Elsevier

Journal of Catalysis

Volume 237, Issue 1, 1 January 2006, Pages 190-196
Journal of Catalysis

Highly reproducible syntheses of active Au/TiO2 catalysts for CO oxidation by deposition–precipitation or impregnation

https://doi.org/10.1016/j.jcat.2005.11.006Get rights and content

Abstract

Gold catalysts supported on TiO2 were prepared by a deposition–precipitation (DP) method to investigate how highly reproducible performance of the gold catalysts in CO oxidation can be achieved. A protocol was established for synthesizing identically performing catalysts by different operators. The results show that for this synthesis route, the calcination step is not needed to form highly active Au/TiO2 catalysts, but leads to decreased activity. Improved catalytic activity was observed when a high solution pH was adjusted during the precipitation. Surprisingly, wet impregnation followed by ammonia steam treatment and a washing step with water also leads to Au/TiO2 with 2- to 4-nm individual gold particles highly dispersed on the TiO2 surface. In addition, this catalyst is active for room temperature CO oxidation. The temperature for 50% conversion of CO is below 25 °C, which is comparable to that of the gold catalyst prepared by the DP method. Therefore, contrary to reports in the literature, the impregnation method can be used in the preparation of high-activity gold catalysts.

Introduction

A tremendous increase in research on gold catalysis was triggered by Haruta's first reports on the unexpectedly high catalytic activity of supported gold nanoparticles in CO oxidation [1], [2]. Several methods for the preparation of highly active gold catalysts have been developed. Besides classical deposition–precipitation (DP) proposed by Haruta [3], other methods, including co-precipitation [4], co-sputtering [5], chemical vapor deposition [6], grafting [7], and adsorption of gold colloids on metal oxides [8] have been investigated. Usually these preparation methods can produce small (<10 nm) Au particles that are strongly attached to the supports. Despite the numerous methods developed, however, the DP method still seems to be the most efficient method of preparing highly active gold catalyst. In particular, using TiO2 as a support, small, highly dispersed gold nanoparticles on the support can be achieved. Notably absent in the list of synthetic methods is the classical impregnation, because this pathway typically results in the formation of large gold particles with correspondingly low activity.

It has been pointed out that the nature of the support materials, as well as the physical state of the support, can strongly influence the activity of the resulting gold catalysts [9], [10], although recent reports have demonstrated that “naked” gold particles can exhibit activity in aerobic glucose oxidation, which is comparable with supported particles [11]. Various metal oxides, including TiO2 [3], Al2O3 [12], ZrO2 [13], MgO [14], Fe2O3 [15], and Co3O4 [16], have been investigated as gold supports, with the goal of creating high catalytic activity. In addition, ordered porous silicas, such as SBA-15 [17] and MCM-48 [18], have been successfully used for the synthesis of active gold-based catalysts. Besides the preparation methods and the types of the supports, various other synthetic conditions, including pH value in the DP method, pretreatment, and calcination temperature, significantly influence the properties of the gold catalysts [13], [19].

Despite the numerous studies published on supported gold catalysts, there is still no clear picture of the origin of the catalytic activity. In fact, the results reported in literature concerning gold catalysis are often contradictory, due to difficulties in reproducing the synthesis of the catalysts as well as problems with the catalytic tests. Thus the nature of the active species of the gold catalysts remains unclear. One reason behind the difficulty in elucidating the origin of catalytic activity is probably the unusual sensitivity of small gold particles to the environment, which leads to difficulties in synthesizing gold catalysts reproducibly. Because reproducibility of preparations and methods is crucial to gaining insight into catalytic systems, we have made a great effort to develop a robust synthesis protocol for the preparation of highly active gold-based catalysts. As reported previously [13], a consistent series of catalysts could normally be produced in one synthesis session, but day-to-day variability in catalyst performance for nominally identical catalysts could be as much as a 100-K difference in T50%, the temperature for 50% CO conversion.

Gold supported on TiO2 prepared by DP for CO oxidation was selected as target catalyst because this system has been widely investigated. As a result of this study, a synthesis procedure with well-controlled steps that allows the preparation of active catalysts with a high level of reproducibility could be developed. In addition, because impregnation is a rather straightforward and simple catalyst synthesis procedure compared with the DP method, we attempted to develop an impregnation method for the synthesis of active gold catalysts. Impregnation has not been considered a good method for producing active gold catalysts, due to the difficulty in creating individual Au particles <10 nm by this method [20]. Our studies led to the discovery that wet impregnation followed by ammonia gas-phase treatment and a final washing step with water results in the formation of gold catalysts containing 2-nm gold particles. Consequently, these gold catalysts show similar catalytic activity in CO oxidation as those materials prepared by the DP method.

Section snippets

Experimental

Reproducible catalysts were obtained through the following procedure. First, 1.8 g of TiO2 (Degussa P25) was suspended in 75 ml of deionized water under vigorous stirring for 0.5 h. Then the pH value of this suspension was adjusted with 0.1 M NaOH solution to the desired value, using a computer-based pump system connected to a pH meter. The suspension was maintained at this pH under stirring for additional 1.5 h. In parallel, 15 ml of deionized H2O and 1.4 ml of 0.1 M NaOH solution were mixed

Reproducibility of Au/TiO2 catalyst

To investigate the reproducibility of gold catalysts, gold supported on TiO2 prepared by the DP method was selected as a model system. Basically, we followed Haruta's standard preparation procedures; however, we provide more details of the synthesis compared with Haruta's report [2]. In addition, the synthesis differs with respect to the calcination step. In Haruta's approach, gold catalysts were calcined at temperatures >200 °C to obtain stable metallic species and strong metal–support

Conclusion

This study has demonstrated that Au/TiO2 catalysts prepared by the DP method show good reproducibility under meticulously controlled synthesis conditions. The crucial factors influencing reproducibility are strict pH control during precipitation, which is facilitated by using a relatively dilute NaOH solution; maintenance of a fixed volume at the end of the synthesis; and exact control of the drying conditions. The catalytic activity of Au/TiO2 depends strongly on the pH during precipitation,

Acknowledgements

The authors are grateful to B. Spliethoff for the TEM studies. Financial support from the DFG via SFB 558 is gratefully acknowledged.

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