Low-Temperature Water–Gas Shift Reaction over Au Supported on Anatase in the Presence of Copper: EXAFS/XANES Analysis of Gold–Copper Ion Mixtures on TiO2

Abstract

Cu-modified Au/TiO2 (anatase, 200 m2/g) has been prepared by the incipient-wetness technique with introduction of the modifier either before or after Au loading. Such catalysts gave higher catalytic activities for the water–gas shift (WGS) reaction when compared to the unmodified catalysts mainly because of the existence of a synergetic interaction between Cu and Au, as the activities of both Cu/TiO2 and Au/TiO2 are lower than that of the bimetallic system. The WGS activities of both the as-prepared Au/TiO2 and a Au–Cuc/TiO2 catalyst were found to be high and stable. The presence of nitrates on Cuc–Au/TiO2 was found to be detrimental to the activity of Au on TiO2, as a result of the poisoning of Au and enhanced Au agglomeration by NO2 formed during reaction. The activities of Au/TiO2 catalysts modified with Cu-containing acetate counterions were found to decrease during the first 30 min on stream, reaching a constant value of (45 ± 2)%. However, when the poisoning by the acetate anion was eliminated through calcination, the activity increased compared to those of the Au/TiO2 catalysts. An increase in Cu loading was found to lower the activity of Au as a result of the loss of the interaction between the Au and the support interface. The XANES spectra of uncalcined as-prepared catalysts exhibited a pre-edge feature (due to the 1s-to-3d transition) characteristic of Au in a tetrahedral environment. The water–gas-shift-treated sample showed a reduction in the intensity of the white line, indicating a reduction of Au (a partial reduction of Au3+ to Au+/Au0). The XANES spectrum of calcined Cuc–Au/TiO2 (0.44 wt % Cu/3.74 wt % Au) confirmed that Cu exists as ions (Cu+/Cu2+) before and during the WGS reaction. The Au catalyst was not stable and continuously deactivated with time, and the white-line region showed a decrease in peak intensity during use. An increase in gold activity on Cu2+-modified TiO2 was achieved because at least 90% of the gold was reduced to the zero-valent state whereas most of the stabilizing metal (Cu2+) remained in the ionic state. We concluded that the addition of a stabilizing metal in ionic form during the preparation of the Au–Cuc/TiO2 system has marked effects in improving the initial dispersion of the reduced gold and also in helping to maintain a highly dispersed state of gold during use. The atomic ratio of stabilizing metal ions to gold metal plays a crucial role in maximizing the dispersion/stabilizing effects. The formation of bimetallic particles was not detected by EXAFS data analysis. The observed effects can be interpreted as a mutual influence of gold and copper ions and reduced species of gold and copper because of their competion for ion-exchange sites. It is unclear at present how this is achieved. However, EXAFS data analysis of both metals suggests that it is not due to a direct interaction between Au and Cu. In contrast to the results for the WGS reaction, copper was found to show no promotional effect on low-temperature CO oxidation or on preferential CO oxidation (PROX) in excess hydrogen.