Uncalcined Catalysts Research Articles

Esterification of acetic acid with propanol isomers under autogeneous pressure: A catalytic activity study of Al-MCM-41 molecular sieves

Catalytic activity of mesoporous Al-MCM-41 (Si/Al = 30, 51, 72 and 97) molecular sieves, which were synthesised and characterised by XRD, BET (surface area) and FT-IR was tested towards esterification of acetic acid with propanol isomers such as isopropanol and n-propanol under autogeneous pressure condition. The effect of Si/Al ratios of Al-MCM-41, reaction temperature, time, mole ratio (alcohol:acid) and catalyst loading on propanol conversion was studied. Al-MCM-41 (Si/Al = 30) was found to be more active than other Si/Al ratios owing to its high density of Brönsted acid sites and the reaction was also influenced by hydrophobic properties of the catalyst. The reaction was also studied over HY zeolite and H mordenite under same condition as Al-MCM-41 and their results are compared. The requirement of Brönsted acid sites for the reaction was clearly established by running the reaction in the absence of catalyst. Occurrence of the reaction mainly within the pores was confirmed by running the reaction over the uncalcined catalyst that provides less conversion than the calcined material.

Effective Au-Au+-Clx/Fe(OH)y catalysts containing Cl− for selective CO oxidations at lower temperatures

Supported Au catalysts Au-Au+-Clx/Fe(OH)y (x<4, y≤3) and Au-Clx/Fe2O3 prepared with co-precipitation without any washing to remove Cl− and without calcining or calcined at 400°C were studied. It was found that the presence of Cl− had little impact on the activity over the unwashed and uncalcined catalysts; however, the activity for CO oxidation would be greatly reduced only after Au-Au+-Clx/Fe(OH)y was further calcined at elevated temperatures, such as 400°C. XPS investigation showed that Au in catalyst without calcining was composed of Au and Au+, while after calcined at 400°C it reduced to Au0 completely. It also showed that catalysts precipitated at 70°C could form more Au+ species than that precipitated at room temperatures. Results of XRD and TEM characterizations indicated that without calcining not only the Au nano-particles but also the supports were highly dispersed, while calcined at 400°C, the Au nano-particles aggregated and the supports changed to lump sinter. Results of UV–vis observation showed that the Fe(NO3)3 and HAuCl4 hydrolyzed partially to form Fe(OH)3 and [AuClx(OH)4−x]− (x=1–3), respectively, at 70°C, and such pre-partially hydrolyzed iron and gold species and the possible interaction between them during the hydrolysis may be favorable for the formation of more active precursor and to avoid the formation of Au–Cl bonds. Results of computer simulation showed that the reaction molecular of CO or O2 were more easily adsorbed on Au+ and Au0, but was very difficultly absorbed on Au−. It also indicated that when Cl− was adsorbed on Au0, the Au atom would mostly take a negative electric charge, which would restrain the adsorption of the reaction molecular severely and restrain the subsequent reactions while when Cl− was adsorbed on Au+ there only a little of the Au atom take negative electric charge, which resulting a little impact on the activity.

Direct synthesis of hydrogen peroxide from H 2 and O 2 using zeolite-supported Au catalysts

The direct synthesis of hydrogen peroxide from H 2 and O 2 using zeolite-supported Au catalysts is described and their activity is contrasted with silica- and alumina-supported Au catalysts. Two zeolites were investigated, ZSM-5 and zeolite Y. The effect of calcination of these catalysts is studied and it is found that for uncalcined catalysts high rates of hydrogen peroxide formation are observed, but these catalysts are unstable and lose Au during use. Consequently, reuse of these catalysts leads to lower rates of hydrogen peroxide formation. However, catalysts calcined at 400 °C are more stable and can be reused without loss of gold. The use of zeolites as a support for Au gives comparable rates of hydrogen peroxide formation to alumina-supported Au catalysts and higher rates when compared with silica-supported catalysts. prepared using a similar method. Zeolite Y-supported catalysts are more active than ZSM-5-supported catalysts for the stable calcined materials. It is considered that the overall activity of these supported catalysts may be related to the aluminium content as the activity increases with increasing aluminium content.

Hydrogen production by partial oxidation of methanol over gold catalysts supported on TiO 2-MO x (M = Fe, Co, Zn) composite oxides

Hydrogen production by partial oxidation of methanol (POM) has been investigated over Au/TiO 2-MO x (M = Fe, Co and Zn) catalysts in the temperature range of 423–548 K. The catalysts were characterized by ICP, BET, XRD, TEM and XPS analyses. The XRD analysis confirms the desired structure and phase purity of Fe 2O 3, Co 3O 4, ZnO and TiO 2 samples and the presence of gold in these materials. TEM observations show that the gold particles are stabilized against sintering during calcination and after catalytic tests, in the presence of MO x in Au/TiO 2 catalysts. The XPS analysis detects the existence of metallic gold (Au 0), non-metallic gold (Au δ+ ) and Au 2O 3 species in the uncalcined catalyst samples both before and after reaction, and the existence of metallic gold (Au 0) and Au 2O 3 species in the calcined catalyst samples. The catalytic activity of Au/TiO 2 for the POM reaction to produce hydrogen is improved by using additional support (MO x ), probably due to a combination of factors, such as increasing the mobility of the lattice oxygen, maintaining the adequate oxidation state of the active gold particles and controlling the sintering of gold particles. Therefore, MO x can act as a structural promoter and/or as a cocatalyst. The most active catalyst is Au/TiO 2-Fe 2O 3. Although Fe 2O 3 in Au/TiO 2 catalysts increases the catalytic activity, a surfeit of Fe 2O 3 lowers the activity for hydrogen formation. Calcination of the catalyst samples results in a decrease of the catalytic activity. The sample dried at 373 K in air exhibits the highest activity for POM reaction. Both methanol conversion and hydrogen selectivity are increased with increase in reaction temperature. The reaction pathway is suggested to consist of consecutive methanol combustion, partial oxidation, steam reforming and decomposition. CO produced by methanol decomposition and/or by reverse water gas shift is subsequently transformed into CO 2 and H 2 by the water gas shift and/or CO oxidation.

Direct synthesis of hydrogen peroxide from H 2 and O 2 using TiO 2-supported Au–Pd catalysts

The direct synthesis of H 2O 2 at low temperature (2 °C) from H 2 and O 2 using TiO 2-supported Au, Pd, and Au–Pd catalysts is discussed. The Au–Pd catalysts performed significantly better than the pure Pd/TiO 2 and Au/TiO 2 materials. Au–Pd particles were found with a core–shell structure, with Pd concentrated on the surface. The highest yields of H 2O 2 were observed with uncalcined catalysts, but these were particularly unstable, losing both metals during use. In contrast, samples calcined at 400 °C were stable and could be reused several times without loss of performance. These catalysts exhibited low activity for CO oxidation at 25 °C; conversely, catalysts effective for low-temperature CO oxidation were inactive for H 2 oxidation to H 2O 2. This anticorrelation is explored in terms of the mechanism by which the catalysts function and the design of catalysts for the selective oxidation of one of these substrates in the presence of the other.

Production of hydrogen via partial oxidation of methanol over Au/TiO 2 catalysts

Selective production of hydrogen by partial oxidation of methanol (CH 3OH + (1/2)O 2 → 2H 2 + CO 2) over Au/TiO 2 catalysts, prepared by a deposition–precipitation method, was studied. The catalysts were characterized by XRD, TEM, and XPS analyses. TEM observations show that the Au/TiO 2 catalysts exhibit hemispherical gold particles, which are strongly attached to the metal oxide support at their flat planes. The size of the gold particles decreases from 3.5 to 1.9 nm during preparation of the catalysts with the rise in pH from 6 to 9 and increases from 2.9 to 4.3 nm with the rise in calcination temperature up to 673 K. XPS analyses demonstrate that in uncalcined catalysts gold existed in three different states: i.e., metallic gold (Au 0), non-metallic gold (Au δ+ ) and Au 2O 3, and in catalysts calcined at 573 K only in metallic state. The catalytic activity is strongly dependent on the gold particle size. The catalyst precipitated at pH 8 and uncalcined catalysts show the highest activity for hydrogen generation. The partial pressure of oxygen plays an important role in determining the product distribution. There is no carbon monoxide detected when the O 2/CH 3OH molar ratio in the feed is 0.3. Both hydrogen selectivity and methanol conversion increase with increasing the reaction temperature. The reaction pathway is suggested to consist of consecutive methanol combustion, partial oxidation and steam reforming.

On the potential role of hydroxyl groups in CO oxidation over Au/Al 2O 3

The deuterium isotope effect in the steady state CO oxidation rate over Au/γ-Al 2O 3 in the presence of H 2 or H 2O and the effect of pretreatment on an uncalcined catalyst were studied. In a reaction feed containing 1% CO, 0.5% O 2, and 40.5% H 2 at room temperature, CO oxidation exhibited a deuterium isotope effect ( k H/ k D) of 1.4±0.2. The rate of D 2 oxidation was also slower than the oxidation of H 2, such that the selectivity for CO oxidation was 86% in the presence of D 2 versus 77% in the presence of H 2. In contrast, there was no deuterium isotope effect in a feed containing 1% CO, 0.5% O 2, and 1.5% H 2O. H 2 was also more effective in regenerating a CO oxidation reaction deactivated catalyst than D 2, whereas H 2O and D 2O were equally effective. The difference was attributed to the different mechanisms with which H 2 or H 2O prevented deactivation of the catalyst during CO oxidation. An uncalcined Au/γ-Al 2O 3 was rather inactive. It could be activated by treatment with a mixture of H 2 and H 2O at 100 °C, although treatment by either H 2 or H 2O alone was ineffective. The observations are consistent with the model of the active site consisting of an ensemble of metallic Au atoms and a cationic Au with a hydroxyl group.

State of platinum in zirconia and sulfated zirconia catalysts

Platinum is present in a metallic state following activation in air at 725‡C of both 5 wt% Pt/ZrO2 and 5 wt% Pt/SO 4 2− /ZrO2. Reduction of either catalyst at 725‡C produces a Pt-Zr alloy, and these reduced catalysts, upon recalcination in air at 725‡C, form metallic Pt crystallites. Likewise, reduction of these uncalcined catalysts at 725‡C in H2 leads to a Pt-Zr alloy formation. However, treatment of these uncalcined catalysts in H2 at 450‡C does not produce Pt crystallites large enough to detect by XRD.

Preparation of molybdenum-based catalysts by new slurry impregnation method: active carbon supported molybdenum sulfide hydrodesulfurization catalyst

Active carbon supported molybdena catalysts (15% MoO 3 ) were prepared by new slurry impregnation method ( SIM ) using carbon of various origin and particle size. The support was refluxed with the slurry of powdered MoO 3 in water. A low solubility of MoO 3 was sufficient for transport of MoO 3 from the powder via the solution to the surface of the support. Calcination was left out because no other chemicals beside MoO 3 and water were used in the impregnation. The catalysts were presulfided and tested in hydrodesulfurization of thiophene at 1.6 MPa and 300–400°C. Their activity was four times higher than that of a commercial molybdena on alumina catalyst and about the same as the activity of a carbon supported catalyst prepared by the conventional impregnation method (CIM) using ammonium heptamolybdate. The advantage of the new SIM over CIM method is: (i) all molybdena species are deposited by adsorption; precipitation during drying is negligible, (ii) oxidative degradation of the carbon support cannot occur during preparation because calcination is left out; ammonium sulfide is not formed during presulfidation of the uncalcined catalysts.

EXAFS, XPS and TEM Characterization of SnO2-Based Emission Control Catalysts

ABSTRACTCatalysts formed by sorption of Cr(VI) on to tin(IV) oxide have been investigated by EXAFS, XPS and TEM. The latter shows that uncalcined catalysts comprise uniform, homogeneous particles of dimensions 2nm which increase to 4nm on calcination at 400°C. Calcination at higher temperatures results in an increase in particle size to 20nm × 30nm at 600°C, ca. 60nm at 800°C, and 90–100nm at 1000°C. XPS data show the evolution of chromium(III) species upon calcination and both chromium(VI) and chromium(III) species exist at all calcination temperatures. Chromium K-edge XANES also shows the presence of tetrahedral chromium(VI) species at all calcination temperatures, but the relative abundance decreases with increase in calcination temperature. Corresponding EXAFS data indicate that the uncalcined catalyst contains sorbed four-coordinated {Cr2O7} species whereas calcination produces six-coordinated {CrO6} species together with some residual tetrahedrally coordinated chromium. EXAFS data show that adsorbed {Cu(H2O)6} is the copper(II) species present prior to calcination of CuII-SnO2 catalysts.

Sulfidation of Co/Al 2O 3 and CoMo/Al 2O 3 Catalysts Studied by Mössbauer Emission Spectroscopy

The sulfidation of uncalcined and calcined alumina-supported cobalt and cobalt-molybdenum catalysts was systematically studied by means of in situ Mössbauer emission spectroscopy (MES) at room temperature. The spectra obtained during the stepwise sulfidation of the uncalcined catalysts clearly resemble those observed for carbon-supported ones. Hence, the interpretation of the spectra of the alumina-supported catalysts is based on the conclusions drawn from the MES studies of the carbon-supported catalysts, which are less complex because Co ions do not diffuse into the support. It is demonstrated that not only in sulfided CoMo/Al 2O 3, but also in sulfided Co/Al 2O 3, catalysts Co-sulfide species with a "Co-Mo-S"-type quadrupole splitting can be formed. It is concluded that the Co-sulfide species formed in sulfided Co/Al 2O 3 and CoMo/Al 2O 3 catalysts are essentially the same, only the particle size and ordering of the Co-sulfide species may differ, as in the case of Co/C and CoMo/C catalysts. The function of the Mo, which is present as MoS 2, is merely to stabilize very small Co-sulfide particles, which in the limit contain only one single Co atom. Furthermore, it turns out that the value of the electric quadrupole splitting (Q.S. value) of the Co-sulfide phase in the sulfided catalysts depends on the sulfiding temperature and Co content. This observation leads to the conclusion that large Q.S. values point to the presence of very small Co-sulfide entities or particles (the lower limit being "particles" containing only one Co atom, such as proposed in the "Co-Mo-S" model), whereas small Q.S. values point to the presence of large Co-sulfide particles (the upper limit being crystalline Co 9S 8).

Effects of starting cobalt salt upon the cobalt-alumina interactions and hydrodesulfurization activity of CoO/Al 2O 3

The effects of the starting cobalt salt were investigated on the cobalt—alumina interaction modes and cobalt dispersion in CoO/Al 2O 3, catalysts. The catalysts were characterized in oxidic, sulfided, and uncalcined states using temperature-programmed reduction, X-ray photoelectron spectroscopy, X-ray diffraction, and nitric oxide adsorption techniques. It was found that the cobalt—alumina interaction modes depend strongly on the starting salt. It was demonstrated that CoO/Al 2O 3 catalysts prepared from cobalt acetate show higher cobalt dispersion and higher catalytic activity for thiophene hydrodesulfurization than the catalysts prepared from conventional cobalt nitrate. The proportion of Co 3+ species on CoO/Al 2O 3 is significantly decreased when cobalt acetate is used instead of cobalt nitrate. On the basis of XPS characterization of uncalcined catalysts and a comparative study on CoO/Al 2O 3 catalysts prepared from other cobalt precursors, it is proposed that with CoO/Al 2O 3, prepared from cobalt nitrate, the oxidation of Co 2+ to Co 3+ during calcination by coexisting NO − 3 anions on uncalcined precursors results in a high Co 3+ proportion and, consequently, poorly dispersed cobalt species.

Effect of cobalt source on the reduction properties of silica-supported cobalt catalysts

The bulk and surface reduction properties of silica-supported cobalt catalysts are influenced by the identity of the cobalt salt employed in catalyst preparation. Temperature-programmed reduction (TPR), X-ray powder diffraction (XRPD), and X-ray photoelectron spectroscopy (XPS) have been used to characterize the reduction, calcination, and catalytic behaviors of a series of 6 wt.% Co/SiO 2 catalysts prepared from nitrate, chloride, and acetate precursors. TPR profiles in hydrogen of the uncalcined catalysts reveal that reduction of Co(NO 3) 2/SiO 2 occurs via an initial reductive decomposition of the nitrate ions, producing CoO x SiO 2 surface species that are much more difficult to reduce to metallic cobalt than is the unsupported nitrate salt. Complete reduction of the silica-supported acetate is also markedly inhibited compared to that of the unsupported salt. By contrast, reduction of CoCl 2/SiO 2 occurs in a single step that is virtually unaffected by the presence of the silica support. XRPD analysis confirms that precalcination of the three catalysts at 500°C prior to reduction leads to the formation of Co 3O 4 with the nitrate- and chloride-derived catalysts, but not with the acetate-derived material. TPR profiles and XPS spectra indicate that isothermal reduction in hydrogen at 400°C is much less complete for the uncalcined catalysts than for the calcined materials, particularly for the nitrate and acetate precursors. Exposure of the uncalcined, hydrogen treated catalysts to a H 2/CO reaction mixture at 250°C results in further reduction of Co 2+ to Co 0 for the nitrate- and acetate-derived catalysts, which had been only slightly reduced by the prior hydrogen treatment, and partial re-oxidation of Co 0 to Co 2+ for the chloride-derived material, which had been largely reduced by the hydrogen treatment.

A molybdenum-95 solid-state NMR study of hydrodesulfurization catalysts. 1. Formation of fresh HDS catalyst precursors by adsorption of polyoxomolybdates onto .gamma.-alumina

The solid-state {sup 95}Mo NMR of oxomolybdates and polyoxomolybdates adsorbed to {gamma}-alumina was utilized to characterize the molybdenum species present at circa monolayer coverages of molybdenum. This represents the first attempt at using solid-state NMR to look directly at the molybdenum species on the surface of 'fresh' hydrodesulfurization catalyst precursors. The selectively excited central {plus minus}1/2 transition line shapes were obtained, by the solid-echo technique, for several polyoxomolybdates, which are considered model compounds of the surface molybdena species, as well as for the uncalcined and calcined catalysts at various loadings. The spectra were obtained for static and magic angle spinning samples. Static sample spectra of the catalysts reveal inhomogeneously broadened lines owing to a range of surface species being present. Magic and angle spinning spectra show the presence of four possible species on the surface of the uncalcined catalysts and two possible species present on the surface of calcined catalysts. Spikelet echo spectra identified static adsorbed species (speculated to be predominantly adsorbed tetrahedral/octahedral molybdena, and perhaps Al{sub 2}(MoO{sub 4}){sub 3}), and a dynamic species (speculated to be disordered, surface-interactive tetrahedral and octahedral molybdates), in the uncalcined catalyst samples. Upon calcination the line widths of the static and MAS spectra aremore » increased, indicating a polymerization of the surface species to form a MoO{sub 3}-like phase. 67 refs.« less

Strong metal-support interactions in nickel/titania catalysts: The importance of interfacial phenomena

Nickel catalysts supported on silica and titania have been prepared by wet impregnation. The uncalcined catalysts, after an initial mild reduction at 623 K, have been treated by heating in hydrogen or argon at successively higher temperatures up to 923 K. The activity and selectivity of the catalysts have been determined under flow conditions at 1 bar pressure for the hydrogenolysis of n-hexane at 548 K, for the hydrogenolysis of ethane over the temperature range 478–673 K, and for the CO/hydrogen reaction at 548 K. Hydrogen adsorption isotherms have been measured and the specific metal surface areas determined using the Langmuir equation for dissociative adsorption to determine the monolayer coverage. Mean crystallite diameters have been determined by X-ray line broadening. Ni/titania catalysts exhibit normal hydrogen chemisorption properties after reduction at 723 K, although adsorption is suppressed after thermal treatment at 923 K. X-Ray line broadening measurements indicate a mean particle size of 8.0 nm after reduction at 723 K, and 13.3 nm after heating to 923 K. Activity measurements, after reduction at 723 K, show excellent agreement between the specific activities of the Ni/silica and Ni/titania catalysts for the hydrogenolysis of n-hexane and ethane. All these results are consistent with literature data for these reactions. However, for the CO/hydrogen reaction the specific activity of the Ni/titania catalyst is a factor of 50 higher than that of the Ni/silica catalyst. After heating at 923 K, there was a large decrease in the activity of the Ni/titania catalysts for all the reactions investigated. It is concluded that the enhanced activity of the Ni/titania catalysts for the CO/hydrogen reaction cannot be due to strong metal-support interactions (SMSI) as defined in the literature. A model of the Ni/titania system is presented which emphasizes the importance of considering the interface between the metal and the partially reduced support. It is proposed that new active sites created at the interface are responsible for the high specific activity of these catalysts for the CO/hydrogen reaction.

Hydrogenation of carbon monoxide over nickel zeolite catalysts — effect of reduction and calcination

AbstractNiNaY zeolite catalysts were prepared by successive ion exchange of NaY. The catalysts were characterized for BET surface area, nickel metal area, nickel crystallite size and crystallinity. The methanation of carbon monoxide was studied in a fixed bed reactor in the temperature range of 523–673 K. Specific activities for methanation were found to vary with nickel concentration and crystallite size. The values of nickel metal area and specific methanation activity for the uncalcined catalysts were found to be higher compared to those of calcined catalysts. The effect of reduction at 723 K on the zeolite base structure and the effects of calcining the catalyst on metal area, crystallite size and activity are discussed.