Session Q34: Focus Session: Nano IV: Nanocatalysis

Cluster size effects on chemical and physical properties of model catalysts

Model catalysts are prepared by deposition of size- and energy-selected metal clusters on well characterized solid supports in an ultrahigh vacuum system. The system provides capabilities to probe the physical properties of the samples by X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS), ion neutralization spectroscopy (INS), and low energy ion scattering (ISS). The chemical properties can be probed by a variety of pulsed, temperature-programmed, and constant temperature mass spectrometric methods, and resulting changes in physical properties from interactions with adsorbates are probed by XPS, UPS, INS, and or ISS. Recently, capabilities for in situ electrochemical characterization were added. Results from several systems will be presented, including gas-surface reactions over Pd/alumina and Pd/titania, and solution phase oxygen reduction over Pt/glassy carbon.   

CO Oxidation on Copper Oxide Cluster Ions

We investigated the reaction of CO and O$_2$ on size-selected copper cluster ions, Cu$_n^+$ ($n$=4--18) and Cu$_n^-$ ($n$=4--11), by use of a tandem mass-spectrometer equipped with octopole ion guides. A coadsorbing product, Cu$_n$O$_2$(CO)$^+$, was observed in the reaction of Cu$_n$O$_2^+$ with CO, and it was found that CO adsorbs onto Cu$_n$O$_2^+$ more efficiently than onto Cu$_n^+$ in $n \geq 9$. This shows the cooperative coadsorption of O$_2$ and CO. On the other hand, in the reaction of Cu$_n$O$_2^-$ with CO, a reduced product, Cu$_n$O$^-$, was obtained instead of the coadsorbing product, Cu$_n$O$_2$(CO)$^-$. In particular, Cu$_5$O$_2^-$ and Cu$_9$O$_2^-$ have relatively high efficiency for the production of Cu$_n$O$^-$. This result suggests the production of CO$_2$ by the oxidation of CO on Cu$_n$O$_2^-$. The DFT calculation indicates that the activation energy in the reaction of Cu$_5$O$_2$(CO)$^-$ $\rightarrow$ Cu$_5$O(CO$_2$)$^-$ is only 0.79 eV while that of the corresponding cation is 1.79 eV. The structure of Cu$_n^-$ is more flexible than that of Cu$_n^+$ probably because of its excess electron. It is concluded that the stabilization of the transition state and the decrease of the activation energy make the CO oxidation proceed on Cu$_n$O$_2^-$.   

Effective rate constants for nanostructured heterogeneous catalysts

There is currently a high level of interest in the use of nanostructured materials for catalysis. For instance, gold, which is largely inert in the bulk, can exhibit strong catalytic activity when in nanoparticle form. With precious metal catalysts such as Pt and Pd in high demand, the use of these materials in nanoparticle form can also substantially reduce costs by exposure of more surface area for the same volume of material. When reactants are plentiful, the effective activity of a nanoparticulate catalyst will increase roughly with its surface area. However, under diffusion-limited conditions, the reactant must diffuse to active sites on the catalyst, so a high surface area and a high density of active sites may bring diminishing returns if reactant is consumed faster than it arrives. Here we apply a mathematical homogenisation approach to derive simple expressions for the effective reactivity of a nanostructured catalyst under diffusion limited conditions that relate the intrinsic rate constants of the surfaces presented by the catalyst to an effective rate constant. When highly active catalytic sites, such as step edges or other defects are present, we show that distinct limiting cases emerge depending on the degree of overlap of the reactant depletion zone about each site. In gases, the size of this depletion zone is approximately the mean free path, so the effective reactivity will depend on the structure of the catalyst on that scale. We discuss implications for the optimal design of nanoparticle catalysts. \newline   

Towards a Molecular Level Understanding of CO and H$_{2}$ Adsorption and Dissociation on Cobalt Nanoparticles

The development of sustainable energy technologies, including the production of synthetic fuels, is of global importance. Fischer-Tropsch synthesis (FTS) has recently gained increased attention as it involves the formation of hydrocarbons via the catalytic conversion of syngas (CO and H$_{2})$, which can be derived from renewable sources. FTS is often performed using Co-based catalysts that are greatly affected by the adsorption state of reactants, as well as nanoparticle shape and size. Here we have used low-temperature scanning tunneling microscopy (LT-STM) to study the interaction of syngas with well-defined Co nanoparticles grown onto Cu {\{}111{\}}, an inert metal for FTS. Hydrogen adsorbs dissociatively on the Co surfaces, resulting in three unique coverage-dependent phases. We demonstrate that these phases can resolve crystal packing ambiguities of the underlying Co nanoparticles, a question that has been debated in the literature. Simultaneous exposure of the Co to H$_{2}$ and CO results in segregated islands of the adsorbates on the nanoparticle surface at 80 K, and we propose that atomic H blocks CO adsorption, causing the build-up of CO at the nanoparticle step edges. With increasing CO coverage, a two-dimensional phase compression of H by CO is observed, providing \textit{the first direct} visualization of this phenomenon in a catalytically relevant system. Our data suggest that FTS reactivity may be dominated by the interface length between the adsorbates and be subject to unforeseen kinetic restraints as a function of particle size.   

Cooperative Effects in the Oxidation of CO by Palladium Oxide Cations

It is shown that cooperative reactivity plays an important role in the oxidation of CO to CO$_{2}$ by palladium oxide cations. Comprehensive studies including guided-ion-beam mass spectrometry and theoretical investigations reveal the reaction products and profiles of PdO$_{2}^{+}$ and PdO$_{3}^{+}$ with CO through oxygen radical centers and dioxygen complexes bound to the Pd atom. We find that the O radical centers are more reactive than the dioxygen complexes, and experimental evidence of both direct and cooperative CO oxidation with the adsorption of two CO molecules are observed. The binding of multiple electron withdrawing CO molecules is found to increase the barrier heights for reactivity due to decreased binding of the secondary CO molecule, however reactivity is enhanced by the increase in kinetic energy available to hurdle the barrier. We examine the effect of oxygen sites, cooperative ligands, and spin including two-state reactivity.   

Design of nanocatalysts for improved selectivity and stability

Several examples from ongoing work in our laboratory on the use of self-assembly to prepare heterogeneous catalysts with novel architectures will be discussed in this presentation. In one case, catalysts consisting of dispersed platinum metal nanoparticles with narrow size distributions and well-defined shapes were prepared and tested for the selective promotion of carbon-carbon double-bond cis-trans isomerization reactions in olefins. It was shown that the selective formation of the cis isomer could be controlled by using tetrahedral particles with exposed (111) facets. In a second example, catalysts based on small platinum nanoparticles of well-defined sizes were made by using dendrimers as scaffolding structures. The organic framework in that case can provide new fuctionality, including chirality as a way to introduce enantioselectivity. The third example involves the control of metal nanoparticle sintering by covering those with a layer of mesoporous silica grown on top. The final case to be discussed is one where yolk@shell metal-semiconductor constructs are being developed for increase stability in oxidation and photocatalytic applications.   

Towards Catalysis by Gold Clusters: reaction cycles and poisons

Nanosized gold particles are good catalysts in a variety of oxidation reactions. These reactions, for which oxidation of CO to CO$_2$ serves as a paradigm, imply a transition in the total spin and therefore do not occur spontaneously in the gas phase. In the catalytic process, the catalyst clusters are exposed to an atmosphere of gas-phase O$_2$ and CO reactants at finite temperature and pressure. We have thus modeled free gold clusters in contact with an atmosphere composed of O$_2$ and CO by means of DFT calculations (PBE functional), and accounted for both temperature and pressure effects employing \textit{ab initio} atomistic thermodynamics. On the basis of this analysis, we could recognize the thermodynamic driving force of the catalytic CO oxidation process and single out the possible ($p,T$)-dependent reaction cycles and those paths leading to stable structures that poison the catalytic process. This as a useful (exploratory) theoretical step, before taking chemical reaction kinetics into consideration. In the proposed reaction paths, the total spin is conserved in each elementary step, and it is the adsorption of an incoming O$_2$ molecule that drives the catalyst cluster from the singlet to the triplet spin state, and \textit{vice versa}.   

Breaking Carbonyl Bonds in Formaldehyde via Complementary Active Sites

We had recently shown that the complementary active sites in homonuclear clusters may stimulate the breaking of polar bonds enabling an atomic level control of reactivity. However, such bond cleavage has only been observed in hydroxyl bonds. In this work, we present experimental and theoretical evidence that demonstrates that the stronger C=O carbonyl bond of formaldehyde is split by complementary active sites on size selective aluminum cluster anions. The resonance structure in which the carbonyl is reduced to a single bond is stabilized by the paired active sites establishing the potential use of these geometrically driven centers in devising precursors for synthesizing chemicals or radicals that might find use in production of fine chemicals.   

Morphological, electronic, and catalytic properties of Pt nanoclusters on defective graphene

The synthesis of well dispersed, size-controlled Pt nanoclusters on carbon supports is highly desirable since such clusters have been shown to possess enhanced catalytic activity and selectivity in a variety of chemical reactions. However, these nanoclusters interact rather weakly with defect-free carbon supports and can coarsen over time leading to loss of surface area and thence catalytic activity. Defects in carbon supports play an important role in enhancing Pt-carbon bonding, thereby reducing the propensity for cluster coalescence. Using a combination of density functional theory and empirical potential simulations, we examine the interaction of Pt nanoclusters with point defects in graphene. We focus on the role of the support defects in controlling the morphology, electronic structure, and CO-tolerance of Pt nanoparticles. Our results suggest possible avenues for controlling the dispersion and activity of Pt nanoclusters on carbon supports via defect engineering.   

Controlled Catalytic Properties of Platinum Clusters on Strained Graphene

We employed biaxially strained graphene as the supporting material for Pt clusters (Pt$_{x}$, x=1, 4 or 6) and studied the molecular adsorption behaviors of H$_{2}$, CO and OH on the cluster using ab initio calculations. It was shown that the applied strain enhances binding of the Pt cluster on the graphene, which lowers the average energy of Pt d electron (d-band center). The binding energies of H$_{2}$, CO and OH on Pt$_{1}$/graphene are strongly correlated with the d-band center modulated by the graphene strain. The calculations with small Pt clusters (Pt$_{4}$ and Pt$_{6}$) also show that the d-band center is a substantial factor for the catalytic activity of the Ptx/graphene system. We also found that the stability of the Pt clusters was enhanced by applying the strain on the graphene support.   

Composition-Dependent Size and Shape Changes of Pt-Rh Alloy Nanoparticles on $\alpha$-Al$_2$O$_3$(0001) during CO Oxidation Reactions

Pt-Rh nanoparticles are widely used in chemical industry and in automotive exhaust control where they catalyze among other reactions the oxidation of CO. Major attention has in recent years been paid to the study of alloy nanoparticles with the aim to identify systems that allow to control the catalyst selectivity and to enhance its activity and lifetime [1]. Sintering is regarded as one of the major causes of catalyst deactivation and it is of utmost scientific and economic interest to find ways to prevent it. Here we present concentration-dependent size and shape changes of epitaxial Pt-Rh alloy nanoparticles on $\alpha$-Al$_2$O$_3$(0001) substrates observed in-situ during CO oxidation at near atmospheric pressures. The experiments were carried out in a flow-reactor at the high energy beamline ID15A (ESRF) by means of grazing incidence x-ray diffraction (E=78.8 keV), x-ray reflectivity measurements and in-situ mass-spectrometry [2]. During the experiments the O$_2$ pressure ranged between 0 and 14 mbar while the temperature and CO pressure were kept at 550 K and 20 mbar, respectively. Our results demonstrate that a higher Rh concentration reduces sintering significantly.\\[4pt] [1] J.Y. Park et al., Nano Lett. 8 673 (2008)\\[0pt] [2] R. v. Rijn et al., Rev. Sci. Instr. 81 014101 (2010)