The ability of ceria to break H 2 in the absence of noble metals has prompted a number of studies because of its potential applications in many technological fields. Most of the theoretical works reported in the literature are focused on the most stable (111) termination. However, recently, the possibility of stabilizing ceria particles with selected terminations has opened new avenues to explore. In the present paper, we investigate the role of termination in H 2 dissociation on stoichiometric ceria. We model (111)-, (110)-, and (100)-terminated slabs together with the stepped (221) and (331) surfaces. Our results support a dissociation mechanism proceeding via the formation of a hydride/hydroxyl CeH/OH intermediate. Both the stability of such an intermediate and the activation energy depend critically on the termination, the (100)-terminated surfaces being the most reactive: the activation energy is 0.16 eV, and the CeH/OH intermediate is stable by −0.64 eV for the (100) slab, whereas the (111) slab presents 0.75 and 0.74 eV, respectively. We provide structural, energetic, electronic, and spectroscopic data, as well as chemical descriptors correlating structure, energy, and reactivity, to guide in the theoretical and experimental characterization of the Ce–H surface intermediate.
A four-step reaction mechanism is proposed for the H 2 dissociation over pure ceria and galliumpromoted mixed oxide materials, in a combined experimental and computational investigation. Two samples of cerium-gallium mixed oxides with Ce/Ga atomic ratios equal to 90/10 and 80/20 were studied by time-resolved diffuse reflectance infrared spectroscopy under H 2 (D 2) flow at isothermal condition in the range of 523-623 K. X-ray photoelectron spectrometry allowed to conclude that only Ce 4+ is reduced to Ce 3+ (Ga 3+ is not reduced), in agreement with density functional theory (DFT) results. The time evolution profiles of gallium hydride ðGaAHÞ species, hydroxyl groups (OH) and Ce 3+ infrared signals were analyzed and kinetic rate parameters for each step were obtained by mathematical modeling. The values for activation energies were in agreement with those calculated by DFT, for the different elementary pathways. A small activation energy ($4 kcal/mol) was found for H 2 dissociation found on Ga Á Á Á OACe sites assuming that the heterolytic cleavage of the HAH bond is the rate determining step. On pure ceria, the experimental activation energy is $23 kcal/mol, showing that the addition of Ga 3+ cations boosts the splitting of H 2. Interestingly, the reduction step of pure CeO 2 surface domains seems to proceed via a CeH/OH pair intermediate, according to DFT calculations. Moreover, 71 Ga NMR experiments indicate the possible presence of gallia nanodomains. It is proposed that the generation of Ga Á Á Á OACe sites in the perimeter of such surface gallia nanodomains is responsible for the enhanced reactivity of the mixed materials. The key role of this new type of sites to improve the efficiency of relevant catalytic reactions such as selective alkyne hydrogenation and light alkane dehydrogenation is then analyzed. q This contribution is part of the virtual issue ''30 years of the International Conferences on Theoretical Aspects of Catalysis (ICTAC)".
IrO 2 is a key material for photocatalytic applications as water oxidation catalyst. Despite its increasing interest, little is known about its molecular structure and reactivity. In this study, the surface properties of stoichiometric rutile IrO 2 are investigated by means of periodic density functional theory (DFT), including the structural, energetic, electronic properties and chemical reactivity towards catechol, a probe molecule mimicking photocatalytic linkers. Our results show that the (110)-IrO 2 rutile termination is the most stable, and we discuss the role of the number and type of surface sites in the relative stability compared with (100), (001) and (101) terminations. Regarding the reactivity of the surfaces with catechol, our results show that the molecule dissociates and binds in bidentate, chelate and monodentate modes.Interestingly, we find the chelated mode selectively favored over the (001)
H 2 dissociation on ceria (CeO 2) has attracted much attention in the last years because of its potential application in catalysis for hydrogenation reactions, as well as for the stabilization of hydride bulk and surface species. The ability of ceria to split hydrogen is strongly dependent on the surface morphology. However, to the best of our knowledge, the reactivity of the cerium sesquioxide Ce 2 O 3 Atype (hexagonal structure with space group 3 1) has not been previously addressed. In the present study, we investigate (i) the formation of oxygen vacancies in ACe 2 O 3 bulk and (ii) the effect of the surface topology in the H 2 dissociation and in the oxygen vacancy formation for the four most stable surfaces of ACe 2 O 3 : (0001), (01-11), (11-20) and (11-21). Our results indicate a significant decrease of the energetic barrier for the hydrogen dissociation compared to stoichiometric CeO 2 , with an activation energy of ~0.1 eV. Interestingly, Ce 2 O 3 surfaces lead to a heterolytic product with hydride species more stable than the homolytic product, which is the opposite behavior found in CeO 2. These results suggest a better performance of Ce 2 O 3 than CeO 2 for H 2 dissociation and provide insight in the nature of hydride-Ce 2 O 3 interfaces that could be important intermediates in the formation of CeH x phases from cerium oxide.
In order to obtain a hydrogen storage system that is cost-effective, safe and energy-viable, in this work we studied the adsorption process of different sites and configurations of the hydrogen molecule in both MOF HKUST-1 and FeBTC structure. For the computational study was used the Ab initio simulation package from Vienna (VASP) with the GGA PBE functional in an SBU of 672 atoms. The exposed metal sites were found to be the best interaction site for the hydrogen molecule. In this work showed adsorption energy of -0.114 eV and -0.93 eV for HKUST-1 and FeBTC, respectively. This work also evidences the modifications of the atomic positions when the hydrogen molecule interacts with the MOF and the network energy.
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