Reactivity and Catalytic Activity of Hydrogen Atom Chemisorbed Silver

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Reactivity and Catalytic Activity of Hydrogen Atom Chemisorbed Silver Clusters Dar Manzoor, and Sourav Pal J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b01987 • Publication Date (Web): 19 May 2015 Downloaded from http://pubs.acs.org on May 21, 2015

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The Journal of Physical Chemistry

Reactivity and Catalytic Activity of Hydrogen Atom Chemisorbed Silver Clusters Dar Manzoor* and Sourav Pal* Physical Chemistry Division, CSIR-National Chemical Laboratory, Pune-411008, India

Abstract: Metal clusters of silver have attracted recent interest of researchers as a result of their potential in different catalytic applications and low cost. However, due to the completely filled d orbital and very high first ionization potential of the silver atom, the silver based catalysts interact very weakly with the reacting molecules. In the current work, density functional theory calculations were carried out to investigate the effect of hydrogen atom chemisorption on the reactivity and catalytic properties of inert silver clusters. Our results affirm that the hydrogen atom chemisorption leads to enhancement in the binding energy of the adsorbed O2 molecule on the inert silver clusters. The increase in the binding energy is also characterized by the decrease in the Ag-O and increase in the O-O bond lengths in the case of the AgnH silver clusters. Pertinent to the increase in the O-O bond length, a significant red shift in the O-O stretching frequency is also noted in the case of the AgnH silver clusters. Moreover, the hydrogen atom chemisorbed silver clusters show low reaction barriers and high heat of formation of the final products, for the environmentally important CO oxidation reaction as compared to the parent catalytically inactive clusters. The obtained results were compared with those of the corresponding gold and hydrogen atom chemisorbed gold clusters obtained at the same level of theory. It is expected the current computational study will provide key insights for future advances in the design of efficient nano silver based catalysts through the adsorption of a small atom or a ligand.

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I. Introduction The catalytic activity of small metal clusters of silver and gold both in the gas phase as well as on different supports has received paramount interest from both experimental and theoretical point of view during the last few years. The metal clusters of silver and gold are known to catalyze wide range of reactions such as hydrogenation and oxidation of various hydrocarbons1-13, C-H activation14,15, C-C coupling reactions16-18, CO oxidation19-24, etc. Among the above mentioned reactions, the CO oxidation finds wide range of applications in CO detectors, CO2 lasers, fuel cells and is particularly important from an environmental point of view25-30. Owing to these reasons, the catalytic efficiency of a variety of catalysts based on a number of transition metals or their alloys such as Pt, Pd, Au, Ag, Cu etc towards CO oxidation has been the focus of the recent research31-35. Among the above mentioned systems, the CO oxidation on the Au and Pt based catalysts both in bare as well as on different oxide supports have been studied extensively. However, the Au and Pt based catalysts are very costly and suffer from some drawbacks vis a vis selective oxidation of CO36. Recent research works have pointed out that silver in different forms can act as an efficient catalyst for the oxidation of CO to CO237,38. The reactivity and catalytic properties of silver clusters depends on a number of complicated factors which include preparation method39, size and morphology of the cluster40-43, oxidation state of the cluster44-46, supporting material15,16,47,48, etc. However the exact mechanism through which one or a combination of these factors influences the reactivity and catalytic behavior of these clusters is not clearly understood and remains to be a subject of debate.


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The catalytic activity of silver clusters for the environmentally important CO oxidation is considered to be one of the important topics in heterogenous catalysis. Based on the preparation method, a number of silver catalysts with profound catalytic activity for CO oxidation have been designed. For example, Bera et al reported highly active Ag/CeO2 catalysts for CO oxidation using combustion technique49. The enhanced catalytic response of Ag/CeO2 system was claimed to be due to the presence of ionically scattered Ag+ on the CeO2 surface. Similarly, Chen et al revealed that the tunnel like structure and nanorod morphology of Ag-OMS-2 catalyst favored enhanced CO adsorption and O2 activation, thereby giving high turnover for CO oxidation at low temperatures50. Further, some recent works using density functional theory have been carried out to investigate the O2 activation and CO oxidation on silver clusters19,20,51,52,53. These studies have validated that clusters with odd number of electrons show enhanced reactivity and catalytic ability with respect to CO oxidation as compared to the clusters with even number of electrons. Silver based catalysts are relatively cheap, but due to the completely filled d orbital and very high first ionization potential of 7.58 eV of the silver atom, the silver based catalysts show weak interaction with the reacting molecules39. Therefore, efforts are currently being made to improve the reactivity and catalytic property of silver based catalysts through a number of ways. One such way is the pre-adsorption of metal clusters with small atoms or molecules. The pre-adsorption of an atom or a molecule can induce significant changes in the electronic structure of the cluster, there by leading to modulation of the reactivity and catalytic ability. For example, a number of studies have shown that hydrogen treatment has been favorable for enhancing catalytic activity of supported silver clusters54,55. Recently, Lang et al56 and, Pal and co-workers57 have shown that preadsorption of molecular and atomic hydrogen respectively improves the reactivity



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and catalytic efficiency of pristine gold clusters using experimental and theoretical methods. Furthermore, in an interesting study, Khairallah et al revealed that the cationic Ag4H cluster is highly active and selective for the C-C coupling with allyl bromide as compared to the pristine Ag4+ cluster58. It was found that the hydrogen atom plays a significant role to initiate the reaction as pure silver clusters where found to be inactive in promoting the C-C coupling. Further, the silver hydride clusters have been synthesized and isolated in the gas phase59-62, and the impact of hydrogen atom chemisorption on the structure and electronic properties of silver clusters has been studied theoretically63,64. However, to the best of our knowledge, the effect of hydrogen atom chemisorption on the reactivity and catalytic activity of silver clusters is unknown. Hence, the objective of this work is to highlight the effect of hydrogen atom chemisorption on the reactivity and catalytic activity of neutral closed shell silver clusters and compare our results with the corresponding gold clusters. The results confirm that the hydrogen atom activates the inert silver clusters, thereby enhancing their reactivity significantly towards molecular oxygen. Further, the hydrogen atom chemisorbed silver clusters are found to catalyze the CO oxidation reaction efficiently with very low activation barriers as compared to the pristine silver clusters.

II. Computational Details All the calculations were carried out by using Density functional Theory (DFT) with PBE exchange and correlation potential as implemented in the Gaussian 0965. In order to locate the ground state geometry, a number of conformers were used as a starting guess for each cluster. The optimization of geometries was carried out by using the Berny algorithm with the default convergence criterion. We considered both the singlet and triplet multiplicities for the Agn, and both doublet and quartet multiplicities for the AgnH (n=2, 4, 6, 8) cluster optimizations. The lowest energy


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structures for the Agn and AgnH clusters were found to have singlet and doublet multiplicities respectively. The LANL2DZ basis set and the corresponding Los Alamos relativistic effective core potential (RECP) was used for the silver atoms. For the hydrogen, oxygen and carbon atoms, the TZVP basis set was used. To find the lowest energy configurations of O2 adsorbed silver cluster complexes, we considered the various possible adsorption patterns, including the bridge (with two Ag-O bonds) and atop (with single Ag-O bond) adsorption of O2 on the Agn and AgnH clusters at the various possible sites. The optimized geometries were guaranteed to be the local minima by carrying out the frequency calculations after the optimizations. The equilibrium geometries of the Agn and AgnH clusters and their O2 adsorbed complexes were also obtained by using the TPSS functional. The O2 binding energies have been calculated as the difference between the energy of constituents of the O2 adsorbed complex ( i.e O2 and cluster ) and the complex. The basis set superposition error (BSSE) has not been considered for calculation of O2 binding energies as it has been demonstrated recently that the BSSE has a negligible effect on the O2 adsorption energies of silver clusters66,67. The transition states were found by using the linear synchronous transit method and were characterized by the presence of one imaginary frequency.

III. Results and Discussion We begin with a discussion on the structure of the Agn and AgnH (n= 2, 4, 6 and 8) clusters. The optimized geometries of the Agn and AgnH clusters are depicted in the Figure 1 and are in line with the earlier reported studies63,64. We find that the hydrogen atom undergoes bridged type of bonding with two Ag-H bonds, with all the silver clusters considered in the current study. As revealed in earlier studies, the oxidation catalysis on silver and gold clusters involves the activation of molecular oxygen as the primary and rate determining step. Therefore, we next look at the



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binding and activation of oxygen molecule on the Agn and AgnH clusters. The optimized geometries of the AgnO2 and AgnHO2 complexes obtained using PBE functional are presented in the Figure 2 and without stated the results correspond to this functional. The TPSS obtained structures are presented in the supporting information (Figure S1) and are similar to the structures obtained using the PBE functional. It is important to mention here that the O2 adsorption was studied for both the singlet and triplet multiplicities on the Agn clusters and doublet and quartet multiplicities on the AgnH clusters. We found that the lowest energy structures of the AgnO2 and AgnHO2 complexes prefer triplet and doublet spin multiplicities respectively. It can be seen from the Figure 2, that O2 molecule adsorbs in atop mode with a single Ag-O bond on the Agn (n = 2, 6, 8). However, the O2 molecule adsorbs via bridged mode with two Ag-O bonds on the Ag4 cluster. This is in agreement with earlier reported results of oxygen adsorption on silver clusters68. Contrary to the pure silver clusters, the oxygen molecule shows bridged type of bonding with two Ag-O bonds with all the AgnH clusters. The interaction of molecular oxygen can be quantitatively understood in terms of the O2 binding energies on the Agn and AgnH clusters. Tables 1 and 2 enlist the computed binding energies of O2 molecule with the Agn and AgnH clusters obtained using the PBE and TPSS functionals. The PBE and TPSS results are in good agreement with each other for all the clusters studied except that the calculated O2 binding energy for the Ag4 cluster is found to be higher in case of PBE functional as compared to the TPSS functional. From the binding energy values, we note that the AgnH clusters shown significantly large binding energies with the O2 molecule as compared to the pure silver clusters. We next plot the relative increase in the binding energies for the O2 molecule on the silver and gold clusters up on hydrogen atom chemisorptions in the Figure 3. The plot of the relative increase in binding energies reveals that the increase in binding energy of the O2 molecule up


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on hydrogen atom chemisorption is higher for the silver clusters with 2 and 4 atoms as compared to the corresponding gold clusters where as the relative increase in the binding energy is more for gold clusters with 6 and 8 atoms with respect to their silver counterparts reported in our earlier study57. The enhanced binding of the O2 molecule can be further qualitatively understood in terms of the relevant geometrical parameters such as Ag-O and O-O bond lengths. Two important conclusions can be made from the Ag-O and O-O bond lengths as presented in the Table 1. First, we note that the hydrogen atom chemisorption leads to a significant decrease in the Ag-O bond lengths on all the AgnH clusters, thereby leading to a strong Ag-O bond in the case of AgnHO2 complexes. Second, the O-O bond distance is found to increase upon hydrogen atom chemisorption, which reflects the enhanced activation of the O2 molecule on the hydrogen atom chemisorbed gold clusters. The O-O bond lengths are 1.24, 1.30, 1.24 and 1.24 Å on the Agn clusters respectively, whereas the O-O bond lengths are 1.31, 1.34, 1.32 and 1.33 Å respectively on the AgnH clusters ( n = 2, 4, 6, 8). The increase in the O-O bond distance and subsequent activation of the O2 molecule in the case of the AgnH clusters is further reflected in the consequential red shift of the O-O stretching frequency (see Table 1). Moreover, it is known that the O2 molecule acts as an electron acceptor and a charge transfer occurs from the cluster to the O2 molecule during the interaction of O2 molecule with the metal clusters. The ionization potential provides a simple way of predicting the ease with which an electron can be donated from a cluster to the adsorbed O2 molecule. Figure 4 depicts the calculated ionization potentials for the Agn and AgnH clusters. The hydrogen atom chemisorption is found to reduce the ionization potential for the Agn clusters and thereby facilitates transfer of electron from the clusters to the O2 molecule. We have also computed the net charge on the O2 molecule on the AgnO2 and AgnHO2 complexes using natural bond orbital



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(NBO) charge analysis (see Table 1). In accordance with the above mentioned results, the hydrogen atom inflects an increase in the net charge transfer from the cluster to the O2 molecule. The net negative charge on the O2 molecule in AgnO2 complexes is very low (~0.14) except for Ag4O2 complex which shows a net negative charge of 0.41. Contrary to the pristine clusters, the net negative charge on the O2 molecule in the AgnHO2 complexes is 0.50, 0.62, 0.56 and 0.59 with n = 2, 4, 6 and 8 respectively. Thus from the above discussion, we conclude that the hydrogen atom chemisorption activates the inert closed shell neutral silver clusters, thereby leading to the enhanced binding and activation of the O2 molecules on the AgnH clusters. It is noteworthy to mention here that the binding and activation of O2 molecule is considered to be of primary importance in the oxidation catalysis by metal clusters. We also investigated the effect of hydrogen atom chemisorption on the O2 activation on anionic counterparts of the above studied silver clusters. However, the hydrogen atom chemisorption was not found to have a significant effect on the O2 activation on anionic silver clusters as can be seen from the similar O-O bond lengths on anionic AgnO2 and AgnHO2 complexes (see Figure S2). Further, the hydrogen atom chemisorbed anionic silver clusters were found to show significantly lower O2 binding energies as compared to parent anionic silver clusters. We next look at the effect of hydrogen atom chemisorption on the catalytic activity of Agn silver clusters by studying the CO oxidation on the Agn and AgnH clusters. CO oxidation on the metal clusters follows two types of reaction mechanisms: a) Langmuir-Hinshelwood mechanism and b) Eley Redel reaction mechanism. In the Langmuir-Hinshelwood type of reaction mechanism, the O2 and CO molecules first co-adsorb on the cluster. This is followed by the elongation of the O-O bond and shortening of the O-C bond, resulting in the formation of the OO-C-O intermediate. The next step involves the cleavage of the O-O bond of the


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O-O-C-O intermediate and subsequent formation of the CO2 molecule. The Eley Redel mechanism involves the interaction of the CO molecule with the already preadsorbed O2 molecule and formation of the CO2 molecule through the cleavage of O-O bond. However, a number of research works69-72 have shown the LangmuirHinshelwood mechanism is the favored mechanism for CO oxidation of the metal clusters. Thus, we have also followed the Langmuir-Hinshelwood type of reaction mechanism to study the CO oxidation on the Agn and AgnH (n = 2, 4, 6, 8) clusters. The reaction pathways following the Langmuir-Hinshelwood mechanism are presented in the Figures 5, 6, 7 and 8. In order to generate the lowest energy isomer of the O2 and CO co-adsorbed Agn and AgnH clusters, the CO molecule was adsorbed on the various sites adjacent to the already adsorbed O2 molecule on the Agn and AgnH clusters. It can be seen from the Figures 5 and 6, that the first step leading to the formation of the O-O-C-O intermediate on the Ag6 and Ag8 involves very high activation barriers of 0.60 and 0.57 eV. The second step which involves the cleavage of O-O bond of the O-O-C-O intermediate and leads to the formation of CO2 molecule with low activation barriers of 0.16 and 0.14 eV on Ag6 and Ag8 respectively. However, it is important to mention here that recently it has been shown that the activation of molecular oxygen and formation of O-O-C-O intermediate is the rate limiting step for CO oxidation on metal clusters72-75. Further, the energies of the TS (I), I(2) and TS (2) are higher than the isolated metal cluster (Ag6 and Ag8) and reacting molecules ( O2 and CO ). As a result of this, the CO oxidation reaction becomes uphill particularly on Ag8 and thus has lesser chances of occurring. In contrast to the Ag6 and Ag8 clusters, the first step leading to the formation of O-O-C-O intermediate on the Ag6H and Ag8H involves very low activation barriers of 0.11 and 0.12 eV. This reduction in the activation barrier for the formation of the O-O-C-O intermediate is in line with the corresponding Au6 and Au8 gold clusters where the activation barriers are found to



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decrease from 0.40 and 0.81 eV to 0.01 and 0.35 eV respectively upon hydrogen atom chemisorptions57. The activation barriers for the second step on the Ag6H ( 0.28 eV ) and Ag8H ( 0.32 eV ) clusters are slightly higher than their pristine counterparts. This is contrary to the results observed for the gold clusters where the hydrogen atom chemisorption is seen to reduce the activation barrier for this step also57. Interestingly, we note that the CO oxidation reaction on the Ag6H and Ag8H clusters involves the intermediates and transition states significantly lower in the energy than the isolated cluster and reacting molecules. This makes the CO oxidation reaction downhill and thereby increasing the probability of the CO oxidation on the Ag6H and Ag8H clusters. Furthermore, the calculated heat of formation ( ∆Hf ) for the final products in the case of Ag6H and Ag8H are -3.60 and -3.93 eV respectively, while as the calculated ∆Hf values for the final product in the case of Ag6 and Ag8 clusters are in -2.66 and -2.90 eV respectively. Thus, the hydrogen atom chemisorption not only enhances the catalytic ability of inert silver clusters by reducing the activation barrier but also makes the CO oxidation process more exothermic. IV. Conclusions In summary, we have explored the effect of hydrogen atom chemisorption on the reactivity and catalytic properties of the closed shell inert silver clusters using density functional theory. The results demonstrate that the hydrogen atom chemisorption leads to significant enhancement in the reactivity of the inert silver clusters towards molecular oxygen. The enhancement in reactivity with molecular oxygen in the case of AgnH silver clusters is confirmed by a notable increase in the O2 binding energy as compared to the Agn clusters. These findings are further validated by a significant decrease in the Ag-O and increase in the O-O bond lengths with a concomitant red shift in the O-O stretching frequency in the case of AgnH clusters. Furthermore, the CO oxidation reaction proceeds with very low


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activation barriers with high heat of formation for the final products on the hydrogen atom chemisorbed silver clusters. Thus, the hydrogen atom chemisorbed silver clusters act as efficient catalysts as compared to the parent catalytically inactive silver clusters. Hence, in short our results suggest that the adsorption of a small atom or a molecule can be an efficient and attractive way of tuning the reactivity and catalytic activity of the metal clusters, and can provide key inputs in the design of efficient nanocatalysts. V. Supporting Information Figures showing the TPSS optimized geometries AgnO2 and AgnHO2, effect of hydrogen atom chemisorption on the anionic counterparts of the Agn clusters , oxygen dissociation barriers on the Agn and AgnH clusters and cartesian coordinates of reactants, intermediates, transition states and products for the CO oxidation on the various Agn and AgnH (n= 6, 8) clusters. This material is available free of charge via the Internet at http://pubs.acs.org. VI. Corresponding Author Dar Manzoor Email:- [email protected] Sourav Pal Email:- [email protected] VII. Acknowledgements The authors acknowledge the Center of Excellence in Computational Chemistry (COESC) at CSIR-NCL, Pune for the calculations presented and CSIR XIIth 5-year plan for Multiscale simulation of materials (MSM) project grant. D.M. acknowledges University Grants Commision (UGC), India for Senior Research Fellowship and Sailaja Krishnamurty for her support and mentorship. S.P. acknowledges grant from SSB project of CSIR and J. C. Bose Fellowship project of DST towards partial completion of the work.



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33. Falsig, H.; Hvolbæk, B.; Kristensen, I. S.; Jiang, T.; Bligaard, T.; Christensen, C. H.; Nørskov, J. K. Trends in the Catalytic CO Oxidation Activity of Nanoparticles. Angew. Chem. 2008, 120, 4913-4917. 34. Manzoor, D.; Krishnamurty, S.; Pal, S. Effect of Silicon Doping on the Reactivity and Catalytic Activity of Gold Clusters. J. Phys. Chem. C 2014, 118, 7501-750. 35. van Rijn, R.; Balmes, O.; Felici, R.; Gustafson, J.; Wermeille, D.; Westerström, R.; Lundgren, E.; Frenken, J. W. M. Comment on “CO Oxidation on Pt-Group Metals from Ultrahigh Vacuum to Near Atmospheric Pressures. 2. Palladium and Platinum. J. Phys. Chem. C 2010, 114, 68756876. 36. Huang, Y.; Wang, A.; Wang, X.; Zhang, T. Preferential Oxidation of CO Under Excess H 2 Conditions over Iridium Catalysts. Int. J. Hydrogen Energy 2007, 32, 3880-3886. 37. Yu, L.; Shi, Y.; Zhao, Z.; Yin, H.; Wei, Y.; Liu, J.; Kang, W.; Jiang, T.; Wang, A. Ultrasmall Silver Nanoparticles Supported on Silica and their Catalytic Performances for Carbon Monoxide Oxidation. Catal. Commun. 2011, 12, 616-620. 38. Tompos, A.; Margitfalvi, J. L.; E. G. Szábo, E. G.; L. Végvári, L. Combinatorial Design of Al2O3 Supported Au Catalysts For Preferential CO Oxidation. Top. Catal. 2010, 53, 108-115. 39. Xiaodong, Z.; Zhenping, Q. U.; Fangli, Y. U.; Yi, W. Progress in Carbon Monoxide Oxidation over Nanosized Ag Catalysts. Chin. J. Catal. 2013, 34, 1277-1290. 40. Liao, M. S.; Watts, J. D.; Huang, M. J. Theoretical Comparative Study of Oxygen Adsorption on Neutral and Anionic Agn and Aun Clusters (n = 2– 25). J. Phys. Chem. C 2014, 118, 21911-21927.


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41. Trinchero, A.; Klacar, S.; Paz-Borbón, L. O.; Hellman, A.; Grönbeck, H. Oxidation at the Subnanometer Scale. J. Phys. Chem. C, DOI: 10.1021/jp508302b 42.Klacar, S.; Hellman, A.; Panas, I.; Grönbeck, H. Oxidation of Small Silver Clusters: A Density Functional Theory Study. J. Phys. Chem. C 2010, 114, 12610-12617. 43. Socaciu, L. D.; Hagen, J.; Roux, J. L.; Popolan, D.; T.M. Bernhardt, T. M.;L. Wöste, L.; Vajda, S. Strongly Cluster Size Dependent Reaction Behavior of CO with O2 on Free Silver Cluster Anions. J. Chem. Phys. 2004, 120, 2078. 44. Zhou, J.; Zhen-Hua Li, Z. H.; Wen-Ning Wang, W. N.; Fan, K. N. Density Functional Study of the Interaction of Carbon Monoxide with Small Neutral and Charged Silver Clusters. J. Phys. Chem. A 2006, 110, 7167-7172. 45. Zhou, J.; Li, Z. H.; Wang, W. N.; Fan, K. N. Density Functional Study of the Interaction of Molecular Oxygen with Small Neutral and Charged Silver Clusters. Chem. Phys. Lett. 2006, 421, 448-452. 46. Zhao, S.; Li, Z. H.; Wang, W. N.; Fan, K. N. Density Functional Study of the Interaction of Chlorine Atom with Small Neutral and Charged Silver Clusters. J. Chem. Phys. 2005,122, 144701. 47. Shimizu, K.; Miyamoto, Y.; Satsuma, A. Size- and Support-dependent Silver

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Nitroaromatics. J. Catal. 2010, 270, 186-194. 48. Shimizu, K.; Ohshima, K.; Satsuma, A. Direct Dehydrogenative Amide Synthesis from Alcohols and Amines Catalyzed by γ-Alumina Supported Silver Cluster. Chem. Eur. J. 2009, 15, 9977-9980.



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49. Bera, P.; Patil, K. C.; Hegde, M. S. NO Reduction, CO and Hydrocarbon Oxidation over Combustion Synthesized Catalyst. Phys. Chem. Chem. Phys. 2000, 2, 3715-3719. 50. Chen, J.; Li, J.; Li, H.; Huang, X.; Shen, W. Facile Synthesis of Ag–OMS-2 Nanorods and their Catalytic Applications in CO Oxidation. Microporous Mesoporous Mater. 2008, 116, 586-592. 51. Kim, H. Y.; Kim, D. H.; Ryu, J. H.; Lee, H. M. Design of Robust and Reactive Nanoparticles with Atomic Precision: 13Ag-Ih and 12Ag-1X (X = Pd, Pt, Au, Ni, or Cu) Core-Shell Nanoparticles. J. Phys. Chem. C 2009, 113, 15559-15564. 52. Kim, D. H.; Shin, K.; Lee, H. M. CO Oxidation on Positively and Negatively Charged Ag13 Nanoparticles. J. Phys. Chem. C 2011, 115, 24771-24777. 53. Tang, D.; Chen, Z.; Hu, J.; Sun, G.; Lua, S.; Hu, C. CO Oxidation Catalyzed by Silver Nanoclusters: Mechanism and Effects of Charge. Phys. Chem. Chem. Phys. 2012, 14, 12829-12837. 54. Yen, C. W.; Lin, M. L.; Wang, A.; Chen, S. A.; Chen, J. M.; Mou, C. Y. CO Oxidation Catalyzed by Au-Ag Bimetallic Nanoparticles Supported in Mesoporous Silica. J. Phys. Chem. C 2009 113, 17831-17839. 55. Liu, X.; Wang, A.; Yang, X.; Zhang , T.; Mou, C Y.; Su, D. S.; Li, J. Synthesis of Thermally Stable and Highly Active Bimetallic Au-Ag Nanoparticles on Inert Supports. Chem. Mater. 2009, 21, 410-418. 56. Lang, S. M.; Bernhardt, T. M.; Barnett, R. N.; Yoon, B.; Landman, U. Hydrogen-Promoted Oxygen Activation by Free Gold Cluster Cations. J. Am. Chem. Soc. 2009, 131, 8939-8951.


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57. Manzoor, D.; Pal, S. Hydrogen Atom Chemisorbed Gold Clusters as Highly Active Catalysts for Oxygen Activation and CO Oxidation. J. Phys. Chem. C 2014, 118, 30057-30062. 58. Khairallah, G. N.; O’Hair, R. A. J. Gas-Phase Synthesis of [Ag4H]+ and Its Mediation of the C-C Coupling of Allyl Bromide. Angew. Chem. 2005, 117, 738-741. 59. Khairallah, G. N.; O’Hair, R. A. J. Gas Phase Synthesis and Reactivity of Agn+ and Agn-1H+ Cluster Cations. Dalton Trans. 2005, 2702-2712. 60. Wang, F. Q.; Khairallah, G. N.; O’Hair, R. A. J. Role of Cluster Size and Substrate in the Gas Phase C-C Bond Coupling Reactions of Allyl Halides Mediated by Agn+ and Agn-1H+ Cluster Cations. Int. J. Mass Spectrom. 2009, 283, 17-25. 61. Mitrić, R.; Petersen, J.; Kulesza, A.; Röhr, M. I. S.; Bonačić-Koutecký , V.; Brunet, C.; Antoine, R.; Dugourd, P.; Broyer, M.; O’Hair, R. A. J. GasPhase Synthesis and Vibronic Action Spectroscopy of Ag2H+. J. Phys. Chem. Lett. 2011, 2, 548-552. 62. Xie, H.; Xing, X.; Liu, Z.; Cong, R.; Qin, Z.; Wu, X.; Tang, Z.; Fan, H. Photoelectron Imaging and Theoretical Calculations of Gold-Silver Hydrides: Comparing the Characteristics of Au, Ag and H in Small Clusters. Phys. Chem. Chem. Phys. 2012, 14, 11666-11672. 63. Otávio, J.; Lins, M. A.; Nascimento, M. A. C. A Density Functional Study of some Silver Cluster Hydrides. Chem. Phys. Lett. 2004, 391, 9-15. 64. Zhao, S.; Liu, Z. P.; Li, Z. H.; Wang, W. N.; Fan, K. N. Density Functional Study of Small Neutral and Charged Silver Cluster Hydrides. J. Phys. Chem. A 2006, 110, 11537-11542.



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65. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A et al. Gaussian 09, Revision A.1; Gaussian, Inc.: Wallingford, CT, 2009. 66. Joshi, A. M.; Delgass, W. N.; Thomson, T. K. Analysis of O2 Adsorption on Binary−Alloy Clusters of Gold: Energetics and Correlations. J. Phys. Chem. B 2006, 110, 23373-23387. 67. Lyalin, A.; Taketsugu, T. Reactant-Promoted Oxygen Dissociation on Gold Clusters. J. Phys. Chem. Lett. 2010, 1, 1752-1757. 68. Klacar, S.; Hellman, A.; Panas, I.; Grönbeck, H. Oxidation of Small Silver Clusters: A Density Functional Theory Study. J. Phys. Chem. C 2010, 114, 12610-12617. 69. An, W.; Pei, Y.; Zeng, X. C. CO Oxidation Catalyzed by Single-Walled Helical Gold Nanotube. Nano Lett. 2008, 8, 195-202. 70. Lopez-Acevedo, O.; Kacprzak, K. A.; Akola, J.; Häkkinen, H. Quantum Size Effects in Ambient CO Oxidation Catalyzed by Ligand Protected Gold Clusters. Nat. Chem. 2010, 2, 329-334. 71. Socaciu, L.; Hagen, J.; Bernhardt, T.; Wöste, L.; Heiz, U.; Häkkinen, H.; Landman, U. Catalytic CO Oxidation by Free Au2−: Experiment and Theory. J. Am. Chem. Soc. 2003, 125, 10437-10445. 72. Liu, C.; Tan, Y.; Lin, S.; Li, H.; Wu, X.; Li, L.; Pei, Y.; Zeng, X. C. CO Self-Promoting Oxidation on Nanosized Gold Clusters: Triangular Au3 Active

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74. Gao, Y.;

Pei, Y.;

Chen, Z.;

Zeng, X.

C.

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Table 1. O2 binding energy (Eb) and relevant optimized geometrical parameters such as Ag-O bond length (RAg-O), O-O bond length (RO-O) and O-O stretching frequency (ʋo-o) of the AgnO2 and AgnHO2 complexes obtained using the PBE functional. qO2 represents the calculated net net NBO charge on the O2 molecule in the AgnO2 and AgnHO2 complexes. System

Eb (eV)

RAg-o (Å)

Ro-o (Å)

ʋo-o (cm-1)

qO2

System

Eb (eV)

RAg-o (Å)

Ro-o (Å)

ʋo-o (cm-1)

qO2

Ag2O2

0.18

2.42

1.24

1368

-0.13

Ag2HO2

1.03

2.39/2.39

1.31

1124

-0.50

Ag4O2

0.39

2.35/2.46

1.30

1137

-0.41

Ag4HO2

1.17

2.27/2.21

1.34

1099

-0.62

Ag6O2

0.16

2.47

1.24

1365

-0.13

Ag6HO2

0.66

2.33/2.24

1.32

1119

-0.56

Ag8O2

0.41

2.42

1.24

1369

-0.14

Ag8HO2

1.11

2.27/2.31

1.33

1110

-0.59

Table 2. O2 binding energy (Eb) and relevant optimized geometrical parameters such as Ag-O bond length (RAg-O), O-O bond length (RO-O) and O-O stretching frequency (ʋo-o) of the AgnO2 and AgnHO2 complexes obtained using the TPSS functional. qO2 represents the calculated net net NBO charge on the O2 molecule in the AgnO2 and AgnHO2 complexes. System

Eb (eV)

RAg-o (Å)

Ro-o (Å)

ʋo-o (cm-1)

qO2

System

Eb (eV)

RAg-o (Å)

Ro-o (Å)

ʋo-o (cm-1)

qO2

Ag2O2

0.14

2.42

1.25

1346

-0.14

Ag2HO2

1.12

2.69/2.36

1.32

1104

-0.57

Ag4O2

0.39

2.31/2.39

1.31

1080

-0.48

Ag4HO2

1.26

2.27/2.19

1.35

1087

-0.74

Ag6O2

0.13

2.44

1.25

1342

-0.14

Ag6HO2

0.64

2.29/2.21

1.34

1089

-0.61

Ag8O2

0.14

2.24

1.25

1336

-0.15

Ag8HO2

1.15

2.26/2.29

1.34

1094

-0.62



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Figure 1. Optimized geometries of Agn and AgnH clusters ( n = 2, 4, 6, 8 ) obtained using the PBE functional. Atomic color codes: Ag, blue; H, sky blue.


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Figure 2. Optimized geometries of Agn-O2 and AgnH-O2 clusters (n = 2, 4, 6, 8) obtained using the PBE functional. Atomic color codes: Ag, blue; H, sky blue; O, red.



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Figure 3. Calculated relative increase in the O2 binding energy (ER) on the Agn and Aun (n=2, 4, 6, 8) clusters up on hydrogen atom chemisorption


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Figure 4. Ionization potential of the Agn and AgnH (n = 2, 4, 6, 8) clusters.



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Figure 5. Reaction pathway for CO oxidation on the Ag6 cluster calculated using the PBE functional.


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Figure 6. Reaction pathway for CO oxidation on the Ag8 cluster calculated using the PBE functional.



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Figure 7. Reaction pathway for CO oxidation on the Ag6H cluster calculated using the PBE functional.


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Figure 8. Reaction pathway for CO oxidation on the Ag8H cluster calculated using the PBE functional.



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Table of Contents Graphic


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Reactivity and Catalytic Activity of Hydrogen Atom Chemisorbed Silver

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