Rapid Kinetic Measurements in Ammonia and Methanol Syntheses

Maike Müller , Stephan Hermes , Kevin Kähler , Maurits W. E. van den Berg , Martin Muhler and Roland A. Fischer. Chemistry of Materials 2008 20 (14)...
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Ind. Eng. Chem. Res. 2001, 40, 2793-2800

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Rapid Kinetic Measurements in Ammonia and Methanol Syntheses Hubert Bielawa,† Melanie Kurtz, Thomas Genger,‡ and Olaf Hinrichsen* Laboratory of Industrial Chemistry, Ruhr-University Bochum, D-44780 Bochum, Germany

Kinetic measurements in heterogeneous catalysis are essential for the derivation of rate expressions that are necessary for the design of reactors and that can serve for data extrapolation beyond the measured data range. To accelerate the acquisition of kinetic data, the method of quasi-isothermal temperature-programming has been applied to a study of the kinetics of the ammonia and methanol syntheses. Both reactions were performed in a microcatalytic reactor setup near industrial reaction conditions. For the ammonia synthesis, the technique turned out to be a useful screening method for Ru-based catalysts, which allowed power-law rate expressions to be derived rapidly. Moreover, the in situ temperature-programmed surface reaction of N-* with gas-phase H2 (N-* TPSR) was studied in detail, allowing for its application as a fast method for catalyst screening within the class of unpromoted Ru-based catalysts. However, in the methanol synthesis, the applicability of the quasi-isothermal temperature-programmed method is limited to qualitative catalyst screening because of the methanol adsorbing capacity and the slow change of the state of the working Cu/ZnO/Al2O3 catalyst. Introduction The improvement of ammonia and methanol synthesis catalysts has an important economic impact because of their large-scale industrial use. Two ways to achieve improved state-of-the-art catalysts can be considered: either fundamental research aiming at a deeper understanding of the working catalyst, which involves in situ and kinetic studies, or trial-and-error-based testing of catalysts. In pioneering research at BASF, Mittasch1 studied more than 8000 compositions in a combinatorial-like way of testing catalysts for ammonia synthesis. To date, more than 100 000 other formulations have been investigated, but the multipromoted iron catalyst, still used today, turned out to be the best commercial catalyst for the Haber process. However, 60 years later at BP, about 500 Ru-based catalysts were tested in a set of laboratory-scale screening reactors and highpressure pilot plants. Finally, Tennison et al.2 developed ruthenium-on-graphite copromoted with barium and cesium into a commercial alternative catalyst, which is now used in the reduced-energy Kellogg Advanced Ammonia Process (KAAP).3,4 Based on the fundamental academic work of the groups led by Aika, Ertl, Muhler, and Kowalczyk in the last three decades, a fundamental and comprehensive study has recently been performed, providing new insights into the ammonia synthesis catalyzed by ruthenium (see ref 5 and references therein). In 1966, ICI introduced a low-pressure, low-temperature version of the methanol synthesis process with a new class of catalysts: a copper/zinc oxide/alumina (Cu/ZnO/Al2O3) catalyst that is now widely used in a variety of compositions.6 Although the process has been successfully commercialized, an inspection of the lit* Author to whom correspondence should be addressed. Tel.: +49 234 3226907. Fax: +49 234 3214115. E-mail: [email protected]. † Present address: Lurgi Oel‚Gas‚Chemie GmbH, A-GPGA, D-60295 Frankfurt, Germany. ‡ Present address: BASF AG, ZAV/B, D-67056 Ludwigshafen, Germany.

erature reveals that open questions still remain regarding the nature of the active sites, the state of the copper in the working catalyst, and the effect of ZnO in methanol synthesis. In recent years, the combinatorial approach has also been applied in heterogeneous catalysis as a means of finding new classes of catalyst compositions for improved activity and selectivity.7 The application of highthroughput synthesis and screening techniques leads to a large number of catalysts. Because measurement time is limited and often just one set of standard operating conditions is applied, the need for rapid kinetic data acquisition close to industrial conditions and over a broad parametric range arises. Furthermore, from an industrial point of view, a detailed kinetic study is considered to be more reliable for extrapolation beyond the measured kinetics when the data are obtained in laboratory-scale reactors near industrial reaction conditions in a classical way. The temperature-scanning reaction method and technology developed and patented by Wojciechowski and co-workers has been applied in recent studies as a promising possibility for performing kinetic experiments more conveniently and rapidly.8-11 This method involves ramping the input temperature to a reactor and recording the composition of the effluent gas and the bed temperature without waiting for isothermal steady state to be established. As a result, reaction rates can be derived directly from the measured concentrations in the effluent with the assumption of an appropriate reactor model. The present work was undertaken to study the applicability of the quasi-isothermal temperatureprogrammed mode for collecting kinetic data more rapidly under industrially relevant reaction conditions in order to obtain a reliable kinetic database and provide experimental guidelines for the usage of this method in catalyst screening. Experimental Section Catalyst Preparation. Magnesia-supported ruthenium catalysts (Ru/MgO) were prepared from ultrapure

10.1021/ie0008225 CCC: $20.00 © 2001 American Chemical Society Published on Web 05/30/2001

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Table 1. Results Obtained Either by Static N2 Physisorption and H2 Chemisorption or by in Situ Temperature-Programmed Surface Reaction of N-* with Gas-Phase H2 (N-* TPSR) for Various Supported Ru Catalysts Ru/MgO BET area H2 chemisorption N-* TPSR Tonset catalyst (m2 gcat-1) (µmol of H gcat-1) (µmol of N gcat-1) (K) a b c d e f g h

45 50 52 53 60 56 64 66

75 111 148 213 242 248 296 314

20 40 53 62 71 82 98 108

389 439 413 385 402 375 377 358

Table 2. Results of the Isothermal N2O Reactive Frontal Chromatography and BET Measurements sample

TG1

TG2

TG3

specific Cu surface area (m2 gcat-1) BET area (m2 gcat-1)

15 39

20 64

23 73

chemicals [99.996% MgO supplied by Johnson Matthey and Ru3(CO)12 supplied by Strem] by wet impregnation following the procedures in ref 12: the catalyst support was heated in high vacuum at 773 K for 6 h, added to a solution of Ru3(CO)12 dissolved in THFabs, and stirred for 16 h at room temperature under an inert gas atmosphere. After the solvent was evaporated in a rotary at 313 K, the slightly orange-colored powder was pressed into cylindrical pellets and then crushed and sieved. Typically, a 1-g catalyst sample of a 250-355 µm sieve fraction was stepwise heated in high vacuum to 723 K to achieve the complete decomposition of the carbonyl precursor. Prior to the kinetic experiments, the catalysts were reduced in situ in a high-purity stoichiometric synthesis gas mixture of H2 and N2, with the temperature ramped slowly to 783 K and then cycled between 588 and 783 K until a steady-state conversion was observed. Zinc- and alumina-supported copper catalysts (Cu/ZnO/Al2O3) were prepared by a conventional coprecipitation method. They were precipitated from an aqueous solution of copper, zinc, and aluminum nitrates. The washed and filtered precipitates were then dried in air at 393 K and calcined in air at 603 K, resulting in the formation of the precursor CuO/ZnO/Al2O3 with a Cu metal loading of approximately 50 wt %. Before use, the catalyst precursor sample was ground and sieved to obtain a fraction of 250-355 µm. Finally, the catalyst was activated by reduction of the CuO using a dilute stream of 2% hydrogen in helium at a temperature of 513 K. The activation process was carried out overnight at atmospheric pressure to ensure complete copper reduction. Surface Area Measurements. Ru metal loadings of the catalysts were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). The BET surface areas of the Ru and Cu catalysts (Tables 1 and 2, respectively) were measured by nitrogen physisorption in an Autosorb 1-C setup (Quantachrome). In the same setup, quantitative static chemisorption of H2 was used to determine the Ru metal surface area of the in situ reduced and treated catalyst, with the generally accepted assumption of a 1:1 stoichiometry for the metal-adsorbate site.13 The results are included in Table 1 for selected catalysts. The total number of active sites was determined in a flow setup described below according to a titration experiment developed by Fas-

trup et al.14 The amount of NH3 generated by the reaction of preadsorbed atomic nitrogen (N-*) in a H2 gas stream (N-* TPSR) was recorded in a temperatureprogrammed way, and thus, in the case of saturation, the total number of active sites was derived by integration of the resulting NH3 signal. The onset temperature, Tonset, of the signal turned out to be an important parameter in the evaluation of different Ru-based catalysts (cf. Discussion section). The adsorptive decomposition of N2O in an isothermal flow experiment [reactive frontal chromatography (N2O RFC)] introduced by Chinchen et al.15 was used under mild conditions16 for the determination of the specific Cu metal surface areas of the supported catalysts presented in Table 2. Apparatus for Kinetic Measurements. Kinetic ammonia synthesis studies were conducted in the flow setup that has been described in detail in ref 12. Downstream, the reactor system, which consists of a glass-lined U-tube with an inner diameter (i.d.) of 3.8 mm surrounded by an oven, with a high-pressure unit was connected to a nondispersive infrared NH3 detector (BINOS, Fisher-Rosemount) where gas analysis was performed on-line using either the low-concentration (ppm region up to 1% NH3 in the effluent) or the highconcentration (700 K), tid is less than 5 s using a low value of SV. Compared to the temporal change of the reactor temperature in a temperature-programmed flow experiment, tid has only a negligible influence on the isothermal reaction conditions. Hence, our moderate reaction conditions justify the assumption of a quasi-isothermal operation mode. The screening experiments with methanol synthesis catalysts were conducted with 200 mg of catalyst fixed in the reactor by means of quartz wool. The standardized 2-day testing included reduction of the catalyst, measurement of the catalytic activity under constant reaction conditions (p ) 0.1 MPa, T ) 493 K, Q ) 10 NmL min-1), and determination of the metal Cu surface area by N2O RFC, followed by the acquisition of kinetic data in a temperature-programmed way (p ) 0.1 MPa, Q ) 10 NmL min-1, β ) (0.2 K min-1). Finally, a second N2O RFC run completed the testing. A significant influence of the initial deactivation on the catalytic activity can be neglected because of the given moderate reaction conditions. A detailed kinetic study was performed with 10 mg of catalyst after initial deactivation. Evaluation of Kinetic Data. The reactor was described as an isothermally operating plug-flow reactor. Because mass and heat transfer limitations could be excluded, a pseudo-homogeneous one-dimensional reactor model was used.26 Previous studies in NH3 synthesis by Aika et al.27 and Rosowski et al.12 showed that the applicability of a power-law given by

r ) kapppNH3RpN2βpH2γ

(3)

is justified for determination of the kinetic parameters, i.e., the apparent activation energy and the reaction orders, which were determined directly from the kinetic

Figure 1. (a) Dependence of the NH3 effluent mole fraction on the feed gas composition as a function of time on stream observed for the Ru/MgO catalyst. The corresponding temperature profile is shown as dashed line. (b) Magnification of boxed area in a.

data.12,27 The simple power-law expression can be easily replaced by more complex models expressed in terms of Hougen-Watson rate expressions. However, a detailed kinetic analysis is omitted in the following as modeling has been done for similar catalyst systems in previous works by our group.5,28-30 Methanol synthesis at differential conversion was measured for a high space velocity of synthesis gas, making use of a very low catalyst mass. Therefore, the presented rates are expressed as average rates of methanol formation rCH3OH (mL gcat-1 s-1), which are obtained from experimental quantities by

rCH3OH )

xCH3OHQ mcat

(4)

where Q is the flow rate of synthesis gas leaving the reactor (mL min-1) measured at standard conditions (T ) 273 K and atmospheric pressure), xCH3OH is the mole fraction in the gas mixture leaving the reactor, and mcat is the catalyst weight. Results Kinetic Measurements in Ammonia Synthesis. Figure 1a displays the results from part of a typical run for kinetic measurements, which included data collection at atmospheric pressure over a wide temperature

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range (300 K). The temperature profile (bottom) and the corresponding effluent NH3 mole fraction observed for the Ru/MgO catalyst (top) are plotted as functions of time on stream for a stoichiometric synthesis gas mixture. The runs comprised isothermal measurements starting at 783 K held under steady-state operating conditions, followed by stepwise lowering of the reaction temperature to 513 K. Subsequently, the temperature was ramped linearly at 5 K min-1 to 783 K. Finally, the reactor was cooled by means of pressurized air, and then the temperature was ramped up linearly at 5 K min-1 to 783 K (in this case, 5 K min-1 was chosen to verify the reproducibility of the experiment). The experimental trace for the gas-phase NH3 can be explained for the time period from 252 to 254 h as follows: The reaction starts at around 500 K, passes through a maximum, and approaches the equilibrium composition of the mixture following the temperature dependency of the overall reaction equilibrium constant. As anticipated for an exothermic reaction, the effluent equilibrium mole fraction decreases with increasing temperature in the regime governed by thermodynamics. Quasi-isothermal operation of the reaction is guaranteed, because the effluent NH3 mole fraction is in good agreement with thermodynamic data. A comparison of the two types of operation clearly demonstrates how time-consuming the classical way of catalytic testing is as a means of obtaining a reliable database for the determination of reaction rates. Moreover, the small set of kinetic data measured during steady-state operation should be compared to the abundance of information obtained in the temperature-programmed mode over the same time period. A closer inspection of the kinetic data reveals that a variation of the reaction temperature results in an immediate change in the effluent NH3 concentration (Figure 1b), which is a basic prerequisite for the use of the quasi-isothermal temperatureprogrammed method. A technique for collecting kinetic data more rapidly should work very efficiently near industrial conditions to provide a safer base for extrapolation beyond the measured data range. The results of three different runs repeated in the quasi-isothermal temperature-programmed mode under the same high-pressure conditions are summarized in Figure 2a. The use of a linear heating rate is not necessary, as the effluent mole fraction is measured as a function of “clock time” (the time since the start of the ramp). Therefore, the value for the heating rate is a mean value of the temperatureprogrammed rate, whereas the negative sign symbolizes a cooling process (ramping down the temperature). Because of the high space time under the reaction conditions studied, which is needed to overcome the dead volumes of the reactor, the high-pressure unit, and the tubings in the experimental setup, a time lag of about 20 min exists for the outlet flow leaving the exit of the catalytic fixed bed and being analyzed by the quadrupole mass spectrometer. Taking this time lag into account, the clock time can be easily replaced by the reaction temperature (Figure 2b) via a simple data transformation. An inspection of the transformed kinetic data reveals that the traces using slower heating rates of either 1 or 2 K min-1 are identical, whereas a small deviation from the trace obtained by 5 K min-1 is visible. The data points of selected temperatures obtained under steady-state reaction conditions have been included in this figure to show the good agreement between the two

Figure 2. (a) High-pressure kinetic data of ammonia synthesis measured for the Ru/MgO catalyst as a function of clock time using various heating rates. (b) NH3 mole fraction in the reactor effluent as a function of reaction temperature after simplified data transformation.

operation modes, i.e., the steady-state and the quasiisothermal temperature-programmed modes. As a further result under the reaction conditions applied here, 5 K min-1 should be regarded as the upper limit of the quasi-isothermal temperature-programmed method using the simplified data transformation described above. However, a faster ramping should lead to a set of raw experimental data, which, in turn, has to be analyzed by means of a more complicated reactor model as addressed in the discussion. Classification of Ru/MgO Catalysts Using PowerLaw Kinetics. Using the results of the quasi-isothermal temperature-programmed method, the evaluation of the reaction orders for simple power-law kinetics (r ) kapppNH3RpN2βpH2γ) is straightforward.12,27 The analysis was carried out in the temperature range specified in Table 3, thus ensuring that the measurements were performed in the kinetically controlled regime. It can be seen in Table 3 that the reaction orders determined by the quasi-isothermal temperature-programmed mode are identical with the values obtained by the steadystate mode of operation. The slight deviation between previous results of our group for a similar catalyst12 and the values presented here might originate from the limitation of the evaluation method ((0.1 for the reaction order and (5 kJ/mol for the apparent activation energy). A minor effect caused by the catalyst (catalyst composition, etc.) might also contribute to the small differences. Contrary to the results of Aika et al.,27 the reaction order for H2 was found to be negative in all cases. The positive reaction order reported by Aika et al.27 might be due to the presence of chlorine originating

Ind. Eng. Chem. Res., Vol. 40, No. 13, 2001 2797 Table 3. Exponents for the Power-Law Rate Expression r ) kapppNH3rpN2βpH2γ and Apparent Activation Energies as a Function of the Total Pressure Determined in the Given Temperature Rangea catalyst

pressure (MPa)

temperature range (K)

R (NH3)

β (N2)

γ (H2)

Ea (kJ/mol)

Ru/MgOb (temperature-programmed) Ru/MgOb (steady-state) Ru/MgOc (steady-state)

0.1 2.0 0.1

543-643 588-673 543-643

-0.6 -0.6 -0.6

0.8 0.8 0.8

-0.5 -0.7 -0.5

76 79 73

0.1 2.0

513-603 573-663

-0.3 -0.3

0.8 1.0

-0.3 -0.5

69 78

a The apparent activation energies were derived from Arrhenius plots at constant flow rate. The accuracy of the determination of the power-law exponents is about (0.1, and that of the apparent activation energies is less than (5 kJ/mol. b This work. c Reference 12.

Figure 3. Ammonia synthesis activity under screening conditions (T ) 588 K, p ) 0.1 MPa, Q ) 40 NmL min-1, xN2:xH2 ) 1:3) as a function of the number of active sites determined by in situ temperature-programmed surface reaction of N-* with gas-phase H2 (inset).

from RuCl3 used in catalyst preparation. The apparent activation energy of the overall reaction has been reported by Rosowski et al.12 to be 69 kJ mol-1 at atmospheric pressure and 78 kJ mol-1 at 2.0 MPa. Both values are in good agreement with the results determined in this study. Temperature-Programmed Reaction of GasPhase H2 with Preadsorbed Atomic Nitrogen. To compare catalysts in terms of catalytic activity, the values of the site time yield (STY) or turnover frequency (TOF) are widely used to specify a reaction rate in heterogeneous catalysis.31,32 The former is defined as the number of molecules of a specified product made per catalytic site and per unit time, and the latter is the number of revolutions of the catalytic cycle per unit time. Note that both specific rates are referred to the total number of active sites. In ammonia synthesis, it is generally accepted that the active site consists of an ensemble of surface sites that enables the dissociative chemisorption of nitrogen. Hence, we determined the number of active sites by using the temperatureprogrammed surface reaction of gas-phase H2 with preadsorbed atomic nitrogen (N-*).14 This in situ technique has the added advantage of rapid data aquisition in the same setup intended to avoid exposure to air. By means of this technique, we determined the number of active sites for the series of Ru/MgO catalysts listed in Table 1. A characteristic profile is shown in the inset of Figure 3; similar profiles can be found in refs 5, 29, and 30. A linear relationship between the catalytic activity and the number of active sites is observed when the production rate for ammonia synthesis determined for a screening point (T ) 588 K, p ) 0.1 MPa, Q ) 40 NmL min-1, xN2:xH2 ) 1:3) is

Figure 4. Arrhenius plot of the effluent CH3OH mole fraction for TG1 (]), TG2 (O), and TG3 (4). Experimental reaction conditions: p ) 5.0 MPa, mcatQ-1 ) 14 800 g s m-3.

displayed as a function of the amount of N-* obtained by N-* TPSR (Figure 3). The scatter of the experimental points, which is higher than would be expected from the experimental errors, leads to the conclusion that there might be a small, second-order effect of the catalyst composition, etc., on the specific ammonia synthesis activity. Moreover, the results of ex situ static H2 chemisorption measurements (cf. Table 1), which serve as a control experiment for the determination of the Ru metal surface area, support the linear relationship. Kinetic Measurements in Methanol Synthesis. In the kinetic study of methanol synthesis catalysts, we measured catalytic activity for three different Cu/ZnO/ Al2O3 catalysts. In situ determination of the specific copper metal surface area was carried out by isothermal N2O RFC, yielding the results summarized in Table 2. Kinetic measurements were performed after the methanol synthesis had been conducted at 5.0 MPa for several days on stream. In all kinetic experiments with the feed gas, which consisted of CO, CO2, and H2, the major products were found to be methanol and water, which were produced by the reverse water-gas shift reaction. The methanol selectivity was remarkably high, in excess of 99%. Steady-state conversions were slowly reached within 2 h after the pressure, temperature, gas flow rate, and feed gas composition were established. Similar behavior observed for binary and ternary Cu systems has been reported in the literature.33-35 The highpressure measurements were conducted over a lowtemperature range from 433 to 513 K. The resulting production rates calculated from the outlet CH3OH concentration assuming differential conditions are displayed versus the inverse temperature in a so-called Arrhenius plot (Figure 4). It can be seen that the differences in the production rates of the three catalysts are small. The differences increase with increasing

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tively. The slow approach to steady-state synthesis activity has been observed also at higher pressure. Discussion

Figure 5. Methanol synthesis activity measured for TG1 (dotted line), TG2 (dashed line), and TG3 (solid line) in the quasiisothermal temperature-programmed mode. Experimental reaction conditions: p ) 0.1 MPa, β ) 0.5 K min-1, Q ) 10 NmL min-1.

Figure 6. Methanol production of Cu/ZnO/Al2O3 from CO2/ CO/H2 as a function of reaction temperature ramping the temperature up (β ) 0.2 K min-1, solid line) and down (β ) -0.2 K min-1, dashed line). The points measured under steady-state conditions are included (filled symbols). Experimental reaction conditions: p ) 0.1 MPa, Q ) 50 NmL min-1.

temperature, and TG3 was found to be the catalyst with the highest catalytic activity over the entire temperature range: TG3 > TG2 > TG1. The same sequence results when the specific Cu metal areas are compared (Table 2), indicating that, within a certain class of catalysts, the rate of methanol formation is correlated linearly with the specific Cu metal area. For rapid catalyst screening, the quasi-isothermal temperature-programming mode was applied to measure the catalytic activity in methanol synthesis. The same sequence for the CH3OH production rate was found at ambient pressure (Figure 5) as was observed for the steady-state mode at high pressure (Figure 4), i.e., TG3 > TG2 > TG1. A relatively moderate linear heating rate of β ) 0.5 K min-1 was used for data acquisition throughout these experiments. However, a closer inspection of the kinetic data measured for catalyst TG1 using a higher flow rate (Figure 6) reveals that the values for the ascending branch (ramping up the temperature, β ) 0.2 K min-1) deviate from those for the descending branch (ramping down the temperature, β ) -0.2 K min-1) over the complete temperature range, whereas at specified temperatures over a period of about 2 h, the CH3OH activity slowly reaches steady state (symbols) by a gradual decline or increase, respec-

For kinetic studies on ammonia synthesis, the quasiisothermal temperature-programmed mode was found to be a useful technique for collecting experimental data over a wide range of conditions more rapidly. Moreover, the method allows for the use of nonlinear temperature changes and the possibility of ramping the temperature up or down. In particular, when the reaction is performed at low conversion, the quasi-isothermal temperature-programmed mode is very effective in achieving a reliable base of kinetic data, which, in turn, can be used for data extrapolation beyond the measured data range. In the absence of temperature gradients within the catalyst bed, a simplified data transformation can be performed. However, it should be emphasized that running the ammonia synthesis reaction at high conversion might result in much higher NH3 mole fractions in the effluent than shown in Figure 2. This clearly leads to an axial temperature profile. In this case, the heat and material balance equations have to be solved simultaneously using a proper reactor model, e.g., a pseudohomogeneous one-dimensional reactor model or, in the case of radial temperature and concentration gradients, a two-dimensional reactor model. Recently, Wojciechowski and co-workers8-10 have put much effort into combining data acquisition and data analysis, leading directly to reaction rates based on complex reactor models. The temperature-programmed surface reaction of preadsorbed atomic nitrogen (N-*) in a H2 gas stream (N-* TPSR) was applied for the determination of the number of active sites.14 It can be seen in Figure 3 that the catalytic activity in ammonia synthesis using unpromoted Ru/MgO catalysts is proportional to the number of active sites. Furthermore, this transient experiment provides new insight into the classification of Ru-based ammonia synthesis catalysts. The temperature of NH3 formation (Tonset) was found to vary between 355 and 440 K (Table 1), but the scatter of the experimental data does not allow for a direct correlation of Tonset with the catalytic activity of the catalyst. However, unpromoted Ru-based catalysts with the highest catalytic activity never fell below a Tonset value of around 355 K. Compared to the experiments performed with the unpromoted Ru/MgO catalysts, previous studies by our group with Cs-promoted Ru/MgO showed a shift in the onset of NH3 formation in the TPSR experiment to lower temperature (for β ) 5 K min-1, Tonset ) 325 K29) because of the effect of alkali promotion. Thus, the N-* TPSR experiment allows both the number of active sites to be derived rapidly, which, in turn, determines the catalytic activity (Figure 3), and the presence of electronic promoters to be detected. A detailed analysis of the TPSR spectra can be found elsewhere.36 In methanol synthesis, the usage of the quasi-isothermal temperature-programmed technique is limited by a slow change in the state of the working Cu/ZnO/ Al2O3 catalyst. A possible explanation is based on the microkinetic model presented by the Topsøe group,37 which has recently been extended.38 In their combined in situ X-ray absorption fine structure and X-ray diffraction experiments over Cu/ZnO, it was found that the Cu-Cu coordination number for Cu metal particles on

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ZnO changed reversibly upon switching from dry to wet CO2/CO/H2 at only 0.1 MPa and 493 K. Cu particles became smaller when water was removed from the synthesis gas stream. These data led to the Topsøe model that correlated the relative interfacial area between Cu metal and ZnO in Cu-ZnO with the reduction of the ZnO component and the consequent spreading of the Cu metal species on ZnO.37 On the other hand, Lee et al.34 reported that steady-state rates were reached in catalytic experiments with a Cu/ZnO/ Al2O3 catalyst after 5 and 20 h on stream for CO2/H2 and CO/H2, respectively. A similar transient behavior has also been observed by Meitzner and Iglesia33,35 for Cu/SiO2. The catalytic activity decreased gradually when they were feeding CO2 to a CO/H2/N2 mixture. Hence, in our study, a minor effect of the catalyst cannot be discarded because of a slow accumulation of a species on the support, which participates in the methanol synthesis reaction. However, further screening results from a variety of supported copper catalysts tested for methanol synthesis in our equipment clearly indicate that the catalyst activity is proportional to the specific copper metal surface area of the working catalyst, which has been determined by isothermal N2O RFC. Evidently, this finding is in good agreement with results in the literature, where a linear relationship between the catalytic activity and the specific copper metal surface area has been observed and reported by other groups for binary and ternary catalyst systems.39-41 In terms of catalyst ranking in methanol synthesis, we put forward a fast screening test in a fully automated flow setup that includes the measurement of the catalytic activity under steady-state conditions and the determination of the Cu metal surface by isothermal N2O RFC performed under mild conditions as a fast and reliable technique. Because the same sequence with respect to the CH3OH production rate was found at ambient pressure as was observed for the steady-state mode at high pressure in terms of catalyst ranking, a scale-down in pressure seems to be safe. However, because of the change in state of the working catalyst, it is not recommended that kinetic data be collected in a quasi-isothermal temperature-programmed mode. Conclusions Compared to the steady-state mode, the method of quasi-isothermal temperature-programming more rapidly provides a reliable kinetic database for studying the kinetics of ammonia synthesis over Ru-based catalysts, setting up rate expressions, and extrapolating beyond the measured data range. Moreover, the in situ temperature-programmed surface reaction of N-* with gasphase H2 (N-* TPSR) turns out to be a fast method for catalyst screening for Ru-based catalysts. However, the use of the fast data acquisition is not without problems. In methanol synthesis over Cu/ZnO/Al2O3, the induction of activity for synthesis from a CO2/CO/H2 mixture restricts the quasi-isothermal temperature-programmed mode of operation to qualitative catalyst screening. Acknowledgment Fruitful discussions with Martin Muhler are gratefully acknowledged. This research was partially supported by the Bundesministerium fu¨r Bildung und Forschung (BMBF), Grant 03C0283B.

Symbols Used mcat ) catalyst mass (g) p ) total pressure (Pa) Q ) flow rate (NmL min-1) R ) ideal gas constant (kJ mol-1 K-1) SV ) space velocity (h-1) tid ) ideal catalyst contact time (s) T ) temperature (K) x ) mole fraction Greek Letters β ) heating rate (K min-1) δ ) compressibility factor b ) bed void fraction

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Received for review September 18, 2000 Revised manuscript received April 5, 2001 Accepted April 9, 2001 IE0008225