Removal of oxygen from bismuth molybdate during prereduction and

Removal of oxygen from bismuth molybdate during prereduction and catalytic oxidation. Initiation and steady-state steps. Eli Ruckenstein, and Dady B...
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THE J O U R N A L OF

PH.YSICAL CHEMISTRY Registered in U.S. Potent Office

0 Copyright, 1980, by the American Chemical Societ:y

VOLUME 84, NUMBER 26

DECEMBER 25, 1980

Removal of Oxygen from Bismuth Molybdate during Prereduction and Catalytic Oxidation. Inntiation and Steady-State Steps Ell Huckenstein Facuky of Engineering and Applied Science, State University of New York at Buffalo, Buffalo, New York 14214

and Dady B. Dadyburjor” Department of Chemical and Environmental Englneering, Rensselaer Polytechnic Institute, Troy, New York 12 18 1 (Received: January 21, 1980; In Final Form: June 9, 1980)

The loss of a bismuth-bound 02-ion out of the lattice of a BizMoOscatalyst is an important step in the mechanism of selective oxidation. Here it is suggested that two kinds of such a process exist. The “initiation” process refiers to the loss of an ion that was initially in the lattice, i.e., before the start of the oxidation process. The latitiice vacancy created by this process may be filled by an ion originally outside the lattice, i.e., as gas-phase 02,The loss of such an ion is termed the “steady-state process”. The energy path method indicates that ion, dlue to a high potential barrier to its removal difficulties may arise in the initiation loss of a Bi-bound 02from a perfect lattice. The potential barrier can be significantly decreased by the loss of local electroneutrality added to the removal of certain labile Mo-bound and intermediate-layer 02-ions. The latter phenomenon occurs at prereduction, and the former is ascribed to the presence of the reactant hydrocarbon. This decrease in potential barrier allows the bismuth-bound 02ion to leave the lattice at the start of the selective oxidatiop reaction. The steady-state loss occurs more easily. The path of an 02-ion into and out of a partially vacant (i.e., imperfect) Bi2Mo06lattice is found to be qualitatively similar to the corresponding path in a perfect lattice. Further, at 110 point in this path is the energy of the 02-ion greater than at the start of the path. Hence the steady-state loss of 02-ions with a BizMoOslattice containing anion vacancies can be considered energetically favorable.

1. Introduction

In the bismuth niolybdate lattice, oxygen ions are found in three types of locations: in a bismuth layer, affiliated with molybdenum, and in a layer intermediate between the other two. It would appear that the bismuth-oxygen neighborhood in t h e bismuth molybdate lattice is particularly relevant in thie selective oxidation of hydrocarbons. For example, Haber and co-workers1 extrapolated from work on bismuth oxide to indicate that, over the bismuth molybdate, the site where the hydrocarbon is oxidized is around the hismuth ion. Haber found that, in the absence of oxygen in the chemical atmosphere, propylene oxidized over bismuth oxidte yields a significant amount of 1,5hexadiene in the product. This compound is postulated 0022-3654/80/2084-3487$0 1.OO/O

to be formed by the dimerization of the propyl species. The propyl intermediate is also found when propylene is oxidized to acrolein in the presence of oxygen over bismuth molybdate. Since the chemical intermediate is the same in both cases, it seems reasonable that over bismuth molybdate, as over bismuth oxide, the catalytic site is at the bismuth layer. The role of the molybdenum layer in the bismuth molybdate would appear to be to provide the bismuth layer with oxygen ions, replenishing its own supply from the chemical atmosphere. Other experimental work confirming this mechanism has been collected in ref 2. Dadyburjor and Ruckenstein3 used the computational technique of minimum energy paths4 to describe the 0 1980 Amerlcan Clhemlcal Society

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The Journal of Physical Chemistty, Vol. 84, No. 26, 1980

motion of 02-ions into and inside the lattice of bismuth molybdate. An idealized lattice was considered, completely filled except for the single anion vacancy to be filled by the mobile 02ion. By comparing the activation energies required for motion of an 02-ion along various paths, it was shown that the following path is energetically favorable:

-

--

02-ion outside the lattice 02-ion affiliated with Mo6+ 02ion in intermediate layer 02-ion affiliated with Bi3+ (1) The pathway described above reinforces the view that the bismuth-bound 02-oxidizes the hydrocarbon, that the anionic vacancy thus formed is replenished via the intermediate layer by a molybdenum-bound 02-moving through the lattice, and that reactant oxygen from the gas phase finally fills the molybdenum-bound anionic vacancy. We recently started to consider6 the process that logically succeeds the pathway described in eq 1, i.e., the motion of an 02-ion, affiliated with bismuth, out of (but near) the Bi2Moo6 lattice. An activation energy of removal, AE, was defined as the difference between the maximum energy in the ion path and the energy at the start of the path. For a completely filled, perfect, Bi2Mo06 lattice, it was found that the minimum energy path for such a motion requires an activation energy that is exceedingly large. On the other hand, it was found that 02ions in the intermediate layer, and those affiliated with molybdenum, require relatively small activation energies for similar paths leaving an otherwise fully occupied lattice. These results appear to indicate that it is much more difficult for a bismuth-bound 02-ion to leave the lattice (and presumably oxidize the hydrocarbon) than for a molybdenum-bound 02-ion to perform the same function. Consequently it would appear that there is a discrepancy between these results and the experimental results described earlier (and the results of ref 3 as summarized by eq l),which indicate that it is the bismuth-bound 02-ion which leaves the lattice and is responsible for the catalytic oxidation. However, note that the lattice of ref 5 was taken as a completely occupied, perfect structure. Particularly after prereduction, a real catalyst will possess a lattice with appreciable vacancy concentrations or even nonstoichiometries. Further, during the catalytic process charge imbalances (local nonelectroneutrality) may occur. All these play a significant part in the performance of the catalyst and must be considered in the evaluation of the energy paths. This is done in the present work. We consider that the prereduction step introduces vacancies in anion positions associated with the molybdenum layer and intermediate layers in the Bi2Moo6,since these 02-ions are relatively easily removed, even from a completely filled latticea6 Concurrent with the formation of anion vacancies is the corresponding loss of positive charge in surrounding cations, in order to keep the lattice electrically neutral. We calculate the activation energy of removal, AE, for the Bi-bound 02-ion from a lattice containing increasing numbers of vacancies. We find that, at the end of the prereduction step, i.e. for an electroneutral lattice containing vacancies, the tendency of the Bi-bound 02-ion to leave the lattice is not significantly altered. During the actual reaction process, however, charge transfers occur between reactants, catalyst, and products. Hence we expect that a number of cations which lost some positive charges during prereduction could return to their

Ruckenstein and Dadyburjor

normal charged state. Of course this would imply that small portions of the lattice have a net charge (the condition of local nonelectroneutrality); however the reactant-catalyst-product system would be electrically neutral. Repeating the calculations of AE under conditions now approximating those found during reaction, i.e., for a nonelectroneutral, vacancy-containing lattice, we find that the interaction energies between ion and lattice are significantly lowered. Consequently, a Bi-bound 02-ion in the “original” lattice, Le., before prereduction, would be able to leave the lattice during the actual reaction, whereas it was prevented from doing so before and after prereduction. The process whereby an “original” Bi-bound 02-ion leaves the lattice we term the initiation loss. Replacements for the ions lost in this initiation step must come eventually from oxygen in the gas phase. We define the steady-state loss as the transfer of 02-ions from the gas phase, through the lattice, to just outside the lattice. It is found that the energy path for this process requires no activation energy, when a nonelectroneutral, vacancy-containing lattice (similar to the lattice used for a favorable initiation step) is used. The energy path for the steady-state loss from the present lattice is consistent with that obtained earlier3 for a perfect lattice, viz., that motion of the external ion occurs via the molybdenum and intermediate layers to the bismuth layer vacancy, and hence to the surface of the lattice. The lattices used to model the presence of various levels of anionic vacancy and local nonelectroneutrality are described quantitatively in section 2. In sections 3-5 are shown the energy paths corresponding to the initiation step for prereduced lattices, and for prereduced lattices under reaction conditions, and the energy path for the steadystate step under reaction conditions. The levels of vacancy formation and nonelectroneutrality required for the initiation step are discussed in section 6, along with mechanisms for the prereduction process, the initiation loss, and the steady-state loss. 2. Lattices Considered

The lattices correspond to the “working lattice” described in ref 3, with the modifications described below. Briefly, the lattices contain a central unit cell, surrounded by unit cells one lattice parameter away in the fx, +y, and fz direction, as shown in Figure 1. The coordinates x , y, and z are nondimensionalized with respect to c1 (=5.5 A), c2 (=cl), and c3 (-16.1 A), respectively. Figure 2 describes the idealized unit cell. In this work, there are vacancies in intermediate-layer and molybdenum-bound positions of oxygen ions in the central unit cell of the working lattice. If the working lattice must be electrically neutral, as is expected during prereduction of the oxide catalyst, the absence of an 02ion implies that one or more ions of the two types of positively charged species (Bi3+or Mo6+)must lose a total of two electronic charge units. Corresponding to each 02vacancy in the molybdenum layer, two nearby Mo ions lose one unit of electronic charge each (e.g., from Mo6+ to Mo6+), Corresponding to each 02-vacancy in the intermediate layer, one nearby Bi ion and one nearby Mo ion each lose one electronic charge unit. (It is shown later that it is not critical which ions lose the charges.) The energy path of a Bi-bound 02-ion leaving a lattice such as that described above is treated in section 3 and is discussed in section 6. If electrons liberated by the removal of 02-ions can be used elsewhere, the Bi and Mo ions would return to their former charges of 3+ and 6+, respectively. When one notes that electron transfer occurs in the Mars-van Krevelen

The Journal of Physical Chemistry, Vol. 84, No. 26, 1980 3489

Removal of Oxygen from Bismuth Molybdate

TABLE I: Interaction Potentials Ejk between an 02-Ion (j) and Various Ions (k) as Functions of Interionic Distances ra k range of r Ajk Bjk c j k-Djk Fj k Gjk Biz+ r . : 0.28 1.33 X lo5 1.18

--

Mo5+

0.2!8 < r < 0.75 r , 0.15 r c 0.3 0.3 < r < 0.84 0.84 < r 1.85 < r r 2.06

.:

Mo4+

Mo2+

r : * 0.45

0.45 < r 1.25 < r r > 1.84 r 1.7 a

< 1.7

2.97 1.26 5.99 1.09 1.45 2.35 55.7

x 104 X lo6 x 104 x 103 x 103 X

loa

6.45 11.5 10.5 4.80 2.38 1.02 0.305

5.42 x 104 3.29 x 103 29 3 7.0 5.42 x 104 6.84 x 103 7.13 X 10'

77.0

9.30 3.10 1.18 0.40 9.40 4.22 1.74 0.44

- 52.1

- 262.4

- 57.6

6.17

- 20.6

- 249.3

-144.0

2.60

- 20.57

- 198.1

-115.2

2.60

- 20.57

-129.9

- 57.6

The constants A,k; to Gjk are used in the equation E j k ( r ) = Ajk exp[-Bjklr -- (Gjk/rZ)I]- Cjk/r6 - Djk/r4 t

2.60

Fjkh

with Ejk

in eV and r in A . z.050

z =O 30

z = 0.33

2.0.25

z=O.17

@ Mo"

2.0

Figure 1. Truncated lattice used in the computation of the interaction energy of a mobile 02-ion. The central unit cell, shaded, lies on the external surface of the' solid lattice at y = 0. Dashed arrows show the directlons that the %truncatedlattice can be extended to form the infinite lattice.

model6 of selective oxidation, such a locally nonelectroneutral lattice is likely to be formed during the catalytic process itself The charge imbalance in the working lattice can however be asslumed corrected by charges of the adsorbed and reacting hydrocarbon molecules somewhere in the appropriately infinite lattice. The difference between the central unit cell of such a working lattice after the transfer of electrons and Figure 2 is the existence of vacancies at thie appropriate Mo-bound and intermediatelayer oxygen ion positions. The energy path of a Bi-bound 02-ion leaving this lattice is treated in section 4 and is discussed in section 6. 3. Energy Paths of Bi-Bound 02-Ions Leaving the Working Lattice after Prereduction I n t e r a c t i o n P o t e n t i a l s . The calculation of energy paths in such a lattice requires the use of interaction potentials of ion pairs riot considered in ref 3. These potentials are calculated in a manner similar to that used to generate Figure 1 of ref 3. In brief, Coulombic, induced-dipole, instantaneous-dipole, and electron-shell repulsion terms

12

2.0

Flgure 2. Lower half of an Idealized unit cell of Bi2MoOB. The upper half Is a mirror image through the plane z = 0.5. Loeations of Bi3+ and Mo" ions are indicated by filled circles and shaded circles, respectively. Position!$of 02-ions are indicated by numbers 1-21, whlch are used to identify the locations in the text.

are obtained as described by us earlier.3 These terms are gathered in Table I for the ion pairs of present interest. E n e r g y Puthtr. Figure 3 illustrates energy paths of a bismuth-bound 02-ion leaving the lattice through the y = 0 face. The ion corresponds to number 11 of Figure 2. The lower curve represents the y coordinate of the ion corresponding t o the path coordinate. The upper curves 0, A, B, and C are the energy paths, describingthe energy of the 02ion asla function of its progress along the path as given by the path coordinate. Curve 0 describes the energy path of the 0'- ion leaving a full Bi2Mo06lattice and is taken from ref 5. As first noted in that work, the energy barrier fix motion of this ion is formidable under these circumstances. Curve A represents the energy path of the 02-ion when the central unit cell has no 02-ions

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The Journal of Physical Chemistty, Vol. 84, No.

26, 1980

Ruckenstein and Dadyburjor

-460~

"""

Yt

c

path coordinate

-+

-007

path coordinate --L

o,2h t \

O.O Figure 3. Energy paths of ion 1 1 leaving eiectroneutrai lattices. Curve

0 describes the energy path from the fully occupied lattice and corresponds to Figure 6.5 The independent varlabie Is an arbitrarily defined "path coordinate". The motion is in the -y direction, and the y coordinate of the moving ion is shown as a function of the path coordinate in the bottommost curve. Curves A, B, and C correspond to energy paths out of lattices which are eiectroneutrai and after different numbers of 02-ions have been removed from the central unit ceii. Curve A corresponds to the lattice when vacancies are present in three intermediate-layer 02-ion positions. As described In the text, one Bi ion and one Mo ion lose one unit of positive charge each for each such vacancy, in order to maintain electroneutrality. For energy path B, SIX intermediate-iayer 02-ion vacancies exist in the central unit cell. In addition to the vacancies of curve A at positions 6, 7,and 15,there are vacancies at positions that are mirror images of these three (1,0.5,0.17), (1,0,0.33), through z = 0.5. Bismuth ions at (0,0.5,0.17), and (1,0.5,0.83) are Bi2+; there is an Mo4+ ion (1,0,0.67), (0,0.5,0.83), and Mo6+ions exist at (O,O,O),(l,O,O), (O,O,l), and (l,O,l). at (0.5,0,0.5), A total of 12 vacancies exist in the central unit cell for curve C. In addition to the six vacancies in curve B, molybdenum-bound 0" ions numbers 2, 3, 19, and 20,and the mirror images of numbers 2 and 3 through z = 0.5 are removed. Six Bi2+ions exist, identical with those of curve B. Mo5+ions exist at (0,0.5,0.5) and (1,0.5,0.5); the ions at (O,O,O),(l,O,O), (0.5,0.5,0), (O,O,l), (l,O,l),and (0.5,0.5,1) are Mo4+; and there is a Mo2+ ion at (0.5,0,0.5).

corresponding to locations 6,7, and 15 of Figure 2. These three ions are in the intermediate layer; hence, in accordance with the electroneutrality condition, three bismuth and three molybdenum ions each lose one unit of positive charge. The three bismuth ions so altered are those at (1,0,0.33), (0,0.5,0.17),and (1,0.5,0.17);the Mo6+ ions are at (O,O,O), (l,O,O), and (0.5,0,0.5). Such a lattice was shown in a previous section to be consistent with conditions after prereduction of the catalyst. Note that the energy level at the start of curve A, given by point Ei,is somewhat less than that at the start of curve 0, given by point Eo. The assumption here is that the number-11 ion is immobile during the removal of the intermediate-layer anions (although its energy level changes from Eo to Ei) and attemps its passage to the surface of the lattice only after the removal of the three labile 02ions is complete. The removal of the labile 02-ions is of course not instantaneous and probably occurs concurrent with small changes in the position of the number-11 ion. However, the treatment of only one mobile ion at a time is a computationally convenient device and is not likely

Yf

Flgure 4. Curves A, B, and C represent energy paths of BI-bound 02ion number 1 1 leaving lattices containing 3,6, and 12 02-vacancles in the central unit ceii, respectlvely. The 02-ion positions that are vacant are the same as those in the corresponding curves of Figure 3. The initial portion of the curves (to E, E, E), represent the decreasing energy levels of the number-1 1 ion in the vacancy-containing lattlce as bismuth and molybdenum ions return to Bi3+ and he+ and the lattice turns locally nonelectroneutral at the start of the reaction.

to result in a significant error in the path. Curves 0 and A and the corresponding energy barriers to the motion of the number-11 ion under these conditions are discussed in section 6. To consider more extreme cases of prereduction, first ions are removed from the central unit 6 and then 12 02cell of the working lattice. The 02-ions removed are all either in the intermediate layer or bound to molybdenum ions; according to ref 5 such ions are relatively easily removed even from a full lattice. As before, bismuth and molybdenum ions lose units of positive charge to maintain electroneutrality. The corresponding energy paths are shown in Figure 3, curves B and C. The initial energy level in these curves is not identically Ei, the corresponding value for curve A, but is very nearly this value, Curves B and C are also discussed in section 6. 4. Energy Paths of Bi-Bound 02-Ions Leaving the Lattice at the Start of Reaction Conditions In this section are presented energy paths for 02-ion number 11, Bi-bound, leaving the lattice with 02-ions previously removed from the central unit cell. Three, six, and twelve 02-ions are successively removed, and those removed are identical with those of section 3. However, the lattice is not electrically neutral; during the process all the bismuth ions return to the Bi3+form, and all the molybdenum ions to Mas+. Except for the 02-ions removed, therefore, the central m i t cell is identical with that of Figure 2. It was shown in section 2 that such a working lattice is consistent with conditions when the reactants are first introduced, i.e., at the start of the reaction. The energy paths can be seen in Figure 4. As in the previous section, curves A, B, and C indicate that 3,6, and 12 02-ions are missing from the central unit cell of the working lattice when the energy path of 02-ion number 11is computed leaving the lattice. The position of the ion in the path, as given by the y coordinate, is shown in the

Removal of Olxygen firom Bismuth Molybdate

poth coordinote -+

02

Figure 5. Energy paths and coordinates for an external 0’-ion entering the lattice in the +ydirection and then moving inside the lattice In the -z direction toward the bismuth-bound site. The central unit cell is noneiectroneiitrai and corresponds to case C in Figure 4. In addition, there is a vacancy corresponding to 0’-ion 1 1, at the bismuth-bound site.

lower figure. As in Figure 3 the computations are carried out by assuming no change in position of the number-11 ion until the electrons have been removed from the bismuth and molybdenum ions, Le., until the condition of local nonelectroneutrality has been met (and after the ions have been removed intermediate- and Mo-layer 02as in section 3). Accordingly, the first portion of curve A is a decrease in the energy level of the mobile ion from Ei to EA,the mobile ion being stationary in the number-11 position while electrons are being removed from the bismuth and molybdate ions. After the transfer of electrons is complete, the remainder of curve A characterizes the energy path of the motion of the number-11 ion toward the surface. Similar observaitions can be made for curves B and C. The differences between Figures 3 and 4 are discussed in section 6. 5. Energy P a t h s of External 02-Ions Entering a

Bi-Bound Site during t h e Reaction Process Figure 5 describes the energy path (above) and corresponding coordinates of the trajectory (below) for an 02ion originally outside the lattice (extreme left of the figure), entering the lattice, and moving inside it toward a Bibound anion vacancy corresponding to number ll (extreme right of figure). The lattice is equivalent to that of the previous section, i.e., is nonelectroneutral with 12 02vacancies, and contruns in addition the 02-vacancy at position 11. Figure 5 is of interest when discussing the steady-state ions and is considered in the replenishment and loss of 02following section. 6. Discussion Processes Considered. Two types of processes are considered in this work, each characteristic to a particular type of 02-ion. The ions are differentiated not by position in the lattice, RS both (areat position 11at some point in their Characteristic process; rather the ions have different histories prior to being at position 11. The first type of 02ion is one which is an “original” member of the lattice of Bi2Mo06,Le., one present in the lattice prior to prereduction and the subsequent reaction. The characteristic process considered for this ion is its removal to the surface of the lattice. This initiates the catalytic oxidation reaction, and hence is termed the “initiation” step. We relate

The Journal of Physlcal Chemistry, Vol. 84, No. 26, 1980 3491

below how changing the number of vacancies and the ionic charges in the lattice alters the energy path of the initiation step, changes the energy barrier of the process, and hence improves the ease with which the process can occur. The vacancy left when the “original” number-11 ion leaves the lattice is filled by the second type of 02-ion considered here. This ion arrives at this position from being initially in the gas phase. The process considered for this ion is its insertion into the lattice near the molybdenum ion, its movement through the lattice to near the bismuth ion, and its removal to the surface of the lattice. This process, occurring after the initiation step, is termed the “steady-state” step. The steady-state step requires the presence of an adsorbed hydrocarbon and is not consideredl in detail. Initiation Step. First consider Figure 3, where curve 0 represents the energy path of the Bi-bound number-11 02ion moving toward the surface of the full working lattice, i.e., one with all other 02-ions present, and curves A, B, and C represent energy paths when the electroneutral lattice contains 3, 6, and 12 02-vacancies, respectively. Then the eneqgy barrier to the motion of the number-11 ion toward the surface is the difference between the maximum energy level in the path and the initial energy level. For the case of curve 0, the initial energy level is E,,, the interaction energy of the ion with the full lattice. It is reasonable to suppose that the motion of the labile 02-ions giving rise to the vacancies is slow. Hence for curve A, with three 02-ion vacancies, the initial energy level will be Ei, the interaction energy of the ion in the number-11 posiition with the rest of the vacancy-containing, electroneutral lattice. Consequently, the energy barrier for motion of this ion to the surface of the lattice is given by AEAin Figure 3. Note that AEA is not significantly different from the energy barrier for curve 0, AEo. For curves B and C, the initial energy is very nearly Ei, and the maximum energy level is only slightly different from that for curve A. Since the energy barriers for curves A, B, and C are not significantly different from that for curve 0 (approximately 80 eV), it may reasonably be concluded that the formation of anion vacancies while the lattice is kept electricaQy neutral does little to facilitate the removal of bismuth-bound 02-ions in the initiation step. Hence at the end of the prereduction process, the Bi-bound 02ion is still unable to leave the lattice. The reformulating of bismuth and molybdenum ions as Bi3+and Mo6+yields the energy paths shown as curves A, B, and C in Figure 4 for the electron-transfer process and the motion of the number-11 ion toward the surface of the lattice. As in Figure 3, an energy barrier for each of these curves, corresponding to nonelectroneutral lattices with increasing anion vacancies, can be given by the difference between the maximum energy level of the path and the energy of the ion at the start of the path. For curve A the latter energy level will be Ei,the energy of interaction of the number-11 ion with the vacancy-containing, electroneutral lattice. The transfer of electrons out of the working lattice probablly occurs much faster than the loss of 02ions to form thle anion vacancies for Figure 3. Hence the drop in potential energy of the mobile ion from Ei to EA during the first part of the energy path is not dissipated, but instead helps the ion to attempt to move over the energy barrier in ita path to the surface of the lattice. For curve A, the rnaximum energy is approximately 30 eV greater than E’k Though this still represents a difficult requirement, the drop in AE from 80 to 30 eV is significant. For curve R (6 vacancies) the value of AE drops to -8 eV, and for the extreme case of 1 2 vacancies in curve C, the

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The Journal of Physical Chemistty, Vol. 84, No. 26, 1980

maximum energy level of the path is below the initial point (approximately Ei). Hence the mobile 02-ion should easily overcome the rise in the portion of the curve between Ec and the maximum energy level and move to the surface of the lattice. Clearly the condition of local nonelectroneutrality is required, along with the presence of anion vacancies, if the initiation step is to be favorable. In terms of interaction potentials such as those of Table I, it is not difficult to understand the small difference between curve 0 (full lattice) and curves A, B, and C in Figure 3 (electroneutral lattice with vacancies) or the large differences between curve 0 and curves A, B, and C in Figure 4 (nonelectroneutral lattice with vacancies). Consider Figure 4 first. The difference in the interaction energy of 02-ion 11 with the two types of lattices arises because of the absence of 02--02pair potentials corresponding to the 3, 6, or 12 anion vacancies. These pair potentials are mainly Coulombic repulsion terms, although there are small electron-shell (repulsion) and polarization (mainly attraction) contributions even at the relatively large interionic distances involved between ion 11and 02ions to be absent. The absence of 3, 6, or 12 repulsion terms makes the lattice interaction energy of the mobile ion more negative. Consequently, the value at the peak of the energy path curve may approach and even be less (more negative) than the energy of the mobile ion at the start of the path. Now consider the curves of Figure 3. As in Figure 4,the lattice is missing several 02-ions and so the total interaction energy of the mobile 02-ion at any position in the lattice is decreased. However, in order to maintain electroneutrality, some ions in the working lattice are less positively charged than in Figure 4 or in the full lattice. For each such ion, the pair potential with the mobile 02ion now carries a weaker (less negative) Coulombic attraction term. For example, as a bismuth ion changes from Bi3+to Bi2+,the product of the charges with respect to 02changes from 6- to 4-. There are additionally small changes in polarization and electron-shell repulsion. The net effect is an increase in the ion-pair potential. The number of positive charges lost (electrons gained) is equal to the number of negative charges disappearing because of the vacancy formation. Consequently, in the interaction energy between the mobile ion and electroneutral working lattice with vacancies, the algebraic sum of all the Coulombic terms is approximately the same as that in the original, filled, working lattice. The sum is not exactly the same, of course, because the charge compensations occur a t different interionic distances from the mobile ion. The small net difference in these two Coulombic terms, together with the changes in the smaller contributors to the interionic potentials-electron-shell, polarization-account for the relatively minor improvement in the ease with which the bismuth-bound 02-ion can be removed from the lattice when anion vacancies are coupled with the electroneutrality condition. Further, note that the changes in the Coulombic terms of anion-anion and anion-cation potentials will cancel to about the same level of approximation, more or less regardless of their position with respect to the mobile 02ion, so long as the cations that lose the positive charges are reasonably close to the location of the anion that becomes a vacancy and consequently "loses" negative charges. Hence, curves A, B, and C in Figure 3 are almost independent of which 02-ions become vacancies and which Bi and/or Mo ions lose how many positive charges, so long as the formation of the vacancies and the loss of charges

Ruckenstein and Dadyburjor

(b)

Figure 6. Slmpiifled representatlon of one possible process during the prereduction of Bi,MoO,. (a) The view of the catalyst is facing the top surface. Subscrlpts I, b, and m denote 02-ions located in an intermediate layer, and bound to bismuth and molybdenum ions, in that order. (b) After prereduction. One anion vacancy (0)is shown, for an Om2-ion. 0;-Ions can also leave the lattice (not shown) but 0;ions cannot leave. Note that electroneutrality Is maintained.

occur at adjacent ionic positions. Mechanism of Prereduction t o Vacuum. Recall now that the electroneutral lattice with vacancies can be expected at the end of a prereduction state and that the electroneutrality condition may not be satisfied in portions of the lattice during the actual oxidation process. (The overall system would, of course, be electroneutral because of the presence of adsorbed intermediates.) Consequently, we can hypothesize the following processes occurring during prereduction and at the start of the oxidation process, leading to the initiation step. Prereduction occurs either into vacuum or in the presence of hydrogen and serves to increase the number of anion vacancies present in the intermediate and molybdenum layers. In the former case, 02-ions from these layers can be removed relatively easily, even from the ideal, fully occupied lattice. The anions leave as O2 and the electrons migrate to cations, reducing their positive charge. Prereduction into vacuum is illustrated in Figure 6. If one recalls the large energy barriers to the motion of ion 11 in Figure 3, the Bi-bound 02-ion is not expected to leave the electroneutral lattice at the end of the prereduction step. Prereduction by hydrogen represents of course a steady-state chemical reaction between the hydrogen and the bismuth molybdate to form water. A discussion of this process is best postponed until after consideration of the steady-state replenishment and loss process below. We discuss first the start of the oxidation process, culminating ion. in the initiation loss of the Bi-bound 02Mechanism of the Initiation Step. When the hydrocarbon and oxygen are introduced in the gas phase, the oxidation process may be considered to start. Now electrons leave the cations of lower charge, Biz+ and MoS+, possibly to help convert gas-phase O2 to lattice 02-ions elsewhere in the lattice. In this manner Bi3+and Mo6+ions are formed once again near the anionic vacancies (Figure 7a,b). The original bismuth-bound 02-ion is able to leave the (nonelectroneutral, vacancy-containing) lattice easily, helped perhaps by the presence of a hydrocarbon near this

The Journal of Physical Chemistry, Vol. 84, No. 26, 1980 3493

Removal of Oxygen from Bismuth Molybdate

0;: 6,”

Bi13

0;

0;‘

Mo”

0;

Blt3

1

Figure 7. One posslblet scheme for the oxldation process. Symbols are as in Figure 6. (a) ,4fter prereduction, a number of anion vacancies are present corresponding to 0:- and 0,’-Ions. Thls figure Is slmilar to Flgure 6b except that an addltional vacancy Is shown. Note also, above the catalyst surface, the presence of gas-phase oxygen, denoted by the ciashed lines. (b) In the presence of gas-phase oxygen, electrons leave cations, perhaps to enable the oxygen to fill excess anion vacanck~.cations are now Mae+. The presence of a hydrocarbon above, the catalyst surface is shown schematically by dasheddotted lines. (c) An 02-Ion Is removed from the lattice, leaving 0,behind. Its electrons move to a molybdenum site, above which gas-phase oxygen is present. (d) Oxygen fills the molybdenum-bound vacancy, and 02-mlgrates to the vacant bismuth-bound position. (e) Om2- migrates to fill 0,.Parts a-d can now be repeated.

site. The electrons from the leaving 02-ion can be transferred, via a Mars-van Krevelen mechanism: to a molybdenum site. Mechanism of Prereduction with Hydrogen. Prereduction with hydrogen is in fact a chemical reaction of the hydrogen with bismuth molybdate. Since hydrogen is expected to be adsorbed on the surface, the mechanism of this prereducrtion probably bears resemblances to Figure 7 as well as 6. I[n this case local nonelectroneutrality, due to the presence of adsorbed forms of hydrogen on the surface, and the presence of naturally occurring anion vacancies in the lattice allow bismuth-bound 02-ions to be removed. Tlhe resulting anion vacancies can be quickly filled with intermediate-layer or molybdenum-bound 0’ions (see ref 3), resulting in vacancies in those positions. In the absence of gas-phase oxygen, the latter vacancies and, as in the are not filled. The anions now leave as H20, case of vacuum prereduction, electrons migrate to cations, reducing their positive charge. Whether in vacuum or by hydrogen, therefore, the prereduced lattice contains anion vacancies in the molybdenum and intermediate layers, and remains electroneutral by adjusting the charge of nearby cations. Further, the initial energy of a bismuth-bound 02-ion after hydrogen prereduction, as after vacuum prereduction, is given by Ei of Figures 3 and 4. Finally, none of the above should be taken to imply that the oxidation process cannot occur in the absence of prereduction of the catalyst. Dissociative adsorption of the hydrocarbon itself leads to the presence of adsorbed hydrogen on the surface, which may have the same effect as prereduction with hydrogen. Further, it is inconceivable that the actual lattice of the bismuth molybdate is perfect, so that 0” vacancies are to be expected even before the reduction process. However, prereduction can reasonably be expected to increase the number of vacancies and thus to accelerate the initiation of the selective oxidation process. This hypothesis is consistent with the recent experimental reports of Yong et a1.7 They found that prereducing the catalyst increases the density of anion vacancies from w1018 to more than 1019per gram of catalyst. Steady-State Loss. Following the loss of the “original” Bi-bound 02-ilon from the vacancy-containinglattice to the outside in the initiation step, its place is taken by another 0’-ion, which in its turn moves to the outside, and so on. When a steady state is reached, the replacement 0’- ion must have originally come from the gas phase. Consequently its initial energy level is not Ei, corresponding to that of the 02-ion inside the electroneutral vacancy-containing lattice, as is that for the “original” Oz ion. Instead the initial energy level is that of an 0’-ion outside the nonelectroneutral lattice containing vacancies, and in the presence of a hydrocarbon, perhaps adsorbed on the surface. Similarly the energy path of the leaving 02-ion must also take the hydrocarbon into account. Then, as in the case of the initiation loss, the ease of the steady-state loss will depend upon the difference between the initial energy of the 0’ion outside the lattice and the maximum energy level on the energy path of the moving ion. Taking into account the presence of a hydrocarbon adsorbed on the surface of the working lattice in computing the energy path of the moving ion would introduce a significantly higher level of complexity to the problem. Instead we consider the working lattice in the absence of the hydrocarbon, and the energy path of interest is one where the 0’-ion enters the lattice from outside the surface, moves inside the lattice, and then leaves the lattice. As

3494

The Journal of Physical Chemistry, Vol. 84, No. 26, 1980

mentioned earlier, for the sake of definiteness the working lattice is taken to be one with 12 anion vacancies plus a vacancy in position 11 (and is locally nonelectroneutral). Since essentially the same lattice is used to obtain curve c of Figure 4, the last Part of the energy Path of Present interest, where the mobile ion leaves the lattice, is given by the Curve of Figure 4 c after Point Ec. The first two Parts of the energy Path of interest, where the mobile ion enters the lattice and moves inside it, are shown in Figure 5. The steady-state process is relatively quick. Hence the potential energy Of the mobile ion arriving near position 11 from the outside (right-hand side of Figure 5 ) is not dissipated before the ion moves to the surface (Figure 4C after Ec). Consequently, the energy level at the Start of the path must be compared with the maximum energy level in the paths Of and Of Figure 4cto determine the energy barrier for the steady-state loss of 02-.For the start of the path as given in Figure 59 there is no energy barrier to the motion of the external ion to the lattice, inside the lattice, and out again. Note that in Figure 5 the path Of the 0'- ion is started at Y = -l, i.e*,5*5 A outside the surface of the lattice, for the sake of definiteness. The actual y coordinate for the start of the path depend upon the position and nature Of the hydrocarbon and the energetic characteristics of the 0 2 molecule and the 0'- ions. In fact, it can be seen from Figure 5 that the initial energy would have to correspond to Y 4 - 5 7 i.e-,closer than 2*8A outside the surface, for any energy barrier to be present for the steady-state loss of 02-.Considering that the van der Waals diameter of an oxygen atom is of that magnitude, it is reasonable to suppose that the formation of the ion would not occur at a distance from the surface that was significantly less than this value. Hence it is likely that there is no barrier to the from a nonelectroneutral, vacansteady-state loss of 02cy-containing lattice. in this Of the hydrocarbon is not The analysis* However note that it is probably adsorbed near the bismuth site and consequently has little effect on the energy path of the mobile ion passing through the molybdenum or intermediate positions. From Figure 5 , the energy path of the mobile ion near the bismuth site is at the start Of the path)' The its highest peak presence Of the nearby hydrocarbon may Serve to decrease the energy level in this region. This would make even more favorable the motion of the 02-ion through the lattice. Recall that for the full lattice, Le., one with no vacancies, it has been shown3that energetically favorable paths exist for (a) an external 0 2 - ion to a Mo-bound site, (b) a M ~ bound 0 2 - ion to an intermediate-layer site, and (c) an intermediate-layer 0 2 - ion to a Bi-bound site, examination of Figure 5 shows differences in the paths from those described above and in ref 3 for the full lattice. In the present case, the 02-ion enters the lattice between the molybdenum-bound site and the intermediate site and travels to between the intermediate site and the hismuth-bound site, Considering the increased numbers of vacancies in the present lattice, as compared to the full lattice, such modifications in the paths can hardly be unexpected. 7. Conclusion

The initiation step considered here refers to the loss, at ion the start of catalytic oxidation, of a bismuth-bound 02that was actually present in the lattice before prereduction and catalytic oxidation, The steady-state step refers to the loss of an 02-ion not bound to bismuth originally (i.e., before prereduction) but one that moves to that location

Ruckenstein and Dadyburjor

from the gas phase, indirectly via a molybdenum-bound site, during the catalytic oxidation itself. The initiation loss of bismuth-bound 02-ions is contingent on three factors. The first is the presence of a sufficient quantity of intermediate-layer and molybdenum-bound anion vacancies, Figure 4 demonstrates the relative ease of loss of the ion when increasing numbers of vacancies are present in the lattice. It is worth noting that the incremental ease of removal of the bismuth-bound 02-ion decreases with an increased number of vacancies. Consequently, there must be an optimum number of vacancies beyond which it is not worthwhile to proceed, The second factor is the (local) nonelectroneutrality of the lattice, The anion vacancies must be surrounded by statebismuth and molybdenum ions in their "normg' ~ i 3 and + ~ ~ 6 comparing + , ~i~~ 4 with ~i~~ 3, one sees that the loss of electrons from cations returning to their statedecreases the potential of the mobile ion and the magnitude of the potential barrier, thus facilitating the approach of the ion to the lattice surface. The final factor influencing the initiation loss process involves the presence of the hydrocarbon near the bismuth site. The exact role of the hydrocarbon is not clear from this analysis. However, it must be significant, because Figure 4 indicates that Over six vacancies are required to be present if the initial bismuth-bound 02- ion is to leave the unit cell in the absence of the hydrocarbon. This is unrealistic. The number can probably be reduced if the presence of the hydrocarbon can increase the of removal of the bismuth-bound 0 2 - ion to the external surface of the lattice and to the hydrocarbon. The role of prereduction is tied to the first two of these requirements. The end result of prereduction, whether under vacuum or with the use of hydrogen, is to increase the number of intermediate-layer and molybdenum-bound 02ion vacancies in the lattice, while keeping it electrically neutral. Under vacuum, the motion of the corresponding ions to the external surface of the lattice is found to be relatively easily accomplished, The presence of hydrogen creates these vacancies in a more indirect manner, Reaction of hydrogen with the bismuth-bound 0'- ions creates vacancies that are filled by anions moving from the intermediate layer and the molybdenum layer of the bismuth molybdate. At the end of the prereduction stage, the lattice is electrically neutral by adjustment of cationic charges, and bismuth-bound 02-ions cannot be easily removed from the lattice. The local electroneutrality condition is lifted during the actual oxidation process, and these bismuth-bound 0" ions can now leave, thus initiating the oxidation of the hydrocarbon. This initiation loss of an 02-ion from the lattice results in a vacancy that is filled by an 02-ion originally from the gas phase. In its turn, this 0'- ion leaves the lattice too, during the steady-state step* As in the case of the initiation process, the role of the hydrocarbon is also significant in the steady-state process. Here the 0'- ion, initially outside the lattice, enters at the molybdenum layer, moves to the bismuth layer, and leaves from there to oxidize the hydrocarbon. However, Figure 5 indicates that, even in the absence of the hydrocarbon, the energy required for such an ion to enter the (vacancy-containing, nonelectroneutral) lattice, move inside it, and leave is never greater than the energy level of the ion outside the lattice. The presence of the hydrocarbon, adsorbed near a position corresponding to the highest peak of the energy path of Figure 5, probably decreases the energy level at that point and hence decreases the energy barrier even further. Consequently this process and the

J. Phys. Chem. 1980,84, 3495-3503

349s

(3) Dadyburjor, D. B.; Ruckenstein, E. J . Phys. Chem. 1978,82,1563. (4) Ruckenstein, E.; Dadyburjor, D. B. AIChE J. 1978, 22,785. (5) Dadyburjor, D. B.; Ruckenstein, E. J . Cafal. 1980, 63, 383. (6) Mars, P.; van Krevelen, D. W. Chem. Eng. Sci. Suppl. 1954,3 , 41. (7)Yong, L. F.; Hlowe, R. F.; Keulks, G. W.; Hall, W. K. J. Catal. 1978,

steady-state replenishment and loss are favorable. - - .. Heferences and Notes - -

(1)Haber, J.; Grzybowska, G.J. Cafal. 1973,28,489. Haber, J. Inf. Chem. €ng. 197!5,75, 21. (21 Dadvburior. D. B.: Jewur, S. S.: Ruckenstein, E. Catal. Rev.-Sci.

Temperature Dependence of the Reaction O('P)

+ OH(*r.[)

-

O2

+H

R. Si. Lewis7 and R. T. Watson" Jet F'ropulslonLaboratory, California Institute of Technology, Pasadena, Californla 9 1 103 (Received: Februaw 8, 1980; In Flnal Form: September 19, 1980)

The low-pressuredischarge flow-resonance fluorescence technique has been utilized to study the kinetic behavior of ground-state atomic oxygen with hydroxyl radicals; pseudo-first-orderconditions were used, [O],> [OH],, in order to minimize complications caused by secondary kinetic processes. The reaction and the temperature dependence of the rate constant, expressed in units of cm3molecule-l s-l, can be written in the following way: 0 + OH H 0,; AHo298 = -16.8 kcal mol-'; kl = (2.01 f 0.18) X lo-', exp((ll2 + 29)/T), 221-499 K; or k , =: (2.37;:;) X T(0.362*0.072), 221-499 K. The experimental technique used affords the possibility of a small (6-10%) error being present in the measurements. This possibility is discussed in detail. The results are compared with previous measurements of kl and k-, (using the thermodynamic equilibrium constant), and the role of this reaction in atmospheric chemistry is discussed.

-

+

Introduction The reactions of HO are of great importance in atmospheric chemdstry, and many of these reactions have been the subject of intense study. The HO radical plays a major role in the oxidation of SOz,NO2,CO, HzS,hydrocarbons, halogenated alkanes, and alkanes in the troposphere and low stratosphere. The title reaction plays a major role in the chemistry of the upper stratosphere and mesosphere where 0 atoms become the dominant form of odd 0xygen.l In this region, the 0 OH reaction partially controls the OH/H02 ratio and in addition to the 0 + HOz reaction provides a sink for 0(3P),thus partially controlling the odd oxygen concentration. In addition, the reaction is a major source of H atoms in the mesosphere. There have been several absolute but somewhat indirect determinations of the O(3P)+ OH rate constant, kl,over a narrow temperature range, 228-340 K, using the lowpressure discharge flow technique.z-s Although the values obtained from these studies for k1(298 K) cover a wide range, 0.5 X 10-'l-5.0 X lo-', cm3molecule -l s-l, the three lowest value^^^^^^ can be rejected on the basis of subsequent studies by the same author^,^'^ resulting in values of kl ranging from 3 x LO-11 to 5 x cm3 molecule-l s-l. There has also been one recent direct kinetic study using pseudo-first-order conditions with a combination discharge flow-flash plhotolysis-resonance fluorescence system at 298 KV9 A value of 3.13 X cm3 molecule-l s-l was determined for kl from this study which is in excellent agreement with the range of evaluated values1@12for k1(298 K) of 3.8 X 1O-ll-4.2 >< lo-', cm3molecule-' s-l based on the more indirect studiles. On the basis of the studies of Clyne3 and Westenberg et d.,8where the reaction was studied over rather limited templerature ranges, it has been concluded in all recent evaluaiti~ons~~~~ that the rate constant exhibits no temperature dependence. In addition to the absolute

+

NASA-NEE Resident Research Associate, 1976-1978.

rate-constant studies, there has been one study where the O(3P) OH rate constant was determined relative to the CO OH rate constant at 425 K.13 The result appears to be consistent with the absolute rate-constant determinations. In addition to the studies of the O(3P) OH reaction, there have been numerous kinetic studies of the H O2 OH -k 0 reaction (reaction -1) between 700 and 2500 K,14the data of which have been the subject of several reviews and e v i z l ~ a t i o n s . ~ Unfortunately, ~J~ combining the experimental values of kl and k-, to determine the experimental equilibrium constant differs from the equilibrium constant determined from thermodynamic data by a factor of f-2. Consequently there is a need for an additional direct kinetic study of the 0 + OH reaction which will hopefully minimize the uncertainty in the absolute value of kl over a range of temperature so that it can be used in atmospheric-modeling calculations, and so the inconsistency in the values of kl,kl, and Keq(thermodynamic) can be explained. In the present study, we have utilized the discharge flow-resonance fluorescence technique to measure directly the absolute rate and temperature dependence of reaction 1 over the temperature range 211-499 K. This study 0 + OH O2 H (1)

+

+

+

+

-

+

+

represents the first direct study of this reaction over a wide range of temperature. Experimental Section The discharge flow-resonance fluorescence system used in this study has been described previ0us1y.l~ Therefore, only essential details will be given. A schematic of the system is shown in Figure 1. The flow tubes used in this study were constructed of quartz, mounted vertically and continuously evacuated by a 50-L s-l rotary pump (Welch 1396) through two traps cooled to 77 K. The flow tubes consisted of three regions: (i) a resonance fluorescence 0 1980 American Chemical Society