SZ Catalyst Research Articles

Enhanced activity and stability of SO42−/ZrO2 by addition of Cu combined with CuZnOZrO2 for direct synthesis of dimethyl ether from CO2 hydrogenation

Cu-modified SO42−/ZrO2 catalysts (XCu-SZ) with different Cu loading contents were prepared by sulfation of ZrOCl2·8H2O with (NH4)2SO4 to form SO42−/ZrO2 (SZ) followed by impregnation of SZ with a Cu precursor. The resulting XCu-SZ catalysts combined with a CuO–ZnO–ZrO2 catalyst were tested for CO2 hydrogenation to dimethyl ether (DME). The results indicated that the unmodified SZ catalyst exhibited the maximum DME yield (3.7%) which was 2.0–2.6 times higher than the DME yield of all XCu-SZ catalysts at the beginning of reaction. However, the DME yield over the unmodified SZ catalyst rapidly decreased, while that of all XCu-SZ catalysts gradually increased during the time-on-stream experiment. After 100 h, the 6 wt% Cu-modified SZ catalyst achieved the maximum DME yield of 3.2% at 260 °C and 20 bar, while the DME yield of the unmodified SZ catalyst was only 2.5%. The NH3-TPD and XPS analyses indicated that more strong acid sites were present on the unmodified SZ catalyst, resulting in a coke formation and thus the rapid deactivation. For the XCu-SZ catalysts, Cu0 was formed on the surface of SZ after the reduction with H2 which accounted for the active site of hydrogenolysis of methanol to methane. During the time-on-stream experiment, Cu0 was progressively transformed to Cu2S through poisoning, leading to the continued increase of DME yield.

Synthesis and Characterization of Ni-WO3/Sulfated Zirconia Nano catalyst for Isomerization of N-Hexane and Iraqi Light Naphtha

This work deals with preparation of Sulfated Zirconia catalyst (SZ) for isomerization of n-hexane model and refinery light naphtha, as well as enhanced the role of promoters to get the target with the mild condition, stability, and to prevent formation of coke precursors on strong acidic sites of the catalyst. The prepared SZ catalysts were characterization by fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Brunauer –Emmett-Teller (BET) surface area analysis, Thermogravimetric Analysis (TGA), Scanning Electron Microscope (SEM) and atomic force microscopy (AFM) Analyzer. The results illustrate that the maximum conversion and selectivity for n-hexane isomerization with Ni-WSZ and operating temperature of 150 °C was 80.1% and 96 % respectively .Other set of experimental with light naphtha , the results show that the maximum conversion and selectivity with Ni-WSZ and operating temperature of 150 °C was 73.6% and 74% respectively.

Catalytic Behavior of La2O3-Promoted SO42-/ZrO2 in the Simultaneous Esterification and Transesterification of Palm Oil

Catalytic behavior of La 2 O 3 promoted SO 4 2- /ZrO 2 (SZ) in the simultaneous esterification and transesterification of palm oil with 10wt% myristic acid was studied. La 2 O 3 (90% purity) derived from domestic monazite ore was introduced into SZ by impregnation method. Effects of La 2 O 3 loading and reaction parameters on the reaction behavior and the conversion were also investigated. The 20%La 2 O 3 -SZ catalyst possessed dual strong basic and acid sites which could efficiently promote both esterification and transesterification reactions. Using the 20%La 2 O 3 -SZ catalyst, the initial reaction rate of esterification and transesterification reactions was faster, and equilibrium was reached faster. The equilibrium of total fatty acid methyl ester (FAME) content of 74.51% and myristic acid methyl ester (MAME) content of 8.41% were achieved. The 20%La 2 O 3 -SZ catalyst maintained a relatively stable catalytic behavior during three reaction cycles. No significant loss in catalyst activity for esterification reaction was observed after reusing the catalyst five times without regeneration.

Synergistic effect of microwave rapid heating and weak mineralizer on silica-stabilized tetragonal zirconia nanoparticles for enhanced photoactivity of Bisphenol A

Silica-doped tetragonal mesoporous zirconia nanoparticles (SZ) were successfully synthesized with various concentrations of ammonia solution(NH4OH) using microwave-assisted method for photodegradation of Bisphenol A (BPA). The phase of SZ catalyst was pure mesoporous tetragonal ZrO2 with nano in size (5–10nm). The increasing concentration of NH4OH solution enhanced the hydrolysis and condensation process, as confirmed by formation of additional ZrOZr, SiOSi and SiOZr bonds. The formation of oxygen vacancy (Vo) and Zr3+ surface defects (ZSD) also increased proportionally with the concentration of NH4OH solution. Both properties and the SiOZr bond are supposed to play important roles in stabilizing the tetragonal phase of ZrO2. The Vo and ZSD also lowered the band-gap energy (from 4.35eV to 3.05eV) and act as electron acceptors. Catalytic testing showed that 8-SZ gave the highest initial rate (0.372× 10−3mMmin−1) of 10mgL−1 BPA at pH7 using 0.5gL−1 catalyst within 4h while under similar conditions, 5-SZ, 3-SZ, 0-SZ, ZrO2 and TiO2 only gave 0.263×10−3, 0.211×10−3, 0.145× 10−3, 0.026× 10−3 and 0.305×10−3mMmin−1, respectively. These results demonstrate that higher numbers of SiOZr, Vo, and ZSD seemed to be the main factors in the high photoactivity of SZ prepared in 8M NH4OH solution (8-SZ). A kinetic study demonstrated that the photodegradation followed the pseudo-first-order Langmuir-Hinshelwood model. The mechanism study using scavenging agents proved that photogenerated hole was the main active species, thus led more hydroxyl radicals formed to degrade the BPA. The 8-SZ also exhibited a great potential on degradation of various phenols and dyes in wastewater.

Sulphated zirconia catalysed conversion of high density polyethylene to value-added products using a fixed-bed reactor

A sulphated zirconia catalyst (SZ) was investigated for catalytic conversion of high density polyethylene (HDPE) to liquid and gaseous hydrocarbons using a fixed-bed reactor. The SZ catalyst reduced the onset of degradation from 337°C for HDPE alone to 187°C with 10wt% SZ added. At 450°C a complete weight loss was obtained with the SZ addition against only 4wt% loss for HDPE only. Fixed-bed reactor experiments using 2g of HDPE with 10wt% SZ catalyst with a 30min residence time showed a 98.0wt% conversion at temperature as low as 380°C. The liquid yield obtained was 39.0wt% with a composition of 16wt% paraffins, 21wt% olefins, 5wt% naphthenes and 58wt% aromatics. The carbon number distribution of the liquid was C7–C12, which is within the gasoline range. Equally, gaseous products ranging from methane up to different isomers of pentane which contained more paraffinic and naphthenic hydrocarbon were obtained. The sulphated zirconia catalyst was found to have high ammonia desorption (337.0μmolNH3g−1), BET surface area (116.0m2g−1), external surface area (112.0m2g−1) and mesoporous structure. The overall results indicate that sulphated zirconia had excellent properties for catalytic conversion at temperature as low as 380°C with significant liquid yield which could offer a solution to plastic waste problem by converting the waste back into value-added chemicals and fuel.

Acidity and catalytic performance of Yb-doped [formula omitted]/Zr in comparison with [formula omitted]/Zr catalysts synthesized via different preparatory conditions for biodiesel production

Highly efficient, robust and mesoporous sulfated zirconia, SZ catalysts were prepared by co-precipitation and incipient-wetness routes with sufficient and in-excess acid. The study investigated the effect of pH, precursor type, and concentration, on SZ and compared their acidity and performance with ytterbium-doped SZ toward transesterification. N2 sorption, elemental analysis, XRD, and ammonia temperature-programmed desorption (NH3-TPD) revealed the properties of the catalytic materials. The specific surface area increases independently of the presence of Ti up to 60.33 m2/g, with increasing pH while pore size of the tetragonal crystallites decreases from 37.04 to 6.83 nm. Soaking the material in sufficient 0.5-M H2SO4 produced 37.04 nm SBET in contrast to 21.21 nm from soaking in excess. Double sulfation ensured sulfate incorporation while moderate precursor amounts enhanced activity of the materials. Interestingly, despite low specific surface area, which was due to short aging period, large mesoporosity, high amount, and dispersion of active sites on Yb-doped SZ, SZr-Ti-Yb-500-4/s ensured remarkable activity. The catalyst converted ca. 99% of used frying oil containing ca. 48 wt. % FFA. Consequently, this highlights the prospect of producing biodiesel at lower cost especially with current dwindling price of crude oil.

Direct Synthesis of Propylene from Ethylene over Palladium Containing Catalysts: Influence of the Support Nature

An influence of support acidity on the dimerization function on Pd-containing catalysts was studied. After addition of Re2O7 to PdO/SZ as ethylene dimerization catalyst with high butene-2 selectivity the obtained PdO-Re2O7/SZ catalyst was tested in direct synthesis of propylene from ethylene under mild conditions (40°C and atmospheric pressure). Propylene yield was approximately 45%, which is close to equilibrium one.

Transesterification of diethyl malonate with benzyl alcohol catalyzed by modified zirconia: Kinetic study

Zirconia and its modified forms such as 10%Mo(VI)/ZrO2, 10%V(V)/ZrO2, 10%W(VI)/ZrO2 and SO42−/ZrO2 were prepared and characterized for their physico-chemical properties such as BET for surface area, NH3-TPD and n-butylamine back titration method for total surface acidity, PXRD technique for crystallinity and ICP-OES technique for elemental analysis. These materials were used as catalysts in liquid phase transesterification reaction of diethyl malonate (DEM) with benzyl alcohol (BA). Optimization of reaction conditions such as reaction time, reaction temperature, weight of the catalyst and molar ratio of reactants were carried out to obtain highest possible transester yield. Dibenzyl malonate (DBM) and benzyl ethyl malonate (BEM) were obtained as major products. Highest total transester yield of (88%) was obtained in the presence of 0.75g of SZ catalyst at a molar ratio of DEM: BA=1:3, reaction temperature of 393K and reaction time 5h. Kinetic studies were carried out to find out the rate of the reaction and energy of activation values for zirconia catalysts, in order to identify a facile catalyst system for this reaction. A possible reaction mechanism was proposed based on the kinetic data and it was observed that Eley-Rideal mechanism fits well for this reaction. Reactivation and reusability studies of the catalysts were also taken up.

Isomerization of n-butane over Pd–SO4/ZrO2 catalyst: Prospects for commercial application

Increase in productivity of the refineries and improvement the fuel quality demands to engage isobutane as a key feedstock for producing both alkylates and oxygenated additives. Substantial growth of isobutane output via isomerization of n-butane poses a real technological challenge. The goal of this study is to investigate performance of Pd promoted sulfated zirconia catalysts (Pd–SZ) in n-butane isomerization. The physical–chemical characteristics of various anion-modified zirconia catalysts (XRD and IR spectra, BET surface) are presented. It was observed that Lewis acid sites of medium and strong strength and two types of Brönsted acid sites exist. Approximately 90–97% of zirconia is in tetragonal phase.The influence of parameters on conversion of n-butane and selectivity towards isobutane was investigated by varying the temperature (90–290°C), pressure (15–26bar), WHSV (0.5–5h−1), and molar ratio H2/C4 (0.08–3). Mainly, temperature and H2/C4 ratio are responsible for increasing the effectiveness of the process. At H2/С4=0.08–0.3 the apparent activity of Pd–SZ catalyst was high enough to reach the equilibrium conversion of n-butane to isobutane at 140°С value, that is, ∼65%. The best results were obtained on Pd–SZ granules in the form of 2mm trilobe. Water impact and the catalyst lifetime were studied in the pilot setup. Endurance testing of Pd–SZ catalyst in course of 90h showed its sustained operation at H2O concentration up to 20ppm without loss of activity.The estimated process specifications on Pd–SZ catalyst were compared to a conventional process; potential benefits of making use Pd–SZ catalyst are emphasized and discussed.

Synthesis of Sulfated Pt/ZrO2 Catalysts Using Different Precipitants for n-Hexane Hydroisomerization

AbstractAmmonia (AOH), dimethylamine (DME) and triethanolamine (TEA) as precipitants were used to prepare Pt-SO42-/ZrO2 catalysts, and the crystalline structures, textural structures and surface acid properties of the catalyst samples were characterized and n-hexane hydroisomerization activity over the samples were investigated. DME and TEA as the precipitants would delay the crystallization process. The organic amines were difficult to remove during the washing process, leading to the rinsing times increase, which probably decreased the catalyst BET area. Super Lewis acid sites in SZ catalysts would play very important roles in catalytic hydroisomerization activity.

Sulfated alumina and zirconia in isobutane/butene alkylation and n-pentane isomerization: Catalysis, acidity, and surface sulfate species

The catalytic behavior of sulfated alumina (SA) and zirconia (SZ) was compared for two acid-catalyzed reactions, i.e., isobutane/butene alkylation and isomerization of n-pentane. For the alkylation reaction, the SZ catalyst gave a higher C 5+ alkane yield and much lower selectivity for trimethylpentanes (TMPs) at 2 h TOS. However, TMPs selectivity was comparable for both catalysts at 5.5 h TOS. For the isomerization, high n-pentane conversion was obtained on SZ at the reaction temperature of 100 °C when SA had no activity. Surface sulfate species were identified by IR spectroscopy. The acidity of sulfated oxides was compared by IR spectroscopy using pyridine and carbon monoxide as probe molecules and TPD–MS of ammonia. An effort was made to explain the difference between catalytic behavior of two sulfated oxides in terms of acidity, the nature of surface sulfate species and the formation of hydride species.

Esterification of Fatty Acid by Zirconic Catalysts

Sulfated zirconia (SO4 2−/ZrO2) was proved as an efficient catalyst for the esterification of fatty acids and alcohols, the alkyl esters of long chain fatty acids are wildly used in industry and daily life. In this paper, the esterification of long chain fatty acid (both saturated and unsaturated) with simple alcohol catalyzed by SZ catalyst was presented. It is found that the degree of esterification reaction of the fatty acids with 1-butanol has a close relationship with the length and unsaturation degree of the chain. The longer the carbon chain is, the more difficult it is to react; the higher the unsaturation level is, the more difficult it is to respond. The related regulation will be confirmed and serve as the dictator for other reactions within this system.

Esterification over rare earth oxide and alumina promoted SO 42−/ZrO 2

Solid superacid catalysts including SO 4 2−/ZrO 2 (SZ), rare earth (RE) oxide-promoted SZ and RE oxides together with alumina-promoted SZ were prepared. Their catalytic performances in the esterification reaction of ethanol and acetic acid were investigated. The textural property, crystalline phase and surface acidity of the prepared catalysts were characterized by using nitrogen adsorption–desorption isotherms, X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy of pyridine adsorption techniques, respectively. Effects of the reaction time and catalyst reuse cycle as well as catalyst regeneration on the catalytic behaviors were studied. Experimental results showed that Yb 2O 3–Al 2O 3 promoted SZ (designated as SZAY) catalyst exhibited an optimal esterification performance; the Lewis acid sites with moderate and super strong strength could mainly be responsible for the esterification reaction; and doping both Yb 2O 3 and Al 2O 3 on SZ not only boosted the esterification activity but also alleviated catalyst deactivation resulted from the surface sulfur loss by solvation.

Temperature imposed textural and surface synergism affecting the isomerization activity of sulfated zirconia catalysts

Using sulfuric acid as the sulfating agent, two catalyst series were obtained from hydroxide and nitrate precursor with a sulfate loading identical to commercial sulfated hydroxide, i.e., 4.2 mass%. After calcination at 500, 600 and 700?C, all nine samples had various contents of residual sulfates depending on the origin of the catalyst. Accordingly, their surface properties were different, which, together with various textural properties, govern the formation of the active phase and their catalytic activity in the n-hexane isomerization reaction. The dominant activity and yield of mainly mono-branched isomers were attained in reaction at 200?C with a commercially sulfated zirconia catalyst calcined at 500?C. Among the SZ catalyst series synthesized from hydroxide and nitrate, the second according to its activity profile was similar to that of the commercially sulfated one, while samples originating from hydroxide showed some activity only after calcination at 600?C. This is due to the poorer textural properties of the hydroxide series, necessitating a higher calcination temperature in order to promote the simultaneous decomposition of S-containing species and their re-adsorption into the zirconia matrix following interaction and active phase formation. It seems that the tetragonal zirconia phase was not responsible for the catalytic activity but a synergistic effect of the textural properties of the samples and the sulfate loadings, which determine different acid strengths on the catalyst surface.

Acidity and catalytic activities of sulfated zirconia inside SBA-15

SBA-15 containing various amount (10–50 wt.%) of highly dispersed ZrO 2 is prepared using urea hydrolysis method. The sulfated zirconia (ZrO 2) in SBA-15 catalysts were prepared using these materials. The materials were characterized by XRD, BET surface area, TEM, TGA, acidity measurements using in situ IR pyridine adsorption, sulfate content and catalytic activities were studied for cumene cracking and esterification of cyclohexanol. The Brönsted acidity, catalytic activity for cumene cracking and esterification and sulfate content were found to pass through a maxima where ZrO 2 loading is 35 wt.%. The 35SZ-SBA catalysts exhibited 6.4 times higher activity for cumene cracking compared to the bulk SZ catalysts. The relationship between acidity, sulfate content and catalytic activity are discussed in the light of existing knowledge on sulfated zirconia.

Direct Synthesis of Mesoporous Sulfated Silica-Zirconia Catalysts with High Catalytic Activity for Biodiesel via Esterification

A solid acid material of high loading of sulfated zirconia on mesoporous silica SBA-15 has been successfully synthesized by a direct-synthesis under strong acidic condition. The materials were characterized by powder X-ray diffraction, N2 adsorption−desorption, UV−visible diffuse reflectance spectroscopy, transmission electron microscopy (TEM), ICP-mass, and NH3 temperature-programmed-desorption (TPD). The zirconia content and well-ordered mesostructure of SiO2−SZ are controllable by tuning the molar ratios of sulfate to zirconia and silica to zirconia. UV−visible spectra and TEM observations confirm the incorporation of zirconium (IV) onto the mesoporoes framework. Acidity is enhanced in comparison with unsupported sulfated zirconia. Much improved catalytic performance under mild temperature condition in the esterification of lauric acid and palmitic acid with the direct-synthesis SiO2−SZ catalyst was observed as compared to un-supported SZ. The good catalytic performance of the sulfated silica-zirconia materials is attributed to a higher dispersion of zirconia to give higher acid site density and also to better tolerance of water. The solid acid catalytic materials have many advantages over liquid acids in its environmentally friendliness and easy separation.

Catalytic behavior of nanostructured sulfated zirconia promoted by alumina: Butane isomerization

Promotion of sulfated zirconia with alumina (ASZ) improves its catalytic activities in n-butane isomerization. The activity and stability of the sulfated zirconia catalysts are investigated in three different nanostructures: ASZ supported on MCM-41, ASZ nanoparticles, and Al-promoted mesoporous sulfated zirconia. The increase of activity was determined primarily by the amount of aluminum addition and the temperature of calcination. The remarkable activity and stability of the Al-promoted catalysts are due to an improved distribution of acid sites strength. The Al loadings in all three catalysts can be adjusted so that optimum catalytic activities for butane isomerization could be found. The increase of butane conversion can be as high as 6 times of that in un-promoted SZ catalysts. This is due to an enhanced amount of weak Brønsted acid sites with intermediate strength on the optimal catalysts. For nanoparticle form of sulfated zirconia, the activity is most steady which is related to the optimum distribution of weak Brønsted acid. On the other hand, too much strong Brønsted acid leads to rapid decay of activity because of coking and cracking. The overall reaction mechanism of the isomerization of n-butane over sulfated zirconia was discussed to understand the details in product distribution.

Promotion of PEM Self-Humidifying Effect by Nanometer-Sized Sulfated Zirconia-Supported Pt Catalyst Hybrid with Sulfonated Poly(Ether Ether Ketone)

A self-humidifying membrane based on low-cost sulfonated poly (ether ether ketone) (SPEEK) hybrid with sulfated zirconia (SO4(2-)/ZrO2, SZ)-supported platinum catalyst (Pt-SZ catalyst) was investigated for fuel cell applications. The SZ particle, a solid-state superacid with hygroscopic and high proton conductivity properties, was employed as the catalyst support. The SPEEK/Pt-SZ self-humidifying membrane was characterized by TEM and SEM coupled with EDX. FT-IR was conducted to verify the effect of SPEEK/Pt-SZ membrane on catalytic combination of crossover hydrogen and oxygen. To display the advantages of Pt-SZ catalyst as the additive, the IEC, water uptake, proton conductivity, single-cell performance, and areal resistance measurements were compared between the plain SPEEK membrane, SPEEK/Pt-SiO2 membrane, and the SPEEK/Pt-SZ membrane. The SPEEK/Pt-SZ membrane exhibited the highest IEC value, proton conductivity, single-cell performance, and the lowest areal resistance relative to the plain SPEEK and SPEEK/Pt-SiO2 membranes. The SPEEK/Pt-SZ self-humidifying membrane exhibited peak power density of 1.0 W/cm2 under dry operation condition compared with 0.89 W/cm2 and 0.58 W/cm2 of SPEEK/Pt-SiO2 and plain SPEEK membranes, respectively. The incorporation of the catalytic, hygroscopic and proton conductive Pt-SZ catalyst in the SPEEK/Pt-SZ self-humidifying membrane facilitated water balance and proton conduction, and accordingly improved its single cell performance under dry operation. In addition, the enhanced OCV and the decreased areal ohmic resistance confirmed the promotion effect of Pt-SZ catalyst in the self-humidifying membrane on suppressing reactant crossover and the membrane self-humidification.

Hydroisomerization and Cracking of n-Octane and n-Hexadecane over Zirconia Catalysts

C8−C16 n-paraffins can be upgraded to fuel-grade gasoline (C5-8) by means of simultaneous hydrocracking and hydroisomerization over bifunctional metal/acid catalysts. The hydroisomerization of n-paraffins over nickel/sulfated-zirconia (NiSZ), nickel/tungsten-zirconia (NiWZ), platinum/sulfate-zirconia (PtSZ), and platinum/tungsten-zirconia (PtWZ) “fresh” catalysts provides between 10 and 113 additional research octane number (RON) points over those of a model feedstock of n-octane. A close linear relation between the production of light gases (C1−C3) and the RON gain indicates that octane improvement can only be reached at the expense of the isomerizate yield. The RON gain stabilizes at a Ni−Pt content of 1% and varies slightly at higher loadings. PtSZ is the most active catalyst and provides a 100−113 RON upgrade depending on the Pt content. PtWZ can only provide 42 RON points at these reaction conditions. Though Ni-based catalysts provide a moderate RON increase of 79−83 points, they are cheaper than Pt catalysts and show an improved stability at high values of time-on-stream. The RON gain is better correlated to the total acidity than to the strong acidity, thus confirming that the production of branched isomers from long alkanes does not require highly acidic sites. On comparison, the reaction of n-hexadecane on the 0.1 and 1.0% Ni-promoted SZ and WZ catalysts indicates that the WZ catalysts provide the highest yield and selectivity to C5−C8 isomers. For the same amount of Ni, SZ catalysts have a higher activity than the WZ catalysts, but their selectivity for the production of light gases is too high.

Ga 2O 3-promoted sulfated zirconia systems: Morphological, structural and redox properties

Ga 2O 3-promotion of ZrO 2/SO 4 (SZ) systems has been shown to influence the catalytic activity of n-butane isomerization. This contribution reports a study on the influence of Ga 2O 3 addition and of calcination temperature on ZrO 2 structure, morphology and activity. Small additions of Ga 2O 3 (1–5 mol%) bring about an increase of specific surface area (SSA). By N 2 physisorption, X-ray diffraction, Raman spectroscopy and TEM microscopy, the increase of an amorphous phase and the decrease of SSA were observed when the Ga 2O 3 content raised above 5 mol%. The catalytic activity went through a maximum with Ga loading of 3–5 mol%. In all cases, the typical spectral features of surface sulfates were observed, but they became more and more confused as the Ga 2O 3 content increased. DTG/DSC measurements showed that Ga 2O 3 addition tends to slow down both the formation of ZrO 2 crystallites and the thermal elimination of surface sulfates. After calcination at temperatures as high as 720 °C, the amount of amorphous phase is drastically reduced and an appreciable fraction of surface sulfates is lost. In particular, 15 mol% Ga 2O 3-doped SZ sample, which is amorphous and inactive in n-butane isomerization after a standard calcination at 650 °C, exhibits tetragonal crystalline features and a dramatic improvement of catalytic activity after a thermal treatment at 720 °C. TPR-MS analyses showed that Ga 2O 3-promoted SZ catalysts possess enhanced redox properties, with respect to plain SZ systems, and this can probably account for the higher catalytic activity.