Hydrogen Chemisorption Measurements Research Articles

Characterization of the Structure and Catalytic Activity of Pt/Sepiolite Catalysts

Four catalysts containing 1% w/w Pt deposited on various sepiolite supports were prepared. Two natural sepiolites and another two obtained by acid treatment of one of them were used. Both the sepiolites used as metal supports and the catalysts were characterized for structure and surface properties, using 1H and 29Si MAS NMR spectroscopy, and 29Si CP/MAS NMR spectroscopy, in addition to hydrogen, pyridine, and CO2 chemisorption measurements. The activity of the catalysts in the hydrogenation of benzene to cyclohexane and the dehydrogenation of cyclohexane to benzene was examined and the catalyst supported on the natural sepiolite called Pangel was found to be the most active of all.

Palladium–vanadium interaction in binary supported catalysts

We have studied the activity and selectivity of Pd/γ-Al 2O 3, VO x /γ-Al 2O 3 and Pd–VO x /γ-Al 2O 3 catalysts for the decomposition of NO and the reduction of NO with CO. Pd–VO x /γ-Al 2O 3 catalysts were prepared by anchoring Pd(AcAc) 2 on VO x /γ-Al 2O 3. Characterization of the binary samples by hydrogen chemisorption and TPR measurements indicated that the reduction of VO x is enhanced by a close contact with palladium and that partially reduced vanadia decorate noble metal particles. This palladium–vanadium interaction alters the catalytic properties of palladium: the activity for NO decomposition is higher for the binary sample and, for the NO–CO reaction, both the activity and the selectivity to N 2 increase when vanadium is in contact with palladium.

Preparation, characterization, and reactivity of Pt/SDBC catalysts for the hydrogen-water isotopic exchange reaction

Pt/SDBC catalysts, which are used for the hydrogen-water isotopic exchange reaction, were prepared. TGA experiments showed that the treatment temperature of Pt/SDBC catalysts in inert gas is limited to 400 °C and the maximum allowable heat treatment temperature in oxygen is 200 °C. From nitrogen adsorption and hydrogen chemisorption measurements, it was shown that the dispersion of platinum particles depended on the physical properties, i.e., specific surface area and pore structure of SDBC. It was found that the heat treatment could not impact the structure of SDBC and the oxygen treatment at 150 °C improved the platinum dispersion. It was shown by XPS analysis that the oxygen treatment of impregnated Pt/SDBC increased the fraction of platinum metal state and platinum dispersion. As the supported platinum area increases, the catalytic activity of Pt/SDBC for the hydrogen-water vapor isotopic exchange reaction increases. It indicates that the hydrogen chemisorption measurement can be used to estimate the catalytic activities of Pt/SDBC catalysts. It was not observed that the particle size of supported platinum affected the specific reaction rate at 60 °C. It implies that this reaction is structure insensitive.

Selective hydrogenation of benzene to cyclohexene using a suspended Ru catalyst in a mechanically agitated tetraphase reactor

The reactivity on unsupported Ru based catalyst in benzene selective hydrogenation to cyclohexene has been studied. The reaction has been carried out in a tetraphase slurry reactor at 423 K, at 5 MPa, in the presence of two liquid phases: benzene and an aqueous solution of ZnSO 4 0.6 mol l −1. A detailed study of the influence of the transport phenomena on the reactivity of the catalyst has been carried out. No correlation has been found between the characteristic numbers of Weeler–Weisz and of Carberry mass transport at external liquid/solid interface or into the catalyst pores for both benzene and hydrogen and the selectivity of the catalyst. The main features of the catalysts are the strong dependence between the catalysts preparation procedure and their activity and selectivity. In particular the influence of the alkaline or the earth alkaline hydroxide, employed in the precipitation of the Ru precursor, on the selectivity, has been studied. Hydrogen chemisorption measurements indicate that the amount of weakly adsorbed hydrogen depends on the nature of the base employed in the precipitation step.

Kinetics of the NO and CO Reaction over Platinum Catalysts: I. Influence of the Support

The kinetics of the CO+NO reaction over Pt-based catalysts was investigated using a fixed bed flow reactor at 300°C, with CO and NO partial pressure ranges of 1.5×10−3to 9×10−3atm. Pt was deposited on various supports: γ-Al2O3, Si3N4, and Cr3C2. XRD, XPS, and hydrogen chemisorption measurements seem to indicate that Pt dispersion decreases as follows: Pt/Al2O3>Pt/Si3N4>Pt/Cr3C2. Kinetic data obtained on these Pt catalysts were interpreted on the basis of four mechanisms as proposed in the literature; only one mechanism can correctly model the reactant partial pressure dependencies of the rate. This mechanism involves nondissociative CO and NO competitive adsorptions followed by a dissociation of adsorbed NO which requires a vacant nearest-neighbor adsorption site. Clearly, the adsorption equilibrium constants, λCOand λNOof CO and NO, together with the rate constant of the NO dissociation step are strongly influenced by the support, particularly λNO, which is consistently lower than λCOon Pt/Al2O3, increases notably for Pt/Si3N4, and increases even more for Pt/Cr3C2. It has been shown that Pt/Si3N4may be the most suitable catalyst if the Pt dispersion on this support can be improved. The nature of the support also has a significant effect on the selectivity of the NO transformation into N2and N2O. Pt/Cr3C2is the most selective catalyst for N2formation (much more than Pt/Si3N4and particularly Pt/Al2O3). This seems to be related to the fact that the rate constant of the step 2Nads→N2is much higher than that of the other step Nads+NOads→N2O.

Acetone hydrogenation over co-precipitated Ni/Al2O3, Co/Al2O3 and Fe/Al2O3 catalysts

Co-precipitated Ni/Al2O3, Co/Al2O3 and Fe/Al2O3 catalysts containing 10–50 wt.% of metal are prepared and characterized by BET surface area, pore volume and XRD. Hydrogen chemisorption and oxygen uptake measurements are used to estimate the extent of reduction and the available surface metal sites. Acetone hydrogenation under vapour phase conditions is carried out over the catalysts in the temperature region 373–523 K. Conversion and selectivity of the catalysts are compared and are correlated with their hydrogen and oxygen uptake characteristics. Nickel catalysts are very active followed by cobalt and iron. A dual function mechanism involving both metal and acidic properties decide the selectivity of the products. The selectivity for propan-2-ol depends only on the metal availability whereas the selectivity for isobutyl methyl ketone depends on both acidic and metallic character of the catalyst. A mechanism explaining the different products formed is postulated.

X-Ray Absorption Characterization of Supported Palladium Catalysts : A Comparison with Hydrogen Chemisorption

X-ray absorption spectroscopy was used to characterize the supported palladium hydrocracking catalysts. Results with regard to the percentage of metal exposed are compared with those from hydrogen chemisorption measurements. Results from XAS indicate that hydrogen chemisorption has underestimated the metal dispersion. Possible explanations for this discrepancy are discussed.

The effect of the precursor on the characteristics of Co/SiO 2 catalysts

The effect of the precursor on the characteristics of the Co/SiO 2 catalysts prepared from Co(NO 3) 2, Co 2(CO) 8 and Co 4(CO) 12 was determined. The near-surface reduction was clearly lower for the Co 2(CO) 8 derived catalyst than for the Co 4(CO) 12 based one. The hydrogen chemisorption, CO desorption, XRD and XPS measurements indicated that the dispersion of the metallic species decreased in the precursor order Co 2(CO) 8 > Co 4(CO) 12 ≫ Co(NO 3) 2. The TPD studies showed that CO was more strongly adsorbed on catalysts derived from carbonyls than on those based on nitrate, resulting in greater initial activity in CO hydrogenation. In pulsed CO hydrogenation reactions, the selectivity of the Co 2(CO) 8 derived catalyst differed from the other two. Thus, despite the well documented transformation of the Co 2(CO) 8 to Co 4(CO) 12 on the silica support, the supported catalysts derived from these two precursors exhibited distinct characteristics and reactivity.

Benzene Hydrogenation as a Tool for the Determination of the Percentage of Metal Exposed on Low Loaded Ceria Supported Rhodium Catalysts

Benzene hydrogenation, a structure insensitive reaction, was used to determine the percentage of metal exposed onlowloadedceriasupported rhodium 0.15 to 0.33 wt% Rh/CeO2catalysts. Catalysts were prepared from nitrated and chlorinated precursors. Freshly prepared catalysts and catalysts aged in air at 1173 K were studied. Hydrogen chemisorption carried out at 313 K on the chlorinated catalysts could be used to confirm the technique since, due to the presence of Cl−ions on the samples, no hydrogen spillover was observed after reduction at 773 K. Data obtained from benzene hydrogenation were found to be in very good agreement with the percentages of metal exposed as calculated from hydrogen chemisorption measurements. Benzene hydrogenation was able to count very low percentages of metal exposed and it was shown from the data obtained for the nonchlorinated catalysts that, unlike hydrogen chemisorption, it was not disturbed by the hydrogen spillover occurring on some Rh/CeO2systems. The stability of the metal phase was investigated when rhodium was deposited on both high and low surface area ceria. It appeared that anionic exchange on a low specific surface area ceria could lead to a catalyst which, after calcination at 1173 K and reduction at 773 K, exhibits the same amount of exposed metallic rhodium atoms compared to a catalyst prepared from a high specific surface area ceria, hence, containing 50% more rhodium.

Modified Activities and Selectivities of Silated–Oxidized–Reduced Pd and Pt Catalysts

Supported nanoscale Pd and Pt particles lose their ability to catalyze hydrogenation and to hydrogenolyze triethylsilane when silated with either trimethyl silane or silane. Subsequently exposing these silated catalysts to oxygen restores their hydrogenation activities to various extents depending on their metal particle sizes. Large particles (<50%D) experience partial restoration, whereas small particles (>50%D) experience virtually complete restoration (85–111%) of their original alkene hydrogenation activity. Benzene hydrogenation activity on Pt and triethyl silane hydrogenolysis activities on Pd are restored less than alkene hydrogenation activities on Pt and Pd. Additional data from XPS, FTIR, DSC, TGA, and hydrogen chemisorption measurements point to a mechanism in which silation produces surface silicon, some of which migrates into the metal particle and some of which remains on the surface and poisons hydrogenation and hydrogenolysis activities. Oxidation produces PdO2(or PtO2) and SiO2at the surface, and because the surface Si concentration has been reduced, interior Si migrates back to the surface where it is also oxidized. Hydrogen does not reduce SiO2but does reduce PdO2to Pd metal, which now presents a new reconstructed surface for hydrogenation. Thereby, Si alloyed to nanoscale Pd particles imparts not only new chemical characteristics, but also new physical characteristics. The reconstructed larger Pd–Si particles are not able to form as many favorable hydrogenation sites as before silation–oxidation–reduction, whereas the reconstructed smaller Pd–Si particles are able to form more hydrogenation sites than before. Likely the smallest Pd particles experience instabilities, rendering them incapable of maintaining many active hydrogenation sites; however, alloying with Si enlarges and stabilizes them such that they are capable of maintaining more active sites than before.

Surface organometallic chemistry on metals: kinetics of the hydrogenolysis of tetra n-butyl tin on silica and alumina supported rhodium catalysts

The stoichiometry and the kinetics of the hydrogenolysis of tetra n-butyl tin on silica and alumina supported rhodium catalysts has been determined at 298 K in n-heptane. Various Rh/SiO 2 and Rh/Al 2O 3 catalysts have been prepared by exchange from [RhCl(NH 3) 5] (OH) 2 and by impregnation from RhCl 3 respectively. After reduction, metallic dispersions were found to vary from 0.45 to 0.9 as determined by hydrogen chemisorption measurements. Concerning the hydrogenolysis of the organotin complex, the amount of grafted tin was found to depend on the amount of tetra n-butyl tin introduced: since this amount is lower than the value corresponding to a ratio Sn/Rh s of 0.7, all the tin complex is grafted and the amount of grafted tin is directly proportional to the number of surface rhodium atoms; on average, each surface rhodium atom is able to graft 0.8 Sn(n-C 4H 9) x fragment. Above this ratio, the amount of grafted tin is almost independent of the amount of complex introduced. These results strongly support the hypothesis that the reaction occurs selectively on the metallic surface. The stoichiometry of the hydrogenolysis, determined by the amount of butane evolved per mole of grafted tin, depends on the coverage of the rhodium surface: for a low coverage, all the butyl groups are hydrogenolyzed, showing the formation of ‘naked’ tin atoms. For a full coverage, some butyl groups remain fixed on the surface showing the presence of grafted organotin species on the rhodium surface. The stoichiometry of the hydrogenolysis also depends on the reaction time for a given concentration of tetra n-butyl tin: at short reaction times which correspond to low coverages of the metallic particles, all the butyl groups are removed whereas this phenomenon is not observed at longer reaction times for which alkyl groups remain on the metallic surface. Kinetics of the hydrogenolysis indicates that the reaction is first order in ‘free surface adsorption site’ and zero order in tetra n-butyl tin.

Characterization of Pd [sbnd]CeO 2 interaction on alumina support and hydrogenation of 1,3-butadiene

The influence of the preparation method and loading of CeO 2 on the interaction of Pd/CeO 2/Al 2O 3 catalysts was studied. The materials were characterized by temperature-programmed reduction, hydrogen chemisorption and X-ray photoelectron spectroscopy measurements. The 1,3-butadiene hydrogenation was used as a model reaction. The Pd Ce interaction affected the reduction behavior of the catalysts. Temperature-programmed reduction (TPR) results showed that the presence of CeO 2 shifts the reduction temperature of PdO to lower values, while palladium similarly facilitates the reduction of the cerium surface species. The hydrogen uptake decreased when increasing the ceria content. XPS results showed that the Pd/Al ratio decreased with CeO 2 addition being affected by the preparation method and content. The formation of catalytic sites at the interface of Pd Ce was postulated from turnover frequency (TOF) results of the 1,3-butadiene hydrogenation. The number of these new sites increases with the addition of ceria. The nature of these sites is discussed in terms of the influence on the selectivity in butadiene hydrogenation.

Effects of the addition of lanthana on the thermal stability of alumina-supported palladium

Promotional effects of lanthana on the catalytic property of Pd/A1 2O 3 toward methane combustion were examined. The results revealed that Pd/A1 2O 3 became active on raising the temperature above 767 K but its activity decayed considerably at high temperatures. Addition of La 2O 3 to the Pd/A1 2O 3 catalyst inhibited this activity decay. Hydrogen chemisorption and BET measurements revealed that the addition of La 2O 3 to Pd/A1 2O 3 not only enhanced the thermal stability of γ-A1 2O 3, but also retarded the supported Pd from sintering upon calcinations at T> 923 K. Temperature-programmed oxidation studies further demonstrated that the retardation in Pd sintering resulted from an increase in the bond strength of Pd-O. Therefore, the increase in the bond strength of Pd-O may account for the promotional effect of lanthana and the long lifetime of La-added Pd/A1 2O 3 catalyst in methane combustion.

Sulfur Poisoning of a Pt/BaK-LTL catalyst: A catalytic and structural study using hydrogen chemisorption and x-ray absorption spectroscopy

The sulfur poisoning of a Pt/BaK-LTL catalyst has been studied with X-ray absorption spectroscopy and hydrogen chemisorption. The fresh catalyst contained highly dispersed platinum inside the zeolite pores. EXAFS analysis determined a Pt-Pt coordination number of 3.7, suggesting an average platinum cluster size of 5 or 6 atoms, consistent with the TEM and chemisorption data (H/Pt = 1.4). The catalyst was poisoned with H 2S until the dehydrocyclization activity of n-hexane decreased to 30% of fresh activity. The first-shell PtPt coordination number increased to 5.5, indicating a growth of the average platinum cluster size to 13 atoms. Hydrogen chemisorption measurements of the poisoned catalyst show a decrease in the H/Pt value to 1.0. The EXAFS data also provide evidence for the presence of sulfur adsorbed on the surface of the platinum particles with a PtS bond distance of 2.27 Å. The high sensitivity of the Pt/LTL catalyst to poisoning by very low levels of sulfur is attributed to the loss of active platinum surface by adsorption of sulfur and the growth of the platinum clusters. Much of the available platinum surface was found to be capable of chemisorbing hydrogen, but with no activity for dehydrocyclization. Growth of the platinum particle was sufficient to block the pore. In the sulfur-poisoned catalyst, only the sulfur-free platinum atoms exposed through the pore windows remain active. The evidence suggests the location of the sulfur was at or near the metal-zeolite interface. Since both high activity and selectivity require extremely small platinum particles, regeneration of sulfur-poisoned catalysts will require removal of the adsorpted sulfur and restoration of the original particle size.

Magnesia-supported nickel catalysts I. Factors affecting the structure and morphological properties

The effects of calcination and reduction temperature on the metal dispersion of MgO-supported Ni in the ranges 400–800°C and 300–800°C, respectively, have been systematically evaluated by hydrogen chemisorption measurements and comparison has been made with a NiOMgO physical mixture. The role of the bulk Ni x Mg (1 − x) O solid solution in controlling NiO reducibility and the metal dispersion of Ni/MgO has been ascertained by XPS and ISS investigations. A mechanistic model accounting for the nickel oxide dispersion across the MgO support and the consequent formation of the bulk Ni x Mg (1 − x) O solid solution has been proposed. A linear relationship between the mean particle size and the reduction temperature, the slope of which increases with Ni loading (2.8–18.0 wt%), has been found. A “multilayer arrangement” of Ni precursor on the support is discussed.

Effects of lanthanide metals in Eu–Ni and Yb–Ni bimetallic catalysts

Lanthanide metals (Eu and Yb) dissolved in liquid ammonia and reacted readily with reduced Ni powders to form Eu–Ni and Yb–Ni bimetallic catalysts with different lanthanide contents. The catalysts were characterized by the hydrogenolysis of ethane and cyclohexane, the hydrogenation of ethene, hydrogen chemisorption and temperature-programmed desorption (TPD) measurements. The rate of hydrogenolysis decreased markedly when Yb and Eu were added to the Ni surface, whereas the hydrogenation activity showed a tendency to increase, especially in the Eu–Ni system. The lanthanide itself showed a low activity for the hydrogenolysis and hydrogenation reactions under the reaction conditions. An analysis of the decrease in the rate of hydrogenolysis with lanthanide addition suggested a decrease in concentration of surface nickel available for structure-sensitive reactions caused by lanthanide coverage. For the hydrogenation of ethene the surface was gradually covered with the lanthanide metals and simultaneously certain interactions occurred to produce active centres; the presence of lanthanide metals on Ni strongly influenced the state of adsorption of hydrogen in the subsequent activation processes, resulting in enhanced capacity of this surface to dissociate hydrogen. For the hydrogenation of ethene and adsorption characteristics, lanthanide and transition metals were more efficient when they were used together.

Activity and selectivity in carbon monoxide hydrogenation over rhodium supported on pure zirconia and on K-, P-, and Y-doped zirconia

Catalysts for the CO/H 2 reaction were prepared by supporting Rh 4(CO) 12 on pure ZrO 2, with different phase compositions, and on ZrO 2 doped at the surface or in the bulk with K 20, P 20 5, or Y 2O 3, and by decomposing the supported carbonyl in a hydrogen stream at 523 or 823 K. The catalysts were characterized by XRD, oxygen and hydrogen chemisorption, TPR, FT-IR/PAS, and catalytic activity measurements both in temperature-programmed experiments and in isothermal condition runs, at atmospheric pressure and with different feed compositions. The phase composition of the support or the doping procedure (bulk or surface) has a minor effect on the physicochemical properties and catalytic activity. Also, the various doping agents, with the exception of K 20, have minor effects. K 2O doping decreases the activity and changes the selectivity of Rh/ZrO 2 in the CO/H 2 reaction, favoring the formation of oxygenated compounds. Activation at high temperature (823 K) decreases the activity but does not change the selectivity of the catalysts. A significant influence of the feed HZ/CO ratio on activity and selectivity has been found: activity decreases with Hz/CO ratio and selectivity to oxygenates increases. The role of the various parameters investigated is discussed.

Characterization of silica-supported ruthenium catalysts by hydrogen chemisorption and nuclear magnetic resonance of adsorbed hydrogen

Adsorbed hydrogen on four silica-supported ruthenium catalysts was measured quantitatively by proton magnetic resonance (PMR). The PMR technique revealed two distinct adsorbed states of hydrogen on the metal: reversible and irreversible. The results from PMR and those from conventional hydrogen chemisorption measurements were compared directly. The observed discrepancy between the PMR and volumetric techniques in the case of total adsorption is attributed to spillover of reversibly adsorbed hydrogen from ruthenium onto the silica support. Good agreement was obtained between the two techniques in the case of irreversible adsorption. The relatively narrow line of PMR spectra on the reversibly adsorbed hydrogen indicates rapid motion for this state of hydrogen on ruthenium surfaces. The variation of spectral lineshift and of the spin-lattice relaxation times for the adsorbed hydrogen with ruthenium particle size suggests a stronger interaction between the adsorbed hydrogen and defect-like ruthenium adsorption sites. The results from PMR intensity measurements also suggest that the reversibly adsorbed hydrogen is at least in part associated with the defect-like ruthenium adsorption sites.

Monte Carlo simulations of supported bimetallic catalysts

Supported bimetallic catalysts are modeled with a Monte Carlo simulation technique that uses a coordination-dependent potential model. Cubo-octahedral particles with dispersions ranging from 30 to 60% are studied as well as particles that have irregular shapes. Systems modeled include the PtIb (Ib = Cu, Ag, Au), AgRu, and PtRh bimetallics. In the PtIb systems, the Ib element segregates to the surface of the catalyst and tends to occupy the lowest coordinated sites first. Differences in the degree of surface segregation among the Ib elements are easily seen at higher Ib concentrations where Au is shown to segregate to the surface more strongly than Ag or Cu. The degree of clustering of Pt atoms on the surface of the catalyst particles depends on which Ib element is present. For a given amount of Ib atoms present on the surface the AuPt system is observed to produce larger ensembles of surface Pt atoms than the AgPt or CuPt systems. Predicted relative platinum dispersions for the AgPt system were compared to hydrogen chemisorption measurements and found to be in good agreement except for one sample (30 wt% Ag) in which the measured value was found to be higher than the predicted value. This difference could be explained by a nonuniform distribution of silver in the catalyst sample or by the effect of chemisorption. In the AgRu system, silver atoms are found to segregate to the surface and cluster together. Predicted relative ruthenium dispersion is compared to hydrogen chemisorption measurements and found to be in agreement. The PtRh system showed that platinum atoms undergo net surface segregation, although rhodium atoms tended to dominate the corner and edge sites of the crystallites.

The effect of oxygen surface groups of the support on platinum dispersion in Pt/carbon catalysts

A series of Pt catalysts supported on carbon black has been prepared; the original support was treated in such a way as to ensure carbons with the same porous texture but with an increasing amount of oxygen surface groups. Pt catalysts (about 1% loading) were prepared with these carbons using the same experimental procedure and the metal dispersion was calculated from hydrogen chemisorption measurements. Both dispersion and resistance to sintering were found to be a function of the number of oxygen surface groups of the support.