|Abstract:||Catalytic upgrading of biomass derivatives (i.e., ethanol from fermentation and bio-oil from pyrolysis) requires increased understanding of reaction mechanisms and catalyst properties in order to improve the design of highly selective, reactive, cost effective, and stable catalysts. In situ characterization methods provide the ability to correlate different catalyst properties (e.g., surface acidity, hydrophobicity, cluster size) under reaction conditions with changes in rates and selectivity. Here, we describe how the combination of kinetic measurements, density functional theory (DFT) calculations, and in situ spectroscopic techniques elucidate reaction mechanisms and probe catalyst properties during reactions of oxygenates over supported metal, metal oxide, and metal phosphide materials. We also detail a combination of analytical techniques and mathematical methods to deconvolute complex spectroscopic data sets in order to identify individual reactive intermediate structures over heterogeneous catalysts.
Selective dehydrogenation catalysts that produce acetaldehyde (C2H4O) from bio-derived ethanol can increase the efficiency of subsequent processes such as C-C coupling over metal oxides to form longer chain oxygenates and hydrocarbons for fuels and chemicals. Copper (Cu) catalysts are selective for dehydrogenation; however, they suffer from esterification as a major side reaction which decreases dehydrogenation yields and reduces the value of the products stream for fuel and lubricant applications. In Chapter 2, we use in situ X-ray absorption spectroscopy, steady-state rate measurements, and reaction inhibition studies to identify the active sites for esterification and dehydrogenation on Cu catalysts as Cuδ+ and Cu, respectively. The number of esterification active sites (i.e., Cuδ+ species) depends on the cluster size (i.e., number of perimeter Cu atoms), and the identity of the support, which affects the extent of charge transfer. This relationship holds for Cu clusters over a wide-range of diameters and catalyst supports, and reveals that dehydrogenation selectivities may be controlled by manipulating either.
The selective hydrogenolysis of C-O bonds in small oxygenates is an important step for the conversion of biomass-pyrolysis oils into fuels and valuable chemicals, such as α,ω-diols from furanic species. Metal phosphides are promising catalysts for C-O bond rupture as they are inexpensive, chemically and thermally stable, and selectively cleave C X bonds (where X=S, N, or O) over C-C bonds. In Chapters 3 and 4, we combine kinetic analysis of reaction networks, modulation excitation in situ infrared spectroscopy (MES), and DFT calculations to describe the mechanism and reactive intermediates for C-O bond rupture over silica supported Ni, Ni12P5, and Ni2P nanoparticles. Rates of C-O bond rupture within 2-methyltetrahydrofuran (MTHF), a model pyrolysis oil component, combined with DFT calculations show adsorption and dehydrogenation are quasi-equilibrated and precede kinetically relevant C-O bond rupture.3 Selectivities for cleaving sterically-hindered C-O (3C-O) bonds are higher than for unhindered C-O (2C-O) bonds over Ni12P5 and Ni2P, and 3C-O bond selectivities are much greater over Ni12P5 and Ni2P than Ni. Measured and DFT-predicted apparent activation enthalpies (ΔH‡) for C-O bond rupture indicate that the phosphorus content decreases the ΔH‡ for 3C-O bond rupture relative to that of 2C-O bond rupture. Sites that bind the reactive intermediates for both 3C-O and 2C-O bond rupture resemble those that bind CO*, NH3*, and H* on Ni, Ni12P5, and Ni2P catalysts, which implies C-O bond rupture selectivity depends on differences between the coordination of reactive intermediates to the same active site and not on differences between the ensemble of surface atoms.3 Selectivity differences between specific C-O bonds within MTHF reflect differences in the H-content of reactive intermediates, activation enthalpy barriers, and phosphorus content of Ni, Ni12P5, and Ni2P.
Direct experimental evidence for the structure of reactive intermediates distinguishes relationships between C-O bond rupture selectivity, surface coordination, and energy barriers over Ni, Ni12P5, and Ni2P catalysts; however, identifying reactive intermediates within an “organometallic zoo” of species that form on catalytic surfaces during reactions is a long standing challenge in heterogeneous catalysis. Sinusoidal modulation of H2 pressure reveals spectral features of reactive intermediates by suppressing those of inactive surface species through the application of phase sensitive detection (PSD). The combined spectra of all reactive species are deconvoluted using singular value decomposition techniques that provide distinct spectra and changes in surface coverages for independent species. These deconvoluted spectra indicate the presence of distinct reactive surface intermediates during C-O bond rupture of MTHF on Ni, Ni12P5, and Ni2P that are consistent with intermediates proposed from measured rates and DFT calculations. Our experiments show that the compositions of the most abundant reactive intermediate (MARI) on Ni, Ni12P5, and Ni2P nanoparticles during C-O bond rupture of MTHF are identical; however, the MARI changes orientation from Ni3(μ3-C5H10O) to Ni3(η5-C5H10O) (i.e., lies more parallel with the catalyst surface) with increasing phosphorus content. The shift in binding configuration with phosphorus content suggests the decrease in steric hindrance to rupture the 3C-O bond is the fundamental cause for increased selectivity towards 3C-O bond rupture. Kinetic measurements and calculations indicate C-O bond rupture occurs on Ni ensembles on Ni, Ni12P5, and Ni2P catalysts; however, the addition of more electronegative phosphorus atoms that withdraw a small charge from Ni ensembles results in the differences in binding configuration, activation enthalpy and selectivity. The derivation of the reaction mechanism, determination of the active site, and identification of binding configuration suggest that manipulation of the electronic structure of metal ensembles by the introduction phosphorus provides strategies to design catalysts for selective cleavage of hindered C-X bonds during hydrogenolysis of bio- or petroleum-derived feedstocks.
We continue to explore methods to deconvolute the “organometallic zoo” in Chapter 5, using operando infrared spectroscopy, selective inhibition, and transient measurements to determine the reaction mechanism for formic acid (HCOOH) on Au and TiO2 surfaces when both are present on the same catalyst. Transient cutoff experiments indicate reactive intermediates bound to TiO2 convert through monodentate and bidentate formates to form CO and CO2 while residual bidentate formates bind strongly to the TiO2 surface. The addition of Au or co-fed water decreases the accumulation of residual bidentate formates, which suggests Au facilitates the diffusion of water molecules across the TiO2 surface. Modulation of CO pressure at shorter time scales than the decomposition of formates on TiO2 surfaces isolate monodentate intermediates in two binding configurations on Au nanoparticles. These data elucidate the different intermediates and mechanism for HCOOH decomposition over Au and TiO2 surfaces on a single catalyst and show the utility of inhibition and transient measurements to isolate reactions occurring at significantly different rates.
Distinguishing the composition and orientation of reactive intermediates provides complimentary evidence to measured rates and DFT calculations to depict reaction mechanisms and active site identities. The combination of these powerful techniques provides insight into how surface properties dictate rate and selectivity, and further the design of cost effective catalysts.