Adsorbed and Subsurface Absorbed Hydrogen Atoms on Bare and MgO(100)-Supported Pd and Pt Nanoparticles

Heterogeneous catalysts customarily contain active metal nanoparticles (NPs) deposited on oxides. It is often difficult to understand in detail the influence of supports on NP properties based solely on experimental data. Here, we study by means of electronic structure calculations the effect of a rather chemically inert defect-free support MgO(100) on adsorption and absorption properties of 1.6 nm large Pd127 and Pt127 NPs representative of bigger species. We show that metal nanostructuring only slightly affects adsorption of single hydrogen atoms on terrace sites. At the same time, structural flexibility of the NPs increases thermodynamic stability of subsurface H in Pt NPs and seems to kinetically assist absorption of H in both Pd and Pt. For H bound to Pd, NPs influence of the support is only noticeable near the metal–oxide interface, while for Pt NPs H atoms more distant from the interface are also affected. Overall, the support is found to change the binding energies of H to Pd127 and Pt127 NPs by less than 0.1 eV. Quantitative estimates of the differences between adsorption and absorption properties of bare and MgO-supported noble metal NPs are important for modeling of catalytic systems not featuring strong metal–support interactions

Energetic Stability of Absorbed H in Pd and Pt Nanoparticles in a More Realistic Environment

Absorbed hydrogen can dramatically increase hydrogenation activity of Pd nanoparticles and was predicted to do so also for Pt. This calls for investigations of the energetic stability of absorbed H in Pd and Pt using nanoparticle models as realistic as possible, i.e., (i) sufficiently large, (ii) supported, and (iii) precovered by hydrogen. Herein, hydrogen absorption is studied in MgO(100)-supported 1.6 nm large Pd and Pt nanoparticles with surfaces saturated by hydrogen. The effect of surface H on the stability of absorbed H is found to be significant and to exceed the effect of the support. H absorption is calculated to be endothermic in Pt, energy neutral in Pd(111) and bare Pd nanoparticles, and exothermic in H-covered Pd nanoparticles. Hence, we identify the abundance of surface H and the nanostructuring of Pd as prerequisites for facile absorption of hydrogen in Pd and for the concomitantly altered catalytic activity

Systematic Characterization of Electronic Metal–Support Interactions in Ceria-Supported Pt Particles

Electronic metal–support interactions affect the chemical and catalytic properties of metal particles supported on reducible metal oxides, but their characterization is challenging due to the complexity of the electronic structure of these systems. These interactions often involve different states with varying numbers and positions of strongly correlated d or f electrons and the corresponding polarons. In this work, we present an approach to characterize electronic metal–support interactions by means of computationally efficient density functional calculations within the projector augmented wave method. We describe Ce3+ cations with potentials that include a Ce4f electron in the frozen core, overcoming prevalent convergence and 4f electron localization issues. We systematically explore the stability and chemical properties of different electronic states for a Pt8/CeO2(111) model system, revealing the predominant effect of electronic metal–support interactions on Pt atoms located directly at the metal–oxide interface. Adsorption energies and the reactivity of these interface Pt atoms vary significantly upon donation of electrons to the oxide support, pointing to a strategy to selectively activate interfacial sites of metal particles supported on reducible metal oxides

Atomic and Electronic Structure of Cerium Oxide Stepped Model Surfaces

The atomic and electronic structure of ceria surfaces exhibiting step edges have been studied by means of periodic density functional (LDA+U and GGA+U) calculations. A variety of stoichiometric and nonstoichiometric models of increasing complexity have been designed. The electronic structure has been explored using the topological Bader analysis, the calculated magnetic moments and the ELF (electron localization function) maps. It is concluded that Ce3+ atoms may exist even in stoichiometric extended ceria samples and that the presence of oxygen vacancies in stepped surfaces also induces the presence of Ce3+ atoms although in both cases, the reduced atoms tend to occupy the sites with smallest possible coordination number

Steric Effects on Dinitrogen Cleavage by Three-Coordinate Molybdenum(III) Complexes: A Molecular Mechanics Study

The universal force field approach is used to investigate the steric demand in nitrogen molecule cleavage by three-coordinate molybdenum complexes MoL3 of different ligand types L (L = NH2, NMe2, N(mesityl)(tert-butyl), O(tert-butyl), Me, tert-butyl, neopentyl). Calculated geometries of the intermediates L3Mo−N2−MoL3, of the products L3MoN, and of the undersirable side product dimers L3MoMoL3 are presented. The primary role of ligand sterics appears to be the prevention of dimerization of MoL3 monomers

Comparative Theoretical Study of Formaldehyde Decomposition on PdZn, Cu, and Pd Surfaces

Methanol steam reforming, catalyzed by Pd/ZnO (PdZn alloy), is a potential source of hydrogen for on-board fuel cells. CO has been reported to be a minor side product of methanol decomposition that occurs in parallel to methanol steam reforming on PdZn catalysts. However, fuel cells currently used in vehicles are very sensitive to CO poisoning. To contribute to the understanding of pertinent reaction mechanisms, we employed density functional slab model calculations to study the decomposition of formaldehyde, a key intermediate in methanol decomposition and steam reforming reactions, on planar surfaces of Pd, Cu, and PdZn as well as on a stepped surface of PdZn. The calculated activation energies indicate that dehydrogenation of formaldehyde is favorable on Pd(111), but unfavorable on Cu(111) and PdZn(111). On the stepped PdZn(221) surface, the dehydrogenation process was calculated to be more competitive to formaldehyde desorption than on PdZn(111). Thus, we ascribe the experimentally observed small amount of CO, formed during steam reforming of methanol on the Pd/ZnO catalyst, to occur at metallic Pd species of the catalyst or at defect sites of PdZn alloy

How the C−O Bond Breaks during Methanol Decomposition on Nanocrystallites of Palladium Catalysts

Experimental findings imply that edge sites (and other defects) on Pd nanocrystallites exposing mainly (111) facets in supported model catalysts are crucial for catalyst modification via deposition of CHx (x = 0−3) byproducts of methanol decomposition. To explore this problem computationally, we applied our recently developed approach to model realistically metal catalyst particles as moderately large three-dimensional crystallites. We present here the first results of this advanced approach where we comprehensively quantify the reactivity of a metal catalyst in an important chemical process. In particular, to unravel the mechanism of how CHx species are formed, we carried out density functional calculations of C−O bond scission in methanol and various dehydrogenated intermediates (CH3O, CH2OH, CH2O, CHO, CO), deposited on the cuboctahedron model particle Pd79. We calculated the lowest activation barriers, ∼130 kJ mol−1, of C−O bond breaking and the most favorable thermodynamics for the adsorbed species CH3O and CH2OH which feature a C−O single bond. In contrast, dissociation of adsorbed CO was characterized as negligibly slow. From the computational result that the decomposition products CH3 and CH2 preferentially adsorb at edge sites of nanoparticles, we rationalize experimental data on catalyst poisoning

Effect of Steps on the Decomposition of CH₃O at PdZn Alloy Surfaces

The decomposition of methoxide (CH3O) on a PdZn alloy is considered to be the rate-limiting step of steam re-forming of methanol over a Pd/ZnO catalyst. Our previous density functional (DF) studies (Langmuir 2004, 20, 8068; Phys. Chem. Chem. Phys. 2004, 6, 4499) revealed only a very low propensity of defect-free flat (111) and (100) PdZn surfaces to promote C−H or C−O bond breaking of CH3O. Thus, we applied the same DF periodic slab-model approach to investigate these two routes of CH3O decomposition on PdZn(221) surfaces that expose Pd, (221)Pd, and Zn, (221)Zn, steps. C−H bond cleavage of CH3O is greatly facilitated on (221)Pd:  the calculated activation energy is dramatically reduced, to ∼50 kJ mol-1 from ∼90 kJ mol-1 on flat PdZn surfaces, increasing the rate constant by a factor of 108. The lower barrier is mainly due to a weaker interaction of the reactant CH3O and an enhanced interaction of the product CH2O with the substrate. The activation energy for C−O bond scission did not decrease on the (221)Pd step. On the (221)Zn step, the calculated reaction barriers of both decomposition routes are even higher than on flat surfaces, because of the stronger adsorption of CH3O. Steps (and other defects) appear to be crucial for methanol steam re-forming on Pd/ZnO catalyst; the stepped surface PdZn(221)Pd is a realistic model for studying the reactivity of this catalyst

CH₃O Decomposition on PdZn(111), Pd(111), and Cu(111). A Theoretical Study

Methanol steam re-forming, catalyzed by Pd/ZnO, is a potential hydrogen source for fuel cells, in particular in pollution-free vehicles. To contribute to the understanding of pertinent reaction mechanisms, density functional slab model studies on two competing decomposition pathways of adsorbed methoxide (CH3O) have been carried out, namely, dehydrogenation to formaldehyde and C−O bond breaking to methyl. For the (111) surfaces of Pd, Cu, and 1:1 Pd−Zn alloy, adsorption complexes of various reactants, intermediates, transition states, and products relevant for the decomposition processes were computationally characterized. On the surface of Pd−Zn alloy, H and all studied C-bound species were found to prefer sites with a majority of Pd atoms, whereas O-bound congeners tend to be located on sites with a majority of Zn atoms. Compared to Pd(111), the adsorption energy of O-bound species was calculated to be larger on PdZn(111), whereas C-bound moieties were less strongly adsorbed. C−H scission of CH3O on various substrates under study was demonstrated to proceed easier than C−O bond breaking. The energy barrier for the dehydrogenation of CH3O on PdZn(111) (113 kJ mol-1) and Cu(111) (112 kJ mol-1) is about 4 times as high as that on Pd(111), due to the fact that CH3O interacts more weakly with Pd than with PdZn and Cu surfaces. Calculated results showed that the decomposition of methoxide to formaldehyde is thermodynamically favored on Pd(111), but it is an endothermic process on PdZn(111) and Cu(111) surfaces

Reassignment of the Vibrational Spectra of Carbonates, Formates, and Related Surface Species on Ceria: A Combined Density Functional and Infrared Spectroscopy Investigation

Using a combination of state-of-the-art computational modeling and Fourier transform infrared (FTIR) spectroscopy study of the surface species formed during interaction of CO2 or CO with activated (stoichiometric), reduced, and hydroxylated ceria, CeO2, we assigned various experimentally observed vibrational modes to individual types of surface species. We considered carbonates CO32–, formates HCO2–, and hydrogen carbonates CO2(OH)− bound in various ways to the surface of a ceria nanoparticle. Since the structure of the surface carbonate species is particularly versatile, we introduced a notation of different types of such species and computationally determined the regions where the characteristic vibrational frequencies of each type of species can be found. The complementary FTIR measurements of the surface species produced under different conditions revealed the actual experimental vibrational peaks and allowed estimation of the accuracy of the computational method to reproduce the frequencies of different vibrational modes. Thus, combining computed and experimental data we suggest a sound, partly new assignment of the vibrational bands in the complex IR spectra of surface (hydrogen)carbonate and formate species on ceria. The proposed reassignment of the vibrational peaks enables reliable detection of the surface species on ceria surface using vibrational spectroscopy. This is critical for the meaningful analysis of the reactivity of these species and the clarification of the mechanisms of the rich variety of surface processes on ceria