Supported nanosized gold catalysi the influence of support morphology and reaction mechanism 6

The performance of Au/iron oxide catalysts towards PROX reaction was found to be strongly affected by catalyst preparation and post-treatment.. Provided that gold particles in all the fo

Chapter 6 Conclusion

In this thesis supported nanogold catalysts on low temperature CO oxidation are intensively studied Three types of easily reduced oxides, i.e iron oxides, titanium oxides and copper oxide are used as the support Also three preparation methods, i.e co-precipitation, deposition-co-precipitation, and colloid-based impregnation are employed for preparing Au catalysts with Au particle size around 2-4 nm The performance of Au/iron oxide catalysts towards PROX reaction was found to be strongly affected by catalyst preparation and post-treatment Among the three methods studied in this chapter, the colloid-based method is better than the co-precipitation and deposition-precipitation methods The AuCH sample (Au catalyst supported on Fe2O3 prepared by the colloid-based method) showed the highest activity, selectivity and stability at temperatures between RT and 100oC The reaction ratercofor the CO+O2CO2at 25oC was measured

to be 12.3 x 10-4 molco* gAu-1* S-1, better than that reported in literature, i.e 4.87 * 10

-4

molCOg-1AuS-1for Au/-Fe2O3, provided by the World Gold Council With the colloid-based method, Au particle size and distribution can be better controlled Provided that gold particles in all the four Au/iron-oxide samples prepared by various methods are small enough to be able to activate CO and O2, the CO oxidation activity was found to remarkably depend on the structure of the iron oxide support, in the order: ferrihydrite > hematite > magnetite XRD, TPR, XPS and SIMS studies showed the presence of OH and COO groups on the AuCH samples Moreover, non-metallic Au has been detected in

the AuCH sample in addition to metallic Au All these would contribute to the enhanced activity of the AuCH sample

In Chapter 4 Au supported on six different types of TiO2 are studied Compared to the commercial anatase TiO2(CB) and 4 kinds of TiO2 samples from KAISHA Ltd, the home-made TiO2 nanotubes (NT) are better catalyst support, on which the Au nanoparticles exhibit significantly improved catalytic activity for the CO oxidation at room temperature Surface area is found to be an important factor for catalysts’ activity, since higher surface area samples show higher activity However, Au/TiO2 (NT) and Au/MC150, one of the Kaisha samples, have similar surface areas but exhibit remarkably different activity In depth studies in this chapter showed that TiO2nanotubes are a better support because they have smaller sizes (<10 nm), higher defects density, and more chemically bonded OH groups or water molecules, which result from the hydrothermal processing for preparing TiO2tubes from TiO2particles

Chapter 5 demonstrates that the catalytic activity of Au/CuO catalysts could be significantly improved for CO oxidation at room temperature using the nanosized supports, i.e home-made CuO (NF) and CuO (NP) as compared to the commercial CuO(CB) The Au catalysts prepared by colloid-based method on the three CuO supports have similar gold particle size and size distribution XRD showed that all the three CuO supports have the same crystalline structure Au/CuO(NP) catalyst with larger support surface area showed better catalytic activities Under the same reaction conditions, the activation energy was measured to be 3.8 KJ/mol in a temperature range of 25-125oC for Au/CuO (NP), and 5.8 kJ/mol for Au/CuO (CB) The reaction rate was 0.22 mmol/gAu·s for Au/CuO (NP) and 0.07 mmol/ gAu·s for Au/CuO (CB) The pre-treatment temperature

was found to affect the samples’ catalytic activity, and 300oC is the optimum temperature for air pre-treatment

DRIFTS study showed that Cu ions on the CuO supports are mainly in +1 oxidation state, indicating there exist high oxygen vacancies on the CuO support surfaces CO was found

to be mainly adsorbed at Au active sites Surface –OH groups were detected on Au/CuO samples prepared by the colloids-based method They were found to be involved in the

CO oxidation reaction via the interaction with adsorbed CO, forming HCOO intermediates Higher concentration of OH groups was measured on Au/CuO(NP) with higher CO oxidation activity (than Au/CuO(CB)) O2 could be adsorbed with the CuO samples, forming O-or O2-species, which might react with surface HCOO to release CO2

via carboxylate or carbonate intervidiates In the presence of CO + O2, Au-, Au0, and

Aun+ could be simultaneously found on the Au/CuO samples, as evidenced by the detection of C-O vibrations at 1978, 2104, 2164 and 2189 cm-1 Aun+ can adsorb and activate CO stronger Strong support-metal interaction was observable on Au/CuO(NP) which showed higher CO oxidation activity The reaction mechanism derived from the in-situ DRIFT studies is basically the same as those proposed by G.C Bond and H.H Kung

In summary, the study in this thesis indicated that the improved colloid-based method (employed in our lab) is better than conventional co-precipitation and deposition-precipitation methods in controlling the Au dispersion and introducing more OH and COO groups Hydrothermal processing of catalyst support could result in more structural defects and more chemically bonded OH groups so that can improve the CO oxidation activity on these oxide-supported Au catalysts The Au/Fe2O3 and Au/TiO2 catalysts

prepared using colloid-based method can achieve very good activity In-situ DRIFTS study of the CO oxidation over Au/CuO could detect the presence of various Au species

as well as a variety of reaction intermediates including formate, carbonate, and hydrocarboxylate, allowing the derivation of a reaction mechanism similar to those proposed by G.C Bond and H.H Kung All these results provide deep insights on the gold catalysis and valuable guidance for future study of it

In this thesis the investigation is started from the one of the most effective metal oxide supported nano-gold catalyst—Au/Fe2O3, whose activity greatly depends on the method

of preparation After comparing the catalytic results of Au/Fe2O3samplesprepared using different methods, it is found that the best preparation method is colloid-based method Then in the next chapter titanium oxide – one of the earliest and most investigated support for gold nano particles is chosen as the support Although both experimental and theoretical researches concerning the effect of gold nanoparticle’s size have already been conducted comprehensively, the research on titanium oxide support still has some gap to fill The catalytic performance of various Au/TiO2 samples, prepared with same method and having similar gold dispersion, similar gold particle size, but different surface area, different morphology and different crystalline structure of support, were carefully studied

By comparing these results, it is found that the properties of oxide support significantly affect the catalytic performance of Au/TiO2samples With the same preparation method, TiO2 having more –OH functional groups shows higher catalytic activities for CO oxidation over Au/TiO2 Also the catalytic performance is found to increase with increasing support surface area These findings provide clear guidelines on how to choose metal oxide for supported nano-gold catalysts Easily reducible metal oxide such

as Fe2O3 which have more defects and larger surface area should have better catalytic performance than less- and non-reducible metal oxide supports, including TiO2, CeO2, ZrO2, MgO, Al2O3and SiO2 In Chapter 5, an oxide that has not attracted much attention and have not been extensively investigated CuO (copper oxide) is chosen to be the support for nano-gold catalyst, and the experimental results in this chapter prove the above conclusion on how to prepare and improve metal oxide supported nano Au catalysts

Due to the novelty and wide applications nano-gold catalysis has received increasing interest and attention More systematic work can be carried out in future to understand how the use of capping agent in colloid-based impregnation method and the post-growth treatment can well control the gold particle distribution and improve its catalytic performance Advanced technologies such as high resolution scanning TEM, in-situ FTIR, XPS can be employed to determine the nature and structure of the active center over the nano-gold catalysts: whether bi-layer gold cluster is essential for CO oxidation, what are the gold oxidation states under reaction conditions, where oxygen is activated, what is the activated oxygen intermediate states, and how water or OH functional groups participate

in the CO oxidation For wider catalytic applications of supported nano-gold catalyst,

some ‘reactive transition metal oxides’ such as NiO that have proved to be beneficial to

the oxidation reaction but have not been often used as the support for nano-Au catalysts can be tested Mixed or composite metal oxide support and alloy catalysts can be considered too For example, mixture of one easily reducible metal oxide and one irreducible metal oxide with more defects may be studied as the support for nano gold particles Gold nano particles with another noble metal as a bimetallic catalyst supported

on single metal oxide support can be another research interest They may be used for water gas shift reaction, selective reduction of nitric oxide, propene oxidation and other chemical processes