No abstract

The adsorption of CO(2) over a set of gallium (III) oxide polymorphs with different crystallographic phases (alpha, beta, and gamma) and surface areas (12-105 m(2) g(-1)) was studied by in situ infrared spectroscopy. On the bare surface of the activated gallias (i.e., partially dehydroxylated under O(2) and D(2) (H(2)) at 723 K), several IR signals of the O-D (O-H) stretching mode were assigned to mono-, di- and tricoordinated OD (OH) groups bonded to gallium cations in tetrahedral and/or octahedral positions. After exposing the surface of the polymorphs to CO(2) at 323 K, a variety of (bi)carbonate species emerged. The more basic hydroxyl groups were able to react with CO(2), to yield two types of bicarbonate species: mono- (m-) and bidentate (b-) [nu(as)(CO(3)) = 1630 cm(-1); nu(s)(CO(3)) = 1431 or 1455 cm(-1) (for m- or b-); delta(OH) = 1225 cm(-1)]. Together with the bicarbonate groups, IR bands assigned to carboxylate [nu(as)(CO(2)) = 1750 cm(-1); nu(s)(CO(2)) = 1170 cm(-1)], bridge carbonate [nu(as)(CO(3)) = 1680 cm(-1); nu(s)(CO(3)) = 1280 cm(-1)], bidentate carbonate [nu(as)(CO(3)) = 1587 cm(-1); nu(s)(CO(3)) = 1325 cm(-1)], and polydentate carbonate [nu(as)(CO(3)) = 1460 cm(-1); nu(s)(CO(3)) = 1406 cm(-1)] species developed, up to approximately 600 Torr of CO(2). However, only the bi- and polydentate carbonate groups still remained on the surface upon outgassing the samples at 323 K. The total amount of adsorbed CO(2), measured by volumetric adsorption (323 K), was approximately 2.0 micromol m(-2) over any of the polymorphs, congruent with an integrated absorbance of (bi)carbonate species proportional to the surface area of the materials. Upon heating under flowing CO(2) (760 Torr), most of the (bi)carbonate species vanished a T > 550 K, but polydentate groups remained on the surface up to the highest temperature used (723 K). A thorough discussion of the more probable surface sites involved in the adsorption of CO(2) is made.

Reducible oxides have been shown to greatly improve the activity of water gas shift (WGS) catalysts. The precise mechanism for this effect is a matter of intense debate, but the dissociation of water is generally considered to be the key step in the reaction. We present here a study of the water activation on oxygen vacancies at the support as part of the mechanism of the WGS reaction on Pt supported on pure and gallium-doped ceria. Doping the ceria with gallium allows tuning the vacancies in the support while maintaining constant the metal dispersion. An inverse relationship was found between the catalytic activity to WGS and the amount of oxygen vacancies. In situ time-resolved X-ray diffraction, mass spectrometry, and diffuse reflectance infrared spectroscopy (DRIFT) showed that the oxygen vacancy filling by water is always fast in either Pt/CeO2 or Pt/CeGa. DFT calculation provides molecular insights to understand the pathway of water reaction with vacancies at the metal–oxide interface sites. Our results suggest that the activation of the water molecule in the WGS mechanism is not the rate-limiting step in these systems. Concentration-modulation spectroscopy in DRIFT mode under WGS reaction conditions allows the selective detection of key reaction intermediates, a monodentate formate (HCOO) and carboxylate (CO2 δ−) species, which suggests the prevalence of a carboxyl (HOCO) mechanism activated at the oxide–metal interface of the catalyst.

The chemisorption of H(2) over a set of gallia polymorphs (alpha-, beta-, and gamma-Ga(2)O(3)) 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 H(2), 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 [nu(Ga(t)-H) and nu(Ga(o)-H), respectively]. Irrespective to the gallium cation geometrical environment, (i) a linear relationship between the integrated intensity of the whole nu(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 Deltah(I) = 155 +/- 25 kJ mol(-1) and Deltas(I) = 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 H(2) 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.

Supported catalytically active liquid metal solutions (SCALMS) represent a class of catalytic materials that have only recently been developed, but have already proven to be highly active, e.g., for dehydrogenation reactions. Previous studies attributed the catalytic activity to isolated noble metal atoms at the surface of a liquid and inert Ga matrix. In this study, we apply diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with CO as a probe molecule to Ga/Al2O3, Pt/Al2O3, and Ga37Pt/Al2O3 catalysts, to investigate in detail the nature of the active Pt species. Comparison of CO adsorption on Pt/Al2O3 and Ga37Pt/Al2O3 shows that isolated Pt atoms are, indeed, present at the surface of the liquid SCALMS. Combining DRIFTS with online gas chromatography (GC), we investigated the Ga/Al2O3, Pt/Al2O3, and Ga37Pt/Al2O3 systems under operando conditions during propane dehydrogenation in CO/propane and in Ar/propane. We find that the Pt/Al2O3 sample is rapidly poisoned by CO adsorption and coke, whereas propane dehydrogenation over Ga37Pt/Al2O3 SCALMS leads to higher conversion with no indication of poisoning effects. We show under operando conditions that isolated Pt atoms are present at the surface of SCALMS during the dehydrogenation reaction. IR spectra and density-functional theory (DFT) suggest that both the Ga matrix and the presence of coadsorbates alter the electronic properties of the surface Pt species.

No abstract

The doping of CeO 2 with different types of cations has been recognized as a significant factor in controlling the oxygen vacancies and improving the oxygen mobility. Thus, the catalytic properties of these materials might be determined by modifying the redox properties of ceria. A combined experimental and theoretical study of the redox properties of gallium-doped cerium dioxide is presented. Infrared spectroscopy and timeresolved X-ray diffraction were used for temperature programmed reduction (H 2 ) and oxidation (with O 2 and H 2 O) studies. Additionally, X-ray absorption near edge spectroscopy shows that only Ce 4+ is reduced to Ce 3+ in the ceria−gallia mixed oxides when annealed up to 623 K. The oxygen storage capacity (OSC) measurements show a pronounced enhancement on the reduction of ceria by gallium doping. Theoretical calculations by density functional theory (DFT) confirm the higher reducibility of gallium-doped ceria oxides and give a molecular description of the stabilization of the doped material. On the basis of infrared spectroscopic measurements, a novel mechanism is proposed for the surface reduction of Ce 4+ to Ce 3+ where Ga−H species are suggested to be directly involved in the process. In addition, the reoxidation by H 2 O was precluded in the galliumdoped ceria oxide.

Gallia (gallium oxide) has been proved to enhance the performance of metal catalysts in a variety of catalytic reactions involving methanol, CO and H(2). The presence of formate species as key intermediates in some of these reactions has been reported, although their role is still a matter of debate. In this work, a combined theoretical and experimental approach has been carried out in order to characterize the formation of such formate species over the gallium oxide surface. Infrared spectroscopy experiments of CO adsorption over H(2) (or D(2)) pretreated beta-Ga(2)O(3) revealed the formation of several formate species. The beta-Ga(2)O(3) (100) surface was modelled by means of periodic DFT calculations. The stability of said species and their vibrational mode assignments are discussed together with the formate interconversion barriers. A possible mechanism is proposed based on the experimental and theoretical results: first CO inserts into surface (monocoordinate) hydroxyl groups leading to monocoordinate formate; this species might evolve to the thermodynamically most stable dicoordinate formate, or might transfer hydrogen to the surface oxidizing to CO(2) creating an oxygen vacancy and a hydride group. The barrier for the first step, CO insertion, is calculated to be significantly higher than that of the monocoordinate formate conversion steps. Monocoordinate formates are thus short-lived intermediates playing a key role in the CO oxidation reaction, while bidentate formates are mainly spectators.