Thesis First-principles Study on Hydrogen Adsorption on Platinum Surfaces TRAN THI THU HANH The Department of Physics The Graduate School of Science The University of Tokyo July 2014 Abstract In recent years, much attention has been paid on hydrogen (H) atoms and molecules on a solid surface interfaced with liquid, especially H at the platinum (Pt) - solution interface. Many properties, including adsorption, diffusion, and vibration have been intensively studied. In spite of such efforts, however, theoretical understanding is still insufficient and there is much room for theoretical advancement. In this thesis the focus is put on removing known theoretical inconsistency regarding H on the Pt(111) surfaces and, through detailed comparison with experiment, justify the ther- modynamic approach based on the density functional theory (DFT).
The approach is then used to explore H on the Pt(110) surfaces. The present theoretical work is motivated by the aforementioned inconsistency regarding the most stable H site on the Pt(111) surfaces. Some calculations predicted the fcc site as the most stable one while others predicted the top site. Experimentally the fcc site was conjectured most stable from the electrochemical measurements while spectroscopic signal from the top site can be detected.
Detailed comparison between theory and experiment is a key to settle this problem but most theory used very small lateral cell and provided only zero temperature properties, which cannot be directly compared with the measured thermodynamic data. Karlberg et al. [1] performed a Monte Carlo simulation using a parameter determined from DFT calculations but only the fcc site was assumed to exist. Our DFT calculation for H/Pt(111) reveals that the H adsorption energy depends very sensitively on the parameters adopted for the calcu- lation and, to obtain reliable energy, large number of k-points and many Pt layers are required, which are much larger than those adopted by many foregoing researches.
Then performing converged DFT calculations, the results were used to construct a lattice gas model with which we perform Monte Carlo simulations. The obtained isothermal adsorption properties were used to calculate the g-value, which reflects the H-H interaction, as a function of the H coverage. The obtained g-value is in good agreement with the precise measurement, with the effective H-H interaction being underestimated only by 10 %. It is emphasized that the theory is most stringently tested by this comparison.
From the comparison dominance of the fcc site is con- firmed. The good agreement with experiment possibly suggests minor contribution of the hydration effect neglected in the present model. This theoretical approach is then applied to H on the missing row Pt(110)-(1×2). The dominant site is found to be the bridge site on the ridge, which is in agreement with the LEED experimental and DFT theoretical results found in the literature.
The calculated g-value is in rea- sonable agreement in the lower coverage ΘH < 1/3 conditions and in fair agreement for ΘH > 1/2, while the theory predicts a distinct peak at ΘH ' 1/3 although no such peak appears experimentally. The inconsistency with experiment will indicate that the present modeling with the missing row structure only is questionable and further calculation is then necessary to explain the experiment. Contents 1 Introduction 1 2 Background 4 2.2 Electrochemical Adsorption Isotherms .3 Determination of Hupd isotherms on Pt(hkl) .1 Density Functional Theory Calculation Method .2 Zero Point Energy Calculation .3 Monte Carlo Method .2 Density Functional Theory (DFT) calculations .2 DFT-GGA description of H on Pt(111) .3 Monte-Carlo (MC) simulation .1 Free-energy and effective H-H interaction .2 MC simulation conditions .3 Results of MC simulations .4 Discussion on voltage dependence of the Pt-H stretching fre- quency. 40 5 The missing row Pt(110)-(1×2) 41 5.2 Density Functional Theory (DFT) calculations .2 DFT-GGA description of H on missing row Pt(110)-(1×2) .3 Monte-Carlo (MC) simulation .1 Free-energy and effective H-H interaction .2 MC simulation conditions .3 Results of MC simulations.
58 6 Conclusion 59 ii Chapter 1 Introduction Materials exhibit wide variety of functionality originating from infinite combina- tions of arranging large number of atoms and molecules. Elucidation of the material functionality includes search for the relationship between the microscopic world and the macroscopic one, which has long been a challenging theme of physics and ma- terials science. Today the research has become more and more quantitative. The material functionality does not only reflect its bulk properties but also, or often more importantly, reflects its surface/interface properties, which fact has motivated researches on the surfaces and interfaces.
This is particularly the case for the study of catalytic functionality, where even a slight change in the surface structure and/or surface stoichiometry can completely change the functionality. Among others, plat- inum surfaces as well as noble metal surfaces offer the most ideal model systems for such research because both the catalytic functionality and the surface/interface structures can be most precisely controlled and measured. Indeed, owing to recent advances in the technology, it is possible to provide atomically flat interface of a solid and a liquid as well as atomically flat interface of a solid and the ultrahigh vacuum (UHV). It is noteworthy that a scanning tunneling microscopy (STM) has confirmed such flat interface is indeed realized between a metal surface and the so- lution [2].
The realized system, called as model catalyst, has opened a way to relate the surface structure and the catalytic functionality. Despite the advances in preparing the interface (or the buried surface), micro- scopic characterization of the interface has been hampered by the intense signal from the bulk. To extract signal from the interface, novel surface sensitive experimen- tal methods have been developed such as infrared resection-absorption spectroscopy (IRAS) [3], the sum frequency generation (SFG) [4], the Fourier transform infrared adsorption spectra (FT-IRAS) [5], and the Raman spectroscopy (RS) [6]. Such ap- paratuses have been combined with the traditional electrochemical methods such as cyclic voltammetry (CV) [7, 8, 9, 10, 11] to significantly advance understanding of the interface structures and atoms/molecules adsorbed at the interfaces.
Yet, it is still extremely difficult to capture atomic processes leading to catalysis because the process is often too fast to detect experimentally. In this context, the first-principles calculation has attracted considerable attention. As a tool to investigate the surfaces in UHV, the first-principles calculation has shown great success. In the case of the surfaces in UHV, one can use the sur- face structures determined experimentally or those optimized within the theory to 1 investigate the properties of the surface.
In the case of the solid/liquid interface, however, things are different. The liquid structures fluctuate rapidly and the theory needs to deal with the statistics of the liquid structures. This is a heavy burden of the calculation and it is still infeasible to take it fully into account. Instead, the UHV surface approach has been applied to the problem of hydrophobic interfaces where interaction with the solution is weak.
The approach has been considered valid for platinum or other noble metal and large number of calculations can be found in the literatures [12, 13]. Although the approach generally provides consistent expla- nation of experiments, detailed comparison with precise measurement (or accurate calculation of the interface) has been lacking. It is very important to show how the UHV approach is accurate or inaccurate in describing the buried surface. One of the aims of the present thesis is to elucidate the hydrogen electroad- sorption from the first-principles calculation, the Monte Carlo simulation, and the electrochemical data.
We are trying to provide an example where theory and exper- iment are seriously compared to examine if the g-value can be accurately predicted. We also want to advance understanding of the electroadsorption. The target of the present study is the effective interaction of adsorbed hydro- gen atoms on platinum surfaces. The interaction depends strongly on the surface structures.
According to the CV experiment [11, 14], the interaction is repulsive on Pt(111) and the repulsion is much weaker on Pt(100) and Pt(110). When interfaced with H2 SO4 solution, the interaction is attractive on Pt(100) and Pt(110). These results were obtained from the CV measurement by determining the Gibbs free- energy of H-adsorption (∆G), and then to obtain the H coverage derivative of ∆G, which corresponds to the energy cost of adsorbing additional H atom. The latter quantity corresponds to the effective H-H interaction, and plays a very important role in determining the surface coverage and the catalytic activity of the surface.
What is important in the present study is that the adsorption isotherm is systemat- ically determined for various surfaces with the zero point energy (ZPE) correction of quantum effect, which has never been calculated in foregoing theoretical stud- ies. Therefore, by comparing these data with theory, it is possible to diagnose the accuracy of theory. When ∆G is calculated accurately using a model that neglects the hydration effect, the comparison provides information on the strength of the hydration. Among others, Pt(111) is the simplest surface where calculation can be done most accurately.
In this context, the problem of H/Pt(111) is used for testing the UHV surface model. In doing the theoretical calculation of H/Pt(111), it is worth mentioning that many forgoing calculation [15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25] did not lead to the same conclusion regarding the most stable adsorption site. Some studies showed that the top site is the most stable site [15, 18, 21], while others found that the fcc is more stable than the top [22, 25]. This happened despite the fact that those calculations commonly used the density functional theory (DFT) within the general- ized gradient approximation (GGA) for the exchange-correlation energy.
This is due to insufficient parameters for the DFT-GGA calculation, in particular, insufficient number of k-points in the Brillouine zone integration and insufficient number of Pt layers for the slab model. In this context the present research started from accurate 2 determination of the H adsorption energy within DFT-GGA. The calculated adsorp- tion energy is then used to compute the effective H-H interaction. We will focus on the comparison of the effective H-H interaction, or the g-value, using a Monte Carlo simulation on a lattice gas model parameterized.
Note that a similar Monte Carlo simulation was done by Karlberg et al. [23] using the fcc site only to compare the theoretical and experimental isotherm, ΘH (U), but here we use both the fcc and the top sites and compare the derivative of the isotherm, which corresponds to the g-value. We examine if the lattice gas model successfully accounts for the experi- ment or it needs adjustment of the parameters. Discrepancy from the experiment should be ascribed to the hydration effect and/or the DFT-GGA error albeit it is not possible to discuss relative importance.
The comparison nevertheless provides important insight into the H-adsorption, which prompts further theoretical investi- gation. For the H/Pt(110), the modeling is more complex. For the face-centered cubic FCC(110) surfaces, the unreconstructed (1×1) phase and the reconstructed (1×2) phase with missing-row exist. In practical applications, the Pt catalyst is often finely dispersed in small particles embedded in a matrix and the active sites can be of various types, such as, edges where crystal facets meet.
The missing row reconstructed Pt(110)-(1×2) surface is a convenient model for the edge sites formed between the most stable facets, or Pt(111). This fact motivated almost all theoretical calculations to use the missing row Pt(110)-(1×2) [27, 28, 29, 30, 31], reproducing thereby reasonable properties of the most stable adsorption site. The modeling, however, has not been seriously tested. It is worth investigating if the effective H-H interaction can be reproduced by the missing row Pt(110)-(1×2) model.
So, this thesis focuses on comparing in detail the effective H-H interaction to diagnose the model. In this thesis, chapter 2 is devoted to the summarization of foregoing studies of hydrogen electroadsorption on the Pt surface. Chapter 3 is devoted to the methods adopted in the present research. In chapter 4, the first-principles thermodynamic study on Pt(111) surface is presented.