Structured cobalt oxide catalyst for VOC combustion. Part I: Catalytic and engineering correlations

Applied Catalysis A: General 366 (2009) 206–211

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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Structured cobalt oxide catalyst for VOC combustion. Part I: Catalytic and engineering correlations J. Łojewska a,*, A. Kołodziej b, T. Łojewski a, R. Kapica c, J. Tyczkowski c a

Jagiellonian University, Faculty of Chemistry, Ingardena 3, 30-060 Krako´w, Poland Institute of Chemical Engineering of the Polish Academy of Sciences, Bałtycka 5, 44-100 Gliwice, Poland c Technical University of Lodz, Faculty of Process and Environmental Engineering, Division of Molecular Engineering, Wo´lczan´ska 213, 90-924 Ło´dz´, Poland b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 19 March 2009 Received in revised form 1 July 2009 Accepted 7 July 2009 Available online 14 July 2009

Structured reactors based on metallic carriers for catalysts of highly enhanced transport properties can be an interesting alternative to monolithic converters. In this study the carriers based on wire gauze have been shown to improve the efficiency of the VOC combustion simultaneously decreasing the converter length. A successful application of metallic microstructures needs proper methods of catalyst layering. This study focuses on non-equilibrium plasma (NEP) deposition technique which is referred to Langmuir–Blodgett (LB) film deposition and to wet impregnation. The cobalt oxide catalyst deposited on wire gauze and steel sheets were characterised by XPS and Raman microscopy and tested in n-hexane oxidation in tubular and gradientless (jet-stirred) reactors. The model cobalt catalysts showed sufficiently high activity in relation with Pt reference catalysts. The most important factor affecting the catalyst activity was the type of the carrier. The best performance was achieved for the catalysts deposited on wire gauzes. ß 2009 Elsevier B.V. All rights reserved.

Keywords: VOC combustion Cobalt spinel Structured reactor Raman XPS Non-equillibrium plasma-deposition Langmuir–Blodgett film deposition

1. Introduction Catalytic combustion appears as the most efficient method for the removal of volatile organic compounds (VOCs) from exhaust gases. From the chemical engineering standpoint, the main problem of VOCs removal is the very high dilution of reactants in large gas streams, which implies that the overall process rate is controlled by diffusion. In our study, the problem of gas transport to catalyst surface has been approached by designing of fine structures of metallic converter fillers whose geometry prevents the development of laminar flow and thus intensifies mass transport [1–4]. Structured catalytic reactors are usually associated with ceramic monoliths [1,2] which have proved a successful solution in automotive catalytic converters. The main drawback of ceramic monoliths is relatively poor mass transport to the catalyst surface. Obviously, a low flow resistance and intense mass transport are against each other. A compromise is difficult to achieve for monoliths due to a fully developed laminar flow in their long straight channels. In such conditions flow resistance (Fanning friction factor) and mass transfer (Sherwood number) depends mainly on the channel cross-sectional shape which is weakly

* Corresponding author. E-mail address: [email protected] (J. Łojewska). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.07.006

affected by for example fluid velocity. The wire gauze reactors and many other structured reactor types offer a great variety of intermediate relationships between flow resistances and mass transfer that could lead to a solution much better than that of monoliths or packed beds. Structured metallic reactors have long been used in chemical industry. The reactors for catalytic ammonia oxidation built of stacked platinum gauzes have been used for about one hundred years. New designs of wire gauzes for industrial applications are described by Perez-Ramirez et al. [5] among which of a particular interest are knitted gauzes [6], such as spatially knitted Multinit described in [7]. Currently, even more sophisticated catalytic gauzes have been developed with the catalysts deposited on them. Good examples of these kind of structures are: the gauzes offered by Katator AB (Sweden) (also cut or slotted) [8] and studied by Ahlstro¨m-Silversand and Odenbrand [9]; Microlith1 described by Lyubovsky et al. [10]; or perforated thin metal sheets [11]. Indeed, catalytic knitted gauzes are becoming more and more interesting for other than ammonia oxidation catalytic processes as stated by Hills et al. [12], and even for heterogeneously catalysed gas–liquid reactive systems such as reactive distillation [13]. The examples are KATAPAK-S or MULTIPAK [14] that contain wire gauze containers filled with catalyst pellets. The fine microstructures need sophisticated methods of catalyst layering that would secure good adhesion to the metallic ground and would not change the elaborated geometry [1,15]. It

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may be achieved by the thorough control of the amount of deposited material, which is usually not guaranteed by classical deposition methods such as sol–gel impregnation [16,17]. Thus only a few techniques can be regarded for the application to metallic surfaces among which plasma or flame spraying [9] seem good candidates. The activity of the catalysts should also be adjusted to the enhanced transport properties of the prospective structural reactors in order not to limit the process yield. An alternative to existing active but vulnerable to coking [9] precious metal catalysts is highly demanded. Cobalt spinel in both pure and bimetallic form seems promising for its low cost and good activity in VOC combustion [18–20]. Cobalt spinel structure has also demonstrated low temperature of combustion of VOC oxygenates [18] as well as sooth particles [21–23]. In fact, cobalt spinel seems to have an interesting structure in terms of the catalytic redox properties. The electron transfer is secured by two oxidation states of Co and open tetrahedral voids with Co+2 in the oxygen crystal sublattice. The spinel structure is also the most thermodynamically stable among the cobalt oxides but at temperatures below 700 8C [24–26] which in practice excludes its application for other than VOC combustion processes. The aim of this work is to demonstrate the influence of the transport phenomena on the catalyst activity under various flow conditions (in the boundary laboratory reactors) and on a chosen metallic carrier of various geometry (wire gauze and sheets). The cobalt oxide catalysts of the desired dispersion and chemical structure were prepared with the non-equillibrium plasma (NEP) method, and compared to the Langmuir–Blogett (LB) and to the impregnation method in terms of the efficiency of deposition, catalyst chemical properties (by Raman and XP spectroscopy) and catalytic performance in n-hexane combustion. The preparation method was adjusted to obtain dispersed cobalt spinel structure treated as a model catalyst surface. 2. Experimental 2.1. Catalyst The catalyst carriers were stainless chromium–aluminium steel sheets, 0.3 mm thick, denoted as CrAl/s (00H20J5, Strzemieszyce, Poland) and knitted wire gauzes made of the same steel, 17.5 mesh/ in., wire diameter d = 0.1 mm denoted as CrAl/g. The reference samples were: (i) cobalt foil, 99.99 + % purity, 0.1 mm thick, denoted as Co/f (Aldrich); (ii) PtRh10 gauze 90 wt% Pt and 10 wt% Rh, 81 mesh/in., wire diameter 0.059 mm, denoted as PtRh/g (Polish State Mint); (iii) commercial Pt/Al2O3 powder catalyst with 0.35 wt % of Pt denoted as Pt/Al2O3.

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Regardless of the layering technique used, the catalyst preparation consisted of three steps: (1) the precalcination of a steel carrier in air at 1000 8C for 24 h (Co foil 700 8C for 5 h), (2) deposition of Co compounds on a precalcined carrier, (3) activation by the oxidation of the deposited samples at 350 8C for 1 h. The precalcination step is used to obtain alumina layer which becomes a support for the catalyst. It has been shown before that precalcination of the steel leads to the segregation of a-Al2O3 which creates 1 mm highly dispersed layer on top of the metallic surface [1,15]. This layer creates the support for the cobalt catalyst. 2.2. Deposition The NEP technique has been described in [27]. Cobalt-based films obtained from cyclopentadienyldicarbonyl-cobalt(I) (Stream Chemicals, Inc.), denoted as CpCo(CO)2, were deposited in a parallel-plate plasma reactor (13.56 MHz) using a total pressure of 45–75 Pa and a glow discharge power of 40 W or 80 W in the flow of the carrier gas: argon and/or oxygen. More details on the reactor and procedures can be found in [28]. The CpCo(CO)2 catalyst precursor–liquid under standard conditions–was supplied into the reactor chamber with the carrier gas passing through a saturator. The films were deposited from various gaseous mixtures on the surface of precalcined CrAl steel. The amounts of the deposited materials were estimated from ellipsometric measurements [28]. The details concerning the preparation of the catalysts using the LB technique can be found in [29]. In brief, a catalyst precursor– cobalt stearate, Co(C18H35O2)2, denoted as CoSA, was transferred onto the precalcined carrier surface from a Langmuir trough. The transfer of compressed monolayers was repeated to obtain the desired amount of cobalt on the catalyst surface. The impregnation was performed using Co(NO3)2 water/ glycerine (1:1) solutions. The glycerine was added to the solutions to increase the viscosity of the solution and improve wettening properties. The appropriate amounts of the solutions were added onto the precalcined surface of the CrAl steel and then the sample was dried at 130 8C for 3 h. The procedure was repeated to achieve a desired amount of deposited catalysts. The symbols of the samples prepared by the methods described above and the amount of deposited material are presented in Table 1. 2.3. Characterisation XPS analyses were carried out in a VG Scientific ESCA-3 photoelectron spectrometer using the Al Ka radiation (1486.6 eV) from an X-ray source operating at 13 kV and 10 mA. The working pressure was lower than 5  107 Pa. The analyses were adjusted for the measurements of electron signals from C1s, O1s, Al2p, Cr2p,

Table 1 Samples composition and preparation methods. Sample name

Carrier

Preparation method

Conditions

Co3O4 (mass%)*

Co3O4 (mol.%)*

CrAl/s 0.1Co/CrAl/s 0.3Co/CrAl/s 0.7Co/CrAl/s

CrAl sheet

Calcination LB

1000 8C, 48 h 10, monolayers CoSA 40, monolayers CoSA 100, monolayers CoSA

0

0

Co0/CrAl/s Co100/CrA/s 70Co/CrAl/s 55Co/CrAl/s 70Co/CrAl/g 55Co/CrAl/g

CrAl sheet

NEP

20 min, Ar 100 mol.%, 80 W 30 min, O2 100 mol.%, 80 W 120 min, O2 20 mol.%, 40 W 60 min, O2 20 mol.%, 40 W 120 min, O2 20 mol.%, 40 W 60 min, O2 20 mol.%, 40 W

– – 70 55 70 55

– – 50 34 50 33

im55Co/CrAl/s im80Co/CrAl/s

CrAl sheet

Impregnation

Co(NO3)2 in H2O/glycerine

70 80

51 67

*

CrAl gauze

0.1 0.3 0.7

Molar fractions of Co3O4 are estimated in relation to the alumina amount in a 1 mm thick layer [1] of an approximate density of 3.9 g/cm3.

4.2  104 1.2  103 2.9  103

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Fe2p and Co2p. More detailed information on the processing of spectra and the quantitative analysis can be found in [1]. Raman microspectra (RM) were collected in a Jobin-Yvon T 64000 instrument equipped with a CCD camera, by projecting a continuous wave laser (514.5 nm) on the surface of the samples (with a 1 cm1 resolution) under ambient conditions. Prior to each analysis, the sample surface was examined with an optical microscope at several spots to check to which extent the Raman results are congruent. 2.4. Catalytic tests The catalytic properties of the materials were tested in oxidation of n-hexane in two types of reactors: a laboratory tubular microreactors of a plug-flow type denoted further as PF (4.25 mm i.d), and a gradientless jet-stirred reactor of a type of continuous stirred tank reactor denoted as CSTR (Microberty, AutoClave Engineers). A bed filling the PF reactor of around 5 mm in height was composed of several pieces of catalyst leaves with a total geometrical surface area of 1 cm2 (around 2 cm2 of working area). In the CSTR reactor, the catalysts were also cut into pieces but the total surface area was in the range of 5–10 cm2 (10–20 cm2 of working area). Both reactors operated in a continuous flow of reactants at atmospheric pressure. A description of the analytical and gas supplying systems can be found in [1,15]. The n-hexane concentration of 0.75 mol.% was used. The reaction was carried out at a total flow rate of 100 ml/min and in the 100–550 8C temperature range. The reaction rate was calculated from mass balances for a differential PF reactor and ideal CSTR. In both cases, the differential regime was secured by low conversion during catalytic tests (<10%). This in turn allowed using the reaction rate values to derive Arrhenius plots from the obtained results. 3. Results and discussion 3.1. Preparation methods versus catalyst surface The catalysts prepared with NEP and LB methods were characterised by Raman microscopy (RM) and XPS. The results are presented in Fig. 1 and Table 2, respectively. In order to place the results in a broader context of supported catalysts the amounts of the catalyst were standardised in the following way: the amount of cobalt was recalculated to cobalt oxide according to the spinel stoichiometry and related to the amount of alumina formed on the surface of CrAl carriers upon precalcination. Except the bulk composition in Table 1, the surface composition including all metal oxides detected by XPS is presented in Table 2. It is worth pointing out that the series of the samples prepared by means of LB and NEP methods significantly differ in cobalt oxide content (Table 1). That is because the LB method is practically limited to low amounts of the materials due to the long layering procedures. This limitation places the LB technique as a comparative method that allows to introduce controlled but low amounts of the materials for laboratory applications at the stage of the modelling of the catalyst structure and composition. As regards Raman spectra, a well-developed spinel structure can be recognised on the sample deposited with NEP in pure Ar (Fig. 1 A curve a) and on the precalcined Co foil used as a reference sample (Fig. 1 B, curve e) where the bands at around 477, 516, 615 and 685 cm1 correspond to Eg, F2g, F2g and A1g vibrational modes, respectively [28,30,31]. The spinel structure is still visible for the catalysts obtained with the NEP method in more oxidized conditions and less evident for the LB series (compare in Fig. 1 A and B). As evidenced in [28], the structure of the oxide deposited with NEP (Fig. 1 A) depends on the oxygen content in the carrier

Fig. 1. Raman spectra of the catalyst samples deposited on precalcined CrAl steel, A—with NEP technique: (a) Co0/CrAl/s (intensity  2), (b) 55Co/CrAl/s, (c) 70Co/ CrAl/s, (d) Co100/CrAl/s,; B—with LB technique: (a) precalcined CrAl/s reference, (b) 0.1Co/CrAl/s, (c) 0.3Co/CrAl/s, (d) 0.7Co/CrAl/s, (e) precalcined Co/f reference (intensity  7).

gas; it changes from the spinel oxide formed in pure Ar to the amorphous oxide obtained in pure O2; the latter of which is represented by the characteristic vibrational mode at 590 cm1 [30]. However, in any case, the strong A1g mode at around 680 cm1 and overlapping bands in the range from 400 to 600 cm1 are present on all the samples. The broadening of the bands, common for both series of the samples, can be accounted for Table 2 XPS surface composition. Sample name

Co3O4, mol. %*

BE Co

2p3/2,

eV, (FW)**

0.1Co/CrAl/s 0.3Co/CrAl/s 0.7Co/CrAl/s 70Co/CrAl/s 55Co/CrAl/s Co0/CrAl/s Co100/CrAl/s Co/f

0.002 0.005 0.001 32 25 55 5 100

781.7 781.2 782.4 780.2 780.1 780.9 780.5 780.3

(3.7) (3.5) (3.7) (2.9), 781.7 (2.9) (2.9), 781.8 (2.9) (2.9) (2.3), 782.0 (2.4) (3.5)

* The amounts of detected Co obtained from the integrals of Co 2p bands according to the formulae given in [1] were recalculated to the Co3O4 molar fractions taking into account all the metal oxides detected on the surface (Cr2O3, Al2O3, Fe(OH)O, etc). ** The values in brackets show the full width in half height of the bands.

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by the high dispersion of the deposited cobalt oxide [28]. Depending on the chemical environment the frequencies of characteristic bands of metal oxides may vary within a broad range reaching even 50 cm1 as has been evidenced for chromium oxides of various dispersion [32,33]. The bands coming from the Co–O vibrations detected on the LB-deposited samples are weak but still visible at 480, 527, 634 and 785 cm1 (Fig. 1 B, curve a). This time, however, the frequencies of Co–O vibrations are shifted towards higher values in comparison with the cobalt spinel vibrational pattern, which may be due to the interactions of the cobalt oxide with the Al2O3 matrix as discussed above. The strongest interactions of that kind could come from CoAl2O4 spinel but it can be excluded in our samples according to the results presented in [34]. The formation of the mixed spinel oxide, which is claimed to decrease the catalyst activity in combustion, is prevented by the corundum type a-Al2O3 [19]. The qualitative analysis of the XPS spectra shows the trends that evolve upon changing conditions used during catalysts deposition. In principle, in the spectra we should expect two bands from Co 2p3/2 electrons representing two oxidation states of cobalt. This is shown for example in the works by Jacobs et al [35] and Ernst et al. [36] for the precalcined Co/Al2O3 and Co/SiO2 catalysts, respectively, where the 779.5 eV maximum was attributed to Co+3 state in the cobalt spinel and higher energy bands at around 781.5 eV that appeared upon further reduction of cobalt oxides to Co+2. The observation reflects a known reverse tendency of binding energies on oxidation states for cobalt oxides though sometimes quite opposite band correlation can be found in the literature [37,38]. For the sample obtained in Ar plasma (Co0/ CrAl/s) and the reference samples of precalcined cobalt foil (Co/f) where cobalt oxide spinel was detected by RM, the two bands apparently overlap giving in result a broad single-maximum profile (see the values of full width in half height of the bands, FW, printed in brackets in Table 2). The chemical nature of the cobalt oxide formed on the samples deposited in O2 containing plasma (70Co/CrAl/s, 55Co/CrAl/s and Co100/CrAl/s) is different. Two oxidation states of cobalt are represented by the two maxima that shift towards higher values of binding energies with increased oxygen content during NEP deposition. This may indicate that the amorphous cobalt oxide formed in pure O2 more interacts with the Al2O3 support than the crystalline form formed in the intermediated oxidizing mixtures. In general, the observed BE values are higher than the typical binding energies reported in the references cited above and for cobalt oxides in the XPS databases. As shown in [28] with NEP technique used by us various size nanoparticles of cobalt oxide can be generated, which apparently have different properties than well-developed cobalt oxide crystallites. The quantitative results obtained from XPS represent the surface coverage of the oxides detected on the samples and at the same time are a measure the efficiency of the depositing methods. The efficiency of the NEP increases with the oxygen concentration in plasma and with depositing time (Table 2). This trend can be observed for the 55Co/CrAl/s and 70Co/CrAl/s samples and is not followed by the Co0/CrAl/s and Co100/CrAl/s samples, which may suggest that a highly dispersed oxide formed in the pure O2 plasma locates in the pores of the Al2O3 matrix which partly precludes the detection of the photoelectrons. The same effect can explain the lowest surface coverage observed for the sample with the highest cobalt loading in the series of the LB deposited samples.

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geometry, the mass transport properties and the observed overall combustion rate. For the first evaluation, we will talk about activity of the catalysts understood threefold as the standardised reaction rate at a given temperature, the activation energy and finally the temperature of the reaction initiation. For the second type of the correlation, the mass transport phenomena will be observed in two types of boundary laboratory reactors: tubular PF and gradientless CSTR (Fig. 2A–C). Considering the reaction rate for the quick and brief comparison of the prepared and reference catalysts, the cobalt oxide catalysts obtained with both methods have occurred as relatively active in n-hexane combustion in confrontation with the commercial Pt/ Al2O3 or PtRh gauze catalysts (Fig. 2 A and C, respectively). The most active among a great number of the samples prepared in NEP with varying plasma power, oxygen content in the carrier gas and

3.2. Activity and mass transfer cross-linking in reactor Comparing the catalytic performance of the samples two correlations can be regarded among (i) the catalyst composition and the activity in n-hexane oxidation and (ii) the carrier

Fig. 2. Arrhenius plots for the samples tested in n-hexane combustion (r–nmol/g/s) in: A—PF reactor for the LB samples; B—CSTR reactor for the LB samples; C—CSTR reactor for the NEP and impregnated samples (* the results obtained for 70Co/CrAl were repeated to check the reproducibility of the catalyst activity), Rln(r)–natural logarithm of the reaction rate multiplied by the ideal gas constant R.

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the time of deposition, are the samples prepared at 20% O2 in Ar denoted as 55Co/CrAl and 70Co/CrAl. There is no impact of cobalt oxide amount on the activity for these two samples. However, the reduced activity of the LB deposited samples can be attributed to a lower amount of the catalyst and thus less number of active centres available for the reaction than for the NEP samples (Fig. 2 A and Table 1). Low performance of the PtRh wire gauze (Fig. 3C), a commercial catalyst for ammonia oxidation and a reference sample for the gauze catalysts, can be explained by its low surface area in comparison with the other catalysts used in this study. No further correlation between the catalyst amount and activity can be noted for the discussed samples. In fact the most prevailing factor affecting the reaction rate in the discussed reaction systems is the diffusion of the reactants into the catalyst active centres. It is not surprising then that coming from the tubular PF reactor to the jet-stirred CSTR reactor this mass flow resistance vanishes. The diffusion impact can be recognized by the underestimated values of the apparent activation energy observed for the LB samples in the PF reactor in comparison with those in the CSTR reactor (Fig. 2 A and B, Table 3). On average, the difference amounts to around 30 kJ mol. The diffusion exerts its effect also on the temperature of the reaction initiation which is higher for the PF reactor than for the CSTR reactor. The most striking observation is, however, that even in the CSTR reactor where practically no resistance could be expected, the diffusion limitation of the reaction rate is still observed for the samples deposited on the steel sheets rather than on the wire gauzes (Fig. 2 C, compare samples 70Co/CrAl/s and 70Co/CrAl/g). The

Table 3 Activation energies calculated from the results obtained in the PF and CSTR reactors. Sample name

Reactor

T range (8C)

Ea (kJ/mol)

0.1Co/CrAl/s 0.3Co/CrAl/s 0.7Co/CrAl/s Co/f 0.4Pt/Al2O3

PF

440–600 360–500 350–740 350–570 210–260

45 54 57 52 86

0.1Co/CrAl/s 0.3Co/CrAl/s 0.7Co/CrAl/s 70Co/CrAl/g 55Co/CrAl/g 70Co/CrAl/s im55Co/CrAl/s im80Co/CrAl/s Co/f PtRh10/g

CSTR

340–380 320–380 320–360 280–325 280–350 300–350 300–375 300–375 300–350 350–475

63 82 86 75 82 81 71 72 83 86

values of the apparent activation energy found for the series of catalysts tested in the CSTR reactor do not differ significantly from one another and drop into the range from 71 to 86 kJ/mol regardless of the carrier type and the preparation method applied (except the sample 0.1Co/CrAl/s with very little amount of the catalyst). The values of this parameter estimated for the samples deposited on the wire gauzes can be treated more as the actual activation energy. The detailed analysis of the impact of the diffusion on the activation energy and infinite reaction rate constant together with modelling and simulation of the results will be presented in the second part of the paper. In light of the above findings the benefits of the gauze structures in the intensification of mass transport seem clear. To demonstrate them in a direct form, more useful from the point of view of the reactor construction the reactor filled with wire gauze is compared with the classic monolith (200 cpsi) using the specified criteria describing mass transport. Reliable performance criteria of such reactors are not easy to assess as they much depend on the selection of the parameters. For this study the parameters describing the reactor length and pressure drop have been chosen as suitable measures also because they directly influence the investment and exploitation costs. These have been derived according to the plug-flow model developed for the structures studied by us and presented in [3,4]. The final equations for the estimators are as follows: L ¼ w0 ln



DP ¼ 2 f

Fig. 3. Temperature dependence of: A—relative reactor length and B—relative pressure drop for the knitted gauze structures 17.5 mesh/in., d = 0.1 mm referred to monolith 25 cpsi, 651 m1 (see [4] for details).

 1 kr þ kC 1  x kr kC a

rw20 L e2 dh

(1)

(2)

where kr is reaction rate constant, (m/s), kC is mass transport coefficient, (m/s) L is reactor length, (m), w0 is superficial gas velocity, (m/s), x is conversion, kC is mass transfer coefficient, (m/s), a is specific surface, (m1), DP is pressure drop (Pa), f is Fanning friction factor, r is gas density, (kg m3), e is void fraction, dh = 4e/a is hydraulic diameter, (m). For the structures evaluation the arbitrary assumed conversion x = 97% and gas velocity w0 = 5 (m/s) was inserted to Eqs. (1) and (2). The temperature dependences of the relative values of the gauze reactor length (L/Lmon), and pressure drop (DP/DPmon) are presented in Fig. 3 A and B, respectively. The values Lmon and DPmon where calculated also from Eqs. (1) and (2) for the classic monolith assumed. These parameters are compared for the rate constant kr derived for the 70Co/CrAl/g catalyst in the CSTR reactor and for the automotive Pt catalyst on the monolith by Bennett et al. [39].

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A real advantage of the gauze reactor is that the reactor length, related to that of a classic monolith (Fig. 3 A), can be significantly decreased even up to around 50 times at 600 8C. Since the specific surface area of the gauze structure is twice as high as that of the monolith a relative reactor length of 0.5 in Fig. 3 A describes similar performance of both carriers. In this representation, the plasmadeposited cobalt oxide catalyst occurred only slightly less active than the Pt catalyst, however, the kinetic parameters taken to compare them are not fully compatible as they were obtained for different catalysts amounts and test reactions (n-hexane in this work and propane oxidation in [39]). The significant shortening of the reactor length for the gauze structure is accompanied by an undesired but acceptable increase in flow resistance in comparison with the corresponding values found for the monolith. At high temperature, above 600 8C for the automotive catalyst, above 800 8C for the cobalt one, Fig. 3 B, it is less than twice. Here, the temperature, higher than that expected for the combustion process, should be treated more arbitrary for the comparison of the gauze reactor. In this representation the temperature range will depend on the assumed values of the infinite rate constants derived for a given catalyst. 4. Conclusions The catalyst deposited with non-equilibrium plasma (NEP) technique on the wire gauze can be regarded as possible alternative for monolithic reactors for oxidation processes. The non-equilibrium plasma-deposition NEP technique gives excellent results for the deposition of materials on the structural reactors for catalytic applications. It allows to control the dispersion of the deposited material, its structure and its surface coverage not changing the geometry of the carriers. In contrast to the LB method whose depositing efficiency was too low to obtain an active catalyst within reasonable time and to the impregnation method which did not allow to control the thickness of the deposited layers. The cobalt oxide catalyst showing dispersed spinel structure as characterised by XPS and Raman microscopy proved active in nhexane combustion as compared to commercial Pt catalysts. The gauze carrier enhances the mass transport in the reactors preventing the diffusional limitation of the reaction rate. Compared to the standard monolithic converter it allow to reduce reactor length by around 50 times with a 20% increase in pressure drop. Acknowledgements The study was partly supported by the Polish State Committee for Scientific Research which funded the project N209144736 (2009–2012). The financial support is gratefully acknowledged.

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