CeO2 Catalyst

Dec 11, 2012 - BASF Corporation, Catalysts Division, 25 Middlesex-Essex Turnpike, ... oxide support was investigated using a differential reactor appr...
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Kinetics of Methanol Steam Reforming with a Pd−Zn−Y/CeO2 Catalyst under Realistic Operating Conditions of a Portable Reformer in Fuel Cell Applications Oleg M. Ilinich,* Ye Liu, Earl M. Waterman, and Robert J. Farrauto‡ BASF Corporation, Catalysts Division, 25 Middlesex-Essex Turnpike, Iselin, New Jersey 08830, United States ABSTRACT: Kinetics of methanol steam reforming (MSR) with a catalyst containing palladium and zinc on a cerium oxide support was investigated using a differential reactor approach. The goal of the study was to develop optimal process conditions for portable fuel reformers in fuel cell applications. Activation energies for the MSR reaction and for the overall methanol conversion, as well as pre-exponential factors and the reaction orders for CH3OH, H2O, H2, CO2, and CO were determined. The study covers the temperature range from 230 to 320 °C, consistent with the operation of a reformer integrated to a portable power fuel cell device. All work was done with a mixture of CH3OH and H2O with a volume ratio 2:1 (molar ratio 0.88:1) at atmospheric pressure, which is representative of the process conditions of practical interest for this application. For the determination of reaction orders the concentration of each reactant under consideration was varied to mimic a broad range of CH3OH conversions. The following power law kinetic equations for methanol steam reforming reaction and for overall methanol conversion were established: (i) For methanol steam reforming CH3OH + H2O = 3H2 + CO2, RMSR = 34.476 × e−54270/(RT) × pCH3OH0.54 × pH2O0.10 × pH2−0.40 × pCO2−0.13 × pCO−0.075. (ii) For overall methanol conversion (MSR plus CO formation), Roverall = 53.309 × e−55860/(RT) × pCH3OH0.54 × pH2O0.10 × pH2−0.40 × pCO2−0.13 × pCO−0.075. In these equations, RMSR and Roverall are the reaction rates (mol·s−1·gPd−1); 34.476 and 53.309 are the pre-exponential factors; 54270 and 55860 are the activation energies of MSR and of the overall methanol conversion, respectively (J·mol−1); R is the molar gas constant (8.314472 J·mol−1·K−1), T is the temperature (K), and pi is the partial pressure of the ith component (kPa).

1. INTRODUCTION Methanol, as a hydrogen storage material, has significant advantage in terms of energy density over other means of hydrogen storage such as hydrides and compressed or liquefied H2, and therefore is an attractive source of hydrogen for fuelcell powered portable devices. Gas mixtures with up to 75% volume H2 can be produced via the reaction of methanol steam reforming

impacts their operation and handling procedures. Copper− zinc−alumina catalysts are also sensitive to steam at high concentrations, which causes additional deactivation by blocking active copper centers and induces the loss of mechanical strength of the catalyst pellets. The latter factor plays an especially important role for portable fuel cell power systems that operate with frequent starts and stops where water condenses on the catalyst.2 The search for a catalyst that is better suited to meet new challenges associated with the duty cycles of fuel cell power systems have led researchers to adopt and modify the catalytic compositions based on palladium and zinc, first discovered by Iwasa et al.3 At BASF Corporation Catalysts Division, a novel MSR catalyst of this type was developed recently to be used as a washcoat to replace pellets in methanol reformers integrated to portable fuel cell power units. The catalyst consists of palladium and zinc on ceria support promoted with yttrium,4 and represents a further evolution of the family of palladium− zinc-based catalysts developed in recent years.5−11 The new MSR catalyst was designed to be coated on metallic substrates to ensure rapid heat transfer to the catalyst, which is essential for a fast startup and efficient performance of fuel cell power systems incorporating the strongly endothermic MSR reaction. A thin adherent catalyst washcoat, not exceeding 150−200 μm

CH3OH + H 2O → 3H 2 + CO2 ΔH 0 = 49.3 kJ/mole

(1)

In the last 20 years or so this reaction became an object of intense studies as an efficient way of generating hydrogen-rich feed gas for fuel cells. A review by Palo et al. provides a comprehensive summary of numerous publications on methanol steam reforming for hydrogen production.1 The catalysts used for MSR reaction fall into two main categories. Historically, the first catalysts were copper-based particulates, typically of the copper−zinc−alumina type widely used for methanol synthesis and water gas shift reactions. However, these catalysts have some significant drawbacks that hamper their application in portable fuel cell power sources. Copper−zinc−alumina catalysts suffer from copper sintering at temperatures above about 260−280 °C, which results in substantial loss of activity. They must be activated by carefully controlled prereduction prior to the use in the target reaction. Serious problems are associated with the catalysts’ dangerous self-heating properties upon air exposure, which negatively © XXXX American Chemical Society

Received: June 18, 2012 Revised: October 23, 2012 Accepted: December 11, 2012

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dx.doi.org/10.1021/ie301606w | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. Schematic of the experimental system.

performed using Agilent Quad microGC, which monitored the reaction products CO2, H2, and CO as well as N2. The microGC was not capable of handling water and methanol, which had to be removed from the reaction mixture prior to the analysis, and therefore the experimental setup included an icecooled knockout-condenser. The feed line connecting the vaporizer with the reactor inlet, the line from the reactor outlet to the knockout, and the nitrogen line to the reactor outlet were heated to prevent vapors from condensing before entering the knockout unit. Reactor temperatures and flow rates of all components in the course of experiments were controlled by a computer. The study was performed using the differential reactor approach by operating at low conversions of methanol and water (ca. 4−8%), to minimize variability of partial pressures of all reactants except for the one being investigated in a given experiment. To maintain low conversions throughout the study, the experiments were carried out at different space velocities to compensate for the reaction rate variation due to changes in the reactants concentrations and catalyst bed temperature. In view of the low conversions, the calculations of the reaction rates were based on concentrations of the products H2, CO2, and CO (no other reaction products were detected). For each reactant, the reaction orders were determined over a broad range of partial pressures to mimic methanol conversions from a few percent up to about 95−98%, while maintaining the partial pressures of all other components constant. The work was focused on the feed mixture of CH3OH and H2O in the volumetric ratio of 2:1 (molar ratio 0.88:1) at atmospheric pressure, which is representative of the process conditions of practical interest. 2.2. Catalyst Preparation Procedure. A zinc-containing solution was prepared by adding 118.98 g Zn(NO3)2 × 6H2O (Aldrich) to 100 g of distilled water and diluting to 200 mL, then stirring to generate a clear solution. A separate palladium nitrate solution containing 20.74 wt % palladium (BASF) was used. An yttrium-containing solution was prepared by adding 76.6 g Y(NO3)3 × 6H2O (Alfa Aesar) to 50 g of distilled water

in thickness, provides mechanical stability and offers no diffusional resistance for the reactants transport in the catalyst pores. The present paper describes the study of MSR reaction kinetics with the catalyst hereafter referred to as Pd−Zn−Y/ CeO2. The study was conducted in the framework of the National Institute of Standards and Technology (NIST) Advanced Technology Program “Hydrogen Generator for a Miniature Fuel-Cell Power Source” (Award No. 70NANB3H3012), and was initiated to provide basis for modeling of the optimized performance of a fuel reformer utilizing this novel catalyst. The open literature has a limited number of publications related to kinetic studies which generated kinetic equations for the MSR reaction. A majority of these studies refer to the copper-based catalysts,12−23 while only a few to the Pd−Zn compositions either with γ-Al2O3 support6,8 or without it.7 The ultimate goal of our present work was to develop power law kinetic equations describing the reaction rates for MSR reaction and overall conversion of methanol with the new Pd−Zn−Y/ CeO2 catalyst. This catalyst was the subject of our several recent publications dealing with various aspects of its use in the MSR reaction.2,4,24

2. EXPERIMENTAL SECTION 2.1. Reactor Setup. The present study was conducted using the experimental setup shown in Figure 1. A mixture of methanol and water was fed through the vaporizer into the quartz tube flow reactor (12.5 mm ID) using the HPLC pump. The reactor was heated by the tube furnace, and the catalyst temperature was controlled by a thermocouple. The inlet and outlet lines of the reactor were equipped with static mixers to ensure homogeneous compositions of the feed and product streams. The experimental setup was equipped with four gas lines, through which the reactant gases CO2, H2, and CO, as well as N2 used as an internal standard for GC analysis were fed into the setup. Flow rates of all gases were regulated with the MKS flow controllers. The analysis of the reaction mixtures was B

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and diluting to 100 mL, then stirring to generate a clear solution. Next, 100.0 g of cerium oxide powder (Aldrich) was placed into a beaker. Then a mixed solution was prepared by combining 122.8 mL of the zinc nitrate solution, 40.7 mL of the palladium nitrate solution, and 22.1 mL of the yttrium nitrate solution. The mixed solution was added to the cerium oxide powder under continuous stirring. The resulting slurry was then dried in an oven at 75 °C for 16 h. The dry powder was finally calcined in air at 540 °C for 3 h and sieved for a desired particle size. 2.3. Catalyst Activation Procedure and Particle Size for the Kinetic Measurements Free from Pore Diffusion Limitations. The Pd−Zn−Y/CeO2 catalyst prepared as described above and consisting of 5 wt % Pd and 20 wt % ZnO on ceria support was used in the present study. Prior to the kinetic measurements the catalyst was activated by reducing in 7% H2/N2 gas mixture at 550 °C for 1 h, followed by purging with a CH3OH + H2O mixture at 230 °C for 10 h. In good agreement with the literature on Pd−Zn MSR catalysts,5,10 this pretreatment resulted in the formation of a CeO2-supported PdZn alloy crystallites and generated a highly selective catalyst for H2 production.24 The catalyst demonstrated stable catalytic performance over the course of our kinetic measurements, which was verified by repeated runs under the same experimental conditions. The weight amount of the Pd−Zn− Y/CeO2 catalyst in all runs was 0.500 g. To establish experimental conditions where the kinetic measurements are free from pore diffusion limitations, MSR reaction rates were measured under identical conditions (feed flow rates and the catalyst temperatures) for the two catalyst fractions with different particle sizes as shown in Table 1.

RMSR = AMSR × e−EMSR /(RT ) × pCH OH a × pH Ob × pH 3

× pCO d × pCOe 2

fraction 150−250 μm 2.6 × 10−3

(2)

Figure 2. MSR reaction order for H2O. Experimental conditions: reaction temperature, 230 °C; molar ratio CH3OH/H2O in the feed from 0.44:1 to 13.69:1; GHSV = 160 000 to 250 000 h−1.

MSR reaction rate at 320 °C, molCH3OH·s−1·gPd−1 2.7 × 10−3

c 2

where RMSR is the reaction rate of methanol steam reforming (mol·s−1·gPd−1), AMSR is the pre-exponential factor, EMSR is the activation enegy (J·mol−1), R is the molar gas constant (8.314472 J·mol−1·K−1), T is the temperature (K), and pi is the partial pressure of the ith component (kPa). Logarithmic forms of the eq 6 were used in this study to determine all kinetic parameters. 3.1.1. Reaction Order for H2O. The rate of methanol steam reforming shows a weak dependence on water in the concentration range of interest, the reaction order for H2O being equal to 0.1 (Figure 2). This value is in good agreement

Table 1. Rates of Methanol Steam Reforming at 320 °C for Pd−Zn−Y/CeO2 catalyst with different particle sizes fraction 250−500 μm

2

with information available in the literature on the kinetic investigations of Pd−Zn MSR catalysts.6−8 The low reaction order for water might originate from the fact that with the feed mixture of CH3OH/H2O in the molar ratio 0.88:1 there is always an excess of water relative to methanol compared to the MSR reaction stoichiometry, and this excess increases with the methanol conversion. From the process engineering standpoint, low reaction order with respect to water is advantageous since it implies that excess water would not accelerate the reaction relative to a nearly stoichiometric water−methanol ratio used in the realworld fuel cell technology that utilizes Pd−Zn−Y/CeO2 catalyst. At the same time, excessive water content would require additional energy to evaporate and heat up to the reaction temperature, which would negatively affect power output from the portable fuel cell power generator. Small excess of water with the currently used water−methanol ratio (1:0.88 molar) is beneficial as it serves to eliminate coke formation. In summary, consideration of kinetic, chemical, and process engineering information on the role of water in MSR process shows that the optimal water−methanol ratio is close to the reaction 5 stoichiometry with a slight excess of water. 3.1.2. Reaction Order for CH3OH. The rate of methanol steam reforming shows relatively strong dependence on the concentration of methanol. The reaction order for CH3OH in the concentration range of interest equals 0.54 (Figure 3). This

Equal reaction rates measured with the different fractions of the catalyst show that there was no pore diffusion limitations for either particle size even at the highest temperature used in the study. The fraction 150−250 μm was further used for the kinetic measurements. This result also implies that for the catalyst coated on metallic substrates with a typical washcoat thickness not exceeding about 150−200 μm, no diffusional resistance within the washcoat should exist under normal operating conditions of portable fuel reformers.

3. RESULTS AND DISCUSSION 3.1. Reaction Orders. Reaction orders were determined at 230 °C by varying the concentrations of each reactant in the range of practical interest while maintaining the partial pressures of all other components constant. For all reactants, the reaction orders for methanol steam reforming and for overall conversion of methanol were essentially the same. The aim of this study was to generate power law kinetic equations for methanol steam reforming, which in general terms can be expressed as follows: C

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Figure 4. MSR reaction order for CO2. Experimental conditions: reaction temperature, 230 °C; molar ratio CH3OH/H2O in the feed 0.88:1; GHSV = 220 000 to 320 000 h−1.

Figure 3. MSR reaction order for CH3OH. Experimental conditions: reaction temperature, 230 °C; molar ratio CH3OH/H2O in the feed from 0.88:1 to 0.0107:1; GHSV = 220 000 to 360 000 h−1.

hydrogen concentration. The reaction order for H2 in the concentration range of interest is equal to about −0.4 (Figure 5). Negative MSR reaction orders for H2 were also reported in the literature for Pd−Zn catalyst7 and for copper-based catalysts.16−18

value is lower than the reaction orders for CH3OH reported in the literature for different catalysts that range from 1 to 0.66.6−8 This difference may result from the different composition of the catalyst used in our study compared to the cited literature references. Thus, the BASF catalyst used in this study consisted of palladium and zinc with yttrium additive on ceria support, while the catalysts used in refs 6 and 8 were prepared by impregnating γ-Al2O3 with the mixture of Pd and ZnO and the catalyst used in ref 7 was prepared by impregnating ZnO with Pd. From a practical standpoint, relatively low reaction orders for methanol and water are a positive feature of the Pd−Zn−Y/ CeO2 catalyst. This implies that the rate of MSR reaction at high conversions should remain substantial and will not decrease as much as it would have decreased in the case of high reaction orders. 3.1.3. Reaction Order for CO2. The reaction orders for CO2 and other gaseous products (H2 and CO) were determined using feeds with different amounts of the corresponding gas added to the CH3OH−H2O mixture at a molar ratio of 0.88:1, to mimic up to 95% methanol conversions to a given product. Under the conditions of the present study, the reaction order for CO2 equals −0.13 (Figure 4). Limited information available in the literature points at nearly zero order in CO2 for the MSR kinetics with Pd−Zn catalyst,7 which generally agrees with our results. Note that this experiment also included a test of stability of the catalytic activity. The test was conducted by progressively increasing the CO2 partial pressure. After completing the activity measurements at the highest partial pressure (30.61 kPa), the partial pressure was decreased to the lowest value used in this set of measurements (4.23 kPa). The values of the reaction rates registered at this lowest pressure in the first and the last measurements, that were conducted within 6 h on stream, differ by only ±0.4% (the last reaction rate value is in fact higher than the first one), which indicates a stable catalytic activity. Stable activity and selectivity of this catalyst operating with the same methanol−water feed mixture in the temperature interval 375−475 °C for a period of 20 h on stream was also demonstrated in our earlier publication.2 3.1.4. Reaction Order for H2. The rate of methanol steam reforming shows relatively strong negative dependence on

Figure 5. MSR reaction order for H2. Experimental conditions: reaction temperature, 230 °C; molar ratio CH3OH/H2O in the feed 0.88:1; GHSV = 220 000 to 540 000 h−1.

The negative reaction order for hydrogen implies that the MSR reaction rate at high methanol conversions should decrease due to high concentrations of hydrogen. One might anticipate that high conversions might be therefore difficult to reach, which would complicate the use of Pd−Zn−Y/CeO2 catalyst in the targeted application for portable fuel cell power systems. However, in other studies with the same Pd−Zn−Y/ CeO2 catalyst we repeatedly operated at methanol conversions in excess of 99%.2,24 This shows that even though kinetically the reaction rate decreases at high conversions when H2 concentrations in the reformate are high (product inhibition), essentially complete methanol conversions are readily achievable with the subject catalyst. 3.1.5. Reaction Order and Selectivity for CO. Under the experimental conditions of the present study, small amounts of CO byproduct were detected in the reaction mixtures in D

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addition to CO2. Carbon monoxide can be generated via direct methanol decomposition CH3OH → CO + 2H 2

(3)

and/or the reverse water gas shift (RWGS) CO2 + H 2 → CO + H 2O

(4)

Reactions 3 and 4 are undesired and should be minimized as much as possible, since they decrease the efficiency of hydrogen production. Besides, carbon monoxide, a known catalytic poison, can only be tolerated by the fuel cell catalysts if its concentration in the reformate feed is low (