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Hydrotalcite-Derived catalysts for removal of nitrogen-containing volatile organic compounds
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
599
Hydrotaicite-Derived Catalysts for Removal of Nitrogen-Containing Volatile Organic Compounds J. Habera , K. Bahranowskib, J. Janas", R. Janika, T. Machej", L. Matachowski", A. Michalik", H. Sadowskaa, E.M. Serwiekaa
"Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, 30-239 Krak6w, ul. Niezapominajek 1, Poland, bFaculty of Geology, Geophysics and Environmental Protection, Academy of Mining and Metallurgy, 30-059 Krak6w, al. Mickiewicza 30, Poland, The catalytic properties of mixed oxides derived from the multi-metallic hydrotalcites (HTs) containing Cu, Cr, V, A1 and Zn were tested in the total oxidation of dimethylformamide with particular stress on to the role of the precursor composition and the temperature of thermal decomposition (400~ and 550~ Optimum performance for the 400 series was given by the material obtained from the Cu,Zn, Cr,A1,V-HT, while in the 550 series the catalyst derived from the Cu, Cr,V-HT precursor gave best results. The observed activity and selectivity patterns are discussed in terms of the catalysts physico-chemical properties. 1. INTRODUCTION Recent studies show that exposure of human beings to air contaminated with N-containing organic vapours induces genotoxic consequences and may increase the cancer risk in the affected population [1,2]. While abatement of hydrocarbons by total oxidation has been widely studied [3,4], the catalytic conversion of nitrogen containing organic compounds remains, as yet, an unexplored field. The process requires oxidation of the organic part to CO2 and reduction of the possibly evolving NOx to molecular nitrogen. Thus the catalyst must be able to perform two functions, oxidative and reductive, both in the presence of excess oxygen. Our experience with design of HT-derived catalysts for the removal of volatile oragnic compounds [5] prompted us to research the possibility of using HT precursors for preparation of mixed-oxide catalysts capable of efficient combustion of N-contaning organics. In view of the fact that Cu-Cr oxides are known to perform well as total oxidation catalysts [3-7], while vanadium is the major component of oxide catalysts used for selective reduction of nitrogen oxides, Cu, Cr and V were chosen as main elements constituting the HT structure with A1 and Zn acting as moderating agents. Dimethylformamide (DMF), commonly used industrial solvent, was used as a probe molecule.
2. EXPERIMENTAL HTs containing Cu, Cr, Zn and A1 as layer-forming elements, were pepared by a copreciptation method at constant pH from the solutions of respective nitrates, described in detail in [5]. Vanadium was introduced into the interlayer as decavanadate or pyrovanadate anions by
600 means of anion exchange, according to the procedure described in [6]. Samples of theoretical general formula [•M(II)0.67ZM(III)o.33(OI-I)2]An'o.33/n, where M(II)=Cu, Zn, MOII)=Cr,AI, and An--NO3", V1oO286"or V2074", were obtained and labeled in a manner showing the atomic ratios of the metal elements in each material: Cu2Cr (nitrate form), Cu2CrVo.5 (pyrovanadate form) Cu2CrVI.7, Cu2Cro.5A]o.5Vi.7, CuZnCro.5A]o.5V1.7 (all decavanadate forms). The parent HTs were calcined for 3 h in air at 400~ and 550~ The calcination products obtained at lower temperature were designated Cu2Cr-400, Cu2CrVo.5-400, Cu2CrV~.7-400, Cu2Cr0.sAlo.5V~.7400, CuZnCro.sAlo.sV~.7-400, those prepared at 550~ were referred to as Cu2Cr-550, Cu2CrV0.5-550, Cu2CrVl.7-550, Cu2Cr0.5Alo.5V1.7-550,CuZnCr0.sAlo.5V1.7-550. Samples were characterized with PXRD (DRON-3 diifractometer, Ni filtered Cu Kcx radiation), BET (nitrogen adsorption, outgassing at 473 K, Micromeritics ASAP 2000V apparatus), and chemical analysis (ICP-AES Plasma 40 Perkin-Elmer spectrometer). The reaction was carried out in a flow system at atmospheric pressure, in the temperature range 175-500~ The catalyst volume was 0.5 cm3 (0.7_+0.05g), particle size 0.2-0.5 ram. DMF concentration was 1.5 g/m3 (~1000 ppm), G.H.S.V.=10000 hq. 3. RESULTS AND DISCUSSION
The real atomic ratios of the metal elements in the synthesized smiles are given in Table 1. Table 1
S~le doo3 (A) Cu2Cr 8.99 Cu2CrVo.5 7.79 Cu2CrVI.7 11.79 Cu2Cro 5Alo5V1.7 11.61 CuZnCro.sAlo.sV1.7 11.61
Cu 2.05 1.99 1.96 1.89 1.02
Zn 0.88
Cr 1.00 1.00 1.00 0.50 0.50
AI 0.49 0.54
V 0.57 1.78 2.01 2.13
A higher than intended vanadium contem is seen in two decavanadate-exchanged samples. It may be due either to the presence of impurity phases (see below) or to the enhanced loading with decavanadate anions caused by partial protonation of the intercalated species. The XRD patterns of fresh samples confirm that the structures expected for the given anions have been obtained [8] (Fig. 1). In the Cu2Cr0.5Al0.5V~.7sample, of poorest crystallinity, a small amount of an impurity, identified as copper orthovanadate, is visible. The PXRD patterns of solids obtained after calcination at 400~ and 550~ are presented in Fig. 2 a and b respectively. Heat treatment at 400~ transforms all decavanadate forms of hydrotalcites into predominantly amorphous solids, while in the Cu~Cr-400 and Cu~CrV0.5-400 samples crystalline phases appear. In the former the reflections of copper oxide CuO, copper chromite CuCr~O4 and copper chromate CuCrO4 can be identified, in the latter the pattern is dominated by several modifications of copper vanadate Cu5V20~0 accompanied by copper chromite. After calcination at 550~ all previously amorphous solids reveal the presence of crystalline phases. The patterns of Cu2Cr-550 and Cu~CrV0.5-550 are similar to those observed after calcination at 400~ except that in the former the chromate phase disappears and in the Cu~CrV0.5-550 a strong peak emerging around 20=40 ~ points to the enhanced crystallization of CuO and the CusV20~0 modification known as stoiberite. The patterns of all other samples are dominated by
601
CuZnCr0.5AI0.5V1.7
Cu2Cr0.5AI0.5Vl.7
Cu2CrV1.7
Cu2CrV0.5
Cu2Cr .
0
.
I0
.
.
;;0
.
20
~
..
30 40 50 Cu Kct [deg]
60
I
70
Fig. 1 XRD patterns of HTs precursors reflections characteristic of various vanadates. Cu2CrV1.7-550 contains mainly Cu2V2OT, CuVO3, CuCr204, and CrO3. Cu2Cr0.sA10.sV1.7-550 shows reflections characteristic of Cu2V2OT, CuV206, CuVO3 and a mixed oxide Cr0.07V1.9304. CuZnCr0.sA10.sV1.7-550 is composed mainly of CUEV207, Zn3(VO4)2, and Cr0.07Vl.9304. It is worthwhile to note that in the Al-containing samples of the 550 series no crystalline CuCr204 is formed.
0
10
20
30
40-
50
60 0 1"0 20 Cu K(x [deg]
2"0
N
Fig. 2 XRD patterns of HTs samples calcined at: a) 400~ b) 550~
40
5"0
60
70
602 BET specific surface areas of all calcined samples are presented in Table 2. Table 2 ,BE T ~ . c ~ C . . . s ~ c e ~ea of.c.flc~cd.,.~s Calcination temperature Cu2Cr Cu2CrVo.5
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
(~
Cu2CrV1.7 Cu2Cro.sAlo.sVx.7CuZnCro.sAlo.WL7
..................................................................................................................................................................................................
400 550
43 20
25 19
54 19
28 8
25 3
All catalysts proved very active in the total oxidation of DMF, reaching 100% conversion in the temperature range between 200~ and 350~ depending on the sample composition and on the temperature of calcination. The order of activities was similar for both series: Cu2Cr-4~>Cu2CrV~.7-4~Cu~CrV~.5-4~>Cu2Cr~.5A~.5V~.7-4~>CuZnCr~.5A~.5V~.7-400 and Cu~Cr-55~>Cu2CrV~5-55~>Cu2CrV~7-55~>Cu2Cr~5A~.5V~.7~55~>CuZnCr~.5A~5V~7~550. All samples which as 400 series displayed amorphous character showed an important loss of activity as 550 catalysts, understandable in view of the decrease in specific surface areas. On the majority of catalysts the organic component was converted exclusively to CO2. Only on samples with half of chromium replaced by aluminium CO could be also seen before the conversion reached 100%. The effect was more significant for CuZnCr0.sAl0.sV1.7-400 and CuZnCr0.sA10.W~.7-550 samples having additionally half of copper replaced with zinc. At low temperature range the nitrogen part of DMF was converted to N2 with 100% selectivity. At higher temperatures nitrogen oxides started to evolve (Fig. 4). The ordering of catalysts according to the increasing temperature of the onset of NOx evolution follows exactly the pattern of activities, i.e. the more active catalyst the sooner NO~ appear in the reaction products. Analysis of the influence of sample composition on the selectivity to NO~ shows that, as expected, addition of increasing amounts of vanadium to the copper-chromium catalyst 100
r'
9i
--~=.-~f~Z-.-.---~.~,~..~~ ,, 9
oj
9
,,
8O
zO o0 n, LM > Z
, ,9
"
60
: l~
~= --=
./
-- 9 -- Cu~Cr-400
,'
- - 9 - - Cu2Cr-550
js S
!i!_cucrv
,,'
- - A - - Cu2CrVl.7-550
/
.C
o,,OO Cu2CrVo s-550
40
o o
// 20
/ ]
--- Cu2Cro ~,1o5Vl 7-400
r
o - - Cu2CrosAIo5V~7-550
6~~
---A--- CuZnCro ~.1o5V1 r400 -- 9
150
200
250
300
350
TEMPERATURE, ~
Fig. 3 DMF conversion over calcined HTs.
CuZnCrosAIosV~r550 400
450
603 100
o~
80
~-" >
60
r UJ ._1 LLI o~
40
b)
20 m
150
200
250
300
350
400
450 150
E
200
~/
250
i
300
350
400
450
TEMPERATURE, *C Fig. 4 Selctivity to NOx over HTs calcined at: a) 400~
Cu2CrVo.5, A -
Cu2CrV1.7, O - Cu2Cro.sAlo.sV1.7,
and b) 550 ~ C; 9 - Cu~Cr, 9 -
A - CuZnCro.sAlo.sVi.7.
results in a gradual retardation of NOx evolution. Similar observation was reported recently for the non-hydrotalcite derived Cu-Cr-V catalyst [9]. Interestingly, the spectrum of various nitrogen oxides is affected in a different way depending on the V content in the sample. Thus, initially the later onset of NOx formation is due to the reduced evolution of N20 while higher amount of V reduces the evolution of NO and NO2. Further modification by replacing half of chromium with aluminium lowers evolution of N20 and so does subsequent replacement of part of copper with zinc. Varying shapes of the conversion and selectivity curves cause that in each of the series different ~nples show best characteristics. Table 3 compares the temperature ranges and the temperature windows in which the catalysts show the activity higher than 80% and the yield of NOx lower than 20%. Table 3 Temperature ranges and temperature windows of>80% conversion and <20% NOx selectivity for calcined HTs. Calcination temperature CuECr CuECrV0.5 Cu2CrV1.7 Cu2Cr0.sAl0.sV1.7CuZnCr0.sAlo.sV1.7 ...................( 0 c )
400 550
.....................................................................................................................................................................
184-192~ 8~ 180-203~ 23~
208-238~ 30~ 216-280~ 64~
208-262~ 54~ 242-291 ~ 49~
246-313~ 67~ 285-356~ 63~
264-360~ 96~ 310-350~ 40~
In the case of the catalysts calcined at 400~ the temperature window increases with enrichment of the composition, simultaneously being shifted towards higher temperatures. The best of the series is CuZnCr0.sAl0.sV~.7-400 which shows good performance over almost 100~ There is no clear trend in the catalysts calcined at 550~ Samples Cu2CrV0.s-550 and Cu:Cr0.sA10.sV~.7-550 show both temperature windows of over 60~ but the former has to be regarded as superior because its maximum falls at lower temperature range. The calcination at
6O4 550~ worsens the properties of previously amorphous solids by shifting their maximum performance towards higher temperatures and reducing the temperature window. The latter is particularly significant for CuZnCr0.sA10.sVL7-550 catalyst. In this sample the crystallization of new phases is most pronounced, as seen by narrow XRD reflections and a strong drop in the specific surface area. On the other hand, the Cu2Cr-550 and Cu2CrV0.5-550 samples, both crystalline already at 400~ improve their characteristics after heat treatment at higher temperature. This effect may be tentatively attributed to the changes in phase composition of the catalysts when passing from one temperature of calcination to another. In the first case the phase of copper chromate disappears, in the second CuO and the stoiberite modification of CusV2O10 can be identified in significant quantities. It is conceivable that the presence of the latter phases makes the Cu2CrV0.5-550 catalyst superior with respect to the richer in vanadium Cu2CrV~.7-550 sample. Investigations into these effects are now in progress in our laboratory. 4. CONCLUSIONS The appropriate choice of elements constituting the hydrotalcite precursors and adjustement of the calcination temperature allow to optimize the balance between the oxid'ging and the reducing functiom of the resulting mixed oxides. The catalysts of higher activity show enhanced evolution of NO~ in a degree depending on the catalyst composition and temperature of calcination. As a result, the ordedering of catalysts according to the best performance differs for the two series derived from the same precursors but calcined at different temperatures. In the 400 series it is the amorphous material doped with AI and Zn, and thus having the lowest oxidizing power. In the 550 catalysts the best is the sample with no A1 and Zn in the structure, revealing the presence of CuO and a significant amount of the stoiberite modification of Cu5V201o.
Acknowledgement: TI~ work was financially supported by the Polish Committee for Scientific Research within the research projects 6 P04D 040 14 and 7 T08D 008 17. REFERENCES
1. J. Major, A. Hudak, G. Kiss, M.G. Jakab, J. Szaniszlo, M. Naray, I. Nagy, A. Tompa, EnvironrrL and Mol. Mutagenesis 31 (1998) 301. 2. R. Wrbitzl~, Int. Arch.ives of Occupational and Environm. Health 72 (1999) 19. 3. J.J. Spivey, Ind. Eng. Chem.. Res. 26 (1987) 2165. 4. J.J. Spivey, J.B. Butt, Catal. Today 11 (1992) 465. 5. K. Bahranowski, E. Bielafiska, R. Janik, T. Machej, E.M. Serwicka, Clay Miner.34 (1999) 67. 6. K. Bahranowski, G. Bueno, V. Cort6s-Corberfin, F. Kooli, E.M. Serwicka, R.X. Valenzuela, K. Wcisto, Appl. Catal. (1999) in print. 7. T. Machej, J. Janas, H. Sadowska, J. Haber, S. C~ckiewicz, L. Matachowski, PL Patent No 166598 (1992) 8. C. Dep6ge, L. Bigey, C. Forano, A. de Roy, J.P. Besse, J. Solid State Chem. 126 (1996) 314. 9. J. Haber, T. Machej, H. Sadowska, J. Janas, L. Matachowski, 16th Meeting of the NAM Catalysis Society, Boston 1999, Book of abstracts, PI-002.
599
Hydrotaicite-Derived Catalysts for Removal of Nitrogen-Containing Volatile Organic Compounds J. Habera , K. Bahranowskib, J. Janas", R. Janika, T. Machej", L. Matachowski", A. Michalik", H. Sadowskaa, E.M. Serwiekaa
"Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, 30-239 Krak6w, ul. Niezapominajek 1, Poland, bFaculty of Geology, Geophysics and Environmental Protection, Academy of Mining and Metallurgy, 30-059 Krak6w, al. Mickiewicza 30, Poland, The catalytic properties of mixed oxides derived from the multi-metallic hydrotalcites (HTs) containing Cu, Cr, V, A1 and Zn were tested in the total oxidation of dimethylformamide with particular stress on to the role of the precursor composition and the temperature of thermal decomposition (400~ and 550~ Optimum performance for the 400 series was given by the material obtained from the Cu,Zn, Cr,A1,V-HT, while in the 550 series the catalyst derived from the Cu, Cr,V-HT precursor gave best results. The observed activity and selectivity patterns are discussed in terms of the catalysts physico-chemical properties. 1. INTRODUCTION Recent studies show that exposure of human beings to air contaminated with N-containing organic vapours induces genotoxic consequences and may increase the cancer risk in the affected population [1,2]. While abatement of hydrocarbons by total oxidation has been widely studied [3,4], the catalytic conversion of nitrogen containing organic compounds remains, as yet, an unexplored field. The process requires oxidation of the organic part to CO2 and reduction of the possibly evolving NOx to molecular nitrogen. Thus the catalyst must be able to perform two functions, oxidative and reductive, both in the presence of excess oxygen. Our experience with design of HT-derived catalysts for the removal of volatile oragnic compounds [5] prompted us to research the possibility of using HT precursors for preparation of mixed-oxide catalysts capable of efficient combustion of N-contaning organics. In view of the fact that Cu-Cr oxides are known to perform well as total oxidation catalysts [3-7], while vanadium is the major component of oxide catalysts used for selective reduction of nitrogen oxides, Cu, Cr and V were chosen as main elements constituting the HT structure with A1 and Zn acting as moderating agents. Dimethylformamide (DMF), commonly used industrial solvent, was used as a probe molecule.
2. EXPERIMENTAL HTs containing Cu, Cr, Zn and A1 as layer-forming elements, were pepared by a copreciptation method at constant pH from the solutions of respective nitrates, described in detail in [5]. Vanadium was introduced into the interlayer as decavanadate or pyrovanadate anions by
600 means of anion exchange, according to the procedure described in [6]. Samples of theoretical general formula [•M(II)0.67ZM(III)o.33(OI-I)2]An'o.33/n, where M(II)=Cu, Zn, MOII)=Cr,AI, and An--NO3", V1oO286"or V2074", were obtained and labeled in a manner showing the atomic ratios of the metal elements in each material: Cu2Cr (nitrate form), Cu2CrVo.5 (pyrovanadate form) Cu2CrVI.7, Cu2Cro.5A]o.5Vi.7, CuZnCro.5A]o.5V1.7 (all decavanadate forms). The parent HTs were calcined for 3 h in air at 400~ and 550~ The calcination products obtained at lower temperature were designated Cu2Cr-400, Cu2CrVo.5-400, Cu2CrV~.7-400, Cu2Cr0.sAlo.5V~.7400, CuZnCro.sAlo.sV~.7-400, those prepared at 550~ were referred to as Cu2Cr-550, Cu2CrV0.5-550, Cu2CrVl.7-550, Cu2Cr0.5Alo.5V1.7-550,CuZnCr0.sAlo.5V1.7-550. Samples were characterized with PXRD (DRON-3 diifractometer, Ni filtered Cu Kcx radiation), BET (nitrogen adsorption, outgassing at 473 K, Micromeritics ASAP 2000V apparatus), and chemical analysis (ICP-AES Plasma 40 Perkin-Elmer spectrometer). The reaction was carried out in a flow system at atmospheric pressure, in the temperature range 175-500~ The catalyst volume was 0.5 cm3 (0.7_+0.05g), particle size 0.2-0.5 ram. DMF concentration was 1.5 g/m3 (~1000 ppm), G.H.S.V.=10000 hq. 3. RESULTS AND DISCUSSION
The real atomic ratios of the metal elements in the synthesized smiles are given in Table 1. Table 1
S~le doo3 (A) Cu2Cr 8.99 Cu2CrVo.5 7.79 Cu2CrVI.7 11.79 Cu2Cro 5Alo5V1.7 11.61 CuZnCro.sAlo.sV1.7 11.61
Cu 2.05 1.99 1.96 1.89 1.02
Zn 0.88
Cr 1.00 1.00 1.00 0.50 0.50
AI 0.49 0.54
V 0.57 1.78 2.01 2.13
A higher than intended vanadium contem is seen in two decavanadate-exchanged samples. It may be due either to the presence of impurity phases (see below) or to the enhanced loading with decavanadate anions caused by partial protonation of the intercalated species. The XRD patterns of fresh samples confirm that the structures expected for the given anions have been obtained [8] (Fig. 1). In the Cu2Cr0.5Al0.5V~.7sample, of poorest crystallinity, a small amount of an impurity, identified as copper orthovanadate, is visible. The PXRD patterns of solids obtained after calcination at 400~ and 550~ are presented in Fig. 2 a and b respectively. Heat treatment at 400~ transforms all decavanadate forms of hydrotalcites into predominantly amorphous solids, while in the Cu~Cr-400 and Cu~CrV0.5-400 samples crystalline phases appear. In the former the reflections of copper oxide CuO, copper chromite CuCr~O4 and copper chromate CuCrO4 can be identified, in the latter the pattern is dominated by several modifications of copper vanadate Cu5V20~0 accompanied by copper chromite. After calcination at 550~ all previously amorphous solids reveal the presence of crystalline phases. The patterns of Cu2Cr-550 and Cu~CrV0.5-550 are similar to those observed after calcination at 400~ except that in the former the chromate phase disappears and in the Cu~CrV0.5-550 a strong peak emerging around 20=40 ~ points to the enhanced crystallization of CuO and the CusV20~0 modification known as stoiberite. The patterns of all other samples are dominated by
601
CuZnCr0.5AI0.5V1.7
Cu2Cr0.5AI0.5Vl.7
Cu2CrV1.7
Cu2CrV0.5
Cu2Cr .
0
.
I0
.
.
;;0
.
20
~
..
30 40 50 Cu Kct [deg]
60
I
70
Fig. 1 XRD patterns of HTs precursors reflections characteristic of various vanadates. Cu2CrV1.7-550 contains mainly Cu2V2OT, CuVO3, CuCr204, and CrO3. Cu2Cr0.sA10.sV1.7-550 shows reflections characteristic of Cu2V2OT, CuV206, CuVO3 and a mixed oxide Cr0.07V1.9304. CuZnCr0.sA10.sV1.7-550 is composed mainly of CUEV207, Zn3(VO4)2, and Cr0.07Vl.9304. It is worthwhile to note that in the Al-containing samples of the 550 series no crystalline CuCr204 is formed.
0
10
20
30
40-
50
60 0 1"0 20 Cu K(x [deg]
2"0
N
Fig. 2 XRD patterns of HTs samples calcined at: a) 400~ b) 550~
40
5"0
60
70
602 BET specific surface areas of all calcined samples are presented in Table 2. Table 2 ,BE T ~ . c ~ C . . . s ~ c e ~ea of.c.flc~cd.,.~s Calcination temperature Cu2Cr Cu2CrVo.5
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
(~
Cu2CrV1.7 Cu2Cro.sAlo.sVx.7CuZnCro.sAlo.WL7
..................................................................................................................................................................................................
400 550
43 20
25 19
54 19
28 8
25 3
All catalysts proved very active in the total oxidation of DMF, reaching 100% conversion in the temperature range between 200~ and 350~ depending on the sample composition and on the temperature of calcination. The order of activities was similar for both series: Cu2Cr-4~>Cu2CrV~.7-4~Cu~CrV~.5-4~>Cu2Cr~.5A~.5V~.7-4~>CuZnCr~.5A~.5V~.7-400 and Cu~Cr-55~>Cu2CrV~5-55~>Cu2CrV~7-55~>Cu2Cr~5A~.5V~.7~55~>CuZnCr~.5A~5V~7~550. All samples which as 400 series displayed amorphous character showed an important loss of activity as 550 catalysts, understandable in view of the decrease in specific surface areas. On the majority of catalysts the organic component was converted exclusively to CO2. Only on samples with half of chromium replaced by aluminium CO could be also seen before the conversion reached 100%. The effect was more significant for CuZnCr0.sAl0.sV1.7-400 and CuZnCr0.sA10.W~.7-550 samples having additionally half of copper replaced with zinc. At low temperature range the nitrogen part of DMF was converted to N2 with 100% selectivity. At higher temperatures nitrogen oxides started to evolve (Fig. 4). The ordering of catalysts according to the increasing temperature of the onset of NOx evolution follows exactly the pattern of activities, i.e. the more active catalyst the sooner NO~ appear in the reaction products. Analysis of the influence of sample composition on the selectivity to NO~ shows that, as expected, addition of increasing amounts of vanadium to the copper-chromium catalyst 100
r'
9i
--~=.-~f~Z-.-.---~.~,~..~~ ,, 9
oj
9
,,
8O
zO o0 n, LM > Z
, ,9
"
60
: l~
~= --=
./
-- 9 -- Cu~Cr-400
,'
- - 9 - - Cu2Cr-550
js S
!i!_cucrv
,,'
- - A - - Cu2CrVl.7-550
/
.C
o,,OO Cu2CrVo s-550
40
o o
// 20
/ ]
--- Cu2Cro ~,1o5Vl 7-400
r
o - - Cu2CrosAIo5V~7-550
6~~
---A--- CuZnCro ~.1o5V1 r400 -- 9
150
200
250
300
350
TEMPERATURE, ~
Fig. 3 DMF conversion over calcined HTs.
CuZnCrosAIosV~r550 400
450
603 100
o~
80
~-" >
60
r UJ ._1 LLI o~
40
b)
20 m
150
200
250
300
350
400
450 150
E
200
~/
250
i
300
350
400
450
TEMPERATURE, *C Fig. 4 Selctivity to NOx over HTs calcined at: a) 400~
Cu2CrVo.5, A -
Cu2CrV1.7, O - Cu2Cro.sAlo.sV1.7,
and b) 550 ~ C; 9 - Cu~Cr, 9 -
A - CuZnCro.sAlo.sVi.7.
results in a gradual retardation of NOx evolution. Similar observation was reported recently for the non-hydrotalcite derived Cu-Cr-V catalyst [9]. Interestingly, the spectrum of various nitrogen oxides is affected in a different way depending on the V content in the sample. Thus, initially the later onset of NOx formation is due to the reduced evolution of N20 while higher amount of V reduces the evolution of NO and NO2. Further modification by replacing half of chromium with aluminium lowers evolution of N20 and so does subsequent replacement of part of copper with zinc. Varying shapes of the conversion and selectivity curves cause that in each of the series different ~nples show best characteristics. Table 3 compares the temperature ranges and the temperature windows in which the catalysts show the activity higher than 80% and the yield of NOx lower than 20%. Table 3 Temperature ranges and temperature windows of>80% conversion and <20% NOx selectivity for calcined HTs. Calcination temperature CuECr CuECrV0.5 Cu2CrV1.7 Cu2Cr0.sAl0.sV1.7CuZnCr0.sAlo.sV1.7 ...................( 0 c )
400 550
.....................................................................................................................................................................
184-192~ 8~ 180-203~ 23~
208-238~ 30~ 216-280~ 64~
208-262~ 54~ 242-291 ~ 49~
246-313~ 67~ 285-356~ 63~
264-360~ 96~ 310-350~ 40~
In the case of the catalysts calcined at 400~ the temperature window increases with enrichment of the composition, simultaneously being shifted towards higher temperatures. The best of the series is CuZnCr0.sAl0.sV~.7-400 which shows good performance over almost 100~ There is no clear trend in the catalysts calcined at 550~ Samples Cu2CrV0.s-550 and Cu:Cr0.sA10.sV~.7-550 show both temperature windows of over 60~ but the former has to be regarded as superior because its maximum falls at lower temperature range. The calcination at
6O4 550~ worsens the properties of previously amorphous solids by shifting their maximum performance towards higher temperatures and reducing the temperature window. The latter is particularly significant for CuZnCr0.sA10.sVL7-550 catalyst. In this sample the crystallization of new phases is most pronounced, as seen by narrow XRD reflections and a strong drop in the specific surface area. On the other hand, the Cu2Cr-550 and Cu2CrV0.5-550 samples, both crystalline already at 400~ improve their characteristics after heat treatment at higher temperature. This effect may be tentatively attributed to the changes in phase composition of the catalysts when passing from one temperature of calcination to another. In the first case the phase of copper chromate disappears, in the second CuO and the stoiberite modification of CusV2O10 can be identified in significant quantities. It is conceivable that the presence of the latter phases makes the Cu2CrV0.5-550 catalyst superior with respect to the richer in vanadium Cu2CrV~.7-550 sample. Investigations into these effects are now in progress in our laboratory. 4. CONCLUSIONS The appropriate choice of elements constituting the hydrotalcite precursors and adjustement of the calcination temperature allow to optimize the balance between the oxid'ging and the reducing functiom of the resulting mixed oxides. The catalysts of higher activity show enhanced evolution of NO~ in a degree depending on the catalyst composition and temperature of calcination. As a result, the ordedering of catalysts according to the best performance differs for the two series derived from the same precursors but calcined at different temperatures. In the 400 series it is the amorphous material doped with AI and Zn, and thus having the lowest oxidizing power. In the 550 catalysts the best is the sample with no A1 and Zn in the structure, revealing the presence of CuO and a significant amount of the stoiberite modification of Cu5V201o.
Acknowledgement: TI~ work was financially supported by the Polish Committee for Scientific Research within the research projects 6 P04D 040 14 and 7 T08D 008 17. REFERENCES
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