Pd surface oxides

The first systematic study on the oxide growth at Pd electrodes under well-defined potentiostatic conditions was reported in [135]. The impact of temperature variation on the development of Pd surface oxides was investigated in aqueous H2SO4. Various theoretical models were applied in order to elucidate the growth kinetics and mechanism in relation to the experimental conditions. 

Even when oxidation reactions are performed at much lower temperature (500 K), Pd surface oxides (oxidation state <+2) may be present and influence the catalytic activity. A number of recent surface science investigations and theoretical smdies of CO oxidation on noble metal (Pt, Pd, Rh) single crystals have suggested not only the involvement of metal-oxide phases, but the oxides were even proposed to exhibit higher activity than the metals (e.g., [60-62]). It is important to clarify whether the formation of a surface oxide (e.g., Pd O ) just accompanies the transition from the inactive to the active state or whether the surface oxide is the cause of high catalytic activity. 

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. 

The electrocatalysis of HCOOH oxidation on Pd constitutes a special case and represents an exception when compared to CH3OH and C2H5OH. Some of the early work on this topic has been presented in Section 4.1. The cyclic voltammogram of Pd(lll) in 0.1 M H2SO4 is shown by Figure 4.32 [167], Key features are i) pronunced underpotentially deposited hydrogen peaks (adsorption/desorption) around -0.05 V vs. SCE, ii) ordered SOd adlayer formation at +0.1 V vs. SCE, and iii) Pd surface oxidation at 0.8 V vs. SCE and reduction of the surface oxides at 0.4 V on the cathodic scan, generating irreversible surface defects [167]. 

The authors further tested the Pt(l 11) and Pd(l 10) surfaces [71, 72] using in situ STM and SXRD. All these single crystals show a similar kinetic behavior in CO oxidation. The gradual roughening of the surface corresponds to the formation of surface oxides and a higher CO oxidation rate. The structure insensitivity observed at high pressure is in contrast with the results obtained in UHV, where the reactivity shows a strong orientational dependence. 

In the case of Pt and Pd, the surface oxidation can also take place, but the resulting metal ions can be easily reduced in the presence of hydrogen. According to this fact, the observed difference in the regioselectivity of the rupture of the oxirane ring on Ni and Cu or Pt and Pd can be explained on the basis of the different stability of the metal ions in a hydrogen atmosphere.289... 

Oxidation of H ds. the so-called H ionization reaction, does not occur on Cu surfaces, although it occurs with essentially 100% efficiency on Pt and Pd surfaces. Van den Meerakker s mechanism is to some extent a milestone mechanism for electroless Cu deposition that many researchers pay attention to, either to support, or, find fault. 

V to +0.7 V vs. RHE for a Pd surface. Normally, this is anodic, or positive, with respect to the Em value of the electroless reaction (Fig. 1). Following removal of the oxide species from the catalyst surface, whether deposition subsequently initiates or not depends on the interplay between the kinetics of the parallel metal ion and O2 reduction reactions, and oxidation of the reducing agent. Once an appropriate Em value is reached, metal deposition will occur. 

Rh, Ru, Pd) and oxides (<4wt% Fe jO4/Cr2O3, La2O3, SnO2, K2O) was recently performed by Lodeng et al. [134]. A comparison with Ni- and Fe-based catalysts was also addressed. It was found that addition of metal promoters, particularly Rh and Pt, enhanced the catalyst activity at low temperatures (which resulted in delayed extinction of the reaction during ramping at —1 Tmin ). However, addition of Ni promoted carbon formation. Addition of surface oxides typically promoted instability, deactivation and combustion (hence the formation of a stable Co metallic phase was hindered). It was found that Ni performed better than Co-based catalysts at all temperatures. However, Fe-based catalysts showed high combustion activity. 

Dynamic effects are a potentially important but easily overlooked aspect of heterogeneous catalysis that can nonetheless impact the accuracy of our knowledge and predictions. For example, multiple co-existing meta-stable surface oxide phases have been identified for Pd and Ag interacting with oxygen, which suggests that the catalyst surfaces may be in a state of flux under reaction conditions, adding new uncertainty to the nature of the... 

In most Al-containing alloys, the shape of the particles was tear-drop like due to the tight surface oxide film. The typical shape was shown in Fig.l. The effect of rapid solidification on microstructures is shown in Fig. 5 for AI2CU (precursor for Raney Cu) with a small amount of Pd (11). In the case of slowly solidified (conventional) precursor, most of the added Pd was solidified as a secondary Pd rich phase shown by white dendritic structure in Fig.5 (a). On the other hand, no such secondary phase was observed in a rapidly solidified precursor as shown in Fig.5 (b). 

Significant improvement of the activity of selectivity of Pd on Si02 could be achieved in the hydrogenation of acetylene by adding Ti, Nd, or Ce oxides to the catalyst.395 The metal oxides modify both geometrically and electronically the Pd surface. They retard the sintering of the dispersed Pd particles, suppress the formation of multiply bound ethylene, and facilitate the desorption of ethylene. The beneficial effect of lead in the hydrogenation of 1,3-butadiene over a Pd-Pb-on-... 

In the experiment, a bare Pd surface was exposed to oxygen, until the surface attained a saturation coverage of O(s) of 0glt=O.4. The oxygen source was then turned off, and the surface was exposed to a constant flux of CO of Fco beginning at time t = 0 s. A quadrupole mass spectrometer was used to monitor the flux of the oxidation product C02, as well as CO, from the surface. The coverages of O(s) and CO(s) were deduced as a function of time through analysis of the data and the surface reaction mechanism above. 

The selectivity towards FFCA is dependent on the type of catalyst used. For example the maximum yield in the case of Pd catalysts is about 50%, whereas Pt and Ru reach a maximum of 75%. In the case of Pd, oxidation of the aldehyde group as first step of the reaction (leading to HFCA) plays an important role. This might be due to a higher concentration of hydrated HMF present on the Pd surface. Also the catalyst support influences the selectivity of the oxidation. For Pd/Al-N the selectivity towards HFCA is higher than for Pd/C, with the maximum yield of FFCA being almost the same. 

Finally, if the role of Au is essentially to act as a structural promoter for Pd, it is expected that the support influences this effect. The observation of Hutchings et al. [ 105 ] that Pd-Au/carboncatalysts showthe presenceofahomogeneous alloy (both Au and Pd on the surface) and improved reactivity with respect to Pd-Au/oxide (Ti02, A1203 and Fe203), where a core-shell structure is observed, is thus not surprising. This result evidences that Au has a structural effect more than an electronic effect [106]. 

In the absence of oxygen, the reaction depicted by (2.5) is reversible. However, when oxygen is present, a competing oxidation takes place at the Pd surface, making the overall reaction irreversible. 

It is interesting also to compare the results of the present experiment, which shows directly that a competitive mechanism occurs in the co-adsorption of NO and CO, with previous studies on several surfaces of the platinum group metals. On Pt(lll) and Pt(110), Lambert and Comrie (65) have inferred from thermal desorption data that gaseous CO displaces molecular NO from the surface and causes also a conversion between two thermal desorption states of molecular NO. Similarly, Campbell and White (55) report that adsorbed CO inhibits the oxidation of CO by NO at low temperature on polycrystalline Rh. They attribute this to the occupation of sites by CO which are required for NO adsorption and dissociation. Conrad et al. (66) have used UV-photoelectron spectroscopy to observe directly the displacement of molecular NO by gaseous CO from Pd(110) and polycrystalline Pd surfaces. Thus, it appears that adsorption of molecular CO and NO is competitive on these... 

These models consider the mechanisms of formation of oscillations a mechanism involving the phase transition of planes Pt(100) (hex) (lxl) and a mechanism with the formation of surface oxides Pd(l 10). The models demonstrate the oscillations of the rate of C02 formation and the concentrations of adsorbed reactants. These oscillations are accompanied by various wave processes on the lattice that models single crystalline surfaces. The effects of the size of the model lattice and the intensity of COads diffusion on the synchronization and the form of oscillations and surface waves are studied. It was shown that it is possible to obtain a wide spectrum of chemical waves (cellular and turbulent structures and spiral and ellipsoid waves) using the lattice models developed [283], Also, the influence of the internal parameters on the shapes of surface concentration waves obtained in simulations under the limited surface diffusion intensity conditions has been studied [284], The hysteresis in oscillatory behavior has been found under step-by-step variation of oxygen partial pressure. Two different oscillatory regimes could exist at one and the same parameters of the reaction. The parameters of oscillations (amplitude, period, and the... 

Infra-red chemiluminescence has been used to measure the vibrational state distributions of CO2 formed by reaction on Pt and Pd surfaces [45-50]. While the detection sensitivity of infrared chemiluminescence does not approach that of LIF or REMPI, it is an attractive way to probe molecules such as CO2 where there is substantial vibrational excitation and REMPI schemes are not available. In some cases Doppler measurement of the translational energy release can be achieved, giving direct information on the translational energy release of vibrationally excited C02 [45]. Recently infrared diode laser spectroscopy has been used to detect vibrationally excited CO2 from CO oxidation over... 

Figure 14 Steady state CO2 production on various Pd surfaces during CO oxidation. The maximum rates have been normalized to the same value. (A) Pd(l 1 1), ( ) Pd(l 00), ( ) Pd(l 1 0), (A) Pd(2 1 0), (O) Pd foil (from Ref. [124]).

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