Nanocatalysis
Gold nanocatalysis
Gold is one of the most interesting and mysterious materials with unique catalytic properties emerging at nanoscale. The large interest to gold nanocatalysis is stipulated by the fact that nanoparticles of gold demonstrate extraordinary catalytic activity and selectivity even at room temperatures. The reactions of oxidation catalyzed by the small gold nanoparticles are of particular interest, because the molecular oxygen from the air can play a role of the oxidant, making the whole process to be an ideal and waist free reaction. This feature is important for many industrial and green chemistry applications. There are several factors that can affect catalytic properties of nanoparticles. Among them the size, the geometry structure and morphology, the charge state of nanoparticle, the support effects, etc. To create new catalytic materials by design at the nano-level it is necessary to predict how these factors affect the catalytic properties of nanoparticles.
Reactivity of free clusters
In our recent works we have focused on theoretical investigation of unusual catalytic properties of gold clusters for oxidation reactions by molecular oxygen. We have demonstrated that catalytic activation of the adsorbed O2 on small pure gold clusters can not lead to its dissociation. However, coadsorption of simple hydrocarbons, such as ethylene, C2H4, results in extra charge transfer from the gold cluster to O2, energetically promoting oxygen dissociation. Therefore, O2 dissociation on the surface of small gold clusters is sensitive to the presence of other adsorbates, including the reactant molecule itself. This effect can be particularly important for understanding the mechanism of catalytic oxidation on gold clusters. We have also found an effect of the cooperative adsorption of O2 and C2H4 on small gold clusters. This finding indicates that the process of oxygen dissociation on the surface of gold clusters is sensitive to the presence of other adsorbates, including the reactant molecule itself.
Figure: Cooperative effect in O2-Au7-C2H4 system (left). Ethylene promoted O2 dissociation on Au5 (right).
Adsorption of ethylene molecules on neutral, anionic, and cationic gold clusters demonstrates very interesting features. Thus, C2H4 can be adsorbed on small gold clusters in two different configurations, corresponding to the π- and di-σ-bonded species. Adsorption in the π-bonded mode dominates over the di-σ mode over all considered cluster sizes n ≤10, with the exception of the neutral C2H4-Au5 system. A striking difference is found in the size dependence of the adsorption energy of C2H4 bonded to the neutral gold clusters in the π and di-σ configurations. The electronic shell effects play an important role for the di-σ mode of ethylene adsorption on neutral gold clusters. The interaction of C2H4 with small gold clusters strongly depends on their charge. The typical shift in the vibrational frequencies of C2H4 adsorbed in the π and the di-σ configurations gives a guidance to experimentally distinguish between the two modes of adsorption.
Figure: Optimized geometries and adsorption energies of C2H4 on small gold clusters for the π (a) and di-σ (b) modes of ethylene adsorption.
In collaboration with the members of the Quantum Chemistry Lab, Hokkaido University we have adapted the global reaction route mapping (GRRM) technique for description of chemical reactions on atomic clusters and applied effective automated reaction path search methods (anharmonic downward following (ADDF) and artificial force induced reaction (AFIR)) to a systematic investigation of H-H bond activation pathways catalyzed by the small gold clusters. It has been demonstrated that the most stable structures of the gold clusters are not always highly reactive and several isomeric structures have to be taken into account for adequate description of the reaction rates at finite temperatures. On the example of H2 dissociation on the gold clusters we have demonstrated that the ADDF and AFIR methods are very useful in systematic exploration of metal cluster catalyzed chemical bond activation. The proposed approach can serve as a promising tool for investigation of the chemical reactions catalyzed by small metal clusters.
Figure: Application of the GRRM technique for automated search of H-H bond activation pathways catalyzed by gold clusters.
Reactivity of supported clusters
Support effect is one of the most important factors in nanocatalysis. Therefore, a large part of our work is devoted to the theoretical analysis of the catalytic activity of the supported clusters. Recently, we have demonstrated that hexagonal boron nitride (h-BN) surface which was traditionally considered as an inert support can strongly modify properties of the gold clusters and act as an “active” support. In particular, the structural, electronic, and catalytic properties of Au and Au2 supported on the pristine and defected h-BN surface have been studied. It is demonstrated that adsorption and catalytic activation of O2 on the h-BN supported Au and Au2 can be affected by the interaction with the support via electron pushing and donor/acceptor mechanisms. It is shown that even weak interaction of Au and Au2 with the defect-free ″inert″ h-BN surface can have an unusually strong influence on the binding and catalytic activation of the molecular oxygen. This effect occurs due to the mixing of the 5d orbitals of the supported Au and Au2 with the N-pz orbitals. Although the defect-free h-BN surface does not act as a good electron donor for the supported O2−Au, it promotes an electron transfer from the Au to O2, pushing electrons from the gold to the adsorbed oxygen. In the case of the defected h-BN surface, Au and Au2 can be trapped effectively by N or B vacancy and impurity point defects. Strong adsorption on the surface defects is accompanied by the large charge transfer to/from the adsorbate. The excess of the positive or negative charge on the supported Au and Au2 can considerably promote their catalytic activity. Therefore, the h-BN surface (pristine or defected) cannot be considered as an inert support for Au and Au2.
Figure: Adsorption and activation of O2 on Au/h-BN (left); H2 dissociation at the perimeter interface between the gold cluster and the rutile TiO2(110) support (right).
Adsorption and dissociation of H2 on gold clusters supported on the rutile TiO2(110) surface has been studied. The molecular and dissociative adsorption of H2 on gold clusters depends on cluster size, geometry structure, cluster flexibility and the interaction with the support material. Rutile TiO2(110) support energetically promotes H2 dissociation on gold clusters. The perimeter interface between gold nanoparticle and TiO2 surface acts as an active site for H2 dissociation. The low coordinated oxygen atoms on the TiO2(110) surface play a crucial role for H2 dissociation. Performed calculations can explain recent experimental findings of T.Fujitani et al, Angew. Chem. Int. Ed., 48, 9515 (2009). The reported theoretical predictions are important for understanding the mechanisms of catalytic reactions on supported gold clusters.
Relevant publications
M. Gao, D. Horita, Y. Ono, A. Lyalin, S. Maeda, and T. Taketsugu, Isomerization in Gold Clusters upon O2 Adsorption, J. Phys. Chem. C, 121, 2661−2668 (2017).
A. Lyalin, M. Gao, and T. Taketsugu, When Inert Becomes Active: A Fascinating Route for Catalyst Design, The Chemical Record, 16, 2324–2337 (2016).
M. Gao, A. Lyalin, M. Takagi, S. Maeda, and T. Taketsugu, Reactivity of gold clusters in the regime of structural fluxionality, J. Phys. Chem. C 119, 11120-11130 (2015).
M. Gao, A. Lyalin, S. Maeda, and T. Taketsugu, Application of automated reaction path search methods to a systematic search of single-bond activation pathways catalyzed by small metal clusters: A case study on H-H activation by gold, J. Chem. Theory Comp., 10, 1623-1630 (2014).
M. Gao, A. Lyalin, and T. Taketsugu, CO oxidation on h-BN supported Au atom, J. Chem. Phys. 138, 034701 (2013).
M. Gao, A. Lyalin, and T. Taketsugu, Oxygen activation and dissociation on h-BN supported Au atoms, Int. J. Quantum Chem. 113, 443-452 (2013).
M. Gao, A. Lyalin, and T. Taketsugu, Catalytic activity of Au and Au2 on h-BN surface: adsorption and activation of O2, J. Phys. Chem. C 116, 9054–9062, (2012).
M. Gao, A. Lyalin, and T. Taketsugu, Role of the support effects on the catalytic activity of gold clusters: a density functional theory study, Catalysts 1, 18-39, (2011).
A. Lyalin and T. Taketsugu, A computational investigation of H2 adsorption and dissociation on Au nanoparticles supported on TiO2 surface, Faraday Discussions 152, 185-201, (2011).
A. Lyalin and T. Taketsugu, Reactant promoted oxygen dissociation on gold clusters, J. Phys. Chem. Lett. 1, 1752-1757, (2010).
A. Lyalin and T. Taketsugu, Adsorption of ethylene on neutral, anionic and cationic gold clusters, J. Phys. Chem. C 114, 2484-2493, (2010).
A. Lyalin and T. Taketsugu, Cooperative adsorption of O2 and C2H4 on small gold clusters, J. Phys. Chem. C 113, 12930-12934, (2009).
Catalysis by clusters different from gold
Reactivity of free clusters
The mechanism of dissociative D2 adsorption on Ti2O4−, which serves as a model for an oxygen vacancy on a titania surface, has been studied using density functional theory calculations and a recently developed single-component artificial force induced reaction (AFIR) method. We have demonstrated that Ti2O4− readily reacts with D2 under multiple collision conditions in a gas-filled ion trap held at 16 K forming a global minimum-energy structure (DO–Ti–(O)2–Ti(D)–O)−. The highly exergonic reaction proceeds quasi barrier-free via several intermediate species, involving heterolytic D2-bond cleavage followed by D-atom migration. Compared to neutral Ti2O4, the excess negative charge in Ti2O4− is responsible for the substantial lowering of the D2 dissociation barrier, but does not affect the molecular D2 adsorption energy in the initial physisorption step.
Figure: Experimental IRPD spectrum of [Ti2O4D2]−·(D2)1–3 (bottom panel) and unscaled UB3PW91/aug-cc-pVDZ harmonic IR spectra (upper panels) of low energy isomers of bare Ti2O4− (Rcis and Rtrans), Ti2O4−(D2) (physisorbed, Icis and Itrans) and [Ti2O4D2]− (chemisorbed, P1–P5) in the fingerprint (400–1300 cm−1) and O–D stretching (2575–2975 cm−1) spectral regions.
Relevant publications
X. Song, M. R. Fagiani, S. Debnath, M. Gao, S. Maeda, T. Taketsugu, S. Gewinner, W. Schöllkopf, K. R. Asmis and A. Lyalin, Excess Charge Driven Dissociative Hydrogen Adsorption on Ti2O4ˉ, Phys. Chem. Chem. Phys., 19, 23154-23161 (2017).
Reactivity of supported clusters
Very often chemical reactions occur at the perimeter interface at the boundary between the supported cluster and the surface. We show such an interface effect the example of catalytic hydrogen elimination from isopropanol (C3H8O) by free and θ-Al2O3(010)-supported Ni13 cluster. We demonstrated that dehydrogenation of C3H8O on the free Ni13 cluster is a two-step process with the first hydrogen elimination from the alcohol hydroxyl group, followed by C–H bond cleavage. Our calculations show that H elimination from OH group of C3H8O to Ni13 cluster is the rate-determining step with the barrier of 0.95 eV, while the C–H bond cleavage requires overcoming the barrier of 0.41 eV. In the case of Ni13 cluster supported on θ-Al2O3(010) the isopropanol molecule adsorbs on top of the surface Al atom in the close vicinity of the nickel cluster, which results in considerable decrease in barrier for H elimination due to formation of the complementary adsorption sites at the metal/support interface. It is demonstrated that intermediate formation of the Ni–C bond considerably promotes C–H bond cleavage. The described mechanism provides fundamental understanding of the process of the oxidant-free catalytic hydrogen elimination from alcohols on supported nickel clusters and can serve as a tool for rational design of novel type of nanocatalysts based on abundant noble-metal-free materials.
Figure: Energy diagram for H elimination from isopropanol adsorbed at the perimeter interface between the supported Ni13 cluster and θ-Al2O3(010) (blue and red lines) and energy profile for dehydrogenation of isopropanol on free Ni13 cluster (black lines).
Relevant publications
A. Lyalin, K-I. Shimizu, and T. Taketsugu, Interface Effects in Hydrogen Elimination Reaction from Isopropanol by Ni13 Cluster on θ-Al2O3(010) Surface, J. Phys. Chem. C, 121, 3488-3495 (2017).