J. Chem. SOC., Faruduy Trans. I , 1982, 78, 21 11-21 19 Infrared Study of Hydrogen Adsorption on MgO, CaO and SrO Possible Mechanism in Promoting 0; Formation BY SALVATORE COLUCCIA,* FLORA BOCCUZZI, GIOVANNA GHIOTTI A N D CLAUDIO MORTERRA Istituto di Chimica Fisica dell’universita di Torino, Corso Massimo D’Azeglio 48, 10125 Torino, Italy Received 23rd July, 1981 Hydrogen adsorption on high-surface-area alkaline-earth oxides has been studied by i.r. spectroscopy at 300 and 78 K. In the former case heterolytic dissociation of H, molecules produces hydride and hydroxy groups which are partially reversible upon evacuation. Low-temperature hydrogen adsorption appears to be undissociative. The active sites are cation-anion couples in low coordination on the surface. On the basis of i.r.evidence a mechanism is proposed for the interaction of oxygen with preadsorbed hydrogen envisaging an electron-transfer process from H- ions to oxygen molecules to give 0; paramagnetic species. Highly dispersed alkaline-earth oxide powders show widespread surface activity. In particular it has long been known that unirradiated MgO exhibits remarkable catalytic activity in promoting the isotopic hydrogen-deuterium equilibration at temperatures as low as 78 K.l This topic has been widely investigated and paramagnetic centres, formed in extremely low concentration during the pretreatment of the sample, were shown to be the catalytic sites., Recently hydrogen preadsorption has been demonstrated to promote the subsequent production of 0; radicals upon oxygen adsorption on Mg03 and Ca0.4 In these cases non-stoichiometric excess oxygen, forming 0- pairs or larger clusters, has been suggested to be responsible for hydrogen adsorption through homolytic dissociation of H, m01ecules.~$~ The possibility that ‘intrinsic’ sites such as metal cations and 0,- anions present at the surface in low coordination (e.g.M2L+C and OL2) could be active for hydrogen uptake on these solids has not been taken into account. A reason may be the absence, up to now, of direct evidence of the nature of the adsorbed hydrogen species. However, preliminary measurements have shown that new bands, although weak owing to the low concentration of surface species,3 appear in the i.r. spectra when hydrogen is adsorbed on Mg0.5 Such data, together with photoluminescence studies,6 suggest that heterolytic dissociation of H, molecules is likely to occur over MgiA>-O‘& surface couples.6 This i.r.study is now extended to confirm the above suggestion and to check if a similar mechanism also takes place in the case of other alkaline-earth oxides. Moreover, a detailed analysis of the hydrogen adsorption could be of some help in the interpretation of more complex reactions, such as the dimerization of pyridine,’? the production of organicg and inorganic3? * radicals,1° and exchange and related reacti0ns.l 211121 12 HYDROGEN ADSORPTION O N ALKALINE-EARTH OXIDES EXPERIMENTAL MgO, CaO and SrO were obtained by thermal decomposition in U ~ C U O ( lop4 N m-2) of either the parent hydroxides or carbonates which had been compressed into pellets of good mechanical resistance and placed in the i.r.cell.12 The oxides were then outgassed up to 1 123 K. The surface areas were 200, 70 and 8 m2 g-l, respectively, for MgO, CaO and SrO. High-purity gases were admitted cia a trap at 78 K without further purification. The i.r. spectra in the 4000-600 cm-l region were obtained with a Beckmann IR 12 spectrophotometer using reference-beam screening to obtain a suitable 100% base-line in ail spectral ranges. The relevant spectra were repeated with a Perkin Elmer 580 spectrophotometer equipped with an i.r. data station, whose computer facilities allowed precise background subtraction and accumulation of spectra. Identical results were obtained in the two cases. The bands being weak, great care was taken in positioning the cell, which was not moved over each adsorpti on-desorp tion cycle. B.E.T.surface areas and hydrogen adsorption on MgO were measured by a small-dead-space conventional volumetric apparatus. RESULTS The bands shown below are weak, and this could well be the reason why they have escaped detection so far. The possibility that the changes observed in the i.r. spectra upon contact with hydrogen were just variations of the intrinsic spectra of the oxides has been checked by allowing onto the samples a number of different gases (He, Ar, N2), and no effect was detected. This definitely demonstrated that the bands appearing upon hydrogen absorption are associated with adsorbed species, as confirmed by the isotopic shift described in the following.HYDROGEN ADSORPTION ON MgO Fig. 1 ( a ) gives the spectrum of the MgO pellet in the 1400-800 cm-1 range. The high-frequency region is not shown in the figure for the sake of brevity. No remaining OH groups are detected in the i.r. spectrum after the standard thermal pretreatment. When the spectrum is run in the presence of 26.6 kN m-2 hydrogen after 20 min contact at beam temperature (b.t.), two broad and complex absorptions are observed in the 1350-800 [curve (b)] and 3800-3000 cm-l ranges. Their intensities increase with time, reaching a maximum within 16 h [curve (c)]. Most components of each absorption are broad, but sharper bands are observed at 1326 and 3750 cm-l. This is more clearly seen in the spectra given in absorbance units obtained by background subtraction from spectrum (c), as shown for the low-frequency region in the inset of As shown elsewhere,j when the pressure of H, is progressively reduced, the overall intensity of the spectrum in the 1350-800 and 3800-3000 cm-l regions decreases, and this effect is enhanced by pumping off at b.t.Most of the variation occurs within 1 min outgassing. The 1326 cm-l band completely disappears, and the better defined, although still broad, components, such as that at 1130 cm-l, tend to decrease preferentially, leaving a much broader band. 1.r. spectra run in the presence of H, at 78 K show that the intensities of the bands already observed at b.t. do not change significantly and that no new bands appear. Hydrogen uptake on MgO is at the limit of sensitivity of the volumetric apparatus and this prevented systematic experiments from being carried out.However, the amount of hydrogen adsorbed at 298 K was found to be ca. molecules nm-2. This is of the same order of magnitude as the data of Cordischi et al.,3 but probably higher than those of Tanaka et al.13 At 78 K the uptake is more than one order of fig. I .S. COLUCCIA, F. BOCCUZZI, G. GHIOTTI AND C. MORTERRA _? ~ ~- __- - 00 ._T ~- -~ I I *.\\ I ! ! 13 1300 1200 1100 1000 900 800 wavenumber/cm-' 21 13 I ---, , -__i_-L 1300 1200 1100 1000 900 wavenumber/cm-' FIG. 1.-1.r. spectra of hydrogen, adsorbed at b.t. on MgO. ( a ) MgO outgassed at 1123 K (-); (h) in the presence of 26.6 kN m-' H, after contact for 20 min (---); (c) as (b) after 16 h ( . - . -).Inset: spectrum (c) in absorbance units, having subtracted the background (a). I I I I I I 1200 1100 1000 900 800 700 2 5 . 1200 1100 1000 900 800 wavenumber/cm-' FIG. 2 . 4 . r . spectra of hydrogen adsorbed at b.t. on CaO. (a) CaO outgassed at 1123 K (-); (b) in the presence of 26.6 kN rn-, H, after contact for 16 h (---); (c) after outgassing 1 min at b.t. ( . -. -). Inset: spectra (6) and (c) in absorbance units, having subtracted the background (a).21 14 HYDROGEN ADSORPTION O N ALKALINE-EARTH OXIDES magnitude larger (ca. 0.3 and ca. 0.4 molecules nmP2 at 2.66 and 13.3 kN mP2, respectively). HYDROGEN ADSORPTION ON CaO AND SrO The effects observed in the spectra of calcium and strontium oxides parallel those described for MgO. After hydrogen adsorption on CaO two broad and complex absorptions are observed at 1200-700 [fig.2(b)] and 3000-3800 cm-l. The intensity increases with pressure (up to 26.6 kN mP2) and time of contact (up to 16 h) and decreases by outgassing [fig. 2(c)]. A sharp component is observed at 3700 cm-'. In the case of SrO the spectrum above 2000 cm-l could not be explored because of high light scattering. Upon hydrogen adsorption [fig. 3(b)J a broad and complex 1oc is 5c K 0 10 1000 900 800 700 wavenumber/cm-' 1000 900 I 800 700 wavenumber/cm-' FIG. 3.-1.r. spectra of hydrogen adsorbed at b.t. on SrO. (a) SrO outgassed at 1123 K ; (b) in the presence of 26.6 kN m-* H, after contact for 16 h; (c) after outgassing 1 min at b.t. Inset: spectra (b) and (c) in absorbance units, having subtracted the background (a).absorption appears at lower frequencies (1000-600 cm-l) than those found with MgO and CaO. The absorbance spectra (fig. 3, inset) show that by outgassing the narrower components are depleted preferentially, this tendency being common to the three oxides.s. COLUCCIA, F. BOCCUZZI, G. GHIOTTI AND c. MORTERRA 21 15 DEUTERIUM ADSORPTION ON MgO, CaO AND SrO When 2H2 is allowed onto the three oxides absorption bands appear in the i.r. spectra, which are similar in shape to those observed upon hydrogen uptake, but shifted to lower frequencies. The observed shifts agree well with the expected values and the sharp band observed at 1326 cm-l in the MgO spectrum upon hydrogen uptake shifts to 950 cm-l when deuterium is adsorbed. INTERACTION OF PREADSORBED HYDROGEN WITH OXYGEN If oxygen is allowed onto a CaO sample which has previously been contacted with hydrogen and then outgassed for 1 min [fig.4(b)] the intensity of the low-frequency 1 1 I I wavenumber/cin-' FIG. 4.--Oxygen interaction with preadsorbed hydrogen on CaO. (a) CaO outgassed at 1123 K; (b) after contact with 26.6 kN m-2 H, and subsequent outgassing at b.t.; ( c ) after allowing 1.33 kN m-2 0,. hydrogen absorption strongly decreases, whereas the absorption above 3000 cm-l is not affected. Similar behaviour is observed in the case of SrO. Parallel experiments with MgO have shown very small, if any, effects of oxygen on the i.r. spectrum of preadsorbed hydrogen. DISCUSSION On the basis of the i.r. spectra at beam temperature two types of adsorbed species can be recognized: one is reversible at b.t.and is related to the bands whose intensity depends on the pressure of gas; the other is irreversible and is related to the bands still present after outgassing at b.t. The latter is desorbed at high temperature. As both species absorb in the same spectral regions, their nature and structure must be similar, their stability only being different. Therefore we shall discuss them together.21 16 HYDROGEN ADSORPTION O N ALKALINE-EARTH OXIDES NATURE OF THE ADSORBED SPECIES The i.r. spectroscopic features associated with hydrogen species on alkaline-earth oxides can be summarised as follows: (a) a broad and complex absorption band at high frequencies (above 3000 cm-l), (6) a broad and complex absorption band in the low-frequency region, observed at 1350-800 cm-l in the spectrum of MgO and at progressively lower frequencies in the spectra of CaO and SrO.As already suggested for the interaction of H, with Mg0,5 absorption (a) can be assigned to stretching modes of surface hydroxy groups and absorption (b) to surface hydride species. The former assignment is straightfor~ard,~~~ l5 whereas the latter requires some comments as, after the original observation by Eischens et aZ.16 on ZnO, the formation of surface hydride groups has not been detected on other highly dispersed oxides. Comparison of our results with spectroscopic data available for molecular complexes support the proposed assignment. Stretching modes of hydride groups in metal complexes are observed at 1900+ 300 cm-' when H is in a terminal position and at 1 loo+ 300 cm-l when H is in a bridged position.17 Similar correlations have been determined for hydride groups on ZnO, a sharp band at 1708cm-l being ascribed to Zn-H and a broad one extending down to 1200 cm-l to Zn-H-Zn surface species, respectively.ls The assignment of bands of type (h) to hydride species is further supported by their shifts to lower energies in the series MgO, CaO, SrO.In fact this is expected for stretching modes of hydride groups, whose frequency moves to lower values as the bond polarity increase~.'~ The enhanced basicity of the matrix in the order MgO, CaO, Sr020 as a function of the cationic radii is bound to bring about increasing polar character of the M-H bond". Indeed, it has been determined that, among alkali metal hydrides, CaH, and SrH, are definitely ionic, whereas some contribution of covalency is present in the case of MgH,.,O From the fact that the stretching-mode frequencies of hydride groups on MgO, CaO and SrO are below 1400 cm-l, hydrogen could be inferred to be shared by two or more surface cations.However, because of the strongly ionic character of the matrix, the M-H bond on alkaline-earth oxides must be much more polar than the analogous one in molecular metal hydrides17 and that of hydride groups on the surface of ZnO.ls Therefore, even a non-bridged surface hydride on the surface of MgO, CaO, SrO is bound to absorb at lower energies19 as compared with the terminally bonded homogeneous complexes and surface Zn-H, which have a high degree of The above consideration, together with the fact that the bands associated with terminal hydride groups are generally sharper than those associated with bridge structure,17 helps in interpreting the complex features of absorption bands of type (b) in the spectra of the three alkaline-earth oxides.Moreover, it has been shown that outgassing at beam temperatures tends to deplete preferentially the sharper components, so that, for all the three oxides, the spectrum run in the presence of H, can be considered as the superimposition of two spectra whose features can be summarised as follows: (1) in the spectrum still present after b.t. outgassing broad components are predominant which must be associated with bridge structures M-H-M ; (2) in the other spectrum, strongly pressure-dependent, sharper compo- nents are present which are probably associated with ' terminal' surface hydrides Me-H.Although many parameters, e.g. kinetic, can play a role in determining the reversibility of the surface species, the different stabilities of the two species seem to support the above assignment, since in the former species, irreversible at b.t., the l8S. COLUCCIA, F. BOCCUZZI, G. GHIOTTI AND C. MORTERRA 21 17 hydrogen anion is stabilised by the electrical field of two or more surface cations, whereas in the latter, reversible at b.t., only one cation is involved. Two stretching modes (symmetric and asymmetric) should be observed for each bridge structure,21 but the weakness of the bands and their heavy overlapping prevents more thorough correlations.However, as the relative intensities of several components vary from sample to sample of the same oxide, the presence of families of slightly different surface hydride species can be inferred. Analogous conclusions can be drawn from a detailed analysis of the high-frequency absorption band A which we have already assigned to stretching modes of the hydroxy groups. On the basis of previous i.r. studies of hydrated Mg02, and Ca023 powders, the sharp bands in the 3700-3750 cm-l region are assignable to ‘free’ OH groups and the broad absorption extending down to 3000 cm-l to hydroxy groups whose H atom is engaged in hydrogen bonding with other anions on the surface. Finally hydrogen adsorption at low temperature (78 K) must be discussed.It is necessary in this case to make use of the following information from different techniques. (1) Photoluminescence spectroscopy has shown that interaction with hydrogen at low temperature enhances quenching of the intrinsic emission of alkaline-earth oxides as compared to the effect produced by hydrogen adsorption at room temperature.6 (2) Gas-volumetric data quoted in the results show a larger uptake of hydrogen at 78 than at 300 K . (3) Neither a significant intensity increase of the bands observed at room temperature nor new bands are shown in the i.r. spectra upon hydrogen adsorption at 78 K. From this the conclusion must be drawn that the extra amount of hydrogen present on the surface at 78 K is adsorbed in a molecular form not detectable in the infrared.ADSORPTION MECHANISM A N D STRUCTURE OF ACTIVE SITES In the above section it has been shown that hydride and hydroxy groups are produced at the same time on the surface of highly dispersed alkaline-earth oxides upon hydrogen adsorption at room temperature. This is consistent with the heterolytic dissociation of hydrogen molecules already suggested in the case of Mg053 6 l 24 and shown here to be operative on CaO and SrO as well: H,-+H++H-. (1) The uptake of molecular hydrogen at low temperature can be thought to occur through H--H bond polarization, not large enough to cause dissociation of the molecule but sufficient to stabilise at 78 K a weak surface complex: Both processes are coherent with the nature of intrinsic sites on the surface of alkaline-earth oxides, e.g.metal cations and oxygen anions whose acidity and basicity are highly enhanced by the low coordination states characteristic of exposed ions.25 The fact that couples of metal and oxygen ions in low coordination states are involved in the hydrogen adsorption has been demonstrated by photoluminescence spectroscopy in the case of Mg0,6 and similar results have been obtained for CaO and Sr0.26 Moreover, by comparison of photoluminescence spectra of MgO powders from different origins, and from the effect of hydrogen adsorption on the different emission bands, idealised models of the surface structure can be drawn.6, 27 Hydrogen has been shown6 to be adsorbed at room temperature only by: ( a ) couples containing both the cation and the anion in the lowest coordination (e.g.three-coordinated magnesium and oxygen ions: Mg:GO;,), (6) regions of high irregularity likely to occur21 18 HYDROGEN ADSORPTION O N ALKALINE-EARTH OXIDES on the very rough surface of MgO ex-hydroxide,28 and possibly (c) a fraction of Mg,2;0;,, although usually this requires U.V. irradiation to react. The surface heterogeneity that can be envisaged from the models6t27 accounts for the multiplicity of adsorbed species which are evidenced by this infrared study. For example, dissociation occurring on a couple whose ions are at the two corners of a microstep protruding over a large (100) plane generates ‘free’ hydride and hydroxy groups. In fact neither H+ nor H- can interact respectively with more than one anion or cation in very low coordination. These OH- and (Mg-H)+ groups may well be associated with the sharpest components in the i.r.spectra. By contrast, when dissociation occurs in a region of high irregularity, where three O$j ions are present in a triangular array which simulates a (1 11) microplane, the H+ fragment is shared by the three anions. This is a bridge structure which contributes to the intensity of the broad bands in the high-frequency region (3700-3000 cm-l). Analogous structures in which three Mg;; are present would generate bridge hydride groups associated with the broader components of band (b). The molecular undissociative adsorption of hydrogen at 78 K occurs on surface couples whose ions are in states of higher coordination. These are certainly the four-coordinated ions on extended edges (e.g.Mgi&0iC)6 and possibly even some of the five-coordinated ions on (100) planes. Such a scheme, although derived from idealized models,6t 27 illustrates situations which are plausible for the real highly irregular and heavily stepped surface of Mg0.28 They can be extended to CaO and SrO, which have the same crystal structure and are obtained by the same decomposition route. Other mechanisms have been proposed involving homolytic dissociation of hydrogen molecules over 0- ions to give hydroxy We believe that those mechanisms may contribute to the hydrogen adsorption on alkaline-earth oxides, but cannot account for the overall process, as the hydride species shown by the i.r. spectra can only be produced through a heterolytic pathway. Very recently such a model has been used to interpret t.p.d.data relative to hydrogen adsorption on Mg0.29S. COLUCCIA, F. BOCCUZZI, G . GHIOTTI AND C. MORTERRA 21 19 PROMOTING ROLE OF PREADSORBED HYDROGEN FOR 0, FORMATION It has been shown by e.s.r. that 0; species are formed when oxygen is allowed onto alkaline-earth oxides which have preadsorbed hydr~gen.~. On the other hand, the infrared spectra indicate that a large fraction of hydride groups is destroyed by interaction with oxygen (fig. 4), whereas no significant changes are observed in the hydroxy bands. 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