surface science

ELSEVIER Surface Science 343 (1995) 261-272

A theoretical study of the adsorption of oxalic acid o n T i O 2

Adil Fahmi a,1, Christian Minot a,,, Patrick Fourr6 b Patrice Nortier b 9

a Laboratoire de Chimie Organique Thdorique, Universit~ P. et M. Curie, Bolte 53, URA 506 CNRS, 4 Place Jussieu, F-75252 Paris Cedex 05, France

b Service de Synth~se Mindrale, RhSne-Poulenc Recherches, 52 rue de la Hale Coq, F-93308 Aubervilliers, France

Received 7 February 1995; accepted for publication 31 July 1995

Abstract

This paper presents results of periodic Hartree-Fock calculations on the adsorption of oxalic acid on rutile and anatase TiO 2 structures. One dimensional polymers are used as models for TiO 2 bare surfaces. Oxalic acid undergoes dissociative adsorption leading to the oxalate bonded to two adjacent Ti atoms, which seems to be the most stable form of oxalic acid on TiO 2 surfaces. The adsorption is stronger on the anatase polymer than on the ruti!e polymer and thus the crystal growth of TiO 2, in the presence of oxalic acid, leads to the rutile structure.

Keywords: Ab initio quantum chemical methods and calculations; Carboxylic acid; Low index single crystal surfaces; Titanium oxide

1. Introduction

The effect of foreign substances on the growth of crystals is a well-known phenomenon [1-3]. There are several possible mechanisms by which an addi- tive can inhibit the crystal growth reaction. Two main mechanisms are often used to describe the inhibitory effect [4]: in the first one, the additive forms stable complexes with one of the precipitating ions and decreases solution supersaturation; in the second one, the additive is adsorbed on the active growth sites. During this later process, the additive poisons the surface where the adsorption takes place. The adsorption inhibits the growth on that face pre- venting the deposition of the crystal molecules. The

* Corresponding author. Present address: Eindhoven University of Technology, De-

partment of Inorganic Chemistry and Catalysis, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

growth is therefore oriented towards the faces that interact weakly with the additive. In the case of the competition between the formation of two polymor- phic structures (for instance rutile and anatase struc- tures for TiO2), the growth is oriented towards the least active structure. Here rises the molecular recog- nition phenomenon at inorganic surfaces [5] that has the potential to provide control over crystal growth processes. A detailed understanding of recognition processes leads to the choice of the additive which can direct the growth towards the desired crystalline structure.

This work was stimulated by the neefi for more effective chemicals to control the titanium dioxide crystallization. Rutile and anatase are the main crys- tal structures of TiO 2, rutile is the most stable one [6]. During TiO 2 crystallization, some carboxylic acids [7] are preferentially adsorbed on futile and the growth is oriented toward~ a~atase. However, the rutile formation is still observed in presence of ox-

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262 A. Fahmi et al. / Surface Science 343 (1995) 261-272

alic acid. The same trend was found in the TiO 2 sol-gel process [8]. It is therefore suggested that oxalic acid is preferentially adsorbed on anatase, blocking the anatase growth.

In this paper we present a theoretical study of the adsorption of the oxalic acid on bare rutile and anatase surfaces. The structure of the oxalic acid on TiO., is analyzed. The crystal growth orientation will in principle be found by the comparison of the adsorption energies of the oxalic acid on the two structures. We first present the oxalic acid molecule (HOOC-COOH), the hydrogen oxalate (HOOC- COO-) and the oxalate (C.,O4 2-) ions (Sections 3-5). We discuss the gas phase acid-base properties of the oxalic acid (Section 6) and we present the models for the bare TiO,. surfaces (Section 7). Next, we report the results of the molecular adsorption (Section 8). The various dissociative adsorptions are analyzed in Sections 9 and 10. The last section is devoted to oxalic acid decomposition in the gas phase and on the TiO2 surface.

2. Method of calculation

The calculations are carried out at the SCF level with a restricted Hartree-Fock hamiltonian using the core pseudopotential from Durand-Barthelat [9]. For the gas phase properties of oxalic acid, we use the MONSTERGAUSS [10] program to optimize the geometry of the various free anions which can be generated by the oxalic acid. Next we use the CRY~,- TAL [11] program to compute the electronic prope~'- ties o: the optimized molecules and also for the adsorption studies. The basis sets are the same as in

our previous studies [6,12,13], a PS-31G [14] basis set for the oxygen and the carbon atoms and the standard 4-31G basis set for the hydrogen atom. For the titanium atom the basis set consists of five d primitives contracted to a (4/1) basis set and a single 4sp shell with an exponent of 0.484; the 4sp shell is almost vacant for the titanium ions. These basis sets were optimized for the TiO 2 bulk structure [6]. One might think that they are of medium quality. However, this represents for a periodic calculation a better level than it would be for a molecular calcula- tion. Indeed, very diffuse orbitals are not required as for a molecular calculation, since they are responsi- ble for the basis set linear dependence [15].

3. The oxalic acid molecule

The oxalic acid has a planar geometry. According to hydrogen atoms orientation, two conformations are very close in energy (see Scheme 1). For the first one, the hydrogen bonding takes place within each carboxyl group, while for the second one, hydrogen atoms connect different carboxyl groups. The first one (conformation A) is more stable by 1.7 kca|/mol with the PS-31G basis sets.

The conformation B, with hydrogen bonding (O- O 2.7 and H. . . O ~ 2.29 ,g,) is believed to be the most stable according to electron diffraction [16] and spectroscopic analysis [16,17]. Despite of numerous theoretical studies, the most stable conformation is still controversial [18-20]. The use of ab initio all electron calculations shows a very small energy dif- ference between the two conformations. At the MP2/4-31G level [19] the conformation A is more

Table 1 Optimized geometrical parameters of the oxalic acid (in ,~ and deg), the hydrogen oxalate and the oxalate ions

Bond length Oxalic acid Hydrogen oxalate Oxalate Oxalate or angle ("A"conformation) (planar) (orthogonai) (planar)

C=C 1,504 (1,536) 1,556 1,530 1.596 C=O 1,206 (1,202) 1,212 - - C-O 1,33 (1,291) 1.361-1,239-1,268 1,274 1.269 O=C-C 124,7 (121,7) I i !.8 117.5 117.7 O-C-C ! 11,5 (112) 127,7-11 l,l-118 117.5 117.7

The experimental values [21] arc indicated in parentheses. The three values for the CO bond in the hydrogen-oxalate correspond to single bonds (the first one is that of the hydroxyl group, the last one is cis with respect to that group). The same convention is used for the OCC angles,

A. Fahmi et al . / Surface Science 343 (1995) 261-272 263

0 O - - H

. - d -

conformation A

Scheme 1.

\H conformation B

stable by 0.8 kcal/mol and at the MP2/6 -31G, level [20] the conformation B is more stable by 2.5 kcal/mol.

The optimized parameters for the conformation A with the PS-31G basis sets are shown in Table 1. The overall agreement with experimental data [21] is satisfactory except the position of the hydrogen atom that gives errors of 0.1 A for the O - H bond length and 10 ° for the C - O - H angle. Errors of the same magnitude were found by other authors [19].

lation is 0.048. The optimized parameters for the planar conformation are shown in Table 1.

5. The oxalate ion, C20 z-

Using the 6-31G, and 6-31 + G basis sets, De- war et al. [22] found that the orthogonal D2d geome- try of the oxalate is more stable than the planar D2h geometry. By a metal-cation complex the geometry can be modified towards the planar oxalate. The energy difference between the two conformations are 4.9 (6-31G,) and 6.3 (6-31 + G) kcal/mol. With the PS-31G basis sets, we found the same trend, a stabilization of the D20 geometry by 2.9 kcal/mol. The optimized parameters are shown in Table 1. The C-C bond length is longer for the planar geometry because of the oxygen-oxygen repulsion.

6. Gas phase properties of oxalic acid

4. The hydrogen oxalate ion, HOOC-COO-

The hydrogen oxalate ion has a planar geometry. The hydrogen atom is oriented in the way that allows

ydrogen bonding (O-O = 2.52 and H. . . O- -2 .04 ) between the two carboxyl groups (see Scheme 2).

This bonding is strong because of the negative charge on the oxygen atom.

The energy difference between the planar and the orthogonal conformations is 9.9 kcal/mol. This later value can be considered as an estimation of the hydrogen bonding energy. The O . . . H overlap popu-

io oxxo___,,,o / - - \

%. o

planar conformation orthogonal conformation

Scheme 2.

6.1. Gas phase proton affinities

1here are two basic sites in the oxalic acid: a carboxylic and a hydroxylic oxygen atom. When a single proton interacts with the oxalic acid molecule, the first proton affinity is involved:

HOOC-COOH + H + ..o HOOC-C 0 (OH) 2,

A Hc,,i,: = - 180.5 kcal/mol

(the proton is attached to a carbonyl group),

HOOC-COOH + H + --* HOOC-CO(OH 2 ) +'

AHc.~lc = - 168.2 kcal/mol

(the proton is attached to a hydroxyl group). The proton is located on a carbonyl group, the

most basic site. Since the second COOH group is an attractor substituent, the oxalic acid proton affinity is small with respect to that of the formic acid (A H - - 184.1 kcal/mol; experimental value: - 182.8 kcal/moi [23]) or that of water ( A H = - 1 8 2 . 5 kcal/mol; experimental value: - 1 7 3 kcal/mol [23,24]).

264 A. Fahmi et ai . / Surface Science 343 (1995) 261-272

When two protons interact with the oxalic acid, the best protonation mode also involves the carbonyl groups:

HOOC-COOH + 2H + --* (HO)2C+-C + (OH)2,

AHcatc -- - 244.1 kcal/mol

(protonation of two carbonyl groups),

HOOC-COOH + 2H + --* + ( H 2 0 ) O C - C + (OH)e,

A Helle = - 231.4 kcai/mol

(protonation of one hydroxyl and one carbonyl groups),

HOOC-COOH + 2H + --* + (H20)OC-CO(OH~,) +,

AHcalc = - 218.3 kcal/mol

(protonation of two hydroxyl groups). From these results we expect several modes for

the molecular adsorption of oxalic acid on TiO2. The mode that should be the best involves two carbonyl groups.

6.2. Acidic cleavages of oxalic acid

Since the COOH group is an attractor, the hydro- gen oxalate ion, HCOO-COO =, is more stable than the formate, HCOO =, or hydroxide, HO °, ions, Therefore, the first acidic cleavage of oxalic acid is

less endothermic than that of formic acid or that of water:

H20 -* HO- + H +, AHcalc - 423.8 kcal/mol

(experimental value: 390.8 kcal/mol [25,26]),

HCOOH -* H C O O - + H +, AHcalc - 356.7 kcal/mol

(experimental value: 345.2 kcal/mol [26]),

HCOO-OOCH -* H O O C - C O O - + H +, AHc~alc- 328.8 kcal/mol.

The second acidic cleavage of oxalic acid is very endothermic:

HCOO-OCC- ---, - OOC-COO- + H +, AHcalc - 459.5 kcal/mol.

7. Models for the TiO z surfaces

We model the TiO 2 surfaces by one dimensional polymers [6,12]: a linear polymer for rutile and a zigzag polymer for anatase (see Scheme 3). These models have been checked by a study on the clean surface [27]; it is concluded that atoms from the TiO 2 polymers do not differ by much from the bulk and are still bound objects relative to the free atoms. We have already used this model for the studies of adsorption on the TiO 2 surfaces [12,13,28].

J 11 11 11 0 C O 0 G 0

j o--T - - o ~ ~ o

ruble polymer anatasc polymer _ . presented as undistorted sys~m ~ormc optimized suucture, Ti-O-Ti is 152 °.

Scheme 3.

A. Fahmi et al./ Surface Science 343 (1995) 261-272 265

The rutile polymer is made of rectangular pieces with a long edge (3.0065 A, the c parameter of the bulk) in the direction of the chain and a short one (2.4875 A) in theoperpendicular direction. The Ti-O distances, 1.951 A, are the equatorial distances from the bulk structure. For the anatase polymer, the Ti-Ti translation vector is the a parameter of the bulk, 3.7629 A. This latter system involves both equatorial Ti-O bonds, 1.939 A, along the axis of the polymer and apical ones, 1.995 A, in the perpen- dicular direction. These parameters are taken from the optimized bulk structures [6]. The coordination of the Ti atoms is four and is the same for the two polymers. For the oxygen atoms, the coordination is two in the linear polymer, while, in the zigzag polymer, half of them is singly-coordinated and the other half is triply-coordinated. On real surfaces, the coordination of the Ti atoms is usually five (it is four on the (001) surface of ruffle) and that of the oxygen atoms is most often two or three. Previous studies [12,13] have shown that the reduction of the coordi- nation of the atoms in the polymer models contribute to overestimate the adsorption energies. The energy shift is comparable for various adsorption modes and thus does not affect the relative stabilities.

These polymers are obviously far from being a complete representation of crystallographic faces al- though they are significantly better than a cluster model. However, they contain titanium and oxygen atoms with the oxidation numbers + IV and -11, respectively. The linear polymer can be recognized as the main feature of the ruffle (110) and (100) faces; it contains the linearity of the TiO 2 sequence that is specific of rutile. The zigzag polymer can be found on the anatase (100) and (010) faces. We consider that the study of the polymer models should give information about the activities of the TiO2 surfaces.

The ruffle polymer is more stable than the anatase polymer; the calculated total bonding energies are 83 and 68 kcal/mol, respectively [12]. The water ad- sorption study [12] shows that the metal cations from the ruffle polymer are the best Lewis acidic sites and the singly-coordinated anions from the anatase poly- mer are the best basic sites.

A basic adsorbing species would approach the titanium atom perpendicularly to the polymer plane and an acidic adsorbing species would app~::oach the

oxygen atom in the polymer plane. We will consider the adsorption of one molecule of oxalic acid on a cell of two TiO 2 units of L~e polymer. Adsorption energies have been calculated according to the ex- pression:

Ead s ffi ETiO2 4" EH2C20, -- E(H2C202/TiO2),

where E(H~C204/TiOe ) is the total energy of the ad- sorbate/substrate system, ETi02 is the total energy of the substrate and Eu2c204 is the total energy of the free oxalic acid molecule in its equilibrium ge- ometry. A positive Ead s value corresponds to a stable adsorbate/substrate system.

For the geometry of adsorbed oxalic acid, the following methodology has been used. We first per- formed a molecular calculation of the adsorbate species (without the substrate) using the MONSTER- GAUSS program. The adsorbate species is defined as the oxalic acid molecule plus one or two protons simulating the Ti metal cations. Positions of oxygen atoms by which the oxalic acid is attached to the surface and those of protons are fixed; the positions of all the other atoms are optimized. Finally, this geometry is transferred to the CRYSTAL calcula- tions. The Ti-O distances between the oxalic acid and the surface are taken from water adsorption [12] ( T i - O - 2.12 ~. for the molecular adsorption and Ti-O = 1.81 A for the dissociative adsorption). The study of the formic acid adsorption shows that these parameters, optimized for H20, are transferable to a carboxylic acid [29].

8. Molecular adsorption on futile

Titanium dioxide is an amphoteric compound, endowed with acidic Ti(+IV) and basic O ( - I I ) sites. Surface chemistry may thus involve Lewis and Brcnsted acid-base reactions. Bare TiO 2 surfaces dissociatively adsorb water to form surface hydroxyl groups [12]. These surfaces correspond to the real catalytic species more often than bare surfaces. The first step in the interaction of a carboxylic acid (RCOOH) with TiO 2 surfaces is the formation of the carboxylate (RCOOad s) [30-34]. The proton is trans- ferred to an oxygen atom of the surface. The second step is the decomposition of the carboxylate. The structure of the carboxylate and the thermodynamic

266 A. Fahmi et al . / Surface Science 343 (1995) 261-272

H ~ ~ ......... ',,,,~:)...,,,I, H Lewis acid-base reaction, the Lewis acid is the metal cation and the Lewis base is an oxygen atom from the oxalic acid. There is a correlation between the molecular adsorption and the gas phase proton affinity of the carboxylic acid. The oxalic acid can interact with the surface by one or two carboxyl groups. These two adsorption modes are investi- gated.

(a) 8.1. Adsorption by one fimction

. . . . "m,llll~..,,,lill H

H

(b) Fig, 1, Molecular adsorption of the oxalic acid through a single function: (a) the ¢arbonyl group, and (b) the hydroxyl group,

of the reaction leading to its formation are not well determined.

in this section we d,~al with the molecular adsorp- tion: the oxalic acid is not dissociated on the surface. The molecular adsorption can be described as a

This function can be either a carbonyl group (Fig. la) or a hydroxyl group (Fig. lb). The geometry of the adsorbed oxalic acid corresponds to that of HOOC- C(OH) + species (a model for the adsorp- tion through the carbonyl) or to that of HOOC- CO(OH:) + species (a model for the adsorption through the hydroxyl), where the proton simulates a metal cation of the polymer. The best conformations for the adsorbed oxalic acid are the orthogonal ones where the non-adsorbed carboxyi group is parallel to the polymer plane. In these conformations, the oxy- gen-oxygen repulsion is the weakest.

As expected from the gas phase proton affinities of oxalic acid, the adsorption by the carbonyl is more favourable than that by the hydroxyl. The adsorption energies are respectively 23.0 and 7.0 kcai/mol (exothermic adsorptions). The molecular adsorption of the oxalic acid is weak in comparison with that of the formic acid (31.2 and 26.2 kcal/m01 for the adsorption by the carbonyl and by the hydroxyl respectively). The deconjugation in the orthogonal

H

H

H

H

(a) (b)

Fig, 2, Molecular adsorption of the oxalic acid through two functions: (a) two carbonyl groups, and (b) the carbonyl group and the hydroxyl group,

A. Fahmi et a l . / Surface Science 343 t1995) 261-272 267

conformation of the oxalic acid increases the differ- ence expected from the proton affinities.

8.2. Adsorption by two Jimctions

From the gas phase proton affinities (Section 6.1), we have seen that the protonation of two hydroxyl groups is less favourable than that involving two carbonyl groups. Thus, the corresponding adsorption should be less favourable than that involving car- bonyl groups. Because of the proton-proton repul- sion between two nearest oxalic acid molecules, the calculation of the adsorption by two hydroxyl groups requires the use of a low coverage (one oxalic acid per 4 TiO 2 units). The geometry of the adsorbed oxalic acid corresponds to that optimized for (HO)2C+-C+(OH)2 species with the constraint O -

O = 3.0065 ,g, (the translation parameter of the poly- mer). The adsorption energy is weak, 25 kcal /mol . At higher coverage (one oxalic acid per 2 TiO 2 units), the two adsorbed functions are either two carbonyl groups (Fig. 2a) or one carbonyl and one hydroxyl groups (Fig. 2b). Adsorption energies are similar, 48.1 kca l /mol for the first adsorption mode and 48.0 kca l /mol for the second one.

The adsorption through two functions for both modes is more favourable than the adsorption through one function.

. . . . I l l l l H

The dissociative adsorption can be described as a Br0nsted acid-base reaction where the proton is

~ ' . , , * , i * * l l l l l ~

(a)

H

H

9. Dissociative adsorption on futile

(b)

(c) Fig. 3. Various adsorption modes of the hydrogen oxalate: (a) The hydrogen oxalate is adsorbed through a single oxygen atom (CO), the conformation is orthogonal. (b) The hydrogen oxalate is adsorbed through the two oxygen atoms of the carboxylate group, the conformation is planar. (c) The hydrogen oxalate is adsorbed through two oxygen atoms, one from the carboxylate group and the other one from qle carboxyl group, the conformation is planar.

268 A. Fahmi et aL /Surface Science 343 (1995) 261-272

transferred tO an oxygen atom of the oxide (a Br¢nsted base) and the carboxylate is adsorbed on a metal cation. On the contrary to molecular adsorp- tion, dissociative adsorption involves simultaneously the acidic and basic properties of both the oxalic acid and the T i O 2 surface.

9.1. Adsorption of the hydrogen oxalate ion, HOOC-CO0 -

The adsorption of the hydrogen oxalate ion can be seen as the result of a dissociative adsorption of the oxalic acid molecule on the TiO_, surface (HOOC- COOHad s ~ HOOC-CO0~ + H + ads)" The hydrogen oxalate is adsorbed on a Ti ion and the proton goes to a bridged oxygen atom of the polymer. When the hydrogen oxalate is adsorbed through a single oxy= gen atom from the carboxylate group (see Fig. 3a, the carboxyl group is parallel to the polymer plane), the adsorption energy is very small, 18.7 kcal/mol. When the other oxygen atom of the carboxylate is also adsorbed (see Fig. 3b), the adsorption energy is 83 kcal/mol. This adsorption mode is general for monocarboxylic acids such as the formic acid [29,35]. The heat of adsorption is slightly inferior to that of the formic acid, 85 kcal/mol, because of the attrac- tive effect of the COOH group. Since the oxalic acid is a diacid, the adsorption may involve the carbonyl of the carboxyl group instead of that of the carboxyl- ate (see Fig. 3c). Then, the adsorption energy be- comes slightly larger, 85.7 kcal/mol.

in this later adsorption mode, the geometry of the adsorbate is planar with a strong internal hydrogen bonding. The oxygen-hydrogen distance is 2.1 A with a Mulliken overlap population, OP--0.023. From this OP, we expect a high value for the hydro- gen bonding. For instance, in the water dimmer (H,O), [12] the hydrogen bond has an OP of 0.017 and a bonding energy of 7.3 kcal/mol.

Clearly, the best adsorption mode for the hydro- gen oxalate involves two oxygen atoms. These oxy- gen atoms can be either from the carboxylate group or from the carboxylate and the carboxyl groups.

9.2. Adsorption of the oxalaw ion, C20 ~ -

H

Fig. 4. Adsorption of tile oxalate in a twisted conformation, the twist angle is 15 ° .

cleavages. The oxalate is complexed in a twisted geometry by two metal cations; the twisted angle is 15 ° . The protons are transferred to oxygen atoms of the polymer (Fig. 4). The complexation of the ox- alate ion by two metal cations was found on hematite [36,37], no data are available for TiO 2.

There are different modes for the protonation of the polymer as already found for the dissociative adsorption of water [12]. In the best proton distribu- tion, protons alternate on each side of the polymer. The adsorption energy of the oxalate is 101.7 kcal/mol; it is larger than that of the hydrogen oxalate.

We have also calculated the adsorption of the oxalate ion through the two oxygen atoms of the same CO, group. This adsorption is not favourable, the adsorption energy is only 44.0 kcal/mol. This system should be unstable and decompose into two CO, molecules). The CC Mulliken overlap popula- tion is very negative, O P - -0.15; this value sug- gests a repulsion between the two CO, groups. The adsorption of the oxalate by one CO, group, (TiO)2C-CO 2, shows similarities with the (HO).,C- CO., system obtained by a proton transfer in the oxalic acid molecule; such system is unstable and undergoes decarboxylation as found from previous calculations [ 19].

From these calculations, we conclude that the oxalate is more stable than the hydrogen oxalate on TiO, surfaces.

9.3. Adsorption formally resulting from basic cleavages, HOOC- + CO and OC +- + CO

The formation of the oxalate on the TiO 2 surface can be seen as the result of two successive acidic

The heterolytic dissociation of an acid can be either acidic (O-H cleavage) or basic (C-OH cleav-

A. Fahmi et a l . / Surface Science 343 (1995) 261-272 269

a)

P

©

I .... 0 H'2~44 A

,o--I- I- O ~ Ti 0 ] / / / Ti

/ H

f b)

Fig. 5. (a) Dissociative adsorption of the oxalic acid associated with a basic cleavage (HOOC-COOH ~ HOOC-CO~., + HO~). HOOC= + CO is adsorbed in a bridging position. (b) Dissociative adsorption of ~he o×alic acid associated with two cleavages (acidic and basic: HOOC=COOH ~ H + - ,,d,, + 02C-CO,,,*t,, + HO,~,). Since the C=C bond is elongated, this adsorption mode leads to the decomposition of the oxalic acid on the surface.

age). The basic cleavage of the oxalic acid on a TiO 2 surface leads to a doubly-coordinated carboxylate and a singly-coordinated hydroxyl group (Fig. 5a). There is an internal hydrogen bonding in the carbox- ylate; the oxygen-hydrogen distance is 2.44 A and the overlap population is 0.011.

Avoiding to speculate on the validity of a mecha- nism involving a basic cleavage for an acidic species, we can conceive this adsorption mode simply as an alternative distribution of fragments that may be generated from an acidic cleavage followed by an exchange between the OR and OH groups on the surface. Let us note however in favour of a basic cleavage mechanism, that, for the oxalic acid in the

gas phase, such cleavage is less endothermic than the acidic cleavage by 35 kcal/mol:

HOOC-COOH ---> HOOC-CO ÷+ HO- ,

AH = 294.13 kcal/mol.

The adsorption energy is 44.4 kcal/mol. When the adsorption is by one function, the basic cleavage (Fig. 5a) is more favourable than the acidic cleavage (Fig. 3a) (see Section 9.1). However, while the singly-coordinated carboxylate that results from an acidic cleavage leads to the adsorption of a second function (Figs. 3b and 3c) and to an increase of the adsorption energy by a factor of five, the absence of similar possibility for the basic cleavage makes this process unfavourable.

The adsorption of the dication +OC-CO + that results from two successive basic cleavages is un- favourable (Ead s = 34.8 kcal/mol). The correspond- ing gas phase reaction is highly endothermic:

HOOC-CO + ~ + OC-CO + + HO -,

AH = 510.9 kcai/mol.

Finally, we have considered the adsorption of the -O2C-CO + species that results from the dehydra- tion of the oxalic acid (acidic and basic cleavages). This would be a possibility to have the adsorption of two functions, one of them resulting from a basic cleavage. The corresponding geometry IS shown in Fig. 5b. The corresponding gas phase reaction is less endothermic than two acidic or basic cleavages.

HOOC-COOH ~ -O2C-CO+ + HO- + H +,

A H = 418.2 kcal/mol.

The calculated adsorption energy is 71.6 kcal/mol; this value is inferior to that obtained for the oxalate. As the -O2C-CO + species is a charged representation of the CO2-CO complex, this adsorp- tion mode raises the question of the decomposition of the oxalic acid on the surface. We shall discuss this matter in Section 11.

10. Adsorption of the oxalate on anatase

Since the oxalate is more stable than the oxalic acid or the hydrogen oxalate on the futile polymer,

270 A. Fahmi et al. / Surface Science 343 (1995) 261-272

C~o H~ ° ! /

H

Fig. 6. Dissociative adsorption of the oxalic acid on the anatase polymer. The geometry of the oxalate is planar and the protons are in the polymer phme.

we limit our discussion in this section to the oxalate adsorption. Dissociative adsorption of the oxalic acid on the zigzag chain of anatase involves the protona- tion of singly-coordinated oxygen atoms (which are more basic than doubly-coordinated oxygen atoms of the rutile polymer [12]). The protons are in the polymer plane (Fig. 6). The geometry of the oxalate is planar. The oxygen-oxygen distance when the oxygen atoms lie above the titanium atoms is long (3.10 ~) as compared with that of the rutile case (3.01/~, the c parameter from the bulk) and closer to that of the oxalate in the gas phase (2.14 J.).

The adsorption energy, 128.2 kcal/mol, is larger than that on the rutile polymer by 26.5 kcal/mol. The anatase structure appears to be more reactive because of the basicity of the singly-coordinated oxygen atom [ 12]. The distortion of the oxalic acid is not the driving force. Indeed, the adequation of the oxalic acid to the surface is not better on the anatase surface than on the rutile one. On the contrary, on the futile polymer the oxalate is twisted and the O-O distance becomes close to that of the oxalate in the gas phase. Such an adaptation is not possible on the anatase polymer since the molecule remains planar. The large O-O distance in the adsorbate is however not that destabilizing, since the calculated adsorption energy is large.

The difference in reactivity between the two phases allows us to explain the crystallization pro- cess of TiO, in presence of oxalic acid. Since the adsorption is stronger on anatase, the oxalic acid is adsorbed on the first crystallites of anatase and by

that the anatase formation is inhibited. The adsorp- tion on rutile is weaker than on anatase, whence the crystallization leads to rutile structure.

11. Decomposition of oxalic acid

11.1. Gas phase decomposition

In the gas pi~as~, the thermal decomposition of oxalic acid yields CO_, + HCOOH [38]. The study of UV photolysis [39] suggests that two processes oc- cur, yielding CO, + HCOOH and CO 2 + CO + H 2 O. The calculated enthalpies for these reactions, in agreement with previous calculations [19], show exothermic reactions:

HOOC-COOH --* CO 2 + HCOOH,

AHcalc = - 12.1 kcal/mol

(experimental value: - 9 . 6 kcal/moi [40]; AHc.qc = -16 .6 kcal/mol when intermolecular interactions between products are allowed),

HOOC-COOH ~ CO., + CO + H 2 O,

A Hc,,i c - - 7.2 kcal / mol

(experimental value: - 3.3 kcal/mol [40]; AH~,,i ,, -- 16.0 kcal/mol when intermolecular interactions

between products are allowed).

1 i.2. Decomposition on the Ti02 surfaces

In order to estimate the enthalpies of these two reactions on TiO., surfaces, we first calculate adsorp- tion energies of all the products, H 20, HCOOH, CO, and CO., independently. For such calculations, we used the rutile polymer and consider the best adsorp- tion mode for each adsorbate: dissociative adsorption for water (H,O ~ H,ds + OHad~) and formic acid (HCOOH ~ H,id, ~ + HCOO, d,~) at saturation ( 0 - 1). For water, the hydroxyl group is adsorbed above a Ti atom and the proton is transferred to an oxygen atom of the polymer [ 12]. For formic acid, the carboxylate is adsorbed between two Ti atoms and the proton is transferred to an oxygen atom of the polymer [29]. For CO [13] and CO 2 [28] the best adsorption modes correspond to perpendicularly adsorbed molecules above Ti atoms. We have considered both the 0 - 1

A. Fahmi et aL / Surface Science 343 (1995) 261-272 271

Table 2 Adsorption energies in kcai/mol for various molecules on the rutile polymer at 0 = 1 and 0 = 1 /2 coverages

Adsorbed system 0 = 1 0 = 1 /2

H 2Odiss 63 a _ HCOOHdiss 85 b _ CO 15.2 c 19.3 c CO 2 10.4 d 19.0 d

CO., + HCOOH diss 95.4 104.0 CO 2 + CO + H20 diss 88.6 101.3

H 2C204 diss 101.7 -

The subscript "diss" refers to dissociatively adsorbed species. a Ref. [12]. b Ref. [29]. c Ref. [13]. d Ref. [281.

and 0 = 1 / 2 coverages. The various heat of adsorp- tions are given in Table 2. The sum of these adsorp- tion energies give numbers that are very close to the value for the adsorption energy of the oxalate, 101.7 kcal/mol. From these values and the gas pha~se values, we derive the following values for the disso- ciation on the surface:

HOOC-COOHdiss - , CO 2 + HCOOHoiss,

AH~a~ = - 5 . 8 kcal/mol ( - 14.4 at 0 = 1/2)

(to be compared with - 16.6 kcal/mol),

HOOC-COOHdi~ -~ CO 2 + CO + H 2Odi.~,,,

AHcalc = +5.9 kcal/mol ( +6.8 at 0 = 1 /2)

(to be compared with - 16 kcal/mol). The surface decomposition does not seem to be

favoured by the adsorption. According to these val- ues, only the decomposition leading to the formate and the carbon dioxide would remain possible.

Finally, we performed a calculation on the coad- sorption of the three molecules CO2ads + COad s + (H + HO),,ds on the polymer; this requires a unit cell containing three titanium atoms and thus, the adsorp- tion energy has to be compared with an adsorption energy for the oxalate larger than 101.7 kcal /mol since the adsorption energy increases when the cov- erage decreases. The calculated value is only 31.8 kcal /mol showing that coadsorption effects are not favourable at this coverage (saturation).

12. Conclusion

In this paper, we have presented a periodic Hartree-Fock ab initio calculation of the adsorption of the oxalic acid on TiO 2 surfaces. We have used polymers as models for rutile and anatase surfaces. Various adsorption modes for oxalic acid molecule, hydrogen oxalate and oxalate ions were investigated. The best adsorption modes always involve two oxy- gen atoms of the adsorbate. The dissociative adsorp- tion is more favourable than the molecular adsorp- tion; our results clearly show that the oxalate com- plexed by two metal cations is the most stable form of the oxalic acid on TiO 2 surfaces. This is a study of the thermodynamically stable adsorption. We rec- ognize that kinetics may limit the accessibility of the stable configurations. The decomposition reactions of the oxalic acid that are exothermic in the gas phase are not favoured by the adsorption on the oxide surface. The comparison of the adsorption energy of the oxalate on rutile and anatase polymers explains crystallization experiments of TiO 2 in pres- ence of oxalic acid. The adsorption energy is larger on anatase. Therefore, the oxalic acid is adsorbed on the growing anatase preventing the formation of the bulk structure. The adsorption on ruffle is weaker and the crystal growth leads to the ruffle structure.

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