Hydrogen Chemisorption Measurements Research Articles

Rare-earth-platinum alloy nanoparticles in mesoporous zeolite for catalysis.

Platinum is a much used catalyst that, in petrochemical processes, is often alloyed with other metals to improve catalytic activity, selectivity and longevity1-5. Such catalysts are usually prepared in the form of metallic nanoparticles supported on porous solids, and their production involves reducing metal precursor compounds under a H2 flow at high temperatures6. The method works well when using easily reducible late transition metals, but Pt alloy formation with rare-earth elements through the H2 reduction route is almost impossible owing to the low chemical potential of rare-earth element oxides6. Here we use as support a mesoporous zeolite that has pore walls with surface framework defects (called 'silanol nests') and show that the zeolite enables alloy formation between Pt and rare-earth elements. We find that the silanol nests enable the rare-earth elements to exist as single atomic species with a substantially higher chemical potential compared with that of the bulk oxide, making it possible for them to diffuse onto Pt. High-resolution transmission electron microscopy and hydrogen chemisorption measurements indicate that the resultant bimetallic nanoparticles supported on the mesoporous zeolite are intermetallic compounds, which we find to be stable, highly active and selective catalysts for the propane dehydrogenation reaction. When used with late transition metals, the same preparation strategy produces Pt alloy catalysts that incorporate an unusually large amount of the second metal and, in the case of the PtCo alloy, show high catalytic activity and selectivity in the preferential oxidation of carbon monoxide in H2.

Production of benzylamine by tandem dehydrogenation/amination/reduction over Cu and Au catalysts

The feasibility of continuous synthesis of benzylamine from benzyl alcohol by tandem dehydrogenation/amination/reduction has been demonstrated over Cu/SiO2 and Au/TiO2. Both catalysts exhibited an equivalent metal particle size range (1−6 nm) and mean (3.1–3.2 nm, from STEM). Hydrogen chemisorption measurements revealed (five-fold) higher uptake on Au/TiO2 than Cu/SiO2 under reaction conditions. Reductive amination of benzaldehyde over Cu/SiO2 generated a 99 % yield of dibenzylamine with no detectable benzylamine. Reaction of benzyl alcohol with NH3 (in N2) over Cu/SiO2 established utilisation of reactive hydrogen from alcohol dehydrogenation to convert intermediate imines to amines. Benzonitrile was the principal product due to the limited hydrogenation capacity of Cu. Inclusion of Au/TiO2 with Cu/SiO2 as a physical mixture promoted benzylamine formation and inhibited condensation-hydrogenation of benzylamine with benzaldehyde. Incorporation of H2 at low partial pressure (PH2= 0.04 atm) in the feed and a lower reaction temperature (498 K → 448 K) enhanced benzylamine production.

Hydrogenation of toluene over nickel nanoparticles supported on SBA-3 and AlSBA-3 materials

Pure siliceous and aluminosilicate mesoporous molecular sieves of SBA-3 type were used as supports for nickel catalysts obtained via wet impregnation (1 wt.%). Characterization of obtained materials was performed by means of X-ray diffraction (XRD), X-ray fluorescence (XRF), low-temperature nitrogen adsorption/desorption (BET), temperature-programmed desorption of ammonia (TPD-NH3), hydrogen chemisorption measurements and temperature-programmed reduction with hydrogen (TPR-H2). The effect of nickel precursor (nickel nitrate or nickel acetate) and chemical composition of support (SBA-3 and AlSBA-3) on catalytic activity of prepared materials in toluene hydrogenation was studied. It was found that the catalysts obtained using nickel nitrate showed better preserved mesoporous structure with higher dispersion of Ni particles than catalysts obtained from nickel acetate. The use of AlSBA-3 as a support causes the formation of Ni particles with lower dispersion in comparison to the catalysts supported on SBA-3. The activity of nickel catalysts supported on the AlSBA-3 (especially the catalyst obtained using nickel nitrate) for toluene hydrogenation was higher than that of the catalyst supported on SBA-3 despite the lower Ni dispersion. The obtained results bring to conclusion that catalytic activity for toluene hydrogenation is significantly influenced by textural and chemical properties of the support.

Hydrogenation of arenes, N-heteroaromatic compounds, and alkenes catalyzed by rhodium nanoparticles supported on magnesium oxide

A new catalyst composed of Rh nanoparticles supported on magnesium oxide was prepared by mild hydrogenation of a pre-adsorbed organometallic precursor, and characterized by TEM, XRD, XPS, and hydrogen chemisorption measurements. The catalyst contains 5wt% Rh as ∼50nm metallic nanoparticles uniformly distributed on the surface of MgO, in turn made up of smaller (∼2nm) particles. This material is an efficient catalyst for the hydrogenation of arenes, N-heteroaromatic compounds, and alkenes under moderate reaction conditions. Analogous catalysts containing Rh nanoparticles on other rare earth oxide supports display similar catalytic properties and the hydrogenation activity correlates with the basicity of the support in the case of N-heterocycles, following the trend Rh/MgO<Rh/CaO<Rh/SrO.

Ceria-Modified Palladium/Activated Carbon as a High-Performance Catalyst for Crude Caprolactam Hydrogenation Purification

A series of Pd/C catalysts modified with CeO2 were characterized by X-ray diffraction (XRD), transmission electron microscopy–energy dispersive X-ray spectroscopy (TEM-EDX), temperature-programmed reduction (TPR), hydrogen chemisorption, and temperature-programmed desorption (TPD) measurements. The influence of CeO2 additive on the physicochemical and catalytic properties of Pd/C catalyst was investigated. The addition of CeO2 facilitated the reduction of PdO, while the presence of Pd shifted the reduction temperature of CeO2 to lower values. Thus, a synergistic effect between Pd and CeO2 was observed on these CeO2-modified Pd/C catalysts. Both the dispersion of Pd and the hydrogen adsorption strength were significantly promoted by the addition of CeO2. Moreover, Pd-CeO2/C catalysts presented high catalytic performance for crude caprolactam (CPL) hydrogenation purification in comparison with Pd/C catalyst, which may be attributed to higher dispersion, stronger hydrogen adsorption strength, and better redu...

Carbon Nanotube Structures as Support for Ethanol Electro-Oxidation Catalysis

In the present study, the effect of support for ethanol electro-oxidation reaction was investigated on 20 percent Pt/C (E-Tek), 20 percent Pt/commercial CNT, and 20 percent Pt/home-made CNTs. Homemade CNTs were prepared by template synthesis method via chemical vapor deposition (CVD) method. Twenty percent Pt/commercial CNTs and 20 percent Pt/home-made CNTs were prepared by polyol method. The metal dispersions were determined from volumetric chemisorption measurements. These catalysts were tested as anode catalysts for the ethanol electro-oxidation reaction at room temperature by cyclic voltammetry. An optimization study was conducted to find out the optimum scan rate and optimum potential change for ethanol electro-oxidation reaction on 20 percent Pt/C (E-Tek) catalyst. Then, ethanol electro-oxidation measurements were performed on 20 percent Pt/C (E-Tek), 20 percent Pt/commercial CNTs, and 20 percent Pt/home-made CNTs catalysts in 0.5 M H2SO4 +0.5 M ethanol solution at 0.05 V/s scan rate and 1.2 V vs. NHE. Although the raw data indicated that the 20 percent Pt/commercial CNTs exhibited the worst performance, the performances of all of the catalysts were identical after normalizing the current values with respect to the exposed Pt site obtained from the volumetric hydrogen chemisorption measurements. These results indicate that only the metal dispersions improved ethanol electro-oxidation reaction and support did not have any effect on ethanol electro-oxidation reaction under the conditions used in this study.

Carbon Nanotube Structures as Support for Ethanol Electro-Oxidation Catalysis

In the present study, the effect of support for ethanol electro-oxidation reaction was investigated on 20 percent Pt/C (E-Tek), 20 percent Pt/commercial CNT, and 20 percent Pt/home-made CNTs. Homemade CNTs were prepared by template synthesis method via chemical vapor deposition (CVD) method. Twenty percent Pt/commercial CNTs and 20 percent Pt/home-made CNTs were prepared by polyol method. The metal dispersions were determined from volumetric chemisorption measurements. These catalysts were tested as anode catalysts for the ethanol electro-oxidation reaction at room temperature by cyclic voltammetry. An optimization study was conducted to find out the optimum scan rate and optimum potential change for ethanol electro-oxidation reaction on 20 percent Pt/C (E-Tek) catalyst. Then, ethanol electro-oxidation measurements were performed on 20 percent Pt/C (E-Tek), 20 percent Pt/commercial CNTs, and 20 percent Pt/home-made CNTs catalysts in 0.5 M H2SO4 +0.5 M ethanol solution at 0.05 V/s scan rate and 1.2 V vs. NHE. Although the raw data indicated that the 20 percent Pt/commercial CNTs exhibited the worst performance, the performances of all of the catalysts were identical after normalizing the current values with respect to the exposed Pt site obtained from the volumetric hydrogen chemisorption measurements. These results indicate that only the metal dispersions improved ethanol electro-oxidation reaction and support did not have any effect on ethanol electro-oxidation reaction under the conditions used in this study.

Application of liquid phase deposition method for preparation of Co/ZrO x/SiO 2 catalyst with enhanced Fischer–Tropsch synthesis activity: Importance of Co–Zr interaction

Liquid phase deposition (LPD) method using (NH 4) 2ZrO(CO 3) 2 was applied to modifying chemical nature of SiO 2 surface with ZrO x species. ZrO x -modified SiO 2 was then impregnated with Co nitrate solution for preparation of Co/ZrO x /SiO 2 catalysts with 5 and 20 mass% Co loadings. Modification of SiO 2 surface with ZrO x using the LPD method enhanced FTS activity of the Co/ZrO x /SiO 2 catalysts under the reaction conditions of 503 K and 1.1 MPa. The LPD method was superior to other method reported previously such as impregnation and sol–gel methods in terms of surface modification of SiO 2 with ZrO x . H 2-TPR, XRD and hydrogen chemisorption measurements revealed that modification of SiO 2 surface with ZrO x using the LPD method enhanced dispersion of Co 0 without loss of high reducibility of Co. However, degree of Co 0 dispersion was less than that expected from enhancement of activity for the catalysts induced by ZrO x modification, indicating that turnover frequency (TOF) increased in the presence of ZrO x . This effect was particularly important for the catalyst with the low Co loading. Furthermore, in situ DRIFT measurement provided evidence for the formation of specific interaction of Co species with Si–OH and Zr–OH species in the Co/ZrO x /SiO 2 catalysts. It was suggested that the formation of such specific interaction was responsible for not only enhancement of Co 0 dispersion but also increase in TOF of the ZrO x modified catalysts.

Novel Cluster-Size Dependent Phenomenon in Fischer-Tropsch Synthesis with SiO2 Supported Co Cluster Catalyst

Co/SiO2 Fischer-Tropsch synthesis (FTS) catalysts were prepared by a novel preparation method, where aqueous Co nitrate solutions containing various organic acids and/or chelating agents were used as impregnating solutions. FTS activities were investigated at 503 K and 1.1 MPa. Preparation using some chelating agents improved the FTS activity of Co/SiO2 catalyst. Especially, the catalyst prepared using nitrilotriacetic acid (NTA) showed the FTS activity ca. 3 times greater than the catalyst without chelating agents. A volcano-type dependency giving a maximum with NTA was found when the FTS activity of these catalysts was plotted against logarithmic complex formation constants with Co ion in the aqueous solution, suggesting that the formation of Co complex with moderate stability (i.e. NTA-Co complex) is crucial for obtaining higher FTS activity. Hydrogen chemisorption measurements on reduced catalysts showed that the preparation of the catalysts using these chelating agents and/or organic acids leads to the formation of fine metallic Co clusters with 1-5 nm size. TOF of these catalysts was independent of the metallic cluster size in 1-17 nm range, which was evidently different from those reported previously. The formation mechanism of fine metallic clusters was also investigated by means of XPS and Co K-edge EXAFS. [DOI: 10.1380/ejssnt.2009.633]

Characterization and reactivity of Nb2O5 supported Ru catalysts

Abstract A series of Ru/Nb 2 O 5 catalysts with Ru loadings from 0.5–7.18 wt.% were prepared and characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption (TPD) of hydrogen and CO chemisorption measurements. XRD patterns reveal the presence of RuO 2 from 3.0 wt.% Ru loading. TPR profiles show only one peak due to reduction of RuO 2 to Ru 0 . The H 2 -TPD exhibits two peaks, one due to desorption of hydrogen from Ru metal and another due to spill over from metal to the support. The catalytic properties were evaluated for the vapor-phase hydrodechlorination of 1,2,4-trichlorobenzene and related to Ru dispersion.

Experimental and numerical investigation of supported rhodium catalysts for partial oxidation of methane in exhaust gas diluted reaction mixtures

The partial oxidation of methane/oxygen mixtures with large exhaust gas dilution (46.3 vol% H 2 O and 23.1 vol% CO 2 ) has been investigated experimentally and numerically over Rh / CeO 2 – ZrO 2 , Rh / ZrO 2 and Rh / α - Al 2 O 3 catalysts. Experiments were carried out in a short-contact-time ( ∼ 8 ms ) reactor at 5 bar and included exhaust gas analysis, temperature measurements along the reactor, and catalyst characterization. Additional experiments were performed in an optically accessible channel-flow reactor and involved in situ Raman measurements of major gas-phase species concentrations over the catalyst boundary layer and laser-induced fluorescence (LIF) of formaldehyde. A full elliptic two-dimensional numerical code that included elementary hetero-/homogeneous chemical reaction schemes and relevant heat transfer mechanisms in the solid was used in the simulations. The employed heterogeneous reaction mechanism, including only active Rh sites, reproduced the experiments with good accuracy. The ratio of active to geometrical surface area, deduced from hydrogen chemisorption measurements, was the single model parameter needed to account for the effect of different supports. This indicated that water activation occurring on support sites, resulting in inverse OH spillover from the support to the noble metal sites, could be neglected under the present conditions with high water dilution. An evident relationship between noble metal dispersion and catalytic behavior, in terms of methane conversion and synthesis gas yields, could be established. Both measurements and predictions indicated that an increasing Rh dispersion (in the order Rh / α - Al 2 O 3 , Rh / ZrO 2 , and Rh / CeO 2 – ZrO 2 ) resulted in higher methane conversions, lower surface temperatures, and higher synthesis gas yields.

Preparation, characterization, activity and selectivity of highly loaded oxide promoted ruthenium catalysts for selective hydrogenation of benzene to cyclohexene

The influence of promoters and precipitants of the catalyst precursor on the activity and selectivity of the hydrogenation of benzene to cyclohexene catalyzed by highly loaded oxide-promoted Ru/ZrO2catalysts, carried out in a tetraphase reactor (in the presence of an aqueous solution of ZnSO4), at 423 K and 5 Mpa, was studied. The effect of hydrogen diffusion on the reaction kinetics and on the selectivity has been taken into consideration, the internal pore diffusion being actually the limiting step. Hydrogen chemisorption measurements indicate that the catalyst activity is not influenced by the Ru dispersion, but rather by weakly chemisorbed species.

Study of the performance of Pt catalysts supported on activated carbon felt and granular carbon for nitrobenzene hydrogenation

Abstract The catalytic behavior of the Pt (1 wt%) catalysts supported on purified activated carbon felt (ACF) and powder carbon (C) has been studied in the nitrobenzene (NBZ) hydrogenation reaction in liquid phase after activation under a H 2 atmosphere between 298 and 623 K. Hydrogen chemisorption measurements, temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) were used to characterize the metallic phase of the catalysts. For Pt/ACF catalysts, the catalytic activity in nitrobenzene hydrogenation after reduction at 298 K was very high and it decreased as the reduction temperature increased up to 373 K. On the other hand, the Pt/C catalyst after reduction at 298 K was inactive. The activity of the Pt/C catalysts increased in a moderate way when the reduction temperature increases up to 373 K. Between 373 and 623 K of reduction temperature, the hydrogenation rate was modified in a minor proportion for both catalysts. The Pt/ACF catalyst previously reduced at 298 K showed a lag step before the reaction ignition. This effect could be attributed to the tendency the metallic centers have to conform the proper arrangement to hydrogenate the nitrobenzene molecule.

Characterization of reduced α-alumina-supported nickel catalysts by spectroscopic and chemisorption measurements

Ni/α-Al 2O 3 catalysts, prepared from nickel acetylacetonate, with low nickel contents (0.4–3 wt.%), have been characterized after a reduction treatment at 400 and 550 °C by infrared spectroscopy of adsorbed CO (IR-CO), X-ray photoelectron spectroscopy (XPS), hydrogen chemisorption measurements, and temperature-programmed reduction (TPR). Relatively high dispersion and particle size smaller than 8 nm were established. It was observed that an increase of the metal loading resulted in an enhancement of the average particle size and a decrease of the dispersion, indicating that suppression of hydrogen adsorption encountered in supported nickel catalysts with low metal contents is not necessarily due to the amount of nickel as reported in the literature, but instead, can be explained in terms of change of the particle size.

Direct synthesis of methyl isobutyl ketone in gas-phase reaction over palladium-loaded hydroxyapatite

Several compositions of palladium-loaded calcium hydroxyapatite Pd( x)/CaHAp were prepared and characterized by XRD, IR, and UV–visible spectroscopy and temperature-programmed reduction (TPR). The acid–base properties of the samples were studied with the use of butan-2-ol conversion and IR spectroscopy coupled with lutidine adsorption/desorption. Calcium hydroxyapatite bears Lewis acid sites of moderate strength and a limited number of Brønsted acid sites, which produce butenes by dehydration. The selectivity of the dehydrogenation reaction (formation of methyl ethyl ketone) is very limited in the absence of oxygen but indicates the existence on the surface of phosphate of basic sites that are as essential as the acid sites in the direct synthesis of methyl isobutyl ketone (MIBK) from acetone and H 2. Dispersion of Pd on the hydroxyapatite introduces redox properties and the bifunctionality needed in the hydrogenation of mesityl oxide (MO). Temperature-programmed reduction (TPR) of Pd( x)/CaHAp catalysts shows a sharp peak of H 2 consumption between 268 and 276 K, depending on palladium loading, followed by a negative peak at about 338 K, which is associated with the decomposition of β-PdH. Hydrogen chemisorption measurements carried out at 353 K allowed the determination of the size of palladium particles, which varies between 8 and 28 nm. The Pd( x)/CaHAp catalysts showed good performance in the low-pressure one-step synthesis of MIBK. They are stable under the experimental conditions, and they exhibit a MIBK and MIBC selectivity that reaches 92% for a conversion of 22% and an optimal Pd loading of 2 wt%. Other parameters of the reaction (such as temperature of reaction, residence time, H 2/acetone ratio) were studied. They all play an important role in acetone conversion and product distribution. However, the reaction temperature has the most significant influence, since a maximum of the global conversion was observed at 423 K.

Low temperature complete combustion of methane over titania-modified alumina-supported palladium

Effects of titania on the catalytic property of Pd/Al 2O 3 towards methane combustion were examined. The results revealed that the catalytic activity of the Pd/Al 2O 3 catalyst was considerably improved by pre-coating the alumina support with titania at low temperature (below 700 °C). Hydrogen chemisorption and BET measurements revealed that the titania-modified alumina supports could modify the support characterization to achieve a high dispersion of palladium. Temperature-programmed reduction and temperature-programmed desorption study further demonstrated that the coating of Pd/Al 2O 3 catalysts with titania can weaken the bond strength of Pd–O and enhance their catalytic activity towards methane combustion at lower temperature.

Effects of the addition of titania on the thermal characterization of alumina-supported palladium

The detailed thermal characterization of Pd/TiO 2–Al 2O 3 catalysts under oxygen and hydrogen atmosphere was conducted by means of thermal gravimetric analysis/differential scanning calorimetry (TG/DSC), temperature-programmed reduction (TPR) and temperature-programmed desorption (TPD). A simultaneous TG/DSC measurement revealed that the heat evolved during oxygen adsorption at 25 °C varied slightly with the supports and had a higher value for the smaller palladium crystallite. Hydrogen chemisorption and BET measurements revealed that the coating of Pd/Al 2O 3 catalysts with titania modified the support character to achieve a high dispersion of palladium. TPR and TPD characterizations of oxidized samples further demonstrated that the coating of Pd/Al 2O 3 catalysts with titania promoted the reduction and decomposition of PdO into palladium.

Structure sensitivity and in situ activation of benzene combustion on Pt/Al 2O 3 catalysts

The structure sensitivity and in situ activation of benzene combustion on Pt/Al 2O 3 catalysts of different platinum and chlorine loadings were studied. The catalyst activities were evaluated through both conversion versus temperature (light-off curves) and conversion versus time catalytic tests. The light-off curves shifted to lower temperature with increasing Pt particle size, thereby suggesting that benzene combustion is a structure sensitive reaction. Kinetically-controlled catalytic tests confirmed that benzene oxidation turnover rates are preferentially promoted by larger Pt crystallites. Kinetic studies showed that the reaction orders and the apparent activation energy are not changed by changing the metallic dispersion. Results are explained by considering that benzene oxidation proceeds via a Langmuir–Hinshelwood mechanism which involves the rapid and strong adsorption of benzene on metallic platinum and assumes that the rate constant of oxygen adsorption is very low compared to the rate constant of the surface reaction. The number of PtO bonds of lower binding energy, i.e. the site density of more reactive surface oxygen, increases on larger Pt particles. Low-conversion catalytic tests performed at constant temperature showed that on well-dispersed Pt/Al 2O 3 catalysts the benzene conversion increases with time, irrespective of the chlorine content on the sample. TEM examination and hydrogen chemisorption measurements suggested that the activity increase parallels a concomitant increase in the platinum particle size. In contrast, sintered samples (platinum dispersions lower than 10%) did not exhibit initial activation periods. It is proposed that the initial in situ activation of well-dispersed Pt catalysts is caused by the sintering of the metallic phase. Hot-spots on the metallic particles together with the presence of gaseous water cause the formation of larger Pt crystallites, even at mild reaction conditions. As a result, the benzene conversion increases with time until the formation of larger steady state Pt particles is completed.

Hydrogenation of Benzene over Alumina-Supported Nickel Catalysts Prepared from Ni(II) Acetylacetonate

Hydrogenation of benzene was used to probe the activity of Ni/Al2O3 catalysts with metal content in the 0.4–3 wt% range prepared with Ni acetylacetonate as a precursor. Average particle sizes in the 1.9–8 nm range were determined from hydrogen chemisorption measurements depending on the nickel content and reduction temperature. A preliminary decomposition of the organic ligands in air at 250°C followed by reduction markedly increased the activity relative to a direct reduction. Treatments at higher temperatures had a detrimental influence on catalyst efficiency. Catalysts reduced at 350–400°C were more active than those reduced at 450°C and above, with the most active catalysts having nickel content of 1 and 1.6%. The dependency of the turnover frequency on particle size suggested that the reaction was structure sensitive for particle sizes below 4 nm and insensitive above this size. The fraction of unreduced nickel also had an effect on the activity. From a tentative comparison of the turnover frequencies (TOF) with values reported in the literature, it may be inferred that Ni(acac)2 is a rather valuable precursor in preparing very efficient and stable catalysts with low nickel content.

Microwave heating during catalyst preparation: influence on the hydrodechlorination activity of alumina-supported palladium–iron bimetallic catalysts

Alumina-supported Pd–Fe mono- and bimetallic catalysts have been prepared by microwave irradiation and by conventional calcination methods. The catalysts were characterised by powder X-ray diffraction (XRD), temperature-programmed reduction (TPR) and nitrogen adsorption measurements. Hydrogen chemisorption measurements were performed after low- and high-temperature reduction (LTR and HTR) of the catalyst. Hydrodechlorination of chlorobenzene was carried out on the reduced catalysts. Microwave heating changed the crystallite size and the susceptibility of the catalysts to alloy formation, with a resultant increase in the activity of the reduced catalysts for the hydrodechlorination of chlorobenzene.