Effect of Oxide Coating on Performance of Copper-Zinc Oxide-Based Catalyst for Methanol Synthesis via Hydrogenation of Carbon Dioxide Tetsuo Umegaki 1, *, Yoshiyuki Kojima 1, : and Kohji Omata 2, : Received: 3 September 2015 ; Accepted: 9 November 2015 ; Published: 16 November 2015 Academic Editor: Alfons Baiker 1 2

* :

Department of Materials & Applied Chemistry, College of Science & Engineering, Nihon University, 1-8-14, Kanda-Surugadai, Chiyoda-Ku, Tokyo 101-8308, Japan; [email protected] Department of Materials Science, Interdisciplinary Faculty of Science and Engineering, Shimane University, 1060, Nishikawatsu-Chou, Matsue, Shimane 690-8504, Japan; [email protected] Correspondence: [email protected]; Tel.: +81-3-3259-0810; Fax: +81-3-3293-7572 These authors contributed equally to this work.

Abstract: The effect of oxide coating on the activity of a copper-zinc oxide–based catalyst for methanol synthesis via the hydrogenation of carbon dioxide was investigated. A commercial catalyst was coated with various oxides by a sol-gel method. The influence of the types of promoters used in the sol-gel reaction was investigated. Temperature-programmed reduction-thermogravimetric analysis revealed that the reduction peak assigned to the copper species in the oxide-coated catalysts prepared using ammonia shifts to lower temperatures than that of the pristine catalyst; in contrast, the reduction peak shifts to higher temperatures for the catalysts prepared using L(+)-arginine. These observations indicated that the copper species were weakly bonded with the oxide and were easily reduced by using ammonia. The catalysts prepared using ammonia show higher CO2 conversion than the catalysts prepared using L(+)-arginine. Among the catalysts prepared using ammonia, the silica-coated catalyst displayed a high activity at high temperatures, while the zirconia-coated catalyst and titania-coated catalyst had high activity at low temperatures. At high temperature the conversion over the silica-coated catalyst does not significantly change with reaction temperature, while the conversion over the zirconia-coated catalyst and titania-coated catalyst decreases with reaction time. From the results of FTIR, the durability depends on hydrophilicity of the oxides. Keywords: oxide coating; copper-zinc oxide based catalyst; methanol synthesis; hydrogenation of carbon dioxide; hydrophilicity of oxides

1. Introduction Special attention has been paid to the sequestration of carbon dioxide by conversion to liquid compounds as a means of mitigating the environmental impact of this gas. In this context, the catalytic conversion of carbon dioxide to methanol via hydrogenation using heterogeneous catalysts has attracted enormous interest for its central role in carbon dioxide utilization [1–3]. Methanol can be used not only as the starting feedstock for many other useful chemicals but also as an alternative source in the production of liquid fuels. On an industrial scale, the production of methanol is generally achieved using Cu/ZnO/Al2 O3 catalysts from synthesis gas, which is obtained via the steam reforming of natural gas and mainly contains carbon monoxide and hydrogen along with a small amount of carbon dioxide [4–8]. When there is no appreciable production of dimethyl ether,

Materials 2015, 8, 7738–7744; doi:10.3390/ma8115414

Materials 2015, 8, 7738–7744

the reactions that this gas mixture undergoes are: the synthesis of methanol from carbon dioxide (Reaction (1)), the reverse water gas shift reaction, RWGS (Reaction (2)), and the “dry” methanol synthesis reaction from carbon monoxide (Reaction (3)) [9]: ∆H298

CO2 ` 3H2 ØCH3 OH ` H2 O

CO2 ` H2 ØH2 O ` CO

Materials 2015, 8, page–page 




“ ´49.58 kJ¨ mol´1

“ 41.12 kJ¨ mol´1

(Reaction  (1)),  the  reverse  water  gas  shift  reaction,  RWGS  (Reaction  (2)),  and  1 “dry”  methanol  CO ` 2H2 ØCH3 OH ∆H298 K “ ´90.55 kJ¨ mol´the  synthesis reaction from carbon monoxide (Reaction (3)) [9]: 


(2) (3)

The stability of the catalytic of the copper-based catalysts is affected by (1)  the feed −1  CO2 + 3Hactivity 2↔CH3OH + H2O  ΔH298 K = −49.58 kJ·mol containing a CO2 /CO molar ratio more than unity. Furthermore, water is produced during methanol CO2 + H2↔H2O + CO  ΔH298 K = 41.12 kJ·mol−1  (2)  synthesis via carbon dioxide hydrogenation (Reaction (1)) and RWGS (Reaction (2)) accelerates the CO + 2H2↔CH3OH  ΔH298 K = −90.55 kJ·mol−1  (3)  oxidation and deactivation of active sites, which is the influence of the water by-product produced The  stability  the  catalytic  activity  of  the  copper‐based  is  affected  by  the  feed  inactive via the hydrogenation ofof  carbon dioxide [10–14]. The number ofcatalysts  active sites decreases when containing a CO2/CO molar ratio more than unity. Furthermore, water is produced during methanol  copper compounds, such as copper oxide, sintered active copper species, and copper with strongly synthesis via carbon dioxide hydrogenation (Reaction (1)) and RWGS (Reaction (2)) accelerates the  adsorbedoxidation and deactivation of active sites, which is the influence of the water by‐product produced  water molecules, are formed. Generally, mixing or coating the catalyst with inorganic materialsvia the hydrogenation of carbon dioxide [10–14]. The number of active sites decreases when inactive  such as silica is a promising technique to reduce the negative effect of water vapor [15–17]. Thecopper compounds, such as copper oxide, sintered active copper species, and copper with strongly  present study investigated the effect of oxide coating on catalytic activity and durability of adsorbed  water  molecules,  are  formed.  Generally,  or  coating  the  inorganic  a commercial copper-zinc oxide-based catalyst for themixing  hydrogenation of catalyst  carbon with  dioxide to methanol. materials such as silica is a promising technique to reduce the negative effect of water vapor [15–17].  The influence of preparation conditions such as the type of promoter (ammonia or L(+)-arginine) The present study investigated the effect of oxide coating on catalytic activity and durability of  used for a commercial copper‐zinc oxide‐based catalyst for the hydrogenation of carbon dioxide to methanol.  the sol-gel reaction for the coating oxide on the commercial catalyst was also investigated. The  influence  of  preparation  conditions  such  as  the  type  of  promoter  (ammonia  or  L(+)‐arginine) 

2. Results and Discussion used for the sol‐gel reaction for the coating oxide on the commercial catalyst was also investigated. 

The2. Results and Discussion  morphologies of oxide-coated F04M catalysts prepared using various promoters were examined using TEM. The pristine F04M catalyst consists of finely dispersed particles with the The  morphologies  of  oxide‐coated  F04M  catalysts  prepared  using  various  promoters  were  diameters of approximately 20 nm (Figure 1a), while the oxide-coated catalysts prepared with examined  using  TEM.  The  pristine  F04M  catalyst  consists  of  finely  dispersed  particles  with  the  ammoniadiameters  consist of  ofapproximately  the particles20 ofnm  F04M catalyst loosely covered with silica with low (Figure  1a),  while  the  oxide‐coated  catalysts  prepared  with contrast ammonia  consist  of  the  particles  F04M  covered  with  silica  with  (Figure 1b,d,f). These results indicateof  that thecatalyst  F04M loosely  catalyst weakly bonded withlow  thecontrast  oxide  coating (Figure 1b,d,f). These results indicate that the F04M catalyst weakly bonded with the oxide coating  with ammonia. However, the oxide-coated catalyst prepared with L(+)-arginine consisted of the with  ammonia.  However,  the  oxide‐coated  catalyst  prepared  with  L(+)‐arginine  consisted  of  the  accumulated fine particles of pristine F04M catalyst covered with a silica (Figure 1c,e,g). These results accumulated fine particles of pristine F04M catalyst covered with a silica (Figure 1c,e,g). These results  indicate that the F04M catalyst strongly bonded with the oxide coating with L(+)-arginine. indicate that the F04M catalyst strongly bonded with the oxide coating with L(+)‐arginine. 


Figure  1.  TEM  images  of  pristine  F04M  catalyst  (a)  and  F04M  catalyst  coated  with  silica  (b,c),   

Figure 1.zirconia (d,e), and titania (f,g) using ammonia (b,d,f) and L(+)‐arginine (c,e,g).  TEM images of pristine F04M catalyst (a) and F04M catalyst coated with silica (b,c), zirconia (d,e), and titania (f,g) using ammonia (b,d,f) and L(+)-arginine (c,e,g). In  order  to  obtain  information  about  the  reducibility  of  active  copper  species,    temperature‐programmed  reduction  (TPR)  profiles  observed  upon  treatment  of  the  as‐prepared  In order to obtain information about the reducibility of active copper species, samples  in  H2  are  obtained  by  thermogravimetric  analysis  (TGA).  Figure  2  shows  the  derivative  temperature-programmed reduction (TPR) profiles observed upon treatment of the as-prepared thermogravimetry (DTG) curves of the as‐prepared samples. The curves contained one main domain  samples near 560 K. From the reduction profile of pristine F04M catalyst in Figure 2, clearly defined peaks  in H2 are obtained by thermogravimetric analysis (TGA). Figure 2 shows the derivative assigned to copper species at around 560 K appeared in the DTG curve [18–24]. Compared with the  thermogravimetry (DTG) curves of the as-prepared samples. The curves contained one main domain result of the pristine F04M catalyst, the peak temperatures in the profile of the oxide‐coated catalysts  using  ammonia  were  lower  (Figure  2A),  while  the  temperatures  in  the  profile  of  oxide‐coated 

7739 2

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near 560 K. From the reduction profile of pristine F04M catalyst in Figure 2, clearly defined peaks assigned to copper species at around 560 K appeared in the DTG curve [18–24]. Compared with the result of the pristine F04M catalyst, the peak temperatures in the profile of the oxide-coated catalysts using ammonia were lower (Figure 2A), while the temperatures in the profile of oxide-coated catalysts using L(+)-arginine were higher (Figure 2B), and the reduction peak of the copper species of the oxide-coated catalysts was very broad. In other words, the temperatures depend on the strength Materials 2015, 8, page–page  of the bond between the oxide and F04M catalyst, and the bond strength of the samples prepared Materials 2015, 8, page–page  with L(+)-arginine was higher than that of the samples prepared with ammonia. Figure 2A,B also catalysts using L(+)‐arginine were higher (Figure 2B), and the reduction peak of the copper species of  the oxide‐coated catalysts was very broad. In other words, the temperatures depend on the strength  show temperature peaks at 553.6, 547.1, and 555.0 K for the catalysts coated with silica, zirconia, and catalysts using L(+)‐arginine were higher (Figure 2B), and the reduction peak of the copper species of  of the bond between the oxide and F04M catalyst, and the bond strength of the samples prepared  the oxide‐coated catalysts was very broad. In other words, the temperatures depend on the strength  titania using ammonia, respectively. On the other hand, the temperature peaks of the catalysts coated with L(+)‐arginine was higher than that of the samples prepared with ammonia. Figure 2A,B also  of the bond between the oxide and F04M catalyst, and the bond strength of the samples prepared  with silica, zirconia, and titania using L(+)-arginine were 585.0, 591.0, and 590.0 K, respectively. It is show temperature peaks at 553.6, 547.1, and 555.0 K for the catalysts coated with silica, zirconia, and  with L(+)‐arginine was higher than that of the samples prepared with ammonia. Figure 2A,B also  reportedtitania using ammonia, respectively. On the other hand, the temperature peaks of the catalysts coated  that reconstruction of the active copper particles occurred with the addition of TiO2 [25–27]. show temperature peaks at 553.6, 547.1, and 555.0 K for the catalysts coated with silica, zirconia, and  with  silica,  zirconia,  titania  using  L(+)‐arginine  were in 585.0,  590.0  K,  respectively.  It  is  using The result indicates thatand  the active copper species the591.0,  TiOand  catalyst prepared 2 -coated titania using ammonia, respectively. On the other hand, the temperature peaks of the catalysts coated  reported that reconstruction of the active copper particles occurred with the addition of TiO 2  [25–27].  with  silica,  zirconia,  and dispersion titania  using than L(+)‐arginine  were  591.0, catalyst. and  590.0 These K,  respectively.  It  is  L(+)-arginine show broader those in the585.0,  pristine results indicate that The  result  indicates  that  the  active  copper  species  in  the  TiO2‐coated  catalyst  prepared  using    reported that reconstruction of the active copper particles occurred with the addition of TiO 2 [25–27].  the temperatures depend on the oxide coating over the catalyst. L(+)‐arginine show broader dispersion than those in the pristine catalyst. These results indicate that  The  result  indicates  that  the  active  copper  species  in  the  TiO2‐coated  catalyst  prepared  using    Methanol synthesis from the hydrogenation of carbon dioxide was tested over the oxide-coated the temperatures depend on the oxide coating over the catalyst.  L(+)‐arginine show broader dispersion than those in the pristine catalyst. These results indicate that  catalyststhe temperatures depend on the oxide coating over the catalyst.  after pre-reduction procedures. Figure 3 shows the temperature dependence of the Methanol synthesis from the hydrogenation of carbon dioxide was tested over the oxide‐coated  catalysts  after  pre‐reduction  procedures.  Figure  3  shows  the  temperature  dependence  of  the  on the conversion of carbon dioxide over the oxide-coated catalysts. The conversion depends Methanol synthesis from the hydrogenation of carbon dioxide was tested over the oxide‐coated  conversion  of  carbon  dioxide  over  the  oxide‐coated  catalysts.  The  conversion  depends  on  pre‐reduction  procedures.  3  shows  the  temperature  dependence catalysts of  the  the  using catalysts.catalysts  In theafter  entire temperature region, Figure  the conversion over the oxide-coated catalysts.  In of  the  entire dioxide  temperature  region,  the  conversion  over The  the  oxide‐coated  catalysts on  using  conversion  carbon  over  the  oxide‐coated  catalysts.  conversion  depends  the  that the ammonia was higher than that over the catalysts using L(+)-arginine. These results show ammonia  higher  than  that  over  region,  the  catalysts  using  L(+)‐arginine.  These  results catalysts  show  that  the  catalysts.  was  In  the  entire  temperature  the  conversion  over  the  oxide‐coated  using  bond strength between the oxide and the commercial catalyst influences the catalytic activity. bond strength between the oxide and the commercial catalyst influences the catalytic activity.    ammonia  was  higher  than  that  over  the  catalysts  using  L(+)‐arginine.  These  results  show  that  the  bond strength between the oxide and the commercial catalyst influences the catalytic activity.    (A)

(B) F04M





DTGDTG [a.u.][a.u.]

F04M DTGDTG [a.u.][a.u.]

(a) (a) (b) (b) (c)

(d) (d) (e) (e) (f) (f)

(c) 373


573 673 373 473 573 673 Temp. [K] Temp. [K] 373 473 573 673 373 473 573 673 Temp. [K] Temp. [K] Figure 2. DTG curve registered upon H 2‐TPR of F04M catalyst coated with silica (a,d), zirconia (b,e),  2. DTG curve registered upon H2 -TPR of F04M catalyst coated with silica (a,d), zirconia and titinia (c,f) using ammonia (A) and L(+)‐arginine (B). 

Figure Figure 2. DTG curve registered upon H2‐TPR of F04M catalyst coated with silica (a,d), zirconia (b,e),  and titinia (c,f) using ammonia (A) and L(+)-arginine (B). and titinia (c,f) using ammonia (A) and L(+)‐arginine (B).  25 25

CO2 conversion CO2 conversion [%] [%]

20 20

(b) (b)



(f) (f) (d)


15 15


(d) (g) (g)


(e) (e)





0 460 0 460

480 500 520 Reaction Temp. [K] 480 500 520 Reaction Temp. [K] Figure 3. CO2 conversion over pristine F04M catalyst (a) and F04M catalyst coated with silica (b,c), 


zirconia (d,e), and titania (f,g) using ammonia (b,d,f) and L(+)‐arginine (c,e,g). Reaction conditions:  Figure 3. CO2 conversion over pristine F04M catalyst (a) and F04M catalyst coated with silica (b,c),   

−1·mol −1, Hpristine Figure 3.3 MPa, W/F = 5 g·h CO2 conversion over F04M catalyst (a) and F04M catalyst coated with silica (b,c), 2/CO2/Ar = 72/24/4 (molar ratio).  zirconia (d,e), and titania (f,g) using ammonia (b,d,f) and L(+)‐arginine (c,e,g). Reaction conditions:    zirconia 3 MPa, W/F = 5 g·h (d,e), and titania (f,g) using ammonia (b,d,f) and L(+)-arginine (c,e,g). Reaction conditions: −1·mol −1, H 2/CO2/Ar = 72/24/4 (molar ratio).  ´1 ¨ conversion  Comparing  among  the  oxide  coatings,  the  silica‐coated  catalyst  showed  the  3 MPa, W/F = 5 g¨ hthe  mol´1 , H2 /CO 2 /Ar = 72/24/4 (molar ratio).

highest activity in the high temperature region, while the zirconia‐coated catalyst and titania‐coated  Comparing  the  conversion  among  the  oxide  coatings,  the  silica‐coated  catalyst  showed  the  highest activity in the high temperature region, while the zirconia‐coated catalyst and titania‐coated  3 3 7740

Materials 2015, 8, 7738–7744

Comparing the conversion Materials 2015, 8, page–page 

among the oxide coatings, the silica-coated catalyst showed the highest activity in the high temperature region, while the zirconia-coated catalyst and titania-coated catalyst had high activity in the low temperature region. These results indicate that the temperature catalyst had high activity in the low temperature region. These results indicate that the temperature  profile of the conversion depends on the nature of the catalyst’s oxide coating. profile of the conversion depends on the nature of the catalyst’s oxide coating.  In order to verify the durability of the oxide-coated catalysts, they were tested for long reaction In order to verify the durability of the oxide‐coated catalysts, they were tested for long reaction  times. Figure shows thethe  conversion of carbon dioxide over theover  oxide-coated catalyst using ammonia. times.  Figure 4 4  shows  conversion  of  carbon  dioxide  the  oxide‐coated  catalyst  using  For comparison, the conversion over pristine F04M catalyst is also shown in this figure. The ammonia. For comparison, the conversion over pristine F04M catalyst is also shown in this figure.  time course of the conversion depends on the catalyst’s oxide coating. The conversion over the The time course of the conversion depends on the catalyst’s oxide coating. The conversion over the  silica-coated catalyst does not significantly change in terms of reaction time, while the conversion silica‐coated catalyst does not significantly change in terms of reaction time, while the conversion  over the zirconia-coated catalyst and the titania-coated catalyst decrease up to 2 h before remaining over the zirconia‐coated catalyst and the titania‐coated catalyst decrease up to 2 h before remaining  constant up to 8 h. These results indicated that the silica-coated catalyst has a suitable activity for constant up to 8 h. These results indicated that the silica‐coated catalyst has a suitable activity for  long reaction silica-coated catalyst shows almost the same conversion per weight of F04 of  M  long  reaction times. times. The The  silica‐coated  catalyst  shows  almost  the  same  conversion  per  weight  («23%) compared with the F04M catalyst, and the silica-coated catalyst did not significantly reduce F04 M (≈23%) compared with the F04M catalyst, and the silica‐coated catalyst did not significantly  the catalytic activity. Table 1 shows product selectivity over the pristine F04M and the oxide-coated reduce the catalytic activity. Table 1 shows product selectivity over the pristine F04M and the oxide‐coated  catalysts. As shown in Table 1, carbon monoxide and methanol are produced over all of the catalysts. catalysts. As shown in Table 1, carbon monoxide and methanol are produced over all of the catalysts.  Methanol selectivity over the silica-coated catalyst and the titania-coated catalyst was higher than Methanol selectivity over the silica‐coated catalyst and the titania‐coated catalyst was higher than  that over the pristine F04M catalyst, while the selectivity over the zirconia-coated catalyst was the that over the pristine F04M catalyst, while the selectivity over the zirconia‐coated catalyst was the  same as that over the pristine F04M catalyst. These results indicated that silica and titania coatings on same as that over the pristine F04M catalyst. These results indicated that silica and titania coatings  the F04M catalyst effectively improved methanol selectivity. It is reported that the addition of TiO2  on the F04M catalyst effectively improved methanol selectivity. It is reported that the addition of TiO improves the methanol selectivity because of the reconstruction of the active copper particles and/or improves the methanol selectivity because of the reconstruction of the active copper particles and/or  its acid-base property, which controls the stability of the reaction intermediate [25–27]. its acid‐base property, which controls the stability of the reaction intermediate [25–27]. 

  Figure 4. Time course of CO Figure 4. Time course of CO22 conversion over pristine F04M catalyst (a) and F04M catalyst coated  conversion over pristine F04M catalyst (a) and F04M catalyst coated −1,  ´1 −1 with  (b), zirconia zirconia (c), (c), and and titania titania  (d).  Reaction  conditions:  523  3  MPa,  W/F  ·mol´1 with silica  silica (b), (d). Reaction conditions: 523 K, K,  3 MPa, W/F = 5= g¨5 hg·h ¨ mol , 2/Ar = 72/24/4 (molar ratio).  H H22/CO /CO /Ar = 72/24/4 (molar ratio). 2 Table 1. Product selectivity over the various catalysts.  Table 1. Product selectivity over the various catalysts.

Catalyst  CO Selectivity (%) Catalyst CO Selectivity (%) F04M  62.1  F04M 62.1 SiO2 coat  53.1  SiO2 coat 53.1 ZrO 2 coat  61.7  ZrO2 coat 61.7 TiOTiO 2 coat  45.3  coat 45.3 2

CH3OH Selectivity (%)  37.9  37.9 43.9  43.9 38.3  38.3 54.7  54.7

CH3 OH Selectivity (%)

−1·mol ´1−1, , H Reaction conditions: 523 K, 3 MPa, W/F = 5 g·h 2/CO2/Ar /Ar = 72/24/4 (molar ratio).  Reaction conditions: 523 K, 3 MPa, W/F = 5 g¨ h´1 ¨ mol H2 /CO = 72/24/4 (molar ratio). 2

In order to determine differences in activity, the oxide‐coated catalysts were characterized by  In order to determine differences in activity, the oxide-coated catalysts were characterized by FTIR spectroscopy to assess their hydrophilicity. Figure 5 shows the FTIR spectra for the oxide‐coated  FTIR spectroscopy to assess their hydrophilicity. Figure 5 shows the FTIR spectra for the oxide-coated catalysts  prepared  with  ammonia.  The  zirconia‐coated  catalyst  and  the  titania‐coated  catalyst  displayed a band of the H–O–H bond at around 1640 cm−1 [28], while the silica‐coated catalyst lacked  this band. These results indicated that the silica‐coated catalyst had a relatively low hydrophilicity  7741 compared with the zirconia‐coated catalyst and the titania‐coated catalyst because the oxide with low  4

Materials 2015, 8, 7738–7744

catalysts prepared with ammonia. The zirconia-coated catalyst and the titania-coated catalyst displayed a band of the H–O–H bond at around 1640 cm´1 [28], while the silica-coated catalyst lacked this band. These results indicated that the silica-coated catalyst had a relatively low hydrophilicity Materials 2015, 8, page–page  compared with the zirconia-coated catalyst and the titania-coated catalyst because the oxide with low hydrophilicity probably reduced the influence of the steam by-product. Therefore, the silica-coated hydrophilicity probably reduced the influence of the steam by‐product. Therefore, the silica‐coated  catalystcatalyst had a higher durability than the zirconia‐coated catalyst or titania‐coated catalyst.  had a higher durability than the zirconia-coated catalyst or titania-coated catalyst. (c)

Intensity [a.u.]

(b) (a)




Wave number [cm-1]



FigureFigure 5. FTIR spectra of F04M catalyst coated with silica (a), zirconia (b), and titania (c).  5. FTIR spectra of F04M catalyst coated with silica (a), zirconia (b), and titania (c). 3. Experimental Section 

3. Experimental Section

3.1. Catalyst Preparation 

3.1. Catalyst Preparation

Various  oxides  (silica,  zirconia,  titania)  were  coated  on  commercial  copper‐zinc  oxide‐based 

catalyst (JGC Catal. Chem. Ltd.; Kanagawa, Japan, F04M) by a sol‐gel method. An aqueous solution  Various oxides (silica, zirconia, titania) were coated on commercial copper-zinc oxide-based catalystof ammonia (Kanto Chemical Co., Tokyo, Japan, 28 wt % solution, >99.0%) or L(+)‐arginine (Wako  (JGC Catal. Chem. Ltd.; Kanagawa, Japan, F04M) by a sol-gel method. An aqueous Chemical  Co.,  Osaka,  Japan,  >99.0%)  were  mixed  with  0.5  g  of  F04M  in  ethyl  alcohol,  then  solution of ammonia (Kanto Chemical Co., Tokyo, Japan, 28 wt % solution, >99.0%) or L(+)-arginine tetraethoxysilane (Wako Pure Chem. Co., Osaka, Japan, >99.0%), zirconium butoxide (Sigma‐Aldrich  (Wako Co. St. Louis, MO, USA, LLC., 80 wt % solution in 1‐butanol), or titanium‐n‐butoxide (Wako Pure  Chemical Co., Osaka, Japan, >99.0%) were mixed with 0.5 g of F04M in ethyl alcohol, then tetraethoxysilane (Wako Pure Chem. Co., Osaka, Japan, >99.0%), zirconium butoxide Chem. Co., Osaka, Japan, >97.0%) was added to the solution, followed by stirring at 323 K for 3 h and  centrifuging at a rotation speed of 6000 rpm for 5 min.  (Sigma-Aldrich Co. St. Louis, MO, USA, LLC., 80 wt  % solution in 1-butanol), or titanium-n-butoxide (Wako Pure Chem. Co., Osaka, Japan, >97.0%) was added to the solution, followed by stirring at 3.2. Characterization  323 K for 3 h and centrifuging at a rotation speed of 6000 rpm for 5 min. The  morphology  of  the  oxide‐coated  catalysts  was  observed  using  a  Hitachi  FE2000  (Hitachi  High‐Tech. Co., Tokyo, Japan) transmission electron microscope (TEM) operated at an acceleration  3.2. Characterization voltage of 200 kV. The temperature‐programmed reduction–thermogravimetric analyses (TPR–TGA)  Thewere performed with a Rigaku TG8120 (Rigaku Co., Tokyo, Japan) instrument. TPR profiles were  morphology of the oxide-coated catalysts was observed using a Hitachi FE2000 by  passing  a  10  vol  %  HJapan) 2  in  Ar  (260  mL·min−1)  through  the microscope sample  (~2  mg)  heated  at  a  (Hitachirecorded  High-Tech. Co., Tokyo, transmission electron (TEM) operated at −1 constant rate of 5 K·min  up to 1173 K.  an acceleration voltage of 200 kV. The temperature-programmed reduction–thermogravimetric

analyses (TPR–TGA) were performed with a Rigaku TG8120 (Rigaku Co., Tokyo, Japan) instrument. 3.3. Catalytic Activity  TPR profiles were recorded by passing a 10 vol % H2 in Ar (260 mL¨ min´1 ) through the sample The catalysts were evaluated in a tubular stainless steel, fixed‐bed reactor (1/2′′ i.d) equipped  (~2 mg) heated at a constant rate of 5 K¨ min´1 up to 1173 K.

with a temperature‐programmed control unit and a K‐type thermocouple. Each sample was loaded  between  two  layers  of  quartz  wool  in  the  reactor.  Before  experiment,  the  catalyst  was  firstly    3.3. Catalytic Activity pre‐reduced at 553 K and atmospheric pressure in a hydrogen stream (1 vol % H2 in Ar) and cooled  2/COsteel, 2/Ar = 72/24/4 (molar ratio)) was introduced  Theto desired temperature. Subsequently, synthesis gas (H catalysts were evaluated in a tubular stainless fixed-bed reactor (1/211 i.d) equipped −1 −1 at W/F = 5 g·h ·mol , at a pressure of 3.0 MPa using a mass flow controller (Brookhaven). The effluent  with a temperature-programmed control unit and a K-type thermocouple. Each sample was loaded gases  were  analyzed  by  two  GC‐TCD  (Shimadzu  GC‐8A,  Shincarbon  ST  column  and  Shimadzu    between two layers of quartz wool in the reactor. Before experiment, the catalyst was firstly GC‐8A,  FlusinT  column).  Conversion  and  selectivity  were  calculated  by  internal  standard  and    pre-reduced at 553 K and atmospheric pressure in a hydrogen stream (1 vol % H2 in Ar) and mass‐balance methods. CO 2 conversion was calculated as follow:   

cooled to desired temperature. Subsequently, synthesis gas (H2 /CO2 /Ar = 72/24/4 (molar ratio)) 5 was introduced at W/F = 5 g¨ h´1 ¨ mol´1 , at a pressure of 3.0 MPa using a mass flow controller 7742

Materials 2015, 8, 7738–7744

(Brookhaven). The effluent gases were analyzed by two GC-TCD (Shimadzu GC-8A, Shincarbon ST column and Shimadzu GC-8A, FlusinT column). Conversion and selectivity were calculated by internal standard and mass-balance methods. CO2 conversion was calculated as follow: CO2 conversion “ 1 ´ FrCO2 sout {FrCO2 sin Maximum error margin of the conversion in this study was ˘ 7%. 4. Conclusions This study investigated the effect of oxide coatings on the activity of copper-zinc oxide-based catalyst for methanol synthesis via the hydrogenation of carbon dioxide. A commercial copper-zinc oxide-based catalyst was coated with various oxides (silica, zirconia, titania) by a sol-gel based method. In this study, the influence of preparation conditions such as kinds of promoters (ammonia or L(+)-arginine) on the sol-gel reaction was investigated. From the result of TPR-TGA, the reduction peak assigned to the copper species of the oxide-coated samples prepared using ammonia shifted to lower temperatures than that of the pristine F04M catalyst, while the copper reduction peaks shifted to higher temperatures for the oxide-coated samples prepared using L(+)-arginine, indicating that the copper species that were more weakly bonded with oxide were more easily reduced. The coated catalyst prepared with ammonia shows a higher conversion of carbon dioxide than the coated catalyst prepared with L(+)-arginine, indicating that the catalyst with a weaker oxide bond has a higher activity than the catalyst with a strong oxide bond. Among the oxide-coated catalysts prepared using ammonia, the silica-coated catalyst displayed the highest activity in the high reaction temperature region, while the zirconia-coated and titania-coated catalysts had high activities at low reaction temperatures. At high reaction temperatures, the conversion over the silica-coated catalyst did not significantly change with the reaction time, while the conversion over the zirconia-coated and titania-coated catalysts decreased during the initial reaction time, indicating that the silica-coated catalysts have a more suitable activity for long reaction times. The zirconia-coated catalyst and the titania-coated catalyst displayed the band of the H–O–H bond, while the silica-coated catalyst did not display the band, indicating that the silica-coated catalyst had a relatively low hydrophilicity compared with the zirconia-coated and the titania-coated catalysts. The silica-coated catalyst, therefore, had a high durability compared with the other oxide-coated catalysts. Acknowledgments: This work was supported by an ALCA (Advanced Low Carbon Technology Research and Development) project of Japan Science Technology Agency (JST) for funding. Author Contributions: Tetsuo Umegaki conceived and designed the experiments; Tetsuo Umegaki performed the experiments; Tetsuo Umegaki, Yoshiyuki Kojima, Kohji Omata analyzed the data. Conflicts of Interest: The authors declare no conflict of interest.

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Effect of Oxide Coating on Performance of Copper-Zinc Oxide-Based Catalyst for Methanol Synthesis via Hydrogenation of Carbon Dioxide.

The effect of oxide coating on the activity of a copper-zinc oxide-based catalyst for methanol synthesis via the hydrogenation of carbon dioxide was i...
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