(141g) Transition Metal Nitride Core-Noble Metal Shell Nanoparticles As Highly CO Tolerant Catalysts

Introduction

Core-shell nanoparticles (NPs)
have become increasingly attractive as a catalytic design platform due to their
unique ability to simultaneously enhance the activity, increase the durability,
and reduce the loading of noble metal (NM) catalysts. However, these materials
have been limited by the lack of independent control over the core and shell
compositions, shell thickness, and particle size [1]. To address these issues,
our group recently developed a novel method for the synthesis of transition
metal carbide (TMC) core-NM shell (denoted NM/TMC) NPs with precise control
over the core-shell architecture [2]. Group 4-6 TMCs are earth-abundant
compounds with high sinter-resistance, electrical conductivity, and chemical
stability, making them ideal candidates for core materials [3].

Transition metal nitrides (TMNs) share all of these
favorable material properties of TMCs but have rarely been investigated in
core-shell materials. Here, we present the ability to synthesize NM/TMN
core-shell NPs with varying core and shell compositions and controllable shell
thickness via nitridation of the corresponding NM/TMC. The impacts of titanium
tungsten nitride (TiWN) and titanium tungsten carbide (TiWC) cores on the
electronic properties of a Pt shell are compared, and the effect of these
differences on catalytic properties was investigated by measuring the activity
for the hydrogen oxidation reaction (HOR) under the presence of CO to assess
the CO tolerance. The results illustrate how the use of nitride versus carbide
cores offers an effective way to tune the reactivity of NM catalysts.

Materials and Methods

NM/TMC NPs were nitrided under
NH₃ at 800°C for 3 h to form
NM/TMN NPs. Synthetic details for NM/TMC are provided elsewhere [2]. All
materials were supported on carbon. Electrochemical measurements were performed
at 30°C on a rotating disk electrode
with a Ag/AgCl reference and platinum counter electrode. All potentials are
reported versus a reversible hydrogen electrode (RHE). For comparison, catalyst
loadings were targeted to have roughly equal Pt surface areas. HOR scans were
performed in H₂-sat 0.1 M HClO₄ at 2 mV/s and 1600
rpm with iR compensation. To test CO tolerance, the working electrode was first
held at 0.025 V until the electrolyte equilibrated with 1000 ppm CO before
starting the HOR scan.

Results and Discussion

Pt/TiWC NPs with varying Pt
coverage were synthesized using the method previously reported by our group and
transformed into Pt/TiWN NPs via nitridation at 800°C. The high sinter-resistance and strong interfacial bonds of
Pt/TiWC prevent particle agglomeration and preserve the core-shell structure
while the core is converted into a nitride, which is confirmed by XRD,
STEM-EDX, TEM, XPS, and CHNS analysis (Figure 1A).

XPS spectra of the core-shell materials reveal shifts in
the core electron binding energies, called core level shifts (CLSs), for both
the W 4f and Pt 4f signals. These CLSs indicate a significant change in the Pt
electronic structure, as shifts in the core level energies of metal overlayers
are reflected in similar shifts in the valence d-band center, which is known to
correlate strongly with adsorbate binding energies and thus the reactivity [4-5].
The XPS results suggest downward shifts in the Pt d-band center for the core-shell
materials compared to a pure Pt surface and an even lower d-band center and
weaker binding for Pt/TiWN compared to Pt/TiWC.

To probe the difference in adsorbate binding and
reactivity among our core-shell and commercial Pt (denoted Pt_comm)
catalysts, we used CO as a model adsorbate and measured the HOR activity in the
presence of CO (Figure 1B). Strong CO binding is a common problem for Pt
catalysts, poisoning the surface and impeding reactivity. In the presence of
1000 ppm CO, the HOR activity of Pt_comm is completely suppressed by
CO adsorption and is not recovered until above 0.5 V when CO can be oxidized
off the surface. In contrast, both core-shell materials retain HOR activity at
potentials as low as 0.1 V, thus demonstrating dramatically improved CO
tolerance and reduced CO bond strength. Furthermore, 2-3 monolayer (ML) Pt/TiWN
displays even greater CO tolerance than 2-3 ML Pt/TiWC, indicative of weaker CO
binding as predicted from the XPS CLSs.

Figure 1. A) STEM-EDX map of 2-3 ML Pt/TiWN. B) HOR
polarization curves in H₂-sat 0.1 M HClO₄ at 1600
rpm with (solid line) and without (dotted line) 1000 ppm CO.

Significance

In addition to Pt/TiWC, the
synthetic method can be applied to core-shell NPs of varying NM/TMC
compositions, demonstrating a promising way of synthesizing new core-shell
architectures with tunable electronic properties that can serve as next
generation catalysts.

References

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974-978 (2016).

3.       
Zhong, Y., Xia, X., Shi, F., Zhan,
J., Tu, J., and Fan, H.J. Adv. Sci.  3, 1500286 (2016).

4.       
Weinert, M., and Watson, R.E. Phys.
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