Langmuir 1990,6, 365-370
365
Adsorption Preferences of NH3 on V205Catalysts: Theoretical Analysis of a Critical Step in Ammoxidation Processes Jonathan Otamiri and Arne Andersson Department of Chemical Technology, University of Lund, P.O. Box 124, S-22100 Lund, Sweden
Susan A. Jansen* Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122-01600 Received January 20,1989. In Final Form: June 16, 1989
Metal oxides utilized as mild oxidation catalysts are industrially important in ammoxidation processes. This reaction involves the activation of NH,, and subsequent oxidation produces a nitrile product. The most important nitriles are obtained by ammoxidation of propylene, toluene, xylenes, and pyridines. Our work focuses on adsorption and activation processes for ammonia on model V,O,(OlO) surfaces. Extended Hiickel/tight-binding calculations have been used to characterize both the active sites of the catalyst and the nucleophilic and electrophilic surface oxygen species. For several oxidation processes, nucleophilic oxygen species are implicated in selective processes, and electrophilic species produce nonselective carbon oxides. Our studies have also evaluated known low-energy adsorption sites and described bonding interactions between surface/adsorbate and coadsorbate interactions. An analysis of such interactions is essential, as competitive adsorption processes are critical for ammonia activation.
Introduction Vanadium pentoxide is known to be a highly seltctive catalyst for oxidation and ammoxidation reactions.' Several experimental and kinetic studies have focused on product formation and distribution, as both selective and nonselective oxidation processes are known to occur on metal oxide surfaces.'-12 The desired ammoxidation reaction requires so-called nucleophilic sites and is described below. The nonselective oxidations produce, predominantly, simple carbon oxides at electrophilic oxygen sites. The nitriles produced are of intrinsic interest and are vital components in many polymeric materials. toluene + NH,
-
propylene + NH,
benzonitrile acrylonitrile
In several cases, competitive adsorption processes affect the product distribution and coadsorbate interactions con(1) Andersson, A.; Lundin, s.T. J. Catal. 1979,58, 383. (2) Mikov, S. R.; Suvorov, B. V.; Solomin, A. V. Proceedings of
the Conference on catalytic Hydrogenation and Oxidation; Izd-vo Akad. Nauk Kaz SSR Alma-Ata; 1955, p 241. (3) Solomin, A. V.; Suvorov, B. V.; Mikov, S. R. Zh. Obsheh. Khim. 1958,28,133. (4) Grasselli, R.K.; Burrington, J. D.;Brazdil, J. F. Faraday SOC. Discuss. 1982, 72, 204. (5) Grasselli, R. K.; Burrington, J. D.Adu. Catal. 1981,30, 133. (6) Burrington, J. D.; Kartisek, C. T.; Grasselli, R. K. J. Catal. 1983,81,489. (7) Burrington, J. D.; Kartisek, C. T.; Grasselli, R. K. J. Catal. -1-9-a-,. 87.363. - ., -. -. (8) Grasselli, R. K.; Burrington, J. D. Ind. Eng. Chem. Prod. Res. Deu. 1984,23, 393. (9) Gueeinov, A. B.; Mamedov, E. A,; Rizaev, R. G. React. Kinet. Catal. Lett. 1985, 27(2), 371. (IO) Niwa, M.; Murakami, Y. J. Catal. 1982, 76,9. (11) Cavalli, P.; Cavani, F.; Manenti, I.; Trifiro, F.; El-Sawi, M. Ind. Eng. Chem. Res. 1987,26, 804. ( 1 2 ) Das, A.; Kar, A. Ind. Eng. Chem. Prod. Res. Deu. 1980,19,689.
0743-7463f 90/2406-0365$02.50/0
tribute to activation of reactive species. For ammoxidation, the sequence of dosing or exposure is critical in surIf ammonia is admitted prior to the face a~tivati0n.l~ hydrocarbon, no nitrile product is formed. If the reactants are admitted simultaneously, then nitrile products are observed, if the hydrocarbon precedes the ammonia species, then nitrile is obtained. These observations are apparently due to competitive adsorption processes and activation of ammonia species adsorbed in alternate sites. A critical analysis of bonding and mechanism of oxidation is lacking in the characterization of many catalytic materials, as little theoretical work accompanies the experimental measurements. This is due to the structural and chemical complexities of the oxides. Our work focuses on the adsorption chemistry of ammonia in lowenergy sites and high-coverage alternate sites with reference to toluene adsorption, as the ammonia adsorption can poison or promote nitrile production depending on the experimental condition. Ammonia, as a simple molecule, also proves an interesting probe for surface site activity as well. The extended Huckel/ tight binding method has been employed to analyze model surfaces of V,O, For this method, the surface is modeled by a semiinfinite slab of finite thickness. For our studies, eight vanadium atoms along with the coordinated oxygen atoms comprise the surface unit cell. The model is discussed in greater detail along with the computational method in the following section. (13) Murakami, Y.; Niwa, M.; Haitori, T.; Osawa, S.; Igushi, I.; Ando,
H.J. Catal. 1977, 49, 83. (14) Silvestre, J. J.Am. Chem. SOC.1987, 109, 594. (15) Andersson, A. J. Solid State Chem. 1982,42,263.
(16) Belokopytov, Yu. V.; Kholyavento, K. M.; Gerei, S. V. J. Catal. 1979,60, 1. (17) Grasselli, R. K.; Brazdil, J. F.; Burrington, J. D. Proceedings of the 8th Int. Congr. on Catalysis; Berlin, FRG, July 24,1984, Verlag Chemie: Weinheim, 1984; pp 5, 369. (18) Otamiri, J. C.;Andersson, A. Catal. Today 1988,3,211.
0 1990 American Chemical Society
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Computational Method The methodology used here has been developed by Hoffmann and several co-w~rkers.'~This methodology has been used to evaluate solid-state structure and catalytic properties of simple metal surfaces and complex carbide and oxide surfaces. For the latter, an evaluation of charge transfer and ionicity of material is of great interest. Wang et al.,' have devised an empirical means to adjust the charge transfer to account for the ionicity in oxides and chlorides. The methodology employed here does not include this empirical correction, however, as the arguments presented are based on relative trends in energy and population. The surface model selected is an infinite two-layer slab as shown in 2 below.
Table I. Density of States Data surface oxidation state" +4 0 -4 Charge Density Distribution for the V,O, Surface atom V
1.835 7.344 6.414 6.811
Ob 0,
1.863 7.403 7.143 7.181
2.816 7.407 7.188 7.188
Overlap Populations for V-O(Nucleophilic) and V-O(Electrophi1ic) bond
v-0, v-0,
a779 0.530 0.218
0.778 0.531 0.261
0.770 0.528 0.249
" Surface oxidation state indicates how many electrons are added to a surface unit of four vanadium atoms. So -4 indicates a single-electron reduction for each vanadium. On,nucleophilic,
'
1'
0.
The parameters necessary for the atoms in the structure are given by the valence-state ionization potential for each atom. These atomic parameters are optimized by a charge iteration for particular applications as a usual part of the extended Huckel approach. The valence orbitals are of the Slater type. The parameters for the vanadium and oxygen of the surface model are given in Table I11 along with the parameters selected for nitrogen and hydrogen of the adsorbate species. This method provides typical density of states (DOS) in which local or projected DOS are obtainable. The bond strength can be evaluated by analysis of the overlap population. The overlap population takes on positive values for bonding interactions and negative values for antibonding interactions. In addition, the system may be divided into logical fragments such as the surface and adsorbate; changes due to interaction can then be evaluated with respect to the pristine surface or free adsorbate molecule by comparison of charge redistribution or orbital population analyses. Comparison of solid-state calculations with molecular orbital calculations on model transition-metal compounds has also proved useful in understanding the coordination of aromatic hydrocarbons with the surface. Results of Calculation Pristine Surface. The structure of V,O, is similar to that of MOO,. The coordination of the oxygen atoms about the central metal atom is roughly octahedral; however, the distortions in the octahedron split the degeneracies of the t, and eg states. The fundamental difference between the vanadium pentoxides and molybdenum trioxide is in the extended structure. The vanadium pentoxide structure has a center of inversion providing ~~
~
(19) Hoffmann, R. J. Chem. Phys. 1963,39, 1397. Hoffmann, R.; Lipscomb,W. M., Ibid. 1962,36,3179; 1962,37,2872. Ammeter, J. H.; Burgi, H.-B.; Thibeault, J. C.; Hoffmann, R. J. Am. Chem. SOC.1978, 100,3686. (20) Wang, Y.; Nordlander, P.; Tolk, N. J . Chem. Phys. 1988, 89, 4163.
1 : ...................................... ,._._..__....... ........ .'
-e..:
/,___...I.
.". '
-", ws
pps
Figure 1. Density of states for the V,O, surface. (a, Left) Vanadium contribution. (b, Right) Oxygen contribution.
weak V-0 bonds along the cleavage plane, and hence "naked" vanadium atoms can be produced at the surface. This may be a critical consideration for the differences in reactivity and selectivity between the molybdenum and vanadium oxide catalysts. Our calculations have provided density of states information similar to that of Si1~estre.l~ The oxygen p states fall far below the vanadium d states, and sizable charge transfer occurs between vanadium and oxygen. The Fermi level falls between oxygen and vanadium states for the theoretically neutral surface. In addition, there are two chemically different oxygen species in the lattice. The so-called nucleophilic oxygen are those coordinated to vanadium with short double bonds, also called terminal oxygens, and the electrophilic oxygen atoms are in the surface plane. Our calculations show a charge distribution consistent with this description. The terminal oxygen atoms are more negatively charged than the inplane oxygens, and the difference in charge between these species becomes exaggerated upon oxidation or reduction of the surface. In addition, the terminal oxygens are more strongly bonded to the metal. This is shown in Table I. Figure 1 shows the density of states for several atomic orbital components. These are described in terms of distortions of the typical octahedral crystal field. The dxy,d,,, and d,, comprise t,, like orbitals which point between the ligands or in this case the oxygen atoms. The dxz;g and d,2 combinations form the egorbitals, which point directly at the ligand or oxygen species. The sym-
Langmuir, Vol. 6, No. 2, 1990 367
Adsorption Preferences of NH, on V,O, Catalysts
-
z
F
W
XZ
t2g
Y2 XY
0
-
I2
XY
-
XI YZ
XY
a1 -
Q
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Figure 3. Orbital interactions responsible for bonding of ammonia to a surface.
14
Ibl
IC1
Figure 2. Reduction of orbital symmetry upon distortion of the octahedron.
metry of the t,, and e, states is reduced by the structural distortions from octahedral symmetry, as shown in Figure 2. Those orbitals with components parallel to the surface plane are those implicated in reactivity; thus the greatest change upon adsorption should occur in the d,z, d,,, and d orbitals. Such states are high in energy when comparedYio the Fermi level of the neutral surface. In addition, the structural distortion and cleavage of the surface suggest that the d, state will be affected more critically by moderate reduction. The atomic orbital population of the d, states of the surface vanadium increases sharply with moderate reduction. The dyzis also affected, but to a much smaller extent. More extreme reduction conditions lead to increased population of the dz2 and d states. Here moderate reduction is addition of one efectron per surface vanadium atom. Furthermore, analysis of the density of states shows some penetration of vanadium character into the oxygen p states and, although much charge transfer is occurring, from the vanadium d states into the p states. Reduction or prereduction of the material activates metal states by increasing the electron density at surface metal sites. This can be achieved by a variety of experimental means, but clearly increased population of the metal d states can lead to a situation in which metal to adsorbate back-bonding can occur. This type of interaction is typically referred to as the Blyholder mechanism for surface-adsorbate interaction.21 This back-bonding leads to dissociation of CO on pristine metal and carbided metal surfaces and is thus necessary in the activation of NH, and other adsorbed species. An important structural feature of the V,O, is the “twistn in the structure of the material which exposes several naked vanadium and reactive vanadium atoms upon surface preparation. For Mo0,(001), there is no twist, and only short Mo-0 bonds are present at the surface plane. The energy required to produce naked metal centers is greater, and hence the surface shows somewhat reduced activity relative to the vanadium oxide catalysts. Addition of dopants increases the activity of the molybdenum oxides. All of these considerations become important when considering simple and complex chemisorption processes. NH, Adsorption. Ammonia adsorption is a critical consideration in the ammoxidation reaction. The production of nitrile during ammoxidation depends on the extent to which ammonia is activated, which in part is (21) Blyholder, G . J. Chem. Phys. 1964,68, 2772.
dependent on several factors. The most important factors are as follows: (a) Nitrile production is inhibited if ammonia is admitted first to the reaction vessel. (b) If toluene and ammonia are admitted simultaneously, then nitrile product is observed. (c) If toluene is admitted prior to ammonia, nitrile product is observed. (d) For nonstoichiometric amounts of ammonia reactant, nitrile production consumes the available ammonia, and then benzaldehyde is obtained from oxidation processes on this surface. Clearly, these observations suggest that competitive adsorption processes are occurring and that the mechanism of ammoxidation and oxidation may occur via a single active adsorption process. The latter conclusion is drawn from d above, which suggests that in the absence of ammonia no nitrile is formed. Experimentally, NH, imido species, are formed at the surface in the nucleophilic oxygen sites. In this discussion,we will focus on the binding of ammonia to the vanadium oxide surfaces and consider stable species which block or poison the surface toward nitrile production. 1. Stable Surface States of Ammonia. Adsorption of ammonia depends on the interaction of its frontier orbitals with metal centers. When the surface is prepared, several naked vanadium metal atoms can be exposed.15 The vanadium centers are electron deficient and can receive electron density from a, lone-pair orbitals of ammonia though interaction of symmetry-related metal and ammonia orbitals. For simple, stable on-top adsorption, the HOMO a1 orbital of ammonia can interact with orbitals of s(d,z) type combinations of the naked vanadium, as shown in Figure 3. This interaction is a typical donor interaction; electron density of the a, states is donated into surface states with a large metal component. This orbital is nonbonding with respect to N-H bonds within the ammonia adsorbate, and hence no activation of the ammonia species is observed.“ This site is the lowest energy site of study and presumably the preferred site for hydrocarbon adsorption. Activation of the ammonia may occur through population of e orbitals of the ammonia. These orbitals are antibonding with respect to nitrogen-hydrogen bonding, and hence occupation of these states can lead to nitrogen-hydrogen bond activation. The symmetry-allowed combinations which can lead to N-H bond activation involve 7 type interactions between the e LUMO set of ammonia orbitals and d,, and d,,, orbitals of the vanadium. If these metal states become occupied through a reduction process, the energetics of electron transfer between vanadium and ammonia are favorable, and population of antibonding states and bond activation occur. Our surface redox studies show only slight additional population of the e states. This may be due to the fact that moderate reduction does not populate the d,, and d,,,
Otamiri et al.
368 Langmuir, Vol. 6, No. 2, 1990
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Figure 4. Orbital interactions responsible for bonding of aromatics to a metal center.
Figure 5. Adsorption geometries of secondary sites.
orbitals of the surface vanadium, those with the appropriate symmetry for interaction. Activation can also occur if bonding e states below the HOMO interact with oxygen or vanadium states of the surface. These possibilities will be discussed for secondary adsorption processes at the high-coverage limit or for processes which involve competitive adsorption. The density of states projections for fragment orbitals of ammonia shown in Figure 6i are consistent with this description. In the on-top position, the a, interaction is responsible for the bonding between surface and adsorbate. The a, orbital shows the greatest dispersion and greatest shift of median energy. This adsorption geometry should be the most stable. It represents the lowest energy site in our calculations and is consistent with the strongly bonded, nonactivated adsorbed species which poisons the ammoxidation reaction. 2. Secondary Adsorption Processes. When the adsorbate concentration is high, the ammonia species are forced into secondary adsorption sites. This situation
occurs when toluene and ammonia are admitted to a reaction vessel simultaneously or when the surface coverage is extremely high. The toluene competes with the ammonia for the naked metal sites at the surface. Generally, the bonding of an aromatic with an electron-deficientmetal is quite strong, as the 7~ system can donate electron density into orbitals of the appropriate symmetry. Typical aromatic complexes of iron and chromium involve n6coordination of a benzene ring with a metal center. The general bonding picture is shown in Figure 4. The bonding between the aromatic ring and the metal is enhanced for two main reasons, as can be seen in this figure. (a) First, there are several donor and acceptor states provided by the aromatic ring. (b) Second, the energetics favor both donation and back-donation for metal-aromatic bonding. The aromatic species compete with ammonia species for the naked metal sites at the surface. Because of the energetic and orbital advantage, toluene binds more strongly to the metal center, and the ammonia is forced into secondary sites. Under these circumstances, the ammonia species become activated; imido species is formed at the surface, and water is given off. The formation of imido species is critical for the ammoxidation." Our studies have included several secondary adsorption sites for ammonia; the proposed geometries involve those in which the ammonia protons can interact with the nucleophilic oxygen species at the surface and in which some interaction with the surface vanadium is preserved. These are shown in Figure 5. These sites represent conditions in which the N of the ammonia interacts with surface oxygen directly and a site in which the ammonia species is bent toward the nucleophilic oxygen as though displaced by a coadsorbate. Each is a realistic adsorption site, predicted by surface IR measurements, thermal desorption studies, product analysis of ammonia adsorption, ammoxidation, and simple oxidation studies.22 No direct structural analysis such as LEED has unambiguously verified these geometries. The species analyzed in our studies are consistent with intermediate formation in the proposed ammoxidation mechanism." Each of these sites shows potential for N-H bond activation as the result of several orbital interactions, which include (1)depopulation of bonding levels, (2) population of antibonding levels, and (3) rehybridization of molecular orbitals. The fragment molecular orbital method is particularly useful in these studies, as it allows one to assess the population of the e antibonding states of the ammonia species as a function of surface oxidation state and adsorption geometry. In addition, it is possible to produce local or projected density of states plots for individual molecular orbitals of the adsorbate and study their interaction with various atoms in the substrate material. In this manner, it is possible to assess which of the three effects is the most significant in the adsorption and reaction chemistry of ammonia on the vanadia catalysts. One further consideration is the surface redox state. When the d states of vanadium become occupied through surface prereduction, the potential for donation into the antibonding e states is enhanced. The secondary adsorption geometries considered show greater population of the e states facilitated through back-donation from the surface as a result of the orientation of the ammonia adsorbate. Table I1 shows the population of the molecular orbitals upon absorption as a function of the orientation and surface redox state. The N-H overlap population for is also provided for the "neutral case". Here, the N-H bond becomes activated through donation into the e antibonding states. (22) Samati, M.; Andersson, A., unpublished results.
Adsorption Preferences of NH, on V,O, Catalysts
Langmuir, Vol. 6, No. 2, 1990 369
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370 Langmuir, Vol. 6, No. 2, 1990
Otamiri et al.
Table 11. Occupation of Fragment Molecular Orbitals for Adsorbed NH, Species surface oxidation state geometry on-top
orbital
+4
a, e
1.638 0.009 0.002 1.020 0.063 0.025 1.070 0.022 0.105 1.074 0.084 0.075
2B
a1 e
30°B
a, e
on-top/30°B
a, e
0 1.718 0.009
0.002 1.674 0.072 0.045 1.848 0.072 0.175 1.848 0.153 0.125
-4
N-H"
1.876 0.010 0.003 1.676 0.072 0.046 1.848 0.072 0.176 1.848 0.154 0.126
0.671 0.460 0.414 0.405
a The overlap populations reported here are for the neutral oxidation state.
Table 111. Extended Hiickel Parameters. atom
orbital
Hii, eV
Slater exponent
v
4s 4P 3d
-6.7 -3.4 -6.7
0
2s
-27.61 -11.01 -23.95 -10.95 -11.15
1.600 1.600 4.750b 1.500' 2.275 2.275 2.280 2.280 1.300
2P
N H
2s 2P 1s
a The valence-state ionization energies or Coulomb integrals for the atoms in the structure were obtained from typical charge iteration procedures. The Slater exponents are given as well. c1 = 0.4558. c2 = 0.7516.
the projected density of states for the highest occupied molecular orbital, HOMO, and the r antibonding e LUMO pair. The dispersion apparent in the orbital projection is indicative of interaction. This so-called dispersion results from component interactions of a particular fragment molecular orbital of the ammonia with a surface metal or oxide state of appropriate symmetry. This dispersion is really a term to describe the energetic distribution of states created through interaction and is easily identified by following the integration line in the density of states plot. For a noninteracting species, the integration shows one sharp primary feature. For a strongly interacting state, the integration shows several inflections in different energy regimes. In each geometry, the a1 orbital interacts most strongly, though each of these geometries shows N-H bond activation necessary for imido species formation and each represents a potentially reactive adsorption state. The importance of the nucleophilic oxygen species at the surface is also realized when considering these higher energy states. For the low-energy sites, no interaction with the nucleophilic oxygen is observed. There is very little population of the LUMO e states and no rehybridization of the orbitals. For those geometries in which there is an interaction with the e type orbitals, there is some rehybridization of the orbit-
als and considerable donation into antibonding states. In the secondary adsorption sites, the 0-H overlap population attains a value of about 0.170 for the geometries of study and suggests the formation of imido species at the surface and desorption of water. This interaction of the hydrogen with the nucleophilic oxygen assists in population of the e orbital by pulling their energy down to a more reactive level, where they can be easily populated. In Figure 6a, the e states of the ammonia for the on-top geometry are centered at a much higher level than in Figure 6ii-iv. The imido species is isoelectronicwith oxygen, and therefore the relative bonding effects should be similar. A comparison of an NH species bound at the nucleophilic oxygen site shows several similarities with the oxide species. The charge distribution shows the same trends for the NH species as the oxygen; that is, the nitrogen is very negative. One difference, however, is that the charge density at the nitrogen is more severely affected by reduction. Our studies included consideration of both linearly bound, upright species and slightly tilted species formed from a secondary adsorption site. In each case, the charge on the imido nitrogen increased with population, and the V-N became increasingly weaker with reduction. This also may demonstrate the importance of reduction or prereduction in activation of the catalytic material.
Conclusion We have discussed the bonding of ammonia in both low-energy and representative secondary adsorption sites, as adsorption and subsequent activation of ammonia are critical components of the ammoxidation reaction. Both processes are dependent on the surface redox state, with simple adsorption and activation being favorable when the surface has been prereduced. Our calculations show that secondary adsorption may lead to imido species formation and hence provide intermediates for ammoxidation. Greater population and rehybridization occur when there is a direct interaction of the ammonia hydrogen species with the nucleophilic oxide species. Strong oxygen-hydrogen interactions are responsible for the production and desorption of water, leaving the imido species at the surface. The imido species is necessary for reaction with the toluene. I t is located adjacent to an active surface site, to which toluene may be bound. In the absence of the imido species, benzaldehyde is produced from the nucleophilic oxygen. Our study demonstrates the importance of nucleophilic oxygen, helps to identify differences in the oxygen species, and shows the resemblance of the imido species to nucleophilic oxygen in the V,O, surface. These studies suggest that surfaces exposing short M-0 bonds may be more active than other prepared metal oxide surfaces, reinforce the idea that surface reduction or prereduction and secondary adsorption processes are necessary for maximum activity, and show the importance of competitive adsorption processes to overall reactivity. Registry No. V,O,, 1314-62-1; NH,, 7664-41-7.
