Untreated Catalyst Research Articles

Molybdenum oxide based partial oxidation catalyst: 1. Thermally induced oxygen deficiency, elemental and structural heterogeneity and the relation to catalytic performance

A mixed oxide catalyst containing Mo, V, and W was used for the partial oxidation of methanol. The relation between the structure and the degree of reduction of this mixed oxide catalyst and its catalytic performance was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), Rutherford backscattering (RBS), XPS, ion scattering spectroscopy (ISS), UPS, X-ray diffraction (XRD) and Raman microspectroscopy. Thermal activation of the MoVW mixed oxide led to an increase in the methanol conversion by a factor of 3 and an increase in selectivity to formaldehyde from 66% to 80%. SEM–EDX characterization of the untreated catalyst revealed the presence of at least two different phases in the sample on a micron range, one with a high V concentration, and another with all three metals present. TEM–EDX showed a homogeneous element distribution in the submicron regime. The thermally activated mixed oxide revealed an inhomogeneous element distribution in the micron and submicron regime as shown by SEM–EDX and TEM–EDX. The activation led to a reduction of the bulk oxide as determined by RBS and its surface as shown by XPS, ISS, and UPS. The formation of Mo 4+ and V 4+ centers was detected at the catalyst surface upon thermal activation. XRD of the starting material showed the presence of nanocrystalline material which was identified as being a mixture of a majority of Mo 5O 14 and a minority of MoO 3-type MoVW oxides. Confocal Raman microspectroscopy confirmed the presence of two different components. The major component could be identified as amorphous Mo 5O 14-type MoVW mixed oxide. The second, minor component was similar to an amorphous MoO 3-type MoVW oxide. XRD showed that the thermally activated mixed oxide consisted of a mixture of a majority of crystalline Mo 5O 14-type oxide and of small amounts of crystalline MoO 3-type and MoO 2-type oxides. The Raman spectrum of the Mo 5O 14-type phase could be identified by statistical data evaluation of 1000 spectra and by comparison with the XRD result. Raman microscopy confirmed the presence of a minority of MoO 3- and MoO 2-type oxide. The formation of Mo 5O 14-type oxide upon loss of oxygen is discussed with respect to the remarkable increase in the catalytic activity and selectivity.

Kinetics and Mechanism of the Heterogeneous Decomposition of Nitric Oxide on Metal Oxides in the Presence of Hydrocarbons

Rates and products of the (3 + α)NO + CH_4 ⇒ 1/2(3 + α)N_2 + (1 − α)CO + αCO_2 + 2H_2O (0 ≤ α ≤ 1) reaction were determined in low-pressure (NO/CH_4/O_2) mixtures ([NO] < 1 μM, [CH_4] < 10[NO], [O_2] ≤ [NO]; 1 μM = 82 ppm at 1 atm, 1000 K) flowing over Sm_2O_3 between 1000 and 1200 K. Samaria pretreated with CH_4 (or H_2) at reaction temperatures instantly releases N_2 when exposed to NO. Prompt CO formation also occurs on methane-conditioned samples. In contrast, stationary outflow gas compositions attain only after several reactor residence times following step (NO + CH_4) injections to the untreated catalyst. Nitric oxide reduction rates R-NO are roughly proportional to ([CH_4] × [NO])^(1/2) but do not extrapolate to zero at [NO] → 0 and always increase with T. We infer that: (1) there is no direct reaction between CH_4 and NO on the catalyst surface; (2) instead, NO is reduced to N_2 by reaction with oxygen vacancies V, and with nonvolatile carbon-containing C_s species created in the heterogeneous oxidation/decomposition of CH_4, respectively; (3) the entire mass, rather than just the surface, of catalyst microparticles participate in this phenomenon. We propose a purely heterogeneous mechanism in which physisorbed NO reacts with either vacancies in equilibrium with the active oxygen OR species responsible for CH_4 oxidation or with C_s species. The derived kinetic law: R-NO = k_A([NO]_s[CH4])^(1/2) + k_B[CH_4], with [NO]_s = [NO]/(K_8^(-1) + [NO]), in conjunction with the reported Arrhenius parameters, closely fits rates measured under anoxic conditions. The fact that R-NO is unaffected by O_2 up to F_(O_2) ∼ 0.3F_(NO) but drops at larger F_(O_2) inflows, even if O_2 is fully consumed in CH_4 oxidation, is consistent with the competition of NO and O_2 for vacancies. The dissimilar observations made in experiments performed in the Torr range strongly suggest that solid catalysts promote combustion at such relatively high pressures.

Enhancement of Residue Hydroprocessing Catalysts by Doping with Alkali Metals

Alkali metal nitrates and hydroxides were added to Ni/Mo on γ-alumina hydroprocessing catalysts in order to neutralize surface acidity, thereby reducing coke deposition on the catalyst and enhancing activity. The initial activity of the catalysts was tested by hydroprocessing Athabasca bitumen in microbatch reactors and a 1 L continuous-feed stirred reactor. Addition of nitrate salts was not effective. Addition of 0.26 mmol alkali metal hydroxide/g of catalyst was sufficient to reduce the concentration of very acidic sites in the catalyst, as measured by thermal desorption of pyridine. At this level of addition, coke deposition was reduced and hydrodesulfurization was increased. Experiments in a batch reactor showed that Li and Na were similar in activity, giving 80% sulfur removal compared to 63% with the untreated commercial catalyst. Coke content was reduced, with carbon concentration decreasing from 17.3% in the untreated catalyst to 14.4% with Li or Na at 26 to 28 mmol/g. The Na-doped catalyst gave a 40% increase in the hydrodesulfurization rate constant over the untreated catalyst in a continuous-feed stirred reactor during a 6 h experiment, consistent with the batch results. Asphaltene removal was also improved, while hydrodenitrogenation, microcarbon residue conversion, and residue conversion were unaffected.

Liquid phase methanol synthesis catalyst

Abstract This work focuses on the investigation of the stability of catalyst activity in the liquid phase methanol synthesis process. The effects of various kinds of metfhods to inhibit the deactivation of catalyst have been experimentally examined. The activity of catalyst was stabilized without lowering of activity by a suitable hydrothermal treatment, although the activity of the untreated catalyst decreased gradually with time. The additive and coating of hydrophobic material were effective for slowing down the crystallite size growth and inhibition of deactivation of catalyst. It was considered that hydrophobic material inhibited the adsorption of produced water onto the hydrophilic catalyst. Hydrophobic treatment of catalyst was found to be effective for liquid phase methanol synthesis from CO2 and H2 using hydrophobic solvent.

Catalysis on microstructured surfaces: Pattern formation during CO oxidation in complex Pt domains.

The exploration of pattern formation by reaction-diffusion systems in complex bounded domains has begun only recently. While theoretical and numerical information points to a strong interaction between patterns and boundaries, experiments are rare and for heterogeneous catalytic reactions practically nonexistent. By constructing (using microlithography) catalytic surfaces of arbitrary shape and size, we are able to study this interaction for the catalytic oxidation of CO on Pt(110). Experiments along these lines shed light on issues such as anisotropic diffusion and the behavior of individual defects. In addition, certain geometries give rise to patterns that have not been observed on the untreated catalyst and bring to light surface mechanisms that have no analog in homogeneous reaction-diffusion systems. Simple domains of controlled size constitute paradigms that make the comparisons between theory and experiment more fruitful, as we demonstrate through modeling and simulation of such examples. This approach opens the way for systematically probing certain aspects of pattern formation unique to heterogeneous catalysis.

Preparation, Structural Properties, and Hydrogenation Activity of Highly Porous Palladium-Titania Aerogels

Mesoporous to macroporous palladium-titania aerogels with high surface area have been synthesized by the sol-gel-aerogel route. A titania gel was prepared by the addition of an acidic hydrolysant to tetrabutoxytitanium(IV) in methanol. The palladium precursor solutions, added after the redispersion of the titania gel, were either Na2PdCl4, (NH4)2PdCl4, Pd(acac)2, or Pd(OAc)2 dissolved in protic or aprotic solvents. The palladium-titania aerogels have a BET surface area of 170-190 m2 g−1 after a thermal treatment up to 673 K and contain well-developed anatase crystallites of about 7-8 nm mean size. Depending on the palladium precursor used, the volume-weighted-mean particle size, determined by TEM, varies significantly in the range 21-224 nm, this being independently consistent with XRD line-broadening results. All aerogel samples showed pronounced structural stability of both the titania matrix and the palladium particles towards the pretreatment media used (air or hydrogen) at temperatures up to 773 K. Thermal analysis, combined with mass spectrometry, revealed that the untreated catalysts contain a considerable amount of entrapped organic impurities after high-temperature supercritical drying. Liquid-phase hydrogenations of trans-stilbene and benzophenone were used as test reactions for characterizing the activity and accessibility of the palladium particles. A comparison of the best dispersed Pd(OAc)2-derived aerogel catalysts with conventionally impregnated titania-supported palladium catalysts in the liquid-phase hydrogenation of 4-methylbenzaldehyde reveals superior activity and selectivity for the aerogel catalysts.

Evidence for intrinsically distinct active sites on rhenium-based metathesis catalysts. Norbornene metathesis with Re 2O 7/Al 2O 3

Treatment of calcined Re 2O 7/Al 2O 3 with norbornene results in extensive polymerization within the catalyst pores. 13C NMR analysis shows the polymer is exclusively cis-poly(norbornene), resulting from ROMP (ring opening metathesis polymerization). In contrast, a Re 2O 7/Al 2O 3 catalyst pretreated with Me 4Sn also polymerizes norbornene but to a ROMP polymer favoring trans double bonds. These observations suggest that intrinsically different active sites are generated from Re 2O 7/Al 2O 3, depending on the presence or absence of organotin cocatalyst. Active sites produced on untreated catalyst are highly selective, preferentially reacting with strained, high energy double bonds. Active sites generated upon treatment with Me 4Sn are much less selective and react both with strained and unstrained double bonds. These observations have resulted in a catalyst for the preparation of cis-1,3-divinylcyclopentanes via ethenolysis of norbomenes.

High Surface Area Platinum-Titania Aerogels: Preparation, Structural Properties, and Hydrogenation Activity

High surface area platinum-titania aerogels with marked meso- to macroporosity have been synthesized via the sol-gel-aerogel route. An acid-catalyzed titania gel was prepared from tetrabutoxy-titanium(IV) with methanol as solvent. The platinum precursor solutions added after the redispersion of the titania gel were either PtCl 4, (NH 4) 2PtCl 6 or Pt(acac) 2 dissolved in protic solvents. Platinum metal particles formed upon high-termperature supercritical drying. The platinum-titania aerogels have a BET surface area of 150 to 190 m 2 g −1 after thermal pretreatments up to 673 K and the titania matrix consists of well-developed anatase crystallites of about 8-9 nm mean size. Depending on the platinum precursor used, the volume-weighted-mean particle size, determined by TEM, varies in the range 3.6 to 68 nm, consistent with XRD results for the platinum component. All aerogel samples showed a pronounced stability of both the titania matrix and the platinum particles towards air or hydrogen at temperatures up to 673 K. Thermal analysis, combined with mass spectroscopy, revealed that the untreated catalysts contain a considerable amount of entrapped organic impurities after the high-temperature supercritical drying. For the characterization of the activity and the accessibility of platinum particles the liquid phase hydrogenations of trans-stilbene and benzophenone were used as test reactions. Compared to a commercial alumina-supported platinum catalyst, the untreated 2-5 wt% platinum-titania catalysts derived from (NH 4) 2PtCl 6- and especially PtCl 4- precursor solutions exhibit a markedly higher catalytic activity. In general, air pretreatments at 573 K or above had either no or promoting influence on activity. In contrast, pretreatments in hydrogen produced either no or detrimental activity change.

Effects of Boundaries on Pattern Formation: Catalytic Oxidation of CO on Platinum

The effect of boundaries on pattern formation was studied for the catalytic oxidation of carbon monoxide on platinum surfaces. Photolithography was used to create microscopic reacting domains on polycrystalline foils and single-crystal platinum (110) surfaces with inert titanium overlayers. Certain domain geometries give rise to patterns that have not been observed on the untreated catalyst and bring to light surface mechanisms that have no analog in homogeneous reaction-diffusion systems.

Characterization and catalytic performance of the platinum KL zeolite treated with chlorotrifluoromethane

Abstract In the aromatization of C 6 feedstock, the platinum catalyst supported on the CF 3 cl-treated KL zeolite (Pt/FKL) exhibits a high stability even at a low H 2 /C 6 mole ratio of 0.5 and a high aromatization activity, compared with the untreated catalyst (Pt/KL). The results of co-FT-IR and 129 Xe NMR show that the platinum particles on the Pt/FKL are rich in electrons, compared with those on Pt/KL and that most of the Pt particles on both catalysts are located on the external surfaces. Therefore, it is thought that the electronic state of platinum particles affects more significantly the catalytic activity than the channel structure of the L-zeolites.

Alkali-metal- and alkaline-earth-promoted catalysts for coal liquefaction applications

The promotion of Co (or Ni) Mo/Alumina hydrotreating catalysts with percentage quantities of alkali and alkaline earth metals has been proposed as a means of reducing carbon formation when the catalyst is subjected to a high coking environment. The authors recently reported the results of a study in which a sodium promoted NiMo catalyst was compared with the untreated catalyst while hydrotreating a coal-derived liquid. In terms of hydrogenation activity, the treated and untreated catalysts were essentially equivalent. Both possessed excellent activity and this activity was well maintained over the 400 hour run duration. Carbon deposition on the used catalyst was substantially reduced by the sodium treatment. The incorporation of sodium into the catalyst did, however, reduce the hydrodenitrogenation activity. The present study was undertaken to ascertain whether similar effects could be realized by promotion with the alkaline earth metals.

Comparison of ethylene with propylene hydroformylation over a Rh-Y zeolite catalyst under atmospheric pressure

Hydroformylation of ethylene and propylene over the Rh-Y zeolite was carried out in a continuous-flow system under atmospheric pressure. The activity of the catalyst used for the formation of propionaldehyde and butyraldehyde was observed to be constant for a period of over 1 month. Pretreatment of the catalyst with HeH 2 (10%) at 127 °C considerably affected the steady-state rate ( r PA) and the apparent activation energy ( E PA) for the propionaldehyde formation. On the Rh-Y zeolite with a rhodium content of less than 110 × 10 −6 mol/g cat (1.13 wt%), the enhancement in r PA was observed with no appreciable change in E PA. On the other hand, on the Rh-Y zeolite (No. 6) with a rhodium content of 280 × 10 −6 mol/g cat (2.88 wt%), r PA was somewhat reduced by the pretreatment while E PA was considerably enhanced i.e., 40 kJ/mol on the untreated catalyst and 56 kJ/mol on the pretreated one. In the propylene hydroformylation, the pretreatment of the Rh-Y (No. 6) remarkably reduced r BA compared with r PA, but it enhanced the n-isomer selectivity ( S n-BA ). The enhancement in S n-BA is closely related to the enhancement in E PA. On the basis of the results obtained, it is concluded that (1) the active sites formed either at the entrance of the pore or the external surface can effectively catalyze both the hydroformylation of ethylene and propylene and (2) the active sites formed in the pore at a very short distance from the entrance can catalyze the hydroformylation of ethylene but not propylene.

Spectroscopic investigation of a polymerization catalyst: Tetra(neophyl)zirconium reacted with alumina

Tetra(neophyl)zirconium (TNZ) reacted with alumina, one of a family of high activity, innocuous, and thermally stable polymerization catalysts, was studied using transmission infrared and uv-visible diffuse reflectance spectroscopy (DRS) with the goal of understanding the synthesis of the catalyst and its evolution at the high temperatures of use. (Neophyl is the 2-methyl-2-phenylpropyl group.) Reaction of TNZ with the free hydroxyls present at the surface of alumina to bind the zirconium was observed. Hydrogen bonded hydroxyls were unreactive during synthesis but can serve as catalyst poison at elevated temperatures. High neophyl contents of catalysts prepared with thermally dehydroxylated aluminas imply other modes of TNZ binding are possible on these aluminas. No organic species other than intact neophyl ligands were observed on the untreated catalyst. Heat treatment of the catalyst in vacuum caused loss of neophyl ligands and reduction of the Zr oxidation state. An absorption at 360 nm is seen in the DRS of reduced catalysts. Heat treatment in hydrogen caused hydrogenation of the phenyl group of the ligand, loss of ligands, but no appearance of DRS features characteristic of catalyst reduction. An infrared absorption at 1900 cm −1 is produced on hydrogenation of the catalyst and is assigned to a Zr hydride species on the basis of its frequency and response to exposure to air, water, and ethylene. Catalysts containing reduced Zr or Zr hydride sites were active in the polymerization of ethylene. The high temperature activity is suggested to result from the preserved activity of the decomposition products of the original catalyst.

Effect of heat treatment on the performance of carbon-supported transition-metal chelates in the electrochemical reduction of oxygen

A study has been made of the changes occurring in carbon-supported metalloporphyrins and metallo-phthalocyanines during heat treatment, which improves both the stability and the activity of these materials in the electrochemical reduction of oxygen in acid electrolyte. A variety of physical techniques was applied, including X-ray photoelectron spectroscopy (X.p.s.), Fourier-transform infrared spectroscopy, electron spin resonance and Mossbauer spectrometry.During the heat treatment a bond can be formed between the carbon surface and both the central metal ion and the macrocyclic ligand, which thereby loses all its peripheral substituents while retaining its inner part. In some cases some of the chelate molecules are destroyed. The active sites formed in the more active catalysts consist of an isolated metal ion surrounded by the thermally modified ligand. A relation between redox potential and O2-reduction activity of the same form as that for untreated catalysts can be used to explain the observed enhancements in activity.

Laboratory experiments evaluating the effects of S and Cu on a PtAl 2O 3 auto exhaust oxidation catalysts

Copper impregnation substantially increases the ethylene oxidation activity in a laboratory gas stream of a PtAl 2O 3 catalyst, while it decreases CO oxidation activity. SO 2 decreases the activity of untreated PtAl 2O 3 catalyst for both CO and C 2H 4 oxidation. The Cu-treated PtAl 2O 3 catalyst shows a decrease in ethylene oxidation activity due to SO 2 about equal to or less than the untreated catalyst, whereas the Cu-treated catalyst shows a larger decrease in CO oxidation activity due to SO 2 than the untreated material. Reduction of the Cu-treated catalyst greatly increased the CO activity and decreased the ethylene activity.