Marked effect of In,Pb and Ce addition upon the reduction of NO by CO over SiO2 supported Pd catalysts

Addition effect of In, Pb and Ce on the NO–CO reaction over SiO₂ supported Pd catalysts was studied, using a closed gas circulation system as well as an in situ infrared spectroscopy. Formation of intermetallic compounds was observed in the cases of Pd–In/SiO₂ and Pd–Pb/SiO₂ catalysts, which caused the drastic enhancement of the reaction rate of N₂O formation. The infrared analyses revealed the weakening of adsorption strength of CO on Pd metal by the formation of intermetallic compounds, which is the main reason for the enhancement of the reaction rate. In the case of Pd–Ce/SiO₂ catalysts, SMSI-like interaction between Pd and CeO₂ would be important for the enhancement effect.     

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.     

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.     

吡啶改性Pd/SiO2催化剂用于H2和O2直接合成H2O2

引言过氧化氢(H2O2)是一种理想的绿色氧化剂,广泛应用于化学品合成、纺织、造纸、环保、食品、医药、冶金和农业等领域[1]。目前,蒽醌法[2-5]是工业上生产H2O2的主要方法。20世纪40年代,德国I.G.Farbenindustrie首先采用蒽醌法(又称Riedl-Pfleiderer法)工业化生产过氧化氢。该方法首先将2-烷基蒽醌(通常是2-乙基蒽醌)溶解于合适的有机溶剂中,溶液中的2-烷基蒽醌经催化剂催化加氢,被还原成蒽氢醌或5,6,7,8-四氢蒽氢醌,再经空气氧化得到蒽醌或四氢蒽醌和     

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.     

Synthesis and Hydrodesulfurization Properties of Noble Metal Phosphides: Ruthenium and Palladium

Silica-supported ruthenium and palladium phosphide catalysts (Ru₂P, RuP, Pd₃P, Pd₅P₂) were investigated for the hydrodesulfurization (HDS) of dibenzothiophene (DBT). The Ru and Pd phosphide catalysts were prepared by temperature-programmed reduction of hypophosphite-based precursors consisting of uncalcined or calcined Ru/SiO₂ or Pd/SiO₂ impregnated with ammonium hyposphosphite (NH₄H₂PO₂). The Ru₂P/SiO₂ and RuP/SiO₂ catalysts prepared from uncalcined precursors had smaller average crystallite sizes, higher CO chemisorption capacities, and higher HDS activities than the catalysts prepared from the calcined precursors, while the effect of preparation method on catalytic properties was less clear for the Pd₃P/SiO₂ and Pd₅P₂/SiO₂ catalysts. Following HDS testing at 673?K, X-ray diffraction analysis revealed that the Pd₅P₂/SiO₂ catalysts decomposed to give Pd₃P on the silica support, while the other phosphides exhibited good stability during the testing period. At temperatures at which high DBT conversion was observed (>598?K), the Ru and Pd phosphide catalysts were less active than sulfided Ru/SiO₂ and Pd/SiO₂ catalysts prepared from the uncalcined metal precursors.     

Detailed Mechanism for Reduction of N2O over Rhodium by CO in Automotive Exhaust

Elementary-steps based mechanisms of CO–O₂ and CO–N₂O over rhodium catalyst were proposed and utilized to simulate experimental data from literature. The results showed that the mechanisms possess good prediction capability. It was found that the dissociation of adsorbed N₂O is the rate limiting step of N₂O reduction under conditions characterized by high CO coverages. The rather high light-off temperature (50 % conversion) of CO–N₂O (638 K) compared to that of CO–O₂ (453 K) is explained by the high temperature to initiate N₂O dissociation to offer surface oxygen needed for CO oxidation. Removing CO out of the reaction system, the oxygen generated via the dissociation of adsorbed N₂O accumulates on the surface of Rh, and finally leads to a poisoned catalyst and termination of the N₂O reduction process. However, increasing the inlet CO concentration inhibits the adsorption of N₂O to some extent, thus the reduction rate of N₂O is lowered on the contrary. Analysis of kinetic parameters showed that facilitating CO desorption or the decomposition of adsorbed N₂O leads to higher conversion of N₂O, with the latter having larger influence.     

An overview: Comparative kinetic behaviour of Pt, Rh and Pd in the NO + CO and NO + H2 reactions

This paper reports a comparative kinetic investigation of the overall reduction of NO in the presence of CO or H₂ over supported Pt-, Rh- and Pd-based catalysts. Different activity sequences have been established for the NO+H₂ reaction Pt/Al₂O₃>Pd/Al₂O₃>Rh/Al₂O₃ and for the NO+CO reaction Rh/Al₂O₃>Pd/Al₂O₃> Pt/Al₂O₃. It was found that both reactions differ from the rate determining step usually ascribed to the dissociation of chemisorbed NO molecules. The rate enhancement observed for the NO+H₂ reaction has been mainly related to the involvement of a dissociation step of chemisorbed NO molecules assisted by adjacent chemisorbed H atoms. The calculation of the kinetic and thermodynamic constants from steady-state rate measurements and subsequent comparisons show that Pd and Rh are predominantly covered by chemisorbed NO molecules in our operating conditions which could explain either changes in activity or in selectivity with the lack of ammonia formation on Rh/Al₂O₃ during the NO+H₂ reaction. Interestingly, Pd and Rh exhibit similar selectivity behaviour towards the production of nitrous oxide (N₂O) irrespective of the nature of the reducing agent (CO or H₂). A weak partial pressure dependency of the selectivity is observed which can be related to the predominant formation of N₂ via a reaction between chemisorbed NO molecules and N atoms, while over Pt-based catalysts the associative desorption of two adjacent N atoms would occur simultaneously. Such tendencies are still observed under lean conditions in the presence of an excess of oxygen. However, a detrimental effect is observed on the selectivity with an enhancement of the competitive H₂+O₂ reaction, and on the activity behaviour with a strong oxygen inhibiting effect on the rate of NO conversion, particularly on Rh.     

Effects of chloride precursors on the palladium valency and surface structures of PdMg2+/SiO2catalysts for carbon monoxide hydrogenation

The effects of chloride precursors on the Pd valence states and surface structures of PdMg²⁺/SiO₂ catalysts for carbon monoxide hydrogenation have been studied by means of electron-spin resonance (ESR) and X-ray photoelectron spectroscopy (XPS) techniques. The Cl-containing catalyst exhibits a much more positive Pd valency and a much more stable Pd surface structure than the Cl-free one. This is attributed to the stronger ability of chlorine anions to stabilize Pd structures and to transfer electrons from Pd to Mg²⁺, rather than to a better Pd dispersion of the Cl-containing catalyst. Over the Cl-containing catalyst, methanol formation and a Pd⁺ ESR signal are found to increase concurrently with reaction time. Accordingly, Pd⁺ ions are claimed to be essential for methanol formation. However, only those Pd⁺ ions which are located in a suitable structure are supposed to be the active center for methanol synthesis, because oxygen or hydrogen-water pretreatments are found to cause (i) a marked decrease of the turnover frequencies (TOF) for methanol formation, (ii) a probable transformation of Pd(Cl,X)Mg²⁺ (X=Cl or O) bridge structures to oxychloropalladium and MgO, and (iii) the formation of more Pd⁺ ions. Furthermore, the Pd⁺ ions formed during the induction period are found not to be mainly due to the catalyst being modified by water or methanol, but probably due both to the high-pressure Pd reconstruction and the oxidation of an HCOO intermediate. In addition, the metal-promoter-support interactions are discussed.     

Infrared studies of reaction of ethylene with syngas on Ni/SiO2

Ni/SiO₂, a methanation catalyst, has been shown to exhibit CO insertion activity. In situ infrared studies of CO/H₂ and C₂H₄/CO/H₂ reactions on Ni/SiO₂ show that carbonylation of Ni/SiO₂ to Ni(CO)₄ leads to an inhibition of methanation in CO hydrogenation but an enhancement of formation of propionaldehyde in the C₂H₄/CO/H₂ reaction.     

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.     

FTIR study of CO adsorption on Pd/SiO2 and Pd-Cu/SiO2 treated with sulphur dioxide

Infrared spectra are reported of Pd/SiO₂ and Pd-Cu/SiO₂ exposed at 295 K to either CO followed by SO₂ or SO₂ followed by CO. Chemisorptive interaction of SO₂ with bridging CO sites on Pd {100} faces created cationic Pd sites which adsorbed CO linearly. Multibonding CO sites on Pd {111} faces and linear CO sites were poisoned by weak non-dissociative SO₂ adsorption, and could be regenerated by evacuation. Pd/SiO₂ was more resistant than Cu/SiO₂ to SO₂ poisoning but for Pd-Cu/SiO₂ the poisoning of Pd was promoted by the presence of Cu. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of metal-support interactions on the hydrogenation of CO over PdSiO2 and PdLa2O3

A study of CO hydrogenation over $\text{Pd}\text{SiO}{}_2$ and $\text{Pd}\text{La}{}_2\text{O}{}_3$ has been carried out for the purpose of identifying the effects of Pd dispersion, Pd morphology, and support composition on the catalytic activity of supported Pd. The specific activity of each catalyst for methanol and methane synthesis was determined from microreactor studies carried out at a fixed set of reaction conditions. Palladium dispersion was measured by H₂O₂ titration, and the morphology of the Pd crystallites, as expressed by the distribution of Pd(100) and Pd(111) planes, was determined from in situ infrared spectra of adsorbed CO. The crystallite morphology of the $\text{Pd}\text{SiO}{}_2$ catalysts is the same, independent of Pd weight loading: 90% of the surface is comprised of Pd(100) planes and 10% of the surface is comprised of Pd(111) planes. By contrast, the crystallite morphology of the $\text{Pd}\text{La}{}_2\text{O}{}_3$ catalysts changes with Pd loading. Primarily Pd(100) planes are exposed at low-weight loadings while Pd(111) planes are exposed at high-weight loadings. The Pd dispersion has little effect on the methanol turnover frequency over both $\text{Pd}\text{SiO}{}_2$ and $\text{Pd}\text{La}{}_2\text{O}{}_3$, for dispersions between 10 and 20%. On the other hand, the methane turnover frequency is independent of Pd dispersion over $\text{Pd}\text{SiO}{}_2$, but increases with decreasing dispersion over $\text{Pd}\text{La}{}_2\text{O}{}_3$. It is further observed that the Pd morphology influences the specific activity of $\text{Pd}\text{La}{}_2\text{O}{}_3$ for methanol synthesis: Pd(100) is nearly threefold more active than Pd(111). For a fixed morphology, the specific methanol synthesis activity of $\text{Pd}\text{La}{}_2\text{O}{}_3$ is a factor of 7.5 greater than that of $\text{Pd}\text{SiO}{}_2$.     

Positive Effect of Water Vapor on CO Oxidation at Low Temperature over Pd/CeO2–TiO2 Catalyst

CO oxidation at low temperature over Pd/CeO₂–TiO₂ catalyst was carried out in the feed containing different contents of water vapor (H₂O). A positive effect of H₂O was observed on the catalytic performance of Pd/CeO₂–TiO₂ in CO oxidation at low temperature. The extent of this effect depends on the content of H₂O in the feed; with a H₂O content being 2.5 vol%, the catalyst Pd/CeO₂–TiO₂ exhibits the highest stability (longest life time for CO oxidation at 80 °C). The results of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temperature-programmed reaction (TPReaction) reaction illustrated that H₂O in the feed supplies sufficient OH groups in the presence of O₂, which can react with adsorbed CO on Pd species to produce CO₂. Moreover, H₂O may also enhance the adsorption of CO and suppress the formation of some carbonate species.     

Adsorbed species and reaction rates for NO-CO-O2 over Rh(111)

We have studied the NO-CO-O₂ reaction over a Rh(111) catalyst by monitoring the reaction products (CO₂, N₂O, and N₂) and the infrared (IR) intensity of surface CO and NO at various partial pressures of NO, CO and O₂, and sample temperatures. The selectivity for N₂O formation, apparent activation energy for product formation, and NO consumption rate during NO-CO-O₂ are identical to those measured during the NO-CO reaction. The IR measurements show that during NO-CO-O₂ the same two adsorbed species, NO at 1640 cm^-1 and linear CO at ~2040 cm^-1, are present in the same surface concentrations as during NO-CO. For this reason the NO-CO-O₂ kinetics are dominated by the NO-CO kinetics, the NO consumption is rate limited by dissociation of adsorbed NO, and the N₂O selectivity is dominated by surface NO coverage. In contrast, O₂ consumption is adsorption rate limited with the NO-CO adsorption-desorption equilibrium controlling the vacant sites required to dissociatively adsorb O₂. These kinetic and IR data of the CO-NO-O₂ reaction and our interpretation of them agree with previous studies over supported Rh catalysts and thus confirm the previously proposed explanation. From RAIRS and kinetic data we estimate the rate constant for the CO+O→CO₂ elementary step. The pre-exponential factor for this rate is 2×10¹⁰ s^-1, a factor of 50 smaller than previous estimates. This rate constant is important to the NO-CO-O₂ kinetics because it affects O coverage, which, under certain conditions, inhibits NO consumption. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of CeO2 and La2O3 on the Activity of CeO2−La2O3/Al2O3-Supported Pd Catalysts for Steam Reforming of Methane

The effect of the addition of CeO₂ or La₂O₃ on the surface properties and catalytic behaviors of Al₂O₃-supported Pd catalysts was studied in the steam reforming of methane. The FTIR spectroscopy of adsorbed CO and the Pd dispersion suggest the partial coverage of Pd⁰ by ceria or lanthana species. This could lead to the formation of an adduct MPd _x O (M = Ce or La) at the surface of the metal crystallites. The addition of ceria or lanthana resulted in an increase of the turnover rate and specific rate for steam reforming of methane. One possible explanation if that the Pd⁰*Pd^δ+O–M interfacial species (M = Ce or La) are oxidized by H₂O or CO₂, promoting the O* transfer to the metal surface. This could facilitate the removal of C* species from the metal surface, resulting in the increase of specific reaction rate and increase of the accessibility of CH₄ to metal active sites.     

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.     

Comparison of H2Adsorption,O2Adsorption,H2Titration,and O2Titration on Supported Palladium Catalysts

Four different methods for Pd dispersion measurement are compared: H₂adsorption, O₂adsorption, H₂titration, and O₂titration. A correct comparison requires the knowledge of H/Pd_sand O/Pd_sstoichiometries. The H/Pd_sstoichiometry is discussed by reviewing published results, which lead us to the conclusion that H/Pd_s=1. Our experimental results demonstrate that the relative O/H stoichiometry is independent of Pd concentration, type of carrier, and preparation method, O/H=1; thus O/Pd_s=1. If correctly performed, the four methods give the same Pd dispersion, but those based on direct adsorption have less applicability, because they require the complete removal of hydrogen used for reduction from the metal surface. A method to evaluate residual hydrogen is given.     

Promoting effect of noble metals to Co/SiO2 catalysts for hydroformylation of 1-hexene

Small amount of Pt, Pd and Ru promoters in Co/SiO₂ led to great improvement of catalyst activity for hydroformylation of 1-hexene. The 1-hexene conversion as high as 89.7% and oxygenate products selectivity of 88.9% were obtained after 2 h reaction over Pd promoted Co/SiO₂ catalyst. The increasing of reduction degree, the minimizing of cobalt particle size and the enhancement of carbonyl and linear CO adsorption were responsible for the improved performance of the catalyst.     

Partial oxidation of ethane over K2MoO4/SiO2 and potassium promoted MoO3/SiO2 catalyst

Silica supported K₂MoO₄ and potassium-promoted MoO₃ were used as catalysts for the partial oxidation of ethane in fix-bed continuous-flow reactor at 770–823 K using N₂O as oxidant. The main products of the oxidation reaction were ethylene, acetaldehyde, CO and CO₂. Addition of various compounds of potassium to the MoO₃/SiO₂ greatly enhanced the conversion of ethane and influenced the product distribution. The highest rate and selectivity for acetaldehyde formation was found on a K₂MoO₄/SiO₂ catalyst.