Article pubs.acs.org/JPCC
Size Dependence of Low-Temperature Catalytic Activity of CO Oxidation Driven by Platinum Clusters Directly Bound to Silicon Substrate Surface Hisato Yasumatsu*,† and Nobuyuki Fukui‡ †
Cluster Research Laboratory, Toyota Technological Institute in ‡East Tokyo Laboratory, Genesis Research Institute, Inc., 717-86 Futamata, Ichikawa, Chiba 272-0001, Japan
ABSTRACT: Studied were cluster-size dependence of catalytic activity of CO oxidation driven by unisized platinum clusters, PtN (N = 10, 30, and 60), directly bound to a silicon substrate surface. Temperature-programmed desorption measurements were repeated for a given PtN/Si catalyst with systematic change of the reaction condition. The CO oxidation on the PtN/Si catalyst is described in a manner similar to the bulk Pt(111) surface; the Langmuir−Hinshelwood mechanism by molecular oxygen activated by the catalyst at 120−140 K (α reaction) and the dissociatively adsorbed atomic oxygen in the temperature range of 130−350 K (β reaction). However, the PtN/Si catalyst has the advantage of a lower-temperature activity compared with the bulk Pt(111) surface. Furthermore, the Pt60/Si catalyst has 1.5 times higher activity per Pt atom than Pt30/Si, while no catalytic activity for the Pt10/Si sample. These results are interpreted in relation to the geometric structure and the electron accumulation of the Pt clusters on the Si surface.
1. INTRODUCTION Owing to elaborate studies on metal clusters in these few decades, it has become known that a metal cluster is considered as a subnano quantum capacitor of almost free electrons, the capacitance and the quantum energy levels of which are described according to the shell and the jellium models.1−3 Therefore, it is straightfoward to employ the rich electrons in a metal cluster for accelerating its functional utilization along expansion of the cluster-science field toward practical applications. One can extract the electrons from the clusters through electronic interaction with a surface of a solid substrate.4−6 Indeed, we have shown successful extraction of the electrons7,8 by accumulating them at a subnano interface between a monatomic-layered platinum cluster disk and a silicon substrate surface.9,10 Focusing on catalysis as one of the practical functions desired for solving the global key issues of energy and materials we are confronted with, the rich electrons can be employed to reductively promote molecules adsorbed there as active species in a catalytic process through electron transfer from the catalyst to the molecules. Numbers of research studies have been reported that catalytic activities are gained by extra electrons in/around supported nanoparticles.11−21 Haruta and his co© XXXX American Chemical Society
workers have discovered catalytic activities of gold nanoparticles, despite the high chemical stability of bulk gold, due to extra negative charges at the periphery of the gold nanoparticles as a result of interaction with the supporting substrate.11 For smaller clusters, it has been reported that the charging state of the gold clusters supported on an MgO(100) thin film relates closely to their catalytic activities.12−14 In the catalytic CO oxidation by platinum clusters on the MgO(100)15 and TiO2(110)16 surfaces, distinct cluster-size dependences observed have indicated the way for tuning their electronic structures. High activity of CO dissociation by nickel clusters supported on the MgO(100) thin film, which is dependent on the cluster size, has been explained by efficient electron transfer from the cluster to antibonding molecular orbitals of CO adsorbed.17 We have also reported efficient catalytic performance of thermal oxidation of CO by O2 on an unisized monatomic Pt Special Issue: Current Trends in Clusters and Nanoparticles Conference Received: December 7, 2014 Revised: January 16, 2015
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DOI: 10.1021/jp512172v J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
reached as low as the back ground level. It was confirmed that no 13CO remains on the catalyst sample after this treatment as no 13CO2 production even with exposure against 2 L of O2 (Taiyo Nippon Sanso Corp. ≥99.999% pure); exposure against 0.05 and 2 L of 13CO and O2, respectively, are sufficient for the detection of 13CO2 produced.22 CO molecules adsorbed on the bulk Pt(111) surface are also completely desorbed at this temperature.32 Then, the reactant gases, O2 and 13CO, were supplied one by one in this order at the substrate temperature of 105 K. While heating this cluster catalyst to 630 K at the rate of 0.3 K s−1, the desorbed 13CO2 product was mass-analyzed by means of a quadrupole mas filter equipped in a differentially pumped chamber maintained at the pressure less than 5 × 10−8 Pa through a skimmer having a hole of 6 mm in diameter placed at ∼1 mm far from the sample surface;26 the acceptance angle of the desorbed species is ranged in 0 to 70 degrees with respect to the normal of the substrate surface. The detection efficiency of the TPD device was calibrated with Ar every day also by means of the residual gas analyzer. The Si(111)−7 × 7 reconstructed surface was prepared from a Si wafer (Nilaco, n-type, 99% pure) at 2 × 10−7 Pa for 300 s (6 × 10−5 Pa s corresponding to ∼0.5 Langmuir, L hereafter) at a substrate temperature of 105 K followed by resistive heating of the Si substrate to 630 K at a ramping rate of 2.5 K s−1; the partial pressure of the reactants (
