Strong modification of Pt-CO interaction caused by alloying with chromium in Pt-Cr/HZSM-5 catalysts

The catalysts Pt/HZSM-5 and Pt-Cr/HZSM-5 are characterized by FTIR spectroscopy using CO as a probe molecule, and by transmission electron microscopy. The major fraction of the metal phase detected in the micrographs exhibits particles in the range of 1–3 nm uniformly distributed in the zeolite crystals. Nanodiffraction analysis of single particles confirmed the formation of fcc Pt-Cr alloy with lattice constanta = 0.386 nm. The infrared spectra at 300 K exhibit bands assigned to CO linearly bonded to zeolite-hosted metal particles at 2089 cm^–1 for Pt/HZSM-5, and at 2120 and 2092 cm^–1 for Pt-Cr/HZSM-5. The temperature increase to 610 K resulted in a strong shift of the 2089 cm^–1 band for Pt/HZSM-5 by 25 cm^–1 and a slight shift of the corresponding bands for Pt-Cr/HZSM-5 by 6 cm^–1. The differences are referred to different electron backdonation capacities of the CO binding Pt surface atoms of the metal particles. The lower capacity for the Pt-Cr/HZSM-5 sample is due to alloying in the metal particles.     

Cerium promoted Pd/HZSM-5 catalyst for methane combustion

A series of Pd/HZSM-5 (Si/Al₂ = 165) catalysts without and with additives of oxides of La, Ce, Sm, Nd and Tb were prepared by the impregnation method, and characterized by XRD, Raman spectra, N₂-adsorption, CO-chemisorption, O₂-TPD and CH₄-TPR techniques. The catalysts were investigated for low-temperature CH₄ combustion, and CeO₂ was found to have a significant promoting effect on the activity of Pd/HZSM-5. Pd–Ce/HZSM-5 showed the best methane combustion activity and the improved thermal/hydrothermal reaction stability among tested catalysts. The characterization results of catalysts indicated that CeO₂ can effectively promote the formation of crystalline PdO and weaken the bond strength of Pd–O on Pd–Ce/HZSM-5, resulting in that Pd–Ce/HZSM-5 possessed lower temperatures for oxygen desorption and CH₄ reduction than Pd/HZSM-5. This could be ascribed to the covalent property and large oxygen storage/supplying capacity of CeO₂. It is believed that more active PdO species on Pd/HZSM-5 for low-temperature methane combustion process could be effectively promoted due to the introduction of CeO₂.     

Direct synthesis of hydrogen peroxide over Pd nanoparticles embedded between HZSM-5 nanosheets layers

Direct synthesis of hydrogen peroxide (DSHP) was studied over Pd loaded on HZSM-5 nanosheets (Pd/ZN). Pd nanoparticles with average size of ca. 4.3 nm were introduced into the adjacent nanosheet layers (thickness of ca. 2.9 nm) by impregnation method. Pd/ZN with theoretical Si/Al molar ratio of 25 showed the highest selectivity for H₂O₂ among the prepared catalysts, together with highest formation rate of H₂O₂ (38.0 mmol·(g cat)⁻¹·h⁻¹), 1.9 times than that of Pd supported on conventional HZSM-5 zeolite (Pd/CZ-50). Better catalytic performance of nanosheet catalysts was attributed to the promoted Pd dispersion which promoted H₂ dissociation, more Brønsted acid sites and stronger metal-support interaction which inhibited the dissociation of OO bond in H₂O₂. The embedded structure sufficiently protected the Pd nanoparticles by space confinement which restrained the Pd leaching, leading to a better catalytic stability with 90% activity retained after 3 cycles, which was almost 3 times than that of Pd/CZ-50 (30.4% activity retained).     

Direct synthesis of hydrogen peroxide from H2 and O2 and in situ oxidation using zeolite-supported catalysts

Four microporous materials, zeolites HZSM-5, Y, Beta and TS-1, were used as the supports to prepare supported gold catalysts using impregnation or deposition precipitation. The gold catalysts were tested in the direct synthesis of hydrogen peroxide from H₂ and O₂ and for CO oxidation. The effect on the catalytic activity of different metal (e.g., Pd, Pt, Cu, Ag, Rh or Ru) on the synthesis of hydrogen peroxide was also tested. Organic substrates, such as cyclohexane or cyclooctene, were introduced to investigate the possibility of in situ H₂O₂ oxidation with these catalysts.     

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.     

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.     

Low-temperature synthesis of DME from CO2/H2 over Pd-modified CuO–ZnO–Al2O3–ZrO2/HZSM-5 catalysts

Three series of Pd-modified CuO–ZnO–Al₂O₃–ZrO₂/HZSM-5 catalysts were prepared and characterized by BET, XRD and TPR analysis. The catalytic system was evaluated in the development of direct synthesis of dimethyl ether (DME) from carbon dioxide hydrogenation at low temperature (T=200 °C, P=3.0 MPa). The results indicated that the addition of palladium markedly enhanced the DME synthesis and retarded the CO formation. An explanation of this promoting effect of Pd on the DME synthesis could be attributed to the spillover of hydrogen from Pd⁰ to the neighboring phase.     

Selective catalytic reduction of nitrogen monoxide with methane over impregnated In/HZSM-5 in the presence of excess oxygen

In/HZSM-5 catalyst prepared by the impregnation method was active for NO reduction with methane. Complete reduction of NO was obtained at 450°C over an In/HZSM-5 catalyst. The presence of oxygen in the feed greatly enhanced the NO reduction activity of In/HZSM-5. Co/HZSM-5 and Ga/HZSM-5 were less effective than In/HZSM-5. Cu/HZSM-5, In/Na-ZSM-5 and In₂O₃/Al₂O₃ were ineffective for NO reduction with CH₄. The NO reduction activity was proportional to the level of indium impregnated onto HZSM-5 but excess amounts of indium were detrimental to the catalytic activity. Phase analysis by XRD measurements demonstrated that there was a threshold value in the indium content, i.e., the maximum dispersion capacity of indium oxides. It is concluded that highly dispersed indium species are the active centers for the selective catalytic reduction of NO with CH₄.     

Characterization of Pd/γ-Al2O3 Catalysts Prepared Using [Pd(hfac)2] in Liquid CO2

A series of Pd/γ-Al₂O₃ catalysts was prepared from [Pd(hfac)₂] (hfac = hexafluoroacetylacetonate) in liquid carbon dioxide using the method reported by Kim et al. [Chem Mater 18:4710 (2006)]. The catalysts were characterized using CO pulse chemisorption, diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS), X-ray absorption fine structure (XAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron microscopy. The catalysts were reduced initially in the high-pressure CO₂ reaction cell using H₂ at 75 °C. Samples were removed, stored in a desiccator, and re-reduced in situ at 250 °C prior to pulse chemisorption, DRIFTS and XAFS. CO pulse chemisorption evidenced that the Pd dispersion decreased from 55% to 5% as the Pd loading increased from 0.58 to 3.94 wt.%. The as-prepared 0.58 and 1.77 wt.% Pd/γ-Al₂O₃ catalysts (after air exposure) contained oxidized Pd species that were converted after in situ reduction to supported Pd particles. The average Pd particle sizes of these two catalysts (16 and 23 Å, respectively) estimated from the first-shell Pd–Pd coordination numbers are in good agreement with the CO chemisorption results. DRIFTS evidenced a prevalence of weakly bound linear CO (ν_CO = 2083 cm^?1) adsorbed on the 0.58 wt.% Pd catalyst. A 2.95 wt% Pd catalyst (49 Å average particle size) also exhibited a strong linear CO band (ν_CO = 2093 cm^?1). In contrast, CO chemisorption on a commercial 1 wt.% Pd/Al₂O₃ catalyst (37 Å average particle size) gave predominantly 2-fold bridging CO species. We infer that the supported Pd particles prepared from [Pd(hfac)₂] are rougher on the atomic scale (with a higher percentage of edge and corner atoms) than equivalently sized particles in conventionally prepared Pd/γ-Al₂O₃ catalysts.     

Catalysts by design: genesis and CO hydrogenation of novel Pd carbonyl clusters in zeolite 5A

The potential of catalyst synthesis by design is demonstrated by comparing Pd/5A to Pd/NaY. While supercage dimensions are similar for both zeolites, the diameter of the supercage windows not only determines the Pd nuclearity of entrapped Pd carbonyl clusters, but also restricts their growth under CO hydrogenation conditions. While Pd₁₃(CO) _x clusters prevail in zeolite Y as a result of migration and coalescence of primary Pd carbonyl clusters after CO exposure at room temperature, cluster growth in zeolite 5A is confined to Pd₆(CO) _x . Under the conditions of syngas conversion, small Pd clusters are stabilized in the supercages of 5A, in contrast to the agglomeration of Pd particles to size larger than 60 Å in NaHY. The catalytic activity of Pd/5A is twice that of Pd/NaHY. The selectivities of CO hydrogenation on both catalysts are also drastically different: on Pd/5A, methanol and dimethylether are the sole products besides methane, but on Pd/NaHY, production of C₂₊ hydrocarbons is significant.     

FeOx supported single‐atom Pd bifunctional catalyst for water gas shift reaction

Water gas shift reaction on supported noble metal catalysts is an essential process for upgrading hydrogen source industrially. Here a series of Pd/FeO_x catalysts were detected for this reaction with Pd/Al₂O₃ as reference. It was found that Pd/FeO_x exhibited higher CO conversion than Pd/Al₂O₃ with a good stability even in the presence of CO₂ and H₂. Along the loading decreasing, the turnover frequency of exposed Pd atoms increased with the dispersion from subnanometer (～1 nm) to single atoms. Various characterizations suggested that Pd single atoms greatly enhanced the reducibility of FeO_x and facilitated the formation of oxygen vacancies, which served as sites to promote the dissociation of H₂O to form H₂ and atomic O. The atomic O was ready to react with the linear adsorbed CO species on Pd single‐atom sites through a redox mechanism, which resulted in low activation energy of ～30 kJ mol^?1. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4022–4031, 2017     

The role of protons in the NO reduction by acetylene over ZSM-5

Intermediate species formed on ZSM-5 zeolites during the selective catalytic reduction of NO by acetylene (C₂H₂-SCR) were investigated by transmittance FTIR and temperature-programmed desorption, to elucidate the different catalytic performance of the proton and sodium forms of ZSM-5. Bidentate nitrates formed exclusively on HZSM-5 were very active toward the reductant, whereas the nitrates associated with the Na⁺ ions on NaZSM-5 were inert. Although the saturation adsorption amount of acetylene on NaZSM-5 was about five times higher than that on HZSM-5 at 80 °C, most of the acetylene on NaZSM-5 was desorbed below 200 °C, whereas a considerable amount of acetylene was detected above 250 °C on HZSM-5 in TPD. The strongly adsorbed species formed on acetylene adsorption was vinyl alcohol (CH₂CHOH) bound to the Brønsted acid sites that could be converted to acetate species at elevated temperatures on HZSM-5. Based on our results, a mechanism of the C₂H₂-SCR on HZSM-5 was proposed in which the bidentate nitrates and acetate species formed exclusively on HZSM-5 react with each other, leading to NO_x elimination, which explains why the behavior of HZSM-5 is quite different from that of NaZSM-5 in the C₂H₂-SCR.     

Enhanced performance of methane dehydro-aromatization on Mo-based HZSM-5 zeolite pretreated by NH4F

Enhanced performance of methane dehydro-aromatization reaction (MDA) were achieved on a Mo-based HZSM-5 zeolite catalyst in which HZSM-5 were pretreated by a proper amount of NH₄F (Mo/HZ(F)). The results of NH₃-TPD and ²⁷Al MAS NMR demonstrated that the number of Brönsted acid sites decreased on the HZSM-5 zeolite and Mo/HZSM-5 catalyst after NH₄F treatment. TGA and TPO measurements showed that the Mo/HZ(F) catalysts were highly resistant to coke deposition, which resulted mainly from the elimination of the Brönsted acid sites after the pretreatment of the HZSM-5 zeolite with NH₄F.     

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.     

Physicochemical properties and catalytic performance of galloaluminosilicate in aromatization of lower alkanes: a comparative study with Ga/HZSM-5

A series of gallium-containing HZSM-5 zeolites with different Ga contents (Ga/(Al+Ga)?=?0.1?C0.6) were prepared by hydrothermal in situ synthesis and post synthesis. Their catalytic performance were compared in the aromatization of propane, butane and propane/butane mixture (1:1?molar). Galloaluminosilicate obtained via hydrothermal in situ synthesis exhibited high fraction of acidic framework Ga³⁺ with few dispersed extracrystalline Ga₂O₃. Ga/HZSM-5 obtained by post synthesis showed the presence of extracrystalline Ga₂O₃ and/or extra framework gallyl ions. The aromatization performance of Ga-containing HZSM-5 followed the following sequence; galloaluminosilicate > Ga/HZSM-5 (ion-exchange) > Ga/HZSM-5 (impregnation) ? HZSM-5. Optimum aromatization performance over galloaluminosilicate was achieved with Ga/(Al+Ga) ratio of 0.3. Propane conversion reached 50.9?wt% over galloaluminosilicate with Ga/(Al+Ga) of 0.3, as compared to 31.8 and 40.7?wt% for the corresponding Ga/HZSM-5 obtained by impregnation and ion-exchange, respectively, at gas hourly space velocity of 1,600?h^?1, and 540?°C. Comparison of aromatic selectivity at the same conversion level (~10.0?wt%) revealed that galloaluminosilicate is more selective than Ga/HZSM-5. The superior performance of galloaluminosilicate was attributed to the presence of highly dispersed-reducible extra-framework Ga₂O₃ (Lewis-dehydrogenating sites) formed by degalliation in close proximity to zeolitic Br?nsted sites. Thus, hydrothermal in situ approach can thus be considered as an effective method for improving the aromatization performance of HZSM-5.     

The electrochemical reduction of PdCl4 and PdCl6 in polyaniline: Influence of Pd deposit morphology on methanol oxidation in alkaline solution

The controlled uptake and electrochemical reduction of metal precursors PdCl₄²⁻ and PdCl₆²⁻ in polyaniline (PANI) is demonstrated. The formation of PANI/Pd composites is achieved with a reduction in proton doping and an increase in the oxidation of the polymer with Pd deposits physically blocking the nitrogen groups. High surface area filaments (PdCl₄²⁻) or a rough encapsulation (PdCl₆²⁻) of Pd metal on PANI are obtained. The structural differences highlight the influence of the metal precursor oxidation state on the morphology of the Pd deposits in PANI. Thermal gravimetric analysis provides an estimate of the Pd content for each composite of ∼40%. X-ray Photoelectron Spectroscopy and X-ray-excited Auger Electron Spectroscopy analyses confirm the deposition of Pd metal. The catalytic oxidation of methanol was demonstrated for both PANI/Pd composites in alkaline solutions that prohibit proton doping of the polymer. The data indicates that Pd metal acts as a solid-state dopant that may delocalize the charge on the polymer backbone to maintain conductivity. Methanol oxidation at PANI/Pd composites produced using PdCl₄²⁻ was enhanced relative to the composite produced using PdCl₆²⁻ and a planar Pd electrode. Comparison of PANI/Pd composite produced using PdCl₄²⁻ with other Pd catalysts from the literature indicates surface poisoning is reduced when Pd is coupled with the polymer. The composite is robust and stable in alkaline solution with the charge density decreasing by 5% on the positive scan and 13% on the negative scan after 200 voltammetric cycles. The data also indicates that the reductive desorption of surface contaminants is possible, minimizing the catalytic loss due to surface poisoning.     

Hydrodechlorination Reactions of CCl4 Over HZSM-5- and HY-Supported Pt and Pd Catalysts

The hydrodechlorination reactions of CCl₄ have been studied over Pt/HZSM-5, Pt/HY, Pd/HZSM-5 and Pd/HY under hydrogen atmosphere in the 393?C673?K temperature range in a pulse microreactor. The catalysts were prepared by impregnation and were characterised by various physico-chemical means. It has been found that all catalysts were active in the partial to full dechlorination reactions. Typically, over the Pt-containing catalysts methane was the major dechlorinated product, while it was ethane over the Pd-containing catalysts. This latter product predominated over the Pd/HZSM-5 and the quantities of the di- and trichlorinated products were minimal and the monochlorinated one was not formed at all. Phosgene formed was the indication of the partial destruction of the support being more serious for the HY-supported ones.     

Effect of acid sites on catalytic destruction of trichloroethylene over solid acid catalysts

The catalytic destruction of trichloroethylene (TCE) over several solid acid catalysts (HZSM-5, γ-Al₂O₃ and SBA-15/P) was evaluated under dry conditions. The activity order was found to be: HZSM-5>SBA-15/P>γ-Al₂O₃. It was reported that Brønsted and Lewis acid sites of catalysts both played an important role on TCE catalytic destruction, while the Brønsted acid sites were more decisive. Additionally, the formation of the polychlorinated by-product (tetrachloroethylene, PCE) over HZSM-5 and γ-Al₂O₃ catalysts was observed and attributed to the presence of Lewis acid sites and basic O^2?, and PCE was not detected over SBA-15/P catalyst due to the presence of only Brønsted acid sites. The TCE/O₂-TPSR studies demonstrated that the main oxidation products during TCE catalytic destruction are CO, CO₂ and Cl₂, and the carbon in TCE was firstly converted to CO and then further oxidized into CO₂ by gas phase O₂.     

Surface Modification of Pd/α-Si3N4 Catalysts Through the Solvent Used During Synthesis. Implications on the CO Chemisorption Properties and Catalytic Performances

Palladium catalysts supported on α-Si₃N₄ were prepared by impregnation with Pd(II)-acetate dissolved either in toluene or in water. The mean metal particle size of ~0.5 wt% Pd catalysts was similar (~5 nm) and independent of the way of preparation. Nevertheless, the two catalysts present very different chemisorption behaviour chemisorptive and catalytic properties. Fourier transformed infrared (FTIR) spectra of adsorbed CO at different temperatures (ranging from room temperature to 300 °C) show a very different behaviour for both catalysts. While the CO adsorption states on the Pd/α-Si₃N₄ prepared in toluene are very similar to those generally measured for silica and/or alumina supported palladium catalysts, CO chemisorbs less strongly on Pd/α-Si₃N₄ prepared in water and on different adsorption sites. The Pd/α-Si₃N₄ catalyst obtained by aqueous impregnation is much less efficient for the methane total oxidation. It is less active and less stable: it deactivates strongly after 3 h on stream at 650 °C. The two catalysts present about the same activity for the 1,3-butadiene hydrogenation after stabilisation at 20 °C. But, the catalyst prepared in water shows a much better selectivity to butenes. The results are discussed in terms of the possible migration of silicon atoms from the silicon nitride support to the surface of the palladium particles, when the catalyst is prepared in water. This is not the case when prepared in an organic solvent.     

Effects of Pd particle size on the rates of oxygen back-spillover and CO oxidation under dynamic oxygen storage and release measurements over Pd/CeO2 catalysts

A kinetic mathematical model has been applied to investigate for the first time the effects of Pd particle size on the rates of oxygen back-spillover and CO oxidation during Oxygen Storage Capacity (OSC) measurements under dynamic conditions over Pd/CeO₂ catalysts in the 500–700 °C range. The dependence of the intrinsic rate constant k₁ of the CO oxidation reaction on PdO, and that of k ₂ ^app of the oxygen back-spillover from ceria to Pd/PdO on the palladium particle size was estimated by performing curve-fitting of the experimental CO and CO₂ pulse transient responses obtained. Activation energies of 8.0, 9.5 and 21.1 kJ/mol were calculated for the Eley–Rideal step of CO oxidation for the 1.3, 1.8 and 16.4 nm Pd particles, respectively, supported on CeO₂. The transient rates of CO oxidation and oxygen back-spillover were found to decrease with increasing Pd particle size.