Formic acid decomposition over palladium based catalysts doped by potassium carbonate

The introduction of potassium carbonate into Pd/Al2O3, Pd/SiO2 and Pd/C catalysts promoted both the catalytic activities and the hydrogen selectivities for the vapor-phase formic acid decomposition, giving values of the turnover frequency (TOF) at 343 K that were 8-33 times higher than those for the undoped samples. The apparent activation energies over all the K-doped samples increased considerably, this showing that there is a difference in the reaction path between the doped and the undoped catalysts. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) has been used to gain an understanding of the nature of the species formed in the Pd/SiO2 catalysts during the reaction. This study showed that a considerable fraction of the HCOOH was condensed in the pores of the catalysts and that the introduction of potassium contributed to the formation of buffer-like solution. The existence of mobile formate ions present in the buffer solution and stabilized by K ions in a K-doped catalyst is an essential factor in the promotion of its activity. (C) 2015 Elsevier B.V. All rights reserved

Hydrogen production from formic acid vapour over a Pd/C catalyst promoted by potassium salts: evidence for participation of buffer-like solution in the pores of the catalyst

Doping a 1 wt.% Pd/C catalyst with alkali metal carbonates has a very significant promotional effect on its activity in hydrogen production from the decomposition of formic acid vapour (2 vol.%, 1 bar), potassium and caesium carbonates giving the largest effects. The K carbonate species present on the fresh catalysts react with formic acid to form formate ions, these being dissolved in a formic acid/water solution condensed in the pores of the support. The steady-state activities of the samples containing formate ions were 1-2 orders of magnitude greater than those of the unpromoted Pd/C and CO content was lower than 30 ppm. The activation energies for the reaction increased with doping from 66 to 88-99 kJ mo1-1, relatively independent of the cation of the dopant. Similar but lesser effects were found with unsupported Pd nanocrystals doped with K carbonate. The rate-determining step for the promoted samples appears to be the decomposition of formate ions on the Pd surface. (C) 2014 Elsevier B.V. All rights reserved

Catalytic properties of PdZn/ZnO in formic acid decomposition for hydrogen production

This is one of the first reports, which is related to hydrogen production through formic acid decomposition over Pd/ZnO catalysts widely used for methanol steam-reforming. These catalysts have been investigated in comparison with Pt/ZnO and Pd/Al2O3 catalysts as well as ZnO support. HAADF/STEM, XRD, XPS and DRIFTS in situ studies of the systems were performed. The measured catalyst activity corresponds to the following order: Pd/Al2O3≥Pd/ZnO>Pt/ZnO>ZnO. Among the studied catalysts, Pd/ZnO showed the highest selectivity to hydrogen (up to 99.3%). This was assigned to the formation of a PdZn alloy during the reductive pre-treatment of the catalyst. An increase of the pre-treatment temperature from 573 to 773 K led to a significant increase of the mean PdZn (PtZn) nanoparticle size. However, the catalyst activity did not change, but the selectivity to hydrogen increased. These features closely remind the behavior of Pd/ZnO catalysts in methanol steam reforming implying that the mechanism of formic acid decomposition involves the same key steps and active sites

Boosting hydrogen production from formic acid over Pd catalysts by deposition of N-Containing precursors on the carbon support

Formic acid is a promising liquid organic hydrogen carrier (LOHC) since it has relatively high hydrogen content (4.4 wt%), low inflammability, low toxicity and can be obtained from biomass or from CO2. The aim of the present research was the creation of efficient 1 wt% Pd catalysts supported on mesoporous graphitic carbon (Sibunit) for the hydrogen production from gas-phase formic acid. For this purpose, the carbon support was modified by pyrolysis of deposited precursors containing pyridinic nitrogen such as melamine (Mel), 2,20-bipyridine (Bpy) or 1,10-phenanthroline (Phen) at 673 K. The following activity trend of the catalysts Pd/Mel/C > Pd/C ~ Pd/Bpy/C > Pd/Phen/C was obtained. The activity of the Pd/Mel/C catalyst was by a factor of 4 higher than the activity of the Pd/C catalyst at about 373 K and the apparent activation energy was significantly lower than those for the other catalysts (32 vs. 42-46 kJ/mol). The high activity of the melamine-based samples was explained by a high dispersion of Pd nanoparticles (~2 nm, HRTEM) and their strong electron-deficient character (XPS) provided by interaction of Pd with pyridinic nitrogen species of the support. The presented results can be used for the development of supported Pd catalysts for hydrogen production from different liquid organic hydrogen carriers

Nanometer-Sized MoS₂ Clusters on Graphene Flakes for Catalytic Formic Acid Decomposition

MoS₂ was deposited on graphene flakes via decomposition of MoS₃ in vacuum at different temperatures (500–800 °C). The materials obtained were tested for catalytic formic acid decomposition, giving mainly hydrogen and carbon dioxide. According to atom-resolved transmission electron microscopy study, a considerable amount of MoS₂ clusters with a mean size of 1 nm was formed on the graphene surface at 500 °C. Simulation of the structure of a cluster revealed the presence of Mo-edge atoms. Raising the preparation temperature up to 800 °C led to agglomeration of MoS₂ clusters and formation of thin crystalline MoS₂ particles 20–30 nm in size. The sample enriched with the MoS₂ clusters showed 6 times higher catalytic activity at 160 °C than the sample with the crystalline MoS₂ particles. This demonstrates that the observed nanometer-sized MoS₂ clusters are responsible for catalysis

Pd Clusters Supported on Amorphous, Low-Porosity Carbon Spheres for Hydrogen Production from Formic Acid

Amorphous, low-porosity carbon spheres on the order of a few micrometers in size were prepared by carbonization of squalane (C₃₀H₆₂) in supercritical CO₂ at 823 K. The spheres were characterized and used as catalysts’ supports for Pd. Near-edge X-ray absorption fine structure studies of the spheres revealed sp² and sp³ hybridized carbon. To activate carbons for interaction with a metal precursor, often oxidative treatment of a support is needed. We showed that boiling of the obtained spheres in 28 wt % HNO₃ did not affect the shape and bulk structure of the spheres, but led to creation of a considerable amount of surface oxygen-containing functional groups and increase of the content of sp² hybridized carbon on the surface. This carbon was seen by scanning transmission electron microscopy in the form of waving graphene flakes. The H/C atomic ratio in the spheres was relatively high (0.4) and did not change with the HNO₃ treatment. Palladium was deposited by impregnation with Pd acetate followed by reduction in H₂. This gave uniform Pd clusters with a size of 2–4 nm. The Pd supported on the original C spheres showed 2–3 times higher catalytic activity in vapor phase formic acid decomposition and higher selectivity for H₂ formation (98–99%) than those for the catalyst based on the HNO₃ treated spheres. Using of such low-porosity spheres as a catalyst support should prevent mass transfer limitations for fast catalytic reactions

Support effect for nanosized Au catalysts in hydrogen production from formic acid decomposition

Catalysts with about 2.5 wt% of gold supported on Al2O3, ZrO2, CeO2, La2O3 and MgO oxides and with the same mean metal particle sizes of 2.4-3.0 nm have been studied in hydrogen production via formic acid decomposition. A strong volcano-type relation of the catalytic activity on the electronegativity of the support's cation was demonstrated with the Au/Al2O3 catalyst on the top. This indicated that the activity is affected by the acid-base properties of the support. A study of the most active Au/Al2O3 catalyst with aberration-corrected HAADF/STEM, XPS and EXAFS proved that gold is in metallic state. The content of single supported gold atoms/cations was negligible. Therefore, the mechanism of the reaction was related to the activation of formic acid on the catalyst's support followed by further decomposition of the formed reaction intermediate on the Au/support interface

Single isolated Pd2+ cations supported on N-Doped carbon as active sites for hydrogen production from formic acid decomposition

Single-site heterogeneous catalysis with isolated Pd atoms was reported earlier, mainly for oxidation reactions and for Pd catalysts supported on oxide surfaces. In the present work, we show that single Pd atoms on nitrogen-functionalized mesoporous carbon, observed by aberration-corrected scanning transmission electron microscopy (ac STEM), contribute significantly to the catalytic activity for hydrogen production from vapor-phase formic acid decomposition, providing an increase by 2-3 times in comparison to Pd catalysts supported on nitrogen-free carbon or unsupported Pd powder. Some gain in selectivity was also achieved. According to X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) studies after ex situ reduction in hydrogen at 573 K, these species exist in a Pd2+ state coordinated by nitrogen species of the support. Extended density functional theory (DFT) calculations confirm that an isolated Pd atom can be the active site for the reaction, giving decomposition of the formic acid molecule into an adsorbed hydrogen atom and a carboxyl fragment, but only if it is coordinated by a pair of pyridinic-type nitrogen atoms located on the open edge of the graphene sheet. Hence, the role of the N-doping of the carbon support is the formation and stabilization of the new active Pd sites. A long-term experiment performed for more than 30 h on stream indicated an excellent stability of these Pd species in the reaction

Single atoms of Pt-group metals stabilized by N-doped carbon nanofibers for efficient hydrogen production from formic acid

Formic acid is a valuable chemical derived from biomass, as it has a high hydrogen-storage capacity and appears to be an attractive source of hydrogen for various applications. Hydrogen production via formic acid decomposition is often based on using supported catalysts with Pt-group metal nanoparticles. In the present paper, we show that the decomposition of the acid proceeds more rapidly on single metal atoms (by up to 1 order of magnitude). These atoms can be obtained by rather simple means through anchoring Pt-group metals onto mesoporous N-functionalized carbon nanofibers. A thorough evaluation of the structure of the active site by aberration-corrected scanning transmission electron microscopy (ac-STEM) in high-angle annular dark field (HAADF) mode and by CO chemisorption, X-ray photoelectron spectroscopy (XPS), and quantum chemical calculations reveals that the metal atom is coordinated by a pair of pyridinic nitrogen atoms at the edge of graphene sheets. The chelate binding provides an ionic/electron-deficient state of these atoms that prevents their aggregation and thereby leads to an excellent stability under the reaction conditions. Catalysts with single atoms have also shown very high selectivity. Evidently, the findings can be extended to hydrogen production from other chemicals and can be helpful for improving other energy-related and environmentally benign catalytic processes

Single Isolated Pd²⁺ Cations Supported on N‑Doped Carbon as Active Sites for Hydrogen Production from Formic Acid Decomposition

Single-site heterogeneous catalysis with isolated Pd atoms was reported earlier, mainly for oxidation reactions and for Pd catalysts supported on oxide surfaces. In the present work, we show that single Pd atoms on nitrogen-functionalized mesoporous carbon, observed by aberration-corrected scanning transmission electron microscopy (ac STEM), contribute significantly to the catalytic activity for hydrogen production from vapor-phase formic acid decomposition, providing an increase by 2–3 times in comparison to Pd catalysts supported on nitrogen-free carbon or unsupported Pd powder. Some gain in selectivity was also achieved. According to X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) studies after ex situ reduction in hydrogen at 573 K, these species exist in a Pd²⁺ state coordinated by nitrogen species of the support. Extended density functional theory (DFT) calculations confirm that an isolated Pd atom can be the active site for the reaction, giving decomposition of the formic acid molecule into an adsorbed hydrogen atom and a carboxyl fragment, but only if it is coordinated by a pair of pyridinic-type nitrogen atoms located on the open edge of the graphene sheet. Hence, the role of the N-doping of the carbon support is the formation and stabilization of the new active Pd sites. A long-term experiment performed for more than 30 h on stream indicated an excellent stability of these Pd species in the reaction