Direct evidence of hydrogen spillover from Ni to Cu on Ni–Cu bimetallic catalysts

https://doi.org/10.1016/j.molcata.2013.12.013Get rights and content

Highlights

  • The spillover of hydrogen from Ni to Cu was observed.

  • Hydrogen selectively adsorbs on Ni sites on Ni–Cu bimetallic catalysts under UHV conditions.

  • Hydrogen spillover could result in failure of active sites measurement by selective hydrogen adsorptions.

Abstract

The spillover of hydrogen from Ni to Cu was directly observed on bimetallic Ni–Cu thin films and nanoparticles by hydrogen temperature programmed desorption (TPD) via depositing Cu atoms onto Ni surfaces precovered with atomic hydrogen. The spillover of hydrogen between metals could result in over counting of active sites by hydrogen selective adsorption methods, and also provides a new concept for catalyst design.

Introduction

Hydrogen spillover has been widely studied in catalysis and hydrogen storage [1], [2]. Most of them focused on the spillover of hydrogen activated on metal clusters to supports [1], [2], [3], [4], [5]. The studies on the spillover of hydrogen between metals are limited, because hydrogen molecules dissociate readily on most of the transition metals. There is one exception that hydrogen is activated adsorption on Group IB metals (Cu, Ag, and Au) with a dissociation barrier larger than 0.4 eV [6]. However, hydrogen adatoms can be formed and stabilized on these IB metals by first dissociating the H2 in the gas phase [7], [8], [9]. Therefore, one question is aroused that what will happen in hydrogen adsorption on the bimetallic catalysts composed with active transition metals (TM) and less active Group IB metals (IBM). One typical alloy system contains Group VIII and Group IB metals [10], [11], [12], [13], like nickel–copper (Ni–Cu). Hydrogen can be activated on TM sites, and then spills over to the nearby IBM sites [14]. Goodman and co-workers first noticed that hydrogen adatoms bound to ruthenium (Ru) sites can spill over onto Cu sites in Ru–Cu bimetallic model catalysts [15]. The spillover of hydrogen from Ru to Cu was also confirmed by an NMR study [16]. Most recently the spillover of atomic hydrogen from palladium (Pd) atoms to nearby Cu atoms was imaged by scanning tunneling microscopy [17], [18]. These observations challenge the interpretations of the catalytic studies based on the surface composition of bimetallic catalysts determined by the selective H2 chemisorptions assuming no uptake of hydrogen on the less active metal sites [13], and also open up a new avenue for catalyst design [17], [18], [19], [20]. Sykes and co-workers have demonstrated that a highly effective and selective hydrogenation catalyst can be prepared by modification of Cu surfaces with minority Pd atoms due to hydrogen spillover from Pd to Cu [17], [18], [21]. Similar catalytic system consisting of an active Group VIII transition metal and a less active Group IB metal has potential applications for the design of highly selective catalyst [12], [22], [23], [24].

In this study, we chose Ni–Cu bimetallic catalysts, as a prototype to study the spillover of hydrogen on TM-IBM bimetallic catalysts. There is no direct evidence reported that hydrogen can spill over from Ni to Cu for this widely studied bimetallic catalyst. Ni and Cu are two typical representative metals for non-activated and activated hydrogen adsorption, respectively. The spillover of hydrogen adatoms on Ni sites to Cu sites usually is not energetically favored due to the higher binding energy of hydrogen to Ni (2.89 eV) than hydrogen to Cu (2.39 eV) [25]. Herein, we studied the possibility of the spillover of hydrogen from Ni to Cu by depositing atomic Cu on Ni precovered with activated atomic hydrogen, and got direct evidence that hydrogen atoms could spill over from Ni to Cu. This observation is against the assumption that no hydrogen uptake on Cu in hydrogen selective adsorptions on Ni–Cu bimetallic catalysts [13], [26].

Section snippets

Experimental

The experiments were performed in a stainless steel ultrahigh vacuum (UHV) system (base pressure of 2 × 10−10 Torr) equipped with Auger electron spectroscopy (AES) and a UTI 100 mass spectrometer [23]. A linear ramp of 10 K/s was used for TPD measurements.

A Mo(1 1 0) used as a substrate for preparation of SiO2 thin films was mounted to tantalum wires at the bottom of the sample manipulator. The sample temperature could be varied from 80 K to 2300 K by combined resistive/e-beam heating and liquid

Energetic analysis of hydrogen spillover from Ni to Cu

In order to experimentally observe a hydrogen spillover, the process should be thermodynamically and kinetically favored. Fig. 1 shows the potential energy diagram of hydrogen adsorption on Ni, Cu and Ni–Cu surfaces. The hydrogen adsorption energies and activation barriers were taken from reported results [17], [25]. Hydrogen molecules readily adsorb dissociatively on Ni surfaces, while there is an about 0.4 eV barrier for activated hydrogen adsorption on Cu surfaces. The large difference of the

Conclusions

In this study, direct evidence of the spillover of hydrogen from Ni to Cu was observed via depositing Cu atoms on Ni precovered with hydrogen atoms. The spillover of hydrogen between metals could result in a failure in the measurement of the active sites by the selective hydrogen adsorption under ambient conditions, especially on TM-IBM bimetallic catalysts, e.g. Ni–Cu. Under UHV conditions, hydrogen still selectively adsorbs at Ni sites on Ni–Cu bimetallic catalysts. The observation of

Acknowledgement

We gratefully acknowledge the support for this work by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Bio-sciences (DE-FG02-95ER-14511).

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