TiO2 Prepared by Deposition Precipitation with Urea: Improved

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Au-Ir/TiO2 Prepared by Deposition Precipitation with Urea: Improved Activity and Stability in CO Oxidation Antonio Go´mez-Corte´s,† Gabriela Dıáz,*,† Rodolfo Zanella,*,‡ Humberto Ramı´rez,‡ Patricia Santiago,† and Jose´ M. Saniger‡ Instituto de Fı´sica, UniVersidad Nacional Autońoma de Me´xico (UNAM), Circuito de la InVestigacioń Cientı´fica S/N, A. P. 20-364, C. P. 01000, Me´xico D. F., Me´xico and Centro de Ciencias Aplicadas y Desarrollo Tecnolo´gico, UniVersidad Nacional Autońoma de Me´xico (UNAM), A. P. 70-186, Me´xico D. F., 04510 Me´xico ReceiVed: December 10, 2008; ReVised Manuscript ReceiVed: April 15, 2009

A series of Ir and Au-Ir supported on TiO2 catalysts were prepared by deposition-precipitation with urea to study the activity and stability of these materials in the CO oxidation reaction. Bimetallic samples were prepared using two approaches: one by codeposition of the metal precursors and the other by sequential deposition being iridium the first to be incorporated on the support. Samples were submitted to calcination in air or reduction in hydrogen thermal treatments. Nominal gold and iridium loading were 4 wt %. Samples were characterized by EDS, H2-TPR, HRTEM, HAADF, and CO + O2 adsorption followed by DRIFTS. It is shown for the first time that deposition-precipitation with urea is able to deposit Ir and Au-Ir nanoparticles on TiO2. Catalyst pretreatment had an important effect on the structure of the iridium phase. In calcined samples, Ir spreads over the TiO2 mainly as a thin layer of IrO2 particles preferentially deposited on the rutile phase of TiO2. In reduced samples, Ir particles were homogeneously dispersed on all of the TiO2 crystals. It is shown that, even in the samples reduced at 300 °C, IrO2 was present in the catalysts. Ir/TiO2 samples prepared by deposition-precipitation with urea calcined or reduced in H2 were not active at room temperature (light-off temperature above 250 °C). The preparation protocol in bimetallic catalysts (codeposition or sequential deposition of the metals and different pretreatments) had a strong influence on the catalytic performance of the catalysts. The most active sample was the one prepared by sequential deposition and thermally treated in hydrogen at 300 °C. An enhanced activity was observed when compared to Au/TiO2. Besides this synergetic effect, the bimetallic catalyst was more stable in time on stream and more stable against sintering after reaction. DRIFTS experiments showed that the interaction of Au and Ir could modify the adsorption properties of catalyst surface. Introduction Since the discovery in the late 1980s that gold can be catalytically active when it is dispersed as small particles (99.5%). Commercial HAuCl4 · 3H2O and IrCl4 · xH2O both from Aldrich were used as gold and iridium precursors. Before preparation, TiO2 was dried in air at 100 °C for at least 24 h. Catalysts were prepared in the absence of light, which is known to decompose and reduce gold precursors. The preparation of gold nanoparticles on TiO2 was performed by deposition-precipitation with urea (DPU) following the previously reported procedure 15,16 and will be identified as Au/TiO2. Ir/TiO2 samples were prepared by deposition-precipitation with urea method as follows: 1 g of TiO2 was added to 50 mL of an aqueous solution containing IrCl4 (4.2 10-3 M) and urea (0.42 M). The initial pH was ∼4. The suspension thermostatted

J. Phys. Chem. C, Vol. 113, No. 22, 2009 9711 at 80 °C was vigorously stirred for 16 h. Urea decomposition leads to a gradual rise in pH from 4 to 7.5. Sample will be identified as Ir/TiO2. The gold or iridium amount in the solutions corresponds to a nominal loading of 4 wt % on the support. After the deposition-precipitation procedure, all samples were centrifuged, washed with water, and then centrifuged four times and dried under vacuum for 2 h at 100 °C. Thermal treatments were performed in a U reactor with a fritted plate of 1.5 cm of diameter; calcinations under a flow of dry air (1 mL · min-1mgsample-1) or reductions under a flow of H2 (1 mL · min-1mgsample-1) were performed at 400 or 300 °C respectively for 2 h. All of the samples were stored at room temperature under vacuum in a desiccator away from light to prevent any alteration.20 2.1.2. Preparation of Bimetallic Samples. Codeposition and sequential deposition methods were used to prepare bimetallic catalysts. In the first method, Au and Ir were codeposited on the TiO2 and for that 1 g of TiO2 was dispersed into 50 mL of an aqueous solution containing HAuCl4 and IrCl4 (4.2 10-3 M) heated to 80 °C. Urea was added to the HAuCl4 and IrCl4 solution to achieve a concentration of 0.42 M. The suspension of TiO2 in the solution was stirred for 16 h. Afterward, the sample was washed, dried, and stored at room temperature under vacuum in a desiccator away from light. This sample will be identified as Au-Ir/TiO2-C. In the second approach, sequential deposition, iridium was first deposited on TiO2 by the DPU method following the procedure previously described for the monometallic Ir/TiO2. The sample was dried and calcined in air at 400 °C for 2 h before gold was deposited by the DPU method. After gold deposition, the sample was washed, dried, and stored as previously described. The sample will be identified as Au-Ir/ TiO2-S. 2.2. Characterization Techniques. Chemical analysis of Au and Ir in the samples to determine the actual loading was performed by energy dispersive X-ray spectroscopy (EDS) using an Oxford-ISIS detector coupled to a scanning electron microscope (JEOL JSM-5900-LV). Chemical analysis was performed after thermal treatment of the samples. To have accurate metal loading values, more than 50 different areas were analyzed and the average metal concentration for every sample is reported. The Au and Ir weight loading is expressed in grams of each metal per gram of sample. Hydrogen temperature programmed reduction (H2-TPR) experiments were performed in a RIG-100 unit under a flow of 5% H2/Ar gas mixture (30 mLmin-1) and a heating rate of 10 °C/min from room temperature to 700 °C. The H2O produced by the reduction process was trapped before the TCD. Bulk CuO was used as reference for calibration of the TCD signal. Thermally treated samples before and after reaction were examined by transmission electron microscopy (HRTEM) in a JEM 2010 FasTem analytical microscope equipped with a Z-contrast annular detector. CO and CO + O2 interactions with the surface of the catalysts were followed by FTIR spectroscopy. Experiments were done in a Nicolet Nexus 470 spectrophotometer equipped with an environmentally controlled high-pressure/high-temperature Spectra Tech DRIFT cell with ZnSe windows. All spectra were collected from 128 scans with a resolution of 4 cm-1. For each experiment, 0.025 g of the dried sample was packed directly in the sample holder and pretreated in situ under H2 flow (30 mLmin-1) at 300 °C for 2 h. Temperature was increased from room temperature at a heating rate of 5 °C/min. After this

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TABLE 1: Metal Loading Determined by EDSa sample

Au loading (wt %)

Au/TiO2 Ir/TiO2 Au-Ir/TiO2-C Au-Ir/TiO2-S

4.0 2.3 4.0

Ir loading (wt %) 3.7 3.5 3.6

a -C ) codeposited samples, -S ) sequential deposition. The nominal gold and iridium loading was 4 wt % in all cases.

treatment, the sample was purged with He (60 mLmin-1) during 30 min and cooled to room temperature in the same gas atmosphere. A reference spectrum of the solid was collected under He flow at 25 °C before admittance of 2.5% CO/He (30 mLmin-1) for 15 min. Afterward, the cell was purged with He for 5 min before a DRIFT spectrum was collected. In all cases, spectra were referenced to the spectrum of the freshly reduced solid prior to CO adsorption. CO + O2 coadsorption was analyzed over freshly in situ reduced samples. After the reduction treatment at 300 °C, the sample was purged with He (60 mLmin-1) during 30 min and cooled to room temperature in the same gas atmosphere. At this time, a gas mixture (2.5%CO/He + 2.5%O2/He) flow (30 mLmin-1) was admitted in the DRIFT cell at 25 °C for 10 min. After this period of time, a spectrum was collected under the same gas atmosphere. The temperature was then increased up to 100 °C and after 10 min a new spectrum was collected. The same procedure was used at each temperature in the range 100-300 °C. All spectra were referenced to the spectrum of the freshly reduced solid at each temperature prior to admittance of the CO + O2 gas mixture. 2.3. Catalytic Activity. The CO + O2 reaction was studied in the temperature range from room temperature to 400 °C in a flow reactor at atmospheric pressure. Typically, 0.010-0.020 g of catalyst was activated for 2 h in either an air flow at 400 °C or H2 at 300 °C before the catalytic run. After this treatment, the sample was cooled to room temperature under He. The composition of the reactant gas mixture was 2.5% CO, 2.5% O2, and balanced He. The total flow of reactant gas mixture was 60 mL/min. At each temperature, the reaction was allowed to stabilize before collecting any data. The exit gases were analyzed by online GC (TCD) using a Carboxen 1000 packed column. Stability of the catalysts versus time on stream was examined at 23 °C during a 20 h run. Before the catalytic run, the bimetallic catalysts were activated in a flow of H2 at 300 °C for 2 h. In the case of Au/TiO2, it was activated at the same temperature but in a flow of air. After these treatments, the samples were cooled to 23 °C in He. Then, 200 mL/min of a reactant gas mixture consistent of 2.5% CO and 2.5% O2 balance He was admitted through the catalyst and conversion data under time on stream were collected. 3. Results and Discussion 3.1. Metal Loading. Table 1 shows the actual Ir loading in Ir/TiO2 samples prepared by DPU. Practically, the nominal metal loading (4 wt %) was obtained as it can be seen in Table 1. In the case of bimetallic samples (Table 1), when metals were codeposited (Au-Ir/TiO2-C), the actual gold loading was 2.3 wt % (55% of the nominal loading), whereas practically all Ir (3.5 wt %) was deposited on the support. It was quite surprising to see that the target loading for gold was not achieved, as it is usually done when DPU method is used to prepare Au/TiO2.15,16 It must be remembered that in the DPU method the progressive

Figure 1. TPR profiles of DPU Au, Ir, and Au-Ir/TiO2-S catalysts. (a) Dry Au/TiO2 sample, (b) Ir/TiO2 calcined at 300 °C, (c) Dry Au-Ir/ TiO2-S, and d) Au-Ir/TiO2-S calcined at 300 °C.

decomposition of urea in solution at temperatures above 60 °C releases OH- ions, which gradually increase the medium pH. This method makes possible the slow precipitation of hydroxides onto the support, and avoids a brutal and local increase of pH, which could induce precipitation in solution. It is possible that the surface of the gold precipitate is charged in solution.16 The presence of a second metal precursor in solution (IrCl4) could have modified the surface potential charge of gold precipitate, inducing repulsions with TiO2 surface species avoiding the total gold deposition. The above result may be explained also in terms of an adsorption competition between Au and Ir species for the TiO2 surface. When Ir and Au were deposited sequentially (Au-Ir-S), practically the target loading for each metal was obtained. 3.2. Reduction Properties. The reduction properties of the catalysts were studied by hydrogen programmed reduction (TPR). Figure 1 displays the H2-TPR profiles of Au/TiO2, Ir/ TiO2, and Au-Ir/TiO2-S samples. Dry Au/TiO2 sample is characterized by a more or less broad reduction peak, with a maximum at T ) 125 °C. A low-temperature reduction peak like this has already been observed for Au/TiO2 samples6 and is assigned to the reduction of oxygen species on the nanosized gold particles. The TPR profile of the Ir sample calcined at 400 °C is characterized by three hydrogen consumption peaks at around 100, 250, and 600 °C. The small one around 100 °C could be assigned to the reduction of some large iridium oxide particles,42 whereas a larger amount of iridium oxide species are reduced around 250 °C, which can be identified as welldispersed particles.43 The high-temperature peak present in the TPR profile could be associated with the reduction either of iridium species not already reduced or eventually to the reduction of the support (Ti4+ f Ti3+). It is well-known that TiO2 is partially reduced to TiO2-x by hydrogen at high temperatures (above 500 °C) and this process is promoted by the presence of dispersed metal crystallites.44,45 Quantitative data of the hydrogen consumption associated only with reduction peaks below 300 °C showed that reduction of the iridium oxide to Ir metal in the calcined Ir/TiO2 sample is not complete assuming an Ir4+ f Ir0 reduction process. In the case of Au-Ir/TiO2-S sample, the TPR profile of the dry sample is characterized by a sharp peak at Tmax ) 106 °C, which can be related with the reduction of gold species and a

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Figure 2. TEM images of Ir/TiO2 calcined in air at 400 °C. (a) Z-contrast image, (b) HRTEM image, (c) amplified HRTEM image showing a TiO2 rutile crystal covered with IrO2 particles. FFT corresponding to the selected area shows the (101) and (110) reflections of IrO2 and (101) reflection of rutile, and (d) TEM image of Ir/TiO2 catalyst.

broad peak at higher temperatures associated with the reduction of iridium species. The TPR profile of the calcined Au-Ir/ TiO2-S sample showed, as expected, practically the same profile as in the case of the calcined iridium monometallic catalyst because reduction of Au was achieved through the calcination treatment. For the calcined bimetallic Au-Ir/TiO2-S sample, quantitative data (not shown) of the hydrogen consumption associated to reduction peaks below 300 °C indicated that the extent of reduction of the iridium species is higher compared to the Ir/TiO2 monometallic catalyst but not complete even in this case. The same holds for Au-Ir/TiO2-C sample, although in this case reduction of iridium is achieved in a lesser extent compared to Au-Ir/TiO2-S sample. 3.3. Microstructure (HRTEM). Figure 2 shows typical TEM and Z-contrast images of Ir/TiO2 sample calcined in air at 400 °C. These images show that Ir spreads over the support mainly as particles that seem to deposit as a thin layer of about 1-2 nm thick on the TiO2 particles (parts a and b of Figure 2). These particles were clearly identified as IrO2 by conventional HRTEM images as shown in part c of Figure 2. The lattice parameter measured from the layer formed on the TiO2 particles corresponds to (101) and (110) reflections of IrO2 as shown in the FFT. In addition, as can be seen in part d of Figure 2 for the same catalyst, IrO2 particles are not homogeneously deposited on all TiO2 particles; some have a high density of IrO2 particles and some others seem to have almost no IrO2 particles. In Figure 3, HRTEM images of the TiO2 support are presented showing that a preferential deposition of IrO2 particles occurred, whereas particles were found on rutile crystals (part a of Figure 3), anatase crystals as the one shown in part b of Figure 3 have no IrO2 particles. This result was observed

previously by Akita et al.46 who studied Ir/TiO2 (Degussa P25) prepared by DP NaOH by means of TEM, SEM, ADF-STEM, and EELS. In our samples, this behavior seems to be dependent on the thermal treatment of the sample. When the samples were thermally treated in hydrogen, iridium particles were homogeneously dispersed on all the TiO2 crystals. Part a of Figure 4 shows a Z-contrast image of the Ir/TiO2 sample prepared by DPU thermally treated in H2 at 300 °C. The average iridium particle size is 1.2 nm. Part b of Figure 4 shows a Z-contrast typical image of Au/TiO2 sample thermally treated in H2 at 300 °C. The average gold particle size is 2.2 nm. Concerning the bimetallic samples, Figure 5 shows a Zcontrast image of the codeposited sample (Au-Ir/TiO2-C) thermally treated in air at 400 °C. Apparently, the same behavior observed in the previous monometallic Ir/TiO2 calcined sample was observed, that is not all TiO2 crystals are covered by nanoparticles. As gold is present in this sample, it is astonishing that the common homogeneous deposition of gold nanoparticles on both anatase and rutile TiO2 phases was not observed. In contrast, the sample prepared using a sequential deposition (Au-Ir/TiO2-S) showed metal nanoparticles on all of the TiO2 crystals (parts a and b of Figures 6) independently of the thermal treatment. When the sample was activated in air at 400 °C (part a of Figure 6), some TiO2 particles showed a higher concentration of IrO2 as a thin layer; however all of the TiO2 particles are covered with both IrO2 and Au particles. The average particle size in this sample is 1.9 nm. When the Au-Ir/TiO2-S sample was thermally treated in hydrogen at 300 °C, the same behavior is observed, part b of Figure 6, although a small increase in the particle size occurred. The average particle size in this sample is 2.4 nm. It is worth noting that, even in the sample reduced at

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Figure 3. HRTEM images of Ir/TiO2 catalyst calcined in air at 400 °C. (a) Image showing IrO2 deposited on a rutile crystal. FFT shows the (210) reflection of IrO2 and (101) reflection of rutile (b) image of an anatase crystal, FFT in the inset. No evidence of IrO2 particles was found on anatase crystals.

300 °C, IrO2 was still present as demonstrated by the measurements in the HRTEM images (part c of Figure 6). Sample Au-Ir/TiO2-C treated in H2 was not characterized using TEM. The presence of iridium oxide in the sample reduced at 300 °C correlates well with the observation issued from TPR experiments, which showed from quantitative analysis of the hydrogen consumption associated with reduction peaks below 300 °C, that, if reduction of the sample is carried out at this temperature, the reduction of the iridium is not complete. Identification of IrO2 in this hydrogen-treated sample is also in good agreement with the published results of Wang et al.47 who have shown that when supported Ir samples are treated in H2 at

300 °C, IrO2 is not completely reduced. Complete reduction of IrO2 particles is achieved at 400 °C. 3.5. Catalytic Performance. Part a of Figure 7 shows the CO conversion as a function of reaction temperature in the CO oxidation displayed by Ir/TiO2 samples, one calcined and the other reduced in H2. As can be seen, none of these samples is active at room temperature (light-off temperature above 250 °C). Reduced Ir/TiO2 is only slightly more active than the calcined sample. The reactivity of the reduced sample seems to be in disagreement with those previously reported by Akita et al.46 for Ir/TiO2 prepared by deposition-precipitation with NaOH and reduced in H2, which were active at room temperature. In


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Figure 4. Z-contrast images of monometallic samples thermally treated in H2 at 300 °C: (a) Ir/TiO2 and (b) Au/TiO2.

Figure 6. HRTEM images of sequentially deposited bimetallic sample (Au-Ir/TiO2-S). (a) Z-contrast image of sample thermally treated in air at 400 °C, (b) Z-contrast image of sample thermally treated in H2 at 300 °C, and (c) HRTEM image of sample treated in H2 at 300 °C. IrO2 is identified as a thin layer on the edges of the TiO2 particles on some TiO2 crystals. FFT of the selected area shows the (101) and (110) reflections of IrO2. Figure 5. Z-contrast image of codeposited bimetallic sample thermally treated in air at 400 °C (Au-Ir/TiO2-C). Metals were not homogeneously deposited on all TiO2 particles.

part b of Figure 7, the activities (molCO · molmetal-1 · s-1) of Au/ TiO2 calcined at 300 °C and Ir/TiO2 reduced at 300 °C are compared. The reactivity of Au/TiO2 catalyst at low temperature is clearly higher than that of Ir/TiO2 catalyst; at 25 °C Au/TiO2 catalyst has an activity of 0.6 (molCO · molmetal-1 · s-1) and at 30

°C it is of 1.0 (molCO · molmetal-1 · s-1). In contrast, at these temperatures Ir/TiO2 catalyst has practically an activity of zero and this holds for temperatures below 150 °C. The activities of the Au/TiO2 catalyst presented in this work are equivalent to those reported previously48,49 for very active gold catalysts. In what it concerns the bimetallic Au-Ir catalysts, Figure 8 shows the light-off curves displayed by the Au-Ir/TiO2-C sample. Practically, the behavior of the monometallic iridium catalyst is retrieved in the case of the codeposited bimetallic

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Figure 9. CO oxidation light-off curves of bimetallic catalysts: Au-Ir/ TiO2-S calcined in air at 300 °C (9) and Au-Ir/TiO2-S reduced in H2 at 300 °C (2). Catalyst sample: 0.010 g, total flow 60 mL/min.

Figure 7. CO oxidation light-off curves displayed by monometallic Ir and Au catalysts. (a) Ir/TiO2 submitted to different pretreatments. (2) Calcined in air at 400 °C or reduced in H2 at 300 °C (9), and (b) activity comparison of calcined Au/TiO2 (9) and reduced Ir/TiO2 (b). Catalyst sample: 0.020 g for Ir/TiO2, 0.010 g for Au/TiO2, total flow 60 mL/min.

Figure 8. Light-off curves in the CO oxidation displayed by codeposited Au-Ir/TiO2 catalysts (Au-Ir/TiO2-C) submitted to different pretreatments (9) calcined in air at 400 °C or reduced in H2 at 300 °C (2).

sample. The pretreatment (calcination in air or reduction in H2) has no significant effect on the performance of the sample. Taking into account the precedent results for Ir/TiO2 catalysts, this behavior suggests that the preparation method and thermal treatments in this case provide an active catalytic surface mostly enriched in iridium. The reactivity of samples prepared by sequential deposition of the metals and submitted to calcination or reduction pretreatments is shown in Figure 9. As it can be observed, the Au-Ir-S sample calcined in air at 300 °C was slightly active at room temperature and the activity rapidly increased as a function of the reaction temperature. The Au-Ir/TiO2-S sample reduced in H2 at 300 °C transformed about 50% of the CO at

Figure 10. Activity per mol of gold displayed by the Au/TiO2, Au-Ir/ TiO2-S, and a mechanical mixture of the monometallic catalysts (MX Au/TiO2 + Ir/TiO2). All samples were reduced in hydrogen at 300 °C.

25 °C. In Figure 10, the activity per mol of gold displayed by the Au-Ir/TiO2-S reduced at 300 °C is compared to that of Au/TiO2. A clear enhancement in the catalytic activity is observed in the case of the Au-Ir/TiO2-S catalyst. Particle size effects may be invoked to explain this result, but because the average particle size is about the same in both samples, Au/ TiO2 (2.2 nm) and Au-Ir/TiO2-S (2.4 nm), this seems not to be the reason for this behavior. To establish if this synergetic effect is due to an interaction between gold and iridium, the catalytic properties of a mechanical mixture (MX) of Au/TiO2 and Ir/TiO2 catalysts with a metal composition as in the Au-Ir/TiO2-S catalyst were studied. The activity curve is included in Figure 10. As observed in this case, the reactivity is closer to that of Au/TiO2. This result supports the idea that in the Au-Ir/TiO2-S catalyst some kind of interaction between the metal phases is responsible for the observed synergism. It is known that gold-based systems gradually deactivate during the catalytic run. To know the stability of the Au-Ir/ TiO2-S catalyst, time on stream experiments during 20 h at 23 °C were performed. Figure 11 shows the results of both Au/ TiO2 and Au-Ir/TiO2-S reduced in H2 at 300 °C. It is important to note that whereas the Au/TiO2 catalyst continues to deactivate after 20 h under stream, the bimetallic Au-Ir/TiO2-S catalyst becomes stable after 15 h. One of the reasons claimed for deactivation in gold catalysts is related to sintering of the gold particles. To verify the extent of sintering in Au/TiO2 and Au-Ir/TiO2-S catalysts, average particle size was determined by TEM on spent catalysts after reaction in the temperature range of room temperature to 300


Figure 11. Evolution of the CO oxidation activity as a function of time on stream for the Au/TiO2 and Au-Ir/TiO2-S catalysts reduced in H2 at 300 °C.

Figure 12. DRIFT spectra of CO adsorbed at 25 °C on Au/TiO2, Ir/ TiO2, and Au-Ir/TiO2-S samples reduced at 300 °C in hydrogen.

°C. Both samples showed an increase in the average particle size; however, it was noticeable the fact that, whereas in Au/ TiO2 the average particle size increased more than 100% (from 2.2 to 4.7 nm), in Au-Ir/TiO2-S sample it was only of about 42% (from 2.4 to 3.4 nm). A theoretical DFT study done by Liu et al.41 showed that the presence of an active Au/IrO2 interface could increase the resistance to sintering of gold nanoparticles increasing their stability. In our bimetallic Au-Ir system, the actual presence of IrO2 evidenced by TPR and HRTEM experiments is in line with this proposal to explain the higher stability of Au-Ir/TiO2 catalyst. 3.4. DRIFTS Experiments. CO adsorption and coadsorption of CO and O2 were studied by DRIFTS to obtain information about the reactive surface in Au/TiO2, Ir/TiO2, and Au-Ir/ TiO2-S catalysts. Figure 12 shows the DRIFT spectra of CO adsorbed at 25 °C on the catalysts reduced in situ in hydrogen flow at 300 °C. In the carbonylic region, gold metallic sites were identified by a CO absorption band at ca. 2116 cm-1.6,50 The iridium-based catalysts (Ir/TiO2 and Au-Ir/TiO2-S) presented strong bands in the region around 2100 cm-1 where bands characteristic of the stretching vibrations of CO molecules adsorbed on Ir metal do appear. The monometallic Ir catalyst showed an intense absorption band with a maximum at ca. 2068 cm-1 and a broad contribution in the low-frequency side. Absorption bands in this region can be assigned to CO linearly adsorbed on different Ir0 sites.51,52 On the other hand, also in the spectral region 2000-2107 cm-1, bands characterizing cationic iridium species are found.53-55 Irδ+-CO species were identified by a CO band at ca. 2100 cm-1 51 on oxidized Ir/Al2O3 after CO adsorption at

J. Phys. Chem. C, Vol. 113, No. 22, 2009 9717 room temperature. The Irδ+ surface species would result from the reduction of iridium oxidized species by CO. The presence of these strong bands shows that iridium sites strongly chemisorb CO at room temperature. The DRIFT spectrum of the bimetallic Au-Ir/TiO2-S catalyst showed also as in the case of Ir/TiO2 sample an asymmetric absorption band with a maximum at ca. 2073 cm-1 and a broadening in the low-frequency side. This band is related to that identified at ca. 2068 cm-1 in the case of the Ir/TiO2 catalyst, although some features in the profile of the band can be observed when compared to that of the monometallic iridium. Differences in dispersion of the iridium phase in both samples could explain this. A small band at ca. 2176 cm-1 appeared in the spectrum of this sample. This could be assigned to CO adsorbed on TiO2.56,57 Finally, no clear evidence of the presence of the band at ca. 2116 cm-1 characterizing CO adsorbed on Au metallic sites was obtained. The coadsorption of CO and O2 followed by DRIFTS in the temperature range 25-300 °C is shown in Figure 13 where the spectral region 1400-2500 cm-1 is presented. As observed for Au/TiO2, part a of Figure 13, the interaction of CO and O2 with the catalyst surface at 25 °C is characterized by a well-defined absorption band at ca. 2114 cm-1, a small one at ca. 2175 cm-1, not resolved bands in the frequency range 1400-1800 cm-1, and finally the signal due to CO2 around 2360 cm-1. The band at 2114 cm-1 identifies CO adsorbed on Au metallic sites. As the temperature increases to 100 °C, the intensity of this band strongly decreases with the concomitant increase of the signal due to CO2. The bands due to CO adsorbed on cationic gold species are observed in the 2188-2160 cm-1 region,57,58 so, the small band at 2175 cm-1 could be assigned to cationic gold; however, because this band is also present in the Ir/TiO2 catalyst, the assignment to CO adsorbed on the support seems more appropriate. This band is still observable at 300 °C. The intensity of the absorption bands in the carbonate region (1400-1600 cm-1) observed upon adsorption at RT decreased also as temperature increases. At 300 °C, a very small signal of the CO band and bands associated with carbonate species are observed. The presence of carbonate species in gold catalysts has been related either to unreactive spectrator species that block the active sites1,59-61 or to intermediate products that might represent a key step in the catalytic oxidation of carbon monoxide.62-64 Part b of Figure 13 shows the DRIFT spectra obtained for Ir/TiO2 catalyst. At 25 °C, the interaction of CO and O2 with the surface produced an intense broad absorption band with a maximum at 2081 cm-1. As reaction temperature increased, the position of the maximum was about the same but the intensity further increased and a shoulder in the low-frequency side at ca. 2010 cm-1 become more evident. At 300 °C, a noticeable change takes place as the intensity of the band of the adsorbed CO clearly decreased and a small blue shift of the maximum (2088 cm-1) is observed. Also, the broadness in the lowfrequency side of the band disappeared. The signal of CO2 is also clearly observed at this temperature. As previously mentioned, absorption bands characterizing iridium metal and cationic iridium species are found in the spectral region 2000-2107 cm-1.51,53-55 In the carbonate region, no clear evidence of the presence of these species in the Ir/TiO2 catalyst was observed, at least under the experimental conditions we used. Some indication of small bands in the region 1500-1800 cm-1 was obtained at 200 °C. Bands in this region have been related to carbonate-like species on iridium-supported catalysts.65 Also, as in the case of Au/TiO2 a small band at ca. 2175

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Figure 13. DRIFTS spectra of coadsorbed CO and O2 over freshly reduced catalysts as a function of reaction temperature: (a) Au/TiO2, (b) Ir/TiO2, (c) Au-Ir/TiO2-S, and (d) a mechanical mixture (MX Au/TiO2 + Ir/TiO2).

cm-1 is observed in this sample and its intensity progressively decreased as reaction temperature increased. The presence of the CO band at high temperature confirms that iridium strongly chemisorbs carbon monoxide and these observations are in line with the results obtained in the catalytic flow reactor (part a of Figure 7), which have shown that a significant CO conversion only occurred at temperatures higher than 200 °C. Part c of Figure 13 presents the DRIFT spectra of the CO + O2 reaction over Au-Ir/TiO2-S catalyst. The coadsorption of the molecules at 25 °C was characterized by the following features: absorption bands around 2114 and 2075 cm-1, the small band at ca. 2176 cm-1 previously observed in Au and Ir samples, practically no observable bands in the region 1400-1800 cm-1, which characterize the presence of adsorbed carbonates,58 and a clear indication of CO2 production characterized by absorption bands around 2360 cm-1. Taking into account the previous DRIFTS results issued from the monometallic samples, the band around 2114 cm-1 would be related to gold sites, whereas that around 2075 cm-1 would be related to iridium sites. The evolution of bands in the carbonyl region as temperature increased roughly resembles to what is observed in the case of the monometallic catalysts, that is the intensity of the band at 2114 cm-1 progressively decreased until practically disappearance at 300 °C (part a of Figure 13) and small changes occurred to the band at ca. 2075 cm-1 (part b of Figure 13). At 300 °C, the intensity of this band is very low and the maximum is blueshifted. Also, a progressive intensity diminution of the band around 2175 cm-1 is observed. In the carbonate region, a small band around 1780 cm-1 also present in the Ir/TiO2 sample was observed in the temperature range 100-200 °C. According to the preparation protocol used in Au-Ir/TiO2-S catalyst and the limited miscibility of Au and Ir in the bulk,66

alloying is not expected to occur. The DRIFT result would be in line with this assumption. However, if the catalyst consisted of segregated Au and Ir particles (metal and oxide species), it would be reasonable to expect that the interaction of CO and O2 with the catalyst surface must reflect a sum of the individual reactivity of each of the components, Au and Ir, which is not the case because it is evident the high production of CO2 at room temperature evidenced by the high intensity CO2 band not observed in the case of the monometallic samples. Moreover, this enhanced reactivity is in line with that observed for this sample in the flow reactor (Figure 10). In addition, it is interesting to note the absence of carbonate species in the DRIFT spectra of the Au-Ir/TiO2-S catalyst; such species were observed in the gold catalyst but not in the case of the iridium sample. Also, the difference in intensity of the bands associated with Au and Ir sites characterizes this sample. To clarify these points, the coadsorption of CO and O2 was followed by DRIFTS (part d of Figure 13) on the mechanical mixture (MX) of Au/TiO2 and Ir/TiO2 catalysts previously tested in the flow reactor. As mentioned before, this sample represents the case of truly segregated Au and Ir particles. As expected, the DRIFT spectra resemble the sum of features found in the monometallic catalysts. Namely, at room temperature the bands in the 2000-2200 cm-1 region identifying CO adsorbed over gold and iridium sites are present and the signal corresponding to CO2 and bands in the carbonate region both present in the case of the Au/TiO2 sample are observed. Even if CO2 is produced by the MX sample at 25 °C (part d of Figure 13), when compared to the bimetallic catalyst (Au-Ir/TiO2-S), it

Au-Ir/TiO2 Prepared by Deposition Precipitation is evident the higher reactivity presented by the Au-Ir/TiO2-S catalyst characterized by the intense CO2 signal (part c of Figure 13). These results support the assumption that, in the bimetallic catalyst (Au-Ir/TiO2-S), Au and Ir may have an interaction that induces a modification in the adsorption properties of the catalyst surface, resulting in a more active surface at low temperature. In addition, the very low carbonate-type bands in the Au-Ir/TiO2-S catalyst could also have some influence on the performance of the catalyst. 4. Conclusions Ir/TiO2 and Au-Ir/TiO2 catalysts were prepared by deposition-precipitation with urea (DPU). In the case of monometallic Ir/TiO2 samples, practically the nominal metal loading (4 wt %) was deposited. In codeposited Au-Ir/TiO2 sample, the nominal loading for gold was not achieved, as it is usually the case when the DPU method is used. This behavior could be explained in terms of an adsorption competition between Au and Ir species for the TiO2 surface. When Ir and Au were deposited sequentially, practically all Au and Ir were deposited on TiO2. Catalyst pretreatment has an important effect on the structure of the iridium phase. When calcined in air, IrO2 preferentially deposits on the rutile phase of TiO2. This behavior was retrieved in the codeposited bimetallic sample (Au-Ir/TiO2-C). In contrast, bimetallic sample prepared by sequential deposition (Au-Ir/TiO2-S) presented metal nanoparticles in practically all of the TiO2 crystals. When the Ir/TiO2 is reduced in hydrogen at 300 °C, a homogeneous distribution of small particles was observed. On their side, the Au-Ir/TiO2-S sample treated at the same temperature showed an apparent redistribution of the metal phases. TPR and HRTEM confirmed that reduction of iridium at this temperature is not complete. The reactivity of Au, Ir, and Au-Ir supported on TiO2 was studied in CO oxidation. Ir/TiO2 samples calcined or reduced in H2 were not active at room temperature (light-off temperature above 250 °C). The low reactivity displayed by iridium catalysts is in line with the DRIFTS results showing that iridium strongly adsorbs CO even at high temperature. The reactivity of the codeposited Au-Ir/TiO2 catalyst was similar to that of Ir/TiO2 monometallic sample, suggesting that the reactive surface is mainly exposing iridium sites. In the sequentially deposited sample, Au-Ir/TiO2-S, thermally treated in hydrogen at 300 °C, a synergetic effect in the activity was observed when compared to Au/TiO2. A close interaction of Au and Ir in the catalyst could modify the adsorption properties of the catalyst surface, resulting in a more reactive surface at low temperature. DRIFTS experiments and results obtained using a mechanical mixture of the Au/TiO2 and Ir/TiO2 catalysts seem to confirm this. Moreover, the stability in time on stream and against sintering is improved in the Au-Ir/TiO2-S catalyst compared to that of Au/TiO2. Acknowledgment. Technical help from Luis Rendoń LCMIF (HRTEM) and Ivan Puente USAI-FQ (EDS) is acknowledged. We are indebted to UNAM Nanoscience and Nanotechnology project (PUNTA) for financial support. R. Zanella, J. M. Saniger and H. Ramı´rez acknowledge the support provided by DGAPA IN106507 and CONACYT 55154 projects. G. Dıáz acknowledges DGAPA IN-117706 and CONACYT 42666-F projects whose infrastructure was used in this work. References and Notes (1) Haruta, M. Cattech 2002, 6, 102.

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