Temperature tunable mesoporous gold deposited CO oxidation catalyst

Abstract

The present invention discloses a novel mesoporous gold deposited oxidation catalyst of formula: XAu-M.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 wherein X=0.01-10%, M is selected from Cu, Co or Mn and process for the preparation thereof by photodeposition method.

Claims

1. A mesoporous gold deposited oxidation catalyst of formula Au-M.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, characterized in that gold deposited on the catalyst is in the form of nanoscale particles and the mesoporous gold deposited oxidation catalyst has between 0.01-10 atomic wt % of gold, wherein M is selected from Cu, Co or Mn and the catalyst is a crystalline compound exhibiting a fluorite cubic crystal lattice structure, and M is doped in the lattice.

2. The mesoporous gold deposited oxidation catalyst according to claim 1, wherein the gold deposited on the catalyst is in the range of 0.1-3 atomic wt % of gold.

3. A process for the preparation of mesoporous gold deposited oxidation catalyst according to claim 1, comprising the steps of: a) adding M(NO.sub.3).sub.2.3H.sub.2O, Ce(NO.sub.3).sub.3.6H.sub.2O and ZrOCl.sub.2.8H.sub.2O into the ethanol solution of triblock copolymer (P-123) with stirring for period in the range of 30 minutes to 2 hours to obtain transparent coloured sol; b) aging the transparent coloured sol for period in the range of 46 to 48 hours at temperature in the range of 40 to 45 C.; c) drying the aged sol as obtained in step (b) at temperature in the range of 90 to 110 C. for period in the range of 20 to 24 hours followed by calcining at temperature in the range of 350 to 400 C. for period in the range of 2 to 4 hours with the ramping rate of 1 C./min to get the M.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 mesoporous mixed oxide; d) adding water containing HAuCl.sub.4.3H.sub.2O solution into methanolic solution of M.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 mesoporous mixed oxide as obtained in step (c); e) irradiating the reaction mixture of step (d) under UV light for period in the range of 30 minutes to 2 hours and the solution is allowed to settle down and then decanted; f) centrifuging the remaining solution of step (e) with water and ethanol at speed in the range of 4000 to 6000 rotations per minute (rpm) for period in the range of 8 to 10 minutes followed by drying to obtain mesoporous gold deposited oxidation catalyst.

4. The process as claimed in claim 3, wherein drying in step (c) is carried out at temperature in the range of 90 to 110 C. for period in the range of 20 to 24 hours.

5. The process as claimed in claim 3, wherein drying in step (f) is carried out at temperature in the range of 40 to 45 C. for period in the range of 8 to 16 hours and at temperature in the range of 80 to 100 C. for period in the range of 8 to 12 hours.

6. The mesoporous gold deposited oxidation catalyst according to claim 1, wherein said catalyst is useful for oxidation of CO.

7. The mesoporous gold deposited oxidation catalyst according to claim 6, wherein said catalyst exhibit CO conversion efficiency in the range of 50 to 100% between 35 and 69 C.

8. The mesoporous gold deposited oxidation catalyst according to claim 1, wherein the mesoporous gold deposited oxidation catalyst has pore diameters ranging from 37.8 to 51.0 .

9. The mesoporous gold deposited oxidation catalyst according to claim 1, wherein the mesoporous gold deposited oxidation catalyst has pore diameters ranging from 4 to 7 nm.

10. The mesoporous gold deposited oxidation catalyst according to claim 1, wherein the mesoporous gold deposited oxidation catalyst has a unimodal pore size distribution.

11. The mesoporous gold deposited oxidation catalyst according to claim 1, wherein the mesoporous gold deposited oxidation catalyst is AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 with 0.1 atomic wt % of gold.

12. The mesoporous gold deposited oxidation catalyst according to claim 1, wherein the mesoporous gold deposited oxidation catalyst is AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 with 0.6 atomic wt % of gold.

13. The mesoporous gold deposited oxidation catalyst according to claim 1, wherein the gold particles are less than 5 nm in size.

14. The process as claimed in claim 3, wherein the obtained mesoporous gold deposited oxidation catalyst has pore diameters ranging from 37.8 to 51.0 .

15. The process as claimed in claim 3, wherein the obtained mesoporous gold deposited oxidation catalyst has pore diameters ranging from 4 to 7 nm.

16. The process as claimed in claim 3, wherein the mesoporous gold deposited oxidation catalyst has a unimodal pore size distribution.

17. A mesoporous oxidation catalyst of formula AuM.sub.0.1Ce.sub.0.85Zr.sub.0.05O .sub.2, wherein M is selected from Cu, Co or Mn; the gold is deposited on the M.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 in the form of nanoscale particles; the oxidation catalyst has between 0.01-10 atomic wt % of gold; the oxidation catalyst exhibits a type IV N.sub.2(g) adsorption-desorption isotherm; the catalyst is a crystalline compound exhibiting a fluorite cubic crystal lattice structure; and M is doped in the lattice.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1: General schematic representation of the synthesis of Mesoporous Cu.sub.0.1Ce.sub.0.9-xZr.sub.xO.sub.2 (x=0.5 to 0.85).

(2) FIG. 2: Low angle XRD of mesoporous Cu.sub.0.1Ce.sub.0.9-xZr.sub.xO.sub.2 (x=0.05 to 0.85) materials.

(3) FIG. 3: Wide angle XRD pattern of mesoporous Cu.sub.0.1Ce.sub.0.9-xZr.sub.xO.sub.2 (x=0.05 to 0.85) materials.

(4) FIG. 4: Raman Spectra of mesoporous Cu.sub.0.1Ce.sub.0.9-xZr.sub.xO.sub.2 (x=0.05 to 0.85) materials

(5) FIG. 5: Temperature programmed reduction (TPR) analysis of mesoporous Cu.sub.0.1Ce.sub.0.9-xZr.sub.xO.sub.2 (x=0.05 to 0.85) materials.

(6) FIG. 6: N.sub.2 adsorption-desorption isotherm and pore size distribution of mesoporous Cu.sub.0.1Ce.sub.0.9-xZr.sub.xO.sub.2 (x=0.05 to 0.85) materials.

(7) FIG. 7: TEM images of mesoporous Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst

(8) FIG. 8: CO oxidation with 1:5 ratio of CO:O.sub.2 at 6000 GHSV (gas hourly space velocity) over mesoporous Cu.sub.0.1Ce.sub.0.9-xZr.sub.xO.sub.2 (x=0.05 to 0.85) materials.

(9) FIG. 9: CO oxidation with 1:2 ratio of CO:O.sub.2 at 6000 GHSV over mesoporous Cu.sub.0.1Ce.sub.0.9-xZr.sub.xO.sub.2 (x=0.05 to 0.85) materials

(10) FIG. 10: CO oxidation with 1:5 ratio of CO:O.sub.2 at different GHSV over mesoporous Cu.sub.0.1Ce.sub.0.9-xZr.sub.xO.sub.2 (x=0.05 to 0.85) materials.

(11) FIG. 11: Wide angle XRD pattern of Au deposited on mesoporous Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalysts.

(12) FIG. 12: Raman Spectra of Au on Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst.

(13) FIG. 13: TPR analysis of Au on Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst.

(14) FIG. 14: N.sub.2 adsorption-desorption isotherm of Au on mesoporous Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst

(15) FIG. 15: TEM images of mesoporous 0.5AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst.

(16) FIG. 16: CO oxidation with 1:5 ratio of CO:O.sub.2 at 6000 GHSV over Au on Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst.

(17) FIG. 17: A cartoon depicting the possible surface structure and mechanistic changes occurring under reaction conditions. Carbon and oxygen in CO, O2 and CO2 is depicted as small grey and red solid circles.

(18) FIG. 18: Recyclability of 0.6AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst with 1:5 ratio of CO:O.sub.2 at 6000 GHSV.

(19) FIG. 19: Powder XRD pattern of Ce.sub.0.9Zr.sub.0.1O.sub.2, Cu Ce0.85Zr.sub.0.05O.sub.2, 0.1 AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, and 0.6AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalysts. Inset shows an increase in line broadening from Ce.sub.0.9Zr.sub.0.1O.sub.2 to 0.6AU-Cu.sub.0.1 Ce.sub.0.85Zr.sub.0.05O.sub.2.

(20) FIG. 20: Raman spectral analysis of Ce.sub.0.9Zr.sub.0.1O.sub.2, Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, 0.1AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, and 0.6AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2.

(21) FIG. 21: (a) N.sub.2 adsorption-desorption isotherms, and (b) pore size distribution of Ce.sub.0.9Zr.sub.0.1O.sub.2) Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, 0.1AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, and 0.5AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 mesoporous catalysts.

(22) FIG. 22: TPR of Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, 0.1AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 and 0.6AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalysts. Dashed line indicates the onset of reduction of all catalysts.

(23) FIG. 23: XPS spectra recorded for virgin and Au-deposited Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, catalysts for (a) Ce 3d, (b) Cu 2p, (c) O 1s core levels. Au 4f core level is shown in inset in panel b for 0.6Au-Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalysts. All Ce.sub.3+ features are indicated by dashed arrows in panel a. Cu.sup.+ and Cu.sup.2+ feature BE is indicated by dotted lines in panel b. It is also to be noted that Au 4f.sub.7/2 core level appears at 83.0 eV, indicating the anionic nature of gold nanoclusters.

(24) FIG. 24: CO oxidation catalytic activity measured for (a) Ce.sub.0.9Zr.sub.0.1O.sub.2 (b) Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 (c) 0.1AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, (d) 0.6AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, and (e) 0.6AuCe.sub.0.9Zr.sub.0.1O.sub.2. CO+O.sub.2 reaction was carried out with 1:5 ratio of CO:O.sub.2 at 6000 GHSV.

(25) FIG. 25: HRTEM images of mesoporous Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 and Au-Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalysts.

DETAILED DESCRIPTION OF THE INVENTION

(26) Present invention provides a mesoporous Au deposited oxidation catalyst of formula XAu-M.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 wherein X=0.01-10%, M is selected from Cu, Co or Mn characterized in that gold deposited on the catalyst in nano form in the range of 0.01-10 atomic wt % of gold, preferably 0.1-3 atomic wt % of gold and demonstrates their application in CO oxidation.

(27) Present invention provides a process for preparation of mesoporous Au deposited oxidation catalyst of formula XAu-M.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 wherein X=0.01-10%, M is selected from Cu, Co, Mn using photodeposition method.

(28) The mesoporous gold deposited oxidation catalysts of formula)(Au-M.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 are selected from 0.1AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, 0.5AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, 1AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 and 3AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, 0.5AuCo.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, 0.1AuMn.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, 0.5AuMn.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, 1AuCo.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2

(29) Present invention provides a process for the preparation of mesoporous gold deposited oxidation catalyst of formula XAu-M.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 wherein X=0.01-10% M is selected from Cu, Co, Mn comprising the steps of: a) adding water containing HAuCl.sub.4.3H.sub.2O solution to M.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst dissolved in methanol; b) irradiating the reaction mixture of step (a) under UV light for period in the range of 30 min. to 2 hrs and the solution is allowed to settle down and then decanted c) centrifuging the remaining solution of step (b) with water and ethanol at speed in the range of 4000 to 6000 rpm for period in the range of 8 to 10 min followed by drying to obtain mesoporous gold deposited oxidation catalyst.

(30) The drying in oven in step (c) is carried out at temperature in the range of 40 to 45 C. for period in the range of 8 to 16 h and at temperature in the range of 80 to 100 C. for period in the range of 8 to 12 h to get desired catalyst.

(31) The gold deposited catalysts (XAu-M.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2) were evaluated for CO oxidation catalysis. The oxidation catalysis begins at ambient temperatures and a steep rise in CO oxidation activity can be witnessed in FIG. 25 for 0.1Au and 0.6Au containing catalysts with 100 (50) % CO conversion temperatures are at 65 (48) C. and 47 (36) C., respectively. As in the earlier case, CO oxidation activity onset can be tuned by varying the gold content from 0 to 0.6 wt %. However, further increase in gold content to 1 wt % increases the CO oxidation onset temperature. 0.6AuCe.sub.0.9Zr.sub.0.1O.sub.2 catalyst shows a different trend in activity. Although ambient temperature activity was observed, like the above catalysts, only 30% CO conversion was observed; 100% (50%) CO conversion was observed at rather 285 (210) C. very high temperatures. Above observation underscores the role of Cu in bringing down 100% CO conversion temperature and its part in facilitating the same in XAuCU.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2.

(32) The recyclability of the 0.6AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst was tested for five cycles by simply cooling down the reactor temperature after each reaction to a maximum temperature of 150 C. and without any further treatment (see FIG. 19). Catalyst was held at 150 C. for 60 min at the end of each cycle. Very similar CO oxidation catalytic activity was observed without any significant difference in the activity in each cycle implies the efficacy of the mesoporous catalysts. It also underscores that the catalysts does not undergo any structural or microstructural changes during the repeated activity evaluation.

(33) In another preferred embodiment, the present invention provides gold deposited mesoporous oxidation catalyst from mesoporous oxidation catalyst wherein the physiochemical characteristics are as shown below in Table 2.

(34) TABLE-US-00001 TABLE 2 Physicochemical characteristics of mesoprous XAu-M.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst Cryst. S.sub.BET Pore V.sub.p CO conversation Au Material Size m.sup.2g.sup.1 size () (mL g.sup.1) T.sub.50 (T.sub.100) C. (Mole %) Ce.sub.0.9Zr.sub.0.1O.sub.2 12.4 140.1 55.45 0.1871 Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 8.4 131.7 52.12 0.1716 77 (120) 0.1AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 7.9 95.8 39.5 0.0912 48 (64) 0.092 0.5AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 5.2 87.2 47.3 0.1028 35 (45) 0.514 1AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 7.3 84.8 49.4 0.1240 39 (55) 0.987 3AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 15.8 79.2 37.8 0.0853 52 (69) 3.126 1AuCo.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 11.1 121.0 44.4 0.121 45 (62) 1.01 0.5AuMnCo.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 8.8 94.7 51.0 0.105 51 (69) 0.48

EXAMPLES

(35) Following examples are given by way of illustration and therefore should not be construed to limit the scope of the invention.

Example 1

(36) a. Synthesis of Meso-Cu.sub.0.1Ce.sub.0.9-xZr.sub.xO.sub.2 (x=0.5 to 0.85)

(37) Following starting materials, Ce(NO.sub.3).sub.3.6H.sub.2O, ZrOCl.sub.2.9H.sub.2O and Cu(NO.sub.3).sub.3.3H.sub.2O, were employed as precursors. Initially, triblock co-polymer, known as P123, has been dissolved completely in 50 ml of ethanol. Then the desired quantity of Cu(NO.sub.3).sub.2.3H.sub.2O, Ce(NO.sub.3).sub.3.6H.sub.2O and ZrOCl.sub.2.8H.sub.2O were added into the ethanol solution (total moles of precursors should be 0.01 mmol). After stirring for 2 h, completely dissolved transparent coloured sol has been kept in the oven for 48 h at 40 C. for controlled solvent evaporation. After aging the gel product has been kept at 100 C. for 24 h for drying. Finally material has been calcined at 400 C. for 4 h with the ramping rate of 1 C./min to get the desired CuCeZr mesoporous mixed oxide. A schematic representation of the above synthesis procedure is given in the flow diagram in FIG. 1.

(38) b. Synthesis of xAu-Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 (x=0.01 to 10)

(39) For the photodeposition of Au nanoparticles, 0.75 g of prepared catalyst was taken in quartz round bottom flask containing 120 mL of methanol and 30 mL of distilled water, calculated amounts of HAuCl.sub.4.3H.sub.2O solution was added to make two different compositions (0.1, 0.5, 1 and 3 mol %) of XAuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2. The prepared mixture was irradiated under UV light (>250 nm) for 2 h; Due to irradiation, Au.sup.3+ from gold solution reduces to metallic gold and gets deposited on metal oxide support in the form of Au-nanoclusters. After photo deposition the solution is allowed to settle down and then decanted, remaining solution was centrifuged with distilled water and ethanol at 6000 rpm for 10 min. Finally solution was dried at 45 C. for 16 h and at 100 C. for 12 h in an oven.

(40) c. Catalytic Test

(41) CO oxidation catalytic testing has been carried out using fixed bed glass reactor with an outer diameter of 14 mm size. In a typical CO oxidation experiment, 250 mg of CuCeZr catalyst has been loaded on the fixed bed reactor and passed by 5% CO containing gas mixture (CO:O.sub.2=1:5) diluted with N.sub.2. Flow rate of the gas mixture was maintained at 25 ml/min and calculated GHSV was 6000 cm.sup.3/g h. Temperature of the reactor was increased at the ramping rate of 2 C./min, and held at different temperature for 10 min for analysis. Composition of the gas was monitored by online GC. CO oxidation catalytic activity was measured from room temperature to 300 C. Rate measurements were carried out under steady state conditions. Catalytic activity was recorded in terms of the % conversion of the CO to CO.sub.2 molecule by using the following formula. In order to check the efficacy of the catalysts, high CO content oxidation measurements were also made with the gas mixture composition of CO:O.sub.2=1:2 and different GHSV of 12,000 and 18,000.

(42) $X_{\mathrm{CO}}=\left(\frac{P_{\mathrm{CO},in}-P_{\mathrm{CO},\mathrm{out}}}{P_{\mathrm{CO},in}}\right)100$
d. Characterization of Mesoporous Cu.sub.0.1Ce.sub.0.9-xZr.sub.xO.sub.2 (x=0.05 to 0.85) and its Catalytic Applications
A. XRD In FIG. 3, LXRD patterns display a Single broad diffraction feature at 2=0.5-3 indicates materials are mesoporous in nature. In FIG. 4, WXRD of Fluorite cubic crystal structure has been observed for all Cu.sub.0.1Ce.sub.0.85-xZr.sub.xO.sub.2 (x=0.05 to 0.85) materials. In WXRD, as the zirconia content increases crystallite size also increases. In WXRD, Shift towards higher 2 value indicates the incorporation of ZrO.sub.2 into lattice of CeO.sub.2. In WXRD, absence of CuO peaks suggests that it could either be incorporated into ceria lattice or highly dispersed nanocrystalline in nature.
B. Raman Analysis: (FIG. 5) Six Raman-active modes (A.sub.1g+2B.sub.1g+3E.sub.g) are observed for tetragonal-ZrO.sub.2, while for cubic CeO.sub.2 only F.sub.2g mode, centered at around 461 cm.sup.1, was observed. Shift in the F2g peak towards lower wavenumber is due to incorporation of Zr.sup.4+ in the Ceria lattice.
C. TPR Analysis: (FIG. 6) Reduction peak of copper oxide appears 150 C. for Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 indicates Cu.sup.2+ is likely incorporated in CeO.sub.2 lattice. CuO reduction peak shifts to high temperature for higher ZrO.sub.2 content, and two peaks were observed indicating lower CuO-support interaction
D. N.sub.2 Adsorption-desorption Isotherms: (FIG. 7) All materials show type IV isotherms with H2 hysteresis loop indicating mesoporosity. All materials exhibit narrow pore size distribution in the mesopore range with pore diameter between 4 and 7 nm. BET surface area observed between 105-146 m.sup.2/g.

EXAMPLE 2

Synthesis of Cu.SUB.0.1.Ce.SUB.0.85.Zr.SUB.0.05.O.SUB.2 .Material

(43) 1 g of P123 has been dissolved completely in 20 ml of ethanol. Then 0.2416 g of Cu (NO.sub.3).sub.2.3H.sub.2O, 3.691 g of Ce(NO.sub.3).sub.3.6H.sub.2O and 0.1611 g of ZrOCl.sub.2.8H.sub.2O were added into the ethanol solution. After stirring for 2 h, completely dissolved transparent coloured sol has been kept in the oven for 48 h at 40 C. for controlled solvent evaporation. After aging the gel product has been kept at 100 C. for 24 h for drying. Finally material has been calcined at 400 C. for 4 h with the ramping rate of 1 C./min to get the desired mesoporous Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 mixed oxide.

EXAMPLE 3

Synthesis of Cu.SUB.0.05.Ce.SUB.0.675.Zr.SUB.0.275.O.SUB.2 .Material

(44) 1 g of P123 has been dissolved completely in 20 ml of ethanol. Then 0.1208 g of Cu(NO.sub.3).sub.2.3H.sub.2O, 2.931 g of Ce(NO.sub.3).sub.3.6H.sub.2O and 0.886 g of ZrOCl.sub.2.8H.sub.2O were added into the ethanol solution. After stirring for 2 h, completely dissolved transparent coloured sol has been kept in the oven for 48 h at 40 C. for controlled solvent evaporation. After aging the gel product has been kept at 100 C. for 24 h for drying. Finally material has been calcined at 400 C. for 4 h with the ramping rate of 1 C./min to get the desired mesoporous Cu.sub.0.05Ce.sub.0.675Zr.sub.0.275O.sub.2 mixed oxide.

EXAMPLE 4

Synthesis of Co.SUB.0.1.Ce.SUB.0.25.Zr.SUB.0.65.O.SUB.2 .material

(45) 1 g of P123 has been dissolved completely in 20 ml of ethanol. Then 0.291 g of Co(NO.sub.3).sub.2.6H.sub.2O, 1.086 g of Ce(NO.sub.3).sub.3.6H.sub.2O and 2.095 g of ZrOCl.sub.2.8H.sub.2O were added into the ethanol solution. After stirring for 2 h, completely dissolved transparent coloured sol has been kept in the oven for 48 h at 40 C. for controlled solvent evaporation. After aging the gel product has been kept at 100 C. for 24 h for drying. Finally material has been calcined at 400 C. for 4 h with the ramping rate of 1 C./min to get the desired mesoporous Co.sub.0.1Ce.sub.0.25Zr.sub.0.65O.sub.2 mixed oxide.

EXAMPLE 5

Synthesis of Mn.SUB.0.1.Ce.SUB.0.05.Zr.SUB.0.85.O.SUB.2 .material

(46) 1 g of P123 has been dissolved completely in 20 ml of ethanol. Then 0.179 g of Mn(NO.sub.3), 0.2171 g of Ce(NO.sub.3).sub.3.6H.sub.2O and 2.7392 g of ZrOCl.sub.2.8H.sub.2O were added into the ethanol solution. After stirring for 2 h, completely dissolved transparent coloured sol has been kept in the oven for 48 h at 40 C. for controlled solvent evaporation. After aging the gel product has been kept at 100 C. for 24 h for drying. Finally material has been calcined at 400 C. for 4 h with the ramping rate of 1 C./min to get the desired mesoporous Mn.sub.0.1Ce.sub.0.05Zr.sub.0.85O.sub.2 mixed oxide.

EXAMPLE 6

Synthesis of 0.1AuCu.SUB.0.1.Ce.SUB.0.85.Zr.SUB.0.05.O.SUB.2 .Material

(47) Initially 0.75 g of prepared catalyst was taken in quartz RB containing 120 mL of methanol. Then 30 mL of distilled water containing 1.85 ml of 0.0025 M HAuCl.sub.4.3H.sub.2O solution was added to make 0.1 mol % of AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst. The prepared mixture was irradiated under UV light (>250 nm, 400 W) for 2 hrs, during irradiation Au.sup.3+ from gold solution reduces to metallic gold and gets deposited on metal oxide support in the form of nanoclusters. After photo deposition the solution is allowed to settle down and then decanted, remaining solution was centrifuged with distilled water and ethanol at 6000 rpm for 10 min. Finally solution was dried at 45 C. for 16 h and at 100 C. for 12 h in an oven.

EXAMPLE 7

Synthesis of 0.5AuCu.SUB.0.1.Ce.SUB.0.85.Zr.SUB.0.05.O.SUB.2 .Material

(48) Initially 0.75 g of prepared catalyst was taken in quartz RB containing 120 mL of methanol. Then 30 mL of distilled water containing 9.5 ml of 0.0025 M HAuCl.sub.4.3H.sub.2O solution was added to make 0.5 mol % of AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst. The prepared mixture was irradiated under UV light (>250 nm, 400 W) for 2 hrs, during irradiation Au.sup.3+ from gold solution reduces to metallic gold and gets deposited on metal oxide support in the form of nanoclusters. After photo deposition the solution is allowed to settle down and then decanted, remaining solution was centrifuged with distilled water and ethanol at 6000 rpm for 10 min. Finally solution was dried at 45 C. for 16 h and at 100 C. for 12 h in an oven.

EXAMPLE 8

Synthesis of 1AuCu.SUB.0.1.Ce.SUB.0.85.Zr.SUB.0.05.O.SUB.2 .Material

(49) Initially 0.75 g of prepared catalyst was taken in quartz RB containing 120 mL of methanol. Then 30 mL of distilled water containing 4.6 ml of 0.01 M HAuCl.sub.4.3H.sub.2O solution was added to make 1 mol % of AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst. The prepared mixture was irradiated under UV light (>250 nm, 400 W) for 2 hrs, during irradiation Au.sup.3+ from gold solution reduces to metallic gold and gets deposited on metal oxide support in the form of nanoclusters. After photo deposition the solution is allowed to settle down and then decanted, remaining solution was centrifuged with distilled water and ethanol at 6000 rpm for 10 min. Finally solution was dried at 45 C. for 16 h and at 100 C. for 12 h in an oven.

EXAMPLE 9

Synthesis of 3AuCu.SUB.0.1.Ce.SUB.0.85.Zr.SUB.0.05.O.SUB.2 .Material

(50) Initially 0.75 g of prepared catalyst was taken in quartz RB containing 120 mL of methanol. Then 30 mL of distilled water containing 13.9 ml of 0.01 M HAuCl.sub.4.3H.sub.2O solution was added to make 3 mol % of AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst. The prepared mixture was irradiated under UV light (>250 nm, 400 W) for 2 hrs, during irradiation Au.sup.3+ from gold solution reduces to metallic gold and gets deposited on metal oxide support in the form of nanoclusters. After photo deposition the solution is allowed to settle down and then decanted, remaining solution was centrifuged with distilled water and ethanol at 6000 rpm for 10 min. Finally solution was dried at 45 C. for 16 h and at 100 C. for 12 h in an oven.

EXAMPLE 10

Synthesis of 3AuCo.SUB.0.1.Ce.SUB.0.85.Zr.SUB.0.05.O.SUB.2 .material

(51) Initially 0.75 g of prepared catalyst was taken in quartz RB containing 120 mL of methanol. Then 30 mL of distilled water containing 13.9 ml of 0.01 M HAuCl.sub.4.3H.sub.2O solution was added to make 3 mol % of AuCo.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst. The prepared mixture was irradiated under UV light (>250 nm, 400 W) for 2 hrs, during irradiation Au.sup.3+ from gold solution reduces to metallic gold and gets deposited on metal oxide support in the form of nanoclusters. After photo deposition the solution is allowed to settle down and then decanted, remaining solution was centrifuged with distilled water and ethanol at 6000 rpm for 10 min. Finally solution was dried at 45 C. for 16 h and at 100 C. for 12 h in an oven.

EXAMPLE 11

Synthesis of 0.5AuMn.SUB.0.1.Ce.SUB.0.85.Zr.SUB.0.05.O.SUB.2 .Material

(52) Initially 0.75 g of prepared catalyst was taken in quartz RB containing 120 mL of methanol. Then 30 mL of distilled water containing 9.5 ml of 0.0025 M HAuCl.sub.4.3H.sub.2O was added to make 0.5 mol % of AuMn.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst. The prepared mixture was irradiated under UV light (>250 nm, 400 W) for 2 hrs, during irradiation Au.sup.3+ from gold solution reduces to metallic gold and gets deposited on metal oxide support in the form of nanoclusters. After photo deposition the solution is allowed to settle down and then decanted, remaining solution was centrifuged with distilled water and ethanol at 6000 rpm for 10 min. Finally solution was dried at 45 C. for 16 h and at 100 C. for 12 h in an oven.

EXAMPLE 12

(53) a. Structural and Spectroscopy Characterization

(54) Powder XRD pattern of Ce.sub.0.9Zr.sub.0.1O.sub.2, Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, 0.1 and 0.6 wt % Au on Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalysts are shown in FIG. 19. All diffraction features were indexed with reference to the cubic fluorite crystal structure of ceria (JCPDS 34-0394) and it is in very good agreement. CZ solid solution can exist in three stable phases, namely cubic (c), tetragonal (t), and monoclinic (m), and two metastable (t, t) phases. Broadening of the wide angle x-ray diffraction peaks indicating the nanocrystalline nature of the prepared catalysts. Absence of CuO, Cu.sub.2O and gold peaks in the above XRD spectra demonstrates the copper is introduced into the lattice of ceria or present in highly dispersed nanocrystalline or amorphous form. The crystallite size of the prepared catalysts varies with Cu doping and gold deposition. Ce.sub.0.9Zr.sub.0.1O.sub.2 catalyst shows the crystallite size as 12 nm and Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, catalyst as 9 nm. However, upon gold deposition, crystallite size decreased further and found to be 8 and 5 nm for 0.1 and 0.6 wt % Au on Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, respectively, as shown by a line-broadening in FIG. 19 inset. Indeed, this observation is puzzling, but indicating the possibility of breaking up of crystallites into smaller size.

(55) Raman analysis of the catalysts is shown in FIG. 20. Ce.sub.0.9Zr.sub.0.1O.sub.2 catalyst exhibit a strong characteristic peak at 462 cm.sup.1 corresponds to F.sub.2g vibration mode of fluorite type structure. Introduction of Cu into CZ lattice broadens and shifts the F.sub.2g peak to 445 cm.sup.1. Above red shift indicates the changes in electronic interaction possibly due to the incorporation of Cu in CZ lattice. Another possible reason could be due to increased oxygen vacancies, which is related to structural defects derived from partial or total incorporation of copper into CZ solid solution, in agreement with the decrease of lattice parameter. However, no features corresponding to CuO or Cu.sub.2O were observed suggesting the absence of crystalline copper oxide; nonetheless, amorphous and/or very small particle size (<2 nm) copper oxide particles cannot be ruled out, which would broaden the corresponding Raman features enormously. Deposition of gold over Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst marginally shifts F.sub.2g peak, but it broadens further. This indicates gold deposition seem to lead a further interaction with Cu-doped CZ. Full width at half maximum (FWHM) of F.sub.2g peak of ceria in the mixed oxide can be used to measure the oxygen vacancies in the catalyst. FWHM of F.sub.2g feature increases from 20 to 51 on Ce.sub.0.9Zr.sub.0.1O.sub.2 through Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 to 0.6 wt % AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, respectively. An increasing amount of copper incorporation in CZ lattice along with increasing oxygen vacancies are possible reasons for the above changes in Raman spectra.

(56) Morphology and textural properties of the Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 and AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalysts have been studied by HRTEM and the representative results are shown in FIG. 25. Average size of the crystallites was apparently reduced, after deposition of gold over Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst, and what causes this size reduction is not clear. A disordered mesoporous structure was observed for Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst. Selected-area electron diffraction (SAED) pattern confirms the crystalline nature of Cu Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst (FIG. 25b). HRTEM image shows the majority of lattice fringes corresponding to CZ (111) (d=0.31 nm) facets of cubic fluorite structure. Absence of any lattice fringes corresponding to CuO and Cu.sub.2O indicates the total Cu-doping in CZ lattice (Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2). These observations are in excellent agreement with XRD and Raman spectral analysis. Disordered mesoporous nature has further advantages like low diffusional barriers, since the depth of mesopores are minimum to a few nanometres, unlike several hundred nanometres in conventional ordered mesoporous materials, like MCM-41 and SBA-15. This disordered pseudo 3D (p3D) mesoporous framework provides an easy route for the diffusion of reactants and products due to low diffusion barriers. In the case of 0.6AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst also disordered mesoporosity observed. The size of the gold particles has been measured and found to be less than 5 nm (FIG. 25d). However, along with CeO.sub.2ZrO.sub.2 (111) lattice, many Au (001) faceted particle (d.sub.001=2.02 ) also has been observed. A careful analysis of HRTEM gold clusters on the above catalysts shows the predominantly Au (001) faceted clusters deposited on CeO.sub.2ZrO.sub.2 (111) facets leading to a distinct interface. This type of interface is crucial for the transport of atomic oxygen across the interface from CZ to gold, possibly by reverse spill-over mechanism.

(57) Textural characteristics of CZ based catalysts were measured by N.sub.2 adsorption isotherms and pore-size distribution analysis. The results are shown in FIG. 21. The results shows type IV adsorption-desorption isotherm which is typical for mesoporous materials. Ce.sub.0.9Zr.sub.0.1O.sub.2 catalyst shows H3 type hysteresis loop which does not level off even at the saturation vapour pressure (P=P.sub.0). However, all other catalyst shows H2 hysteresis loop. All the catalysts show narrow pore size distribution. BET surface area of all materials were calculated. Surface area decreases from 140 to 132 m.sup.2/g after the doping with copper in Ce.sub.0.9Zr.sub.0.1O.sub.2 catalyst. Further the surface area decreases to 96 and 87 m.sup.2/g for 0.1AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 and 0.6AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, respectively, Decrease in the surface area of the catalyst at the expense of lower crystallite size could be due to pore blockage by gold clusters. Unimodel pore size distribution was observed with an average pore diameter around 51 nm for all catalysts. The TPR profiles of Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, and AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalysts are shown in FIG. 23. Zirconia could not be reduced in H.sub.2 up to 900 C. According to various literature reports pure ceria shows two peaks around 500 and 800 C. corresponds to the reduction of surface and bulk species, respectively. Reducibility of ceria increased in the presence of Zr.sup.4+ and thereby reduction temperature has decreased further. However, Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 shows two strong Cu-reduction peaks at 168 and 248 C. Literature reports suggests that both copper reduction peaks in CZ lattice occur at lower temperatures than pure CuO A careful analysis reveals a decreasing onset of first reduction peak from 110 C. on Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, to 65 and 48 C., respectively with 0.1 and 0.6 wt % Au (FIG. 22) demonstrates the Cu-reducibility increases in the above order; this also underscores an easily reducible character of copper due to gold deposition. This could be due to the synergistic electronic interaction between copper and CZ. Low temperature copper reduction peak is attributed to Cu introduced in the CZ lattice, which are believed to be active sites for CO oxidation. The high temperature copper reduction peak is attributed to the CuO species interacting with the CZ support. TPR results suggests the lattice doping of majority of Cu.sup.2+ in CZ lattice and in good agreement with spectral and structural analysis results.

(58) Electronic structure of the catalyst was analysed by XPS and the results are shown in FIG. 23. Ce 3d spectrum of virgin and gold deposited Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalysts (FIG. 23a) exhibits characteristic features of Ce.sup.4+. All peaks, except v (885.7 eV) and u (904.2 eV), correspond to Ce.sup.4+ oxidation state; however, reduction in intensity of high BE peak at 917 eV (u) demonstrates an increasing amount of Ce.sup.3+. v and u features are characteristic of Ce.sup.3+ oxidation state, and corresponds to the Ce(III) 3d.sup.94f.sup.2-O 2p.sup.5 configuration. A careful analysis of the Ce 3d spectra reveals the following point: A decrease in intensity, FWHM as well as area of u feature with increasing Au-content underscores a relatively increasing amount of Ce.sup.3+ from Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 to 0.6AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2. This is further supported by increasing intensity of other Ce.sup.3+ features (v and u) (dashed lines). Above points directly suggests an electronic interaction between Cu and nano Au-clusters with ceria-zirconia lattice, especially to increase the Ce.sup.3+ content. Cu 2p XPS results are shown in FIG. 23b for virgin and Au-deposited Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalysts. Cu 2p.sub.3/2 core level shows predominant Cu.sup.+ (932.20.1 eV) and some Cu.sup.2+ (934.2 eV) oxidation state on Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2. Presence of Cu.sup.2+ state is confirmed from the observation of satellite features around 940 eV. The formation of Cu.sup.+ species may result from a strong interaction of Cu with the high-surface area CZ support. However, on 0.1Au-deposition, Cu.sup.2+ features disappears and only Cu.sup.+ feature is observed at 932 eV. On increasing the gold deposition to 0.6 wt %, small amount of Cu.sup.+ was observed and the low S/N level hints the masking of Cu, possibly by gold. Inset in FIG. 23b shows the Au 4f core level spectra from 0.6AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2. Au 4f.sub.7/2 core level appears at a BE=83 eV. It is well known that metallic gold BE appears at 84 eV for Au 4f.sub.7/2 core level. Even though gold content is not high and it is observed as nanoclusters in HRTEM, observation of Au 4f level hints the possibility of gold deposition preferably on Cu-sites. Nano gold clusters prefer to deposit on the oxygen vacancies of CuCeZr (111) plane and involves the formation of anionic gold Au.sup.. It is very likely that lower coordination number Au atoms on nano gold surfaces deposited on oxygen vacancy sites of CZ enhances the electron transfer from the latter to Au; this makes the electron density higher on gold and apparently Au behaves like anionic gold. Ionic radii consideration (Ce.sup.4+ (1.01 ), Ce.sup.3+ (1.15 ), Cu.sup.+ (0.91 ), Cu.sup.2+ (0.87 ), Zr.sup.4+ (0.86 ), O.sup.2 (1.26 )) suggests the possibility of Cu-doping near Zr-sites to minimize the lattice distortion; however, this would also induce more oxygen vacancies (O.sub.v) and hence more Ce.sup.3+, which make the composite redox mechanism enabled at ambient conditions, and required for oxidation reactions. We also suggest Zr.sup.4+OCu.sup.+O.sub.v-Ce.sup.3+ linkages could be available predominantly. Gold deposition on such Cu-sites might be a reason for decrease in the intensity of Cu 2p features on AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2. Importantly, this would generate an interface, where all metal ions are present in close proximity to help for various elementary steps of catalysis reaction to occur in a tandem manner. XPS spectra for O 1s core level shows a main peak centred at 529.50.1 eV for Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 and gold deposited catalysts. A distinct shoulder feature was observed at 531.5 eV on gold-deposited catalysts is attributed to hydroxyl features. (FIG. 23)

(59) b. Activity Results

(60) FIG. 24 shows the catalytic performance of Ce.sub.0.9Zr.sub.0.1O.sub.2, Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, 0.1AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2, and 0.6AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 for CO oxidation. 0.6AuCe.sub.0.9Zr.sub.0.1O.sub.2 was also evaluated for comparison, especially to underscore the role of Cu. Among all the catalysts, Ce.sub.0.9Zr.sub.0.1O.sub.2 catalyst shows the lowest oxidation activity with activity onset at 240 C. and 17% CO conversion at 300 C. A quantum jump in the CO oxidation catalytic activity of Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst is evident from T100 (T50) value 120 C. (77 C.) than that of Ce.sub.0.9Zr.sub.0.1O.sub.2. It is also to be emphasized that the onset of CO oxidation begins at ambient temperatures. Higher activity of Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst could be due to the presence of active copper species near the O.sub.v sites of CZ present in the catalyst for CO oxidation. Although not shown in FIG. 24, by varying Cu and Zr-contents in the Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst, onset of oxidation activity could be systematically brought down to ambient temperatures.

(61) Gold deposited catalysts (AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2) were evaluated for CO oxidation catalysis. Onset of oxidation catalysis begins at ambient temperatures and a steep rise in CO oxidation activity can be witnessed in FIG. 24 for 0.1Au and 0.6Au containing catalysts with 100 (50) % CO conversion temperatures are at 65 (48 C. and 47 (36 C., respectively. As in the earlier case, CO oxidation activity onset can be tuned by varying the gold content from 0 to 0.6 wt %. However, further increase in gold content to 1 wt % increases the CO oxidation onset temperature. 0.6AuCe.sub.0.9Zr.sub.0.1O.sub.2 catalyst shows a different trend in activity. Although ambient temperature activity was observed, like the above catalysts, only 30% CO conversion was observed; 100% (50%) CO conversion was observed at rather 285 (210 C. very high temperatures. Above observation underscores the role of Cu, and hence O.sub.v, in bringing down 100% CO conversion temperature and its part in facilitating the same in AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2.

(62) Recyclability of the 0.6 wt % AuCu.sub.0.1Ce.sub.0.85Zr.sub.0.05O.sub.2 catalyst was tested for five cycles by simply cooling down the reactor temperature after each reaction to a maximum temperature of 150 C. and without any further treatment (see FIG. 18). Catalyst was held at 150 C. for 60 min at the end of each cycle. Very similar CO oxidation catalytic activity was observed without any significant difference in the activity in each cycle implies the efficacy of the mesoporous catalysts. It also underscores that the catalysts does not undergo any structural or microstructural changes during the repeated activity evaluation.

Advantages of Invention

(63) a. Carbon monoxide (CO) oxidation can be performed from near room temperatures to high temperatures by varying the composition of Au deposited Cu.sub.0.1Ce.sub.0.85Zr.sub.0.05 catalysts. b. Novel material with sustainable reaction under variety of CO:O.sub.2 ratios, temperature and space velocity conditions. c. The mesoporous channels allow high volume of gases and gold allows near room temperature CO oxidation. d. Easily recyclable