(85b) Oxidation Properties of Pt-Ni Surface Alloys

Nickel is the generally accepted industrial catalyst metal used in reforming, due to its abundance in nature and low cost. However, traditional Ni/Al2O3 catalysts have difficulties, like deactivation due to coke deposition, requirement of reactivation under hydrogen flow as a result of strong oxidation tendency, hot-spot formation and additionally, sintering during oxidative steam reforming (OSR) reactions. In recent experimental studies, Pt?Ni/Al2O3 was tested in OSR of propane and LPG [1-3]. It was shown that (i) addition of Pt to Ni/Al2O3 as a second metal enhanced OSR activity at reduced temperatures (ca. 673 K), (ii) Pt/Ni ratio has an influence on the catalytic activity and (iii) Pt-Ni catalyst has pronounced secondary WGS activity, which is an advantage for reducing CO in the hydrogen production process. In OSR of methane, addition of Pt to Ni has also been reported to prevent hot-spot formation, along with increased reforming activity [4]. These studies proved that the catalytic activity and the temperature profile are both strongly related to the Ni/Pt ratio and preparation method. Lower Ni/Pt ratio exposed to high temperature treatment resulted in Ni-Pt substitutional alloy formation, whereas seperate Pt and Ni phases each of which have a stronger tendency for catalyzing oxidation and reforming reactions, respectively, has been reported with high Ni/Pt ratios at lower temperatures.

In this study, the stability and oxidation properties of pure Ni(111), Pt(111), Ni-Pt substitutional surface alloys with different Ni/Pt ratios on Ni(111) and additionally, Pt pseudomorphic layers on Ni(111) are tested using periodic computational density functional theory methods. Additionally, considering the higher catalytic activity of lower-coordinated sites, i.e. step-edges, in steam reforming of methane [5], (211) surface termination of monometallic Ni and Ni-Pt surface alloy with Ni/Pt:4 has been tested based on their oxidation tendency.

It was shown that Pt-Ni surface alloys at different Ni/Pt ratios are very stable structures based on their surface energy, and in correspondence with experimental studies, they are more probable to exist after high temperature treatment on the catalyst surface. The high stability of Pt-Ni substitutional surface alloy compared to both monometallic Ni and Pt/Ni(111) layers are related to their lower tensile stress reduced as a result of substitution of a metal with a larger metallic radius on the surface. The results also confirmed that Pt-Ni surface alloys have lower oxygen chemisorption strength, compared to both monometallic Ni and Pt. The stability of oxygen chemisorption is affected by the ratio of Ni/Pt, because (i) weaker Pt-O bonds start to substitute Ni-O bonds as the surface Pt concentration increases and (ii) as proven by Local Density of States analysis and Mulliken charge analysis of surface atoms, charge transfer between Ni and Pt causes weaker ionic interaction and stronger Pauli repulsion between Ni and O atoms amd this electronic reconstruction of the substrate is affected by surface Ni/Pt ratio. It was also shown that Pt atoms have a tendency to substitute lower-coordinated step-edge sites on the Ni surface, more than flat terrace sites. As these lower coordinated sites are the main active sites in steam reforming reaction, Pt substitution on step-edges and a correspondent decrease in the oxidation tendency of these sites have a direct effect on the catalytic activity.

Acknowledgements

This work is financially supported by Turkish State Planning Organization (DPT) through projects DPT 07K120630 and DPT 03K120250.

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