CO Chemisorption Capacity Research Articles

Investigation of activity losses of gold nanoparticles in the CO selective oxidation

Gold catalysts nanometrically dispersed over Al 2O 3, ZrO 2 and ZrO 2/Al 2O 3 are prepared and tested in selective CO oxidation. XRD, DRS, Raman and in situ DRIFTS techniques are used to investigate the main causes of activity loss after long period storage, evaluating the physical–chemical properties of the fresh catalyst and after 2 years storage. Storing in darkness is not efficient to maintain the catalytic activity. The gold particle sizes increase 13–54% depending on the support. The auto-reduction of cationic gold nanoparticles causes the decrease of CO chemisorption capacity and hence a drastically decrease in the activity.

Effect of indium in trimetallic Pt/Al 2O 3SnIn–Cl naphtha-reforming catalysts

Trimetallic Pt/Al 2O 3SnIn–Cl naphtha-reforming catalysts were characterized by 119Sn Mössbauer spectroscopy, TPR, CO chemisorption, FTIR of adsorbed CO and NH 3-TPD. TPR traces indicate that indium facilitates tin reduction and strongly interacts with platinum as well. 119Sn Mössbauer spectroscopy results show that addition of even small amount of In (0.06 wt.%) leads to the formation of a Pt 3Sn alloy. At higher indium loadings, higher amounts of Pt x Sn alloys of almost equal Pt and Sn atomic concentrations were detected. Indium also decreases the overall CO chemisorption capacity and provokes a modification of Pt electronic properties at the highest In loadings. A decrease in the acidity of the support in trimetallic catalysts is also observed by NH 3-TPD upon indium addition. These results correlate well with those from n-heptane-reforming reaction, showing that indium decreases catalyst’s overall conversion and selectivity to C 1 and C 3–C 4 paraffins and increases isomerization selectivity.

Thermokinetic Analysis of the CO2 Chemisorption on Li4SiO4 by Using Different Gas Flow Rates and Particle Sizes

Lithium orthosilicate (Li(4)SiO(4)) was synthesized by solid-state reaction and then its CO(2) chemisorption capacity was evaluated as a function of the CO(2) flow rate and particle size. Initially, a Li(4)SiO(4) sample, with a total surface area of 0.4 m(2)/g, was used to analyze the CO(2) chemisorption, varying the CO(2) flow between 30 and 200 mL/min. Results showed that CO(2) flows modify the kinetic regime from which CO(2) capture is controlled. In the first moments and at low CO(2) flows, the CO(2) capture is controlled by the CO(2) diffusion through the gas-film system, whereas at high CO(2) flows it is controlled by the CO(2) chemisorption reaction rate. Later, at larger times, once the carbonate-oxide external shell has been produced the whole process depends on the CO(2) chemisorption kinetically controlled by the lithium diffusion process, independently of the CO(2) flow. Additionally, thermokinetic analyses suggest that temperature induces a CO(2) particle surface saturation, due to an increment of CO(2) diffusion through the gas-film interface. To elucidate this hypothesis, the Li(4)SiO(4) sample was pulverized to increase the surface area (1.5 m(2)/g). Results showed that increasing the surface particle area, the saturation was not reached. Finally, the enthalpy activation (DeltaH(double dagger)) values were estimated for the two CO(2) chemisorption processes, the CO(2) direct chemisorption produced at the Li(4)SiO(4) surface, and the CO(2) chemisorption kinetically controlled by the lithium diffusion, once the carbonate-oxide shell has been produced.

Support effect on the low-temperature hydrogenation of benzene over PtCo bimetallic and the corresponding monometallic catalysts

PtCo bimetallic and Co, Pt monometallic catalysts supported on γ-Al2O3, SiO2, TiO2 and activated carbon (AC) were prepared and evaluated for the hydrogenation of benzene at relatively low temperatures (343K) and atmospheric pressure. Results from flow reactor studies showed that supports strongly affected the catalytic properties of different bimetallic catalysts. AC supported PtCo bimetallic catalysts exhibited significantly better performance than the other bimetallic catalysts, and all the bimetallic catalysts possessed higher activity than the corresponding monometallic catalysts. Results from CO chemisorption and H2-temperature-programmed reduction (H2-TPR) studies suggested that different catalysts possessed different properties in chemisorption capacity and reduction behavior, and AC supported PtCo catalysts possessed significantly higher CO chemisorption capacity compared to the other catalysts. Extended X-ray absorption fine structure (EXAFS) and transmission electron microscopy (TEM) analysis provided additional information regarding the formation of Pt–Co bimetallic bonds and metallic particle size distribution in the PtCo bimetallic catalysts on different supports.

Effect of Ge content on the metal and acid properties of Pt-Re-Ge/Al2O3-Cl catalysts for naphtha reforming

Abstract The effect of the Ge content on the properties of the metal and acid functions of trimetallic naphtha reforming catalysts of platinum-rhenium-germanium supported on chloride alumina was studied. Pt, Re and Ge were loaded using the coimpregnation method. The elemental concentration of the catalysts used was: Pt = Re = 0.3%; Ge = 0.1%, 0.3%, 0.6% and 0.9% (weight basis). In order to make a comparison Pt/Al 2 O 3 and Pt-Re/Al 2 O 3 catalysts were prepared. The properties of the acid and metal functions were studied by physicochemical techniques and using test reactions. Reforming of n-heptane (450 °C, 0.1 MPa, H 2 / n C 7 = 4, WHSV = 7.3) was used to evaluate the catalytic properties of the samples. It was found that the addition of small quantities of Ge (0.1%) decreased both the hydrogenolytic and dehydrogenation activities by breaking Pt-Re ensembles and modifying the electronic density of Pt. The latter was revealed by the decrease in the CO chemisorption capacity and the shift of the bands of linearly adsorbed CO. With respect to the acid function Ge addition produced a change in the acid strength distribution, increasing the amount of mild and weak acid sites and decreasing the amount of strong ones. This modification of the metal/acid balance had an effect over the distribution of products of n-heptane reforming. Both the isomerization activity and the stability of the catalyst were enhanced while the cracking and dehydrocyclization activities were decreased. The isoparaffins/aromatics ratio and the octane gain were improved and as a result the reformate product better adjusted to environmental regulations.

Surface and structural effects in the hydrogenation of citral over RuCu/KL catalysts

Four 2 wt% Ru/KL catalysts, doped with copper, were prepared by successive impregnation of a KL zeolite with Ru 3(CO) 12 and Cu(NO 3) 2 · 3H 2O aqueous solutions in an adequate concentration to obtain 0, 0.5, 1 and 1.5 wt% of copper. The samples Ru/ KL, RuCu0.5/ KL, RuCu1.0/ KL and RuCu1.5/ KL were characterized by X-ray diffraction, temperature-programmed reduction, chemisorption of CO and H 2, BET surface area, infrared spectroscopy of adsorbed CO and microcalorimetry of CO adsorption. Catalytic activity measurements in the selective hydrogenation of citral, at 323 K and 5 MPa in the liquid phase, showed that copper inhibits the overall hydrogenation activity of Ru/KL catalysts, the higher the copper concentration the larger the activity reduction, without significant modification of the hydrogenation turnover frequencies. Selectivity toward unsaturated alcohols decreases as the copper content increases in favor of that of citronellal. The presence of bimetallic entities on the doped catalysts was suggested by the TPR measurements and the occurrence of hydrogen spillover phenomenon evidenced by the results of the chemisorption measurements of hydrogen and CO. The catalyst behavior is consistent with both, decoration/encapsulation of ruthenium by copper particles and occlusion of Ru inside the zeolite pores. This is suggested by the strong decrease of CO chemisorption capacity of the catalysts in the order Ru/ KL > RuCu0.5/ KL > RuCu1.0/ KL > RuCu1.5/ KL and strengthened by the CO-FTIR and microcalorimetry results.

Tungsten Carbide Microspheres as a Noble‐Metal‐Economic Electrocatalyst for Methanol Oxidation

Thrifty catalysis: Tungsten carbide microspheres with high surface area and CO chemisorption capacity were prepared by heating mixtures of a resorcinol–formaldehyde polymer and ammonium metatungstate. Platinum supported on these microspheres (see electron micrograph) shows higher activity than the commercial catalyst with 20 % Pt–Ru (1:1) on C for the electrooxidation of methanol.

Oxide-supported Rh particle structure probed with carbon monoxide

Rh deposits in a wide range of metal exposures have been grown on a thin, well-ordered alumina film under ultra-high vacuum conditions. Their morphologies range from individual Rh particles with sizes below ten atoms to closed metal films, as determined by scanning tunneling microscopy (STM). CO chemisorption on these deposits has been studied by infrared reflection absorption spectroscopy (IRAS) and X-ray photoelectron spectroscopy (XPS). At low metal coverages, the formation of isolated Rh carbonyl species associated with surface defects is observed. Changes in the infrared spectra of CO adsorbed on annealed Rh aggregates are attributed to a thermally induced ordering of the Rh particle surface on a local scale, leading to a reduction in the number of low coordinated metal atoms. This is accompanied by a marked decrease of the CO chemisorption capacity. Adsorbed CO reduces the extent of this ordering process, while the adlayer itself reorganizes.

FTIR Spectroscopic Study and CO Hydrogenation on V, Nb, and Ta Oxide Promoted Rh/SiO2Catalysts

The CO hydrogenation over Rh/SiO2catalysts promoted with oxides of Vb transition metals was studied. Two different kinds of promoter effects occurred which were established by high temperature reduction (HT reduction, Tred≥ 673 K) and HT calcination (Tcalc≥ 973 K), followed by reduction at 673 K, respectively. While HT reduction favors formation of ethanol, HT calcination, followed by reduction at 673 K enhances the selectivity toward methanol. The overall activity increased in the order V < Nb < Ta. The impact of promotion on the metal state was characterized by FTIR spectroscopy of adsorbed CO at 85 K. Modification of Rh/SiO2with promoter oxide causes a slight high frequency shift for linearly bonded CO on rhodium. Reduction at 673 K leads to a decrease in CO chemisorption capacity. In this state the rhodium metal surface is thought to be partially covered by promoter oxide and hence compares to the conventional SMSI state. This effect is enhanced if the reduction is preceded by HT calcination. The capability of the promoter oxide to spread over the metal surface increases in the order Ta < Nb < V. While the total CO chemisorption capacity is lowered in the SMSI state, the relative amount of linearly to bridge-bonded CO increases and characteristic changes in the band shape of bridge-bonded CO occur. A high frequency component at around 1920 cm−1characteristic of bridge-bonded CO which is present after LT reduction (Tred=573 K) disappears after HT reduction.

Strong Metal–Promoter Oxide Interactions Induced by Calcination in V, Nb, and Ta Oxide Promoted Rh/SiO2Catalysts

The influence of the calcination temperature on the structure of Rh/SiO2catalysts promoted by Vb transition metal oxides has been studied. Calcination atT>973 K leads to the formation of crystalline RhXO4mixed oxides with increasing abundance in the order RhTaO4<RhNbO4<RhVO4. Their reduction atT=773 K yields highly dispersed Rh°metal particles which seem to be embedded in a matrix of partially reduced promoter oxide. The presence of Vb metal oxides enhances the noble metal dispersion as compared to unpromoted Rh/SiO2catalysts. Calcination atT=773 K establishes an interaction between Rh2O3and promoter oxide which manifests itself in a decreased CO-chemisorption capacity after low temperature reduction (LTR=523 K). This calcination induced metal–promoter interaction decreases with increasing atomic number of the promoter element.

Palladium-lanthanum interaction phenomena in Pd-LaCl 3 /SiO 2 and Pd-La 2 O 3 /SiO 2 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.

CO chemisorption on TiO2(110): Oxygen vacancy site influence on CO adsorption

CO chemisorption has been studied on TiO2(110) under surface conditions where oxygen anion vacancy sites are not present (oxidized surface), compared to conditions where the vacancy sites are present (annealed surface). The binding energy of CO on the nondefective TiO2(110) surface is 9.9 kcal/mole in the limit of zero coverage. CO...CO repulsive interactions have been observed at higher coverages. When anion vacancy sites are produced under controlled annealing conditions in vacuum at 900 K, a significant increase in the desorption temperature of a portion of the chemisorbed CO is observed. This observation, coupled with measurements showing that defective TiO2(110) does not have enhanced CO chemisorption capacity, suggests that CO adsorbs more strongly on lattice Ti sites in the vicinity of anion vacancy sites. It is postulated that enhanced CO bonding occurs via the interaction of the O moiety of CO with the anion vacancy site while primary adsorbate bonding occurs via the C moiety to Ti lattice sites. Neither CO2 production nor oxygen exchange in CO occurs when CO desorbs from defective TiO2(110).

Modification of the catalytic properties of supported noble metal catalysts by carrier doping

AbstractThe influence of altervalent cation doping of TiO2 carriers on the chemisorptive and catalytic properties of supported Pt and Rh crystallites has been investigated. It was observed that doping of the carrier with higher valence cations leads to suppression of the H2 and CO chemisorption capacity of Pt catalysts, while their activity in hydrogenation and oxidation reactions is significantly reduced. The opposite effects were observed in the case of Rh catalysts supported on higher valence doped TiO2. These catalysts were found to possess higher activity in CO and CO2 hydrogenation, in aromatics hydrogenation and in CO and C2H4 oxidation. Their stability characteristics were also found to be superior to those of the undoped Rh/TiO2 catalyst. These effects are believed to originate from an electronic type interaction at the metal‐support interface, induced by doping, which results in electron transfer from the support to the metal crystallites.

An isotopic tracer study of the deactivation of Ru/Ti0 2 catalysts during Fischer-Tropsch synthesis

An investigation of the causes of catalyst deactivation during Fischer-Tropsch synthesis over Ru/Ti0 2 catalysts has been carried out. The effects of Ru dispersion, TiO 2 phase, and TiO 2 surface area on catalyst activity and selectivity were examined. CO chemisorption capacity and carbon species accumulation were determined as a function of reaction time using isotopic tracer techniques in conjunction with temperature-programmed surface reaction (TPSR). The results of this investigation show that all of the catalysts undergo deactivation with time, with no change in product selectivity. Initially, deactivation is very rapid followed by a slower activity loss. The long-term rate of deactivation is proportional to the initial CO turnover frequency, obtained by extrapolation of the long-term activity data. Deactivation is accompanied by a progressive loss in CO chemisorption capacity as well as an accumulation of various types of carbon species. Carbidic carbon, Cα, and alkyl carbon chains, C β , were observed to accumulate as reaction proceeds. C β consists of two species, C′ β which is the precursor to C 2+ hydrocarbon products and C″ β which consists of longer alkyl chains and does not participate in the production of gas phase products. The rapid initial loss in activity in the first 10 min correlates with the C α + C β accumulation; the long-term loss in CO uptake and catalyst activity are probably due to C″ β . The alkyl chains comprising C″ β do not undergo hydrogenolysis under reaction conditions, probably due to inaccessibility to hydrogen.

Effects of dopants on performance of metal crystallites: 2. Further characterization of doped supports and catalysts

The effects of dopants on chemisorptive and catalytic properties of metal crystallites are further investigated by additional characterization of supports and catalysts. Alterations of the Fermi energy level of TiO 2 doped with altervalent cations are detected by measurements of specific electrical conductivity and activation energy of electron conduction. It is established that under any environment the specific electrical conductivity of higher-valence doped TiO 2 is significantly higher than that of undoped, equal-, and lower-valence doped TiO 2. The CO chemisorption capacity of Pt supported on higher-valence doped carriers is found to be significantly reduced and to follow the same general trends as those of O 2 and H 2. Shifts in the vibrational frequency of CO adsorbed on these catalysts indicate that the metal-carbon bond is weakened, probably due to increased saturation of Pt d orbitals as a result of electronic interactions at the metal-support interface.

The influence of TiO x deposits on a polycrystalline Rh foil on CO adsorption and deposition

Overlayers of TiO x on Rh foil have been grown by Ti evaporation in vacuo followed by oxidation in 100 Torr of O 2 at 420 K. Auger electron spectra taken as a function of Ti dosing show a two-dimensional growth of TiO x until completion of one monolayer; thereafter, growth is three-dimensional. The decrease in CO chemisorption capacity of the foil is substantially greater than that expected for physical blockage of adsorption sites by TiO x . This behavior is enhanced by H 2 reduction at 770 K. Temperature-programmed desorption of CO shows a weakening in the strength of CO adsorption on Rh for TiO x coverages exceeding 0.5 ML. The data can be interpreted in terms of a model in which CO chemisorption is inhibited at Rh sites adjacent to TiO x species in addition to the Rh sites covered by TiO x .

Metal-support interaction in automotive exhaust catalysts: Rh-washcoat interaction

This work compares the effect of three washcoat materials, ..gamma..-Al/sub 2/O/sub 3/, ..cap alpha..-Al/sub 2/O/sub 3/, and ZrO/sub 2/, on the CO chemisorption capacity, thermostability, and catalytic activity for the H/sub 2/-NO reaction of Rh on a monolithic support. The Rh loading was analyzed by the conventional x-ray fluorescence method and by the inductive coupled plasma technique. The ZrO/sub 2/ and ..gamma..-Al/sub 2/O/sub 3/ washcoat show resistance to thermal deactivation. Since the BET area of ZrO/sub 2/ and ..gamma..-Al/sub 2/O/sub 3/ washcoats was five to seven times lower than that of ..gamma..-Al/sub 2/O/sub 3/ washcoat it was necessary to compare the chemical resistance of these catalysts for Pb, P, and S in fuel. Experiments were carried out in a pulse-flame reactor with isooctane fuel containing either 6 mg Pb/gallon (certification fuel) or 60 mg Pb/gallon with 0.8 mg P/gallon and 0.03 wt. % S. The results showed that using a fuel containing a ten times excess of lead over the certification fuel, the Rh/..cap alpha..-Al/sub 2/O/sub 3/ and Rh/ZrO/sub 2/ catalysts show significantly higher deactivation compared with a Rh/..gamma..-Al/sub 2/O/sub 3/ catalyst after 15,000 simulated miles of pulsator aging. These results indicate that the catalysts prepared on thermally stable waschcoatsmore » may be compatible only with fuels of low lead level (Pb < 6 mg/gallon). 1 figure, 2 tables. (DP)« less