An In Situ ESR Study of Pd/H-ZSM-5 Interaction with Different Adsorbents

In situ ESR at 120–473 K permits to monitor formation of transient paramagnetic ions/complexes (isolated Pd⁺ sites; Pd⁺/H₂O; Pd⁺/C₆H₆) upon interaction of isolated Pd²⁺ cations stabilized by the H-ZSM-5 matrix with different organic compounds and gas mixtures (NO, O₂, H₂O, H₂, propene, benzene). The in situ study provides insight into the elementary steps of redox processes on isolated Pd species in H-ZSM-5 zeolite under realistic conditions. Adsorbed water stabilizes the transient paramagnetic complex and decreases the rate of Pd²⁺ to Pd⁰ reduction by H₂. Strong bonding of NO _x ligands to Pd²⁺ species suppresses the reduction of Pd(II) ions. Sorption of benzene on preoxidized Pd²⁺/HZSM-5 is accompanied by an easy formation of organic cation-radicals and of a Pd⁺/benzene complex, the paramagnetic Pd⁺/benzene structure indicating a surprisingly high resistance to further reduction to Pd⁰. Illumination of the Pd/HZSM-5 by UV-visible light causes no measurable change in the redox properties of the catalyst.     

Methanol decomposition to synthesis gas over supported Pd catalysts prepared from synthetic anionic clays

Supported Pd or Rh catalysts were prepared by the solid-phase crystallization method starting from hydrotalcite anionic clay minerals based on [Mg₆Al₂(OH)₁₆CO ₂ ²⁻ ]·4H₂O as the precursors. The precursors were prepared by a coprecipitation method from the raw materials containing Pd²⁺ and various trivalent metal ions which can replace each site of Mg²⁺ and Al³⁺ in the hydrotalcite. Rh³⁺ was also used for preparing the catalyst as comparison. The precursors were then thermally decomposed and reduced to form supported Pd or Rh catalysts and used for the methanol decomposition to synthesis gas. Among the precursors tested, use of Mg–Cr hydrotalcite containing Pd²⁺ resulted in the formation of efficient Pd supported catalysts for the production of synthesis gas by selective decomposition of methanol at low temperature. Although Pd²⁺ cannot well replace the Mg²⁺ site in the hydrotalcite, the Pd supported catalyst (Pd/Mg–Cr) prepared by the solid-phase crystallization method formed highly dispersed Pd metal particles and showed much higher activity than that prepared by the conventional impregnation method. When the precursor was prepared under mild conditions, more fine particles of Pd metal were formed over the catalyst, resulting in high activity. It is likely that the high activity may be due to the highly dispersed and stable Pd metal particles assisted by the role of Cr as the co-catalyst. This revised version was published online in November 2006 with corrections to the Cover Date.     

Methanol Decomposition to Synthesis Gas Over Supported Pd Catalysts Prepared from Hydrotalcite Precursors

Supported Pd catalysts were prepared by solid-phase crystallization (spc) starting from MgAl hydrotalcite anionic clay minerals as the precursors, and were tested for the methanol decomposition to synthesis gas. The precursors based on [Mg₆Al₂(OH)₁₆CO₃ ^2-] · 4H₂O were prepared by coprecipitation from raw materials containing Pd²⁺, Mg²⁺ and Al³⁺ ions as the components of hydrotalcite. The precursors were thermally decomposed and reduced to afford supported Pd catalysts on MgAl mixed oxide. Pd-supported catalysts as a reference were also prepared by the impregnation (imp) method. The spc-Pd catalysts thus prepared afforded highly dispersed Pd metal particles and showed higher activity as well as lower activation energy than the imp-Pd catalysts. When the precursor was prepared under mild conditions, finer particles of Pd metal were formed over the catalyst, resulting in a high activity. In the case of spc-Pd catalysts, carbon monoxide adsorbed on Pd easily desorbed, compared with imp-Pd catalysts. It is likely that the high activity is due to the highly dispersed and stable Pd metal particles and the easy desorption of carbon monoxide.     

Enhanced reductibility of Ni and Ni + Pd in zeolite Y: Blocking of small cages with Ca2+ or Mg2+ ions

Reducibility of zeolite encaged Ni²⁺ was found by temperature programmed reduction to increase in the following order Ni/NaY < Ni/MgY < Ni/CaY < (Pd + Ni)/NaY < (Pd + Ni)/MgY < (Pd + Ni)/CaY. In comparison with NaY supported samples, bimetallic MgY and CaY supported samples exhibited enhanced Pd²⁺ and Ni²⁺ reducibility as well as higher H_ads/Me ° ratios after reduction below 500 °C. Temperature programmed reduction (TPR) and desorption (TPD) results indicate that Mg²⁺ and Ca²⁺ ions are occupying the small cages in the zeolite thus preventing the migration of the majority of Ni²⁺ and Pd²⁺ ions to these cages.     

ESR study of alumina-supported palladium catalyst

The ESR spectra of differently pretreated 0.97 wt% Pd/Al₂O₃ catalyst showed very broad signal atg 2.10 assigned to Pd⁺ ions. The intensity of this signal is stronger after pretreatments at higher temperatures (500–600 °C). This result appears to support our earlier idea (ref. [2]) as to an important role of electron-deficient palladium as an active centre in catalyzing the reaction of neopentane hydrogenolysis.     

Study of polymer-bound mixed valence palladium catalysts. I. Preparation and characterization of polymer-bound mixed valence palladium catalysts

Polymer-bound palladium catalysts of different valence have been prepared by reducing PdCl₂ with a proper reductant, i.e., hydrazine hydrate, under certain conditions. ESR, XPS and IR were used to characterize the catalysts. Pd ⁺ was prepared for the first time and found to be stable on the polymersupport. The authors also investigated the influences of the reducing conditions, the surface properties of polymer supports and the effectsof reducing agents on the degree of reduction of divalent palladium bound onto the polymers, and the state of the palladium after reduction. The experimental results showed that the coexistence of different valence states of palladium in the supports may be Pd ²⁺, Pd^O; Pd²⁺, Pd⁺ (or Pd^δ+); Pd^O and Pd²⁺ Pd⁺ (or Pd^δ+), Pd^O. The method of preparing catalysts of mixed valence is described.     

The Active Phase in the Direct Synthesis of H2O2 from H2 and O2 over Pd/SiO2 Catalyst in a H2SO4/Ethanol System

In an effort to determine the active state of supported palladium for the direct formation of H₂O₂ from H₂ and O₂, the catalytic behavior of Pd⁰/SiO₂, PdO/SiO₂ and partially reduced PdO/SiO₂ was determined. The results obtained in an ethanol slurry, with chloride ions and H₂SO₄ being present, showed that the PdO/SiO₂ catalyst was almost completely inactive for the formation of H₂O₂ at 10 °C. The Pd⁰/SiO₂ catalyst exhibited the highest activity for H₂O₂ formation, and the PdO/SiO₂ material, reduced under very mild conditions, exhibited an intermediate activity. The state of Pd on the three catalysts was characterized by XRD, TEM and XPS methods. Only Pd⁰ (the metal phase) and PdO were observed on Pd⁰/SiO₂ and PdO/SiO₂, respectively. As expected, with the partially reduced PdO/SiO₂ catalyst, both Pd⁰ and PdO phases were evident. The TEM results revealed that the Pd⁰ particles decorated the larger PdO particles. The results reported here support the role of metallic palladium, rather than the oxide, as the active phase for the direct formation of H₂O₂.     

Structure and activity of Au-Pd/SiO2 bimetallic catalyst for thiophene hydrodesulfurization

Monometallic Au, Pd and bimetallic Au-Pd catalysts supported on SiO₂ were prepared by an impregnation method. Their activities on thiophene hydrodesulfurization (HDS) at atmospheric pressure are found to be as a function of calcination temperature of these catalysts. The bimetallic catalyst calcined in air at 400 °C gives the highest activity among them. The techniques including nitrogen physical adsorption, X-ray diffraction, transmission electron microscopy, and X-ray absorption near edge structure were employed to characterize the structure of these catalysts. The results indicate that the effect of gold particles in AuPd/SiO₂ catalyst can facilitate the reduction of PdO phase as well as inhibit the formation of less active Pd₄S phase. The promotional effect of partially oxidative gold and a little of Pd⁰ in AuPd/SiO₂ catalyst is suggested to enhance the HDS activity. The formation of Au_xPd_y alloy phase improves the resistance to sulfur-poisoning of the bimetallic catalyst. The presence of partially oxidized gold particles is considered to be due to the inter-atomic charge transfer from the Au 5d to the Pd 5d band.     

Palladium Supported on an Acidic Resin: A Unique Bifunctional Catalyst for the Continuous Catalytic Hydrogenation of Organic Compounds in Supercritical Carbon Dioxide

1% Palladium‐doped acidic resin (Amberlyst^® 15; styrene‐divinylbenzene matrix with sulfonic acid groups) is shown to be a highly active catalyst for the continuous catalytic hydrogenation of CC bonds in supercritical carbon dioxide (scCO₂) without affecting CO bonds. This 1% Pd/Amberlyst‐15 catalyst promotes the industrially important selective formation of 2‐ethylhexanal from crotonaldehyde in a “one‐pot” pathway involving hydrogenation and aldol condensation with a number of merits. The selectivity behavior of 1% Pd/Amberlyst‐15 is strikingly different compared to that of 1% Pd/C and 1% Pd/Al₂O₃ due to its prominent bifunctional nature based on sulfonic acid groups adjacent to metallic Pd sites. Hybrid “[Pd_n–H]⁺” sites are suggested to act as both metal and acid sites promoting the bifunctional catalysis.     

Novel Pd–Au/TiO2 catalyst for the selective catalytic reduction of NOx by H2

Novel Pd–Au/TiO₂ catalyst exhibited high catalytic activity with a wide temperature window for the selective catalytic reduction of NO_x by H₂ in the presence of oxygen. The synergetic effect between Pd and Au contributes to the formation of Pd⁰ and Pd–Au alloy, thus promoting the NO_x reduction to proceed.     

A novel SiO2 supported Pd metal catalyst for the Heck reaction

A ligand-free heterogeneous metal catalyst system (represented as Pd/SiO₂ (O)) derived by calcination of Pd(acac)₂/SiO₂ in air and its catalytic properties toward the Heck coupling of bromobenzene (PhBr) and styrene have been studied. X-ray photoelectron spectroscopy (XPS) and catalytic results demonstrate that most of Pd²⁺ is reduced to Pd⁰ on SiO₂ by N,N-dimethylacetamide (DMA) during the Heck reaction and that the resulting Pd⁰/SiO₂ is highly active for the Heck reaction, the remaining Pd²⁺/SiO₂ is not responsible for the high activity. Pd/SiO₂ (O) possesses incomparable advantages over a heterogeneous homolog (represented as Pd/SiO₂ (H)) prepared by reduction of Pd(acac)₂/SiO₂ in H₂ as a pre-catalyst in both activity and catalyst recycling. The activity over Pd/SiO₂ (O) is comparable to that over a homogeneous Pd system. Transmission electron microscopy (TEM) analysis illustrates that the high activity over Pd/SiO₂ (O) consists in the small size of supported Pd particles generated in-situ with gentle reducing agents at a mild temperature.     

NO Reduction by CH4in the Presence of O2over Pd-H-ZSM-5

An investigation of the interaction of NO and NO₂with Pd-H-ZSM-5, as well as the reduction of NO by CH₄, has been conducted using mass spectrometry andin situinfrared spectroscopy. Prior to reaction most of the Pd in Pd-H-ZSM-5 (Pd/Al=0.048) is present as Pd²⁺cations. NO reduction by CH₄in the absence of O₂results in the progressive reduction of Pd²⁺cations above 610 K and the formation of small Pd particles. Reduction of Pd²⁺cations is significantly suppressed when O₂is added to the feed of NO and CH₄.In situinfrared spectroscopy reveals the presence of NO⁺and NO as the principal adsorbed species. NO⁺is present as a charge-compensating cation (e.g., Z⁻NO⁺) and is believed to be formed via the reaction 2 Z⁻H⁺+2 NO+1/2 O₂=2 Z⁻NO⁺+H₂O. NO⁺does not react with CH₄at temperatures up to 773 K. Adsorbed NO reacts with CH₄above 650 K and CN species are observed as intermediates. The latter species react with both NO, O₂, and presumably NO₂. Based on the accumulated data, a mechanism is proposed to explain the reduction of NO by CH₄both in the presence and absence of O₂.     

Identification of Pd3+ ions in zeolite L by temperature programmed reduction and electron paramagnetic resonance spectroscopy

Pd supported on KL zeolite has been studied by temperature programmed reduction (TPR) and electron paramagnetic resonance (EPR) spectroscopy. It has been found that upon calcination in pure oxygen Pd³⁺ and Pd²⁺ ions are formed; the Pd³⁺ ions are identified by their EPR signal. As Pd³⁺ ions in the cancrinite cages are difficult to reduce, the H₂ consumption in conventional TPR is below the stoichiometric amount.     

Enhanced stability of hydrochlorination of acetylene using polyaniline-modified Pd/HY catalysts

A polyaniline (PANI)-modified zeolite Y supported Pd catalyst (Pd/PANI-HY) for acetylene hydrochlorination was successfully synthesized by ultrasonic-assisted impregnation. The catalyst exhibited excellent stability, and acetylene conversion was maintained at > 95% more than 300 h. The excellent stability of the Pd/PANI-HY catalyst was attributed to the presence of the polyaniline that partly weakens the occurrence of carbon deposition and also inhibited the reduction of the Pd^2 + active component on the catalyst surface.     

Palladium-lanthanum interaction phenomena in Pd-LaCl3/SiO2 and Pd-La2O3/SiO2 catalysts

The effect of La addition and the nature of the precursors on the surface properties of Pd/SiO₂ are studied. Samples containing 0.5 wt% Pd were prepared by incipient wetness impregnation and characterized by H₂ and CO chemisorption, TEM, TPR, EDX, XPS, Ar⁺-sputtering and CO/FTIR. The use of nitrate precursors improves both reducibility and dispersion of Pd. Lanthanum addition makes the metal reduction more difficult, indicating that a Pd-La interaction takes place. When starting from Cl^- precursors, Pdⁿ⁺ species are formed at the surface, whereas only Pd⁰ is found when nitrate precursors are used. The addition of LaCl₃ increases the Pd dispersion and hinders the formation of Pdⁿ⁺ species through a dilution effect. In a sample prepared from Pd(NO₃)₂₊La(NO₃)₃, the La₂O₃ formed upon calcination originates a SMSI-like effect on the palladium. This effect is suggested by the strong reduction of the H₂ and CO chemisorption capacity of Pd in this sample, in spite of the fact that the dispersion of palladium calculated from TEM increases in the presence of La. From XPS results this SMSI state appears to be due to a physical decoration of Pd by La₂O₃ rather than to an electronic Pd-La₂O₃ interaction. The ``decoration model' is further confirmed by the progressive increase of the Pd/La atomic ratio observed upon XPS/Ar⁺-sputtering at different times the sample containing La. The hypotheses of both dilution and decoration effects caused by LaCl₃ and La₂O₃, respectively, are strengthened by the CO/FTIR results. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the deactivation of supported palladium hydrogenation catalysts by thiophene poisoning

The deactivation of supported palladium catalysts for ethylbenzene hydrogenation by thiophene was studied. The presence of Pd ⁿ⁺ species on the catalyst surface, in samples prepared from acid solutions of PdCl₂, and reduced at temperatures below 450 °C, was evidenced by XPS. A correlation between the concentration of electron-deficient Pd species and the sulfur resistance was found. Thus, the higher the value of the Pd ⁿ⁺/Pd⁰ ratio, the higher the sulfur resistance. Catalysts prepared from Pd(NO₃)₂ or from PdCl₂ reduced at 450 °C, which essentially contain Pd⁰, are more quickly deactivated. Furthermore, our results also suggest that while the influence of the metal dispersion on the stability of the palladium catalysts toward sulfur is not significant, the nature of the support, on the other hand, plays an important role.     

NO reduction with H2 in the presence of excess O2 over Pd/MFI catalyst

The NO SCR (selective catalytic reduction) activity with H₂ in the presence of excess O₂ was investigated over Pd/MFI catalyst prepared by sublimation method. With GHSV=90?000 h⁻¹, a very high steady-state conversion of NO to N₂ (70%) is achieved at 100 °C. Significant reorganizations take place inside the catalyst upon its first contact with all reactants and products at the reaction temperature. Pd⁰, which has a significant role in the NO-H₂-O₂ reaction, is possibly the active site for NO reduction. The formation of Pd-β hydride deactivates the catalyst for NO reduction. Throughout the entire NO-H₂-O₂ reaction, no N₂O or NO₂ is formed; N₂ is the only N-containing product. The presence of O₂ inhibits the formation of undesirable NH₃. The rate of the NO+H₂ reaction is fast or comparable to that of the H₂+O₂ reaction. The oxidation of Pd⁰ and subsequent agglomeration of PdO are responsible for the decreased NO reduction activity at high temperature.     

Macrocycle-Mediated Transport in a Bulk 1.5 M HNO3-CHCI3-0.01 M HNO3 Membrane System of Pd2+ and Mn+ from Pd2+Mn+ Mixtures

Abstract

Pd²⁺ has been transported using sulfur substituted macrocycles as carriers and several Mⁿ⁺ (Mⁿ⁺ = Lⁱ⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Ag⁺, TL⁺, Cd²⁺, and Pb²⁺) have been transported using 18-crown-6 (18C6) and sulfur substituted macrocycles as carriers in a 1.5M HN0₃/CHCL₃3/0.01M HNO₃ bulk liquid membrane system. Competitive Pd²⁺-Mⁿ⁺ transport studies have also been carried out for the same systems. The cyclic polyether 18C6 transports Mⁿ⁺ selectively over Pd²⁺ for all Mⁿ⁺ except Li⁺, Mg²⁺, and Cd²⁺. In the cases of these three cations, no transport was found for either Pd²⁺ or Mⁿ⁺. Generally, the sulfur substituted macrocycles transport Pd²⁺ selectively over Mⁿ⁺.     

Surface restructuring of palladium particles induced by CO adsorption

In order to characterize chemisorption induced reconstruction the surface of supported palladium particles (model catalyst) has been investigated during CO adsorption by SSIMS (static secondary ion mass spectrometry). The SSIMS signal ratio _n Pd _n CO⁺/Pd _n ⁺ has been used to monitor the CO adsorption kinetics. The relative occupancy of linear and bridged CO sites has been determined by the value of PdCO⁺/( _n Pd _n CO⁺) and the variation of the Pd-Pd next neighbour distance by plotting Pd ₂ ⁺ /Pd⁺ during CO exposure. It has been shown that CO chemisorption induces a surface restructuring by increasing the Pd-Pd distance in the particle surface. This phenomenon has the features of a cooperative phenomenon. On the reconstructed particle one bonding state has been identified which appeared to be a precursor of the CO dissociation.     

Catalysts Based Upon Organoclay with Tunable Polarity and Dispersion Behavior: New Catalysts for Hydrogenation, C–C Coupling Reactions and Fluorous Biphase Catalysis

Spherical Pd nanoparticles (2–8 nm) were immobilized on the nanometer-scaled platelets of montmorillonite (MMT) by means of cation exchange of Pd²⁺ for Na⁺ followed by chemical reduction. The resulting Pd catalysts were readily dispersible in water and polar solvents. Polarity design and thus, variation of the dispersion behavior was achieved by subsequent organophilic MMT modification involving cation exchange with N-alkyl and N-perfluoroalkylethyl pyridinium cations. This allowed for easy catalyst dispersion in a wide range of organic solvents. According to characterization by TEM, EDXS, AAS and WAXS the nanoparticle formation did not destroy the superstructure of unmodified and organophilic MMT. Catalysts were applied to hydrogenation, Suzuki–Miyaura C–C coupling and fluorous biphase catalysis. The tunable dispersion behavior of MMT catalysts was successfully employed to guarantee easy catalyst recovery and recycling.