PdO bulk oxide

A significant research effort has been devoted to the characterization of Pd surface oxides [17, 63]. In situ studies under non-UHV conditions are most relevant, because surface oxides may only be present under reaction conditions (and not accessible by post-reaction (ex situ) characterization). A stepwise oxidation of Pd(lll) was reported [64-66], starting from the well-known (2x2) chemisorbed oxygen adlayer, followed by the formation of a two-dimensional Pd O surface oxide, which eventually transforms to a PdO bulk oxide. At high temperature, PdO decomposes with the concurrent dissolution of oxygen atoms into the bulk. In situ synchrotron HP-XPS [64, 65] indicated that a two-dimensional Pd O surface oxide was formed on Pd(lll) at 0.4 mbar O and >470 K, which transformed to PdO at temperatures >660 K. PdO was found to decompose at temperatures >720 K. 

Proceeding to Pd(lll) one observes in most aspects a continuation of the identified trends. As apparent in Fig. 5.10, the again reduced thermal stability of the PdO bulk oxide shows up by a further decreased stability range, which commences nevertheless already at an O chemical potential below the one required to stabilize higher on-surface adsorbate coverages with 0 1/4 ML. 

PdO interacting with the PdO bulk oxide or metallic Pd crystallites and... 

Fig. 9.19 XP spectra of O li/Pd 3/ 3,2 (a) and Pd 3 5,2 (b) regions for a Pd(lll) surface taken at various temperatures under the presence of O2 gas at 200 mTorr. The XP spectra are deconvoluted into several components as indicated by different colors. Structure models of the Pd504 surface oxide (c) and the PdO bulk oxide with the (101) surface orientation (d). Reproduced from ref. [56] with modification...

Figure 21 provides an example of the use of ESCA to define an oxidation state of a freshly reduced palladium-on-carbon hydrogenation catalyst exposed to the air. The metallic palladium peaks (Fig. 21a) are quite evident, indicating no bulk oxidation occurred. There is a strong peak for carbon, probably due to adsorbed CO2 from the air. The presence of a small amount of PdO is suggested at 337 eV in Fig. 21B. This peak is a shoulder on the palladium 3 5/2 peak and most likely represents a surface layer of oxide on the palladium. This information could not be conveniently obtained by XRD because small palladium (or PdO) crystallites cannot diffract X rays. Furthermore, XRD measures bulk properties and would not see surface oxides even if the crystallite sizes were sufficiently large to be XRD sensitive. We can therefore expect to see more frequent use of ESCA or other surface sensitive techniques to monitor the surface of catalytic materials.

A number of oxides can be modeled by vacuum-grown thin oxide films and oxide nanoparticles which are amenable to atomic level characterization with the application of surface-sensitive (surface science) methods. This is crucial for the advancement of the field because in many cases oxide surfaces are not simple truncations of the bulk oxide structure. Defects such as oxygen vacancies, despite being minority surface species, play an important role in the catalytic properties of an oxide surface. In this chapter, we have presented results on the synthesis, characterization and catalytic properties of Ga Oj, In Oj, and V Oj, Nb Oj, Pd O and PdO vacuum-grown thin oxide films and oxide nanoparticles. The catalytic activity of the model systems can be examined at atmospheric pressure and compared to those... 

Very similar results were also obtained by Farrauto et al. [51] from a study of the high-temperature catalytic chemistry of supported Pd for the combustion of methane. Palladium oxide supported on alumina decomposes in two distinct steps in air at atmospheric pressure. The first step occurs between 750 and 800X and is believed to be a decomposition of Pd-O species dispersed on bulk Pd metal, designated (PdO /Pd). The second decomposition occurs between 800 and 850" C, and it behaves like crystalline palladium oxide (PdO). To form the oxide once again, metallic Pd has to be cooled down to 650°C, thus causing a hysteresis gap of 150°C. Above 500°C, catalytic methane oxidation can occur only as long as the palladium oxide phase is still present. Above 650 C, metallic Pd cannot chemisorb oxygen, and hence it is catalytically inactive toward methane oxidation. 

Fig. 5.11. Top- and side view of the pseudo-commensurate, so-called ( /6 X /6) surface oxide structure determined on Pd(lll). The atomic positions do not seem to resemble the bulk PdO arrangement but exhibit roughly an O-Pd-O trilayer type structure (from [33])...

Fig. 5.13. DFT-GGA surface phase diagram of stable surface structures at a Pd(lOO) surface in constrained equilibrium with an O2 and CO environment. The stability range of bulk PdO is the same as the one shown in Fig. 5.12. Phases involving the (. 5 X. 5)R27° surface oxide exhibit a largely increased stability range, which now comprises gas phase conditions representative of technological CO oxidation catalysis (po2 = Pco = 1 atm. and 300 K < T < 600 K, marked by the black line, as in Fig. 5.12) (from [46])...

By quantification of the oxygen content it was possible to estimate the thickness of the oxide layer, related to the exposed surface of Pd (whose size was estimated after complete reduction of the sample). It was observed that the more active catalysts had a larger number of oxygen layers which suggested the participation of bulk Pd in the reaction. Therefore, for methane oxidation, the Pd/Al2C>3 catalytic properties seem to be controlled by a redox mechanism, where the oxidation and reduction rates of the active phase depend on the size and stability of the Pd and PdO species. The small PdO particles seemed to be poorly reactive, due to their higher stability or to a greater interaction with the support. In fact, analysis of TPO and TPD data revealed that the small particles of PdO decompose at... 

Heats of chemisorption of oxygen have been measured on palladium black and on palladium-silver alloys, in the temperature range 500-700° K, and fraction of surface coverages, 0, 1 X 10" to 0.7. Heat of chemisorption data were obtained by means of measurements of reaction equilibrium of water decomposition on palladium and palladium-silver alloys. For palladium blaek, a value of the adsorption heat of —24 kcal. /mole oxide was obtained. This value is not much different from the value corresponding to the formation of bulk palladium oxide (PdO). 

The XRD patterns of the in situ (PdZr-i, trace 2) and air-oxidized (PdZr-a, trace 3) alloys do not indicate significant bulk structural differences between these catalysts. After oxidation of the alloy imder reaction conditions in situ) as well as in air, a solid made up of PdO and monoclinic and tetragonal Zr02 was formed. After reduction in hydrogen only reflections due to metallic palladium and zirconia phases were detected by XRD (trace 4). 

On the other hand, on etching for 5 and 10 min the major surface components were Ni and Nb in oxide state. Pd became the prevailing component only after 15 min etching in agreement with XRD studies. After reaction Pd remained the major component on the surface mainly in PdO form and the other components disappeared from the surface. However this was not in contradiction with the XRD measurements because the bulk could be a partially crystallized NiNb alloy embedded in amorphous NiNb alloy the surface of which was fiilly covered with highly dispersed Pd oxide. 

The presence of two distinct states of Pd has also been demonstrated by TPR studies which usually show two or more H2 consumption peaks. The consumption peak observed below room temperature has been ascribed to the reduction of bulk PdO clusters, while consumptions at higher temperatures have generally been related to metal-support interactions. The negative peak typically observed near 60 °C is due to H2 release from the decomposition of P Pd-hydride. TPR has been used to correlate the amount of H2 consumption with methane oxidation activity. It has been observed that the fresh catalyst that exhibited low activity had a H2/Pd ratio of 0.3. As the catalyst activated and reached its maximum rate the H2/Pd ratio increased to 1.6, and finally after 7 h on stream the partially deactivated catalyst exhibited a H2/Pd ratio of 1.25. 

Pd-doped catalysts have been produced by USS [82]. The fingerprint of Pd adopting the octahedral coordination of Fe in LaFeo,95Pdo,o503 has been observed in the XANES spectra of the material prepared by spray synthesis (27m /g) similarly to the preparation by the amorphous citrate method (14m /g) [17,82]. In contrast, the flame-made material of the same composition (22m /g) exposed metallic Pd particles on LaFeOs similarly to preparation by solution combustion. The different nature of the Pd species obtained by changing the synthesis method dramatically influences their catalytic performance, since PdO nanoparticles exposed at the surface of the mixed oxide exhibit catalytic activity, whereas Pd—O species in the bulk of the mixed oxide are inactive, at least in the case of methane oxidation [27]. In contrast to LaFeOs, LaMnOs did not allow Pd to adopt the octahedral coordination irrespective of synthesis method. Therefore, the coordination of Pd strongly depends on both the composition of the perovskite-type oxide and the synthesis method. 

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