Accepted Manuscript Title: Synthesis and characterization of Nanocrystalline Copper-chromium catalyst and its application in the oxidation of carbon monoxide Authors: Sajad Mobini, Fereshteh Meshkani, Mehran Rezaei PII: DOI: Reference:
S0957-5820(17)30051-4 http://dx.doi.org/doi:10.1016/j.psep.2017.02.009 PSEP 976
To appear in:
Process Safety and Environment Protection
Received date: Revised date: Accepted date:
30-12-2016 7-2-2017 10-2-2017
Please cite this article as: Mobini, Sajad, Meshkani, Fereshteh, Rezaei, Mehran, Synthesis and characterization of Nanocrystalline Copper-chromium catalyst and its application in the oxidation of carbon monoxide.Process Safety and Environment Protection http://dx.doi.org/10.1016/j.psep.2017.02.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and characterization of Nanocrystalline Copper-chromium catalyst and its application in the oxidation of carbon monoxide Sajad Mobinia, Fereshteh Meshkania, Mehran Rezaeia,b,*
a
Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty
of Engineering, University of Kashan, Kashan, Iran b
Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, Iran
*
Corresponding author: University of Kashan, Chemical Engineering Department, Catalyst and Advanced Materials Research Laboratory, Km 6 Ravand Road, Kashan, Iran. Tel.: +98 31 55912469, fax: +98 31 55559930. E-mail address:
[email protected] (M. Rezaei).
1
Graphical abstract
Research highlights The catalysts were prepared using modify surfactant-assisted hydrothermal method The effect of different molar ratios of Cu/Cr on catalyst performance was studied The catalyst with the spinel structure have the best performance for CO oxidation The spinel CuCr2O4 have a good stability under difficult conditions
Abstract Nanocrystalline Cu-Cr mixed oxides catalysts with different molar ratios of Cu/Cr were synthesized by the hydrothermal method and employed in the catalytic CO oxidation. The physicochemical properties of these catalysts were characterized by powder X-ray diffraction (XRD), N2 adsorption (BET), temperature programmed reduction (TPR), scanning and transmission electron microscopy (SEM and TEM) methods. Also, for a better comparison of catalysts the reaction rate and TOF values were calculated at two different temperatures. The results represented that the powders were mainly consisted of macropores with wide pore-size distribution. The catalytic results
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showed that the activity of the sample with a Cu/Cr molar ratio of 2 was significantly higher than the other samples due to the formation of CuCr2O4 spinel structure, which was more active than the CuO and Cr2O3 phases in CO oxidation reaction.
Keywords: CO oxidation; spinel structure; copper chromite; hydrothermal synthesis
1. Introduction The oxidation of CO is one of the main ways to control air pollution, especially in terms of the automobile and industrial emission control. Many researchers have investigated the oxidation of carbon monoxide over different types of catalysts (Biabani-Ravandi et al., 2013a; Einaga et al., 2016; Faure and Alphonse, 2015; Reina et al., 2016; Zhang et al., 2015). Because of the high catalytic activity and stability of noble metal catalysts in CO oxidation at low-temperature, these catalysts are widely studied in this reaction. However, wide applications of noble metals are limited due to the high cost and inaccessibility of them. Therefore, today using the base metal oxides and their mixed oxides as catalyst are mostly expanded (Biabani-Ravandi et al., 2013b). Among the different mixed-oxides catalysts, the copper-based ones are the most important catalysts for different reactions, such as CO oxidation. Amini et al. (Amini and Rezaei, 2015) showed that with the addition of CuO to iron oxide, the textural properties and performance of the iron oxide catalyst were changed. The catalyst with 15 mol % CuO exhibited the highest specific surface area and CO conversion and showed a high catalytic stability for CO oxidation at low temperature. Zhou et al. (Zhou et al., 2015) improved the catalytic activity of Co3O4 by replacing the Co2+ ions in the lattice with Cu2+. The presence of Cu slightly changes the CO adsorption on the Co3+ surface sites, but the oxygen vacancy is more favorably formed in the bonding of Co3+−O−Cu2+ than in Co3+−O−Co2+. 3
In Cu-based catalysts, at high enough temperatures, the reaction of adsorbed CO molecules with a lattice oxygen atom leads to the occurrence of CO oxidation. During this interaction, firstly the oxide surface is reduced and then oxidized by ambient oxygen. The study of this interaction on CuO revealed that with the contribution of surface oxygen obtained from the surface oxide, the intermediate compounds such as carbonates are formed during the CO oxidation. At room temperature (273±293 K), CO as a carbonyl complex is absorbed on the surface of copper oxide, which is rapidly oxidized to the carbonate complex by reaction with surface and atmospheric oxygen. This interaction between CuO and CO causes the reduction of CuO to Cu2O. However, simultaneously with this reaction, Cu2O is oxidized back to the CuO by oxygen atoms in the framework (Xanthopoulou and Vekinis, 1998). It is known that the existence of Cr ions on the surface can improve the CuO reduction (Laine et al., 1990; Stegenga et al., 1990). One of the most attractive features in catalysis research is synergistic or cooperative effects of two-metal based catalysts (Laine et al., 1990). A mixture of copper and chromium has been known as one of the most effective and inexpensive catalysts for CO oxidation. The copper chromite catalysts have been investigated since the 1930s for CO oxidation (Prasad and Singh, 2012). In Cu-Cr catalysts, it is proposed that the copper is considered as the main active species for CO oxidation, and chromium is considered as the regulator of the reduction through the formation of the CuCr2O4 phase (Laine et al., 1990; Severino et al., 1986; Severlno and Lalne, 1983). The high chemical and thermal stability of the CuCr2O4 spinel cause that this spinel has been considered as an appropriate catalyst for CO oxidation (Zhang
et al., 2013). For preparation of CuCr catalyst there are various synthesis methods, including coprecipitation (Edrissi et al., 2011), thermal decomposition (Rajeev et al., 1995; Sanoop et al., 2015), sol-gel (Habibi and Fakhri, 2014; Hosseini et al., 2014a), ceramic method (Beshkar et 4
al., 2015), hydrolysis (Patron et al., 2001), template preparation (Liang et al., 2009; Zhang et al., 2013), etc. However, few studies by Acharyya et al. (Acharyya et al., 2015, 2014, 2014; Shankha Shubhra Acharyya et al., 2014) have been conducted on the synthesis of copper chromite using the hydrothermal method. The hydrothermal method has been known as one of the most efficient routes to achieve nanoparticles with well-defined and controlled morphologies. In this work, the catalytic performance of Cu-Cr mixed oxides powders with different Cu/Cr molar ratios prepared by hydrothermal method was investigated in CO oxidation reaction. 2. Experimental section 2.1.
Catalyst preparation
Cu-Cr mixed oxides catalysts with various Cu-Cr molar ratios were prepared using the hydrothermal method. According to the composition of the catalyst, the certain amounts of copper (II) nitrate trihydrate (DAEJUNG, 99%) and chromium (III) nitrate nonahydrate (SAMCHUN, 98%) were dissolved in distilled water containing cetyltrimethylammonium bromide (CTAB, Merck). The molar ratio of CTAB/Metal cations was chosen 0.27. After that, the pH of the solution was adjusted to 12 by a drop wise addition of 1 M sodium hydroxide aqueous solution. After precipitation, the suspension was aged at 80°C for 4h under stirring and then transferred to a stainless steel autoclave vessel with an internal lining of Teflon and maintained at 180°C for 11 h. After aging, the dark brown product was filtered and washed with distilled water and then dried at 80°C and calcined at 500°C for 4 h. 2.2.
Catalyst characterization
The nitrogen adsorption/desorption analysis was performed at -196°C to determine the BET surface area by an automated gas adsorption analyzer (Belsorp mini II). For evaluation of the reducibility of the catalysts, the temperature-programmed reduction (TPR) analysis was done 5
using an automatic apparatus (Chemisorb 2750, Micromeritics). For the TPR analysis, 50 mg of the fresh sample was heat treated under an inert gas stream at 250°C for 3h, and then exposed to a reducing gas stream (a mixture of 90 % Ar and 10 % H2, 30 ml/min) and the temperature raised with a ramp rate of 10°C/min. The X-ray diffraction (XRD) technique was used for evaluation of the crystalline structure of the catalysts using a PANalytical X’Pert-Pro diffractometer. The Scherrer’s equation (Eq. 1) was used to measure the crystallite sizes of the catalysts:
)1( Where D is the crystalline size, λ is the wavelength of incident X-rays (0.15405 nm), β is the peak width at half height and θ relates to the peak position. Scanning and transmission electron microscopes (SEM, Vega@Tescan and TEM, JEOL TEM-1400) were used to observe the surface morphology of the samples and the changes in the morphology of the fresh and spent catalysts. 2.3.
Catalytic evaluation
The catalytic reaction tests were conducted in a tubular fixed bed flow reactor made of quartz (ID 7mm x 500mm long) under atmospheric pressure. The reactor was placed vertically inside a programmable tubular furnace, which was heated electrically. The total catalyst charged for each reaction was held constant (100 mg and with particle size of 0.25-0.5 mm) without any solid diluent. The temperature of the catalyst bed was measured and controlled with a flexible K-type thermocouple and a programmable thermometer (Hanyoung PX9). The feed stream containing 20% O2 and 10% CO balanced with argon at desired GHSV was passed over the catalyst bed. The flow rates of the gases were controlled by the Brooks 5850S mass flow controllers. Prior to the reaction tests, the catalysts were pretreated in 20%O2
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balanced with Ar at 300°C for 1h. The activity of the catalysts was measured in the temperature range of 50-400°C. The outlet gas composition was determined by a gas chromatograph (Varian 3400) equipped with a thermal conductivity detector (TCD) and a Carboxen 1000 column. 3. Result and discussion 3.1.
Crystal structure and morphology
The X-ray diffraction patterns of Cu-Cr mixed oxides catalysts with various Cu-Cr molar ratios are shown in Fig 1. As can be seen, the monoclinic CuO is the main phase formed in the sample with CuCr=1:0 (ICDD# 00-001-1117, 01-074-1021). In this sample, the diffraction peaks at the 2θ values of 36.6, 42.5, 61.7 and 73.69° are probably due to the cubic crystal lattice of CuO or Cu2O (ICDD# 01-078-0428, 00-001-1142). With the addition of chromium to copper oxide (CuCr=1:0.5), the major phase was still CuO, and a small amount of CuCr2O4 with tetragonal crystal lattice was formed (ICDD# 01-088-0110). Also due to the presence of two diffraction peaks at 32.7° and 41.1°, the Cr2O3 was probably formed in this sample. In CuCr=1:1 sample, Cu and Cr atoms have been placed on the rhombohedral structure of delafossite-type CuCrO2, which indicates the presence of Cu1+ and Cr3+ in its structure (ICDD# 00-039-0247). Although, the reaction between CuO and Cr2O3 in the air is proceeded to form CuCr2O4 spinel type crystal at low temperature (Chiu et al., 2011; Jacob et al., 1986), however, the hydrothermal synthesis under high pressure led to the formation of delafossite-type CuCrO2 (Sheets et al., 2006). A low amount of monoclinic CuO was also observed in this sample. Two peaks at position of 35.4° and 38.9° are corresponded to this phase (ICDD# 00-001-1117). The presence of the diffraction peak at 2θ=35.5° confirmed the formation of spinel type CuCr2O4 (ICDD# 01-085-2313) structure in CuCr=1:2 sample and showed the position of Cu
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and Cr on the tetrahedral and octahedral spaces, respectively (Hosseini et al., 2014b). In this sample, all the diffraction peaks were shifted to the higher angles compared to reference peaks, which could be due to the presence of the phases in the sample and representing that when the crystal and particle sizes decrease, the lattice parameter contracts (Meshkani and Rezaei, 2015). The presence of an additional peak at 2θ=42.7° is indicative of Cu2O cubic phase in this sample. In CuCr:1:3 sample, Cr2O3 with rhombohedral crystal structure was formed (ICDD# 01-084-0313), but monoclinic CuO and tetragonal CuCr2O4 phases can be identified in the structure. In the last sample with CuCr=0:1 molar ratio, pure Cr2O3 phase with rhombohedral crystal structure was obtained, which indicates the presence of Cr3+ in the structure.
The average crystal sizes determined by the Scherrer formula are reported in Table 1. The pure copper oxide catalyst exhibited the biggest crystallite size due to the low thermal stability of the copper oxide at high temperature. The addition of chromium up to CuCr=1:2, decreased the crystallite size and the further increase in chromium content had a negative effect and increased the crystallite size.
3.2.
Pore structure and surface area
The distribution of pores and the nitrogen adsorption/desorption isotherms of the synthesized catalysts are illustrated in Fig. 2a and 2b, respectively. The isotherms can be categorized as type II with a H3 type hysteresis loop. The hysteresis loop was seen at a high relative pressure in the range of 0.8-1, representing the presence of large pores on the prepared samples. Macropores filling occurs only at very high relative pressure. The type II isotherms are related to macroporous solids. In this case, the formation of a monolayer of the nitrogen molecules adsorbed on the surface of the solid is the prevailing process, while at high P/P0, a 8
multilayer adsorption takes place specially in macropores. The H3 type hysteresis loop is usually observed on powders containing aggregated or agglomerated particles with slitshaped pores with nonuniform size (Arandiyan et al., 2013). The occurrence of this hysteresis confirmed the presence of some mesopores in the prepared powders. In addition, for the pure copper oxide catalysts and also the catalyst with a Cu-Cr ratio of 1:0.5 small hysteresis loops were observed at very high relative pressure compared to other samples, indicating the macroporous structure of these catalysts. The pore size distributions are also showed a wide distribution of the pores in meso and macro-regions. As can be seen in Fig. 2b, the catalysts with Cu/Cr ratios of 1:0, 1:0.5 and 1:3 exhibited no distribution of pores in meso region, which is in agreement with the type of the isotherms. The catalysts with Cu/Cr ratios of 1:1 and 1:2 showed narrower pore size distributions in meso and macro-regions, Fig. 2b.
The textural properties of the prepared catalysts are also reported in Table 1. The catalysts containing macropores (with Cu/Cr ratios of 1:0, 1:0.5 and 1:3) possessed low BET area and pore volume. The lowest BET area was observed for the pure copper oxide catalyst. There is an optimum Cu/Cr molar ratio, in which the sample displayed the highest surface area. The theoretical particle size with an assumption of spherical particles can be determined from the following equation: DBET =
(2)
Where DBET is the equivalent particle diameter (nm),
is the density of the material (g/cm3)
and S is the specific surface area (m2/g). The results are reported in Table 1. As can be seen, the copper oxide catalyst showed the biggest particle size due to severe particle sintering at high temperature. The smallest particle size was observed for the catalyst with the highest surface area (Cu/Cr=1:2). 9
Fig. 3 presents the SEM images of the prepared Cu-Cr catalysts. As can be seen, the catalyst with CuCr=1:0 has large and agglomerated particles. With the addition of chromium to the catalyst, the particle sizes decreased. The samples with CuCr=1:1 and CuCr=1:2 display the formation of more distributed uniform particles by nearly spherical shape with a size less than 50 nm. In the CuCr=1:3 sample the particles are similar to 2D thin nanosheets with an average thickness of 20nm. The CuCr=0:1 sample has relatively small particles with uniform size distribution. In general, these figures show that the addition of chromium to copper improves the distribution of particles and makes their size smaller.
3.3.
TPR analysis
The reducibility of the catalysts has a large impact on the catalytic performance. Fig. 4 illustrates the H2-TPR analysis of the catalysts with different Cu/Cr molar ratios. In the catalyst with CuCr=1:0, a wide reduction peak with a Tmax located at 340
was detected,
which is related to the reduction of Cu2+ to Cu0 (Liang et al., 2009). The temperature related to the maximum of reduction peak in pure copper oxide catalyst was observed at a higher temperature compared to those reported in other published papers (Tanaka et al., 2003; Xiao et al., 2013a), suggesting that this sample is bulk and nonporous as indicated in Table 1. The reducibility of Cr2O3 due to the extremely negative energy is difficult (Xiao et al., 2013b), which is in agreement with the observed TPR profile for CuCr=0:1. Two reduction peaks with Tmax=204 and Tmax=252°C are related to the reduction of Cr2O3 to Cr3O4, which have a low area due to the minor reduction of the sample. The existence of these two peaks indicates the existence of both amorphous and crystalline phases in the sample (Ilieva, 1995). The first peak is attributed to the reduction of an amorphous phase and the second peak is assigned to the reduction of crystalline phase (Ilieva, 1995). Amrute et al. (Amrute et al., 2012) observed two reduction peaks at 277 and 497°C, corresponding to the reduction of Cr6+ or Cr5+ to Cr3+, 10
which are present in the near-surface level. But this is not acceptable in this work because in XRD analysis the Cr6+ or Cr5+ species were not observed. The hydrogen consumption profile related to the sample with a CuCr=1:2 shows two main reduction peaks. The first peak at Tmax=181°C is related to the reduction of Cu2+ with small particle size located on the surface of CuCr2O4, which is reduced at a lower temperature compared to pure CuO. A shoulder observed at about 210°C could be also related to the reduction of pure CuO to Cu0, which its existence was confirmed by the XRD results. The weak peak observed at Tmax=390°C refers to the reduction of CuCr2O4 with large particles size. Due to the penetration limitations in these particles rather than the surface state of the particles, the reduction is difficult (Xiao et al., 2013a). For CuCr=1:0.5, an apparent peak observed in the temperature range of 220 to 390°C was related to the reduction of CuO and Cu2+ on the surface of CuCr2O4 (Xiao et al., 2013b). The results indicate that the reduction temperature of this sample is higher than CuCr=1:2. Accordingly, there is a significant interaction between the CuO and CuCr2O4 phases in this sample. In CuCr=1:1 sample four peaks were observed at Tmax=273, Tmax=297, Tmax=327 and Tmax=628°C. According to the XRD results, almost a pure phase of CuCrO2 with a minor content of CuO was obtained in this sample. It seems that the first three reduction peaks were related to the reduction of surface Cu2+ to Cu0, Cu+ existing on the surface of CuCrO2 to Cu0 and pure CuO to metallic Cu, respectively (Liang et al., 2009; Ma et al., 2010; Xiao et al., 2013a). The reduction peak observed at Tmax=628°C was related to the gradual reduction of CuCrO2. For CuCr=1:3 sample, the peak at Tmax=311°C with a shoulder at 410°C was assigned to the reduction of Cu2+ to Cu0, which is placed on the surface of CuCr2O4 and CuO, respectively (Xiao et al., 2013a). In comparison with CuCr=1:2 sample, the reduction temperature was shifted to higher temperatures, which reflects the strong interaction between Cr2O3 and CuCr2O4 in this sample.
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In general, the results indicate that with increasing the amount of chromium in the samples the reduction temperature of CuO decreased, which is due to the formation of CuCr2O4 and distribution of CuO on the surface of this phase. By increasing the chromium content from CuCr=1:2 molar ratio, the reduction temperature of the surface Cu2+ species increased because of the increase in the particle size of the CuCr2O4. 3.4.
Catalytic performance The effect of reaction temperature on the CO conversion of different samples with
various Cu:Cr compositions is shown in Fig. 5a. In addition, the CO conversion as a function of catalyst composition at different temperatures is illustrated in Fig. 5b. It is clear that the activity of the catalysts with CuCr=1:2 is much higher than the other catalysts. Unlike many reports (Laine et al., 1990; Severino et al., 1998; Stegenga et al., 1993; Worn Park and S. Ledford, 1998), pure catalysts (CuCr=1:0 and CuCr=0:1) exhibited higher activity compared to the other mixed oxide catalysts. According to the TPR results, a direct relation between the reducibility and activity of the catalysts can be achieved. Increasing in the reducibility improved the activity of the catalysts. According to the XRD results, in CuCr=1:0 catalyst, copper molecules are in Cu2+ oxidation state and in a cubic structure. By addition of Cr to Cu-based catalyst, the activity decreased in CuCr=1:0, CuCr=1:0.5 and CuCr=1:3 series. This indicates that the copper species in low oxidation states (Cu+ and/or Cu0), are more active for carbon monoxide oxidation (Severino et al., 1998). Excess chromium can cover the active sites and reduce the activity of Cr-rich catalysts (Ma et al., 2010). In CuCr=1:1 and CuCr=1:2 samples the catalytic activity increased due to the formation of delafossite and spinel structure, respectively. It is confirmed that CuCr2O4 spinel phase has higher activity than CuO phase for CO oxidation. It is known that the redox characteristic is an important factor to carry out the CO oxidation reaction on the surface of oxides. Therefore, due to this fact that the CuCr2O4 spinel phase can generate surface oxygen easier than CuO or copper on the 12
surface, the reaction is more progressed over the CuCr2O4 phase (Ma et al., 2010). Thus, according to the XRD, BET and TPR analyses and the above mentioned explanations, the formation of relatively pure spinel CuCr2O4 along with the presence of CuO surface species, high surface area and high reducibility, are the main reasons for the higher activity of CuCr=1:2 sample compared to other samples. The higher activity of CuCr=0:1 than the other catalysts is a little strange, which could be due to the high surface area of this sample compared to other samples.
For comparison of the catalytic performance of the samples, T50% and T90% were calculated and the results are summarized in Table 2. The comparison of these data indicates that the catalytic activity of the samples decreased in the sequence of CuCr=1:2 > CuCr=0:1 > CuCr=1:0 > CuCr=1:1 > CuCr=1:0.5 > CuCr=1:3. Obviously, the CuCr=1:2 and CuCr=0:1 catalysts have the highest catalytic performance compared to other samples. Compared with the other samples, these catalysts possessed the highest specific surface area. However, the CuCr=1:0 sample with the lowest specific surface area (0.99 m2/g) exhibited relatively high activity due to the high activity of copper for this reaction. The turnover frequencies (TOFCuO and TOFCr2O3) and the reaction rates (μmol/g.s) were calculated based on the activity results and the moles of CuO and Cr2O3 in the different CuCr synthesized catalysts according to the reported procedure (Arandiyan et al., 2014; Liu et al., 2013). The TOF (mol/molAg.s) was calculated according to the number of CO molecules converted by CuO or Cr2O3 sites per second. In fact, each mole of the CuO or Cr2O3 present in the catalyst was considered as an active site. The TOFCuO and TOFCr2O3 of the samples were calculated under the conditions of CO/O2=2/20 molar ratio, GHSV = 60000 mL/g.h, and reaction temperature of 150 and 250 °C, as summarized in Table 2. As can be seen, in the carbon monoxide oxidation at 150 and 250°C the TOF of CuO and Cr2O3 reached the highest
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value over CuCr=1:2 catalyst. For CO oxidation at 250°C the Cu/Cr=1:2 and CuCr=0:1 samples showed the highest CO reaction rate.
The effects of pretreatment process on the CO conversion of the catalyst with CuCr=1:2 are illustrated in Fig 6. The catalyst with CuCr=1:2 was pretreated in oxidative, inert and reductive atmospheres. For example, to investigate the effect of pretreatment with the oxidative atmosphere (O2-pretreatment), prior to the reaction a gas stream (a mixture of 20% O2 in Ar) was passed through the catalyst bed at 300°C for 1 h. In addition, the pure N2 and 20% H2 in Ar were also used for inert (N2-pretreatment) and reductive (H2-pretreatment) atmospheres, respectively. As can be seen, the pretreatment conditions dramatically affect the catalytic performance in terms of CO conversion, Fig 6. The results showed that the highest activity was obtained for the sample treated under an oxidative atmosphere. The O2pretreatment condition can modify the oxygen atoms on the surface and leads to the adsorption of active superficial oxygen species, which consequently leads to higher catalytic performance. The CO conversion of the catalyst pretreated in H2 atmosphere was low due to partial reduction of Cu–Cr mixed oxides and formation of species with lower oxygen capacity (Biabani-ravandi et al., 2013).
The effect of GHSV on the CO conversion over the catalyst with a CuCr=1:2 was investigated and the results are presented in Fig 7a. It is seen that the CO conversion decreased with the increase of GHSV at a constant reaction temperature. Increasing in GHSV decreased the contact time of the reactants with the sample surface, which caused a decrease in activity of the catalyst for CO oxidation reaction. The influence of the feed ratio (O2/CO molar ratio) on the CO conversion of the catalyst with a Cu/Cr=1:2 is shown in Fig. 7b. As
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can be seen, the CO conversion was not significantly affected by the feed ratio. However, the lowest amount of CO conversion was observed under the lowest O2/CO molar ratio. According to “asymmetric inhibition” of CO oxidation, expressed by Ertl et al. (Royer and Duprez, 2011), the presence of additional oxygen had no inhabitation effect on CO adsorption and CO molecules can always find adsorption sites. In the stoichiometric feed composition, because of the limited access to oxygen, the CO conversion was slightly decreased. In previous studies, it was observed that increasing in O2/CO ratio improved the CO conversion (Singh and Madras, 2015).
For investigation of the catalytic stability, different reactant feeds with various compositions were passed over the catalyst (CuCr=1:2) and the stability of CO conversion was monitored during 36 h time on stream. The obtained results are presented in Fig 8. It is seen that the catalyst exhibited high stability without any decline in CO conversion in dry feed condition (20% O2 and 10% CO). The addition of water vapor to the reactant feed slightly decreased the catalytic stability. However, simultaneous addition of water vapor and CO2 to the reactant feed has a dramatic effect on the stability of CO conversion. Fig. 9a and 9b illustrate the SEM images of the calcined and spent catalysts with a CuCr=1:2, respectively. The comparison of the morphology of the catalyst before and after dry stability test shows no significant changes in the structure of nanoparticles, which represents the high stability of the catalyst in this reaction. These results are also confirmed by TEM analysis (Fig. 10a and 10b). In Fig 10a, the nanorods, nano-particles and nano plates were observed and the particle size was less than 50nm. After dry stability test (Fig 10b), no significant changes have been created in morphology and size of particles. The addition of trace amounts of moisture in reactant feed, not only reduce the initial conversion but also reduce the CO conversion by 10% during 36 h. In comparison with previous works (Biabani-ravandi et al., 2013; Royer
15
and Duprez, 2011) a very low decline observed in CO conversion indicates the high stability of the catalyst in the existence of water vapor. In the simultaneous presence of CO2 and water, more serious deactivation was observed. The presence of water and CO2 in the feed simultaneously occupies the surface-active sites, which hinders the adsorption of CO and O2 on the surface active sites (Biabani-ravandi et al., 2013).
4. Conclusion To determine the optimum ratio of Cu/Cr on the catalytic performance of the mixed Cu-Cr oxides, various Cu-Cr catalysts with different Cu/Cr ratios were prepared by hydrothermal method. The sample with a CuCr=1:0 showed bigger particle size with a low specific surface area compared to other prepared samples. According to the SEM, TPR and BET analysis, by the addition of chromium to catalyst, the particles size, and reduction temperature decreased and the surface area increased. The sample with a Cu-Cr molar ratio of 1:2 possessed almost pure spinel structure and exhibited the highest specific surface area among the other samples and showed the formation of more distributed uniform particles by nearly spherical shape. The catalytic results presented that the CO conversion strongly depends on the composition of the catalyst and the catalyst with CuCr=1:2 showed the highest activity. The high catalytic activity of this sample was due to the formation of spinel phase and also the presence of CuO active sites on the surface of the catalyst. Also, the TOF values displayed that the copper species are more active than the chromium species. The results also showed that working under the oxidative atmosphere improved the catalytic activity of CuCr2O4 catalysts. The results of the long-term stability test showed the high stability of the catalyst in the presence of water vapor. The simultaneous presence of CO2 and water vapor in the reactant feed has a negative effect on the catalytic stability due to coverage of active sites by CO2 and H2O. 16
Acknowledgments The authors gratefully acknowledge the support from University of Kashan by Grant No. 158426/149.
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Fig. 1: XRD patterns of Cu-Cr catalysts with various Cu-Cr molar ratios
23
Fig. 2: (a) N2 adsorption/desorption isotherms and (b) pore-size distributions of the Cu-Cr based catalysts with various Cu/Cr molar ratios.
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(a)
(b)
(c)
(d)
(e)
(f)
Fig. 3: SEM analysis of catalysts with different Cu/Cr molar ratios, a) CuCr=1:0, b) CuCr=1:0.5, c) CuCr=1:1, d) CuCr=1:2, e) CuCr=1:3 and CuCr=0:1.
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Fig. 4: H2-TPR profiles of Cu-Cr based catalysts with different Cu/Cr molar ratios.
Fig. 5: a: CO conversion as a function of reaction temperature and b: CO conversion as a function of catalyst composition at different temperatures over the prepared catalysts. Reaction conditions: 10% CO and 20% O2 balanced with Ar, GHSV=60,000ml/g.h, pretreated in oxidative atmosphere.
26
Fig. 6: Effect of different pretreatment conditions on the activity of CuCr=1:2 sample, reaction conditions: 10% CO and 20% O2 balanced with Ar, GHSV=60,000ml/g.h.
27
Fig. 7: a) Effect of GHSV on CO conversion (at CO/O2=2/20 molar ratio) and b) Effect of feed composition on CO conversion (GHSV=60000 ml/h.gcat), Cu/Cr=1:2 catalyst, reaction temperature=110°C
Fig. 8: Long-term stability test of CO oxidation reaction over Cu/Cr=1:2 catalyst under dry feed (10%CO and 20%O2), wet feed (4%H2O, 10%CO and 20%O2) and presence of 10%CO2 and 4%H2O
28
in the dry feed
Fig. 9: SEM images of the catalyst (Cu/Cr=1:2): a) before and b)after dry stability test
Fig. 10: TEM images of the catalyst (Cu/Cr=1:2): a) before and b)after dry stability test
29
Table 1: The textural properties of the catalysts with various Cu/Cr molar ratios.
Molar ratio
Surface area (m2/g) CuCr=1:0 0.9 CuCr=1:0.5 4.5 CuCr=1:1 21.6 CuCr=1:2 46.9 CuCr=1:3 9.7 CuCr=0:1 34.4
Mesopore volume (cm3g-1) 0.01 0.05 0.38 0.49 0.09 0.42
Mesopore Dparticle size (nm) (from BET data) (nm) 25.8 960.4 41.6 215.9 69.5 46.5 41.7 22.1 38.1 108.5 49.7 33.4
Crystal size (from scherrer equation) (nm) 24.8 23.0 18.8 15.4 23.2 21.2
Table 2: Catalytic activity, reaction rate and TOFCuO and TOFCr2O3 values at 150 and 250°C for CO oxidation reaction
sample
CuCr=1:0 CuCr=1:0.5 CuCr=1:1 CuCr=1:2 CuCr=1:3 CuCr=0:1
CO oxidation at 150°C
CO oxidation at 250°C
CO oxidation activity
Reaction rate ( 10-6 mol/g.s)
T50%(°C)
T90%(°C)
( 10 s )
(
Reaction rate ( 10-6 mol/g.s)
( 10-5 s-1)
( 10-5 s-1)
7.0 7.2 8.4 70.6 7.3 8.9
0.70 1.06 1.65 20.76 2.79 -
2.25 1.71 10.70 0.98 0.89
65.3 34.1 47.8 72.6 17.0 73.8
6.53 5.02 9.37 21.38 6.54 -
10.67 9.76 11.01 2.30 7.38
215 257 242 110 272 172
260 350 304 141 344 195
-5
-1
10-5 s-1)
30
