JOUF~ of Mohxdar Catalysts, 62 (1990) 199-213 199

THEORETICAL STUDY OF HYDROGEN ACTIVATION IN A HEMATJTE CLUSTER

L J RODRfGUEZ

Departamento o!e Quimlca, Facultad Experzmental & Ctencurs, Unrversdad de1 Z&z, Maraca& (Venezuela)

F RuE’IlX

Centro de Quimrca, Znstrtuto Venezoho de Znvestzgacwnes Czent@cas, MC, Apartado 21827, Caracas 102a4 (Venezuela)

and M ROSA-BRUSSIN

EscueLa de Quimzca, Facultad de Crencras, Unwerszdad Centml de Venezuela, Apartado 47102,

Caracas Nenezuela)

(Recenwd September 22,1989, revwed February 28,199O)

Summary

The MINDO/SR method has been employed m order to study adsorp- tion, dissociation and activation of hydrogen on a hydrodemetallation catalyst (hematite) A sunple cluster model for hematite (FezO,) was utilized, and its calculated properties were compared with theoretical and experimental results The effect of electronic density on the iron adsorption site was examined by changmg the cluster charge ([Fez031q, Q = 0, -1). The results reveal that Hz is chenusorbed on an iron atom m a non-dissociative way, with a charge transfer from hydrogen to hematite Hz interaction with oxygen atoms leads to deoxygenation and reduction of the hematite cluster A negative charge on the cluster decreases Hz adsorption energy and the Fe--O bonds A noticeable decrease m H-H diatomic energy with respect to the free Hz suggests that molecular hydrogen is activated on an iron atom site

Introduction

Hydrogen activation is a fundamental step m a series of mdustrml processes that involve crude oil hydrotreatment, such as hydrodesulphunza- tion, hydrocrackmg, hydrodemtrogenation, hydrodemetalhzation, etc The hydrodemetallization (HDM) process is an efficient way to extract metals from heavy oil. The most common metals found 111 petroleum are nickel and vanadmm, and these are present principally m porphyruuc structures In the HDM process, the metalloporphyrms adsorbed by the catalyst surface undergo a series of sequential reactions 111 The first step, which mvolves hydrogenation of peripheral double bonds, is followed by the final hydroge- nolysis step m which the rmg is fragmented and the metal is removed In both cases, an activated hydrogen molecule coordinated to the surface is

200

necessary, with very high reactivity, capable of hydrogenating the porphyn- me structure The term uckuat~~ here means the ‘softenmg’ of the H-H bond m an adsorbed hydrogen molecule as result of the filhng a, antibondmg or voidmg up bondmg hydrogen molecular orbitals

Recently, experimental stu&es by Rosa-Brussm et al 121 have shown that the natural clays have very high hydrodemetalhzation (HDM) activity, m winch the iron contamed m these clays plays an important role A Mossbauer analysis of the final phase of these catalysts mdmates that about 55% of the iron atoms are present under hematite configuration 133, and another work confirms that iron oxides demetalhze crude oil 141

As far as we know, this IS the first theoretical work concernmg H2 adsorption over Fe203 clusters, and represents the first step m a series of studies m progress concerning hematite mterackon with hydrogen and hydrocarbons, such as porphyrms, where hydrogenation and hydrogenolysis of these molecules are studied in the presence of activated Hz Moreover, it also constitutes a part of several works [51 on the bond activation of small molecules (Hz, N2 and CH,) on iron, m order to understand the electronic distribution effect on an iron site m the activation, adsorption and &ssocia- tion of H-H, N-N and C-H bonds.

The present investigation uses the MINDO/SR 161 procedure to study the H2 adsorption and dissociation over a hematite cluster model, with different iron oxidation states. This method has been successfully applied to study the H2 adsorption and dissociation on difIerent metallic clusters [73. Oxidation states over Fe atoms were simulated by changmg the total charge of the cluster. Several H2 adsorption pathways were explored over Fe and 0 atoms Hz adsorption and dissociation potential energy curves were elabo- rated for the neutral and charged clusters, using the most adequate mteraction pathway with an iron atom.

In the next section, we briefly describe the methodology employed here A comparative analysis, with respect to previous calculations of Fe-O diatomc molecule and hematite cluster, is presented below Finally, the results and discussion of the hydrogen mteraction with different adsorption sites are given m the last part of this work, emphasizing the activation of the hydrogen molecule

Calculation procedure

Calculations were done usmg a semi-empirical self-consistent-field method referred to as MINDO/SR 161, which represents a modification of the MIND0/3 method [S] to mclude transition metals The computer program used here is based on QCPE-290 algorithm, described by Rmaldi [9], which contams automatic geometry optimization usmg analytically calculated gradients The Rmaldi version was modified to embody symmetry, selective orbital filling and transference of HF-matrix fragments from previous calculations The latter is useful in mamtaming the same occupa-

201

TABLE 1

Bondmg parametera

Dlatormc

P-

Fe-Fe” Fe--O”

FeHb o-o” C&H’ H-H’

Molecular parameters

Cr

0 44317 175703

1257 0 82604 0 47890

148945

B

0 47027 106944

0 41000 0 64466 0 41776 0 24477

Bond lengthd

(A)

2 02 1626 1500 120741 0 9706 0 74116

Bmdmg energy (kcal mol-‘)’

-24 O(-24 2) -90 3(-90 3) -82 0 -118 O(-117 8) -94 9(-1010) -103 2(-104 0)

* Tb work

b [12al

= [81 d [lOI * Data m parentheses are experunental values from [lOI

tional symmetry, or molecular state, throughout a calculation pathway, and also in accelerating electronic convergency

The Slater-Condon parameters and the Slater exponent orbit& for Fe, 0 and H, used m this work, have been taken from the literature [ 8,11,12a] The bondmg parameters for the correspondmg diatonuc pairs Fe--Fe, Fe-O and O-O were adJusted after a search for the different electromc cotigura- tions of the diatomic molecules The results showed that it is possible to obtain more stable states than found m previous results 1121 The final parameters from this analysis are given in Table 1

In order to analyze the nature and strengths of the bonds involved m the surface reaction, the well-known energy partitiomng technique 1131 (ET = Z Eab + Z E,; ET = total energy; Eab = dlatomic energy for atoms a and b; E, = monoatomic energy of atom a > was employed m these MINDO/SR calculations, together with the Mulhken population analysis [ 141

Results and discussion

Before settmg out to examme the hydrogen adsorption results over a hematite cluster, an analysis of the method and parameters IS presented This analysis was performed by comparmg results from the Fe0 diatomic molecule and our hematite cluster, with theoretical calculations, and exper- mental results reported m the literature

Fe0 duttomu: molecule The ground state of Fe0 was established from a series of calculations

that start wrth electromc configurations obtamed from shlftmg the electronic occupation of several high occupied and low unoccupied molecular orbitals

202

Thus procedure was carrred out for CY as well as for 6 electrons The energy for each configuration was obtained after electromc convergency and the chosen ground state configuratron is, of course, the most stable m total energy The ground state configuration obtamed m thrs work corresponds to 1~?2~?1~r~@23~~3a~ which 1s associated to a ‘A state. This result 1s in excellent agreement with calculations using multlconfiguratron self- consistent-field wave timctrons wrth relatlvistlc effective potentmls 1151, and other ab mrtzo calculations [161

The Fe0 molecular orbitals energy levels are presented m the correla- tion diagram shown in Fig. 1, consldermg separately CY and /3 spms The top number on the right side of each molecular orbital symbol corresponds to its occupation number Some of the Fe and 0 atomic levels are taken as average of (Y and /3 values, m order to slmpllfy the diagram The la(Fe(sp),O(s)), Za(Fe(d), O(p)) and 3a(Fe(sd), O(sp)) occupied orbrtals are 76, 86 and 68% localized on 0, Fe and 0 atoms, respectively The ln(Fe(d),O(p)) and Zn(Fe(pd),O(p)) are 96 and 80% centered on u-on and oxygen atoms, respectively. The correlation lines mdicate that the connected atomic orbltals contribute more than 10% to the Fe0 molecular orbital considered For example, the correlation hne between Fe0 and oxygen for the 2a((u) orbital 1s included, notwrthstandmg that the contnbutlon from oxygen 1s only 14%. The iron electronic population found was (sp)’ 43d6 38, which means an electronic charge transfer to the oxygen of 0 19 electrons and an u-on oxidation state close to +2. Thrs charge over the oxygen is relatively small, compared with the value of 0.51 based on electronegativrty differences El71

00

h-05 3 0

G f5 2

-10

-I 5

4- - I;:-;

I/ \ ‘\

//

\ i/

0 Fe0 Fe Rg 1 Fe0 molecular orbhl diagram

Hemutrte cluster Hematite has a corundum structure 1181, which is rhombohedral with

space group D63d (R3c) 1191 and it consists of an approximately hexagonal closed-packed (hcp) array of oxygen atoms m which the metal atoms occupy two-thuds of the octahedral holes However, electrostatic repulsion between the iron atoms along the c axes results in a trigonal distortion of the oxygen environment of each metal. In order to represent the hematite surface in a simple way, a cluster of five atoms (FezO,) is sketched m Fig. 2 The mam reasons for this selection are.

(a) A complete deoxygenated surface is assumed because, durmg the HDM reaction, contmual hydrogen flow over the catalyst causes removal of the superficial oxygen atoms, leavmg them available for a direct mteraction with the adsorbate.

(b) A small size cluster is suitable for calculations of chenusorption and dissociation reaction paths of Ha, especially for future calculations of hydrogenation and hydrogenolysis of porphyrnuc compounds on hematite

(c) The fact that the total charge on our model is zero gives more realistic energy levels, because highly charged clusters produce, as m other calculations [20bl, occupied electromc levels with positive energy values At tlus pomt, it is worthwhile mentromng that our model is quite different from the usual hematite bulk models selected m the literature 120,213, where a &storted octahedral structure is represented by a central iron atom and snt oxygen atoms with a total charge of -9 to simulate the oxidation state of +3 of the iron atom.

The Fe-Fe, FM and O-O bond lengths and mteratomic angles used for this cluster were provided from the crystal structure data of Blake et al. [22] and Fasiska 1231. In order to study oxidation state changes, calculations wrth the model charged cluster of -1 were also carried out Here, the same geometry of the hematite cluster was kept

Previous hematite theoretical works have been performed m connection with electronic structure and metal passivatlon [ 241, spectroscopic properties [203, and hype&me mteraction of the iron atoms 1211 However, theoretical

0 OXYGEN

E’lg 2 Hematlte model cluster

204

calculations on Fe203 related with chemisorption and catalysis are very hmited

In order to analyze the hematite properties and the effect of oxidation state changes, m Table 2 we present the Mulhken bond orders (MBO), diatomic enewes (DE) and electromc configurations for the FeZ03’, FezO,-l and Fe0 diatonuc molecule The MBOs and DES indicate that the strongest bond m hematite corresponds to F& (DE = -0 351 au , MB0 = 0 55). However, the Fe0 bond m hematite is considerably weaker than that m the diatomic molecule (DE = -1089, MB0 = 1 28) Tlus is due to a smaller interatomic overlap (small MBO) ansmg from a longer Fe-O equihbnum mternuclear &stance m hematite (2 116 A) than m the Fe0 molecule (1626 A>, and to greater coordmation numbers m the oxygen as well as m the iron atoms On the other hand, the O-O mteraction is very small (DE = +0 010 au.) and repulsive, which is m agreement with previous results [21bl

The charges over iron and oxygen atoms are +0 21 and -0 14, respectively This reveals that the Fc+O bond is relatively covalent with some ionic character, which is somewhat smaller than that obtamed by Nagel 121al (ionic charges Fe= +183 and 0 = -171) and Sherman [2Obl (Fe charge = + 13). One should, however, consider that our charge separations obtained by Mulhken population analysis are deficient, as has been established m the literature [251 For instance, MgO and LiO molecules calculated by ab cnrtzo methods 1261 using Bader’s formalism 1271 showed a considerable difference between the charge calculated with Mulhken’s ap- proach (q(Mg) = 0.81, q(Li) = 0 63) and that obtamed by density mtegration m the atomic regron (q(Mg) = 133, q(Li) = 0 94) The calculated iron orbital population of FezO, is (sp)l 78d6 01, m which the 3d population is larger than the formal value of 5, and comparable to Nagel’s value of 5 7 121al It is good to note that the oxidation number formahsm and the expected formal d

TABLE 2

Bond orders, dlatomlc enerees and orbital populations of Fe,O,‘, Fe,O,-’ clusters

Cluster Bond Mull&en Y-X bond orders

Dlatomlc enerpes (a u )

Valence orbkal population on atom X (e-j

sP d total

FezOSo FA 0 55 -0 351 614 - 6 14 000 +o 010 614 - 6 14

Fe-Fe 045 -0 018 178 6 01 7 79 Fe,O,-’ Fe--O 0 51 -0 274 625 - 6 25

0 00 +o 014 625 - 6 25 Fe-Fe 049 -0043 1 13 699 8 12

Fe0 (I--Fe 128 -1089 143 638 7 81

205

occupancy are just convement pictures that contam valuable information, so any discrepancy with the calculated values is anticipated

All calculations were carried out by considering the most stable spm multiphcity. In this case, the optimal multiplicity for the hematite cluster IS 9, which corresponds to a spm magnetic moment of 4.0 PB wluch is low compared to other theoretical calculations by Sherman 120bl (4 69 PB) and to the experimental value of 5 0 PB for the a-FeaOs bulk [ 281.

The values for the FeaOa(-), properties presented m Table 2, clearly indicate that the negative charge on the cluster conduces to an expected decrease m the iron oxidation state, reflected by an mcrease of 0 98e- m the d population (6.99e-) with respect to the neutral case (6 01 e-) In addition, the total negative charge on the oxygen atoms IS augmented by 0 33 e- Therefore, a decrease of 129 e- m the total Fe(sp1 population occurs to offset the total Fe(d) population mcrease of 1.96 e- The extra charge added to the cluster also produces a decrease m the Fe-O diatonuc energy, from -0 351 to -0.274 au., which indicates a possible geometrical relaxation of the Fe203- cluster Therefore, calculations with substrate optimization should be convenient Nevertheless, tlus IS not considered here, because the nature of this work is qualitative

Hydrogen adsorptzon Hydrogen adsorption over surfaces has been extensively studied, due to

its importance m corrosion and catalysis phenomena In this field, theoretical calculations have provided valuable information about the surface electromc structure and the processes that appear m the adsorbate-adsorbent mterac- tion. The method employed here, MINDO/SR, has previously been applied to hydrogen adsorption over Ni [7a, 7c, 131 and iron 112al clusters, and smgle metallic atoms 1291.

A theoretical study of H2 adsorption over a surface requires a very large number of calculations, because the gaseous molecule can approach several adsorption sites m different ways A variety of Hz paths of mteraction with the Fe adsorption site are shown m Fig 3 The dotted lines represent the direction m which the H2 approaches. A and B geometries differ m the

I E I .F

.

c’ l G

*Fe -H

Fig 3 H, mteract~on paths with won adsorption s1t.e on hematite

orientation of the H-H molecule with respect to an Fe-G bond. Hz IS echpsed with one Fe+G bond m geometry A, and it is staggered m the case of geometry B All of these initial calculations were carried out at a fixed H-H distance (H-H equihbnum bond length), varymg the distance between Hz and the adsorption site

In order to analyze the electronic and bmdmg properties of Fe203H2 for the different geometries shown in Fig 3, the optimized F-Hz bond distances, Hz adsorption energies and the diatomic enerees (DE) for H-H, Fe-H and Fe-G bonds are required, as shown m Table 3 In this work, the term adsorption energy stands for the &fference between the total energy of the Hz-cluster system and the sum of the total energies of the cluster and the hydrogen molecule separated at mfimty A stable adsorption state of the Hz molecule would have a negative value of adsorption energy The DE values for F-H bonds are those for the shortest Fe-H distance, and an average value is considered for the Fe-G bonds. The Fe-Hz distances correspond to those mdicated by the dotted lines shown in Fig 3

Results shown m Table 3 clearly indicate that hydrogen chemisorbs on hematite with the geometries A-E, the side-on geometry A bemg the most favored The H2 molecule can rotate freely on the top of the iron atom, as mdicated by the small difference between A and B geometries. The end-on adsorption (E) is also possible, but less stable, because the iron-hydrogen mteraction is only with one hydrogen atom. On the other hand, adsorptions with F and G geometries are not favored, because hydrogen interaction is repulsive By analyzing the DE terms, one can conclude that A-E geometries lead to hydrogen activation, because the H-H diatomic energies have diminished with respect to the free Hz molecule ( -0.402 a.u 1 and reasonable F-H bonding mteractions (from -0.103 to -0 061 a.u.) appear With respect to the Fc+G bonds, a small decrease of the Fe0 bondmg interaction takes place as compared with free cluster (-0 351 a.u.1. This result suggests that a small cluster relaxation is expected, owing to hydrogen adsorption Our results also reveal an electromc charge transfer from the hydrogen

TABLE 3

Electromc and bmdmg propertlee for &fferent geometmes of H, on won adsorptlon site shown m Fig 3

Geometry Fe-H, Adeorpt distance energy” (A) (kcal mol-‘)

Dlatonuc Energy (a u ) HZ charge

H-H F+H Fe-O

A 180 -34 72 -0 291 -0 103 -0 348 +o 30 B 181 -34 68 -0 293 -0 102 -0346 +o 30 C 179 -33 34 -0 295 -0098 -0 342 +0 28 D 185 -34 06 -0 293 -0 101 -0 336 +o 30 E 179 -24 51 -0 328 -0 061 -0 334 +o 19 F - rl - - - - G - rl - - - -

a r i = repulsive mteraction

207

molecule to the cluster (0.30 e - 1, which is very hrgh as compared with previous results for a single iron atom (0 OSe-1 [5al. The value of oxygen population on the free cluster (6.14e-1 compared with Fez09Hz with geometry A (6.29e-) seems to indicate that the electrons transferred from hydrogen go in great part to the oxygen atoms, due to their more electronegative character Thrs transfer of charge from the H-H u bond to the cluster and the F-H bond formation causes H-H bond weakenmg, indicated above

It IS mteresting to note here that, m all calculations, the optimal multlpliclty for the H zFez03 neutral clusters was 7 (neutral cluster multlphaty = 9). Thrs indicates that hydrogen adsorption mduces a decrease of 1 PB m the hematite magnetic moment.

To search for other possible angles that could favor H2 chemmorptlon, 111 Frg 4 we present the potentml energy curve that results from the adsorption energy variation obtained by H2 rotatron, with respect to the top posltlon shown m geometry A of Fig. 3. As we can see, there is a potential well as the top of the non atom, so that a Hz molecule approaching the iron atom tends to be located dnectly on top of it.

Before obtanung further details about hydrogen adsorption on iron atoms, we wrll study the Hz mteraction with the hematite oxygen atoms, as shown in Fig. 5. The results are presented in Table 4. Direct adsorption on an oxygen atom 1s well favored, as is the case for J and K geometnes w&h

^ 30, I

20 40 60 60

Angle ( Degree)

hg 4 Potentml energy curve for leamng H, adsorbed on top of an won atom

l%g 5 Hz mteractlon paths with oxygen adsorption s~tee on &math

208

TABLE 4

Ekctromc and bmdmg properties for dflerent geometnes of H, on oxygen adsorption site shown m Fig 5

Geometry C&H, Adsorpt &stance energy (A) &xl mol-‘)

J 0 95 -45 61 K 096 -4181 L 069 +16 96 M 082 +59 00

Dlatomx energy (a u ) H, charge

H-H O-H FlZ-0

-0 038 -0 337 -0 195 +0 56 -0 049 -0 325 -0 187 +0 58 +o 002 -0 322 -0 298 +046 -0 103 -0090 -0 259 +066

adsorption energies of -45.6 and -41 8 kcal mol-‘, respectively On the other hand, the adsorption on two oxygen atom sites is not stable Two important factors m the mteraction of Hz with oxygen atoms are: the very small H-H diatomc energy of the J and K geometries (DE(H-H) = -0 05 a.u.) m comparison with the DE of free Hz (DE(H-H) = -0 4021, and the formation of a strong O-H bond (DECO-H) < -0.3 a u.) This means that Hz tends to be dissociated on oxygen atoms, because the H-H weakenmg is offset by the formation of two O-H bonds A considerable decrease m DE(Fe-0) wrth respect to the clean cluster, the formation of strong O-H bonds, and the fact that the total energy (TE) of Fez,O, + 3H2 (TE = -76 49 a u ) is lower m absolute value than that for 2Fe + 3Hz0 systems (TE = -76 92 a.u. 1, suggest that a deoxygenation process and the concurrent hematite reduction may occur, as has been experimentally observed 1301 Finally, Hz adsorption over Fe and 0 atoms was also studied by approachmg Hz m a orientation parallel to a Fe-O bond Our calculations reveal that this mteraction pathway is repulsive

Once the most adequate H2 adsorption site (t.e geometry A m Fig 3) has been obtained, the H2 adsorption and dissociation processes were studied 111 more detail at this site. The graph of adsorption energy us. FsHz distance (h) 1s presented in Fig 6 The iron oxidation effect on the hydrogen activation was considered by including the potential energy curve for the Hz adsorption over [Fez03]-, shown also m Fig 6 In order to mvestigate the Hz dissociation, calculations of adsorption energy at different H-H distances were carried out, and the results are depicted in Fig. 7 for the neutral and negative species The H-Fe distances and the H-F-H angle were optimized for each H-H distance Bonding and electronic properties of the most stable [Fez03H210 and lFezO,H,l- systems, shown m Fig 7, are presented in Table 5 The iron atom considered here is that m which hydrogen is chemisorbed.

A comparison with calculations of hydrogen activation on a smgle iron atom Fe9 (q = +l, 0, -1) [5al may be important m discovermg the oqgen hgand effects on the adsorption site and consequently, on the H-H adsorption and dissociation Calculations of H2 interaction with one smgle positively-charged iron atom show that the side-bonded geometry is stabilized

5 -4d I I I I I I 07 09 II 13

h H-H d 5 -30

Distance ( 1

d -Charge = ---H

0 H

-35 16 16 20 22 T ------C,,,,,ge z - , Distance h (1) ie

fig 6 Potential energy curves for H, adsorptlon on Fe,Os’ and Fe,O,- systems

Fx 7 Potential energy curves for H, &ssoclation on Fe,OSo and FezO,- systems

TABLE 5

Electromc and bmdmg properties for Fe,O,H,’ and Fe,OsH,- at the numma of the potentml energy curves presented m Fg 7 The won atom corresponds to that on whxh H, IS chermsorbed The properties that mvolve 0 or H atoms are taken as average

Molecule Fe electromc popuiatlon Fe-H Mulhken bond orders

8 P d sp-s d-s

FezOsHzo 058 118 600 058 000 Fe,O,H,-’ 0 35 094 698 045 000

Optmed bond &stance (A) Atonw charge (e-j

H-H F-H H Fe 0

Fe,O,HzO 080 182 0 16 020 -0 29 Fe,OsHSwl 0 75 193 009 -0 27 -026

Duatomx eneqpes (a u ) H, adsorption

H-H Fe-H Fe-O energy (kcal mol-’ )

Fe,OsH,’ -0286 -0 109 -0 335 -37 6 Fe20sH,-’ -0 335 -0 083 -0264 -101

210

m such a way that Hz dissociation IS not a thermodynamically allowed process. These results are equivalent to those of H2 adsorption on neutral hematite cluster iron sites, because the side-bonded adsorption is quite stable and dissociation is not favored, as shown m Figs. 6 and 7 The adsorption energy for the neutral system (-37.5 kcal mol-‘1 is comparable with the value obtained for a positively charged iron atom (-33.8 kcal mol-l) [5al In the case of neutral hematite, iron atoms are positively charged, due to oxygen withdrawing electronic charge The stability of this non-dissociated Hz molecule has also been reported on transition metal complexes, where electron-vvlthdrawmg hgands and positive charge facilitate the formation of a dihydrogen adduct [ 313

The effect of a negative charge on hematite clearly indicates a reduction m the hydrogen adsorption energy, see Figs 6 and 7. In the case of one smgle iron atom, a negative charge on iron facilitates the H-H dissociation, because the electron transfer to the a, weakens the H-H bond On the other hand, a negative charge on hematite does not produce the same effect, because this extra electron density is mainly driven to oxygen and iron atoms, as is shown by the atomic charges presented 111 Table 5 This last case is similar to a single neutral iron atom in which the hydrogen charge is +0 08 compared with +0.09 for the [Fe203H21- cluster. Furthermore, the adsorption energy is reduced to -10 1 kcal mol-‘, which is close to the value of -5 0 kcal mol-’ obtained for a single neutral iron atom [5al It would be valuable to compare the results of the [Fe2031-2 system (oxidation state = 2) with those of one smgle negatively-charged iron atom, because it induces a spontaneous Hz dissociation. Unfortunately, our calculations with charge -2 cannot be reported here because electromc convergency leads to spurious results, m which occupied molecular orbitals have positive energy.

Mulhken bond orders indicate that the F-H bond is an sp-s type with a very small direct participation of d orbit&. This result is m agreement with calculations for the FeH+ diatonuc molecule [32al and experimental considerations 132bl The optunized bond distances mdicate that the negative charge (iron reduction) increases the Fe-H distance and shortens the H-H bond with respect to the neutral system These bond changes are reflected in the H-H and Fe-H diatomic energies (DE). The DE(H-H) is m absolute value higher m the negative system (-0.335 a.u ) than in the neutral one (-0 286 a.u ) On the contrary, the F-H bond is weakened m the negative case (DE(Fe-H) = -0 083 au.) m comparison with the neutral case (DE(Fe-H) = -0.109 a.u.) These changes are the result of less electronic charge transfer from the H,(a,) molecular orbital to iron as a consequence of the negative charge on the iron atom

Based upon the results from Table 3 and Fig. 3, we can conclude that the geometries capable of activating hydrogen are those that contain Hz close to and above the non atom. A regon near to a plane cleaving the iron atom, and parallel to the plane containing the oxygen atoms, is completely repulsive Around this plane (nodal plane), there is no electron density for bond formation, as would be expected m an octahedral geometry We can

211

conclude, therefore, that besides the fact that oxygen hgands create an electron-deficient adsorption site, a dehrmted bonding region is formed in the space around this site.

The fact that Hz is non-dissociatively chemisorbed, and the fact that the H-H bond is weakened durmg the adsorption process (DE = -0.286 a u. for chemisorbed H2 compared wrth DE = -0 402 a.u for free Hz), implies that Hz is fairly activated on hematite. This activated hydrogen may be respon- sible for the hydrogenation and hydrogenolysis of the organometalhc com- pounds present in heavy oil. However, catalytic hydrogenation of hydrocar- bons is an energetic compromise between bond formation (C-H) and bond breaking (H-H and Fe-H) Therefore, the optimal degree of hydrogen activation must be determined by considering the mteraction of the Hz chermsorbed with the molecule to be hydrogenated or hydrogenolyzed.

Finally, we must add here that our hematite model is oversimplified and only represents a particular adsorption site (one-center site). The possibdity of dissociation on multicenter sites must be taken mto account, even though a weaker multicenter H-Fe interaction is expected, because the Fc+Fe nearest neighbor distance in hematite (2 96 A) 1221 is longer than in the bulk iron (2.48A) 1333. Furthermore, there is experimental evidence that mononuclear transition metal-dihydrogen complexes are able to hydro- genate hydrocarbons 1341

As far as we know, there are no experimental studies that deal with Hz adsorption on hematite m which the surface has been previously deoxyge- nated. Studies of Hz adsorption on metal oxide surfaces such as CrzOJsllica 1351 and [MoO~l-2/alumma 1361 conclude that a heterolytic dissociation of hydrogen is possible, with the formation of M--O--Ha+ and M-Ha- (M = Cr, MO) species Our calculations reveal that the H2 mteractions with oxygen-metal and oxygen-oxygen bridge adsorption sites are repulsive. However, we have not considered the case in which the surface is partially deoxygenated (t.e unsaturated with terminal oxygens), because it is assumed that the hydrogen pretreatment has eliminated all super- ficial oxygens. More work must be done in order to consider several intermediate deoxygenated models of the surface as starting pomts for H2 activation.

Another possibility that cannot be discarded is the hydrogen activation on small iron clusters produced by previous reduction of the catalyst surface It is well known that H2 is dissociated on iron 1371 and H atoms can freely migrate on the surface. According to Ware and Wei El], the rate-limiting step m hydrodemetalhzation of metalloporphyrms is an initial hydrogenation that occurs on a meso bridge position. This implies that the hydrogenation would take place on adJacent carbons. This situation may be dynamically favored by two nearby hydrogens with low mobility, as the case of the H2 activated molecule presented here.

One experimental fact that also must be considered m modeling of hydrodemetallization of heavy crude oil is the formation of pyrrhotites due to the presence of sulfured compounds in heavy crudes [21. Our group is

212

currently working m this direction, because of the importance of this study not only m hydrodemetalhzation but also m metallurgic processes 1381 and carbon gasification 1391

Conclusions

(a) Hydrogen chemisorption on hematite one-iron-center site IS more stable in a side-bonded geometry and presents an adsorption energy of -37.6 kcal mol-’ This is a non-dissociative process with a charge transfer from hydrogen to hematite oxygen atoms Activation of hydrogen is observed, as shown m the decrease m H-H diatonuc energy and bond order m the cluster with respect to free Hz. This IS due to the transfer of electron charge density from the bonding Hz(a,) molecular orbital to hematite

(b) The addition of electronic charge to hematite produces a &mimsh- ing of hydrogen adsorption energy

(c) Hydrogen chemisorption induces a decrease m the magnetic moment of the hematite cluster

(d) Adsorption and dissociation of Hz on hematite oxygen atoms can occur but lead to deoxygenation (water formation) and reduction of the solid.

Acknowledgements

The authors would like to express their gratitude to Drs Juan Rivero, Claudio Mendoza, Miguel Luna and Walter Cunto of the IBM Venezuelan Scientific Center for assistance with computer programs and Dr Fernando Gonzalez for his helpful discussions A computer time grant on the 3081 computer at IBM Venezuela Computmg Centre is gratefully acknowledged L. J. R. would like to thank the ConseJo National de Investigaciones Cientfficas y Tecnologicas, CONICIT, for financial support.

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