Methanol synthesis from carbon monoxide and hydrogen over platinum-iron and platinum catalysts
Abstract
The active phase requirement for production of methanol from 3H$\sb2$/CO at 31.6 atm to 1 atm pressure over supported PtFe catalysts was studied with X-Ray Photoelectron Spectroscopy (XPS), temperature programmed reduction (TPR), and Mossbauer spectroscopy. The results from the carbon and silica supported catalysts as well as a 3Pt1Fe powder indicate that the Fe$\sp{3+}$ phase observed with Mossbauer spectroscopy and XPS over silica-supported catalysts is not a required species for the methanol enhancement of PtFe catalysts versus Pt only catalysts. XPS was used to correlate the specific catalytic activities to the surface ratios of Pt$\sp0$/Fe$\sp0$. XPS analyses show that the optimum surface Pt$\sp0$/Fe$\sp0$ ratio for methanol production is in the range of 3 to 10. When the surface becomes Pt-rich (Pt$\sp0$/Fe$\sp0$ $>$ 15), then the methanol activity and selectivity decline, however, they are still higher than those from Pt only catalysts. For a low Pt alloy (Pt$\sp0$/Fe$\sp0$ $<$ 2), high hydrocarbon activity, typical of Fe, dominates. For the transition region between the hydrocarbon active and the optimum surface ratios (2 $>$ Pt$\sp0$/Fe$\sp0$ $>$ 3), the surface arrangement of Pt and Fe appears to control the selectivity of the catalysts. The alloy surface in the transition region can become activated to produce hydrocarbons that suppress the methanol selectivity of the catalysts when the dispersion is low (i.e. 23%). TPR studies indicate that the hydrocarbon active PtFe catalyst is free of unalloyed Fe and, therefore, that a PtFe surface with large ensembles of Fe is the active phase for the hydrocarbon production. With a high dispersion of metals (i.e. 43%), the alloy surface in the transition region can suppress formation of large Fe ensembles, thereby maintaining high methanol selectivity. Furthermore, XPS and Mossbauer spectroscopy show that there is an electronic interaction between two metals as indicated by a positive binding energy shift of Fe in XPS and a positive isomer shift in Mossbauer spectroscopy. Mossbauer spectroscopy and XPS measurements indicate that there is a net 4s electron transfer from Fe to Pt. The possible causes of the enhanced methanol activity are suppression of Pt ensembles for CO dissociation and formation of Fe$\sp{\delta+}$ active sites generated by the alloy formation.
Degree
Ph.D.
Advisors
Delgass, Purdue University.
Subject Area
Chemical engineering
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