Particle size effect in methane activation over supported palladium nanoparticles

A synthesis method for producing MgAl oxide supported uniform palladium nanoparticles with varying diameters has been developed. The method consists of reductive-thermal decomposition of a PdMgAl hydrotalcite-like compound, formed via co-precipitation of metal nitrate salts and sodium carbonate. The hydrotalcite–like precursors were characterized by XRD, TG-MS and SEM, and were found to contain a well-defined crystalline structure and a uniform distribution of all constituent elements. The resulting catalysts were characterized by XRD, TEM, Chemisorption of CO and in situ IR measurements of CO, and were found to consist of partially oxide-embedded Pd nanoparticles with diameters ranging from d = 1.7 to 3.3 nm and correspond dispersions of 67–14%. Furthermore, the particle size was found to be inversely related to Pd loading. The palladium catalysts were studied for methane activation via chemisorption at 200 and 400 °C followed by a temperature programmed surface hydrogenation. The most disperse catalyst (d = 1.7 nm) possessed an intrinsic methane adsorption capacity, which was an order of magnitude larger than that of other catalysts in the series, indicating a strong structure sensitivity in this reaction. Additionally, the methane adsorption capacity of the hydrotalcite-derived Pd catalysts was nearly two orders of magnitude higher than that of catalysts derived through other synthesis pathways such as colloidal deposition or sonochemical reduction

Comparative study of hydrotalcite-derived supported Pd₂Ga and PdZn intermetallic nanoparticles as methanol synthesis and methanol steam reforming catalysts

An effective and versatile synthetic approach to produce well-dispersed supported intermetallic nanoparticles is presented that allows a comparative study of the catalytic properties of different intermetallic phases while minimizing the influence of differences in preparation history. Supported PdZn, Pd2Ga, and Pd catalysts were synthesized by reductive decomposition of ternary Hydrotalcite-like compounds obtained by co-precipitation from aqueous solutions. The precursors and resulting catalysts were characterized by HRTEM, XRD, XAS, and CO-IR spectroscopy. The Pd2+ cations were found to be at least partially incorporated into the cationic slabs of the precursor. Full incorporation was confirmed for the PdZnAl-Hydrotalcite-like precursor. After reduction of Ga- and Zn-containing precursors, the intermetallic compounds Pd2Ga and PdZn were present in the form of nanoparticles with an average diameter of 6 nm or less. Tests of catalytic performance in methanol steam reforming and methanol synthesis from CO2 have shown that the presence of Zn and Ga improves the selectivity to CO2 and methanol, respectively. The catalysts containing intermetallic compounds were 100 and 200 times, respectively, more active for methanol synthesis than the monometallic Pd catalyst. The beneficial effect of Ga in the active phase was found to be more pronounced in methanol synthesis compared with steam reforming of methanol, which is likely related to insufficient stability of the reduced Ga species in the more oxidizing feed of the latter reaction. Although the intermetallic catalysts were in general less active than a Cu-/ZnO-based material prepared by a similar procedure, the marked changes in Pd reactivity upon formation of intermetallic compounds and to study the tunability of Pd-based catalysts for different reactions

Catalytic reactivity of face centered cubic PdZn_α for the steam reforming of methanol

Addition of Zn to Pd changes its catalytic behavior for steam reforming of methanol. Previous work shows that improved catalytic behavior (high selectivity to CO2) is achieved by the intermetallic, tetragonal L10 phase PdZnβ1, where the Pd:Zn ratio is near 1:1. The Pd–Zn phase diagram shows a number of other phases, but their steady-state reactivity has not been determined due to the difficulty of precisely controlling composition and phase in supported catalysts. Hence, the role of Zn on Pd has generally been studied only on model single crystals where Zn was deposited on Pd(1 1 1) with techniques such as TPD and TPR of methanol or CO. The role of small amounts of Zn on the steady-state reactivity of Pd–Zn remains unknown. Therefore, in this work, we have synthesized unsupported powders of phase pure PdZnα, a solid solution of Zn in fcc Pd, using a spray pyrolysis technique. The surface composition and chemical state were studied using Ambient Pressure-XPS (AP-XPS) and were found to match the bulk composition and remain so during methanol steam reforming (MSR) (Ptot = 0.25 mbar). Unlike the PdZnβ11 phase, we find that PdZnα is 100% selective to CO during methanol steam reforming with TOF at 250 °C of 0.12 s−1. Steady-state ambient pressure micro-reactor experiments and vacuum TPD of methanol and CO show that the α phase behaves much like Pd, but Zn addition to Pd improves TOF since it weakens the Pd–CO bond, eliminating the poisoning of Pd by CO during MSR over Pd. The measured selectivity for fcc PdZnα therefore confirms that adding small amounts of Zn to Pd is not enough to modify the selectivity during MSR and that the PdZnβ1 tetragonal structure is essential for CO2 formation during MSR

Strong metal-support interactions between palladium and iron oxide and their effect on CO oxidation

Pd/FeOx catalysts were prepared by co-precipitation and characterized before and after reduction using X-ray powder diffraction, thermal analysis, CO chemisorption, electron microscopy, and X-ray photoelectron spectroscopy. Results give evidence for the encapsulation of palladium particles by iron oxide after reduction at high temperatures (523 K). Oxidation of carbon monoxide was applied as test reaction to characterize catalyst samples in different states. Strong metal-–support interactions significantly enhance catalytic activity for oxidation of carbon monoxide. However, this state is not stable under the applied reaction conditions. Catalyst deactivation occurs in two ways: (1) via changes in the oxidation state of iron species and (2) due to sintering of palladium particle

Insights into the ceria-catalyzed ketonization reaction for biofuels applications

The ketonization of small organic acids is a valuable reaction for biorenewable applications. Ceria has long been used as a catalyst for this reaction; however, under both liquid and vapor phase conditions, it was found that given the right temperature regime of about 150-300 °C, cerium oxide, which was previously believed to be a stable catalyst for ketonization, can undergo bulk transformations. This result, along with other literature reports, suggest that the long held belief of two separate reaction pathways for either bulk or surface ketonization reactions are not required to explain the interaction of cerium oxide with organic acids. X-ray photon spectroscopy, scanning electron microscopy, and temperature programmed decomposition results supported the formation of metal acetates and explained the occurrence of cerium reduction as well as the formation of cerium oxide/acetate whiskers. After thermogravimetry/mass spectrometry and FT-IR experiments, a single reaction sequence is proposed that can be applied to either surface or bulk reactions with ceria

Density functional theory based screening of ternary alkali-transition metal borohydrides: A computational material design project

The dissociation of molecules, even the most simple hydrogen molecule, cannot be described accurately within density functional theory because none of the currently available functionals accounts for strong on-site correlation. This problem led to a discussion of properties that the local Kohn-Sham potential has to satisfy in order to correctly describe strongly correlated systems. We derive an analytic expression for the nontrivial form of the Kohn-Sham potential in between the two fragments for the dissociation of a single bond. We show that the numerical calculations for a one-dimensional two-electron model system indeed approach and reach this limit. It is shown that the functional form of the potential is universal, i.e., independent of the details of the two fragments.We acknowledge funding by the Spanish MEC (Grant No. FIS2007-65702-C02-01), “Grupos Consolidados UPV/EHU del Gobierno Vasco” (Grant No. IT-319-07), and the European Community through e-I3 ETSF project (Grant Agreement No. 211956).Peer reviewe