Hydrogen Chemisorption on Gallium Oxide Polymorphs - Langmuir

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Langmuir 2005, 21, 962-970

Hydrogen Chemisorption on Gallium Oxide Polymorphs Sebastia´n E. Collins, Miguel A. Baltana´s, and Adrian L. Bonivardi* Instituto de Desarrollo Tecnolo´ gico para la Industria Quı´mica (CONICET, UNL) Gu¨ emes 3450, S3000GLN Santa Fe, Argentina Received July 23, 2004. In Final Form: October 29, 2004 The chemisorption of H2 over a set of gallia polymorphs (R-, β-, and γ-Ga2O3) has been studied by temperature-programmed adsorption equilibrium and desorption (TPA and TPD, respectively) experiments, using in situ transmission infrared spectroscopy. Upon heating the gallium oxides above 500 K in 101.3 kPa of H2, two overlapped infrared signals developed. The 2003- and 1980-cm-1 bands were assigned to the stretching frequencies of H bonded to coordinatively unsaturated (cus) gallium cations in tetrahedral and octahedral positions [ν(Ga(t)-H) and ν(Ga(o)-H), respectively]. Irrespective to the gallium cation geometrical environment, (i) a linear relationship between the integrated intensity of the whole ν(Ga-H) infrared band versus the Brunauer-Emmett-Teller surface area of the gallia was found and (ii) TPA and TPD results revealed that molecular hydrogen is dissociatively chemisorbed on any bulk gallium oxide polymorph following two reaction pathways. An endothermal, homolytic dissociation occurs over surface cus-gallium sites at T > 450 K, giving rise to Ga-H(I) bonds. The heat and entropy of this type I hydrogen adsorption were determined by the Langmuir’s adsorption model as ∆hI ) 155 ( 25 kJ mol-1 and ∆sI ) 0.27 ( 0.11 kJ mol-1 K-1. In addition, another exothermic, heterolytic adsorption sets in already in the low-temperature region. This type of hydrogen chemisorption involves surface Ga-O-Ga species, originating GaO-H and Ga-H(II) bonds which can only be removed from the gallia surface after heating under evacuation at T > 650 K. The measured desorption energy of this last, second-order process was equal to 77 ( 10 kJ mol-1. The potential of the H2 chemisorption as a tool to measure or estimate the specific surface area of gallia and to discern the nature and proportion of gallium cation coordination sites on the surface of bulk gallium oxides is also analyzed.

1. Introduction Gallium oxide has become an important material in several processes involving breaking or formation of hydrogen-containing bonds: polycrystalline semiconducting Ga2O3 thin films represent a promising new basic material for sensors used to detect hydrogen,1-5 and catalysts containing supported gallium are known to be active in light alkane dehydrogenation and aromatization (Cyclar process).6-9 Moreover, some of us have reported a 500-fold enhancement of the turnover rate to methanol from H2/CO2 using Ga2O3-Pd/SiO2 catalysts as compared to Pd/SiO2 from H2/CO210 and gallia alone is able to dissociate dihydrogen at temperatures higher than 500 K to hydrogenate adsorbed CO2, stepwise, from formate to methoxy groups.11,12 * To whom correspondence should be addressed: Dr. Adrian L. Bonivardi, INTEC, Gu¨emes 3450, S3000GLN Santa Fe, Argentina. Telephone: +54(342)4559175. Fax: +54(342)4550944. E-mail: [email protected]. (1) Fleischer, M.; Meixner, H. Sens. Actuators, B 1992, 6, 257. (2) Pohle, R.; Fleischer, M.; Meixner, H. Sens. Actuators, B 2000, 68, 151. (3) Weh, T.; Frank, J.; Fleischer, M.; Meixner, H. Sens. Actuators, B 2001, 78, 202. (4) Trinchi, A.; Wlodarski, W.; Li, Y. X. Sens. Actuators, B 2004, 100, 94. (5) Trinchi, A.; Kaciulis, S.; Pandolfi, L.; Ghantasala, M. K.; Li, Y. X.; Wlodarski, W.; Viticoli, S.; Comini, E.; Sberveglieri, G. Sens. Actuators, B 2004, 103, 129. (6) Ono, Y. Catal. Rev.sSci. Eng. 1992, 34, 179. (7) Carli, R.; Le Van Mao, R.; Bianchi, C.; Ragaini, V. Catal. Lett. 1993, 21, 265. (8) Takahara, I.; Saito, M.; Inaba, M.; Murata, K. Catal. Lett. 2004, 96, 29. (9) Meitzner, G. D.; Iglesia, E.; Baumgartner, J. E.; Huang, E. S. J. Catal. 1993, 140, 209. (10) Bonivardi, A. L.; Chiavassa, D. L.; Querini, C. A.; Baltana´s, M. A. Stud. Surf. Sci. Catal. 2000, 130D, 3747. (11) Collins, S. E.; Baltana´s, M. L.; Garcı´a Fierro, J. L.; Bonivardi, A. L. J. Catal. 2002, 211, 252.

The oxidation state, location, and type of Ga sites during those catalytic reactions have also been of concern in the literature. Some authors have postulated the formation of mobile Ga2O species on Ga-doped H-ZSM5, assuming some interaction of these gallium suboxide moieties with the lattice of the zeolite.13,14 Meitzner and co-workers9 have posited that the active form of Ga, that is, the GaHx species during the conversion of light alkanes to aromatics, is only achieved under a reductive atmosphere and at a high temperature. In the case of CO2 hydrogenation on Ga2O3/SiO2, Collins et al.11 claim that Gaδ+-H surface species (δ < 2) react to give the hydrogenated oxycarbonaceous intermediates from chemisorbed carbon dioxide. The specific role of the geometric environment or coordination of gallium ions has been less emphasized in catalytic materials. Only bulk spectroscopic techniques, such as X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), have been used to study the coordination of gallium ions either on pure or on supported gallium oxides.15,16 Particularly, Shimizu et al. found that NO reduction with methane was a structure-sensitive reaction, linked to the presence of GaO4 tetrahedral active species.16 However, no surface technique has been developed at present to discriminate the coordination of surface gallium ions. (12) Collins, S. E.; Baltana´s, M. A.; Bonivardi, A. L. J. Catal. 2004, 226, 410. (13) Me´riaudeau, P.; Naccache, C. Appl. Catal. 1991, 73, L13. (14) Carli, R.; Le Van Mao, R.; Bianchi, C. L.; Ragaini, V. Catal. Lett. 1993, 21, 265. (15) Nishi, K.; Shimizu, K.; Takamatsu, M.; Yoshida, H.; Satusma, A.; Tanaka, T.; Yoshida, S.; Hattori, T. J. Phys. Chem. B 1998, 102, 10190. (16) Shimizu, K.; Takamatsu, M.; Nishi, K.; Yoshida, H.; Satsuma, A.; Tanaka, T.; Yoshida, S.; Hattori, T. J. Phys. Chem. B 1999, 103, 7542.

10.1021/la0481389 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/24/2004

H2 Chemisorption on Gallium Oxide Polymorphs

Langmuir, Vol. 21, No. 3, 2005 963 Table 1. Ga2O3 Polymorphs

sample R-Ga2O3 γ-Ga2O3c β-Ga2O3(N)c β-Ga2O3(O) b

precursora

calcination crystal surface area time phase BET-N2 T (K) (h) (XRD) (m2 g-1)

Ga(NO3)3‚xH2O 823 Ga(NO3)3‚xH2O 823 Ga(NO3)3‚xH2O 923 1073 Ga2O3

6 8 6 5

R γ β β

20 105 64 12

a Both Ga(NO ) ‚xH O and commercial gallium oxide were 3 3 2 supplied by Strem Chemicals (99.99 and 99.998% Ga, respectively). b Precipitated from gallium nitrate with ammonia in aqueous solution. c Precipitated from gallium nitrate with ammonia in ethanol.

Figure 1. Powder XRD patterns (Cu KR radiation) of gallium oxide samples.

This work examines the interaction of molecular hydrogen over different gallium oxides crystal phases by in situ Fourier transform infrared (FTIR) spectroscopy, between room temperature and 723 K, focusing on the coordination of surface gallium ions and the mechanism of formation of Ga-H species. 2. Experimental Section 2.1. Materials. A pure phase of R-Ga2O3 was prepared by adding a NH4OH aqueous solution (pH ) 10) to 4 wt % Ga(NO3)3‚ xH2O in doubly distilled (DD) water, at room temperature until no further precipitated was formed.17 The precipitated gel was immediately filtrated under vacuum and washed several times with DD water until no residual NO3- anions were detected in the washing water by UV spectroscopy (λ ) 270 nm, optical path ) 1 cm). The gel was then aged overnight at 373 K, and, finally, it was calcined in air at 823 K (6 h) to give the R-Ga2O3 sample. γ- and β-Ga2O3 phases were synthesized following procedures similar to those reported by Otero Area´n and co-workers.17-20 Hydrated gallium hydroxide gel was obtained from the addition of an ammonia ethanolic solution (50% v/v) to 7 wt % Ga(NO3)3‚ xH2O in ethanol. This gel was filtered under a vacuum and washed with ethanol at room temperature until no NO3- anions were detected in the washing solution by UV spectroscopy. The washed gel was then dried at 343 K (1 h) and next calcined at 823 K (8 h), in air, to obtain the γ-Ga2O3 polymorph. A portion of this last material was further calcined in air at 923 K (6 h) to obtain the most stable form of gallium oxide, that is, β-Ga2O3 [labeled as β-Ga2O3(N)]. Another β-gallia sample was prepared by calcination of a commercial gallium oxide in air at 1073 K during 6 h [labeled as β-Ga2O3(O)]. The crystallographic phase of each gallium oxide type was determined by X-ray diffraction spectrometry (XRD) using a Shimadzu XD-D1 apparatus (Cu KR radiation). The obtained diffraction patterns are shown in Figure 1. A Micromeritics Accusorb 200 unit was used to determine the Brunauer-Emmett-Teller (BET) surface areas (SBET) of the gallium oxides. The samples were previously outgassed at 473 K for 3 h under dynamic vacuum (base pressure ) 1.33 × 10-4 Pa), and the nitrogen adsorption isotherms were measured at 77 K. Table 1 shows the main characteristics of the gallia polymorphs used in this study. (17) Lavalley, J. C.; Daturi, M.; Montouillout, V.; Clet, G.; Otero Area´n, C.; Rodrı´guez Delgado, M.; Sahibed-dine, A. Phys. Chem. Chem. Phys. 2003, 5, 1301. (18) Rodrı´guez Delgado, M.; Otero Area´n, C. Mater. Lett. 2003, 57, 2292. (19) Otero Area´n, C.; Lo´pez Bellan, A.; Pen˜arroya Mentruit, M.; Rodrı´guez Delgado, M.; Turnes Palomino, G. Microporous Mesoporous Mater. 2000, 40, 35. (20) Rodrı´guez Delgado, M.; Morterra, C.; Cerrato, G.; Magnacca, G.; Otero Area´n, C. Langmuir 2002, 18, 10255.

2.2. In Situ FTIR Studies. Pressing 30 mg of powder at 5 ton cm-2 made self-supported wafers of each gallia sample of 13 mm in diameter. These wafers were placed in turn into an infrared Pyrex cell with water-cooled NaCl windows, which was attached to a conventional manifold system, as previously described.21 To eliminate the artificial bands in the 3000-2800 cm-1 region that arise from oil contamination during wafer preparation, which are attributed to the C-H stretching modes,11 an in situ cleaning pretreatment of each wafer had to be performed before the hydrogen adsorption-desorption experiments took place. This was accomplished by (a) heating under O2 (100 cm3 min-1, 5 K min-1) from 298 to 723 K and then (b) cooling to 298 K under vacuum (base pressure ) 1.33 × 10-4 Pa). After the cleaning procedure, temperature-programmed adsorption equilibrium (TPA) experiments were carried out by flowing H2 or D2 (100 cm3 min-1) through the cell at atmospheric pressure (101.3 kPa), using the following ramps of temperature: (R1) heating from 298 to 723 K (5 K min-1); (R2) cooling to 373 K (3 K min-1); and (R3) heating again from 373 to 723 K (5 K min-1). Subsequently, each sample was cooled to 373 K under H2 (or D2), then it was evacuated (base pressure ) 1.33 × 10-4 Pa), and a temperatureprogrammed desorption (TPD) experiment was performed, by heating each wafer from 373 to 723 K (10 K min-1) under vacuum. After 60 min under vacuum at this last temperature, the sample was gradually cooled to allow reference IR spectra of the clean wafers to be taken. Additionally, isothermic adsorption experiments were carried out at 723 K by flowing H2/N2 mixtures at atmospheric pressure. A Shimadzu 8210 FTIR spectrometer was employed to acquire in situ infrared spectra in transmission mode using a DLATGS detector (4 cm-1 resolution, 100 scans). The spectra were further processed by employing the Microcal Origin 4.1 software (with a peak fitting module): background correction of the spectra was achieved by subtracting the spectra of the “clean wafers” at each temperature, and a Lorentzian sum function was used to fit the overlapping bands and measure peak areas and intensities.22 It is worth mentioning that no change in the original XRD pattern of any gallia sample was detected after the in situ FTIR experiments. Hydrogen (AGA ultrahigh-purity grade 99.999%) and nitrogen (AGA high-purity 99.998%) were further purified through MnO/ Al2O3 and molecular sieve (3 Å, Fisher Co.) traps to eliminate oxygen and water impurities, respectively. Oxygen (AGA research grade 99.999%) was passed through a molecular sieve (3 Å, Fisher) and Ascarite traps to remove water and carbon dioxide, respectively. Deuterium (Scott C.P. grade 99.7%) was introduced into the cell without further purification.

3. Results and Discussion 3.1. Surface Gallium Sites after Hydrogen Adsorption. Infrared spectra collected during the TPA of H2 on each gallia sample revealed in all cases, from 500 K (R1) onward, the evolution of two partially overlapped IR signals at 2003 and 1980 cm-1. Both bands grew together, reaching a plateau from 673 K. The origin of (21) Cabilla, G. C.; Bonivardi, A. L.; Baltana´s, M. A. Catal. Lett. 1998, 55, 147. (22) Ingle, J. D.; Crouch, S. R. Spectrochemical Analysis, 1st ed.; Prentice Hall: Upper Saddle River, NJ, 1988; p 211.

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Collins et al.

Figure 4. Total integrated infrared absorbance of the stretching Ga-H band versus the BET surface area of the gallium oxide samples.

Figure 2. Infrared spectra of the 2200-1800-cm-1 region under flowing H2 (100 cm3 min-1, 101.3 kPa) at 723 K, for the full set of gallium oxide polymorphs. The insets show the 1500-1250cm-1 IR region using D2 under identical experimental conditions.

Figure 3. Adsorption isotherm of H2 on the R-Ga2O3 sample at 723 K, under H2/N2 flowing mixtures (100 cm3 min-1).

these bands was attributed to gallium-hydrogen bond formation and was assigned to the stretching mode of Ga-H, ν(Ga-H).11 Figure 2 shows the IR spectra of each gallia polymorph and its computational resolution at 723 K, the highest temperature that was used. Besides those bands, a broad band around 3500 cm-1 always developed in all spectra. This broad band was assigned to the stretching mode of O-H bonded to Ga sites [ν(GaO-H)] and will be considered later. The TPA experiments, using D2 instead of H2, showed two bands at 1430 and 1420 cm-1 with an evolution similar to those of the TPA of hydrogen. The experimental ratios of the ν(Ga-H)/ν(Ga-D) frequencies were equal to 1.39 and 1.40, respectively, in agreement with the theoretical ratio of 1.40 expected for the H-D isotopic exchange of Ga-H species [ν(Ga-H)/ν(Ga-D) ) (µGaD/µGaH)1/2, where µGaD and µGaH are the reduced masses of GaD and GaH, respectively]. Additionally, the isothermal adsorption of H2 at 723 K was studied at different hydrogen partial pressures, by flowing H2/N2 mixtures. As an example, Figure 3 shows the total integrated absorbance for the ν(Ga-H) band on R-gallia plotted against the partial pressure of H2. It is

clear that above 50 kPa of H2 the saturation value (i.e., the highest concentration of Ga-H species) was already achieved. For the saturation value of the Ga-H species on the full set of gallium oxide polymorphs, Figure 4 nicely shows that the total integrated absorbance of the ν(Ga-H) signals is proportional to the BET surface area of each sample (723 K, 101.3 kPa of H2). These last results prove unambiguously that the Ga-H species formed after the adsorption of H2 on gallia polymorphs are located over the surface of these metal oxides. In a previous work, some of us studied the adsorption of H2 over gallium oxide silica-supported catalysts, for example, Ga2O3/SiO2 and Ga2O3-Pd/SiO2. The formation of Gaδ+-H species (δ < 2) was identified and characterized by means of X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy.11 Hence, on the basis of those findings and the present results, it is possible to postulate that hydrogen is dissociatively chemisorbed over the surface of each of the polymorphs of gallium oxide studied here, producing terminal Ga-H species on the surface with stretching frequencies at 2003 and 1980 cm-1. The percentage of each of the ν(Ga-H) bands formed upon hydrogen adsorption was clearly different for each gallium oxide polymorph (Figure 2). We also found that the percentage of each Ga-H infrared band and for each gallia crystal phase remained constant along the thermal evolutions. However, the assignment of these two signals coming from Ga-H bonds is not yet obvious. Certainly, the position of these bands cannot be attributed to the naturally occurring isotopes Ga69 (60.1%) and Ga71 (39.9%), because the difference between the wavenumbers for ν(Ga69-H) and ν(Ga71-H) should be just about 1 cm-1 and the integrated absorbance of these bands should parallel the statistical, natural mixture of the gallium isotopes regardless of the gallia polymorph. To attempt an explanation for the appearance of two bands at about 2000 cm-1 it is also possible to invoke the formation of Ga-H-Ga-, H-GaOH- or H-Ga-H-like bonds. Yet, in Ga-H-Ga bridges, the antisymmetric and symmetric stretching frequencies usually lie below 1700 cm-1, that is, far away from 2000 cm-1. For example, Himmel et al.23 synthesized and characterized dihydrobridged species, Ga(µ-H)2Ga, in solid Ar matrixes, and they found that the main IR frequency was located at 1002 cm-1, which was in close agreement with the results (23) Himmel, H. J.; Manceron, L.; Downs, A. J.; Pullumbi, P. J. Am. Chem. Soc. 2002, 124, 4448.

H2 Chemisorption on Gallium Oxide Polymorphs

of Xiao et al. for said species.24 Notwithstanding, digallane, H2Ga(µ-H)2GaH2, was succefully synthesized by Downs’ group from gallane oligomers [(GaH3)n with n g 4]. The measured stretching frequencies of Ga-H-Ga were 1273 and 1202 cm-1 for digallane and 1705 and 950 cm-1 for the oligomers (antisymmetric and symmetric modes, respectively).25,26 Qi et al.27 studied the hydrogen adsorption on GaAs and found that bridged gallium hydride species had a broad IR band around 1400 cm-1. Hence, the Ga-H-Ga bond formation should be ruled out in our case, and a similar conclusion is applicable for the H-GaOH species formation, which has a stretching ν(Ga-H) band at 1670 cm-1.24,28 As for the presence of two terminal hydrogen atoms bonded to one Ga atom, H-Ga-H, Xiao et al. reported that gallium dihydride species, GaH2, showed symmetric and asymmetric stretching signals at 1799 and 1727 cm-1, that is, a frequency splitting close to 80 cm-1.24 However, Pulham et al. reported two terminal hydrogen vibrational bands in digallane: 1993 and 1976 cm-1 attributed to dipole changes in the directions perpendicular and parallel to the Ga‚‚‚Ga axis.26 Thus, it is unclear to preclude the formation of H-Ga-H species by just only considering the separation between these signals. A careful examination of the spectra in Figure 2 allows us to recognize that the relative percentage of each IR band at 2003 and 1980 cm-1 changes according to the gallia-bulk crystal phase type, which suggests the formation of terminal Ga-H bonds on coordinatively unsaturated Ga (cus-Ga) surface sites with different geometric environments. From a structural standpoint, the monoclinic crystalline structure of β-Ga2O3 has two kinds of coordinated Ga3+ ions, each one in the same bulk proportion, namely, octahedral and tetrahedral, hereafter referred to as Ga(o) and Ga(t).29,30 β-Ga2O3 presents two cleavage planes, the far more frequent and stable is the (100), the second being the (001).31,32 The first plane is mainly formed by oxygen bonded to Ga(o) and Ga(t) in identical surface proportion.33,34 In the case of R-Ga2O3, which has a corundum structure, all Ga3+ ions are in a sixfold coordination.17,35 Then, we can only expect Ga(o) on the surface for any cleavage plane of this last oxide. Finally, for γ-Ga2O3 which features a spinel structure with predominance of Ga3+ octahedrally coordinated,15,17 there is no preponderance reported, to the best of our knowledge, of any cleavage plane. Our infrared data show that the low-frequency band at 1980 cm-1 largely prevails on R-gallia and constitutes the major contributor to the integrated absorbance of the (24) Xiao, Z. L.; Hauge, R. H.; Margrave, J. L. Inorg. Chem. 1993, 32, 642. (25) Downs, A. J.; Goode, M. J.; Pulham, C. R. J. Am. Chem. Soc. 1989, 111, 1936. (26) Pulham, C. R.; Downs, A. J.; Goode, M. J.; Rankin, D. W. H.; Robertson, H. E. J. Am. Chem. Soc. 1991, 113, 5149. (27) Qi, H.; Gee, P. E.; Nguyen, T.; Hicks, R. F. Surf. Sci. 1995, 323, 6. (28) Hauge, R. H.; Kauffman, J. W.; Margrave, J. L. J. Am. Chem. Soc. 1980, 102, 6005. (29) Geller, S. J. Chem. Phys. 1960, 33, 676. (30) Massiot, D.; Farnan, I.; Gautier, N.; Trumeau, D.; Trokiner, A.; Coutures, J. P. Solid State Nucl. Magn. Reson. 1995, 4, 241. (31) Tomm, Y.; Reiche, P.; Klimm, D.; Fukuda, T. J. Cryst. Growth 2000, 220, 510. (32) Vı´llora, E. G.; Murakami, Y.; Sugawara, T.; Atou, T.; Kikuchi, M.; Shindo, D.; Fukuda, T. Mater. Res. Bull. 2002, 37, 769. (33) Gonza´lez, E. A.; Jasen, P. V.; Juan, A.; Collins, S. E.; Baltana´s, M. A.; Bonivardi, A. L. Surf. Sci., published online Nov 28, 2004 http:// dx.doi.org/10.1016/j.susc.2004.11.018. (34) Khol, D.; Ochs, Th.; Geyer, W.; Fleischer, M.; Meixner, H. Sens. Actuators, B 1999, 59, 140. (35) Marezio, M.; Remeika, J. P. J. Chem. Phys. 1962, 46, 1862.

Langmuir, Vol. 21, No. 3, 2005 965 Table 2. Fraction of Gallium Surface Sites on Octahedral (o) and Tetrahedral (t) Coordination Ga(o)

Ga(t)

sample

surfacea

bulk

surfacea

bulk

R-Ga2O3 γ-Ga2O3 β-Ga2O3(N) β-Ga2O3(O)

91 73 54 53

100b 62c 50b 50b

9 27 46 47

0b 38c 50b 50b

a This work. b From crystallographic data.29,35 XANES.15

c

Measured by

Ga-H signal in γ-gallia, while both bands have a similar extension on the two β-gallia samples (Figure 1). On the basis of the previous observations, it is reasonable to assign the bands at 2003 and 1980 cm-1 to the stretching mode of terminal Ga(t)-H and Ga(o)-H surface bonds, respectively. Table 2 presents the percentage of each Ga surface site calculated from the integrated absorbance of the 2003 and 1980 cm-1 signals of each sample. An excellent correlation between the Ga site coordination in the bulk for the gallia polymorphs and the assigned ν(Ga(o)-H) or ν(Ga(t)-H) surface signal is observed. Small deviations of the surface Ga coordination from the ideal or theoretical bulk composition are expected, after relaxation or reconstruction due to surface reordering. Also, in real powder samples, impurities of other crystal phases can be found. For example, the 9% of Ga(t) in our R-Ga2O3 sample can be attributed to the presence of -Ga2O3 impurities not detected by XRD.15 Nishi et al. also reported a tiny enrichment of Ga(o) in their synthetic β-gallia powders using X-ray absorption spectrometries, that is, XANES and EXAFS.15 Also, the broad bands in the XRD pattern for high-surface-area β-Ga2O3(N) (Figure 1) might be indicating the presence of a small remaining amount of the γ crystal phase. In any case, the correlation depicted in Table 2 is indeed remarkable. There is another aspect we judge critical to the unambiguous assignment of these IR signals to the Ga(o)-H and Ga(t)-H bonds, given by the quantum chemical calculation of the adsorption of hydrogen on the (100) surface of β-Ga2O3. Gonza´lez et al.33 modeled the (100) plane where, to provide adsorption sites for the adsorbate, the hydrogen adsorption process was conducted over oxygen vacancies. After H adsorption, the surface Ga-O overlap decreased while a Ga-H bond was formed on top of both Ga(o) and Ga(t). Moreover, the Ga-H overlap was found to be higher for Ga(t)-H than for Ga(o)-H. The bond energy of the former was predicted to be approximately 24% stronger than the last one.33 This, again, is consistent with the assigned higher stretching frequency for the Ga(t)-H vibration. In summary, we have identified and characterized by IR spectroscopy two kinds of Ga-H surface bonds, as endorsed by the comparison of the IR measured spectra over the surface of different well-defined Ga2O3 polymorphs with the spectra of known gallium hydride molecules and with the properties forecast by quantum chemical calculations. Furthermore, the characterization of these surface species is in agreement with predicted structural data. 3.2. Mechanistic Aspects of the H2 Adsorption. Figures 5 and 6 show the evolution of the infrared signals of ν(Ga-H) and ν(GaO-H) stretching modes recorded during the TPA and TPD experiments of H2 on the γ-Ga2O3 and R-Ga2O3 samples after the heating and cooling ramps. Identical qualitative infrared results were obtained for all crystallographic phases of gallium oxide. Upon heating from 323 to 723 K under hydrogen flow, the onset of the formation of the Ga-H bonds takes place

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Figure 5. Infrared spectra of the 4000-2600-cm-1 and 22001800-cm-1 regions during the TPA and TPD experiments of H2 on the γ-Ga2O3 sample: (A) heating from 298 to 723 K (R1), cooling to 373 K (R2), and heating again from 373 to 723 K (R3) under flowing H2 (100 cm3 min-1, 101.3 kPa) and (B) subsequently heating from 373 to 723 K under vacuum (TPD).

Collins et al.

in this first heating step, namely: oxygen vacancies over both octahedral and tetrahedral cationic sites are left over the surface of the gallium oxides in the flowing hydrogen atmosphere during this procedure, like on Ga2O3/SiO2 materials.11 After decreasing the temperature from 723 to 323 K, the intensities of the Ga(t)-H and Ga(o)-H stretching infrared signals decreased monotonically, in parallel, to a constant value of about 57% of the maximum intensity of the Ga-H band. This intensity value remained constant below 473 K (see R2 in Figures 5 and 6). The Ga-H band was fully regenerated upon heating against the gallium oxide samples under flowing hydrogen (see R3 in Figures 5 and 6). Hence, approximately 43% of both Ga(o)-H and Ga(t)-H bonds over the surface of Ga2O3 can be named type I, high-temperature adsorbed species [hereafter, GaH(I)], whereas the surface concentration of another “refractory”, residual Ga-H species [i.e., the 57% of the signal still remaining at low temperature, hereafter indicated as type II Ga-H species, or Ga-H(II), indistinctly] keeps constant under the steady pressure of molecular hydrogen gas flow. It is important to underline that the intensity of the ν(GaO-H) band remained constant while the Ga-H(I) formation/decomposition occurred (see the hydroxyl infrared region in Figure 5, R2 and R3), so that we unequivocally can conclude that the fraction of Ga-H(I) bonds is not directly associated to any hydroxylation/dehydroxylation of the gallia surface. (Such dehydroxylation, though, may well constitute a precondition for cus-Ga sites to become available). Results identical to the previous ones were obtained when similar experiments were carried out using different heating rates (3 K min-1 and 10 K min-1) or, moreover, when stepped heating was applied (15 min at each temperature), which allowed us to discard any hidden kinetic effect in the formation/decomposition of Ga-H species. Thus, it is possible to postulate that molecular hydrogen can be partially adsorbed on coordinatively unsaturated surface Ga(o) or Ga(t) cationic sites via a homolytic, reversible reaction

H2 + 2cus-Ga T 2Ga-H(I)

Figure 6. Evolution of the normalized intensity of the Ga-H stretching band as a function of the temperature during the TPA and TPD experiments of H2 on the R-Ga2O3 sample (nomenclature: idem Figure 5). The inset shows the normalized coverages for the Ga-H(II) and GaO-H bands (θGaH(II) and θGaOH, respectively) during the TPD experiment (see text for details).

over the gallium oxide surface at temperatures higher than 500 K (Figure 5). During this first heating ramp (R1) both octahedral and tetrahedral gallium surface sites are able to form Ga-H bonds (Figure 5, R1); the maximum Ga-H concentration is reached above 650 K. While increasing the temperature, a decrease in the intensity of the IR band belonging to hydroxyl surface groups,17-19,36 between 3750 and 2600 cm-1, was also observed, owing to a partial dehydroxylation (or reduction) of the gallia surface.11 Probably, cus-Ga surface cations are generated (36) Me´riaudeau, P.; Primet, M. J. Mol. Catal. 1990, 61, 227.

(1)

in an endothermic process, because this Ga-H(I) species decomposes at a low temperature. The evolution of the Ga-H(I) fractional coverage (θGaH(I)) as a function of the temperature for all the gallium oxide polymorphs used in this work and for both adsorption/ desorption processes (R3 and R2) is shown in Figure 7. Again, it is important to emphasize that the evolutions of both Ga(o)-H(I) and Ga(t)-H(I) fractional coverages were identical. Then, the θGaH(I) versus temperature plot suffices to represent the overall chemisorption of type I hydrogen and to measure the heat of H(I) adsorption on gallia (see below). Bianchi’s group has showed in recent studies that the heat of adsorption of different CO species chemisorbed on supported metal catalysts at several coverages can be successfully determined by studying the evolution of the intensity of the characteristic infrared bands with the adsorption temperature for a given pressure.37-43 Basi(37) Derrouiche, S.; Bianchi, D. Langmuir 2004, 20, 4489. (38) Bourane, A.; Dulaurent, O.; Bianchi, D. Langmuir 2001, 17, 5496. (39) Chafik, T.; Dulaurent, O.; Gass, J. L.; Bianchi, D. J. Catal. 1998, 179, 503. (40) Dulaurent, O.; Chandes, K.; Bouly, C.; Bianchi, D. J. Catal. 1999, 188, 237.

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The heat of type I hydrogen adsorption, as measured in our work, is in good agreement with the theoretical value estimated from bond dissociation energies (D0):

∆hI ) 2D0(Ga-H) - D0(H2) ) 149 kJ (mol H2)-1 (3)

Figure 7. Evolution of the fractional coverage of Ga-H(I) species (θGaH(I)) versus temperature during the heating and cooling ramps (R2 and R3), for the full set of gallia samples, under flowing H2 (100 cm3 min-1). Solid line: theoretical coverage according to the homolytic adsorption model.

cally, they performed simple isobaric experiments. So, assuming that the type I hydrogen equilibrium on the gallia surface corresponds to a simple elementary step (adsorption/desorption) rather than to a succession of elementary steps, we followed Bianchi’s approach, using Langmuir’s equation for dissociative chemisorption44 to calculate the heat (∆hI) and entropy (∆sI) of adsorption of type I hydrogen on gallia:

( ) ( ( ) ( pH2

θGaH(I) )

1/2

p0 pH2 1+ p0

exp

1/2

)

∆hI ∆sI 2R 2RT

)

∆hI ∆sI exp 2R 2RT

(2)

where pH2 stands for the hydrogen equilibrium pressure, taking the standard state of the gas as 101.3 kPa (p0), that is, perfect gas, and R stands for the universal gas constant. Because reversibility as well as the existence of a monolayer asymptote are necessary but not sufficient conditions for Langmuir behavior, we also performed a volumetric hydrogen adsorption experiment at 723 K, on γ-Ga2O3, and a linear Langmuir plot was obtained. The solid line in Figure 7 represents the best fit for the proposed adsorption model. The object function minimization was carried out by the Nelder-Mead down-simplex method.45 The criterion for optimization was the residual sum of squares between the measured fraction of surface hydrogen (θGaH(I)) and the corresponding model output. All computations were performed in the GNU Octave 2.1.36 (GPL) environment. The confidence interval for each parameter was estimated using the residual sum of squares following the procedure described by Bard.46 The obtained parameters for Langmuir’s adsorption model were ∆hI ) 155 ( 25 kJ mol H2-1 and ∆sI ) 0.27 ( 0.11 kJ mol H2-1 K-1. (41) Dulaurent, O.; Chandes, K.; Bouly, C.; Bianchi, D. J. Catal. 2000, 192, 237. (42) Dularent, O.; Courtois, X.; Perrichon, V.; Bianchi, D. J. Phys. Chem. B 2000, 104, 6001. (43) Bourane, A.; Nawdali, M.; Bianchi, D. J. Phys. Chem. B 2002, 106, 2665. (44) Clark, A. The Theory of Adsorption and Catalysis; Academic Press: New York, 1970; pp 35-67. (45) Nelder, J. A.; Mead, R. Comput. J. 1965, 7, 308. (46) Bard, Y. Nonlinear Parameter Estimation; Academic Press: New York, 1974; Chapter 5.

where D0 of Ga-H, as calculated using the average of the IR stretching for Ga(o)-H and Ga(t)-H following the procedure reported by Bradford and Vannice,47 is equal to 292 kJ mol-1 and D0 of H2 is equal to 435 kJ (mol H2)-1.48 Using B3LYP calculations, Wang and Andrews determined that the GaH molecule is generated from the endothermic reaction of Ga with H2 with a difference in energy equal to 155 kJ (mol H2)-1, that is, close to our experimental heat of adsorption for type I hydrogen.49 The value of ∆sI can be converted into the partial molar entropy of the adsorbed phase (sad), if the standard entropy of gaseous H2 (sH2) is taken into account. The sad value is referred to 1 mol of H2 and has to be divided by a factor of 2 to obtain an entropy value, sad′, per mole of adsorbed H atoms [i.e., sad′ ) (∆sI + sH2)/2]. The sH2 is almost constant in the range of temperature where H2 adsorption takes place on gallia [between 473 and 700 K, sH2 goes from 144 to 155 J (mol H2)-1 K-1 ],48,50 so that an average value of sad′ equal to 210 J mol H-1 K-1 is obtained. This sad′ value is significantly higher than those reported for hydrogen chemisorption on metals [e.g., a value equal to 70 J mol H-1 K-1 was reported for H on Pd(100) at low coverage],51 and, likewise, sad′ values for uncharged adsorbates higher than about 120 J mol-1 K-1 are uncommon in metals.52,53 However, β-Ga2O3 is a known n-type semiconductor with a relatively high band gap energy value (Eg ) 4.8 eV at 300 K)54 and R-Ga2O3 seams to have similar semiconductor behavior.55 Unlike metals, semiconductor materials can support ionization and other forms of charge localization. For example, silicon can support charges ranging between +1 and -2, and the ionization of such bulk defects results in the production of electron-hole pairs.56,57 If both charge carriers roam the crystal structure freely, no significant change in the entropy of the solid is produced. Conversely, if one of the carriers remains tightly localized near the defect, the lattice vibrational modes in the vicinity can soften, thus, resulting in a positive entropy of ionization (sion); the entropy increases regardless of the sign of the localized charge carrier.56,57 Such effects are rarely considered in experimental or theoretical treatments of adsorption/desorption. Actually, the β-Ga2O3 sensing property to H2 was tested on different devices, either in its pure form4,58 or covered by a layer of SiO2.3 Both types of devices have shown that the conductivity drastically increases after exposing the (47) Bradford, M. C. J.; Vannice, M. A. Ind. Eng. Chem. Res. 1996, 35, 3171. (48) CRC Handbook of Chemistry and Physics, 72nd ed.; Lide, D. L., Ed.; CRC Press: Boca Raton, FL, 1992; pp 9-107. (49) Wang, X.; Andrews, L. J. Phys. Chem. A 2003, 107, 11371. (50) Cox, J. D.; Medvedev, A. A. CODATA Key Values for Thermodynamics; Hemisphere Publishing Corp.: New York, 1984 (http:// webbook.nist.gov). (51) Behm, R. J.; Christmann, K.; Ertl, G. Surf. Sci. 1980, 99, 320. (52) Kuhn, W. K.; Szangi, J.; Goodman, D. W. Surf. Sci. 1994, 303, 377. (53) Christmann, K. Introduction to Surface Physical Chemistry; Steinkopff Verlag: Darmstadt, 1991; p 26. (54) Tippins, H. H. Phys. Rev. 1965, 140, 316. (55) Cho, S.; Lee, J.; Park, I.-Y.; Kim, S. Mater. Lett. 2002, 57, 1004. (56) Van Vechten, J. A.; Thurmond, C. D. Phys. Rev. B 1976, 14, 3539. (57) Van Vechten, J. A.; Thurmond, C. D. Phys. Rev. B 1976, 14, 3551. (58) Re´ti, F.; Fleischer, M.; Meixner, H.; Giber, J. Sens. Actuactors, B 1994, 19, 573.

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sensors to molecular hydrogen at temperatures higher than 673 K. It is known that an adsorbed electron donor, such as hydrogen, can act on the surface of crystalline n-type semiconductors such as Ga2O3, causing a band bending due to the partial charge transfer to the solid. Thus, the band bending creates an accumulation of charge near the surface, increasing the conductivity of the reactive oxide.59,60 The net effect is similar to a charge transfer, regardless of the sign, and an increase of the entropy of adsorption for the polarized/ionized species can be anticipated, that is, the entropy of formation of the partially ionized species exceeds that of the neutral species. The magnitude of sion for Ga2O3 is, to the best of our knowledge, unknown, but for a semiconductor the entropy of formation of these hole-electron pairs is equal to the negative change of the band gap energy with the temperature (sion ) -∂Eg/ ∂T).56,57 The sion/R value generally increases as Eg widens. With the assumption of a linear relationship between ∂Eg/ ∂T and Eg for several semiconductor materials,55-57,61 values of sion/R near 12 are quite likely for gallium oxide. This effect would result in a sad′ value approximately 100 J (mol H)-1 K-1 higher than those reported for chemisorption on metal substrates. Therefore, our experimental value of sad′ equal to 210 J (mol H)-1 K-1 for the endothermic, high-temperature (type I) hydrogen chemisorption on gallia may be now justified. Notwithstanding the simplicity of this physical picture, in the case of real applications such as the current case, it is difficult to ascertain the sign of the charge, the degree of charge localization, and the nature of the polarized/ ionized species. Nonetheless, our earlier XPS data showed that Gaδ+ (δ < 2) exists over the surface of silica-supported gallium oxide after dihydrogen adsorption.11 In this regard, it is important to highlight that Seebauer’s group has formerly proposed this physical picture, invoking adsorbate ionization on a semiconductor surface, to rationalize their unusually low pre-exponential factor for the desorption of NO from Cl-treated Fe2O3.62-64 Finally, the type I hydrogen adsorption being an endothermic process and considering the estimated errors of the calculated values for ∆hI and ∆sI, the mean value of ∆gI (∆gI ) ∆hI - T∆sI) at temperatures higher than 650 K is satisfactory (i.e., ∆gI < 0), which allows a θGa(I) ) 1 over the gallium oxides. However, great caution must be exercised when dealing with temperature-independent regression parameters (∆hI, ∆sI) in semiconductors. The type II chemisorbed hydrogen that still remains on the surface of gallia under 450 K down to room temperature can only be removed by evacuating at high temperature: the TPD trace of H2 presorbed on gallia shows that the intensity of the ν(Ga-H) bands decreases monotonically while heating under vacuum and the bands vanish above 650 K (e.g., see TPD from γ-Ga2O3 in Figure 5). Concurrently, the ν(GaO-H) band loses almost all of its intensity during the TPD ramp, in parallel with the Ga-H(II) intensity decay. The inset in Figure 6, where the normalized absorbances for the Ga-H(II) and the GaO-H bands on R-gallia (θGaH(II) and θGaOH, respectively) were plotted alongside, details this last finding. Nonetheless, the variations of the intensity of the IR bands of the OH groups deserve a closer examination. (59) Tanielian, M. Philos. Mag. B 1982, 45, 435. (60) Morrison, S. R. The Chemical Physics of Surfaces; Plenum Press: New York, 1977; p 36. (61) Ashcroft, N. W.; Mermin, N. D. Solid State Physics, 1st ed.; Holt, Rinehart & Winston: New York, 1976; p 566. (62) Blomiley, E. R.; Seebauer, E. G. Langmuir 1999, 15, 5970. (63) Dev, K.; Seebauer, E. G. Phys. Rev. B 2003, 67, 035312-1. (64) Seebauer, E. G.; Allen, C. E. Prog. Surf. Sci. 1995, 49, 265.

Collins et al.

The IR spectrum taken at 723 K during the TPD process (Figure 5, TPD), that is, where the sample had been repeatedly activated or prereduced under molecular hydrogen (steps R1 through R3), clearly shows that the intensity of the IR bands of the OH groups in the 38003000 cm-1 range is significantly lower than the intensity displayed after the first cooling stage, under vacuum, with further exposure to flowing hydrogen (Figure 5, R1, 373 K), that is, after the gallia sample was first preoxidized. Moreover, the intensity of the ν(GaO-H) bands for the prereduced sample at 373 K under vacuum (Figure 5, TPD, 373 K) was similar to the intensity of the hydroxyl bands under flowing hydrogen pressure (Figure 5, R2 or R3, 373 K), which rules out contamination from water traces in the H2 stream. Then, the R1 treatment of the preoxidized gallium oxide under flowing hydrogen leads to the partial dehydroxylation of the gallia surface. Admittedly, one could be tempted to think that a readsorption of residual H2O (residual partial pressure under the evacuation of the IR cell) during the cooling stage of the preoxidized gallia sample, leading to rehydroxylation of the oxide surface, might have occurred. However, no IR band at 1640 cm-1 [δ(HOH)] was observed (at any temperature nor during any stage), either. After depleting the gallium oxide surface of the GaH(I), Ga-H(II), and Ga-OH species at 723 K under vacuum (60 min), we re-admitted H2 gas into the cell. All these surface species were completely regenerated at 723 K. However, when the temperature of the cell was decreased, under vacuum, to 473 K and the gallium oxides were exposed again to molecular hydrogen, we recovered only the 57% of the total ν(Ga-H) signal, together with the full Ga-OH band similar in shape and intensity to the one depicted in the TPD at 373 K (Figure 5). Furthermore, by means of a temperature-programmed reaction experiment of CO over γ-Ga2O3, from 373 to 723 K, we were able to completely remove the hydroxyl layer of the oxide (this procedure was chosen, instead of the conventional heating in H2 followed by annealing in vacuo above 723 K, because this last method can totally remove the surface OH groups but simultaneously it can modify the structure of the material, e.g., decreasing the surface area). The flowing CO gas reacted with the OH groups to form formate species, which further decomposed to H2O and CO2. The resulting dehydroxylated sample was evacuated at 723 K and cooled to 473 K. After admitting H2 into the IR cell, only the ν(Ga-H) and ν(GaO-H) signals at ∼2000 and 3800-3000 cm-1, respectively, appeared. Again, only 57% of the Ga-H infrared signal was recovered at 473 K. So, we are certain that the GaH(II) species are correlated with the development of accompanying Ga-OH species. Therefore, we propose that the type II chemisorbed hydrogen on the gallia surface is formed via an exothermic, heterolytic chemisorption of molecular hydrogen on GaO-Ga sites produced during the first reduction/dehydroxylation under flowing H2 (R1 treatment):

The heterolytic adsorption of H2 involves different structural OH groups, such as mono-, bi-, and tricoordinated hydroxyl groups,17-19,36 without a specific preference to one of them, because at least three bands in the 3500 cm-1 region are observed (see Figure 5). Moreover, because the fractional coverage of both Ga(o)-H and Ga(t)-H evolved identically, like in the

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Langmuir, Vol. 21, No. 3, 2005 969

based on previous findings of Eischens and co-workers.74 At room temperature a fast H2 adsorption, followed by a slower process, was found.69-74 The first process was rapid and reversible, and appeared to be responsible for the hydrogen-deuterium exchange and ethylene hydrogenation reactions.69-73 By means of transmission IR spectroscopy both research teams agreed that H2 dissociates over activated ZnO yielding Zn-H and Zn-OH species. The overall heterolytic desorption model of dihydrogen (H2) on gallia can be modeled as a second-order process (the reverse of eq 4), as follows:75

dθGaH(II) dT Figure 8. Evolution of the fractional coverage of Ga-H(II) species (θGaH(II)) during the TPD of H2, under vacuum (base pressure ) 1.33 × 10-4 Pa). Solid line: theoretical coverage according to second-order heterolytic desorption model.

homolytic adsorption, this process seems to be insensitive to the coordination of surface gallium cations. In agreement with the stability pattern of our type II surface Ga hydride species, Me´riaudeau and Primet36 suggested that the process of Ga-H(II) bond formation is exothermic. These authors proposed that H2 chemisorbs heterolytically on R-Ga2O3, at 573 K, to produce Ga-H and Ga-OH species. They also studied the desorption of adsorbed H2 on R-Ga2O3 by increasing the temperature from 350 K until 773 K under vacuum and found that half of the normalized absorbance of the Ga-H and GaO-H signals was lost at about 580 K (Figure 4 in their paper), that is, close to the inflection point for the desorption of H2 in our gallium oxides samples at 573 K (Figure 8). The heterolytic chemisorption of H2 on other catalytically relevant metal oxides (such as Al2O3 and ZnO) has been proposed by some research groups as the usual (and only) chemisorption pathway, leading to the formation of M-H (M hydrides) and M-OH (hydroxyls) bonds. A particularly interesting case is the H2 adsorption on activated transition aluminas. Morterra and Magnacca,65 which have thoroughly discussed the main works published on this subject, conclude that the types of adsorbed H2 on alumina strictly depend on the contact temperature, as also highlighted by Amenomiya66 and Kazansky et al.67,68 Kazansky’s group conclusively demonstrated using an in situ IR diffuse reflectance technique that, at 77 K, molecular H2 exists on the surface of activated η-Al2O3. By increasing the temperature to 300 K, the alumina dissociatively chemisorbed H2 (or D2) by means of a heterolytic mechanism, giving several bands corresponding to Al-H and Al-OH species over the surface. Those surface species could be removed as molecular H2 by evacuation at temperatures higher than 370 K. However, at temperatures as high as 500 K, the dissociative chemisorption of hydrogen produced the same typical Al-H and Al-OH stretching bands but was accompanied by the formation of more stable aluminum hydride species, which, nevertheless, were absorbing IR radiation at the same frequency as the previous Al-H groups.67,68 The hydrogenation of olefins on ZnO inspired a series of studies of hydrogen activation to Kokes and Dent,69-73 (65) Morterra, C.; Magnacca, G. Catal. Today 1996, 27, 497. (66) Amenomiya, Y. J. Catal. 1971, 22, 109. (67) Kazansky, V. B.; Borovkov, V. Yu.; Kustov, M. L. Proc. 8th Int. Congr. Catal. 1984, 3, 3. (68) Kazansky, V. B.; Borovkov, V. Yu.; Zaitsev, A. V. Proc. 9th Int. Congr. Catal. 1988, 3, 1426. (69) Dent, A. L.; Kokes, R. J. J. Phys. Chem. 1969, 73, 3772. (70) Dent, A. L.; Kokes, R. J. J. Phys. Chem. 1969, 73, 3781.

)-

( )

2AdNs Ed θGaH(II) 2 exp β RT

(5)

where Ad refers to the Arrhenius preexponential factor, Ns is the total surface concentration of sites able to form Ga-H(II) and Ga-OH bonds, β stands for the heating rate, and Ed indicates the desorption energy. We estimated the Ns value from the volumetric hydrogen adsorption isotherm on γ-Ga2O3 at 723 K. An average total surface concentration of sites able to form Ga-H(II) and Ga-OH bonds equal to 1.3 × 1014 (H atom) cm-2 (which corresponds to 73% of the total amount of chemisorbed H2) was obtained.76 The kinetic parameters (Ad and Ed) were estimated by nonlinear regression. The ordinary differential equation (eq 5) was numerically solved using an implicit fourth-order Runge-Kutta routine, after reparametrizing the temperature dependence according to Kittrel’s method.77 The procedure to minimize the object function, the optimization criteria, and the methodology for calculating the confidence interval for each parameter were the same as those already described for Langmuir’s model for type I hydrogen adsorption. The agreement between the heterolytic desorption model and the experimental data is shown in Figure 8, where Ed ) 77 ( 10 kJ mol H2-1 and Ad ) (0.4 ( 0.1) × 10-9 cm2 (H atom)-1 s-1. Our calculated Ad is an unusually small value, as compared to the pre-exponential desorption factor expected from the collision theory between two identical particles in the case of associative desorption [∼10-4 cm2 particle-1 s-1].53,75 A prefactor depression by such a large amount has been rarely reported in metals, but a pre-exponential factor value equal to 10-7 cm2 (H atom)-1 s-1 was observed for H2 desorption from GaAs,78 that is, a semiconductor material. Yet, although these discrepancies are uncommon, and the underlying physics of chemisorption in oxide semiconductors is still poorly understood, mostly because general models have been developed for adsorption/ desorption particles on metals; again, this significant deviation from the usual simple estimate value for a (71) Dent, A. L.; Kokes, R. J. Adv. Catal. Relat. Subj. 1972, 22, 1. (72) Kokes, R. J.; Dent, A. L.; Chang, C. C. J. Am. Chem. Soc. 1972, 94, 4429. (73) Kokes, R. J. Acc. Chem. Res. 1973, 6, 226. (74) Einschens, R. P.; Pliskin, W. A.; Low, M. J. D. J. Catal. 1962, 1, 180. (75) Boudart, M.; Dje´ga-Mariadassou, G. Kinetics of Heterogeneous Catalytic Reactions; Princeton University Press: Princeton, NJ, 1984; Chapter 2. (76) Ns ) NGa-H(II) + NGa-OH, where NCa-H(II) and NGa-OH stand for the surface concentration of said species at maximun coverage, respectively. Also, Ns ) 2NT - NGa-H(I), where NT is the total amount of H2 adsorbed (moles) per unit surface area and NGa-H(I) is the total surface concentration of sites able to form Ga-H(I) bonds. From Figure 5, NGa-H(I)/NGa-H(II) ) 0.43/0.57, and NGa-H(II) ) NGa-OH (heterolytic mechanism of adsorption). Thus, 73% of the total H2 is adsorbed via the heterolytic adsorption pathway and the remaining 27% goes to the homolytic one. (77) Kittrell, R. J. Adv. Chem. Eng. 1970, 8, 97. (78) Lu¨th, H.; Matz, R. Phys. Rev. Lett. 1981, 46, 1652.

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second-order reaction deserves an attempt at an explanation. Because we are dealing with an associative desorption process, in which recombination of individual atoms is the rate-determining step during desorption, we must consider the various (if any) possible situations portrayed by the transition state theory. So, in our situation and from a statistical thermodynamic approach Ad ) (kT/h)(q-1#/qGa-HqGaO-H), where q-1# denotes the complete partition function of the activated complex and qGa-H and qGaO-H stand for the molecular partition functions of the adsorbed H. But only in the case of sterically demanding reactions with strained transition complexes, q-1#/qGa-HqGaO-H may be considered smaller than unity, thus, leading to a very low preexponential factor.53 Another explanation can be found from the thermodynamic formulation of the transition state theory itself, where Ad is proportional to exp(∆s°#/R) and ∆s°# refers to the standard entropy difference between the transition state and the adsorbed species.75 Thus, and once again, considering the semiconducting property of gallium oxide, the contribution of the entropy of ionization62 to this ∆s°# term could account for an ∼105 decrease of the theoretical pre-exponential factor for second-order reactions, suggesting that the H2 desorption reaction from the Ga-H(II) and the GaO-H adjacent moiety requires a change in the net charge transfer between the adsorbate and the substrate. Next, we can try to compare our experimental Ed value with some simple theoretical calculations. As explained before, the increase in the H coverage with the temperature of adsorption in the low-temperature range during the first reduction/dehydroxylation of the surface (Figure 6, R1) is provoked by the formation of the adsorption site. Indeed, the Ga-H(II) and GaO-H bonds could be easily and fully regenerated after exposing the activated surface of gallium oxide to molecular hydrogen at 473 K. It seems logical to assume, then, that the heterolytic hydrogen chemisorption is not an activated process, that is, that its adsorption energy (Ea) is close to 0. Thus, the desorption energy value can be justifiably compared to a theoretical heat of adsorption (Ed ∼ -∆hII). This heat of type II hydrogen adsorption can be equated, approximately, to the difference between the dissociation energies of the involved elementary bonds, as follows:

∆hII ) D0(Ga-H) + D0(GaO-H) - D0(H2) D0(Ga-O) ) -66 kJ mol H2-1 (6) where D0 of GaO-H, as calculated using the IR stretching for terminal GaO-H bonds (∼3670 cm-1) following the procedure reported by Bradford and Vannice,47 is equal to 415 kJ mol-1 and D0 of Ga-O is equal to 353 kJ mol-1.48 So, our Ed value is in close agreement with the theoretical value of -∆hII. We believe that the observed difference between exothermic and endothermic adsorbed states of hydrogen on gallia polymorphs (albeit unusual) is not hard to explain. By IR spectroscopy we could follow the evolution of the Ga-H and GaO-H signals in a wide range of temperature

Collins et al.

values and for well-established experimental conditions, namely, preoxidized, prereduced, and partially or fully dehydroxylated surfaces. So, molecular hydrogen can be readily chemisorbed near room temperature, heterolytically, over an activated (prereduced, partially dehydroxylated) gallium oxide. The amount of chemisorbed hydrogen over the activated oxide surface can be increased, however, by raising the temperature of adsorption above 500 K while keeping the hydrogen partial pressure constant. This second process does not involve the formation of extra O-H bonds, and H2 is adsorbed homolytically over still-vacant cus-Ga sites, most likely causing surface reconstruction or relaxation (because the process is endothermic). Further experiments are currently under way to clarify this last picture. 4. Conclusions After the adsorption of H2 on bulk gallium oxide polymorphs two overlapped signals owing to the Ga-H stretching vibration are observed by infrared spectroscopy, and their relative intensities depend clearly on the surface coordination of the gallium cations: the first one, at 2003 cm-1, is assigned to the Ga-H bonds on the gallium cation in the tetrahedral sites, while the second one, at 1980 cm-1, corresponds to H bonded to a gallium cation in an octahedral position. A linear relationship between the total integrated absorbance of the ν(Ga-H) band at 723 K and the specific surface area of the activated gallia polymorphs was found, which indicates that H2 chemisorption can be envisioned as a potential tool to measure the specific surface area of gallium oxides and to discern the nature and proportion of gallium cation coordination on the surface of bulk gallia samples. The dissociative hydrogen chemisorption observed on gallia follows two distinct, but complementary, mechanisms. At 723 K approximately 27% of the H2 (43% of Ga-H species) is adsorbed via an endothermal, homolytic reversible reaction pathway, giving rise to adsorbed gallium-hydrogen(I) species [Ga-H(I)] that can be easily removed by cooling under 473 K or by evacuating the molecular hydrogen from the gas phase. The remaining 73% of the sorbed H2 (57% of Ga-H species) corresponds to Ga-H(II) species formed via exothermal, heterolytic adsorption of H2 on Ga-O-Ga sites, which produces additional OH groups over the surface. Both Ga-H(II) and GaO-H can recombine to desorb as molecular hydrogen only after evacuation at 673 K, or higher temperatures. Finally, both types of chemisorption mechanisms appear to be insensitive to the surface gallium cation coordination type. Acknowledgment. The financial support of the Universidad Nacional del Litoral (UNL), the National Council for Scientific and Technical Research (CONICET), and the National Agency for the Promotion of Science and Technology (ANPCyT) of Argentina is gratefully acknowledged. LA0481389