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Hydrogenation of CO2 over metal supported fine particles
T. Inui et al. (Editors), N e w Aspects of Spillover Effect in Catalysis 0 1993 Elsevier Science Publishers B.V. All rights reserved.
391
Hydrogenation of COz over metal supported fine particles Takayoshi Uernatsu, Masatoshi Fukuda , Tsuyoshi Iijirna, and Shogo Shimazu Department of Applied Chemistry, School of Engineering, Chiba University, Yayoi-Cho, Inage-Ku, Chiba, 263, JAPAN
Abstract
Superfine particles of nickel supported metal oxide were prepared by spray reaction method and characterized by means of S M , XRD, XPS, TPR, and adsorption measurements. The metal clusters developped were highly dispersed and stabilized by strong interaction with oxide suppots. The catalyic activity for the hydrogenation of C02 was much higher than those by conventional impregnation method. Alkali metal addition enhanced high catalytic activity and promoted CO formation. 1,
INTRODUCTION
The characteristics of superfine particles have attracted much interests as new potential catalysts. Spray reaction method (SPR) is an excellent preparation metod for multi-component superfine perticles such as high temperature superconductors and ferrites. We have applied this method to metal-supporting oxide particles111 . Catalytic properties were studied for the hydrogenation of CO,, comparing with those conventional impregnation methods. The effects of oxides supports and alkali aetal addition were discussed in terms of SIISI effects and roles of spilled-over hydrogen.
2 , EXPERIMENTAL Mixed solutions of Ni- and aetal salts for oxide supports were atomized by ultrasonic device and calcined at 1273K. The submicron ccmplexed oxide particles were treated with H2 to develop Nio clusters on the supports, ZiO, (sN) and Nio/h0y. Hybrid fine particles (HYB) and conventional impregnation catalysts (IMP) were prepared, respectively, by iapregnation of SPR particles (sN) and a comercia1 ZrO, powder (iN) with solitions of alkalimetal salts. A closed cycle reactor was applied for catalytic tests.
398
3 - RESULTS AND DICUSSION 3 - 1 - XRD powder patterns XRD powder patterns for as-sprayed binary oxides classified them into two types according to their coaposition and distribution state of Ni-O bonds, which depended on the supports and metal concentration. The binary oxide paraticles, such as NiO/ZrO,, NiO/ZnO, NiO/MgO and NiO/A1,0, (Group I), showed only traces of diffraction peaks due to the segregated NiO poly-crystalline, suggesting the minor component NiO ( 515 ml-X) solved in the matrix of major oxides or distributed in amorphous states. While in the cases of NiO/TiO, and NiO/Nb,05 (Group 11), Ni-O bonds were detected only in binary oxide phases, NiTiO, and NiNbxOy. NiO segregation was not observed. H, treatment at elevated temperatures developped Nio peaks for Group I with disappearance of NiO. However, as for 11, Ni-O bonds of NiTiO, was reduced above 77% to form Nio clustes (Figure 1). Diffraction intensity for TiO,(mtil) increased with the decoaposition of NiTiO, and decreased again with the fomation of NiO. Further, a reduced type of T i m was produced, suggesting a role of spilled-over hydrogen. Similarly, all diffraction peaks for Ni-Nb-O diminished by H, reduction at 1013K, but no trace of Ni' clusters was detected by XRD. XPS technique gives average Ni concentrations in the uppermost surface layer in the depth. The concentration obtained was 4.Iml-X for Nio/ZrO,SPR and 3 2 . 1 ~ 1 - X for NiO/ZrO,-HYB, respectively. Since Ni content in the bulk was 15 ml-X,the reduction treatment decreased the surface distribution in SPR particles and increased on HYB particles. This observation is in consistent with those for S M study demonstrating homogenous higher dispersion of Nio cluster with lean diaaeter 5 20A on SPR particles and localized large Nio particles on HYB particles as given in Figure 2. .:
.-C
100
E
..r &
20
ti
a
0
before
773 073 073 1073 1 173 1273
Reduction temperature/K Figure 1. Change in XRD profiles by reduction of Ni-Ti-0-SPR. reductn.
Figure 2. SEM photographs.
399
3 - 2 -TPR The reducibility of Ni-O bonds on as-sprayed binary systems NiO/hOy were evaluated by TPR measurerent. Higher shifts of reduction temperature were observed froa 589K of pure NiO-SPR, upto 87% (Ni-Ti*) and 90% (NiNb-O) as well as for Ni-Zn-O (658K) and Ni-Hg-0 (77%). This fact MY suggest that Ni-O bonds in the binary SPR paeticles are much stabilaized by the interaction with the texture of the supporting oxides. This stabilization effects were more prominent for SPR than HYB particles. 3 - 3 , CO and H 2 adsorption The amounts of CO adsorption over Ni0/hOy after H,-treatment at 67% were in the order: Al,0s(680K)> znO(658K)> ngO(673K)) Nb2O,(903K)> ZrO, (72310) TiO, (87%). This can be explained interns of the easiness of the reduction of Ni-O bonds at 67% which is indicated by TPR reduction peak given in the parenthesis. Figure 3 caapares amounts of CO and HL adsorption over Nio/ZrO2-HYB and Nio/ZrO,-SPR particles varying H, treatment temperature. Apparently, the numbers of adsorption site on SPR particles are much saaller than those on HYB particles, in spite of smaller sizes of Nio cluster. This might be firstly due to the differece in the Nio distribution; secondly the difference in the surface areas: 1.45 n'/g for SPR catalyst and 21.9 n'/g for HYB catalyst. The presence of the optima reduction temperatures for adsoption may probably relate more to SHSI effects than to the sintering as described below. The optimum temperature was much higer over Ni-ZrQ SPR (69811 for H, and 723K for CO) than Ni-Zr-O-HYB (62% for H, and CO). These shifts MY be explained by stronger stabilization on SPR particles. The aaounts of H, adsorption obtained for Nio/ZrO, were much larger than those of CO adsorption. The presence of spill-over hydrogen right play a role again over these superfine particles.
773
773 1023 673 773 873
Redctn.TempJK Figure 4. SMSl effects on Con conversion.
Figure 3. Adsorption of CO and H2 over Nio/Zr02sereies (273K).
400
3 - 4 , M e t h a n a t i o n over N i o / M x O y Figure 4 sunarizes catalytic behavior in the methanation of CO, on Nio /MxOy-SPR. HI pretreatment temperatures were decided taking into account of TPR results. Over group I catalysts of Ni0/A1,O3 and Nio/HgO, the high tempereture reduction (HTR = 1043, 102%) was effective to increase the activities upto 9.6 and 22 times, respectivly of those subjected low temperature reduction (LTR = 77310; while for Group I1 catalysts of Nio/ TiO, and NiO/Nb,O,, HTR depressed it reversibly. However, after HTR, further successive low temperature oxidation and reduction, LTR, at 598K (H/L) their activities recovered again very dramatically. The latter two oxides are known as so-called typical S E I oxides where the catalyst support is easily reduced by the spilled over hydrogen leading to the decoration of Nio clusters with inactive partially reduced type support like TiO,.x or Nb,O,.y. 3 - 5 , Effects of a l k a l i m e t a l s Effects of alkali metal addition on CO, methanation is shown in Figure 5. Alkali containing SPR particles, Z-Nio/ZrO,-SPR, was prepared by spray reaction of ternary mixture solution of Zr-, Ni- and Z(alka1i metal)salts. HYB and IMP type particles prepared by impregnation of NiO/ZrO,-SPR (sN) and on Nio/ZrO,-lMP (iN) with appropriate solutions were treated also with H, prior to use. Interestingly, BET surface areas of SPR particles increased remarkably by a l h i addition: Na(26m*/g) and K(32 m'/g). The effects on the catalytic behavior were sumarized in Fig. 5. Over HYB series, dramatical increases in the activity for CO, conversion were obtained in the order: Li< Na < K < Rb < Cs, though CO formation became predorinated. As for IMP series the highest activity was observed for Na added one. The most reaarkable effect was obtained for Z-SPR where the maximum activity was enhanced on Na-NiO-ZrO,-SPR with 10 times higher of non-promoted NiO-ZrO,-SPR. Evidently, SPR particles extended great prolotion effects by alkalili .-'c 60 metals and still maintained high c selectivity for methanation. The - 50 pronoting effect of K-NiO/ZrO,SPR attained by the decrease in 40 E the activation energy from 80.6 dE 30 to 66.2kJ/nol, while the effects on K-Ni"/ZrO, were caused by the $ 20 % increase in pre-exponential terra. The mechanism must be interesing $10 subject for further study. a 0 23 m y o m y o "23 m y o U) z ao z 60.- z 60 4 REFERENCE c
f
v
SPR
)
I
v)
HYB
IMP
Figure 5. Effects of alkalirnetal addition on C02hydrogenation over Nio/Zr02(573K).
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1 T.Ueaatsn, S. Shiaazu, Proc. Ist ~ A T (1990) , p.329.
391
Hydrogenation of COz over metal supported fine particles Takayoshi Uernatsu, Masatoshi Fukuda , Tsuyoshi Iijirna, and Shogo Shimazu Department of Applied Chemistry, School of Engineering, Chiba University, Yayoi-Cho, Inage-Ku, Chiba, 263, JAPAN
Abstract
Superfine particles of nickel supported metal oxide were prepared by spray reaction method and characterized by means of S M , XRD, XPS, TPR, and adsorption measurements. The metal clusters developped were highly dispersed and stabilized by strong interaction with oxide suppots. The catalyic activity for the hydrogenation of C02 was much higher than those by conventional impregnation method. Alkali metal addition enhanced high catalytic activity and promoted CO formation. 1,
INTRODUCTION
The characteristics of superfine particles have attracted much interests as new potential catalysts. Spray reaction method (SPR) is an excellent preparation metod for multi-component superfine perticles such as high temperature superconductors and ferrites. We have applied this method to metal-supporting oxide particles111 . Catalytic properties were studied for the hydrogenation of CO,, comparing with those conventional impregnation methods. The effects of oxides supports and alkali aetal addition were discussed in terms of SIISI effects and roles of spilled-over hydrogen.
2 , EXPERIMENTAL Mixed solutions of Ni- and aetal salts for oxide supports were atomized by ultrasonic device and calcined at 1273K. The submicron ccmplexed oxide particles were treated with H2 to develop Nio clusters on the supports, ZiO, (sN) and Nio/h0y. Hybrid fine particles (HYB) and conventional impregnation catalysts (IMP) were prepared, respectively, by iapregnation of SPR particles (sN) and a comercia1 ZrO, powder (iN) with solitions of alkalimetal salts. A closed cycle reactor was applied for catalytic tests.
398
3 - RESULTS AND DICUSSION 3 - 1 - XRD powder patterns XRD powder patterns for as-sprayed binary oxides classified them into two types according to their coaposition and distribution state of Ni-O bonds, which depended on the supports and metal concentration. The binary oxide paraticles, such as NiO/ZrO,, NiO/ZnO, NiO/MgO and NiO/A1,0, (Group I), showed only traces of diffraction peaks due to the segregated NiO poly-crystalline, suggesting the minor component NiO ( 515 ml-X) solved in the matrix of major oxides or distributed in amorphous states. While in the cases of NiO/TiO, and NiO/Nb,05 (Group 11), Ni-O bonds were detected only in binary oxide phases, NiTiO, and NiNbxOy. NiO segregation was not observed. H, treatment at elevated temperatures developped Nio peaks for Group I with disappearance of NiO. However, as for 11, Ni-O bonds of NiTiO, was reduced above 77% to form Nio clustes (Figure 1). Diffraction intensity for TiO,(mtil) increased with the decoaposition of NiTiO, and decreased again with the fomation of NiO. Further, a reduced type of T i m was produced, suggesting a role of spilled-over hydrogen. Similarly, all diffraction peaks for Ni-Nb-O diminished by H, reduction at 1013K, but no trace of Ni' clusters was detected by XRD. XPS technique gives average Ni concentrations in the uppermost surface layer in the depth. The concentration obtained was 4.Iml-X for Nio/ZrO,SPR and 3 2 . 1 ~ 1 - X for NiO/ZrO,-HYB, respectively. Since Ni content in the bulk was 15 ml-X,the reduction treatment decreased the surface distribution in SPR particles and increased on HYB particles. This observation is in consistent with those for S M study demonstrating homogenous higher dispersion of Nio cluster with lean diaaeter 5 20A on SPR particles and localized large Nio particles on HYB particles as given in Figure 2. .:
.-C
100
E
..r &
20
ti
a
0
before
773 073 073 1073 1 173 1273
Reduction temperature/K Figure 1. Change in XRD profiles by reduction of Ni-Ti-0-SPR. reductn.
Figure 2. SEM photographs.
399
3 - 2 -TPR The reducibility of Ni-O bonds on as-sprayed binary systems NiO/hOy were evaluated by TPR measurerent. Higher shifts of reduction temperature were observed froa 589K of pure NiO-SPR, upto 87% (Ni-Ti*) and 90% (NiNb-O) as well as for Ni-Zn-O (658K) and Ni-Hg-0 (77%). This fact MY suggest that Ni-O bonds in the binary SPR paeticles are much stabilaized by the interaction with the texture of the supporting oxides. This stabilization effects were more prominent for SPR than HYB particles. 3 - 3 , CO and H 2 adsorption The amounts of CO adsorption over Ni0/hOy after H,-treatment at 67% were in the order: Al,0s(680K)> znO(658K)> ngO(673K)) Nb2O,(903K)> ZrO, (72310) TiO, (87%). This can be explained interns of the easiness of the reduction of Ni-O bonds at 67% which is indicated by TPR reduction peak given in the parenthesis. Figure 3 caapares amounts of CO and HL adsorption over Nio/ZrO2-HYB and Nio/ZrO,-SPR particles varying H, treatment temperature. Apparently, the numbers of adsorption site on SPR particles are much saaller than those on HYB particles, in spite of smaller sizes of Nio cluster. This might be firstly due to the differece in the Nio distribution; secondly the difference in the surface areas: 1.45 n'/g for SPR catalyst and 21.9 n'/g for HYB catalyst. The presence of the optima reduction temperatures for adsoption may probably relate more to SHSI effects than to the sintering as described below. The optimum temperature was much higer over Ni-ZrQ SPR (69811 for H, and 723K for CO) than Ni-Zr-O-HYB (62% for H, and CO). These shifts MY be explained by stronger stabilization on SPR particles. The aaounts of H, adsorption obtained for Nio/ZrO, were much larger than those of CO adsorption. The presence of spill-over hydrogen right play a role again over these superfine particles.
773
773 1023 673 773 873
Redctn.TempJK Figure 4. SMSl effects on Con conversion.
Figure 3. Adsorption of CO and H2 over Nio/Zr02sereies (273K).
400
3 - 4 , M e t h a n a t i o n over N i o / M x O y Figure 4 sunarizes catalytic behavior in the methanation of CO, on Nio /MxOy-SPR. HI pretreatment temperatures were decided taking into account of TPR results. Over group I catalysts of Ni0/A1,O3 and Nio/HgO, the high tempereture reduction (HTR = 1043, 102%) was effective to increase the activities upto 9.6 and 22 times, respectivly of those subjected low temperature reduction (LTR = 77310; while for Group I1 catalysts of Nio/ TiO, and NiO/Nb,O,, HTR depressed it reversibly. However, after HTR, further successive low temperature oxidation and reduction, LTR, at 598K (H/L) their activities recovered again very dramatically. The latter two oxides are known as so-called typical S E I oxides where the catalyst support is easily reduced by the spilled over hydrogen leading to the decoration of Nio clusters with inactive partially reduced type support like TiO,.x or Nb,O,.y. 3 - 5 , Effects of a l k a l i m e t a l s Effects of alkali metal addition on CO, methanation is shown in Figure 5. Alkali containing SPR particles, Z-Nio/ZrO,-SPR, was prepared by spray reaction of ternary mixture solution of Zr-, Ni- and Z(alka1i metal)salts. HYB and IMP type particles prepared by impregnation of NiO/ZrO,-SPR (sN) and on Nio/ZrO,-lMP (iN) with appropriate solutions were treated also with H, prior to use. Interestingly, BET surface areas of SPR particles increased remarkably by a l h i addition: Na(26m*/g) and K(32 m'/g). The effects on the catalytic behavior were sumarized in Fig. 5. Over HYB series, dramatical increases in the activity for CO, conversion were obtained in the order: Li< Na < K < Rb < Cs, though CO formation became predorinated. As for IMP series the highest activity was observed for Na added one. The most reaarkable effect was obtained for Z-SPR where the maximum activity was enhanced on Na-NiO-ZrO,-SPR with 10 times higher of non-promoted NiO-ZrO,-SPR. Evidently, SPR particles extended great prolotion effects by alkalili .-'c 60 metals and still maintained high c selectivity for methanation. The - 50 pronoting effect of K-NiO/ZrO,SPR attained by the decrease in 40 E the activation energy from 80.6 dE 30 to 66.2kJ/nol, while the effects on K-Ni"/ZrO, were caused by the $ 20 % increase in pre-exponential terra. The mechanism must be interesing $10 subject for further study. a 0 23 m y o m y o "23 m y o U) z ao z 60.- z 60 4 REFERENCE c
f
v
SPR
)
I
v)
HYB
IMP
Figure 5. Effects of alkalirnetal addition on C02hydrogenation over Nio/Zr02(573K).
-
1 T.Ueaatsn, S. Shiaazu, Proc. Ist ~ A T (1990) , p.329.