NH3 adsorption on PtM (Fe, Co, Ni) surfaces: Cooperating effects of charge transfer, magnetic ordering and lattice strain

Abstract

Adsorption of a molecule or group with an atom which is less electronegative than oxygen (O) and directly interacting with the surface is very relevant to development of PtM (M=3d-transition metal) catalysts with high activity. Here, we present theoretical analysis of the adsorption of NH 3 molecule (N being less electronegative than O) on (111) surfaces of PtM(Fe,Co,Ni) alloys using the first principles density functional approach. We find that, while NH 3 -Pt interaction is stronger than that of NH 3 with the elemental M-surfaces, it is weaker than the strength of interaction of NH 3 with M-site on the surface of PtM alloy.Keywords: Bi-metallic alloys, Fuel cell PACS: 82.65. My, 82.20.Pm, 82.30.Lp, 82.65.Jv Bimetallic surfaces of Pt alloyed with first row transition metals (M) such as Fe, Co, Ni etc. are interesting and promising for their potential applications as catalytic cathodes in Proton Exchange Membrane Fuel Cell (PEMFC). These alloys not only ensure the cost effectiveness but also reduce the over-potential and increase the oxidation reduction reaction (ORR) activity [1,2,3]. In a cathode which is made up of pure Pt, the accumulation of oxygen species, such as O, OH etc, decrease the availability of active sites, as well as increase the activation barrier for the ORR. In recent study it was shown that carbon supported PtM alloys exhibit improved catalytic activity by preventing the oxidation and the dissolution of the Pt in the aqueous medium [2]. However, yet the catalytic activity is much weaker than what is expected theoretically. This is because, the oxophilic nature of M atoms results in the formation of surface M-oxides leading to degradation of catalytic activity of these bi-metals. To prevent such effects, it was proposed that selective coverage of M-sites with ligands containing lesser electro-negative elements such as nitrogen (N) would reduce the oxidation of M species and thereby help more Pt sites to remain active [4]. The enhancement of catalytic activity along this route involves two key steps: (1) proper selection of M which can provide Pt-sites suitable environment to retain their ORR activity and (2) tuning the electronic structure of M-sites in order to prevent formation of M-oxides.The first step is emphasized as follows: the electro negativity difference between M and Pt causes the charge transfer from M to Pt site, resulting a down-shift of the Pt-d states via filling of the Pt d-band, thereby reducing the accumulation of oxygen species over Pt sites. To achieve the second step, one requires preferential adsorption of N-containing ligand on M-sites, i,e M should be more N-philic than Pt, in a PtM alloy condition.The electronic, chemical and structural properties of the transition metal (M) are therefore very important to achieving this two mechanisms. The predictability of the modification of electronic and chemical properties of M in an alloy environment with respect to its elemental state is a challenging task. It is therefore necessary to find the descriptor(s) which...

Identifying the critical surface descriptors responsible for the appearance of negative slopes in the adsorption energy scaling relationships

“…This pushes the majority spin states of Co to lower energy, resulting in the enhancement of their occupation and hence in the magnetic moment. 36,47,48 In the other hand, the charge transfer from the Co atoms to Pt is accompanied by a gain in the magnetic moment of Pt. This is evident from the DOS as we can see this transferred electrons asymmetrically occupied the majority and minority band of Pt and induced non-zero magnetic moment at the Pt-site (see Figure -4).…”

Cooperation and competition between magnetism and chemisorption

“…The focus has, however, turned in recent years towards simple 3d-transition metals to be used as catalysts [14]. In most of the cases 3d transition metals are used as alloying elements in either binary or ternary alloys-catalysts [15,16]. These materials offer additional degrees of freedom such as internal strain and magnetic moment when alloyed with traditional heavy metals.…”

Controlling Oxygen-Based Electrochemical Reactions through Spin Orientation

“…In order to improve the sluggish reaction kinetics at the electrodes, efforts have been made in terms of manipulating various factors such as charge transfer and strain by introducing bimetallic alloys of Pt (platinum) and Pd (palladium), such as Pt­(Pd)­Fe, Pt­(Pd)­Co, Pt­(Pd)­Ni, etc., which are the best known catalysts for ORR. − Because of their relatively lower costs as well as the ability to improve the catalytic activity through faster ORR activity, many researchers are focusing on studying the competence of these materials as replacements of Pt in proton exchange membrane fuel cells (PEMFC). − The main reason for the growing interest in these alloy catalysts from a materials perspective is the charge transfer between the constituent metals . Because of the large electronegativity difference between the 3d (Fe, Co, Ni) and 5d (Pt,Pd) transition metals, electrons are transferred from the former to the latter, which is an important factor for tuning the chemical reactivity of oxygen to these surfaces. , However, an overpotential as big as about 0.3 V is still observed, and efforts have been made to improve their activity.…”