A Theoretical Study of the Mechanism of the Adsorptive

Chapter 20

A Theoretical Study of the Mechanism of the Adsorptive Decomposition of Nitrous Oxide on Copper 1

1

2

Peter Wolohan , William J . Welsh , Robert Mark Friedman , and Jerry R. Ebner 2

1

Department of Chemistry and Center for Molecular Electronics, University of Missouri, St. Louis, MO 63121 Monsanto Corporate Research, Monsanto Company, St. Louis, MO 63167

Downloaded by COLUMBIA UNIV on August 7, 2012 | http://pubs.acs.org Publication Date: April 8, 1999 | doi: 10.1021/bk-1999-0721.ch020

2

We present results from a Density Functional Theory (DFT) study of the mechanism of the dissociative adsorption of nitrous oxide on the principal low index crystal planes of copper ( and ) by applying the generalized gradient approximation (GGA) to three -dimensional supercell representations of these crystal planes. We have studied the "molecular adsorption" of N O on these surfaces and the site and structure of the "atomic" oxygen product. The transition state of the reaction mechanism has been located using the four-fold hollow site as the primary catalytic site on both planes. The calculated activation energy to dissociation is 0.67 eV lower on the Cu surface than on the Cu surface. Furthermore, adsorption on the Cu surface results in a greater weakening of the N=0 bond of N O. The present calculations indicate that adsorption of N O in this site on the Cu surface is extremely favorable, which is consistent with the highly reactive nature of this surface. 2

2

2

Copper, often highly dispersed on a variety of supports, is widely used as a catalyst in many industrially important catalytic processes including, for example, methanol synthesis, the water gas shift reaction, and the oxidative dehydrogenation of alcohols. One method for quantification of the free-copper surface area of supported and unsupported copper catalysts uses adsorptive decomposition of N 0 as described by Dell el al. (1). In this procedure, the number of exposed surface atoms of Cu is estimated from the amount of N released in the adsorptive decomposition of nitrous oxide according to the reaction: 2

2

N 0 (g) + 2Cu 2

( s )

= N (g) + -(Cu-O-Cu)2

(ads)

(1)

Numerous analytical techniques have been published for monitoring the course of this reaction and the completion of the monolayer (2-7). General

© 1999 American Chemical Society

In Transition State Modeling for Catalysis; Truhlar, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

259

260 agreement exists regarding the stoichiometry of the chemisorbed layer: Cu(s)Oads, corresponding to full coverage [0 = 1]. Many investigations of this reaction have been reported including a recent detailed study by G. Sankar et al (8) who employed a specially designed in-situ reaction cell that also accommodated both X-ray absorption and X-ray diffraction measurements. These workers identified two preferred configurations for the chemisorbed atomic oxygen culminating from N 0 dissociation: (1) a two-fold long bridge site in which the oxygen atom bridges two Cu atoms 2

2

O(ads)

/ \ Downloaded by COLUMBIA UNIV on August 7, 2012 | http://pubs.acs.org Publication Date: April 8, 1999 | doi: 10.1021/bk-1999-0721.ch020

(-Cu-Cu-) separated by a distance of 3.62 A, and (2) chemisorption of the atomic oxygen in the four-fold hollow site. The two-fold long bridge site is unique to the Cu surface, which is known to be the most reactive of the three principal low-index surfaces: , , and (9-10). This site exhibits a primary Cu-0 distance of 1.87 A and a secondary Cu-0 distance of 3.30 A . Adsorption at the alternative four-fold hollow site gives rise to a primary Cu-O distance of 2.23 A . Comparable adsorption sites have been studied and discussed for O k on the various crystal planes of copper generated by reaction of the metal with 0 (11-14). Still, many questions remain unanswered with respect to the N 0 reaction, including: What is the mechanism of the adsorptive decomposition of N 0 on copper? What is the activation energy for dissociation on the principal crystallographic planes and at the different adsorption sites? What is the transition state of the mechanism and is it independent of the crystallographic plane? What are the implications of the structural specificity for the general application of the dissociative adsorption for quantification of the catalytic active? The bonding of a molecule to an extended metal surface or to the surface of a supported metal crystallite is a fundamental issue. Although the bonding within a molecular species or within a metal has been well studied, many aspects regarding the bonding of a molecule with a metal surface such as that of a catalyst remain unclear and largely unexplored beyond the cluster model. Ab initio theoretical techniques allow us to study such important chemical processes at a level not possible with available experimental techniques. Density functional theory (15-21) (DFT), in particular the gradient corrected DFT (GC-DFT) procedures allow us to calculate the properties of interacting species. a(

2

2

2

Computational Methods One particular implementation of DFT is that used in the quantum chemistry program CASTEP (22) which allows one to study the interaction of a discrete molecular species with a periodic representation of the extended metal surface (2326). In a chemical system, the molecular orbitals are eigenstates of the system. In order to describe the bonding and chemistry of the molecule, it is essential to understand such properties as the shape, symmetry and energy of these eigenstates. Charge densities of the eigenstates of a system are calculated as


261 p ,(r)= H p .

Ar)

(2)

2 pJLr)

(3)

= 1 1 ib&ndki

where /band is the band number, k is thefc-point,and the summations run over all occupied bands and sampledfc-points,respectively^ The term W (r) represents }

ibmdtkj

a Bloch state that is an eigenstate of the extended periodic system. The charge densities of all the sampled Bloch states, p , then give a representation of the molecular orbitals of the system being studied. In the present study, the adsorption of N 0 and atomic O were modeled on both the Cu surface (the most reactive for dissociation) and Cu surface (much less reactive for dissociation) using three-dimensional cell representations that contained three atomic layers of the copper catalyst. For the Cu surface, the O atom was placed above the two-fold long bridge site at a Cu-0 distance of 1.87 A and above the four-fold hollow site at a Cu-O distance of 2.23 A in accordance with those observed by Sankar et al (8) (Figure 1). The former distance compares favorably with 1.84 A for Cu-0 in the two-fold long bridge site as determined from angular dependent surface extended X-ray absorption fine structure (SEXAFS) of (2x1) O on Cu (11). In the absence of experimental geometric data with regard to positioning the N 0 molecule, it was placed at the same distance given above for the O atom in each case. Similar placement of the N 0 molecule and atomic oxygen with respect to their O atoms allowed interpretation of differences in calculated properties between the two systems as due to perturbative effects arising from differences in structure between the intact N 0 molecule and atomic oxygen. In particular, their similar placement enables comparison of the electron density distributions relative to the adsorptive decomposition reaction and, thus, facilitates an understanding of the adsorptive mechanism. Similar geometric data from experiment were again to our knowledge unavailable in the case of N 0 on the Cu surface, although this is not surprising given the less reactive nature of the Cu surface. As a result, suitable Cu-O distances were estimated using molecular mechanics (MM) to carry out an extensive study of N 0 adsorption on both copper surfaces and adsorption sites using much larger representations of the extended Cu surface. Values for the equilibrium adsorbed Cu-0 distance given by these MM calculations were 1.78 A on the two-fold site and 2.11 A on the four-fold hollow site of the Cu surface. We used our MM-calculated distances for both the N 0 and 0 ds) for consistency with calculations performed on the Cu surface. These results can be compared with data from an extensive array of studies on the dissociative adsorption of 0 on the Cu surface (12-14). The chemisorption of O at a four-fold hollow site on the Cu surface yielded Cu-0 distances of » 1.9 A from multiple scattering (MS) analysis of near-edge X-ray absorption fine structure (NEXAFS) (12), 1.94 A from surface extended X-ray absorption fine structure


itmJtk

2

2

2

2

2

2

2

2


(a


262

Figure 1. Ball and stick illustration of the calculated geometry of the transition state of the N 0 adsorptive decomposition mechanism when N 0 is adsorbed in (a) the four-fold hollow site on Cu(llO) surface and (b) the four-fold hollow site on Cu -- N £ )

-14859.

Cu --o+

-14863 _

OK110>--N£>

-14867-

Cu--O + N

2

Reaction coordinate Figure 2. Schematic of the energetics of the various species along the reaction coordinate of the adsorptive decomposition mechanism of N 0 in the four-fold hollow site on the Cu and Cu surfaces. 2


269

-

7.2942 6.92014 6.54608 6.17202 5.79795 5.42389 5.04983 4.67577 .4.30171 . 3.92765 .3.55359 . 3.17952 . 2.80546 . 2.4314 , 2.05734 1.68328 1.30922 0.93516 0.56109 .0.18703 .


(a)

00

Figure 3. Volumetric illustration of the shape and symmetry of the calculated highest occupied band (HB) of a N 0 adsorbate in the four-fold hollow site (a) on the Cu(110) surface and (b) on the Cu(100) surface. The contour plot in the upper righthand corner is taken from a slice of the charge density through the LUMO orbital in the x-direction. For color, see the color insert. 2



270

Figure 3. Continued.


271 antibonding orbital of the N 0 (Table HI). This observation is further corroborated by the significant weakening of the N=0 bond (r =1.121 A) and to a lesser extent the N=N bond (r = 1.153 A) of the N 0 adsorbate which had started at the experimental distance for the N=0 bond in gas-phase N 0 (Table I). The same type of frontier orbital interaction is more difficult to identify for the corresponding orbital of the N 0-Cu complex. The present calculations indicate that only the atomic orbitals of the O atom in the N 0 adsorbate contribute to the HB of the complex. This conclusion is consistent with the calculated geometry which shows that the N 0 adsorbate is too distant from the Cu surface to develop a significant interaction. Indeed, the N=0 bond of the adsorbed system is only slightly contracted in the energy-minimized model (r = 1.091 A) relative to its starting point corresponding to the experimental distance for N=0 in gas-phase N 0 (Table I). It thus appears that the primary effect of the Cu surface is to perturb the geometry of the N 0 adsorbate in preparation for the exothermic process of dissociation (Table HI). 2

0

0

2

2

2

2

2


D

2

2

Conclusions From the present DFT study of the mechanism of the adsorptive decomposition of N 0 on copper, we can conclude the following: 2

• N 0 adsorption is energetically more favorable in the two-fold sites than in the corresponding four-fold sites, particularly adsorption in the short two fold site on the Cu surface which is exothermic at the distances modeled in the single point calculations. • Adsorption of atomic oxygen, placed at the experimental distances for the Cu surface and at the estimated distances for the Cu surface, is exothermic in all cases. Adsorption is energetically more favorable in the four-fold sites than in the corresponding two-fold sites, particularly the four fold site on the Cu surface which is the most energetically favorable of the sites investigated. This finding is not supported by the experimental results of Sankar et al. (8) in which the long two-fold bridge is assigned as the location of the product O from the N 0 decomposition. However, it is important to remember that the experiments were carried out at saturation whereas the present calculations relate to very low coverage without inclusion of adsorbate-adsorbate interactions (32). These effects are the focus of future work. At the same time, the binding energy of atomic O calculated in the present work (-4.90 eV from the single-point calculations and -5.46 eV from the optimized results) is consistent with estimates of 5-6 eV reported elsewhere. • Dissociation of N 0 was calculated to be exothermic only in the four-fold hollow site on the Cu surface which is observed as the most reactive surface by far. Upon optimization of the supercell models, these conclusions are confirmed but the magnitude of the difference as a ratio of dissociation energies is reduced. 2

2

33

2


272 • The calculated activation energy for dissociation of N 0 in the four-fold hollow site on the Cu surface is 2.80 eV while the corresponding energy on the Cu surface is 2.40 eV. • In the transition state of the dissociation process in the four-fold site, the N=N bond is strengthened and the N=0 bond is weakened to a greater extent on the Cu surface than on the Cu surface despite costing less energy to achieve this strained conformation. • The adsorptive decomposition of N 0 on copper is initiated by reduction of the N 0 whereby electron density is transferred from the metal surface to the LUMO of the adsorbate molecule. However, this interaction must be energetically favorable to occur. 2

2


2

Further studies using the DFT dynamics capabilities of CASTEP are being carried out using these optimized copper supercells. Besides accounting for the effects of temperature, these extended studies will allow us to explore the mechanism of the dissociative adsorption of N 0 on the Cu and principal crystallographic planes. They should provide further evidence of the more reactive nature of the Cu surface. 2

Acknowledgments PW and WJW wish to thank the Monsanto Company for funding of this research. Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15)

Chinchen, G. C.; Hay, C. M.; Vanderveil H. D.; Waugh, K. C. J. Catalysis 1987, 79, 103. Dell, R, M.; Stone, S.; Tiley, P. F. Trans. Faraday Soc. 1953, 49, 159. Scholten, J. J. F.; Konvalinka, J. A. Trans Faraday Soc. 1969, 65, 2465. Dvorak, B.; Pasek, J. J. Catal. 1970, 18, 108. Evans, J. W.; Wainwright, M . S.; Bridgewater, A.; Young, D. J. Appl. Catal. 1983, 7, 75. Chinchen, G. C.; Hay, C. M.; Vanderveil, H. D.; Waugh, K. C. J. Catal. 1987, 103, 79. Giamello, E.; Fubini, B.; Lauro, P.; Bossi, A. J. Catal. 1984, 87, 443. Sankar, G.; Thomas, J. M.; Waller, D.; Couves, J. W.; Catlow, c. R. A.; Greaves, G. N. J. Phys. Chem. 1992, 96, 7485. Arlow, J. S.; Woodruff, D. P. Surf. Sci. 1985, 157, 327. Arlow, J. S.; Woodruff, D. P. Surf. Sci. 1985, 162, 310. Döbler,, U.; Baberschke, K.; Haase, J.; Pushmann, A. Phys. Rev. Lett. 1984, 52, 1437. Vvedensky, D. D.; Pendry, J. B.; Döbler, U.; Baberschke, K. Phys. Rev. B 1987, 35, 7756. Döbler, U.; Baberschke, K.; Stöhr,J.;Outka, D. A. Phys. Rev. B 1985, 31, 2532. Kono, S.; Goldberg, S. M.; Hall, N. F. T.; Fadley, C. S. Phys. Rev. B 1980, 22, 6085. Hohenberg, P.; Kohn, W. Phys. Rev. B 1964, 136, 864.


273 (16) (17) (18) (19)


(20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33)

Kohn, W.; Sham, L. J. Phys. Rev. A 1965, 140, 1133. Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and Molecules; Oxford University Press: New York, 1989. Jones, R. O.; Gunnarsson, O. Rev. Mod. Phys. 1989, 61, 689. Density Functional Methods in Chemistry; Labanowski, J. K.; Andzelm, J., Eds.; Springer-Verlag: New York, 1991. Ziegler, T. Chem. Rev. 1991, 91, 651. Wigner, E. P. Trans. Faraday Soc. 1938, 34, 678. Cambridge serial total energy package, University of Cambridge, Madingley Road, Cambridge, United Kingdom. White, J. A.; Bird, D. M.; Payne, M. C. Phys. Rev. B 1996, 53, 1667. Owen, J. H. G.; Bowler, D. R.; Goringe, C. M.; Miki, K.; Briggs, G. A. D. Phys. Rev. B 1996, 54, 14153. Milman, V.; Pennycook, S. J.; Jesson, D. E. Thin Solid Films 1996, 272, 375. Kantorovich, L. N.; Gillan, M. J.; White, J. A. J. Chem. Soc. 1996, 92, 2075. Car, R.; Parrinello, M. Phys. Rev. Lett. 1985, 55, 2471. Lin, J. S.; Qteish, A.; Payne, M. C.; Heine, V. Phys. Rev. B 1993, 47, 4174. Perdew, J. P.; Wang, L. Phys Rev. B 1991, 40, 3425. Wolohan, P.; Welsh, W. J.; Friedman, R. M.; Ebner, J. R. (unpublished work). The Surface Chemistry of Metals and Semiconductors; Gatos, H.C.,Ed.; John Wiley and Sons, Inc.: New York, 1959. Chemistry and Physics of Solid Surfaces, Vanselow, R., Ed.; CRC Press: Boca Raton, Florida, 1979, Vol. 2. Madhaven, P. V.; Newton, M. D. J. Chem. Phys. 1987, 86, 4030, and references cited therein.


A Theoretical Study of the Mechanism of the Adsorptive

Recommend Documents