Methanol decomposition

Methanol decomposition is an on-board source of H2 and CO for chemical processes and fuel cells  

The reaction is endothermic and can be performed over metals from Group 10, among which Ni and Pd have been those most widely studied. These metals have been supported on different oxide substrates such as AI2O3, Ti02, Si02, Ce02, Zr02 and Pr2 03.112-123 Palladium seems to be the most effective for methanoi decomposition and in the case of 

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At high alkali coverages (near monolayer coverage), when the adsorbed alkali overlayer shows a metal-like character, alkali-methoxy species are formed. As shown by TPD experiments in the system K/Ru(001) these alkali-methoxy species are more stable than the methoxy species on clean Ru(001) and adsorbed methanol on 0.1K/Ru(001). On metal surfaces inactive for methanol decomposition, e.g. Cu(lll), these alkali-methoxy species are formed even at low alkali coverages, due to the weaker interaction of the alkali atoms with the metal surface. The formation of these species stabilizes the methoxy species on the metal surface and enhances the activity of the metal surface for methanol decomposition. 

Ab initio methods allow the nature of active sites to be elucidated and the influence of supports or solvents on the catalytic kinetics to be predicted. Neurock and coworkers have successfully coupled theory with atomic-scale simulations and have tracked the molecular transformations that occur over different surfaces to assess their catalytic activity and selectivity [95-98]. Relevant examples are the Pt-catalyzed NO decomposition and methanol oxidation. In case of NO decomposition, density functional theory calculations and kinetic Monte Carlo simulations substantially helped to optimize the composition of the nanocatalyst by alloying Pt with Au and creating a specific structure of the PtgAu7 particles. In catalytic methanol decomposition the elementary pathways were identified... 

Cao D, Lu G-Q, Wieckowski A, Wasileski SA, Neurock M. 2005. Mechanisms of methanol decomposition on platinum A combined experimental and an initio approach. J Phys ChemB 109 11622-11633. 

Without the direct pathway contribution, this equation may either yield an increasing or decreasing current transient, depending on the value of A ox/ dec- If this ratio is larger than 4, i.e., if methanol decomposition is slow compared with CO oxidation, then the current is predicted to increase with time. Experimentally, this simation has been observed for a low methanol concentration and an almost perfect Pt(l 11) electrode [Housmans and Koper, 2003], which both lead to a low methanol decomposition rate. Typically, however, current transients decrease with time, suggesting that the rate... 

Figure 12.13 Electrochemistry and kinetics of CO resulting from methanol decomposition on polycrystalline Pt with O.IM H2SO4 electrol3de and 0.1 M methanol, (a-d) Current, SFG amphtude, frequency, and width of adsorbed CO, scanning the potential in both directions as indicated with the solid hne and fiUed circles denoting the forward (anodic) scan and the dashed hne and unfilled circles denoting the back (cathodic) scan, (e-g) Starting at 0.6 V, where the adsorbed CO is rapidly electro-oxidized, the potential is suddenly jumped to 0.2 V. The reformation of the CO layer (CO chemisorption) due to methanol decomposition occurs in about 20 s. The adsorbed CO molecules are redshifted, and have a broader spectrum at shorter times, when the adlayer coverage is low.

A similar question may also be asked for the indirect pathway Is COad directly formed by methanol decomposition, or does it result from a follow-up reaction. 

The final coal product in the MeOH/KOH experiments was 20%-25% soluble in the methanol. When the methanol was removed, the resultant product was a room temperature liquid with the properties described in Table V. Apparently the polymethylphenol fraction is formed by the cleavage of phenolic ethers and subsequent methyla-tion by the CO that is present in the reaction mixture as a result of methanol decomposition. The methylation reaction has been observed before for similar systems (3). 

A CO and H2 space time yield comparison of recent literature on methanol decomposition... 

Partial dehydrogenation of methanol can yield formaldehyde and this can potentially react with itself or unconverted methanol to yield another commonly reported by-product of methanol decomposition, methyl formate 1,12... 

However, it is possible to rationalise this apparent pathway as a combination of methanol decomposition, methanation and the water-gas shift reaction. 

In a comparison of supports, Nakagawa et al.21 have reported that oxidised diamond is the most effective support for Ni in terms of surface area normalised hydrogen production via methanol decomposition. In addition, they have compared the activities of a number of metals supported on oxidised diamond and have reported that Ni is a better catalyst than Co, Pd, Rh, Ir, Cu, Pt and Ru. In this context, oxidised diamond refers to natural or diamond powder which was initially hydrogenated under pure hydrogen at 900 °C and then oxidised using 20% oxygen in the 300 °C-1000 °C temperature range.76... 

The most active Pd based catalysts in the table have been reported by Shishido et al.51 who have employed hydrotalcite precursors, using a solid phase crystallisation (SPC) method. Comparisons made with catalysts prepared via impregnation showed SPC to afford higher activity catalysts, which was ascribed to better dispersion and also easy desorption of CO. The inclusion of a Cr component in these types of catalyst has also been reported to be beneficial.52 In terms of the desorption of CO, isotopic studies performed by McKee53 over ruthenium and platinum catalysts have indicated that the rate determining step of methanol decomposition is the fission of a... 

Within the literature, FTIR has been used to identify the various intermediate species formed over Au/Ti02 under both methanol decomposition and methanol reforming conditions.65 In the case of methanol decomposition, unlike methanol reforming, only methoxy species associated with Ti02 were observed. 

Imamura, Kaito, and coworkers—metal-support effects observed after calcination. Imamura et al 9X reported a strong metal-support interaction between Rh and Ce02, whereby high surface area ceria calcined at low temperature (550 °C) was able to transport Rh particles to the bulk, as measured by XPS. They suggested that despite the low degree of exposure of the Rh particle at the surface, the exposed Rh was highly active for the methanol decomposition reaction. 

The reason for the lack of methanol decomposition on Pt(lll) has been explained using DFT results. On a 1/4 ML covered surface, the activation barriers for the formation of hydroxymethyl (CH2OH) and methoxy (CH30) have been calculated to be 0.96 eV and 1.47 eV, respectively.92 For a 1/9 ML covered surface, the barriers have been calculated as 0.63 eV and 0.81 eV, respectively.96 These barriers are significantly larger than the desorption barriers. Thus, adsorbed methanol will desorb prior to decomposition in UHV. 

As was previously mentioned, PtRu alloys exhibit improved performance over pure Pt alloys.117,118 This is primarily a result of the ability of Ru to dissociate H20 for reaction with CO adsorbed on Pt sites.115,116 That CO oxidation on pure Ru is unfavorable indicates that on the bimetallic surface, CO is oxidized only on the Pt sites.119 Thus, CO is oxidized on Pt sites adjacent to Ru sites, where water is activated.120,121 This is known as a bifunctional mechanism. In addition, the presence of Ru atoms reduces the adsorption energy of CO on neighboring Pt atoms, lowering the activation energy of CO oxidation.122 This effect is purely electronic and is less significant than the bifunctional effect of Ru.123 One significant limitation of PtRu is the weak adsorption of methanol on Ru, particularly at room temperature.117,124 The weak adsorption severely hinders methanol decomposition, which is evident in Fig. 7 by the drop in current density for PtRu electrodes with high Ru composition.125... 

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