Catalytic combustion is a highly effective method of removing volatile organic compounds. Noble metals such as Pt and Pd [1-11] are mainly used in this process. Because of their high price it is important that they show high activity and operational stability at their low content in the catalyst. The activity of noble metals in oxidation processes increases in the order: Rh < Pd < Pt . Pt and Pd exhibit different activity in different reactions. Pd is more active than Pt  in the oxidation of CO, CH4 and olefins, whereas Pt is more active  in the oxidation of higher aliphatic hydrocarbons. The two metals show considerable activity in the oxidation of aromatic hydrocarbons . Platinum is usually deposited on the ɣ-Al2O3 support since it more easily undergoes sintering and partial deactivation on this support . The stability of the structure and texture of ɣ -Al2O3 and its activity in hydrocarbon oxidation can be increased by adding rare earths, e.g. CeO2 and La2O3 [4,5], to ɣ -Al2O3 or by adding metallic oxides, e.g. NiO , Fe2O3, Co3O4 , to the Pt catalyst. Palladium catalysts show the highest activity in the oxidation of methane. Their activity depends on the Pd content in the catalyst, the type of precursor used and the methods of preparing and pre-treating the catalyst [8-12]. Pd(NO3)2, PdCl2, H2PdCl4 [8-10], or organic Pd salts dissolved in various solvents, are most often used as Pd precursors [11,12]. The type of precursor used determines Pd dispersion and the kind of palladium compound (Pd, PdO, PdOx/Pd) on the catalyst surface [8-10,12].
In addition, bimetallic catalysts containing Pd and Pt exhibit high activity in the oxidation of organic compounds [1,2,13,14]. The activity of the monolithic bimetallic Pt-Pd catalyst, on a ceramic cordierite support, in the oxidation of aromatic hydrocarbons decreases in the order: styrene>toluene>benzene.The temperatures required for 95% conversion of the above compounds are 210°C, 220°C and 230°C, respectively . As the amount of the noble metals is increased from 0.1% to 0.2%, the temperature of the 99% conversion of toluene decreases by 40°C. A further increase in the Pd and Pt content to 0.4% no longer leads to a significant increase in catalyst activity. Vigneron et al.  found that a Pt/Pd (1/5) catalyst on a corrugated metallic support is more active in toluene oxidation than catalysts containing only Pt or Pd. Furthermore, it is easier to remove silicon compounds - a catalytic poison - by washing this catalyst in NaOH solution. Owing to the synergistic effects of Pd and Pt, the bimetallic Pd-Pt-SAPO-5 catalyst shows better catalytic properties in methane oxidation than the Pd-SAPO-5 catalyst and higher thermal stability than the Pd/Al2O3 catalyst and the Pd/zeolite catalyst . In the initial period of operation of this catalyst, methane conversion amounts to 55% at 400°C and it is higher than the other catalysts. After 20 h of catalyst work, it decreases to 40% and no further deactivation is observed when the operation time is extended. The combustion of methane over Pd-Ru and Pd-Rh showed that at Ru/Pd=1.5/3% wt. the Ru addition increases the catalytic activity of Pd, even after the catalyst is poisoned by H2S. Rh addition does not increase the activity of the Pd catalyst .
At high rates of gas flow through the catalyst, it is advantageous to use catalysts on a ceramic or metallic monolithic support instead of granular catalysts. The use of monolithic supports is advantageous due to the low resistance of the flow of the gas through the catalyst, even at gas flow rates higher than 105 h-1, whereby high loads can be applied without large pressure drops on the catalyst. Other advantages of this type of catalyst are: good mass and heat transport and high catalytic efficiency per unit mass of the catalyst active phase, owing to which smaller reactors can be built than in the case of granular catalysts [15-17].
This paper presents the results of investigations into the effect of the use of different Pt and Pd precursors in the monolithic catalysts: 0.1% Pt/Al2O3, 0.5% Pd/Al2O3 and 0.5% Pd/0.1% Pt/Al2O3, in terms of their activity in the oxidation of hexane. Also, the effect of calcination conditions on the activity of the 0.15% Pt/Al2O3 catalyst, with H2PtCl6 used as the Pt precursor, is presented. Monolithic catalysts made of heat-resistant foil with an Al2O3 washcoat were used to oxidize hexane. Up to now, there have been numerous publications on noble metal catalysts (Pt, Pd, Rh) on granular metal oxide supports. However, there are only a few reports about the methods and conditions of preparation of Pt and Pt-Pd catalysts on monolith metallic supports. These type catalysts are important due to their advantages that allow their universal application in the protection of the environment to reduce emission of pollutants into the atmosphere. The most important of these advantages include the low resistance of gas flow through the catalyst and a large contact surface of reactants with the catalysts, enabling reduction of the reactor size.
2 Materials and methods
2.1 Preparation of catalysts and methodology of their investigation
0.05 mm thick heat resistant foil FeCr20%Al5% was used as the catalyst support. The catalyst supports had the shape of a cylinder, 26 mm in diameter and 70 mm high, and the structure of a honeycomb with triangular channels. The supports were degreased and etched in 10% H2SO4 at 60°C for 2 min. The specific surface area of the monolithic support was coated with an Al2O3 washcoat, using the sol-gel method. The sols for washcoating were obtained by dissolving proper amounts of aluminium hydroxide gel, glycerol and lanthanum nitrite. The sols were gelated by adding ammonium hydroxide, applied to the support and calcined at 400°C for 3 h.
Pt or Pd was applied to the support coated with the Al2O3 washcoat, using the impregnation method. Bimetallic catalysts containing Pt and Pd were made in the same way. H2PtCl6 (catalysts denoted as Pt(Cl)) or platinum nitrate solution (catalysts denoted as Pt(A)) was used as the platinum precursor. Palladium nitrate (catalysts denoted as Pd(A)) or palladium chloride (catalysts denoted as Pd(Cl)) was used as the palladium precursor. All Pd and Pt precursors were from the Mint of Poland. In all the catalysts the noble metals were deposited with an aluminium hydroxide gel addition containing 0.15% wt. Al(OH)3. The catalyst containing 0.15% wt. Pt, in which H2PtCl6 was the platinum precursor, was calcined in the conditions described in Table 1. All the bimetallic Pd/Pt catalysts were calcined at 500°C in static air (in a furnace without airflow) for 3 h. The characteristics and denotations of the tested catalysts are presented in Table 1.
The activity of the catalysts in hexane oxidation was investigated in a laboratory flow reactor placed in a heating furnace with a controlled temperature rise of 3°C/min. 1500 ppm hexane was oxidized over the catalysts under air in a temperature range of 140-500°C at a catalyst load of 11 000 h-1.
2.2 Analytical methods
Hexane and carbon dioxide concentrations in the gas before and after the reaction were measured using a Horiba MEXA-574GE analyser. This analyser worked in the concentration range 0-2000 ppm for hexane and 0.00-20.00% for carbon dioxide.
A FEI Quanta 250 scanning microscope was employed to examine the surface of catalyst samples. A BSE detector was used to assess the variation in the composition of the surface layer of the individual catalysts. The microscope was equipped with an EDS system enabling one to determine the average composition on the catalyst surface or the composition in selected points on the surface.
3 Results and discussion
Since the catalysts could be poisoned with the chlorine left after their calcination, it was necessary to select optimum calcination conditions for the 0.15% Pt(Cl) catalyst. Calcination was conducted at the temperature of 400-600oC in static air (without airflow) or at 500°C in a flow of air in a catalytic reactor. The results obtained show that the catalyst calcined at 500°C is the most active from among the catalysts calcined in static air. Either increasing the temperature to 600°C, or lowering it to 400oC, results in a reduction in the activity of the catalyst in hexane oxidation. As the temperature changes, the activity of the catalysts decreases in the order: 500°C > 400°C > 600°C (Fig. 1). For the catalysts calcined at the above temperatures, the 90% hexane conversion temperatures amount to 360°C, 395°C and 425°C, respectively (Table 2). The differences in the hexane conversion temperature should be linked to the changes taking place on the surface of the Al2O3 washcoat at high temperatures (Fig. 2), such as the formation of platinum crystallites and their agglomeration, and with changes in the catalyst washcoat itself, such as a change in the pore structure of the aluminium oxide layer. The action of high temperature during calcination causes the segregation of some of the platinum on the surface of the Al2O3 layer. This is indicated by the several times higher Pt content in the clusters (visible as bright points near the BSE detector in Fig. 2) in comparison with the average platinum content on the surface (the area marked in Table 3). The changes in the morphology of the Al2O3 layer coated with platinum, visible in Figs 2a, 2b and 2c, associated with the temperature rise from 400°C to 600°C, affect the size of the specific surface area and porous structure of the Al2O3 layer with deposited Pt. In the case of the catalysts calcined in a flow of air at 260°C and then in static air at 500°C, the morphology of the Al2O3 washcoat is similar to the one observed in the catalyst calcined only at 500°C in static air (Fig. 2b). An increase in calcination temperature usually results in a reduction in the specific surface area of the Al2O3 layer . Also Lu et al. found that an increase of temperature of calcination of the 0.1% Pt-Pd/1% Ce0.75Zr0.25/γAl2O3/cordierite catalyst above 500°C causes a decrease of the catalyst activity in oxidation of toluene.
On the other hand, if the calcination temperature is lower than 500°C, γ-Al2O3 is not transformed into α-Al2O3 and noble metal crystallites do not grow .
The EDS microanalysis results presented in Table 3 indicate the presence of elements coming from the support made of FeCr20%Al5% foil (Fe, Cr and Al) and from the Al2O3 washcoat (Al and O) in all of the catalysts, which is due to the considerable depth of penetration of the electron beam into the examined material. Also, chemical composition analyses made in different areas of the catalysts indicate the presence of Pt, and in some of the catalysts, the presence of small amounts of chlorine coming from H2PtCl6, being the platinum precursor. The distribution and size of the platinum crystallites on the Al2O3 washcoat was approximately determined on the basis of these analyses. The higher Pt content in the bright places on the surface of Al2O3 indicates at least partial segregation of Pt on the surface of the catalysts. In the case of the catalyst calcined at 400°C for 4.5 h, the platinum content in the bright places (Fig. 2a) amounts to 1.65% at., while the chlorine content is small (up to 0.09% at.) (Table 3). When the calcination temperature is increased to 600°C, chlorine is practically completely removed from the catalyst – its content falls below 0.02% at. (the limit of detection by the apparatus). At the same time the platinum content amounts to 1.07% at. A vestigial amount of chlorine (0.00-0.05% at.) was also found on the surface of the 0.15% Pt(Cl)500 catalyst. In the bright places observed on its surface the platinum content amounted to 0.51% at., being 3 to 4 times higher than the one measured in catalyst areas marked in Fig. 2b (0.13-0.17% at.).
In order to facilitate the removal of the chlorine produced during the decomposition of the Pt precursor (H2PtCl6), the catalyst was also calcined in a flow of air at 500°C for 3.5 h (Fig. 2d) or in a flow of air at 260°C for 3 h and then in static air at 500°C for 3 h. As Fig. 1 and Table 2 show, only the calcination of the catalyst in an airflow of 80 dm3/h at 500°C for 3.5 h leads to a small decrease (by 2-3°C) in T50 and T90 in comparison with the catalyst calcined at 500°C in static air. For the airflow calcination of the catalyst, the chemical composition analysis results presented in Table 3 indicate similar Pt content values in the bright places (0.22-0.23% at.) and on the rest of the catalyst surface (e.g. area 1 in Fig. 2d) (0.23-0.28%), which is evidence of uniform Pt distribution on the surface of the catalyst.
The highest activity of the 0.15% Pt(Cl) catalyst calcined at 500oC in static air or in a flow of air should be linked to not only the small chlorine residue on the surface of, or within, the Al2O3 layer, but also to the degree of agglomeration of platinum crystallites. The ratio of the Pt content in the agglomerates to the Pt content in the larger surface area of the catalyst, given as Ptpoint/Ptarea in Table 3, can be an indication of the degree of Pt agglomeration. This ratio decreases from 10.7 to 1.0 in the order: 0.15% Pt(Cl)600 > 0.15% Pt(Cl)400 > 0.15% Pt(Cl)500 > 0.15% Pt(Cl)500p, whereas the activity of the catalysts in hexane oxidation increases as the value of Ptpoint/Ptarea decreases. As the calcination temperature increases from 500°C to 600°C, the Pt content in large crystallites significantly increases (Ptpoint/Ptarea increases from about 3 to 10.7), which means that the Pt content in small crystallites (not visible under SEM) decreases. As a result, the activity of the catalyst decreases. The optimum of the 0.15% Pt(Cl) catalyst calcination temperature is 500°C.
From among the catalysts: 0.1% Pt(Cl), 0.5% Pd(Cl) and 0.5% Pd(A) (calcined at 500°C in static air) the 0.5% Pd(A) catalyst, in which palladium nitrate was the Pd precursor, shows the highest activity in hexane oxidation. The oxidation of hexane considerably deteriorates when palladium chloride is used as the Pd precursor. The temperature of 50% hexane oxidation is 53°C higher and the temperature of 90% hexane oxidation is 68°C higher (Table 2) in comparison with the 0.5% Pd(A) catalyst. The 0.1% Pt(Cl) catalyst exhibits very similar activity to that of the 0.5% Pd(Cl) catalyst (Fig. 3). The temperatures of 50% hexane conversion over the above catalysts are similar, amounting to 380°C and 378°C, respectively, while 90% hexane conversion over the catalysts occurs at the temperature of 438°C. The similar activity of the catalysts containing 0.1% Pt and 0.5% Pd shows that the hexane oxidation activity of platinum (considering its lower content in the catalyst) is higher than that of palladium.
The oxidation of hexane was also conducted over bimetallic catalysts containing 0.5% Pd and 0.1% Pt. Precursors with and without chlorine were used to prepare the catalysts. The test results show that in the case of the Pd and Pt precursors without chlorine (Pd and Pt nitrates) the activity of the bimetallic 0.5% Pd(A)/0.1% Pt(A) catalyst is higher than that of the 0.5% Pd(A) catalyst. By about 15°C lower temperature T10 and by 5°C lower temperature T50 were obtained in the case of the bimetallic catalyst. The 90% hexane conversion temperatures for the two catalysts were the same, amounting to 370°C. The hexane oxidation activity order for the Pd and Pt containing catalysts for the different noble metal precursors is as follows: 0.5% Pd(Cl) 0.1% Pt(A) > 0.5% Pd(A) 0.1% Pt(Cl) > 0.5% Pd(Cl) 0.1% Pt(A). The higher hexane oxidation activity of the 0.5% Pd(A) 0.1% Pt(Cl) catalyst than that of the 0.5% Pd(Cl) 0.1% Pt(A) catalyst can be due to the fact that a smaller amount of chlorine was introduced into the catalyst with the Pt (H2PtCl6) precursor than into the catalyst with the Pd (PdCl2) precursor. The catalysts: 0.5% Pd(Cl), 0.5% Pd(Cl) 0.1% Pt(A) and 0.1% Pt(Cl) show the lowest and quite similar activity in hexane oxidation (Fig. 3). In the case of these catalysts, T50 amounts to about 380°C and T90 to 438°C, 445°C and 438°C, respectively (Table 2).
The monolithic palladium and bimetallic Pd-Pt catalysts obtained from the precursors containing chlorine (PdCl2 and H2PtCl6) exhibit lower activity in hexane oxidation than the catalysts obtained from the precursors without chlorine (Pd(NO3)2 and Pt(NO3)4).
The activity of the 0.15% Pt(Cl) catalyst depends on calcination temperature and decreases with a change in calcination temperature in the order: 500°C > 400°C > 600°C. This is due not only to the fact that a small amount of chlorine remains in the catalyst at low calcination temperature, but also to the size of Pt crystallites.
The hexane oxidation activity of the 0.5% Pd(A) 0.1% Pt(A) catalyst is higher than that of the catalysts containing only Pd or Pt, i.e., 0.5% Pd(A) and 0.1% Pt(Cl).
The work was financed by a statutory activity subsidy from the Polish Ministry of Science and Higher Education in 2016/17 for the Faculty of Chemistry of Wroclaw University of Technology (Department of Advanced Material Technologies) No. 0401/0262/16.
Lu J., Na Y., Jiqin Z., Chunyu S., Preparation of monolithic Pt-Pd bimetallic catalyst and its performance in catalytic combustion of benzene series, Catal. Today, 2013, 216, 71-75. Web of ScienceCrossrefGoogle Scholar
Nomura K., Noro K., Nakamura Y., Yazawa Y., Yoshida H., Satsuma A., et al., Pd-Pt bimetallic catalyst supported on SAPO-5 for catalytic combustion of diluted methane in the presence of water vapor, Catal. Lett., 1998, 53, 167-169. CrossrefGoogle Scholar
Galisteo F.C., Mariscal R., Granados M.L., Fierro J.L.G., Daley R.A., Anderson J.A., Reactivation of sintered Pt/Al2O3 oxidation catalysts, Appl. Catal. B-Environ., 2005, 59, 227-233. CrossrefGoogle Scholar
Kucharczyk B., Monolithic catalysts for hydrocarbons removal from combustion gases, Przem. Chem., 2000, 79 (3), 83-86, (in Polish). Google Scholar
Kucharczyk B., Zabrzeski J., Influence of lanthanum oxide (III) on activity of monolithic catalysts in process of hydrocarbons combustion, Przem. Chem., 2001, 80 (3), 109-112, (in Polish). Google Scholar
Kucharczyk B., Tylus W., Okal J., Chęcmanowski J., Szczygieł B., The Pt-NiO catalysts over the metallic monolithic support for oxidation of carbon monoxide and hexane, Chem. Eng. J., 2017, 309, 288-297. Web of ScienceCrossrefGoogle Scholar
Kucharczyk B., Oxidation of carbon oxide over monolithic platinum catalysts doped with metal oxides, Environ. Prot. Eng., 2008, 34 (4), 69-74. Google Scholar
Kinnunen N.M., Suvanto M., Moreno M.A., Savimaki A., Kallinen K., Kinnunen T.J.J, et al., Methane oxidation on alumina supported palladium catalysts: Effect of Pd precursor and solvent, Appl. Catal. A-Gen., 2009, 370, 78-87. Web of ScienceCrossrefGoogle Scholar
Roth D., Gelin P., Kaddouri A., Garbowski E, Primet M., Tena E., Oxidation behaviour and catalytic properties of Pd/Al2O3 catalysts in the total oxidation of methane, Catal. Today, 2006, 112, 134-138. CrossrefGoogle Scholar
Vigneron S., Deprelle P., Hermia J., Comparison of precious metals and base metal oxides for catalytic deep oxidation of volatile organic compounds from coating plants: test results on an industrial pilot scale incinerator, Catal. Today, 1996, 27, 229-236. CrossrefGoogle Scholar
Konieczyński J., Ochrona powietrza przed szkodliwymi gazami, Wydawnictwo Politechniki Śląskiej, Gliwice, 2004, (in Polish). Google Scholar
Sorbak Z., Kataliza w ochronie środowiska, Wydawnictwo Naukowe Uniwersytetu im Adama Mickiewicza w Poznaniu, Poznań, 2004, (in Polish).Google Scholar
Brinker C.J., Scherer G.W., Sol-gel science, Academic Press, San Diego, 1990. Google Scholar
About the article
Published Online: 2017-07-26
Citation Information: Open Chemistry, Volume 15, Issue 1, Pages 182–188, ISSN (Online) 2391-5420, DOI: https://doi.org/10.1515/chem-2017-0020.
© 2017 Barbara Kucharczyk et al.. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License. BY-NC-ND 3.0