Cellulose-derived tetrahydrofuran-dimethanol (THFDM) can be converted over Pt-WO x /TiO2 catalysts to 1,6-hexanediol (1,6-HDO) with up to 70% yield. This reaction involves ring-opening of THFDM to 1,2,6-hexanetriol (HTO) and then hydrogenolysis of HTO to 1,6-HDO. Hydrogen atoms spill over from Pt sites onto WO x /TiO2 to reduce the WO functional group and create Brønsted acid sites. Similar catalytic activity for THFDM conversion can be been obtained with a physical mixture of Pt/TiO2 and WO x /TiO2 due to hydrogen spillover over spatially separate Pt and WO x when a reducible support (TiO2) is used.

Microkinetic analysis plays an important role in catalyst design because it provides insight into the fundamental surface chemistry that controls catalyst performance. In this review, we summarize the development of microkinetic models and the inclusion of scaling relationships in these models. We discuss the importance of achieving stoichiometric and thermodynamic consistency in developing microkinetic models. We also outline how analysis of the maximum rates of elementary steps can be used to determine which transition states and adsorbed intermediates are kinetically significant, allowing the derivation of general reaction kinetics rate expressions in terms of changes in binding energies of the relevant transition states and intermediates. Through these analyses, we present how to predict optimal surface coverages and binding energies of adsorbed species, as well as the extent of potential rate improvement for a catalytic system. For systems in which the extent of potential rate improvement is small because of limitations imposed by scaling relations, different approaches, including the addition of promoters and formation of catalysts containing multiple functionalities, can be used to break the scaling relations and obtain further rate enhancement.

Well-defined Cu catalysts containing different amounts of zirconia were synthesized by controlled surface reactions (CSRs) and atomic layer deposition methods and studied for the selective conversion of ethanol to ethyl acetate and for methanol synthesis. Selective deposition of ZrO2 on undercoordinated Cu sites or near Cu nanoparticles via the CSR method was evidenced by UV–vis absorption spectroscopy, scanning transmission electron microscopy, and inductively coupled plasma absorption emission spectroscopy. The concentrations of Cu and Cu-ZrO2 interfacial sites were quantified using a combination of subambient CO Fourier transform infrared spectroscopy and reactive N2O chemisorption measurements. The oxidation states of the Cu and ZrO2 species for these catalysts were determined using X-ray absorption near edge structure measurements, showing that these species were present primarily as Cu0 and Zr4+, respectively. It was found that the formation of Cu-ZrO2 interfacial sites increased the turnover frequency by an order of magnitude in both the conversion of ethanol to ethyl acetate and the synthesis of methanol from CO2 and H2.

The selective hydrogenation of acetylene has been studied over AgPd and CuPd catalysts. Controlled surface reactions were used to synthesize these bimetallic nanoparticles on both TiO2 and SiO2 supports. Chemisorption measurements of the bimetallic catalysts indicate that Pd prefers to be on the nanoparticle surface with a Cu parent catalyst, while Pd prefers to be subsurface with a Ag parent catalyst. From energy-dispersive X-ray spectroscopy analysis, the composition of the nanoparticles is determined to be more uniform on the SiO2 support compared to that on the TiO2 support. X-ray absorption spectroscopy results indicate that, after reduction, the CuPd bimetallic catalysts have some Pd–Pd bonds, but the average number of Pd–Pd bonds decreases after reaction. Infrared spectra of the adsorbed CO show that an increased fraction of isolated Pd species are present on the bimetallic catalysts compared to those on the monometallic catalysts. Adsorption of acetylene and ethylene, however, indicates adsorbed surface species that require contiguous Pd ensembles. These results suggest that the surface structure of the catalyst is highly dynamic and influenced by the gas environment, as well as the support. The catalysts are active for the selective hydrogenation of acetylene in an ethylene-rich environment under mild conditions. Over all catalysts, the ethylene selectivity is greater than 92%; however, improved selectivity is observed over the bimetallic catalysts compared to that over the monometallic Pd catalysts. An ethylene selectivity of 100% is observed over the CuPd0.08/TiO2 catalyst. The highest acetylene conversion rate per gram of Pd is observed over the CuPd0.02/TiO2 catalyst, while the highest turnover frequency is found over the AgPd0.64/TiO2 catalyst. The bimetallic SiO2-supported catalysts have lower rates than Pd/SiO2 but still show improved selectivity. The combined characterization measurements and reaction kinetics studies indicate that the performance improvements of the bimetallic catalysts may be attributed to both electronic and geometric modifications of Pd by the parent Cu or Ag metal.

Core-shell architectures offer an effective way to tune and enhance the properties of noble-metal catalysts. Herein, we demonstrate the synthesis of Pt shell on titanium tungsten nitride core nanoparticles (Pt/TiWN) by high temperature ammonia nitridation of a parent core-shell carbide material (Pt/TiWC). X-ray photoelectron spectroscopy revealed significant core-level shifts for Pt shells supported on TiWN cores, corresponding to increased stabilization of the Pt valence d-states. The modulation of the electronic structure of the Pt shell by the nitride core translated into enhanced CO tolerance during hydrogen electrooxidation in the presence of CO. The ability to control shell coverage and vary the heterometallic composition of the shell and nitride core opens up attractive opportunities to synthesize a broad range of new materials with tunable catalytic properties.

Rh/SiO 2 catalysts promoted with Fe and Mn are selective for synthesis gas conversion to oxygenates and light hydrocarbons at 523 K and 580 psi. Selective anchoring of Fe and Mn species on Rh nanoparticles was achieved by controlled surface reactions and was evidenced by ultraviolet−visible absorption spectroscopy, scanning transmission electron microscopy, and inductively coupled plasma absorption emission spectroscopy. The interaction between Rh and Fe promotes the selective production of ethanol through hydrogenation of acetaldehyde and enhances the selectivity toward C 2 oxygenates, which include ethanol and acetaldehyde. The interaction between Rh and Mn increases the overall reaction rate and the selectivity toward C 2+ hydrocarbons. The combination of Fe and Mn on Rh/SiO 2 results in trimetallic Rh-Fe-Mn catalysts that surpass the performance of their bimetallic counterparts. The highest selectivities toward ethanol (36.9%) and C 2 oxygenates (39.6%) were achieved over the Rh-Fe-Mn ternary system with a molar ratio of 1:0.15:0.10, as opposed to the selectivities obtained over Rh/SiO 2 , which were 3.5% and 20.4%, respectively. The production of value-added oxygenates and C 2+ hydrocarbons over this trimetallic catalyst accounted for 55% of the total products. X-ray photoelectron spectroscopy measurements suggest that significant fractions of the Fe and Mn species exist as metallic iron and manganese oxides on the Rh surface upon reduction. These findings are rationalized by density functional theory (DFT) calculations, which reveal that the exact state of metals on the surfaces is condition-dependent, with Mn present as Mn(I) and Mn(II) oxide on the Rh (211) step edges and Fe present as Fe(I) oxide on the step edge and metallic subsurface iron on both Rh steps and terraces. CO Fourier transform infrared spectroscopy and DFT calculations suggest that the binding of CO to Rh (211) step edges modified by Fe and/or manganese oxide is altered in comparison to CO adsorption on a clean Rh (211) surface. These results suggest that Mn 2 O x species and Fe and Fe 2 O modify bonding at Rh step edges and shift reaction selectivity away from CH 4 .

Supported Pt-Mo catalysts were prepared with different Mo contents by a controlled surface reaction (CSR) method and studied for the reverse water gas shift (RWGS) reaction under dark and visible light irradiation conditions. Characterization results from Raman spectroscopy, scanning transmission electron microscopy (STEM), CO chemisorption, and inductively coupled plasma-absorption emission spectroscopy (ICP-AES) indicate that selective Mo deposition onto Pt was achieved at low Mo loading (Mo/Pt ratio < 0.3). Mo deposition changed the apparent activation energy and CO 2 and CO reaction orders. Visible light irradiation changed the apparent activation energy and reaction orders of CO 2 , CO, and H 2 , ascribed to direct photoexcitation. The intrinsic activity of a Pt site is 0.4 and 4.1 min-1 under dark and light conditions at 473 K, respectively, whereas the intrinsic activity of a Pt-MoO x site is increased to 22.7 and 160 min-1 under dark and light conditions at 473 K, respectively.

Conversion of synthesis gas into value-added products, including oxygenates and C2+ hydrocarbons, was studied at 523 K, 580 psi, and a 1/1 CO/H2 ratio over Rh catalysts on catalyst supports prepared by atomic layer deposition (ALD) of molybdenum and tungsten species on silica. The reactivity measurements showed that coating the silica support with molybdenum and tungsten species helped to suppress the methane selectivity and promote the overall conversion rate. When the silica support was coated with five cycles of β-Mo2C, the methane selectivity decreased from 32% (1% Rh/SiO2) to 13% (1% Rh/5c-Mo2C/SiO2), and the overall product rate increased 33 times from 0.4 to 12.7 mmol min–1 (g of Rh)−1. CO-FTIR results showed that supporting Rh on silica led to the formation of Rh(211) facets, which favored the formation of methane and had a higher CO conversion rate. Rh on a MoO3/SiO2 support prepared by ALD was found to be the most active catalyst while maintaining the suppression of methane selectivity, showing an overall rate ∼60 times higher than that of 1% Rh/SiO2. A reaction pathway is proposed, in which hydrogenation steps are promoted most significantly by Mo and W species, followed by promotion of CO insertion steps for ethanol synthesis and C–C coupling steps for hydrocarbon formation. CO-FTIR results showed that 1% Rh/MoO3/SiO2 has the highest proportion of gem-dicarbonyl adsorption sites and the lowest proportion of bridge-bonded CO adsorption sites. The rate of methanol formation shows a positive correlation with the number of sites that form gem-dicarbonyl species.