Theoretical Study on Pt Particle Adsorbate Bonding:  Influence of Support Ionicity and Implications for Catalysis

Abstract

The influence of the support ionicity on the adsorption of H, CH x (x ) 2, 3), and O on supported Pt particles is investigated using density functional theory (DFT). The DFT calculations reveal that the importance of the 6s,p states for the Pt adsorbate bonding increases in the order Pt-O < Pt-CH 2 < Pt-CH 3 < Pt-H. Both the Pt-H and Pt-O bond strengths are strongly affected by changes in the support ionicity whereas the difference for the Pt-CH x bond strength is much smaller. The Pt-H bond strength is higher for ionic supports, whereas the Pt-O bond strength is higher for acidic or more covalent supports. These bond strength changes occur because the support ionicity has a large impact on both the Pt 6s,p and 5d states. On ionic supports with electron rich oxygen atoms, the 6s,p states are pushed toward the surface of a metal cluster where it can participate in H bonding, whereas on supports with electron poor oxygen atoms (existing in acidic supports or for supports with more covalent cations) the 6s,p states are attracted toward the metal-support interface. The Pt 5d states shift to lower binding energy (lower ionization potential) on ionic supports, where it bonds less effectively with the O adsorbate. These findings indicate that the hydrogen and oxygen coverage on Pt particles are strongly determined by the ionicity and acid/base properties of the support and therefore may be the most important for producing changes in the reactivity of Pt wit support ionicity in various hydrogenolysis, hydrogenation, and oxidation reactions.

Influence of support ionicity on the hydrogen chemisorption of Pt particles dispersed in Y zeolite: consequences for Pt particle size determination using the H/M method

“…The hydrogen chemisorption capacity of Pt is directly influenced by the ionicity of the support oxygen. The H/M results presented in this paper are completely in line with previously published Pt AXAFS, XANES, XPS, and CO FTIR data [7,8,23] and provide an experimental proof of our recently published DFT calculations [22]. They confirm the interatomic potential model [17] that we have proposed as a basis for metal-support interaction.…”

“…The H/Pt s data presented in Table 4 show that the number of hydrogen atoms chemisorbed per surface Pt is highest for Pt particles on the most ionic (basic) support (NaY) and lowest for Pt particles on the most covalent (acidic) support (H-USY). This implies that the H/M data presented in this paper are a direct experimental proof of recently published DFT calculations [22]. Moreover, they are completely in line with previously published Pt EXAFS, XANES, XPS, and CO FTIR [7,8,19,23] data and confirm our interatomic potential model that was put forward as a basis for metal-support interaction [17].…”

“…A proposed MO scheme to explain the observed changes in the EXAFS data in more detail [17] was based on the idea that the location of the 6s, p interstitial bonding orbitals moves from the surface of the Pt particle (Pt supported on electron-rich O atoms, for ionic or basic supports) to the Pt-support interface (Pt supported on electron-poor O atoms, for acidic supports with protons or supports with more covalent cations). This concept was confirmed by recent DFT calculations showing that the support-induced changes of the electronic structure within a supported Pt particle affects the chemisorption of such adsorbates as H 2 , CH x , and O 2 on the surface of Pt particle [22]. According to the DFT calculations, the Pt-H bond strength is larger on supports with electron-rich oxygen atoms (basic), which implies greater H coverage on ionic supports than on covalent supports (acidic, with electronpoor O atoms).…”

“…DFT calculations show that the Pt-H bond strength is greater on ionic supports (basic, with electron-rich O atoms), implying higher H coverage compared with covalent supports (acidic, with electron-poor O atoms) [22]. The combination of DFT calculations with the new XANES analysis procedure applied to EXAFS data on small supported Pt particles (n < 6.5) led to detailed insights into the correlation between the ionicity of the support and the type of active sites for H chemisorption [23].…”

Application of AXAFS Spectroscopy to Transition‐Metal Oxides: Influence of the Nearest and Next Nearest Neighbour Shells in Vanadium Oxides

“…[10] The complete density of states (DOS) of the Pt is shifted to higher binding energy, and the location of the 6s,p interstitial bonding orbital (IBO) moves from the surface to the metal-support interface of the Pt particles with decreasing ionicity of the support 1 (with decreasing electronic charge on the oxygen atoms of the support). [26] The variations in support oxygen charge density are often the result of promotion effects or alterations in the support cations (e.g., Si/Al ratio), and so these are next nearest neighbour effects. [8,15] The results mentioned above for bulk metal systems, organometallic systems, and supported metal catalysts have shown that AXAFS is a viable tool for probing the influence of the local surroundings on the electronic properties of a central atom.…”