Particle size effect in methane activation over supported palladium nanoparticles

doi:10.1016/j.apcata.2012.11.021

Applied Catalysis A: General

Volume 452, 15 February 2013, Pages 203-213

https://doi.org/10.1016/j.apcata.2012.11.021 Get rights and content

Abstract

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.

Graphical abstract

Highlights

► A series of supported Pd catalysts was prepared from Hydrotalcite precursors. ► The particle size of Pd Nanoparticles have been tuned from 1.7 to 3.8 nm. ► The methane chemisorptions capacity was largely enhanced for particles <2 nm. ► For 2 nm the capacity scaled with the Pd loading.

Introduction

In our previous work [1] we have shown that the catalytic activity in the non-oxidative coupling of methane over Pd nanoparticles strongly depends on the preparation method and the resulting particle size. A sonochemically reduced Pd/α-Al₂O₃ catalyst and a Pd/α-Al₂O₃ sample prepared by colloidal deposition differed in particle size of Pd, widths of their particle size distributions and amount of carbon incorporation in the Pd lattice after synthesis. PdC_x was found in the colloidal sample due to carbon incorporation from the protecting ligand shell upon thermal treatment. It was shown that a large particle size has detrimental impact on the methane adsorption property and the ability to form higher hydrocarbons from methane. To further investigate the aforementioned particle size effect, in the current work we aimed at developing a synthesis concept to obtain size controlled Pd-nanoparticles and to study their catalytic performance in non-oxidative methane coupling. In our synthesis approach, hydrotalcite-like compounds (HTlc's) are used as well-defined precursor materials, whose general formula is [M²⁺_1−xM³⁺_x(OH)₂](Aⁿ⁻)_x/n × mH₂O [2], [3] with M²⁺ = Mg²⁺, Pd²⁺, and M³⁺ = Al³⁺. This synthesis concept has been previously applied for the preparation of oxide-supported intermetallic Pd₂Ga (M³⁺ = Ga³⁺) nanoparticles [4]. Divalent and trivalent metal cations are incorporated in brucite-like layers and between these positively charged layers, charge-balancing anions, typically carbonate, maintain the electro-neutrality of the lattice. Furthermore the M²⁺/M³⁺ ratio can be varied from approximately 0.2–0.4 to gain phase-pure materials.

Upon heating, HTlc's decompose into mixed oxides exhibiting high specific surface area, homogeneous metal distribution, and strong interaction between the individual elements. During reduction in H₂, noble metals such as Pd segregate out of the mixed-oxide matrix to form well-define nanoparticles, whose size tends to depend on metal loading. PdMgAl hydroxycarbonates have already been studied by several groups and were found to be active in phenol hydrogenation, oxidation of toluene, acetone condensation, hydrodechlorination of 1,2,4-trichlorobenzene and total oxidation of methane [5], [6], [7], [8], [9].

The catalytic methane homologation into higher hydrocarbons under non-oxidative condition was proposed to be an alternative reaction to the oxidative coupling of methane, which also leads to CO₂ formation via partial combustion [10], [11]. Direct conversion of methane into higher alkanes or alkenes is thermodynamically disfavored. To homologate methane it is necessary to dissociate methane by chemisorption on metal surfaces [12], [13], [14]. The adsorption and decomposition mechanism of methane on metallic surfaces is an exothermic activation process of dissociative and homolytic nature. Indeed, the kinetics of CH₄ adsorption on metallic surfaces has been studied using surface analyses, showing that atomic and electronic surface properties affect the adsorption and formation of intermediate species, and indicating that the methane dissociation is the determining step in this process [15].

The dissociation of methane on transition metals may occur directly (DDM—direct dissociative mechanism) or through a precursor mediated mechanism (PMM). In the first case the dissociation occurs during the impact at the surface while for the PMM mechanism desorption or dissociation occurs after adsorption and accommodation of the molecule at the surface. In this case, the reaction depends on the initial probability of adsorption. Most studies concluded that the DDM mechanism on transition metals is preferred [16], [17], [18]. Yang et al. [18] and Beebe et al. [19] showed that the methane dissociation on single crystal surfaces can form intermediate hydrocarbon species with one or two carbons, such as methylidene (CH), vinylidene (CCH₂) and ethylidene (CCH₃), besides graphitic carbon [20]. Lee et al. [16] observed on Ni surfaces CH fragments at very low temperatures which are recombined to C₂H₂ at temperatures around 230 K with simultaneous desorption of H₂ at 395 K. Heating favors the trimerization of the adsorbed C₂H₂ species [18].

Thus, chemisorption can lead to different carbonaceous intermediates some of which can further be hydrogenated to C₂₊ hydrocarbons. The nature and reactivity of the formed adspecies toward coupling are expected to be different dependent on the reaction conditions [21], [22]. Furthermore the morphology, crystallinity, and abundance of vertices and kinks on the Pd surface may affect activity and selectivity in this reaction. For nanoparticles, these properties are closely associated with particle size, and related to the phenomenon of structure sensitivity in catalytic reactions. A straightforward correlation between single crystal surfaces and supported catalysts exists only for structure insensitive reactions, however, for structure sensitive reactions no direct correlation is possible below a certain particle size [23].

The main objective of this work is to study the influence of the particle size of nanostructured supported palladium catalysts in the activation of methane by chemisorption. Catalysts with different Pd particle sizes were prepared by co-precipitation, comprehensively characterized and their methane chemisorptions capacity was determined. The quantity and nature of methane adsorption sites were measured via adsorption followed by temperature programmed surface hydrogenation (TPSH), as outlined previously[1]. Additionally, we have compared the current results with the sonochemical and colloidal synthesis approaches reported in our earlier work [1] and the new catalysts turn out to have greater methane chemisorptions capacity. Regarding the non-oxidative coupling of methane in two steps, unfortunately no significant activity could be detected.

Section snippets

Catalyst preparation

PdMgAl HTlc's were synthesized by controlled co-precipitation at pH 8.5 and a temperature of 55 °C by co-feeding appropriate amounts of mixed metal nitrate and sodium carbonate solutions. Appropriate ratio of magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O), aluminum nitrate (Al(NO₃)₃·4H₂O) and palladium nitrate solution (Pd(NO₃)₂·2H₂O) were mixed with deionized water. The Pd:Mg:Al atomic ratio was x:70 − x:30 (x = 0.1, 0.5, 1.0, 1.5 and 2.5). A 0.345 M sodium carbonate solution was used as

Catalyst preparation and characterization

We have used a ternary HTl precursor consisting of Pd²⁺, Mg²⁺ and Al³⁺ and carbonate as interlayer anions. Mg²⁺ was partially substituted by Pd²⁺ and the Pd content was varied from 0 to 2.5 mol% of all metal species in order to vary the Pd particle sizes in the final catalysts. The Al³⁺ content was constant at 30 mol% to obtain phase-pure HTlc's. The precursor materials were prepared by controlled co-precipitation and thermally reduced in hydrogen at 500 °C. Table 1 presents an overview of all

Conclusions

A synthesis method for supported Pd catalysts was presented, which allows a certain control over the particle size by variation of the Pd substitution in the cationic lattice of the HTlc precursor material. A series of catalysts with Pd loadings between 0.3 and 8 wt% has been prepared and the resulting Pd/MgO/MgAl₂O₄ catalysts exhibit a homogenous microstructure, high specific surface area, a relatively uniform particle size distribution and average Pd particle sizes between <1.9 and 3.5 nm,

Acknowledgments

The authors thank Frank Girgsdies for help with XRD pattern refinement, Gisela Weinberg for SEM investigations, Dirk Rosenthal for the help with the CO chemisorption measurement, and Neil G. Hamilton for the IR experiments, Norbert Pfänder and Wei Zhang for recording the TEM images and Igor Kasatkin for the HRTEM analysis. Robert Schlögl is acknowledged for valuable discussions and continuous support. Furthermore, the authors would like to thank Fabio S. Toniolo, Maria Auxiliadora S. Baldanza,

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Particle size effect in methane activation over supported palladium nanoparticles

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Catalyst preparation

Catalyst preparation and characterization

Conclusions

Acknowledgments

Appl. Catal. A: Gen.

Appl. Catal. A: Gen.

Appl. Catal. A: Gen.

Appl. Catal. A: Gen.

Appl. Clay Sci.

Appl. Catal. B: Environ.

J. Catal.

J. Catal.

J. Mol. Catal. A: Chem.

Chem. Phys. Lett.

J. Catal.

Appl. Catal. A: Gen.

Catal. Today

J. Catal.

J. Mol. Struc.-Theochem.

React. Funct. Polym.

J. Catal.

Stud. Surf. Sci. Catal.

J. Catal.