains almost constant with a value of 190 " 6 kJ moly1 . At the end of this second plateau, the heat of adsorption undergoes another sudden change to drop off to its steady state value of 110 " 15 kJ moly1 , marking the beginning of the third plateau Žfor an apparent coverage above 1 ML.. The sticking probability in the zero coverage limit is 0.82 " 0.02. It falls to a value of 0.56 at a coverage of 0.5 ML, and suddenly changes slope at a coverage of ; 0.55 ML to fall at a higher rate until the saturation coverage of ; 1 ML is established. The sticking probability at the end of the second regime, at the point where steady state is reached, is 0.03.
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4. Discussion 4.1. Surface reconstruction The observed Ž1 = 2. LEED pattern demonstrates a significant surface reconstruction of the clean Pt 3114 substrate, which could be very similar to the missing row reconstruction observed on Pt 1104 . In Fig. 5, we compare these two surfaces in their Ž1 = 1. and Ž1 = 2. phases, clearly showing that there are only a few steric differences between Pt 1104 and Pt 3114 . These can be best expressed in terms of atomic coordination numbers. For both surfaces in their Ž1 = 1. and Ž1 = 2. phases, the coordination number for the top layer atoms is 7. For Pt 3114 , the rows have a different alignment than for
Pt 1104 , resulting in second layer coordination numbers of 8 and 9 and a third layer coordination number of 10 for Pt 3114 -Ž1 = 2. , whereas for Pt 1104 -Ž1 = 2. these are 9 Žsecond layer. and 11 Žthird layer.. While Pt 1104 exhibits 1114 facets only, Pt 3114 shows a mixture of 1004 and 1114 facets. The model for a Ž1 = 2. reconstruction of the clean Pt 3114 surface is not only accompanied by the observation of a similar reconstruction on the sterically similar Pt 1104 surface w2x, but strongly supported by observations made by Gaussmann and Kruse using FIM w4x. They have recently observed a Ž1 = 2. missing row reconstruction of the clean Pt 3114 facets on an FIM tip which is the only previous study further substantiating the reconstruc-
Fig. 5. CO induced lifting of the Pt1104-Ž1 = 2. and Pt3114-Ž1 = 2. structures. The numbers indicate corresponding atomic coordination numbers.
R. Kose, D.A. King r Chemical Physics Letters 313 (1999) 1–6
tion model for clean Pt 3114 . These results suggest that the Ž1 = 1. structure for the Pt 3114 substrate, observed by Blakely and Somorjai w3x, could be due to the presence of residual carbon andror poor UHV conditions. After adsorbing oxygen a Ž1 = 2. structure formed, which could be due to a clean-off of surface impurities, establishing the original Žclean. reconstructed Ž1 = 2. phase. This interpretation would be in agreement with our heat of adsorption measurements for O 2 on a Pt 3114 crystal covered with a small amount of carbon: an initial higher adsorption heat was observed, which is attributed to the formation of gaseous CO 2 from the surface reaction between oxygen and carbon w22x. It is noted, however, that there are no studies concerning the structure of adsorbed oxygen on Pt 3114 . 4.2. Adsorption kinetics, heats of adsorption and the process of reconstruction lifting The Pt 3114 -Ž1 = 2. reconstruction is presumably lifted with the formation of randomly distributed Ž1 = 1. islands with adsorbed CO once the adsorbate coverage exceeds 0.16 ML. As can be seen from the measurement of fractional LEED spot intensity ŽFig. 2., the lifting of the reconstruction occurs over the CO coverage range 0.2 to 0.5 ML. At 0.5 ML, the reconstruction is completely lifted and the substrate exhibits a Pt 3114 -Ž1 = 1. structure with an ordered CO layer, whereby every other top-layer Pt atom is occupied by a CO molecule. At coverages above 0.5 ML, the heat of CO adsorption is lowered due to nn CO–CO interactions, which is reflected by a second plateau in the heat of adsorption. The adsorption of CO molecules on sites other than on Pt atoms on the ridges would be highly unfavourable, as was clearly shown by density functional theory ŽDFT. calculations for CO adsorbed on Pd 1104 w23x and on Pt 1104 w24x. Assuming that the observed drop in adsorption heat Ž16 kJ moly1 . is entirely due to nn interactions, the CO–CO nn pairwise interaction energy can be estimated as ; 8 kJ moly1 which is in very close agreement with the CO nn interaction energy determined by SCAC for COrRh 1004 w25x. This reconstruction process is comparable to that observed on Pt 1104 , where there is a clear steric similarity of the structures in their respective Ž1 = 1. and Ž1 = 2. phases ŽFig. 5.. However, surface Pt
5
atom coordination numbers Žexcept the top layer. are generally lower on Pt 3114 , compared with Pt 1104 w21,26x, which agrees with the overall higher heat of adsorption measured for CO adsorption on Pt 3114 . Although Gaussmann and Kruse observed a Ž1 = 2. reconstruction of the clean Pt 3114 surface, they reported no evidence for its CO-induced lifting. This does not necessarily imply a disagreement between the FIM observations and our experiments, as there are fundamental differences between FIM studies and studies on bulk single crystals. In FIM, the crystal is essentially three-dimensional, extending over a range of different crystal faces, each small in extension. Between these, material exchange can easily occur, especially if this lowers the total energy of the tip. Additionally, the extremely strong fields required for cleaning and imaging can potentially inhibit or favour specific processes that would otherwise not occur on a two-dimensional crystal w27x. Discrepancies between FIM and conventional surface science studies have also been found for CO adsorbed on Pt 1104 , where Gaussmann and Kruse found only a partial lifting of the Pt 1104 -Ž1 = 2. structure, whereas for Pt 1104 bulk crystals, the reconstruction is completely lifted w28–30x. As the drop in the adsorption heat for COrPt 3114 at a critical coverage of ; 0.16 ML is entirely due to the lifting of the surface reconstruction, the energy difference between the two phases can be estimated. Extrapolating the heats of adsorption at G 0.16 ML back to the zero coverage limit yields an energy difference between the Pt 3114 -Ž1 = 2. and Ž1 = 1. phases of ; 10 kJ moly1 . This energy compares well with the previously calorimetrically determined energy difference of 12 kJ moly1 between Pt 1004 hex and Ž1 = 1. w26,31x, and the theoretically estimated energy difference between Ir 1004 -Ž1 = 5. and Ž1 = 1. of 6 kJ moly1 w32x.
5. Summary and conclusions The clean Pt 3114 substrate is Ž1 = 2. reconstructed and CO adsorption leads to a lifting of the reconstruction. This reconstruction and the lifting process are similar to those observed for Pt 1104 and we note the steric similarities of the two bulk terminations, Pt 1104 and Pt 3114 . The energy difference
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R. Kose, D.A. King r Chemical Physics Letters 313 (1999) 1–6
between Pt 3114 -Ž1 = 2. and Pt 3114 -Ž1 = 1. is estimated as 10 kJ moly1 .
Acknowledgements Qingfeng Ge and Simon Titmuss are acknowledged for stimulating discussions, Andrew Karmazyn for technical assistance, and Jacques Chevallier for supplying and mounting the Pt crystals. The Oppenheimer Trust is acknowledged for a Fellowship ŽR.K.., and the EPSRC for an equipment grant.
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