Faisal Mehmood | University of Management and Technology (original) (raw)

Papers by Faisal Mehmood

Science, 2010

Production of the industrial chemical propylene oxide is energy-intensive and environmentally unf... more Production of the industrial chemical propylene oxide is energy-intensive and environmentally unfriendly. Catalysts based on bulk silver surfaces with direct propylene epoxidation by molecular oxygen have not resolved these problems because of substantial formation of carbon dioxide. We found that unpromoted, size-selected Ag3 clusters and approximately 3.5-nanometer Ag nanoparticles on alumina supports can catalyze this reaction with only a negligible amount of carbon dioxide formation and with high activity at low temperatures. Density functional calculations show that, relative to extended silver surfaces, oxidized silver trimers are more active and selective for epoxidation because of the open-shell nature of their electronic structure. The results suggest that new architectures based on ultrasmall silver particles may provide highly efficient catalysts for propylene epoxidation.

Density functional theory (DFT) calculations were carried out to investigate the fundamental chem... more Density functional theory (DFT) calculations were carried out to investigate the fundamental chemical mechanisms underlying the selective oxidation of propene (CH3--CH=CH2) to propylene oxide (CH3--CH--CH2O) or acrolein (CH2=CH--CHO). Silver which is a known catalyst for these reactions, is modeled as a silver trimer supported on θ-Al2O3 surface to investigate reaction mechanisms. Each reaction step starts with O2 dissociation on interface of Ag3 and alumina with an activation barrier that was found to be significantly smaller than what was reported on an Ag(111) surface. Propylene oxide formation was achieved through oxametallocyle formation with a much small barrier of 0.12 eV compared to 0.70 eV on Ag(111). We will also show how acrolein formation can be triggered by the abstraction of first H atom with essentially no apparent barrier on a Ag3 cluster. Finally, we rationalize our results in comparison to experiments that show an enormous increase in reactivity for propylene epoxidation on subnanometer silver cluster. We will also discuss the preference of Ag trimers over tetramers for O2 dissociation and how the microscopic understanding of such information can help better design new catalysts.

Journal of Physical Chemistry B, 2010

A density functional theory study of the decomposition of methanol on Cu(4) and Co(4) clusters is... more A density functional theory study of the decomposition of methanol on Cu(4) and Co(4) clusters is presented. The reaction intermediates and activation barriers have been determined for reaction steps to form H(2) and CO. For both clusters, methanol decomposition initiated by C-H and O-H bond breaking was investigated. In the case of a Cu(4) cluster, methanol dehydrogenation through hydroxymethyl (CH(2)OH), hydroxymethylene (CHOH), formyl (CHO), and carbon monoxide (CO) is found to be slightly more favorable. For a Co(4) cluster, the dehydrogenation pathway through methoxy (CH(3)O) and formaldehyde (CH(2)O) is slightly more favorable. Each of these pathways results in formation of CO and H(2). The Co cluster pathway is very favorable thermodynamically and kinetically for dehydrogenation. However, since CO binds strongly, it is likely to poison methanol decomposition to H(2) and CO at low temperatures. In contrast, for the Cu cluster, CO poisoning is not likely to be a problem since it does not bind strongly, but the dehydrogenation steps are not energetically favorable. Pathways involving C-O bond cleavage are even less energetically favorable. The results are compared to our previous study of methanol decomposition on Pd(4) and Pd(8) clusters. Finally, all reaction energy changes and transition state energies, including those for the Pd clusters, are related in a linear, Brønsted-Evans-Polanyi plot.

Nature Materials, 2009

Small clusters are known to possess reactivity not observed in their bulk analogues, which can ma... more Small clusters are known to possess reactivity not observed in their bulk analogues, which can make them attractive for catalysis 1-6 . Their distinct catalytic properties are often hypothesized to result from the large fraction of undercoordinated surface atoms 7-9 . Here, we show that sizepreselected Pt 8−10 clusters stabilized on high-surface-area supports are 40-100 times more active for the oxidative dehydrogenation of propane than previously studied platinum and vanadia catalysts, while at the same time maintaining high selectivity towards formation of propylene over byproducts. Quantum chemical calculations indicate that undercoordination of the Pt atoms in the clusters is responsible for the surprisingly high reactivity compared with extended surfaces. We anticipate that these results will form the basis for development of a new class of catalysts by providing a route to bond-specific chemistry, ranging from energy-efficient and environmentally friendly synthesis strategies to the replacement of petrochemical feedstocks by abundant small alkanes 10,11 .

Nature Materials, 2009

Small clusters are known to possess reactivity not observed in their bulk analogues, which can ma... more Small clusters are known to possess reactivity not observed in their bulk analogues, which can make them attractive for catalysis 1-6 . Their distinct catalytic properties are often hypothesized to result from the large fraction of undercoordinated surface atoms 7-9 . Here, we show that sizepreselected Pt 8−10 clusters stabilized on high-surface-area supports are 40-100 times more active for the oxidative dehydrogenation of propane than previously studied platinum and vanadia catalysts, while at the same time maintaining high selectivity towards formation of propylene over byproducts. Quantum chemical calculations indicate that undercoordination of the Pt atoms in the clusters is responsible for the surprisingly high reactivity compared with extended surfaces. We anticipate that these results will form the basis for development of a new class of catalysts by providing a route to bond-specific chemistry, ranging from energy-efficient and environmentally friendly synthesis strategies to the replacement of petrochemical feedstocks by abundant small alkanes 10,11 .

The mechanistic studies of Pd-based catalysts and its interaction with methanol have attracted hu... more The mechanistic studies of Pd-based catalysts and its interaction with methanol have attracted huge attention because of the possibility of using methanol as an on-board source of hydrogen for fuel cells. Stabilizing subnanometer metal clusters is a challenging process that has exhibited novel catalytic properties for various industrially important reactions such as production of hydrogen from hydrogen-rich molecules. One such reaction is methanol decomposition that was modeled by applying DFT methods on metal clusters. The thermodynamics and kinetics of three decomposition routes involving C-O, C-H and O-H scission were investigated; activation energy barriers were determined with the nudged elastic band method on Pd clusters with a comparison to Co and Cu clusters. A detailed analysis of the PES for methanol decomposition shows C-O activation to be the least favorable step on all three metal clusters. However we find activation to be ˜0.30 eV smaller on Co cluster. In addition, estimated thermodynamical data for a large number of transition metals has been generated from linear correlations constructed from the binding energies of Pd, Cu and Co to broaden our understanding of the role such metal clusters can play as catalyst for such reactions.

Extensive experimental and theoretical work has been done to understand the decomposition of meth... more Extensive experimental and theoretical work has been done to understand the decomposition of methanol on various metal and metal oxide nanoparticles for hydrogen production. The activity of sub-nanometer sized particles < 1nm however is not very well known, primarily because of technical challenges involved in preparation and stabilization of the clusters. To explore the properties of the Pd clusters computationally, we have carried out density functional calculations for the methanol decomposition reaction on Pd4 and Pd8 clusters. The thermodynamics and kinetics of three decomposition routes involving C--O, C--H and O--H scission were investigated; activation energy barriers were determined with the nudged elastic band method. A detailed analysis of the PES for methanol decomposition shows C--O activation to be the least favorable step. In addition, all possible reaction paths for the Pd4 cluster are much lower in comparison to single crystal surface and large nanoparticles. To understand how particle size affects the elementary reaction steps, we also present a comparison of methanol decomposition on Pd4 with Pd8 clusters. Finally, we will discuss the implication of a linear correlation between the transition state and final state energies that is followed for all elementary reaction steps on Pd4 and Pd8 clusters.

Journal of Physical Chemistry C, 2009

A density functional theory study of the decomposition of methanol on subnanometer palladium clus... more A density functional theory study of the decomposition of methanol on subnanometer palladium clusters (primarily Pd 4 ) is presented. Methanol dehydrogenation through C−H bond breaking to form hydroxymethyl (CH 2 OH) as the initial step, followed by steps involving formation ...

Journal of Physical Chemistry C, 2010

Size and support effects in the oxidative decomposition of methanol on amorphous alumina supporte... more Size and support effects in the oxidative decomposition of methanol on amorphous alumina supported subnanometer palladium clusters were studied under realistic reaction conditions of pressure and temperature. The smaller Pd 8-12 clusters were found to promote the decomposition channel to CO and hydrogen, however with mediocre activity due to poisoning. The larger Pd 15-18 clusters preferentially produce dimethyl ether and formaldehyde, without signs of posioning. A thin titania overcoat applied on the Pd 15-18 improves the sintering-resistance of the catalyst. Accompanying density functional calculations confirm the posioning of small Pd clusters by CO.

We present results of a detailed study of the vibrational dynamics and thermodynamics of disorder... more We present results of a detailed study of the vibrational dynamics and thermodynamics of disordered Cu_3Au surfaces for structural configurations determined by Monte Carlo simulations at temperatures between 0.4 and 1.6 Tc (the bulk order-disorder transition temperature). Vibrational dynamics of the systems are determined using a real space Green's function method with interaction potentials from the embedded atom method. Thermodynamic functions are calculated in the harmonic approximation. We find the layer-by-layer segregation profiles of each of the three low Miller index surfaces to change in characteristic manners with increasing temperature, thereby leading to striking differences in the local vibrational density of states for each surface. Phonons for Au and Cu atoms are thus calculated for a large variety of surroundings. The role of vibrational entropy in the order-disorder transition for each surface is discussed.

Our ab initio calculations of CO adsorption on several low and high Miller index surfaces of Cu s... more Our ab initio calculations of CO adsorption on several low and high Miller index surfaces of Cu show that the adsorption energy increases as the coordination of the adsorption site decreases from 9 to 6, in qualitative agreement with experimental observations. On each surface the adsorption energy is also found to decrease with increase in coverage, although the decrement is not uniform. Calculated vibrational properties show an increase in the frequency of the metal-C mode with decrease in coordination, whereas no such effect is found for the frequency of the CO stretch mode. Examination of the surface electronic structure shows a strong local effect of CO adsorption on the local density of state of the substrate atoms. We also provide some energetics of CO diffusion on Cu(111) and Cu(211).

Physical Review B, 2009

Our ab initio calculations of CO adsorption energies on low miller index (111), (100), stepped (2... more Our ab initio calculations of CO adsorption energies on low miller index (111), (100), stepped (211), and kinked (532) gold surfaces show a strong dependence on local coordination with a reduction in Au atom coordination leading to higher binding energies. We find trends in adsorption energies to be similar to those reported in experiments and calculations for other metal surfaces. The (532) surface provides insights into these trends because of the availability of a large number of kink sites which naturally have the lowest coordination (6). We also find that, for all surfaces, an increase in CO coverage triggers a decrease in the adsorption energy. Changes in the workfunction upon CO adsorption, as well as the frequencies of the CO vibrational modes are calculated, and their coverage dependence is reported.

The chemisorption of CO on Cu surfaces is regarded as a prototype system to understand molecular ... more The chemisorption of CO on Cu surfaces is regarded as a prototype system to understand molecular adsorption on transition metals surfaces. To understand the observed trends^1 in the site specific (flat, stepped, and kinked) adsorption energies of CO on Cu surfaces, we have performed a density functional theory based first principle electronic structure calculations on several low and high Miller index surfaces of Cu with CO adsorbed on-top site. Our calculated values show that adsorption energies increase with decrease in the local coordination of CO with the substrate, as seen in the experiment, although this dependence is not trivial. For in depth understanding of this, we have made a detailed study of electronic structure of these systems. Our calculated vibrational frequencies of CO-molecule on these surfaces are very similar, as expected. However, the frequency of the CO-metal stretch mode shows a small increase for the lower coordinated surfaces as compared to that on the low Miller index surfaces. Similar trends are also found for workfunctions, charge densities and the local density of states. Results will also be presented for CO adsorption on Au(111), Au(211), Au(532) and the behavior compared to that on the Cu surfaces. This work is supported in part by NSF Grant No. CHE0205064. ^1S. Vollmer, G. Witte, C. Wöll, Cat. Lett., 77, 97 (2001).

Journal of Physics-condensed Matter, 2006

We have performed ab initio electronic structure calculations of C and S adsorption on two vicina... more We have performed ab initio electronic structure calculations of C and S adsorption on two vicinal surfaces of Pd with different terrace geometry and width. We find both adsorbates to induce a significant perturbation of the surface electronic and geometric structure of Pd(533) and Pd(320). In particular C adsorbed at the bridge site at the edge of a Pd chain in Pd(320) is found to penetrate the surface to form a sub-surface structure. The adsorption energies show almost linear dependence on the number of adsorbate-metal bonds, and lie in the ranges of 5.31eV to 8.58eV for C and 2.89eV to 5.40eV for S. A strong hybridization between adsorbate and surface electronic states causes a large splitting of the bands leading to a drastic decrease in the local densities of electronic states at the Fermi-level for Pd surface atoms neighboring the adsorbate which may poison catalytic activity of the surface. Comparison of the results for Pd(533) with those obtained earlier for Pd(211) suggests the local character of the impact of the adsorbate on the geometric and electronic structures of Pd surfaces.

In the course of catalytic oxidation of CO, carbon may atomically adsorb on catalyst surface and ... more In the course of catalytic oxidation of CO, carbon may atomically adsorb on catalyst surface and change (presumably poison) its reactivity. On the other hand, the reactivity of many catalysts is enhanced upon alkali co-adsorption. To gain insight into the nature of these effects, we carried out ab initio electronic structure calculations based on density functional theory with the generalized gradient approximation. The energetics and reaction pathways for CO oxidation on the clean Pd(111) and on Pd(111) co-adsorbed with C or K are calculated. We find that even at coverages as small as 1/12 ML, co-adsorbed C increases the activation energy barrier for the reaction by 20%, while K reduces it by 10%. This clearly shows the role of K as a promoter and C as a poison for this catalytic reaction. To understand the microscopic mechanism of these phenomena, we analyze the effects of the co-adsorbate coverage and the CO --- co-adsorbate distance on the activation energy barriers and the local densities of electronic states and valence charge densities calculated for the initial and transition states of the reaction.

Physical Review B, 2006

Our ab initio calculations of CO adsorption on several low and high Miller index surfaces of Cu s... more Our ab initio calculations of CO adsorption on several low and high Miller index surfaces of Cu show that the adsorption energy increases as the coordination of the adsorption site decreases from 9 to 6, in qualitative agreement with experimental observations. On each surface the adsorption energy is also found to decrease with increase in coverage, although the decrement is not uniform. Calculated vibrational properties show an increase in the frequency of the metal-C mode with decrease in coordination, but no such effect is found for the frequency of the CO stretch mode. Examination of the surface electronic structure shows CO adsorption to have a strong effect on the local density of state of the substrate atoms. We also report calculated energetics of CO diffusion on Cu(111) and Cu(211) .

Applied Physics A-materials Science & Processing, 2007

We present results of ab initio electronic structure calculations based on density functional the... more We present results of ab initio electronic structure calculations based on density functional theory which show in detail several effects of alkali adsorption on metal substrates and on molecules coadsorbed on the substrate. First, calculations of the isoelectronic reactivity index demonstrate a dramatic enhancement of the electronic polarizability of the metal substrate extending it several angstroms into the vacuum. This phenomenon is traceable to an unusual feature induced in the surface potential on alkali adsorption. The effect appears to be general and helps explain the observed substantial decrease in the vibrational frequency of molecules such as CO and O2 when co-adsorbed with alkalis on metal surfaces. Next, for the oxidation of CO on Pd(111), we illustrate the changes in the reaction pathway and activation energy barriers induced in the presence of coadsorbed K.

Physical Review B, 2009

Our ab initio calculations of CO adsorption energies on low miller index (111), (100), stepped (2... more Our ab initio calculations of CO adsorption energies on low miller index (111), (100), stepped (211), and kinked (532) gold surfaces show a strong dependence on local coordination with a reduction in Au atom coordination leading to higher binding energies. We find trends in adsorption energies to be similar to those reported in experiments and calculations for other metal surfaces. The (532) surface provides insights into these trends because of the availability of a large number of kink sites which naturally have the lowest coordination (6). We also find that, for all surfaces, an increase in CO coverage triggers a decrease in the adsorption energy. Changes in the workfunction upon CO adsorption, as well as the frequencies of the CO vibrational modes are calculated, and their coverage dependence is reported.

Physical Review B, 2004

Results of ab initio electronic structure calculations of C on Pd(211), a vicinal of Pd(111), sho... more Results of ab initio electronic structure calculations of C on Pd(211), a vicinal of Pd(111), show a hierarchy of adsorption sites with adsorption energy ranging from −8.21 to − 5.46 eV (0.33 ML coverage) and scaling almost linearly with the effective coordination at the site. In the most preferred site, C atoms sit almost under the Pd step atoms and influence drastically multilayer relaxations and surface registry of Pd(211). The adsorption energy at this site is −9.10 eV for 0.17 ML coverage. The local densities of electronic states indicate strong hybridization between C p states and neighboring Pd d states leading to the formation of strong covalent C u Pd bonds. The depletion of the electronic density of states of the Pd surface atoms at the Fermi level suggests a poisoning effect of C which is found to be coverage dependent. We compare the changes in the electronic structure of Pd(211) on C adsorption with those on S adsorption.

Surface Science, 2006

We have performed ab initio electronic structure calculations of C and S adsorption on two vicina... more We have performed ab initio electronic structure calculations of C and S adsorption on two vicinal surfaces of Pd with different terrace geometry and width. We find both adsorbates to induce a significant perturbation of the surface electronic and geometric structure of Pd(533) and Pd(320). In particular C adsorbed at the bridge site at the edge of a Pd chain in Pd(320) is found to penetrate the surface to form a sub-surface structure. The adsorption energies show almost linear dependence on the number of adsorbate-metal bonds, and lie in the ranges of 5.31eV to 8.58eV for C and 2.89eV to 5.40eV for S. A strong hybridization between adsorbate and surface electronic states causes a large splitting of the bands leading to a drastic decrease in the local densities of electronic states at the Fermi-level for Pd surface atoms neighboring the adsorbate which may poison catalytic activity of the surface. Comparison of the results for Pd(533) with those obtained earlier for Pd(211) suggests the local character of the impact of the adsorbate on the geometric and electronic structures of Pd surfaces.

Science, 2010

Production of the industrial chemical propylene oxide is energy-intensive and environmentally unf... more Production of the industrial chemical propylene oxide is energy-intensive and environmentally unfriendly. Catalysts based on bulk silver surfaces with direct propylene epoxidation by molecular oxygen have not resolved these problems because of substantial formation of carbon dioxide. We found that unpromoted, size-selected Ag3 clusters and approximately 3.5-nanometer Ag nanoparticles on alumina supports can catalyze this reaction with only a negligible amount of carbon dioxide formation and with high activity at low temperatures. Density functional calculations show that, relative to extended silver surfaces, oxidized silver trimers are more active and selective for epoxidation because of the open-shell nature of their electronic structure. The results suggest that new architectures based on ultrasmall silver particles may provide highly efficient catalysts for propylene epoxidation.

Density functional theory (DFT) calculations were carried out to investigate the fundamental chem... more Density functional theory (DFT) calculations were carried out to investigate the fundamental chemical mechanisms underlying the selective oxidation of propene (CH3--CH=CH2) to propylene oxide (CH3--CH--CH2O) or acrolein (CH2=CH--CHO). Silver which is a known catalyst for these reactions, is modeled as a silver trimer supported on θ-Al2O3 surface to investigate reaction mechanisms. Each reaction step starts with O2 dissociation on interface of Ag3 and alumina with an activation barrier that was found to be significantly smaller than what was reported on an Ag(111) surface. Propylene oxide formation was achieved through oxametallocyle formation with a much small barrier of 0.12 eV compared to 0.70 eV on Ag(111). We will also show how acrolein formation can be triggered by the abstraction of first H atom with essentially no apparent barrier on a Ag3 cluster. Finally, we rationalize our results in comparison to experiments that show an enormous increase in reactivity for propylene epoxidation on subnanometer silver cluster. We will also discuss the preference of Ag trimers over tetramers for O2 dissociation and how the microscopic understanding of such information can help better design new catalysts.

Journal of Physical Chemistry B, 2010

A density functional theory study of the decomposition of methanol on Cu(4) and Co(4) clusters is... more A density functional theory study of the decomposition of methanol on Cu(4) and Co(4) clusters is presented. The reaction intermediates and activation barriers have been determined for reaction steps to form H(2) and CO. For both clusters, methanol decomposition initiated by C-H and O-H bond breaking was investigated. In the case of a Cu(4) cluster, methanol dehydrogenation through hydroxymethyl (CH(2)OH), hydroxymethylene (CHOH), formyl (CHO), and carbon monoxide (CO) is found to be slightly more favorable. For a Co(4) cluster, the dehydrogenation pathway through methoxy (CH(3)O) and formaldehyde (CH(2)O) is slightly more favorable. Each of these pathways results in formation of CO and H(2). The Co cluster pathway is very favorable thermodynamically and kinetically for dehydrogenation. However, since CO binds strongly, it is likely to poison methanol decomposition to H(2) and CO at low temperatures. In contrast, for the Cu cluster, CO poisoning is not likely to be a problem since it does not bind strongly, but the dehydrogenation steps are not energetically favorable. Pathways involving C-O bond cleavage are even less energetically favorable. The results are compared to our previous study of methanol decomposition on Pd(4) and Pd(8) clusters. Finally, all reaction energy changes and transition state energies, including those for the Pd clusters, are related in a linear, Brønsted-Evans-Polanyi plot.

Nature Materials, 2009

Small clusters are known to possess reactivity not observed in their bulk analogues, which can ma... more Small clusters are known to possess reactivity not observed in their bulk analogues, which can make them attractive for catalysis 1-6 . Their distinct catalytic properties are often hypothesized to result from the large fraction of undercoordinated surface atoms 7-9 . Here, we show that sizepreselected Pt 8−10 clusters stabilized on high-surface-area supports are 40-100 times more active for the oxidative dehydrogenation of propane than previously studied platinum and vanadia catalysts, while at the same time maintaining high selectivity towards formation of propylene over byproducts. Quantum chemical calculations indicate that undercoordination of the Pt atoms in the clusters is responsible for the surprisingly high reactivity compared with extended surfaces. We anticipate that these results will form the basis for development of a new class of catalysts by providing a route to bond-specific chemistry, ranging from energy-efficient and environmentally friendly synthesis strategies to the replacement of petrochemical feedstocks by abundant small alkanes 10,11 .

Nature Materials, 2009

Small clusters are known to possess reactivity not observed in their bulk analogues, which can ma... more Small clusters are known to possess reactivity not observed in their bulk analogues, which can make them attractive for catalysis 1-6 . Their distinct catalytic properties are often hypothesized to result from the large fraction of undercoordinated surface atoms 7-9 . Here, we show that sizepreselected Pt 8−10 clusters stabilized on high-surface-area supports are 40-100 times more active for the oxidative dehydrogenation of propane than previously studied platinum and vanadia catalysts, while at the same time maintaining high selectivity towards formation of propylene over byproducts. Quantum chemical calculations indicate that undercoordination of the Pt atoms in the clusters is responsible for the surprisingly high reactivity compared with extended surfaces. We anticipate that these results will form the basis for development of a new class of catalysts by providing a route to bond-specific chemistry, ranging from energy-efficient and environmentally friendly synthesis strategies to the replacement of petrochemical feedstocks by abundant small alkanes 10,11 .

The mechanistic studies of Pd-based catalysts and its interaction with methanol have attracted hu... more The mechanistic studies of Pd-based catalysts and its interaction with methanol have attracted huge attention because of the possibility of using methanol as an on-board source of hydrogen for fuel cells. Stabilizing subnanometer metal clusters is a challenging process that has exhibited novel catalytic properties for various industrially important reactions such as production of hydrogen from hydrogen-rich molecules. One such reaction is methanol decomposition that was modeled by applying DFT methods on metal clusters. The thermodynamics and kinetics of three decomposition routes involving C-O, C-H and O-H scission were investigated; activation energy barriers were determined with the nudged elastic band method on Pd clusters with a comparison to Co and Cu clusters. A detailed analysis of the PES for methanol decomposition shows C-O activation to be the least favorable step on all three metal clusters. However we find activation to be ˜0.30 eV smaller on Co cluster. In addition, estimated thermodynamical data for a large number of transition metals has been generated from linear correlations constructed from the binding energies of Pd, Cu and Co to broaden our understanding of the role such metal clusters can play as catalyst for such reactions.

Extensive experimental and theoretical work has been done to understand the decomposition of meth... more Extensive experimental and theoretical work has been done to understand the decomposition of methanol on various metal and metal oxide nanoparticles for hydrogen production. The activity of sub-nanometer sized particles < 1nm however is not very well known, primarily because of technical challenges involved in preparation and stabilization of the clusters. To explore the properties of the Pd clusters computationally, we have carried out density functional calculations for the methanol decomposition reaction on Pd4 and Pd8 clusters. The thermodynamics and kinetics of three decomposition routes involving C--O, C--H and O--H scission were investigated; activation energy barriers were determined with the nudged elastic band method. A detailed analysis of the PES for methanol decomposition shows C--O activation to be the least favorable step. In addition, all possible reaction paths for the Pd4 cluster are much lower in comparison to single crystal surface and large nanoparticles. To understand how particle size affects the elementary reaction steps, we also present a comparison of methanol decomposition on Pd4 with Pd8 clusters. Finally, we will discuss the implication of a linear correlation between the transition state and final state energies that is followed for all elementary reaction steps on Pd4 and Pd8 clusters.

Journal of Physical Chemistry C, 2009

A density functional theory study of the decomposition of methanol on subnanometer palladium clus... more A density functional theory study of the decomposition of methanol on subnanometer palladium clusters (primarily Pd 4 ) is presented. Methanol dehydrogenation through C−H bond breaking to form hydroxymethyl (CH 2 OH) as the initial step, followed by steps involving formation ...

Journal of Physical Chemistry C, 2010

Size and support effects in the oxidative decomposition of methanol on amorphous alumina supporte... more Size and support effects in the oxidative decomposition of methanol on amorphous alumina supported subnanometer palladium clusters were studied under realistic reaction conditions of pressure and temperature. The smaller Pd 8-12 clusters were found to promote the decomposition channel to CO and hydrogen, however with mediocre activity due to poisoning. The larger Pd 15-18 clusters preferentially produce dimethyl ether and formaldehyde, without signs of posioning. A thin titania overcoat applied on the Pd 15-18 improves the sintering-resistance of the catalyst. Accompanying density functional calculations confirm the posioning of small Pd clusters by CO.

We present results of a detailed study of the vibrational dynamics and thermodynamics of disorder... more We present results of a detailed study of the vibrational dynamics and thermodynamics of disordered Cu_3Au surfaces for structural configurations determined by Monte Carlo simulations at temperatures between 0.4 and 1.6 Tc (the bulk order-disorder transition temperature). Vibrational dynamics of the systems are determined using a real space Green's function method with interaction potentials from the embedded atom method. Thermodynamic functions are calculated in the harmonic approximation. We find the layer-by-layer segregation profiles of each of the three low Miller index surfaces to change in characteristic manners with increasing temperature, thereby leading to striking differences in the local vibrational density of states for each surface. Phonons for Au and Cu atoms are thus calculated for a large variety of surroundings. The role of vibrational entropy in the order-disorder transition for each surface is discussed.

Our ab initio calculations of CO adsorption on several low and high Miller index surfaces of Cu s... more Our ab initio calculations of CO adsorption on several low and high Miller index surfaces of Cu show that the adsorption energy increases as the coordination of the adsorption site decreases from 9 to 6, in qualitative agreement with experimental observations. On each surface the adsorption energy is also found to decrease with increase in coverage, although the decrement is not uniform. Calculated vibrational properties show an increase in the frequency of the metal-C mode with decrease in coordination, whereas no such effect is found for the frequency of the CO stretch mode. Examination of the surface electronic structure shows a strong local effect of CO adsorption on the local density of state of the substrate atoms. We also provide some energetics of CO diffusion on Cu(111) and Cu(211).

Physical Review B, 2009

Our ab initio calculations of CO adsorption energies on low miller index (111), (100), stepped (2... more Our ab initio calculations of CO adsorption energies on low miller index (111), (100), stepped (211), and kinked (532) gold surfaces show a strong dependence on local coordination with a reduction in Au atom coordination leading to higher binding energies. We find trends in adsorption energies to be similar to those reported in experiments and calculations for other metal surfaces. The (532) surface provides insights into these trends because of the availability of a large number of kink sites which naturally have the lowest coordination (6). We also find that, for all surfaces, an increase in CO coverage triggers a decrease in the adsorption energy. Changes in the workfunction upon CO adsorption, as well as the frequencies of the CO vibrational modes are calculated, and their coverage dependence is reported.

The chemisorption of CO on Cu surfaces is regarded as a prototype system to understand molecular ... more The chemisorption of CO on Cu surfaces is regarded as a prototype system to understand molecular adsorption on transition metals surfaces. To understand the observed trends^1 in the site specific (flat, stepped, and kinked) adsorption energies of CO on Cu surfaces, we have performed a density functional theory based first principle electronic structure calculations on several low and high Miller index surfaces of Cu with CO adsorbed on-top site. Our calculated values show that adsorption energies increase with decrease in the local coordination of CO with the substrate, as seen in the experiment, although this dependence is not trivial. For in depth understanding of this, we have made a detailed study of electronic structure of these systems. Our calculated vibrational frequencies of CO-molecule on these surfaces are very similar, as expected. However, the frequency of the CO-metal stretch mode shows a small increase for the lower coordinated surfaces as compared to that on the low Miller index surfaces. Similar trends are also found for workfunctions, charge densities and the local density of states. Results will also be presented for CO adsorption on Au(111), Au(211), Au(532) and the behavior compared to that on the Cu surfaces. This work is supported in part by NSF Grant No. CHE0205064. ^1S. Vollmer, G. Witte, C. Wöll, Cat. Lett., 77, 97 (2001).

Journal of Physics-condensed Matter, 2006

We have performed ab initio electronic structure calculations of C and S adsorption on two vicina... more We have performed ab initio electronic structure calculations of C and S adsorption on two vicinal surfaces of Pd with different terrace geometry and width. We find both adsorbates to induce a significant perturbation of the surface electronic and geometric structure of Pd(533) and Pd(320). In particular C adsorbed at the bridge site at the edge of a Pd chain in Pd(320) is found to penetrate the surface to form a sub-surface structure. The adsorption energies show almost linear dependence on the number of adsorbate-metal bonds, and lie in the ranges of 5.31eV to 8.58eV for C and 2.89eV to 5.40eV for S. A strong hybridization between adsorbate and surface electronic states causes a large splitting of the bands leading to a drastic decrease in the local densities of electronic states at the Fermi-level for Pd surface atoms neighboring the adsorbate which may poison catalytic activity of the surface. Comparison of the results for Pd(533) with those obtained earlier for Pd(211) suggests the local character of the impact of the adsorbate on the geometric and electronic structures of Pd surfaces.

In the course of catalytic oxidation of CO, carbon may atomically adsorb on catalyst surface and ... more In the course of catalytic oxidation of CO, carbon may atomically adsorb on catalyst surface and change (presumably poison) its reactivity. On the other hand, the reactivity of many catalysts is enhanced upon alkali co-adsorption. To gain insight into the nature of these effects, we carried out ab initio electronic structure calculations based on density functional theory with the generalized gradient approximation. The energetics and reaction pathways for CO oxidation on the clean Pd(111) and on Pd(111) co-adsorbed with C or K are calculated. We find that even at coverages as small as 1/12 ML, co-adsorbed C increases the activation energy barrier for the reaction by 20%, while K reduces it by 10%. This clearly shows the role of K as a promoter and C as a poison for this catalytic reaction. To understand the microscopic mechanism of these phenomena, we analyze the effects of the co-adsorbate coverage and the CO --- co-adsorbate distance on the activation energy barriers and the local densities of electronic states and valence charge densities calculated for the initial and transition states of the reaction.

Physical Review B, 2006

Our ab initio calculations of CO adsorption on several low and high Miller index surfaces of Cu s... more Our ab initio calculations of CO adsorption on several low and high Miller index surfaces of Cu show that the adsorption energy increases as the coordination of the adsorption site decreases from 9 to 6, in qualitative agreement with experimental observations. On each surface the adsorption energy is also found to decrease with increase in coverage, although the decrement is not uniform. Calculated vibrational properties show an increase in the frequency of the metal-C mode with decrease in coordination, but no such effect is found for the frequency of the CO stretch mode. Examination of the surface electronic structure shows CO adsorption to have a strong effect on the local density of state of the substrate atoms. We also report calculated energetics of CO diffusion on Cu(111) and Cu(211) .

Applied Physics A-materials Science & Processing, 2007

We present results of ab initio electronic structure calculations based on density functional the... more We present results of ab initio electronic structure calculations based on density functional theory which show in detail several effects of alkali adsorption on metal substrates and on molecules coadsorbed on the substrate. First, calculations of the isoelectronic reactivity index demonstrate a dramatic enhancement of the electronic polarizability of the metal substrate extending it several angstroms into the vacuum. This phenomenon is traceable to an unusual feature induced in the surface potential on alkali adsorption. The effect appears to be general and helps explain the observed substantial decrease in the vibrational frequency of molecules such as CO and O2 when co-adsorbed with alkalis on metal surfaces. Next, for the oxidation of CO on Pd(111), we illustrate the changes in the reaction pathway and activation energy barriers induced in the presence of coadsorbed K.

Physical Review B, 2009

Our ab initio calculations of CO adsorption energies on low miller index (111), (100), stepped (2... more Our ab initio calculations of CO adsorption energies on low miller index (111), (100), stepped (211), and kinked (532) gold surfaces show a strong dependence on local coordination with a reduction in Au atom coordination leading to higher binding energies. We find trends in adsorption energies to be similar to those reported in experiments and calculations for other metal surfaces. The (532) surface provides insights into these trends because of the availability of a large number of kink sites which naturally have the lowest coordination (6). We also find that, for all surfaces, an increase in CO coverage triggers a decrease in the adsorption energy. Changes in the workfunction upon CO adsorption, as well as the frequencies of the CO vibrational modes are calculated, and their coverage dependence is reported.

Physical Review B, 2004

Results of ab initio electronic structure calculations of C on Pd(211), a vicinal of Pd(111), sho... more Results of ab initio electronic structure calculations of C on Pd(211), a vicinal of Pd(111), show a hierarchy of adsorption sites with adsorption energy ranging from −8.21 to − 5.46 eV (0.33 ML coverage) and scaling almost linearly with the effective coordination at the site. In the most preferred site, C atoms sit almost under the Pd step atoms and influence drastically multilayer relaxations and surface registry of Pd(211). The adsorption energy at this site is −9.10 eV for 0.17 ML coverage. The local densities of electronic states indicate strong hybridization between C p states and neighboring Pd d states leading to the formation of strong covalent C u Pd bonds. The depletion of the electronic density of states of the Pd surface atoms at the Fermi level suggests a poisoning effect of C which is found to be coverage dependent. We compare the changes in the electronic structure of Pd(211) on C adsorption with those on S adsorption.

Surface Science, 2006

We have performed ab initio electronic structure calculations of C and S adsorption on two vicina... more We have performed ab initio electronic structure calculations of C and S adsorption on two vicinal surfaces of Pd with different terrace geometry and width. We find both adsorbates to induce a significant perturbation of the surface electronic and geometric structure of Pd(533) and Pd(320). In particular C adsorbed at the bridge site at the edge of a Pd chain in Pd(320) is found to penetrate the surface to form a sub-surface structure. The adsorption energies show almost linear dependence on the number of adsorbate-metal bonds, and lie in the ranges of 5.31eV to 8.58eV for C and 2.89eV to 5.40eV for S. A strong hybridization between adsorbate and surface electronic states causes a large splitting of the bands leading to a drastic decrease in the local densities of electronic states at the Fermi-level for Pd surface atoms neighboring the adsorbate which may poison catalytic activity of the surface. Comparison of the results for Pd(533) with those obtained earlier for Pd(211) suggests the local character of the impact of the adsorbate on the geometric and electronic structures of Pd surfaces.